CN114865680A - Specific resonant frequency suppression method of modular multilevel converter - Google Patents
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- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
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- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
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- H—ELECTRICITY
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- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
- H02M7/53873—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current with digital control
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
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Abstract
The invention discloses a specific resonance frequency suppression method of a modular multilevel converter, which comprises the following steps: establishing a mathematical model of the modular multilevel converter according to the topological structure and the working principle of the modular multilevel converter, establishing an impedance relation between the modular multilevel converter and a power grid interconnection system, and analyzing the impedance characteristic of the modular multilevel converter; according to the impedance characteristic of the modular multilevel converter, a method of adding a quasi-proportional resonant controller is provided for restraining system broadband oscillation; and optimizing parameters of the additional quasi-proportional resonant controller according to the impedance characteristic of the modular multilevel converter, so as to realize the suppression of the specific resonant frequency of the modular multilevel converter. The method inhibits the broadband resonance phenomenon which may occur to the MMC, thereby fully playing the advantages of the flexible direct current technology, ensuring the safe and stable operation of a power grid, and improving the working efficiency and the service life of devices in the MMC. The method is convenient to be applied to practical power systems.
Description
Technical Field
The invention relates to the technical field of flexible direct current transmission, in particular to a specific resonant frequency suppression method of a modular multilevel converter.
Background
The flexible direct-current transmission technology based on Modular Multilevel Converters (MMC) is widely applied to a high-voltage direct-current transmission system due to the advantages of modularization, low switching frequency, small distortion of output voltage waveform and the like. At present, the flexible direct current transmission system established in China comprises: zhejiang Zhoushan soft straight system, Shanghai Hui soft straight system, Guangdong south Australia, Fujian Xiamen, Luxi south back-to-back engineering, Yubei back-to-back and other direct current engineering soft straight systems. Along with the use of a plurality of flexible direct current system projects, the stability problem related to the converter is gradually highlighted, reports of different types of oscillation phenomena are increased, such as the phenomenon of secondary synchronous oscillation occurs in the process that the delivery output of the flexible direct current project of Shanghai Hui and the flexible direct current project of south China, Guangdong and Australia increases, the oscillation frequency is about 30Hz, and the high-frequency oscillation occurs in the process of converting the five-end flexible direct current project of the Zhoushan into an island in a single-station networking operation mode to cause tripping; luxi back-to-back flexible DC engineering has seen 1.2kHz high frequency oscillations; high-frequency oscillation up to 1.3kHz occurs in the debugging process of the Yuhuo back-to-back flexible direct current engineering. The resonance problem of the flexible direct-current transmission system and the alternating-current power grid can excite the alternating-current system to generate harmonic waves with large amplitude, alternating-current voltage and alternating current are seriously distorted, the running loss of the system is increased, and primary equipment can be punctured to enable the system to be locked and shut down.
Due to the lack of a method for inhibiting the MMC broadband resonance problem, part of back-to-back direct current engineering in China can only be operated in a derating mode (50% of the underrated capacity), and meanwhile, a large safety risk also exists. Therefore, a method for suppressing the specific resonant frequency of the direct current transmission system is urgently needed, and the method can suppress the broadband resonance in the flexible direct current engineering, so that the technical advantages of the flexible direct current are fully exerted, and the safe and stable operation of a power grid is guaranteed.
Disclosure of Invention
In view of this, the present invention provides a specific resonant frequency suppression method for a modular multilevel converter, which can provide positive damping for a small-range frequency band at a known resonant frequency without affecting impedance of other frequency bands, and solve the problem of MMC broadband resonance in the prior art.
The invention discloses a specific resonance frequency suppression method of a modular multilevel converter, which comprises the following steps:
step 1: establishing a mathematical model of the modular multilevel converter according to the topological structure and the working principle of the modular multilevel converter, establishing an impedance relation between the modular multilevel converter and a power grid interconnection system, and analyzing the impedance characteristic of the modular multilevel converter;
step 2: according to the impedance characteristic of the modular multilevel converter, a method of adding a quasi-proportional resonant controller is provided for restraining system broadband oscillation;
and step 3: and (3) according to the impedance characteristic of the modular multilevel converter in the step (1), optimizing the parameters of the additional quasi-proportional resonance controller in the step (2) to realize the suppression of the specific resonance frequency of the modular multilevel converter.
Furthermore, the topological structure of the modular multilevel converter is a bipolar framework, and an equivalent model of the modular multilevel converter is composed of half-bridge sub-modules, bridge arm equivalent resistors and bridge arm reactances; the control system of the modular multilevel converter topological structure comprises a power outer ring, a current control ring and a circulating current suppressor; according to the topological structure and the control system, a mathematical model of the modular multilevel converter is established, the impedance relation between the modular multilevel converter and the power grid interconnection system is established, and the impedance characteristic of the modular multilevel converter is analyzed.
Further, the mathematical model of the modular multilevel converter is as follows:
in the formula, L is bridge arm inductance, R is resistance, i x Ac side phase current of three phases, i xt Is a three-phase bridge arm circulation current, v ac Is an alternating side phase voltage, v xu 、v xl Total capacitance voltage of upper and lower bridge arms, m xu 、m xl Respectively being an upper and a lower bridge determined by the control systemAnd C is an equivalent bridge arm capacitance, a superscript s represents a steady-state value of a variable, and a delta represents a small signal component, and the impedance characteristic of the modular multilevel converter can be obtained through calculation according to a mathematical model.
Further, an impedance relation between the modular multilevel converter system and the power grid interconnection system is obtained according to kirchhoff's law by using a relation graph between the modular multilevel converter system and the alternating current power grid interface, and is as follows:
in the formula, the alternating current system is equivalent to a voltage source V g And an equivalent impedance Z g The flexible-straight side subsystem is equivalent to a current source I c And an equivalent impedance Z mmc And the grid-connected point current is I.
Further, the analysis of the impedance characteristics of the modular multilevel converter includes the influence of a delay link and the influence of changing parameters of a current inner loop, a circulating current suppression link and a power outer loop control link on the impedance characteristics under the condition of time delay.
Further, the step 2 specifically includes:
the modularized multi-level converter adopts dq axis voltage signals output by double-loop control to obtain three-phase voltage signals through park inverse transformation, samples the voltage signals at the point of common coupling, obtains oscillation components through an additional quasi-proportional resonant controller, and superposes the opposite oscillation components and the three-phase voltage signals to provide positive damping for a small-range frequency band at a specific resonant frequency, so that system broadband oscillation is suppressed.
Further, the step 3 specifically includes:
after the impedance characteristics of the modular multilevel converter are analyzed by using an impedance analysis method, a parameter optimization method aiming at the specific oscillation frequency of the current modular multilevel converter is provided, namely, the specific resonance frequency is restrained by setting internal parameters of an additional quasi-proportional resonance controller.
Further, the suppression of the specific resonant frequency is realized by setting internal parameters of the additional quasi-proportional resonant controller, and specifically includes: sampling the voltage at the point of common coupling, adding a negative value output by the additional quasi-proportional resonant controller into a modulation index, and outputting a voltage opposite to the oscillation direction of the system by the modular multilevel converter end to reduce oscillation harmonics; the impedance of the modular multilevel converter is changed by adjusting the control parameters of the additional quasi-proportional resonant controller, so that the phase margin of the impedance intersection point is changed.
Further, the specific resonance frequency is a specific frequency causing the system to resonate, and is obtained when amplitude-frequency and phase-frequency characteristics are analyzed according to the impedance relation between the modular multilevel converter and the power grid interconnection system.
Due to the adoption of the technical scheme, the invention has the following advantages: (1) broadband resonance phenomenon that probably takes place to the MMC restraines to the flexible direct current technological advantage of full play, guarantee electric wire netting safety and stability operation promote the work efficiency and the life of device among the MMC. The method is convenient to be applied to practical power systems. The method has the advantages of being scientific and reasonable, strong in applicability, high in reliability and good in effect. (2) Can provide positive damping for the known resonant frequency department minizone frequency channel on the basis that does not influence other frequency channel impedance, solve the problem of the broadband resonance of MMC that prior art exists.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments described in the embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings.
Fig. 1 is a schematic diagram of a topology structure of a modular multilevel converter according to an embodiment of the present invention;
fig. 2 is a schematic diagram of a topology structure of an MMC and power grid interconnection system according to an embodiment of the present invention;
fig. 3 is a simplified topology structure diagram of an MMC and power grid interconnection system according to an embodiment of the present invention;
fig. 4 is a block diagram of an additional quasi-proportional resonant controller according to an embodiment of the invention.
Detailed Description
The present invention will be further described with reference to the accompanying drawings and examples, it being understood that the examples described are only some of the examples and are not intended to limit the invention to the embodiments described herein. All other embodiments available to those of ordinary skill in the art are intended to be within the scope of the embodiments of the present invention.
Referring to fig. 1 and 2, a topological structure diagram of a modular multilevel converter and a topological structure diagram of an MMC and power grid interconnection system are shown. The modular multilevel converter topology is a bipolar framework, and an equivalent model comprises a half-bridge submodule SM, a bridge arm equivalent resistor, a bridge arm reactance and the like. The control system comprises a power outer ring, a current control ring, a circulating current suppressor and the like.
When the MMC is connected to the power grid, the MMC generally operates symmetrically according to three phases, the modulation wave signals of bridge arms of all phases sequentially lag behind by 120 degrees, and besides, the three-phase bridge arms operate in the same mode, so that a mathematical model of other two phases can be deduced by analyzing one phase. According to the structure of the grid-connected MMC, the inductive current of an upper bridge arm and a lower bridge arm of an x-phase (x ═ a, b and c) has
L and R on the left of the equation represent bridge arm inductance and its resistance, i xu And i xl The currents of the upper and lower arms are shown, respectively. Equality right side v p And v n Respectively representing the voltages of the positive and negative poles at the DC side of the MMC to the midpoint at the AC side, vxu, i and vxl, i respectively representing the capacitance voltages of the ith sub-module in the upper and lower bridge arms, s xu,i And s xl,i Used for representing the switching state of the ith sub-module in the upper and lower bridge arms, and s is used for showing the switching state of the ith sub-module in the upper and lower bridge arms when the corresponding module is switched in xu,i And s xl,i Is 1, when the corresponding module is cut out, s xu,i And s xl,i The value of (d) is 0. v. of x Representing the voltage from the x-phase PCC point to the ac side midpoint.
The capacitance of the ith sub-module in the upper and lower bridge arms is
Wherein C is SM Is the capacitance value of the sub-module. According to the KCL theorem, there are also bridge arm currents
In the formula i x And i xl The phase current and the arm circulating current of the x phase on the ac side are shown.
Note the coefficient s representing the switching state of the submodule xu,i And s xl,i The modeling analysis is difficult due to the fact that the model is not a continuous function of time, and the capacitance and voltage of the bridge arm sub-modules are basically the same by considering that an additional voltage-sharing control strategy, namely the following formula is approximately satisfied
In the formula v xu And v xl And respectively representing the total voltage of the upper bridge arm capacitor and the lower bridge arm capacitor of the x phase. If the ratio of the number of the submodules put into the upper and lower bridge arms at a certain time to the total number of the submodules of the bridge arms is defined as a switching coefficient m xu And m xl Then there is
Since the grid-connected MMC has a large number of sub-modules, m xu And m xl At this point it can be considered to be approximately continuously varying. In addition, according to KVL theorem, there are
In the formula v dc Is a DC side anode and cathodeVoltage between, v m The voltage from the midpoint of the dc side to the midpoint of the ac side. Therefore, a dynamic equation set with respect to capacitance and inductance in the grid-connected MMC can be established as
C is the concentrated equivalent capacitance of each bridge arm, and has the following relationship with the submodule capacitance of MMC
In addition, a dynamic equation set related to alternating-current side current and bridge arm circulation current can be established by sorting the upper bridge arm inductance current equation and the lower bridge arm inductance current equation in the step (4) according to the step (3)
According to the above equations (1) to (9), the mathematical model of the modular multilevel converter can be obtained as follows:
in the formula, L is bridge arm inductance, R is resistance, i x Ac side phase current of three phases, i xt Is a three-phase bridge arm circulation current, v ac Is an alternating side phase voltage, v xu 、v xl Total capacitance voltage, m, of upper and lower bridge arms, respectively xu 、m xl The modulation functions of an upper bridge arm and a lower bridge arm determined by a control system are respectively shown, C is equivalent bridge arm capacitance, an upper standard s represents a steady-state value of a variable, and delta represents a small signal component, and the impedance characteristic of the modular multilevel converter can be obtained through calculation according to a mathematical model.
In conclusion, the average mathematical model of the grid-connected MMC is established through the equation, and it can be found that the dynamic equation of the capacitor and the inductor comprises the product terms of the switching coefficient, the capacitor voltage and the bridge arm current, so that the average mathematical model of the MMC has a nonlinear characteristic, and in addition, the averaged MMC mathematical model ignores the switching process of a power device in a submodule and the influence caused by the difference of the discrete level and the submodule, but completely keeps the dynamic characteristics of the capacitor and the inductor in the MMC, and considers that the main harmonic frequency is lower than the switching frequency when the MMC operates in a steady state, so that the mathematical model not only can better reflect the multi-frequency harmonic characteristic of the MMC, but also can simplify the mathematical analysis process of the MMC, and accelerate the progress of simulation research.
Referring to fig. 3, it is a simplified topology structure diagram of the MMC and grid interconnection system.
In the formula, Z g And Z mmc The impedances of the power grid and the MMC respectively, and the equivalent of the alternating current system is a voltage source V g And an equivalent impedance Z g The flexible-straight side subsystem is equivalent to a current source I c And an equivalent impedance Z mmc And the grid-connected point current is I.
From the above formula, the current stability mainly depends on the soft side impedance and the ac grid equivalent impedance. And linear control theory, when Z g And Z mmc When the amplitude-frequency and phase-frequency characteristics are satisfied, the system is stable.
Wherein, U PCC (s) is the PCC voltage, I g (s) is grid-connected current, and the grid-connected inverter is equivalent to an ideal current source is(s) and an output impedance Z inv (s) parallel connection, the power grid is equivalent to an ideal voltage source U g (s) and the grid impedance Z g (s) in series. Z inv (s) not only integrates the frequency characteristics of the phase-locked loop, current control loop, and filter, but also integrates Z with g (s) are not affected each other, and both of them are presented as independent unified whole. When the interactive stability of the grid-connected inverter and the power grid is analyzed, the grid-connected current I g (s) is represented by
Assuming that both the grid-connected inverter and the grid subsystem can operate stably and independently, the stability of the grid-connected current Ig(s) depends on the second term on the right side of the equation, i.e. 1/[1+ Z ] g (s)/Z inv (s)]Similar to the closed loop transfer function of a negative feedback control system. The gain of the forward channel of the system is 1, and the gain of the negative feedback channel is Z g (s)/Z inv (s). Therefore, in order to ensure that the grid-connected inverter and the grid interaction system are stable, if and only if the ratio Z of the grid impedance to the output impedance of the grid-connected inverter is g (s)/Z inv (s) satisfies the Nyquist criterion. Therefore, the interactive stability of the grid-connected inverter and the power grid can be effectively analyzed and quantitatively characterized by the frequency characteristics of the output impedance Zinv(s) of the inverter and the impedance Zg(s) of the power grid.
Referring to fig. 4, there is shown a block diagram of an additional quasi-proportional resonant controller. The PCC voltage vi (i ═ a, b, c) is sampled and the inverse of the QPR output is added to the modulation index by the quasi-proportional resonant controller. Namely, the MMC terminal outputs voltage opposite to the oscillation direction of the system so as to reduce harmonic oscillation of the system.
The transfer function of QPR is defined as
Wherein k is r As a proportional parameter, ω c Is the resonant frequency. By adjusting QPR control parameters, the impedance characteristics of the MMC can be changed, so that the phase margin of the impedance intersection point is changed, and the risk of oscillation near a resonant frequency point is restrained.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is to be covered by the claims.
Claims (9)
1. A specific resonant frequency suppression method of a modular multilevel converter is characterized by comprising the following steps:
step 1: establishing a mathematical model of the modular multilevel converter according to the topological structure and the working principle of the modular multilevel converter, establishing an impedance relation between the modular multilevel converter and a power grid interconnection system, and analyzing the impedance characteristic of the modular multilevel converter;
step 2: according to the impedance characteristic of the modular multilevel converter, a method of adding a quasi-proportional resonant controller is provided for restraining system broadband oscillation;
and step 3: and (3) according to the impedance characteristic of the modular multilevel converter in the step (1), optimizing the parameters of the additional quasi-proportional resonance controller in the step (2) to realize the suppression of the specific resonance frequency of the modular multilevel converter.
2. The method for suppressing specific resonant frequency of the modular multilevel converter according to claim 1, wherein the topology of the modular multilevel converter is a bipolar architecture, and an equivalent model of the topology is composed of half-bridge sub-modules, bridge arm equivalent resistors and bridge arm reactances; the control system of the modular multilevel converter topological structure comprises a power outer ring, a current control ring and a circulating current suppressor; according to the topological structure and the control system, a mathematical model of the modular multilevel converter is established, the impedance relation between the modular multilevel converter and the power grid interconnection system is established, and the impedance characteristic of the modular multilevel converter is analyzed.
3. The method for specific resonance frequency suppression of a modular multilevel converter according to claim 1, wherein the mathematical model of the modular multilevel converter is:
in the formula, L is bridge arm inductance, R is resistance, i x Ac side phase current of three phases, i xt Is a three-phase bridge arm circulation current, v ac Is an alternating side phase voltage, v xu 、v xl Total capacitance voltage, m, of upper and lower bridge arms, respectively xu 、m xl The modulation functions of an upper bridge arm and a lower bridge arm determined by a control system are respectively shown, C is equivalent bridge arm capacitance, an upper standard s represents a steady-state value of a variable, and delta represents a small signal component, and the impedance characteristic of the modular multilevel converter can be obtained through calculation according to a mathematical model.
4. The method for suppressing the specific resonant frequency of the modular multilevel converter according to claim 3, wherein the impedance relationship between the modular multilevel converter and the grid interconnection system obtained by the modularized multilevel converter system and the AC grid interface relationship diagram according to kirchhoff's law is as follows:
in the formula, the alternating current system is equivalent to a voltage source V g And an equivalent impedance Z g The flexible-straight side subsystem is equivalent to a current source I c And an equivalent impedance Z mmc And the grid-connected point current is I.
5. The method according to claim 1, wherein the analyzing the impedance characteristics of the modular multilevel converter comprises the influence of a delay element and the influence of changing parameters of a current inner loop, a circulating current suppression element and a power outer loop control element on the impedance characteristics in the presence or absence of delay.
6. The method for suppressing specific resonant frequency of a modular multilevel converter according to claim 1, wherein the step 2 comprises:
the modularized multi-level converter adopts dq-axis voltage signals output by double-loop control to obtain three-phase voltage signals through park inverse transformation, samples the voltage signals at a public coupling point, obtains oscillation components through an additional quasi-proportional resonance controller, superposes the opposite oscillation components and the three-phase voltage signals, and provides positive damping for a small-range frequency band at a specific resonance frequency, so that system broadband oscillation is suppressed.
7. The method for suppressing specific resonant frequency of a modular multilevel converter according to claim 1, wherein the step 3 comprises:
after the impedance characteristics of the modular multilevel converter are analyzed by using an impedance analysis method, a parameter optimization method aiming at the specific oscillation frequency of the current modular multilevel converter is provided, namely, the specific resonance frequency is restrained by setting internal parameters of an additional quasi-proportional resonance controller.
8. The method for suppressing the specific resonant frequency of the modular multilevel converter according to claim 7, wherein the suppressing the specific resonant frequency is realized by setting internal parameters of an additional quasi-proportional resonant controller, and specifically comprises: sampling the voltage at the point of common coupling, adding a negative value output by the additional quasi-proportional resonant controller into a modulation index, and outputting a voltage opposite to the system oscillation direction by the end of the modular multilevel converter so as to reduce oscillation harmonics; the impedance of the modular multilevel converter is changed by adjusting the control parameters of the additional quasi-proportional resonant controller, so that the phase margin of the impedance intersection point is changed.
9. The method for suppressing the specific resonant frequency of the modular multilevel converter according to claim 1, wherein the specific resonant frequency is a specific frequency causing a system to resonate, and is obtained when analyzing amplitude-frequency and phase-frequency characteristics according to an impedance relationship between the modular multilevel converter and a grid interconnection system.
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| CN115173460A (en) * | 2022-08-26 | 2022-10-11 | 南昌工程学院 | Ripple optimization method, device and equipment in flexible direct current transmission system |
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| CN115173460A (en) * | 2022-08-26 | 2022-10-11 | 南昌工程学院 | Ripple optimization method, device and equipment in flexible direct current transmission system |
| CN115173460B (en) * | 2022-08-26 | 2023-03-10 | 南昌工程学院 | Ripple optimization method, device and equipment in flexible direct current transmission system |
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