Multi-working-condition electromagnetic transient equivalent modeling method of lithium battery energy storage type MMC

Abstract

The invention discloses a multi-working-condition electromagnetic transient equivalent modeling method of a lithium battery energy storage type MMC, and relates to the technical field of high-voltage flexible direct-current power transmission. The method comprises the following steps: firstly, analyzing an energy storage access mode of MMC-BESS and a modeling method of a lithium battery, and selecting a topology and lithium battery electromagnetic transient simulation model according to working conditions and requirements; secondly, analyzing the operation mode of the MMC-BESS, and selecting a switching group modeling method; according to capacitance and inductance discretization and a switching group binary resistance method, obtaining an adjoint circuit of the sub-module, obtaining a sub-module equivalent circuit through a nested fast solution method, and carrying out series connection and superposition to obtain an MMC-BESS equivalent model; analyzing the operation mode of the MMC-BESS under the locking working condition, determining a locking loop, and integrating the locking loop with the equivalent model; and finally, according to the proposed modeling method and the PSCAD / EMTDC, constructing a model overall code framework. The method is simple and easy to implement, compared with a detailed model, the simulation speed can be remarkably increased, simulation resources are saved, and the MMC-BESS related internal characteristics can be reflected through an output interface.

Description

Technical Field

[0001] The present invention relates to the field of high-voltage flexible direct current (HVDC) power transmission technology, and in particular to a multi-operating-condition electromagnetic transient equivalent modeling method for a lithium battery energy storage (MMC) system. More specifically, the present invention relates to a multi-operating-condition electromagnetic transient equivalent modeling method for a modular multilevel converter with battery-based energy storage system (MMC-BESS) system. Background Art

[0002] my country's energy supply is gradually transitioning from a system dominated by thermal power to a green energy system based on renewable energy sources such as wind and solar power. However, traditional power system equipment cannot address the challenges posed by the intermittent, volatile, and uncertain nature of renewable energy sources to grid stability and power quality.

[0003] Modular multilevel converters (MMCs), with their high voltage, large capacity, flexible control methods, and excellent output characteristics, have become a key technology for addressing the integration of renewable energy into the grid. Combined with energy storage technologies, they can effectively shift peaks and valleys, smooth fluctuations, and improve the economic efficiency and reliability of grid operations. The modular design of lithium battery energy storage MMCs (MMC-BESS) not only allows for flexible adjustment of output voltage and power, but also effectively reduces energy conversion losses, improving grid stability and power quality.

[0004] However, the widespread application of MMC-BESS faces a significant technical bottleneck: the efficiency of electromagnetic transient simulation. Traditional simulation modeling methods, when dealing with MMC-BESSs with complex topologies, require simulating every switch action and energy storage unit state. This results in high computational complexity, low simulation efficiency, and long simulation times, and is unable to adapt to changing operating conditions. Hardware parallelization methods improve simulation speed through subsystem partitioning, multi-core parallelization, or master-slave collaborative simulation, but these methods are often limited by hardware resources and the original circuit structure, lacking universality and ease of use.

[0005] To address the above issues, current research has proposed equivalent modeling methods that are independent of hardware and topological constraints. For example, the capacitors and inductors are discretized using the Dommel method, and the sub-modules are simplified to obtain the Thevenin equivalent circuit of the bridge arm. However, this method cannot accurately reflect the internal characteristics of the MMC and the differences between the sub-modules in the bridge arm, making it difficult to use for studying the charge state of the energy storage module and the fault characteristics of the converter.

[0006] Based on the above content, the present invention proposes a multi-operating-condition electromagnetic transient equivalent modeling method for a lithium battery energy storage type MMC. Summary of the Invention

[0007] The purpose of the present invention is to propose a multi-condition electromagnetic transient equivalent modeling method for lithium battery energy storage MMC to solve the problems raised in the background technology. Compared with the existing technology, the present invention is simpler and easier to implement, and is convenient for expanding the voltage level and the number of sub-modules. Compared with detailed models, it can significantly improve the simulation speed and save simulation resources.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A multi-operating-condition electromagnetic transient equivalent modeling method for a lithium battery energy storage MMC comprises the following steps:

[0010] S1. Analyze the energy storage access method of MMC-BESS and the modeling method of lithium batteries, and select the topology and lithium battery electromagnetic transient simulation model according to the working conditions and requirements;

[0011] S2. Analyze the operating modes of the MMC-BESS and select the switch group modeling method;

[0012] S3. Obtain the accompanying circuit of the submodule based on the discretization of capacitance and inductance and the binary resistance method of the switch group. Obtain the equivalent circuit of the submodule through the nested fast solution method, and then superimpose them in series to obtain the equivalent model of the MMC-BESS.

[0013] S4. Analyze the operating mode of the MMC-BESS under the locking condition, determine the locking loop, and integrate it with the equivalent model;

[0014] S5. Based on the proposed modeling method and the usage of the PSCAD / EMTDC custom components Branch, Begin, Dsdyn, and Dsout code segments, build the overall model code framework.

[0015] Preferably, the S1 specifically includes the following contents:

[0016] The structure of the MMC-BESS is identical to a conventional half-bridge MMC, except for the energy storage component. A battery-based energy storage module (BESM) is connected to a conventional half-bridge submodule, forming a submodule of the MMC-BESS. Several half-bridge submodules and a bridge arm inductor are connected in series to form a bridge arm of the MMC-BESS. Each phase consists of two upper and lower bridge arms, for a total of three phases, forming a distributed energy storage MMC topology. Several lithium batteries are connected in series and parallel to form a lithium battery pack, which is put into operation as the energy storage component of the lithium battery energy storage module (BESM).

[0017] The ways to connect a lithium battery energy storage module (BESM) to a conventional half-bridge submodule include direct parallel connection of submodule capacitors, parallel connection via a non-isolated DC / DC circuit, and parallel connection via an isolated DC / DC circuit.

[0018] If the lithium battery energy storage module (BESM) and the submodule capacitor are directly connected in parallel, the voltage of the lithium battery energy storage module (BESM) and the submodule capacitor are the same; if they are connected in parallel through a non-isolated DC / DC circuit, the voltage of the lithium battery energy storage module (BESM) and the voltage of the submodule capacitor are decoupled and will not affect each other; if they are connected in parallel through an isolated DC / DC circuit, the lithium battery energy storage module (BESM) and the submodule capacitor are electrically isolated;

[0019] In the field of electromagnetic transient simulation, modeling of lithium batteries is achieved through equivalent circuit models, which include Sheffield model, Thevenin model, PNGV (partnership for a new generation of vehicles) model and neural network model.

[0020] Preferably, the operating modes of the MMC-BESS in S2 include: an engaged state, a disconnected state, and a locked state.

[0021] Preferably, the S3 specifically includes the following contents:

[0022] Before establishing the equivalent circuit, the capacitor and inductor components are discretized, and their discretized equivalent parameters are shown in the following formula:

[0023]

[0024] Where Δt represents the simulation step of electromagnetic transient simulation; R L It represents the equivalent resistance value obtained after the inductor element is discretized; L represents the inductance value of the inductor element; R C Represents the equivalent resistance value obtained after the discretization of the capacitor element; C represents the capacitance value of the capacitor element; I Leq V represents the current flowing through the circuit obtained by discretizing the inductor element; L Represents the voltage across the circuit obtained by discretizing the inductor element; I L I represents the current of the parallel historical current source in the circuit obtained by discretizing the inductor element; Ceq Represents the current flowing through the circuit obtained by discretizing the capacitor element; V C Represents the voltage across the circuit obtained by discretizing the capacitor element; I C represents the current of the parallel historical current source in the circuit obtained by discretizing the capacitor element;

[0025] Combining the binary resistance model of the switch group and the discretized model of the capacitor and inductor, the accompanying circuit of the submodule is obtained; then, based on the accompanying circuit, node number and direction, the corresponding node voltage equation is obtained:

[0026] G SM V SM =J SM +I SM (2)

[0027] Where G SM represents the node admittance matrix of the submodule companion circuit; V SM represents the node voltage matrix of the submodule accompanying circuit; J SM Represents the injection current source of the submodule accompanying circuit; I SM Represents the branch current matrix of the submodule companion circuit;

[0028] According to the node regulations of the equivalent circuit, the matrices in equation (2) are obtained as follows and rewritten as block matrices:

[0029]

[0030] Where G C G represents the admittance of the equivalent resistance obtained after the discretization of the capacitor element; L G represents the admittance of the equivalent resistance obtained after the inductor element is discretized; bat Indicates the admittance of the battery internal resistance; E bat Indicates the voltage across the battery; I ARM Indicates the bridge arm current flowing through this submodule;

[0031] Formula (2) is expressed in the form of a block matrix as:

[0032]

[0033] According to the nested fast simultaneous solution method, formula (3) is rewritten as:

[0034] Y 11 V EX +Y 12 V IN =J EX +I EX (4)

[0035] Y 21 V EX +Y 22 V IN =J IN +I IN =J IN (5)

[0036] Set VIN in the initial step length to 0 or other initial values, and then find VEX in the next step length in the formula:

[0037]

[0038] Through formula (6), the synchronous VIN, that is, the voltage value of each node, including the submodule capacitor voltage, is inversely calculated:

[0039]

[0040] Regulation:

[0041]

[0042] Therefore, formula (7) is further simplified as:

[0043]

[0044] Where, is the Norton equivalent admittance, is the Norton equivalent current source;

[0045] The submodules are further simplified into Thevenin form. After obtaining the Thevenin equivalent circuit of the submodule, the Thevenin equivalent circuits of each submodule are superimposed to obtain the Thevenin equivalent circuit of each bridge arm. The equivalent circuits of the upper and lower bridge arms of each phase are connected to obtain the equivalent circuit of the MMC-BESS.

[0046] Preferably, the S4 specifically includes the following contents:

[0047] During the startup phase of the MMC-BESS, the IGBT is locked and the submodule capacitor is charged through the anti-parallel diode;

[0048] When a DC fault occurs, the IGBT is also locked;

[0049] The operating modes of the MMC-BESS submodules under locked-out conditions were analyzed, dividing the submodules into a half-bridge submodule and an energy storage system. The locked-out operating mode of the half-bridge submodule was implemented by adding a locking loop to the equivalent model. The operating mode of the energy storage system, which depends on the voltage between the BESM and the submodule capacitors, is independent of the locked-out mode of the half-bridge submodule and satisfies the binary resistor equivalence.

[0050] Based on the above analysis, a locking loop is designed and integrated with the established MMC-BESS equivalent model to obtain a locking equivalent model.

[0051] Preferably, the S5 specifically includes the following contents:

[0052] In the PSCAD interactive interface, use the component wizard tool to create a custom component and set the component's electrical nodes and signal interfaces in its Graphic interface; create the parameters required for the equivalent model in the Parameters interface; and finally, write the interface code in the Script interface.

[0053] The interface code includes Branch, Begin, Dsdyn, and Dsout. First, the branch name between the electrical nodes of the equivalent model is set in Branch. Secondly, the equivalent value calculation of the discretized components is performed in the Begin section. Then, the node admittance matrix is ​​generated in the Dsdyn section, and the circuit is solved to obtain the voltage values ​​of each node and branch current value of the equivalent model. Finally, the circuit is inversely solved in the Dsout section to obtain its internal electrical information.

[0054] Compared with the prior art, the present invention provides a multi-operating-condition electromagnetic transient equivalent modeling method for lithium battery energy storage MMC, which has the following beneficial effects:

[0055] The present invention proposes a multi-operating-condition electromagnetic transient equivalent modeling method for lithium battery energy storage type MMC. Compared with the existing technology, the present invention is simpler and easier to implement, and is convenient for expanding voltage levels and the number of sub-modules. Compared with detailed models, it can significantly improve simulation speed and save simulation resources, effectively improving the simulation speed of large-capacity MMC-BESS and providing a basis for real-time simulation of converters. In addition, while ensuring simulation efficiency, the present invention accurately simulates the external and internal characteristics of MMC-BESS under multiple operating conditions, which is conducive to in-depth research and optimization of its control and protection measures, improving its reliability in new energy grid connection, and providing strong technical support for simulation research and engineering applications of MMC-BESS. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is a flow chart of a multi-operating-condition electromagnetic transient equivalent modeling method for a lithium battery energy storage MMC mentioned in Example 1 of the present invention;

[0057] Figure 2 Schematic diagram of the MMC-BESS system structure mentioned in Example 1 of the present invention;

[0058] Figure 3 This is a schematic diagram of the battery-based energy storage module (BESM) access method mentioned in Example 1 of the present invention;

[0059] Figure 4 This is a schematic diagram of the lithium battery equivalent circuit model mentioned in Example 1 of the present invention;

[0060] Figure 5This is a schematic diagram of the MMC-BESS submodule modes and current directions mentioned in Example 1 of the present invention;

[0061] Figure 6 This is a schematic diagram of the accompanying circuit of the submodule mentioned in Example 1 of the present invention;

[0062] Figure 7 This is a schematic diagram of the equivalent Thevenin circuit of the submodule mentioned in Example 1 of the present invention;

[0063] Figure 8 This is a schematic diagram of the equivalent circuit of the MMC-BESS mentioned in Example 1 of the present invention;

[0064] Figure 9 This is a schematic diagram of the equivalent model locking loop mentioned in Example 1 of the present invention;

[0065] Figure 10 This is a waveform comparison diagram of the locked start condition mentioned in Example 1 of the present invention;

[0066] Figure 11 This is a comparison diagram of the AC fault condition waveforms mentioned in Example 1 of the present invention;

[0067] Figure 12 This is a comparison diagram of the steady-state operating waveforms mentioned in Example 1 of the present invention. DETAILED DESCRIPTION

[0068] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0069] Example 1:

[0070] See also Figure 1 The present invention proposes a multi-operating-condition electromagnetic transient equivalent modeling method for a lithium battery energy storage MMC, which specifically includes the following contents:

[0071] Step 1: Analyze the energy storage access method of the MMC-BESS and the modeling method of the lithium battery, and select the topology and lithium battery electromagnetic transient simulation model based on the operating conditions and requirements:

[0072] In this step, the structure of MMC-BESS is as follows Figure 2 As shown. Figure 2As can be seen, the structure of the MMC-BESS is identical to that of a conventional half-bridge MMC, except for the energy storage component. A lithium-ion battery-based energy storage module (BESM) is connected to a conventional half-bridge submodule, forming a submodule of the MMC-BESS. Furthermore, a bridge arm of the MMC-BESS is formed by connecting several half-bridge submodules and a bridge arm inductor in series. Each phase consists of two upper and lower bridge arms, for a total of three phases, forming a distributed energy storage MMC topology. Because MMC-BESS typically operates under high voltage and high current conditions, the voltage and current to which each BESM is subjected far exceeds the tolerance of a single lithium battery. Therefore, a large number of lithium batteries are connected in series and parallel to form a lithium battery pack, which serves as the energy storage component of the BESM.

[0073] The main ways to connect BESM to conventional half-bridge submodules are direct parallel connection of submodule capacitors, parallel connection via non-isolated DC / DC circuits, and parallel connection via isolated DC / DC circuits, such as Figure 3 If the BESM is connected in parallel with the submodule capacitor, as shown in Figure 3 As shown in (a), the BESM and submodule capacitor voltages are the same. When the submodule capacitor voltage fluctuates greatly, the BESM is easily damaged. At the same time, due to the parallel connection of the BESM, the modulation of the converter will also be affected. If connected in parallel through an isolated DC / DC circuit, such as Figure 3 (b) and Figure 3 As shown in (c), the BESM voltage is decoupled from the submodule capacitor voltage and does not affect each other. An isolated DC / DC circuit can achieve electrical isolation between the BESM and the submodule capacitor, thereby allowing the BESM to have a larger power throughput range. However, its cost is significantly higher than that of a non-isolated DC / DC circuit, and the large number of nodes also affects its simulation speed to a certain extent. A non-isolated DC / DC circuit can decouple the BESM and submodule capacitor voltages at a relatively low cost. Therefore, a non-isolated DC / DC circuit is selected as the connection interface between the BESM and the submodule capacitor.

[0074] In the field of electromagnetic transient simulation, the modeling of lithium batteries is usually based on equivalent circuit models. Commonly used models include Sheffield model, Thevenin model, PNGV (partnership for a new generation of vehicles) model and neural network model. Figure 4As shown, a controlled source is connected in series with a resistor representing the battery's internal resistance. The controlled source voltage is affected by the battery's real-time charge and discharge current and the remaining battery charge. The accuracy of the four commonly used models varies slightly. However, compared to the Sheffield model, the Thevenin model and PNGV model have more nodes and components and include energy storage elements. The neural network model requires longer training time and is subject to local optima. To balance accuracy and simplicity, the present invention adopts the Sheffield model for battery modeling.

[0075] Step 2: Analyze the operating mode of the MMC-BESS and select the switch group modeling method:

[0076] During the operation of MMC-BESS, the possible operating states of the submodules are divided into the input state, the cut-off state, and the blocking state. The current states are as follows: Figure 5 (a), (b), (c) are shown. Figure 5 It can be seen that the IGBT and diode in each state switch group are not turned on at the same time, and the switch group is not turned on when the switch signal is 0, so there is no need to consider the diode freewheeling. Therefore, a binary resistor can be used to equivalent the switch group.

[0077] Step 3: Obtain the accompanying circuit of the submodule based on the discretization of capacitors and inductors and the binary resistance method of the switch group. Obtain the equivalent circuit of the submodule through the nested fast solution method, and then superimpose them in series to obtain the equivalent model of the MMC-BESS:

[0078] Before building the equivalent circuit, it is necessary to discretize components such as capacitors and inductors. The discretized equivalent parameters are shown in formula (1).

[0079]

[0080] Where Δt represents the simulation step of electromagnetic transient simulation; R L It represents the equivalent resistance value obtained after the inductor element is discretized; L represents the inductance value of the inductor element; R C Represents the equivalent resistance value obtained after the discretization of the capacitor element; C represents the capacitance value of the capacitor element; I Leq V represents the current flowing through the circuit obtained by discretizing the inductor element; L Represents the voltage across the circuit obtained by discretizing the inductor element; I L I represents the current of the parallel historical current source in the circuit obtained by discretizing the inductor element; Ceq Represents the current flowing through the circuit obtained by discretizing the capacitor element; V C Represents the voltage across the circuit obtained by discretizing the capacitor element; I C represents the current of the parallel historical current source in the circuit obtained by discretizing the capacitor element;

[0081] Combining the binary resistance model of the switch group and the discrete model of the capacitor and inductor, the accompanying circuit of the submodule can be obtained as follows: Figure 6 According to the accompanying circuit, node number and direction, the corresponding node voltage equation can be obtained, namely equation (2).

[0082] G SM V SM =J SM +I SM (2)

[0083] Where G SM represents the node admittance matrix of the submodule companion circuit; V SM represents the node voltage matrix of the submodule accompanying circuit; J SM Represents the injection current source of the submodule accompanying circuit; I SM represents the branch current matrix of the submodule accompanying circuit; according to the node regulations of the equivalent circuit, the matrices in formula (2) can be obtained as follows:

[0084]

[0085] Where G C G represents the admittance of the equivalent resistance obtained after the discretization of the capacitor element; L G represents the admittance of the equivalent resistance obtained after the inductor element is discretized; bat Represents the admittance of the battery's internal resistance; E bat Indicates the voltage across the battery; I ARM Indicates the bridge arm current flowing through this submodule;

[0086] Formula (2) is converted into a block matrix to obtain formula (3):

[0087]

[0088] According to the nested fast simultaneous solution method, the above equations can be rewritten as equations (4) and (5).

[0089] Y 11 V EX +Y 12 V IN =J EX +I EX (4)

[0090] Y 21 V EX +Y 22 V IN =J IN +I IN =J IN (5)

[0091] Initial step length V INIt can be 0 or other initial values, based on which the V in the next step can be obtained in the formula EX , as shown in formula (6).

[0092]

[0093] Through the above formula, we can further inversely calculate the synchronous length V IN , that is, the voltage value of each node, which includes the sub-module capacitor voltage.

[0094]

[0095] Regulation:

[0096]

[0097] Therefore, formula (7) can be further simplified to formula (10).

[0098]

[0099] in, is the Norton equivalent admittance, It is the Norton equivalent current source, so the submodule can be further simplified into the Thevenin form, such as Figure 7 After obtaining the Thevenin equivalent circuit of the submodule, the Thevenin equivalent circuits of each submodule are superimposed to obtain the Thevenin equivalent circuit of each bridge arm. The equivalent circuits of the upper and lower bridge arms of each phase are connected to obtain the equivalent circuit of the MMC-BESS, as shown in Figure 8 shown.

[0100] Step 4: Analyze the operating mode of the MMC-BESS under the locking condition, determine the locking loop, and integrate it with the equivalent model:

[0101] The startup phase of an MMC-BESS is similar to that of a conventional MMC. The IGBTs must be locked out, and the submodule capacitors must be charged through their antiparallel diodes. Furthermore, the IGBTs must be locked out in the event of a DC fault. Under these locking conditions, the antiparallel diodes of the switch group are involved, so the binary resistance model in step 3 is no longer applicable. A new locking model must be developed for these conditions.

[0102] right Figure 5 The operating mode of the MMC-BESS submodule under the blocking condition is analyzed. The submodule can be divided into the half-bridge submodule part and the energy storage system part. The blocking operating mode of the half-bridge part can be analyzed by adding Figure 9 The operating mode of the energy storage system depends on the voltage between the BESM and the submodule capacitors and is therefore independent of the latching mode of the half-bridge. The binary resistor equivalence in step 3 still holds.

[0103] Based on the above analysis, a closed loop is designed and integrated with the MMC-BESS equivalent model established in step 3 to obtain the following: Figure 9 The latch-off equivalent model is shown.

[0104] Step 5: Based on the proposed modeling method and the use of the PSCAD / EMTDC custom components Branch, Begin, Dsdyn, and Dsout code segments, build the overall model code framework:

[0105] In the PSCAD interactive interface, use the component wizard tool to create a custom component. Set the component's electrical nodes and signal interfaces in the Graphic interface. Create the parameters required for the equivalent model in the Parameters interface. Finally, write the interface code in the Script interface. The interface code consists of four sections: Branch, Begin, Dsdyn, and Dsout. First, in the Branch section, set the branch names between the electrical nodes of the equivalent model. Next, perform the equivalent value calculations for the discretized components in the Begin section. Then, in the Dsdyn section, generate the node admittance matrix and solve the circuit to obtain the node voltages and branch currents of the equivalent model. Finally, in the Dsout section, perform the circuit inverse solution to obtain the internal electrical information.

[0106] In summary, the purpose of the present invention is to provide a multi-condition electromagnetic transient equivalent modeling method suitable for MMC-BESS. This method is simple and effective, easy to modularize, and can significantly improve the simulation speed of the converter station under multiple working conditions such as normal working conditions and locked working conditions, saving computer resources, effectively improving the simulation speed of large-capacity MMC-BESS, and providing a basis for real-time simulation of converters.

[0107] The algorithm used for node elimination in the multi-condition electromagnetic transient equivalent modeling method of the MMC-BESS proposed in this application is a nested fast solution method. While eliminating nodes, completing matrix reduction, and improving simulation speed, it can also retain internal electrical characteristics by inversely solving internal information after the current step simulation is completed. Specifically, according to the specific topology, formula (3) is obtained, and formula (3) is expanded and simplified to obtain formula (10), and the Norton form equivalent circuit parameters are obtained. The order reduction of the high-order matrix corresponding to the detailed model is completed, and after completing the circuit solution of the current step, the information obtained by solving the solution can be substituted into formula (6) to inversely solve the topology internal node voltage to achieve the simulation of the internal characteristics. Therefore, the multi-condition electromagnetic transient equivalent modeling method of the MMC-BESS proposed in this application can achieve an improvement in simulation speed while retaining the simulation of the internal characteristics of the equipment.

[0108] The MMC-BESS multi-condition equivalent model established in this application has good simulation accuracy and acceleration effect under both locked and non-locked conditions. Figure 10 、 11 The following are waveform comparisons of the blocking start and AC fault conditions in the 11-level MMC-HVDC test system to verify the accuracy of the equivalent model in reflecting the internal and external characteristics of the system. The power supply at both ends is set to increase to the rated voltage after 0.05s. After blocking start and entering the steady state and an AC fault condition, the active power, DC power and A-phase upper arm current of the MMC-BESS converter station are tested during blocking, as well as the active power, DC voltage and A-phase upper arm current during AC fault. Figure 10 and 11 It can be seen that the waveforms have a good degree of consistency, so the proposed modeling method has high accuracy in both locked and non-locked conditions. Figure 12 Figure 2 shows a comparison of steady-state waveforms for an 11-level MMC-HVDC test system. Based on the steady-state error calculation formula, the steady-state errors for active power, DC power, and phase A upper arm current are calculated to be 1.7%, 2.7%, and 11.5%, respectively. This verifies the high accuracy of the proposed modeling method under steady-state conditions. Furthermore, with a simulation step size of 50μs and a total simulation time of 5s, the detailed model and equivalent model take 615.485s and 62.625s, respectively. The equivalent model achieves a speedup ratio of 9.83, demonstrating excellent simulation speed-up. This speed-up increases significantly with increasing MMC levels.

[0109] In summary, the purpose of the present invention is to provide a multi-condition electromagnetic transient equivalent modeling method suitable for MMC-BESS. This method is simple and effective, easy to modularize, and can significantly improve the simulation speed of the converter station under multiple working conditions such as normal working conditions and locked working conditions, saving computer resources, effectively improving the simulation speed of large-capacity MMC-BESS, and providing a basis for real-time simulation of converters.

[0110] The algorithm used for node elimination in the multi-condition electromagnetic transient equivalent modeling method of the MMC-BESS proposed in this application is a nested fast solution method. While eliminating nodes, completing matrix reduction, and improving simulation speed, it can also retain internal electrical characteristics by inversely solving internal information after the current step simulation is completed. Specifically, according to the specific topology, formula (3) is obtained, and formula (3) is expanded and simplified to obtain formula (10), and the Norton form equivalent circuit parameters are obtained. The order reduction of the high-order matrix corresponding to the detailed model is completed, and after completing the circuit solution of the current step, the information obtained by solving the solution can be substituted into formula (6) to inversely solve the topology internal node voltage to achieve the simulation of the internal characteristics. Therefore, the multi-condition electromagnetic transient equivalent modeling method of the MMC-BESS proposed in this application can achieve an improvement in simulation speed while retaining the simulation of the internal characteristics of the equipment.

[0111] From step 1 to step 5, the previous step is the basis for the execution of the next step. These 5 steps are closely linked to each other and executed in sequence, forming an organic and indivisible whole.

[0112] It should be noted that the contents not described in detail in the embodiments of the present invention belong to the prior art known to those skilled in the art.

[0113] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims. The information disclosed in the background technology section of the present invention is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or any form of implication that the information constitutes prior art already known to those skilled in the art.

Claims

1. A multi-operating-condition electromagnetic transient equivalent modeling method for lithium battery energy storage MMC, characterized in that: The steps include: S1. Analyze the energy storage access method of MMC-BESS and the modeling method of lithium batteries, and select the topology and lithium battery electromagnetic transient simulation model according to the working conditions and requirements; S2. Analyze the operating modes of the MMC-BESS and select the switch group modeling method; S3. Obtain the accompanying circuit of the submodule based on the discretization of capacitance and inductance and the binary resistance method of the switch group. Obtain the equivalent circuit of the submodule through the nested fast solution method, and then superimpose them in series to obtain the equivalent model of the MMC-BESS. S4. Analyze the operating mode of the MMC-BESS under the locking condition, determine the locking loop, and integrate it with the equivalent model; S5. Based on the proposed modeling method and the usage of the PSCAD / EMTDC custom components Branch, Begin, Dsdyn, and Dsout code segments, build the overall model code framework.

2. The multi-operating-condition electromagnetic transient equivalent modeling method of a lithium battery energy storage MMC according to claim 1 is characterized in that: The S1 specifically includes the following contents: The structure of the MMC-BESS is identical to a conventional half-bridge MMC, except for the energy storage component. A lithium battery energy storage module is connected to a conventional half-bridge sub-module, forming a sub-module of the MMC-BESS. Several half-bridge sub-modules and a bridge arm inductor are connected in series to form a bridge arm of the MMC-BESS. Each phase consists of two upper and lower bridge arms, for a total of three phases, forming a distributed energy storage MMC topology. Several lithium batteries are connected in series and parallel to form a lithium battery pack, which is put into operation as the energy storage component of the lithium battery energy storage module. The lithium battery energy storage module can be connected to a conventional half-bridge sub-module in two ways: direct parallel connection of sub-module capacitors, parallel connection via a non-isolated DC / DC circuit, and parallel connection via an isolated DC / DC circuit. If the lithium battery energy storage module and the sub-module capacitor are directly connected in parallel, the voltage of the lithium battery energy storage module and the sub-module capacitor will be the same; if they are connected in parallel through a non-isolated DC / DC circuit, the voltage of the lithium battery energy storage module and the voltage of the sub-module capacitor will be decoupled and will not affect each other; if they are connected in parallel through an isolated DC / DC circuit, the lithium battery energy storage module and the sub-module capacitor will be electrically isolated; In the field of electromagnetic transient simulation, modeling of lithium batteries is achieved through equivalent circuit models, which include Sheffield model, Thevenin model, PNGV model and neural network model.

3. The multi-operating-condition electromagnetic transient equivalent modeling method of a lithium battery energy storage type MMC according to claim 2 is characterized in that: The operation modes of the MMC-BESS described in S2 include: an on-state, a cut-off state, and a locked state.

4. The multi-operating-condition electromagnetic transient equivalent modeling method of a lithium battery energy storage MMC according to claim 3 is characterized in that: The S3 specifically includes the following contents: Before establishing the equivalent circuit, the capacitor and inductor components are discretized, and their discretized equivalent parameters are shown in the following formula: Where Δt represents the simulation step of electromagnetic transient simulation; R L It represents the equivalent resistance value obtained after the inductor element is discretized; L represents the inductance value of the inductor element; R C Represents the equivalent resistance value obtained after the discretization of the capacitor element; C represents the capacitance value of the capacitor element; I Leq V represents the current flowing through the circuit obtained by discretizing the inductor element; L Represents the voltage across the circuit obtained by discretizing the inductor element; I L I represents the current of the parallel historical current source in the circuit obtained by discretizing the inductor element; Ceq Represents the current flowing through the circuit obtained by discretizing the capacitor element; V C Represents the voltage across the circuit obtained by discretizing the capacitor element; I C represents the current of the parallel historical current source in the circuit obtained by discretizing the capacitor element; Combining the binary resistance model of the switch group and the discretized model of the capacitor and inductor, the accompanying circuit of the submodule is obtained; then, based on the accompanying circuit, node number and direction, the corresponding node voltage equation is obtained: G SM V SM =J SM +I SM (2) Where G SM represents the node admittance matrix of the submodule companion circuit; V SM represents the node voltage matrix of the submodule accompanying circuit; J SM Represents the injection current source of the submodule accompanying circuit; I SM represents the branch current matrix of the accompanying circuit of the submodule; according to the node regulations of the equivalent circuit, the matrices in formula (2) are obtained as follows and rewritten as block matrices: Where G C G represents the admittance of the equivalent resistance obtained after the discretization of the capacitor element; L G represents the admittance of the equivalent resistance obtained after the inductor element is discretized; bat Represents the admittance of the battery's internal resistance; E bat Indicates the voltage across the battery; I ARM Indicates the bridge arm current flowing through this submodule; Formula (2) is expressed in the form of a block matrix as: According to the nested fast simultaneous solution method, formula (3) is rewritten as: Y 11 V EX +Y 12 V IN =J EX +I EX (4) Y 21 V EX +Y 22 V IN =J IN +I IN =J IN (5) Set VIN in the initial step length to 0 or other initial values, and then find VEX in the next step length in the formula: Through formula (6), the synchronous VIN, that is, the voltage value of each node, including the submodule capacitor voltage, is inversely calculated: Regulation: Therefore, formula (7) is further simplified as: Where, is the Norton equivalent admittance, is the Norton equivalent current source; The submodules are further simplified into Thevenin form. After obtaining the Thevenin equivalent circuit of the submodule, the Thevenin equivalent circuits of each submodule are superimposed to obtain the Thevenin equivalent circuit of each bridge arm. The equivalent circuits of the upper and lower bridge arms of each phase are connected to obtain the equivalent circuit of the MMC-BESS.

5. The multi-operating-condition electromagnetic transient equivalent modeling method of a lithium battery energy storage MMC according to claim 4 is characterized in that: The S4 specifically includes the following contents: During the startup phase of the MMC-BESS, the IGBT is locked and the submodule capacitor is charged through the anti-parallel diode; When a DC fault occurs, the IGBT is also locked; The operating modes of the MMC-BESS submodules under locked-out conditions were analyzed, dividing the submodules into a half-bridge submodule and an energy storage system. The locked-out operating mode of the half-bridge submodule was implemented by adding a locking loop to the equivalent model. The operating mode of the energy storage system, which depends on the voltage between the BESM and the submodule capacitors, is independent of the locked-out mode of the half-bridge submodule and satisfies the binary resistor equivalence. Based on the above analysis, a locking loop is designed and integrated with the established MMC-BESS equivalent model to obtain a locking equivalent model.

6. The multi-operating-condition electromagnetic transient equivalent modeling method of a lithium battery energy storage MMC according to claim 5, characterized in that: The S5 specifically includes the following contents: In the PSCAD interactive interface, use the component wizard tool to create a custom component and set the component's electrical nodes and signal interfaces in its Graphic interface; create the parameters required for the equivalent model in the Parameters interface; and finally, write the interface code in the Script interface. The interface code includes Branch, Begin, Dsdyn, and Dsout. First, the branch name between the electrical nodes of the equivalent model is set in Branch. Secondly, the equivalent value calculation of the discretized components is performed in the Begin section. Then, the node admittance matrix is ​​generated in the Dsdyn section, and the circuit is solved to obtain the voltage values ​​of each node and branch current value of the equivalent model. Finally, the circuit is inversely solved in the Dsout section to obtain its internal electrical information.

Images