CN111404191B - MMC converter station low-voltage ride through method and system based on resonance controller - Google Patents
MMC converter station low-voltage ride through method and system based on resonance controller Download PDFInfo
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- CN111404191B CN111404191B CN202010309136.6A CN202010309136A CN111404191B CN 111404191 B CN111404191 B CN 111404191B CN 202010309136 A CN202010309136 A CN 202010309136A CN 111404191 B CN111404191 B CN 111404191B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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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
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Abstract
The invention discloses a resonant controller-based MMC converter station low-voltage ride through method and a control system. The method aims at the problem of distortion of output current of the MMC converter station when the asymmetric fault of the power grid occurs, and the three resonance controllers respectively realize the inhibition of negative sequence, zero sequence components and internal circulation in the output current. Compared with the existing method for respectively controlling the positive and negative sequence components of the current under the forward and reverse synchronous coordinate system, the method has a simpler control structure, can inhibit the zero sequence current, and further improves the power sending capacity of the MMC converter station, thereby reducing the design capacity of the DC chopper, reducing the engineering construction cost, and even realizing the fault ride-through operation of the system under the condition of not needing to input the DC chopper when the fault is light. The MMC converter station low-voltage ride through method based on the resonant controller is simple in structure, remarkable in effect and high in engineering practical value.
Description
Technical Field
The invention relates to a resonance controller-based MMC converter station low-voltage ride-through method and system, which are suitable for the technical field of power electronics.
Background
In recent years, with the vigorous development of offshore wind power industry, the construction of offshore wind power plants gradually progresses from short distance and small capacity to deep and far sea in a large scale. The traditional alternating current transmission technology is suitable for offshore wind power plants with lower capacity and shorter distance due to the advantages of mature technology, low equipment cost and the like. However, when the wind power station is oriented to deep and far sea, the problems of high line cost, large reactive loss, weak wind power station supporting capability and the like exist. Therefore, when the routing distance exceeds 70-80 km, the adoption of a more economical and effective high-voltage direct-current transmission technology is a current development consensus. The flexible direct current transmission technology based on the Modular Multilevel Converter (MMC) topology is an optimal selection for accessing a large onshore power grid of a long-distance offshore wind farm by virtue of the advantages of high waveform quality, low operation loss, capability of reversing power flow and the like.
However, when a large grid connected to the grid fails, the flexible dc transmission system is also severely affected. Take the gentle direct delivery system of marine wind-powered electricity generation as an example, when the voltage drop accident takes place for land big electric wire netting, the power delivery ability of land MMC converter station will seriously reduce, and because the output power of fan can't attenuate in the short time, the input power of marine MMC converter station also hardly attenuates at the very first time, surplus power will make the direct current bus voltage of flexible direct current transmission system rise rapidly, if do not take effective measure, will lead to whole direct current system to be off-network, seriously influence the safety and stability of generating efficiency and local electric wire netting. Therefore, the research on the fault ride-through strategy of the MMC converter station under the grid fault has important significance.
At present, research on a fault ride-through strategy of an MMC converter station under the fault of an alternating current power grid is limited, a method for inputting a DC chopper to consume surplus power when the alternating current power grid has the fault is provided in the existing research on the fault ride-through strategy of the MMC converter station, the effectiveness of the method depends on the capacity of an energy consumption resistor, and for a high-power direct current transmission system, the cost is high and the method can be input only intermittently in a short time.
Actually, for a large ac power grid, the frequency of asymmetric faults such as single-phase short-circuit faults is high, and at this time, the MMC converter station still has a certain power transmission capability. How to improve the control strategy to enhance the power output capacity of the MMC converter station when the power grid has an asymmetric fault is considered, the investment time of the DC chopper can be effectively shortened, and even the fault ride-through operation of the system is realized under the condition that the DC chopper is not needed, so that the method has important significance.
Aiming at the asymmetric fault of the power grid, a control strategy for respectively controlling the positive sequence component and the negative sequence component of the current under a positive and negative rotation synchronous coordinate system is developed, so that the negative sequence current can be effectively inhibited. However, this method requires positive and negative sequence separation of voltage and current, two sets of control systems are also required to be used to control the positive and negative sequence components respectively in two coordinate systems, and in addition, the common-mode current needs to be transformed to a third coordinate system to be suppressed, so that the control structure is very complex, and the control parameters need to be redesigned. In addition, because the grid fault point is connected with the MMC through a connecting transformer, when the connecting transformer adopts Y0/Y0When the connection is carried out, an additional zero sequence loop is generated. At this time, the output current of the MMC current converter will contain a large amount of zero sequence components, which further causes the distortion of current waveform and seriously affects the power output capability of the current converter. In conclusion, the improved control strategy of the MMC converter station under the power grid voltage asymmetric drop fault is researched, the control structure is simplified,the power sending capacity of the MMC converter station is improved when a fault occurs, and the method has important significance for the whole flexible direct current transmission system to smoothly realize fault ride-through operation.
Disclosure of Invention
Aiming at the existing problems, the resonant controller-based MMC converter station low-voltage ride-through method and system with a simple control structure are provided, so that the power sending capacity of the MMC converter station is effectively improved when a fault occurs, the design capacity of a DC chopper is reduced, and the engineering construction cost is reduced.
The technical scheme adopted by the invention is as follows: a low voltage ride through method of an MMC converter station based on a resonance controller is characterized in that: when the asymmetric fault of the power grid occurs, the three resonance controllers respectively realize the inhibition of negative sequence, zero sequence and internal circulation in the output current, so that the power output capacity of the MMC converter station is improved.
The method comprises the following steps:
acquisition of three-phase current I on side of MMC alternating current networksabcAnd three phase voltage UsabcValve side three-phase current I of paired connection transformervabcMMC upper and lower bridge arm current IpabcAnd InabcAnd a DC-side DC bus voltage UdcCollecting;
according to the MMC upper and lower bridge arm current IpabcAnd InabcCalculating to obtain MMC three-phase internal circulation Icabc(ii) a Acquiring three-phase voltage U on power grid side by using phase-locked loopsabcPhase angle theta ofvAnd angular frequency ωv(ii) a According to three-phase voltage and current U at power grid sidesabcAnd IsabcCalculating reactive power Q output by MMCs；
To the three-phase voltage U of the power grid sidesabcThree-phase current I on valve sidevabcCarrying out Park conversion to obtain a corresponding voltage vector U under a synchronous rotation d-q-0 coordinate systemsdq0Sum current vector Ivdq0；
Direct current bus voltage U using PI controllerdcAnd reactive power QsControl is performed so as to follow given reference values U, respectivelydcrefAnd QsrefOutput of two PI controllersAs reference values I of d-and q-axis currents, respectivelyvdqref(ii) a Reference value UdcrefReference value Q for rated DC bus voltage of systemsrefAccording to the system instruction, the unit power factor is set to be 0 when in operation, and the corresponding reactive compensation can be carried out on the power grid according to the reactive requirement of the power grid;
d-axis and q-axis currents I are controlled by adopting PI controllervdqThe d-axis and q-axis reference values are IvdqrefThe output of the PI controller is used as the DC component U of the MMC reference differential mode voltagedifrefdq1；
Using a resonant controller with a resonant frequency of 100Hz to couple d-and q-axis currents IvdqThe reference value of the resonance controller is set to be 0, and the output of the resonance controller is used as a frequency-doubling pulsating component U of the MMC reference differential mode voltagedifrefdq2；
Zero sequence current component I by using resonant controller with resonant frequency at 50Hzv0Performing individual control, setting the reference value to zero to realize zero-sequence current suppression, and using the output of the resonance controller as the zero-sequence component U of the MMC reference differential mode voltagedifref0；
Referring the DC component U of the differential mode voltage to the MMCdifrefdq1And a double frequency ripple component Udifrefdq2Adding to obtain d and q axis components UdifrefdqAnd then with the zero sequence component U of the MMC reference differential mode voltagedifref0Combining to obtain MMC reference differential mode voltage Udifrefdq0(ii) a Then, the reference differential mode voltage U is addeddifrefdq0Carrying out inverse Park conversion to obtain a reference differential mode voltage U in a static three-phase coordinate systemdifrefabc；
Three-phase internal circulation I by adopting resonance controller with resonance frequency of 100HzcabcControlling to set the reference value to zero, and using the output of the resonance controller as the MMC reference common mode voltage Ucomrefabc；
According to a reference differential mode voltage UdifrefabcAnd a reference common mode voltage UcomrefabcAnd calculating to obtain the reference voltage U of the upper bridge arm and the lower bridge arm of the MMCprefabcAnd UnrefabcReuse of recent level approximationAnd modulating by a near method to realize the control of the MMC.
Direct current component U of MMC reference differential mode voltagedifrefdq1Obtained according to the following method:
wherein, FPI(s) is the transfer function of the PI controller, Ivdref、IvqrefCorresponding to a current vector IvdqrefD-axis, q-axis components of (I)vd、IvqCorresponding to a current vector Ivdq0D-axis, q-axis components of (1); l is an equivalent inductance including a coupling transformer and a bridge arm reactor, UsdAnd UsqAre respectively a voltage vector Usdq0D-axis, q-axis components of (U)difrefd1、Udifrefq1Corresponding to a voltage vector Udifrefdq1D-axis, q-axis components.
The double-frequency pulsating component U of the MMC reference differential mode voltagedifrefdq2Obtained according to the following method:
wherein: fR100(s) is the transfer function of the resonant controller, kgIs the gain factor, omega, of the resonant controllercTo cut-off the frequency, Udifrefd2、Udifrefq2Corresponding to a voltage vector Udifrefdq2D-axis, q-axis components.
The MMC reference differential mode voltage vector Udifrefdq0Obtained according to the following method:
wherein, Udifrefd1、Udifrefq1Corresponding to a voltage vector Udifrefdq1D-axis, q-axis components of (U)difrefd2、Udifrefq2Corresponding to a voltage vector Udifrefdq2D-axis, q-axis components. Wherein, Udifref0And the zero sequence component of the differential mode voltage is referred to by the MMC.
The MMC references a common mode voltage UcomrefabcObtained according to the following method:
wherein, FR100(s) is the transfer function of the resonant controller, kgIs the gain factor, omega, of the resonant controllercTo cut-off the frequency, Ucomrefa、UcomrefbAnd UcomrefcCorresponding to a voltage vector UcomrefabcA-axis, b-axis and c-axis components of (I)ca、IcbAnd IccCorresponding to three-phase internal circulating current IcabcThe a-axis, b-axis and c-axis components of (a).
A control system based on the control method is characterized by comprising the following steps:
a voltage sensor for detecting three-phase voltage U at power grid sidesabc；
A current sensor I for detecting three-phase current I on the side of the power gridsabc；
A current sensor II for detecting the three-phase current I on the valve side of the connecting transformervabc；
A phase-locked loop module for obtaining three-phase voltage U at power grid sidesabcPhase angle thetavAnd angular frequency ωv；
A power calculation module for calculating the three-phase voltage and current U according to the power gridsabcAnd IsabcCalculating reactive power Q output by MMCs；
Coordinate transformationModule for three-phase voltage U on power grid sidesabcThree-phase current I on valve sidevabcCarrying out Park conversion to obtain a corresponding voltage vector U under a synchronous rotation d-q-0 coordinate systemsdq0Sum current vector Ivdq0；
A DC bus voltage and reactive power controller for using PI controller to DC bus voltage UdcAnd reactive power QsControl is performed so as to follow given reference values U, respectivelydcrefAnd QsrefThe outputs of the two PI controllers are respectively used as reference values I of d-axis and q-axis currentsvdqref；
A differential mode current controller for controlling d and q axis currents I by using PI controllervdqThe d-axis and q-axis reference values are IvdqrefThe output of the PI controller is used as the DC component U of the MMC reference differential mode voltagedifrefdq1；
A negative sequence current suppression module for adopting a resonance controller with the resonance frequency of 100Hz to perform d-axis and q-axis current IvdqThe reference value of the resonance controller is set to be 0, and the output of the resonance controller is used as a frequency-doubling pulsating component U of the MMC reference differential mode voltagedifrefdq2；
A zero sequence current suppression module for eliminating zero sequence current component I by using a resonance controller with resonance frequency of 50Hzv0Performing individual control, setting the reference value to zero to realize zero-sequence current suppression, and using the output of the resonance controller as the zero-sequence component U of the MMC reference differential mode voltagedifref0；
A reference differential mode voltage calculation module for referring the DC component U of the differential mode voltage to the MMCdifrefdq1And a double frequency ripple component Udifrefdq2Adding to obtain d and q axis components UdifrefdqAnd then with the zero sequence component U of the MMC reference differential mode voltagedifref0Combining to obtain MMC reference differential mode voltage Udifrefdq0(ii) a Then, the reference differential mode voltage U is addeddifrefdq0Carrying out inverse Park conversion to obtain a reference differential mode voltage U in a static three-phase coordinate systemdifrefabc；
Internal loop controllers using resonant frequencyResonant controller at 100Hz for three-phase internal circulating current IcabcControlling to set the reference value to zero, and using the output of the resonance controller as the MMC reference common mode voltage Ucomrefabc；
A bridge arm voltage calculation module for calculating the voltage according to UdifrefabcAnd UcomrefabcObtaining the reference voltage U of the upper bridge arm and the lower bridge arm of the MMC through calculationprefabcAnd Unrefabc；
A modulation module for modulating the reference voltage U according to the MMC upper and lower bridge armsprefabcAnd UnrefabcAnd outputting a switch on-off signal by adopting a recent level approximation method to realize the control of the MMC.
The invention has the beneficial effects that: when the voltage of a power grid has an asymmetric drop fault, the three resonance controllers respectively realize the inhibition of negative sequence, zero sequence and internal circulation in output current, and can effectively improve the power sending capacity of the MMC converter station, thereby reducing the design capacity of the DC chopper, reducing the engineering construction cost, and even realizing the fault ride-through operation of a system under the condition of not inputting the DC chopper when the fault is light.
The three resonance controllers respectively realize the inhibition of negative sequence, zero sequence and internal circulation in the output current, and the control structure is very simple and has strong practicability.
Drawings
Fig. 1 is a schematic diagram of a structure of an MMC converter station.
Fig. 2 is a schematic diagram of an implementation principle of the system according to the embodiment.
FIG. 3 is a diagram of a model structure based on a PSCAD/EMTDC simulation environment for verifying effectiveness of an embodiment.
Fig. 4 shows a simulation waveform using a conventional control strategy.
FIG. 5 is a simulation waveform of the MMC converter station low-voltage ride through method based on the resonant controller in the embodiment.
Detailed Description
As shown in fig. 1 and fig. 2, the resonant controller based MMC converter station low voltage pass-through under the power grid voltage asymmetric drop fault in the present inventionThe system comprises an MMC converter station 1 and a connection group Y0/Y0The system comprises a connecting transformer 2, a voltage sensor 3 for detecting three-phase voltage at the side of a power grid, a current sensor 4 for detecting three-phase current at the side of the power grid and a current sensor 5 for detecting three-phase current at the side of a valve of the connecting transformer; the control link of the system comprises a phase-locked loop module 6, a power calculation module 7, a coordinate transformation module 8, a direct current bus voltage and reactive power controller 9, a differential mode current controller 10, a negative sequence current suppression module 11, a zero sequence current suppression module 12, a reference differential mode voltage calculation module 13, an internal loop controller 14, a bridge arm voltage calculation module 15 and a modulation module 16.
The MMC converter station low-voltage ride through method based on the resonant controller in the system in the embodiment comprises the following steps:
acquisition of three-phase voltage U on MMC alternating current network side by using voltage sensor 3sabc(ii) a Three-phase current I acquired by current sensor I4sabcCollecting and connecting three-phase current I at transformer valve side by using current sensor II 5vabcCollecting the current I of the upper and lower bridge arms of the MMC by using a sensor in the MMC converter stationpabcAnd InabcAnd a DC-side DC bus voltage UdcAnd all of them are converted into per unit values.
The three-phase voltage U of the power grid side is obtained through the phase-locked loop 6sabcPhase angle theta ofvAnd angular frequency ωv(ii) a According to the MMC upper and lower bridge arm current IpabcAnd InabcCalculating to obtain MMC three-phase internal circulation IcabcThe calculation method is as follows:
wherein: i ispa、Ipb、IpcRespectively is an upper bridge arm current IpabcCorresponding to the currents on the A, B and C phases, Ina、Inb、IncRespectively, the lower bridge arm current InabcCorresponding to the currents on the A, B and C phases, Ica、Icb、IccRespectively three-phase internal circulation IcabcCorresponding to the current on the A phase, the B phase and the C phase.
According to three-phase voltage and current U at power grid sidesabcAnd IsabcAnd the active power P output by the MMC is calculated by using the power calculation module 7sAnd reactive power Qs(ii) a The calculation method is as follows:
wherein: u shapesa，UsbAnd UscAre respectively a voltage UsabcCorresponding to voltages on A, B and C phases, Isa，IsbAnd IscAre respectively current IsabcCorresponding to the current on the A phase, the B phase and the C phase.
The coordinate transformation module 8 is utilized to respectively carry out three-phase voltage UsabcAnd three-phase current IvabcPerforming Park conversion to correspondingly obtain a voltage vector U under a synchronous rotation d-q-0 coordinate systemsdq0Sum current vector Ivdq0The calculation method is as follows:
wherein: u shapesd,UsqAnd Us0Corresponding to a voltage vector Usdq0D-axis, q-axis and 0-axis components of (I)vd,IvqAnd Iv0Corresponding to a current vector Ivdq0The d-axis, q-axis and 0-axis components of (1).
DC bus voltage U is controlled by DC bus voltage and reactive power controller 9dcAnd reactive power QsControl is performed so as to respectively follow a given reference value UdcrefAnd QsrefThe outputs of the two controllers are respectively used as reference values I of d and q axes of currentvdqrefThe controller is implemented as follows:
wherein: fPI(s) is the transfer function of the PI controller, kpIs a proportionality coefficient, kiWhen the control variables are different, the proportional and integral coefficients are adjusted according to the actual conditions, Ivdref,IvqrefCorresponding to a current vector IvdqrefD-axis, q-axis component.
Using differential mode current controller 10 to control current I under synchronous rotating coordinate systemvdqControl is carried out, and the d and q axis reference values are IvdqrefThe output of the controller is used as the DC component U of the MMC reference differential mode voltagedifrefdq1The controller is implemented as follows:
wherein L is equivalent inductance including a coupling transformer and a bridge arm reactor, and U is equivalent inductancesdAnd UsqAre respectively a voltage vector Usdq0D-axis, q-axis components of (U)difrefd1、Udifrefq1Corresponding to a voltage vector Udifrefdq1D-axis, q-axis component.
In the negative sequence current suppression module 11, a resonance controller with a resonance frequency of +/-100 Hz is adopted for the current IvdqControlling to set the reference value to zero to realize the suppression of negative sequence current, and taking the output of the resonance controller as the frequency doubling pulsating component U of the MMC reference differential mode voltagedifrefdq2The controller is implemented as follows:
wherein: fR100(s) is the transfer function of the resonant controller, kgIs the gain factor, omega, of the resonant controllercIn the embodiment, the resonant frequency is selected to be +/-100 Hz, and the cut-off frequency is selected to be 12 Hz; u shapedifrefd2、Udifrefq2Corresponding to a voltage vector Udifrefdq2D-axis, q-axis components.
In the zero sequence current suppression module 12, a resonance controller with a resonance frequency of +/-50 Hz is adopted to control the zero sequence current component Iv0Performing individual control, setting the reference value to zero to realize zero-sequence current suppression, and using the output of the resonance controller as the zero-sequence component U of the MMC reference differential mode voltagedifref0The controller is implemented as follows:
Udifref0＝FR50(s)(0-Iv0)
wherein: fR50(s) is the transfer function of the resonant controller, in this step the resonant frequency is chosen to be ± 50Hz and the cut-off frequency is chosen to be 12 Hz.
In the reference differential mode voltage calculation module 13, the direct current component U of the reference value differential mode voltage is calculateddifrefdq1And a double frequency ripple component Udifrefdq2Adding to obtain final reference differential mode voltage UdifrefdqAnd then with the zero sequence component U of the MMC reference differential mode voltagedifref0Combining to obtain the final reference differential mode voltage Udifrefdq0The calculation process is as follows:
then, the reference differential mode voltage U is addeddifrefdq0Performing inverse Park transformationAnd obtaining a reference differential mode voltage U in a static three-phase coordinate systemdifrefabcThe calculation method is as follows:
wherein, Udifrefd,UdifrefqAnd Udifref0Corresponding to a voltage vector Udifrefdq0D-axis, q-axis and 0-axis components of (1), Udifrefa,UdifrefbAnd UdifrefcCorresponding to a voltage vector UdifrefabcThe a-axis, b-axis and c-axis components of (a).
In the internal circulation current suppression module 14, a resonance controller with a resonance frequency of +/-100 Hz is adopted for three-phase internal circulation current IcabcControlling to set the reference value to zero to realize the suppression of internal circulation, and taking the output of the resonance controller as the MMC reference common mode voltage UcomrefabcThe controller is implemented as follows:
wherein, Ucomrefa,UcomrefbAnd UcomrefcCorresponding to a voltage vector UcomrefabcThe a-axis, b-axis and c-axis components of (a).
Using the bridge arm voltage calculation module 15, according to the reference differential mode voltage UdifrefabcAnd a reference common mode voltage UcomrefabcCalculating to obtain the reference voltage U of the upper and lower bridge arms of the MMCprefabcAnd UnrefabcAnd then, a modulation module 16 adopting a nearest level approximation method is used for outputting a switch on-off signal to realize the control of the MMC, wherein the calculation method of the reference voltages of the upper bridge arm and the lower bridge arm is as follows:
wherein, Uprefa,UprefbAnd UprefcCorresponding to a voltage vector UprefabcA-axis, b-axis and c-axis components of (1), Unrefa,UnrefbAnd UnrefcCorresponding to a voltage vector UprefabcThe a-axis, b-axis and c-axis components of (a).
FIG. 3 is a model structure diagram of a PSCAD/EMTDC simulation environment based on verifying the effectiveness of the present invention. In the simulation process, the marine MMC converter sends constant power to the onshore side, the onshore alternating current power grid has single-phase voltage short-circuit fault within 3s, the fault lasts for 1s, and the alternating current power grid recovers to be normal within 4 s.
Fig. 4 shows a simulation waveform using a conventional control strategy. In fig. 4(a), the active power waveform, the voltage waveform, and the current waveform sent from the MMC converter station to the ac power grid are from top to bottom, respectively; fig. 4(b) shows an operation signal of the DC chopper. According to simulation results, after the voltage drop fault occurs to the power grid voltage, more zero sequence and negative sequence components exist in the output current of the MMC converter station, the output power of the MMC converter station is limited and drops from 930MW to 850MW, in order to prevent the excessive power from causing the voltage of the direct current bus to excessively rise to cause the system to be disconnected, a DC chopper needs to be input to consume a part of energy, and 4 times of DC chopper is input in 1 s.
FIG. 5 is a simulation waveform of the MMC converter station low-voltage ride through method based on the resonant controller in the invention. In fig. 5(a), the active power waveform, the voltage waveform, and the current waveform sent from the MMC converter station to the ac power grid are from top to bottom, respectively; fig. 5(b) shows an operation signal of the DC chopper. According to simulation results, when a fault occurs, the zero sequence and negative sequence components in the output current are effectively inhibited, the output power capability of the MMC converter station is not obviously reduced, and the output power capability is obviously improved compared with that of the traditional method, so that the voltage of a direct-current bus cannot be increased to exceed a threshold value within 1s of fault time, and fault ride-through can be realized without inputting a DC chopper.
In summary, by using the resonant controller-based MMC converter station low voltage ride through method in this embodiment, when an asymmetric voltage drop fault occurs in a power grid, negative sequence and zero sequence components in output current can be effectively suppressed, and the output power capability of the MMC converter station is enhanced, so that the design capacity of a DC chopper is reduced, the engineering construction cost is reduced, and even fault ride through operation of a system can be realized without investing the DC chopper; the low voltage ride through method is very simple to realize and has strong engineering practical value.
The embodiments described above are presented to enable a person having ordinary skill in the art to make and use the invention. It will be readily apparent to those skilled in the art that various modifications to the above-described embodiments may be made, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications to the present invention based on the disclosure of the present invention within the protection scope of the present invention.
Claims (6)
1. A low voltage ride through method of an MMC converter station based on a resonance controller is characterized in that: when the asymmetric fault of the power grid occurs, the three resonance controllers respectively realize the inhibition of negative sequence, zero sequence component and internal circulation in the output current, thereby improving the power output capacity of the MMC converter station;
the resonant controller-based MMC converter station low-voltage ride-through method comprises the following steps:
acquisition of three-phase current I on side of MMC alternating current networksabcAnd three phase voltage UsabcValve side three-phase current I of paired connection transformervabcMMC upper and lower bridge arm current IpabcAnd InabcAnd a DC-side DC bus voltage UdcCollecting;
according to the MMC upper and lower bridge arm current IpabcAnd InabcCalculating to obtain MMC three-phase internal circulation Icabc(ii) a Acquiring three-phase voltage U on power grid side by using phase-locked loopsabcPhase angle theta ofvAnd angular frequency ωv(ii) a According to three-phase voltage and current U at power grid sidesabcAnd IsabcComputing MMC outputsReactive power Q ofs；
To the three-phase voltage U of the power grid sidesabcThree-phase current I on valve sidevabcCarrying out Park conversion to obtain a corresponding voltage vector U under a synchronous rotation d-q-0 coordinate systemsdq0Sum current vector Ivdq0；
Direct current bus voltage U using PI controllerdcAnd reactive power QsControl is performed so as to follow given reference values U, respectivelydcrefAnd QsrefThe outputs of the two PI controllers are respectively used as reference values I of d-axis and q-axis currentsvdqref(ii) a Reference value UdcrefReference value Q for rated DC bus voltage of systemsrefAccording to the system instruction, the unit power factor is set to be 0 when in operation, and the corresponding reactive compensation can be carried out on the power grid according to the reactive requirement of the power grid;
d-axis and q-axis currents I are controlled by adopting PI controllervdqThe d-axis and q-axis reference values are IvdqrefThe output of the PI controller is used as the DC component U of the MMC reference differential mode voltagedifrefdq1；
Using a resonant controller with a resonant frequency of 100Hz to couple d-and q-axis currents IvdqThe reference value of the resonance controller is set to be 0, and the output of the resonance controller is used as a frequency-doubling pulsating component U of the MMC reference differential mode voltagedifrefdq2；
Zero sequence current component I by using resonant controller with resonant frequency at 50Hzv0Performing individual control, setting the reference value to zero to realize zero-sequence current suppression, and using the output of the resonance controller as the zero-sequence component U of the MMC reference differential mode voltagedifref0；
Referring the DC component U of the differential mode voltage to the MMCdifrefdq1And a double frequency ripple component Udifrefdq2Adding to obtain d and q axis components UdifrefdqAnd then with the zero sequence component U of the MMC reference differential mode voltagedifref0Combining to obtain MMC reference differential mode voltage Udifrefdq0(ii) a Then, the reference differential mode voltage U is addeddifrefdq0Carrying out inverse Park conversion to obtain a reference differential mode voltage U in a static three-phase coordinate systemdifrefabc；
Three-phase internal circulation I by adopting resonance controller with resonance frequency of 100HzcabcControlling to set the reference value to zero, and using the output of the resonance controller as the MMC reference common mode voltage Ucomrefabc；
According to a reference differential mode voltage UdifrefabcAnd a reference common mode voltage UcomrefabcAnd calculating to obtain the reference voltage U of the upper bridge arm and the lower bridge arm of the MMCprefabcAnd UnrefabcAnd modulating by using a nearest level approximation method to realize the control of the MMC.
2. The resonant controller-based MMC converter station low-voltage ride-through method of claim 1, characterized in that, the MMC references the direct current component U of the differential mode voltagedifrefdq1Obtained according to the following method:
wherein, FPI(s) is the transfer function of the PI controller, Ivdref、IvqrefCorresponding to a current vector IvdqrefD-axis, q-axis components of (I)vd、IvqCorresponding to a current vector Ivdq0D-axis, q-axis components of (1); l is an equivalent inductance including a coupling transformer and a bridge arm reactor, UsdAnd UsqAre respectively a voltage vector Usdq0D-axis, q-axis components of (U)difrefd1、Udifrefq1Corresponding to a voltage vector Udifrefdq1D-axis, q-axis components.
3. The resonant controller-based MMC converter station low-voltage ride-through method of claim 1, characterized in that, the MMC references a double frequency ripple component U of the differential mode voltagedifrefdq2Obtained according to the following method:
wherein: fR100(s) is the transfer function of the resonant controller, kgIs the gain factor, omega, of the resonant controllercTo cut-off the frequency, Ivd、IvqCorresponding to a current vector Ivdq0D-axis, q-axis components of (U)difrefd2、Udifrefq2Corresponding to a voltage vector Udifrefdq2D-axis, q-axis components.
4. The resonant controller-based MMC converter station low-voltage ride-through method of claim 1, characterized in that the MMC references a differential mode voltage vector Udifrefdq0Obtained according to the following method:
wherein, Udifrefd1、Udifrefq1Corresponding to a voltage vector Udifrefdq1D-axis, q-axis components of (U)difrefd2、Udifrefq2Corresponding to a voltage vector Udifrefdq2D-axis, q-axis components of (U)difref0And the zero sequence component of the differential mode voltage is referred to by the MMC.
5. The resonant controller-based MMC converter station low-voltage ride-through method of claim 1, characterized in that the MMC references a common mode voltage UcomrefabcObtained according to the following method:
wherein, FR100(s) is the transfer function of the resonant controller, kgIs the gain factor, omega, of the resonant controllercTo cut-off the frequency, Ucomrefa、UcomrefbAnd UcomrefcCorresponding to a voltage vector UcomrefabcA-axis, b-axis and c-axis components of (I)ca、IcbAnd IccCorresponding to three-phase internal circulating current IcabcThe a-axis, b-axis and c-axis components of (a).
6. A control system based on the low voltage ride through method of any one of claims 1 to 5, comprising:
a voltage sensor for detecting three-phase voltage U at power grid sidesabc；
A current sensor I for detecting three-phase current I on the side of the power gridsabc；
A current sensor II for detecting the three-phase current I on the valve side of the connecting transformervabc；
A phase-locked loop module for obtaining three-phase voltage U at power grid sidesabcPhase angle thetavAnd angular frequency ωv；
A power calculation module for calculating the three-phase voltage and current U according to the power gridsabcAnd IsabcCalculating reactive power Q output by MMCs；
A coordinate transformation module for three-phase voltage U on the power grid sidesabcThree-phase current I on valve sidevabcCarrying out Park conversion to obtain a corresponding voltage vector U under a synchronous rotation d-q-0 coordinate systemsdq0Sum current vector Ivdq0；
A DC bus voltage and reactive power controller for using PI controller to DC bus voltage UdcAnd reactive power QsControl is performed so as to follow given reference values U, respectivelydcrefAnd QsrefThe outputs of the two PI controllers are respectively used as reference values I of d-axis and q-axis currentsvdqref；
A differential mode current controller for controlling d and q axis currents I by using PI controllervdqThe d-axis and q-axis reference values are IvdqrefThe output of the PI controller is used as the DC component U of the MMC reference differential mode voltagedifrefdq1；
A negative sequence current suppression module for adopting a resonance controller with the resonance frequency of 100Hz to perform d-axis and q-axis current IvdqThe reference value of the resonance controller is set to be 0, and the output of the resonance controller is used as a frequency-doubling pulsating component U of the MMC reference differential mode voltagedifrefdq2；
A zero sequence current suppression module for eliminating zero sequence current component I by using a resonance controller with resonance frequency of 50Hzv0Performing individual control, setting the reference value to zero to realize zero-sequence current suppression, and using the output of the resonance controller as the zero-sequence component U of the MMC reference differential mode voltagedifref0；
A reference differential mode voltage calculation module for referring the DC component U of the differential mode voltage to the MMCdifrefdq1And a double frequency ripple component Udifrefdq2Adding to obtain d and q axis components UdifrefdqAnd then with the zero sequence component U of the MMC reference differential mode voltagedifref0Combining to obtain MMC reference differential mode voltage Udifrefdq0(ii) a Then, the reference differential mode voltage U is addeddifrefdq0Carrying out inverse Park conversion to obtain a reference differential mode voltage U in a static three-phase coordinate systemdifrefabc；
Internal circulation controller for three-phase internal circulation I by using resonant controller with resonant frequency of 100HzcabcControlling to set the reference value to zero, and using the output of the resonance controller as the MMC reference common mode voltage Ucomrefabc；
A bridge arm voltage calculation module for calculating the voltage according to UdifrefabcAnd UcomrefabcObtaining the reference voltage U of the upper bridge arm and the lower bridge arm of the MMC through calculationprefabcAnd Unrefabc；
A modulation module for modulating the reference voltage U according to the MMC upper and lower bridge armsprefabcAnd UnrefabcAnd outputting a switch on-off signal by adopting a recent level approximation method to realize the control of the MMC.
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| CN112086987B (en) * | 2020-08-26 | 2022-06-14 | 中国电建集团华东勘测设计研究院有限公司 | MMC fault current suppression method based on model predictive control algorithm |
| CN112366745B (en) * | 2020-10-10 | 2023-06-27 | 中国电建集团华东勘测设计研究院有限公司 | Centralized modularized DC Chopper topology and control method |
| CN112600452B (en) * | 2020-12-09 | 2022-07-05 | 中国电建集团华东勘测设计研究院有限公司 | MMC finite set model prediction control method and system based on bridge arm current control |
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