CN103337980A - Modular multilevel converter (MMC) circulating current suppression method - Google Patents

Abstract

The invention relates to a method for suppressing circulating current of a modularized multilevel converter, which belongs to the technical field of electric engineering, and in particular relates to a method for suppressing the internal circulating current of a modularized multilevel converter. The present invention solves the problem that the existing multilevel converter circulation suppression method cannot be compatible with single-phase and three-phase systems, and the circulation suppression ability is poor and the control method is complicated. The present invention uses a trap to extract the second harmonic in the phase current Signal, subtract the second harmonic signal of the phase current from the average current of the phase current to obtain the average current signal of the upper and lower bridge arms without the second harmonic, and remove the average current signal of the upper and lower bridge arms of the second harmonic The difference between the current signal and the reference current is obtained to obtain the second harmonic signal of the circulating current; the proportional resonance controller is used to track the current of the second harmonic signal of the circulating current to obtain a voltage compensation signal, and the compensation signal is processed and fed back to the modular multiple The sub-module of the upper bridge arm of the level converter realizes circulation suppression. The invention is applicable to the field of electrotechnical technology.

Description

Circulating Current Suppression Method for Modular Multilevel Converter

technical field

The invention belongs to the field of electrotechnical technology, and in particular relates to a suppression method for suppressing internal circulation of a modular multilevel converter. the

Background technique

With the increasing number of new energy supply channels, the voltage source converter based DC transmission technology (Voltage Source Converter Based HVDC, VSC-HVDC) has received more and more attention. The traditional two-level or three-level VSC topology has problems such as high switching loss, high current harmonics, and low reliability. Compared with other converters, the modular multilevel converter (MMC) proposed by German scholar Rainer Marquardt has advantages such as redundant control, more output levels, and common DC Advantages such as bus bars provide a new solution for high-voltage and high-power energy conversion applications. However, due to the large number of modules in the modular multilevel converter, there are technical obstacles such as complex control strategies, difficult voltage equalization of sub-module capacitors, and large switching losses. The above problems are closely related to the internal circulating current of the modular multilevel converter, especially its double fundamental frequency component. Therefore, it is particularly important to conduct research on the internal circulating current of the MMC. the

There are the following suppression schemes for the internal circulation of MMC:

1) Increase the inductance of the MMC bridge arm. This method is effective to a certain extent, but only relying on increasing the inductance of the bridge arm to suppress the internal circulation has limited effect, and will also reduce the frequency response speed of the system, and will greatly increase the size and cost of the MMC. the

2) The three-phase circulating current inside the converter is decomposed into two DC components through the double frequency negative sequence rotation coordinate transformation, and the corresponding circulating current suppression controller is designed. This method requires double frequency negative sequence coordinate transformation and phase-to-phase The decoupling link increases the calculation amount of the MMC control system, and this method is only applicable to the three-phase system. the

3) The circulating current suppressor is designed by compensating the circulating current voltage drop on the upper and lower bridge arm inductance in the upper and lower bridge arm voltage commands respectively. The principle of this method is relatively simple, but due to the introduction of the differential link, The difficulty of actual control increases. the

However, the above method has the problems of incompatibility with single-phase and three-phase systems, poor circulation suppression ability and complicated control methods. the

Invention content:

In order to solve the problem that the existing multi-level converter circulation suppression method cannot be compatible with single-phase and three-phase systems, and the circulation suppression ability is poor and the control method is complex, a modular multi-level converter circulation suppression method is proposed. . the

The method for suppressing the circulating current of the modularized multilevel converter of the present invention is realized based on the modularized multilevel converter. The modularized multilevel converter is composed of three-phase circuits, and each phase circuit It is composed of upper and lower bridge arm inductors and n sub-modules in series, wherein, 1≤n<+∞, each sub-module is composed of a half-bridge converter and an energy storage capacitor, and the half-bridge converter includes a diode No. , No. 1 controllable switch, No. 2 controllable switch and No. 2 diode; the cathode of No. 1 diode is connected to the collector of No. 1 controllable switch and one end of the energy storage capacitor; the emitter of No. 1 controllable switch is connected to the No. 2 controllable switch The collector of the control switch is connected to the cathode of the second diode, and the anode of the second diode is connected to the emitter of the second control switch and the other end of the energy storage capacitor; the emitter of the first control switch is the signal input of the sub-module terminal; the emitter of the second controllable switch is the signal output terminal of the sub-module;

It is characterized in that: the method is used to suppress the circulating current of the modularized multilevel converter, and the specific steps of the method for suppressing the circulating current of the modularized multilevel converter are:

Step 1. Use phase-locked loop PLL to modulate the AC voltage of the power grid to obtain the reference current i _ref . The reference current i _ref uses a subtractor to subtract the phase current i of the modular multilevel converter to obtain the No. 1 current difference signal ;

Step 2. Use a proportional resonance controller to track and compensate the obtained No. 1 current difference signal to obtain a compensation signal;

Step 3: Multiply the grid AC current by the proportional coefficient k, and use an adder to add the k-fold grid AC current signal to the obtained compensation signal to obtain the reference voltage u _ref ;

The reference voltage u _ref obtained in step 4 and step 3 is added to the DC voltage E of the modular multilevel converter through an adder to obtain the No. 1 voltage and signal;

Step 5, using the subtractor to subtract the DC voltage E of the modular multilevel converter from the reference voltage u _ref obtained in step 3 to obtain the No. 2 voltage difference signal;

The DC voltage E= _Udc /2 of the modularized multilevel converter, where _Udc is the total voltage of the DC side of the modularized multilevel converter;

Step 6. Multiply the phase current output by the modular multilevel converter by the proportional coefficient 1/2 to obtain the average current of the upper and lower bridge arms;

Step 7: Using a circulating current suppressor to suppress and compensate the obtained average currents of the upper and lower bridge arms to obtain a compensation signal u _cc ;

Use the circulating current suppressor to suppress and compensate the average current of the upper and lower bridge arms, and the specific method to obtain the compensation signal u _cc is as follows:

Step 71. Using a notch filter to extract the second harmonic of the average current signal of the upper and lower bridge arms obtained in step 6 to obtain a second harmonic current signal;

Step 72: Use a subtractor to make a difference between the average current signal of the upper and lower bridge arms obtained in step 6 and the current of the second harmonic obtained in step 71 to obtain the average value of the upper and lower bridge arms with the second harmonic removed current signal;

Step 73, using a subtractor to make a difference between the average current signal of the upper and lower bridge arms with the second harmonic removed and the reference current, to obtain the second harmonic signal of the circulating current;

Step 74: Use a proportional resonance controller to perform current tracking on the second harmonic signal of the circulating current obtained in Step 73 to obtain a voltage compensation signal u _cc ; and perform Step 8;

Step 8, using a subtractor to make a difference between the No. 2 voltage difference signal obtained in step 5 and the compensation signal u _cc obtained in step 7 to obtain No. 3 voltage difference signal;

Use the subtractor to make a difference between the No. 1 voltage sum signal obtained in step 4 and the compensation signal obtained in step 7 to obtain the No. 4 voltage difference signal;

Step 9. Multiply the No. 3 voltage difference signal and the No. 4 voltage difference signal obtained in step 8 by the proportional coefficient 1/2E respectively; obtain the No. 3 voltage difference signal of 1/2E and the No. 4 voltage difference signal of 1/2E;

Step 10. Use the carrier phase shift modulator to perform carrier phase shift modulation on the 1/2E No. 3 voltage difference signal obtained in step 9, and feed back the signal obtained after carrier modulation to the upper bridge of the modular multilevel converter In the sub-module of the arm, the circulation suppression of the upper bridge arm circuit is realized;

Use the carrier phase shift modulator to perform carrier phase shift modulation on the 1/2E No. 4 voltage difference signal obtained in step 9, and feed back the signal obtained after carrier modulation to the sub-section of the upper bridge arm of the modular multilevel converter In the module, the circulation suppression of the lower bridge arm circuit is realized. the

The invention suppresses the circulation of any phase of the three-phase output of the modular multilevel converter, and is compatible with single-phase and three-phase systems. Moreover, the present invention reduces the circulating current inside the circuit, reduces the fluctuation of the capacitor voltage of the MMC sub-module, reduces the switching loss, and improves the circulating current suppression ability by more than 10% compared with the existing circulating current suppression method. the

Description of drawings

Figure 1 is a structural diagram of a modular multilevel converter, in which a, b, and c represent the three-phase output of the modular multilevel converter, respectively, and SM1, SM2...SMn represent sub-modules;

Figure 2 is a circuit structure diagram of the sub-module in the modular multi-level converter, in which Uc is the capacitor voltage value of the sub-module;

Figure 3 is a block diagram of the circulating current control of the circulating current suppressor. In the figure, i _z is the circulating current output by the phase of the modular multilevel converter, and i _z2 is the secondary component of the circulating current of this phase;

Fig. 4 is the control block diagram that comprises the modularized multilevel converter circulation suppression method of the present invention;

Figure 5 is the effect diagram of circulation suppression; the dotted line on the left side of the figure is the MMC startup time, and the dotted line on the right side is the NCCS startup time;

Figure 6 is a comparison effect diagram of circulation suppression; curve 1 in the figure is the current curve before circulation suppression, and curve 2 is the current curve after circulation suppression;

Figure 7 is a comparison diagram of sub-module voltage suppression effects; curve 3 in the figure is the voltage curve before circulation suppression, and curve 4 is the voltage curve after circulation suppression;

Figure 8 is a diagram of the power effect of phase a after the circulation is suppressed. the

Detailed ways

Specific Embodiment 1: This embodiment is described in conjunction with Fig. 1, Fig. 2, Fig. 3, and Fig. 4. The method for suppressing circulating current of a modular multilevel converter described in this embodiment is based on a modular multilevel converter The multi-level converter based on modularization is composed of three-phase circuits, and each phase circuit is composed of upper and lower bridge arm inductors and n sub-modules connected in series, wherein, 1≤n<+∞, each sub-module Both are composed of a half-bridge converter and an energy storage capacitor 1-1. The half-bridge converter includes a No. 1 diode 1-2, a No. 1 controllable switch 1-3, a No. 2 controllable switch 1-4 and a No. 2 No. diode 1-5; the cathode of No. 1 diode 1-2 is connected to the collector of No. 1 controllable switch 1-3 and an end of energy storage capacitor 1-1; the emitter of No. 1 controllable switch 1-3 is connected to No. 2 The collector of the controllable switch 1-4 is connected to the cathode of the second diode 1-5, and the anode of the second diode 1-5 is connected to the emitter of the second controllable switch 1-4 and the other end of the energy storage capacitor 1-1 ; The emitter of No. 1 controllable switch 1-3 is the signal input end of the sub-module; the emitter of No. 2 controllable switch 1-4 is the signal output end of the sub-module;

It is characterized in that: the method is used to suppress the circulating current of the modularized multilevel converter, and the specific steps of the method for suppressing the circulating current of the modularized multilevel converter are:

Step 1. Use phase-locked loop PLL to modulate the AC voltage of the power grid to obtain the reference current i _ref . The reference current i _ref uses a subtractor to subtract the phase current i of the modular multilevel converter to obtain the No. 1 current difference signal ;

Step 2. Use a proportional resonance controller to track and compensate the obtained No. 1 current difference signal to obtain a compensation signal;

Step 3: Multiply the grid AC current by the proportional coefficient k, and use an adder to add the k-fold grid AC current signal to the obtained compensation signal to obtain the reference voltage u _ref ;

The reference voltage u _ref obtained in step 4 and step 3 is added to the DC voltage E of the modular multilevel converter through an adder to obtain the No. 1 voltage and signal;

Step 5, using the subtractor to subtract the DC voltage E of the modular multilevel converter from the reference voltage u _ref obtained in step 3 to obtain the No. 2 voltage difference signal;

The DC voltage E= _Udc /2 of the modularized multilevel converter, _Udc is the total voltage of the DC side of the modularized multilevel converter;

Step 6. Multiply the phase current output by the modular multilevel converter by the proportional coefficient 1/2 to obtain the average current of the upper and lower bridge arms;

Step 7: Using a circulating current suppressor to suppress and compensate the obtained average currents of the upper and lower bridge arms to obtain a compensation signal u _cc ;

Use the circulating current suppressor to suppress and compensate the average current of the upper and lower bridge arms, and the specific method to obtain the compensation signal u _cc is as follows:

Step 71. Using a notch filter to extract the second harmonic of the average current signal of the upper and lower bridge arms obtained in step 6 to obtain a second harmonic current signal;

Step 72: Use a subtractor to make a difference between the average current signal of the upper and lower bridge arms obtained in step 6 and the current of the second harmonic obtained in step 71 to obtain the average value of the upper and lower bridge arms with the second harmonic removed current signal;

Step 73, using a subtractor to make a difference between the average current signal of the upper and lower bridge arms with the second harmonic removed and the reference current, to obtain the second harmonic signal of the circulating current;

Step 74: Use a proportional resonance controller to perform current tracking on the second harmonic signal of the circulating current obtained in Step 73 to obtain a voltage compensation signal u _cc ; and perform Step 8;

Step 8, using a subtractor to make a difference between the No. 2 voltage difference signal obtained in step 5 and the compensation signal u _cc obtained in step 7 to obtain No. 3 voltage difference signal;

Use the subtractor to subtract the No. 1 voltage sum signal obtained in step 4 and the compensation signal obtained in step 7 to obtain the No. 4 voltage difference signal;

Step 9. Multiply the No. 3 voltage difference signal and the No. 4 voltage difference signal obtained in step 8 by the proportional coefficient 1/2E respectively; obtain the No. 3 voltage difference signal of 1/2E and the No. 4 voltage difference signal of 1/2E;

Step 10. Use the carrier phase shift modulator to perform carrier phase shift modulation on the 1/2E No. 3 voltage difference signal obtained in step 9, and feed back the signal obtained after carrier modulation to the upper bridge of the modular multilevel converter In the sub-module of the arm, the circulation suppression of the upper bridge arm circuit is realized;

Use the carrier phase shift modulator to perform carrier phase shift modulation on the 1/2E No. 4 voltage difference signal obtained in step 9, and feed back the signal obtained after carrier modulation to the sub-section of the upper bridge arm of the modular multilevel converter In the module, the circulation suppression of the lower bridge arm circuit is realized. the

Embodiment 2: This embodiment is a further description of the method for suppressing the circulating current of a modular multilevel converter described in Embodiment 1. The notch filter described in step 71 adopts an infinite impulse response digital filter. The transfer function of the digital filter is:

$\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&tau;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Where ω ₀ is the resonant frequency, s is the Laplacian operator, τ ₀ =1/2Q, Q is the quality factor of the trap circuit;

And the formula (1) is given by the formula

$\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; z</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i</span>}}}{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; z</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i</span>}}}<span\; class="notranslate">\; =</span>\frac{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Pi;</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; z</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; )</span>}{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Pi;</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

When there is only a pair of conjugate zeros located on the imaginary axis, and a pair of conjugate poles corresponding to the zeros, it is a single frequency notch filter, which is obtained when a notch appears at frequency ω ₀ ;

In formula 2, a _i and b _i are the constant coefficients of the filter function, z _i is the zero point of the transfer function, p _i is the pole point of the transfer function, M is the number of zero points, N is the number of pole points, and z is the Z transformation operator z ^{- i} represents the i power of the reciprocal of z.

Embodiment 3: This embodiment is a further description of the method for suppressing the circulating current of a modular multilevel converter described in Embodiment 1. The transfer function of the proportional resonance controller is: The transfer function of the proportional resonance controller is:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PR</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; quasi</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}\mathrm{<span\; class="notranslate">\; the\; s</span>}}{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

In the formula, K _p is the proportional item coefficient, K _r is the resonance item coefficient, ω ₀ is the resonance frequency, ω _c is the cut-off angular frequency, and when the grid voltage frequency is allowed to fluctuate within ±0.8Hz, then ω _c /π=1.6 Hz, so ω _c =5Hz.

The transfer function of the PR controller described in this embodiment overcomes the transfer function of

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PR</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; ideal</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}\mathrm{<span\; class="notranslate">\; the\; s</span>}}{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

There are two main problems in the actual system application of the ideal PR controller: (1) due to the limitation of the accuracy of the parameters of the analog system components and the accuracy of the digital system, the proportional resonant controller is not easy to realize; (2) the PR controller is The gain at the resonance point is very small, and when the grid frequency shifts, the harmonics generated by the grid cannot be effectively suppressed; the gain at the resonance point tends to infinity, and there is almost no attenuation outside this frequency point. Although the ideal PR controller can achieve zero steady-state error, it improves the ability to selectively resist grid voltage disturbance. the

At the same time, the proportional resonant controller needs to meet the following points: the quality factor should be as large as possible to reduce the steady-state error; the cut-off frequency of the open-loop transfer function of the control system should be less than 1/5 of the switching frequency to reduce switching ripple; The phase stability margin should be as large as possible to reduce the overshoot and oscillation times of the output current in each control cycle. the

Taking phase a as an example, the overall control block diagram of the system based on the above suppression strategy is shown in Figure 4: According to the system block diagram, in the qualitative analysis, taking PR as the ideal proportional resonance link, the output of the AC side of the modular multilevel converter can be obtained The transfer functions of the current i _a circulating current i _za and its secondary fundamental frequency component i _{za_2} are:

${\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; z</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; s\; L</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; s\; L</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 5</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="G\; z"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">G</figure-callout></span><figure-callout\; id="2"\; label="G\; z"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">\; </figure-callout>}}_{\mathrm{<span\; class="notranslate">\; </span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 5</span>}{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 50</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 5</span>}\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; sL</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; sL</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 5</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

L and R are the equivalent inductance and resistance of the grid side, L _c is the bridge arm inductance, I _a , I _z and I _{z_2} are the image functions of i _a , i _za and i _{za_2} respectively, G _z,o (s) and G _z2,o (s) is the transfer function of the output current I _a to the circulating current I _z and the secondary fundamental frequency component i _{za_2} .

From formulas (1) and (2), the transfer function of the secondary fundamental frequency component of the circulating current to the circulating current is:

${\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="G\; z"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">G</figure-callout></span><figure-callout\; id="2"\; label="G\; z"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">\; </figure-callout>}}_{\mathrm{<span\; class="notranslate">\; </span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="G\; z"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">G</figure-callout></span><figure-callout\; id="2"\; label="G\; z"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">\; </figure-callout>}}_{\mathrm{<span\; class="notranslate">\; </span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; [</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; sL</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; the\; s</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 5</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span>}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 50</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 5</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; sL</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="HDA00003276685600041.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 5</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

When s=jω ₀ , G _z2,z (s)=0, in the circulation, the secondary fundamental frequency component has been fully suppressed, where j is the imaginary unit.

Embodiment 4. This embodiment is a further description of the method for suppressing the circulating current of a modular multilevel converter described in Embodiment 2, ω ₀ =100π.

In this embodiment, ω ₀ is taken as 100π because the target to be suppressed is twice the fundamental frequency component.

Embodiment 5. This embodiment is a further description of the method for suppressing the circulating current of a modular multilevel converter described in Embodiment 2, τ ₀ =0.2.

From τ ₀ =1/2Q, which is the quality factor of the notch filter circuit, it can be seen that the smaller τ ₀ is, the closer the pole is to the unit circle, the deeper the frequency response curve is, and the width of the sag is narrower. The present invention needs to filter out the secondary fundamental frequency current, so take ω ₀ =100π, and take τ ₀ as 0.2 to meet the accuracy requirement.

The single-ended MMC grid-connected simulation platform has been built using the method of the present invention to verify the suppression effect of the method proposed by the present invention on the circulation. The system parameters are as shown in Table 1, and the modulation strategy is selected from the dual modulation wave with a phase difference of 180 degrees. Carrier phase-shifting sinusoidal pulse width modulation technology controls the upper and lower bridge arms respectively, and the simulation results are shown in Figures 5, 6, 7 and 8. the

Table 1 Simulation parameter table

The comparison of circulation suppression effects is shown in Figure 5 and Figure 6. Before adding NCCS, the unsuppressed circulation fluctuated between -30A and 30A, but after adding CCS to suppress and stabilize, the peak value of the circulation did not exceed 3.5A, only the original About 5%. It can be seen that the effect of the suppression method described in the present invention is very obvious. the

Figure 7 and Figure 8 show the comparison of the sub-module voltage waveform and the total power of phase a before and after suppression. After adding the CCS proposed by the present invention, the sub-module capacitor voltage waveform has been significantly improved, and the phase power fluctuation has been significantly reduced at the same time. The service life of the capacitor is extended, and the reliability of the MMC is improved. the

Claims (5)

1. A method for suppressing circulating current of a modular multilevel converter, which is realized based on a modular multilevel converter, said modular multilevel converter is composed of three-phase circuits, each phase circuit It is composed of upper and lower bridge arm inductors and n sub-modules in series, where 1≤n<+∞, each sub-module is composed of a half-bridge converter and an energy storage capacitor (1-1), the half-bridge converter The device includes No. 1 diode (1-2), No. 1 controllable switch (1-3), No. 2 controllable switch (1-4) and No. 2 diode (1-5); No. 1 diode (1-2) The cathode of the first controllable switch (1-3) is connected to the collector of the first controllable switch (1-3) and one end of the energy storage capacitor (1-1); the emitter of the first controllable switch (1-3) is connected to the second controllable switch (1-1 -4) The collector is connected to the cathode of the second diode (1-5), and the anode of the second diode (1-5) is connected to the emitter of the second controllable switch (1-4) and the energy storage capacitor (1 The other end of -1); the emitter of the first controllable switch (1-3) is the signal input end of the sub-module; the emitter of the second controllable switch (1-4) is the signal output end of the sub-module ; It is characterized in that: the method is used to suppress the circulating current of the modularized multilevel converter, and the specific steps of the method for suppressing the circulating current of the modularized multilevel converter are: Step 1. Use phase-locked loop PLL to modulate the AC voltage of the power grid to obtain the reference current i _ref . The reference current i _ref uses a subtractor to subtract the phase current i of the modular multilevel converter to obtain the No. 1 current difference signal ; Step 2. Use a proportional resonance controller to track and compensate the obtained No. 1 current difference signal to obtain a compensation signal; Step 3: Multiply the grid AC current by the proportional coefficient k, and use an adder to add the k-fold grid AC current signal to the obtained compensation signal to obtain the reference voltage u _ref ; The reference voltage u _ref obtained in step 4 and step 3 is added to the DC voltage E of the modular multilevel converter through an adder to obtain the No. 1 voltage and signal; Step 5, using the subtractor to subtract the DC voltage E of the modular multilevel converter from the reference voltage u _ref obtained in step 3 to obtain the No. 2 voltage difference signal; The DC voltage E= _Udc /2 of the modularized multilevel converter, where _Udc is the total voltage of the DC side of the modularized multilevel converter; Step 6. Multiply the phase current output by the modular multilevel converter by the proportional coefficient 1/2 to obtain the average current of the upper and lower bridge arms; Step 7: Using a circulating current suppressor to suppress and compensate the obtained average currents of the upper and lower bridge arms to obtain a compensation signal u _cc ; Use the circulating current suppressor to suppress and compensate the average current of the upper and lower bridge arms, and the specific method to obtain the compensation signal u _cc is as follows: Step 71. Using a notch filter to extract the second harmonic of the average current signal of the upper and lower bridge arms obtained in step 6 to obtain a second harmonic current signal; Step 72: Use a subtractor to make a difference between the average current signal of the upper and lower bridge arms obtained in step 6 and the current of the second harmonic obtained in step 71 to obtain the average value of the upper and lower bridge arms with the second harmonic removed current signal; Step 73, using a subtractor to make a difference between the average current signal of the upper and lower bridge arms with the second harmonic removed and the reference current, to obtain the second harmonic signal of the circulating current; Step 74: Use a proportional resonance controller to perform current tracking on the second harmonic signal of the circulating current obtained in Step 73 to obtain a voltage compensation signal u _cc ; and perform Step 8; Step 8, using a subtractor to make a difference between the No. 2 voltage difference signal obtained in step 5 and the compensation signal u _cc obtained in step 7 to obtain No. 3 voltage difference signal; Use the subtractor to make a difference between the No. 1 voltage sum signal obtained in step 4 and the compensation signal obtained in step 7 to obtain the No. 4 voltage difference signal; Step 9. Multiply the No. 3 voltage difference signal and the No. 4 voltage difference signal obtained in step 8 by the proportional coefficient 1/2E respectively; obtain the No. 3 voltage difference signal of 1/2E and the No. 4 voltage difference signal of 1/2E; Step 10. Use the carrier phase shift modulator to perform carrier phase shift modulation on the 1/2E No. 3 voltage difference signal obtained in step 9, and feed back the signal obtained after carrier modulation to the upper bridge of the modular multilevel converter In the sub-module of the arm, the circulation suppression of the upper bridge arm circuit is realized; Use the carrier phase shift modulator to perform carrier phase shift modulation on the 1/2E No. 4 voltage difference signal obtained in step 9, and feed back the signal obtained after carrier modulation to the sub-section of the upper bridge arm of the modular multilevel converter In the module, the circulation suppression of the lower bridge arm circuit is realized. the 2. The modular multilevel converter circulating current suppression method according to claim 1, wherein the notch filter described in step 71 adopts an infinite impulse response digital filter, and the transfer function of this digital filter is :

Where ω ₀ is the resonant frequency, s is the Laplacian operator, τ ₀ =1/2Q, Q is the quality factor of the trap circuit; And the formula (1) is given by the formula

When there is only a pair of conjugate zeros located on the imaginary axis, and a pair of conjugate poles corresponding to the zeros, it is a single frequency notch filter, which is obtained when a notch appears at frequency ω ₀ ; In the formula, a _i and b _i are the constant coefficients of the filter function, z _i is the zero point of the transfer function, p _i is the pole point of the transfer function, M is the number of zero points, N is the number of pole points, and z is the Z transformation operator z ^-i Indicates the i power of the reciprocal of z. 3. The modularized multilevel converter circulating current suppression method according to claim 1, wherein the transfer function of the proportional resonance controller is: the transfer function of the proportional resonance controller is:

In the formula, K _p is the coefficient of the proportional term, K _r is the coefficient of the resonance term, ω ₀ is the resonance frequency, and ω _c is the cut-off angular frequency. 4. The method for suppressing circulating current of a modular multilevel converter according to claim 1, characterized in that ω ₀ =100π. 5. The method for suppressing circulating current of a modular multilevel converter according to claim 1, characterized in that τ ₀ =0.2.

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