Bidirectional hybrid output direct current transformer and fault current limiting design method thereof

Abstract

The invention discloses a bidirectional hybrid output direct-current transformer and a fault current limiting design method thereof, and the medium-voltage side and the high-voltage side of the bidirectional hybrid output direct-current transformer are respectively composed of three thyristor valve groups and three sub-module bridge arms, and are connected through a medium-frequency transformer. The thyristor valve group is formed by reversely connecting two groups of series thyristors in parallel, and the medium-voltage side sub-module bridge arm and the high-voltage side sub-module bridge arm are respectively composed of NM half-bridge sub-modules and NH half-bridge sub-modules. According to the invention, the sub-module bridge arms with strong control capability are combined with the thyristors and the diode valve groups which are low in cost, low in loss and high in power density, bidirectional power transmission and reliable turn-off of the thyristors can be realized through coordination and cooperation of the middle-voltage side bridge arms and the high-voltage side bridge arms, smooth direct current is ensured, and a filter does not need to be installed. When a short-circuit fault occurs in the outgoing submarine cable, a fault current active suppression strategy is provided, and the scheme can effectively reduce the peak value of the fault current and eliminate the current-limiting inductance.

Description

Technical Field

[0001] The invention belongs to the field of power electronics and relates to a DC transmission transformer, in particular to a bidirectional hybrid DC transmission transformer and a fault current limiting method thereof. Background Art

[0002] As offshore wind energy resources are gradually saturated, offshore wind power is moving towards deep waters and offshore. For large-capacity, long-distance deep-sea wind power bases, full DC technology with medium-voltage DC collection and high-voltage DC transmission has become a new generation of solutions. The advantages of this solution are that it uses DC cables to expand the collection range, reduce collection losses, and ensure transmission capacity. There are no problems such as reactive power, negative sequence, and phase-locked stability. The transmission DC transformer is one of the two core equipment of the full DC offshore wind farm, and it faces the limitations of space and load capacity of the offshore platform. To this end, the study of new high-gain, high-efficiency, lightweight, and compact transmission DC transformers and related basic issues has important academic research significance and engineering application prospects for promoting the large-scale development of offshore wind power in my country. In addition, the full DC system presents low inertia and small damping characteristics, and the DC fault develops and propagates quickly. At the same time, the overcurrent capacity of power electronic equipment such as the transmission DC transformer is limited, which brings challenges to the fault protection of the full DC offshore wind farm. Therefore, how to give full play to the active control capability of the DC transformer, avoid installing bulky and expensive DC circuit breakers at sea, and reduce transient overcurrent caused by faults is a major problem that needs to be solved urgently. Summary of the invention

[0003] In view of the deficiencies in the prior art, the present invention provides a bidirectional hybrid DC transmission transformer and a fault current limiting design method thereof. The present invention combines a sub-module bridge arm with strong control capability with a low-cost, low-loss, high-power-density thyristor and diode valve group, and proposes a bidirectional hybrid DC transmission transformer topology. The bidirectional hybrid DC transmission transformer can achieve bidirectional power transmission and reliable shutdown of the thyristor through the coordination of the middle and high voltage side bridge arms, and ensure smooth DC current without the need to install a filter. When a short circuit fault occurs in the transmission submarine cable, a fault current active suppression strategy is proposed. This solution can effectively reduce the fault current peak and eliminate the current limiting inductor. The present invention has the characteristics of high efficiency, low cost, and lightweight, and provides a new idea for the development of all-DC offshore wind farms.

[0004] The objective of the present invention is achieved through the following technical solutions:

[0005] A bidirectional hybrid DC transformer, the medium voltage side and the high voltage side are composed of three thyristor valve groups and three sub-module bridge arms, the medium voltage side and the high voltage side are connected through a medium frequency transformer, wherein:

[0006] The thyristor valve group is composed of two groups of series thyristors connected in reverse parallel.j+ and T J+ They are the forward thyristor valve groups on the medium voltage side and the high voltage side, T j- and T J- They are reverse thyristor valve groups on the medium voltage side and high voltage side, j = a, b, c, J = A, B, C;

[0007] The bridge arm of the medium voltage side submodule and the bridge arm of the high voltage side submodule are respectively composed of N M and N H Half-bridge sub-modules, where L M and L H are the bridge arm inductances of the medium voltage side and high voltage side submodules, U M and I M is the DC voltage and current on the medium voltage side, U H and I H is the DC voltage and current on the high voltage side, u Pj and i Pj They represent the bridge arm voltage and current of the medium voltage side submodule, u PJ and i PJ Respectively represent the voltage and current of the high-voltage side submodule bridge arm, i Tj and i TJ are the thyristor valve group currents on the medium voltage side and high voltage side, i j is the transformer primary current;

[0008] The bidirectional hybrid transmission transformer has two operating modes:

[0009] Forward operation mode:

[0010] In each operation cycle T, the medium voltage side forward thyristor valve group T a+ ~T c+ And the high voltage side forward thyristor valve group T A+ ~T C+ The valves are triggered to conduct in sequence, and the conduction overlap time T is designed between the adjacent two-phase valve groups. c To achieve commutation, the current of the thyristor valve group on the medium and high voltage sides is controlled to an amplitude of I respectively under the cooperation of the three-phase sub-module bridge arm. M and I H The three-phase waveforms are staggered by 120° electrical angle to obtain a smooth DC current. The medium frequency transformer current i a ~i c The bridge arm of the quilt module is controlled to have an amplitude of I AC And interlaced trapezoidal waves;

[0011] Due to the interlacing of the three-phase waveforms, there are 6 commutation states I in each operating cycle T. c ~VI c ,in:

[0012] Commutation state I c :Through the coordination of the bridge arms of the c and a phase submodules on the medium voltage side and the bridge arms of the c and a phase submodules on the high voltage side, the current I M The corresponding c+ Switch to T a+ , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pc and u Pa Adjust to U M -U0 and U M +U0, applying a commutation voltage U0 of equal magnitude and opposite direction to the inductance of the bridge arm of the c and a phase submodules on the medium voltage side, thereby making i Pc From I M -I AC rises to 0, and i Pa Decrease from 0 to I at the same rate M -I AC ; For the high voltage side, u PC and u PA The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the c and a phase submodules on the high voltage side, so that the current i c From I AC Drop to 0, i a Increase from 0 to I at the same rate AC , so i Tc From I M Down to 0, i Ta Increase from 0 to I at the same rate M , complete the commutation;

[0013] Commutation State II c :Through the coordination of the b and c phase submodule bridge arms on the high voltage side and the b and c phase submodule bridge arms on the medium voltage side, the high voltage side current I H The corresponding B+ Switch to T C+ , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PB and u PC Adjust to U H -U1 and U H +U1, applying a commutation voltage U1 of equal magnitude and opposite direction to the inductance of the bridge arm of the b and c phase submodules on the high-voltage side, thereby making i PB From I AC / nI H drops to 0, and i PC Increase from 0 to I at the same rate AC / nI H ; For the medium voltage side, u Pb and uPc The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the b and c phase submodules on the medium voltage side, so that the medium frequency transformer current i b From-I AC Raised to 0, i c Decrease from 0 to -I at the same rate AC , so i TB From I H Drop to 0, i TC Increase from 0 to I at the same rate H , complete the commutation;

[0014] Commutation state III c :Through the coordination of the bridge arms of the a and b phase submodules on the medium voltage side and the bridge arms of the a and b phase submodules on the high voltage side, the current I M The corresponding a+ Switch to T b+ , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pa and u Pb Adjust to U M -U0 and U M +U0, applying commutation voltage U0 of equal magnitude and opposite direction to the bridge arm inductance of phase a and phase b submodules on the medium voltage side, thereby making i Pa From I M -I AC rises to 0, and i Pb Decrease from 0 to I at the same rate M -I AC ; For the high voltage side, u PA and u PB The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the a and b phase submodules on the high voltage side, so that the current i a From I AC drops to 0, and i b Increase from 0 to I at the same rate AC , so i Ta From I M Down to 0, i Tb Increase from 0 to I at the same rate M , complete the commutation;

[0015] Commutation State IV c :Through the coordination of the c and a phase submodule bridge arms on the high voltage side and the c and a phase submodule bridge arms on the medium voltage side, the high voltage side current I H The corresponding C+ Switch to T A+ , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PC and uPA Adjust to U H +U1 and U H -U1, applying a commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of the a and c phase submodules on the high voltage side, thereby making i PC From I AC / nI H drops to 0, and i PA Increase from 0 to I at the same rate AC / nI H ; For the medium voltage side, u Pc and u Pa The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the c-phase and a-phase submodules on the medium voltage side, so that the medium frequency transformer current i c From-I AC Raised to 0, i a Decrease from 0 to -I at the same rate AC , so i TC From I H Drop to 0, i TA Increase from 0 to I at the same rate H , complete the commutation;

[0016] Commutation state V c :Through the cooperation of the bridge arms of the b and c phase submodules on the medium voltage side and the bridge arms of the b and c phase submodules on the high voltage side, the current I M The corresponding b+ Switch to T c+ , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pb and u Pc Adjust to U M -U0 and U M +U0, applying a commutation voltage U0 of equal magnitude and opposite direction to the bridge arm inductance of the b and c phase submodules on the medium voltage side, thereby making i Pb From I M -I AC rises to 0, and i Pc Decrease from 0 to I at the same rate M -I AC ; For the high voltage side, u PB and u PC The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the b and c phase submodules on the high voltage side, so that the current i b From I AC drops to 0, and i c Increase from 0 to I at the same rate AC , so i Tb From I M Down to 0, iTc Increase from 0 to I at the same rate M , complete the commutation;

[0017] Commutation Status VI c :Through the coordination of the bridge arms of the a and b phase submodules on the high voltage side and the bridge arms of the a and b phase submodules on the medium voltage side, the high voltage side current I H The corresponding A+ Switch to T B+ , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PA and u PB Adjust to U H +U1 and U H -U1, applying a commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of the a and b phase submodules on the high voltage side, thereby making i PA From I AC / nI H drops to 0, and i PB Increase from 0 to I at the same rate AC / nI H ; For the medium voltage side, u Pa and u Pb The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the a and b phase submodules on the medium voltage side, so that the medium frequency transformer current i a From-I AC rises to 0, i b Decrease from 0 to -I at the same rate AC , so i TA From I H Down to 0, i TB Increase from 0 to I at the same rate H , complete the commutation;

[0018] There is a constant current state after each commutation state ends. There are 6 constant current states I to VI in one cycle, among which:

[0019] Constant current state I: In commutation state I c After the end, in order to turn off the thyristor T c+ , c phase medium voltage side submodule bridge arm u Pc Adjust to U M +U q , so that T c+ Bearing size U q The reverse voltage promotes the full recombination of residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pc With a fixed slope from U M Descend to U M -UAC , to prevent the thyristor from being triggered by mistake due to excessive du / dt;

[0020] Constant current state II: In commutation state II c After the end, in order to turn off the thyristor T B+ , b-phase high voltage side submodule bridge arm u PB Adjust to U H +U q , so that T B+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PB With a fixed slope from U H Descend to U H -nU AC ;

[0021] Constant current state III: In commutation state III c After the end, in order to turn off the thyristor T a+ , a phase medium voltage side submodule bridge arm u Pa Adjust to U M +U q , so that T a+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pa With a fixed slope from U M Descend to U M -U AC ;

[0022] Constant current state IV: In commutation state IV c After the end, in order to turn off the thyristor T C+ , c-phase high voltage side submodule bridge arm u Pc Adjust to U H +U q , so that T C+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PC With a fixed slope from U H Descend to U H -nU AC ;

[0023] Constant current state V: In commutation state V c After the end, in order to turn off the thyristor T b+ , b phase medium voltage side submodule bridge arm u PaAdjust to U M +U q , so that T b+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pb With a fixed slope from U M Descend to U M -U AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt;

[0024] Constant current state V: In commutation state V c After the end, in order to turn off the thyristor T A+ , a-phase high voltage side submodule bridge arm u Pa Adjust to U H +U q , so that T A+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PA With a fixed slope from U M Descend to U H -nU AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt;

[0025] Reverse operation mode:

[0026] In each operation cycle T, the medium voltage side reverse thyristor valve group T a+ ~T c+ And the high voltage side reverse thyristor valve group T A- ~T C- The valves are triggered to conduct in sequence, and the conduction overlap time T is designed between the adjacent two-phase valve groups. c To achieve commutation, the current of the thyristor valve group on the medium and high voltage sides is controlled to an amplitude of I respectively under the cooperation of the three-phase sub-module bridge arm. M and I H The three-phase waveforms are staggered by 120° electrical angle to obtain a smooth DC current. The medium frequency transformer current i a ~i c The bridge arm of the quilt module is controlled to have an amplitude of I AC And interlaced trapezoidal waves;

[0027] Due to the interlacing of the three-phase waveforms, there are 6 commutation states I in each operating cycle T. c ~VI c ,in:

[0028] Commutation state I c:Through the coordination of the bridge arms of the c and a phase submodules on the medium voltage side and the bridge arms of the c and a phase submodules on the high voltage side, the current -I M The corresponding c- Switch to T a- , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pc and u Pa Adjust to U M +U0 and U M -U0, applying commutation voltage U0 of equal magnitude and opposite direction to the inductance of the bridge arm of the c and a phase submodules on the medium voltage side, thereby making i Pc from-(I M -I AC ) drops to 0, and i Pa At the same rate from 0 to -(I M -I AC );For the high voltage side, u PC and u PA The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the c and a phase submodules on the high voltage side, so that the current i c From-I AC Raised to 0, i a Decrease from 0 to -I at the same rate AC , so i Tc From-I M Raised to 0, i Ta From 0 to -I at the same rate M , complete the commutation;

[0029] Commutation State II c :Through the coordination of the b and c phase submodule bridge arms on the high voltage side and the b and c phase submodule bridge arms on the medium voltage side, the high voltage side current -I H The corresponding B- Switch to T C- , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PB and u PC Adjust to U H +U1 and U H -U1, applying a commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of the b and c phase submodules on the high voltage side, thereby making i PB from-(I AC / nI H ) rises to 0, and i PC At the same rate, it decreases from 0 to -(I AC / nI H ); for the medium voltage side, u Pb and u PcThe commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the b and c phase submodules on the medium voltage side, so that the medium frequency transformer current i b From I AC Drop to 0, i c Increase from 0 to I at the same rate AC , so i TB From-I H Raised to 0, i TC Decrease from 0 to -I at the same rate H , complete the commutation;

[0030] Commutation state III c :Through the coordination of the bridge arms of the a and b phase submodules on the medium voltage side and the bridge arms of the a and b phase submodules on the high voltage side, the current -I M The corresponding a- Switch to T b- , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pa and u Pb Adjust to U M +U0 and U M -U0, applying commutation voltage U0 of equal magnitude and opposite direction to the bridge arm inductance of phase a and phase b submodules on the medium voltage side, thereby making i Pa from-(I M -I AC ) drops to 0, and i Pb At the same rate from 0 to -(I M -I AC );For the high voltage side, u PA and u PB The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the a and b phase submodules on the high voltage side, so that the current i a From-I AC rises to 0, and i b Decrease from 0 to -I at the same rate AC , so i Ta From-I M Raised to 0, i Tb From 0 to -I at the same rate M , complete the commutation;

[0031] Commutation State IV c :Through the coordination of the c and a phase submodule bridge arms on the high voltage side and the c and a phase submodule bridge arms on the medium voltage side, the high voltage side current -I H The corresponding C- Switch to T A- , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PC and uPA Adjust to U H -U1 and U H +U1, applies a commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of the a and c phase submodules on the high voltage side, thereby making i PC from-(I AC / nI H ) rises to 0, and i PA At the same rate, it decreases from 0 to -(I AC / nI H ); for the medium voltage side, u Pc and u Pa The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the c-phase and a-phase submodules on the medium voltage side, so that the medium frequency transformer current i c From I AC Drop to 0, i a Increase from 0 to I at the same rate AC , so i TC From-I H Raised to 0, i TA Decrease from 0 to -I at the same rate H , complete the commutation;

[0032] Commutation state V c :Through the cooperation of the bridge arms of the b and c phase submodules on the medium voltage side and the bridge arms of the b and c phase submodules on the high voltage side, the current -I M The corresponding b- Switch to T c- , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pb and u Pc Adjust to U M +U0 and U M -U0, applying commutation voltage U0 of equal magnitude and opposite direction to the bridge arm inductance of phase b and c submodules on the medium voltage side, thereby making i Pb from-(I M -I AC ) drops to 0, and i Pc At the same rate from 0 to -(I M -I AC );For the high voltage side, u PB and u PC The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the b and c phase submodules on the high voltage side, so that the current i b From-I AC rises to 0, and i c Decrease from 0 to -I at the same rate AC , so i Tb From-IM Raised to 0, i Tc From 0 to -I at the same rate M , complete the commutation;

[0033] Commutation Status VI c :Through the coordination of the bridge arms of the a and b phase submodules on the high voltage side and the bridge arms of the a and b phase submodules on the medium voltage side, the high voltage side current -I H The corresponding A- Switch to T B- , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PA and u PB Adjust to U H -U1 and U H +U1, applies a commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of the a and b phase submodules on the high voltage side, thereby making i PA from-(I AC / nI H ) drops to 0, and i PB At the same rate, it decreases from 0 to -(I AC / nI H ); for the medium voltage side, u Pa and u Pb The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the a and b phase submodules on the medium voltage side, so that the medium frequency transformer current i a From I AC Drop to 0, i b Increase from 0 to I at the same rate AC , so i TA From-I H Raised to 0, i TB Decrease from 0 to -I at the same rate H , complete the commutation;

[0034] There is a constant current state after each commutation state ends. There are 6 constant current states I to VI in one cycle, among which:

[0035] Constant current state I: In commutation state I c After the end, in order to turn off the thyristor T c- , c phase medium voltage side submodule bridge arm u Pc Adjust to U M -U q , so that T c- Bearing size U q The reverse voltage promotes the full recombination of residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pc With a fixed slope from U M Descend to UM -U AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt;

[0036] Constant current state II: In commutation state II c After the end, in order to turn off the thyristor T B- , b-phase high voltage side submodule bridge arm u PB Adjust to U H -U q , so that T B- Bearing size U q The reverse voltage promotes the full recombination of residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PB With a fixed slope from U H Descend to U H -nU AC ;

[0037] Constant current state III: In commutation state III c After the end, in order to turn off the thyristor T a- , a phase medium voltage side submodule bridge arm u Pa Adjust to U M -U q , so that T a- Bearing size U q The reverse voltage promotes the full recombination of residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pa With a fixed slope from U M Descend to U M -U AC ;

[0038] Constant current state IV: In commutation state IV c After the end, in order to turn off the thyristor T C- , c-phase high voltage side submodule bridge arm u Pc Adjust to U H -U q , so that T C- Bearing size U q The reverse voltage promotes the full recombination of residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PC With a fixed slope from U H Descend to U H -nU AC ;

[0039] Constant current state V: In commutation state V c After the end, in order to turn off the thyristor T b-, b phase medium voltage side submodule bridge arm u Pb Adjust to U M -U q , so that T b- Bearing size U q The reverse voltage promotes the full recombination of residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pb With a fixed slope from U M Descend to U M -U AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt;

[0040] Constant current state VI: In the commutation state VI c After the end, in order to turn off the thyristor T A- , a-phase high voltage side submodule bridge arm u Pa Adjust to U H -U q , so that T A- Bearing size U q The reverse voltage promotes the full recombination of residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PA With a fixed slope from U M Descend to U H -nU AC , to prevent the thyristor from being triggered incorrectly due to excessive du / dt.

[0041] A fault current limiting design method for the above-mentioned bidirectional hybrid DC output transformer comprises the following steps:

[0042] Step 1: Propose a low-cost fault isolation solution

[0043] A mechanical switch S is used at the outlet of the wind turbine t , a low-cost diode isolation device is configured for each collection line inside the offshore platform. During normal operation, the mechanical switch S t The diode isolation device only allows current to flow from the wind turbine to the offshore platform. During the startup phase, the mechanical switch S t Close to realize power reverse transmission; configure mechanical switch S at the outlet of the DC transformer s1 ; For onshore MMC converter stations, configure AC circuit breaker S on the AC side of the MMC ac , configure a mechanical switch S on the DC side s2 ;

[0044] When a short circuit fault occurs in the outgoing submarine cable, the onshore MMC converter station and the outgoing DC transformer will inject fault current into the short circuit point. For the onshore MMC converter station, the fault isolation scheme commonly used in existing flexible DC transmission projects is adopted. After the fault is detected, the onshore MMC converter station is locked and the AC side circuit breaker S ac Disconnect to prevent the AC main grid from feeding fault current to the fault point. After the submarine cable current decays to 0, disconnect the mechanical switch S s2 Realize fault isolation; for bidirectional hybrid DC transformer, the fault process is divided into two stages:

[0045] 1) Fault current rising stage: After the fault occurs but before the control system detects the fault, the DC transformer will maintain the normal control strategy and continue to operate, and the medium voltage busbar and bridge arm submodule capacitors will discharge to the fault point;

[0046] 2) Fault current natural decay stage: When the control system detects a fault, it immediately locks the DC transformer and the submodule capacitor discharge circuit is cut off. The DC fault current decays naturally from the maximum value. The decay speed depends on the fault current peak value and line impedance parameters. When the line current decays to 0, the mechanical switch S is disconnected. s1 , to achieve fault isolation;

[0047] Step 2: Propose a fault current active suppression strategy

[0048] (1) Analysis of short-circuit characteristics of bidirectional hybrid DC transformer

[0049] For the short-circuit fault of the outgoing submarine cable, there are three situations according to the time of the fault: ① the fault occurs in the constant current stage, ② the fault occurs in the thyristor commutation stage on the medium voltage side, and ③ the fault occurs in the thyristor commutation stage on the high voltage side;

[0050] When the outgoing submarine cable fails, the bidirectional hybrid outgoing DC transformer is equivalent to the equivalent circuit diagram of the outgoing submarine cable failure, where L uf is the equivalent inductance of the discharge circuit on the medium voltage collection side, L P is the equivalent inductance of the bridge arm discharge circuit connected in parallel with the high voltage side, L o For the above three situations ①, ② and ③, the corresponding circuit equivalent inductance L uf and L P They are:

[0051]

[0052] Where, L M and L H are the equivalent inductance of the medium voltage bridge arm and the equivalent inductance of the high voltage bridge arm, respectively. n is the transformer ratio. According to the equivalent circuit and equivalent inductance expression, the peak value of the short-circuit fault current before blocking i is calculated.f,max for:

[0053]

[0054] Among them U H and I H is the voltage and current of the high-voltage DC side under rated conditions, T d The time required for fault detection;

[0055] (2) Active fault current suppression method

[0056] When the output DC transformer operates in the forward operation mode, if the submodule bridge arm is connected in parallel with the high-voltage DC port, the IGBT drive signal of the submodule bridge arm is not enabled, otherwise it is enabled normally;

[0057] When the output DC transformer operates in the reverse operation mode, if the bridge arm of the submodule is connected in parallel with the medium voltage DC port, the IGBT drive signal of the bridge arm of the submodule is not enabled, otherwise it is enabled normally.

[0058] Compared with the prior art, the present invention has the following advantages:

[0059] 1. The bidirectional hybrid output DC transformer of the present invention has fewer components, lower device cost, higher operating efficiency and smaller capacitor energy storage requirements, and the overall volume and weight can be significantly reduced. It has the advantages of low cost, low device loss and simple structure.

[0060] 2. The bidirectional hybrid DC transformer of the present invention can realize bidirectional power transmission and reliable shutdown of thyristors through the coordination of the middle and high voltage side bridge arms, and ensure smooth DC current without the need to install filters, thereby reducing the cost and volume of the device.

[0061] 3. The bidirectional hybrid output DC transformer of the present invention realizes fault current suppression through active control, thereby reducing the overcurrent risk of the submodule IGBT and shortening the fault isolation time. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 This is a circuit diagram of a bidirectional hybrid DC transformer according to the present invention;

[0063] Figure 2 This is the principle waveform of the forward operation mode of the bidirectional hybrid DC transformer of the present invention;

[0064] Figure 3 The equivalent circuit of the bidirectional hybrid DC transformer of the present invention in the commutation state IIIc;

[0065] Figure 4The equivalent circuit of the bidirectional hybrid DC transformer of the present invention in constant current state III;

[0066] Figure 5 This is the operating principle waveform of the reverse operation mode of the bidirectional hybrid DC transformer of the present invention;

[0067] Figure 6 This is the fault protection configuration diagram for the entire DC system;

[0068] Figure 7 This is a typical fault current waveform of a short circuit fault;

[0069] Figure 8 This is a circuit state diagram of the bidirectional hybrid sending DC transformer of the present invention when a sending submarine cable failure occurs;

[0070] Fig. 9 This is the equivalent circuit diagram when the submarine cable fails;

[0071] Fig.10 For different fault conditions, f,max and i P With the current limiting inductor L o The change curve of

[0072] Fig.11 It is a schematic diagram of the operating waveform when a short circuit fault occurs in the bidirectional hybrid DC transformer of the present invention;

[0073] Fig.12 It is a schematic diagram of the operating waveform of the method for actively suppressing fault current of the present invention;

[0074] Fig.13 This is a flow chart of the method for actively suppressing fault current of the present invention;

[0075] Fig.14 is the peak value of the fault current when the fault current active suppression method of the present invention is running. f,max Change curve. DETAILED DESCRIPTION

[0076] The technical solution of the present invention is further described below in conjunction with the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0077] The present invention provides a bidirectional hybrid DC transmission transformer and a fault current limiting design method thereof, comprising the following steps:

[0078] Step 1: Bidirectional hybrid DC transformer topology and operation principle

[0079] The bidirectional hybrid DC transmission transformer is composed of three-phase identical circuits, in which the medium voltage side is a three-phase six-arm bridge, the upper bridge arm is composed of a thyristor valve group, and the lower bridge arm is composed of IGBT; the high voltage side is a diode uncontrolled rectifier bridge, and the two are connected through a medium frequency transformer.

[0080] The bidirectional hybrid DC transformer topology proposed in the present invention is as follows: Figure 1 As shown in the figure, both the medium voltage side and the high voltage side are composed of three thyristor valve groups and three submodule bridge arms. At the same time, in order to achieve bidirectional power transmission, each thyristor valve group is composed of two groups of series thyristors connected in reverse parallel, where T j+ (j=a,b,c) and T J+ (J=A, B, C) are the forward thyristor valve groups on the medium voltage side and the high voltage side respectively, T j- and T J- The bridge arms of the submodules on the medium voltage side and the bridge arms of the submodules on the high voltage side are respectively composed of N M and N H Half-bridge sub-modules, where L M and L H are the bridge arm inductances of the medium voltage side and high voltage side submodules, U M and I M is the DC voltage and current on the medium voltage side, U H and I H is the DC voltage and current on the high voltage side, u Pj and i Pj They represent the bridge arm voltage and current of the medium voltage side submodule, u PJ and i PJ Respectively represent the voltage and current of the high-voltage side submodule bridge arm, i Tj and i TJ are the thyristor valve group currents on the medium voltage side and high voltage side, i j is the primary current of the transformer.

[0081] The bidirectional hybrid transmission transformer has two operating modes, namely the forward operating mode (power is transferred from the medium voltage side to the high voltage side) and the reverse operating mode (power is transferred from the high voltage side to the medium voltage side). The working principle waveform in the forward operating mode is as follows Figure 2 In each operation cycle T, the medium voltage side forward thyristor valve group T a+ ~T c+ And the high voltage side forward thyristor valve group T A+ ~T C+ The valves are triggered to conduct in sequence, and a certain conduction overlap time T is designed between the adjacent two-phase valve groups. c The current of the thyristor valve group on the medium and high voltage sides is controlled to an amplitude of I M and I HThe three-phase waveforms are staggered 120 degrees to obtain a smooth DC current without the need for an additional DC filter. a ~i c It is also controlled by the submodule bridge arm as follows Figure 2 The amplitude shown is I AC In general, in order to synthesize a smooth and continuous DC current, at any time, there is always one phase submodule bridge arm supporting the medium voltage DC port voltage, one phase submodule bridge arm supporting the high voltage DC port voltage, and the third phase submodule bridge arm is used for commutation.

[0082] Due to the interlacing of the three-phase waveforms, there are 6 commutation states I in each operating cycle T. c ~VI c Among them, the commutation state III c The circuit diagram is as follows Figure 3 As shown, in order to realize the current commutation from phase a to phase b, the bridge arm voltage u Pa and u Pb Adjust to U M -U0 and U M +U0, thereby applying a commutation voltage U0 of equal magnitude and opposite direction to the bridge arm inductance of the a and b phase submodules on the medium voltage side, so that i Pa From I M -I AC drops to 0, and i Pb Increase from 0 to I at the same rate M -I AC For the high voltage side, u PA and u PB The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the a and b phase submodules on the high voltage side, so that the transformer current i a From I AC drops to 0, and i b Increase from 0 to I at the same rate AC Through the coordination of the bridge arms of the medium-voltage side and high-voltage side submodules, the medium-voltage side current I M Correspondingly, from T a Switch to T b , the interphase commutation is completed. And the thyristor current and the transformer current are commutated together.

[0083] In addition, another function of the submodule bridge arm is to achieve reliable shutdown of the thyristor valve group. There is a constant current state after each commutation state ends. There are 6 constant current states I to VI in one cycle. Among them, the circuit state of constant current state III is as follows Figure 4 As shown, in the commutation state III c After the end, in order to turn off the thyristor T a+, a phase medium voltage side submodule bridge arm u Pa Adjust to U M +U q , so that T a+ Bearing size U q The reverse voltage promotes the full recombination of residual carriers inside it and ensures the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the submodule bridge arm voltage u Pa With a certain slope from U M Descend to U M -U AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt. It is worth mentioning that during the thyristor reverse voltage shutdown period, although the sub-module bridge arm voltage is high, the sub-module bridge arm current of this phase is 0, and no sub-module bridge arm energy fluctuation will occur.

[0084] The corresponding principle waveform of the reverse operation mode is as follows Figure 5 As shown. It is similar to the forward waveform, but all the forward thyristor valve groups are in the off state, while the reverse thyristor valve groups are triggered to conduct alternately, and all current waveforms are the same as the forward operation, but the current polarity changes. The detailed process is similar to the forward operation and will not be repeated.

[0085] Step 2: Propose a low-cost fault isolation solution

[0086] In order to avoid installing bulky and expensive DC circuit breakers at sea, the use of Figure 6 The low-cost fault isolation solution shown in Figure 1 is shown in Figure 2. Specifically, a mechanical switch S can be used at the outlet of the wind turbine. t A low-cost diode isolation device (S h1 and S h2 ), each diode isolation device consists of a series diode valve group and a mechanical switch connected in parallel. In normal operation, the mechanical switch S t The diode isolation device only allows current to flow from the wind turbine to the offshore platform. During the startup phase, the mechanical switch S t In addition, since the output DC transformer also has a fault isolation function, only a mechanical switch S is required at its output position. s1 For the onshore MMC converter station, the existing DC transmission technology solution is adopted, and an AC circuit breaker S is configured on the AC side of the MMC. ac , and a mechanical switch S is configured on the DC side s2 .

[0087] When a short circuit occurs in the outgoing submarine cable, the onshore MMC converter station and the outgoing DC transformer will inject fault current into the short circuit point. For the onshore MMC converter station, the fault isolation scheme commonly used in existing flexible DC transmission projects is adopted. After detecting the fault, the onshore MMC converter station is locked, and the AC side circuit breaker S ac Disconnect to prevent the AC main grid from feeding fault current to the fault point. After the submarine cable current decays to 0, disconnect the mechanical switch S s2 To achieve fault isolation. For a bidirectional hybrid DC transformer, the typical short-circuit fault current waveform is as follows Figure 7 As shown, the failure process is divided into two stages:

[0088] 1) Fault current rising stage: Since fault detection requires a certain amount of time, the typical value of fault detection time for existing flexible DC transmission projects is about 100μs. Therefore, after the fault occurs but before the control system detects the fault, the sending DC transformer will maintain the normal control strategy and continue to operate, and the medium-voltage busbar and bridge arm submodule capacitors will discharge to the fault point;

[0089] 2) Fault current natural decay stage: When the control system detects a fault, it immediately locks the DC transformer and the submodule capacitor discharge circuit is cut off. The DC fault current decays naturally from the maximum value. The decay speed depends on the fault current peak value and line impedance parameters. When the line current decays to 0, the mechanical switch S is disconnected. s1 , to achieve fault isolation.

[0090] In summary, for faults in outgoing submarine cables, mechanical switches can also be used to achieve fault isolation. However, the fault ride-through time depends on the short-circuit fault current peak. On the one hand, an excessively large fault current peak makes the fault isolation time longer, affecting the system recovery and reclosing time. On the other hand, the submodule IGBT needs to withstand the short-circuit fault current for a long time, and there is a risk of overcurrent damage. Therefore, it is necessary to study the fault characteristics of the outgoing DC transformer, and to suppress the fault current through active control of the outgoing DC transformer, thereby reducing the overcurrent risk of the submodule IGBT, shortening the fault isolation time, and even eliminating the current limiting reactor, reducing the weight and cost of offshore platform equipment.

[0091] Step 3: Propose a strategy for active fault current suppression

[0092] (1) Analysis of short-circuit characteristics of bidirectional hybrid DC transformer

[0093] For a short-circuit fault in the outgoing submarine cable, there are Figure 8 The following three situations are shown: ① The fault occurs in the constant current stage, corresponding to Figure 8 (a) ② The fault occurs during the thyristor commutation stage on the medium voltage side. Figure 8(b) ③ The fault occurs during the thyristor commutation phase on the high-voltage side. Figure 8 (c) For cases ① and ②, only one bridge arm is connected in parallel with the high-voltage DC port when a fault occurs, while for case ③, two bridge arms are connected in parallel with the high-voltage DC port. Figure 8 It can be seen that in either case, the short-circuit fault current has two sources. One part is the fault current injected into the short-circuit point by the medium voltage collection side through the transformer (corresponding to i in the figure). uf ), and the other part is caused by the discharge of the capacitor of the bridge arm submodule connected in parallel on the high voltage side (corresponding to i in the figure P ). Therefore, for the bidirectional hybrid DC transformer, when the submarine cable short-circuit fault occurs, it can be equivalent to Fig. 9 (a) shows the equivalent circuit, where L uf is the equivalent inductance of the discharge circuit on the medium voltage collection side, L P is the equivalent inductance of the bridge arm discharge circuit connected in parallel with the high voltage side, L o is the current limiting inductor. Figure 8 The three cases ①, ② and ③ shown in the figure correspond to the equivalent inductance L of the circuit. uf and L P They are:

[0094]

[0095] In formulas (1) to (3), L M and L H are the equivalent inductance of the medium voltage bridge arm and the equivalent inductance of the high voltage bridge arm, respectively. n is the transformer ratio. According to the equivalent circuit and equivalent inductance expression, the short-circuit fault current peak value i before blocking can be obtained. f,max for:

[0096]

[0097] Among them U H and I H is the voltage and current of the high-voltage DC side under rated conditions, T d is the time required for fault detection, and its typical value is 100μs.

[0098] Fig.10 The fault current peak value i is shown under the typical working conditions of 300MW, ±50kV / ±200kV. f,max And the high voltage side parallel bridge arm discharge current i P About the current limiting inductor L o The change curve of the fault current peak value i f,max With rated current I H The reference value is normalized. Fig.10The fault current peak of the traditional MMC-FTF under the same parameters is also shown in Figure 2. For the convenience of analysis, only the fault current i caused by the capacitor discharge of the bridge arm submodule on the fault side of the MMC-FTF is considered here. P , its corresponding equivalent circuit is as follows Fig. 9 As shown in (b), since each phase bridge arm of the MMC-FTF contains two bridge arm inductors, the equivalent inductance of the three-phase bridge arms connected in parallel is 2 / 3L. H .

[0099] from Fig.10 The relationship curve shows that:

[0100] 1) In case ① and case ②, only one bridge arm is connected in parallel with the high-voltage DC port, and the curves of the two are similar. In case ③, two bridge arms are connected in parallel with the high-voltage DC port, and the fault current peak value i f,max The worst of the three situations;

[0101] 2) When no current limiting inductor L is added o When L o =0, fault current peak value i f,max Up to 5.78pu;

[0102] 3) The bridge arm discharge current i connected in parallel with the high voltage port P At the peak fault current i f,max In order to further analyze the bridge arm discharge circuit in parallel with the high voltage port, Fig.11 The operation diagram of the outgoing submarine cable failure is given. Before the failure occurs, the high voltage side bridge arm of phase c is connected in parallel with the high voltage DC port. When the outgoing submarine cable failure occurs, the high voltage DC voltage U H The current quickly drops to 0, and the capacitor of the phase C submodule discharges to the short-circuit point through the upper tube IGBT, causing a large discharge current i in the bridge arm. P After the fault is detected and the DC transformer is locked, i P The main reason for the slow decay of fault current is that the anti-parallel diode of the IGBT under the submodule is connected to the current. P The only way to prevent the discharge of the capacitor is to disconnect the upper IGBT of the bridge arm connected in parallel with the high-voltage port during normal operation, thereby cutting off the discharge circuit.

[0103] (2) Active fault current suppression method

[0104] The operating principle waveform corresponding to the proposed fault current active suppression method is as follows: Fig.12 Compared to Fig.11 The difference is that when the bridge arm is connected in parallel with the high-voltage DC port, the IGBT drive signal S on the bridge arm is not enabled. u , and the lower IGBT drive signal Sl and Fig.11 When a fault occurs in the outgoing submarine cable, there is no discharge path for the submodule capacitor in the bridge arm connected in parallel with the high-voltage DC port, and no discharge current will be generated. It is worth pointing out that in normal operation, the current in the bridge arm connected in parallel with the high-voltage DC port is always positive, which means that the bridge arm current always flows through the anti-parallel diode of the upper tube IGBT of the submodule, rather than the upper tube IGBT. Therefore, the upper tube IGBT drive signal S of this bridge arm is not enabled. u It will not affect the normal operation of the DC transformer, but eliminates the discharge current i of the bridge arm in the event of a fault. P .

[0105] Fig.13 The flow chart of the proposed fault current active suppression method is given. When the DC transformer is running in the forward operation mode, if the bridge arm is connected in parallel with the high-voltage DC port, the IGBT drive signal of the bridge arm is not enabled, otherwise it is enabled normally. It is worth noting that this method is also applicable to the reverse operation mode, and the control logic is the same.

[0106] Fig.14 The fault current peak value i of the bidirectional hybrid DC transformer under the proposed fault current active suppression method is further given. f,max With the current limiting inductor L o It can be seen that due to the elimination of the high-voltage DC port parallel bridge arm discharge current i P , fault current peak value i f,max Even without an external current limiting inductor, i.e. L o =0, fault current peak value i f,max It can be reduced from the original 5.78pu to 2.3pu. At the same time, compared with the traditional MMC-FTF, the fault current peak i f,max The proposed scheme can effectively reduce the fault current peak value and eliminate the current limiting inductor.

[0107] In the present invention, thyristors and IGBTs are used on the medium voltage side to carry current and transmit power, which has the characteristics of high efficiency, low cost and high reliability; the output voltage and transmission power are adjusted by controlling the voltage at both ends of the bridge arm of the sub-module in series, which has the characteristics of wide voltage regulation range and fault current limiting. In addition, the medium frequency transformer is smaller in size and weight than the industrial frequency transformer of MMC, which can realize the lightweight and compact design of the offshore platform. At the same time, the present invention also proposes an active fault current suppression strategy to reduce the risk of overcurrent of the sub-module IGBT, shorten the fault isolation time and even cancel the current limiting reactor, further reducing the weight and cost of the offshore platform equipment.

[0108] On the whole, the bidirectional hybrid DC transmission transformer proposed in the present invention has fewer components, lower device cost, higher operating efficiency and smaller capacitor energy storage requirements, and the proposed fault current active suppression strategy can effectively reduce the fault current peak and eliminate the current limiting inductor. The overall volume and weight can be significantly reduced. It has the advantages of low cost, low device loss and simple structure, and is very suitable for new DC loads and energy storage systems, new energy DC transmission systems and other fields.

Claims

1. A bidirectional hybrid DC transformer, characterized in that The medium voltage side and high voltage side of the bidirectional hybrid DC transformer are both composed of three thyristor valve groups and three submodule bridge arms, and the medium voltage side and high voltage side are connected through a medium frequency transformer, wherein: The thyristor valve group is composed of two groups of series thyristors connected in reverse parallel. j+ and T J+ They are the forward thyristor valve groups on the medium voltage side and the high voltage side, T j- and T J- They are reverse thyristor valve groups on the medium voltage side and high voltage side, j = a, b, c, J = A, B, C; The bridge arm of the medium voltage side submodule and the bridge arm of the high voltage side submodule are respectively composed of N M and N H Half-bridge sub-modules, where L M and L H are the bridge arm inductances of the submodules on the medium voltage side and the high voltage side, respectively, U M and I M is the DC voltage and current on the medium voltage side, U H and I H is the DC voltage and current on the high voltage side, u Pj and i Pj They represent the bridge arm voltage and current of the medium voltage side submodule, u PJ and i PJ Respectively represent the voltage and current of the high-voltage side submodule bridge arm, i Tj and i TJ are the thyristor valve group currents on the medium voltage side and high voltage side, i j is the primary current of the transformer.

2. The bidirectional hybrid DC transformer according to claim 1 is characterized in that The bidirectional hybrid transmission transformer has two operating modes: Forward operation mode: In each operation cycle T, the medium voltage side forward thyristor valve group T a+ ~T c+ And the high voltage side forward thyristor valve group T A+ ~T C+ The valves are triggered to conduct in sequence, and the conduction overlap time T is designed between the adjacent two-phase valve groups. c To achieve commutation, the current of the thyristor valve group on the medium and high voltage sides is controlled to an amplitude of I respectively under the cooperation of the three-phase sub-module bridge arm. M and I H The three-phase waveforms are staggered by 120° electrical angle to obtain a smooth DC current. The medium frequency transformer current i a ~i c The bridge arm of the quilt module is controlled to have an amplitude of I AC And interlaced trapezoidal waves; Due to the interlacing of the three-phase waveforms, there are 6 commutation states I in each operating cycle T. c ~VI c ,in: Commutation state I c :Through the coordination of the bridge arms of the c and a phase submodules on the medium voltage side and the bridge arms of the c and a phase submodules on the high voltage side, the current I M The corresponding c+ Switch to T a+ , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pc and u Pa Adjust to U M -U0 and U M +U0, applying a commutation voltage U0 of equal magnitude and opposite direction to the inductance of the bridge arm of the c and a phase submodules on the medium voltage side, thereby making i Pc From I M -I AC rises to 0, and i Pa Decrease from 0 to I at the same rate M -I AC ; For the high voltage side, u PC and u PA The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the c and a phase submodules on the high voltage side, so that the current i c From I AC Drop to 0, i a Increase from 0 to I at the same rate AC , so i Tc From I M Down to 0, i Ta Increase from 0 to I at the same rate M , complete the commutation; Commutation State II c :Through the coordination of the b and c phase submodule bridge arms on the high voltage side and the b and c phase submodule bridge arms on the medium voltage side, the high voltage side current I H The corresponding B+ Switch to T C+ , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PB and u PC Adjust to U H -U1 and U H +U1, applying a commutation voltage U1 of equal magnitude and opposite direction to the inductance of the bridge arm of the b and c phase submodules on the high-voltage side, thereby making i PB From I AC / nI H drops to 0, and i PC Increase from 0 to I at the same rate AC / nI H ; For the medium voltage side, u Pb and u Pc The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the b and c phase submodules on the medium voltage side, so that the medium frequency transformer current i b From-I AC Raised to 0, i c Decrease from 0 to -I at the same rate AC , so i TB From I H Drop to 0, i TC Increase from 0 to I at the same rate H , complete the commutation; Commutation state III c :Through the coordination of the bridge arms of the a and b phase submodules on the medium voltage side and the bridge arms of the a and b phase submodules on the high voltage side, the current I M The corresponding a+ Switch to T b+ , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pa and u Pb Adjust to U M -U0 and U M +U0, applying commutation voltage U0 of equal magnitude and opposite direction to the bridge arm inductance of phase a and phase b submodules on the medium voltage side, thereby making i Pa From I M -I AC rises to 0, and i Pb Decrease from 0 to I at the same rate M -I AC ; For the high voltage side, u PA and u PB The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the a and b phase submodules on the high voltage side, so that the current i a From I AC drops to 0, and i b Increase from 0 to I at the same rate AC , so i Ta From I M Down to 0, i Tb Increase from 0 to I at the same rate M , complete the commutation; Commutation State IV c :Through the coordination of the c and a phase submodule bridge arms on the high voltage side and the c and a phase submodule bridge arms on the medium voltage side, the high voltage side current I H The corresponding C+ Switch to T A+ , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PC and u PA Adjust to U H +U1 and U H -U1, applying a commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of the a and c phase submodules on the high voltage side, thereby making i PC From I AC / nI H drops to 0, and i PA Increase from 0 to I at the same rate AC / nI H ; For the medium voltage side, u Pc and u Pa The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the c-phase and a-phase submodules on the medium voltage side, so that the medium frequency transformer current i c From-I AC Raised to 0, i a Decrease from 0 to -I at the same rate AC , so i TC From I H Drop to 0, i TA Increase from 0 to I at the same rate H , complete the commutation; Commutation state V c :Through the cooperation of the bridge arms of the b and c phase submodules on the medium voltage side and the bridge arms of the b and c phase submodules on the high voltage side, the current I M The corresponding b+ Switch to T c+ , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pb and u Pc Adjust to U M -U0 and U M +U0, applying a commutation voltage U0 of equal magnitude and opposite direction to the bridge arm inductance of the b and c phase submodules on the medium voltage side, thereby making i Pb From I M -I AC rises to 0, and i Pc Decrease from 0 to I at the same rate M -I AC ; For the high voltage side, u PB and u PC The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the b and c phase submodules on the high voltage side, so that the current i b From I AC drops to 0, and i c Increase from 0 to I at the same rate AC , so i Tb From I M Down to 0, i Tc Increase from 0 to I at the same rate M , complete the commutation; Commutation Status VI c :Through the coordination of the bridge arms of the a and b phase submodules on the high voltage side and the bridge arms of the a and b phase submodules on the medium voltage side, the high voltage side current I H The corresponding A+ Switch to T B+ , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PA and u PB Adjust to U H +U1 and U H -U1, applying commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of phase a and phase b submodules on the high voltage side, thereby making i PA From I AC / nI H drops to 0, and i PB Increase from 0 to I at the same rate AC / nI H ; For the medium voltage side, u Pa and u Pb The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the a and b phase submodules on the medium voltage side, so that the medium frequency transformer current i a From-I AC rises to 0, i b Decrease from 0 to -I at the same rate AC , so i TA From I H Down to 0, i TB Increase from 0 to I at the same rate H , complete the commutation; There is a constant current state after each commutation state ends. There are 6 constant current states I to VI in one cycle, among which: Constant current state I: In commutation state I c After the end, in order to turn off the thyristor T c+ , c phase medium voltage side submodule bridge arm u Pc Adjust to U M +U q , so that T c+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pc With a fixed slope from U M Descend to U M -U AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt; Constant current state II: In commutation state II c After the end, in order to turn off the thyristor T B+ , b-phase high voltage side submodule bridge arm u PB Adjust to U H +U q , so that T B+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PB With a fixed slope from U H Descend to U H -nU AC ; Constant current state III: In commutation state III c After the end, in order to turn off the thyristor T a+ , a phase medium voltage side submodule bridge arm u Pa Adjust to U M +U q , so that T a+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pa With a fixed slope from U M Descend to U M -U AC ; Constant current state IV: In commutation state IV c After the end, in order to turn off the thyristor T C+ , c-phase high voltage side submodule bridge arm u Pc Adjust to U H +U q , so that T C+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PC With a fixed slope from U H Descend to U H -nU AC ; Constant current state V: In commutation state V c After the end, in order to turn off the thyristor T b+ , b phase medium voltage side submodule bridge arm u Pa Adjust to U M +U q , so that T b+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pb With a fixed slope from U M Descend to U M -U AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt; Constant current state V: In commutation state V c After the end, in order to turn off the thyristor T A+ , a-phase high voltage side submodule bridge arm u Pa Adjust to U H +U q , so that T A+ Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PA With a fixed slope from U M Descend to U H -nU AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt; Reverse operation mode: In each operation cycle T, the medium voltage side reverse thyristor valve group T a+ ~T c+ And the high voltage side reverse thyristor valve group T A- ~T C- The valves are triggered to conduct in sequence, and the conduction overlap time T is designed between the adjacent two-phase valve groups. c To achieve commutation, the current of the thyristor valve group on the medium and high voltage sides is controlled to an amplitude of I respectively under the cooperation of the three-phase sub-module bridge arm. M and I H The three-phase waveforms are staggered by 120° electrical angle to obtain a smooth DC current. The medium frequency transformer current i a ~i c The bridge arm of the quilt module is controlled to have an amplitude of I AC And interlaced trapezoidal waves; Due to the interlacing of the three-phase waveforms, there are 6 commutation states I in each operating cycle T. c ~VI c ,in: Commutation state I c :Through the coordination of the bridge arms of the c and a phase submodules on the medium voltage side and the bridge arms of the c and a phase submodules on the high voltage side, the current -I M The corresponding c- Switch to T a- , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pc and u Pa Adjust to U M +U0 and U M -U0, applying commutation voltage U0 of equal magnitude and opposite direction to the inductance of the bridge arm of the c and a phase submodules on the medium voltage side, thereby making i Pc from-(I M -I AC ) drops to 0, and i Pa At the same rate from 0 to -(I M -I AC );For the high voltage side, u PC and u PA The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the c and a phase submodules on the high voltage side, so that the current i c From-I AC Raised to 0, i a Decrease from 0 to -I at the same rate AC , so i Tc From-I M Raised to 0, i Ta From 0 to -I at the same rate M , complete the commutation; Commutation State II c :Through the coordination of the b and c phase submodule bridge arms on the high voltage side and the b and c phase submodule bridge arms on the medium voltage side, the high voltage side current -I H The corresponding B- Switch to T C- , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PB and u PC Adjust to U H +U1 and U H -U1, applying a commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of the b and c phase submodules on the high voltage side, thereby making i PB from-(I AC / nI H ) rises to 0, and i PC At the same rate from 0 to -(I AC / nI H ); for the medium voltage side, u Pb and u Pc The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the b and c phase submodules on the medium voltage side, so that the medium frequency transformer current i b From I AC Drop to 0, i c Increase from 0 to I at the same rate AC , so i TB From-I H Raised to 0, i TC Decrease from 0 to -I at the same rate H , complete the commutation; Commutation state III c :Through the coordination of the bridge arms of the a and b phase submodules on the medium voltage side and the bridge arms of the a and b phase submodules on the high voltage side, the current -I M The corresponding a- Switch to T b- , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pa and u Pb Adjust to U M +U0 and U M -U0, applying commutation voltage U0 of equal magnitude and opposite direction to the bridge arm inductance of phase a and phase b submodules on the medium voltage side, thereby making i Pa from-(I M -I AC ) drops to 0, and i Pb At the same rate from 0 to -(I M -I AC );For the high voltage side, u PA and u PB The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the a and b phase submodules on the high voltage side, so that the current i a From-I AC rises to 0, and i b Decrease from 0 to -I at the same rate AC , so i Ta From-I M Raised to 0, i Tb From 0 to -I at the same rate M , complete the commutation; Commutation State IV c :Through the coordination of the bridge arms of the c and a phase submodules on the high voltage side and the bridge arms of the c and a phase submodules on the medium voltage side, the high voltage side current -I H The corresponding C- Switch to T A- , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PC and u PA Adjust to U H -U1 and U H +U1, applying a commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of the a and c phase submodules on the high voltage side, thereby making i PC from-(I AC / nI H ) rises to 0, and i PA At the same rate from 0 to -(I AC / nI H ); for the medium voltage side, u Pc and u Pa The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the c-phase and a-phase submodules on the medium voltage side, so that the medium frequency transformer current i c From I AC Drop to 0, i a Increase from 0 to I at the same rate AC , so i TC From-I H Raised to 0, i TA Decrease from 0 to -I at the same rate H , complete the commutation; Commutation state V c :Through the cooperation of the bridge arms of the b and c phase submodules on the medium voltage side and the bridge arms of the b and c phase submodules on the high voltage side, the current -I M The corresponding b- Switch to T c- , the interphase commutation is completed, and the thyristor current and the medium frequency transformer current are commutated together; the bridge arm voltage u Pb and u Pc Adjust to U M +U0 and U M -U0, applying commutation voltage U0 of equal magnitude and opposite direction to the bridge arm inductance of phase b and c submodules on the medium voltage side, thereby making i Pb from-(I M -I AC ) drops to 0, and i Pc At the same rate from 0 to -(I M -I AC );For the high voltage side, u PB and u PC The commutation voltage U1 of equal magnitude and opposite direction is applied to the bridge arm inductance of the b and c phase submodules on the high voltage side, so that the current i b From-I AC rises to 0, and i c Decrease from 0 to -I at the same rate AC , so i Tb From-I M Raised to 0, i Tc From 0 to -I at the same rate M , complete the commutation; Commutation Status VI c :Through the coordination of the bridge arms of the a and b phase submodules on the high voltage side and the bridge arms of the a and b phase submodules on the medium voltage side, the high voltage side current -I H The corresponding A- Switch to T B- , and the thyristor current and the medium frequency transformer current are commutated together; the high-voltage side submodule bridge arm voltage u PA and u PB Adjust to U H -U1 and U H +U1, applies a commutation voltage U1 of equal magnitude and opposite direction to the bridge arm inductance of the a and b phase submodules on the high voltage side, thereby making i PA from-(I AC / nI H ) drops to 0, and i PB At the same rate from 0 to -(I AC / nI H ); for the medium voltage side, u Pa and u Pb The commutation voltage U0 of equal magnitude and opposite direction is applied to the bridge arm inductance of the a and b phase submodules on the medium voltage side, so that the medium frequency transformer current i a From I AC Drop to 0, i b Increase from 0 to I at the same rate AC , so i TA From-I H Raised to 0, i TB Decrease from 0 to -I at the same rate H , complete the commutation; There is a constant current state after each commutation state ends. There are 6 constant current states I to VI in one cycle, among which: Constant current state I: In commutation state I c After the end, in order to turn off the thyristor T c- , c phase medium voltage side submodule bridge arm u Pc Adjust to U M -U q , so that T c- Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pc With a fixed slope from U M Descend to U M -U AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt; Constant current state II: In commutation state II c After the end, in order to turn off the thyristor T B- , b-phase high voltage side submodule bridge arm u PB Adjust to U H -U q , so that T B- Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PB With a fixed slope from U H Descend to U H -nU AC ; Constant current state III: In commutation state III c After the end, in order to turn off the thyristor T a- , a phase medium voltage side submodule bridge arm u Pa Adjust to U M -U q , so that T a- Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pa With a fixed slope from U M Descend to U M -U AC ; Constant current state IV: In commutation state IV c After the end, in order to turn off the thyristor T C- , c-phase high voltage side submodule bridge arm u Pc Adjust to U H -U q , so that T C- Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PC With a fixed slope from U H Descend to U H -nU AC ; Constant current state V: In commutation state V c After the end, in order to turn off the thyristor T b- , b phase medium voltage side submodule bridge arm u Pb Adjust to U M -U q , so that T b- Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u Pb With a fixed slope from U M Descend to U M -U AC , to prevent the thyristor from being triggered by mistake due to excessive du / dt; Constant current state VI: In the commutation state VI c After the end, in order to turn off the thyristor T A- , a-phase high voltage side submodule bridge arm u Pa Adjust to U H -U q , so that T A- Bearing size U q The reverse voltage promotes the full recombination of the residual carriers inside to ensure the reliable shutdown of the thyristor. After the thyristor restores its forward blocking capability, the bridge arm voltage u PA With a fixed slope from U M Descend to U H -nU AC , to prevent the thyristor from being triggered incorrectly due to excessive du / dt.

3. A fault current limiting design method for a bidirectional hybrid DC transformer according to any one of claims 1 to 2, characterized in that The method comprises the following steps: Step 1: Propose a low-cost fault isolation solution A mechanical switch S is used at the outlet of the wind turbine t , a low-cost diode isolation device is configured for each collection line inside the offshore platform. During normal operation, the mechanical switch S t The diode isolation device only allows current to flow from the wind turbine to the offshore platform. During the startup phase, the mechanical switch S t Close to realize power reverse transmission; configure mechanical switch S at the outlet of the DC transformer s1 ; For onshore MMC converter stations, configure AC circuit breaker S on the AC side of the MMC ac , configure a mechanical switch S on the DC side s2 ; When a short circuit fault occurs in the outgoing submarine cable, the onshore MMC converter station and the outgoing DC transformer will inject fault current into the short circuit point. For the onshore MMC converter station, the fault isolation scheme commonly used in existing flexible DC transmission projects is adopted. After the fault is detected, the onshore MMC converter station is locked and the AC side circuit breaker S ac Disconnect to prevent the AC main grid from feeding fault current to the fault point. After the submarine cable current decays to 0, disconnect the mechanical switch S s2 Realize fault isolation; for bidirectional hybrid DC transformer, the fault process is divided into two stages: 1) Fault current rising stage: After the fault occurs but before the control system detects the fault, the DC transformer will maintain the normal control strategy and continue to operate, and the medium voltage busbar and bridge arm submodule capacitors will discharge to the fault point; 2) Fault current natural decay stage: When the control system detects a fault, it immediately locks the DC transformer and the submodule capacitor discharge circuit is cut off. The DC fault current decays naturally from the maximum value. The decay speed depends on the fault current peak value and line impedance parameters. When the line current decays to 0, the mechanical switch S is disconnected. s1 , to achieve fault isolation; Step 2: Propose a fault current active suppression strategy (1) Analysis of short-circuit characteristics of bidirectional hybrid DC transformer For the short-circuit fault of the outgoing submarine cable, there are three situations according to the time of the fault: ① the fault occurs in the constant current stage, ② the fault occurs in the thyristor commutation stage on the medium voltage side, and ③ the fault occurs in the thyristor commutation stage on the high voltage side; When the outgoing submarine cable fails, the bidirectional hybrid outgoing DC transformer is equivalent to the equivalent circuit diagram of the outgoing submarine cable failure, where L uf is the equivalent inductance of the discharge circuit on the medium voltage collection side, L P is the equivalent inductance of the bridge arm discharge circuit connected in parallel with the high voltage side, L o is the current limiting inductor. For the above three situations ①, ② and ③, the corresponding circuit equivalent inductance L uf and L P They are: Where, L M and L H are the equivalent inductance of the medium voltage bridge arm and the equivalent inductance of the high voltage bridge arm, respectively. n is the transformer ratio. According to the equivalent circuit and equivalent inductance expression, the peak value of the short-circuit fault current before blocking i is calculated. f,max for: Among them U H and I H is the voltage and current of the high-voltage DC side under rated conditions, T d The time required for fault detection; (2) Active fault current suppression method When the output DC transformer operates in the forward operation mode, if the submodule bridge arm is connected in parallel with the high-voltage DC port, the IGBT drive signal of the submodule bridge arm is not enabled, otherwise it is enabled normally; When the output DC transformer operates in the reverse operation mode, if the bridge arm of the submodule is connected in parallel with the medium voltage DC port, the IGBT drive signal of the bridge arm of the submodule is not enabled, otherwise it is enabled normally.

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