Multi-mode simplified operation control method for photovoltaic-hybrid energy storage-grid-connected and off-grid systems

Abstract

The present invention discloses a multi-modal simplified operation control method for a farmer's photovoltaic-hybrid energy storage-grid-connected and off-grid system, and relates to the field of new energy. It can simplify the operation mode of the farmer's photovoltaic storage system, improve the system operation performance, and make full use of photovoltaic power. The present invention includes: proposing an energy management method, simplifying the system operation mode into three modes dominated by photovoltaic, energy storage and load according to the energy management logic; based on the three operation modes, proposing an operation control method that can simultaneously realize the power distribution of the hybrid energy storage system, the power distribution of the hybrid energy storage system and the grid-connected inverter in parallel, and the smoothing of the secondary ripple of the bus voltage caused by the single-phase bridge inverter. The present invention is suitable for the design of energy management operation control methods for photovoltaic power generation systems.

Description

Technical Field

[0001] The present invention relates to the field of new energy, and in particular to a multi-modal simplified operation control method for a photovoltaic-hybrid energy storage-on-grid and off-grid system. Background Art

[0002] Photovoltaic power generation, as the most promising renewable energy in the future, has attracted widespread attention due to its cleanliness, high reliability, maintenance and low maintenance cost. Photovoltaic power generation provides farmers in remote areas with the convenience of electricity use, and they are not troubled by the infrastructure of power transmission. However, photovoltaic power generation is affected by the intensity of light, and the power generation has a certain degree of intermittency and randomness. Whether the photovoltaic system is connected to the grid or off-grid, the fluctuation characteristics of its power generation will affect the stability of the system, so the energy storage system has become an indispensable part of the photovoltaic system. In order to make full use of the photovoltaic power generation power, the system structure represented by photovoltaic-energy storage-connected and off-grid has been widely used in photovoltaic power generation. Due to the limitations of its own characteristics, a single energy storage system is difficult to meet the complex characteristics of photovoltaic power generation and load power consumption. Therefore, the hybrid energy storage system composed of lithium batteries and supercapacitors can have the advantages of complementary characteristics and low cost, and has become an important development direction of energy storage systems.

[0003] System energy management is the core of system control, and its goal is to make the system fully utilize photovoltaic power in a stable and efficient state. Usually, predetermined rule control requires pre-judgment of the system and setting of system operation control logic. This method is difficult to cope with sudden system operation, and there will also be frequent switching of the system in the predetermined control logic. Intelligent algorithm research is relatively popular because it can take into account the overall state of the system, but it needs to detect more state information, has high computing pressure, has certain delays, and has single point failures. The principle of conventional control methods is relatively simple and relies on the system's own control strategy, but it is difficult to take into account the system state information to achieve system optimization control.

[0004] In the control of different modes of operation of the system, the hybrid energy storage system needs to distribute power to match its own characteristics; when the hybrid energy storage system and the single-phase full-bridge inverter control the DC bus at the same time, the two voltage sources are connected in parallel, and power distribution is usually based on droop control; the single-phase full-bridge inverter causes the DC bus voltage to fluctuate at double frequency due to its own structure. In the photovoltaic storage and off-grid system, these problems are concentrated. Summary of the invention

[0005] In order to solve the problems of the prior art, the present invention provides a multi-modal simplified operation control method for a photovoltaic-hybrid energy storage-grid-connected and off-grid system. By analyzing the multi-modal under the system energy management method, the system operation mode is simplified into three dominant modes, and an operation control method is proposed to simultaneously realize the power distribution between the system hybrid energy storage and the single-phase full-bridge inverter, the power distribution of the hybrid energy storage, and the smoothing of the system bus voltage fluctuations.

[0006] In order to achieve the above object, the present invention adopts the following technical scheme:

[0007] (1) Simplification of multi-mode system operation

[0008] The system operation mode is mainly judged by comparing the size of photovoltaic power generation and load demand power, while considering the size of the lithium battery state of charge SoCe in the hybrid energy storage, and switching the operation control. When the photovoltaic power has output, in order to fully absorb the power generation, the photovoltaic system is the dominant system operation core. When there is no photovoltaic power generation and there is load power consumption, the load power consumption is dominant. When there is neither photovoltaic power generation nor load demand power, the energy storage system is dominant. Therefore, it is necessary to clarify the control strategy of each unit and design the system operation control method according to the simplified dominant unit.

[0009] (2) Single-phase full-bridge inverter control droop coefficient

[0010] When photovoltaic power is output, the DC bus power will increase, causing the DC bus voltage to increase. At this time, the full-bridge inverter absorbs photovoltaic power and connects to the grid by controlling the DC bus voltage. When the photovoltaic power is less, the DC bus power needs to charge the energy storage system, and the bus voltage will decrease as the DC bus power decreases. At this time, it is necessary to absorb power from the grid to the DC bus to maintain the bus voltage stable, and the full-bridge inverter works in the rectification state. The hybrid energy storage and the single-phase full-bridge inverter control the bus voltage at the same time, which is equivalent to two voltage sources in parallel. Therefore, in order to achieve power distribution between the hybrid energy storage and the inverter, it is also necessary to consider the charge state of the lithium battery in the hybrid energy storage. The droop coefficient R of the inverter and rectification of the single-phase full-bridge inverter G It can be expressed as:

[0011]

[0012] In the formula, r0 is the initial coefficient and n is the adjustment coefficient.

[0013] (3) Hybrid energy storage operation control droop coefficient

[0014] Hybrid energy storage controls the DC bus voltage through a shared bus voltage outer loop, and frequency division is achieved by connecting a low-pass filter in series as the reference of the current loop, with the low-frequency component given to the lithium battery and the high-frequency component given to the supercapacitor. At the same time, considering the bus voltage fluctuation caused by the double frequency power fluctuation of the single-phase full-bridge inverter, it is proposed to incorporate the DC bus voltage fluctuation smoothing control link into the supercapacitor current loop output end, and directly adjust the duty cycle to achieve bus voltage fluctuation smoothing.

[0015] Hybrid energy storage and single-phase full-bridge inverter control the bus voltage at the same time, and droop control is needed to achieve power distribution. Lithium batteries in hybrid energy storage are energy storage media, which can provide support for long-term stable operation of the system. Therefore, the charge and discharge droop coefficient of hybrid energy storage can be expressed as

[0016]

[0017] In the formula, r1 is the initial coefficient and n is the adjustment coefficient.

[0018] The beneficial effects of the present invention are:

[0019] 1. The present invention starts from the system energy management process, analyzes different modes, and simplifies them based on the dominant unit. There are mainly three dominant modes: photovoltaic, energy storage and load, which facilitates system optimization operation control.

[0020] 2. The present invention proposes a system operation control strategy based on different dominant modes, adjusts the droop coefficient based on the charge state of the energy storage system, realizes reasonable power distribution, and smoothes the DC bus fluctuation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0022] Figure 1 It is a photovoltaic-hybrid energy storage-on-grid and off-grid system;

[0023] Figure 2 It is the energy management flow chart;

[0024] Figure 3 It is the DC bus voltage control block diagram of a single-phase full-bridge inverter;

[0025] Figure 4 It is the block diagram of AC bus voltage control of single-phase full-bridge inverter;

[0026] Figure 5It is the block diagram of DC bus voltage control for hybrid energy storage operation;

[0027] Figure 6 is the relationship between the corresponding droop coefficients Rhessd and Rgac and SoCe;

[0028] Figure 7 is the relationship between the corresponding droop coefficients Rhessc and Rgdc and SoCe;

[0029] Figure 8 is the relationship between the corresponding droop coefficients Rhessc and Rgac and SoCe;

[0030] Fig. 9 It is the system current under initial control when connected to the grid;

[0031] Fig.10 It is the DC and AC bus voltage under initial operation control when connected to the grid;

[0032] Fig.11 It is the system current under the control of DC bus voltage compensation operation when connected to the grid;

[0033] Fig.12 It is the DC and AC bus voltages under the DC bus voltage compensation operation control when connected to the grid;

[0034] Fig.13 It is the system current under DC bus voltage compensation and droop control when connected to the grid;

[0035] Fig.14 It is the DC and AC bus voltages that are added with DC bus voltage compensation and droop control when connected to the grid;

[0036] Fig.15 It is the SOCe and droop coefficient under DC bus voltage compensation and droop control added during grid connection;

[0037] Fig.16 It is the system current under initial operation control when off-grid;

[0038] Fig.17 It is the DC and AC bus voltage under initial operation control when off-grid;

[0039] Fig.18 It is the system current under the control of adding DC bus voltage compensation operation when off-grid;

[0040] Fig.19 It is the DC bus voltage compensation that is added when off-grid to control the DC and AC bus voltages under operation. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] (1) System structure

[0043] In order to be compatible with the existing electricity systems of farmers, while saving costs and improving system scalability, the following Figure 1 The structure of the farmer's photovoltaic energy storage and off-grid system is shown. Figure 1 The photovoltaic cells are connected to the 380V DC bus using the BOOST converter. The BOOST uses the disturbance observation method for maximum power point tracking control, and always works at the maximum power point to fully absorb the photovoltaic power generation power. The lithium battery and supercapacitor interface converter in the hybrid energy storage system uses the BUCK / BOOST bidirectional converter, and the energy storage medium is connected to the low-voltage side of the bidirectional converter to reduce the impact of current fluctuations on the energy storage medium. The single-phase bridge converter of the AC / DC bus mainly transfers the photovoltaic output power to the AC bus for grid connection or to meet the AC load requirements. At the same time, it can also transfer power from the AC bus to charge the DC bus energy storage system. The on-grid switch is used for the on-grid and off-grid control of the farmer's photovoltaic storage system, and can also be used to simulate the operation control of the system under power outages. Common household electrical appliances are mainly single-phase AC input, so the load access position remains unchanged.

[0044] (2) System Energy Management Process

[0045] The core of the operation and control of the farmer's photovoltaic energy storage and off-grid system is to use the energy storage system to fully absorb the photovoltaic power generation power. Therefore, the energy management process is proposed as follows: Figure 2 As shown. During the day, when there is photovoltaic power generation, the first step is to determine whether there is load power consumption. If not, the photovoltaic power is directly connected to the grid Ppv=Pe; if there is power for the electric load, the power comparison is performed. If they are equal, the photovoltaic power is directly transmitted to the load Ppv=Pl; if they are not equal, the photovoltaic power is greater than the load power, and the SoCe value of the lithium battery is read. If it is less than 0.8, the photovoltaic power meets the load power demand and the excess power is used to charge the energy storage system Ppv=Pe+Pl. If it is not less than 0.8, the photovoltaic power meets the load power demand and the excess power is connected to the grid Ppv=Pg+Pl; if the photovoltaic power is less than the load power, the SoCe value of the lithium battery is read. If it is greater than 0.2, the photovoltaic power and the energy storage system simultaneously supply power to the load Ppv+Pe=Pl. If it is not greater than 0.2, the photovoltaic power and the grid simultaneously supply power to the load Ppv+Pg=Pl.

[0046] At night, when the photovoltaic power generation has no power output, it is also necessary to first determine whether the electrical load has power demand. If so, the SoCo value of the lithium battery is read. If it is greater than 0.2, the energy storage directly supplies power to the load Pe=Pl. If it is not greater than 0.2, the grid directly supplies power to the load Pg=Pl. If there is no electrical load, the SoCo value of the lithium battery is read. If it is greater than the set initial value A, the energy storage will connect the power to the grid. If it is less than A, the grid will charge the energy storage. If it is equal to A, the entire system is in an idle state.

[0047] Analyzing the energy management process of the above system, the judgment of system operation is mainly through comparing the size of photovoltaic power generation and load demand power, while considering the size of the SoCe of the lithium battery in the HESS, and switching the operation control. When the photovoltaic has output, in order to fully absorb the power generation, the control core is dominated by the photovoltaic system. When there is no photovoltaic power generation and there is load power consumption, the load power consumption is dominant. When there is neither photovoltaic power generation nor load demand power, the energy storage system is dominant. Therefore, it is necessary to first clarify the control strategy of each unit, and simplify the design of the photovoltaic storage and off-grid operation control strategy according to the dominant unit of the system energy management process.

[0048] (3) System operation control method

[0049] The DC bus voltage control block diagram of the single-phase full-bridge inverter is as follows: Figure 3 As shown, it is grid-connected operation control. Controlling the DC bus voltage and the grid-connected inductor current as the inner loop improves the dynamic performance of the system. At the same time, it is necessary to detect the frequency and phase of the grid voltage.

[0050] When there is a power outage or the system is off-grid, the solar storage system needs to provide power to the household AC load. The single-phase full-bridge inverter needs to be used as a voltage source to support the AC voltage. The AC bus voltage control block diagram is shown in the figure below. Figure 4 As shown in the figure, the effective value of the AC bus voltage is collected as the feedback control of the voltage loop, and the current loop improves the dynamic performance of the system to meet the load power demand.

[0051] The DC bus voltage control of hybrid energy storage is shown in Figure 5. The common voltage outer loop controls the DC bus voltage, and power distribution is achieved through the filter. The low-frequency component is the reference value of the battery current loop, and the high-frequency component is the reference value of the supercapacitor current loop. At the same time, the bus voltage fluctuation caused by the double frequency power fluctuation of the single-phase full-bridge inverter is considered, and the high-frequency power fluctuation is introduced into the supercapacitor control loop to smooth the bus voltage fluctuation. The duty cycle of the supercapacitor is directly adjusted to achieve fast power compensation of the bus voltage fluctuation.

[0052] Figure 6This is the relationship between the droop coefficients Rhessd and Rgac and SoCo when the hybrid energy storage is discharged and the single-phase full-bridge inverter is connected to the grid. When the SoCo of the lithium battery is smaller, it means that the stored energy is getting less and less. At this time, the value of Rhessd is getting larger and larger, and the output power of the hybrid energy storage is getting smaller and smaller.

[0053] like Figure 7 As shown in the figure, the relationship between the droop coefficients Rhessc and Rgdc and SoCo during hybrid energy storage charging and single-phase full-bridge inverter rectification. When the SoCo of the lithium battery is larger, it means that more and more energy is stored. At this time, the value of Rhessc needs to be increased to reduce the charging power, and the droop coefficient Rgdc also needs to be increased to reduce the rectification power.

[0054] like Figure 8 As shown in the figure, the relationship between the droop coefficients Rhessc and Rgac and SoCo when the hybrid energy storage is charged and the single-phase full-bridge inverter is connected to the grid, when the SoCo of the lithium battery is larger, it means that more and more energy is stored. At this time, it is necessary to increase the value of Rhessc to reduce the charging power, and reduce the value of the droop coefficient Rgac to increase the grid-connected power.

[0055] like Fig. 9 This is the system current waveform under initial control when connected to the grid, which mainly includes the single-phase full-bridge inverter output current iG, the grid-connected current igrid, the load current iload, the battery current ib and the supercapacitor current isc. The randomness of the power load and photovoltaic power generation leads to large fluctuations in the system power. Therefore, in order to balance the system power and achieve stable operation of the system, it is necessary to reasonably control the energy storage and grid-connected units. Zooming in on the time scale, between 1.4s and 1.6s, iG, igrid and iload are in phase. At this time, the single-phase full-bridge inverter works in the grid-connected mode, and ib is less than the charging mode. Between 4.4s and 4.6s, iG and iload are in phase, but the phase difference with igrid is 180°. At this time, the single-phase full-bridge inverter works in the rectification mode, and ib is less than the charging mode.

[0056] Fig.10 The DC bus and AC bus voltage waveforms of the system, where the DC bus voltage udcbus has a large ripple and is affected by the fluctuation of photovoltaic power generation and load switching. Since it is connected to the grid, the AC bus voltage uacbus is supported by the grid and the voltage waveform is relatively stable.

[0057] Fig.11 The system current waveform after adding DC bus voltage compensation control, the DC bus voltage fluctuation is added to the supercapacitor current control to directly adjust the duty cycle to achieve bus voltage fluctuation smoothing. In the figure, the current of the supercapacitor fluctuates with a large amplitude to achieve DC bus power fluctuation smoothing.

[0058] Fig.12 For the AC / DC bus voltage fluctuation, it can be found that after adding the DC bus fluctuation smoothing control, the DC bus voltage udcbus ripple is reduced and the bus voltage is relatively stable.

[0059] When the hybrid energy storage and the single-phase full-bridge inverter simultaneously control the DC bus voltage, the power distribution is achieved by adjusting the droop coefficient based on the droop control method. Fig.13 It is the system current under the action of bus voltage compensation and droop control when connected to the grid.

[0060] Fig.14 The AC and DC bus voltages are stable after adding DC bus voltage compensation and power distribution control.

[0061] Fig.15 The curves of the power distribution droop coefficient of the system hybrid energy storage and the single-phase full-bridge inverter are shown. When the SOCe rises, the power distribution droop coefficient of the hybrid energy storage increases accordingly. The power distribution droop coefficient of the single-phase full-bridge inverter is first determined by its working state (inversion / rectification), and then the droop coefficient changes with the size of the SOCe.

[0062] Fig.16 This is the system current waveform under the initial operation control when off-grid, the off-grid operation igrid is 0, and the grid-connected switch is disconnected. The system provides power to the load through the single-phase full-bridge inverter from the photovoltaic power generation and the hybrid energy storage power, iG = iload.

[0063] Fig.17 is the system AC and DC bus voltage. The DC bus voltage has large ripples and is affected by the fluctuation of photovoltaic power generation. In the off-grid state, the single-phase full-bridge inverter needs to control the AC bus voltage. In the figure, the AC bus voltage is stable.

[0064] Fig.18 When adding DC bus voltage compensation to the system current for operation control when off-grid, it can be found that the supercapacitor current has high-amplitude and high-frequency fluctuations, which not only absorbs the fluctuations in photovoltaic power generation, but also absorbs the high-frequency fluctuations in the DC bus voltage.

[0065] Fig.19 This is the system AC / DC bus voltage waveform. After adding the DC bus voltage compensation operation control, the high-frequency ripple of the DC bus voltage is reduced.

[0066] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the device embodiment, the above is only a preferred embodiment of the present invention. Since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment. The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited to this. Any technician familiar with the technical field is within the technical scope disclosed by the present invention. For ordinary technicians in the technical field, changes or replacements that can be easily thought of should be covered within the protection scope of the present invention without departing from the principle of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A multi-modal simplified operation control method for a photovoltaic-hybrid energy storage-on-grid and off-grid system, characterized in that: Based on the energy management method, a hybrid energy storage system composed of lithium batteries and supercapacitors is used to simplify the system operation mode into three dominant modes: photovoltaic, energy storage and load. The droop coefficient is adjusted based on the charge state of the energy storage system. At the same time, the power distribution between the system hybrid energy storage and the single-phase full-bridge inverter is realized, and the power distribution of the hybrid energy storage is realized, and the DC bus fluctuation is smoothed and controlled; The power distribution process between the hybrid energy storage and the single-phase full-bridge inverter of the system is as follows: when photovoltaic power is output, the DC bus power will increase, causing the DC bus voltage to increase. At this time, the full-bridge inverter absorbs the photovoltaic power and connects to the grid by controlling the DC bus voltage to stabilize; when the photovoltaic power is less, the DC bus power needs to charge the energy storage system, and the bus voltage will decrease as the DC bus power decreases. At this time, it is necessary to absorb power from the grid to the DC bus to maintain the bus voltage stable, and the full-bridge inverter works in a rectifying state; the hybrid energy storage and the single-phase full-bridge inverter control the bus voltage at the same time, which is equivalent to two voltage sources in parallel. In order to achieve power distribution between the hybrid energy storage and the inverter, the charge state of the lithium battery in the hybrid energy storage is considered at the same time, thereby obtaining the control droop coefficient of the single-phase full-bridge inverter; Among them, the single-phase full-bridge inverter controls the droop coefficient R G It is expressed as: In the formula, r0 is the initial coefficient of the single-phase full-bridge inverter control, and n is the adjustment coefficient; The charge and discharge droop coefficient of hybrid energy storage can be expressed as In the formula, r1 is the initial coefficient of charging and discharging of hybrid energy storage, and n is the adjustment coefficient.

2. The multi-modal simplified operation control method of the photovoltaic-hybrid energy storage-on-grid and off-grid system according to claim 1 is characterized in that: The specific process of simplifying the system operation mode is as follows: the judgment of the system operation mode is mainly through comparing the size of photovoltaic power generation and load demand power, and considering the size of lithium battery charge state SoCe in hybrid energy storage, and switching the operation control; when photovoltaic has output, in order to fully absorb the power generation, the photovoltaic system is the dominant system operation core; when there is no photovoltaic power generation and there is load power consumption, the load power consumption is dominant; when there is neither photovoltaic power generation nor load demand power, the energy storage system is dominant.

3. The multi-modal simplified operation control method of the photovoltaic-hybrid energy storage-on-grid and off-grid system according to claim 1 is characterized in that: The power distribution process of the hybrid energy storage is as follows: the hybrid energy storage controls the DC bus voltage through a common bus voltage outer loop, and realizes frequency division by connecting a low-pass filter in series as the reference of the current loop, with the low-frequency component given to the lithium battery and the high-frequency component given to the supercapacitor; at the same time, considering the bus voltage fluctuation caused by the double frequency power fluctuation of the single-phase full-bridge inverter, it is proposed to incorporate the DC bus voltage fluctuation smoothing control link into the output end of the supercapacitor current loop, and directly adjust the duty cycle to realize the bus voltage fluctuation smoothing; the hybrid energy storage and the single-phase full-bridge inverter control the bus voltage at the same time, and realize power distribution in combination with droop control.

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