Full-bridge submodule type MMC rapid simulation method and system

Abstract

The application discloses a full-bridge submodule type MMC (Modular Multilevel Converter) rapid simulation method and system. The modular multilevel converter contains a large number of switching elements, which brings huge calculation burden to electromagnetic transient simulation. In view of the problem, the application provides a full-bridge submodule type MMC rapid simulation method, which comprises the following steps: constructing a full-state equivalent circuit of each bridge arm of the MMC, wherein the bridge arm of the MMC adopts full-bridge self-module cascade; 2) establishing an MMC simulation model based on the full-state equivalent circuit, and gradually solving the parameter value at t time according to the known parameter of the MMC submodule at t-ΔT time. The application accelerates the simulation speed under the premise of ensuring the accuracy.

Description

Technical Field

[0001] This invention belongs to the field of DC power transmission technology, specifically a rapid simulation method and system for full-bridge sub-module type MMC based on bridge arm equivalent capacitance. Background Technology

[0002] DC transmission technology based on modular multilevel converters (MMCs) has significant advantages. To study the transient characteristics of MMCs, extensive research has been conducted on developing MMC models for different applications in electromagnetic transient simulation programs. Most electromagnetic transient simulation programs use Dommel's algorithm. This algorithm applies the trapezoidal integral method, transforming all dynamic components into a Norton equivalent current source in parallel with the conductance, and then establishes the admittance matrix equation for the entire circuit to solve for the circuit. Detailed switching models can accurately reflect the switching characteristics of IGBTs and the transient characteristics of MMCs. However, due to the high switching frequency (typically at the kilohertz level), the large-scale circuit admittance matrix must be frequently updated and manipulated with each switching action, resulting in low computational efficiency. Detailed switching models impose a huge computational burden and excessively long computation time. Average value models are mainly suitable for studying system-level controllers, but not for the detailed response of MMC-HVDC, especially for the transient characteristic analysis of DC voltage and current.

[0003] To overcome the shortcomings of detailed modeling in simulation efficiency, reference 1 [Gnanarathna UN, Gole AM, Jayasinghe RPEfficient modeling of modular multilevel HVDC converters (MMC) on electromagnetic transient simulation programs[J].IEEE Trans. PowerDeliv.2011,26,316–324] proposed a method of equivalence for detailed models. Based on the Thevenin equivalent circuit, the detailed equivalent model eliminates intermediate nodes inside the circuit, greatly improving simulation efficiency. However, since the number of sub-modules is not reduced, the computational efficiency of the equivalent model is still low. To further improve simulation efficiency, reference 2 [Xu J., Gole A., Zhao C..The use of averaged-value model of modular multilevel converter in DC grid[J].IEEE Transactions on Power Delivery,2014,30(2):519-528.] established an averaged-value model. Due to its oversimplification, the averaged-value model ignores the switching action of power devices and the influence of capacitor voltage fluctuations, and cannot fully simulate the transient characteristics of the bridge arm. Reference 3 [Pei X., Tang G., Pang H., et al. A general modeling approach for the MMC averaged-value model in large-scale DC grid [C]. IEEE Conference on Energy Internet and Energy System Integration (EI2). 2017] extends the half-bridge averaged-value model to various MMC submodule topologies such as full-bridge and double-clamped types, but the averaged-value model is difficult to obtain accurate results in the locked-down mode. Summary of the Invention

[0004] In view of the problems existing in the prior art, the present invention provides a fast simulation method and system for full-bridge sub-module type MMC based on bridge arm equivalent capacitance, which improves the simulation speed while ensuring calculation accuracy.

[0005] Therefore, the present invention adopts the following technical solution: a fast simulation method for full-bridge submodule type MMC, comprising:

[0006] 1) Construct the full-state equivalent circuit of each bridge arm of MMC. The bridge arms of MMC adopt full-bridge self-module cascade.

[0007] 2) Based on the full-state equivalent circuit, establish an MMC simulation model. According to the known parameters of the MMC submodule at time t-ΔT, solve the parameter values ​​at time t step by step; t represents the simulation time and ΔT represents the simulation step size.

[0008] Furthermore, the full-state equivalent circuit of each bridge arm of the MMC is used to simulate the operating state of the bridge arm under normal working conditions and under locked conditions.

[0009] Furthermore, the full-state equivalent circuit of each bridge arm of the MMC is composed of an equivalent voltage source and an equivalent resistor connected in series.

[0010] Furthermore, the known parameters of the MMC submodule are the bridge arm current, capacitor voltage, and switching state.

[0011] Furthermore, in the full-state equivalent circuit of each arm of the MMC, the parameters when it is in normal operation are calculated using the following formula:

[0012]

[0013] Among them, U Arm,EQ (t-ΔT) represents the voltage magnitude of the equivalent historical voltage source of the bridge arm at time t-ΔT, R2 is the resistance when the switching device is off, R1 is the resistance when the switching device is on, S is the switching function, and U... CEQ (t-ΔT) is the equivalent capacitance C EQ2 Based on the trapezoidal integral method, the equivalent voltage source, R CEQ2 The second equivalent capacitance C EQ2 The equivalent resistance R based on the trapezoidal integral method Σ =R1+R2+R CEQ2 R Arm,EQ i is the equivalent resistance of the bridge arm. Arm (t) represents the bridge arm current obtained by substituting the equivalent circuit into the network and solving the simultaneous equations, i′ Arm (t) represents the current in the capacitor branch, i C (t) represents the average capacitance current of the submodule at time t.

[0014] Furthermore, in the full-state equivalent circuit of each bridge arm of the MMC, when the negative input is in the normal state, the positions of R1 and R2 can be interchanged.

[0015] Furthermore, the second equivalent capacitance C EQ2 The relationship between the bridge arm output voltage and the bridge arm current is reflected by the following formula:

[0016]

[0017] Where C0 is the capacitance value of the bridge arm submodule, S rjx This is the switching function of the x-th submodule of the j-phase r-arm bridge, where N is the number of bridge arm submodules, and S... rj It is the average switching function of the j-phase r-arm bridge.

[0018] Furthermore, through the first equivalent capacitance C EQ1 This reflects the dynamic process of charging and discharging of the bridge arm equivalent capacitance, and the equivalent capacitance voltage u of the bridge arm is calculated. CEQ (t):

[0019]

[0020] Among them, u CEQ (t-ΔT) and i C (t-ΔT) represent the equivalent capacitance voltage and current at time t-ΔT, respectively, R CEQ1 The first equivalent capacitance C EQ1 The equivalent resistance based on the trapezoidal integral method, i C (t) represents the average capacitance current of the submodule at time t.

[0021] Furthermore, in the full-state equivalent circuit of the MMC submodule, the parameters of each component in the latched state are calculated using the following formula:

[0022]

[0023] Among them, U Arm，Blk (t-ΔT) represents the voltage magnitude of the equivalent historical voltage source of the bridge arm at time t-ΔT, R C，Blk u is the equivalent resistance of the bridge arm. C0,EQ,x (t-ΔT) is the Thevenin equivalent voltage source of the x-th submodule capacitor at time t-ΔT, u C0 (t-ΔT) is the average capacitor voltage of the submodule at time t-ΔT, u C0,EQ (t-ΔT)=u C0,EQ,x (t-ΔT), R C0,x It is the equivalent resistance of the submodule capacitor based on the trapezoidal integration method, i Arm (t-ΔT) is the bridge arm current obtained by substituting the equivalent circuit into the network and solving the simultaneous equations.

[0024] This invention also provides a rapid simulation system for full-bridge submodule-type MMC, which is used to implement the above-mentioned simulation method, and includes:

[0025] The full-state equivalent circuit construction unit is used to construct the full-state equivalent circuit of each bridge arm of the MMC. The bridge arms of the MMC adopt full-bridge self-module cascade.

[0026] The MMC simulation model building unit establishes an MMC simulation model based on the full-state equivalent circuit. Based on the known parameters of the MMC submodule at time t-ΔT, it gradually solves for the parameter values ​​at time t; t represents the simulation time, and ΔT represents the simulation step size.

[0027] Based on the above technical solution, the present invention has the following beneficial technical effects:

[0028] 1. Based on the bridge arm equivalence principle, this invention proposes a bridge arm equivalent circuit modeling method under normal working conditions, locked conditions, and full-state conditions, and establishes a simulation model that can accurately simulate the electromagnetic transient response of the full-bridge MMC.

[0029] 2. The fast simulation method for full-bridge sub-module MMC described in this invention has the same computational accuracy as the Thevenin equivalent model, and the simulation speed is faster with the same number of sub-modules. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the basic structure of MMC in a specific embodiment of the present invention;

[0031] Figure 2 This is a schematic diagram of the operation mode of the full-bridge submodule in a specific embodiment of the present invention;

[0032] Figure 3 This is a schematic diagram of the equivalent circuit of the bridge arm of the full-bridge submodule in a specific embodiment of the present invention;

[0033] Figure 4 This is a schematic diagram illustrating the solution process of the Thevenin circuit for the full-bridge submodule in a specific embodiment of the present invention;

[0034] Figure 5 This is a schematic diagram of the equivalent circuit for the full-bridge submodule bridge arm interlocking in a specific embodiment of the present invention;

[0035] Figure 6 This is a schematic diagram of the equivalent circuit of the full-bridge submodule bridge arm in a specific embodiment of the present invention.

[0036] Figure 7 This is a schematic diagram of the MMC-HVDC system in a specific embodiment of the present invention;

[0037] Figure 8 This is a schematic diagram of the power step of two equivalent models in a specific embodiment of the present invention;

[0038] Figure 9 This is a schematic diagram of the pressure reduction operation of two equivalent models in a specific embodiment of the present invention;

[0039] Figure 10 This is a schematic diagram of two equivalent models for AC faults in a specific embodiment of the present invention;

[0040] Figure 11 This is a schematic diagram of two equivalent DC fault models in a specific embodiment of the present invention. Detailed Implementation

[0041] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] Example

[0043] This embodiment provides a fast simulation method for a full-bridge submodule type MMC, which includes:

[0044] 1) Construct the full-state equivalent circuit of each bridge arm of MMC. The bridge arms of MMC adopt full-bridge self-module cascade.

[0045] 2) Based on the full-state equivalent circuit, establish the MMC simulation model (i.e., the following equations (18), (23) and (24)). According to the known parameters of the MMC submodule at time t-ΔT, solve the parameter values ​​at time t step by step; t represents the simulation time and ΔT represents the simulation step size.

[0046] The full-state equivalent circuit of each bridge arm of the MMC is used to simulate the operating state of the bridge arm under normal working conditions and under locked conditions.

[0047] The full-state equivalent circuit of each bridge arm of the MMC consists of an equivalent voltage source and an equivalent resistor connected in series.

[0048] The known parameters of the MMC submodule are the bridge arm current, capacitor voltage, and switching state.

[0049] The specific details of the above method are as follows:

[0050] like Figure 1 As shown, the three-phase MMC consists of six arms, each arm being composed of multiple full-bridge submodules and an arm inductor L0 connected in series. N represents the number of submodules per phase arm. The arm voltage and arm current are u0 and u0, respectively. rj and i rj Where r (r = p, n) represents the upper arm and lower arm, and j (j = a, b, c) represents the three phases a, b, and c. sj and i vj These are the grid-side voltage and valve-side current of the J-phase converter transformer, respectively. ac It is the equivalent reactance of the AC system. SM,rjx and u C,rjx These are the output voltage and capacitor voltage of the xth full-bridge submodule in the j-phase r-arm, respectively. For example... Figure 2As shown, the full-bridge submodule consists of four IGBT capacitors C0 with anti-parallel diodes. The first three operating states are normal states and can be distinguished according to the polarity of the submodule's output voltage. The latched state is generally used to clear faults or start the system.

[0051] 1) "Actively engaged" state, such as Figure 2 As shown in (a), when a conduction signal is applied to T1 and T4 while a turn-off signal is applied to T2 and T3, the submodule output level is the rated capacitor voltage +U. c .

[0052] 2) "Negative input" state, such as Figure 2 As shown in (b), when a turn-off signal is applied to T1 and T4 while a turn-on signal is applied to T2 and T3, the submodule output level is -U. c .

[0053] 3) "Resected" state, such as Figure 2 As shown in (c), when a conduction signal is applied to T1 and T3 or T2 and T4 simultaneously, the submodule output level is 0.

[0054] 4) "Locked" state, such as Figure 2 As shown in (d), when a turn-off signal is applied to all IGBTs simultaneously or no trigger signal is applied, the submodule capacitor will be charged regardless of the current direction of the submodule.

[0055] Based on the four operating states of the full-bridge submodule, the following properties can be derived: (1) The “on” state of the full-bridge submodule is only related to the conduction of a fixed pair of switches and is not related to the direction of the current; (2) The full-bridge submodule will not charge the capacitor in the “off” state.

[0056] 1. Derivation of the equivalent circuit of the bridge arm

[0057] First, the average capacitance current of the bridge arm submodule is derived.

[0058] The positive directions of the bridge arm voltage and current are as follows: Figure 1 The full-bridge submodule is shown. When the submodule is in a positive or negative input state, the bridge arm current will flow through the capacitor branch, and the capacitor current is i. C,rjx , and i C,rjx =i rj When the submodule is in the cut-off state, i C,rjx =0. Since a submodule can only be in one state at any given time, then:

[0059] i C,rjx =S rjx i rj (1)

[0060] In the formula, S rjxIt is the switching function of the x-th submodule of the j-phase r-arm bridge, and its specific definition is:

[0061]

[0062] By summing the capacitive currents of all submodules in a given bridge arm, we can obtain:

[0063]

[0064] Dividing both sides by N yields:

[0065]

[0066] Introducing the average switching function S rj Its expression is:

[0067]

[0068] According to equations (4) and (5), the average capacitive current i of a submodule of a certain bridge arm C,rj for:

[0069]

[0070] Similarly, the equivalent voltage of the bridge arm is derived.

[0071] The output voltage u of the xth submodule of a certain bridge arm SM,rjx It can be represented as:

[0072] u SM,rjx =S rjx u C,rjx (7)

[0073] Sum the capacitor voltages of all submodules in a given bridge arm, i.e.:

[0074]

[0075] Assuming all submodules on a bridge arm are identical and their capacitor voltages are perfectly balanced, then for a given bridge arm, the capacitor voltage u of a single submodule on that bridge arm is... C,rjx Equal to the average capacitor voltage u of all submodules C,rj , that is u C,rjx =u C,rj Substituting equation (5) into equation (8), we get:

[0076] u rj =S rj (Nu C,rj ) = S rj u CEQ,rj (9)

[0077] In the formula, urj U is the equivalent voltage of the bridge arm. CEQ,rj It is the equivalent capacitance voltage of the bridge arm, with a value of Nu. C,rj .

[0078] After obtaining the expressions for the bridge arm capacitor current and capacitor voltage, the equivalent circuit of the bridge arm is derived.

[0079] Based on the relationship between capacitor voltage and current, i C,rjx The expression is:

[0080]

[0081] Sum the capacitive currents of all submodules in a given bridge arm, i.e.:

[0082]

[0083] Based on the aforementioned assumption u C,rjx =u C,rj Equation (11) can be expressed as:

[0084]

[0085] Substituting equation (6) into equation (12), we get:

[0086]

[0087] Substituting equation (6) back into equation (13), we get:

[0088]

[0089] According to equation (13), the bridge arm equivalent circuit based on the average capacitor current is as follows: Figure 3 As shown in (a), the first equivalent capacitance C of the bridge arm is used to simulate the bridge arm. EQ1 The charging and discharging dynamic process. According to equation (14), the bridge arm equivalent circuit based on the bridge arm current is shown in (b) of 3, reflecting the relationship between the bridge arm output voltage and the bridge arm current, and the second equivalent capacitor C EQ2 The dynamic characteristics of the bridge arm currents can be simulated, and their expressions are as follows:

[0090]

[0091] Figure 3 R1 and R2 in the diagram are used to simulate the power device losses: R1 represents the resistance when the switching device is on, and its resistance is very small; R2 represents the resistance when the switching device is off, and its resistance is very large. Therefore, the bridge arm current is approximately equal to the capacitor current. During normal operation, the number of power devices on the bridge arm at any given time is always 2N, and the number of devices in the off state is also always 2N. If R... On and ROff R1 and R2 are the on-resistance and off-resistance of the switching device, respectively, and are expressed as follows:

[0092]

[0093] It is important to note that Figure 3 This is the equivalent circuit of the bridge arm when positive connection is applied. When negative connection is applied, the positions of R1 and R2 are interchanged.

[0094] 2. Electromagnetic transient modeling of full-bridge modular MMC

[0095] The energy storage element (such as a capacitor) is transformed into a discretized adjoint model using trapezoidal integrals, i.e., a voltage source in series with a resistor. This allows the network equations, expressed as differential equations, to be converted into algebraic equations, and the circuit can be solved relatively easily using the nodal voltage method. Based on the block-by-block variable method, the six arms of the MMC are divided into six subsystems. Based on trapezoidal integrals, the energy storage element (such as a capacitor) is further transformed into a discretized adjoint model, i.e., a voltage source in series with a resistor. Figure 3 Capacitor C in (b) EQ2 The circuit is converted into a discretized adjoint circuit, and the Thevenin equivalent circuit of each bridge arm subsystem is calculated. Then, the six equivalent circuits are substituted into the network for simultaneous solution, yielding all cross-variables (such as bridge arm currents) for the six bridge arm subsystems. Finally, based on the obtained bridge arm currents... Figure 3 (a) Solve independently for the variables within the subsystem. According to the principle of the trapezoidal integral formula, all physical quantities at time t-ΔT are known. For clarity, the subscript rj will be omitted in the following analysis.

[0096] 2.1 Equivalent circuit modeling of bridge arm under normal operating conditions

[0097] Figure 3 (b) Second equivalent C EQ2 Discretized adjoint models such as Figure 4 As shown in (b) above. From the trapezoidal integral, we can deduce that C... EQ2 Represented as a voltage source U CEQ A resistor R is connected in series with (t-ΔT). CEQ2 The resistance value is:

[0098]

[0099] In the formula, U CEQ (t-ΔT) is given in equation (22). Figure 4 in, u Arm (t) and i Arm (t) represents the voltage and current of the bridge arm at time t, respectively. The Thevenin equivalent circuit at time t is shown below. Figure 4 As shown in (c), the equivalent historical voltage source U Arm,EQ (t-ΔT) and resistance R Arm,EQ They are represented as follows:

[0100]

[0101] In the formula, R Σ =R1+R2+R CEQ2 By substituting the six equivalent circuits into the network and solving the simultaneous equations, the bridge arm current i at time t can be calculated. Arm (t), such as Figure 4 As shown in (d) in the diagram. Then, according to... Figure 4 For the circuit shown in (b), the current in the capacitor branch is:

[0102]

[0103] According to equation (6), the average capacitance current i of the submodule at time t is... C (t) is:

[0104] i C (t)=Si′ Arm (t) (20)

[0105] according to Figure 3 In (a), solve for the bridge arm equivalent capacitance voltage u at time t. CEQ (t), represented as:

[0106] u CEQ (t)=R CEQ1 i C (t)+U CEQ (t-ΔT) (21)

[0107] R CEQ1 The first equivalent capacitance C EQ1 Equivalent resistance based on trapezoidal integral method;

[0108] The parameters in the formula are expressed as follows:

[0109]

[0110] In the formula, u CEQ (t-ΔT) and i C (t-ΔT) represents the equivalent capacitance voltage and current at time t-ΔT, respectively.

[0111] As the above analysis shows, by using the known parameters at time t-ΔT, the parameter values ​​at time t can be solved step by step. Then, based on the parameter values ​​at time t, the values ​​at time t+ΔT can be calculated. This allows us to obtain the transient response of the MMC throughout the entire simulation time. It should be noted that... Figure 4 This represents the equivalent circuit of the bridge arm when positively connected. When negatively connected, simply interchange the positions of R1 and R2. The equivalent historical voltage source U... Arm,EQ (t-ΔT) and resistance RArm,EQ They are represented as follows:

[0112]

[0113] 2.2 Equivalent Circuit Modeling of Bridge Arm under Locked-Out State

[0114] When the MMC is in the latched state, none of the IGBTs in the submodule can be turned on. The current flow path of the bridge arm is determined by the direction of the current. Therefore, the equivalent circuit of the submodule at this time is as follows: Figure 5 As shown in the figure. The parameters in the figure are represented as follows:

[0115]

[0116] In the formula, u C0,EQ,x (t-ΔT) is the Thevenin equivalent voltage source of the x-th submodule capacitor at time t-ΔT, similar to equation (9), u C0,EQ (t-ΔT)=u C0,EQ,x (t-ΔT); u C0 (t-ΔT) is the average capacitor voltage of the submodule at time t-ΔT, u C0 (t-ΔT)=u C0,x (t-ΔT). When the bridge arm current flows in the positive direction, it flows only through diodes D1 and D4. When the bridge arm current flows in the negative direction, it flows only through diodes D2 and D3. Regardless of the current direction, the on-state resistance is represented by R. Blk Equivalent to 2NR On All four diodes are ideal (unresisted). The conduction and cutoff of the diode H-bridge are determined directly by the simulation software based on the direction of current flow in the bridge arms.

[0117] The process of solving for the submodule capacitor voltage at time t under the locked state is similar to that in Section 2.1 and will not be repeated here.

[0118] 2.3 Equivalent Circuit Modeling of Bridge Arms in All States

[0119] Based on the foregoing analysis of the equivalent circuits of the full-bridge submodule arms in normal operation and locked states, the full-state bridge arm equivalent circuit model is as follows: Figure 6 As shown. The parameters of the full-state equivalent circuit are shown in Table 1. All six arms of the MMC adopt the following... Figure 6 The full-state equivalent circuit is shown.

[0120] Table 1. Parameters of the full-state equivalent circuit

[0121]

[0122] This embodiment also provides a full-bridge submodule type MMC rapid simulation system, which is used to implement the above simulation method, and includes:

[0123] The full-state equivalent circuit construction unit is used to construct the full-state equivalent circuit of each bridge arm of the MMC. The bridge arms of the MMC adopt full-bridge self-module cascade.

[0124] The MMC simulation model building unit establishes an MMC simulation model based on the full-state equivalent circuit. Based on the known parameters of the MMC submodule at time t-ΔT, it gradually solves for the parameter values ​​at time t; t represents the simulation time, and ΔT represents the simulation step size.

[0125] Simulation Example

[0126] Based on the PSCAD / EMTDC simulation platform, this invention establishes the following... Figure 7 The full-bridge submodule MMC-HVDC system shown is used to verify the correctness of the MMC simulation model described in this invention. System parameters are shown in Table 2.

[0127] 3.1 Comparison of Simulation Working Conditions

[0128] The Thevenin equivalent model can accurately simulate the transient characteristics of the MMC; therefore, the MMC simulation model of this invention will be compared and verified with the Thevenin equivalent model. All variables are measured at the MMC1 terminal: the submodule capacitor voltage, bridge arm current, and converter transformer valve side voltage are all measured on phase a.

[0129] For four different operating conditions—power step, buck operation, AC fault, and DC fault—the MMC simulation model and Thevenin equivalent model of this invention still have almost the same transient response characteristics.

[0130] (1) Power step: At t = 1.0s, the active power command value of the inverter-side MMC2 jumps from 120MW to 400MW, and its transient response characteristics are as follows: Figure 8 As shown.

[0131] (2) Step-down operation: At t = 1.0s, the DC voltage command value of MM1 on the rectifier side changes from 400kV to 280kV for step-down operation. Its transient response characteristics are as follows: Figure 9 As shown.

[0132] (3) AC fault: At t = 1.0s, a three-phase ground fault occurs for 0.05s on the AC side busbar of the rectifier MMC1. Its transient response characteristics are as follows: Figure 10 As shown.

[0133] (4) DC Fault: At t = 1.0s, a permanent pole-to-ground direct ground fault occurs at the midpoint of the DC line. The rectifier-side MMC1 and inverter-side MMC2 are blocked at t = 1.02s. At t = 1.1s, the AC circuit breakers on both sides are opened, and their transient response characteristics are as follows: Figure 11 As shown.

[0134] Table 2 System Simulation Parameters

[0135]

[0136] For example Figure 7 The full-bridge submodule MMC-HVDC system shown was simulated for 1.0s. The computer used for simulation was configured as follows: Intel(R) Core(TM) i7-10700 CPU@2.90GHz, 16GB RAM, Windows 10, PSCAD 4.5.4. The simulation step size was 20μs, and the simulation time for the two models is shown in Table 3.

[0137] As shown in Table 3, the MMC simulation model of this invention not only has the same computational accuracy as the Thevenin equivalent model, but also has a faster simulation speed with the same number of submodules.

[0138] Table 3 Comparison of simulation time for the two equivalent models

[0139]

[0140] A two-end MMC DC power grid test system was built on the PSCAD / EMTDC platform, and its feasibility was verified through simulation. Simulation waveforms and simulation times were compared between the model described in this invention and the Thevenin equivalent model. The results show that the model described in this invention accelerates the simulation speed while maintaining accuracy, verifying the accuracy and speed of the proposed fast simulation method for full-bridge sub-module MMC.

[0141] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A rapid simulation method for a full-bridge sub-module type MMC, characterized in that, include: 1) Construct the full-state equivalent circuit of each bridge arm of MMC. The bridge arms of MMC adopt full-bridge self-module cascade. 2) Establish an MMC simulation model based on the full-state equivalent circuit, and according to the MMC sub-modules t -Δ T Given the parameters at time points, solve step by step. t The parameter value at time; t Δ represents the simulation time. T Indicates the simulation step size; The full-state equivalent circuit of each bridge arm of the MMC consists of an equivalent voltage source and an equivalent resistor connected in series. In the full-state equivalent circuit of each bridge arm of the MMC, the parameters when it is in normal operation are calculated using the following formulas: in, for t -Δ T The voltage magnitude of the equivalent historical voltage source of the bridge arm at any given time. This is the resistance when the switching device is turned off. This is the resistance when the switching device is turned on. S For switching functions, U CEQ ( t -Δ T () is the second equivalent capacitance C EQ2 Equivalent voltage source based on trapezoidal integration method R CEQ2 The second equivalent capacitance C EQ2 Equivalent resistance based on trapezoidal integration method , The equivalent resistance of the bridge arm. The bridge arm currents are obtained by substituting the equivalent circuit into the network and solving the simultaneous equations. The current in the capacitor branch is... for t The average capacitor current of the submodule at any given time; The second equivalent capacitance C EQ2 The relationship between the bridge arm output voltage and the bridge arm current is reflected by the following formula: in, It is the capacitance value of the bridge arm submodule. S rjx yes j Mutually r Bridge arm number x The switch functions for each submodule, N It is the number of bridge arm sub-modules. S rj yes j Mutually r Average switching function of the bridge arm; In the full-state equivalent circuit of the MMC submodule, the parameters of each component in the latched state are calculated using the following formula: in, for t -Δ T The voltage magnitude of the equivalent historical voltage source of the bridge arm at any given time. The equivalent resistance of the bridge arm. u C0,EQ,x ( t- Δ T )yes t -Δ T Time of the first x Thevenin equivalent voltage source for each submodule capacitor. u C0 ( t- Δ T )yes t -Δ T The average capacitor voltage of the time submodule, N It is the number of bridge arm sub-modules. It is the capacitance value of the bridge arm submodule. u C0,EQ ( t- Δ T )= u C0,EQ,x ( t- Δ T ), It is the equivalent resistance of the submodule capacitor based on the trapezoidal integration method. It is the bridge arm current obtained by substituting the equivalent circuit into the network and solving the simultaneous equations.

2. The rapid simulation method for full-bridge sub-module type MMC according to claim 1, characterized in that, The full-state equivalent circuit of each bridge arm of the MMC is used to simulate the operating state of the bridge arm under normal working conditions and under locked conditions.

3. The rapid simulation method for full-bridge sub-module type MMC according to claim 1, characterized in that, The known parameters of the MMC submodule are the bridge arm current, capacitor voltage, and switching state.

4. The rapid simulation method for full-bridge sub-module type MMC according to claim 1, characterized in that, In the full-state equivalent circuit of each bridge arm of the MMC, under normal conditions when negative input, R 1 and R Simply swap the positions of 2.

5. The rapid simulation method for full-bridge sub-module type MMC according to claim 1, characterized in that, Through the first equivalent capacitance C EQ1 This reflects the dynamic process of charging and discharging of the bridge arm equivalent capacitance, and the equivalent capacitance voltage of the bridge arm is solved. : in, It is the capacitance value of the bridge arm submodule. N It is the number of bridge arm sub-modules. u CEQ ( t -Δ T )and i C ( t -Δ T ) are respectively t -Δ T The equivalent capacitance voltage and current at time t. R CEQ1 The first equivalent capacitance C EQ1 Equivalent resistance based on trapezoidal integration method for t The average capacitor current of the submodule at time t.

6. A full-bridge sub-module type MMC rapid simulation system, used to implement the simulation method according to any one of claims 1-5, characterized in that, include: The full-state equivalent circuit construction unit is used to construct the full-state equivalent circuit of each bridge arm of the MMC. The bridge arms of the MMC adopt full-bridge self-module cascade. The MMC simulation model building unit establishes an MMC simulation model based on the full-state equivalent circuit, according to the MMC sub-modules. t -Δ T Given the parameters at time points, solve step by step. t The parameter value at time; t Δ represents the simulation time. T This indicates the simulation step size.

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