Off-line inter-phase SOC self-balancing method for high-voltage direct-hanging energy storage converter system

Abstract

The invention relates to the technical field of power electronic application, and discloses an off-line inter-phase SOC self-balancing method for a high-voltage direct-hanging energy storage converter system, and the method comprises the following steps: an energy storage converter control system drives a three-phase energy storage converter power unit to output in-phase AC voltage to a neutral point at a test connection point; the average value of the three-phase charge states and the average value of the charge states of the high-voltage direct-hanging energy storage converter system are calculated; respectively calculating a charge state average maximum value, a charge state average minimum value and a charge state average intermediate value; and according to the distribution interval of the three-phase charge state mean value, a three-phase current instruction is calculated by using the charge state of the high-voltage direct-hanging energy storage converter system, and inter-phase charge state equalization is realized. On the basis that additional hardware and circuits are not added, the problem of off-line battery SOC equalization of the high-voltage direct-hanging cascade type energy storage converter in the prior art under the condition that no power grid exists is solved, and the method has the advantages of being simple in structure and simple and flexible in method.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics applications, and in particular to an off-line phase-to-phase SOC self-balancing method for a high-voltage direct-mounted energy storage converter system. Background Art

[0002] Due to the uncertainty of natural conditions, renewable energy generation, such as wind and solar energy, is intermittent and unstable. Large-capacity battery energy storage systems can store excess electricity during peak power generation of these energy sources and release electricity during low power generation or peak demand, thereby balancing the supply and demand of the power grid and reducing power fluctuations caused by supply and demand imbalances. In addition, when power demand peaks or power generation capacity is insufficient, energy storage systems can provide necessary power support and enhance the stability of the power grid.

[0003] Large-capacity high-voltage direct-mounted cascade energy storage technology has the technical characteristics of low harmonic content and high modularity, and is suitable for many application scenarios. In particular, it can quickly start the generator sets in the power grid and restore the power supply of the power grid in the event of a power grid failure. Compared with the black start of traditional hydropower units, large-capacity high-voltage direct-mounted cascade energy storage has the advantages of low site requirements, flexible mobility, no resource restrictions, and fast mode switching.

[0004] However, large-capacity battery energy storage systems also face some challenges, such as cost, battery life, environmental impact and safety issues. In the absence of a power grid, the battery SOC imbalance of the large-capacity high-voltage direct-mounted cascade energy storage device itself or the low SOC of some batteries will make it impossible to achieve the black start function. Therefore, the present invention proposes an offline phase-to-phase SOC self-balancing method for a high-voltage direct-mounted energy storage converter system. This technology has strong practical value for the black start function, offline testing and other aspects of large-capacity high-voltage direct-mounted cascade energy storage systems. Summary of the invention

[0005] In view of the problems in the related art, the present invention proposes an offline interphase SOC self-balancing method for a high-voltage direct-mounted energy storage converter system. Without adding additional hardware and circuits, the method solves the offline interphase SOC self-balancing problem of a high-voltage direct-mounted cascade energy storage converter in the absence of a power grid. The method has the advantages of simple structure, simple and flexible method, and wide application occasions. At the same time, the method solves the black start problem caused by interphase imbalance of a high-voltage direct-mounted cascade energy storage converter.

[0006] To this end, the specific technical solution adopted by the present invention is as follows:

[0007] A method for off-line interphase SOC self-balancing of a high-voltage direct-mounted energy storage conversion system is implemented based on a high-voltage direct-mounted energy storage conversion system, wherein the high-voltage direct-mounted energy storage conversion system includes a three-phase structure composed of n-level energy storage conversion power units and an energy storage converter control system, and the energy storage converter control system communicates with the control unit of each energy storage conversion power unit via optical fiber to transmit status information, fault information, mode information and instructions of the energy storage conversion power unit, as well as battery cluster status information of the corresponding charge state;

[0008] Among them, each phase in the three-phase structure includes n-stage energy storage conversion power units connected in series in sequence, and the ports of the first stage of the three-phase that are not connected to other energy storage conversion power units are connected to each other to form a neutral point; the ports of the nth stage of the three-phase that are not connected to other energy storage conversion power units are disconnected from the high-voltage power supply, and are connected to each other through the reactor to form a test connection point;

[0009] The off-line phase-to-phase SOC self-balancing method of the high-voltage direct-mounted energy storage converter system comprises the following steps:

[0010] S1, the energy storage converter control system drives the three-phase energy storage converter power unit to output the same-phase AC voltage to the neutral point at the test connection point;

[0011] S2, obtaining the charge state information of the three-phase energy storage conversion power unit, and calculating the average value of the three-phase charge state and the average value of the charge state of the high-voltage direct-mounted energy storage conversion system;

[0012] S3, based on the average value of the three-phase charge state, respectively calculating the average maximum value of the charge state, the average minimum value of the charge state and the average middle value of the charge state;

[0013] S4. According to the distribution interval of the mean value of the three-phase charge state, the charge state of the high-voltage direct-mounted energy storage conversion system is used to calculate the three-phase current command to achieve inter-phase charge state balance.

[0014] Furthermore, the energy storage converter control system drives the three-phase energy storage converter power unit to output a same-phase AC voltage to the neutral point at the test connection point, including:

[0015] Through the phase current loop where the average maximum value of the state of charge is located, the absolute difference between the average value of the state of charge of the phase and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is controlled to be less than a preset balance allowable limit value;

[0016] Through the phase current loop where the average intermediate value of the state of charge is located, the absolute difference between the average value of the state of charge of the phase and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is controlled to be less than a preset balance allowable limit value;

[0017] Through the phase current loop where the minimum average value of the state of charge is located, the absolute difference between the average value of the state of charge of the phase and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is controlled to be less than a preset balance allowable limit.

[0018] Furthermore, the calculation formula for the average value of the three-phase charge state is:

[0019]

[0020] In the formula, SOC i_avg Represents the average value of the three-phase state of charge, SOC ij It represents the charge state information of the three-phase energy storage conversion power unit, n represents the level of the energy storage conversion power unit in the high-voltage direct-mounted energy storage conversion system, i represents the phase, j represents the level, and A, B, and C represent the three phases A, B, and C respectively.

[0021] Furthermore, the calculation formula for the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is:

[0022]

[0023] In the formula, SOC sys Represents the average value of the charge state of the high-voltage direct-mounted energy storage conversion system.

[0024] Furthermore, the calculation formula for the average maximum value of the charge state is:

[0025] SOC MAX =MAX(SOC i_avg ), i=A,B,C

[0026] The calculation formula for the average minimum value of the charge state is:

[0027] SOC MIN =MIN(SOC i_avg ), i=A,B,C

[0028] The calculation formula for the average median state of charge is:

[0029] SOC MID =∑SOC i_avg -SOC MAX -SOC MIN , i=A,B,C

[0030] In the formula, SOC MAX Indicates the average maximum value of the state of charge, SOC MIN Indicates the average minimum value of the state of charge, SOC MID Represents the average median state of charge.

[0031] Furthermore, the calculation of the three-phase current instruction using the charge state of the high-voltage direct-mounted energy storage conversion system according to the distribution interval of the mean value of the three-phase charge state includes:

[0032] When the difference between the average middle value of the state of charge and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is greater than the preset balance allowable limit, the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge and the average minimum value of the state of charge are located are calculated respectively by using the first current instruction formula;

[0033] When the difference between the average state of charge value of the high-voltage direct-mounted energy storage conversion system and the average middle state of charge value is greater than the preset balance allowable limit, the second current command formula is used to calculate the phase current commands of the phases where the average maximum state of charge value, the average middle state of charge value and the average minimum state of charge value are located;

[0034] When the absolute value of the difference between the average middle value of the state of charge and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is less than the preset balance allowable limit, the third current command formula is used to calculate the phase current commands of the phases where the average maximum value of the state of charge, the average middle value of the state of charge and the average minimum value of the state of charge are located.

[0035] Furthermore, the calculation formula of the first current command formula is:

[0036]

[0037] In the formula, They represent the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge, and the average minimum value of the state of charge are located, respectively. p represents the proportionality coefficient, S represents the complex frequency variable, K int represents the integration parameter.

[0038] Furthermore, the calculation formula of the second current command formula is:

[0039]

[0040] In the formula, They represent the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge, and the average minimum value of the state of charge are located, respectively. p represents the proportionality coefficient, S represents the complex frequency variable, K int Represents the integration parameter.

[0041] Furthermore, the calculation formula of the third current command formula is:

[0042]

[0043] In the formula, They represent the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge, and the average minimum value of the state of charge are located, respectively. p represents the proportionality coefficient, S represents the complex frequency variable, K int Represents the integration parameter.

[0044] Furthermore, the offline interphase SOC self-balancing method of the high-voltage direct-mounted energy storage conversion system also includes: after completing the offline interphase SOC self-balancing, the high-voltage direct-mounted energy storage conversion system disconnects the three phases from the test connection point to restore the normal use state of the high-voltage direct-mounted energy storage conversion system.

[0045] The beneficial effects of the present invention are as follows: the present invention proposes an offline phase-to-phase SOC self-balancing method for a high-voltage direct-mounted energy storage converter system, which solves the problem of offline battery SOC balancing of a high-voltage direct-mounted cascaded energy storage converter in the absence of a power grid in the prior art, as well as the black start problem caused by unbalanced battery SOC of the high-voltage direct-mounted cascaded energy storage converter or too low SOC of several batteries, without adding additional hardware and circuits, and has the advantages of simple structure, simple and flexible method, providing convenience for offline battery SOC balancing, black start function, offline testing and the like of a high-voltage direct-mounted cascaded energy storage converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, 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.

[0047] Figure 1 This is a flow chart of an offline phase-to-phase SOC self-balancing method for a high-voltage direct-mounted energy storage converter system according to an embodiment of the present invention;

[0048] Figure 2 It is a principle block diagram of a high-voltage direct-mounted energy storage conversion system in an offline phase-to-phase SOC self-balancing method of a high-voltage direct-mounted energy storage conversion system according to an embodiment of the present invention;

[0049] Figure 3 The present invention is a block diagram of the principle of an energy storage conversion power unit in an offline phase-to-phase SOC self-balancing method of a high-voltage direct-mounted energy storage conversion system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0050] To further illustrate each embodiment, the present invention provides drawings, which are part of the disclosure of the present invention and are mainly used to illustrate the embodiments and can be used in conjunction with the relevant descriptions in the specification to explain the operating principles of the embodiments. With reference to these contents, ordinary technicians in the field should be able to understand other possible implementations and advantages of the present invention. The components in the figures are not drawn to scale, and similar component symbols are generally used to represent similar components.

[0051] According to an embodiment of the present invention, a method for self-balancing off-line phase-to-phase SOC of a high-voltage direct-mounted energy storage converter system is provided.

[0052] The present invention is further described with reference to the accompanying drawings and specific embodiments. Figure 1-Figure 3 As shown, the off-line interphase SOC self-balancing method of the high-voltage direct-mounted energy storage conversion system according to the embodiment of the present invention is implemented based on the high-voltage direct-mounted energy storage conversion system, wherein the high-voltage direct-mounted energy storage conversion system includes a three-phase structure composed of n-level energy storage conversion power units and an energy storage converter (PCS) control system, wherein each phase includes n-level energy storage conversion power units connected in series in sequence, and the ports of the first stage of the three-phase that are not connected to other energy storage conversion power units are interconnected to form a neutral point; the ports of the nth stage of the three-phase that are not connected to other energy storage conversion power units are disconnected from the high-voltage power supply, and are interconnected after passing through a reactor to form a test connection point; the energy storage converter (PCS) control system performs optical fiber communication with the control unit of each energy storage conversion power unit to transmit status information, fault information, mode information and instructions of the energy storage conversion power unit, and battery cluster status information such as the corresponding charge state;

[0053] The PCS control system described in the embodiment of the present invention includes the required processor (CPU), program memory (ROM / Flash), data memory (RAM) and peripheral interface (I / O), and the internal communication bus realizes high-speed data interaction. The processor (CPU) is responsible for executing program instructions and processing data. The program memory (ROM / Flash) is used to store the firmware or operating system of the embedded system. The data memory (RAM) is used to temporarily store data, such as variables and intermediate calculation results. The peripheral interface (I / O) interface realizes communication between the control system and external devices or sensors. Among them, the processor (CPU) can be a processing device such as a microprocessor MCU, a programmable logic device FPGA, etc. The processor (CPU) completes various functions and control logic by running the software program stored in the program memory (ROM / Flash), and realizes the offline phase-to-phase SOC self-balancing method of the high-voltage direct-mounted energy storage converter system introduced in the method embodiment of the present invention.

[0054] Peripheral interfaces (I / O) include but are not limited to: serial communication interfaces (such as UART, SPI, I2C), parallel interfaces, USB interfaces, Ethernet interfaces, wireless communication interfaces (such as Wi-Fi, Bluetooth, ZigBee, NFC, etc.), sensor interfaces (temperature, humidity, pressure, photosensitivity, etc.).

[0055] The off-line phase-to-phase SOC self-balancing method of the high-voltage direct-mounted energy storage converter system comprises the following steps:

[0056] S1, the energy storage converter control system drives the three-phase energy storage converter power unit to output the same-phase AC voltage to the neutral point at the test connection point;

[0057] The energy storage converter control system drives the three-phase energy storage converter power unit to output a same-phase AC voltage to the neutral point at the test connection point, including:

[0058] Through the phase current loop where the average maximum value of the state of charge is located, the absolute difference between the average value of the state of charge of the phase and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is controlled to be less than the preset balance allowable limit, that is, |SOC i_avg -SOC sys |<SOC * , i=SOC MAX Phase, A / B / C;

[0059] Through the phase current loop where the average intermediate value of the state of charge is located, the absolute difference between the average value of the state of charge of the phase and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is controlled to be less than the preset balance allowable limit, that is, |SOC i_avg -SOC sys |<SOC * , i=SOC MID Phase, A / B / C;

[0060] Through the phase current loop where the average SOC minimum value is located, the absolute difference between the average SOC value of the phase and the average SOC value of the high-voltage direct-mounted energy storage converter system is controlled to be less than the preset balance allowable limit, that is, |SOC i_avg -SOC sys |<SOC * , i=SOC MIN Phase: A / B / C.

[0061] S2, obtaining the charge state information of the three-phase energy storage conversion power unit, and calculating the average value of the three-phase charge state and the average value of the charge state of the high-voltage direct-mounted energy storage conversion system;

[0062] Among them, the calculation formula for the average value of the three-phase charge state is:

[0063]

[0064] The calculation formula for the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is:

[0065]

[0066] In the formula, SOC i_avg Represents the average value of the three-phase state of charge, SOC ij Indicates the charge state information of the three-phase energy storage conversion power unit, n indicates the level of the energy storage conversion power unit in the high-voltage direct-mounted energy storage conversion system, i indicates the phase, j indicates the level, A, B, C indicate the three phases A, B, C respectively, SOC sys Represents the average value of the charge state of the high-voltage direct-mounted energy storage conversion system.

[0067] S3, based on the average value of the three-phase charge state, respectively calculating the average maximum value of the charge state, the average minimum value of the charge state and the average middle value of the charge state;

[0068] The calculation formula for the average maximum value of the charge state is:

[0069] SOC MAX =MAX(SOC i_avg ), i=A,B,C

[0070] The calculation formula for the average minimum value of the charge state is:

[0071] SOC MIN =MIN(SOC i_avg ), i=A,B,C

[0072] The calculation formula for the average median state of charge is:

[0073] SOC MID =∑SOC i_avg -SOC MAX -SOC MIN , i=A,B,C

[0074] In the formula, SOC MAX Indicates the average maximum value of the state of charge, SOC MIN Indicates the average minimum value of the state of charge, SOC MID Represents the average median state of charge.

[0075] S4, according to the distribution interval of the mean value of the three-phase charge state, the charge state of the high-voltage direct-mounted energy storage conversion system is used to calculate the three-phase current command to achieve inter-phase charge state balance;

[0076] The method of calculating the three-phase current instruction by using the charge state of the high-voltage direct-mounted energy storage converter system according to the distribution interval of the mean value of the three-phase charge state includes:

[0077] When SOC MID -SOC sys >SOC * , that is, when the difference between the average middle value of the state of charge and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is greater than the preset balance allowable limit, the first current command formula is used to calculate the phase current commands of the phases where the average maximum value of the state of charge, the average middle value of the state of charge and the average minimum value of the state of charge are located;

[0078] Specifically, the calculation formula of the first current command formula is:

[0079]

[0080] In the formula, They represent the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge, and the average minimum value of the state of charge are located, respectively. p represents the proportionality coefficient, S represents the complex frequency variable, K int Represents the integral parameter, i = SOC MAX The i in A / B / C indicates the phase in which the average maximum value of the charge state belongs, A, B, or C.

[0081] When SOC sys -SOC MID >SOC * , that is, when the difference between the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system and the average intermediate value of the state of charge is greater than the preset balance allowable limit, the second current command formula is used to calculate the phase current commands of the phases where the average maximum value of the state of charge, the average intermediate value of the state of charge and the average minimum value of the state of charge are located;

[0082] Specifically, the calculation formula of the second current command formula is:

[0083]

[0084] In the formula, They represent the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge, and the average minimum value of the state of charge are located, respectively. p represents the proportionality coefficient, S represents the complex frequency variable, K int Represents the integration parameter.

[0085] When | SOC MID -SOC sys |<SOC *, that is, when the absolute value of the difference between the average middle value of the state of charge and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is less than the preset balance allowable limit, the third current command formula is used to calculate the phase current commands of the phases where the average maximum value of the state of charge, the average middle value of the state of charge and the average minimum value of the state of charge are located.

[0086] Specifically, the calculation formula of the third current command formula is:

[0087]

[0088] In the formula, They represent the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge, and the average minimum value of the state of charge are located, respectively. p represents the proportionality coefficient, S represents the complex frequency variable, K int represents the integration parameter.

[0089] In addition, the offline phase-to-phase SOC self-balancing method of the high-voltage direct-mounted energy storage conversion system also includes: after completing the offline phase-to-phase SOC self-balancing, the high-voltage direct-mounted energy storage conversion system disconnects the three phases from the test connection point to restore the normal use state of the high-voltage direct-mounted energy storage conversion system.

[0090] To sum up, with the help of the above-mentioned technical scheme of the present invention, the present invention proposes a method for offline phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage converter system, which solves the problem of offline battery SOC balancing of the high-voltage direct-mounted cascade energy storage converter in the existing technical scheme without a power grid, as well as the black start problem caused by the unbalanced battery SOC of the high-voltage direct-mounted cascade energy storage converter or the excessively low SOC of several batteries. It does not require the addition of additional hardware and circuits, has the advantages of simple structure, simple and flexible methods, and provides convenience for offline battery SOC balancing, black start function, offline testing, etc. of the high-voltage direct-mounted cascade energy storage converter.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage conversion system, which is implemented based on a high-voltage direct-mounted energy storage conversion system and is characterized in that: The high-voltage direct-mounted energy storage conversion system includes a three-phase structure composed of n-level energy storage conversion power units and an energy storage converter control system, and the energy storage converter control system communicates with the control unit of each energy storage conversion power unit via optical fiber to transmit status information, fault information, mode information and instructions of the energy storage conversion power unit, as well as the battery cluster status information of the corresponding charge state; Among them, each phase in the three-phase structure includes n-stage energy storage conversion power units connected in series in sequence, and the ports of the first stage of the three-phase that are not connected to other energy storage conversion power units are connected to each other to form a neutral point; the ports of the nth stage of the three-phase that are not connected to other energy storage conversion power units are disconnected from the high-voltage power supply, and are connected to each other through the reactor to form a test connection point; The off-line phase-to-phase SOC self-balancing method of the high-voltage direct-mounted energy storage converter system comprises the following steps: S1, the energy storage converter control system drives the three-phase energy storage converter power unit to output the same-phase AC voltage to the neutral point at the test connection point; S2, obtaining the charge state information of the three-phase energy storage conversion power unit, and calculating the average value of the three-phase charge state and the average value of the charge state of the high-voltage direct-mounted energy storage conversion system; S3, based on the average value of the three-phase charge state, respectively calculating the average maximum value of the charge state, the average minimum value of the charge state and the average middle value of the charge state; S4. According to the distribution interval of the mean value of the three-phase charge state, the charge state of the high-voltage direct-mounted energy storage conversion system is used to calculate the three-phase current command to achieve inter-phase charge state balance.

2. According to claim 1, a method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage conversion system is characterized in that: The energy storage converter control system drives the three-phase energy storage converter power unit to output a same-phase AC voltage to the neutral point at the test connection point, including: Through the phase current loop where the average maximum value of the state of charge is located, the absolute difference between the average value of the state of charge of the phase and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is controlled to be less than a preset balance allowable limit value; Through the phase current loop where the average intermediate value of the state of charge is located, the absolute difference between the average value of the state of charge of the phase and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is controlled to be less than a preset balance allowable limit value; Through the phase current loop where the minimum average value of the state of charge is located, the absolute difference between the average value of the state of charge of the phase and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is controlled to be less than a preset balance allowable limit.

3. The method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage conversion system according to claim 1 is characterized in that: The calculation formula for the average value of the three-phase charge state is: In the formula, SOC i_avg Represents the average value of the three-phase state of charge, SOC ij It represents the charge state information of the three-phase energy storage conversion power unit, n represents the level of the energy storage conversion power unit in the high-voltage direct-mounted energy storage conversion system, i represents the phase, j represents the level, and A, B, and C represent the three phases A, B, and C respectively.

4. The method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage conversion system according to claim 3 is characterized in that: The calculation formula for the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is: In the formula, SOC sys Represents the average value of the charge state of the high-voltage direct-mounted energy storage conversion system.

5. The method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage conversion system according to claim 4 is characterized in that: The calculation formula for the average maximum value of the charge state is: SOC MAX =MAX(SOC i_avg ),i=A,B,C The calculation formula for the average minimum value of the charge state is: SOC MIN =MIN(SOC i_avg ),i=A,B,C The calculation formula for the average median state of charge is: SOC MID =ΣSOC i_avg -SOC MAX -SOC MIN ,i=A,B,C In the formula, SOC MAX Indicates the average maximum value of the state of charge, SOC MIN Indicates the average minimum value of the state of charge, SOC MID Represents the average median state of charge.

6. A method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage converter system according to claim 5, characterized in that: The method of calculating the three-phase current instruction using the charge state of the high-voltage direct-mounted energy storage conversion system according to the distribution interval of the mean value of the three-phase charge state includes: When the difference between the average middle value of the state of charge and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is greater than the preset balance allowable limit, the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge and the average minimum value of the state of charge are located are calculated respectively by using the first current instruction formula; When the difference between the average state of charge value of the high-voltage direct-mounted energy storage conversion system and the average middle state of charge value is greater than the preset balance allowable limit, the second current command formula is used to calculate the phase current commands of the phases where the average maximum state of charge value, the average middle state of charge value and the average minimum state of charge value are located; When the absolute value of the difference between the average middle value of the state of charge and the average value of the state of charge of the high-voltage direct-mounted energy storage conversion system is less than the preset balance allowable limit, the third current command formula is used to calculate the phase current commands of the phases where the average maximum value of the state of charge, the average middle value of the state of charge and the average minimum value of the state of charge are located.

7. The method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage conversion system according to claim 6 is characterized in that: The calculation formula of the first current command formula is: In the formula, They represent the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge, and the average minimum value of the state of charge are located, respectively. p represents the proportionality coefficient, S represents the complex frequency variable, K int Represents the integration parameter.

8. The method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage conversion system according to claim 6 is characterized in that: The calculation formula of the second current command formula is: In the formula, They represent the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge, and the average minimum value of the state of charge are located, respectively. p represents the proportionality coefficient, S represents the complex frequency variable, K int Represents the integration parameter.

9. The method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage converter system according to claim 6, characterized in that: The calculation formula of the third current command formula is: In the formula, They represent the phase current instructions of the phases where the average maximum value of the state of charge, the average middle value of the state of charge, and the average minimum value of the state of charge are located, respectively. p represents the proportionality coefficient, S represents the complex frequency variable, K int Represents the integration parameter.

10. The method for off-line phase-to-phase SOC self-balancing of a high-voltage direct-mounted energy storage converter system according to claim 1, characterized in that: The method for offline interphase SOC self-balancing of the high-voltage direct-mounted energy storage conversion system also includes: after completing the offline interphase SOC self-balancing, the high-voltage direct-mounted energy storage conversion system disconnects the three phases from the test connection point to restore the normal use state of the high-voltage direct-mounted energy storage conversion system.

Images