Control system and method for improving reactive output capacity of MMC-STATCOM based on variable DC voltage

Abstract

The present invention relates to a control system and method for improving the reactive output capacity of MMC-STATCOM based on variable DC voltage, including an AC power grid, a power grid load, an MMC-STATCOM and an MMC-STATCOM control system, wherein the AC power grid is connected to the power grid load through an AC transmission line, the MMC-STATCOM is connected to the AC power grid by connecting a transformer, and the MMC-STATCOM control system is connected to the MMC-STATCOM. The MMC-STATCOM control system of the present invention can generate a control signal to start the modulation ratio controller through a simple mode judgment when the modulation ratio signal reaches its maximum value, and maintain the modulation ratio at its maximum value, thereby avoiding the system from entering an overmodulation state and improving the operating stability of the MMC-STATCOM. At the same time, compared with traditional fixed DC voltage control, the present invention can increase the DC voltage within an allowable range when the system reactive power demand is large, thereby improving the output AC voltage and reactive output capacity of the MMC-STATCOM. At the same time, compared with methods such as third harmonic voltage injection, the control structure of the method proposed in this article is simple, easy to implement, and not easily affected by the original control of the MMC.

Description

Technical Field

[0001] The present invention belongs to the technical field of reactive power compensation of flexible AC power transmission system, and in particular to a control system and method for improving reactive power output capacity of MMC-STATCOM based on variable DC voltage. Background Art

[0002] Static Synchronous Compensator (STATCOM) has the characteristics of fast reactive compensation adjustment speed and wide range. It can effectively improve the system power factor and improve the voltage quality of the power grid. It is one of the important technologies of flexible AC transmission system. With the continuous expansion of the scale of power systems, the development of STATCOM towards high voltage and high power has become a trend. The Modular Multilevel Converter (MMC) has the characteristics of not requiring a large number of IGBTs to be directly connected in series, low filter device requirements and easy expansion, and is suitable for high voltage and high power scenarios. Therefore, the high-voltage and high-power STATCOM based on MMC has received widespread attention in recent years.

[0003] The reactive power output capacity of MMC-STATCOM is crucial to maintaining the voltage stability of the system. However, under the traditional constant DC voltage control, the maximum reactive power that can be output is significantly weakened because the operating characteristics of MMC are restricted by various constraints. Existing studies have shown that the reactive power output capacity of MMC is mainly constrained by the output AC voltage. The output AC voltage of MMC is affected by the voltage fluctuation range of its submodule capacitors and the operating range of the modulation ratio. Among them, the operating range of the modulation ratio has a more significant impact on the reactive power output capacity of MMC. Based on this, existing literature points out that increasing the modulation ratio of MMC can improve its output AC voltage and reactive power output capacity. However, simply increasing the modulation ratio will cause MMC to enter the overmodulation area, forcing the output AC voltage to "cut the top", resulting in a sharp increase in the total harmonic distortion rate of the grid-connected point voltage and failing to meet the grid-connected standard requirements. To address this problem, existing studies mainly increase the MMC modulation ratio and improve the output AC voltage and reactive power output capacity of MMC by optimizing modulation strategies such as third harmonic voltage injection, while ensuring that the system does not overmodulate. However, these methods have problems such as complex control methods and difficulty in implementation.

[0004] In fact, as an important factor affecting the output AC voltage of MMC, the restrictive effect of DC voltage on the reactive output capacity of MMC cannot be ignored. However, existing studies have not analyzed the restrictive factor of DC voltage. At the same time, considering the insulation requirements of the DC system, the DC voltage of MMC can vary within the range of ±10% of its rated value. Therefore, increasing the DC voltage can also be used as an effective way to improve the reactive output capacity of MMC. However, there is no literature that studies the feasibility of improving the reactive output capacity and the design of control schemes from the perspective of changing the DC voltage of MMC-STATCOM.

[0005] In summary, it is urgent to explore the impact of DC voltage on the reactive output capacity of MMC and study the design of control schemes to increase the DC voltage to improve the reactive output capacity of MMC. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a control system and method for improving the reactive power output capacity of MMC-STATCOM based on variable DC voltage, which can solve the problems of small adjustable range of MMC-STATCOM output AC voltage and insufficient reactive power output capacity under traditional fixed DC voltage control. At the same time, compared with traditional fixed DC voltage control, the present invention can realize flexible adjustment of DC voltage when the system reactive power demand is large, thereby effectively improving the MMC-STATCOM output AC voltage and reactive output capacity, while avoiding the occurrence of overmodulation, thereby ensuring the reliability of MMC-STATCOM operation.

[0007] The present invention solves the technical problem by adopting the following technical solutions:

[0008] The invention discloses a control system for improving reactive output capacity of MMC-STATCOM based on variable DC voltage, comprising an AC power grid, a power grid load, an MMC-STATCOM and an MMC-STATCOM control system, wherein the AC power grid is connected to the power grid load through an AC transmission line, the MMC-STATCOM is connected to the AC power grid through a connecting transformer, and the MMC-STATCOM control system acts on the MMC-STATCOM; each phase of the MMC in the MMC-STATCOM comprises two upper and lower bridge arms, each bridge arm comprises a bridge arm inductor and N cascaded half-bridge submodules; each submodule comprises a DC side capacitor and four semiconductor switching devices; and the coupling points of the upper and lower bridge arms are connected to the grid through a three-phase transformer.

[0009] Moreover, the MMC-STATCOM control system includes a carrier phase shift modulation module, a circulating current suppression module, a capacitor voltage balancing module, a current inner loop module, a droop module, a first coordinate conversion module, a second coordinate conversion module, a first PI controller, a second PI controller, a limiter, an adder, a first inverting adder, a second inverting adder, a mode determiner, a mode selector, a divider, a gain module, and an RMS module;

[0010] The RMS module collects the output AC voltage signal of the MMC-STATCOM to obtain the output AC voltage effective value signal U v And input it to the divider; the DC voltage signal of MMC-STATCOM is connected to the divider through the gain module; the output signal of the divider is divided into two paths, one of which outputs a control signal to the mode selector through the mode judge, and the other is connected to the mode selector through the first inverting adder, the first PI controller, the adder, and the limiter as the input of the second mode of the mode selector; the rated DC voltage setting value signal U dc0 It is divided into two paths, one of which is connected to the adder and the other is connected to the mode selector as the input of the first mode of the mode selector; the selection end of the mode selector is connected to the second PI controller through the second inverting adder; the current inner loop module includes four input signals, the first is connected to the second PI controller, the second is connected to the droop module, and the third and fourth are connected to the second coordinate conversion module; the two input signals of the second coordinate conversion module are the three-phase voltage signal and current signal of the grid connection point; the carrier phase shift modulation module includes two input signals, one is connected to the circulating current suppression module, and the other is connected to the current inner loop module through the first coordinate conversion module; the output signal of the carrier phase shift modulation module is connected to the capacitor voltage balancing module and the MMC-STATCOM in sequence.

[0011] A control system for improving reactive output capacity of MMC-STATCOM based on variable DC voltage includes the following steps:

[0012] Step 1: Calculate and update the modulation ratio signal m according to the real-time state of the power system;

[0013] Step 2: According to the modulation ratio signal and selecting the system working mode, update the DC voltage reference value;

[0014] Step 3: Calculate the current reference value i of the current inner loop module according to the DC voltage reference value d * and i q * ;

[0015] Step 4: Set the inner loop current reference value i d * and i q* and the actual measured value i d and i q The input current inner loop control module obtains the output AC three-phase voltage modulation signal; the modulation signal is input into the carrier phase shift modulation module to generate a pulse signal for opening and closing the switch device of the MMC-STATCOM.

[0016] Furthermore, the step 1 comprises the following steps:

[0017] Step 1.1: Measure the three-phase AC voltage signal u output by MMC-STATCOM through the RMS module vj Get the output AC voltage effective value U v ;

[0018] Step 1.2: Output AC voltage effective value U v Input one of the input terminals of the divider and convert the actual DC voltage measurement value U dc After being input to the other end through the gain module, the MMC modulation ratio signal m is obtained.

[0019] Furthermore, the step 2 comprises the following steps:

[0020] Step 2.1, input the modulation ratio signal m into the mode determiner to obtain a control signal; input the control signal into the mode selector, so that the mode selector selects the system working mode and outputs the corresponding DC voltage reference value U dc * ;

[0021] Step 2.2: Set the rated DC voltage to U dc0 As the DC voltage reference value U in the first mode of the mode selector dc * and input it into the mode selector;

[0022] Step 2.3: Modulation ratio signal m and modulation ratio signal maximum value m max The difference is input into the first inverting adder respectively, and the DC voltage reference value correction ΔU is obtained by the first PI controller. dc * ; ΔU dc * and U dc0 are respectively input into the adder to obtain the DC voltage reference value U in the second mode of the mode selector dc * and input to the mode selector.

[0023] Moreover, the specific implementation method of the mode selector selecting the system working mode in step 2.1 is:

[0024]

[0025] When m≤m max When MMC-STATCOM is in normal modulation state, the conventional constant DC voltage control is adopted to make the DC voltage reference value U dc * Maintain its rated value U dc0 and connecting the selection end of the mode selector to the first mode input of the mode selector;

[0026] When m>m max When MMC-STATCOM enters the overmodulation state, the dc0 Add DC voltage correction ΔU on this basis dc * Realize U dc * Dynamic adjustment of the mode selector and connecting the selected end of the mode selector to the second mode input of the mode selector.

[0027] Moreover, the U dc * The specific implementation method of the dynamic adjustment is: through the modulation ratio controller, and the modulation ratio controller is based on PI control:

[0028]

[0029] Among them, k p and k i are the proportional coefficient and integral coefficient of the modulation ratio controller respectively; U dcmax It is the maximum value of DC voltage reference.

[0030] Furthermore, the step 3 comprises the following steps:

[0031] Step 3.1: Set the DC voltage reference value U output by the mode selector dc * and the actual DC voltage measurement value U dc The difference is input into the second inverting adder respectively, and the d-axis current reference value i is obtained by the second PI controller. d * ;

[0032] Step 3.2: The droop module uses reactive power-AC voltage droop control to generate a q-axis current reference value i according to the grid connection point voltage. q * .

[0033] The advantages and positive effects of the present invention are:

[0034] The present invention includes an AC power grid, a power grid load, an MMC-STATCOM and an MMC-STATCOM control system, wherein the AC power grid is connected to the power grid load through an AC transmission line, the MMC-STATCOM is connected to the AC power grid through a connecting transformer, and the MMC-STATCOM control system is connected to the MMC-STATCOM. The MMC-STATCOM control system of the present invention can generate a control signal to start the modulation ratio controller through a simple mode judgment when the modulation ratio signal reaches its maximum value, and maintain the modulation ratio at its maximum value, thereby avoiding the system from entering an overmodulation state and improving the operating stability of the MMC-STATCOM. At the same time, compared with the traditional fixed DC voltage control, the present invention can increase the DC voltage within the allowable range when the system reactive power demand is large, thereby improving the output AC voltage and reactive power output capacity of the MMC-STATCOM. At the same time, compared with methods such as third harmonic voltage injection, the control structure of the method proposed in this article is simple, easy to implement, and not easily affected by the original control of the MMC. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a structural block diagram of the MMC-STATCOM grid-connected system of the present invention;

[0036] Figure 2 is a block diagram of the control method of the present invention;

[0037] Figure 3 This is a diagram showing the improvement of reactive output capacity of the present invention compared with traditional control.

[0038] in, Figure 1 In: X0 is the bridge arm reactance; X t is the transformer equivalent reactance; S1, S2, D1, D2 are submodule switch devices; C is the submodule capacitance.

[0039] Figure 2 Medium: u vj (j=a, b, c) is the output AC three-phase voltage signal; U v is the effective value of the output AC voltage; u ac,j and i j are the three-phase voltage and current signals at the grid connection point; m and m max are the modulation ratio and its maximum value respectively; ΔU dc * is the correction value of DC voltage reference value; U dc0 is the rated DC voltage setting value; U dc and U dc * are the actual DC voltage measurement value and the DC voltage reference value respectively; u ac,dq and i dq are the voltage and current signals of the grid connection point in the dq coordinate system; idq * is the inner loop current reference signal in the dq coordinate system; u vdq * is the output AC voltage reference signal in the dq coordinate system; u vj * Output AC three-phase voltage reference signal; X eq =X t +X0 / 2 is the equivalent reactance between the grid connection point and the MMC output AC voltage point. DETAILED DESCRIPTION

[0040] The present invention is further described in detail below with reference to the accompanying drawings.

[0041] MMC-STATCOM reactive power output capacity improvement control system based on variable DC voltage, such as Figure 1 As shown, it includes an AC power grid, a power grid load, an MMC-STATCOM and an MMC-STATCOM control system, wherein the AC power grid is connected to the power grid load through an AC transmission line, the MMC-STATCOM is connected to the AC power grid through a connecting transformer, and the MMC-STATCOM control system acts on the MMC-STATCOM; each phase of the MMC in the MMC-STATCOM includes two upper and lower bridge arms, each bridge arm includes a bridge arm inductor and N cascaded half-bridge sub-modules; each sub-module includes a DC side capacitor and four semiconductor switching devices; the coupling points of the upper and lower bridge arms are connected to the grid through a three-phase transformer. The MMC-STATCOM can control the upper and lower bridge arm voltage output by controlling the conduction state of the sub-module switch device, and further control the MMC-STATCOM output AC voltage and the reactive power exchanged with the AC system.

[0042] The MMC-STATCOM control system includes a carrier phase shift modulation module, a circulating current suppression module, a capacitor voltage balancing module, a current inner loop module, a droop module, a first coordinate conversion module, a second coordinate conversion module, a first PI controller, a second PI controller, a limiter, an adder, a first inverting adder, a second inverting adder, a mode judger, a mode selector, a divider, a gain module, and an RMS module.

[0043] The RMS module collects the output AC voltage signal of MMC-STATCOM to obtain the output AC voltage effective value signal U v And input it to the divider; the DC voltage signal of MMC-STATCOM is connected to the divider through the gain module; the output signal of the divider is divided into two paths, one of which outputs a control signal to the mode selector through the mode judge, and the other is connected to the mode selector through the first inverting adder, the first PI controller, the adder, and the limiter as the input of the second mode of the mode selector; the rated DC voltage setting value signal Udc0 It is divided into two paths, one of which is connected to the adder and the other is connected to the mode selector as the input of the first mode of the mode selector; the selection end of the mode selector is connected to the second PI controller through the second inverting adder; the current inner loop module includes four input signals, the first is connected to the second PI controller, the second is connected to the droop module, and the third and fourth are connected to the second coordinate conversion module; the two input signals of the second coordinate conversion module are the three-phase voltage signal and current signal of the grid connection point; the carrier phase shift modulation module includes two input signals, one is connected to the circulating current suppression module, and the other is connected to the current inner loop module through the first coordinate conversion module; the output signal of the carrier phase shift modulation module is connected to the capacitor voltage balancing module and the MMC-STATCOM in sequence.

[0044] A control system for improving reactive output capacity of MMC-STATCOM based on variable DC voltage includes the following steps:

[0045] Step 1: Calculate and update the modulation ratio signal m according to the real-time status of the power system.

[0046] Step 1.1: Measure the three-phase AC voltage signal u output by MMC-STATCOM through the RMS module vj Get the output AC voltage effective value U v ;

[0047] Step 1.2: Output AC voltage effective value U v Input one of the input terminals of the divider and convert the actual DC voltage measurement value U dc After being input to the other end through the gain module, the MMC modulation ratio signal m is obtained.

[0048] Step 2: According to the modulation ratio signal and by selecting the system operation mode, the DC voltage reference value is updated.

[0049] Step 2.1, input the modulation ratio signal m into the mode determiner to obtain a control signal; input the control signal into the mode selector, so that the mode selector selects the system working mode and outputs the corresponding DC voltage reference value U dc * .

[0050] The modulation ratio m of MMC and the output AC output voltage U v , DC voltage U dc The relationship between them is:

[0051]

[0052] Where K = U dcn / U vn is a constant coefficient, U dcn and U vnThey are the DC voltage and output AC voltage reference values ​​respectively.

[0053] The specific implementation method of the mode selector to select the system working mode is:

[0054]

[0055] When m≤m max When MMC-STATCOM is in normal modulation state, the conventional constant DC voltage control is adopted to make the DC voltage reference value U dc * Maintain its rated value U dc0 (1pu), and connecting the selection end of the mode selector to the first mode input of the mode selector;

[0056] When m>m max When MMC-STATCOM enters the overmodulation state, the dc0 Add DC voltage correction ΔU on this basis dc * Realize U dc * Dynamic adjustment of the mode selector and connecting the selected end of the mode selector to the second mode input of the mode selector.

[0057] The DC voltage reference value is U dc * Dynamic adjustment: This is achieved through a modulation ratio controller, which is based on PI control:

[0058]

[0059] Among them, k p and k i are the proportional coefficient and integral coefficient of the modulation ratio controller respectively; U dcmax is the maximum DC voltage reference value, and the modulation ratio controller is m max =1 is the reference value, and its output ΔU dc * is greater than 0, thereby increasing U dc * , and control m to m max .

[0060] Step 2.2: Set the rated DC voltage to U dc0 As the DC voltage reference value U in the first mode of the mode selector dc * and input it into the mode selector.

[0061] Step 2.3: Modulation ratio signal m and modulation ratio signal maximum value m maxThe difference is input into the first inverting adder respectively, and the DC voltage reference value correction ΔU is obtained by the first PI controller. dc * ; ΔU dc * and U dc0 are respectively input into the adder to obtain the DC voltage reference value U in the second mode of the mode selector dc * and input to the mode selector.

[0062] Step 3: Calculate the current reference value i of the current inner loop module according to the DC voltage reference value d * and i q * .

[0063] Step 3.1: Set the DC voltage reference value U output by the mode selector dc * and the actual DC voltage measurement value U dc The difference is input into the second inverting adder respectively, and the d-axis current reference value i is obtained by the second PI controller. d * .

[0064] Step 3.2: The droop module uses reactive power-AC voltage droop control to generate a q-axis current reference value i according to the grid connection point voltage. q * .

[0065] Step 4: Set the inner loop current reference value i d * and i q * and the actual measured value i d and i q The input current inner loop control module obtains the output AC three-phase voltage modulation signal; the modulation signal is input into the carrier phase shift modulation module to generate a pulse signal for opening and closing the switch device of the MMC-STATCOM.

[0066] like Figure 3 As shown, the AC grid voltage U s Reduction and reactive load Q L The increase will lead to an increase in the MMC-STATCOM output reactive power Q. s Lower and Q L When the increase is small, such as Figure 3 As shown in region I, the MMC-STATCOM under both control modes can satisfy the output AC voltage constraint and thus output the same Q. s Less than 0.88pu or Q LWhen it is higher than 0.95pu, Figure 3 As shown in region II, the reactive power output of MMC-STATCOM under the two control modes is significantly different: under the traditional constant DC voltage control, MMC-STATCOM reaches the output AC voltage constraint, so Q reaches its maximum value and cannot be further increased; in contrast, the control proposed in the present invention increases the DC voltage by starting the modulation ratio controller, expanding U v Therefore, under the control of the present invention, the reactive power that can be output by MMC-STATCOM is significantly higher than that of the traditional constant DC voltage control.

[0067] It should be emphasized that the embodiments described in the present invention are illustrative rather than restrictive. Therefore, the present invention includes but is not limited to the embodiments described in the specific implementation manner. Any other implementation manners derived by those skilled in the art based on the technical solution of the present invention also fall within the scope of protection of the present invention.

Claims

1. The MMC-STATCOM reactive output capacity improvement control system based on variable DC voltage is characterized by: It includes an AC power grid, a power grid load, an MMC-STATCOM and an MMC-STATCOM control system, wherein the AC power grid is connected to the power grid load through an AC transmission line, the MMC-STATCOM is connected to the AC power grid through a connecting transformer, and the MMC-STATCOM control system acts on the MMC-STATCOM; each phase of the MMC in the MMC-STATCOM includes two upper and lower bridge arms, each bridge arm includes a bridge arm inductor and N cascaded half-bridge submodules; each submodule includes a DC side capacitor and four semiconductor switch devices; the coupling points of the upper and lower bridge arms are connected to the grid through a three-phase transformer; The MMC-STATCOM control system includes a carrier phase shift modulation module, a circulating current suppression module, a capacitor voltage balancing module, a current inner loop module, a droop module, a first coordinate conversion module, a second coordinate conversion module, a first PI controller, a second PI controller, a limiter, an adder, a first inverting adder, a second inverting adder, a mode determiner, a mode selector, a divider, a gain module, and an RMS module; The RMS module collects the output AC voltage signal of the MMC-STATCOM to obtain the output AC voltage effective value signal U v And input it to the divider; the DC voltage signal of MMC-STATCOM is connected to the divider through the gain module; the output signal of the divider is divided into two paths, one of which outputs a control signal to the mode selector through the mode judge, and the other is connected to the mode selector through the first inverting adder, the first PI controller, the adder, and the limiter as the input of the second mode of the mode selector; the rated DC voltage setting value signal U dc0 It is divided into two paths, one of which is connected to the adder and the other is connected to the mode selector as the input of the first mode of the mode selector; the selection end of the mode selector is connected to the second PI controller through the second inverting adder; the current inner loop module includes four input signals, the first is connected to the second PI controller, the second is connected to the droop module, and the third and fourth are connected to the second coordinate conversion module; the two input signals of the second coordinate conversion module are the three-phase voltage signal and current signal of the grid connection point; the carrier phase shift modulation module includes two input signals, one is connected to the circulating current suppression module, and the other is connected to the current inner loop module through the first coordinate conversion module; the output signal of the carrier phase shift modulation module is connected to the capacitor voltage balancing module and the MMC-STATCOM in sequence.

2. A control method for improving the reactive output capacity of MMC-STATCOM based on variable DC voltage according to claim 1, characterized in that: The following steps are involved: Step 1: Calculate and update the modulation ratio signal m according to the real-time state of the power system; Step 2: According to the modulation ratio signal and selecting the system working mode, the DC voltage reference value is updated; Step 3: Calculate the current reference value i of the current inner loop module according to the DC voltage reference value d * and i q * ; Step 4: Set the inner loop current reference value i d * and i q * and the actual measured value i d and i q The input current inner loop control module obtains the output AC three-phase voltage modulation signal; the modulation signal is input into the carrier phase shift modulation module to generate a pulse signal for opening and closing the switch device of the MMC-STATCOM.

3. The control method of the MMC-STATCOM reactive output capacity improvement control system based on variable DC voltage according to claim 2 is characterized in that: The step 1 comprises the following steps: Step 1.1: Measure the three-phase AC voltage signal u output by MMC-STATCOM through the RMS module vj Get the output AC voltage effective value U v ; Step 1.2: Output AC voltage effective value U v Input one of the input terminals of the divider and convert the actual DC voltage measurement value U dc After being input to the other end through the gain module, the MMC modulation ratio signal m is obtained.

4. The control method of the MMC-STATCOM reactive output capacity improvement control system based on variable DC voltage according to claim 2 is characterized in that: The step 2 comprises the following steps: Step 2.1, input the modulation ratio signal m into the mode determiner to obtain a control signal; input the control signal into the mode selector, so that the mode selector selects the system working mode and outputs the corresponding DC voltage reference value U dc * ; Step 2.2: Set the rated DC voltage to U dc0 As the DC voltage reference value U in the first mode of the mode selector dc * and input it into the mode selector; Step 2.3: Modulation ratio signal m and modulation ratio signal maximum value m max The difference is input into the first inverting adder respectively, and the DC voltage reference value correction ΔU is obtained by the first PI controller. dc * ; ΔU dc * and U dc0 are respectively input into the adder to obtain the DC voltage reference value U in the second mode of the mode selector dc * and input to the mode selector.

5. The control method of the MMC-STATCOM reactive output capacity improvement control system based on variable DC voltage according to claim 4 is characterized in that: The specific implementation method of the mode selector selecting the system working mode in step 2.1 is: When m≤m max When MMC-STATCOM is in normal modulation state, the conventional constant DC voltage control is adopted to make the DC voltage reference value U dc * Maintain its rated value U dc0 and connecting the selection end of the mode selector to the first mode input of the mode selector; When m>m max When MMC-STATCOM enters the overmodulation state, the dc0 Add DC voltage correction ΔU on this basis dc * Realize U dc * Dynamic adjustment of the mode selector and connecting the selected end of the mode selector to the second mode input of the mode selector.

6. The control method of the MMC-STATCOM reactive output capacity improvement control system based on variable DC voltage according to claim 5 is characterized in that: The U dc * The specific implementation method of the dynamic adjustment is: through the modulation ratio controller, and the modulation ratio controller is based on PI control: Among them, k p and k i are the proportional coefficient and integral coefficient of the modulation ratio controller respectively; U dcmax It is the maximum value of DC voltage reference.

7. The control method of the MMC-STATCOM reactive output capacity improvement control system based on variable DC voltage according to claim 2 is characterized in that: The step 3 comprises the following steps: Step 3.1: Set the DC voltage reference value U output by the mode selector dc * and the actual DC voltage measurement value U dc The difference is input into the second inverting adder respectively, and the d-axis current reference value i is obtained by the second PI controller. d * ; Step 3.2: The droop module uses reactive power-AC voltage droop control to generate a q-axis current reference value i according to the grid connection point voltage. q * .

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