CN111162562A - Coordinated fault ride-through method suitable for wind power MMC-MTDC system - Google Patents

Coordinated fault ride-through method suitable for wind power MMC-MTDC system Download PDF

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CN111162562A
CN111162562A CN202010079516.5A CN202010079516A CN111162562A CN 111162562 A CN111162562 A CN 111162562A CN 202010079516 A CN202010079516 A CN 202010079516A CN 111162562 A CN111162562 A CN 111162562A
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CN111162562B (en
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贾科
秦继朔
毕天姝
郑黎明
方煜
杨哲
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North China Electric Power University
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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
    • 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]

Abstract

The invention discloses a coordinated fault ride-through method suitable for a wind power MMC-MTDC system, which comprises the steps of deducing coordinated fault ride-through control parameters based on a wind field side MMC converter WFMMC and a wind side VSC converter WVSC according to input and output power of each node of an MMC-MTDC; deducing a steady-state short-circuit current expression when a short-circuit fault occurs at an alternating current outlet of a receiving end converter GSMMC from a power angle according to the deduced coordinated fault ride-through control parameter; and finally, providing reference for selection of the specification of the submodule of the MMC converter and setting of protection of the alternating current side of the converter according to the deduced steady-state short-circuit current expression, so that coordinated fault ride-through of the wind power MMC-MTDC system is realized.

Description

Coordinated fault ride-through method suitable for wind power MMC-MTDC system
Technical Field
The invention relates to the field of power systems, in particular to a coordinated fault ride-through method suitable for a wind power MMC-MTDC system.
Background
The development and utilization of new energy represented by wind energy have important significance for social and economic development, environmental protection, coping with the current situation of energy shortage and the like. In recent years, great progress is made in development and utilization of wind power, and a large-scale wind power plant attracts wide attention through High Voltage Direct Current (HVDC) mode grid connection. The high-voltage direct-current transmission technology based on the Modular Multilevel Converter (MMC) combines a Voltage Source Converter (VSC) technology and a pulse width modulation technology, has the advantages of high modularization degree, good waveform quality, small occupied area and the like, and is an effective mode for large-scale wind power grid connection. Compared with an offshore wind farm, onshore wind energy resources are dispersed, and a multi-terminal wind power flexible direct-transmission system (MMC-MTDC) based on a direct-current transmission network is derived.
At present, the MMC-MTDC and wind field coordination low-pass research is less, fault passing is realized only through voltage reduction of a traditional wind field side MMC converter, unbalanced power during fault is not eliminated substantially, and the unbalanced power is transferred to the wind field side. This can make the fan bear great pressure back to back system, and Chopper circuit is frequently, the long-time is opened, may lead to the circuit to burn out, therefore it is necessary to study new wind-powered electricity generation MMC-MTDC system coordination fault through tactics. In addition, with the development of multi-terminal flexible direct current engineering represented by the north-tensioned flexible direct current transmission exemplary engineering, each node of the direct current transmission network is usually power-equivalent, that is, the possible input and output power of each node is considered in the planning process. On the basis, in order to provide reference for selection of the specification of the submodule of the MMC converter and setting of protection of the alternating current side of the converter, steady-state short-circuit current of each node when the alternating current side is in short circuit needs to be estimated in advance, so that a steady-state short-circuit current expression of the wind power MMC-MTDC system needs to be researched.
Disclosure of Invention
According to one aspect of the invention, a coordinated fault ride-through method suitable for a wind power MMC-MTDC system is provided, which comprises the following steps:
step 1: deducing a coordinated fault ride-through control parameter based on a wind field side MMC converter WFMMC and a wind side VSC converter WVSC according to input and output power of each node of the MMC-MTDC;
step 2: according to the coordinated fault ride-through control parameter deduced in the step 1, deducing a steady-state short-circuit current expression when a short-circuit fault occurs at an alternating current outlet of a receiver converter GSMMC from a power angle;
and step 3: and (3) providing reference for selection of the specification of the submodule of the MMC converter and setting of protection of the alternating current side of the converter according to the steady-state short-circuit current expression deduced in the step (2), so that coordinated fault ride-through of the wind power MMC-MTDC system is realized.
Further, in step 1, when a short-circuit fault occurs at an alternating current outlet of the receiving end converter GSMMC, reducing the alternating current input voltage of the WFMMC through WFMMC voltage reduction control, so as to reduce the input power and reduce the unbalanced power on the direct current transmission network; and simultaneously, designing a fan side VSC converter WTDSC (wind turbine converter) step-down control coordinated with the WFMMC step-down control to eliminate unbalanced power.
Preferably, the WFMMC step-down control adopts a V/f control method, and controls the voltage amplitude and the frequency of the input converter to be constant values, and the control process includes: during a fault, the ac voltage reference in the inverter control loop will be lowered, which is expressed as equation (1),
Figure BDA0002379762320000021
in the formula (I), the compound is shown in the specification,
Figure BDA0002379762320000022
is an alternating current voltage reference value, U 'in V/f control under normal working condition'ac_WFMMCReference value of AC voltage, U, redesigned for fault conditionsdcFor the MMC-MTDC direct voltage during a fault,
Figure BDA0002379762320000023
is a preset MMC-MTDC direct current upper limit value, KdcControlling parameters for WFMMC pressure reduction;
neglecting the active power loss of the MMC converter, the relation between the input power and the output power of the MMC-MTDC during the fault period is expressed as an expression (2),
PS＝PW-△P (2)，
wherein, PSOutput power, P, for GSMMCW△ P is the difference between input power and output power of MMC-MTDC, which is the sum of input power of all WFMMCs;
further expanding the formula (2) to the formula (3),
Figure BDA0002379762320000031
in the formula (3), I is the effective value of the alternating current input by WFMMC,
Figure BDA0002379762320000032
the value of the equivalent capacitance of the direct current transmission line is the sum of HBSM capacitance values of all sub modules on a bridge arm of the converter;
further, the formula (3) is represented by the formula (4),
Figure BDA0002379762320000033
by integrating the formula (1) and the formula (4), the obtained value of the WFMMC pressure reduction control parameter is shown in the formula (5):
Figure BDA0002379762320000034
preferably, the WTVSC load shedding control adopts PQ control to ensure that the active power and the reactive power output by the fan are constant, and the PQ control process includes: the active power reference value P' WTVSC redesigned in case of a fault is represented as equation (6),
Figure BDA0002379762320000035
in the formula (I), the compound is shown in the specification,
Figure BDA0002379762320000036
is an active power reference value P 'in WVSC control under normal working condition'WTVSCActive power redesigned for fault conditionsReference value, KPLoad reduction control parameters are provided for WVSC;
neglecting the active power loss of the MMC converter, the relation between the input power and the output power of the MMC-MTDC during the fault period is shown as the formula (2), for each fan, the formula (2) can be expressed as the formula (7),
Figure BDA0002379762320000037
in the formula, n is the sum of the number of fans connecting all the MMC-MTDC wind fields;
equation (7) can be further written as equation (8),
Figure BDA0002379762320000038
the value of the WTSC load reduction control parameter obtained by integrating the formula (6) and the formula (8) is shown as the formula (9),
Figure BDA0002379762320000041
the process of deriving the steady-state short-circuit current expression in the step 2 comprises the following steps:
the power of the WFMMC input is expressed as equation (10),
Figure BDA0002379762320000042
in the formula, PWFMMCAnd QWFMMCActive power and reactive power input by WFMMC respectively; u. ofWd、uWqD-axis voltage and q-axis voltage of the WFMMC alternating current side are respectively, and the values of the d-axis voltage and the q-axis voltage are determined by an alternating voltage reference value in WFMMC control; i.e. iWd、iWqThe d-axis current and the q-axis current on the alternating current side of WFMMC respectively, the values of which are determined by the power output by the wind field, are expressed as an expression (11),
Figure BDA0002379762320000043
meanwhile, for the MMC-MTDC system, the relation between the input power and the output power is shown as the formula (12),
Figure BDA0002379762320000044
in the formula IdcIs the current of a DC transmission network, RdcIs a direct current transmission network resistor;
combining equations (10), (11), (12), the GSMMC output power is represented as equation (13) in view of the setting of the MMC-MTDC direct current transmission network topology,
Figure BDA0002379762320000045
in the formula, n1、n2、n3The number of 1#, 2#, 3# wind field fans, l01Is the length of the transmission line between 1# WFMMC and GSMMC02Is the length of the transmission line between 2# WFMMC and GSMMC13Is the length of the transmission line between 1# WFMMC and 3# WFMMC23Is the length of the transmission line between 2# WFMMC and 3# WFMMC, rdcThe resistance value of the direct current transmission line is a unit length;
for the symmetric short-circuit fault on the alternating current side of the GSMMC, the expression formula of the steady-state short-circuit current is shown as a formula (14),
Figure BDA0002379762320000051
for the asymmetric short-circuit fault on the alternating current side of the GSMMC, the expression of the steady-state short-circuit current is shown as the formula (15) due to the existence of negative sequence voltage and negative sequence current,
Figure BDA0002379762320000052
in the formula, D1And D2The values are respectively shown in the formula (16),
Figure BDA0002379762320000053
drawings
FIG. 1 is a flow chart of a coordinated fault ride-through method applicable to a wind power MMC-MTDC system according to the present invention;
FIG. 2 is a wind power MMC-MTDC system topological diagram;
FIG. 3 is a permanent magnet wind turbine topology;
FIG. 4 is a block diagram of a voltage reduction control system for WFMMC during fault ride through;
FIG. 5 is a comparison graph of DC voltage waveforms of a conventional fault ride-through control system and a coordinated fault ride-through control system according to the present invention during a BC phase-to-phase short circuit fault;
FIG. 6 is a comparison graph of MMC-MTDC input and output active power waveforms during a BC phase-to-phase short circuit fault;
fig. 7 is a graph of input power and output power of a back-to-back system of a permanent magnet fan during a two-phase short circuit.
FIG. 8 is a graph of Chopper power consumption.
FIG. 9 is a comparison of simulated values and calculated values of steady-state short-circuit current under a three-phase short-circuit fault;
fig. 10 is a comparison of steady-state short-circuit current simulation values and calculated values under the BC-phase short-circuit fault.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings.
FIG. 1 is a flowchart of a coordinated fault ride-through strategy and fault current analysis method of a wind power MMC-MTDC system according to the present invention. In this embodiment, a topological diagram of a wind power MMC-MTDC system to which the fault ride-through and fault current analysis method is applied is shown in fig. 2.
The wind power MMC-MTDC system comprises an alternating current power grid, 3 wind power plants, 1 GSMMC converter station and 3 WFMMC converter stations. The alternating-current transmission voltage class of an alternating-current power grid is 220kV, the direct-current transmission voltage class is +/-500 kV, the number of MMC half-bridge sub-modules is 76, the number of permanent magnet fans in respective wind fields of 1#, 2#, and 3# of 3 wind power plants is 102, 92, and 82 respectively, the rated capacity of a single fan is 5.2MW, and the length l of a direct-current transmission line is01、l02、l13、l23Respectively 100km, 120km, 80km and 60 km.
The permanent magnet fans in the three wind power plants of the invention mainly comprise fan mechanical parts and back-to-back systems with Chopper circuits, and the topological diagram is shown in fig. 3.
The method for coordinating fault ride-through and fault current analysis of the wind power MMC-MTDC system comprises the following steps of:
step 1: deducing a coordinated fault ride-through control parameter based on a wind field side MMC converter WFMMC and a wind side VSC converter WVSC according to input and output power of each node of the MMC-MTDC;
step 2: according to the coordinated fault ride-through control parameter deduced in the step 1, deducing a steady-state short-circuit current expression when a short-circuit fault occurs at an alternating current outlet of a receiver converter GSMMC from a power angle;
and step 3: and (3) providing reference for selection of the specification of the submodule of the MMC converter and setting of protection of the alternating current side of the converter according to the steady-state short-circuit current expression deduced in the step (2), so that coordinated fault ride-through of the wind power MMC-MTDC system is realized.
In the step 1, when a short-circuit fault occurs at an alternating current outlet of a receiving end converter (GSMMC), the output power of the GSMMC is sharply reduced, and the input power of the WFMMC is unchanged, which may cause a large amount of power accumulation on a direct current transmission network of the MMC-MTDC system, resulting in a rapid rise of the direct current voltage. The method adopted by the invention is to reduce the input power by reducing the alternating current input voltage of the WFMMC, thereby achieving the purpose of reducing the unbalanced power on the direct current transmission network.
The control mode adopted by the WFMMC is V/f control, namely the voltage amplitude and the frequency of the input converter are controlled to be constant values. During a fault, the AC voltage reference in the control loop of the converter is lowered, and the value thereof can be expressed as
Figure BDA0002379762320000071
In the formula (I), the compound is shown in the specification,
Figure BDA0002379762320000072
is an alternating current voltage reference value, U 'in V/f control under normal working condition'ac_WFMMCReference value of AC voltage, U, redesigned for fault conditionsdcFor the MMC-MTDC direct voltage during a fault,
Figure BDA0002379762320000073
is a preset MMC-MTDC direct current upper limit value, KdcThe value derivation process of the WFMMC pressure reduction control parameter is as follows.
Neglecting the MMC converter active power loss, the MMC-MTDC input and output power relation during the fault period can be expressed as
PS＝PW-△P (2)
Wherein, PSOutput power, P, for GSMMCW△ P is the MMC-MTDC input to output power difference for the sum of the input power of all WFMMCs.
The formula (2) can be further developed into
Figure BDA0002379762320000074
In the formula (3), I is the effective value of the alternating current input by WFMMC,
Figure BDA0002379762320000075
the value of the equivalent capacitance of the direct current transmission line is the sum of capacitance values of all sub modules (HBSM) on a bridge arm of the converter, and the increase of direct current voltage during a fault period is caused by the charging effect of the equivalent capacitance.
Formula (3) can be further written as
Figure BDA0002379762320000076
Comparing the formula (1) and the formula (4), it can be seen that the value of the pressure-reducing control parameter of WFMMC is
Figure BDA0002379762320000077
Because WFMMC step-down control is essentially to shift the unbalanced power of MMC-MTDC transmission network to the wind field and not to eliminate the unbalanced power, the wind machine side VSC converter (WTSCC) load reduction control which is coordinated with the WFMMC step-down control needs to be designed to eliminate the unbalanced power. The WTVVSC adopts PQ control to ensure that the active power and the reactive power output by the fan are constant, so that the load reduction control principle of the WTVVSC is to redesign the reference value of the active power, and the value can be expressed as
Figure BDA0002379762320000081
In the formula (I), the compound is shown in the specification,
Figure BDA0002379762320000082
is an active power reference value P 'in WVSC control under normal working condition'WTVSCActive power reference value, K, redesigned for fault conditionsPFor the load reduction control parameter of the WVSC, the derivation process is as follows.
Neglecting the active power loss of the MMC converter, the relation between the input power and the output power of the MMC-MTDC during the fault period is shown as the formula (2), and for each fan, the formula (2) can be written into the form of the formula (7).
Figure BDA0002379762320000083
In the formula, n is the sum of the number of fans connecting all the MMC-MTDC wind fields.
Formula (7) can be further written as
Figure BDA0002379762320000084
Comparing the formula (6) and the formula (8), it can be seen that the value of the WTSC load reduction control parameter is
Figure BDA0002379762320000085
In the step 2, the process of deducing the steady-state short-circuit current expression when the short-circuit fault occurs at the alternating current outlet of the receiving end converter (GSMMC) is as follows:
the power input by WFMMC can be expressed as
Figure BDA0002379762320000086
In the formula, PWFMMCAnd QWFMMCActive power and reactive power input by WFMMC respectively; u. ofWd、uWqD-axis voltage and q-axis voltage of the WFMMC alternating current side are respectively, and the values of the d-axis voltage and the q-axis voltage are determined by an alternating voltage reference value in WFMMC control; i.e. iWd、iWqThe d-axis current and the q-axis current on the alternating current side of the WFMMC respectively have values determined by the power output by a wind field and can be expressed as
Figure BDA0002379762320000091
Meanwhile, for the MMC-MTDC system, the relation between the input power and the output power is
Figure BDA0002379762320000092
In the formula IdcIs the current of a DC transmission network, RdcIs a direct current transmission network resistor.
Combining equations (10), (11), (12), and considering the MMC-MTDC transmission network topology adopted by the present invention, the GSMMC output power can be finally expressed as
Figure BDA0002379762320000093
In the formula, n1、n2、n3The number of 1#, 2#, 3# wind field fans, l01Is the length of the transmission line between 1# WFMMC and GSMMC02Is the length of the transmission line between 2# WFMMC and GSMMC13Is the length of the transmission line between 1# WFMMC and 3# WFMMC23Is the length of the transmission line between 2# WFMMC and 3# WFMMC, rdcIs the resistance value of the direct current transmission line with unit length.
For symmetric short-circuit fault on GSMMC AC side, the expression of steady-state short-circuit current is (14)
Figure BDA0002379762320000094
For the asymmetric short-circuit fault on the alternating current side of the GSMMC, the expression of the steady-state short-circuit current is shown as a formula (15) due to the existence of negative sequence voltage and negative sequence current
Figure BDA0002379762320000101
In the formula, D1And D2The values are respectively shown in formula (16),
Figure BDA0002379762320000102
it can be seen from the equations (13), (14) and (15) that after the short-circuit fault occurs on the ac side of the GSMMC, the steady-state short-circuit current is related to the length of each line of the MMC-MTDC dc transmission network, the number of wind field fans, the reference values of active power and reactive power of the fans, and the fault ride-through control system adopted by the whole system. It is noted that formula (13) does not have the WFMMC step-down control parameter KdcOnly the WTSCC load shedding control parameter K existsPThe steady-state short-circuit current output by the converter during the fault is not related to the voltage reduction degree of the WFMMC but related to the voltage reduction degree of the WTDCC.
FIG. 4 is a block diagram of a WFMMC buck control system during a fault. It should be noted that, since the configuration of the WTVSC load shedding control system is similar to the figure, the configuration of the WTVSC load shedding control system is not listed here.
Fig. 5 is a comparison graph of dc voltage waveforms for configuring a conventional fault ride-through system and for configuring a coordinated fault ride-through control system according to the present invention during a two-phase interphase short circuit fault. As is apparent from fig. 5, after 4.66 seconds of the fault, the dc voltage suppression effect of the coordinated fault ride-through method of the present invention is superior to that of the conventional fault ride-through method. The comparison graph proves the effectiveness of the wind power MMC-MTDC system coordination fault ride-through method provided by the invention.
Fig. 6 is a graph comparing MMC-MTDC input and output active power waveforms during a two-phase short circuit fault. As can be seen from the figure, the reduction of the MMC-MTDC input power is the root cause of the limitation of the dc voltage rise.
Fig. 7 is a graph of input power and output power of a back-to-back system of a permanent magnet fan during a two-phase short circuit. Under the influence of the fan load reduction control system, the input power of a back-to-back system of the fan is reduced along with the reduction of the output power during the fault, so that the unloading pressure of a Chopper circuit is reduced.
FIG. 8 is a graph of Chopper power consumption. As can be seen from the figure, the Chopper circuit under the traditional fault ride-through control has long switching-on time, high switching-on frequency and risk of burning; the on-time and the on-frequency of the Chopper circuit under the coordinated fault ride-through control are obviously reduced, and the strategy proves that the risk of burning the Chopper circuit can be effectively reduced.
Fig. 9 is a comparison of the simulated value and the calculated value of the steady-state short-circuit current under the three-phase short-circuit fault. The failure start time was 4.5s, the failure duration was 0.3s, and the transition resistance was 5 Ω. It can be seen from the figure that after about 10ms of fault, the A, B, C three-phase current simulated value waveform matches with the calculated value waveform, which proves that the steady-state short-circuit current expression provided by the invention is correct under the condition of symmetrical short-circuit fault.
Fig. 10 is a comparison of simulated and calculated steady-state short-circuit currents in a BC phase-to-phase short-circuit fault. The failure start time was 4.5s, the failure duration was 0.3s, and the transition resistance was 5 Ω. It can be seen from the figure that after about 17ms of fault, the A, B, C three-phase current simulation value waveform matches with the calculated value waveform, which proves that the steady-state short-circuit current expression provided by the invention is also correct under the condition of asymmetric short-circuit fault.
It should be noted that the above-mentioned embodiments are only preferred embodiments of the present invention, and should not be construed as limiting the scope of the present invention, and any minor changes and modifications to the present invention are within the scope of the present invention without departing from the spirit of the present invention.

Claims (6)

1. A coordinated fault ride-through method suitable for a wind power MMC-MTDC system comprises the following steps:
step 1: deducing a coordinated fault ride-through control parameter based on a wind field side MMC converter WFMMC and a wind side VSC converter WVSC according to input and output power of each node of the MMC-MTDC;
step 2: according to the coordinated fault ride-through control parameter deduced in the step 1, deducing a steady-state short-circuit current expression when a short-circuit fault occurs at an alternating current outlet of a receiver converter GSMMC from a power angle;
and step 3: and (3) providing reference for selection of the specification of the submodule of the MMC converter and setting of protection of the alternating current side of the converter according to the steady-state short-circuit current expression deduced in the step (2), so that coordinated fault ride-through of the wind power MMC-MTDC system is realized.
2. The method for coordinated fault ride-through of a wind power MMC-MTDC system according to claim 1, wherein the wind power MMC-MTDC system comprises:
the system comprises an alternating current power grid, 3 wind power plants 1#, 2#, 3#, 1 GSMMC converter station and 3 WFMMC converter stations;
the alternating-current transmission voltage class of the alternating-current power grid is 220kV, the direct-current transmission voltage class is +/-500 kV, and the number of the MMC half-bridge sub-modules is 76;
in the respective wind fields of the 3 wind power plants 1#, 2#, 3#, the number of the permanent magnet fans is respectively 102, 92, 82, the rated capacity of each fan is 5.2MW, and the length l of the direct current transmission line01、l02、l13、l23Respectively 100km, 120km, 80km and 60 km.
3. The coordinated fault ride-through method suitable for the wind power MMC-MTDC system according to claim 2, characterized in that in step 1, when a short-circuit fault occurs at an alternating current outlet of a receiving end converter GSMMC, the alternating current input voltage of the WFMMC is reduced through WFMMC voltage reduction control, so that the input power is reduced, and the unbalanced power on the direct current transmission network is reduced; and simultaneously, designing a fan side VSC converter WTDSC (wind turbine converter) step-down control coordinated with the WFMMC step-down control to eliminate unbalanced power.
4. The method for coordinated fault ride-through of a wind power MMC-MTDC system according to claim 3, wherein the WFMMC voltage reduction control adopts a V/f control mode, the voltage amplitude and the frequency of the input converter are controlled to be constant values, and the control process comprises the following steps:
during a fault, the ac voltage reference in the inverter control loop will be lowered, which is expressed as equation (1),
Figure FDA0002379762310000011
in the formula (I), the compound is shown in the specification,
Figure FDA0002379762310000012
is an alternating current voltage reference value, U 'in V/f control under normal working condition'ac_WFMMCReference value of AC voltage, U, redesigned for fault conditionsdcFor the MMC-MTDC direct voltage during a fault,
Figure FDA0002379762310000021
is a preset MMC-MTDC direct current upper limit value, KdcControlling parameters for WFMMC pressure reduction;
neglecting the active power loss of the MMC converter, the relation between the input power and the output power of the MMC-MTDC during the fault period is expressed as an expression (2),
PS＝PW-ΔP (2)，
wherein, PSOutput power, P, for GSMMCWThe sum of input power of all WFMMCs is shown as delta P, and the difference between input power and output power of the MMC-MTDC is shown as delta P;
further expanding the formula (2) to the formula (3),
Figure FDA0002379762310000022
in the formula (3), I is the effective value of the alternating current input by WFMMC,
Figure FDA0002379762310000023
the value of the equivalent capacitance of the direct current transmission line is the sum of HBSM capacitance values of all sub modules on a bridge arm of the converter;
further, the formula (3) is represented by the formula (4),
Figure FDA0002379762310000024
by integrating the formula (1) and the formula (4), the obtained value of the WFMMC pressure reduction control parameter is shown in the formula (5):
Figure FDA0002379762310000025
5. the coordinated fault ride-through method suitable for the wind power MMC-MTDC system according to claim 3, wherein the WTVCS load shedding control adopts PQ control to ensure that the active power and the reactive power output by the wind turbine are constant, and the PQ control process comprises: the active power reference value P' WTVSC redesigned in case of a fault is represented as equation (6),
Figure FDA0002379762310000026
in the formula (I), the compound is shown in the specification,
Figure FDA0002379762310000027
is an active power reference value P 'in WVSC control under normal working condition'WTVSCActive power reference value, K, redesigned for fault conditionsPLoad reduction control parameters are provided for WVSC;
neglecting the active power loss of the MMC converter, the relation between the input power and the output power of the MMC-MTDC during the fault period is shown as the formula (2), for each fan, the formula (2) can be expressed as the formula (7),
Figure FDA0002379762310000031
in the formula, n is the sum of the number of fans connecting all the MMC-MTDC wind fields;
equation (7) can be further written as equation (8),
Figure FDA0002379762310000032
the value of the WTSC load reduction control parameter obtained by integrating the formula (6) and the formula (8) is shown as the formula (9),
Figure FDA0002379762310000033
6. the method for coordinated fault ride-through of a wind power MMC-MTDC system according to claim 2, wherein the step 2 of deriving the steady-state short-circuit current expression comprises:
the power of the WFMMC input is expressed as equation (10),
Figure FDA0002379762310000034
in the formula, PWFMMCAnd QWFMMCActive power and reactive power input by WFMMC respectively; u. ofWd、uWqD-axis voltage and q-axis voltage of the WFMMC alternating current side are respectively, and the values of the d-axis voltage and the q-axis voltage are determined by an alternating voltage reference value in WFMMC control; i.e. iWd、iWqThe d-axis current and the q-axis current on the alternating current side of WFMMC respectively, the values of which are determined by the power output by the wind field, are expressed as an expression (11),
Figure FDA0002379762310000035
meanwhile, for the MMC-MTDC system, the relation between the input power and the output power is shown as the formula (12),
Figure FDA0002379762310000036
in the formula IdcIs the current of a DC transmission network, RdcIs a direct current transmission network resistor;
combining equations (10), (11), (12), the GSMMC output power is represented as equation (13) in view of the setting of the MMC-MTDC direct current transmission network topology,
Figure FDA0002379762310000041
in the formula, n1、n2、n3The number of 1#, 2#, 3# wind field fans, l01Is the length of the transmission line between 1# WFMMC and GSMMC02Is the length of the transmission line between 2# WFMMC and GSMMC13Is the length of the transmission line between 1# WFMMC and 3# WFMMC23Is the length of the transmission line between 2# WFMMC and 3# WFMMC, rdcThe resistance value of the direct current transmission line is a unit length;
for the symmetric short-circuit fault on the alternating current side of the GSMMC, the expression formula of the steady-state short-circuit current is shown as a formula (14),
Figure FDA0002379762310000042
for the asymmetric short-circuit fault on the alternating current side of the GSMMC, the expression of the steady-state short-circuit current is shown as the formula (15) due to the existence of negative sequence voltage and negative sequence current,
Figure FDA0002379762310000043
in the formula, D1And D2The values are respectively shown in the formula (16),
Figure FDA0002379762310000044
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