CN110571840B - Method and device for fault ride-through control at sending end of LCC-MMC power transmission system - Google Patents

Method and device for fault ride-through control at sending end of LCC-MMC power transmission system Download PDF

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CN110571840B
CN110571840B CN201910832212.9A CN201910832212A CN110571840B CN 110571840 B CN110571840 B CN 110571840B CN 201910832212 A CN201910832212 A CN 201910832212A CN 110571840 B CN110571840 B CN 110571840B
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mmc
voltage
control link
control
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CN110571840A (en
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陈争光
郝亮亮
詹清清
王兴国
杜丁香
郭雅蓉
曹虹
王书扬
戴飞扬
蔡文瑞
程琪
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Beijing Jiaotong University
China Electric Power Research Institute Co Ltd CEPRI
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China Electric Power Research Institute Co Ltd CEPRI
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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
    • H02J3/12—Circuit arrangements for AC mains or AC distribution networks for adjusting voltage in AC networks by changing a characteristic of the network load
    • H02J3/16—Circuit arrangements for AC mains or AC distribution networks for adjusting voltage in AC networks by changing a characteristic of the network load by adjustment of reactive power
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

本发明公开了LCC‑MMC输电系统送端故障穿越控制方法和装置。该LCC‑MMC输电系统送端故障穿越控制方法，包括：在确定输电系统送端发生交流故障时，受端MMC根据交流故障程度生成故障穿越控制策略，所述故障穿越控制策略包括增加至少一个控制环节；其中，所述受端MMC已经配置有定电压控制环节；所述定电压控制环节的输入包括：直流电压参考值

Figure DDA0002191082350000011
受端MMC执行所述定电压控制环节和增加的至少一个控制环节，以使得直流电压跟随LCC送端直流电压下降。该送端故障穿越控制方法实现了输电系统在送端交流侧不同程度故障时的穿越能力。

Figure 201910832212

The invention discloses a fault ride-through control method and device at the sending end of an LCC-MMC power transmission system. The method for fault ride-through control at the sending end of the LCC-MMC power transmission system includes: when it is determined that an AC fault occurs at the sending end of the power transmission system, the MMC at the receiving end generates a fault ride-through control strategy according to the degree of the AC fault, and the fault ride-through control strategy includes adding at least one control link; wherein, the receiving end MMC has been configured with a constant voltage control link; the input of the constant voltage control link includes: a DC voltage reference value

Figure DDA0002191082350000011
The receiving end MMC executes the constant voltage control link and the added at least one control link, so that the DC voltage follows the LCC sending end DC voltage to decrease. The transmission-side fault ride-through control method realizes the ride-through capability of the transmission system when the transmission-side AC side has different degrees of faults.

Figure 201910832212

Description

Sending end fault ride-through control method and device for LCC-MMC power transmission system
Technical Field
The invention relates to the technical field of direct current transmission, in particular to a method and a device for controlling fault ride-through of a sending end of an LCC-MMC power transmission system.
Background
The conventional direct current transmission has the advantages of low manufacturing cost, small loss, large transmission capacity, unlimited transmission distance and the like, but also has the problems of easy phase change failure at a receiving end, strong dependence on an alternating current system, large reactive power consumption in operation and the like. On the other hand, the direct current receiving end drop points are often concentrated; the failure of the sending-end alternating current system may cause the simultaneous phase change failure of a plurality of converter stations at the receiving end, so that the plurality of converter stations are locked, the transmission power is interrupted, and the stable operation of a power grid is seriously threatened.
The flexible direct current transmission fundamentally avoids the problem of commutation failure, can respectively and independently control active power and reactive power, can supply power to a passive network, and has small occupied area. But the cost is higher and the loss is larger.
The sending end of a conventional direct current transmission system is reserved, and the receiving end is replaced by a high-capacity flexible direct current Converter valve (flexible direct current Converter valve for short) based on a Modular Multilevel Converter (MMC for short), so that an LCC-MMC hybrid direct current transmission system is formed, and the method is a feasible scheme for solving the problem of phase conversion failure of the traditional direct current receiving end.
The LCC-MMC hybrid direct-current power transmission system combines the cost advantage of conventional direct-current power transmission and the technical advantage of flexible direct-current power transmission, fundamentally avoids the phenomenon of receiving end phase commutation failure of the conventional direct-current power transmission, but introduces the new problem that the fault of a sending end alternating-current system is difficult to pass through.
Disclosure of Invention
The invention provides a method and a device for controlling fault ride-through of a sending end of an LCC-MMC power transmission system, which aim to solve the problem that the alternating current fault ride-through capability of the sending end of the existing LCC-MMC hybrid direct current power transmission system is insufficient.
In a first aspect, the invention provides a method for controlling fault ride-through of a sending end of an LCC-MMC power transmission system, which comprises the following steps:
when determining that the transmission end of the power transmission system has an alternating current fault, receivingThe method comprises the steps that a fault ride-through control strategy is generated by an end MMC according to alternating current fault degree, wherein the fault ride-through control strategy comprises the step of adding at least one control link; the receiving end MMC is provided with a constant voltage control link; the input of the constant voltage control link comprises: reference value of DC voltage
Figure BDA0002191082330000022
And the receiving end MMC executes the constant voltage control link and the added at least one control link so that the direct current voltage is reduced along with the direct current voltage of the LCC sending end, and fault ride-through is realized.
Further, the generating of the fault ride-through control strategy according to the ac fault degree includes:
comparing the rate of change k of the DC currentIdAnd a predetermined DC threshold kIset；
At the rate of change k of the direct currentIdIs not greater than a predetermined threshold kIsetThen, a constant current control link is additionally configured on the MMC at the receiving end;
at the rate of change k of the direct currentIdIs greater than a preset critical value kIsetThen, a receiving end MMC is additionally provided with a constant current control link and a reactive power control link;
wherein, the input of the constant current control link comprises a preset DC reference value
Figure BDA0002191082330000023
And the value of the direct current I at the current sampling momentd；
The output of the constant current control link is the deviation delta U of the direct current voltage reference valuedcFor correcting the voltage reference value of the constant voltage control element
Figure BDA0002191082330000024
The input of the reactive power control link comprises a preset reactive power reference value Q*And the reactive power value Q at the current sampling instant.
Further, the output of the reactive power control link is a q-axis current reference value of the inner loop current controller;
receiving end MMC direct current voltage UdcAnd network side reactive power QwSatisfies the following formula:
Figure BDA0002191082330000021
wherein, UwFundamental line voltage of an alternating current power grid connected with the MMC;
omega is the fundamental frequency of an alternating current power grid connected with the MMC;
L0bridge arm inductance of MMC;
XTthe converter transformer leakage reactance is connected between the MMC and an alternating current power grid.
Further, the preset DC current critical value kIsetDetermined according to the following formula:
Figure BDA0002191082330000031
wherein R isLIs a direct current line resistor;
l is the inductance of the smoothing reactor;
Xrthe phase-change reactance is equivalent to the sending end;
Iddirect current for steady state operation;
E′d1sending an internal potential for the end after the fault;
Ud2is a receiving end direct current voltage.
Further, the step of determining that the ac fault occurs at the transmission end of the power transmission system includes:
obtaining the direct current change rate k at the current sampling momentId；
Rate of change k of the direct current at the current sampling instantIdGreater than a predetermined DC threshold kItheshAnd determining that the transmission end of the power transmission system has an alternating current fault.
Further, the method can be used for preparing a novel materialThe direct current change rate k at the current sampling moment is obtainedIdThe method comprises the following steps:
the direct current I at the current sampling moment is measuredd(k) With the direct current I of the last sampling instantd(k-1) and dividing the difference by the time difference value delta t between two adjacent sampling moments to obtain the direct current change rate kId：
Figure BDA0002191082330000032
Further, the constant current control link comprises a current PI controller;
the reactive power control link comprises a reactive power PI controller.
In a second aspect, the present invention further provides a device for controlling a fault ride-through at a transmission end of an LCC-MMC transmission system, including:
the fault ride-through control strategy generating unit is used for generating a fault ride-through control strategy according to the alternating current fault degree when the alternating current fault of the transmission end of the power transmission system is determined, wherein the fault ride-through control strategy comprises adding at least one control link; wherein, the receiving end MMC is configured with a constant voltage control link; the input of the constant voltage control link comprises: reference value of DC voltage
Figure BDA0002191082330000041
And the fault ride-through control strategy execution unit is used for executing the constant voltage control link and at least one added control link so as to enable the direct current voltage to be reduced along with the direct current voltage at the sending end of the LCC, thereby realizing fault ride-through.
Further, the fault-ride-through control strategy generation unit includes:
a threshold value comparison unit for comparing the DC current change rate kIdAnd a predetermined DC threshold kIset；
A constant current control unit for controlling the constant current at a DC current change rate kIdIs not greater than a predetermined threshold kIsetMeanwhile, a constant current control link is additionally configured;
a reactive power control link configuration unit for controlling the DC current change rate kIdIs greater than a preset critical value kIsetThen, a receiving end MMC is additionally provided with a constant current control link and a reactive power control link;
wherein, the input of the constant current control link comprises a preset DC reference value
Figure BDA0002191082330000042
And the value of the direct current I at the current sampling momentd；
The output of the constant current control link is the deviation delta U of the direct current voltage reference valuedcFor correcting the voltage reference value of the constant voltage control element
Figure BDA0002191082330000043
The input of the reactive power control link comprises a preset reactive power reference value Q*And the reactive power value Q at the current sampling instant.
In a third aspect, the present invention provides an LCC-MMC power transmission system comprising:
the LCC is arranged at the sending end and the MMC is arranged at the receiving end;
the MMC provided at the receiving end is used to execute the sending end fault ride-through control method described in the first aspect.
Compared with the prior art, the sending end fault ride-through control method and the device for the LCC-MMC direct current transmission system provided by the invention combine the constant current control, the MMC constant voltage control and the reactive power control when the sending end alternating current fault of the system is monitored, so that the purpose that the voltage of a direct current receiving end is reduced along with the voltage of the direct current sending end is realized, the power transmission of the transmission system is not interrupted when the voltage drop depth of the sending end alternating current fault is increased, and the ride-through capability of the transmission system at different degrees of faults on the sending end alternating current side is realized.
Drawings
A more complete understanding of exemplary embodiments of the present invention may be had by reference to the following drawings in which:
fig. 1 is a schematic flow chart of a sending-end fault ride-through control method of an LCC-MMC power transmission system according to a preferred embodiment of the present invention;
fig. 2 is a schematic composition diagram of a sending-end fault ride-through control device of an LCC-MMC power transmission system in accordance with a preferred embodiment of the present invention;
FIG. 3 is a functional block diagram of a method of controlling a fault crossing of an LCC-MMC power transmission system in accordance with a preferred embodiment of the present invention;
FIG. 4 is a schematic flow chart of a method for controlling a fault ride-through of a sending end of an LCC-MMC hybrid direct current transmission system in accordance with a preferred embodiment of the present invention;
FIG. 5 is a simplified equivalent circuit schematic before and after a fault in an LCC-MMC hybrid DC power transmission system in a preferred embodiment of the present invention;
fig. 6 is a schematic diagram of an MMC single-phase fundamental wave equivalent circuit in an LCC-MMC hybrid dc transmission system in a preferred embodiment of the present invention;
FIG. 7 is a DC simulation curve for a typical constant voltage control strategy for an MMC in a preferred embodiment of the present invention;
FIG. 8 is a DC current simulation curve for the MMC in the preferred embodiment of the present invention with the addition of a fault-ride-through control strategy.
Detailed Description
The exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, however, the present invention may be embodied in many different forms and is not limited to the embodiments described herein, which are provided for complete and complete disclosure of the present invention and to fully convey the scope of the present invention to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, the same units/elements are denoted by the same reference numerals.
Unless otherwise defined, terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Further, it will be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense.
Some terms are defined as follows:
a Line communated converter (LCC for short);
a Modular Multilevel Converter (MMC for short).
Theoretical analysis and operation practice show that in an LCC-MMC hybrid direct-current power transmission system, when an alternating-current fault occurs at a system sending end (provided with an LCC), a direct-current voltage U of the system sending endd1Will drop with the drop of the ac voltage at the transmitting end. The flexible direct current converter valve of the system receiving end (provided with MMC) generally adopts a constant voltage control strategy, and the output direct current voltage U isd2Maintaining the reference value of the DC voltage preset according to the working parameters of the flexible DC converter valve or the parameters of the power transmission system
Figure BDA0002191082330000061
It should be understood that the operation of adjusting the modulation ratio of the converter valves is well known to those skilled in the art and will not be described in detail herein. Besides adopting a constant voltage control strategy, the flexible direct current converter valve is also generally provided with a typical reactive power control link. When the LCC-MMC hybrid direct-current power transmission system normally operates, the reactive reference value of the MMC typical reactive control link is 0, so that the MMC typical reactive control link is not usually used.
When a slight fault occurs on the alternating current side of the sending end, the alternating voltage of the sending end does not drop greatly; reducing a receive side DC voltage U by increasing a modulation ratio of a receive side MMCd2The continuous transmission of the direct current power can be maintained. When the drop depth of the AC fault voltage at the transmitting end is increased to be beyond the modulation ratio space of the flexible direct current converter valve, the DC voltage U at the receiving end occursd2Can not follow the direct current voltage U of the sending end in timed1The drop results in the phenomena of interruption of the direct current and interruption of the power transmission.
That is, when the ac side of the transmitting side is seriously failed, the receiving side dc voltage U occurs even if the modulation ratio of the MMC is increased to the maximum valued2The magnitude of the reduction is not sufficient to dimensionThe phenomenon of continuous transmission of direct current and power is maintained. At this time, the transmission end AC fault ride-through of the power transmission system fails.
As shown in fig. 6, when ac fault occurs at the sending end, the receiving end dc voltage U can be reduced by changing the modulation ratio of the receiving end MMCdc(i.e., the above-mentioned U)d2) And the AC fault ride-through of the sending end is realized. But is limited by the modulation ratio operation space of the MMC valve, and the modulation ratio of the flexible direct current converter valve is changed to receive-end direct current voltage Ud2Or UdcThe adjustment capability and the adjustment range of (2) are limited.
For convenience of expression, in the ac-dc equivalent circuit shown in fig. 6, the receiving-end dc voltage U isd2Can be recorded as Udc。
In order to ensure that the power of the LCC-MMC hybrid direct-current power transmission system continues to be transmitted when the alternating current of the transmitting end fails, the transmitting end alternating-current fault ride-through control method provided by the embodiment of the invention further expands the adjustment range of the direct-current voltage of the receiving end on the basis of the voltage control of the MMC through the MMC reactive power control and the MMC constant current control on the basis of keeping the MMC at the maximum modulation ratio, thereby effectively improving the ride-through capability of the alternating-current fault of the transmitting end.
As shown in fig. 1, a sending-end ac fault ride-through control method according to an embodiment of the present invention includes:
step S10: when the alternating current fault of a transmission end of a power transmission system is determined, a receiving end MMC generates a fault ride-through control strategy according to the alternating current fault degree, wherein the fault ride-through control strategy comprises the addition of at least one control link; the receiving end MMC is provided with a constant voltage control link; the input of the constant voltage control link comprises: reference value of DC voltage
Figure BDA0002191082330000071
Step S20, the receiving end MMC executes the constant voltage control link and the added at least one control link, so that the dc voltage drops along with the dc voltage at the sending end of the LCC, and fault ride-through is realized.
As shown in fig. 2, the sending-end ac fault ride-through control apparatus according to the embodiment of the present invention includes:
the fault ride-through control strategy generation unit 1000 is configured to, when it is determined that an ac fault occurs at a transmitting end of a power transmission system, generate a fault ride-through control strategy by a receiving end MMC according to an ac fault degree, where the fault ride-through control strategy includes adding at least one control link; the receiving end MMC is provided with a constant voltage control link; the input of the constant voltage control link comprises: reference value of DC voltage
Figure BDA0002191082330000072
And a fault ride-through control strategy execution unit 2000, configured to execute the constant voltage control link and at least one control link added, so that the dc voltage drops along with the dc voltage at the sending end of the LCC, thereby implementing fault ride-through.
According to the sending end alternating current fault ride-through control method or device, on the basis that the MMC is kept at the maximum modulation ratio, the adjustment range of the receiving end direct current voltage is further expanded on the basis of MMC voltage control through MMC reactive power control and MMC constant current control, and therefore the ride-through capability of the sending end alternating current fault is effectively improved.
The schematic diagram of the control method represented by a block diagram is shown in FIG. 3, and a constant current control link is added on the basis of a typical constant voltage control link of an MMC; on the basis of the MMC reactive power control link, a reactive power control opening and closing link is additionally arranged.
When the change rate of the direct current is not more than the threshold value kItheshIn time, MMC typical constant voltage control link is adopted to ensure that U is connected with the power supplyd2Or UdcFollowing a predetermined DC voltage reference value
Figure BDA0002191082330000081
When the DC current change rate is larger than a threshold value kItheshAnd is not greater than a threshold value kIsetBefore the typical constant voltage control link of the MMC, the constant current control link of the MMC is added, so that U is enabledd2Or UdcFollowing the corrected DC voltage reference value
Figure BDA0002191082330000082
When the DC current change rate is larger than a critical value kIsetIn time, on the basis of the typical constant voltage control link of the MMC, the constant current control link of the MMC is added, and the starting and stopping link is controlled by reactive power to be put into the typical reactive power control link of the MMC, so that U is enabled to be controlled by the reactive powerd2Or UdcFollowing the corrected DC voltage reference value
Figure BDA0002191082330000083
Therefore, the ride-through capability of the power transmission system is realized when the alternating current side of the transmitting end has faults of different degrees.
It should be understood that the threshold value k for the rate of change of the direct currentItheshThe positive number is greater than zero, and during specific implementation, the method in the prior art is adopted to comprehensively determine according to the parameters of each device; critical value k for rate of change of DC currentIsetGreater than threshold k of DC current change rateIthesh。
Preferably, as shown in the upper left of fig. 3, the MMC constant current control element includes a current PI regulator 210. Measured value of DC current IdAnd a DC current reference value
Figure BDA0002191082330000084
After the difference is made, the deviation value delta U of the direct current voltage reference value is obtained through a PI regulatordc：
Figure BDA0002191082330000085
It should be understood that the current PI regulator in the above equation is only one example. Those skilled in the art can use other control laws in the prior art to utilize the measured value I of the dc currentdAnd a DC current reference value
Figure BDA0002191082330000086
The difference of (D) is used to obtain the deviation amount Delta U of the DC voltage reference valuedcAnd has a corresponding gain factor.
In specific implementation, the PI parameter of the current PI regulator is determined through repeated debugging by a method known to those skilled in the art.
It should be noted that the direct current reference value of the MMC constant current control link
Figure BDA0002191082330000087
The predetermined value is kept constant during fault ride-through, i.e. during the transient to steady state transition of the fault, during each control cycle.
In specific implementation, in order to avoid voltage regulation instability caused by simultaneous working of constant current control of a receiving end MMC and constant current control of a sending end LCC, a direct current reference value in an MMC constant current control link is preset
Figure BDA0002191082330000088
Is 0.9p.u. (per unit value) smaller than a reference value (denoted as 1p.u. (per unit value)) for constant current control of the transmitting terminal LCC by 0.1p.u. (per unit value).
Alternatively, when the dc current change rate is calculated, the time interval Δ t between two adjacent sampling instants is 0.0001 s.
It should be understood that the MMC constant voltage control link enables the direct current voltage output by the MMC to follow the direct current voltage reference value
Figure BDA0002191082330000091
The voltage reference value
Figure BDA0002191082330000092
The predetermined value is kept constant during fault ride-through, i.e. during the transient to steady state transition of the fault, during each control cycle. After the MMC constant current control link is put into operation, the measured value I of the direct current is useddAnd a DC current reference value
Figure BDA0002191082330000093
Generating a deviation Δ U of a DC voltage reference valuedcFor correcting the DC voltage reference value
Figure BDA0002191082330000094
Thereby leading the receiving end to have direct current voltage UdcFurther follows the drop of the AC voltage at the sending end and drops.
As shown in fig. 3, the MMC exemplary constant-voltage control link preferably includes a voltage PI controller 111 and an inner-loop current PI controller 112; the mathematical expressions of the voltage PI controller and the inner loop current PI controller may be written according to the dc current PI regulator 210, and are not described herein again. MMC typical constant voltage control link enables direct current voltage U output by MMCd2Can follow the further decline of the corrected voltage reference value, thereby obtaining the voltage reference value
Figure BDA0002191082330000095
On the basis of the voltage of the receiving end direct current U, the direct current U of the receiving end is further reduceddcAnd fault ride-through is realized.
It should be understood that the "inner loop" herein refers to the control link that the MMC has set, i.e. 100 within the dashed right-hand box in fig. 3; accordingly, the "outer loop" control refers to the constant current control and the constant reactive power control proposed in the present method, i.e. 200 within the left dashed box in fig. 3.
In order to further reduce the receiving end direct current voltage UdcAt a DC current change rate k, as shown in the lower left of FIG. 3IdGreater than a critical value kIsetWhen the power is switched on, the network side reactive reference value output by the reactive power control on-off link is-1 p.u. (per unit value), and the network side reactive reference value is further input into an MMC reactive control link besides a constant voltage control link and a constant current control link;
if the rate of change k of the DC currentIdIs not greater than a critical value kIsetAnd if the network side reactive reference value output by the reactive power control starting and stopping link is 0, the MMC reactive control link is not input outside the constant voltage control link and the constant current control link.
As shown in the lower right of fig. 3, similar to the constant voltage control link, the MMC reactive control link includes a reactive power PI controller 121 and an inner loop current PI controller 122; the mathematical expressions of the reactive power PI controller and the inner loop current PI controller may be written according to the dc current PI regulator 210, and are not described herein again. MMC reactive power control link inputThen, using the reactive power reference value Q*And a real value of the reactive power Q at a voltage reference value
Figure BDA0002191082330000101
On the basis of the voltage of the receiving end direct current U, the direct current voltage U of the receiving end can be further reduceddc。
The input of the MMC reactive power control link is a reactive power reference value Q*And the measured value Q, the output is the Q-axis current reference value
Figure BDA0002191082330000102
Specifically, as shown in fig. 4, the control steps adopted in the control method include:
before or after the modulation ratio m of the flexible direct current converter valve reaches the maximum value, a MMC constant current and constant voltage control link is put into, and the direct current voltage reference value is reduced by utilizing the constant current control so as to reduce the direct current voltage of the MMC;
after the modulation ratio m of the flexible direct current converter valve reaches the maximum, the MMC reactive power control link, the constant current control link and the constant voltage control link are put into, and the direct current voltage is further reduced by utilizing the constant current control and the reactive power control.
The influence analysis of the reactive power control link on the direct-current voltage of the MMC is as follows. According to the MMC single-phase fundamental equivalent circuit shown in fig. 6, it can be derived that the fundamental phase voltage at the Δ point is:
Figure BDA0002191082330000103
the voltage modulation ratio m of the MMC is defined as:
Figure BDA0002191082330000104
in the formula (2), UdiffmIs a delta point fundamental phase voltage udiffjAmplitude of (U)dcIs a direct current voltage of MMC.
When the power transmission system is in normal operation, the electricity of the alternating current systemThe voltage is stable and constant, and the inner loop current is controlled to enable the delta point voltage udiffjRemains unchanged, its amplitude UdiffmAlso, the dc voltage of the MMC is constant, so the dc voltage is inversely proportional to the modulation ratio.
Generally, the modulation ratio of the MMC is 0.8-0.9, and the maximum modulation ratio is 1. Therefore, the adjustable range of the voltage modulation ratio of the MMC is small, and the direct-current voltage of the MMC is limited by the modulation ratio and can only be reduced by 20 percent at most. Therefore, in the LCC-MMC hybrid DC power transmission system, for the serious fault at the AC side of the sending end, the DC voltage U of the receiving end is reduced by only increasing the MMC modulation ratiodcContinued transfer of power may not be achieved.
In order to improve the ride-through capability of the AC fault at the sending end, the DC voltage U of the MMC at the receiving end needs to be further wideneddcThe adjustable range of (2).
As is clear from equation (2), the dc voltage of the MMC is affected by the voltage at the Δ point in addition to the modulation ratio. Because the MMC is connected with a large power grid through a converter transformer, the alternating voltage u of the MMC iswAnd remain constant.
As can be seen from FIG. 6, if uwConstant, delta point fundamental phase voltage udiffjDirectly dependent on bridge arm inductance L0And current conversion leakage reactance XTVoltage loss over the capacitor. Therefore, the Δ point voltage u can be reduced by increasing the voltage loss on the reactancediffjTo further enable the receiving end MMC direct current voltage UdcAnd decreases.
The capital I indicates direct current, and the small I indicates alternating current. Similarly, upper case U and lower case U have corresponding meanings. That is, case is used to distinguish between ac and dc.
In ac systems, it is customary to represent voltage losses by power instead of current. If shown as I in FIG. 6dcThe current direction of (1) is positive direction, delta point fundamental phase voltage udiffjAlternating voltage u to the receiving endwThe voltage loss Δ U between can be approximated as:
Figure BDA0002191082330000111
in formula (3): u shapediff、UwRespectively, a delta point fundamental phase voltage udiffjEffective value of line voltage, AC voltage uwA line voltage effective value of;
Qwis the net side reactive power (not shown in fig. 6).
The DC voltage U can be obtained by combining the vertical type (2) and the formula (3)dcAnd network side reactive power QwThe relationship of (1) is:
Figure BDA0002191082330000112
wherein, UwFundamental line voltage of an alternating current power grid connected with the MMC;
omega is the fundamental frequency of an alternating current power grid connected with the MMC;
L0bridge arm inductance of MMC;
XTthe converter transformer leakage reactance is connected between the MMC and an alternating current power grid.
From the equation (4), the net side reactive power QwThe level of (c) determines the lowest dc voltage that the MMC can reach at this net-side reactive power level. That is, the regulation range of the dc voltage changes with the change of the reactive power between the receiving-end converter MMC and the ac system. When the modulation ratio is 1 (namely the maximum modulation ratio of the MMC), the more reactive power the converter absorbs from the receiving-end alternating-current system, the larger the range in which the direct-current voltage can be reduced; therefore, when Q iswAt-1 p.u. (per unit value), the DC voltage UdcThe adjustable range of (a) is maximized.
Determining a current rate of change threshold
Figure BDA00021910823300001210
The method comprises the following steps:
current reference value according to MMC constant current control
Figure BDA0002191082330000121
Obtaining a sending end direct current voltage U 'in a steady state after a fault'd1；
After failureSending end direct current voltage U 'in steady state'd1Can obtain fault alternating current voltage U'LL；
According to the simplified equivalent circuit of the mixed direct-current power transmission system before and after the fault shown in the figure 5, an equation is established, and the direct current i 'after the alternating-current fault of the sending end can be obtained'd；
For direct current i 'after fault'dObtaining a critical value by derivation
Figure BDA0002191082330000122
In the above symbols, the prime symbol in the superscript is used to indicate the physical quantity in the transition stage after the fault, so as to distinguish the physical quantity from the steady-state physical quantity before the fault.
Note here that i'dIs the transient DC current after the fault, and the d-axis current i in the inner-loop current controller in the MMC on the right side of the figure 3dIs different.
Specifically, a reactive start threshold is determined
Figure BDA0002191082330000129
The principle of (1) is as follows: first, as shown in FIG. 5, a current reference value according to MMC constant current control
Figure BDA0002191082330000123
Obtaining the sending end direct-current voltage in the steady state after the fault:
Figure BDA0002191082330000124
in the formula (5), R is the line impedance R of the hybrid direct-current transmission systemL；
Secondly, the DC voltage U 'of the fault rear end is used'd1Obtaining the internal potential E 'of the sending end direct current voltage'd1Comprises the following steps:
Figure BDA0002191082330000125
in the formula (6), XrCommutation reactance (not shown in fig. 5) of the sending-end LCC equivalent;
then, the equivalent circuit is simplified according to the mixed direct-current power transmission system before and after the fault shown in fig. 5, and direct current i 'after the fault is obtained'dComprises the following steps:
Figure BDA0002191082330000126
in conclusion, the current critical value of the reactive start is obtained
Figure BDA0002191082330000128
Comprises the following steps:
Figure BDA0002191082330000127
wherein L is the inductance of the smoothing reactor;
Idis a direct current in steady state operation.
Taking a puerarian direct current transformation project as an example, a three-phase short-circuit fault is set at a sending end of a simulation model, and a typical constant voltage control strategy and a fault ride-through control strategy (namely a constant current control link and a reactive power control link) are simulated respectively so as to verify the effectiveness of the fault ride-through control method.
The simulation result of fig. 7 shows that, when a three-phase short-circuit fault occurs at the sending end, the phenomena of dc current interruption and power transmission interruption occur by using a typical constant voltage control strategy of MMC, and at this time, the fault ride-through fails.
The simulation result of fig. 8 shows that, after the constant current control link and the reactive power control link provided by the present invention are utilized comprehensively, when a three-phase short circuit fault occurs at the sending end, the passing through of the sending end alternating current fault is realized.
In conclusion, the fault ride-through control method and the fault ride-through control device are effective in improving the fault ride-through capability, high in feasibility, low in cost and convenient for engineering implementation.
The invention has been described above by reference to a few embodiments. However, other embodiments of the invention than the one disclosed above are equally possible within the scope of the invention, as would be apparent to a person skilled in the art from the appended patent claims.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [ device, component, etc ]" are to be interpreted openly as referring to at least one instance of said device, component, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

Claims (5)

1. A sending end fault ride-through control method of an LCC-MMC power transmission system is characterized by comprising the following steps:
when the alternating current fault of a transmission end of a power transmission system is determined, a receiving end MMC generates a fault ride-through control strategy according to the alternating current fault degree, wherein the fault ride-through control strategy comprises the addition of at least one control link; the receiving end MMC is provided with a constant voltage control link; the input of the constant voltage control link comprises: reference value of DC voltage
Figure FDA0003243466610000011
The generating of the fault ride-through control strategy according to the alternating current fault degree comprises the following steps:
comparing the rate of change k of the DC currentIdAnd a predetermined DC threshold kIset；
At the rate of change k of the direct currentIdIs not greater than a predetermined threshold kIsetThen, a constant current control link is additionally configured on the MMC at the receiving end;
at the rate of change k of the direct currentIdIs greater than a preset critical value kIsetThen, a receiving end MMC is additionally provided with a constant current control link and a reactive power control link;
wherein the constant current control sectionThe input includes a predetermined DC current reference value
Figure FDA0003243466610000012
And the value of the direct current I at the current sampling momentd；
The output of the constant current control link is the deviation delta U of the direct current voltage reference valuedcFor correcting the voltage reference value of the constant voltage control element
Figure FDA0003243466610000013
The input of the reactive power control link comprises a preset reactive power reference value Q*And a reactive power value Q at the current sampling moment;
and the receiving end MMC executes the constant voltage control link and the added at least one control link so that the direct current voltage is reduced along with the direct current voltage of the LCC sending end, and fault ride-through is realized.
2. The LCC-MMC power transmission system send-end fault ride-through control method of claim 1,
the output of the reactive power control link is a q-axis current reference value of the inner loop current controller;
receiving end MMC direct current voltage UdcAnd network side reactive power QwSatisfies the following formula:
Figure FDA0003243466610000014
wherein, UwFundamental line voltage of an alternating current power grid connected with the MMC;
omega is the fundamental frequency of an alternating current power grid connected with the MMC;
L0bridge arm inductance of MMC;
XTthe converter transformer leakage reactance is connected between the MMC and an alternating current power grid.
3. The LCC-MMC power transmission system send-end fault ride-through control method of claim 1,
the preset DC critical value kIsetDetermined according to the following formula:
Figure FDA0003243466610000021
wherein R isLIs a direct current line resistor;
l is the inductance of the smoothing reactor;
Xrthe phase-change reactance is equivalent to the sending end;
Iddirect current for steady state operation;
E′d1voltage internal potential of the voltage is sent after the fault; u shaped2Is a receiving end direct current voltage.
4. The LCC-MMC power transmission system send-end fault ride-through control method of claim 1,
the constant current control link comprises a current PI controller;
the reactive power control link comprises a reactive power PI controller.
5. A LCC-MMC power transmission system send end fault ride-through control device is characterized by comprising:
the fault ride-through control strategy generating unit is used for generating a fault ride-through control strategy according to the alternating current fault degree when the alternating current fault of the transmission end of the power transmission system is determined, wherein the fault ride-through control strategy comprises adding at least one control link; wherein, the receiving end MMC is configured with a constant voltage control link; the input of the constant voltage control link comprises: reference value of DC voltage
Figure FDA0003243466610000022
The fault ride-through control strategy generation unit comprises:
a critical value comparison unit for comparing the direct currentRate of change of current kIdAnd a predetermined DC threshold kIset；
A constant current control unit for controlling the constant current at a DC current change rate kIdIs not greater than a predetermined threshold kIsetMeanwhile, a constant current control link is additionally configured;
a reactive power control link configuration unit for controlling the DC current change rate kIdIs greater than a preset critical value kIsetThen, a receiving end MMC is additionally provided with a constant current control link and a reactive power control link;
wherein, the input of the constant current control link comprises a preset DC reference value
Figure FDA0003243466610000031
And the value of the direct current I at the current sampling momentd；
The output of the constant current control link is the deviation delta U of the direct current voltage reference valuedcFor correcting the voltage reference value of the constant voltage control element
Figure FDA0003243466610000032
The input of the reactive power control link comprises a preset reactive power reference value Q*And a reactive power value Q at the current sampling moment;
and the fault ride-through control strategy execution unit is used for executing the constant voltage control link and at least one added control link so as to enable the direct current voltage to be reduced along with the direct current voltage at the sending end of the LCC, thereby realizing fault ride-through.
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