WO2018176599A1

WO2018176599A1 - Mmc-based electromechanical transient simulation method and system for voltage source converter-based high voltage direct current and direct current power grid

Info

Publication number: WO2018176599A1
Application number: PCT/CN2017/084529
Authority: WO
Inventors: 刘栋; 寇龙泽; 庞辉; 贺之渊; 朱琳; 吴学光; 林畅
Original assignee: 全球能源互联网研究院有限公司; 国家电网公司
Priority date: 2017-03-31
Filing date: 2017-05-16
Publication date: 2018-10-04
Also published as: CN107123981B; CN107123981A

Abstract

An MMC-based electromechanical transient simulation and modeling method and system for a voltage source converter-based high voltage direct current (VSC-HVDC) and direct current power grid. The method comprises: establishing a transient simulation model comprising a steady-state trend model based on a controlled current source, an alternating current transient model, and a direct current transient model, wherein the steady-state trend model based on the controlled current source is employed to generate initial trend data required for a transient simulation (S1); and simulating, by means of the transient simulation model, an electromechanical transient operation characteristic of a VSC-HVDC and direct current power grid in different operation conditions (S2). By not taking initial value computation of a direct current power grid into consideration, the method and system simplify the steady-state trend model without influencing accuracy and stability of transient simulation, and improve expandability of the model.

Description

MMC-based flexible DC and DC grid electromechanical transient simulation method and system

Technical field

The invention relates to a digital simulation technology of flexible direct current transmission, in particular to an MMC-based flexible DC and DC grid electromechanical transient simulation method and system.

Background technique

Voltage Source Converter Based High Voltage Direct Current (VSC-HVDC) has the advantages of no commutation failure, active reactive power, fast independent control, etc., in large-scale distributed renewable energy access. Asynchronous grid interconnection and other aspects are considered to be the most effective technical solutions. VSC-HVDC has different topologies. Modular Multilevel Converter (MMC) has become a research hotspot with its low switching frequency, low harmonic content and easy expansion. Application.

MMC contains a large number of power electronic switching devices, which are suitable for simulating the internal dynamic process and corresponding control strategies using electromagnetic transient simulation tools, such as bridge arm circulation control and sub-module capacitor voltage balance control. At present, some related MMC efficient electromagnetic transient modeling methods have been proposed and widely used. When the flexible DC or DC grid is connected to the AC grid, it will bring about changes in the transient stability of the system. At the beginning of engineering planning and design, it is necessary to conduct an in-depth study on the interaction between the AC and DC systems and the possible stability problems. However, due to the limitations of current computer technology and simulation hardware resources, electromagnetic transient simulation must perform equivalent simplification of the AC system when performing large-scale AC-DC grid simulation, which will lead to the difference between the dynamic characteristics of the AC system model and the actual grid. It also affects the accuracy of the final simulation results.

Summary of the invention

In order to solve the above deficiencies in the prior art, the object of the present invention is to provide an MMC-based flexible DC and DC grid electromechanical transient simulation method and system, and the invention establishes an accurate and practical flexible DC and DC grid electromechanical transient simulation model. Through the joint simulation with the actual grid operation data, a more accurate transient stability result of the AC-DC grid can be obtained.

The object of the present invention is achieved by the following technical solutions:

The invention provides an electromechanical transient simulation method for a flexible DC and DC grid based on an MMC converter, which is improved in that it comprises:

Establish a transient simulation model, including a steady-state tidal current model based on a controlled current source, an AC-side transient model, and a DC-side transient model. The steady-state tidal current model based on the controlled current source is used to generate transient simulations. beginning of Trend data

The transient simulation model is used to simulate the electromechanical transient operation characteristics of flexible DC and DC grid under different working conditions.

Further, the injected bus current is used as a steady state power flow model, and the MMC converter is equivalent to a PQ node or a PV node;

When the MMC converter is equivalent to a PQ node, the current phasor injected into the bus is:

Where V _R +jV _I is the injected bus voltage phasor; P _ref and Q _ref are the active power and reactive power of the positive injection into the bus; I _R and I _I respectively represent the current phasors injected into the bus. Part and imaginary part; V _R and V _I respectively represent the real part and the imaginary part of the voltage phasor injected into the bus bar;

When the MMC converter is equivalent to a PV node, the direction of the injected bus is positive. The value of the reactive power reference value Q _ref is determined by integrating the difference between the bus voltage reference value and the feedback value, and the remaining parameters are changed with MMC. The streamer equivalent should be the same for the PQ node.

Further, the AC side transient model is:

For MMC inverters, the expressions for R _c and L _c are:

Where i _d and i _q represent the components of the three-phase current on the d and q axes, respectively, v _c represents the voltage at the outlet of the converter valve, v _p represents the bus voltage of the common junction PCC, and i is the current on the AC bus, R _c and L _c are the equivalent resistance and equivalent reactance of the AC side; L _t , R _t are the equivalent reactance and equivalent resistance of the converter transformer, respectively, and the equivalent resistance and equivalent reactance of the converter transformer Close to 0, L _arm and R _arm are the bridge arm reactance and the bridge arm equivalent resistance respectively; v _pd and v _pq are the voltage of the common junction bus bar PCC on the d axis and the voltage on the q axis, respectively.

Further, the DC-side transient model includes establishing a controlled DC current source I _dc and an equivalent capacitance C _eq ;

The expression for the equivalent capacitance C _eq is as follows:

The expression of the controlled DC current source I _dc is as follows:

Where: N is the number of sub-modules of the MMC single bridge arm; C _sm is the capacitance in the sub-module; P _dc is the power of the controlled current source; U _dc is the voltage of the controlled current source; P _ac and P _loss are respectively AC The active power of the side input and the power loss of the MMC inverter; v _cd and v _cq are the outlet voltages of the d-axis and the q-axis of the MMC converter, respectively; i _d and i _q are the components of the alternating current on the d and q axes, respectively. .

Further, the method further includes establishing a control system model, and adopting a dq-axis decoupling control mode of a double closed-loop control structure, including an outer loop controller and an inner loop controller; the outer loop controller is based on an initial power of active power and reactive power Data and additional reference increments for generating reference values i _dref and i _qref of the alternating current dq axis component of the inner loop controller;

The inner loop current controller takes the output of the outer loop controller as a reference value, and uses the current measured value after the dq transform as a feedback amount, and uses the decoupling negative feedback PI control structure to implement tracking control of the instantaneous value of the PCC current.

Further, the inner loop current controller is represented by an outlet voltage of the d-q axis of the MMC converter, respectively:

Where i _d and i _q are the components of the alternating current on the d and q axes, respectively, v _p is the amplitude of the PCC bus voltage of the common junction point; P _ref and Q _ref are the reference values of the active power and the reactive power, respectively; i _dref and i _qref are reference values of the d-axis component and reference values of the q-axis component, respectively;

The reference values of the outlet voltages v _cd and v _{cq of the} d-axis and the q-axis of the MMC converter respectively; v _pd , v _pq the voltage of the common junction bus bar PCC on the d-axis, the voltage on the q-axis; U _dc is the current DC Voltage, U _dc0 is the initial DC voltage after each MMC converter is stabilized, T _c is the delay generated by the modulation process, and is ignored after phase compensation; K _pd and K _pq are the proportional coefficients of d and q axes, respectively, K _id K _iq is the integral coefficient of the d and q axes, respectively; s represents a complex number; and ω represents a fundamental frequency angular velocity.

Further, the alternating current phasor in the control system model is converted into a synchronous rotating dq coordinate system based on the PCC bus voltage, and is used by the inner loop controller to control the alternating currents i _d , i _q , including:

The expression of the alternating current dq axis is:

Obtained after injection bus controller and the inner side of the AC alternating current model obtained in d, q-axis component i _d and i _q, the alternating current component of the d, q-axis I _d, i _q of phasor The inverse transformation to a synchronous rotating coordinate system based on the system slack node, the transformed expression is:

Where: I _PI and I _PR are the real and imaginary parts of the components i _d and i _q of the alternating current on the d and q axes, respectively; V _PI and V _PR are the real parts of the phasors of the PCC bus voltage, respectively. The imaginary part.

Further, the method further includes at least one additional module for performing function expansion based on the outer loop control structure, the add-on module providing latching signal generation logic on the AC side and the DC side of the flexible DC and DC grid, through the flexible DC and DC grid The outer ring control structure of the AC side and the DC side is connected.

Further, the transient simulation model is used to simulate the electromechanical transient operation characteristics of the flexible DC and DC grid under different working conditions, including:

The steady state power flow model is used to generate initial power flow data required for transient simulation;

The AC side transient model is used to simulate the dynamic characteristics of the flexible DC and DC grid AC side;

The control system model is used to simulate the control characteristics of the AC side of the flexible DC and DC grid, including: constant AC/DC voltage control, active/reactive power emergency boost/return control function;

The DC side transient model is used to simulate a transient steady state process of voltage and current on a DC side of a flexible DC and DC power grid;

The add-on module is used to simulate the MMC inverter blocking feature and additional control functions.

The invention also provides an electromechanical transient simulation system for a flexible DC and DC grid, the improvement comprising the following:

Modeling module: Establishing a transient simulation model, including a steady-state tidal current model based on a controlled current source, an AC-side transient model, and a DC-side transient model, wherein the steady-state tidal current model based on the controlled current source is used to generate a temporary Initial trend data required for state simulation;

Simulation module: It is used to simulate the electromechanical transient operation characteristics of flexible DC and DC grid under different working conditions by using transient simulation model.

Further, the modeling module further includes:

Power flow model modeling module: used to establish initial power flow data required for generating transient simulation, steady state power flow model based on controlled current source;

AC model modeling module: used to establish an AC-side transient model for simulating the dynamic characteristics of the flexible DC and DC grid AC side;

Control system modeling module: used to establish the control characteristics of the analog flexible DC and DC grid AC side, including: control system model of constant AC/DC voltage control, active/reactive power emergency boost/return control function;

DC model modeling module: a DC-side transient model for establishing a transient steady-state process for simulating the DC side voltage and current of a flexible DC and DC grid;

Additional module modeling module: used to establish the blocking characteristics and additional control functions of the analog MMC inverter.

Compared with the closest prior art, the technical solution provided by the present invention has the excellent effects of:

The invention provides an MMC-based flexible DC and DC grid electromechanical transient simulation modeling method, which can accurately simulate the transient characteristics and control characteristics of the flexible DC AC side and the DC side, and can realize fault commutation Features such as latching and additional controls. The user-defined function module based on commercial electromechanical transient simulation software establishes the electro-mechanical transient simulation model of flexible DC, and compares it with the detailed electromagnetic transient simulation model in PSCAD/EMTDC to verify the simulation accuracy of the electromechanical model. The proposed method provides a guiding reference for the electromechanical transient simulation of large-scale AC/DC systems, and plays a technical support role for the preliminary planning and system design of practical flexible DC and DC grid projects. specific:

1. The invention proposes a steady-state tidal current model based on a controlled current source. Without considering the initial value calculation of the DC network, the calculation process of the steady-state tidal current model is simplified without affecting the accuracy and stability of the transient simulation. Improve the scalability of the model.

2. The invention proposes a model optimization method that considers the dynamic characteristics of the MMC modulation process and improves the electromechanical mode. Type simulation accuracy for transient processes.

Based on the modeling method proposed by the invention, the corresponding double-ended flexible DC model and the four-terminal DC grid model are established in the commercial electromechanical transient simulation software, and simulated by the detailed electromagnetic transient model in PSCAD/EMTDC. Contrast, verifying the accuracy and effectiveness of the model.

DRAWINGS

1 is a flow chart of an MMC-based flexible DC and DC grid electromechanical transient simulation modeling method provided by the present invention;

2 is a schematic diagram of a steady-state tidal current model with a two-end system as an example of the present invention; (a) is a schematic diagram of a steady-state tidal current model of the busbar 1; (b) is a schematic diagram of a steady-state tidal current model of the busbar 2;

3 is a schematic diagram of an equivalent circuit of an AC side of an MMC converter provided by the present invention;

4 is a schematic diagram of a user-defined model and a main program interface circuit provided by the present invention;

Figure 5 is a structural diagram of an outer ring controller provided by the present invention;

Figure 6 is a structural diagram of an inner ring controller provided by the present invention;

7 is a schematic diagram of a DC network equivalent model of a two-end system provided by the present invention;

FIG. 8 is a logic diagram of the latching signal generation provided by the present invention.

detailed description

The specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

The detailed description of the embodiments of the invention are set forth in the description Other embodiments may include structural, logical, electrical, process, and other changes. The examples represent only possible variations. Individual components and functions are optional unless explicitly required, and the order of operations may vary. Portions and features of some embodiments may be included or substituted for portions and features of other embodiments. The scope of the embodiments of the invention includes the full scope of the claims, and all equivalents of the claims. These embodiments of the invention may be referred to herein, individually or collectively, by the term "invention", merely for convenience, and if more than one invention is disclosed, it is not intended to automatically limit the application. The scope is any single invention or inventive concept.

Embodiment 1

The flow chart of the electromechanical transient simulation modeling method for the flexible DC and DC grid proposed by the present invention is shown in FIG. 1 and includes:

S1: Establish 1) steady-state tidal current model based on controlled current source; 2) AC-side transient model; 3) control system model; 4) DC-side transient model and 5) other additional functional modules;

S2: Using the above model to simulate the electromechanical transient operating characteristics of flexible DC and DC grids under different operating conditions.

Before establishing a steady-state tidal current model based on the controlled current source, the method further includes: determining a steady-state tidal current parameter, that is, determining an injection bus voltage, current, active power, and reactive power of the MMC converter.

among them:

S101: Establish a steady-state power flow model based on a controlled current source, and the schematic diagram is as shown in FIG. 2(a) and (b), including:

For an AC system, each converter station can be equivalent to one PQ node or PV node. In the custom modeling, the converter station uses the controlled current source for equivalent, and controls the power interaction with the AC system by controlling the current injected into the bus. Assuming that the converter station on one side is a PQ node, the given values of active power and reactive power are P _ref and Q _ref (positive to the direction of the injected bus), and the bus voltage phasor is V _R +jV _I , then the injection The phasor of the bus current is:

Where: I _R and I _I represent the real and imaginary parts of the current phasor injected into the bus, respectively; V _R and V _I represent the real and imaginary parts of the voltage phasor injected into the bus, respectively.

If the converter station is a PV node, the reactive power setpoint Q _ref can be determined by integrating the difference between the bus voltage setpoint and the feedback value. If the losses of the converter and the DC line are ignored, the active power of each converter station satisfies the law of conservation of power. Ignoring the power loss may bring some errors in the power flow calculation, but it does not affect the subsequent transient stability simulation (transient stability simulation may take a period of oscillation to re-adjust the power flow calculation results), and simplify the steady state model. .

S102: Establish an AC side transient model, and the schematic diagram is as shown in FIG. 3, including:

According to the equivalent circuit of the AC side of the converter station, the mathematical model under the three-phase stationary coordinate system is established, and then the mathematical model under the three-phase stationary coordinate system is converted to the dq synchronous rotating coordinate system by Park-Clark transformation, and the three-phase time-varying The variable is converted to a constant amount of DC for easy controller design. The mathematical model of the MMC AC side in the synchronous rotating coordinate system is:

Where i _d and i _q represent the components of the three-phase current on the d and q axes, respectively, v _c represents the voltage at the outlet of the converter valve, and v _p represents the bus voltage of the Point of Common Connection (PCC). i is the current on the AC bus, R _c and L _c are the equivalent resistance and equivalent reactance on the AC side; L _t , R _t are the equivalent reactance and equivalent resistance of the converter transformer, respectively, and the commutation The equivalent resistance and equivalent reactance of the transformer are close to 0, L _arm and R _arm are the bridge arm reactance and the bridge arm equivalent resistance respectively; v _pd and v _pq are the voltages of the common junction bus bar PCC on the d axis, respectively. The voltage of the shaft. In Figure 3, Rs and Ls are the equivalent resistance and equivalent reactance of the AC side power supply, respectively.

For MMC converters, the expressions for Rc and Lc are:

Where L _t and R _t are the equivalent reactance and resistance of the converter transformer respectively, L _arm and R _arm are the bridge arm reactance and the bridge arm equivalent resistance respectively; v _pd and v _pq are the common joint point bus bar PCC at d respectively The voltage of the shaft and the voltage at the q-axis.

The interface between the AC side model and the PSASP main program is implemented by setting a transformer branch. The schematic diagram of the user-defined model and the main program interface circuit is shown in Figure 4. The function of the transformer branch is as follows: 1) provide the feedback data of the main program for the custom model; 2) simulate the wiring mode of the actual engineering converter transformer to ensure Simulation accuracy under failure. Since the actual transformer parameters are already included in equation (3), in order to avoid affecting the accuracy of the control system, the equivalent resistance and equivalent reactance of the transformer should be set close to zero.

S103: Establish a control system model, including:

The control system of the model adopts the dq axis decoupling control method of the double closed loop control structure. The function of the outer loop controller is to generate reference values i _dref and i _{qref for the} dq axis component of the alternating current in the inner loop controller. The design of the outer loop controller has different methods. The method is based on the reference values of active power and reactive power, and directly calculates the reference value of the dq axis component of the alternating current by using equation (4), where V _p is the amplitude of the PCC bus voltage. value. This control method has a relatively fast dynamic response. When the converter station adopts constant DC voltage control or constant AC voltage control, the PI feedback control can be used to generate a reference value of the corresponding active power or reactive power. The initial reference values P0, Q0, Uac0 of the outer loop controller are derived from the calculation of steady state power flow, and the initial set value Udc0 of the DC voltage is usually set to 1 pu. In addition, the outer loop controller also adds a current limiting link based on the power original image to prevent overcurrent from damaging the converter, and at the same time limits the active power and reactive power interacting with the AC system, making the electromechanical transient simulation more Close to the actual operating conditions of the project. The structure of the outer ring controller is shown in Figure 5.

The inner loop current controller takes the output of the outer loop controller as a reference value, and uses the current measured value after the dq transform as the feedback amount, and uses the decoupling negative feedback PI control structure to realize the tracking control of the instantaneous value of the PCC current. The structure of the inner loop controller is shown in Figure 6. Only the positive sequence current control is considered. The calculation formula of the converter outlet voltage v _cd , v _cq is as follows:

Usually in electromechanical simulation, assuming that the converter's outlet voltage follows the given value well, the MMC and the valve-based control system are equivalent to a first-order inertia. However, in actual engineering, when the DC voltage fluctuates due to faults, the converter outlet voltage does not follow the given value well due to control methods and modulation strategies, etc., so the active power and reactive power of the system will Generate corresponding fluctuations. Therefore, considering the effect of DC voltage fluctuations, the converter outlet voltage can be expressed as:

The reference values of the outlet voltages v _cd and v _{cq of the} d-axis and the q-axis of the MMC converter respectively; v _pd , v _pq the voltage of the common junction bus bar PCC on the d-axis, the voltage on the q-axis; U _dc is the current DC Voltage, U _dc0 is the initial DC voltage after each MMC converter is stabilized, T _c is the delay generated by the modulation process, and is ignored after phase compensation; K _pd and K _pq are the proportional coefficients of d and q axes, respectively, K _id K _iq is the integral coefficient of the d and q axes, respectively; s represents a complex number, and ω represents a fundamental frequency angular velocity. When the DC voltage fluctuates due to the fault, the converter outlet voltage can transmit this ripple to the AC side to simulate the fluctuation of the AC line power.

In electromechanical transient simulation, the dynamic process of PLL control can be neglected, and the phase angle of the PCC point bus voltage can be directly obtained from the main program of the system. Since the inner loop controller needs to control the alternating current, it is necessary to convert the phasor of the alternating current into a coordinate system based on the PCC bus voltage. Suppose RI is a synchronous rotating coordinate system based on the system slack node, and dq is a synchronous rotating coordinate system based on the PCC bus voltage. The phasor of the PCC bus voltage obtained from the main program is V _PR +jV _PI , alternating current The phasor is I _PR +jI _PI , then the expression of the alternating current dq axis is:

After passing through the inner loop controller and the AC side model, the current components i _d and i _q injected into the bus are obtained, which need to be inversely transformed into a synchronous rotating coordinate system based on the system slack node. The transformed expression is:

S104: Establishing a DC-side transient state model, taking the two-end system as an example. The schematic diagram of the DC network equivalent model is shown in FIG. 7 and includes:

The MMC DC-side transient model consists of a controlled DC current source Idc and an equivalent capacitor Ceq. Since the dynamic characteristics of the DC side of the MMC are determined by the capacitance C _sm distributed in the submodule, the value of the DC side equivalent capacitance Ceq needs to be obtained from C _sm . According to the law of conservation of energy:

Where N is the number of submodules of the MMC single bridge arm. According to the law of conservation of power, the active power of the MMC AC side should be equal to the active power of the DC side plus the loss of the converter, so the equivalent DC current source can be expressed as:

For the modeling of DC lines, since the high-frequency response characteristics of the system are usually not considered in the electromechanical transient simulation, in order to simplify the analysis process of the DC network, a π-type RLC circuit can be used to simulate the DC line. According to the actual engineering design experience, the capacitance of the DC cable to the ground is usually smaller than the equivalent capacitance on the DC side. The influence on the external characteristics and control characteristics of the inverter is limited, so it can be neglected during the modeling process. Directly incorporated into the equivalent capacitor, the DC network can be further reduced to an RL circuit. For the two-terminal DC transmission system or the multi-terminal DC grid, according to different topologies, the corresponding DC-side circuit equations can be written and the corresponding model can be established according to the equation.

S105 establishes additional functional modules, including:

The modeling method proposed by the invention realizes the function of MMC blocking. In the electromechanical transient simulation, the opening and closing of the MMC sub-module cannot be realized, and there is no electrical direct connection between the AC side and the DC side. Therefore, the locking process of the MMC can only be simulated by modifying the corresponding mathematical model. . When the AC current rises rapidly after the system fails on the AC or DC side, the inverter current should exceed the threshold. The inverter should be quickly blocked. Similarly, when the DC voltage rises or falls rapidly and exceeds the threshold, the inverter should also be quickly blocked. Therefore, the logic of the blocking signal generation can be added to the model according to the actual control protection system. The blocking signal generation logic is as shown in FIG. 8. After receiving the blocking signal, the current injected into the AC bus is set to zero, that is, i _d =0, i _q =0. After the blocking occurs for 60ms, the impedance of the transformer branch is set to infinity to simulate the action of the AC side circuit breaker tripping. The above implementation method is based on the characteristics of electromechanical transient simulation, and the process of the converter blocking is equivalently simplified, which is beneficial to analyze the influence of the converter blocking on the AC system.

In step S2, in the electromechanical transient simulation, the steady state power flow model is the basis of the transient simulation to generate the initial power flow data required for the transient simulation. The main function of the AC side transient model and the control system model is to simulate the dynamic characteristics and control performance of the flexible DC AC side. The control system adopts the double closed-loop vector control method based on dq rotating coordinate system which is common in engineering, which can realize the control functions of constant AC/DC voltage control and active/reactive power emergency boost/return. The DC side model is used to simulate the transient steady-state process of the DC side voltage and current of the flexible DC and DC grid. The dynamic characteristics are determined by the DC side equivalent capacitance. The AC-DC side models are connected by power balance, that is, the AC side active power is equal to the sum of the DC side active power and the converter loss. Additional functions include DC side fault simulation, converter blocking simulation and other additional control strategies to ensure that the model can more accurately simulate the operating characteristics of the actual system under different operating conditions. In addition, the modeling method proposed by the present invention has a modulus The block-like features can be easily extended to any end-point multi-terminal DC system and DC grid.

In addition, the modeling method has good scalability, and different additional control strategies can be added by modifying the outer loop control structure. For example, a corresponding additional frequency control measure may be added for the frequency stabilization and low frequency oscillation suppression of the AC system, or a corresponding multi-commutation station coordinated control strategy may be added for the safe and stable operation of the DC power grid.

Embodiment 2

Based on the same inventive concept, the present invention also provides an electromechanical transient simulation system for a flexible DC and DC grid based on an MMC converter, comprising:

Modeling module: used to establish a steady-state tidal current model based on a controlled current source, an AC-side transient model, a control system model, a DC-side transient model, and an additional functional model;

Simulation module: used to simulate the electromechanical transient operation characteristics of flexible DC and DC grid under different working conditions by using the above model.

The modeling module further includes:

Additional Model Modeling Module: An additional functional model for establishing analog DC side faults and MMC converter blocking characteristics.

The invention develops a double-ended flexible DC model and a four-terminal DC grid model based on the user-defined model function of the commercial electromechanical transient simulation software PSASP, and has been used for system simulation analysis of multiple practical projects, engineering planning and Construction has guiding significance. This modeling method can also be applied to other commercial software such as PSS/E.

The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to be limiting, and the present invention will be described in detail with reference to the embodiments described herein. It is to be understood that the appended claims are intended to be within the scope of the appended claims.

Claims

An electromechanical transient simulation method for flexible DC and DC grid based on MMC converter, characterized in that it comprises:

Establish a transient simulation model, including a steady-state tidal current model based on a controlled current source, an AC-side transient model, and a DC-side transient model. The steady-state tidal current model based on the controlled current source is used to generate transient simulations. Initial trend data;

The transient simulation model is used to simulate the electromechanical transient operation characteristics of flexible DC and DC grid under different working conditions.
The electromechanical transient modeling method for a flexible DC and DC grid according to claim 1, wherein the injected bus current is used as a steady-state tidal current model, and the MMC converter is equivalent to a PQ node or a PV node;

When the MMC converter is equivalent to a PQ node, the current phasor injected into the bus is:

Where V R +jV I is the injected bus voltage phasor; P ref and Q ref are the active power and reactive power of the positive injection into the bus; I R and I I respectively represent the current phasors injected into the bus. Part and imaginary part; V R and V I respectively represent the real part and the imaginary part of the voltage phasor injected into the bus bar;

When the MMC converter is equivalent to a PV node, the direction of the injected bus is positive. The value of the reactive power reference value Q ref is determined by integrating the difference between the bus voltage reference value and the feedback value, and the remaining parameters are changed with MMC. The streamer equivalent should be the same for the PQ node.
The electromechanical transient modeling method for a flexible DC and DC grid according to claim 1, wherein the AC side transient model is:

For MMC inverters, the expressions for R c and L c are:

Where i d and i q represent the components of the three-phase current on the d and q axes, respectively, v c represents the voltage at the outlet of the converter valve, v p represents the bus voltage of the common junction PCC, and i is the current on the AC bus, R c and L c are the equivalent resistance and equivalent reactance of the AC side; L t , R t are the equivalent reactance and equivalent resistance of the converter transformer, respectively, and the equivalent resistance and equivalent reactance of the converter transformer Close to 0, L arm and R arm are the bridge arm reactance and the bridge arm equivalent resistance respectively; v pd and v pq are the voltage of the common junction bus bar PCC on the d-axis and the voltage on the q-axis, respectively.
The electromechanical transient modeling method for a flexible DC and DC grid according to claim 1, wherein the DC side transient model comprises establishing a controlled DC current source I dc and an equivalent capacitance C eq ;

The expression for the equivalent capacitance C eq is as follows:

The expression of the controlled DC current source I dc is as follows:

Where: N is the number of sub-modules of the MMC single bridge arm; C sm is the capacitance in the sub-module; P dc is the power of the controlled current source; U dc is the voltage of the controlled current source; P ac and P loss are respectively AC The active power of the side input and the power loss of the MMC inverter; v cd and v cq are the outlet voltages of the d-axis and the q-axis of the MMC converter, respectively; i d and i q are the components of the alternating current on the d and q axes, respectively. .
The electromechanical transient modeling method for a flexible DC and DC grid according to claim 1, further comprising: establishing a control system model, using a dq-axis decoupling control mode of the double closed-loop control structure, including an outer loop controller and a ring controller; the outer loop controller is configured to generate reference values i dref and i qref of an alternating current dq axis component in the inner loop controller based on initial power flow data of active power and reactive power and additional reference value increments;

The inner loop current controller takes the output of the outer loop controller as a reference value, and uses the current measured value after the dq transform as a feedback amount, and uses the decoupling negative feedback PI control structure to implement tracking control of the instantaneous value of the PCC current.
The electromechanical transient modeling method for a flexible DC and DC grid according to claim 5, wherein the inner loop current controller is represented by an outlet voltage of the d and q axes of the MMC inverter, respectively:

Where i d and i q are the components of the alternating current on the d and q axes, respectively, v p is the amplitude of the PCC bus voltage of the common junction point; P ref and Q ref are the reference values of the active power and the reactive power, respectively; i dref and i qref are reference values of the d-axis component and reference values of the q-axis component, respectively;
The reference values of the outlet voltages v cd and v cq of the d-axis and the q-axis of the MMC converter respectively; v pd , v pq the voltage of the common junction bus bar PCC on the d-axis, the voltage on the q-axis; U dc is the current DC Voltage, U dc0 is the initial DC voltage after each MMC converter is stabilized, T c is the delay generated by the modulation process, and is ignored after phase compensation; K pd and K pq are the proportional coefficients of d and q axes, respectively, K id K iq is the integral coefficient of the d and q axes, respectively; s represents a complex number; and ω represents a fundamental frequency angular velocity.
The electromechanical transient modeling method for a flexible DC and DC grid according to claim 6, wherein the alternating current phasor in the control system model is converted into a synchronous rotating dq coordinate system based on the PCC bus voltage, The inner loop controller controls the alternating currents i d , i q , including:

The expression of the alternating current dq axis is:

Obtained after injection bus controller and the inner side of the AC alternating current model obtained in d, q-axis component i d and i q, the alternating current component of the d, q-axis I d, i q of phasor The inverse transformation to a synchronous rotating coordinate system based on the system slack node, the transformed expression is:

Where: I PI and I PR are the real and imaginary parts of the components i d and i q of the alternating current on the d and q axes, respectively; V PI and V PR are the real parts of the phasors of the PCC bus voltage, respectively. The imaginary part.
The electromechanical transient modeling method for a flexible DC and DC grid according to claim 1, further comprising at least one additional module for function expansion based on an outer loop control structure, the additional module being in a flexible DC and DC grid The AC side and the DC side are provided with blocking signal generation logic, which is connected through the outer ring control structure of the AC side and the DC side of the flexible DC and DC power grids.
The electromechanical transient modeling method for a flexible DC and DC grid according to any one of claims 1 to 8, wherein the transient simulation model is used to simulate electromechanical transients under different working conditions of a flexible DC and DC grid. Operational features, including:

The steady state power flow model is used to generate initial power flow data required for transient simulation;

The AC side transient model is used to simulate the dynamic characteristics of the flexible DC and DC grid AC side;

The control system model is used to simulate the control characteristics of the AC side of the flexible DC and DC grid, including: constant AC/DC voltage control, active/reactive power emergency boost/return control function;

The DC side transient model is used to simulate a transient steady state process of voltage and current on a DC side of a flexible DC and DC power grid;

The add-on module is used to simulate the MMC inverter blocking feature and additional control functions.
An electromechanical transient simulation system for a flexible DC and DC grid, characterized in that the system comprises:

Modeling module: Establish transient simulation model, including steady-state tidal current model based on controlled current source, AC side State model, DC side transient model, the steady state power flow model based on the controlled current source is used to generate initial power flow data required for transient simulation;

Simulation module: It is used to simulate the electromechanical transient operation characteristics of flexible DC and DC grid under different working conditions by using transient simulation model.
The electromechanical transient simulation system for a flexible DC and DC grid according to claim 10, wherein the modeling module further comprises:

Power flow model modeling module: used to establish initial power flow data required for generating transient simulation, steady state power flow model based on controlled current source;

AC model modeling module: used to establish an AC-side transient model for simulating the dynamic characteristics of the flexible DC and DC grid AC side;

Control system modeling module: used to establish the control characteristics of the analog flexible DC and DC grid AC side, including: control system model of constant AC/DC voltage control, active/reactive power emergency boost/return control function;

DC model modeling module: DC-side transient model for establishing transient steady-state process of DC side and voltage on DC side of DC grid and DC grid; additional module modeling module: used to establish the blocking characteristics of analog MMC inverter and Additional control features.