Modular multilevel matrix converter capacitor voltage control method and system

Abstract

The application provides a modular multilevel matrix converter capacitor voltage control method and system, and belongs to the field of power system relay protection. The method comprises the following steps: when an asymmetric fault occurs in a line, the negative sequence current of the fault line is suppressed to zero, and three-phase voltage and current at the low-frequency side and the power-frequency side are obtained, then the bridge arm power of each bridge arm is calculated, and the direct current component in the bridge arm power is extracted; the bridge arm power imbalance component is determined according to the direct current component, and then the circulating current compensation component is determined; the circulating current compensation component is converted into a circulating current reference value, the circulating current outer loop control of the modular multilevel matrix converter is carried out, the outer loop control of the power-frequency side and the low-frequency side is carried out, and the bridge arm voltage of the modular multilevel matrix converter is controlled according to the overall bridge arm voltage reference value, so that the capacitor voltage of the modular multilevel matrix converter is controlled under the asymmetric fault state, and the safe and stable operation of the modular multilevel matrix converter is ensured.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection, and in particular relates to a modular multilevel matrix converter capacitor voltage control method and system. Background Technology

[0002] Offshore wind power, as a renewable energy technology with open potential, boasts numerous advantages such as abundant resources and independence from onshore construction area limitations, attracting widespread attention in the global energy industry. In recent years, driven by the development goals of "carbon neutrality and carbon peaking," China has successively established large-scale offshore wind power development plans, gradually advancing towards medium- and long-term offshore wind power construction.

[0003] Currently, the most widely used and technologically mature grid connection methods for offshore wind power are high-voltage AC transmission and HVDC (high-voltage DC). However, with the increase in transmission distance, high-voltage AC transmission brings significant voltage deviation and losses, as well as the impact of line capacity increase. High-voltage DC transmission faces challenges such as fault current interruption, DC transformer difficulties, and the economic issues associated with building converter stations. LFTS (low-frequency transmission system), without changing the voltage level, reduces the transmission system frequency, decreases line reactance and charging power, reduces long-distance transmission losses, and increases line transmission capacity. Simultaneously, offshore wind turbines can directly generate low-frequency electricity, reducing the number of required converter stations and mitigating the economic impact of construction and maintenance costs.

[0004] High-voltage, high-capacity AC-AC converters are the core devices for frequency conversion and power transmission in offshore low-frequency power transmission systems. M3C (modular multilevel matrix converter) is better suited for high-voltage, high-capacity flexible low-frequency power transmission systems due to its advantages in power quality, reliability, and economy.

[0005] Current research on modular multilevel matrix converters (M3Cs) is mostly based on control strategies under steady-state operation, with limited research on fault ride-through characteristics and control strategies under fault conditions. Furthermore, there is a lack of research on the control of the internal capacitor voltage of the M3C under asymmetrical fault conditions.

[0006] Based on the above problems, there is an urgent need for a new control strategy for modular multilevel matrix converters under asymmetric fault conditions to ensure the safe and stable operation of modular multilevel matrix converters. Summary of the Invention

[0007] The purpose of this invention is to provide a method and system for controlling the capacitor voltage of a modular multilevel matrix converter, which can control the capacitor voltage of the modular multilevel matrix converter under asymmetrical fault conditions to ensure the safe and stable operation of the modular multilevel matrix converter.

[0008] To achieve the above objectives, the present invention provides the following solution:

[0009] A method for controlling the capacitor voltage of a modular multilevel matrix converter, wherein the two ends of the modular multilevel matrix converter are respectively connected to the power frequency side and the low frequency side, and the modular multilevel matrix converter includes multiple bridge arms; the method for controlling the capacitor voltage of the modular multilevel matrix converter includes:

[0010] When an asymmetrical fault occurs on a line, negative sequence control is added to the modular multilevel matrix converter to suppress the negative sequence current of the faulty line to zero, and to obtain the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side; the line with the asymmetrical fault is either the low-frequency side line or the power frequency side line.

[0011] Based on the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side, calculate the bridge arm power of each bridge arm of the modular multilevel matrix converter, and extract the DC component of each bridge arm power.

[0012] The unbalanced component of the bridge arm power is determined based on the DC component of the power of each bridge arm.

[0013] Determine the circulating current compensation component based on the unbalanced component of the bridge arm power;

[0014] The circulation compensation component is converted into a circulation reference value under the dual αβ0 transformation;

[0015] Based on the circulating current reference value, the modular multilevel matrix converter is subjected to circulating current outer loop control, and outer loop control is performed on the power frequency side and the low frequency side respectively, and the overall reference value of the bridge arm voltage is determined.

[0016] Based on the overall reference value of the bridge arm voltage, the bridge arm voltage of the modular multilevel matrix converter is controlled to control the capacitor voltage of the modular multilevel matrix converter under asymmetrical fault conditions.

[0017] To achieve the above objectives, the present invention also provides the following solution:

[0018] A modular multilevel matrix converter capacitor voltage control system includes:

[0019] The negative sequence control unit, connected to the modular multilevel matrix converter, is used to add negative sequence control to the modular multilevel matrix converter when an asymmetrical fault occurs in the line, so as to suppress the negative sequence current of the faulty line to zero, and obtain the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side; the line where the asymmetrical fault occurs is either the low-frequency side line or the power frequency side line.

[0020] The power calculation unit, connected to the negative sequence control unit, is used to calculate the bridge arm power of each bridge arm of the modular multilevel matrix converter based on the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side, and to extract the DC component in the power of each bridge arm.

[0021] An unbalanced component determination unit, connected to the power calculation unit, is used to determine the unbalanced component of the bridge arm power based on the DC component in the power of each bridge arm.

[0022] The compensation component determination unit, connected to the unbalanced component determination unit, is used to determine the circulating current compensation component based on the unbalanced component of the bridge arm power.

[0023] A circulation reference value determination unit, connected to the compensation component determination unit, is used to convert the circulation compensation component into a circulation reference value under the double αβ0 transformation.

[0024] A voltage reference value determination unit is connected to the circulating current reference value determination unit, the modular multilevel matrix converter, the power frequency side, and the low frequency side, respectively. It is used to perform circulating current outer loop control on the modular multilevel matrix converter according to the circulating current reference value, perform outer loop control on the power frequency side and the low frequency side respectively, and determine the overall reference value of the bridge arm voltage.

[0025] The voltage control unit is connected to the voltage reference value determination unit and the modular multilevel matrix converter respectively. It is used to control the bridge arm voltage of the modular multilevel matrix converter according to the overall reference value of the bridge arm voltage, so as to control the capacitor voltage of the modular multilevel matrix converter under asymmetrical fault conditions.

[0026] According to specific embodiments provided by the present invention, the following technical effects are disclosed: When an asymmetrical fault occurs in a line, negative sequence control is added to the modular multilevel matrix converter to suppress the negative sequence current of the faulty line to zero, and the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side are obtained; based on the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side, the bridge arm power of each bridge arm of the modular multilevel matrix converter is calculated, and the DC component in the power of each bridge arm is extracted; the bridge arm power is determined based on the DC component in the power of each bridge arm. The unbalanced power component is identified; the circulating current compensation component is determined based on the unbalanced power component of the bridge arm; the circulating current compensation component is converted into a circulating current reference value under dual αβ0 transformation; the circulating current outer loop control of the modular multilevel matrix converter is performed based on the circulating current reference value, and the outer loop control is performed on the power frequency side and the low frequency side respectively, and the overall reference value of the bridge arm voltage is determined; the bridge arm voltage of the modular multilevel matrix converter is controlled based on the overall reference value of the bridge arm voltage, thereby realizing the control of the capacitor voltage of the modular multilevel matrix converter under asymmetrical fault conditions, thus ensuring the safe and stable operation of the modular multilevel matrix converter. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a structural diagram of a low-frequency power transmission system based on a modular multilevel matrix converter;

[0029] Figure 2 This is a schematic diagram of the modular multilevel matrix converter.

[0030] Figure 3 This is a flowchart of the modular multilevel matrix converter capacitor voltage control method of the present invention;

[0031] Figure 4 This is a schematic diagram of the capacitor voltage control process;

[0032] Figure 5 A comparison chart of the average voltage of the sub-module for a single-phase ground fault (phase a) on a low-frequency line;

[0033] Figure 6 A comparison chart of the average voltage of the sub-modules for two-phase ground faults (phases a and b) on a low-frequency line;

[0034] Figure 7 A comparison chart of the average voltage of the submodules for two-phase phase-to-phase faults (phases a and b) in low-frequency lines;

[0035] Figure 8 This is a schematic diagram of the modular multilevel matrix converter capacitor voltage control system of the present invention.

[0036] Symbol explanation:

[0037] Negative sequence control unit-1, power calculation unit-2, unbalanced component determination unit-3, compensation component determination unit-4, circulating current reference value determination unit-5, voltage reference value determination unit-6, voltage control unit-7. Detailed Implementation

[0038] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] The purpose of this invention is to provide a modular multilevel matrix converter capacitor voltage control method and system. Based on the control objective of suppressing the negative sequence current of the faulted line to zero, it can improve the safe and stable operation capability of wind farms under fault conditions and improve the stability of the power transmission system to a certain extent.

[0040] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0041] Example 1

[0042] In this embodiment, as Figure 1 As shown, the two ends of the modular multilevel matrix converter are connected to the power frequency side and the low frequency side, respectively. Figure 2 As shown, the modular multilevel matrix converter includes nine bridge arms, each consisting of a bridge arm reactor and multiple full-bridge modules connected in series. From the power frequency side, the modular multilevel matrix converter is divided into three sub-converters; from the low-frequency side, it is also divided into three sub-converters. In the figure, i... au i bu i cu i av i bv i cv i aw i bw i cw For the bridge arm currents of the 9 bridge arms, u au u bu u cu u av ubv u cv u aw u bw u cw For the bridge arm voltages of the 9 arms, O s For the neutral point of the power frequency side voltage, O r The low-frequency side voltage neutral point is used, and D1, D2, D3, and D4 are all IGBT anti-parallel diodes.

[0043] like Figure 3 As shown, this embodiment provides a method for controlling the capacitor voltage of a modular multilevel matrix converter, including:

[0044] S1: When an asymmetrical fault occurs on the line, negative sequence control is added to the modular multilevel matrix converter to suppress the negative sequence current of the faulty line to zero, and to obtain the three-phase voltage and current on the low-frequency side and the power frequency side. In other words, negative sequence control is added to the modular multilevel matrix converter under asymmetrical fault conditions. The line experiencing the asymmetrical fault is either the low-frequency side line or the power frequency side line.

[0045] When the line experiencing an asymmetrical fault is a low-frequency side line, the three-phase voltage on the low-frequency side is:

[0046]

[0047] Among them, e a e b e c For the three-phase voltage on the low-frequency side, ω l Let λ be the low-frequency side angular frequency, λ be the angle between the positive and negative sequences of phase a on the low-frequency side, t be the time, and e be the angular frequency. dl P e represents the d-axis component of the low-frequency positive sequence voltage. dl N This represents the d-axis component of the negative sequence voltage on the low-frequency side.

[0048] Before negative sequence current suppression is applied to the faulty line, the three-phase currents on the low-frequency side are:

[0049]

[0050] After suppressing the negative sequence current of the faulty line, the three-phase current on the low-frequency side is:

[0051]

[0052] Among them, i a i b i c For the three-phase current on the low-frequency side, i dl P i represents the d-axis component of the positive sequence current on the low-frequency side.ql P i represents the q-axis component of the positive sequence current on the low-frequency side. dl N i represents the d-axis component of the low-frequency side negative sequence current. ql N This represents the q-axis component of the negative sequence current on the low-frequency side.

[0053] The three-phase voltages on the power frequency side are:

[0054]

[0055] Among them, e u e v e w The three-phase voltage on the power frequency side, ω s e is the power frequency side angular frequency. ds P This represents the d-axis component of the positive sequence voltage on the power frequency side.

[0056] The three-phase currents on the power frequency side are:

[0057]

[0058] Among them, i u i v i w i represents the three-phase current on the power frequency side. ds P i represents the d-axis component of the positive sequence current on the power frequency side. qs P This represents the q-axis component of the positive sequence current on the power frequency side.

[0059] S2: Based on the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side, calculate the bridge arm power of each bridge arm of the modular multilevel matrix converter, and extract the DC component of each bridge arm power.

[0060] Specifically, the bridge arm power of each bridge arm of the modular multilevel matrix converter is calculated using the following formula:

[0061]

[0062] in, Let e ​​be the bridge arm power of bridge arm xy. x For the three-phase voltage on the low-frequency side, e y For the three-phase voltage on the power frequency side, i x For the three-phase current on the low-frequency side, i y Let x = a, b, c and y = u, v, w, where a, b, c are the three phases on the low-frequency side, and u, v, w are the three phases on the power frequency side.

[0063] The DC component of the power in each bridge arm is determined using the following formula:

[0064]

[0065] P au =P av =P aw ;

[0066]

[0067] P bu =P bv =P bw ;

[0068]

[0069] P cu =P cv =P cw ;

[0070] Among them, P xy Let x be the DC component of the arm power of the bridge arm xy, where x = a, b, c, y = u, v, w, e dl P i represents the d-axis component of the positive sequence voltage on the low-frequency side. dl P e represents the d-axis component of the positive sequence current on the low-frequency side. ds P i represents the d-axis component of the positive sequence voltage on the power frequency side. ds P e represents the d-axis component of the positive sequence current on the power frequency side. dl N i represents the d-axis component of the low-frequency side negative sequence voltage. ql P λ represents the q-axis component of the positive sequence current on the low-frequency side, and λ is the angle between the positive and negative sequence currents of phase a on the low-frequency side.

[0071] S3: Determine the unbalanced component of the bridge arm power based on the DC component of the power in each bridge arm. That is, analyze the unbalanced component of the bridge arm power caused by the unbalanced state based on the DC component of the power in each bridge arm.

[0072] Specifically, firstly, a double αβ transformation is performed on the DC component of the power in each bridge arm to obtain the transformed DC component. Then, the unbalanced power components of the bridge arm are determined based on the transformed DC components. The unbalanced power components of the bridge arm include the unbalanced power components along the α-axis and the unbalanced power components along the β-axis in the two-phase stationary coordinate system.

[0073] The DC component of the power in each bridge arm is transformed using the following formula:

[0074]

[0075]

[0076] Among them, T abc / αβ0 Let P be the equal power transformation matrix from a three-phase stationary coordinate system to a two-phase stationary coordinate system, where T represents the transpose and P is the power transformation matrix. αα P βα P αβ P ββ This indicates a power imbalance between the bridge arms within the M3C sub-converter, P 0α P 0β P α0 P β0 This indicates a power imbalance between the M3C sub-converters, P 00 It is related to the total input / output power of the M3C.

[0077] The following formula is used to determine the power imbalance component of the bridge arm:

[0078]

[0079] Wherein, ΔP α0 Let ΔP be the power imbalance component along the α-axis in a two-phase stationary coordinate system. β0 e represents the power imbalance component along the β-axis in a two-phase stationary coordinate system. dl N i represents the d-axis component of the low-frequency side negative sequence voltage. dl P i represents the d-axis component of the positive sequence current on the low-frequency side. dl P λ represents the q-axis component of the positive sequence current on the low-frequency side, and λ is the angle between the positive and negative sequence currents of phase a on the low-frequency side.

[0080] S4: Determine the circulating current compensation component based on the arm power imbalance component. The circulating current compensation component includes the α-axis circulating current compensation component and the β-axis circulating current compensation component in the two-phase stationary coordinate system. Specifically, the circulating current compensation component is introduced by calculating the input and output power of the modular multilevel matrix converter based on the obtained arm power imbalance component:

[0081]

[0082]

[0083]

[0084] Among them, P Suba For the DC power of the low-frequency side converter a, P Subb For the DC power of the low-frequency side converter b, P Subc For the DC power of the low-frequency side converter c, Δi dαLet Δi be the circulation compensation component along the α-axis in a two-phase stationary coordinate system. dβ For the circulation compensation component along the β-axis in a two-phase stationary coordinate system, ΔP α0 Let ΔP be the power imbalance component along the α-axis in a two-phase stationary coordinate system. β0 e represents the power imbalance component along the β-axis in a two-phase stationary coordinate system. ds i represents the d-axis component of the power frequency side voltage. da For the d-axis current component of the low-frequency side converter a, i db For the d-axis current component of the low-frequency side converter b, i dc For the d-axis current component of the low-frequency side converter c, i dα Let i be the bridge arm current along the α axis in a two-phase stationary coordinate system. dβ Let represent the bridge arm current along the β axis in a two-phase stationary coordinate system.

[0085] S5: Convert the circulating compensation component into a circulating reference value under the double αβ0 transformation.

[0086] Further, step S5 includes:

[0087] S51: Using the equal power transformation matrix from the two-phase stationary coordinate system to the three-phase stationary coordinate system, the circulating current compensation component is transformed into the three-phase stationary coordinate system to obtain the three-phase d-axis current compensation component.

[0088] Specifically, the circulating compensation components are transformed to the three-phase stationary coordinate system using the following formula:

[0089]

[0090] Among them, T αβ / abc Let Δi be the equal power transformation matrix from a two-phase stationary coordinate system to a three-phase stationary coordinate system. da , Δi db , Δi dc This refers to the three-phase d-axis current compensation component.

[0091] S52: Determine the three-phase d-axis current compensation components based on the three-phase d-axis current compensation components and the three-phase q-axis current compensation components. The three-phase q-axis current compensation component is 0.

[0092] Specifically, the three-phase dq-axis current compensation components are:

[0093]

[0094] Where, Δi qa The q-axis current compensation component introduced by sub-converter a, Δi qb The q-axis current compensation component introduced for sub-converter b, Δi qcThe q-axis current compensation component introduced for the sub-converter c.

[0095] S53: Using a transformation matrix from the power frequency rotating coordinate system to the two-phase stationary coordinate system, the three-phase dq-axis current compensation components are transformed into the two-phase stationary coordinate system, resulting in the three-phase current compensation components in the two-phase stationary coordinate system. The obtained three-phase current compensation components in the two-phase stationary coordinate system include the α-axis three-phase current compensation components and the β-axis three-phase current compensation components.

[0096] Specifically, the three-phase current compensation components are transformed into a two-phase stationary coordinate system using the following formula:

[0097]

[0098] Among them, T dq_s / αβ Let Δi be the transformation matrix from the power frequency rotating coordinate system to the two-phase stationary coordinate system. aα , Δi bα , Δi cα For the three-phase current compensation component along the α-axis, Δi aβ , Δi bβ , Δi cβ This represents the three-phase current compensation component along the β axis.

[0099] S54: Using the equal power transformation matrix from the three-phase stationary coordinate system to the two-phase stationary coordinate system, the three-phase current compensation components in the two-phase stationary coordinate system are transformed to obtain the circulating current reference value under the double αβ0 transformation.

[0100] Specifically, the following formula is used to transform the three-phase current compensation components in the two-phase stationary coordinate system:

[0101]

[0102] Among them, T abc / αβ0 This is the equal power transformation matrix from a three-phase stationary coordinate system to a two-phase stationary coordinate system.

[0103] S6: Based on the circulating current reference value, perform circulating current outer loop control on the modular multilevel matrix converter, and perform outer loop control on the power frequency side and low frequency side respectively, and determine the overall reference value of the bridge arm voltage.

[0104] Specifically, the circulating current reference value is obtained by controlling the outer loop of the circulating current to obtain the bridge arm voltage reference value under the dual αβ0 transformation:

[0105]

[0106] Among them, K P The proportional coefficient for proportional control, u mn * Here are the reference values ​​for the bridge arm voltages under the dual αβ0 transformation, where m, n = α, β, and iαα i αβ i βα i ββ This refers to the bridge arm current under the double αβ0 transformation.

[0107] S7: Based on the overall reference value of the bridge arm voltage, control the bridge arm voltage of the modular multilevel matrix converter to control the capacitor voltage of the modular multilevel matrix converter under asymmetrical fault conditions.

[0108] Specifically, by combining the input and output side control of the modular multilevel matrix converter (MMC) to obtain the overall reference target for the bridge arm voltage, the MMC's control system controls the bridge arm voltage to achieve capacitor voltage control under asymmetrical fault conditions. For example... Figure 4 The diagram illustrates the capacitor voltage control process of this invention. The superscript * indicates the reference value for the corresponding component, U... c * Q is the reference value for the bridge arm capacitor voltage. * P is the reactive power reference value. * The active power reference value is ΔP. xy To calculate the power imbalance of the bridge arm, This is the reference value for the d-axis component of the power frequency side current. This is the reference value for the q-axis component of the power frequency side current. This is the reference value for the bridge arm voltage of the modular multilevel matrix converter.

[0109] This invention, based on suppressing the negative sequence current of a faulty line, achieves converter capacitor voltage control through internal circulating current control. First, the negative sequence current of the faulty line is suppressed to zero. Then, the DC power of the bridge arm is calculated based on the line voltage and current under asymmetrical conditions after suppressing the negative sequence current. The unbalanced power caused by line asymmetry is then obtained from the bridge arm DC power. Finally, circulating current compensation is introduced based on the unbalanced power, and the converter capacitor voltage is controlled through circulating current control. This reduces the impact of negative sequence current on the system under asymmetrical fault conditions, and on this basis, achieves converter capacitor voltage control under asymmetrical fault conditions, ensuring the safe and stable operation of the converter under fault conditions.

[0110] To demonstrate that the present invention can achieve capacitor voltage control of a modular multilevel matrix converter under asymmetric fault conditions, the following experimental simulation verifies the effect achievable by the present invention. The experimental parameters are shown in Table 1.

[0111] Table 1 Experimental parameters

[0112] parameter numerical values parameter numerical values System transmission power P / MW 50 Bridge arm inductance L / mH 15.3 <![CDATA[Power frequency side frequency f s / Hz]]> 50 Submodule capacitor C / mF 8.4 <![CDATA[Low-frequency side frequency f r / Hz]]> 20 Number of submodules N / each 108 <![CDATA[Power frequency side voltage U s / kV]]> 220 <![CDATA[Rated value of sub-module capacitor voltage V c / kV]]> 1.6 <![CDATA[Low-frequency side voltage U r / kV]]> 220

[0113] Simulation results show that without appropriate fault control, the fault current increases significantly under asymmetrical fault conditions, and the three-phase asymmetry is quite pronounced. After adding fault control, both the magnitude of the fault current and the three-phase asymmetry are effectively improved, enhancing the stable operation capability of the power system. Figures 5-7 The figure shows the average value of the submodule capacitor voltage under asymmetrical fault conditions when using steady-state operation control and the control of the present invention. As can be seen from the figure, when using steady-state operation control, the capacitor voltage under asymmetrical fault conditions increases significantly compared to the rated value, while the average value of the capacitor voltage controlled by the present invention increases less and can remain stable near the rated value.

[0114] Example 2

[0115] In order to implement the method corresponding to Embodiment 1 above and achieve the corresponding functions and technical effects, a modular multilevel matrix converter capacitor voltage control system is provided below.

[0116] like Figure 8 As shown, the modular multilevel matrix converter capacitor voltage control system provided in this embodiment includes: a negative sequence control unit 1, a power calculation unit 2, an unbalanced component determination unit 3, a compensation component determination unit 4, a circulating current reference value determination unit 5, a voltage reference value determination unit 6, and a voltage control unit 7.

[0117] The negative sequence control unit 1 is connected to the modular multilevel matrix converter. When an asymmetrical fault occurs on the low-frequency side of the line, the negative sequence control unit 1 adds negative sequence control to the modular multilevel matrix converter to suppress the negative sequence current of the faulty line to zero, and obtains the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side.

[0118] The power calculation unit 2 is connected to the negative sequence control unit 1. The power calculation unit 2 is used to calculate the bridge arm power of each bridge arm of the modular multilevel matrix converter based on the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side, and to extract the DC component in the power of each bridge arm.

[0119] The unbalanced component determination unit 3 is connected to the power calculation unit 2. The unbalanced component determination unit 3 is used to determine the unbalanced component of the bridge arm power based on the DC component in the power of each bridge arm.

[0120] The compensation component determination unit 4 is connected to the unbalanced component determination unit 3. The compensation component determination unit 4 is used to determine the circulating compensation component based on the unbalanced component of the bridge arm power.

[0121] The circulation reference value determination unit 5 is connected to the compensation component determination unit 4. The circulation reference value determination unit 5 is used to convert the circulation compensation component into a circulation reference value under the double αβ0 transformation.

[0122] The voltage reference value determination unit 6 is connected to the circulating current reference value determination unit 5, the modular multilevel matrix converter, the power frequency side, and the low frequency side, respectively. The voltage reference value determination unit 6 is used to perform circulating current outer loop control on the modular multilevel matrix converter according to the circulating current reference value, perform outer loop control on the power frequency side and the low frequency side respectively, and determine the overall reference value of the bridge arm voltage.

[0123] The voltage control unit 7 is connected to the voltage reference value determination unit 6 and the modular multilevel matrix converter respectively. The voltage control unit 7 is used to control the bridge arm voltage of the modular multilevel matrix converter according to the overall reference value of the bridge arm voltage, so as to control the capacitor voltage of the modular multilevel matrix converter under asymmetrical fault conditions.

[0124] Compared to existing technologies, the modular multilevel matrix converter capacitor voltage control system provided in this embodiment has the same beneficial effects as the modular multilevel matrix converter capacitor voltage control method provided in Embodiment 1, and will not be repeated here.

[0125] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0126] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for controlling the capacitor voltage of a modular multilevel matrix converter, wherein the two ends of the modular multilevel matrix converter are respectively connected to the power frequency side and the low frequency side, and the modular multilevel matrix converter includes multiple bridge arms; characterized in that, The modular multilevel matrix converter capacitor voltage control method includes: When an asymmetrical fault occurs on a line, negative sequence control is added to the modular multilevel matrix converter to suppress the negative sequence current of the faulty line to zero, and to obtain the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side; the line with the asymmetrical fault is either the low-frequency side line or the power frequency side line. Based on the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side, calculate the bridge arm power of each bridge arm of the modular multilevel matrix converter, and extract the DC component of each bridge arm power. The unbalanced component of the bridge arm power is determined based on the DC component of the power of each bridge arm. Determine the circulating current compensation component based on the unbalanced component of the bridge arm power; The circulation compensation component is converted into a circulation reference value under the dual αβ0 transformation; Based on the circulating current reference value, the modular multilevel matrix converter is subjected to circulating current outer loop control, and outer loop control is performed on the power frequency side and the low frequency side respectively, and the overall reference value of the bridge arm voltage is determined. Based on the overall reference value of the bridge arm voltage, the bridge arm voltage of the modular multilevel matrix converter is controlled to control the capacitor voltage of the modular multilevel matrix converter under asymmetrical fault conditions.

2. The modular multilevel matrix converter capacitor voltage control method according to claim 1, characterized in that, When the line experiencing an asymmetrical fault is a low-frequency side line, the three-phase voltage on the low-frequency side is: Among them, e a e b e c For the three-phase voltage on the low-frequency side, ω l Let λ be the low-frequency side angular frequency, λ be the angle between the positive and negative sequences of phase a on the low-frequency side, t be the time, and e be the angular frequency. dl P e represents the d-axis component of the low-frequency positive sequence voltage. dl N This represents the d-axis component of the negative sequence voltage on the low-frequency side. The three-phase current on the low-frequency side is: Among them, i a i b i c For the three-phase current on the low-frequency side, i dl P i represents the d-axis component of the positive sequence current on the low-frequency side. ql P This represents the q-axis component of the positive sequence current on the low-frequency side. The three-phase voltage on the power frequency side is: Among them, e u e v e w The three-phase voltage on the power frequency side, ω s e is the power frequency side angular frequency. ds P This represents the d-axis component of the positive sequence voltage on the power frequency side. The three-phase current on the power frequency side is: Among them, i u i v i w i represents the three-phase current on the power frequency side. ds P i represents the d-axis component of the positive sequence current on the power frequency side. qs P This represents the q-axis component of the positive sequence current on the power frequency side.

3. The modular multilevel matrix converter capacitor voltage control method according to claim 1, characterized in that, The modular multilevel matrix converter has 9 bridge arms. The bridge arm power of each bridge arm of the modular multilevel matrix converter is calculated using the following formula: in, Let e ​​be the bridge arm power of bridge arm xy. x For the three-phase voltage on the low-frequency side, e y For the three-phase voltage on the power frequency side, i x For the three-phase current on the low-frequency side, i y Let x = a, b, c and y = u, v, w, where a, b, c are the three phases on the low-frequency side, and u, v, w are the three phases on the power frequency side.

4. The modular multilevel matrix converter capacitor voltage control method according to claim 1, characterized in that, The DC component of the power in each bridge arm is determined using the following formula: P au =P av =P aw ; P bu =P bv =P bw ; P cu =P cv =P cw ; Among them, P xy Let x be the DC component of the arm power of the bridge arm xy, where x = a, b, c, y = u, v, w, e dl P i represents the d-axis component of the positive sequence voltage on the low-frequency side. dl P e represents the d-axis component of the positive sequence current on the low-frequency side. ds P i represents the d-axis component of the positive sequence voltage on the power frequency side. ds P e represents the d-axis component of the positive sequence current on the power frequency side. dl N i represents the d-axis component of the low-frequency side negative sequence voltage. ql P λ represents the q-axis component of the positive sequence current on the low-frequency side, and λ is the angle between the positive and negative sequence currents of phase a on the low-frequency side.

5. The modular multilevel matrix converter capacitor voltage control method according to claim 1, characterized in that, The determination of the bridge arm power imbalance component based on the DC component of each bridge arm power specifically includes: The DC component of the power in each bridge arm is subjected to a double αβ transformation to obtain the transformed DC component. Based on the transformed DC component, the power imbalance component of the bridge arm is determined.

6. The modular multilevel matrix converter capacitor voltage control method according to claim 5, characterized in that, The power imbalance components of the bridge arm include the power imbalance components along the α-axis and the power imbalance components along the β-axis in the two-phase stationary coordinate system. The following formula is used to determine the power imbalance component of the bridge arm: Wherein, ΔP α0 Let ΔP be the power imbalance component along the α-axis in a two-phase stationary coordinate system. β0 e represents the power imbalance component along the β-axis in a two-phase stationary coordinate system. dl N i represents the d-axis component of the low-frequency side negative sequence voltage. dl P i represents the d-axis component of the positive sequence current on the low-frequency side. ql P λ represents the q-axis component of the positive sequence current on the low-frequency side, and λ is the angle between the positive and negative sequence currents of phase a on the low-frequency side.

7. The modular multilevel matrix converter capacitor voltage control method according to claim 1, characterized in that, The bridge arm power imbalance component includes the power imbalance component along the α-axis and the power imbalance component along the β-axis in a two-phase stationary coordinate system; the circulating compensation component includes the circulating compensation component along the α-axis and the circulating compensation component along the β-axis in a two-phase stationary coordinate system. The circulation compensation component is determined using the following formula: Where, Δi dα Let Δi be the circulation compensation component along the α-axis in a two-phase stationary coordinate system. dβ For the circulation compensation component along the β-axis in a two-phase stationary coordinate system, ΔP α0 Let ΔP be the power imbalance component along the α-axis in a two-phase stationary coordinate system. β0 e represents the power imbalance component along the β-axis in a two-phase stationary coordinate system. ds This represents the d-axis component of the power frequency side voltage.

8. The modular multilevel matrix converter capacitor voltage control method according to claim 1, characterized in that, Converting the circulation compensation component into a circulation reference value under the dual αβ0 transform specifically includes: Using an equal power transformation matrix from a two-phase stationary coordinate system to a three-phase stationary coordinate system, the circulating current compensation component is transformed into a three-phase stationary coordinate system to obtain the three-phase d-axis current compensation component. Based on the three-phase d-axis current compensation components and the three-phase q-axis current compensation components, the three-phase dq-axis current compensation components are determined; the three-phase q-axis current compensation components are 0. The transformation matrix from the power frequency rotating coordinate system to the two-phase stationary coordinate system is used to transform the three-phase dq axis current compensation components into the two-phase stationary coordinate system, thus obtaining the three-phase current compensation components in the two-phase stationary coordinate system. Using an equal power transformation matrix from a three-phase stationary coordinate system to a two-phase stationary coordinate system, the three-phase two-phase current compensation components in the two-phase stationary coordinate system are transformed to obtain the circulating current reference value under the double αβ0 transformation.

9. A modular multilevel matrix converter capacitor voltage control system, applied to the modular multilevel matrix converter capacitor voltage control method according to any one of claims 1-8, characterized in that, The modular multilevel matrix converter capacitor voltage control system includes: The negative sequence control unit, connected to the modular multilevel matrix converter, is used to add negative sequence control to the modular multilevel matrix converter when an asymmetrical fault occurs in the line, so as to suppress the negative sequence current of the faulty line to zero, and obtain the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side; the line where the asymmetrical fault occurs is either the low-frequency side line or the power frequency side line. The power calculation unit, connected to the negative sequence control unit, is used to calculate the bridge arm power of each bridge arm of the modular multilevel matrix converter based on the three-phase voltage and three-phase current on the low-frequency side and the three-phase voltage and three-phase current on the power frequency side, and to extract the DC component in the power of each bridge arm. An unbalanced component determination unit, connected to the power calculation unit, is used to determine the unbalanced component of the bridge arm power based on the DC component in the power of each bridge arm. The compensation component determination unit, connected to the unbalanced component determination unit, is used to determine the circulating current compensation component based on the unbalanced component of the bridge arm power. A circulation reference value determination unit, connected to the compensation component determination unit, is used to convert the circulation compensation component into a circulation reference value under the double αβ0 transformation. A voltage reference value determination unit is connected to the circulating current reference value determination unit, the modular multilevel matrix converter, the power frequency side, and the low frequency side, respectively. It is used to perform circulating current outer loop control on the modular multilevel matrix converter according to the circulating current reference value, perform outer loop control on the power frequency side and the low frequency side respectively, and determine the overall reference value of the bridge arm voltage. The voltage control unit is connected to the voltage reference value determination unit and the modular multilevel matrix converter respectively. It is used to control the bridge arm voltage of the modular multilevel matrix converter according to the overall reference value of the bridge arm voltage, so as to control the capacitor voltage of the modular multilevel matrix converter under asymmetrical fault conditions.

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