Fault-tolerant control method and system for high-voltage direct-hung chain energy storage conversion system

Abstract

The application discloses a fault-tolerant control method and system of a high-voltage direct-hanging chain type energy storage conversion system, when a fault occurs in an energy storage battery of a sub-module, a bypass is performed on the faulty part, and the system-level active power and the module-level active power output are adaptively adjusted according to the total number of system fault modules and the upper-level power grid scheduling requirements. Meanwhile, the problem of uneven state of charge of each battery pack under the sub-module fault is considered, when the maximum deviation of the battery pack SOC exceeds a threshold value, the SOC balancing control is added in the sub-module power control to suppress the SOC deviation of each module. The system can have the fault-tolerant operation ability when the number of fault sub-modules exceeds the number of redundant sub-modules, improves the operation ability of the power grid under the unbalanced working condition, provides the power guarantee for the safe and stable operation of the new type power system, and has the advantages of high reliability, high efficiency, high dynamic response and the like.

Description

Technical Field

[0001] This invention relates to the field of battery energy storage conversion technology, and in particular to a fault-tolerant control method for a high-voltage direct-connection chain energy storage conversion system. Background Technology

[0002] Cascaded H-bridge converters offer advantages such as high efficiency, good scalability, and modularity, and are widely used in high-voltage motor drives and high-power reactive power compensation. The cascaded H-bridge type high-voltage direct-connected chain energy storage conversion system (Transformerless High-voltage Power Conversion System, THPCS) distributes energy storage batteries into the DC capacitor side of the cascaded H-bridge, enabling "segmented management" of the batteries. This avoids the circulating current problem associated with multiple parallel battery banks, significantly reducing the design complexity of the battery management system (BMS). It also allows for high-voltage direct grid connection without a power frequency transformer, offering advantages such as modularity, large capacity, high reliability, and high efficiency. It can improve the grid's ability to absorb new energy sources, maintain voltage stability over short timescales, and is a feasible measure to suppress commutation failures in high-voltage DC transmission under unbalanced operating conditions. Because cascaded energy storage conversion systems have a large number of sub-modules, research on system reliability issues caused by module battery bank failures is crucial.

[0003] To address submodule failures, the current primary fault-tolerant operation method is to deploy redundant submodules. While a higher number of redundant modules generally leads to higher reliability, the redundancy of energy storage conversion systems in practical applications is limited by cost, losses, and control complexity. When the number of faulty submodules in a phase link exceeds the redundancy limit, there will be no backup resources to compensate for the faulty phase link, resulting in uneven three-phase output, deviations in the State of Charge (SOC) status between battery packs, and, in severe cases, grid disconnection and shutdown. Therefore, there is an urgent need to improve the fault-tolerant operation capability of energy storage conversion systems through effective methods, ensuring normal operation even without additional redundant modules and enabling comprehensive battery pack management. This approach has broad application prospects in the engineering field. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a fault-tolerant control method and system for a high-voltage direct-connected chain energy storage conversion system, which addresses the shortcomings of the existing technology and ensures that the high-voltage direct-connected chain energy storage conversion system can still operate normally under fault conditions where no additional available redundant modules are available.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a fault-tolerant control method for a high-voltage direct-connected chain energy storage conversion system, wherein each phase link of the high-voltage direct-connected chain energy storage conversion system includes multiple cascaded sub-modules; the multiple cascaded sub-modules include n sub-modules currently in use and several redundant sub-modules; each sub-module includes a battery pack, a dual active bridge DC / DC converter, and a single-phase H-bridge inverter connected in sequence; the method includes the following steps:

[0006] S1) Determine if any submodule has failed. If a faulty submodule exists, bypass it and calculate the number of faulty submodules m using the following formula: in Where, m x_flt m represents the number of faulty submodules in the x-th phase link. x_rdd This represents the number of redundant submodules in the x-th phase link; x = ab, bc, ca; ab, bc, ca represent the three-phase links of the high-voltage direct-connected chain energy storage conversion system, respectively.

[0007] S2) Calculate the system-level active power command P using the following formula. ref : Among them, P N This represents the rated capacity of the high-voltage direct-connected chain energy storage conversion system; the min function represents the function that takes the minimum value of the input; P disp For the given active power command;

[0008] Using the system-level active power command P ref The calculated active power command P at the link level is obtained. x_ref :

[0009]

[0010] The maximum deviation ΔSOC of the battery pack in a high-voltage direct-connected energy storage conversion system is calculated using the following formula. max : Wherein, ΔSOC T ΔSOC is the threshold value for the state of charge deviation of the energy storage battery pack. max The maximum deviation of SOC, SOC xj This represents the SOC state of the energy storage battery in the j-th submodule of the x-th phase link in the energy storage conversion system, and n represents the number of modules in each phase link of the high-voltage direct-connected chain energy storage conversion system; F xj This represents the operating state of the j-th submodule in the x-th phase link of the energy storage conversion system. When the submodule fails, F... xj =1, F when the submodule is working normally xj =0; the max function represents the function that takes the maximum value of the input value;

[0011] The active power command of the j-th submodule in the x-th phase link is calculated using the following formula. in,

[0012] This invention is based on the power command P from the superior power grid dispatch. disp In addition to information such as the working status of submodules and SOC, the top-level control command P is calculated in real time. x_ref P xj_ref and I qx_ref When a submodule's energy storage battery fails, the faulty section is bypassed, and the system-level and module-level active power outputs are adaptively adjusted based on the total number of faulty modules and the requirements of the upper-level power grid dispatch. Simultaneously, considering the uneven state of charge (SOC) of each battery pack under submodule failure, when the maximum SOC deviation of the battery pack exceeds a threshold, SOC balancing control is added to the submodule power control to suppress the SOC deviation of each module. Therefore, the method of this invention can ensure the energy storage conversion system has fault-tolerant operation capability when the number of faulty submodules exceeds the number of redundant submodules, improve the grid's operation capability under unbalanced conditions, and provide power assurance for the safe and stable operation of new power systems. It has advantages such as high reliability, high efficiency, and high dynamic response. The energy storage conversion system can cope with emergency conditions where all redundant submodules in a phase link are engaged, resulting in a secondary failure of the battery pack in that phase link, and can maintain external tracking power commands, achieving balanced active power output for each phase. Without increasing hardware costs, it improves the continuous operating time of the energy storage conversion system and its ability to withstand disturbances under severe conditions.

[0013] The method of the present invention further includes:

[0014] Reactive current command I qx_ref With cosθ x The product of these is used as the reactive component of the AC current command, and the active current command i is used as the reactive component. px_ref With sinθ x The product of these components is the active component of the AC current command, which is then superimposed with the reactive component, active component, and zero-sequence current command i. 0_ref Receive AC current input command i x_ref ;

[0015] Input the alternating current command i x_ref The current i in the xth phase link x The difference is used as the input to the PR controller, and the output of the PR controller is compared with the voltage feedforward command u. xj_f The commands are added together, and the added commands are subjected to carrier phase shift modulation to obtain the first modulation wave signal, which controls the on and off of the switching transistors of the single-phase H-bridge inverter.

[0016] Among them, I qx_ref =-K Q (Usx -U ref ); where U sx U represents the phase voltage amplitude of the power grid. ref K represents the rated terminal voltage of the energy storage conversion system. Q This represents the reactive power-voltage droop coefficient.

[0017] Furthermore, the zero-sequence current command i of the present invention 0_ref The calculation formula is:

[0018]

[0019] Where I0 represents the zero-sequence current amplitude, θ ab Indicates the grid line voltage u ab phase, This indicates that the zero-sequence current lags behind the grid line voltage u. ab Phase difference; I 0d and I 0q They are respectively based on the grid line voltage u ab phase θ ab Using the reference phase, the d-axis and q-axis components of the zero-sequence current are obtained by performing a Park transformation on the zero-sequence current i0.

[0020] Furthermore, in this invention, I 0d and I 0q The calculation formula is:

[0021]

[0022] Among them, U 1d and U 1q These are the d-axis and q-axis components of the positive sequence voltage, respectively. 2d and U 2q These are the d-axis and q-axis components of the negative sequence voltage, respectively.

[0023] Furthermore, in this invention, U 1d U 1q U 2d U 2q The extraction method is: grid line voltage u ab u bc and u ca After θ ab After Park transform with reference phase, the positive-sequence voltage component is transformed into a DC component, and the negative-sequence component is transformed into a second harmonic component. After filtering out the second harmonic component using a notch filter, the d-axis component U of the positive-sequence voltage can be extracted. 1d and q-axis component U 1q Similarly, after passing through -θ abAfter Park transform and notch filter processing with reference phase, the d-axis component U of the negative sequence voltage can be extracted. 2d and q-axis component U 2q .

[0024] The method of the present invention further includes: transmitting the active power command P of the j-th submodule of the x-th phase link. xj_ref The output active power P of the j-th submodule in the x-th phase link is xj Subtracting the two values ​​yields a first difference, which is then fed into a first PI controller. The output of the first PI controller is subtracted from the output current of the j-th submodule battery pack in the x-th phase link to obtain a second difference. This second difference is then fed into a second PI controller to obtain the DAB external phase shift angle θ of the j-th submodule battery pack in the x-th phase link. xj_ps , for θ xj_ps Single-phase-shift modulation is performed to obtain a second modulation wave signal, which controls the on / off state of the switching transistors of the dual active bridge DC / DC converter.

[0025] This invention also provides a fault-tolerant control system for a high-voltage direct-connected chain energy storage conversion system. Each phase link of the high-voltage direct-connected chain energy storage conversion system includes multiple cascaded sub-modules; the multiple cascaded sub-modules include n sub-modules currently in use and several redundant sub-modules; each sub-module includes a battery pack, a dual active bridge DC / DC converter, and a single-phase H-bridge inverter connected in sequence; it includes a main controller and multiple sub-controllers; each sub-module's dual active bridge DC / DC converter and single-phase H-bridge inverter are connected to a sub-controller; all sub-controllers are connected to the main controller; the main controller is configured to execute the steps of the method described above and output signal P. x_ref P xj_ref and I qx_ref Distribute the information to each sub-controller.

[0026] Each sub-controller is configured to perform the steps of the present invention for acquiring the first modulated wave signal and the second modulated wave signal.

[0027] In this invention, the number of redundant sub-modules can be set according to the actual situation, for example, one or two.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1) This invention proposes a fault-tolerant control method for a high-voltage direct-connected chain energy storage conversion system, based on the power command P from the upper-level power grid dispatch. disp And real-time calculation of top-level power command P based on submodule operating status, SOC and other information. x_ref P xj_ref and I qx_refThe system then passes the instructions to the underlying controllers of each submodule for execution. Based on the control method of this invention, the energy storage conversion system can cope with emergency conditions such as secondary failures of the battery pack in a phase link when all redundant submodules of that phase link are engaged, and can maintain external tracking power commands. This achieves balanced active power output for each phase, improves the continuous operating time of the energy storage conversion system without increasing hardware costs, and enhances its ability to withstand disturbances under harsh conditions. As a highly reliable energy storage carrier, it provides a safety guarantee for the stable operation of new power systems.

[0030] 2) This invention can quickly track reactive current commands. Based on the reactive-voltage droop characteristic curve, the energy storage conversion system can quickly output reactive current when the power grid experiences fault conditions such as voltage dips, thereby providing emergency reactive support to the power grid and effectively suppressing the occurrence of commutation failure.

[0031] 3) In this invention, the battery management system monitors the SOC status of the sub-module battery packs in real time. When the deviation of the battery packs exceeds the allowable range, the active power control of the sub-modules adjusts the SOC status of each module battery pack, thereby achieving SOC balancing of multiple battery packs and improving the normal operation time of the energy storage device and the service life of the battery packs. Attached Figure Description

[0032] Figure 1(a) shows the overall topology of the high-voltage direct-connected chain energy storage and conversion system; Figure 1(b) shows the sub-module structure of the system.

[0033] Figure 2 This is the overall control block diagram of the high-voltage direct-connected chain energy storage and conversion system according to an embodiment of the present invention;

[0034] Figure 3 This is a flowchart of the fault-tolerant control method according to an embodiment of the present invention;

[0035] Figure 4 This is a block diagram of the phase-to-phase power transfer control layer of the high-voltage direct-connected chain energy storage conversion system according to an embodiment of the present invention.

[0036] Figure 5(a) is a simulation verification diagram of the system output line current when each phase has a faulty submodule using the control method of the present invention; Figure 5(b) is a chain link phase current waveform diagram under the same simulation conditions as Figure 5(a); Figure 5(c) is a chain link SOC state simulation diagram under the same simulation conditions as Figure 5(a).

[0037] Figure 6(a) is a simulation verification diagram of the system output line current when the AB phase link battery pack is completely faulted using the control method of the embodiment of the present invention; Figure 6(b) is a link phase current waveform diagram under the same simulation conditions as Figure 6(a); Figure 6(c) is a simulation diagram of the link SOC state under the same simulation conditions as Figure 6(a). Detailed Implementation

[0038] The technical solution of the present invention will be specifically described below with reference to the accompanying drawings in the embodiments of the present invention.

[0039] Figures 1(a) and 1(b) are topology diagrams of the cascaded H-bridge high-voltage direct-connected chain energy storage conversion system according to an embodiment of the present invention, including an energy storage battery pack and a high-voltage direct-connected chain energy storage conversion system. Figure 1(a) is the overall system structure diagram, which mainly includes an energy storage battery pack, an isolated DC-DC converter (DAB isolation stage), a DC bus capacitor, a chain H-bridge inverter, a filter inductor L, and a main controller. In the high-voltage direct-connected chain energy storage conversion system, the number of modules and energy storage batteries used in each phase link is the same, which is n (n is a natural number and n>1). The entire system has 3n modules. The energy storage batteries are connected to the grid through the energy storage conversion system. In addition, each phase link is equipped with a few redundant sub-modules, which are replaced when the operating sub-module fails. The energy storage conversion system submodule has a two-stage structure, as shown in Figure 1(b). The front stage of the submodule is a dual active bridge DC / DC converter, mainly responsible for the rapid adjustment of the submodule's active power output. The rear stage is a single-phase H-bridge inverter, whose control objectives are DC bus capacitor voltage control and reactive current control. The output terminals of the single-phase H-bridge inverter are connected in series to form phase links, and the three phase links are connected in a delta configuration to form the energy storage conversion system.

[0040] Figure 2 This is the overall control block diagram of a high-voltage direct-connected chain energy storage conversion system. The control structure can be divided into fault-tolerant control (top-level control) and bottom-level control. The bottom-level control mainly includes inter-phase power transfer control, sub-module capacitor voltage equalization control (Gao Chaoyue. Research on control strategy of cascaded H-bridge STATCOM [D]. Harbin Institute of Technology, 2019.), phase-splitting current control (Yao Weizheng, Liu Gang, Hu Siquan, et al. Research on improved control strategy of chain STATCOM applied to weak receiving end of LCC-HVDC [J]. Proceedings of the CSEE, 2018, 38(12):3662-3670+26.), and single-phase shift control (Yang Min. Research on dual active full-bridge bidirectional DC-DC converter with PWM plus phase shift control [D]. Nanjing University of Aeronautics and Astronautics, 2013.). The front-end dual active bridge DC / DC converter employs single-phase-shift control, enabling rapid adjustment of the submodule's output active power and balancing when the battery pack's SOC deviation exceeds the limit. The rear-end single-phase H-bridge inverter uses dual closed-loop control for both voltage and current. The outer voltage loop uses a PI controller to stabilize the DC bus voltage, while the inner current loop employs phase-by-phase current control based on a PR controller. In the voltage loop, the DC voltage command U of each phase link of the energy storage conversion system is transmitted. cx_ref The sum of the capacitor voltages of the corresponding sub-modules in the chain, U cx The difference between the two voltage signals is calculated, and after passing through a PI regulator, an active current command i is generated. px_refBy acquiring the phase signal θ of the grid voltage. x Construct a sinusoidal signal sinθ x Sum of cosine signals cosθ x The active current command i px_ref With sinθ x The product of these components forms the active component of the AC current command, while the reactive current command i output by the top-level control is the reactive current command. qx_ref With cosθ x The product of these is used as the reactive component of the AC current command, combined with the zero-sequence current command i obtained from interphase power transfer. 0_ref This yields the AC current input command i for phase-separated current control. x_ref Input AC current using command i x_ref With the link phase current i of the energy storage conversion system x The error signal obtained by subtraction is sent to the PR controller, and the voltage feedforward command u from the submodule capacitor voltage equalization control is added to its output signal. xj_f After balancing the capacitor voltages of each normally operating submodule, the modulation wave required for carrier phase-shift modulation is finally formed. This involves adding a zero-sequence current command i to the output of the outer voltage loop. 0_ref It can achieve power transfer between phase links without affecting the external output power of the energy storage conversion system, ensuring that the top-level control objective is achieved—balancing the active power output of each phase—even when the number of faulty submodules exceeds the number of redundant submodules. The module-level power command of the top-level control adjusts the output active power of the submodules through the power-current dual closed-loop control of the dual active bridge DC / DC converter. The outer power loop is activated by the submodule's active power command P. xj_ref The actual active power P of the submodule xj The error value is processed by the PI controller to obtain the input command for the inner current loop. The current command is then compared with the output current I of the submodule battery. xjdc_BESS After the difference is calculated, the PI controller outputs the required external phase shift angle θ for the dual active bridge DC / DC converter. xj_ps When the battery pack fails, an emergency bypass can be performed using a dual active bridge converter. The faulty submodule can still operate in a STATCOM-like mode, providing emergency reactive power support when necessary, thereby improving the overall fault tolerance capability of the system.

[0041] Figure 3 This is a flowchart of the fault-tolerant control method for a high-voltage direct-connected chain energy storage conversion system according to an embodiment of the present invention. The specific steps are as follows:

[0042] 1) Sample the grid line voltage u at the beginning of each switching cycle. x Energy storage conversion system link phase current i x The SOC (State of Charge) of each battery pack is obtained through the battery management system. xj(x = ab, bc, ca; j = 1, 2, ..., n), and obtain the active power command P from the superior power grid dispatching department. disp ;

[0043] 2) Real-time monitoring of the operating status of the energy storage conversion system submodules F xj (F during fault) xj =1, F during normal operation xj =0), detect whether a submodule has failed. If a faulty submodule exists, bypass the faulty part of the submodule, and statistically calculate the module's operating status F. xj The total number of faulty modules m in the energy storage conversion system is calculated based on the number of faulty modules and the number of redundant modules. The SOC state of the faulty modules is then set to zero and they are not included in subsequent calculations. Otherwise, the operating state F of the module is changed. xj Set the total number of faulty modules m to zero. The total number of faulty modules m is...

[0044] in

[0045] m represents the total number of faulty submodules in the energy storage conversion system (faulty submodules that are replaced by redundant submodules are not included in the calculation of m; that is, a phase link only enters the fault-tolerant control state if a submodule fails after all redundant submodules in a phase link are put into operation). x_flt m represents the number of faulty submodules in the x-th phase link. x_rdd This represents the number of redundant submodules in the x-th phase link;

[0046] 3) Calculate the system-level active power command P based on the active power command requirements of the superior power grid dispatch and the remaining system capacity after the faulty module is removed. ref The calculation formula is as follows:

[0047]

[0048] P disp This indicates a positive instruction from the higher-level dispatching department, P N This represents the rated capacity of the energy storage conversion system, and the min function represents the function that takes the minimum value of the input.

[0049] Considering that the overall power grid load is in a balanced state, and the active power output of each phase link is evenly distributed according to the system active power command, the link-level active power command P is calculated. x_ref The calculation formula is as follows:

[0050]

[0051] P x_ref P represents the active power command (x = ab, bc, ca) allocated to the x-th phase link in the energy storage conversion system.ref This refers to the active power commands that the system sends to the outside world.

[0052] 4) The Battery Management System (BMS) monitors the SOC status of each module's battery pack in real time and feeds back the SOC status of all modules to the main controller. The main controller sums and averages the SOC status of all normally operating modules, compares the SOC status of the normally operating sub-modules with the system's average SOC, and calculates the maximum SOC deviation ΔSOC. max The calculation formula is as follows:

[0053]

[0054] SOC xj Let F represent the SOC state of the energy storage battery in the j-th module of the x-th phase in the energy storage conversion system (x = ab, bc, ca, j = 1, 2, ..., n), where n represents the number of modules in use per phase link in the system (excluding redundant modules for hot standby). xj This indicates the operating state of the j-th module in the x-th phase of the energy storage conversion system (F during a fault). xj =1, F during normal operation xj =0, increase 1-F xj Factors are used to ensure that faulty submodules do not participate in ΔSOC. max (Calculation of input values), the max function represents the function that takes the maximum value of the input. ab, bc, ca correspond to the three phases of the energy storage conversion system, respectively.

[0055] 5) Determine the maximum deviation of SOC ΔSOC max Is it greater than the State of Charge (SOC) deviation threshold ΔSOC? T If it does not exceed ΔSOC T If the SOC deviation of each battery pack is within the allowable range, or if the SOC of each battery pack is approximately considered balanced, then power is evenly distributed to the normally operating modules, and the active power command of the sub-modules is adjusted according to the total number of faulty modules m. Otherwise, based on the total number of faulty modules m, the active power command of the sub-modules is adjusted using 10% of the SOC deviation of the sub-modules as a benchmark. Variable deviation factor K xj_err for:

[0056]

[0057] K xj_err This represents the variable deviation factor of the j-th module in phase x, which is related to the state of charge (SOC) of the battery pack. ΔSOC T The state-of-charge (SOC) deviation threshold for energy storage battery packs represents the maximum permissible deviation of the SOC of each module battery pack in the energy storage conversion system, numerically expressed as ΔSOC. TThe value can be 1%. The sign function is a sign function (outputs 1 when the input is positive, 0 when the input is 0, and -1 when the input is negative).

[0058] Combined with variable deviation factor K xj_err Given the total number of faulty modules m in the system, the module-level active power command P is obtained. xj_ref The specific calculation formula is as follows:

[0059]

[0060] 6) Reactive power commands are obtained based on the grid voltage. The magnitude of the command is related to the deviation between the grid voltage and the terminal voltage of the energy storage conversion system. When a short-term voltage drop occurs in the grid due to imbalance, the reactive power command increases. The energy storage conversion system provides reactive power support to the grid to stabilize the voltage and suppress commutation failure. The specific calculation formula is shown below:

[0061] I qx_ref =-K Q (U sx -U ref (7)

[0062] I qx_ref This represents the reactive current command for phase x (x = ab, bc, ca), U sx U represents the phase voltage amplitude of the power grid. x U represents the amplitude of the output phase voltage of the energy storage conversion system. ref K represents the rated terminal voltage of the energy storage conversion system. Q U represents the reactive power-voltage droop factor. sx U ref I qx_ref After standardization, K Q The value range is 1.5 to 2.5.

[0063] 7) Transfer the power command P from the top-level control in steps 1)-6) x_ref P xj_ref and I qx_ref The main controller sends commands to each sub-controller, and each sub-controller is responsible for the underlying control of its corresponding sub-module. The front-end uses a single-phase-shift modulation method, and the back-end uses a carrier-phase-shift modulation method.

[0064] When a battery pack failure occurs even after all redundant submodules of a certain phase link are operational, the energy storage conversion system locks out the faulty submodule, resulting in a reduction in the total output power of the submodules on that phase link, leading to a numerical inconsistency with the active power command of that phase link. The fault-tolerant control method of this invention calculates the top-level power command P based on the fault condition and the battery pack's SOC state. x_ref P xj_ref and I qx_refIt then issues commands to each sub-controller for low-level control.

[0065] Figure 4 This is a block diagram of the phase-to-phase power transfer control in a high-voltage direct-connected chain energy storage conversion system. The control method utilizes P... x_ref and P xj_ref The command constructs a zero-sequence current i0, such that the zero-sequence power generated by this zero-sequence current and the positive and negative sequence components of the grid line voltage can compensate for power deviations without affecting the overall external output performance of the system, thereby ensuring that the difference between the active power command and the actual active power output of each phase link is 0. Specifically, the relationship is as follows:

[0066]

[0067] P 0_x U represents the zero-sequence current transfer power of the x-phase link (x = ab, bc, ca). x Let x represent the grid line voltage (x = ab, bc, ca), and i0 represent the zero-sequence current that compensates for the power deviation of the link.

[0068] For ease of zero-sequence power calculation, the zero-sequence current and grid voltage are subjected to Park transformation to convert the electrical quantities into DC quantities in a synchronous rotating coordinate system. The Park transformation matrix M and the inverse transformation matrix M' are shown below. -1 The specific calculation formula is as follows (T represents the matrix transpose operation):

[0069]

[0070] Specifically, assume the expression for the zero-sequence current i0 is:

[0071]

[0072] I0 represents the zero-sequence current amplitude, θ ab Indicates the grid line voltage u ab phase, This indicates that the zero-sequence current lags behind the grid line voltage u. ab The phase difference.

[0073] With grid line voltage u ab phase θ ab Using the zero-sequence current i0 as a reference phase, a Park transform is performed to obtain the d-axis component I of the zero-sequence current. 0d and q-axis component I 0q According to the inverse Park transform, the zero-sequence current command i 0_ref It can be accessed via I 0d and I 0q The specific calculation formula is as follows:

[0074]

[0075] Under asymmetrical operating conditions, the grid line voltage exhibits a negative sequence component. Therefore, when calculating zero-sequence power, it is necessary to provide the d-axis and q-axis components of the grid's positive and negative sequence line voltages. After passing through θ... ab After Park transform with reference phase, the positive-sequence voltage component is transformed into a DC component, and the negative-sequence component is transformed into a second harmonic component. After filtering out the second harmonic component using a notch filter, the d-axis component U of the positive-sequence voltage can be extracted. 1d and q-axis component U 1q Similarly, after passing through -θ ab After Park transform and notch filter processing with reference phase, the d-axis component U of the negative sequence voltage can be extracted. 2d and q-axis component U 2q According to the inverse Park transform, the three-phase grid line voltage can be obtained through U 1d U 1q U 2d and U 2q The specific calculation formula is shown below:

[0076]

[0077] Combining equations (11) and (12), the average zero-sequence power of the three-phase link is

[0078]

[0079] The `lowpass` function performs a low-pass filter on the zero-sequence power, removing the second harmonic fluctuation component of the zero-sequence power, and calculates the average value of the zero-sequence power.

[0080] Equation (13) shows that the zero-sequence power generated by the zero-sequence current and each line voltage is different, but the sum of the zero-sequence power of the three-phase link is 0. That is, the zero-sequence power inside the energy storage conversion system will not affect its external power output performance, and the zero-sequence current can be calculated using only the zero-sequence power of two-phase links. In this invention, P is used as... 0_ab and P 0_bc As the input for interphase power transfer control, the zero-sequence current command is obtained by performing the inverse operation on equation (13). The specific calculation formula is shown below:

[0081]

[0082] Get I 0d and I 0q Then, the final zero-sequence current command i is calculated according to equation (11). 0_ref The zero-sequence current command is then sent to the phase current control section.

[0083] The effectiveness and advancement of the control method proposed in this invention embodiment were verified using PSCAD simulation software.

[0084] Figures 5(a) to 5(c) Figure 5(a) shows the simulation results of the fault-tolerant control method of this invention when each phase has a faulty submodule. The overall system capacity is 90kVA. The simulation model simulates a 0.5PU voltage drop fault in phase A at 0.7s. Figure 5(a) shows the system output line current waveform. Each phase has a different number of faulty modules, and there are no extra redundant modules. The energy storage conversion system can track the power command and achieve balanced power output in each phase. It can also quickly track the reactive current command after a single-phase voltage drop at 0.7s and complete the output adjustment within 20ms (one grid cycle) to achieve emergency reactive power support. Figure 5(b) shows the link phase current waveform. Zero-sequence circulating current is used for inter-phase power transfer to achieve phase-by-phase current control and ensure the balance of external output line current. Figure 5(c) shows the link SOC state simulation diagram. The system SOC has an initial deviation. After running for 2.7s, the battery pack SOC balance control of the normally operating module is achieved. It can be seen that the device of this invention responds quickly and can quickly achieve fault-tolerant control to ensure the SOC balance of the battery packs of all normally operating modules.

[0085] Figures 6(a) to 6(c) Figure 6(a) shows the simulation results of the fault-tolerant control method proposed in this invention when the battery pack of phase AB is completely faulted. This simulates the battery pack maintenance or grounding fault of the corresponding phase battery pack in actual engineering. Phase AB can act as a STATCOM-like system to exchange reactive power with the grid. The simulation model simulates a 0.5PU voltage drop fault in phase A at 0.7s. Figure 6(a) shows the output line current waveform of the system. All modules on the DC side of phase AB are faulty and there are no additional redundant modules. The system can respond quickly to the power command within 25ms and maintain the balanced output of power in each phase. Figure 6(b) shows the phase current waveform of the link. The active current of the link of phase AB is 0. Under this extreme condition, the power difference between phases is compensated by zero-sequence current. In addition, after a single-phase drop fault occurs at 0.7s, phase AB acts as a STATCOM-like system to provide reactive current to the grid and support the voltage for a short time. Figure 6(c) shows the simulation diagram of the SOC state of the link. The SOC of the system has an initial deviation. After running for 4.5s, the SOC of the three-phase link converges. SOC balance can still be achieved under extreme conditions. It is evident that the fault-tolerant control method proposed in this embodiment of the invention is effective in improving the reliability and control flexibility of device operation, and enabling rapid power command tracking and SOC management of the battery pack.

Claims

1. A fault-tolerant control method for a high-voltage direct-connected chain energy storage conversion system, wherein each phase link of the high-voltage direct-connected chain energy storage conversion system includes multiple cascaded sub-modules; the multiple cascaded sub-modules include n sub-modules currently in use and several redundant sub-modules; each sub-module includes a battery pack, a dual active bridge DC / DC converter, and a single-phase H-bridge inverter connected in sequence; characterized in that, The method includes the following steps: S1. Determine if any submodule has failed. If a faulty submodule exists, bypass it. Calculate the number of faulty submodules, m, using the following formula: ; where m x_flt m represents the number of faulty submodules in the x-th phase link. x_rdd This represents the number of redundant submodules in the x-th phase link; x = ab, bc, ca; ab, bc, ca represent the three-phase links of the high-voltage direct-connected chain energy storage conversion system, respectively. S2. Calculate the system-level active power command P using the following formula. ref : Among them, P N This represents the rated capacity of the high-voltage direct-connected chain energy storage conversion system; the min function represents the function that takes the minimum value of the input; P disp For the given active power command; Using the system-level active power command P ref The calculated active power command P at the link level is obtained. x_ref : ; The maximum deviation ΔSOC of the battery pack in a high-voltage direct-connected energy storage conversion system is calculated using the following formula. max : ; where ΔSOC T ΔSOC is the threshold value for the state of charge deviation of the energy storage battery pack. max The maximum deviation of SOC, SOC xj This represents the SOC state of the energy storage battery in the j-th submodule of the x-th phase link in the energy storage conversion system, and n represents the number of modules in each phase link of the high-voltage direct-connected chain energy storage conversion system; F xj This represents the operating state of the j-th submodule in the x-th phase link of the energy storage conversion system. When the submodule fails, F... xj =1, F is normal when the submodule is working. xj =0; the max function represents the function that takes the maximum value of the input value; The active power command P of the j-th submodule in the x-th phase link is calculated using the following formula. xj_ref : ;in, ; The reactive current command I of the xth phase link qx_ref With cosθ x The product of these is used as the reactive component of the AC current command, and the active current command i is used as the reactive component. px_ref With sinθ x The product of these components is the active component of the AC current command, which is then superimposed with the reactive component, active component, and zero-sequence current command i. 0_ref Receive AC current input command i x_ref ; Input the alternating current command i x_ref The current i in the xth phase link x The difference is used as the input to the PR controller, and the output of the PR controller is compared with the voltage feedforward command u. xj_f The summed commands are then subjected to carrier phase-shift modulation to obtain the first modulated wave signal, which controls the on / off switching of the single-phase H-bridge inverter transistors; where θ x This is the phase signal of the grid voltage; where, Among them, U sx U represents the phase voltage amplitude of the power grid. ref K represents the rated terminal voltage of the energy storage conversion system. Q This represents the reactive power-voltage droop coefficient.

2. The fault-tolerant control method for a high-voltage direct-connected chain energy storage conversion system according to claim 1, characterized in that, Zero-sequence current command i 0_ref The calculation formula is: ； Where I0 represents the zero-sequence current amplitude, θ ab Indicates the grid line voltage u ab The phase, φ, indicates that the zero-sequence current lags behind the grid line voltage u. ab Phase difference; I 0d and I 0q They are respectively based on the grid line voltage u ab phase θ ab Using the reference phase, the d-axis and q-axis components of the zero-sequence current are obtained by performing a Park transformation on the zero-sequence current i0.

3. The fault-tolerant control method for a high-voltage direct-connected chain energy storage conversion system according to claim 2, characterized in that, I 0d and I 0q The calculation formula is: ； Among them, U 1d and U 1q These are the d-axis and q-axis components of the positive sequence voltage, respectively. 2d and U 2q These are the d-axis and q-axis components of the negative sequence voltage, respectively.

4. The fault-tolerant control method for a high-voltage direct-connected chain energy storage conversion system according to any one of claims 1 to 3, characterized in that, Also includes: The active power command P of the j-th submodule of the x-th phase link is... xj_ref The output active power P of the j-th submodule in the x-th phase link is xj Subtracting the two values ​​yields a first difference, which is then fed into a first PI controller. The output of the first PI controller is subtracted from the output current of the j-th submodule battery pack in the x-th phase link to obtain a second difference. This second difference is then fed into a second PI controller to obtain the DAB external phase shift angle θ of the j-th submodule battery pack in the x-th phase link. xj_ps , for θ xj_ps Single-phase-shift modulation is performed to obtain a second modulation wave signal, which controls the on / off state of the switching transistors of the dual active bridge DC / DC converter.

5. A fault-tolerant control system for a high-voltage direct-connected chain energy storage conversion system, wherein each phase link of the high-voltage direct-connected chain energy storage conversion system includes multiple cascaded sub-modules; the multiple cascaded sub-modules include n sub-modules currently in use and several redundant sub-modules; each sub-module includes a battery pack, a dual active bridge DC / DC converter, and a single-phase H-bridge inverter connected in sequence; characterized in that, It includes a main controller and multiple sub-controllers; each of the sub-modules' dual active bridge DC / DC converters and single-phase H-bridge inverters is connected to a sub-controller; all sub-controllers are connected to the main controller; the main controller is configured to perform the steps of the method of claim 1 and output signal P. x_ref P xj_ref and I qx_ref Distribute the information to each sub-controller.

6. The control system according to claim 5, characterized in that, Each sub-controller acquires P x_ref P xj_ref and I qx_ref Subsequently, it is configured to perform the steps of the method described in any one of claims 2 to 4.

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