Multi-port DC-direct-connected battery energy storage system and method applicable to high voltage and large capacity

Abstract

This application is a multi-port DC-directly-connected battery energy storage system and method applicable to high voltage and large capacity, belonging to the field of power grid energy storage. Aiming at the problems of low energy storage capacity, poor expandability, poor battery-friendly characteristics, poor grid connection flexibility, and low reliability in the mainstream energy storage device topologies at present, a multi-port DC-directly-connected battery energy storage system and method applicable to high voltage and large capacity are provided, which includes multiple parallel single-string devices. The first end of the single-string device forms a high-voltage DC port, and the second end forms a medium-voltage DC port. Each single-string device includes: an energy storage unit, which includes multiple cascaded energy storage sub-modules. The energy storage sub-module includes a first half-bridge structure and a first capacitor connected in parallel, and a battery pack connected in parallel with the first capacitor; a voltage control unit, which includes multiple cascaded voltage control sub-modules. The voltage control sub-module includes a second half-bridge structure and a second capacitor connected in parallel. Both the first half-bridge structure and the second half-bridge structure are switching devices composed of IGBTs and anti-parallel diodes.

Description

Technical Field

[0001] The present invention belongs to the field of power grid energy storage, and particularly relates to a multi-port DC-directly-connected battery energy storage system and method suitable for high voltage and large capacity. Background Art

[0002] Traditional new energy aggregation technologies only interconnect grid-connected new energy power stations and then centrally connect them to the power grid. They are "channel-type" aggregation stations, which are highly dependent on the power grid. In case of a fault, power will be lost, causing a secondary impact on the power grid and being unable to quickly provide restorative support to the power grid. The electrical distances between a large number of sea winds in the aggregation system are shortened. If connected to a system with a low short-circuit ratio, new energy power stations are more vulnerable to the transient process of the power grid, and the risk of broadband oscillation in the aggregation system is greatly increased. At the same time, submarine cables have a large amplification effect on power grid harmonics. After the sea winds are aggregated, the amplification effect mechanism will become more complex, increasing the difficulty of governance. Therefore, there is an urgent need to conduct research on the grid-forming active support control technology for sea wind aggregation stations.

[0003] Currently, some scholars have proposed adding energy storage devices at the new energy aggregation point to use the energy storage to suppress the power fluctuations of new energy, and also to endow the energy storage with grid-forming characteristics through control means to enhance the inertia of new energy power stations and provide restorative support during faults. In the face of large-scale new energy grid connection, small-capacity distributed energy storage devices have disadvantages such as difficult unified energy scheduling and slow power output. Therefore, it is necessary to equip large-capacity centralized energy storage devices at new energy aggregation stations. However, the energy storage device topologies currently used in engineering applications not only have certain disadvantages in high-voltage and large-capacity application scenarios, but also have a single grid connection port and poor grid connection flexibility. For example, the low-voltage energy storage converter parallel boosting scheme used in a multi-megawatt energy storage power station in a certain place in Shandong is equipped with 37 step-up transformers to connect 37 battery clusters and power conversion systems. The multi-transformer boosting grid connection causes problems such as large floor area, low transmission efficiency, and slow power response speed, making the economy and various technical indicators of this scheme poor. Although there are many grid connection ports, it is not suitable for the shared energy storage business model. To reduce the cost investment in the conversion link and improve the transmission efficiency, some scholars have conducted detailed research on the high-voltage directly-connected chain-type energy storage structure. Some scholars have detailedly studied the AC-DC decoupling control, SOC balancing, and common-mode current suppression control strategies of the chain-type energy storage topology, and evaluated factors such as the efficiency and reliability of the system. Although the chain-type energy storage topology has improved in terms of economy and transmission efficiency, the large number of cascaded modules increases the failure rate of this scheme, and the disconnector and circuit breaker can only be added in the phase unit of the chain-type energy storage topology. If a faulty sub-module is repaired or replaced, the entire energy storage device needs to be taken out of service, reducing the reliability. Moreover, the sub-module utilization rate of the chain-type energy storage topology is high, and the number of sub-modules required under the same voltage level is small, resulting in limited energy storage capacity configuration. Currently, there is no actual project with an energy storage capacity exceeding 30 MW.

[0004] To increase the energy storage configuration and improve the grid connection flexibility of energy storage devices, many scholars have incorporated energy storage batteries into the sub-modules of a modular multilevel converter (MMC) to form a distributed energy storage type MMC. Its energy storage capacity is equivalent to twice that of the cascaded energy storage topology. The additional DC ports can achieve diverse grid connections and have the potential for energy storage sharing. However, the capacitor voltage in the internal sub-modules of the MMC has a high second-harmonic ripple, which severely reduces the battery life. Some scholars have reduced the second-harmonic current flowing into the battery by improving the battery connection method, but at the expense of economy. Similar to the cascaded topology, this topology has low reliability and has not been applied in engineering yet.

[0005] Therefore, it is necessary to improve the existing topology to balance large energy storage capacity, battery-friendly characteristics, high energy transfer efficiency, and high reliability. Summary of the Invention

[0006] Aiming at the problems of low energy storage capacity, poor scalability, poor battery-friendly characteristics, poor grid connection flexibility, and low reliability in the current mainstream energy storage device topologies, a multi-port DC direct-attached battery energy storage system and method applicable to high voltage and large capacity are provided. It has DC ports for grid connection at multiple voltage levels, and a multi-port DC direct-attached battery energy storage system (Multi-port DC direct-attached battery storage energy system, MDC-BESS) with large energy storage capacity and high reliability. It has a large energy storage capacity, good reliability, and a cascaded form of multiple energy storage units, which can achieve battery stack segmentation, effectively reduce the circulating current and losses caused by the barrel effect in large battery stacks, and can also effectively reduce the impact of low-frequency ripple flowing into the battery on the battery life.

[0007] To this end, the present application provides a multi-port DC direct-attached battery energy storage system applicable to high voltage and large capacity, including a plurality of parallel single-string devices. The first end of the single-string device forms a high-voltage DC port, and the second end forms a medium-voltage DC port. Each single-string device includes the following structure:

[0008] Energy storage unit, where the energy storage unit is at least one and includes a plurality of cascaded energy storage sub-modules. The energy storage sub-module includes a first half-bridge structure and a first capacitor connected in parallel, and a battery pack connected in parallel with the first capacitor. The voltages at the ends of all the first capacitors and the battery packs connected in parallel with the first capacitors in the energy storage unit are equal;

[0009] Voltage control unit, where the voltage control unit is one and includes a plurality of cascaded voltage control sub-modules. The voltage control sub-module includes a second half-bridge structure and a second capacitor connected in parallel. The capacitor voltages of the voltage control sub-module and the energy storage sub-module, and the voltage at the end of the battery pack connected in parallel with the first capacitor are equal;

[0010] Both the first half-bridge structure and the second half-bridge structure are switching devices composed of IGBTs and anti-parallel diodes; the first end of the voltage control unit is connected to the energy storage unit, and the second end of the voltage control unit extends backward, forming the medium-voltage DC port at the second end of the voltage control unit.

[0011] Further, each of the energy storage sub-modules internally has a first on-off switch and a second on-off switch that conduct complementarily. When the first on-off switch is turned on and the second on-off switch is turned off, the corresponding energy storage sub-module is put into use, thereby providing the capacitor voltage of one energy storage sub-module.

[0012] Further, the voltage control sub-module internally has a first conduction switch and a second conduction switch that conduct complementarily. When the first conduction switch is turned on and the second conduction switch is turned off, the voltage control sub-module is put into use, thereby providing the capacitor voltage of one voltage control sub-module.

[0013] Further, DC circuit breakers are provided at both ends of the single-string device for online switching during maintenance and operation. This topology has a large energy storage capacity configuration and easy expandability, and the presence of multiple ports makes it flexible to be connected to the grid. The energy storage can be shared by multiple load users, and the application scenarios are extensive.

[0014] In the medium-voltage control unit of the topology structure of the multi-port DC directly-connected battery energy storage system suitable for high voltage and large capacity, medium-voltage DC ports are led out at both ends. The functional requirements on the medium-voltage outlet side will directly affect the control method of the voltage control unit. The sending-end and receiving-end converters on the high-voltage side have coordinated control means. One converter can be set to a constant DC voltage and the other to a constant power. Therefore, the functional requirements for the high-voltage DC port are relatively small, and it is only necessary to keep the output voltage of the high-voltage DC port unchanged. Therefore, the control strategies are divided into two categories. One is that there is a constant voltage requirement at the medium-voltage DC port, and the voltage control unit will provide DC voltage support for the medium-voltage port; the other is that there is a constant power requirement at the medium-voltage DC port. At this time, the voltage control unit will transfer the energy in the multi-port DC directly-connected battery energy storage system to the medium-voltage side.

[0015] A multi-port DC directly-connected battery energy storage method suitable for high voltage and large capacity includes the following steps:

[0016] Determine whether it is in the constant voltage mode according to the requirements of the medium-voltage DC port. If so, control the voltage control unit to operate at a fixed voltage and execute the constant voltage mode;

[0017] Obtain the reference value of the output voltage of the energy storage unit according to the difference between the reference value of the high-voltage DC port voltage and the total capacitor voltage of the voltage control sub-modules put into use. Then, obtain the number of energy storage sub-modules that should be put into use according to the quotient of the reference value of the output voltage of the energy storage unit and the average capacitor voltage of the energy storage sub-modules put into use. After rounding by the round function, obtain the actual number of energy storage sub-modules that should be put into use. At this time, the energy storage unit is in open-loop control and outputs a fixed number of energy storage sub-modulesN H ;

[0018] Based on the obtained number of energy storage sub - modules to be put into operation N H , the SOC - NLM sorting algorithm is used to output the actual valve control signal G H , to control the switches of the energy storage sub - modules, so as to realize the input and cut - off of the energy storage sub - modules;

[0019] The reference value of the medium - voltage DC port voltage is obtained according to the difference between the reference value of the high - voltage DC port voltage and the total voltage of the actually - to - be - put - into - operation energy storage sub - modules; after the difference between the reference value of the medium - voltage DC port voltage and the actual value of the medium - voltage DC port voltage is input into the PI link, the reference value of the capacitor current flowing through the voltage - controlled unit is obtained; the difference between the average SOC of each single - string device and the average SOC of the multi - port DC - directly - connected battery energy storage system is input into the PI link to obtain the unbalanced current, and the unbalanced current is accumulated into the error between the reference value of the capacitor current flowing through the voltage - controlled unit and the actual value of the capacitor current flowing through the voltage - controlled unit. After being processed by the PI link and the round - off function in turn, the actually - to - be - put - into - operation number N of the voltage - controlled sub - modules is obtained V ;

[0020] Based on the obtained actually - to - be - put - into - operation number of the voltage - controlled sub - modules, the NLM capacitor voltage sorting algorithm is used to output the valve control signal to control the switches of the voltage - controlled sub - modules, so as to realize the input and cut - off of the voltage - controlled sub - modules.

[0021] When the medium - voltage port is a self - constructed network user (with a certain amount of new - energy aggregation), constant - voltage control can be adopted.

[0022] Furthermore, the calculation formula for the actually - to - be - put - into - operation number of the energy storage sub - modules is:

[0023] (4);

[0024] In the formula: N H is the actually - to - be - put - into - operation number of the voltage - controlled sub - modules, is the average capacitor voltage of the voltage - controlled sub - modules, is the average capacitor voltage of the energy storage sub - modules, U dc1ref The reference value of the output voltage of the energy storage unit.

[0025] Furthermore, it is determined whether it is in the constant - voltage mode according to the demand of the medium - voltage DC port. Otherwise, the voltage - controlled unit is controlled to work at a given power, specifically:

[0026] Subtract the total capacitance voltage value of the energy storage sub-modules put into operation from the difference between the reference value of the high-voltage DC port voltage and the total capacitance voltage of the voltage-controlled sub-modules put into operation. Then, after processing through a PI link, obtain the reference value of the current flowing through the voltage control unit. The difference between the actual value of the current flowing through the voltage control unit and the reference value of the current flowing through the voltage control unit is processed through a PI link and rounded by a round function in sequence to obtain the number of energy storage sub-modules to be put into operation;

[0027] Output a valve control signal through the SOC-NLM sorting algorithm to control the switches of the energy storage sub-modules, so as to realize the input and removal of the energy storage sub-modules;

[0028] Inject the unbalanced power output by the SOC equalization loop into the difference between the reference value of the output power of the medium-voltage DC port and the actual value of the output power of the medium-voltage DC port. Then, perform PI link processing to obtain the reference value of the battery current flowing through the voltage control unit. The error between the reference value of the battery current flowing through the voltage control unit and the actual value of the capacitance current flowing through the voltage control unit is processed through a PI link and rounded by a round function in sequence to obtain the number of voltage-controlled sub-modules to be actually put into operation; where the unbalanced power Δ P is obtained after processing through a PI link for the difference between the average SOC of each single-string device and the average SOC of the multi-port DC directly-connected battery energy storage system;

[0029] Output a valve control signal through the NLM sorting algorithm to control the switches of the voltage-controlled sub-modules, so as to realize the input and removal of the voltage-controlled sub-modules.

[0030] Furthermore, the SOC-NLM sorting algorithm is an energy storage sub-module battery charge quantity equalization sorting algorithm. The specific process is as follows: Set the sorting frequency f in and the sorting period T in , and add a SOC difference ΔSOC threshold μ max limit, that is, only when the difference between the maximum SOC and the minimum SOC of the sub-module battery is greater than μ max and the next sorting period T in (k + 1) is reached will re-sorting and switching be performed. Otherwise, the working modes of each sub-module remain unchanged.

[0031] The sorting frequency f in is related to the SOC equalization time of the sub-modules. The faster the SOC equalization speed is required, f inThe larger it is. The SOC-NLM algorithm sorts and switches the battery SOC in the energy storage sub-module. The basic principle is as follows: when the current flows into the energy storage device, it judges the maximum value and preferentially inputs the half-bridge energy storage unit with a smaller SOC; when the current flows out of the energy storage device, it judges the maximum value and the minimum SOC and preferentially inputs the half-bridge energy storage unit with a larger SOC.

[0032] Furthermore, the expression of the NLM sorting algorithm is:

[0033] (6);

[0034] Where χ is the ratio of the DC voltage fluctuation magnitude to the capacitor voltage of a single energy storage sub-module, is the maximum value of the current flowing through the energy storage battery, L d is the DC side smoothing reactor, f con is the switching frequency of the energy storage sub-module, U dc1 is the high-voltage DC port voltage. It can be seen from this formula that the higher the switching frequency, the smaller the DC voltage fluctuation rate. The switching frequency f con of the energy storage sub-module is related to the power quality of the outputs of the high-voltage DC port and the medium-voltage DC port.

[0035] This control method adopts partial open-loop control, and the voltage control unit cooperates with the energy storage unit to achieve closed-loop constant voltage control. The high-frequency switching generated by the PI control is borne by the VC-SM, which can reduce the switching frequency of the HB-SM containing the battery, reduce the high-frequency current flowing into the battery due to high-frequency switching, and improve the battery life.

[0036] Advantages of this application: This application is applicable to a multi-port DC directly-connected battery energy storage system and method for high voltage and large capacity. The use of multiple strings of energy storage devices in parallel in the system can greatly improve the energy storage capacity configuration, and the multi-string parallel structure has higher reliability compared to the chain-type high-voltage directly-connected energy storage topology; it has the ability to transform voltage levels, outputs at both high-voltage and medium-voltage DC ports, and is more flexible in grid connection. Moreover, it saves transformers and improves the energy transmission efficiency, effectively enhancing the energy storage sharing ability; there is no low-frequency pulsation on the DC side, and the constant voltage control of the proposed voltage control unit can eliminate the high-frequency pulsation flowing into the energy storage battery, effectively improving the battery life. The two control modes can meet the requirements of conventional users at the medium-voltage DC port. When the medium-voltage port is a self-constructed grid user (with a certain amount of new energy aggregation), constant voltage control can be adopted; when the medium-voltage port is a conventional user (such as some factories, residential users, etc.), constant power control can be adopted; and when constant voltage control is adopted, the energy storage sub-module operates in open loop without high-frequency switching, which can effectively improve the battery life. Brief Description of the Drawings

[0037] Figure 1 Schematic diagram of the access position of a multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity;

[0038] Figure 2 Schematic diagram of the structure of a multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity;

[0039] Figure 3 Equivalent circuit diagram of a multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity;

[0040] Figure 4 Flow chart of constant voltage control;

[0041] Figure 5 Flow chart of constant power control;

[0042] Figure 6 Flow chart of SOC-NLM sorting algorithm;

[0043] Figure 7 DC voltage waveform diagram of medium voltage constant voltage control;

[0044] Figure 8 DC voltage waveform diagram of medium voltage constant power control. Detailed implementation manner

[0045] Example 1

[0046] This example is a multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity, as Figure 1 , Figure 2 shown, including multiple parallel single-string devices, the first end of the single-string device forms a high-voltage DC port, and the second end forms a medium-voltage DC port. Each single-string device includes the following structure:

[0047] Energy storage unit, the energy storage unit is at least one, and it includes N H cascaded energy storage sub-modules. The energy storage sub-module includes a first half-bridge structure and a first capacitor connected in parallel, and a battery pack connected in parallel with the first capacitor;

[0048] Voltage control unit, the voltage control unit is one, and it includes N V cascaded voltage control sub-modules. The voltage control sub-module includes a second half-bridge structure and a second capacitor connected in parallel. The capacitor voltages of the voltage control sub-module and the energy storage sub-module are equal;

[0049] The first end of the voltage control unit is connected to the energy storage unit, and the second end of the voltage control unit extends backward, and the medium-voltage DC port is formed at the second end of the voltage control unit.

[0050] Each of the energy storage sub - modules internally incorporates a first on - off switch T and a second on - off switch T which are complementary - conducting. When the first on - off switch T is turned on and the second on - off switch T is turned off, the corresponding energy storage sub - module is put into use, thereby providing the capacitor voltage of an energy storage sub - module. h1 and a second on - off switch T h2 , when the first on - off switch T h1 is turned on and the second on - off switch T h2 is turned off, the corresponding energy storage sub - module is put into use, thereby providing the capacitor voltage of an energy storage sub - module.

[0051] The voltage - controlled sub - module internally incorporates a first conducting switch T and a second conducting switch T which are complementary - conducting. When the first conducting switch T is turned on and the second conducting switch T is turned off, the voltage - controlled sub - module is put into use, thereby providing the capacitor voltage of a voltage - controlled sub - module. v1 and a second conducting switch T v2 , when the first conducting switch T v1 is turned on and the second conducting switch T v2 is turned off, the voltage - controlled sub - module is put into use, thereby providing the capacitor voltage of a voltage - controlled sub - module.

[0052] The voltage ratio between the high - voltage side and the medium - voltage side is determined by the quantity ratio η of the two types of sub - modules:

[0053] (1);

[0054] In the formula, U dc1 is the high - voltage DC port voltage, U dc2 is the medium - voltage DC port voltage, N H is the actual number of voltage - controlled sub - modules to be put into use, N V is the actual number of energy storage sub - modules to be put into use. Among them, the medium - voltage DC port is led out from both ends of the voltage - controlled unit cascade, and the high - voltage DC port voltage is led out from both ends after the two types of sub - module units are connected in series.

[0055] The relationship expression between the high - voltage DC port voltage and the medium - voltage DC port voltage is:

[0056] (2);

[0057] In the formula, U H is the first capacitor voltage, U V is the second capacitor voltage, and U H = U V .

[0058] DC circuit breakers are provided at both ends of the single - string device for online switching during maintenance and operation. This topological structure has a large energy storage capacity configuration and easy expandability, and the presence of multiple ports makes its grid connection flexible. The energy storage can be shared by multiple load users, and the application scenarios are extensive.

[0059] In the topology structure of a multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity, medium voltage DC ports are led out at both ends of the voltage control unit, and the functional requirements on the medium voltage outlet side will directly affect the control mode of the voltage control unit. The sending-end and receiving-end converters on the high voltage side have coordinated control means, and one converter can be set to a constant DC voltage and the other to a constant power. Therefore, the functional requirements for the high voltage DC port are small, and it is only necessary to keep the output voltage of the high voltage DC port unchanged. Therefore, the control strategies are divided into two categories. One is that there is a constant voltage requirement at the medium voltage DC port, and the voltage control unit will provide DC voltage support for the medium voltage port; the other is that there is a constant power requirement at the medium voltage DC port, and at this time, the pressure difference unit will transfer the energy in the multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity to the medium voltage side.

[0060] Embodiment 2

[0061] This embodiment is a method for a multi-port DC directly-connected battery energy storage applicable to high voltage and large capacity, which is implemented by using the system of Embodiment 1, as Figure 4 、 Figure 5 shown, and includes the following steps:

[0062] Judge whether it is in the constant voltage mode according to the requirements of the medium voltage DC port. If so, control the voltage control unit to operate at a fixed voltage. The flow chart is as Figure 4 shown. When actually judging that the medium voltage port is a self-constructed network user (with a certain amount of new energy aggregation), the specific process is as follows;

[0063] According to the high voltage DC port voltage reference value U dc1ref and the total capacitance voltage of the voltage control sub-modules put into operation to obtain the output voltage reference value of the energy storage unit. Then, according to the quotient of the output voltage reference value of the energy storage unit and the average capacitance voltage of the energy storage sub-modules put into operation (i.e., ), obtain the number N Href of energy storage sub-modules that should be put into operation. After rounding by the round function, obtain the actual number N H of energy storage sub-modules that should be put into operation;

[0064] Based on the obtained actual number N H of energy storage sub-modules that should be put into operation, use the SOC-NLM sorting algorithm to output the actual valve control signal G H to control the switches of the energy storage sub-modules, so as to realize the input and cut-off of the energy storage sub-modules;

[0065] According to the high voltage DC port voltage reference value U dc1refThe difference from the total voltage of the energy storage sub-modules that should actually be input is used to obtain the reference value of the medium-voltage DC port voltage U dc2ref ; The reference value of the medium-voltage DC port voltage U dc2ref is subtracted from the actual value of the medium-voltage DC port voltage U dc2 and then input into the PI link to obtain the reference value of the capacitor current flowing through the voltage control unit ; The difference between the average SOC of each single-string device and the average SOC (SOC) of the multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity avg is input into the PI link to obtain the unbalanced current . The unbalanced current is accumulated into the error between the reference value of the capacitor current flowing through the voltage control unit and the actual value of the capacitor current flowing through the voltage control unit i V . After being processed by the PI link and rounded by the round function in sequence, the actual number N of voltage control sub-modules to be input is obtained V ;

[0066] Based on the obtained actual number N of voltage control sub-modules to be input V , the valve control signal G V is output by using the NLM capacitor voltage sorting algorithm to control the switches of the voltage control sub-modules, thereby realizing the input and removal of the voltage control sub-modules

[0067] In the case of constant voltage control of the voltage control unit, the open-loop control of the energy storage unit only plays a voltage support role. The open-loop control is equivalent to a controlled current source that changes with the state of charge (SOC) of the battery. When the voltage control unit performs constant voltage closed-loop control, it can also be equivalent to a controlled voltage source. The equivalent circuit diagram of this topology can be represented by Figure 3 , where according to the Shepherd equivalent model of the lithium iron phosphate battery R b is the equivalent internal resistance, and the capacitor of the HB-SM energy storage unit is clamped by the battery port voltage. N H U H is equivalent to the battery port voltage E b , which is affected by the battery charge and discharge current and the remaining charge amount, and can be expressed by the following formula

[0068] (3);

[0069] In the formula E 0 is the no-load electromotive force of the fully charged battery Kis the battery polarization constant, which is related to the battery material. Q m is the battery capacity, with the unit of Ah. A and B are the voltage amplitude and time constant under the charge-discharge exponential state. i b is the battery charge-discharge current. The equivalent circuit model of the lithium iron phosphate battery can be obtained from this formula. The expression for the actual number of HB-SM energy storage sub-modules to be put into operation is as follows.

[0070] (4);

[0071] For the overall operation function of the multi-port DC directly-connected battery energy storage system topology suitable for high voltage and large capacity, the priority is to ensure the stability of the high-voltage DC port voltage. Therefore, it is necessary to coordinate and cooperate between the energy storage unit and the voltage control unit to control the high-voltage DC voltage.

[0072] The expression for the reference value of the output voltage of the energy storage unit is:

[0073] (5);

[0074] The reference value of the output voltage of the energy storage unit is subtracted from the sum of the actual values of the capacitor voltages of the sub-modules put into the energy storage unit. The resulting error passes through the PI to output the reference value of the current flowing through the HB-SM battery , and then it is subtracted from the actual value of the current flowing through the HB-SM battery i b The difference is output through the current inner loop PI to obtain the reference value of the HB-SM to be put into operation, and finally the valve control signal is output through the round function and the SOC-NLM sorting algorithm.

[0075] Since the device adopted in this application is connected to the high-voltage DC port in multiple series and parallel, after multiple single-series devices are connected in parallel, there will be deviations in the overall SOC of the batteries in each single-series device. To balance the SOC between single-series devices, this control adds an SOC balance loop. The SOC balance loop can eliminate the error between SOCs through the PI, achieving the function of balancing the SOC between multiple series.

[0076] This control strategy adopts partial open-loop control, and the voltage control unit cooperates with the energy storage unit to achieve closed-loop constant voltage control. The high-frequency switching generated by the PI control is borne by the VC-SM, which can reduce the switching frequency of the HB-SM containing the battery, reduce the high-frequency current flowing into the battery due to high-frequency switching, and improve the battery life.

[0077] When determining whether it is in the constant voltage mode according to the requirements of the medium-voltage DC port, otherwise control the voltage control unit to work at the given power. In actual judgment, the medium-voltage port is used as a regular user (such as some factories, residential users, etc.). The flow chart is as Figure 5 shown, specifically:

[0078] From the difference between the reference value of the high-voltage DC port voltage U dc1ref and the total capacitor voltage of the voltage-controlled sub-modules that are put into operation subtract the total capacitor voltage value of the energy storage sub-modules that are put into operation , and then obtain the reference current flowing through the voltage control unit after processing by a PI link , take the reference current flowing through the voltage control unit and the actual current flowing through the voltage control unit i b , and perform rounding processing on the difference through a PI link and the round function in sequence to obtain the number of energy storage sub-modules that should be put into operation N H ;

[0079] Based on the obtained number of energy storage sub-modules that should be put into operation N H , output a valve control signal through the SOC-NLM sorting algorithm G H , to control the switches of the energy storage sub-modules, so as to realize the input and cut-off of the energy storage sub-modules;

[0080] Inject the unbalanced power Δ P output by the SOC balancing loop into the difference between the reference value of the output power of the medium-voltage DC port and the actual output power of the medium-voltage DC port P 2, and then perform PI link processing to obtain the reference battery current flowing through the voltage control unit , take the reference battery current flowing through the voltage control unit and the actual capacitor current flowing through the voltage control unit i V , and perform PI link and rounding processing on the error through the round function in sequence to obtain the actual number N of voltage-controlled sub-modules that should be put into operation V ; among them, the unbalanced power Δ P output by the SOC balancing loop is the difference between the average SOC of each string SOC avg and the average SOC applicable to the multi-port DC directly-connected battery energy storage system for high-voltage and large-capacity applications, and is obtained after processing by a PI link;

[0081] Based on the obtained actual number of voltage-controlled sub-modules that should be put into operation N V , output a valve control signal through the NLM sorting algorithm G V , to control the switches of the voltage-controlled sub-modules, so as to realize the input and cut-off of the voltage-controlled sub-modules.

[0082] When the network connected to the medium-voltage DC port needs to be applicable to the given power input and output of a high-voltage and large-capacity multi-port DC directly-connected battery energy storage system, the voltage control unit needs to adopt constant-power control to transfer the energy in the energy storage battery applicable to the high-voltage and large-capacity multi-port DC directly-connected battery energy storage system to the medium-voltage terminal. At this time, to ensure the stability of the high-voltage DC port voltage, the energy storage unit needs to participate in the constant-voltage closed-loop control.

[0083] The voltage control unit adopts constant-power control and uses a power-current double-loop control. The energy storage unit operates in the constant-voltage control operation mode at this time and uses a voltage-current double-loop control. The high-frequency switching generated by the PI control is borne by the VC-SM, which can reduce the switching frequency of the HB-SM containing the battery, reduce the high-frequency current flowing into the battery due to high-frequency switching, and improve the battery life. At this time, the active power required by the medium-voltage DC port is given by the voltage control unit, and the DC voltage of the high-voltage DC port is controlled by the energy storage unit.

[0084] The SOC-NLM sorting algorithm is the battery charge quantity equalization sorting algorithm for the energy storage sub-module: Its flow chart is as attached Figure 6 as shown, set the sorting frequency f in and the sorting period T in , and add a SOC difference ΔSOC threshold μ max limit, that is, only when the difference between the maximum SOC and the minimum SOC of the sub-module battery is greater than μ max and the clock reaches the next sorting period T in (k + 1) will it be re-sorted and switched, otherwise the working mode of each sub-module remains unchanged. During charging, the HB-SM with a smaller SOC is preferentially inserted according to the SOC sequence, and vice versa during discharging. f in The value is only determined by the SOC equalization duration between the HB-SM sub-modules. This algorithm is an existing technology.

[0085] The sorting frequency f in is related to the SOC equalization time of the sub-module. The faster the SOC equalization speed is required, f in the greater it is. The SOC-NLM algorithm sorts and switches according to the SOC size of the batteries in the energy storage sub-module. The basic principle is: when the current flows into the energy storage device, judge the maximum value and preferentially insert the half-bridge energy storage unit with a smaller SOC; when the current flows out of the energy storage device, judge the maximum value and the minimum SOC and preferentially insert the half-bridge energy storage unit with a larger SOC.

[0086] The expression of the NLM sorting algorithm is:

[0087] (6);

[0088] Wherein χ is the ratio of the magnitude of the DC voltage fluctuation to the capacitor voltage of a single energy storage sub-module U H of; is the maximum value of the current flowing through the energy storage battery, L d is the DC side smoothing reactor, f con is the switching frequency of the energy storage sub-module, U dc1 is the HVDC port voltage. It can be seen from this formula that the higher the switching frequency, the smaller the DC voltage fluctuation rate, and the switching frequency f con is related to the power quality of the output of the HVDC port and the MVDC port. This algorithm is an existing technology.

[0089] The parameters of the simulation system are shown in Table 1. As Figure 7 and Figure 8 shown, Figure 7 (a) in is the DC voltage output diagram of the high-voltage port in the constant voltage mode; Figure 7 (b) in is the DC voltage output diagram of the medium-voltage port in the constant voltage mode; Figure 7 It can be seen from that in the constant voltage control mode of the medium-voltage port, the overall voltage of the HVDC port is controlled and maintained by the high-frequency switching of the voltage control unit, and the magnitude of the voltage fluctuation of the HVDC port is also related to the high-frequency switching of the voltage control unit. Figure 8 (a) in is the DC voltage output diagram of the high-voltage port in the constant power mode; Figure 8 (b) in is the DC voltage output diagram of the medium-voltage port in the constant power mode; Figure 8 It can be seen from that in the constant power control mode of the medium-voltage port, the change in the output power of the medium-voltage port can mobilize the DC voltage fluctuation of the overall HVDC port, because at this time the energy storage unit and the voltage control unit jointly maintain the voltage of the HVDC port.

[0090] Table 1

[0091]

[0092] As described above, only the specific embodiments of the present invention are described, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that the present invention includes but is not limited to the content described in the drawings and the above specific embodiments. Any modification that does not deviate from the functional and structural principles of the present invention will be included in the scope of the claims.

Claims

1. A multi-port DC directly-connected battery energy storage method applicable to high voltage and large capacity, characterized in that Adopt a multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity. The multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity includes multiple parallel single-string devices. The first end of the single-string device forms a high-voltage DC port, and the second end forms a medium-voltage DC port. Each single-string device includes the following structure: Energy storage unit. The energy storage unit is at least one and includes multiple cascaded energy storage sub-modules. The energy storage sub-module includes a first half-bridge structure and a first capacitor connected in parallel, and a battery pack connected in parallel with the first capacitor. The voltages at the ends of all the first capacitors and the battery packs connected in parallel with the first capacitors in the energy storage unit are equal; Voltage control unit. The voltage control unit is one and includes multiple cascaded voltage control sub-modules. The voltage control sub-module includes a second half-bridge structure and a second capacitor connected in parallel. The capacitor voltages of the voltage control sub-module and the energy storage sub-module and the voltage at the end of the battery pack connected in parallel with the first capacitor are equal; Both the first half-bridge structure and the second half-bridge structure are switching devices composed of IGBTs and anti-parallel diodes. The first end of the voltage control unit is connected to the energy storage unit, and the second end of the voltage control unit extends backward to form the medium-voltage DC port; The method includes the following steps: Determine whether it is in the constant voltage mode according to the requirements of the medium-voltage DC port. If so, control the voltage control unit to operate at a fixed voltage, otherwise control the voltage control unit to work at a given power. Specifically: Subtract the total capacitance voltage of the input energy storage sub-modules from the difference between the high-voltage DC port voltage reference value and the total capacitance voltage of the input voltage control sub-modules, and then obtain the current reference value flowing through the voltage control unit after PI link processing. The difference between the actual value of the current flowing through the voltage control unit and the current reference value flowing through the voltage control unit is processed by PI link and round function rounding in sequence to obtain the number of energy storage sub-modules to be input; Output a valve control signal through the SOC-NLM sorting algorithm to control the switch of the energy storage sub-module, so as to realize the input and cut-off of the energy storage sub-module; Input the unbalanced power output by the SOC balance loop into the difference between the medium-voltage DC port output power reference value and the actual value of the medium-voltage DC port output power, and then perform PI link processing to obtain the battery current reference value flowing through the voltage control unit. The error between the battery current reference value flowing through the voltage control unit and the actual value of the capacitor current flowing through the voltage control unit is processed by PI link and round function rounding in sequence to obtain the number of actual voltage control sub-modules to be input; among them, the unbalanced power output by the SOC balance loop is obtained after PI link processing of the difference between the average SOC value of each single-string device and the average SOC value of the multi-port DC directly-connected battery energy storage system applicable to high voltage and large capacity; Output a valve control signal through the NLM sorting algorithm to control the switch of the voltage control sub-module, so as to realize the input and cut-off of the voltage control sub-module; The reference value of the energy storage unit output voltage is obtained according to the difference between the reference value of the high-voltage DC port voltage and the total capacitor voltage of the voltage-controlled sub-modules put into operation. Then, the number of energy storage sub-modules to be put into operation is obtained according to the quotient of the reference value of the energy storage unit output voltage and the average capacitor voltage of the energy storage sub-modules put into operation. After rounding by the round function, the actual number of energy storage sub-modules to be put into operation is obtained; Based on the obtained number of energy storage sub-modules to be put into operation, the actual valve control signal is output by using the SOC-NLM sorting algorithm to control the switches of the energy storage sub-modules, so as to realize the input and cut-off of the energy storage sub-modules; The reference value of the medium-voltage DC port voltage is obtained according to the difference between the reference value of the high-voltage DC port voltage and the total voltage of the energy storage sub-modules that should be actually put into operation; the difference between the reference value of the medium-voltage DC port voltage and the actual value of the medium-voltage DC port voltage is input into the PI link to obtain the reference value of the capacitor current flowing through the voltage-controlled unit; the difference between the average SOC of each single-string device and the average SOC of the multi-port DC directly-connected battery energy storage system applicable to high-voltage and large-capacity is input into the PI link to obtain the unbalanced current, and the unbalanced current is accumulated into the error between the reference value of the capacitor current flowing through the voltage-controlled unit and the actual value of the capacitor current flowing through the voltage-controlled unit. After being processed by the PI link and rounding by the round function in turn, the actual number of voltage-controlled sub-modules to be put into operation is obtained; Based on the obtained actual number of voltage-controlled sub-modules to be put into operation, the valve control signal is output by using the NLM capacitor voltage sorting algorithm to control the switches of the voltage-controlled sub-modules, so as to realize the input and cut-off of the voltage-controlled sub-modules.

2. The multi-port DC directly-connected battery energy storage method applicable to high voltage and large capacity according to claim 1, characterized in that The calculation formula for the actual number of energy storage sub-modules to be put into operation is: ； Wherein: N H is the number of actual voltage-controlled sub-modules to be put into operation, is the average capacitor voltage of the voltage-controlled sub-module, N V is the number of actual voltage-controlled sub-modules to be put into operation, is the total capacitor voltage of the voltage-controlled sub-modules put into operation, is the average capacitor voltage of the energy storage sub-module, U dc1ref is the reference value of the output voltage of the energy storage unit.

3. The multi-port DC directly-connected battery energy storage method applicable to high voltage and large capacity according to claim 1, characterized in that The SOC-NLM sorting algorithm is the charge quantity equalization sorting algorithm for the energy storage sub-module batteries. The specific process is as follows: Set the sorting frequency and sorting period, and additionally set the SOC difference threshold μ max restriction, that is, re-sorting and switching are only performed when the difference between the maximum SOC and the minimum SOC of the sub-module batteries is greater than the threshold and the next sorting period is reached; otherwise, the working modes of each sub-module remain unchanged.

4. The multi-port DC directly-connected battery energy storage method applicable to high voltage and large capacity according to claim 1, characterized in that The expression of the NLM sorting algorithm is: ； wherein χ is the ratio of the magnitude of the DC voltage fluctuation to the capacitor voltage of a single energy storage sub-module, is the maximum value of the current flowing through the energy storage battery, L d is the DC-side smoothing reactor, f con is the switching frequency of the energy storage sub-module, U dc1 is the HVDC port voltage.

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