Standby power supply containing virtual energy storage photovoltaic and method for supporting direct current system voltage

Abstract

The invention discloses a standby power supply containing a virtual energy storage photovoltaic and a method for supporting a direct current system voltage. The standby power supply comprises a photovoltaic power generation assembly, a hybrid energy storage device, a direct current bus, a direct current bus voltage detection device, a voltage and current acquisition device and a distributed control device. When the voltage of the substation DC system is detected to exceed the limit, the DC bus is connected with the bus of the DC system, and the standby power supply supplies power to the fixed load and the impact load through the DC bus; and the distributed control device sends control signals to the power converter and the bidirectional DC-DC power converter respectively, so that a dual-stage composite control strategy for automatically switching a following network type and constructing network type control according to the direct current bus voltage of the standby power supply is realized, and the capacity configuration of hybrid energy storage is reduced while the voltage of a direct current system is supported.

Description

Technical Field

[0001] The present invention belongs to the technical field of DC system voltage control, and more specifically, relates to a standby power supply including a virtual energy storage photovoltaic and a method for supporting the voltage of a DC system by the same. Background Art

[0002] In a substation of a power system, the DC system is the power supply for key equipment, and the safety, reliability and stability of its power supply are crucial. The DC system of a substation is mainly responsible for providing power support for control signals, relay protection devices, automation equipment, and the operation of circuit breakers. These DC loads are complex, have a wide range of variations, and equipment such as circuit breakers are high-power impact loads, requiring the DC system to maintain voltage stability under high-power impact loads. At present, the DC systems of many 10 - 220 kV substations usually use 220 V lead-acid battery standby power supplies. As the service life increases, individual lead-acid batteries have problems of aging and serious damage, resulting in insufficient voltage support capacity of the entire battery pack. When dealing with relatively large impact loads of the DC system, especially in response to the large closing power of electromagnetic circuit breakers, it is very easy to cause a short-term voltage loss of the DC system, affecting the power supply reliability of the DC system.

[0003] When a standby power supply composed of a battery pack fails, a set of standby power supply systems needs to be quickly put into use to restore power supply to the DC system of the substation and support the DC bus voltage. Currently, the DC system standby power supplies mainly include three types: battery standby power supply, supercapacitor standby power supply, and supercapacitor-battery hybrid energy storage standby power supply. When responding to the impact load of the DC system, in order to prevent the bus voltage from dropping below the lower limit, the capacity of the battery is usually configured to be relatively high, resulting in a relatively high cost of the standby power supply. Although the supercapacitor standby power supply can quickly respond to impact loads, the low energy density of the supercapacitor causes its voltage to drop too fast without charging, being insufficient to support the bus voltage within a long time range. The supercapacitor-battery hybrid energy storage standby power supply often uses a first-order low-pass filter or a second-order low-pass filter to distribute high-frequency power to the supercapacitor and low-frequency power to the battery, but this centralized control process is complex and requires a high communication requirement, and it is necessary to solve the power distribution and the capacity optimization configuration problem between each energy storage element. Moreover, the existing methods all design standby power supplies using traditional energy storage elements, without considering adding clean energy such as photovoltaic power generation to jointly supply power to the DC system, resulting in low economy and being unable to charge the battery and the supercapacitor, which reduces the power supply reliability of the standby power supply to a certain extent. Moreover, the traditional photovoltaic power generation control strategy is MPPT grid-following control, and the grid-following control does not have the ability to adjust the bus voltage level. Summary of the Invention

[0004] To address the deficiencies in the existing technologies, the present invention provides a backup power supply with virtual energy storage photovoltaics and a method for supporting the DC system voltage. The backup power supply for the existing substation DC system is improved, and a new type of backup power supply combining photovoltaic power generation, batteries, and supercapacitors is designed. A two-stage composite control strategy of automatically switching between grid-following and grid-forming controls according to the DC bus voltage of the backup power supply is implemented, while supporting the DC system voltage and reducing the capacity configuration of the hybrid energy storage.

[0005] The present invention adopts the following technical solutions.

[0006] The present invention proposes a backup power supply with virtual energy storage photovoltaics, which supplies power to the fixed load and impact load of the DC system. It includes: photovoltaic power generation components, hybrid energy storage, DC bus, DC bus voltage detection device, voltage and current acquisition device, and distributed control device.

[0007] The photovoltaic power generation components are connected to the DC bus through a power converter, and the hybrid energy storage is connected to the DC bus through a bidirectional DC-DC power converter. The DC bus voltage detection device sends the detected DC bus voltage to the voltage and current acquisition device. When the voltage of the substation DC system is detected to be out of limit, the DC bus is connected to the bus of the DC system, and the backup power supply supplies power to the fixed load and impact load through the DC bus.

[0008] The voltage and current acquisition device sends the output voltage and current of the photovoltaic power generation components and the power converter, and the output voltage and current of the hybrid energy storage and the bidirectional DC-DC power converter to the distributed control device, and the distributed control device sends control signals to the power converter and the bidirectional DC-DC power converter respectively.

[0009] When the backup power supply is not put into operation and the photovoltaic power generation components charge the hybrid energy storage, the power converter of the photovoltaic power generation components adopts grid-following control, and the grid-following control includes MPPT control.

[0010] When the backup power supply is put into operation, the power converter of the photovoltaic power generation components adopts grid-forming control, and the grid-forming control includes droop control based on virtual resistance.

[0011] The hybrid energy storage includes a battery and a supercapacitor.

[0012] The battery and the supercapacitor are respectively connected to the DC bus through corresponding bidirectional DC-DC power converters.

[0013] When the backup power supply is put into operation, the bidirectional DC-DC power converter of the storage battery adopts droop control based on virtual resistance, and the virtual resistance in the droop control of the bidirectional DC-DC power converter of the storage battery is equal to the virtual resistance in the droop control of the power converter of the photovoltaic power generation module. The storage battery and the photovoltaic power generation module supply power to the regular load and accident load of the DC system.

[0014] When the backup power supply is put into operation, the bidirectional DC-DC power converter of the supercapacitor adopts droop control based on virtual capacitance to supply power to the impact load.

[0015] The power converter of the photovoltaic power generation module includes a boost converter and a buck converter; among them, the photovoltaic module is connected to the boost converter through a connecting wire, a photovoltaic-side inductor is connected in series on the positive connection line between the photovoltaic power generation module and the boost converter, and a photovoltaic-side capacitor is connected in parallel on the photovoltaic side of the boost converter; the boost converter and the buck converter are connected through a transmission line, and a transmission-line equivalent capacitor is connected in parallel between the positive and negative poles of the transmission line; the buck converter is connected to the DC bus through a connecting wire, an output-side inductor is connected in series on the connection line between the buck converter and the positive pole of the DC bus, and a DC-side capacitor is connected in parallel on the output side of the buck converter;

[0016] The output voltage of the photovoltaic power generation module is increased by the boost converter and then transmitted to the buck converter, and the buck converter reduces the voltage to the DC bus voltage.

[0017] The distributed control device includes a boost converter control device, and the boost converter control device adopts a three-loop control structure, including a photovoltaic-side voltage outer loop, a photovoltaic-side intermediate voltage loop, and a photovoltaic-side current inner loop;

[0018] In the photovoltaic-side voltage outer loop, the difference between the maximum power point reference operating voltage and the photovoltaic-side voltage is obtained as the transmission-line reference operating voltage after photovoltaic-side voltage proportional control and the first limiting control. Among them, the photovoltaic-side voltage is collected by a voltage transformer connected to the branch where the photovoltaic-side capacitor is located; in the photovoltaic-side intermediate voltage loop, the difference between the transmission-line reference operating voltage and the transmission-line voltage is obtained as the maximum power point reference operating current after transmission-line voltage proportional control and the second limiting control. Among them, the transmission-line voltage is collected by a voltage transformer connected to the branch where the transmission-line equivalent capacitor is located; in the photovoltaic-side current inner loop, the difference between the maximum power point reference operating current and the photovoltaic-side current is obtained as the duty ratio of the boost converter after photovoltaic-side current proportional control and the third limiting control. Among them, the photovoltaic-side current is collected by a current transformer connected to the branch where the photovoltaic-side inductor is located.

[0019] The distributed control device includes a buck converter control device, and the buck converter control device adopts a double-loop control structure, including an output-side voltage outer loop and an output-side current inner loop;

[0020] In the output - side voltage outer loop, the difference between the DC - bus reference voltage and the DC - bus voltage is processed through output - side voltage proportional control and the second limiting control to obtain the output reference current. Among them, the DC - bus voltage is collected by a voltage transformer connected to the branch where the DC - side capacitor is located. In the output - side current inner loop, the difference between the output reference current and the output current is processed through output - side current proportional control and the third limiting control to obtain the duty cycle of the buck converter. Among them, the output current is collected by a current transformer connected to the branch where the output - side inductor is located.

[0021] The upper limit of the first limiting control is 400, the lower limit is 0; the upper limit of the second limiting control is 100, the lower limit is - 100; the upper limit of the third limiting control is 1, the lower limit is - 1.

[0022] The bidirectional DC - DC power converter of the battery adopts droop control based on virtual resistance, including: the difference between the DC - bus voltage after passing through the virtual resistance and the DC - bus reference voltage is processed through the current - inner - loop PI controller to obtain the output reference current of the battery's power converter. The difference between the output reference current and the output current of the battery's power converter is processed through the voltage - outer - loop PI controller to obtain a voltage signal, and the voltage signal is modulated by PWM to obtain the duty cycle of the battery's power converter.

[0023] The bidirectional DC - DC power converter of the supercapacitor adopts droop control based on virtual capacitance, including: the difference between the DC - bus voltage after passing through the virtual resistance and the DC - bus reference voltage is processed through the current - inner - loop PI controller to obtain the output reference current of the supercapacitor's power converter. The difference between the output reference current and the output current of the supercapacitor's power converter is processed through the voltage - outer - loop PI controller to obtain a voltage signal, and the voltage signal is modulated by PWM to obtain the duty cycle of the supercapacitor's power converter.

[0024] The present invention also proposes a method for a standby power source containing virtual energy storage photovoltaic to support the DC - system voltage, including:

[0025] Obtain the virtual resistance in the droop control of the bidirectional DC - DC power converter of the battery and the power converter of the photovoltaic power generation component, the rated value and the allowable minimum value of the DC - system voltage; determine the turning value of the output current of the photovoltaic power generation component by using the allowable maximum value of the DC - system voltage drop and the virtual resistance; when the output current of the photovoltaic power generation component is less than the turning value and the DC - system voltage is greater than the allowable minimum value, the power converter of the photovoltaic power generation component adopts droop control based on virtual resistance; when the output current of the photovoltaic power generation component is not less than the turning value and the DC - system voltage is not greater than the allowable minimum value, the power converter of the photovoltaic power generation component adopts MPPT control;

[0026] Determine the design parameters of the photovoltaic power generation module based on the turning value of the output current of the photovoltaic power generation module; determine the closed-loop transfer function of the boost converter and the closed-loop transfer function of the buck converter based on the design parameters of the photovoltaic power generation module; optimize the control parameters of the boost converter and the buck converter based on the closed-loop transfer function with the minimum output current fluctuation amplitude of the photovoltaic power generation module as the optimization target;

[0027] Determine the transfer function of the bidirectional DC-DC power converter of the storage battery and the transfer function of the bidirectional DC-DC power converter of the super capacitor; based on the droop characteristic model of the standby power supply, allocate the load power of the DC system to the photovoltaic power generation module, the storage battery and the super capacitor according to the load type of the DC system; determine the virtual resistance and the virtual capacitance according to the output total current characteristics of the storage battery and the photovoltaic power generation module and the output current characteristics of the super capacitor; optimize the control parameters of the bidirectional DC-DC power converters of the storage battery and the super capacitor based on the closed-loop transfer function with the minimum output current fluctuation amplitude of the storage battery and the super capacitor as the optimization target.

[0028] Preferably, the closed-loop transfer function of the boost converter includes:

[0029]

[0030] In the formula, G ipv-dboost is the closed-loop transfer function of the output current and the duty ratio of the photovoltaic power generation module in the boost converter, V ref-mid is the reference working voltage of the transmission line, C mid is the equivalent capacitance of the transmission line, R mid is the equivalent resistance of the transmission line, D boost is the duty ratio of the boost converter, L pv is the inductor on the photovoltaic side;

[0031]

[0032] In the formula, G vmid-ipv is the closed-loop transfer function of the reference working voltage of the transmission line to the output current of the photovoltaic panel in the boost converter, V ref-pv is the reference working voltage of the maximum power point;

[0033]

[0034] In the formula, G vpv-vmid is the closed-loop transfer function of the output voltage of the photovoltaic panel to the reference working voltage of the transmission line in the boost converter,

[0035]

[0036] In the formula, G boostis the closed-loop transfer function of the overall boost part of the boost converter, K vpv is the photovoltaic-side voltage proportionality coefficient, K vmid is the transmission-line voltage proportionality coefficient, K ipv is the photovoltaic-side current proportionality coefficient.

[0037] Preferably, the closed-loop transfer function of the buck converter includes:

[0038]

[0039] In the formula, G iout-dbuck is the closed-loop transfer function of the output current to the duty cycle in the buck converter, R is the equivalent DC load of the backup power supply, C dc is the DC-side capacitor, L out is the output-side inductor;

[0040]

[0041] In the formula, G vmid-iout is the closed-loop transfer function of the transmission-line reference operating voltage to the output current in the buck converter;

[0042]

[0043] In the formula, G buck is the closed-loop transfer function of the buck converter, K vdc is the output-side voltage proportionality coefficient, K iout is the output-side current proportionality coefficient.

[0044] Preferably, the transfer function of the bidirectional DC-DC power converter of the battery includes:

[0045]

[0046] In the formula, G ibat is the transfer function of the current inner-loop PI controller in the power converter of the battery, k ipbat is the proportional control parameter of the current inner-loop control in the power converter of the battery, k iibat is the integral control parameter of the current inner-loop control in the power converter of the battery;

[0047]

[0048] In the formula, G vbat is the transfer function of the voltage outer-loop PI controller in the power converter of the battery, k vpbat is the proportional control parameter of the voltage outer-loop control in the power converter of the battery, k vibat is the integral control parameter of the voltage outer-loop control in the power converter of the battery;

[0049]

[0050] Wherein, G ibat-dbat is the closed-loop transfer function of the output current of the storage battery to the duty ratio of the power converter, D bat is the duty ratio of the power converter of the storage battery, L bat is the inductance value on the low-voltage side of the storage battery, C bat is the capacitance value on the high-voltage side of the storage battery, and R is the equivalent DC load of the backup power supply;

[0051]

[0052] Wherein, G vbat-ibat is the closed-loop transfer function of the output voltage of the storage battery to the output current, V ref-bat is the output reference voltage of the power converter of the storage battery, is the current value flowing through the inductor on the low-voltage side of the storage battery converter;

[0053]

[0054] Wherein, G bat is the transfer function of the bidirectional DC-DC power converter of the storage battery.

[0055] Preferably, the transfer function of the bidirectional DC-DC power converter of the supercapacitor includes:

[0056]

[0057] Wherein, G isc is the transfer function of the current inner-loop PI controller in the power converter of the supercapacitor, k ipsc is the proportional control parameter of the voltage outer-loop control in the power converter of the supercapacitor, k iisc is the integral control parameter of the current inner-loop control in the power converter of the supercapacitor;

[0058]

[0059] Wherein, G vsc is the transfer function of the voltage outer-loop PI controller in the power converter of the supercapacitor, k vpsc is the proportional control parameter of the voltage outer-loop control in the power converter of the supercapacitor, k visc is the integral control parameter of the voltage outer-loop control in the power converter of the supercapacitor;

[0060]

[0061] Wherein, G isc-dscis the closed-loop transfer function of the output voltage to the output current of the power converter for the supercapacitor, D sc is the duty cycle of the power converter for the supercapacitor, L sc is the inductance value on the low-voltage side of the supercapacitor, C sc is the capacitance value on the high-voltage side of the supercapacitor, R is the equivalent DC load of the backup power supply;

[0062]

[0063] In the formula, G vsc-isc is the closed-loop transfer function of the voltage on the high-voltage side of the supercapacitor to the output current of the supercapacitor, V ref-sc is the output reference voltage of the power converter for the supercapacitor, is the current flowing through the inductance on the low-voltage side of the supercapacitor;

[0064]

[0065] In the formula, G sc is the transfer function of the bidirectional DC-DC power converter for the supercapacitor.

[0066] Preferably, an equivalent circuit of the backup power supply is established, and various parameters in the equivalent circuit are determined based on the determined transfer function; based on the equivalent circuit, a droop characteristic model of the backup power supply is determined, satisfying the following relationship:

[0067] V dc = V ref - R vbat ·I bat

[0068]

[0069] V dc = V ref - R vpv ·I pv

[0070] I load = I bat + I sc + I pv

[0071] In the formula, V dc is the DC bus voltage, V ref is the DC system reference voltage, R vbat is the virtual resistance in the power converter of the battery, I bat is the output current of the power converter of the battery, C vsc is the virtual capacitance in the power converter of the supercapacitor, I sc is the output current of the power converter of the supercapacitor, Rvpv is the virtual resistance in the power converter of the photovoltaic power generation module, I pv is the output current of the photovoltaic power generation module, I load is the total load current of the DC system.

[0072] Preferably, the virtual resistances of the photovoltaic power generation module and the storage battery are in a parallel state, which is equivalent to a parallel virtual resistance R vbp , and the total output current of the storage battery and the photovoltaic power generation module and the output current characteristics of the supercapacitor respectively satisfy the following relational expressions:

[0073]

[0074] In the formula, I vbp is the total output current of the storage battery and the photovoltaic power generation module.

[0075] Preferably, the total output current characteristics of the storage battery and the photovoltaic power generation module are equivalent to a first-order low-pass filter, and the output current characteristics of the supercapacitor are equivalent to a first-order high-pass filter. The cut-off frequencies of both filters satisfy the following relational expressions:

[0076]

[0077] Taking ω C as the frequency division point, the parallel virtual resistance R vbp of the photovoltaic power generation module and the storage battery and the virtual capacitance C vsc of the supercapacitor are determined.

[0078] The present invention is also a terminal, including a processor and a storage medium; the storage medium is used for storing instructions; the processor is used for operating according to the instructions to execute the steps of the method.

[0079] The present invention is also a computer-readable storage medium, on which a computer program is stored, and when the program is executed by a processor, the steps of the method are implemented.

[0080] Compared with the prior art, the beneficial effects of the present invention at least include: The present invention proposes a backup power supply with virtual energy storage photovoltaic, and at the same time designs a two-stage photovoltaic composite control strategy for automatic switching between grid-following and grid-forming control. When the hybrid energy storage has insufficient power, the photovoltaic power generation components use the MPPT control technology to charge the hybrid energy storage, increasing the DC bus voltage of the backup power supply and quickly putting it into use, avoiding the problem of insufficient efficiency caused by self-discharge or too fast voltage drop of the supercapacitor. When the hybrid energy storage is fully charged and the DC bus voltage level of the backup power supply returns to the normal voltage fluctuation range of the DC system, the photovoltaic is regarded as "virtual energy storage" and uses droop control to play a voltage support role together with the hybrid energy storage to ensure the reliable operation of protection and tripping devices. When the novel backup power supply proposed by the present invention is put into use, the photovoltaic power generation components work in the grid-forming droop control stage. As "virtual energy storage", it not only saves the capacity configuration of the hybrid energy storage, but also enhances the voltage support ability of the backup power supply. Description of the Drawings

[0081] Figure 1 is a schematic diagram of the system structure of a backup power supply with virtual energy storage photovoltaic proposed by the present invention; Figure 1 The reference numerals in are described as follows: 1 - Photovoltaic power generation components, 2 - Storage battery, 3 - Supercapacitor, 4 - Power converter, 5 - First bidirectional DC-DC power converter, 6 - Second bidirectional DC-DC power converter, 7 - DC bus, 8 - DC bus voltage detection device, 9 - Voltage and current acquisition device, 10 - Distributed control device;

[0082] Figure 2 is an equivalent circuit diagram in which the photovoltaic power generation components are connected to the DC bus through a Boost converter and a Buck converter in the embodiment of the present invention;

[0083] Figure 3 is a schematic diagram of the three-loop control structure of the Boost converter control device proposed by the present invention;

[0084] Figure 4 is a schematic diagram of the double-loop control structure of the Buck converter control device proposed by the present invention;

[0085] Figure 5 is a droop control block diagram of the storage battery and the supercapacitor in the embodiment of the present invention;

[0086] Figure 6 is a volt-ampere characteristic curve diagram of the output of the photovoltaic power generation components in the embodiment of the present invention;

[0087] Figure 7 is an I-V curve diagram of the photovoltaic power generation output in the embodiment of the present invention;

[0088] Figure 8It is the P-V curve diagram of the photovoltaic power generation output in the embodiment of the present invention;

[0089] Figure 9 It is the simulation output characteristic curve diagram of the photovoltaic power generation system in the embodiment of the present invention;

[0090] Figure 10 It is the equivalent circuit diagram of the standby power supply in the embodiment of the present invention;

[0091] Figure 11 It is the power curve diagram of the regular load and the impact load in the embodiment of the present invention;

[0092] Figure 12 It is the schematic diagram of the DC bus voltage comparison in the embodiment of the present invention;

[0093] Figure 13 It is the schematic diagram of the power distribution of the hybrid energy storage standby power supply in the embodiment of the present invention;

[0094] Figure 14 It is the schematic diagram of the power distribution of the photovoltaic hybrid energy storage standby power supply in the embodiment of the present invention;

[0095] Figure 15 It is the schematic diagram of the breaker closing bus voltage comparison in the embodiment of the present invention;

[0096] Figure 16 It is the schematic diagram of the breaker closing power distribution in the embodiment of the present invention. Specific embodiments

[0097] To make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

[0098] The system structure of a standby power supply with virtual energy storage photovoltaic proposed by the present invention is as Figure 1 shown, including: photovoltaic power generation module 1, storage battery 2, super capacitor 3, power converter 4, first bidirectional DC-DC power converter 5, second bidirectional DC-DC power converter 6, DC bus 7, DC bus voltage detection device 8, voltage and current acquisition device 9, distributed control device 10; the photovoltaic power generation module is connected to the DC bus through the power converter, the storage battery is connected to the DC bus through the first bidirectional DC-DC power converter, and the super capacitor is connected to the DC bus through the second bidirectional DC-DC power converter;

[0099] The DC bus voltage detection device sends the detected DC bus voltage to the voltage and current acquisition device; when the voltage of the substation DC system is detected to be out of limit, the DC bus is connected to the bus of the DC system, and after the connection, the standby power supply supplies power to the fixed load and the impact load through the DC bus.

[0100] Meanwhile, the output currents of the photovoltaic power generation components, the storage battery and the supercapacitor are respectively sent to the voltage and current acquisition device; the voltage and current acquisition device provides input signals to the distributed control device, and the distributed control device respectively sends control signals to the power converter, the first bidirectional DC-DC power converter and the second bidirectional DC-DC power converter.

[0101] Based on the standby power supply with the hybrid energy storage form of the storage battery - supercapacitor, the present invention adds photovoltaic power generation components. When the hybrid energy storage has insufficient power and the voltage level of the standby power supply is lower than the normal fluctuation range of the DC bus voltage, the photovoltaic power generation components charge the hybrid energy storage.

[0102] When the standby power supply is put into use, if the photovoltaic power generation components continue to charge the hybrid energy storage, then at this time the photovoltaic power generation components have been in a low voltage and large current state, and due to the inability to match the DC bus voltage, the photovoltaic power generation components are disconnected from the DC bus, resulting in light abandonment and being unable to support the voltage of the standby power supply.

[0103] The loads of the substation DC system include regular loads, accident loads and impact loads. Among them, the regular loads and accident loads (such as the lighting load during an accident, etc.) are both small and relatively stable, and will not have a great impact on the DC bus voltage level.

[0104] An impact load refers to a load that causes a large impact on the DC system of a substation, which is likely to cause a sudden drop in the DC system voltage, exceeding the normal voltage range and directly affecting the stability of the DC system. For example, during an AC system fault, the reclosing operation of an electromagnetic circuit breaker is likely to impact the DC bus voltage level. The normal voltage fluctuation range of the 220V DC system in a substation is 85% to 110% of the rated voltage, that is, 187.5V to 242V. Therefore, when the DC system voltage exceeds the limit, the designed backup power supply should be able to handle impact loads such as circuit breaker closing, ensuring that the bus voltage is not lower than 187.5V. The instantaneous power of an electromagnetic circuit breaker during closing is large and the duration is short. When an electromagnetic circuit breaker in a 35kV substation closes, the maximum impact current during closing is about 100A and the closing time is about 0.1s. Suppose there is an AC system fault in the substation, and at this time the voltage of the original DC system is below the lower limit due to the aging or damage of lead-acid batteries and cannot provide sufficient power for the circuit breaker reclosing. Then it is necessary to put into the backup power supply system to achieve the reliable operation of impact loads such as circuit breaker closing. If a backup power supply with a limited capacity only uses batteries, it is difficult to provide power support for this impact load, and there is also a risk of voltage exceeding the limit. And there is a self-discharge phenomenon in supercapacitors. If the backup power supply only uses supercapacitors to support the DC voltage, the DC bus voltage of the backup power supply will decrease if it is not charged in time. Therefore, the backup power supply of the present invention adopts a hybrid energy storage form of supercapacitor-battery. The supercapacitor provides power for impact loads such as circuit breaker closing, and at the same time the battery maintains the stability of the backup power supply voltage.

[0105] The backup power supply with a hybrid energy storage form adopts virtual resistor-capacitor droop control. The impact load is mainly powered by the supercapacitor, while the low-frequency regular and accident loads are mainly powered by the battery. The supercapacitor has a long cycle service life and a high power density. Therefore, the droop control strategy of the virtual capacitor is adopted. When the system power fluctuates at a high frequency, such as when an electromagnetic circuit breaker closes, the virtual capacitor is equivalent to a short circuit and can quickly provide large current support for the DC bus impact load. When the impact load ends and only the regular fixed load remains in the system, the load is relatively stable at this time, and the virtual capacitor is equivalent to an open circuit. At this time, the discharge power of the supercapacitor gradually decreases to zero. The battery has the characteristics of high energy density and low power density, and has strong endurance. Therefore, it is suitable for powering loads with a long duration and relatively stable power. The droop control strategy of the virtual resistor can effectively divide the part of the small power fluctuation of the DC bus into the battery, reducing the charge and discharge times of the battery and extending its service life. A photovoltaic power generation component is added as "virtual energy storage" in the structural design of the hybrid energy storage backup power supply including the battery and the supercapacitor to cope with the impact of the substation DC system load on the bus voltage level and reduce the capacity configuration of the hybrid energy storage.

[0106] Specifically, in the backup power supply, the bidirectional DC-DC power converter for the storage battery adopts droop control based on virtual resistance to supply power to regular loads and accident loads, and the bidirectional DC-DC power converter for the supercapacitor adopts droop control based on virtual capacitance to supply power to impact loads;

[0107] When the backup power supply is not put into operation and the photovoltaic power generation module charges the storage battery and the supercapacitor, the power converter of the photovoltaic power generation module adopts grid-following control, and the grid-following control includes MPPT control to ensure that the output power of the photovoltaic power generation module reaches the maximum;

[0108] When the backup power supply is put into operation, the power converter of the photovoltaic power generation module adopts grid-forming control, and the grid-forming control includes droop control based on virtual resistance to effectively limit the output power and improve the voltage support ability; among them, the virtual resistance in the droop control of the bidirectional DC-DC power converter of the storage battery is equal to that of the power converter of the photovoltaic power generation module, and the storage battery and the photovoltaic power generation module jointly supply power to regular loads and accident loads; the power converter of the supercapacitor adopts droop control based on virtual capacitance, and the supercapacitor supplies power to impact loads;

[0109] The present invention combines MPPT control and droop control based on virtual resistance, which can give full play to the respective advantages of the two control strategies of grid-following and grid-forming, enabling the photovoltaic power generation module to simultaneously have the functions of adjusting the bus voltage and maximum power output to ensure stable power supply for the DC system. However, MPPT control and droop control based on virtual resistance belong to two independent control loops. Although the setting of the virtual resistance helps to suppress the sudden change of the output power, in actual operation, when the distributed controller switches between these two control loops, it may still cause large transient fluctuations and even trigger misoperation of the protection circuit, thus interfering with the smooth operation of the microgrid. The present invention unifies MPPT control and droop control based on virtual resistance into the same control loop, and realizes the maximum power output and current-limiting voltage-regulating functions of the photovoltaic power generation module without relying on communication components.

[0110] Droop control and MPPT control belong to different types of control. In the prior art, switches are often used to complete the switching between the two control loops, and there will be large transient fluctuations during the switching process. Therefore, the power converter of the photovoltaic power generation module proposed by the present invention includes a boost converter and a buck converter; in the embodiment, the boost converter adopts a Boost converter, and the buck converter adopts a Buck converter. The Boost converter and the Buck converter form a buck-boost Boost-Buck converter. The output voltage of the photovoltaic power generation module is increased by the boost converter and then transmitted to the buck converter, and the buck converter reduces the voltage to the DC bus voltage; the photovoltaic power generation module is connected to the DC bus through the Boost converter and the Buck converter in sequence, and the equivalent circuit is asFigure 2 As shown Figure 2 The dashed line in Figure 2 shows the transmission relationship of the signal; in pv , the photovoltaic module is connected to the Boost converter through a connecting wire, and a photovoltaic-side inductor L is connected in series on the positive connection wire between the photovoltaic power generation module and the Boost converter pv ; a photovoltaic-side capacitor C is connected in parallel on the photovoltaic side of the Boost converter mid ; the Boost converter and the Buck converter are connected through a transmission line, and an equivalent capacitance C of the transmission line is connected in parallel between the positive and negative poles of the transmission line out ; the Buck converter is connected to the DC bus through a connecting wire, and an output-side inductor L is connected in series on the connection wire between the Buck converter and the positive pole of the DC bus dc .

[0111] In the present invention, a photovoltaic power generation module is added to the backup power supply of the substation DC system, and a grid-following type and a grid-forming type control are designed to automatically switch to support the DC system bus voltage; when the backup power supply is put into use, the photovoltaic power generation module and the hybrid energy storage jointly support the DC bus voltage as a "virtual energy storage"; a three-loop and two-loop control of the Boost-Buck buck-boost converter is adopted to realize a shockless automatic switch between the grid-following type control and the grid-forming type control; a two-stage automatic switch is realized through a control including only a proportional and a limiting link, without adding a differential link, so as to avoid system oscillation or instability caused by unreasonable parameter settings

[0112] The distributed control device includes a boost converter control device and a buck converter control device

[0113] Specifically, the Boost converter control device adopts a three-loop control structure, including a photovoltaic-side voltage outer loop, a photovoltaic-side intermediate voltage loop, and a photovoltaic-side current inner loop

[0114] As Figure 2 and Figure 3 shown, in the photovoltaic-side voltage outer loop, the difference between the maximum power point reference operating voltage V ref-pv and the photovoltaic-side voltage V pv is obtained as the transmission line reference operating voltage V ref-mid after passing through the photovoltaic-side voltage proportional control and the first limiting control, where K vpv is the photovoltaic-side voltage proportional coefficient, and the photovoltaic-side voltage V pv is collected through a voltage transformer connected to the branch where the photovoltaic-side capacitor C pv is located; in the photovoltaic-side intermediate voltage loop, the difference between the transmission line reference operating voltage V ref-mid and the transmission line voltage V mid is obtained as the maximum power point reference operating current I ref-pv, where K vmid is the transmission line voltage proportionality coefficient, and is connected to the transmission line equivalent capacitance C mid The voltage transformer of the branch collects the transmission line voltage V mid ; In the inner loop of the photovoltaic side current, the maximum power point refers to the working current I ref-pv and the photovoltaic side current I pv The duty cycle D of the Boost converter is obtained after the photovoltaic side current proportional control and the third limit control. boost , where K ipv is the photovoltaic side current proportionality coefficient, which is connected to the photovoltaic side inductor L pv The current transformer of the branch collects the photovoltaic side current I pv ; Photovoltaic side capacitor C pv Connected in parallel on the photovoltaic side of the Boost converter, the photovoltaic side inductor L pv Connected in series to the positive connection line between the photovoltaic power generation component and the Boost converter, the equivalent capacitance C of the transmission line is connected in parallel between the positive and negative poles of the transmission line. mid ;

[0115] Specifically, the Buck converter adopts a dual-loop control structure, including an output-side voltage outer loop and an output-side current inner loop;

[0116] like Figure 2 and Figure 4 As shown, in the output side voltage outer loop, the DC bus reference voltage V ref-dc With DC bus voltage V dc The difference between the output voltage ratio control and the second limiting control is used to obtain the output reference current I ref-out , where K vdc is the output voltage proportional coefficient, which is connected to the DC side capacitor C dc The voltage transformer of the branch collects the DC bus voltage V dc ; In the output current inner loop, the output reference current I ref-out With the output current I out The difference between the output current ratio control and the third limit control is used to obtain the duty cycle D of the Buck converter. buck , where K iout is the output current proportionality coefficient, which is connected to the output inductor L out The current transformer of the branch collects the output current I out ; DC side capacitor C dc Connected in parallel on the output side of the Buck converter, the output side inductor L out Connected in series to the connection line between the Buck converter and the positive pole of the DC bus;

[0117] In the embodiment, the upper limit of the first amplitude limiting control is 400, the lower limit is 0, the upper limit of the second amplitude limiting control is 100, the lower limit is -100, and the upper limit of the third amplitude limiting control is 1, the lower limit is -1.

[0118] The Boost-Boost converter control circuit designed by the present invention integrates the two control methods of grid-following type and grid-forming type in a complete set of control systems in stages, realizing the automatic switching between the two control methods. This control method can not only reduce the loss in the process of photovoltaic power transmission, but also realize the two-stage automatic switching through the control including only the proportional and amplitude limiting links. The control strategy does not add a differential link, avoiding system oscillation or instability caused by unreasonable parameter settings and enhancing the robustness of the control system.

[0119] In order to make the output voltage of the power converters of the battery and the supercapacitor follow the reference value without static error and improve the response speed, the power converters of the battery and the supercapacitor both adopt the voltage-current double closed-loop PI control; as Figure 5 shown, the bidirectional DC-DC power converter of the battery adopts the droop control based on the virtual resistance, including: the difference between the DC bus voltage V dc after passing through the virtual resistance R vbat and the DC bus reference voltage V ref-dc , after passing through the current inner-loop PI controller, the output reference current I ref-bat of the power converter of the battery is obtained, and the difference between the output reference current I ref-bat and the output current I bat of the power converter of the battery passes through the voltage outer-loop PI controller to obtain a voltage signal, and the voltage signal is modulated by PWM to obtain the duty cycle D bat of the power converter of the battery;

[0120] Figure 5 In, the bidirectional DC-DC power converter of the supercapacitor adopts the droop control based on the virtual capacitance, including: the difference between the DC bus voltage V dc after passing through the virtual resistance C vsc and the DC bus reference voltage V ref-dc , after passing through the current inner-loop PI controller, the output reference current I ref-sc of the power converter of the supercapacitor is obtained, and the difference between the output reference current I ref-sc and the output current I sc of the power converter of the supercapacitor passes through the voltage outer-loop PI controller to obtain a voltage signal, and the voltage signal is modulated by PWM to obtain the duty cycle D sc of the power converter of the supercapacitor;

[0121] The present invention also provides a method for a backup power supply containing virtual energy storage to support the DC system voltage, including:

[0122] Step 1: Obtain the virtual resistance in the droop control of the power converter of the storage battery and the power converter of the photovoltaic power generation module, the rated value and the allowable minimum value of the DC system voltage; determine the turning point of the output current of the photovoltaic power generation module by using the allowable maximum value of the DC system voltage drop and the virtual resistance; when the output current of the photovoltaic power generation module is less than the turning point and the DC system voltage is greater than the allowable minimum value, the power converter of the photovoltaic power generation module adopts droop control based on the virtual resistance; when the output current of the photovoltaic power generation module is not less than the turning point and the DC system voltage is not greater than the allowable minimum value, the power converter of the photovoltaic power generation module adopts MPPT control.

[0123] In the embodiment, the current range in the standby power supply input stage, i.e., the droop control stage, is designed according to the characteristics of the photovoltaic and the storage battery, and the virtual resistance value is calculated. Specifically: According to the load statistics data of a 35 kV substation DC system, the long-term stable load current is about 44 A. After considering redundancy, the stable output current of the standby power supply is designed to be 50 A, and the storage battery and the photovoltaic power generation module each bear 25 A. In the standby power supply input stage, both the photovoltaic power generation module and the storage battery adopt droop control based on the virtual resistance, and the virtual resistances are the same, which is 0.8 Ω. According to the regulations and specifications, the rated value of the DC system voltage is 220 V, and the allowable minimum value is 187.5 V. Therefore, the range is 187.5 V to 220 V. When the DC system voltage drops to 187.5 V, the allowable maximum value of the DC system voltage drop is 32.5 V, and the turning value of the output current of the photovoltaic power generation module is 40.625 A, which is the output current when switching from the droop mode to the MPPT mode.

[0124] The present invention designs a two-stage composite control strategy for the power converter of the photovoltaic power generation module, automatically switches the grid-connected type and grid-forming type control according to the standby power supply bus voltage level; and designs the converters of the storage battery and the super capacitor and their control strategies.

[0125] Specifically, different working mode switches are realized by setting the outer-loop voltage parameters in the control loop, so as to improve the stability and reliability of the system. The volt-ampere characteristic curve of the designed two-stage composite control is as Figure 6 shown. Figure 6Among them, Vdc is the DC bus voltage, and Ipv is the output current of the photovoltaic power generation module. In the first stage, that is, when the backup power supply is not put into use, the photovoltaic power generation module uses MPPT control to charge the hybrid energy storage. If the photovoltaic power generation is too small, it will lead to too long charging time and affect the power supply reliability of the backup power supply; if the photovoltaic power generation is too large, the economy of the whole system will be reduced. At this time, the output current of the photovoltaic power generation module is 40.625A to 52A, and the DC bus voltage is 187.5V to 150V. In the second stage, that is, when the hybrid energy storage is fully charged and the backup power supply is put into use, it is necessary to provide voltage support within the normal voltage range for the DC load, that is, the backup power supply voltage range needs to be 187.5V to 220V. At this time, the output current of the photovoltaic power generation module is 40.625A to 0A, and the DC bus voltage is 187.5V to 220V. At this time, the photovoltaic power generation module adopts droop control, and the setting of the virtual resistance value parameter needs to meet the designed droop curve and can share the low-frequency DC load power with the battery together.

[0126] In the present invention, the virtual resistance values in the droop control of the photovoltaic power generation module and the battery are designed to be the same, that is, when the backup power supply voltage is 187.5V - 220V, the battery and the photovoltaic power generation module have the same output characteristics, and the photovoltaic power generation module forms a "virtual energy storage" unit to jointly support the DC system voltage with the battery. When the DC system voltage is higher than 220V, the photovoltaic power generation module no longer outputs power.

[0127] The present invention designs a two-stage composite control strategy for the photovoltaic power generation module. When the backup power supply hybrid energy storage has insufficient power, MPPT control is adopted to charge the hybrid energy storage at a constant power; when the backup power supply is put into use, the photovoltaic uses virtual resistance droop control to provide voltage support for the DC system as a "virtual energy storage", and at the same time can share part of the low-frequency load power with the battery, reducing the capacity of the battery and optimizing the design of the whole system in terms of economy and stability. The designed two-stage composite control realizes the automatic switching from the grid-following type MPPT control to the grid-forming type droop control according to the voltage threshold. There is no differential link in the voltage-current double-loop control, and the control is simple and has strong stability.

[0128] Step 2: Determine the design parameters of the photovoltaic power generation module based on the turning value of the output current of the photovoltaic power generation module; determine the closed-loop transfer function of the boost converter and the closed-loop transfer function of the buck converter based on the design parameters of the photovoltaic power generation module; optimize the control parameters of the boost converter and the buck converter based on the closed-loop transfer function with the minimum fluctuation amplitude of the output current of the photovoltaic power generation module as the optimization goal.

[0129] Specifically, Step 2 includes:

[0130] Step 2.1: Determine the design parameters of the photovoltaic power generation module based on the turning value of the output current of the photovoltaic power generation module;

[0131] The specifications and quantity of the photovoltaic panels are selected based on the output power at the turning point when switching from the droop mode to the MPPT mode. Specifically: It can be calculated that the output power at the turning point is 7,617.18 W. Photovoltaic panels with a maximum output power of 200 W per panel are used. Therefore, the backup power supply designed in the present invention requires 39 photovoltaic panels. It can be calculated that the maximum output current of the photovoltaic power generation components is 52 A, the maximum output power is 7,800 W, and the corresponding minimum output voltage is 150 V. The detailed design parameters of the photovoltaic power generation components are shown in Table 1.

[0132] Table 1 Design Parameters of Photovoltaic Power Generation Components

[0133]

[0134]

[0135] Step 2.2: Determine the reference operating voltage at the maximum power point according to the design parameters of the photovoltaic power generation components.

[0136] According to the detailed design parameters of the photovoltaic power generation components, determine the I-V curve and P-V curve output by the photovoltaic power generation components, as shown in Figure 7 and Figure 8 respectively. Find the maximum power operating area according to the output P-V curve of the photovoltaic panel, and design the converter and its control strategy of the photovoltaic power generation components in combination with the I-V curve.

[0137] In the embodiment, it can be seen from the P-V curve that the operating voltage at the maximum power point is 96 V. When the operating voltage is 91 V - 96 V, the P-V curve is a non-linear curve, that is, the maximum power operating area. At this time, the photovoltaic power generation components operate in the MPPT control mode to charge the hybrid energy storage system; when the voltage is lower than 91 V, the P-V curve is an approximately linear curve. At this time, the photovoltaic power generation components operate in the droop control mode based on the virtual resistance to provide voltage support for the DC system bus. On the P-V curve, during the process of switching from the maximum power operating area to the low-power operating area on the left, the current remains basically constant, while the voltage drops, and it cannot be directly connected to the backup power supply to achieve voltage support. In order to achieve the goal of the photovoltaic outputting a low voltage in the maximum power operating area and a high voltage in the low-power operating area, it is necessary to design the converter and its control strategy.

[0138] Step 2.3: Select the voltage of the boost converter and the voltage of the buck converter according to the reference operating voltage at the maximum power point.

[0139] From the MPPT control region to the droop control region, the photovoltaic can be equivalent to a constant current source, and the voltage drops. Therefore, the converter should ensure that as the power gradually decreases, the voltage gradually increases, and the non-linear volt-ampere characteristic becomes linear. Since the maximum power point voltage of the photovoltaic is only 96V, which is relatively low, the power loss on the cable is relatively large. Therefore, a boost-boost converter is designed. First, the photovoltaic voltage is boosted from 96V to 400V through a boost converter to reduce the transmission line loss, and then a buck converter is used to drop the voltage to 220V.

[0140] Step 2.4, build an equivalent circuit in which the photovoltaic power generation components are connected to the DC bus through a boost converter and a buck converter in sequence. Based on the equivalent circuit, determine the closed-loop transfer function of the boost converter and the closed-loop transfer function of the buck converter; with the minimum output current fluctuation amplitude of the photovoltaic power generation components as the optimization goal, optimize the control parameters of the boost converter and the buck converter based on the closed-loop transfer function.

[0141] In the embodiment, the reference operating voltage V ref-pv of the maximum power point is 96V, the voltage proportionality coefficient K vpv of the photovoltaic side is 50, the upper limit of the first limiter control is 400V, the lower limit is 0V, and the reference operating voltage V ref-mid of the transmission line is the output of the outer loop of the photovoltaic side voltage, and the voltage proportionality coefficient K vmid of the transmission line is 4, the upper limit of the second limiter control is 100A, the lower limit is -100A, and the reference operating current I ref-pv of the maximum power point is the output of the voltage loop of the transmission line, and the current proportionality coefficient K ipv of the photovoltaic side is 100, the upper limit of the third limiter control is 1, the lower limit is -1; the capacitance C pv of the photovoltaic side is 10 μF, the inductance L pv of the photovoltaic side is 500 μH, and the equivalent capacitance C midis 10 μF; The control device of the Boost converter is designed with a three-loop structure. The outermost loop is for photovoltaic voltage control. The reference working voltage of the maximum power point is set to 96 V. The difference between the collected photovoltaic-side voltage and the reference working voltage of the maximum power point gradually increases after passing through the photovoltaic-side voltage proportional control and the first amplitude limiting control. Therefore, the photovoltaic-side voltage gradually decreases to 88 V, and at the same time, the reference voltage of the high-voltage side of the Boost is output. It can be seen that through this link, the reference voltage of the high-voltage side output by the Boost gradually increases until about 400 V. Therefore, the photovoltaic-side voltage outer loop realizes the transformation of the input link with gradually decreasing voltage into the output link with gradually increasing voltage, meeting the requirements of the design goal. The droop interval operating point of the photovoltaic is set on the left side of the maximum power point because the current and power change greatly on the right side of the maximum power point, and it is not easy to achieve stable control. The intermediate voltage loop on the photovoltaic side also outputs the reference current of the current inner loop through the transmission line voltage proportional control and the second amplitude limiting control, approaching the photovoltaic constant current of 83 A. Since the proportional coefficient of the outer loop is large and the voltage increases rapidly, although the proportional coefficient of the intermediate link is limited, the Boost output is likely to exceed the reference voltage of 400 V. Therefore, amplitude limiting is used to limit the voltage overshoot. The current inner loop adjusts the duty cycle according to the difference between the reference current and the actual current.

[0142] 1), The closed-loop transfer function of the boost converter, including:

[0143]

[0144] In the formula, G ipv-dboost is the closed-loop transfer function of the output current and duty cycle of the photovoltaic power generation component in the Boost converter, V ref-mid is the reference working voltage of the transmission line, C mid is the equivalent capacitance of the transmission line, R mid is the equivalent resistance of the transmission line, D boost is the duty cycle of the Boost converter, L pv is the inductor on the photovoltaic side;

[0145]

[0146] In the formula, G vmid-ipv is the closed-loop transfer function of the reference working voltage of the transmission line to the output current of the photovoltaic panel in the Boost converter, V ref-pv is the reference working voltage of the maximum power point;

[0147]

[0148] In the formula, G vpv-vmid is the closed-loop transfer function of the output voltage of the photovoltaic panel to the reference working voltage of the transmission line in the Boost converter;

[0149]

[0150] Wherein, G boost is the closed-loop transfer function of the overall boost part of the Boost converter, K vpv is the photovoltaic-side voltage proportionality coefficient, K vmid is the transmission-line voltage proportionality coefficient, K ipv is the photovoltaic-side current proportionality coefficient;

[0151] In the embodiment, the DC bus reference voltage V ref-dc is 220V, the output-side voltage proportionality coefficient K vdc is 1.25, the output reference current I ref-out is the output of the voltage outer loop, the output-side current proportionality coefficient K iout is 100, the DC-side capacitor C dc is 10 μF, the output-side inductor L out is 100 μH;

[0152] 2) The closed-loop transfer function of the buck converter includes:

[0153]

[0154] Wherein, G iout-dbuck is the closed-loop transfer function of the output current to the duty cycle in the Buck converter, R is the equivalent DC load of the backup power supply, C dc is the DC-side capacitor, L out is the output-side inductor;

[0155]

[0156] Wherein, G vmid-iout is the closed-loop transfer function of the transmission-line reference operating voltage to the output current in the Buck converter;

[0157]

[0158] Wherein, G buck is the closed-loop transfer function of the Buck converter, K vdc is the output-side voltage proportionality coefficient, K iout is the output-side current proportionality coefficient.

[0159] The Boost converter adopts a double-loop control of a voltage outer loop and a current inner loop to reduce the voltage to 220V. In addition, adding a Buck circuit after the Boost circuit can, to a certain extent, prevent the problems of Boost open circuit and infinite rise of the output voltage. The Buck circuit can limit the output voltage below the input voltage, thus avoiding this problem.

[0160] During the process of the photovoltaic voltage changing on the left side of the maximum power point, the current can be regarded as constant. The voltage has experienced a non-linear region from 96V to 91V and a linear region from 91V to 88V, corresponding to the MPPT and droop control regions, realizing the automatic switching between the two regions. After passing through the converter, the voltage turning point of 91V in the two photovoltaic regions becomes the output voltage of 187.5V.

[0161] When the standby power bus voltage is lower than 187.5V, the photovoltaic panel is in the maximum power working area, that is, the MPPT mode, and charges the hybrid energy storage. However, the output voltage is relatively low and insufficient to provide voltage support for the DC bus; when the output voltage of the photovoltaic plus converter is 220V - 187.5V, the bus voltage support can be realized, and the output voltage and current are in the droop mode. Although the ideal output power of the photovoltaic power generation component in the droop mode is less than the maximum output power of the photovoltaic panel, it can still provide a relatively large power. After passing through the converter, the output current decreases and the voltage increases, giving full play to the bus voltage regulation ability of the photovoltaic as a "virtual energy storage". Design the system according to the above parameters and obtain the simulation output characteristic curve as Figure 9 shown. From Figure 9 it can be seen that when the bus voltage is higher than 187.5V, the photovoltaic power generation component is in the droop control mode; at this time, the output current of the photovoltaic power generation component can be obtained according to the designed droop characteristic curve. Figure 8 In the setting in, the droop coefficient is 0.8Ω. When the voltage is equal to 187.5V, the calculated output current is 40.625A, which is consistent with the turning point of the ideal output characteristic curve; when the bus voltage is lower than 187.5V, the photovoltaic power generation component is in the MPPT mode. At this time, the output current of the photovoltaic power generation is calculated by dividing the maximum power of the photovoltaic power generation by the photovoltaic output voltage. Taking the minimum fluctuation amplitude of the output current of the photovoltaic power generation component as the optimization goal, the control parameters of the boost converter and the buck converter are optimized based on the closed-loop transfer function to realize the stable support of the DC system voltage.

[0162] Step 3, determine the transfer function of the bidirectional DC-DC power converter of the battery and the transfer function of the bidirectional DC-DC power converter of the supercapacitor; based on the droop characteristic model of the standby power supply, according to the load type of the DC system, distribute the load power of the DC system to the photovoltaic power generation component, the battery and the supercapacitor; according to the total output current characteristics of the battery and the photovoltaic power generation component and the output current characteristics of the supercapacitor, determine the virtual resistance and virtual capacitance; according to the determined virtual resistance and virtual capacitance, taking the minimum fluctuation amplitude of the output current of the battery and the supercapacitor as the optimization goal, optimize the control parameters of the bidirectional DC-DC power converters of the battery and the supercapacitor based on the closed-loop transfer function.

[0163] When the backup power supply is normally put into use, the supercapacitor adopts a droop control strategy of virtual capacitance to respond to high-frequency power fluctuations; the photovoltaic power generation module and the storage battery adopt a droop control strategy of virtual resistance to respond to low-frequency power fluctuations, realizing the power distribution of the backup power supply.

[0164] Specifically, step 3 includes:

[0165] Step 3.1, determining the transfer function of the bidirectional DC-DC power converter of the storage battery, including:

[0166]

[0167] In the formula, G ibat is the transfer function of the current inner-loop PI controller in the power converter of the storage battery, k ipbat is the proportional control parameter of the current inner-loop control in the power converter of the storage battery, k iibat is the integral control parameter of the current inner-loop control in the power converter of the storage battery;

[0168]

[0169] In the formula, G vbat is the transfer function of the voltage outer-loop PI controller in the power converter of the storage battery, k vpbat is the proportional control parameter of the voltage outer-loop control in the power converter of the storage battery, k vibat is the integral control parameter of the voltage outer-loop control in the power converter of the storage battery;

[0170]

[0171] In the formula, G ibat-dbat is the closed-loop transfer function of the output current of the storage battery to the duty ratio of the power converter, D bat is the duty ratio of the power converter of the storage battery, L bat is the inductance value on the low-voltage side of the storage battery, C bat is the capacitance value on the high-voltage side of the storage battery, and R is the equivalent DC load of the backup power supply;

[0172]

[0173] In the formula, G vbat-ibat is the closed-loop transfer function of the output voltage of the storage battery to the output current, V ref-bat is the output reference voltage of the power converter of the storage battery. In the embodiment, the reference voltage of the DC system is 220V, is the current value flowing through the inductor on the low-voltage side of the storage battery converter;

[0174]

[0175] In the formula, Gbat The transfer function of the bidirectional DC-DC power converter for the storage battery.

[0176] Step 3.2, determine the transfer function of the bidirectional DC-DC power converter for the supercapacitor, including:

[0177]

[0178] In the formula, G isc is the transfer function of the current inner-loop PI controller in the power converter of the supercapacitor, k ipsc is the proportional control parameter of the voltage outer-loop control in the power converter of the supercapacitor, k iisc is the integral control parameter of the current inner-loop control in the power converter of the supercapacitor;

[0179]

[0180] In the formula, G vsc is the transfer function of the voltage outer-loop PI controller in the power converter of the supercapacitor, k vpsc is the proportional control parameter of the voltage outer-loop control in the power converter of the supercapacitor, k visc is the integral control parameter of the voltage outer-loop control in the power converter of the supercapacitor;

[0181]

[0182] In the formula, G isc-dsc is the closed-loop transfer function of the output voltage of the power converter of the supercapacitor to the output current, D sc is the duty ratio of the power converter of the supercapacitor, L sc is the inductance value of the low-voltage side of the supercapacitor, C sc is the capacitance value of the high-voltage side of the supercapacitor, and R is the equivalent DC load of the backup power supply;

[0183]

[0184] In the formula, G vsc-isc is the closed-loop transfer function of the high-voltage side voltage of the supercapacitor to the output current of the supercapacitor, V ref-sc is the output reference voltage of the power converter of the supercapacitor. In the embodiment, the DC system reference voltage is 220V. is the current value flowing through the inductance of the low-voltage side of the supercapacitor;

[0185]

[0186] In the formula, G sc is the transfer function of the bidirectional DC-DC power converter of the supercapacitor.

[0187] When the backup power supply system is put into use in the present invention, the supercapacitor, the storage battery and the photovoltaic power generation module are connected to the DC system bus in parallel. The realization of the high-frequency and low-frequency power distribution control methods between different units includes centralized control and distributed control. In the preferred but non-limiting embodiment of the present invention, distributed control is adopted. Between each unit, there is no need to communicate by the communication method in centralized control, but the output power and voltage are automatically adjusted according to the system conditions, improving the economy and reliability of control.

[0188] Step 3.3: Based on the droop characteristic model of the backup power supply, according to the load type of the DC system, distribute the load power of the DC system to the photovoltaic power generation module, the storage battery and the supercapacitor; according to the total output current characteristics of the storage battery and the photovoltaic power generation module and the output current characteristics of the supercapacitor, determine the virtual resistance and virtual capacitance; based on the determined virtual resistance and virtual capacitance, optimize the control parameters of the bidirectional DC-DC power converters of the storage battery and the supercapacitor based on the closed-loop transfer function to realize the stable support for the DC system voltage.

[0189] Specifically, step 3.3 includes:

[0190] Step 3.3.1: Establish the equivalent circuit of the backup power supply, and determine each parameter in the equivalent circuit based on the determined transfer function;

[0191] In the backup power supply power distribution control strategy of the present invention, the supercapacitor responds to the high-frequency power fluctuation of the system and adopts the droop control strategy of virtual capacitance; the storage battery responds to the low-frequency power fluctuation of the system and adopts the droop control strategy of virtual resistance; when the photovoltaic power generation module works in the droop mode, it also adopts the droop control strategy of virtual resistance. According to the structure and control strategy of the backup power supply, build the equivalent circuit when the backup power supply is normally put into use, as Figure 10 shown. This control method can better coordinate the functions of the supercapacitor, the storage battery and the photovoltaic power generation module, and optimize the performance of the entire backup power supply. Figure 10 where, V ref is the reference voltage of the DC system, R vbat is the virtual resistance in the power converter of the storage battery, I bat is the output current of the power converter of the storage battery, C vsc is the virtual capacitance in the power converter of the supercapacitor, I sc is the output current of the power converter of the supercapacitor, R vpv is the virtual resistance in the power converter of the photovoltaic power generation module, I pv is the output current of the photovoltaic power generation module, V dc is the DC bus voltage, I load is the total load current of the DC system, Rload is the equivalent resistance of the DC system load. Since the line impedance is much smaller than the droop coefficient, the influence brought by the line impedance is ignored, and it is approximately considered that the output voltages of the battery, the supercapacitor, and the photovoltaic power converter are equal to the DC bus voltage; the parameters of each item in the equivalent circuit can be calculated according to the determined transfer function.

[0192] Step 3.3.2: Based on the equivalent circuit, determine the droop characteristic model of the backup power supply;

[0193] According to Figure 10 the equivalent circuit, the droop characteristic model of the backup power supply is obtained, satisfying the following relational expression:

[0194] V dc = V ref - R vbat · I bat

[0195]

[0196] V dc = V ref - R vpv · I pv

[0197] I load = I bat + I sc + I pv

[0198] Step 3.3.3: Based on the droop characteristic model of the backup power supply, according to the load type of the DC system, distribute the load power of the DC system to the photovoltaic power generation components, the battery, and the supercapacitor; including:

[0199] 1), when the DC system load is a regular load, the virtual capacitor in the supercapacitor is equivalent to an open circuit. Therefore, the supercapacitor does not output power externally, and I sc = 0. At this time, the distribution of the load power of the DC system is determined by the virtual resistances of the battery and the photovoltaic power generation components, satisfying the following relational expression:

[0200]

[0201] When the virtual resistances in the droop control of the power converter of the battery and the power converter of the photovoltaic power generation components are equal, the load of the DC system is evenly distributed to the battery and the photovoltaic power generation components.

[0202] 2) When the load of the DC system is an impact load, from the perspective of frequency response, the virtual capacitance in the supercapacitor is equivalent to a short circuit, and the power converter of the supercapacitor adopts an improved droop control strategy based on virtual capacitance. The discharge power of the supercapacitor increases rapidly to respond to the impact load, and the battery and the photovoltaic power generation component adopt the droop control strategy of virtual resistance for discharging. The discharge power of the battery and the photovoltaic power generation component increases slowly until the system load stabilizes. Since the virtual resistances of the photovoltaic power generation component and the battery are in parallel, they can be equivalently regarded as a parallel virtual resistance R vbp , the total output current of the battery and the photovoltaic power generation component is I vbp , and the output currents of each converter satisfy the following relationship:

[0203]

[0204] Step 3.3.4, equivalent the output total current characteristics of the battery and the photovoltaic power generation component to a first-order low-pass filter, equivalent the output current characteristics of the supercapacitor to a first-order high-pass filter, and determine the cut-off frequencies of the two filters; use the cut-off frequencies and the virtual resistance to determine the virtual capacitance in the power converter of the supercapacitor;

[0205] The cut-off frequencies satisfy the following relationship:

[0206]

[0207] According to the power distribution of the standby power supply, select the frequency division point, and determine the parallel virtual resistance R vbp of the photovoltaic power generation component and the battery vsc and the virtual capacitance C

[0208] of the supercapacitor. C Taking ω load as the frequency division point, I

[0209] as the total current output by the DC system, including the low-frequency component and the high-frequency component, which are provided by the photovoltaic power generation component - battery and the supercapacitor respectively, realizing the power distribution within the standby power supply. The low-frequency power fluctuation of the DC system is provided by the photovoltaic power generation component - battery, giving full play to the advantage of the large energy density of the battery. The high-frequency power fluctuation of the DC system is provided by the supercapacitor. When the impact load of the DC system causes the DC bus voltage to drop, the supercapacitor quickly responds to support the DC system voltage, giving full play to the advantage of the large power density of the supercapacitor. vbp and C vsc jointly determine the cut-off frequency of the system. In order to avoid frequent charging and discharging of the battery, the cut-off frequency should not be set too high. In this set of standby power supply settings, take ω C = 0.1×2π rad / s. After selecting the appropriate frequency division point, it is necessary to determine Rvbp and C vsc 。The virtual resistance value will reduce the level of the DC bus voltage. Therefore, the selected virtual resistance value should not be too small. According to the droop control curves of the photovoltaic and the battery, and the load demand, the virtual resistance values of the photovoltaic and the battery are determined to be 0.8 Ω. It can be obtained through calculation that R vbp = 0.4 Ω. After determining R vbp , C can be determined vsc 。

[0210] Step 3.3.5, according to the determined virtual resistance and virtual capacitance, with the minimum output current fluctuation amplitude of the battery and the supercapacitor as the optimization goal, optimize the control parameters of the bidirectional DC-DC power converters of the battery and the supercapacitor based on the closed-loop transfer function to achieve stable support for the DC system voltage.

[0211] The power distribution control strategy of the backup power supply inherits the advantages of plug-and-play of droop control, without the need to exchange information with other energy storage modules, and can achieve frequency division and distribution of power fluctuations without a communication network and a central controller.

[0212] Verify the power distribution and voltage support capabilities of the virtual energy storage backup power supply based on photovoltaic in response to regular loads and impact loads. To verify the method for supporting the DC system voltage of the substation and the power distribution strategy, build a simulation model of the above-mentioned backup power supply and the equivalent load in MATLAB / Simulink. In this model, a lithium-ion battery model in Simulink is used for the battery, a supercapacitor module in Simulink is used for the supercapacitor, and a PV array module in Simulink is used for the photovoltaic power generation components. The bidirectional Buck / Boost circuit is selected as the topology of the battery converter and the supercapacitor. The relevant parameters of the converter are shown in Table 2.

[0213] Table 2 Design parameters of the backup power supply system

[0214]

[0215]

[0216] During the operation of the backup power supply, virtual resistance-capacitance droop control is adopted. Set the following three groups of different backup power supplies to compare and verify the voltage support capability and power distribution effect of the virtual energy storage backup power supply based on photovoltaic in response to regular loads and impact loads.

[0217] The first group: The battery backup power supply only contains a 9.6 kW battery energy storage. Since there is no need to distribute the DC load power, the V in the droop curve is set ref= 220V, the virtual resistance R of the battery vbat = 0.8 Ω.

[0218] The second group: Hybrid energy storage backup power supply. Set V in the droop curve ref = 220V, the virtual resistance R of the battery vbat = 0.8 Ω, set ω C = 0.1 × 2π rad / s, there is no photovoltaic virtual energy storage R in this group of backup power supply vpv , and the virtual capacitance C can be calculated vsc = 1.984 F.

[0219] The third group: Photovoltaic-based virtual energy storage backup power supply. Set V ref = 220V, the virtual resistance R of the battery vbat = 0.8 Ω, the photovoltaic virtual resistance R vpv = 0.8 Ω, set ω C = 0.1 × 2π rad / s, and R can be calculated vbp = 0.4 Ω, the virtual capacitance C vsc = 0.992 F.

[0220] At 1 s, a 22 kW impact load is connected, and its equivalent resistance value is 2.2 Ω, and the impact load is cut off after 0.2 s; at 2 s, a 12.1 kW regular load is connected, and its equivalent resistance value is 4 Ω. If the DC bus voltage level remains unchanged, the ideal power curves of the regular load and the impact load are as Figure 11 shown.

[0221] The regular load of the substation DC system has a long duration and is relatively stable. In order to verify the voltage support ability and power distribution effect of the photovoltaic-based virtual energy storage backup power supply when dealing with regular loads, at 2 s, a regular load is added to the load side of the three groups of backup power supplies respectively, Figure 11 which are the simulation results of the voltage changes of the three groups of backup power supplies.

[0222] From Figure 12It can be seen that after the load is connected, the voltage of the backup power supply containing only the battery drops rapidly, reaching the lowest voltage value of 100V at 2.3s and reaching 180V and tending to be stable at 4.5s. The backup power supply containing only the battery has the problem of a sudden voltage drop when the load is connected, and its voltage support ability is weak; the voltage of the hybrid energy storage backup power supply drops more slowly, reaching the lowest voltage value of 177V at 3.2s and reaching 180V and tending to be stable at 5s. Compared with the first group, the voltage drop of the hybrid energy storage backup power supply is smaller, and the time taken to reach stability is shorter, and its voltage support ability is stronger; the voltage of the virtual energy storage backup power supply based on photovoltaic drops most slowly, reaching the lowest voltage value of 198V at 3s and rising to 200V and tending to be stable at 4.3s. The virtual energy storage backup power supply based on photovoltaic has the least voltage drop, and the voltage value after stability is the highest, and its voltage support ability is the strongest.

[0223] The power distribution curve of the hybrid energy storage backup power supply when dealing with regular loads is as Figure 13 shown. It can be seen from Figure 13 that after the load is connected, the discharge power of the supercapacitor increases rapidly and then decreases slowly, and the discharge power of the battery increases slowly. At 5s, the discharge power of the supercapacitor drops to 0, and the load power is completely borne by the battery.

[0224] The power distribution curve of the virtual energy storage backup power supply based on photovoltaic when dealing with regular loads is as Figure 14 shown. It can be seen from Figure 14 that after the load is connected, the discharge power of the supercapacitor increases rapidly and then decreases slowly, and the output powers of the battery and photovoltaic power generation increase slowly. At 4.3s, the discharge power of the supercapacitor drops to 0, and the battery and photovoltaic jointly bear the load power. Compared with the hybrid energy storage backup power supply, the virtual energy storage backup power supply based on photovoltaic has a faster adjustment speed, and the stable output power of the battery is lower. This further verifies that the new type of backup power supply designed by the present invention has stronger voltage support and power support capabilities when dealing with regular loads.

[0225] The characteristics of the impulse load of the substation DC system are large load power and short duration. Taking the closing power of the electromagnetic circuit breaker as an example, the maximum closing current of the circuit breaker is 100A, and the duration is about 0.1s. In the actual operation of the substation DC system, there may be conditions such as continuous reclosing of a certain circuit breaker and simultaneous closing of multiple circuit breakers. Therefore, in order to verify the voltage support ability and power distribution effect of the virtual energy storage backup power supply based on photovoltaic when dealing with impulse loads, at 1s, an impulse load is added to the load side of the backup power supplies in the second group and the third group respectively, and the load is cut off after 0.2s. Figure 15 This is the simulation result of the voltage change of the backup power supplies in the second group and the third group.

[0226] It can be seen from Figure 15It can be seen that after the impact load is connected, the voltage of the hybrid energy storage backup power supply drops rapidly and reaches the lowest voltage value of 187V at 1.2s. After the load is cut off, the voltage of the backup power supply recovers slowly, but the regulation time is long; the voltage of the virtual energy storage backup power supply based on photovoltaic drops relatively slowly and reaches the lowest voltage value of 197V at 1.2s. After the load is cut off, the voltage of the backup power supply recovers relatively quickly and reaches 220V at 3s. Through comparison, it can be concluded that the virtual energy storage backup power supply based on photovoltaic can meet the normal voltage fluctuation range of 187.5V - 242V for the 220V DC system, and has stronger voltage support ability when dealing with impact loads.

[0227] The power distribution curve of the virtual energy storage backup power supply based on photovoltaic when dealing with impact loads is as Figure 16 shown. It can be Figure 16 seen that after the impact load is connected, the discharge power of the supercapacitor increases rapidly, and the output powers of the battery and photovoltaic increase slowly; at 1.2s, after the load is cut off, the discharge power of the supercapacitor decreases rapidly, and the output powers of the battery and photovoltaic also start to decrease slowly. At 3s, the output powers of the supercapacitor, battery, and photovoltaic recover to 0. The simulation verifies the control strategy of using the supercapacitor to respond to the impact load of the DC system and avoids the frequent charge and discharge of the battery, thus prolonging the service life of the battery.

[0228] Compared with the prior art, the beneficial effects of the present invention at least include: The traditional photovoltaic power generation control strategy is MPPT grid-following control, and the grid-following control does not have the ability to adjust the bus voltage level. Therefore, the present invention designs a two-stage photovoltaic composite control strategy with automatic switching between grid-following control and grid-forming control. When the hybrid energy storage has insufficient power, the photovoltaic power generation components use the MPPT control technology to charge the hybrid energy storage, increasing the standby power bus voltage and quickly putting it into use, avoiding the problem of insufficient efficiency of the supercapacitor in the prior art due to self-discharge or too rapid voltage drop. When the hybrid energy storage is fully charged and the standby power bus voltage level returns to the normal voltage fluctuation range of the DC system, the photovoltaic is regarded as a "virtual energy storage" and uses droop control to play a voltage support role together with the hybrid energy storage to ensure the reliable operation of the protection and tripping devices. The existing standby power uses batteries or supercapacitors to provide voltage support for the DC system, and a sufficiently large energy storage capacity needs to be configured, resulting in a relatively high cost. When the novel standby power in this patent is put into use, the photovoltaic power generation components work in the grid-forming droop control stage. As a "virtual energy storage", it not only saves the capacity configuration of the hybrid energy storage but also enhances the voltage support ability of the standby power. Droop control and MPPT control belong to different types of control. In the prior art, switches are often used to complete the switching between the two control loops, and there will be large transient fluctuations during the switching process. Therefore, this patent innovatively designs a Boost-Boost converter control circuit. This control method can integrate the grid-following control and grid-forming control in a set of control in stages, and realizes the automatic switching between the two control methods by setting voltage thresholds. This control method can not only reduce the loss in the process of photovoltaic power transmission but also achieve two-stage automatic switching through a control with only proportional and limiting links. The control strategy does not add a differential link, avoiding system oscillation or instability caused by unreasonable parameter settings and enhancing the robustness of the control system.

[0229] The present disclosure can be a system, method, and / or computer program product. The computer program product can include a computer-readable storage medium having computer-readable program instructions thereon for causing a processor to implement various aspects of the present disclosure.

[0230] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: It is still possible to modify the specific implementation manners of the present invention or make equivalent replacements. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A backup power supply with virtual energy storage photovoltaics, which supplies power to fixed loads and impact loads of a DC system; characterized in that, Including: Photovoltaic power generation components, hybrid energy storage, DC bus, DC bus voltage detection device, voltage and current acquisition device, distributed control device; The photovoltaic power generation components are connected to the DC bus through a power converter, and the hybrid energy storage is connected to the DC bus through a bidirectional DC-DC power converter; the DC bus voltage detection device sends the detected DC bus voltage to the voltage and current acquisition device; when the voltage of the substation DC system is detected to be out of limit, the DC bus is connected to the bus of the DC system, and the standby power supply supplies power to the fixed load and impact load through the DC bus; The voltage and current acquisition device sends the output voltage and current of the photovoltaic power generation components and the power converter, and the output voltage and current of the hybrid energy storage and the bidirectional DC-DC power converter to the distributed control device, and the distributed control device sends control signals to the power converter and the bidirectional DC-DC power converter respectively.

2. The standby power supply with virtual energy storage photovoltaic according to claim 1, characterized in that When the standby power supply is not put into use and the photovoltaic power generation components charge the hybrid energy storage, the power converter of the photovoltaic power generation components adopts grid-following control, and the grid-following control includes MPPT control; When the standby power supply is put into use, the power converter of the photovoltaic power generation components adopts grid-forming control, and the grid-forming control includes droop control based on virtual resistance.

3. The standby power supply with virtual energy storage photovoltaic according to claim 2, characterized in that The hybrid energy storage includes a battery and a supercapacitor; The battery and the supercapacitor are respectively connected to the DC bus through corresponding bidirectional DC-DC power converters; When the standby power supply is put into use, the bidirectional DC-DC power converter of the battery adopts droop control based on virtual resistance, and the virtual resistance in the droop control of the bidirectional DC-DC power converter of the battery is equal to the virtual resistance in the droop control of the power converter of the photovoltaic power generation components. The battery and the photovoltaic power generation components supply power to the regular load and accident load of the DC system. When the standby power supply is put into use, the bidirectional DC-DC power converter of the supercapacitor adopts droop control based on virtual capacitance to supply power to the impact load.

4. The standby power supply with virtual energy storage photovoltaic according to claim 2, characterized in that The power converter of the photovoltaic power generation components includes a boost converter and a buck converter; among them, the photovoltaic module is connected to the boost converter through a connection line, a photovoltaic side inductor is connected in series on the positive connection line between the photovoltaic power generation components and the boost converter, and a photovoltaic side capacitor is connected in parallel on the photovoltaic side of the boost converter; the boost converter and the buck converter are connected through a transmission line, and a transmission line equivalent capacitor is connected in parallel between the positive and negative poles of the transmission line; the buck converter is connected to the DC bus through a connection line, an output side inductor is connected in series on the connection line between the buck converter and the positive pole of the DC bus, and a DC side capacitor is connected in parallel on the output side of the buck converter; The output voltage of the photovoltaic power generation components is increased by the boost converter and then transmitted to the buck converter, and the buck converter reduces the voltage to the DC bus voltage.

5. The standby power supply with virtual energy storage photovoltaic according to claim 4, characterized in that The distributed control device includes a boost converter control device. The boost converter control device adopts a three-loop control structure, including a photovoltaic-side voltage outer loop, a photovoltaic-side intermediate voltage loop, and a photovoltaic-side current inner loop; In the photovoltaic-side voltage outer loop, the difference between the maximum power point reference operating voltage and the photovoltaic-side voltage is obtained as the transmission line reference operating voltage after photovoltaic-side voltage proportional control and the first amplitude limiting control. Among them, the photovoltaic-side voltage is collected by a voltage transformer connected to the branch where the photovoltaic-side capacitor is located; in the photovoltaic-side intermediate voltage loop, the difference between the transmission line reference operating voltage and the transmission line voltage is obtained as the maximum power point reference operating current after transmission line voltage proportional control and the second amplitude limiting control. Among them, the transmission line voltage is collected by a voltage transformer connected to the branch where the transmission line equivalent capacitor is located; in the photovoltaic-side current inner loop, the difference between the maximum power point reference operating current and the photovoltaic-side current is obtained as the duty ratio of the boost converter after photovoltaic-side current proportional control and the third amplitude limiting control. Among them, the photovoltaic-side current is collected by a current transformer connected to the branch where the photovoltaic-side inductor is located.

6. The backup power supply with virtual energy storage photovoltaic according to claim 5, characterized in that, The distributed control device includes a buck converter control device. The buck converter control device adopts a double-loop control structure, including an output-side voltage outer loop and an output-side current inner loop; In the output-side voltage outer loop, the difference between the DC bus reference voltage and the DC bus voltage is obtained as the output reference current after output-side voltage proportional control and the second amplitude limiting control. Among them, the DC bus voltage is collected by a voltage transformer connected to the branch where the DC-side capacitor is located; in the output-side current inner loop, the difference between the output reference current and the output current is obtained as the duty ratio of the buck converter after output-side current proportional control and the third amplitude limiting control. Among them, the output current is collected by a current transformer connected to the branch where the output-side inductor is located.

7. The backup power supply with virtual energy storage photovoltaic according to claim 6, characterized in that, The upper limit of the first amplitude limiting control is 400 and the lower limit is 0. The upper limit of the second amplitude limiting control is 100 and the lower limit is -100. The upper limit of the third amplitude limiting control is 1 and the lower limit is -1.

8. The backup power supply with virtual energy storage photovoltaic according to claim 3, characterized in that, The bidirectional DC-DC power converter of the battery adopts droop control based on a virtual resistor, including: the difference between the DC bus voltage after passing through the virtual resistor and the DC bus reference voltage is obtained as the output reference current of the battery's power converter after passing through the current inner loop PI controller. The difference between the output reference current and the output current of the battery's power converter is obtained as a voltage signal after passing through the voltage outer loop PI controller. The voltage signal is modulated by PWM to obtain the duty ratio of the battery's power converter.

9. The backup power supply with virtual energy storage photovoltaic according to claim 3, characterized in that, The bidirectional DC-DC power converter of the supercapacitor adopts droop control based on virtual capacitance, including: the difference between the DC bus voltage after passing through a virtual resistor and the DC bus reference voltage, which is processed by the current inner-loop PI controller to obtain the output reference current of the power converter of the supercapacitor. The difference between the output reference current and the output current of the power converter of the supercapacitor is processed by the voltage outer-loop PI controller to obtain a voltage signal, and the voltage signal is modulated by PWM to obtain the duty cycle of the power converter of the supercapacitor.

10. A method for a backup power supply to support the DC system voltage with virtual energy storage photovoltaics, characterized in that obtain the virtual resistors in the droop control of the bidirectional DC-DC power converter of the battery and the power converter of the photovoltaic power generation component, the rated value and the allowable minimum value of the DC system voltage; determine the turning value of the output current of the photovoltaic power generation component by using the allowable maximum value of the DC system voltage drop and the virtual resistor; when the output current of the photovoltaic power generation component is less than the turning value and the DC system voltage is greater than the allowable minimum value, the power converter of the photovoltaic power generation component adopts droop control based on the virtual resistor; when the output current of the photovoltaic power generation component is not less than the turning value and the DC system voltage is not greater than the allowable minimum value, the power converter of the photovoltaic power generation component adopts MPPT control; Based on the turning value of the output current of the photovoltaic power generation component, determine the design parameters of the photovoltaic power generation component; based on the design parameters of the photovoltaic power generation component, determine the closed-loop transfer function of the boost converter and the closed-loop transfer function of the buck converter; with the minimum fluctuation amplitude of the output current of the photovoltaic power generation component as the optimization goal, optimize the control parameters of the boost converter and the buck converter based on the closed-loop transfer function; Determine the transfer function of the bidirectional DC-DC power converter of the battery and the transfer function of the bidirectional DC-DC power converter of the supercapacitor; based on the droop characteristic model of the backup power supply, according to the load type of the DC system, distribute the load power of the DC system to the photovoltaic power generation component, the battery and the supercapacitor; according to the output total current characteristics of the battery and the photovoltaic power generation component and the output current characteristics of the supercapacitor, determine the virtual resistor and the virtual capacitance; according to the determined virtual resistor and virtual capacitance, with the minimum fluctuation amplitude of the output currents of the battery and the supercapacitor as the optimization goal, optimize the control parameters of the bidirectional DC-DC power converters of the battery and the supercapacitor based on the closed-loop transfer function.

11. The method for a backup power supply to support the DC system voltage with virtual energy storage photovoltaics according to claim 10, characterized in that The closed-loop transfer function of the boost converter includes: Wherein, G ipv-dboost is the closed-loop transfer function of the output current and duty ratio of the photovoltaic power generation module in the boost converter, V ref-mid is the reference working voltage of the transmission line, C mid is the equivalent capacitance of the transmission line, R mid is the equivalent resistance of the transmission line, D boost is the duty ratio of the boost converter, L pv is the inductor on the photovoltaic side; where G vmid-ipv is the closed-loop transfer function of the reference working voltage of the transmission line in the boost converter with respect to the output current of the photovoltaic panel, and V ref-pv is the reference working voltage at the maximum power point; Where G vpv-vmid is the closed-loop transfer function of the output voltage of the photovoltaic panel to the reference operating voltage of the transmission line in the boost converter, Where, G boost is the closed-loop transfer function of the overall boost part of the boost converter, K vpv is the voltage proportionality coefficient on the PV side, K vmid is the voltage proportionality coefficient of the transmission line, K ipv is the current proportionality coefficient on the PV side.

12. The method for a backup power supply to support the DC system voltage with virtual energy storage photovoltaics according to claim 10, characterized in that The closed-loop transfer function of the buck converter includes: where G iout-dbuck is the closed-loop transfer function of the output current to the duty cycle in the buck converter, R is the equivalent DC load of the backup power supply, C dc is the DC-side capacitor, and L out is the output-side inductor; where G vmid-iout is the closed-loop transfer function of the output current with respect to the reference working voltage of the transmission line in the buck converter; Wherein, G buck is the closed-loop transfer function of the buck converter, K vdc is the voltage proportionality coefficient on the output side, and K iout is the current proportionality coefficient on the output side.

13. The method for a backup power supply to support the DC system voltage with virtual energy storage photovoltaics according to claim 10, characterized in that The transfer function of the bidirectional DC-DC power converter of the battery includes: Wherein, G ibat is the transfer function of the current inner-loop PI controller in the power converter of the storage battery, and k ipbat is the proportional control parameter of the current inner-loop control in the power converter of the storage battery, and k iibat is the integral control parameter of the current inner-loop control in the power converter of the storage battery; Where G vbat is the transfer function of the voltage outer-loop PI controller in the power converter of the battery, and k vpbat is the proportional control parameter of the voltage outer-loop control in the power converter of the battery, and k vibat is the integral control parameter of the voltage outer-loop control in the power converter of the battery; Where, G ibat-dbat is the closed-loop transfer function of the output current of the storage battery to the duty ratio of the power converter, D bat is the duty ratio of the power converter of the storage battery, L bat is the inductance value of the low-voltage side of the storage battery, C bat is the capacitance value of the high-voltage side of the storage battery, and R is the equivalent DC load of the backup power supply; Where, G vbat-ibat is the closed-loop transfer function of the output voltage of the storage battery to the output current, V ref-bat is the output reference voltage of the power converter of the storage battery, is the current value flowing through the inductor on the low-voltage side of the storage battery converter; where G bat is the transfer function of the bidirectional DC-DC power converter of the storage battery.

14. The method for a backup power supply to support the DC system voltage with virtual energy storage photovoltaics according to claim 10, characterized in that The transfer function of the bidirectional DC-DC power converter for supercapacitors includes: where G isc is the transfer function of the current inner loop PI controller in the power converter of the supercapacitor, k ipsc is the proportional control parameter of the voltage outer loop control in the power converter of the supercapacitor, k iisc is the integral control parameter of the current inner loop control in the power converter of the supercapacitor; Where G vsc is the transfer function of the voltage outer loop PI controller in the power converter of the supercapacitor, and k vpsc is the proportional control parameter of the voltage outer loop control in the power converter of the supercapacitor, and k visc is the integral control parameter of the voltage outer loop control in the power converter of the supercapacitor; Where, G isc-dsc is the closed-loop transfer function of the output voltage of the power converter of the supercapacitor with respect to the output current, D sc is the duty cycle of the power converter of the supercapacitor, L sc is the inductance value on the low-voltage side of the supercapacitor, C sc is the capacitance value on the high-voltage side of the supercapacitor, and R is the equivalent DC load of the backup power supply; where G vsc-isc is the closed-loop transfer function of the high-voltage side voltage of the supercapacitor to the output current of the supercapacitor, V ref-sc is the output reference voltage of the power converter of the supercapacitor, is the current value flowing through the inductor on the low-voltage side of the supercapacitor; Where, G sc is the transfer function of the bidirectional DC-DC power converter of the supercapacitor.

15. The method for a backup power source to support the DC system voltage with virtual energy storage photovoltaics according to claim 10, wherein: An equivalent circuit of the backup power source is established, and various parameters in the equivalent circuit are determined based on the determined transfer function; based on the equivalent circuit, a droop characteristic model of the backup power source is determined, satisfying the following relationship: V dc = V ref - R vbat · I bat V dc = V ref - R vpv · I pv I load = I bat + I sc + I pv Where, V dc is the DC bus voltage, V ref is the reference voltage of the DC system, R vbat is the virtual resistance in the power converter of the storage battery, I bat is the output current of the power converter of the storage battery, C vsc is the virtual capacitance in the power converter of the supercapacitor, I sc is the output current of the power converter of the supercapacitor, R vpv is the virtual resistance in the power converter of the photovoltaic power generation module, I pv is the output current of the photovoltaic power generation module, I load is the total load current of the DC system.

16. The method for a backup power source to support the DC system voltage with virtual energy storage photovoltaics according to claim 15, wherein: The virtual resistances of the photovoltaic power generation module and the storage battery are in a parallel state, which is equivalent to a parallel virtual resistance R vbp , and the output total current of the storage battery and the photovoltaic power generation module and the output current characteristics of the supercapacitor respectively satisfy the following relational expressions: Where, I vbp is the total output current of the storage battery and the photovoltaic power generation module.

17. The method for a backup power source to support the DC system voltage with virtual energy storage photovoltaics according to claim 16, wherein: The total output current characteristics of the battery and the photovoltaic power generation components are equivalent to a first-order low-pass filter, and the output current characteristics of the supercapacitor are equivalent to a first-order high-pass filter. The cut-off frequencies of both filters satisfy the following relationship: With ω C as the frequency division point, determine the parallel virtual resistance R vbp of the photovoltaic power generation module and the battery, and the virtual capacitance C vsc of the supercapacitor.

18. A terminal, comprising a processor and a storage medium; wherein: The storage medium is used to store instructions; The processor is used to operate according to the instructions to execute the steps of the method according to any one of claims 10-17.

19. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, the steps of the method according to any one of claims 10-17 are implemented.

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