CN103972924B - Permanent magnet direct-drive wind power system low voltage traversing control method under unbalanced electric grid voltage - Google Patents
Permanent magnet direct-drive wind power system low voltage traversing control method under unbalanced electric grid voltage Download PDFInfo
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Abstract
The present invention relates to permanent magnet direct-drive wind power system low voltage traversing control method under a kind of unbalanced electric grid voltage, the control method specifically includes following steps：1) obtain the phase theta of positive and negative sequence voltagep、θn；2) using positive-negative sequence voltage-phase, by under three-phase power grid voltage current transformation to dq coordinate systems, obtain the positive-negative sequence voltage x current under dq coordinate systems；3) active power calculated using positive-negative sequence power network current and grid-connected reactor parameter on grid-connected reactor consumes DC component Δ P0, cosine component Δ Pc2, sinusoidal component Δ Ps2；4) reference value of reference value and reactive power DC component of the DC component of inverter active power of output is obtained 5) using the value and power reference for obtaining, calculate the positive and negative order components of inverter output current；6) switching signal is obtained using the positive and negative order components of output current, and control inverter output current and power output with switching signal.Compared with prior art, the present invention has the advantages that efficient, easy.
Description
Technical Field
The invention relates to a control method of a wind power system, in particular to a low voltage ride through control method of a permanent magnet direct-drive wind power system under unbalanced grid voltage.
Background
With the continuous increase of the power of the wind power plant connected to the power grid, the interaction between the wind power plant and the power grid is more and more serious, and in order to improve the stability and the power quality of the power grid, the power grid provides a series of requirements for the grid connection of the wind power plant, wherein the requirements comprise a low voltage ride through requirement.
The permanent magnet direct-drive synchronous wind driven generator adopting the full-power converter grid connection realizes the complete decoupling of the wind driven generator and the power grid, is flexible to control, and can greatly improve the low-voltage ride through capability through reasonable control.
Grid voltage imbalance can be caused by asymmetric faults of the power grid, asymmetric loads, non-full transposition of the power transmission line and the like. When the voltage of the power grid is unbalanced, due to the generation of a negative sequence component, if the converter adopts a traditional positive sequence control strategy, the voltage of a direct current side generates 2-frequency multiplication fluctuation, so that the direct current capacitor is frequently charged and discharged, and the service life of the capacitor is shortened. While the 2 nd harmonic voltage and current on the dc side of the converter will generate 2n +1 th non-specific harmonics on the ac side by the modulating action of the pulse width modulation. Therefore, it is important to research a stable control strategy of the dc side voltage under the unbalanced grid voltage condition.
Gunn, xiachangliang, Hades, Hawain, etc. and Zhao Zilong, Wuweining and Wang Wei respectively in the text of "PWM rectifier constant frequency direct power control under unbalanced grid voltage" and "direct drive wind turbine generator set low voltage ride through technology under asymmetric grid fault" directly suppress the 2-frequency multiplication component of the direct current end voltage by stabilizing the active power transmitted to the grid. Most documents utilize an improved dual-current control strategy based on a symmetric component method to suppress the 2-fold frequency component at the direct-current end. Xiaoliu, Huangdao, Huang Keyuan and the like provide a double-current control strategy for respectively orienting positive and negative sequence voltages in direct-drive permanent magnet wind power generation system direct-current bus voltage stability control under asymmetric power grid faults, and the direct-current side voltage is stabilized by combining an energy release loop. According to the control strategy of the full-power converter wind turbine generator set in the asymmetric fault of the power grid, such as Chenyidong, Yangyin, Wang Liqiao and the like, a phase-locked loop frequency correction link is added aiming at the frequency fluctuation condition of the power grid, and a double-current control strategy of a direct-current side sampling voltage with a frequency doubling notch function of 2 is provided. The control strategy is to realize the control of the active power transmitted to the power grid by injecting a proper positive-negative sequence current into the power grid under the condition of neglecting the power fluctuation on the grid-connected reactor, thereby realizing the control of the direct-current voltage. However, in a megawatt wind turbine generator, the switching frequency of a full-power converter is low (about 2kHz), and the inductance value of a grid-connected reactor is relatively large, so that the control effect is adversely affected by neglecting power fluctuation on the grid-connected reactor. Yongsuh respectively introduces a bridge arm output voltage variable of a Network side converter in Control scheme in a hybrid synchronous static frame for PWM AC-DC converter and generated Unbalanced operation and regulation and analysis of instant Active and Reactive Power for PWM AC/DCConverter under general Unbalanced Network, although the 2-frequency multiplication component of a direct current end can be eliminated, the Control system is complex. A control strategy that power fluctuation on a grid-connected reactor is considered and feedforward is introduced into a voltage outer ring is provided in a control strategy of a full-power wind power grid-connected converter when grid voltage is unbalanced through Liao, Zhuankai and Yaojun, but the calculation process is complex.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provide a low voltage ride through control method for a permanent magnet direct-drive wind power system under the unbalanced grid voltage.
The purpose of the invention can be realized by the following technical scheme:
a low voltage ride through control method of a permanent magnet direct-drive wind power system under unbalanced grid voltage is characterized by specifically comprising the following steps of:
1) obtaining the phase theta of positive and negative sequence voltagep、θn；
2) Converting the three-phase network voltage and current into dq coordinate system by using positive and negative sequence voltage phases to obtain positive and negative sequence voltage and current in the dq coordinate system,
the positive sequence is:
negative sequence:
wherein,andrespectively, a positive and negative sequence voltage component in dq coordinate system, T (theta)p)、T(θn) Respectively positive and negative αβ coordinate systems to positive and negative dq coordinate systems,andrespectively, positive and negative sequence voltage components under αβ coordinate system;
3) calculating active power consumption direct current component delta P on grid-connected reactor by using positive and negative sequence grid current and grid-connected reactor parameters0Cosine component Δ Pc2Sinusoidal component Δ Ps2；
4) Obtaining a reference value of a direct current component of the output active power of the inverterAnd reference value of DC component of reactive power
5) Calculating positive and negative sequence components of the output current of the inverter by using the obtained power reference value;
6) and acquiring a switching signal by using the positive and negative sequence components of the output current, and controlling the output current and the output power of the inverter by using the switching signal.
The step 1) is specifically as follows:
11) converting the three-phase power grid voltage into an alpha beta coordinate system, and obtaining positive and negative sequence components of the power grid voltage by adopting a T/4 time delay method under the alpha beta coordinate system;
12) utilizing the positive and negative sequence components in the αβ coordinate system obtained in the step 11) to adopt the phase lock with respectively oriented positive and negative sequencesMethod for obtaining positive and negative sequence voltage phase thetap、θn。
The step 4) is specifically as follows:
41) the error between the DC side voltage reference value and the measured value is multiplied by the DC voltage reference value after passing through the PI controller to obtain the reference value of the DC component of the active power output by the inverter
42) The error between the voltage reference value and the measured value at the common connection point is subjected to PI controller to obtain the reference value of the DC component of the output reactive power of the inverter
If the unit power factor control is to be maintained, the control unit is used to control the power factorIs set to 0.
The step 6) is specifically as follows:
61) in the dq coordinate system, positive and negative sequence voltage signals are obtained after positive and negative sequence currents flow through a PI controller;
62) converting the positive and negative sequence voltage signals to obtain voltage signals under an alpha beta coordinate system;
63) and obtaining a switching signal of the inverter through SVPWM, wherein the switching signal controls the output current and the output power of the inverter.
According to the control method, on the basis of a double-current control method, the power consumption of a grid-connected reactor is introduced to serve as the correction quantity of the reference power output of an inverter, positive and negative reference currents are corrected, further, the suppression of frequency doubling components of direct-current side voltage 2 is achieved, an energy storage element is introduced to a direct-current end, the power generated by a generator and the power output to a power grid by the inverter are balanced, and the voltage stability of the direct-current end of the inverter is achieved.
The energy storage element is a super capacitor or a storage battery and the like and is connected in parallel to the direct current side, when the output power of the inverter is lower than the output power of the generator, the super capacitor stores the residual power, and when the output power of the generator is lower than the output capacity of the inverter, the power is transmitted to the power grid.
Compared with the prior art, the invention has the following advantages:
1) the super capacitor device is introduced into the direct-current end, so that the power generated by the generator and the power output to the power grid by the inverter are balanced, the voltage stability of the direct-current end of the inverter is realized, and the low-voltage ride through capability of the fan is improved.
2) By adopting the control method, the grid-connected current three-phase is subjected to Fourier analysis, and the Total Harmonic Distortion (THD) of the current is respectively reduced from 4.89%, 5.13% and 5.15% to 1.19%, 2.92% and 1.59%.
Drawings
FIG. 1 is a flow chart of the method of the present invention;
FIG. 2 is a main circuit diagram of a direct-drive permanent magnet synchronous wind power generator system based on full-power converter grid connection;
FIG. 3 is a schematic diagram of a grid-side inverter control strategy in the case of grid imbalance;
FIG. 4 is a schematic diagram of positive and negative sequence voltage phase locking respectively;
FIG. 5 is a supercapacitor control block diagram;
FIG. 6 is a diagram of a simulation result of a conventional balancing control strategy;
FIG. 7 is a diagram illustrating a simulation result of the control strategy proposed by the method of the present invention;
FIG. 8 is a graph of grid-connected current spectrum analysis under a conventional control strategy;
fig. 9 is a grid-connected current spectrum analysis diagram under the control strategy of the invention.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the scope of the present invention is not limited to the following embodiments.
As shown in fig. 1, a control method for low voltage ride through of a permanent magnet direct-drive wind power system under unbalanced grid voltage specifically includes the following steps:
1) obtaining the phase theta of positive and negative sequence voltagep、θn。
2) Converting the three-phase power grid voltage current into a dq coordinate system by using positive and negative sequence voltage phases to obtain positive and negative sequence voltage currents in the dq coordinate system;
3) calculating active power consumption direct current component delta P on grid-connected reactor by using positive and negative sequence grid current and grid-connected reactor parameters0Cosine component Δ Pc2Sinusoidal component Δ Ps2；
4) Obtaining a reference value of a direct current component of the output active power of the inverterAnd reference value of DC component of reactive power
5) Calculating positive and negative sequence components of the output current of the inverter by using the obtained power reference value;
6) and acquiring a switching signal by using the positive and negative sequence components of the output current, and controlling the output current and the output power of the inverter by using the switching signal.
Step 1) comprises the sub-steps of:
11) converting the three-phase power grid voltage to a two-phase static coordinate system (alpha beta coordinate system), and obtaining positive and negative sequence components of the power grid voltage by adopting a T/4 time delay method under a d beta coordinate system;
12) utilizing the positive and negative sequence components in the αβ coordinate system obtained in the step 11), and obtaining the phase theta of the positive and negative sequence voltages by adopting a phase locking method of respectively orienting the positive and negative sequencesp、θn。
Step 4) comprises the sub-steps of:
41) the error between the DC side voltage reference value and the measured value is multiplied by the DC voltage reference value after passing through the PI controller,
obtaining the reference value of the DC component of the active power output by the inverter
42) The error between the voltage reference value and the measured value at the PCC point is processed by the PI controller to obtain the reference value of the DC component of the output reactive power of the inverter
If the unit power factor control is to be maintained, the control unit is used to control the power factorIs set to 0.
Step 6) includes the steps of:
61) in the dq coordinate system, positive and negative sequence voltage signals are obtained after positive and negative sequence currents flow through a PI controller;
62) converting the positive and negative sequence voltage signals to obtain voltage signals under an alpha beta coordinate system;
63) and obtaining a switching signal of the inverter through SVPWM, wherein the switching signal controls the output current and the output power of the inverter.
The control method introduces the power consumption on the grid-connected reactor as the correction quantity of the inverter reference power output on the basis of the double-current control method, corrects the positive and negative reference currents, further realizes the suppression of the frequency doubling component of the direct-current side voltage 2, introduces an energy storage element at the direct-current end, balances the power generated by the generator and the power output by the inverter to the power grid, and realizes the voltage stabilization of the direct-current end of the inverter.
The energy storage element is a super capacitor or a storage battery and the like and is connected in parallel to the direct current side, when the output power of the inverter is lower than the output power of the generator, the super capacitor stores the residual power, and when the output power of the generator is lower than the output capacity of the inverter, the power is transmitted to the power grid.
The main circuit of the direct-drive permanent magnet synchronous wind driven generator system based on the full-power converter grid connection is shown in fig. 2. The power generated by the generator is transmitted to the power grid through the converter, and the power grid and the generator are completely decoupled through a direct current link. The machine side converter mainly controls active power and stator voltage emitted by the fan; the grid-side converter mainly controls the direct-current terminal voltage and the reactive power output to the power grid.
Three-phase neutral-line-free grid connection and zero-sequence component circulation paths are generally adopted in a wind power system, so that only positive-sequence components and negative-sequence components are considered when the grid voltage is asymmetric. Therefore, when the voltage of the power grid is unbalanced, a decoupling model of the grid-side converter of the PMSG unit is as follows:
where the superscripts p and n denote the positive and negative sequence dq axis components, respectively.
When the voltage of the power grid is asymmetric, the active power and the reactive power fed into the power grid by the wind turbine generator are respectively as follows:
Pl=Pl_0+Pl_c2cos2ωt+Pl_s2sin2ωt (2)
Ql=Ql_0+Ql_c2cos2ωt+Ql_s2sin2ωt (3)
in the formula: pl_0、Ql_0Respectively an active and a reactive power DC component, P, of the feed-in networklc2、Pls2、Qlc2、Qls2The active power and the reactive power are respectively 2 times frequency cosine components and sine components.
In order to simplify the operation process and the control structure, the positive and negative sequence voltages are respectively oriented by the network side voltage phase-locked loop. In a positive sequence network, d ispWith axis oriented in the direction of the positive sequence voltage vector, in a negative sequence networknThe axis is oriented in the negative sequence voltage vector direction. Therefore, in the synchronous rotating coordinate (dq coordinate) system, the grid voltage vector is:
wherein,andpositive and negative sequence voltage amplitudes.
The direct current side power balance equation is:
in the formula: pcActive power, P, for output at the port of the grid-side convertersActive power output from rectifier at generator side to DC side。
It is considered to remain unchanged during the fault. P under the condition of neglecting power consumption on the grid-connected reactorc=PlSubstituting formula (2) for formula (6) to obtain:
as long as reasonably controllingMake 2 frequency multiplication component P of net side active powerlc2、Pls2A voltage of 0 ensures a constant voltage across the dc side capacitor.
In this embodiment, a network-side voltage phase-locked loop is used to realize the respective orientation of positive and negative sequence voltages, the first 4 equations in the formula (4) are selected, and the formula (5) is substituted to obtain:
obtaining by solution:
the active power absorbed by the inductance and the resistance of the grid-connected reactor is as follows:
when considering the active power consumed by the grid-connected reactor, the following are available:
substituting the formula (10) into the formula (6) to obtain:
when considering power fluctuation on a grid-connected reactor, in order to eliminate the 2-frequency multiplication component of the direct-current side voltage, Pc has no 2-frequency multiplication component, namely Pc_c2=0、Pc_s2And = 0. To further simplify the operation, equation (9) can be further simplified as:
then (8) in the formula:
when the PI controller is used to regulate the dc terminal voltage, the dc component of the active power output by the port of the grid-side converter can be expressed as:
wherein: kp and Ki are respectively proportional coefficient and integral coefficient of the PI controller.
When the power grid has an asymmetric fault and the voltage drops greatly, the power grid may need the wind farm to provide reactive support, and then reactive control may be implemented by the reactive additional control in fig. 3, but in order to facilitate control, the embodiment directly sets the reactive reference value to 0, that is, the unit power factor control is adopted in the event of the fault.
When the voltage of the power grid is unbalanced, due to the generation of a negative sequence, if the voltage and current signals are directly subjected to Park conversion, the voltage and current components in the dq coordinate system contain 2 frequency multiplication components. The invention separates the positive and negative sequence components of the voltage and current under a two-phase static coordinate system (alpha beta coordinate system) by using a T/4 time delay method, and then converts the voltage and current signals from the alpha beta coordinate system to a dq coordinate system by using a phase angle obtained by a phase-locked loop, so that the voltage and current signals under the dq coordinate system are all direct current quantities, and the tracking of a PI controller is facilitated. Taking voltage as an example, then:
and:
wherein: t is the period of the power grid; t (theta)p)、T(θn) Respectively positive and negative αβ coordinate systems to positive and negative dq coordinate systems.
The positive and negative sequence voltage components separated by the formulas (15) to (17) are utilized, the positive and negative sequence power grid voltage phase locking strategy shown in fig. 4 is adopted, the phases of the positive and negative sequence power grid voltages can be accurately obtained, and then the positive and negative sequence voltages can be respectively oriented by carrying out dq conversion by utilizing the phase angle. In order to accelerate the response speed and the control effect of the PI controller, the input quantity of the PI controller adopts the ratio of a q-axis component and a d-axis component of voltage.
The reference values of the corrected positive and negative sequence currents can be obtained by substituting the equations (13) and (14) into the equation (8), and then the control of the grid-side converter can be realized according to the equation (1). By combining the above processes, a control structure as shown in fig. 3 can be established. The power fluctuation correction on the grid-connected reactor in the control strategy provided by the invention actually uses the power fluctuation quantity on the grid-connected reactor in the last sampling period to correct the reference value of the power in the next control period, thereby achieving the purpose of correcting the reference current.
The super capacitor in fig. 2 is a simplified model.
The severity of the power imbalance condition will directly determine the operating state of the supercapacitor. Under the condition of neglecting the switching loss, when the system is stable and Pc = Ps, no power difference exists, and the super capacitor does not work at the moment; when the system fails and Pc < Ps, S1 is triggered, and the super capacitor absorbs power; when Pc > Ps, S2 is triggered and the super capacitor discharges power. The super capacitor control block diagram is shown in fig. 5.
The super capacitor device is arranged on the direct current side, the power difference between the power generated by the fan and the power fed to the power grid by the inverter is absorbed, and the direct current end voltage can be stabilized better, so that the low voltage ride through capability of the wind power system is improved, for example, the super capacitor in fig. 2 is a simplified model.
The severity of the power imbalance condition will directly determine the operating state of the supercapacitor. Under the condition of neglecting the switching loss, when the system is stable and Pc = Ps, no power difference exists, and the super capacitor does not work at the moment; when the system is in fault and Pc < Ps, S1 is triggered, and the super capacitor absorbs power; when Pc > Ps, s2 is triggered, the super capacitor discharges power. The super capacitor control block diagram is shown in fig. 5.
And establishing a model in a Matlab/Simulink environment for simulation. The main parameters of the system are as follows; the rated power of the permanent magnet synchronous generator is 2 MW; the rated voltage of the power grid is 690V; the inductance of the grid-connected reactor at the grid side is 0.3mH, and the resistance is 0.01 omega; the direct current side capacitance is 50mF, and the direct current side voltage rated value is 1200V; the converter switching frequency is 2 kHz.
The single-phase earth fault occurs in the power grid within 1.2s, the A-phase voltage drops to 30%, and the fault is cleared within 1.6 s.
As shown in fig. 6, when the grid voltage is unbalanced, the conventional control algorithm only considers the positive sequence, and cannot effectively control the negative sequence. Therefore, the voltage of the direct current terminal, the output active power Pc of the inverter and the feed-in active power P of the power gridlThe feed-in power grid has noWork power QlThere is a frequency doubling of 2 component, which will all seriously affect the stable operation of the wind farm.
As shown in fig. 7, when the grid voltage is unbalanced, in order to suppress the dc-side voltage 2 frequency multiplication component, the control strategy provided by the present invention considers the power fluctuation on the grid-connected reactor based on the conventional dual-current control strategy, so that the 2 frequency multiplication fluctuation of the net-side converter port output active power Pc is effectively suppressed, and the dc-side voltage is stabilized. Since the control strategy provided by the invention aims to control the voltage at the direct current end to have no frequency multiplication component of 2, but due to the power fluctuation on the grid-connected reactor, the active power P fed into the power grid is enabled to belStill containing part 2 of the frequency doubled component.
As can be seen from the waveforms of the currents in fig. 6 and 7, the asymmetry of the current is increased in the case of a fault by using the control strategy proposed by the present invention; however, the control strategy provided by the invention can effectively control the negative sequence current, and the harmonic content of the network side current is reduced while the frequency multiplication component of the direct current side voltage 2 is inhibited. Fourier analysis was performed on one of the phase currents, as shown in fig. 8 and 9. The Fourier analysis is carried out on three phases of grid-connected current, and the Total Harmonic Distortion (THD) of the current is respectively reduced from 4.89%, 5.13% and 5.15% to 1.19%, 2.92% and 1.59%.
Claims (5)
1. A low voltage ride through control method of a permanent magnet direct-drive wind power system under unbalanced grid voltage is characterized by specifically comprising the following steps of:
1) obtaining the phase theta of positive and negative sequence voltagep、θn；
2) Converting the three-phase network voltage and current into dq coordinate system by using positive and negative sequence voltage phases to obtain positive and negative sequence voltage and current in the dq coordinate system,
the positive sequence is:
negative sequence:
wherein,andrespectively positive and negative sequence voltage components in dq coordinate system,T(θp)、T(θn) Respectively positive and negative αβ coordinate systems to positive and negative dq coordinate systems,andrespectively, positive and negative sequence voltage components under αβ coordinate system;
3) calculating active power consumption direct current component delta P on grid-connected reactor by using positive and negative sequence currents and grid-connected reactor parameters under dq coordinate system0Cosine component Δ Pc2Sinusoidal component Δ Ps2；
4) Obtaining a reference value of a direct current component of the output active power of the inverterAnd reference value of DC component of reactive power
5) Calculating positive and negative sequence components of the output current of the inverter by using the obtained power reference value;
6) acquiring a switching signal by using the positive and negative sequence components of the output current, and controlling the output current and the output power of the inverter by using the switching signal;
on the basis of a double-current control method, the control method introduces the power consumption of a grid-connected reactor as the correction quantity of the reference power output of an inverter to correct positive and negative reference currents, further realizes the suppression of frequency doubling components of direct-current side voltage 2, introduces an energy storage element at a direct-current end, balances the power generated by a generator and the power output to a power grid by the inverter, and realizes the voltage stability of the direct-current end of the inverter;
the step 6) is specifically as follows:
61) in the dq coordinate system, positive and negative sequence currents of the output current of the inverter flow through the PI controller to obtain positive and negative sequence voltage signals;
62) converting the positive and negative sequence voltage signals to obtain voltage signals under an alpha beta coordinate system;
63) and obtaining a switching signal of the inverter through SVPWM, wherein the switching signal controls the output current and the output power of the inverter.
2. The method for controlling the low voltage ride through of the permanent magnet direct-drive wind power system under the unbalanced grid voltage according to claim 1, wherein the step 1) specifically comprises the following steps:
11) converting the three-phase power grid voltage into an alpha beta coordinate system, and obtaining positive and negative sequence components of the power grid voltage by adopting a T/4 time delay method under the alpha beta coordinate system;
12) utilizing the positive and negative sequence components in the αβ coordinate system obtained in the step 11), and obtaining the phase theta of the positive and negative sequence voltages by adopting a phase locking method of respectively orienting the positive and negative sequencesp、θn。
3. The method for controlling the low voltage ride through of the permanent magnet direct-drive wind power system under the unbalanced grid voltage according to claim 1, wherein the step 4) specifically comprises the following steps:
41) the error between the DC side voltage reference value and the measured value is multiplied by the DC side voltage reference value after passing through the PI controller to obtain the reference value of the DC component of the active power output by the inverter
42) The error between the voltage reference value and the measured value at the common connection point is subjected to PI controller to obtain the reference value of the DC component of the output reactive power of the inverter
4. The method for controlling the low voltage ride through of the permanent magnet direct-drive wind power system under the unbalanced grid voltage according to claim 3, wherein if the control of unit power factor is to be maintained, the method comprises the step ofIs set to 0.
5. The method for controlling the low voltage ride through of the permanent magnet direct-drive wind power system under the unbalanced grid voltage according to claim 1, wherein the energy storage element is a super capacitor or a storage battery and is connected in parallel to a direct current side, when the output power of the inverter is lower than the output power of the generator, the super capacitor stores the residual power, and when the output power of the generator is lower than the output capacity of the inverter, the super capacitor transmits the power to the grid.
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| PT2993753T (en) * | 2014-09-03 | 2020-01-20 | Siemens Ag | Wind turbine generator with fault behavior like a synchronous generator |
| CN104993520A (en) * | 2015-07-30 | 2015-10-21 | 国网浙江省电力公司电力科学研究院 | Frequency converter low-voltage ride-through (LVRT) supporting apparatus based on super capacitor |
| CN111384716B (en) * | 2020-03-31 | 2023-04-11 | 国网黑龙江省电力有限公司哈尔滨供电公司 | Seven-level static synchronous compensator based on unbalanced power grid |
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