MMC dynamic control method and device based on bridge arm energy control and storage medium

Abstract

The invention relates to an MMC dynamic control method and device based on bridge arm energy control and a storage medium, and belongs to the technical field of intelligent power grids. According to the method, the MMC is subjected to optimization control under the asymmetric alternating current power grid, so that the negative sequence alternating current side current can be completely suppressed, and meanwhile, the double frequency fluctuation of the direct current side voltage and current can be avoided, so that the MMC plays a role of a fluctuation power'firewall 'between an alternating current system and a direct current system; meanwhile, a capacitor voltage optimization control model based on disturbance power feed-forward compensation is designed based on a dynamic equation of the capacitor voltage of the MMC sub-module under the asymmetric fault working condition of the alternating current power grid, the internal and external dynamic characteristics of the MMC under the fault working condition can be effectively improved, the control dimension of the MMC is increased through energy control, and the control precision of the MMC is improved. Therefore, the capacitor voltage in the MMC sub-module can be independently controlled, so that the dynamic performance of the capacitor voltage of the MMC sub-module under the fault condition is enhanced, and the conditions that the current control of the MMC is not decoupled and the current dynamic performance of the direct current side is poor are avoided.

Description

Technical Field

[0001] The present application belongs to the field of smart grid technology, and specifically relates to an MMC dynamic control method, device and storage medium based on bridge arm energy control. Background Art

[0002] When an asymmetric fault occurs in the AC main grid, the voltage amplitude of the PCC at the receiving converter station will also decrease, resulting in a decrease in the AC power transmission capacity of the receiving converter station. If the voltage amplitude at the PCC point drops too much, the wind power will be greater than the transmission capacity of the receiving converter station. At this time, the inner loop of the current of the receiving converter station will be saturated and the control ability of the DC voltage will be lost. If the surplus power cannot be processed in time, the DC voltage will rise rapidly, which will have a serious impact on the stable operation of the entire DC system. Therefore, it is crucial to study the fault detection and ride-through strategy of the entire wind farm flexible DC system under AC faults.

[0003] In addition, the power fed into the DC line by the wind farm can be quickly reduced by unified coordination of the control of the receiving station, the sending station and the wind turbine converter. Direct communication can be used to reduce the output of the wind farm. That is, the sending-end converter station determines whether a fault has occurred through DC voltage detection, and reduces the wind power by lowering the output voltage or increasing its output frequency. This method does not require additional equipment, but on the one hand, the communication and control delay makes the wind farm power reduction not timely enough, which may cause the DC voltage to be too high in a short time; on the other hand, in the process of quickly reducing the voltage of the wind farm aggregation line, overcurrent and other problems may occur on the AC side of the sending-end converter station.

[0004] Compared with the two-level topology, MMC has more complex internal and external dynamic behaviors and has additional control degrees of freedom. The six bridge arms can be controlled independently. However, the traditional control strategy of existing MMC completely inherits the control of traditional two-level converters, and the additional control degrees of freedom of MMC are not utilized.

[0005] In addition, traditional strategies do not consider the impact of DC-side inductance on the system's dynamic performance. However, in MMC-HVDC systems, a series current-limiting inductor is usually required on the DC side to suppress the rate of rise of the fault current. In this case, the MMC DC side contains a large inductor, which will have a significant impact on the modeling and control design of the MMC system, resulting in the MMC's current control not being decoupled and poor DC-side current dynamic performance. Summary of the Invention

[0006] To this end, the present application provides an MMC dynamic control method, device and storage medium based on bridge arm energy control to solve the problem in the prior art that the traditional control strategy of MMC has low utilization of the additional control freedom of MMC, does not consider the impact of DC side inductance on the dynamic performance of the system, resulting in the current control of MMC not being decoupled and the DC side current dynamic performance being poor.

[0007] According to a first aspect of an embodiment of the present invention, a method for dynamic control of an MMC based on bridge arm energy control is provided, the method comprising:

[0008] Obtain the dynamic equation of the capacitor voltage of the MMC submodule under the AC grid asymmetric fault condition;

[0009] Based on the dynamic equation of the MMC submodule capacitor voltage under the AC power grid asymmetric fault condition, an energy balance control model between MMC phase units under the AC power grid asymmetric fault condition and an energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition are respectively constructed by defining an auxiliary control input and adopting a power feedforward compensation disturbance component;

[0010] Tracking the DC circulating current reference value without static error through the MMC inter-phase unit energy balance control model under the AC grid asymmetric fault condition; tracking the fundamental frequency AC circulating current reference value without static error through the energy balance control model between the upper and lower bridge arms under the AC grid asymmetric fault condition;

[0011] Obtaining a circulating current suppression reference voltage according to the DC circulating current output by the energy balance control model between the MMC phase units under the asymmetric fault condition of the AC power grid and the AC circulating current output by the energy balance control model between the upper and lower bridge arms under the asymmetric fault condition of the AC power grid;

[0012] Converting the circulating current suppression reference voltage into a three-phase circulating current suppression reference voltage in a three-phase stationary coordinate system;

[0013] If the control type of the MMC is a constant power control MMC, the DC voltage is controlled by the MMC total energy balance control method. If the control type of the MMC is a constant voltage MMC, the DC voltage is controlled by the DC voltage control method under the normal working conditions of the AC grid.

[0014] Obtaining a DC side current reference value according to the DC voltage control process; obtaining a DC voltage reference value according to the DC side current reference value;

[0015] If the control type of the MMC is a constant power or constant voltage MMC, the AC current is controlled by combining a phase-locked loop and power control;

[0016] Acquire an AC side current reference value according to the AC current control process; acquire an AC differential mode voltage according to the AC side current reference value, and convert the AC differential mode voltage into a three-phase stationary coordinate system to acquire a three-phase AC differential mode voltage;

[0017] The three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm are obtained according to the three-phase AC differential mode voltage, the DC voltage reference value and the three-phase circulating current suppression reference voltage, and the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm are controlled by a sub-module voltage balancing control method based on voltage sorting.

[0018] Preferably,

[0019] The dynamic equation for obtaining the capacitor voltage of the MMC submodule under the AC grid asymmetric fault condition includes:

[0020] Obtaining the dynamic equation of the MMC submodule capacitor voltage under the AC grid asymmetric fault condition according to the dynamic equation of the MMC submodule capacitor voltage under the AC grid normal working condition and the space vector representation of the AC side voltage and current under the AC grid asymmetric fault working condition;

[0021] The dynamic equation of the MMC submodule capacitor voltage under normal operating conditions of the AC power grid includes: a dynamic equation of the DC amount in the sum of the energy stored in the first upper and lower bridge arm capacitors and a dynamic equation of the DC amount in the difference of the energy stored in the first upper and lower bridge arm capacitors;

[0022] The dynamic equation of the MMC submodule capacitor voltage under the AC grid asymmetric fault condition includes: a dynamic equation of the DC amount in the sum of the energy stored in the second upper and lower bridge arm capacitors and a dynamic equation of the DC amount in the difference of the energy stored in the second upper and lower bridge arm capacitors.

[0023] Preferably,

[0024] The space vector representation of the AC side voltage and current under the AC grid asymmetric fault condition includes:

[0025] Construct the average value model of MMC under AC grid asymmetric fault conditions;

[0026] Obtaining expressions for the voltage and current on the AC side of the MMC according to the average value model of the MMC under the asymmetric fault condition of the AC power grid;

[0027] According to the average value model of the MMC under the asymmetric fault condition of the AC power grid, the dynamic equation of the AC side current is obtained by Kirchhoff's voltage law and superposition theorem;

[0028] Transforming the expressions of the voltage and current on the AC side of the MMC into a two-phase stationary αβ coordinate system to obtain the positive sequence αβ axis component and the negative sequence αβ axis component respectively;

[0029] Transforming the expressions of the voltage and current on the AC side of the MMC into a dual synchronous rotating dq coordinate system to obtain the positive sequence dq axis components and the negative sequence dq axis components respectively;

[0030] The space vector representation of the AC side voltage and current under the asymmetric fault condition of the AC grid is obtained according to the expressions of the MMC AC side voltage and current, the dynamic equation of the AC side current, the positive-sequence αβ-axis component, the negative-sequence αβ-axis component, the positive-sequence dq-axis component, and the negative-sequence dq-axis component.

[0031] Preferably,

[0032] The dynamic equation of the capacitor voltage of the MMC submodule under normal operating conditions of the AC grid includes:

[0033] Obtain the relationship between the AC current and the common-mode current of the bridge arm, and obtain the reference voltage of the MMC submodule;

[0034] Obtaining the instantaneous power of the upper and lower bridge arm cascade submodules according to the relationship between the AC current and the bridge arm common mode current and the reference voltage of the MMC submodule;

[0035] Obtaining the sum of the instantaneous powers of the upper and lower bridge arm cascade submodules and the difference between the instantaneous powers of the upper and lower bridge arm cascade submodules according to the instantaneous powers of the upper and lower bridge arm cascade submodules;

[0036] Obtaining a DC amount in the sum and difference of the energy stored in the upper and lower bridge arm capacitors according to the sum of the instantaneous powers of the upper and lower bridge arm cascade submodules and the difference between the instantaneous powers of the upper and lower bridge arm cascade submodules;

[0037] The DC amount in the sum of the energy stored in the upper and lower bridge arm capacitors and the difference in the energy stored in the upper and lower bridge arm capacitors is converted to the αβ0 coordinate system, and the dynamic equation of the DC amount in the sum of the energy stored in the first upper and lower bridge arm capacitors and the dynamic equation of the DC amount in the difference in the energy stored in the first upper and lower bridge arm capacitors are respectively obtained through algebraic operations.

[0038] Preferably,

[0039] The acquisition of the dynamic equation of the DC amount in the sum of the energy stored in the second upper and lower bridge arm capacitors includes:

[0040] Substituting the space vector representation of the AC side voltage and current under the AC grid asymmetric fault condition into the dynamic equation of the DC quantity in the sum of the energy stored in the first upper and lower bridge arm capacitors, to obtain the dynamic equation of the DC quantity in the sum of the energy stored in the second upper and lower bridge arm capacitors;

[0041] The dynamic equation for obtaining the DC amount in the difference between the energy stored in the second upper and lower bridge arm capacitors includes:

[0042] Substituting the space vector representation of the AC side voltage and current under the asymmetric fault condition of the AC power grid into the dynamic equation of the DC quantity in the difference between the energy stored in the first upper and lower bridge arm capacitors, the dynamic equation of the DC quantity in the difference between the energy stored in the second upper and lower bridge arm capacitors is obtained.

[0043] Preferably,

[0044] The method of constructing an MMC inter-phase unit energy balance control model under an asymmetric AC grid fault condition by defining an auxiliary control input and adopting power feedforward compensation for a disturbance component based on the dynamic equation of the MMC submodule capacitor voltage under the AC grid asymmetric fault condition includes:

[0045] Obtaining a first auxiliary control input quantity expression according to a dynamic equation of a DC quantity in the sum of the energy stored in the second upper and lower bridge arm capacitors;

[0046] By defining the first auxiliary control input, the positive and negative sequence power feedforward method is used to compensate the MMC positive and negative sequence AC voltages and the power generated by the AC, and the DC circulating current reference value expression is obtained;

[0047] Obtaining an energy balance control model between MMC phase units under the asymmetric fault condition of the AC power grid through the first auxiliary control input expression and the DC circulating current reference value expression;

[0048] The energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition is constructed by defining an auxiliary control input and adopting power feedforward compensation for the disturbance component based on the dynamic equation of the MMC submodule capacitor voltage under the AC power grid asymmetric fault condition, including:

[0049] Obtaining a second auxiliary control input quantity expression according to a dynamic equation of a DC quantity in the difference between the energy stored in the second upper and lower bridge arm capacitors;

[0050] Set the limiting conditions for the reference value of the control quantity;

[0051] By defining a second auxiliary control input, using positive and negative sequence power feedforward to compensate for the disturbance component generated by the MMC, and combining the expression of the second auxiliary control input and the limiting conditions of the control reference value, the expression of the fundamental frequency AC circulating current reference value is obtained;

[0052] The energy balance control model between the upper and lower bridge arms under the asymmetric fault condition of the AC power grid is obtained through the second auxiliary control input expression and the fundamental frequency AC circulating current reference value expression.

[0053] Preferably,

[0054] The obtaining of the DC side current reference value according to the DC voltage control process includes:

[0055] Obtaining a third auxiliary control input quantity expression according to a dynamic equation of a DC quantity in the sum of the energy stored in the second upper and lower bridge arm capacitors;

[0056] By defining the third auxiliary control input, the positive and negative sequence power feedforward method is used to compensate for the disturbance component generated by the MMC, and the DC side current reference value expression is obtained.

[0057] The DC side current reference value is obtained according to the third auxiliary control input quantity expression and the DC side current reference value expression.

[0058] Preferably,

[0059] The obtaining of the AC side current reference value according to the AC current control process includes:

[0060] Obtain the two output closed-loop transfer functions of the second-order generalized integrator;

[0061] Performing algebraic operations on the two outputs of the second-order generalized integrator to obtain the positive sequence component and the negative sequence component of the AC side voltage of the receiving converter station;

[0062] A positive-sequence current reference value and a negative-sequence current reference value are respectively obtained according to the positive-sequence component and the negative-sequence component of the AC side voltage of the receiving-end converter station.

[0063] Preferably,

[0064] Obtaining a positive-sequence current reference value and a negative-sequence current reference value according to a positive-sequence component and a negative-sequence component of the AC side voltage of the receiving-end converter station includes:

[0065] Constructing an MMC average value model under an asymmetric fault AC power grid, and obtaining expressions for AC side voltage and current according to the MMC average value model;

[0066] The dynamic equation of the AC side current is constructed using Kirchhoff's voltage law and superposition theorem;

[0067] Transforming the dynamic equation of the AC side current into a two-phase static αβ coordinate system, and obtaining the positive and negative sequence αβ axis components of the AC side according to the positive sequence component and the negative sequence component;

[0068] The dynamic equation of the AC side current is transformed into the dual synchronous rotating dq coordinate system to obtain the positive and negative sequence d q Axis component;

[0069] Obtaining space vector expressions of the voltage and current on the AC side according to the dynamic equation of the AC side current, the positive and negative sequence αβ axis components, and the positive and negative sequence dq axis components;

[0070] Obtain the expressions of instantaneous active and reactive power of asymmetric AC grid according to instantaneous power theory;

[0071] Substituting the dynamic equation of the AC side current, the positive and negative sequence αβ axis components, the positive and negative sequence dq axis components, and the space vector expressions of the voltage and current on the AC side into the expressions of the instantaneous active and reactive power of the asymmetric AC power grid to obtain the instantaneous power expression of the AC power grid;

[0072] A positive sequence current reference value and a negative sequence current reference value are obtained according to the instantaneous power expression of the AC power grid.

[0073] Preferably,

[0074] The acquiring of the AC differential mode voltage according to the AC side current reference value includes:

[0075] Transforming the AC side current reference value into a dual synchronous rotating αβ coordinate system to obtain the AC side current reference value in the dual synchronous rotating αβ coordinate system;

[0076] The AC differential mode voltage is obtained according to the AC side current reference value in the dual synchronous rotating αβ coordinate system.

[0077] According to a second aspect of an embodiment of the present invention, there is provided an MMC dynamic control device based on bridge arm energy control, the device comprising:

[0078] Dynamic equation acquisition module: used to obtain the dynamic equation of the capacitor voltage of the MMC submodule under the condition of asymmetric fault of the AC grid;

[0079] A model construction module is used to construct an energy balance control model between MMC phase units under the AC power grid asymmetric fault condition and an energy balance control model between upper and lower bridge arms under the AC power grid asymmetric fault condition by defining an auxiliary control input and using power feedforward compensation for the disturbance component based on the dynamic equation of the MMC submodule capacitor voltage under the AC power grid asymmetric fault condition;

[0080] Tracking module: used to track the DC circulating current reference value without static error through the MMC inter-phase unit energy balance control model under the AC power grid asymmetric fault condition; and to track the fundamental frequency AC circulating current reference value without static error through the energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition;

[0081] A circulating current suppression reference voltage acquisition module is configured to acquire a circulating current suppression reference voltage based on the DC circulating current output by the energy balance control model between the MMC phase units under the asymmetric fault condition of the AC power grid, and the AC circulating current output by the energy balance control model between the upper and lower bridge arms under the asymmetric fault condition of the AC power grid;

[0082] Three-phase circulating current suppression reference voltage acquisition module: used to convert the circulating current suppression reference voltage into the three-phase circulating current suppression reference voltage in the three-phase stationary coordinate system;

[0083] DC voltage control module: If the MMC control type is constant power control, the DC voltage is controlled by adopting the MMC total energy balance control method; if the MMC control type is constant voltage MMC, the DC voltage is controlled by adopting the DC voltage control method under the normal working condition of the AC grid;

[0084] A DC voltage reference value acquisition module is configured to acquire a DC side current reference value according to the DC voltage control process; and acquire a DC voltage reference value according to the DC side current reference value.

[0085] AC current control module: used to control the AC current by combining phase-locked loop and power control if the control type of the MMC is constant power or constant voltage MMC;

[0086] A three-phase AC differential mode voltage acquisition module is used to obtain an AC side current reference value according to the AC current control process; obtain an AC differential mode voltage according to the AC side current reference value, and convert the AC differential mode voltage into a three-phase stationary coordinate system to obtain a three-phase AC differential mode voltage;

[0087] Bridge arm voltage control module: used to obtain the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm according to the three-phase AC differential mode voltage, DC voltage reference value and three-phase circulating current suppression reference voltage, and use the sub-module voltage balancing control method based on voltage sorting to control the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm.

[0088] According to a third aspect of an embodiment of the present invention, a storage medium is provided, wherein the storage medium stores a computer program, and when the computer program is executed by a host controller, each step in the above method is implemented.

[0089] The technical solutions provided by the embodiments of the present invention may have the following beneficial effects:

[0090] The present application can completely suppress the negative-sequence AC side current by optimizing and controlling the MMC under an asymmetric AC power grid, and can also ensure that the DC side voltage and current do not fluctuate by double frequency, so that the MMC plays the role of a "firewall" for fluctuating power between the AC and DC systems; at the same time, the present application is based on the dynamic equation of the capacitor voltage of the MMC sub-module under the asymmetric fault condition of the AC power grid, and designs a capacitor voltage optimization control model based on disturbance power feedforward compensation, which can effectively improve the internal and external dynamic characteristics of the MMC under fault conditions, and increase the control dimension of the MMC through energy control, so that the capacitor voltage in the MMC sub-module can be independently controlled, thereby enhancing the dynamic performance of the capacitor voltage of the MMC sub-module under fault conditions, and avoiding the situation where the current control of the MMC is not decoupled and the DC side current dynamic performance is poor.

[0091] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0092] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0093] Figure 1 is a flow chart of a method for dynamic control of an MMC based on bridge arm energy control according to an exemplary embodiment;

[0094] Figure 2 is a schematic diagram of an average value model of MMC under an AC power grid asymmetric fault condition according to an exemplary embodiment;

[0095] Figure 3 1 is a block diagram of total energy balance control of a constant power control MMC under an asymmetric AC power grid according to an exemplary embodiment;

[0096] Figure 4 is a block diagram of total energy balance control of a constant voltage controlled MMC under an asymmetric AC grid according to an exemplary embodiment;

[0097] Figure 5 is a schematic diagram of an energy balance control model between MMC phase units in an asymmetric AC power grid according to an exemplary embodiment;

[0098] Figure 6 1 is a schematic diagram of an energy balance control model between upper and lower bridge arms of an MMC in an asymmetric AC power grid according to an exemplary embodiment;

[0099] Figure 7 is a simulation block diagram of a wind farm transmission system via MMC-HVDC according to an exemplary embodiment;

[0100] Figure 8 is a block diagram of an MMC internal circulation control according to an exemplary embodiment;

[0101] Figure 9 is a block diagram of a phase-locked loop structure based on a second-order generalized integrator (SOGI) according to an exemplary embodiment;

[0102] Figure 10 is a structural block diagram of SOGI according to an exemplary embodiment;

[0103] Figure 11 is a block diagram of positive and negative sequence decomposition based on SOGI according to an exemplary embodiment;

[0104] Figure 12 is a schematic diagram of a submodule voltage balancing control principle based on voltage sorting according to an exemplary embodiment;

[0105] Figure 13 is a block diagram of dynamic control of a constant power MMC under an asymmetric AC power grid according to an exemplary embodiment;

[0106] Figure 14 is a block diagram of dynamic control of a constant voltage MMC under an asymmetric AC grid according to an exemplary embodiment;

[0107] Figure 15 is a system schematic diagram of an MMC dynamic control device based on bridge arm energy control according to an exemplary embodiment;

[0108] In the accompanying drawings: 1-dynamic equation acquisition module, 2-model construction module, 3-tracking module, 4-circulating current suppression reference voltage acquisition module, 5-three-phase circulating current suppression reference voltage acquisition module, 6-DC voltage control module, 7-DC voltage reference value acquisition module, 8-AC current control module, 9-three-phase AC differential mode voltage acquisition module, 10-bridge arm voltage control module. DETAILED DESCRIPTION

[0109] To make the purpose, technical solutions, and advantages of this application more clear, the technical solutions of this application will be described in detail below. Obviously, the embodiments described are only some of the embodiments of this application, rather than all of them. Based on the embodiments in this application, all other implementation methods obtained by ordinary technicians in this field without making any creative work are within the scope of protection of this application.

[0110] Example 1

[0111] Figure 1 FIG is a flow chart of an MMC dynamic control method based on bridge arm energy control according to an exemplary embodiment. Figure 1 As shown, the method includes:

[0112] S1, obtain the dynamic equation of the capacitor voltage of the MMC submodule under the AC grid asymmetric fault condition;

[0113] S2, based on the dynamic equation of the MMC submodule capacitor voltage under the AC power grid asymmetric fault condition, by defining an auxiliary control input and using power feedforward compensation for the disturbance component, respectively constructing an MMC inter-phase unit energy balance control model under the AC power grid asymmetric fault condition and an energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition;

[0114] S3, tracking the DC circulating current reference value without static error through the MMC inter-phase unit energy balance control model under the AC power grid asymmetric fault condition; tracking the fundamental frequency AC circulating current reference value without static error through the energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition;

[0115] S4, obtaining a circulating current suppression reference voltage according to the DC circulating current output by the energy balance control model between the MMC phase units under the asymmetric fault condition of the AC power grid, and the AC circulating current output by the energy balance control model between the upper and lower bridge arms under the asymmetric fault condition of the AC power grid;

[0116] S5, converting the circulating current suppression reference voltage into a three-phase circulating current suppression reference voltage in a three-phase stationary coordinate system;

[0117] S6, if the control type of the MMC is a constant power control MMC, the DC voltage is controlled by using the MMC total energy balance control method; if the control type of the MMC is a constant voltage MMC, the DC voltage is controlled by using the DC voltage control method under normal operating conditions of the AC grid;

[0118] S7, obtaining a DC side current reference value according to the DC voltage control process; obtaining a DC voltage reference value according to the DC side current reference value;

[0119] S8, if the control type of the MMC is a constant power or constant voltage MMC, the AC current is controlled by combining a phase-locked loop and power control;

[0120] S9, obtaining an AC side current reference value according to the AC current control process; obtaining an AC differential mode voltage according to the AC side current reference value, and converting the AC differential mode voltage into a three-phase stationary coordinate system to obtain a three-phase AC differential mode voltage;

[0121] S10, obtaining the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm according to the three-phase AC differential mode voltage, the DC voltage reference value, and the three-phase circulating current suppression reference voltage, and controlling the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm using a sub-module voltage balancing control method based on voltage sorting;

[0122] It can be understood that the steps for obtaining the space vector representation of the AC side voltage and current under the AC grid asymmetric fault condition are:

[0123] Construct the average value model of MMC under the AC power grid asymmetric fault condition, as shown in the attached figure. Figure 2 As shown in the figure; when the AC grid voltage is asymmetric, since there is no zero-sequence voltage and zero-sequence current on the valve side, according to the average value model of the MMC, the voltage and current on the MMC AC side are expressed as:

[0124]

[0125] Where: h = 1, 2, 3; and are the positive and negative sequence voltages on the AC side of phase j respectively; and are the positive and negative sequence currents of the AC side of phase j respectively; and are the values ​​of the positive and negative sequence voltage amplitudes on the AC side respectively; θ + and θ - are the initial phases of the positive and negative sequence voltages on the AC side respectively; and are the amplitudes of the positive and negative sequence currents on the AC side respectively; φ + and φ - are the initial phases of the positive and negative sequence currents on the AC side respectively; ω is the grid angular frequency; j = a, b, c represents the three phases a, b, c;

[0126] According to the attached Figure 2 For the average value model of the MMC shown, the dynamic equation of the AC side current can be obtained from Kirchhoff's voltage law and superposition theorem:

[0127]

[0128] Transforming equation (1) into the two-phase stationary αβ coordinate system, we can obtain:

[0129]

[0130] Where, and and and are the α and β axis components of the MMC positive sequence AC internal potential, AC side voltage, and AC side current, respectively. and and and They are the α and β axis components of the negative sequence AC internal potential, AC side voltage and AC side current of the MMC respectively;

[0131] Transforming equation (1) into the dual synchronous rotating dq coordinate system, we can obtain:

[0132]

[0133] Where, and and and are the dq axis components of the MMC AC internal potential, AC side voltage, and AC side current in the positive sequence rotating coordinate system, respectively. and and and are the dq axis components of the MMC AC internal potential, AC side voltage and AC side current in the negative sequence rotating coordinate system respectively;

[0134] Combining equations (1) to (6), the space vectors of the AC side voltage and current can be expressed as:

[0135]

[0136] In the formula, j is the imaginary unit, and e is the natural constant;

[0137] Then, a dynamic equation of the MMC submodule capacitor voltage under normal AC grid conditions is obtained (including a dynamic equation of the DC amount in the sum of the energy stored in the first upper and lower bridge arm capacitors and a dynamic equation of the DC amount in the difference between the energy stored in the first upper and lower bridge arm capacitors under normal AC grid conditions);

[0138] The steps to obtain the dynamic equation of the capacitor voltage of the MMC submodule under normal AC grid conditions are:

[0139] According to the alternating current i sj and the bridge arm common mode current i comj The instantaneous power of the upper and lower bridge arm cascade sub-modules is calculated by the relationship between and the sub-module reference voltage;

[0140] AC current i sj and the bridge arm common mode current i comj The relationship between them is:

[0141]

[0142] The calculation formula for the submodule reference voltage is:

[0143]

[0144] Where i pj and i nj They represent the current flowing into the upper and lower bridge arm cascade submodules of phase j, u pj * and u nj * Respectively represent the total output voltage reference value of the j-phase upper and lower bridge arm cascade sub-modules, is the AC internal potential (bridge

[0145] The reference value of the differential mode voltage of the arm, U dc is the DC side voltage, is the j-phase double frequency modulation voltage output by the circulating current suppressor;

[0146] According to equations (9) and (10), the instantaneous power of the upper and lower bridge arm cascade submodules can be obtained as follows:

[0147]

[0148] Where: p pj and p nj are the instantaneous power of the upper and lower bridge arm cascade submodules, respectively, w pj and w nj are the energy stored in the upper and lower bridge arm capacitors, e comj is the common mode component of the j-phase bridge arm modulation voltage (i.e., the sum of the DC modulation voltage and the doubled frequency circulating current modulation voltage), e j is the differential mode component (i.e. fundamental frequency modulation voltage) in the j-phase bridge arm modulation voltage, i dc is the DC bus voltage of MMC, i cirj is the circulation component of phase j, i j is the AC side current of phase j (i.e. );

[0149] According to equations (11) and (12), the sum of the instantaneous power of the upper and lower bridge arm cascade submodules p can be obtained: jΣ The difference between the power p jΔ They are:

[0150]

[0151] Where: w jΣ and w jΔ are the sum and difference of the energy stored in the upper and lower bridge arm capacitors respectively;

[0152] Since the balance of capacitor energy depends only on p jΣ and p jΔ The DC flow in the jΣ and w jΔThe DC variation in is independent of the AC variation. Therefore, combining equations (13) and (14) and ignoring the losses of the MMC system, we can obtain:

[0153]

[0154] Where: w jΣdc and w jΔdc are the DC values ​​of the sum and difference of the energy stored in the upper and lower bridge arm capacitors, i cirjdc is the DC amount in the bridge arm circulation, that is, the DC circulation, (v j i j ) dc and (2e j i cirj ) dc v j i j and 2e j i cirj The DC flow in the dc is the DC side voltage;

[0155] Ignoring the influence of system loss and equivalent inductance on the AC side, transforming equations (15) and (16) to the αβ0 coordinate system and performing algebraic operations, we can obtain the dynamic equations of the DC quantity in the sum of the energy stored in the first upper and lower bridge arm capacitors and the dynamic equations of the DC quantity in the difference of the energy stored in the first upper and lower bridge arm capacitors under normal operating conditions of the AC grid;

[0156] Under normal operating conditions of the AC grid, the dynamic equation for the DC quantity in the sum of the energy stored in the first upper and lower bridge arm capacitors is:

[0157]

[0158] Under normal operating conditions of the AC grid, the dynamic equation for the DC amount in the difference between the energy stored in the first upper and lower bridge arm capacitors is:

[0159]

[0160] Where: i cirαdc and i cirβdc are the DC circulating current in the αβ coordinate system, and and and are respectively the positive and negative sequence fundamental frequency AC circulating currents in the αβ coordinate system, u d and i d are the d-axis components of the AC side voltage and current, i dc is the DC side current, w 0Σdc is the total energy stored in the MMC capacitor, w αΣdc and w βΣdcare the energy between MMC phase units, w αΔdc 、w βΔdc and w 0Δdc are the energies between the upper and lower bridge arms of the MMC respectively;

[0161] Then, a dynamic equation for the DC amount in the sum of the energy stored in the second upper and lower bridge arm capacitors under an asymmetric AC grid fault condition is calculated. The calculation steps for the dynamic equation for the DC amount in the sum of the energy stored in the second upper and lower bridge arm capacitors under an asymmetric AC grid fault condition are as follows:

[0162] Substituting the space vector representation of the voltage and current on the AC side under the asymmetric fault condition of the AC grid (Equations (1) to (8)) into the dynamic equation (17) of the DC quantity in the sum of the energy stored in the first upper and lower bridge arm capacitors under the normal condition of the AC grid, the dynamic equation of the DC quantity in the sum of the energy stored in the second upper and lower bridge arm capacitors under the asymmetric fault condition of the AC grid can be obtained as follows:

[0163]

[0164] According to the calculation rule Re(X)Re(Y)=[Re(XY)+Re(XY*)] / 2, formula (19) can be further expressed as:

[0165]

[0166] Where: is the conjugate current space vector;

[0167] The space vector representation of the AC side voltage and current under the AC grid asymmetric fault condition (Equation (1) to Equation (8)) is further expressed as:

[0168]

[0169] Converting Equation (20) to the αβ0 coordinate system and combining Equations (21) and (22) yields:

[0170]

[0171] Where, and and and are the α and β axis components of the MMC AC side voltage and AC side current, and and and are the α and β axis components of the MMC AC side voltage and AC side current, and and and are the dq axis components of the MMC AC side voltage and AC side current in the positive sequence rotating coordinate system, and and and are the dq axis components of the MMC AC side voltage and AC side current in the negative sequence rotating coordinate system, U dc is the DC side voltage, i dc is the DC side current, i cirαdc and i cirβdc They are the DC circulating current in αβ coordinate system respectively;

[0172] Then, the dynamic equation of the DC amount in the difference between the energy stored in the second upper and lower bridge arm capacitors under the AC grid asymmetric fault condition is calculated. The calculation steps of the dynamic equation of the DC amount in the difference between the energy stored in the second upper and lower bridge arm capacitors under the AC grid asymmetric fault condition are as follows:

[0173] Substituting the space vector representation of the AC side voltage and current under the AC grid asymmetric fault condition (Equation (7) and Equation (8)) into the dynamic equation of the DC amount in the difference between the energy stored in the second upper and lower bridge arm capacitors under the AC grid normal condition (Equation 16), the dynamic equation of the DC amount in the difference between the energy stored in the second upper and lower bridge arm capacitors under the AC grid asymmetric fault condition can be obtained as follows:

[0174]

[0175] Converting Equation (24) to the αβ0 coordinate system and performing algebraic operations yields:

[0176]

[0177] Where: and are the dq axis components of the MMC fundamental frequency AC circulating current in the positive sequence rotating coordinate system, and are the dq axis components of the MMC fundamental frequency AC circulating current in the negative sequence rotating coordinate system, and and and are the positive and negative sequence fundamental frequency AC circulating currents in the αβ coordinate system, and are the α and β axis components of the MMC AC side voltage, and are the α and β axis components of the AC side voltage, and point

[0178] are the dq axis components of the MMC AC side voltage in the positive sequence rotating coordinate system, and are the dq axis components of the MMC AC side voltage in the negative sequence rotating coordinate system;

[0179] Equations (23) and (25) describe the dynamic behavior of the DC quantity in the sum and difference of the energy stored in the second upper and lower bridge arm capacitors, respectively. It can be seen that the positive and negative sequence voltages and currents on the AC side will have a disturbing effect on the dynamic behavior of the DC quantity in the sum of energy; the positive and negative sequence voltages and fundamental frequency AC circulating current on the AC side will have a disturbing effect on the dynamic behavior of the DC quantity in the difference of energy; therefore, in order to improve the transient response characteristics of the MMC capacitor voltage, the disturbance quantity can be feedforward compensated in the capacitor voltage balance control loop according to equations (20) and (24);

[0180] The specific steps of constructing the energy balance control model between MMC phase units under the AC power grid asymmetric fault condition are as follows:

[0181] According to the first two equations in the dynamic equation (23) of the sum of the energy stored in the upper and lower bridge arm capacitors under the condition of asymmetric fault in the AC grid, the formula is obtained:

[0182]

[0183] Where u sumα and u sumβ Defined as the first auxiliary control input;

[0184] According to formula (26), the DC circulating current can be used to balance the capacitance energy between phase units. At this time, the positive and negative sequence AC voltages and the power generated by the AC are the disturbance components. By defining the first auxiliary control input u sumα and u sumβ , the positive and negative sequence power feedforward method is used to compensate the MMC positive and negative sequence AC voltage and the power generated by the AC, thereby obtaining the DC circulating current reference value:

[0185]

[0186] According to equations (26) and (27), the PI regulator is used for regulation control, and the design is as shown in the following: Figure 5 The energy balance control model between MMC phase units in the asymmetric AC power grid shown in the figure can track the DC circulating current reference value without static error.

[0187] Since the energy feedback value between MMC phase units under asymmetric AC power grid is w αΣ and w βΣ In addition to the DC quantity, it also contains a 2nd frequency fluctuation component. In order to eliminate the impact of the fluctuation component on the control system, a 100Hz notch filter needs to be added to the feedback link to eliminate the impact of the 2nd frequency fluctuation component on the power grid system.

[0188] The specific steps for constructing the energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition are as follows:

[0189] According to the dynamic equation of the DC quantity in the difference between the energy stored in the second upper and lower bridge arm capacitors under the condition of asymmetric fault in the AC grid (Equation (25)), the formula is obtained:

[0190]

[0191] According to formula (28), the fundamental frequency AC circulating current can be used to balance the energy of the capacitor between the upper and lower bridge arms. There are three controlled quantities and four control quantities. Therefore, the reference value setting of the control quantity is not unique. A constraint condition needs to be introduced: the reactive interaction between the injected fundamental frequency positive sequence circulating current and the AC side positive sequence voltage is zero to ensure the high efficiency operation of the MMC, that is:

[0192]

[0193] Define u difα 、u difβ and u dif0 is the second auxiliary control input. The positive and negative sequence power feedforward method is used to compensate for the disturbance component generated by the MMC. Combining equations (28) and (29), the fundamental frequency AC circulating current reference value is obtained:

[0194]

[0195] According to equations (28) and (30), the PI regulator is used for regulation control, and the design is as shown in the following: Figure 6 The energy balance control model between the upper and lower bridge arms under the AC grid asymmetric fault condition shown in the figure can track the fundamental frequency AC circulating current reference value without static error.

[0196] Since the energy feedback value between phase units in the asymmetric AC power grid w αΔ 、w βΔ and w 0Δ In addition to the DC quantity, the energy also contains fundamental frequency fluctuation components. To eliminate the impact of fundamental frequency fluctuation components on the control system, a 50Hz notch filter is added to the feedback link of the energy balance control model between the upper and lower bridge arms to eliminate the impact of fundamental frequency fluctuation components on the power grid system.

[0197] Using the method described in the present invention to optimize and control the MMC under an asymmetric AC power grid can completely suppress the negative-sequence AC-side current, while also ensuring that the DC-side voltage and current do not experience double-frequency fluctuations, allowing the MMC to act as a fluctuating power "firewall" between the AC and DC systems. At the same time, based on the dynamic equations of the MMC submodule capacitor voltage under asymmetric AC power grid fault conditions, the present invention designs a capacitor voltage optimization control model based on disturbance power feedforward compensation, which can effectively improve the internal and external dynamic characteristics of the MMC under fault conditions.

[0198] The DC circulating current output by the energy balance control model between the MMC phase units and the fundamental frequency AC circulating current output by the energy balance control model between the upper and lower bridge arms are input into the bridge arm circulating current control system to track the bridge arm circulating current reference value without static error to suppress the double frequency circulating current inside the MMC.

[0199] As attached Figure 8 As shown, the DC loop current i output by the MMC phase unit energy balance control model is cirαdcref and i cirβdcref And the AC circulating current output by the energy balance control module between the upper and lower bridge arms and After making the difference, they are used as the input of the bridge arm loop controller (a controller based on proportional-integral-resonance (PIR) regulator). The bridge arm loop controller finally outputs the loop suppression reference voltage e cirαref and e cirβref ;

[0200] Set the circulating current suppression reference voltage e cirαref and e cirβref Converted into the three-phase circulating current suppression reference voltage e in the three-phase stationary coordinate system ciraref 、e cirbref and e circref ;

[0201] When the MMC is a constant power controlled MMC, the DC voltage is controlled by the MMC total energy balance control method, and the AC current is controlled by a combination of a phase-locked loop and power control. Figure 13 As shown;

[0202] When the MMC is a constant voltage (DC side voltage) controlled MMC, the DC voltage is controlled by the DC voltage control method under the normal working condition of the AC grid, and the AC current is controlled by a combination of a phase-locked loop and power control, as shown in the attached figure. Figure 14 As shown;

[0203] When the MMC is a constant power controlled MMC, the specific steps for controlling the DC voltage using the MMC total energy balance control method are as follows:

[0204] According to the dynamic equation of the DC amount in the sum of the energy stored in the second upper and lower bridge arm capacitors under the AC grid asymmetric fault condition (the third equation in Equation 23), the formula is obtained:

[0205]

[0206] Where u sum0 Defined as the third auxiliary control input;

[0207] For the MMC with constant power control, the DC side current can be used to control the balance of total energy. At this time, the power generated by the positive and negative sequence AC voltage and current can be regarded as the disturbance component. By defining the third auxiliary control input u sum0 , the positive and negative sequence power feedforward method is used to compensate for the disturbance component generated by the MMC, thereby obtaining the DC side current reference value:

[0208]

[0209] According to formulas (31) and (32), the PI regulator is used for regulation control and the design is as shown in the attached figure. Figure 3 The total energy balance control block diagram of the constant power control MMC under the asymmetric AC grid is shown in the attached figure. Similarly, the total energy balance control block diagram of the constant voltage control MMC under the asymmetric AC grid is shown in the attached figure. Figure 4 As shown, the DC side current reference value i dcref Tracking without static error, thus achieving control of DC voltage;

[0210] According to the DC side current reference value i dcref The DC voltage reference value e can be calculated dcref ;

[0211] When the MMC is a constant power or constant voltage controlled MMC, the specific steps for controlling the AC current by combining a phase-locked loop and power control are as follows:

[0212] As attached Figure 10 The block diagram of the second-order generalized integrator (SOGI) is shown in the figure, and the two output closed-loop transfer functions of the second-order generalized integrator are obtained:

[0213]

[0214] From formula (33), we can see that Hd(s) shows the characteristics of a frequency selector, which can track signals with frequencies of ±ω; H q (s) shows the characteristics of a low-pass filter, which can output the signal with a frequency of +ω after π / 2 lag and output the signal with a frequency of -ω before π / 2 lag. k determines the bandwidth of the closed-loop system and is usually taken as

[0215] From Equation (33), we can see that the output du of SOGI has exactly the same amplitude and phase angle as the input signal u, while the output qu has the same amplitude as the input signal u, with a phase lag of π / 2. qu and u always maintain an orthogonal relationship and are independent of the parameter k.

[0216] As attached Figure 10 As shown, the outputs du and qu of the second-order generalized integrator are algebraically operated to obtain the positive sequence component of the AC side voltage of the receiving converter station. and Negative sequence component and As attached Figure 11 As shown; the positive sequence component and The input to the PLL can track the frequency ω of the positive sequence grid voltage, and then feed it back to the SOGI, so that the SOGI has frequency adaptive characteristics, thus obtaining the SOGI-based phase-locked loop structure block diagram, as shown in the attached figure. Figure 9 As shown;

[0217] Then, according to the positive sequence component of the AC side voltage of the receiving converter station and Negative sequence component and The calculation process of the positive sequence current reference value and the negative sequence current reference value is as follows:

[0218] Construct the MMC average value model under asymmetric fault AC power grid, as shown in the attached Figure 2 As shown in the figure, since there is no zero-sequence voltage and zero-sequence current on the valve side, the voltage and current on the AC side of the MMC can be expressed as:

[0219]

[0220] Where: h = 1, 2, 3, and are the positive and negative sequence voltage amplitudes on the AC side, θ + and θ - are the initial phases of the positive and negative sequence voltages on the AC side, and are the positive and negative sequence current amplitudes on the AC side, φ + and φ - are the initial phases of the positive and negative sequence currents on the AC side, respectively, and ω is the frequency of the positive sequence grid voltage;

[0221] According to Kirchhoff's voltage law and superposition theorem, the dynamic equation of the AC side current is:

[0222]

[0223] Transforming Equation (35) into the two-phase stationary αβ coordinate system, we can obtain:

[0224]

[0225] Where: and and and are the α and β axis components of the MMC positive sequence AC internal potential, AC side voltage, and AC side current, respectively. and and and They are the α and β axis components of the negative sequence AC internal potential, AC side voltage and AC side current of the MMC respectively;

[0226] Transforming Equation (35) into the dual synchronous rotating dq coordinate system, we can obtain:

[0227]

[0228] Where: and and and are the dq axis components of the MMC AC internal potential, AC side voltage, and AC side current in the positive sequence rotating coordinate system, respectively. and and and are the dq axis components of the MMC AC internal potential, AC side voltage and AC side current in the negative sequence rotating coordinate system respectively;

[0229] Combining Equations (35) and (39), the space vectors of the AC side voltage and current can be expressed as:

[0230]

[0231] According to the instantaneous power theory, the instantaneous active and reactive power of an asymmetric AC grid can be expressed as:

[0232] p(t)=P0+P c2 cos(2ωt)+P s2 sin(2ωt)(42)

[0233] q(t)=Q0+Q c2 cos(2ωt)+Q s2 sin(2ωt)(43)

[0234] Where: p(t) and q(t) are the instantaneous active and reactive power of the asymmetric AC grid respectively; P0 and Q0 are the average values ​​of the instantaneous active and reactive power respectively; P c2 and P s2 are the cosine and sine amplitudes of the instantaneous active power respectively; Q c2 and Qs2 are the cosine and sine amplitudes of the instantaneous reactive power, respectively;

[0235] Substituting equations (35) to (41) into equations (42) and (43), we can obtain the instantaneous power expression of the AC grid:

[0236]

[0237] The converter in an asymmetric AC grid usually has three control objectives: (1) suppressing negative sequence current control; (2) suppressing active power double frequency fluctuation control; and (3) suppressing reactive power double frequency fluctuation control. According to the AC grid instantaneous power expression shown in equation (44), to achieve the above control objectives, the AC side current reference value should be set as:

[0238]

[0239] Where r = 0 corresponds to target (1), r = -1 corresponds to target (2), and r = 1 corresponds to target (3). D1 and D2 can be expressed as:

[0240]

[0241] Under the asymmetric AC grid, if the MMC adopts any of the three control strategies mentioned above, its DC side will not have double frequency fluctuations. However, as a voltage source converter, the MMC only outputs positive sequence fundamental voltage during normal operation. When there are negative sequence voltage components and zero sequence voltage components in the AC bus of the converter station, if the MMC does not provide the corresponding back electromotive force, a large negative sequence current component will be generated on the valve side of the MMC. After superposition with the positive sequence current component and the DC current component, it may greatly exceed the current tolerance range of the power device, thus seriously threatening the safe operation of the MMC. Therefore, in order to make the MMC transmit as much active power as possible under the asymmetric grid and meet the application scenarios with special requirements for the grid-connected negative sequence current, the control target of the MMC can be set to target (1), that is, to suppress negative sequence current control.

[0242] Therefore, in order to achieve the control goal of suppressing negative sequence current, according to formula (44), the reference value of the AC side current is:

[0243]

[0244] The reference value of the AC side current The reference value of the AC side current Transformed to the dual synchronous rotating αβ coordinate system, the AC side current reference value in the dual synchronous rotating αβ coordinate system can be obtained

[0245] According to the AC side current reference value in the dual synchronous rotating αβ coordinate system The AC differential mode voltage e can be calculated αref and e βref , the AC differential mode voltage e αref and e βref Convert to the three-phase stationary coordinate system to obtain the three-phase AC differential mode voltage e in this coordinate system aref 、e bref and e cref ;

[0246] According to the three-phase AC differential mode voltage e aref 、e bref and e cref , DC voltage reference value e dcref And the three-phase circulating current suppression reference voltage e ciraref 、e cirbref and e circref Calculate the three-phase voltage u of the upper bridge arm paref 、u pbref and u pcref And the three-phase voltage u of the lower bridge arm naref 、u nbref and u ncref :

[0247]

[0248] u naref =e dcref +e aref +e ciraref (51)

[0249] u nbref =e dcref +e bref +e cirbref (52)

[0250] u ncref =e dcref +e cref +e circref (53)

[0251] u paref is the a-phase voltage of the upper bridge arm, u pbref is the b-phase voltage of the upper bridge arm, u pcref is the c-phase voltage of the upper bridge arm, u naref is the a-phase voltage of the lower bridge arm, u nbref is the b-phase voltage of the lower bridge arm, u ncref is the c-phase voltage of the lower bridge arm;

[0252] Then, the above 6 voltages are controlled by using the submodule voltage balancing control method based on voltage sorting, as shown in the attached figure. Figure 12 As shown:

[0253] The submodule voltage balancing control principle based on voltage sorting is shown in the attached Figure 12 As shown in the figure, this method mainly determines which submodules need to be put into operation based on the current submodule voltage and the direction of the bridge arm current. When the bridge arm current flows in the positive direction, the bridge arm current will charge the capacitors of the submodules put into operation; otherwise, the submodules put into operation will be discharged. When the number of submodules required to be put into operation is N ij (i=p,n), if the bridge arm current i ij >0, the N with the lowest voltage on the bridge arm ij If all sub-modules are put into operation and all other sub-modules are removed, the capacitors of the low-voltage sub-modules put into operation will be charged and the voltage will rise, while the voltage of the removed sub-modules will remain unchanged. Similarly, if the bridge arm current i ij <0, the N with the highest voltage on the bridge arm ij If all submodules are put into operation and all other submodules are removed, the capacitors of the high-voltage submodules put into operation will discharge and the voltage will drop, while the voltage of the removed submodules will remain unchanged. In this way, the voltages of all submodules in the same bridge arm can be kept almost equal within a certain range.

[0254] It should be noted that if carrier phase shift modulation is used, the number of submodules N ij It needs to be generated indirectly, that is, the number of sub-modules put into operation is equal to the number of times the modulation wave exceeds the carrier at the same time. Since for each sub-module, when the modulation wave exceeds the carrier of the sub-module, the sub-module is put into operation, therefore, for a bridge arm, the number of sub-modules required to be put into operation is the number of times the modulation wave exceeds all phase-shifted carriers. The number of sub-modules generated in this way participates in the selection and sorting to generate the pulse signal, which can not only ensure that the number of input levels is the same as that generated by ordinary CPS-PWM, but also ensure that the sub-module voltages participate in the selection and sorting and remain almost equal within a range;

[0255] The optimized control method for MMCs in asymmetric AC power grids of this application can completely suppress negative-sequence AC-side current while ensuring that DC-side voltage and current do not experience double-frequency fluctuations, enabling the MMC to act as a fluctuating power "firewall" between the AC and DC systems. Furthermore, based on the established precise capacitor energy dynamic model of the MMC in asymmetric AC power grids, a capacitor voltage optimization control strategy based on disturbance power feedforward compensation is designed, which can improve the internal and external dynamic characteristics of the MMC under fault conditions.

[0256] In order to verify the effectiveness of the method of the present invention for MMC dynamic control, a system simulation model was built based on PSCAD, as shown in the attached figure. Figure 7 As shown, attached Figure 7The simulation block diagram of the wind farm transmission system via MMC-HVDC is shown in Table 1, Table 2 and Table 3 respectively.

[0257] The simulation system mainly includes two wind farms, each with a rated capacity of 500MW. Both wind farms use a single full-power wind turbine with a rated power of 6.25MW as the equivalent switch model. The topology adopts a symmetrical single-pole connection method, and the grounding method adopts a DC pole line through a large resistance grounding method. The matching transformers of the MMCs at both ends adopt a Y / D connection method.

[0258] Table 1 Receiving MMC electrical parameters

[0259]

[0260]

[0261] Table 2 Electrical parameters of the sending end MMC

[0262] parameter symbol Numerical Rated apparent power <![CDATA[S n ]]> 1000MVA Rated active power <![CDATA[P n ]]> 1000MW Rated reactive power <![CDATA[Q n ]]> 0Mvar Rated DC voltage <![CDATA[U dc ]]> ±320kV Rated AC voltage <![CDATA[V ac ]]> 230 / 333.13kV Rated AC current <![CDATA[I N ]]> 2.45kA AC system frequency f 50Hz Number of bridge arm submodules N 412 Number of redundant submodules <![CDATA[N r ]]> 31 Submodule capacitance C 11mF Submodule capacitor voltage <![CDATA[V c ]]> 2kV Bridge arm inductor L 80mH DC side current limiting reactor <![CDATA[L dc ]]> 30mH Transformer leakage inductance <![CDATA[L ac ]]> 14％ Transformer capacity <![CDATA[S T ]]> 1400MVA

[0263] Table 3 Parameters of the receiving MMC controller

[0264] parameter Numerical DC side voltage controller <![CDATA[k p =0.01k i =0.4]]> AC side current controller <h2 style=";text-align:left;direction:ltr"><![CDATA[k <h2 style=";text-align:left;direction:ltr"> p <h2 style=";text-align:left;direction:ltr"> =75k<h2 style=";text-align:left;direction:ltr"> i <h2 style=";text-align:left;direction:ltr"> 8000<h2 style=";text-align:left;direction:ltr"> Bridge arm circulation controller <![CDATA[k p =70k i =10000]]> Reactive power controller <![CDATA[k p =0.01k i =0.25]]>

[0265] According to the simulation results, the dynamic performance of the receiving-end MMC's AC side current, active power, reactive power, DC side voltage, and DC side current and bridge arm current under fault transients has been greatly improved. The oscillation and overshoot during the transient process have been effectively suppressed, and the stabilization speed has been greatly improved. At the same time, the balancing speed of the sub-module capacitor voltage has also been greatly improved. During the fault, the DC value of the sub-module capacitor voltage remains unchanged, is not affected by the fault condition, and is decoupled from the DC side bus voltage. This shows that the MMC optimization design based on the energy control strategy can enable the MMC to have better fault ride-through characteristics.

[0266] Example 2

[0267] Figure 15 1 is a system schematic diagram of an MMC dynamic control device based on bridge arm energy control according to an exemplary embodiment, the device comprising:

[0268] Dynamic equation acquisition module 1: used to obtain the dynamic equation of the capacitor voltage of the MMC submodule under the AC grid asymmetric fault condition;

[0269] Model construction module 2: used to construct an energy balance control model between MMC phase units under the AC power grid asymmetric fault condition and an energy balance control model between upper and lower bridge arms under the AC power grid asymmetric fault condition by defining auxiliary control inputs and using power feedforward compensation for disturbance components based on the dynamic equation of the MMC submodule capacitor voltage under the AC power grid asymmetric fault condition;

[0270] Tracking module 3: used to track the DC circulating current reference value without static error through the MMC inter-phase unit energy balance control model under the AC power grid asymmetric fault condition; and to track the fundamental frequency AC circulating current reference value without static error through the energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition;

[0271] Circulating current suppression reference voltage acquisition module 4: used to obtain a circulating current suppression reference voltage based on the DC circulating current output by the energy balance control model between the MMC phase units under the asymmetric fault condition of the AC power grid, and the AC circulating current output by the energy balance control model between the upper and lower bridge arms under the asymmetric fault condition of the AC power grid;

[0272] Three-phase circulating current suppression reference voltage acquisition module 5: used for converting the circulating current suppression reference voltage into a three-phase circulating current suppression reference voltage in a three-phase stationary coordinate system;

[0273] DC voltage control module 6: used to control the DC voltage by adopting the MMC total energy balance control method if the MMC control type is constant power control MMC; if the MMC control type is constant voltage MMC, it controls the DC voltage by adopting the DC voltage control method under the normal working condition of the AC grid;

[0274] DC voltage reference value acquisition module 7: used to acquire a DC side current reference value according to the DC voltage control process; and acquire a DC voltage reference value according to the DC side current reference value;

[0275] AC current control module 8: used to control the AC current by combining phase-locked loop and power control if the control type of the MMC is constant power or constant voltage MMC;

[0276] Three-phase AC differential mode voltage acquisition module 9: used to obtain an AC side current reference value according to the AC current control process; obtain an AC differential mode voltage according to the AC side current reference value, and convert the AC differential mode voltage into a three-phase stationary coordinate system to obtain a three-phase AC differential mode voltage;

[0277] Bridge arm voltage control module 10: used to obtain the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm according to the three-phase AC differential mode voltage, the DC voltage reference value and the three-phase circulating current suppression reference voltage, and use the sub-module voltage balancing control method based on voltage sorting to control the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm.

[0278] Example 3:

[0279] This embodiment provides a storage medium, wherein the storage medium stores a computer program, and when the computer program is executed by a host controller, each step in the above method is implemented;

[0280] It is understandable that the storage medium mentioned above can be a read-only memory, a magnetic disk or an optical disk, etc.

[0281] It can be understood that the same or similar parts of the above embodiments can be referenced to each other, and the contents not described in detail in some embodiments can refer to the same or similar contents in other embodiments.

[0282] It should be noted that, in the description of the present invention, the terms "first", "second", etc. are used for descriptive purposes only and should not be understood as indicating or implying relative importance. In addition, in the description of the present invention, unless otherwise specified, the meaning of "plurality" is at least two.

[0283] Any process or method description in a flowchart or otherwise described herein may be understood to represent a module, segment or portion of code comprising one or more executable instructions for implementing the steps of a specific logical function or process, and the scope of the preferred embodiments of the present invention includes alternative implementations in which functions may be performed out of the order shown or discussed, including performing functions in a substantially simultaneous manner or in the reverse order depending on the functions involved, which should be understood by those skilled in the art to which the embodiments of the present invention pertain.

[0284] It should be understood that various parts of the present invention can be implemented using hardware, software, firmware, or a combination thereof. In the above-described embodiments, multiple steps or methods can be implemented using software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented using hardware, as in another embodiment, any one of the following technologies known in the art or a combination thereof can be used: a discrete logic circuit having a logic gate circuit for implementing a logic function on a data signal, an application-specific integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0285] Those skilled in the art will understand that all or part of the steps in the method of the above embodiment can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable storage medium. When the program is executed, it includes one or a combination of the steps of the method embodiment.

[0286] In addition, the functional units in the various embodiments of the present invention may be integrated into a single processing module, or each unit may exist physically separately, or two or more units may be integrated into a single module. The aforementioned integrated modules may be implemented in the form of hardware or in the form of software functional modules. If the integrated modules are implemented in the form of software functional modules and sold or used as independent products, they may also be stored in a computer-readable storage medium.

[0287] The storage medium mentioned above can be a read-only memory, a magnetic disk or an optical disk, etc.

[0288] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "examples," "specific examples," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0289] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. The MMC dynamic control method based on bridge arm energy control is characterized by: The method comprises: Obtain the dynamic equation of the capacitor voltage of the MMC submodule under the AC grid asymmetric fault condition; Based on the dynamic equation of the MMC submodule capacitor voltage under the AC power grid asymmetric fault condition, an energy balance control model between MMC phase units under the AC power grid asymmetric fault condition and an energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition are respectively constructed by defining an auxiliary control input and adopting a power feedforward compensation disturbance component; Tracking the DC circulating current reference value without static error through the MMC inter-phase unit energy balance control model under the AC grid asymmetric fault condition; tracking the fundamental frequency AC circulating current reference value without static error through the energy balance control model between the upper and lower bridge arms under the AC grid asymmetric fault condition; Obtaining a circulating current suppression reference voltage according to the DC circulating current output by the energy balance control model between the MMC phase units under the asymmetric fault condition of the AC power grid and the AC circulating current output by the energy balance control model between the upper and lower bridge arms under the asymmetric fault condition of the AC power grid; Converting the circulating current suppression reference voltage into a three-phase circulating current suppression reference voltage in a three-phase stationary coordinate system; If the control type of the MMC is a constant power control MMC, the DC voltage is controlled by the MMC total energy balance control method. If the control type of the MMC is a constant voltage MMC, the DC voltage is controlled by the DC voltage control method under the normal working conditions of the AC grid. Obtaining a DC side current reference value according to the DC voltage control process; obtaining a DC voltage reference value according to the DC side current reference value; If the control type of the MMC is a constant power or constant voltage MMC, the AC current is controlled by combining a phase-locked loop and power control; Acquire an AC side current reference value according to the AC current control process; acquire an AC differential mode voltage according to the AC side current reference value, and convert the AC differential mode voltage into a three-phase stationary coordinate system to acquire a three-phase AC differential mode voltage; The three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm are obtained according to the three-phase AC differential mode voltage, the DC voltage reference value and the three-phase circulating current suppression reference voltage, and the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm are controlled by a sub-module voltage balancing control method based on voltage sorting.

2. The method according to claim 1, characterized in that The dynamic equation for obtaining the capacitor voltage of the MMC submodule under the AC grid asymmetric fault condition includes: Obtaining the dynamic equation of the MMC submodule capacitor voltage under the AC grid asymmetric fault condition according to the dynamic equation of the MMC submodule capacitor voltage under the AC grid normal working condition and the space vector representation of the AC side voltage and current under the AC grid asymmetric fault working condition; The dynamic equation of the MMC submodule capacitor voltage under normal operating conditions of the AC power grid includes: a dynamic equation of the DC amount in the sum of the energy stored in the first upper and lower bridge arm capacitors and a dynamic equation of the DC amount in the difference of the energy stored in the first upper and lower bridge arm capacitors; The dynamic equation of the MMC submodule capacitor voltage under the AC grid asymmetric fault condition includes: a dynamic equation of the DC amount in the sum of the energy stored in the second upper and lower bridge arm capacitors and a dynamic equation of the DC amount in the difference of the energy stored in the second upper and lower bridge arm capacitors.

3. The method according to claim 2, characterized in that The space vector representation of the AC side voltage and current under the AC grid asymmetric fault condition includes: Construct the average value model of MMC under AC grid asymmetric fault conditions; Obtaining expressions for the voltage and current on the AC side of the MMC according to the average value model of the MMC under the asymmetric fault condition of the AC power grid; According to the average value model of the MMC under the asymmetric fault condition of the AC power grid, the dynamic equation of the AC side current is obtained by Kirchhoff's voltage law and superposition theorem; Transforming the expressions of the voltage and current on the AC side of the MMC into a two-phase stationary αβ coordinate system to obtain the positive sequence αβ axis component and the negative sequence αβ axis component respectively; Transforming the expressions of the voltage and current on the AC side of the MMC into a dual synchronous rotating dq coordinate system to obtain the positive sequence dq axis components and the negative sequence dq axis components respectively; The space vector representation of the AC side voltage and current under the asymmetric fault condition of the AC grid is obtained according to the expressions of the MMC AC side voltage and current, the dynamic equation of the AC side current, the positive-sequence αβ-axis component, the negative-sequence αβ-axis component, the positive-sequence dq-axis component, and the negative-sequence dq-axis component.

4. The method according to claim 3, characterized in that The dynamic equation of the capacitor voltage of the MMC submodule under normal operating conditions of the AC grid includes: Obtain the relationship between the AC current and the common-mode current of the bridge arm, and obtain the reference voltage of the MMC submodule; Obtaining the instantaneous power of the upper and lower bridge arm cascade submodules according to the relationship between the AC current and the bridge arm common mode current and the reference voltage of the MMC submodule; Obtaining the sum of the instantaneous powers of the upper and lower bridge arm cascade submodules and the difference between the instantaneous powers of the upper and lower bridge arm cascade submodules according to the instantaneous powers of the upper and lower bridge arm cascade submodules; Obtaining a DC amount in the sum and difference of the energy stored in the upper and lower bridge arm capacitors according to the sum of the instantaneous powers of the upper and lower bridge arm cascade submodules and the difference between the instantaneous powers of the upper and lower bridge arm cascade submodules; The DC amount in the sum of the energy stored in the upper and lower bridge arm capacitors and the difference in the energy stored in the upper and lower bridge arm capacitors is converted to the αβ0 coordinate system, and the dynamic equation of the DC amount in the sum of the energy stored in the first upper and lower bridge arm capacitors and the dynamic equation of the DC amount in the difference in the energy stored in the first upper and lower bridge arm capacitors are respectively obtained through algebraic operations.

5. The method according to claim 4, characterized in that The acquisition of the dynamic equation of the DC amount in the sum of the energy stored in the second upper and lower bridge arm capacitors includes: Substituting the space vector representation of the AC side voltage and current under the AC grid asymmetric fault condition into the dynamic equation of the DC quantity in the sum of the energy stored in the first upper and lower bridge arm capacitors, to obtain the dynamic equation of the DC quantity in the sum of the energy stored in the second upper and lower bridge arm capacitors; The dynamic equation for obtaining the DC amount in the difference between the energy stored in the second upper and lower bridge arm capacitors includes: Substituting the space vector representation of the AC side voltage and current under the asymmetric fault condition of the AC power grid into the dynamic equation of the DC quantity in the difference between the energy stored in the first upper and lower bridge arm capacitors, the dynamic equation of the DC quantity in the difference between the energy stored in the second upper and lower bridge arm capacitors is obtained.

6. The method according to claim 5, characterized in that The method of constructing an MMC inter-phase unit energy balance control model under an asymmetric AC grid fault condition by defining an auxiliary control input and adopting power feedforward compensation for a disturbance component based on the dynamic equation of the MMC submodule capacitor voltage under the AC grid asymmetric fault condition includes: Obtaining a first auxiliary control input quantity expression according to a dynamic equation of a DC quantity in the sum of the energy stored in the second upper and lower bridge arm capacitors; By defining the first auxiliary control input, the positive and negative sequence power feedforward method is used to compensate the MMC positive and negative sequence AC voltages and the power generated by the AC, and the DC circulating current reference value expression is obtained; Obtaining an energy balance control model between MMC phase units under the asymmetric fault condition of the AC power grid through the first auxiliary control input expression and the DC circulating current reference value expression; The energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition is constructed by defining an auxiliary control input and adopting power feedforward compensation for the disturbance component based on the dynamic equation of the MMC submodule capacitor voltage under the AC power grid asymmetric fault condition, including: Obtaining a second auxiliary control input quantity expression according to a dynamic equation of a DC quantity in the difference between the energy stored in the second upper and lower bridge arm capacitors; Set the limiting conditions for the reference value of the control quantity; By defining a second auxiliary control input, using positive and negative sequence power feedforward to compensate for the disturbance component generated by the MMC, and combining the expression of the second auxiliary control input and the limiting conditions of the control reference value, the expression of the fundamental frequency AC circulating current reference value is obtained; The energy balance control model between the upper and lower bridge arms under the asymmetric fault condition of the AC power grid is obtained through the second auxiliary control input expression and the fundamental frequency AC circulating current reference value expression.

7. The method according to claim 6, characterized in that The obtaining of the DC side current reference value according to the DC voltage control process includes: Obtaining a third auxiliary control input quantity expression according to a dynamic equation of a DC quantity in the sum of the energy stored in the second upper and lower bridge arm capacitors; By defining the third auxiliary control input, the positive and negative sequence power feedforward method is used to compensate for the disturbance component generated by the MMC, and the DC side current reference value expression is obtained. The DC side current reference value is obtained according to the third auxiliary control input quantity expression and the DC side current reference value expression.

8. The method according to claim 7, characterized in that The obtaining of the AC side current reference value according to the AC current control process includes: Obtain the two output closed-loop transfer functions of the second-order generalized integrator; Performing algebraic operations on the two outputs of the second-order generalized integrator to obtain the positive sequence component and the negative sequence component of the AC side voltage of the receiving converter station; A positive-sequence current reference value and a negative-sequence current reference value are respectively obtained according to the positive-sequence component and the negative-sequence component of the AC side voltage of the receiving-end converter station.

9. The method according to claim 8, characterized in that Obtaining a positive-sequence current reference value and a negative-sequence current reference value according to a positive-sequence component and a negative-sequence component of the AC side voltage of the receiving-end converter station includes: Constructing an MMC average value model under an asymmetric fault AC power grid, and obtaining expressions for AC side voltage and current according to the MMC average value model; The dynamic equation of the AC side current is constructed using Kirchhoff's voltage law and superposition theorem; Transforming the dynamic equation of the AC side current into a two-phase static αβ coordinate system, and obtaining the positive and negative sequence αβ axis components of the AC side according to the positive sequence component and the negative sequence component; The dynamic equation of the AC side current is transformed into the dual synchronous rotating dq coordinate system to obtain the positive and negative sequence of the AC side respectively. d q Axis component; Obtaining space vector expressions of the voltage and current on the AC side according to the dynamic equation of the AC side current, the positive and negative sequence αβ axis components, and the positive and negative sequence dq axis components; Obtain the expressions of instantaneous active and reactive power of asymmetric AC grid according to instantaneous power theory; Substituting the dynamic equation of the AC side current, the positive and negative sequence αβ axis components, the positive and negative sequence dq axis components, and the space vector expressions of the voltage and current on the AC side into the expressions of the instantaneous active and reactive power of the asymmetric AC power grid to obtain the instantaneous power expression of the AC power grid; A positive sequence current reference value and a negative sequence current reference value are obtained according to the instantaneous power expression of the AC power grid.

10. The method according to claim 9, characterized in that The acquiring of the AC differential mode voltage according to the AC side current reference value includes: Transforming the AC side current reference value into a dual synchronous rotating αβ coordinate system to obtain the AC side current reference value in the dual synchronous rotating αβ coordinate system; The AC differential mode voltage is obtained according to the AC side current reference value in the dual synchronous rotating αβ coordinate system.

11. The MMC dynamic control device based on bridge arm energy control is characterized in that: The device comprises: Dynamic equation acquisition module: used to obtain the dynamic equation of the capacitor voltage of the MMC submodule under the condition of asymmetric fault of the AC grid; A model construction module is used to construct an energy balance control model between MMC phase units under the AC power grid asymmetric fault condition and an energy balance control model between upper and lower bridge arms under the AC power grid asymmetric fault condition by defining an auxiliary control input and using power feedforward compensation for the disturbance component based on the dynamic equation of the MMC submodule capacitor voltage under the AC power grid asymmetric fault condition; Tracking module: used to track the DC circulating current reference value without static error through the MMC inter-phase unit energy balance control model under the AC power grid asymmetric fault condition; and to track the fundamental frequency AC circulating current reference value without static error through the energy balance control model between the upper and lower bridge arms under the AC power grid asymmetric fault condition; A circulating current suppression reference voltage acquisition module is configured to acquire a circulating current suppression reference voltage based on the DC circulating current output by the energy balance control model between the MMC phase units under the asymmetric fault condition of the AC power grid, and the AC circulating current output by the energy balance control model between the upper and lower bridge arms under the asymmetric fault condition of the AC power grid; Three-phase circulating current suppression reference voltage acquisition module: used to convert the circulating current suppression reference voltage into the three-phase circulating current suppression reference voltage in the three-phase stationary coordinate system; DC voltage control module: If the MMC control type is constant power control, the DC voltage is controlled by adopting the MMC total energy balance control method; if the MMC control type is constant voltage MMC, the DC voltage is controlled by adopting the DC voltage control method under the normal working condition of the AC grid; A DC voltage reference value acquisition module is configured to acquire a DC side current reference value according to the DC voltage control process; and acquire a DC voltage reference value according to the DC side current reference value. AC current control module: used to control the AC current by combining phase-locked loop and power control if the control type of the MMC is constant power or constant voltage MMC; A three-phase AC differential mode voltage acquisition module is used to obtain an AC side current reference value according to the AC current control process; obtain an AC differential mode voltage according to the AC side current reference value, and convert the AC differential mode voltage into a three-phase stationary coordinate system to obtain a three-phase AC differential mode voltage; Bridge arm voltage control module: used to obtain the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm according to the three-phase AC differential mode voltage, DC voltage reference value and three-phase circulating current suppression reference voltage, and use the sub-module voltage balancing control method based on voltage sorting to control the three-phase voltage of the upper bridge arm and the three-phase voltage of the lower bridge arm.

12. A storage medium, characterized in that: The storage medium stores a computer program, and when the computer program is executed by the processor, each step of the MMC dynamic control method based on bridge arm energy control according to any one of claims 1 to 10 is implemented.

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