A method of MMC control

Abstract

The application discloses a MMC control method, comprising: obtaining the average value of the sub-module capacitor voltage of a three-phase bridge circuit MMC and the reference value of the sub-module capacitor voltage; inputting the average value of the sub-module capacitor voltage and the reference value of the sub-module capacitor voltage into a DQ-axis voltage controller to obtain a DQ-axis voltage reference signal; inputting the DC voltage reference value, the DC voltage measurement value and the DC current measurement value of the three-phase bridge circuit MMC into a zero-axis voltage controller adopting a DC voltage and DC current double-loop control based on a PI controller to obtain a zero-axis voltage reference signal; inputting the DQ-axis voltage reference signal and the zero-axis voltage reference signal into a signal converter to generate a three-phase voltage modulation signal for driving the MMC three-phase bridge circuit. The application adopts the output of the zero-axis voltage controller as the zero-axis component of the three-phase voltage modulation signal, and adopts a DC voltage feedforward control to improve the dynamic characteristics of the DC voltage, thereby effectively suppressing the DC fault current.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power grid control, and more particularly, to an MMC control method. BACKGROUND

[0002] With the rapid development of wind, light and other new energy power generation technologies, the penetration rate of new energy in power systems is continuously increasing. Flexible DC transmission has achieved rapid development worldwide due to its advantages of convenient long-distance power transmission, low loss and large transmission capacity.

[0003] However, in the MMC structure of power control, due to the low line impedance value on the DC side and the absence of natural zero point of DC current, when a fault occurs on the DC side, there are problems such as large short-circuit fault current peak value and short rise time.

[0004] Therefore, when a short-circuit fault occurs on the DC line of the flexible DC system, how to reduce the DC side short-circuit fault current has become one of the research hotspots in engineering and academic circles. SUMMARY

[0005] Therefore, the present application provides an MMC control method, which uses the output of the zero voltage controller as the zero axis component of the generated three-phase voltage modulation signal, and uses DC voltage feedforward control to improve the dynamic characteristics of the DC voltage, thereby effectively suppressing the DC fault current.

[0006] In order to achieve the above-mentioned purpose, the present scheme is as follows:

[0007] An MMC control method, comprising:

[0008] Obtaining the sub-module capacitor voltage average value and the sub-module capacitor voltage reference value of each sub-module of a three-phase bridge circuit MMC;

[0009] Inputting the sub-module capacitor voltage average value and the sub-module capacitor voltage reference value of the three-phase bridge circuit MMC into a DQ axis voltage controller to obtain a DQ axis voltage reference signal output by the DQ axis voltage controller;

[0010] Inputting the DC voltage reference value, the DC voltage measurement value and the DC current measurement value of the three-phase bridge circuit MMC into a zero axis voltage controller using a DC voltage and DC current double-loop control based on a PI controller to obtain a zero axis voltage reference signal output by the zero axis voltage controller;

[0011] Inputting the DQ axis voltage reference signal and the zero axis voltage reference signal into a signal converter to generate a three-phase voltage modulation signal for driving the MMC three-phase bridge circuit.

[0012] Preferably, the DC voltage reference value and the DC voltage measurement value of the three-phase bridge circuit MMC are input into a zero-axis voltage controller with a DC voltage and DC current double-loop control based on a PI controller to obtain a zero-axis voltage reference signal output by the zero-axis voltage controller, which includes:

[0013] The DC voltage reference value and the DC voltage measurement value of the three-phase bridge circuit MMC are input into a first PI controller of the DC voltage and DC current double-loop control to obtain a DC current reference value.

[0014] The DC current measurement value of the three-phase bridge circuit MMC and the DC current reference value are input into a second PI controller of the DC voltage and DC current double-loop control, and the DC voltage measurement value is combined to obtain a zero-axis voltage reference signal.

[0015] Preferably, it further includes:

[0016] In the DC voltage and DC current double-loop control of the zero-axis voltage controller, a DC current threshold range is introduced to limit the DC current reference value.

[0017] Preferably, the calculation strategy of the first PI controller is:

[0018] i dcref = k pudc (u dcref -u dc )+k iudc ∫(u dcref -u dc )dt

[0019] Wherein, i dcref is the DC current reference value, u dcref is the DC voltage reference value, u dc is the DC voltage measurement value, k pudc and k iudc are the proportional coefficient and the integral coefficient of the first PI controller, respectively.

[0020] Preferably, the calculation strategy of the second PI controller is:

[0021] e dc = u dc +[k pidc (i dcref -i dc )+k iidc ∫(i dcref -i dc )dt]

[0022] Wherein, e dc is the zero-axis voltage reference signal, u dcis a direct current measurement value, i dc is a direct current measurement value, i dcref is a direct current reference value, k pidc , k iidc are proportional coefficient and integral coefficient of the second PI controller, respectively.

[0023] Preferably, the maximum value of the positive direct current and the negative direct current of the three-phase bridge circuit MMC is determined as the direct current measurement value.

[0024] Preferably, the average value of the sub-module capacitor voltage of the three-phase bridge circuit MMC and the sub-module capacitor voltage reference value are input into the DQ-axis voltage controller to obtain the DQ-axis voltage reference signal output by the DQ-axis voltage controller, which includes:

[0025] The average value of the sub-module capacitor voltage of the three-phase bridge circuit MMC and the sub-module capacitor voltage reference value are input into the third PI controller of the DQ-axis voltage controller to obtain the D-axis current reference value.

[0026] The Q-axis current reference value, the DQ-axis current measurement value and the D-axis current reference value of the three-phase bridge circuit MMC are input into the closed-loop negative feedback controller of the DQ-axis voltage controller, and combined with the DQ-axis voltage measurement value to generate the DQ-axis voltage reference signal.

[0027] Preferably, the calculation strategy of the third PI controller is:

[0028] i dref =k pc (u cref -u cave )+k ic ∫(u cref -u cave )dt

[0029] wherein u cave is the average value of the sub-module capacitor voltage, u cref is the sub-module capacitor voltage reference value, k pc , k ic are proportional coefficient and integral coefficient of the third PI controller, respectively, and i dref is the D-axis current reference value.

[0030] Preferably, the control strategy of the closed-loop negative feedback controller is:

[0031]

[0032] wherein i dref is the D-axis current reference value, i qref is the Q-axis current reference value, i d , iq is the DQ axis current measurement value, w is the power frequency angular frequency, L is the bridge arm reactance, V d , V q is the DQ axis voltage measurement value, e dpwm , e qpwm is the DQ axis voltage reference signal, k pd , k id are the proportional coefficient and integral coefficient of the closed-loop negative feedback controller for D-axis control, respectively, k pq , k iq are the proportional coefficient and integral coefficient of the closed-loop negative feedback controller for Q-axis control, respectively.

[0033] Preferably, the DQ axis voltage reference signal and the zero axis voltage reference signal are input into a signal converter, and the strategy for generating a three-phase voltage modulation signal is:

[0034]

[0035]

[0036] wherein θ is the grid-connected point voltage phase angle, e dpwm , e qpwm is the DQ axis voltage reference signal, e dc is the zero axis voltage reference signal, u pa,ref , u pb,ref , u pc,ref is the upper bridge arm phase voltage reference value, u na,ref , u nb,ref , u nc,ref is the lower bridge arm phase voltage reference value.

[0037] As can be seen from the above technical solution, the MMC control method provided by the embodiment of the application obtains the sub-module capacitor voltage average value and the sub-module capacitor voltage reference value of each sub-module of the three-phase bridge circuit MMC, inputs the sub-module capacitor voltage average value and the sub-module capacitor voltage reference value into a DQ axis voltage controller to obtain a DQ axis voltage reference signal, and inputs the DC voltage reference value, the DC voltage measurement value and the DC current measurement value of the three-phase bridge circuit MMC into a zero axis voltage controller adopting a DC voltage and DC current double-loop control based on a PI controller to obtain a zero axis voltage reference signal. Finally, the DQ axis voltage reference signal and the zero axis voltage reference signal are input into a signal converter to generate a three-phase voltage modulation signal, which is used to drive the MMC three-phase bridge circuit.

[0038] The application obtains a DQ-axis voltage reference signal through a DQ-axis voltage controller, and obtains a zero-axis voltage reference signal through a zero-axis voltage controller adopting a direct-current voltage and direct-current double-loop control based on a PI controller, and uses the zero-axis voltage reference signal as a zero-axis component for generating a full-bridge MMC three-phase voltage modulation signal, and finally generates an ABC three-phase voltage modulation signal through DQ0 to abc conversion, which is used for driving an MMC three-phase bridge circuit. In order to improve the dynamic response characteristics of the direct-current voltage, the direct-current voltage measurement value is introduced as a feedforward in the zero-axis voltage controller, so as to realize effective suppression of the direct-current fault current. When a ground fault occurs at the direct-current side, the direct-current voltage measurement value will be 0 quickly, and the size of the zero-axis component in the zero-axis voltage reference signal will also quickly decrease, thereby playing a role in suppressing the amplitude of the direct-current current. BRIEF DESCRIPTION OF DRAWINGS

[0039] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only constitute a part of the embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of the provided drawings.

[0040] Figure 1 A basic topology diagram of an MMC disclosed by the embodiments of the present application;

[0041] Figure 2 A flowchart of an MMC control method disclosed by the embodiments of the present application;

[0042] Figure 3 A schematic diagram of a control strategy of an MMC control method disclosed by the embodiments of the present application;

[0043] Figure 4 A schematic diagram of a control strategy of a DQ-axis voltage controller disclosed by the embodiments of the present application;

[0044] Figure 5 A schematic diagram of a control strategy of a zero-axis voltage controller disclosed by the embodiments of the present application. DETAILED DESCRIPTION

[0045] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments only constitute a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0046] The embodiment of the present application provides a MMC control method, which can be applied to a full-bridge MMC circuit, the MMC is a three-phase bridge circuit composed of six bridge arms, each bridge arm comprises an inductor L and N series-connected sub-modules (SM), and a phase unit is composed of upper and lower bridge arms of each phase.

[0047] Figure 1 The figure is a basic topology structure diagram of the MMC. Figure 1 In the formula, L is a bridge arm inductance, and N is a full-bridge sub-module number. pa pb pc is an upper bridge arm phase voltage, u na nb nc is a lower bridge arm phase voltage, i pa pb pc is an upper bridge arm phase current, u na nb nc is a lower bridge arm phase current, i sa sb sc is a grid-side phase current, v sa sb sc is a grid-side phase voltage, u dc is a DC bus voltage, i dcp is a positive DC current, and i dcn is a negative DC current.

[0048] The MMC control method provided by the present application is applied to an MMC structure as shown in Figure 1 , and a DC voltage feedforward control is adopted to improve the dynamic characteristics of the DC voltage, so that the DC fault current is effectively suppressed.

[0049] Next, the present application scheme is introduced, and the present application proposes the following technical scheme, which is specifically described below.

[0050] Figure 2 The figure is a flowchart of the MMC control method disclosed by the embodiment of the present application, Figure 3 is a schematic diagram of a control strategy of the MMC control method, and the present application method is introduced in combination with Figure 2 and Figure 3 The present application method can include the following steps.

[0051] In step S1, the average value of the sub-module capacitor voltage and the reference value of the sub-module capacitor voltage of each sub-module of the three-phase bridge circuit MMC are obtained.

[0052] ​​​​​​​​​​​​Specifically, the application obtains the sub-module capacitor voltage average value of each sub-module of the three-phase bridge circuit MMC, that is, the sub-module capacitor voltage average value obtained by averaging the capacitor voltage of each sub-module, and the sub-module capacitor voltage reference value of each sub-module of the three-phase bridge circuit MMC, that is, the sub-module capacitor voltage reference value obtained by averaging the capacitor voltage reference value of each sub-module, and takes the sub-module capacitor voltage average value and the sub-module capacitor voltage reference value as the input of the DQ-axis voltage controller for generating the DQ-axis voltage reference signal.

[0053] As shown in the three-phase bridge circuit MMC, Figure 1 there are N full-bridge sub-modules, and the sub-module capacitor voltage average value and the sub-module capacitor voltage reference value are the capacitor voltage average value of the N full-bridge sub-modules and the capacitor voltage reference value of the N full-bridge sub-modules respectively.

[0054] Step S2, input the sub-module capacitor voltage average value and the sub-module capacitor voltage reference value of the three-phase bridge circuit MMC into the DQ-axis voltage controller to obtain the DQ-axis voltage reference signal output by the DQ-axis voltage controller.

[0055] Specifically, Figure 4 is a schematic diagram of the control strategy of the DQ-axis voltage controller.

[0056] As shown in the three-phase bridge circuit MMC, Figure 4 the DQ-axis voltage reference signal includes the D-axis voltage reference signal and the Q-axis voltage reference signal. The obtained sub-module capacitor voltage average value and the sub-module capacitor voltage reference value of each sub-module of the three-phase bridge circuit MMC are input into the DQ-axis voltage controller adopting closed-loop negative feedback control, and the D-axis voltage measurement value, the Q-axis voltage measurement value, and the D-axis current measurement value and the Q-axis current measurement value are combined to generate the D-axis voltage reference signal and the Q-axis voltage reference signal.

[0057] Step S3, input the DC voltage reference value, the DC voltage measurement value and the DC current measurement value of the three-phase bridge circuit MMC into the zero-axis voltage controller adopting the DC voltage and DC current double-loop control based on the PI controller to obtain the zero-axis voltage reference signal output by the zero-axis voltage controller.

[0058] Specifically, Figure 5 is a schematic diagram of the control strategy of the zero-axis voltage controller.

[0059] As shown in the three-phase bridge circuit MMC, Figure 5As shown, the zero-axis voltage controller adopts a PI controller-based direct current voltage and direct current double-loop control structure. The direct current voltage reference value, direct current voltage measurement value and direct current measurement value of the three-phase bridge circuit MMC are input into the zero-axis voltage controller adopting the PI controller-based direct current voltage and direct current double-loop control to obtain the zero-axis voltage reference signal output by the zero-axis voltage controller. Meanwhile, in the direct current voltage and direct current double-loop control structure, the direct current voltage measurement value is introduced as a feedforward for improving the dynamic response characteristic of the direct current voltage. When a pole-to-ground fault occurs, the direct current voltage feedforward is rapidly reduced, so that the zero-axis voltage reference signal, i.e. the zero-axis component, is rapidly reduced, thereby playing a role in limiting the direct current short-circuit current.

[0060] Step S4: inputting the DQ-axis voltage reference signal and the zero-axis voltage reference signal into a signal converter to generate a three-phase voltage modulation signal for driving the MMC three-phase bridge circuit.

[0061] Specifically, after obtaining the DQ-axis voltage reference signal through the DQ-axis voltage controller and obtaining the zero-axis voltage reference signal through the zero-axis voltage controller, the DQ-axis voltage reference signal and the zero-axis voltage reference signal are input into a signal converter. The signal converter can realize DQ0-axis to ABC-axis conversion to obtain a final three-phase voltage modulation signal in the ABC-axis for driving the MMC three-phase bridge circuit.

[0062] The manner in which the three-phase voltage modulation signal drives the MMC three-phase bridge circuit can further be to obtain the modulation signal of the MMC through a nearest level approximation or the like for turning on or off the IGBT of the MMC sub-module.

[0063] The strategy for inputting the DQ-axis voltage reference signal and the zero-axis voltage reference signal into a signal converter to generate a three-phase voltage modulation signal is as follows:

[0064]

[0065]

[0066] wherein θ is the phase angle of the grid-connected point voltage, e dpwm , e qpwm is the DQ-axis voltage reference signal, e dc is the zero-axis voltage reference signal, u pa,ref , u pb,ref , u pc,ref is the upper bridge arm phase voltage reference value, u na,ref , u nb,ref , u nc,ref is the lower bridge arm phase voltage reference value.

[0067] From the above technical solutions, the MMC control method provided by the embodiment of the application can obtain the sub-module capacitor voltage average value and the sub-module capacitor voltage reference value of each sub-module of the three-phase bridge circuit MMC, input the sub-module capacitor voltage average value and the sub-module capacitor voltage reference value into a DQ-axis voltage controller to obtain a DQ-axis voltage reference signal, input the DC voltage reference value, the DC voltage measurement value and the DC current measurement value of the three-phase bridge circuit MMC into a zero-axis voltage controller adopting a DC voltage and DC current double-loop control based on a PI controller to obtain a zero-axis voltage reference signal. Finally, the DQ-axis voltage reference signal and the zero-axis voltage reference signal are input into a signal converter to generate a three-phase voltage modulation signal for driving the MMC three-phase bridge circuit.

[0068] The DQ-axis voltage reference signal is obtained through the DQ-axis voltage controller, and the zero-axis voltage reference signal is obtained through the zero-axis voltage controller adopting the DC voltage and DC current double-loop control based on the PI controller, and is used as a zero-axis component for generating the full-bridge MMC three-phase voltage modulation signal. Finally, the ABC three-phase voltage modulation signal is generated through the conversion from DQ0 to abc, and is used for driving the MMC three-phase bridge circuit. In order to improve the dynamic response characteristics of the DC voltage, the DC voltage measurement value is introduced as a feedforward in the zero-axis voltage controller, so that the DC fault current can be effectively suppressed. When the ground fault occurs at the DC side, the DC voltage measurement value will be 0, and the size of the zero-axis component in the zero-axis voltage reference signal will also rapidly decrease, thereby suppressing the amplitude of the DC current.

[0069] In some embodiments of the application, the process of step S3, inputting the DC voltage reference value and the DC voltage measurement value of the three-phase bridge circuit MMC into the zero-axis voltage controller adopting the DC voltage and DC current double-loop control based on the PI controller to obtain the zero-axis voltage reference signal output by the zero-axis voltage controller, can specifically include:

[0070] Step S31, inputting the DC voltage reference value and the DC voltage measurement value of the three-phase bridge circuit MMC into the first PI controller of the DC voltage and DC current double-loop control to obtain a DC current reference value.

[0071] Specifically, as shown in the schematic diagram of the control strategy of the zero-axis voltage controller, Figure 5 the DC voltage reference value u dcref and the DC voltage measurement value u dc of the three-phase bridge circuit MMC are input into the first PI controller to obtain a DC current reference value i dcref .

[0072] The calculation strategy of the first PI controller is:

[0073] i dcref =k pudc (u dcref -u dc )+k iudc ∫(u dcref -u dc )dt

[0074] Among them, i dcref This is the reference value for DC current, u dcref The reference value for DC voltage, u dc This is a DC voltage measurement value, k pudc k iudc These are the proportional and integral coefficients of the first PI controller, respectively.

[0075] Step S32: Input the DC current measurement value of the three-phase bridge circuit MMC and the DC current reference value to the second PI controller of the DC voltage and DC current dual-loop control, and combine the DC voltage measurement value to obtain the zero-axis voltage reference signal.

[0076] Specifically, such as Figure 5 In the schematic diagram of the control strategy of the zero-axis voltage controller shown, the DC current reference value i output by the first PI controller is... dcref The DC current measurement value i of the three-phase bridge circuit MMC dc The DC voltage measurement value u of the three-phase bridge circuit MMC. dc The input is fed into the second PI controller, which is controlled by a dual-loop DC voltage and DC current loop, to generate a zero-axis voltage reference signal e. dc .

[0077] The calculation strategy of the second PI controller is as follows:

[0078] e dc =u dc +[k pidc (i dcref -i dc )+k iidc ∫(i dcref -i dc )dt]

[0079] Among them, e dc The zero-axis voltage reference signal, u dc For DC voltage measurements, i dc i is the measured value of DC current. dcref k is the reference value for DC current. pidc k iidc These are the proportional and integral coefficients of the second PI controller, respectively.

[0080] Optionally, in order to limit the direct current, an amplitude limiting link can also be added in the process of obtaining the direct current reference value by the first PI controller of the direct voltage and direct current double-loop control, specifically:

[0081] In the direct voltage and direct current double-loop control of the zero-axis voltage controller, a direct current threshold range is introduced to limit the direct current reference value.

[0082] Specifically, in the direct voltage and direct current double-loop control structure, by setting a direct current threshold range, a direct current upper limit and a direct current lower limit are introduced to limit the direct current reference value, specifically, a first PI controller is set to generate a direct current reference value i dcref Before that, this link can quickly limit the output of the direct voltage control loop, so that the output direct current reference value is within the preset direct current threshold range, thereby indirectly affecting the output value of the direct current loop, and ultimately playing a role in limiting the direct short-circuit current.

[0083] Optionally, in addition to adding the amplitude limiting link, the maximum value of the positive direct current and the negative direct current of the three-phase bridge circuit MMC can be determined as the direct current measurement value, so as to limit the fault current value of the pole-to-ground fault.

[0084] In some embodiments of the present application, the process of step S2, the average value of the sub-module capacitor voltage of the three-phase bridge circuit MMC and the sub-module capacitor voltage reference value are input into the DQ-axis voltage controller to obtain the DQ-axis voltage reference signal output by the DQ-axis voltage controller, can specifically include:

[0085] Step S21, input the average value of the sub-module capacitor voltage of the three-phase bridge circuit MMC and the sub-module capacitor voltage reference value into the third PI controller of the DQ-axis voltage controller to obtain the D-axis current reference value.

[0086] Specifically, as shown in the schematic diagram of the control strategy of the DQ-axis voltage controller, Figure 4 The DQ-axis voltage controller is composed of a third PI controller and a closed-loop negative feedback controller, and the average value of the sub-module capacitor voltage u cave of the three-phase bridge circuit MMC and the sub-module capacitor voltage reference value u cref are input into the third PI controller of the DQ-axis voltage controller to obtain the D-axis current reference value i dref .

[0087] The calculation strategy of the third PI controller is:

[0088] i dref = k pc (ucref -u cave )+k ic ∫(u cref -u cave )dt

[0089] wherein, u cave is a submodule capacitor voltage average value, u cref is a submodule capacitor voltage reference value, k pc , k ic are a proportional coefficient and an integral coefficient of the third PI controller respectively, i dref is a D-axis current reference value.

[0090] Step S22, inputting the Q-axis current reference value of the three-phase bridge circuit MMC, the DQ-axis current measurement value and the D-axis current reference value into a closed-loop negative feedback controller of the DQ-axis voltage controller, and combining the DQ-axis voltage measurement value to generate a DQ-axis voltage reference signal.

[0091] Specifically, inputting the D-axis current reference value i dref output by the third PI controller and the obtained Q-axis current reference value i qref into the closed-loop negative feedback controller, and combining the D-axis current measurement value i d , the Q-axis current measurement value i q , the D-axis voltage measurement value V d and the Q-axis voltage measurement value V q to generate the D-axis voltage reference signal and the Q-axis voltage reference signal.

[0092] The control strategy of the closed-loop negative feedback controller is:

[0093]

[0094] wherein, i dref is the D-axis current reference value, i qref is the Q-axis current reference value, i d , i q are the DQ-axis current measurement values, w is a power frequency angular frequency, L is a bridge arm reactance, V d , V q are the DQ-axis voltage measurement values, e dpwm , e qpwm are the DQ-axis voltage reference signals, k pd , k id are a proportional coefficient and an integral coefficient of the closed-loop negative feedback controller for D-axis control respectively, k pq , k iq are a proportional coefficient and an integral coefficient of the closed-loop negative feedback controller for Q-axis control respectively.

[0095] Finally, it should be noted that the terms "first", "second", and the like, herein do not denote any order, quantity, combination, or importance, but rather are used to distinguish one element from another, and are not intended to denote the presence of any such actual relationship or order. Moreover, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can also include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "comprises... a" does not, without more constraints, exclude the presence of additional identical elements in the process, method, article, or apparatus that comprises the element.

[0096] The various embodiments in the specification are described in progressive order with reference to each embodiment, each embodiment highlighting differences from other embodiments, and the same or similar elements in the various embodiments are described with reference to each other where possible.

[0097] The above description of disclosed embodiments provides enabling concepts for a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of controlling MMC, characterized by, The method comprises the following steps: obtaining the average value of the sub-module capacitor voltage and the reference value of the sub-module capacitor voltage of each sub-module of the three-phase bridge circuit MMC; inputting the average value of the sub-module capacitor voltage and the reference value of the sub-module capacitor voltage of the three-phase bridge circuit MMC into a DQ-axis voltage controller to obtain a DQ-axis voltage reference signal output by the DQ-axis voltage controller; inputting the DC voltage reference value, the DC voltage measurement value and the DC current measurement value of the three-phase bridge circuit MMC into a zero-axis voltage controller adopting a DC voltage and DC current double-loop control based on a PI controller to obtain a zero-axis voltage reference signal output by the zero-axis voltage controller; inputting the DQ-axis voltage reference signal and the zero-axis voltage reference signal into a signal converter to generate a three-phase voltage modulation signal for driving the MMC three-phase bridge circuit; inputting the DC voltage reference value and the DC voltage measurement value of the three-phase bridge circuit MMC into a zero-axis voltage controller adopting a DC voltage and DC current double-loop control based on a PI controller to obtain a zero-axis voltage reference signal output by the zero-axis voltage controller, comprising: inputting the DC voltage reference value and the DC voltage measurement value of the three-phase bridge circuit MMC into a first PI controller of the DC voltage and DC current double-loop control to obtain a DC current reference value; inputting the DC current measurement value of the three-phase bridge circuit MMC and the DC current reference value into a second PI controller of the DC voltage and DC current double-loop control and combining the DC voltage measurement value to obtain a zero-axis voltage reference signal.

2. The method of claim 1, wherein, Further comprising: in the DC voltage and DC current double-loop control of the zero-axis voltage controller, a DC current threshold range is introduced to limit the DC current reference value.

3. The method of claim 1, wherein, The calculation strategy of the first PI controller is: ； wherein i dcref is a direct current reference value, u dcref is a direct voltage reference value, u dc is a direct voltage measurement value, k pudc , k iudc are a proportional coefficient and an integral coefficient of the first PI controller, respectively.

4. The method of claim 1, wherein, The calculation strategy of the second PI controller is: ； wherein e dc is a zero-axis voltage reference signal, u dc is a DC voltage measurement value, i dc is a DC current measurement value, i dcref is a DC current reference value, k pidc , k iidc are a proportional coefficient and an integral coefficient of the second PI controller, respectively.

5. The method of claim 1, wherein, the maximum value of the positive DC current and the negative DC current of the three-phase bridge circuit MMC is determined as the DC current measurement value.

6. The method of claim 1, wherein, The average value of the sub-module capacitor voltage and the reference value of the sub-module capacitor voltage of the three-phase bridge circuit MMC are input into a DQ-axis voltage controller to obtain a DQ-axis voltage reference signal output by the DQ-axis voltage controller, comprising: the average value of the sub-module capacitor voltage and the reference value of the sub-module capacitor voltage of the three-phase bridge circuit MMC are input into a third PI controller of the DQ-axis voltage controller to obtain a D-axis current reference value; the Q-axis current reference value, the DQ-axis current measurement value and the D-axis current reference value of the three-phase bridge circuit MMC are input into a closed-loop negative feedback controller of the DQ-axis voltage controller and combined with the DQ-axis voltage measurement value to generate a DQ-axis voltage reference signal.

7. The method of claim 6, wherein, The calculation strategy of the third PI controller is: ； Wherein, u cave is the average value of the capacitor voltage of the sub-module, u cref is the reference value of the capacitor voltage of the sub-module, k pc , k ic are the proportional coefficient and the integral coefficient of the third PI controller, respectively, i dref is the D-axis current reference value.

8. The method of claim 6, wherein, The control strategy of the closed-loop negative feedback controller is: ； wherein, i dref is the D-axis current reference value, i qref is the Q-axis current reference value, i d , i q is the DQ-axis current measurement value, w is the power frequency angular frequency, L is the bridge arm reactance, V d , V q is the DQ-axis voltage measurement value, e dpwm , e qpwm is the DQ-axis voltage reference signal, k pd , k id are the proportional coefficient and integral coefficient of the closed-loop negative feedback controller for D-axis control, respectively, k pq , k iq are the proportional coefficient and integral coefficient of the closed-loop negative feedback controller for Q-axis control, respectively.

9. The method of claim 1, wherein, the strategy for inputting the DQ-axis voltage reference signal and the zero-axis voltage reference signal into a signal converter to generate a three-phase voltage modulation signal is: ； Wherein, θ is the grid-connected point voltage phase angle, e dpwm , e qpwm is the DQ axis voltage reference signal, e dc is the zero axis voltage reference signal, u pa,ref , u pb,ref , u pc,ref is the upper bridge arm phase voltage reference value, u na,ref , u nb,ref , u nc,ref is the lower bridge arm phase voltage reference value.

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