CN111600325A - Fault ride-through method and system for hybrid cascaded direct current transmission system - Google Patents
Fault ride-through method and system for hybrid cascaded direct current transmission system Download PDFInfo
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- CN111600325A CN111600325A CN202010341552.4A CN202010341552A CN111600325A CN 111600325 A CN111600325 A CN 111600325A CN 202010341552 A CN202010341552 A CN 202010341552A CN 111600325 A CN111600325 A CN 111600325A
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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
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
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- 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 fault ride-through method of a hybrid cascade direct current transmission system, which comprises the following steps: collecting AC voltage u of AC network connected with voltage source type current converteravAC voltage u of AC network connected with current source type converterac(ii) a Calculating the maximum power which can be transmitted by the direct current system at the moment according to the collected alternating current voltage, and processing the maximum power to obtain a direct current power reference value PREF(ii) a Reference value P of DC powerREFOutputting the signal to a rectifying station as a converter control signal; the rectification converter station controls the DC transmission power to follow the DC power reference value P according to the received control signalREF. The invention can effectively solve the problem of direct current overvoltage caused by the fault of an alternating current power grid connected with an inverter side voltage source type converter in a hybrid cascade direct current transmission system, prevent the overvoltage of direct current bus and submodule capacitors, reliably pass through the alternating current fault and better protectAnd protecting the equipment safety.
Description
Technical Field
The invention belongs to the field of direct current transmission, and particularly relates to a fault ride-through method and system for a hybrid cascade direct current transmission system.
Background
High voltage direct current transmission systems can be divided into two types: a conventional direct current transmission system (LCC-HVDC) based on thyristor technology and a Flexible direct current transmission system (flex-HVDC) based on fully-controlled power electronics technology. The conventional direct-current transmission system (LCC-HVDC) is low in cost, low in loss and mature in operation technology, at present, the direct-current transmission systems in operation in the world are almost LCC-HVDC systems, but the conventional direct-current transmission system (LCC-HVDC) has the defects that commutation failure easily occurs on an inverter side, dependence on an alternating-current system is strong, a large amount of reactive power is absorbed, the occupied area of a converter station is large, and the like. The new generation of Flexible direct current transmission system (Flexible-HVDC) can realize active power and reactive power decoupling control, can supply power to a passive network, has the advantages of compact structure, small occupied area, no problem of inversion side commutation failure and the like, but has the defects of high cost, large loss and the like.
Therefore, the hybrid direct-current power transmission system combining conventional direct-current power transmission and flexible direct-current power transmission has good engineering application prospect. The topology of the current hybrid dc transmission system mainly includes a hybrid two-terminal dc transmission system with symmetrical single-pole connection as shown in fig. 1 and a hybrid two-terminal dc transmission system with symmetrical double-pole connection as shown in fig. 2. The two systems combine the advantages of small loss of conventional direct current transmission, mature operation technology and capability of supplying power to a passive network by flexible direct current transmission without phase change failure.
However, in the hybrid dc transmission system shown in fig. 1 and fig. 2, when an ac power grid connected to the voltage source converter on the inverter side fails, the active power of the dc system cannot be output to the ac side, and the current source converter in the rectification state still transmits power to the dc system according to a predetermined power reference value, at this time, the voltage on the dc side will increase rapidly due to the continuous accumulation of energy, and the safety of the dc equipment will be endangered finally.
In the prior art, in a conventional direct-current transmission system based on a thyristor technology, when an alternating-current power grid connected to an inverter side fails, a phase commutation failure occurs on an inverter side converter, which is equivalent to a short-circuit failure occurring on a direct-current side, so that a large direct-current overvoltage cannot be caused. According to the flexible direct-current transmission system based on the fully-controlled power electronic device technology, when an alternating-current power grid connected to an inverter side fails, active power of a direct-current system cannot be output to an alternating-current side, at the moment, voltage of the direct-current side can be rapidly increased due to continuous accumulation of energy, and the voltage of the direct-current side is generally kept within a controllable range through direct-current energy consumption devices such as a DC chopper and the like, so that the voltage is not too high. However, the method is high in cost and poor in applicability to the hybrid direct-current power transmission system.
Disclosure of Invention
The purpose of the invention is: the fault ride-through method and the fault ride-through system for the hybrid cascaded direct current transmission system can effectively solve the direct current overvoltage problem caused by the fault of an alternating current power grid connected with an inversion side voltage source type converter in the hybrid direct current transmission system, prevent the overvoltage of capacitors of a direct current bus and a submodule, reliably ride through the alternating current fault and better protect the safety of equipment.
In order to achieve the purpose, the invention adopts the following technical scheme:
a fault ride-through method for a hybrid cascaded direct current transmission system comprises a rectifying converter station and an inverting converter station, wherein the rectifying converter station comprises at least one group of thyristor converter units, the inverting converter station comprises at least one group of hybrid cascaded converters, the hybrid cascaded converters comprise a current source type converter and a voltage source type converter which are connected in series, the current source type converter comprises a thyristor converter, and the voltage source type converter comprises a modular multilevel converter; the fault ride-through method comprises the following steps:
(1) collecting AC voltage u of AC network connected with voltage source type current converteravAC voltage u of AC network connected with current source type converterac；
(2) According to the AC voltage uavCalculating a first DC power reference value PREF1(ii) a According to the AC voltage uacCalculating to obtain a second DC power reference value PREF2The first DC power reference value PREF1And the second DC power reference value PREF2Adding to obtain a total DC power reference value PREF；
(3) Reference value P of total DC powerREFOutputting the signal to a rectifying station as a converter control signal;
(4) the rectification converter station controls the DC transmission power to follow the total DC power reference value P according to the received control signalREF。
In a preferred embodiment, step (2) is based on the AC voltage uavCalculating a first DC power reference value PREF1The method comprises the following steps: according to the AC voltage uavCalculating the maximum alternating current power Pacmax which can be transmitted by the direct current system at the moment, and calculating the maximum alternating current power Pacmax to obtain a first direct current power reference value PREF1。
In a preferred embodiment, step (2) is based on the AC voltage uacCalculating to obtain a second DC power reference value PREF2The method comprises the following steps: according to the AC voltage uacCalculating the maximum direct current power Pdcmax which can be transmitted by the direct current system at the moment, and calculating the maximum direct current power Pdcmax to obtain a second direct current power reference value PREF2。
In a preferred embodiment, the maximum ac power Pacmax is calculated to obtain a first dc power reference value PREF1The specific method comprises the following steps: converting the maximum alternating current power Pacmax according to the conversion relation between the alternating current active power and the direct current power to obtain a direct current power conversion value, and taking the direct current power conversion value as the secondA DC power reference value PREF1。
In a preferred embodiment, the maximum ac power Pacmax is calculated to obtain a first dc power reference value PREF1The specific method comprises the following steps: converting the maximum alternating current power Pacmax according to the conversion relation between the alternating current active power and the direct current power to obtain a direct current power conversion value, and sending the deviation between the actually measured alternating current power value and the alternating current power instruction value or the deviation between the actually measured alternating current value and the alternating current instruction value into a regulator to regulate to obtain a calculated power balance quantity; taking the sum of the DC power reduced value and the calculated power balance amount as a first DC power reference value PREF1。
In a preferred embodiment, the regulator is a proportional integral regulator or a proportional regulator.
In a preferred embodiment, the conversion relationship between the ac active power and the dc power includes: the power loss relationship exists when the power on the direct current side is transmitted to the alternating current side or when the power on the alternating current side is transmitted to the direct current side.
In a preferred embodiment, the reduced relation between the ac active power and the dc power further includes: the power loss relationship exists when the power on the rectifying side is transmitted to the inverting side.
In a preferred embodiment, the maximum dc power Pdcmax is calculated to obtain a second dc power reference value PREF2The method comprises the following steps: according to the AC voltage uacPredicting commutation failure, and when the commutation failure is predicted, carrying out amplitude limiting on the maximum direct current power Pdcmax to obtain a second direct current power reference value PREF2(ii) a Otherwise, the second DC power reference value P is not limitedREF2Is equal to Pdcmax.
In a preferred embodiment, the limiting the maximum dc power Pdcmax is: limiting the value Pd by DC powerlimLimiting the maximum direct current power Pdcmax as an upper limit; the DC power limit value PdlimResults from one of two approaches:
i) the DC power limit value PdlimIs a preset value ranging from 0 to the current sourceThe maximum transferable direct current power of the type converter;
ii) the DC power limit value PdlimFor the rated value of the AC voltage of the converter and the actually collected AC voltage uacThe deviation of (2) is generated by proportional integral regulator modulation.
In a preferred embodiment, when a commutation failure is predicted to occur, the maximum dc power Pdcmax is clipped to 0, i.e. the dc power reference value PREF2Is 0.
The invention also provides a fault ride-through system of the hybrid cascade direct-current transmission system, which comprises a rectification converter station and an inversion converter station, wherein the rectification converter station comprises at least one group of thyristor converter units, the inversion converter station comprises at least one group of hybrid cascade converters, the hybrid cascade converters comprise a current source type converter and a voltage source type converter which are connected in series, the current source type converter comprises a thyristor converter, and the voltage source type converter comprises a modular multilevel converter; wherein the fault ride-through system comprises:
an acquisition module for acquiring the AC voltage u of the AC network connected to the voltage source converteravAC voltage u of AC network connected with current source type converterac；
A reference value calculation module for calculating a reference value based on the AC voltage uavCalculating a first DC power reference value PREF1(ii) a According to the AC voltage uacCalculating to obtain a second DC power reference value PREF2The first DC power reference value PREF1And the second DC power reference value PREF2Adding to obtain a total DC power reference value PREF；
A transmission module for converting the total DC power reference value PREFOutputting the signal to a rectifying station as a converter control signal;
a power adjusting module for controlling the rectifying converter station to adjust the DC transmission power to follow the total DC power reference value P according to the received control signalREF。
The invention has the beneficial effects that:
1) the invention can effectively process the alternating current fault of the inversion converter station end in the hybrid cascade direct current transmission system, prevent the overvoltage of the direct current bus and the submodule capacitor, reliably pass through the alternating current fault and better protect the equipment safety.
2) The invention can effectively solve the problem of direct current overvoltage caused by the fault of the alternating current power grid connected with the inversion converter station in the hybrid cascade direct current transmission system, and can effectively keep the voltage at the direct current side in a controllable range during the fault.
3) The invention has simple structure and convenient operation, can effectively prevent the rise of the direct current voltage and keep the voltage at the direct current side in a controllable range when the alternating current power grid connected with the inversion converter station fails, thereby effectively passing through the alternating current fault.
Drawings
Figure 1 is a schematic diagram of a hybrid two terminal dc transmission system with symmetrical single pole connections;
fig. 2 is a schematic diagram of a hybrid two-terminal dc transmission system with symmetrical bipolar connections;
FIG. 3 is a schematic diagram of a hybrid cascaded DC power transmission system fault ride-through method of the present invention;
FIG. 4 is a hybrid cascade multi-terminal DC transmission system, in which a rectifying station is formed by connecting two thyristor converters in series, and an inverting station is formed by connecting 3 modular multi-level converters in parallel and then connecting the 3 modular multi-level converters in series with one thyristor converter through a DC transmission line;
fig. 5 is a schematic diagram of a fault ride-through system of a hybrid cascaded dc power transmission system of the present invention.
Detailed Description
The present invention will be better understood and implemented by those skilled in the art by the following detailed description of the technical solution of the present invention with reference to the accompanying drawings and specific examples, which are not intended to limit the present invention. Wherein like components are given like reference numerals.
Fig. 3 shows a specific embodiment of the fault ride-through method for the hybrid cascaded dc power transmission system of the present invention, which is applied to the hybrid cascaded dc power transmission system shown in fig. 4, and can effectively handle the dc overvoltage problem caused by the fault of the ac power grid connected to the inverter-side voltage source converter in the hybrid cascaded dc power transmission system, prevent the overvoltage of the dc bus and the sub-module capacitor, reliably ride through the ac fault, and better protect the safety of the device. The fault ride-through method comprises the following steps: s1, collecting alternating-current voltage, S2, calculating a total direct-current power reference value, S3, transmitting the total direct-current power reference value, S4: the dc transmission power is adjusted according to the received command.
In S1, an ac voltage u of an ac power grid connected to the voltage source converter is collectedavAC voltage u of AC network connected with current source type converterac。
In S2, according to the AC voltage uavCalculating a first DC power reference value PREF1(ii) a According to the AC voltage uacCalculating to obtain a second DC power reference value PREF2The first DC power reference value PREF1And the second DC power reference value PREF2Adding to obtain a total DC power reference value PREF。
According to the alternating voltage u in step S2avCalculating a first DC power reference value PREF1The method comprises the following steps: according to the AC voltage uavCalculating the maximum alternating current power Pacmax which can be transmitted by the direct current system at the moment, and calculating the maximum alternating current power Pacmax to obtain a first direct current power reference value PREF1。
In some embodiments, the maximum ac power Pacmax is calculated to obtain the first dc power reference value PREF1The specific method comprises the following steps: converting the maximum alternating current power Pacmax according to the conversion relation between the alternating current active power and the direct current power to obtain a direct current power conversion value, and taking the direct current power conversion value as a first direct current power reference value PREF1。
In some embodiments, the maximum ac power Pacmax is calculated to obtain the first dc power reference value PREF1The specific method comprises the following steps: converting the maximum alternating current power Pacmax according to the conversion relation between the alternating current active power and the direct current power to obtain a direct current power conversion valueThe deviation between the actually measured alternating current power value and the alternating current power instruction value or the deviation between the actually measured alternating current value and the alternating current instruction value is sent to a regulator to be regulated to obtain the calculated power balance; taking the sum of the DC power reduced value and the calculated power balance amount as a first DC power reference value PREF1. Wherein the regulator is a proportional-integral regulator or a proportional regulator.
In some embodiments, the reduced relationship between the ac active power and the dc power includes: the power loss relationship exists when the power on the direct current side is transmitted to the alternating current side or when the power on the alternating current side is transmitted to the direct current side.
In some embodiments, the reduced relationship between the ac active power and the dc power further includes: the power loss relationship exists when the power on the rectifying side is transmitted to the inverting side.
In step S2, the alternating voltage u is used as the basisacCalculating to obtain a second DC power reference value PREF2The method comprises the following steps: according to the AC voltage uacCalculating the maximum direct current power Pdcmax which can be transmitted by the direct current system at the moment, and calculating the maximum direct current power Pdcmax to obtain a second direct current power reference value PREF2。
In some embodiments, the maximum dc power Pdcmax is calculated to obtain a second dc power reference value PREF2The method comprises the following steps: according to the AC voltage uacPredicting commutation failure, and when the commutation failure is predicted, carrying out amplitude limiting on the maximum direct current power Pdcmax to obtain a second direct current power reference value PREF2(ii) a Otherwise, the second DC power reference value P is not limitedREF2Is equal to Pdcmax.
In some embodiments, the limiting the maximum dc power Pdcmax refers to: limiting the value Pd by DC powerlimLimiting the maximum direct current power Pdcmax as an upper limit; the DC power limit value PdlimResults from one of two approaches:
i) the DC power limit value PdlimThe value range is from 0 to the maximum transferable direct current power of the current source type converter for a preset value;
ii) the DC power limit value PdlimFor the rated value of the AC voltage of the converter and the actually collected AC voltage uacThe deviation of (2) is generated by proportional integral regulator modulation.
In some embodiments, when a commutation failure is predicted to occur, the maximum dc power Pdcmax is clipped to 0, i.e. the dc power reference value PREF2Is 0.
In S3, the total dc power reference value P is setREFAnd outputting the signal to the rectifying station as a converter control signal.
In S4, the rectifying converter station controls the dc transmission power to follow the total dc power reference value P according to the received control signalREF。
The fault ride-through method of the hybrid cascaded direct current transmission system according to the present invention is specifically described below with reference to fig. 4 as an example.
As shown in fig. 4, the hybrid cascade dc power transmission system includes: rectification current conversion station and contravariant current conversion station, both link to each other through two direct current transmission line, wherein: the rectification converter station is used for converting three-phase alternating current of a sending end alternating current grid into direct current and then transmitting the direct current to the inversion converter station through a direct current transmission line, a bus of the sending end alternating current grid entering the station can be connected with a passive filter or not, the passive filter or not can be determined according to system engineering conditions, when the sending end is composed of a thyristor converter, the passive filter generally needs to be installed, and a reactive compensation capacitor needs to be installed sometimes. In fig. 4, the rectification converter station is composed of two groups of thyristor converter units connected in series, the serial node of the rectification converter station is connected with a grounding electrode, and the positive end and the negative end of the rectification converter station after being connected in series are both connected with a direct current transmission line through a smoothing reactor; and a DC filter is arranged between the DC line and the ground.
The thyristor converter unit adopts a twelve-pulse bridge circuit; each bridge arm is formed by connecting a plurality of thyristors in series, and the thyristor converter is controlled by a constant direct current power control strategy. The thyristor converter is connected with a transmission end alternating current power grid through a three-winding transformer with the wiring mode of Y0/Y/delta respectively. The transformer can carry out voltage grade conversion on three-phase alternating current of a sending end alternating current system so as to adapt to required direct current voltage grade, and the difference of the secondary side wiring modes of the transformer is that upper and lower six-pulse converter bridges of the twelve-pulse bridge thyristor converter provide three-phase alternating current with a phase angle difference of 30 degrees so as to reduce harmonic current flowing into a power grid.
The inversion converter station is used for converting direct current into three-phase alternating current and then transmitting the three-phase alternating current to a receiving end alternating current power grid, and comprises four converter stations including a station 2, a station 3, a station 4 and a station 5, wherein the station 2 is connected with the station 3, the station 4 is connected with the station 5 in series, and the station 3 is connected with the station 4 is connected with the station 5 in parallel. The station 2 is composed of two groups of thyristor converters, the thyristor converters are connected with a receiving end alternating current power grid through a three-winding transformer with a wiring mode of Y0/Y/delta respectively, and the thyristor converters are controlled by constant direct current voltage. Each of the stations 3, 4 and 5 is formed by connecting two groups of voltage source type converters in series, the series node of the voltage source type converter is connected with a grounding electrode, the voltage source type converter is connected with a receiving end alternating current power grid through a double-winding transformer with a connection mode of Y0/delta, the voltage source type converter of the station 3 is controlled by a constant direct current voltage and constant reactive power control strategy, the voltage source type converter of the station 4 is controlled by a constant alternating current side active power and constant reactive power control strategy, and the voltage source type converter of the station 5 is controlled by a constant alternating current side active power and constant reactive power control strategy. The voltage source type converter adopts a modularized multi-level converter, the converter with the active power control mode at the fixed alternating current side adopts current vector control, and an active current reference value and a reactive current reference value are obtained by modulating a given active power reference value and a given reactive power reference value through a proportional-integral controller.
When an ac system connected to the inverter converter station 3 has a serious ac fault, such as a three-phase short circuit fault, if appropriate control measures are not taken, the voltage source type converter of the station 3 and the voltage source type converters of the station 4 and the station 5 will be subjected to serious overvoltage, so that the following control method is adopted in addition to the conventional control strategy:
(1) alternating voltage u of alternating current network connected with 3 voltage source type current converter of acquisition stationavAC voltage u of AC network connected with current source type converter of station 2ac；
(2) According toCollected AC voltage uavAnd the maximum alternating current active power Pacmax which can be transmitted by the station 3 at the moment is calculated by combining the maximum alternating current which can be operated by the voltage source type converter, wherein the maximum alternating current active power Pacmax which can be transmitted by the station 3 is calculated by combining the conversion relation between the alternating current active power and the direct current power to obtain a first direct current power reference value PREF1The conversion relationship between the dc power and the ac active power refers to the conversion of the dc power command value and the ac power command value according to the power loss relationship between the dc power transmission line, the converter transformer, the converter valve, and the like, which exists when the dc power is transmitted to the ac side or the ac power is transmitted to the dc side and the power at the rectification side is transmitted to the inverter side, and these losses can reach 6.0% to 8.5%. Assuming 6%, P isREF1＝Pacmax/0.94。
According to the collected AC voltage uacAnd the maximum direct current power Pdcmax which can be transmitted by the thyristor converter of the maximum direct current calculation station 2 which can operate by combining the thyristor converter at the moment is calculated according to the collected alternating voltage uacAfter phase commutation failure prediction is carried out, the maximum direct current power Pdcmax which can be transmitted is limited and is less than or equal to the direct current power limiting value PdlimWherein is PdlimThe value range is from 0 to the maximum transferable direct current power of the current source type converter for a preset value; when the phase commutation failure is predicted to occur, limiting the maximum deliverable direct current power Pdcmax to 0, namely a direct current power reference value PREF2Is 0. Obtaining a second DC power reference value P after processingREF2The first DC power reference value PREF1And a second DC power reference value PREF2Adding to obtain a total DC power reference value PREF。
(3) Reference value P of total DC powerREFAs a converter control signal, transmitting the signal to a rectifying station through inter-station communication;
(4) the thyristor converter of the rectification converter station immediately controls the hybrid cascade direct-current power transmission system to adjust the transmission power to the total direct-current power reference value P according to the received control signalREFThereby avoiding the inverter station from generating DC powerThe excess causes a dc overvoltage.
Fig. 5 shows a fault ride-through system of a hybrid cascaded dc power transmission system according to the present invention, where the hybrid cascaded dc power transmission system includes a rectifying converter station and an inverting converter station, the rectifying converter station includes at least one group of thyristor converter units, the inverting converter station includes at least one group of hybrid cascaded converters, the hybrid cascaded converters include a current source converter and a voltage source converter connected in series, the current source converter includes a thyristor converter, and the voltage source converter includes a modular multilevel converter. The fault ride-through system comprises: the device comprises an acquisition module, a reference value calculation module, a transmission module and a power adjustment module. Wherein:
an acquisition module for acquiring the AC voltage u of the AC network connected to the voltage source converteravAC voltage u of AC network connected with current source type converterac。
A reference value calculation module for calculating a reference value based on the AC voltage uavCalculating a first DC power reference value PREF1(ii) a According to the AC voltage uacCalculating to obtain a second DC power reference value PREF2The first DC power reference value PREF1And the second DC power reference value PREF2Adding to obtain a total DC power reference value PREF。
A transmission module for converting the total DC power reference value PREFAnd outputting the signal to the rectifying station as a converter control signal.
A power adjusting module for controlling the rectifying converter station to adjust the DC transmission power to follow the total DC power reference value P according to the received control signalREF。
The above description is only a preferred embodiment of the present invention, and not intended to limit the scope of the present invention, and all modifications of equivalent structures and equivalent processes, which are made by using the contents of the present specification and the accompanying drawings, or directly or indirectly applied to other related technical fields, are included in the scope of the present invention.
Claims (12)
1. A fault ride-through method for a hybrid cascaded direct current transmission system comprises a rectifying converter station and an inverting converter station, wherein the rectifying converter station comprises at least one group of thyristor converter units, the inverting converter station comprises at least one group of hybrid cascaded converters, the hybrid cascaded converters comprise a current source type converter and a voltage source type converter which are connected in series, the current source type converter comprises a thyristor converter, and the voltage source type converter comprises a modular multilevel converter; the fault ride-through method is characterized by comprising the following steps of:
(1) collecting AC voltage u of AC network connected with voltage source type current converteravAC voltage u of AC network connected with current source type converterac；
(2) According to the AC voltage uavCalculating a first DC power reference value PREF1(ii) a According to the AC voltage uacCalculating to obtain a second DC power reference value PREF2The first DC power reference value PREF1And the second DC power reference value PREF2Adding to obtain a total DC power reference value PREF；
(3) Reference value P of total DC powerREFOutputting the signal to a rectifying station as a converter control signal;
(4) the rectification converter station controls the DC transmission power to follow the total DC power reference value P according to the received control signalREF。
2. The fault ride-through method for a hybrid cascaded dc power transmission system according to claim 1, wherein step (2) is performed according to the ac voltage uavCalculating a first DC power reference value PREF1The method comprises the following steps: according to the AC voltage uavCalculating the maximum alternating current power Pacmax which can be transmitted by the direct current system at the moment, and calculating the maximum alternating current power Pacmax to obtain a first direct current power reference value PREF1。
3. A hybrid cascaded direct current transmission as defined in claim 1The system fault ride-through method is characterized in that the step (2) is carried out according to the alternating voltage uacCalculating to obtain a second DC power reference value PREF2The method comprises the following steps: according to the AC voltage uacCalculating the maximum direct current power Pdcmax which can be transmitted by the direct current system at the moment, and calculating the maximum direct current power Pdcmax to obtain a second direct current power reference value PREF2。
4. The fault ride-through method for the hybrid cascaded direct current transmission system according to claim 2, wherein the maximum alternating current power Pacmax is calculated to obtain a first direct current power reference value PREF1The specific method comprises the following steps: converting the maximum alternating current power Pacmax according to the conversion relation between the alternating current active power and the direct current power to obtain a direct current power conversion value, and taking the direct current power conversion value as a first direct current power reference value PREF1。
5. The fault ride-through method for the hybrid cascaded direct current transmission system according to claim 2, wherein the maximum alternating current power Pacmax is calculated to obtain a first direct current power reference value PREF1The specific method comprises the following steps: converting the maximum alternating current power Pacmax according to the conversion relation between the alternating current active power and the direct current power to obtain a direct current power conversion value, and sending the deviation between the actually measured alternating current power value and the alternating current power instruction value or the deviation between the actually measured alternating current value and the alternating current instruction value into a regulator to regulate to obtain a calculated power balance quantity;
taking the sum of the DC power reduced value and the calculated power balance amount as a first DC power reference value PREF1。
6. The method of fault ride-through of a hybrid cascaded direct current power transmission system of claim 5, wherein the regulator is a proportional integral regulator or a proportional regulator.
7. A hybrid cascaded DC power transmission system fault ride-through method according to claim 4 or 5, wherein the reduced relation of AC active power to DC power comprises: the power loss relationship exists when the power on the direct current side is transmitted to the alternating current side or when the power on the alternating current side is transmitted to the direct current side.
8. The method of fault ride-through in a hybrid cascaded direct current transmission system of claim 7, wherein the reduced relationship of alternating current active power to direct current power further comprises: the power loss relationship exists when the power on the rectifying side is transmitted to the inverting side.
9. The fault ride-through method for a hybrid cascaded direct current transmission system according to claim 3, wherein the maximum direct current power Pdcmax is calculated to obtain a second direct current power reference value PREF2The method comprises the following steps: according to the AC voltage uacPredicting commutation failure, and when the commutation failure is predicted, carrying out amplitude limiting on the maximum direct current power Pdcmax to obtain a second direct current power reference value PREF2(ii) a Otherwise, the second DC power reference value P is not limitedREF2Is equal to Pdcmax.
10. The fault ride-through method for a hybrid cascaded direct current transmission system of claim 9, wherein the limiting the maximum direct current power Pdcmax is by: limiting the value Pd by DC powerlimLimiting the maximum direct current power Pdcmax as an upper limit; the DC power limit value PdlimResults from one of two approaches:
i) the DC power limit value PdlimThe value range is from 0 to the maximum transferable direct current power of the current source type converter for a preset value;
ii) the DC power limit value PdlimFor the rated value of the AC voltage of the converter and the actually collected AC voltage uacThe deviation of (2) is generated by proportional integral regulator modulation.
11. A hybrid cascaded dc transmission system fault as claimed in claim 9The traversing method is characterized in that when the occurrence of commutation failure is predicted, the maximum direct current power Pdcmax is limited to 0, namely a direct current power reference value PREF2Is 0.
12. A fault ride-through system of a hybrid cascaded direct current transmission system comprises a rectifying converter station and an inverting converter station, wherein the rectifying converter station comprises at least one group of thyristor converter units, the inverting converter station comprises at least one group of hybrid cascaded converters, the hybrid cascaded converters comprise a current source type converter and a voltage source type converter which are connected in series, the current source type converter comprises a thyristor converter, and the voltage source type converter comprises a modular multilevel converter; wherein the fault ride-through system comprises:
an acquisition module for acquiring the AC voltage u of the AC network connected to the voltage source converteravAC voltage u of AC network connected with current source type converterac；
A reference value calculation module for calculating a reference value based on the AC voltage uavCalculating a first DC power reference value PREF1(ii) a According to the AC voltage uacCalculating to obtain a second DC power reference value PREF2The first DC power reference value PREF1And the second DC power reference value PREF2Adding to obtain a total DC power reference value PREF；
A transmission module for converting the total DC power reference value PREFOutputting the signal to a rectifying station as a converter control signal;
a power adjusting module for controlling the rectifying converter station to adjust the DC transmission power to follow the total DC power reference value P according to the received control signalREF。
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