Full-power variable-speed pumped storage unit grid-connected equivalent simulation modeling method and system

Abstract

The invention discloses a full-power variable-speed pumped storage unit grid-connected equivalent simulation modeling method and system, and the method comprises the steps: constructing a power frequency network side equivalent simulation module, a low-frequency machine side equivalent simulation module and an M3C internal loop equivalent simulation module, processing the electrical quantity through coordinate transformation, a phase-locked loop and a double-closed-loop controller, generating a reference voltage, and carrying out the calculation of the reference voltage. A controlled voltage source circuit is used for carrying out equivalent phase-shifting carrier modulation to generate a switching sequence of the H full-bridge subunits, and finally, all modules are integrated through a dynamic interface to form a system simulation model, so that rapid and high-precision grid-connected simulation and stability analysis are realized. According to the method, through module decoupling and step length coordination, the simulation speed and the voltage utilization rate are remarkably improved, and broadband oscillation risk assessment is supported.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power systems and power electronics, and particularly relates to a full-power variable-speed pumped storage unit grid-connected equivalent simulation modeling method and system. BACKGROUND

[0002] In recent years, with the continuous increase of the penetration rate of random power sources such as wind power and photovoltaic power in power systems, new power systems show the development trend of "high proportion of new energy and high power electronics", and the system operation faces new challenges such as the aggravation of wide-band oscillation risk and the decline of inertia support capability. Under this background, full-power variable-speed pumped storage units become one of the important devices to support the stable operation of new power systems because of their bidirectional fast power regulation capability and wide-range operation characteristics. This type of unit usually realizes flexible grid connection through a power electronic converter, and the modular multilevel matrix converter (M3C) gradually becomes the preferred solution for realizing the grid-connected operation of full-power variable-speed pumped storage units because of its compact structure, high output voltage level, excellent harmonic characteristics and other advantages.

[0003] In the prior art, full-power variable-speed pumped storage units usually use two-level or three-level converters for grid-connected operation, but due to the limited number of power modules in series, the output current harmonic content is high, and in the high-voltage grid-connected scenario, an additional transformer needs to be configured to meet the voltage matching requirement, thereby increasing the system complexity and cost. In contrast, M3C can realize multi-level output through the carrier phase-shifted modulation strategy, effectively reduce the harmonic content, and improve the voltage utilization rate, providing a good technical foundation for the efficient grid-connected operation of pumped storage units. However, current modeling research on M3C mainly focuses on offshore converter stations, flexible low-frequency power transmission and other application scenarios, and there is no equivalent simulation modeling method suitable for the grid-connected characteristics of full-power variable-speed pumped storage units.

[0004] When full-power variable-speed pumped storage units are connected to the grid through M3C, their operating characteristics are significantly complex. M3C needs to be connected to both the power grid and the low-frequency rotating generator / motor, and its control strategy needs to coordinate factors such as pumped storage / generation operating condition switching, grid dispatching instructions and unit speed regulation. In addition, M3C needs to implement voltage balancing and circulating current suppression control internally, making the control objectives and methods of the three sides of the machine side, the grid side and the internal loop significantly different and the coupling relationship complex. The existing M3C detailed simulation model needs to model multiple coupled links in detail during electromagnetic transient simulation, resulting in a large overall calculation amount and long simulation time, which makes it difficult to meet the needs of fast simulation and parameter sensitivity evaluation in actual grid-connected stability analysis.

[0005] In summary, for the special operating scenario of the full-power variable-speed pumped storage unit connected to the grid through M3C, an equivalent modeling method and system capable of effectively decoupling the machine side, grid side and internal loop control logic and improving simulation efficiency under the premise of ensuring simulation accuracy is urgently needed to support rapid analysis and evaluation of key issues such as grid connection stability and wideband oscillation risk. SUMMARY

[0006] The embodiment of the application provides a full-power variable-speed pumped storage unit grid connection equivalent simulation modeling method and system based on a modular multilevel matrix converter, which comprises the following steps: constructing a power frequency grid side equivalent simulation module, including a Park transformation module, a Clark inverse transformation module, a phase-locked loop module and a double closed-loop controller module, the Park transformation module is used for converting the grid side electrical quantity from the uvw three-phase coordinate system to the power frequency rotating dq coordinate system, the Clark inverse transformation module is used for converting the electrical quantity in the dq coordinate system to the alpha-beta-zero coordinate system, the phase-locked loop module provides a reference angle for coordinate transformation by directing the grid connection point voltage, the double closed-loop controller module comprises a voltage controller and a current controller, the voltage controller controls the current controller based on the conversion value of the grid side active / reactive power scheduling target, and the output of the current controller is converted into the reference voltage in the alpha-beta-zero coordinate system through the Clark inverse transformation module; constructing a low-frequency machine side equivalent simulation module, including a low-frequency Park transformation module, a low-frequency Clark inverse transformation module, a flux and speed observation module and a double closed-loop controller module, the low-frequency Park transformation module is used for converting the machine side electrical quantity from the abc three-phase coordinate system to the low-frequency rotating dq coordinate system, the flux and speed observation module provides a reference angle for the machine side coordinate transformation, and the double closed-loop controller module controls the output according to the machine side active / reactive power control target, and the output is converted into the reference voltage in the alpha-beta-zero coordinate system through the low-frequency Clark inverse transformation module; constructing an M3C internal loop equivalent simulation module, using a controlled voltage source circuit to equivalently modulate the phase-shifted carrier, generating the switching sequence of the H full-bridge subunit according to the reference voltage output by the grid side module and the machine side module, so as to realize multi-level output; dynamically interfacing and integrating the power frequency grid side module, the low-frequency machine side module and the M3C internal loop module to form a system simulation model, the system simulation model outputs the simulation waveform results of the machine side, the grid side and the internal loop according to the corresponding control instructions and parameters input according to the actual working condition, and realizes accelerated simulation and stability analysis by classifying and adjusting the simulation step length of each module.

[0007] In an embodiment of the application, the Park transformation module in the power frequency grid side equivalent simulation module adopts synchronous coordinate transformation based on the power frequency, which is used for converting the three-phase grid side electrical quantity into the direct current in the dq coordinate system, facilitating the input processing of the controller.

[0008] In one embodiment of the present application, the power frequency Clark inverse transformation module adopts decoupling processing of three-phase voltage and current in αβ0 coordinate system, and converts the control signal in dq coordinate system into reference voltage in αβ0 coordinate system to match the input format of the internal loop of M3C.

[0009] In one embodiment of the present application, the grid-side phase-locked loop module adopts a synchronization strategy based on the grid voltage amplitude and phase, and realizes angle locking through a digital signal processing algorithm to ensure the real-time performance and accuracy of the grid-side coordinate transformation.

[0010] In one embodiment of the present application, in the grid-side double-loop controller module, the output of the voltage controller is the instruction input of the current controller, and the output of the current controller is the input signal of the Clark inverse transformation module, to realize voltage-current closed-loop control and feedback regulation.

[0011] In one embodiment of the present application, the low-frequency Park transformation module converts the machine-side electrical quantities from abc three-phase coordinate system to low-frequency rotating dq coordinate system to match the low-frequency operating characteristics of the machine-side controller.

[0012] In one embodiment of the present application, the flux linkage and speed observation module adopts an algorithm based on motor stator flux linkage estimation and rotor speed detection to provide real-time reference angle for the low-frequency Park transformation module to ensure the dynamic response of the machine-side control.

[0013] In one embodiment of the present application, in the M3C internal loop equivalent simulation module, the switching sequence of each H full-bridge subunit is determined according to the amplitude and phase of the reference voltage, and the IGBTs under the same bridge arm are not allowed to be turned on at the same time to avoid short circuit risk.

[0014] In one embodiment of the present application, the switching sequence of the H full-bridge subunit is only allowed to turn on two groups of IGBTs alternately under normal operation, corresponding to output capacitor voltages +UC and -UC, and the rest is 0 voltage.

[0015] In one embodiment of the present application, the dynamic interface module adopts an equal period time synchronization method to coordinate the simulation step lengths of the grid-side, machine-side and internal loop modules to ensure data synchronization of each module while reducing the overall calculation amount.

[0016] In one embodiment of the present application, in the system simulation model, the simulation step lengths of each module are classified and set according to their dominant influence on oscillation stability, wherein the dominant influence module adopts a smaller simulation step length, and the weak influence module adopts a larger simulation step length to improve the simulation efficiency.

[0017] In one embodiment of the present application, the M3C internal loop equivalent simulation module adopts a controlled voltage source circuit to realize multi-level output when simulating phase-shifted carrier modulation, and ignores the oscillation between the capacitor voltages of each layer of sub-units to simplify the model and improve simulation speed.

[0018] In one embodiment of the present application, the dual closed-loop controllers of the power frequency network side module and the low frequency machine side module both adopt a PI control strategy based on unit negative feedback, wherein the PI parameters of the controller are set in a unit standard to adapt to the control requirements under different operating conditions.

[0019] In one embodiment of the present application, the inputs of the system simulation model include the grid-side active / reactive power instructions, the machine-side active / reactive power instructions, the grid point voltage, the M3C parameters (such as bridge arm inductance, sub-module capacitance value, sub-module voltage, etc.), and the rated capacity and power of the pumped storage unit.

[0020] In one embodiment of the present application, the outputs of the system simulation model include the machine-side three-phase current, the machine-side active power and reactive power, the grid-side three-phase current, the grid-side active power and reactive power, and the switch sequence and capacitor voltage waveform of the M3C internal loop, which are used for analyzing the grid stability and operating characteristics.

[0021] The full-power variable-speed pumped storage unit grid connection equivalent simulation modeling method and system of the embodiments of the present application significantly improve the simulation speed and voltage utilization rate through module decoupling and step-by-step coordination, and support wide frequency oscillation risk assessment.

[0022] Additional aspects and advantages of the present application will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0023] The above and / or additional aspects and advantages of the present application will become apparent and be readily appreciated from the following description, taken in conjunction with the accompanying drawings, in which:

[0024] Figure 1 The flowchart of the full-power variable-speed pumped storage unit grid connection equivalent simulation modeling method of the embodiments of the present application;

[0025] Figure 2 The structural block diagram of the M3C-internal loop equivalent controlled voltage source simulation module in the embodiments of the present application;

[0026] Figure 3 The structural diagram of the full-power variable-speed pumped storage unit grid connection equivalent simulation system based on M3C in the embodiments of the present application;

[0027] Figure 4 The first result graph of the equivalent modeling and simulation test in the embodiments of the present application;

[0028] Figure 5 Figure 2 shows a second result of equivalent modeling and simulation test in the embodiment of the present application. DETAILED DESCRIPTION

[0029] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0030] The purpose of the present application is to propose a M3C-based equivalent simulation modeling method and system for full-power variable-speed pumped storage unit grid connection.

[0031] Figure 1 Figure 1 shows a flow chart of the equivalent simulation modeling method for full-power variable-speed pumped storage unit grid connection in the embodiment of the present application. Figure 1

[0032] S1, construct an equivalent simulation model of M3C-grid side, including: 1) a power frequency Park transformation module, realizing transformation of grid side electrical quantities from a uvw three-phase coordinate system to a power frequency rotating dq coordinate system; 2) a power frequency Clark inverse transformation module, used for realizing conversion of grid side electrical quantities in the power frequency dq coordinate system to a static αβ0 coordinate system; 3) a grid side phase-locked loop module: through directional grid connection point voltage, reference angles are provided for grid side coordinate transformation; 4) a M3C-grid side double-loop controller module: the outer ring is a voltage controller, and the inner ring is a current controller, both of which adopt unit negative feedback PI control, and PI parameters adopt standard value setting. The dq axis input of the outer ring controller is the conversion value of the grid active / reactive power scheduling target through the power frequency Park transformation module, the output of the outer ring controller is the input of the inner ring current controller, and the output of the inner ring current controller is converted to the αβ0 coordinate system through the power frequency Clark inverse transformation module, to provide a reference voltage for the M3C internal loop module.

[0033] S2, construct an equivalent simulation model of M3C-machine side, including: 1) a low-frequency Park transformation module, realizing transformation of pumped storage unit side electrical quantities from an abc three-phase coordinate system to a low-frequency rotating dq coordinate system; 2) a low-frequency Clark inverse transformation module, used for converting machine side electrical quantities in the low-frequency dq coordinate system to a static αβ0 coordinate system; 3) a flux linkage and speed observation module: through observation of the stator flux linkage and rotor speed of the pumped storage unit, reference angles are provided for machine side coordinate transformation; 4) a M3C-machine side double-loop controller: similar to the grid side, the outer ring is a voltage controller, and the inner ring is a current controller. The dq axis input of the outer ring controller is the conversion value of the motor side active / reactive power control target (and the water pump / turbine state) through the low-frequency Park transformation module, and the output is the input of the inner ring current controller. The output of the machine side inner ring current controller is converted to the αβ0 coordinate system through the low-frequency Clark inverse transformation module, to provide a reference voltage for the M3C internal loop module.​

[0034] S3, constructing the equivalent simulation model of M3C-internal loop: after obtaining the 9-layer reference voltage output by the M3C-machine side and network side equivalent simulation modules, the controlled voltage source circuit is used to perform equivalent on the phase-shifted carrier modulation, and then output the switching sequence of each H full-bridge subunit, which improves the simulation speed while ensuring the voltage utilization and realizing multi-level output, as shown in Figure 2 .

[0035] Under normal operation, Figure 2 Each H-bridge subunit has only two IGBTs that can be turned on, and the IGBTs under the same bridge arm (T1 and T2, T3 and T4) are not allowed to be turned on at the same time. (T1, T4) and (T2, T3) are two groups of IGBTs that are alternately turned on, corresponding to output capacitor voltages +U C and -U C , respectively. Therefore, the output of the equivalent controlled voltage source is as follows:

[0036]

[0037] If the oscillation between the capacitor voltages of each subunit in a layer is ignored, the capacitor voltages of all subunits are determined according to the flow of energy in the corresponding layer.

[0038]

[0039] where U C0 and C sm are the initial voltage and capacitance value of the sub-module capacitor, respectively, and are the voltage and current of each layer. The equivalent process of the remaining 8 layers of the M3C internal loop is consistent.

[0040] S4, due to the decoupling and equivalent of each simulation module, when considering the oscillation stability evaluation requirements of different frequency bands, small simulation step length can be used for the "main influence" module, large simulation step length can be used for the "weak influence" module, and "equal period time" dynamic interface method is used between modules. Finally, the overall system simulation model is constructed as shown in Figure 3 .

[0041] Based on Figure 3, according to the actual need to simulate the pumped storage unit grid-connected condition, input the corresponding M3C-network side parameters (network side active / reactive power instruction, controller PI parameter, grid point voltage, etc.), M3C-machine side parameters (machine side active / reactive power instruction, controller PI parameter, speed, flux, etc.), M3C-internal loop parameters (capacitance value of each layer, bridge arm inductance, capacitor voltage, etc.), and obtain the simulation waveform results of machine / network / inner loop. When it is necessary to classify and evaluate the influence of a certain type of control parameter or operating condition, only the input of the corresponding module needs to be modified.

[0042] Further, the equivalent simulation modeling method and system test results.

[0043] In the MATLAB / Simulink simulation platform, the above idea is referred to, and the corresponding grid-connected equivalent simulation model and system are built. Figure 3 The effectiveness of the equivalent modeling method is verified. The simulation input parameters of M3C refer to Table 1 parameter settings, the rated capacity / power and rated voltage of the pumped storage unit are set to 2kVA / 1.5kW and 400V respectively, and the rated voltage and short circuit capacity ratio of the grid side equivalent voltage source are set to 400V and SCR=3.26∠79° respectively.

[0044] Table 1

[0045]

[0046]

[0047] Figure 4 and Figure 5 respectively show the simulation results of the system model constructed in the grid side frequency of 50Hz, and the motor side input frequency of 16.67Hz and 5Hz respectively. The simulation duration is set to 300ms, and the actual simulation time consumed by the MATLAB timing module is 22.3s, which is greatly improved compared with 154.6s of the refined model.

[0048] In addition, as shown in (a)(b) of Figure 4 and (a)(b) of Figure 5 , the three-phase currents I Ma,b,c and I Su,v,w output by the M3C machine side and network side can reach steady state in about 40ms of two power frequency cycles. At the same time, in the two variable frequency grid-connected scenarios, for the simulation performance of the M3C machine side, as shown in (c)(d) of Figure 4 and (c)(d) of Figure 5 : the active power P M and the reactive power Q M output by the machine side can track the input instruction value (set the machine side active reference value and reactive reference values );for the grid-side simulation performance of M3C, such as Figure 4 (e) (f) and Figure 5 (e) (f) in the active power P S and the reactive power Q S also coincide with the instruction value (set the grid-side active reference value and reactive reference values

[0049] The above test results show that the equivalent modeling method proposed in the application can simulate the M3C-based full-power variable-speed pumped storage unit grid-connected scene, and has good accelerated simulation and equivalent performance.

[0050] In summary, the application first considers the differences in operating frequency and control targets of the M3C machine-grid side, respectively constructs the modular equivalent simulation modules of the power frequency grid side and the low frequency machine side, simulates the input / output characteristics of the machine-grid side; secondly, an equivalent controlled voltage source module of the M3C internal loop is constructed to simulate the carrier phase shift modulation characteristics of the internal loop; finally, by integrating the equivalent modules connecting the machine side-grid side-internal loop, the simulation step of each module is determined, and the simulation system is constructed. After inputting the instruction value corresponding to the simulation working condition and the parameters of each module into the system, the simulation result is obtained.

[0051] The proposed grid-connected equivalent modeling simulation method can overcome the problem of long time-consuming of fine M3C model electromagnetic transient simulation, and the mismatch between simulation efficiency and actual grid-connected stability analysis speed demand through the modular equivalent modeling of M3C machine side-grid side-internal loop and the decoupling simulation idea of the dynamic interface of each sub-module, and provides accelerated simulation model support for subsequent variable-speed pumped storage unit grid-connected M3C wide-frequency oscillation risk assessment.

[0052] Further, the application proposes a full-power variable-speed pumped storage unit grid-connected equivalent simulation system, comprising:

[0053] a power frequency grid side equivalent simulation module, comprising a Park transformation module, a Clark inverse transformation module, a phase-locked loop module and a double closed-loop controller module, for outputting the active / reactive power dispatching target of the grid to the reference voltage of the M3C internal loop module after Park transformation and Clark inverse transformation;

[0054] a low-frequency machine side equivalent simulation module, comprising a low-frequency Park transformation module, a low-frequency Clark inverse transformation module, a flux linkage and speed observation module and a double closed-loop controller module, for outputting the active / reactive control target of the machine side to the reference voltage of the M3C internal loop module after low-frequency Park transformation and Clark inverse transformation;

[0055] The M3C internal loop equivalent simulation module adopts a controlled voltage source circuit to simulate the phase-shifted carrier modulation characteristics, generates the switching sequence of the H full-bridge subunit according to the reference voltage output by the power frequency network side module and the low-frequency machine side module, and realizes multi-level output.

[0056] The dynamic interface module is used for coordinating the simulation step lengths of the power frequency network side equivalent simulation module, the low-frequency machine side equivalent simulation module and the M3C internal loop equivalent simulation module, and performing data synchronization between the modules through the equal period time method, so as to construct the overall grid-connected equivalent simulation system model.

[0057] Further, when simulating the phase-shifted carrier modulation, the M3C internal loop equivalent simulation module adopts a controlled voltage source circuit to realize multi-level output, and ignores the oscillation between the capacitor voltages of each layer of subunits, so as to simplify the model and improve the simulation speed.

[0058] Further, the double closed-loop controllers of the power frequency network side module and the low-frequency machine side module both adopt a PI control strategy based on unit negative feedback, wherein the PI parameters of the controllers are set in a standard unit, so as to adapt to the control requirements under different operating conditions.

[0059] Further, the inputs of the system simulation model include the active / reactive power instructions of the network side, the active / reactive power instructions of the machine side, the grid point voltage, the M3C parameters, and the rated capacity and power of the pumped storage unit.

[0060] Further, the outputs of the system simulation model include the three-phase currents of the machine side, the active power and the reactive power of the machine side, the three-phase currents of the network side, the active power and the reactive power of the network side, and the switching sequence and the capacitor voltage waveform of the M3C internal loop, which are used for analyzing the grid stability and operating characteristics.

[0061] The grid-connected equivalent modeling simulation system proposed in the application can overcome the problem that the fine M3C model electromagnetic transient simulation is time-consuming and the simulation efficiency and the actual grid stability analysis speed demand are not matched, by means of the sub-module equivalent modeling of the M3C machine side-network side-internal loop, the decoupling simulation idea of the dynamic interface of each sub-module, and can provide an accelerated simulation model support for the subsequent wide-frequency oscillation risk assessment of the variable-speed pumped storage unit connected through the M3C.

[0062] In order for those skilled in the art to better understand the technical scheme of the present application, the technical scheme in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should belong to the scope of protection of the present application.

[0063] In the description of the specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the description of the specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, the person skilled in the art can combine and combine the different embodiments or examples described in the specification and the features of the different embodiments or examples, without contradiction.

[0064] In addition, the terms "first", "second" are only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise explicitly specified.

Claims

1. A method for grid-connected equivalent simulation modeling of a full-power variable-speed pumped-storage unit, characterized in that, include: An equivalent simulation module for the power frequency grid side is constructed. The Park transformation module transforms the grid-side electrical quantities from the uvw three-phase coordinate system to the power frequency rotating dq coordinate system. The Clark inverse transformation module transforms the electrical quantities in the dq coordinate system to the αβ0 coordinate system. The phase-locked loop module provides a reference angle through the directional grid connection point voltage. The dual closed-loop controller module includes a voltage controller and a current controller. The voltage controller controls the current controller based on the conversion value of the grid-side active / reactive power dispatch target. The output of the current controller is converted into a reference voltage in the αβ0 coordinate system by the Clark inverse transformation module. A low-frequency machine-side equivalent simulation module is constructed, in which the low-frequency Park transformation module transforms the machine-side electrical quantities from the abc three-phase coordinate system to the low-frequency rotating dq coordinate system, the flux linkage and speed observation module provides the machine-side reference angle, and the dual closed-loop controller module controls the output according to the machine-side active / reactive power control target. Its output is converted into the reference voltage in the αβ0 coordinate system by the low-frequency Clark inverse transformation module. An equivalent simulation module for the internal loop of M3C is constructed. A controlled voltage source circuit is used to perform equivalent phase-shift carrier modulation. The switching sequence of the H full-bridge sub-unit is generated based on the reference voltage output by the power frequency network-side equivalent simulation module and the low-frequency machine-side equivalent simulation module to achieve multi-level output. A dynamic interface module is constructed to dynamically integrate the power frequency network-side equivalent simulation module, the low-frequency machine-side equivalent simulation module, and the M3C internal loop equivalent simulation module to form a system simulation model. The system simulation model inputs corresponding control commands and parameters according to the actual operating conditions and outputs simulation waveform results of the machine-side, network-side, and internal loop. By classifying and adjusting the simulation step size of each module, simulation and stability analysis are accelerated.

2. The method according to claim 1, characterized in that, In the equivalent simulation module of the power frequency grid side, the Park transformation module adopts synchronous coordinate transformation based on the power frequency to convert the electrical quantities of the three-phase grid side into DC quantities in the dq coordinate system. The Clark inverse converter module employs decoupling of three-phase voltage and current in the αβ0 coordinate system to convert the control signal in the dq coordinate system into a reference voltage in the αβ0 coordinate system to match the input format of the M3C internal loop. The phase-locked loop module adopts a synchronization strategy based on the voltage amplitude and phase at the grid connection point, and achieves angle locking through digital signal processing algorithms; In the dual closed-loop controller module, the output of the voltage controller serves as the command input to the current controller, and the output of the current controller serves as the input signal to the Clark inverse converter module, thereby realizing closed-loop control and feedback regulation of voltage and current.

3. The method according to claim 1, characterized in that, The low-frequency Park conversion module converts the machine-side electrical quantities from the abc three-phase coordinate system to the low-frequency rotating dq coordinate system to match the low-frequency operating characteristics of the machine-side controller. The flux linkage and speed observation module uses an algorithm based on motor stator flux linkage estimation and rotor speed detection to provide a real-time reference angle for the low-frequency Park transformation module.

4. The method according to claim 1, characterized in that, In the M3C internal loop equivalent simulation module, the switching sequence of each H full-bridge sub-unit is determined according to the amplitude and phase of the reference voltage, wherein IGBTs under the same bridge arm are not allowed to be turned on simultaneously. Under normal operation, the switching sequence of the H-bridge sub-unit only allows two sets of IGBTs to conduct alternately, corresponding to the output capacitor voltages +UC and -UC respectively. In other cases, the output voltage is 0.

5. The method according to claim 1, characterized in that, The dynamic interface module adopts an equal-period time synchronization method to coordinate the simulation step size of the network side, machine side and internal loop modules; In the system simulation model, the simulation step size of each module is set according to the degree of its dominant influence on oscillation stability. The dominant influence module adopts a simulation step size less than the preset threshold, while the weak influence module adopts a simulation step size greater than the preset threshold.

6. A grid-connected equivalent simulation system for a full-power variable-speed pumped-storage unit, characterized in that, include: The power frequency grid-side equivalent simulation module includes a Park transformation module, a Clark inverse transformation module, a phase-locked loop module, and a dual closed-loop controller module. It is used to output the power grid active / reactive power dispatch target to the reference voltage of the M3C internal loop module after Park transformation and Clark inverse transformation. The low-frequency machine-side equivalent simulation module includes a low-frequency Park transformation module, a low-frequency Clark inverse transformation module, a flux linkage and speed observation module, and a dual closed-loop controller module. It is used to output the machine-side active / reactive control target to the reference voltage of the M3C internal loop module after low-frequency Park transformation and Clark inverse transformation. The M3C internal loop equivalent simulation module uses a controlled voltage source circuit to simulate the phase-shift carrier modulation characteristics. It generates the switching sequence of the H full-bridge sub-unit based on the reference voltage output from the power frequency network side module and the low frequency machine side module to achieve multi-level output. The dynamic interface module is used to coordinate the simulation step size of the power frequency grid-side equivalent simulation module, the low-frequency machine-side equivalent simulation module, and the M3C internal loop equivalent simulation module, and to synchronize data between modules through the equal periodic time synchronization method in order to build an overall grid-connected equivalent simulation system model.

7. The system according to claim 6, characterized in that, When simulating phase-shifted carrier modulation, the M3C internal loop equivalent simulation module uses a controlled voltage source circuit to achieve multi-level output and ignores the oscillation between capacitor voltages of each sub-unit in order to simplify the model and improve simulation speed.

8. The system according to claim 6, characterized in that, Both the power frequency grid-side module and the low-frequency machine-side module's dual closed-loop controllers adopt a PI control strategy based on unity negative feedback. The PI parameters of the controller are set using per-unit values ​​to adapt to the control requirements under different operating conditions.

9. The system according to claim 6, characterized in that, The inputs to the system simulation model include grid-side active / reactive power commands, generator-side active / reactive power commands, grid connection point voltage, M3C parameters, and the rated capacity and power of the pumped-storage unit.

10. The system according to claim 6, characterized in that, The output of the system simulation model includes the three-phase current on the generator side, the active and reactive power on the generator side, the three-phase current on the grid side, the active and reactive power on the grid side, as well as the switching sequence and capacitor voltage waveform of the M3C internal loop, which are used to analyze grid connection stability and operating characteristics.

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