CN105048488A - Flexible DC network DC short-circuit fault ride through method - Google Patents

Abstract

The invention discloses a flexible DC network DC short-circuit fault ride through method realized by cooperative control of an MMC with negative level output capability and a mechanical DC circuit breaker, and belongs to the field of flexible DC network DC fault protection. There are three means for processing a DC fault in the prior art mainly, including cut-off of connection with a DC system by means of AC equipment, DC fault elimination by means of a current converter and DC fault isolation by means of DC equipment, while the three methods cannot effectively realize flexible DC network DC fault protection. The MMC comprises submodules with negative level output capability, and voltage of the DC port of the current converter is enabled to be negative-level by flexibly regulating output voltage of upper and lower bridge arms so that objectives that DC short-circuit fault current is controllable and is rapidly reduced to zero can be achieved, and the requirement of isolating the DC fault can be met by the mechanical DC circuit breaker.

Description

A kind of flexible direct current network DC short trouble traversing method

Technical field

The invention belongs to flexible direct current network DC error protection field; more specifically, relate to a kind of module Multilevel Inverters MMC (ModularMultilevelConverter) based on possessing negative level fan-out capability and mechanical DC circuit breaker cooperation control and realize the method that flexible direct current network DC short trouble passes through.

Background technology

In recent years, the generation of electricity by new energy of offshore wind farm and remote districts is developed fast.

Compare ac transmission and Traditional DC transmission of electricity, Technology of HVDC based Voltage Source Converter can independently control to gain merit, reactive power, strong to the defensive ability/resistance ability of voltage disturbance, the electrical network of composition can realize the decoupling zero of voltage problem and the support of frequency issues between interconnected systems, being an active and controlled electrical network, is realize the interconnected optimal selection of many drop points new forms of energy.But direct-current short circuit fault is the unavoidable problem of flexible DC power transmission.Compare Traditional DC transmission system, direct-current short circuit fault in flexible direct current power transmission system has the advantages that fault current is large, the rate of climb is fast, especially in flexible direct current network, single-point DC Line Fault easily develops into multiple spot AC fault, the safe operation of serious threat electrical network.Based on the flexible direct current network of overhead wire, impermanency short trouble probability of happening is high, and therefore the problems referred to above are particularly outstanding.At present, the means processing DC Line Fault mainly contain three kinds:

1) alternating current equipment (as AC circuit breaker etc.) is relied on to disconnect the connection with direct current system.The method does not need to rely on DC circuit breaker, and by converter blocking, DC Line Fault is removed in AC circuit breaker action.After faulty line is isolated, converter deblocking, AC circuit breaker reclosing, System recover power delivery.But due to AC circuit breaker operate time long (≈ 100ms), and current conversion station cannot regulate meritorious and reactive power between age at failure, therefore large on AC system impact.

2) converter (modularization multi-level converter MMC) is relied on itself to carry out DC Line Fault removing, namely based on the direct current lock-out scheme of full-bridge MMC.But the program needs to block IGBT and drives, and sacrifices the controllability of current conversion station, and cannot to meritorious and reactive power flexible between age at failure.

3) DC equipment (as DC circuit breaker) is relied on to carry out DC Line Fault isolation.At present, DC circuit breaker can be divided three classes: mechanical type, solid-state version and mixed type.Mechanical type Circuit breaker technology is comparatively ripe, but this circuit breaker exists the deficiency of cutout ability (self-oscillation ≈ 30 ~ 40ms, precharge type ≈ 10ms), response speed slow (self-oscillation≤4kA, precharge type≤8kA).Solid-state version circuit breaker fast response time (≈ 1ms), but the deficiency having that dynamic voltage balancing is difficult and steady operation loss is high, only apply in low pressure, middle pressure field at present to some extent.Mixed type primary cut-out has good response speed (≈ 3 ~ 5ms) and cutout ability (≤9kA), is considered to the high voltage DC breaker of most application prospect.But this technology is not yet verified in Practical Project, and with current research speed, in Future Ten year, this circuit breaker is difficult to break through to the field being greater than 500kV.Therefore, the high voltage DC breaker of present stage is difficult to be applied directly to flexible direct current network.At present, the protection scheme based on DC circuit breaker increases smoothing reactor in DC line, reduces the demand to circuit breaker response speed, cutout ability by the suppression fault current rate of climb.But the method inevitably increases the loss (loss brought of smoothing reactor) of DC line.

In sum, above-mentioned three kinds of schemes all effectively cannot realize multiterminal flexible high pressure DC transmission system DC error protection.

Summary of the invention

For problems of the prior art, the application provides a kind of MMC and mechanical DC circuit breaker cooperation control based on possessing negative level fan-out capability and realizes the method that flexible direct current network DC short trouble passes through, wherein by the control of flow port negative level output voltage of standing erectly to the change of current, achieve and reduce the object that direct fault current rapidly drops to zero.

For achieving the above object, according to one aspect of the present invention, provide a kind of radial flexible direct current network DC short trouble traversing method, described flexible direct current network comprises n current conversion station port, wherein each port corresponding current conversion station respectively, , DC power transmission line and a mechanical DC circuit breaker, during normal work, one of them current conversion station is used for determining direct voltage, all the other current conversion stations are used for determining active power, described current conversion station comprises A, B, C three-phase, every by upper, lower two brachium pontis composition, it is characterized in that, the method comprises the following steps:

(1) each current conversion station detects and judges that whether DC side is short-circuited fault, is that order performs step (2), otherwise proceeds detection;

(2) electric parameters detected according to described each current conversion station DC port carries out fault location;

(3) in described flexible direct current network, described each current conversion station is switched to and determines active power controller, active power instruction is decided to be zero, reactive power instruction remains unchanged when relatively normally working, and obtains the every required alternating voltage reference value e exported mutually of MMC between age at failure according to meritorious and reactive power command calculations _vij;

All the other all current conversion stations except the corresponding current conversion station of fault location place circuit, according to the reference instruction value of direct fault current the detected value direct fault current i that described current conversion station DC port is checked through _dciwith DC port voltage U ' _dcicalculate the required negative level voltage U exported of DC port of the corresponding current conversion station of non-fault line _zi, and the upper and lower brachium pontis reference voltage u of the corresponding current conversion station of the described non-fault line that is added to _pij, u _nijin common mode operation part, add rear reference voltage u through the stack _pij, u _nijmodulation obtain upper and lower brachium pontis submodule triggering and conducting signal; The current conversion station that faulty line is corresponding is Static Var Compensator STATCOM and runs, and reduces the impact on connected AC network; The fault current in faulty line is finally made to decay to zero, wherein, i=1,2..., n, represent current conversion station 1,2..., n in described flexible direct current network respectively, subscript j=a, b, c, represent a, b, c three-phase respectively;

(4) all described mechanical DC circuit breakers are by detecting the fault current obtained in circuit, if decay to zero, order performs step (5), otherwise proceed to detect;

(5) the mechanical DC circuit breaker that tripping zero failure current circuit is corresponding;

(6) wait for a period of time after all the other all current conversion stations described carry out cooperation control, the current conversion station that non-fault line is corresponding is switched to normal control mode and completes and pass through.

Preferably, in described step (3), the reference instruction value i of described direct fault current _d ^* _ciobtain according to following formula

$\left\{\begin{array}{c}\begin{array}{cc}{i}_{dci}^{*}=i_{dci\_rated}& (i\≠1)\end{array}\\ i_{dc1}^{*}=\underset{i=2}{\overset{k-1}{\Σ}}{i}_{dci\_rated}+\underset{i=k+1}{\overset{n}{\Σ}}{i}_{dci\_rated}\end{array}\right.$

Wherein, assuming that fault occurs in kth bar circuit, i _{dci_rated}for rated current when non-faulty line i normally works, wherein represent described for determining the reference instruction value of the direct fault current of the current conversion station of direct voltage.

Preferably, in described step (3), the negative level voltage U of the DC port output of all the other all current conversion stations described _ziobtain according to following formula

$U_{zi}=U_{dci}^{\′}-\[K_p(i_{dci}^{*}-i_{dci})+K_I\∫(i_{dci}^{*}-i_{dci})dt\]$

Wherein, U' _dcifor current conversion station DC port voltage detecting value, K _p, K _ibe respectively the PI parameter of fault current controllers.

Preferably, in described step (3), described upper and lower brachium pontis reference voltage u _pij, u _nijobtain according to following formula

$\left\{\begin{array}{c}{u}_{pij}=\frac{1}{2}{U}_{dci}^{\′}-\frac{1}{2}{U}_{zi}-e_{vij}\\ u_{nij}=\frac{1}{2}{U}_{dci}^{\′}-\frac{1}{2}{U}_{zi}+e_{vij}\end{array}\right.$

Wherein, U' _dcifor current conversion station DC port voltage detecting value, e _vijfor the every required alternating voltage reference value exported mutually of MMC, U _zifor the negative level voltage that all current conversion station DC port except the corresponding current conversion station of fault location place circuit export.

According to another aspect of the present invention, provide a kind of looped network shape flexible direct current network DC short trouble traversing method, described flexible direct current network comprises several current conversion station ports and several mechanical choppers, the wherein corresponding current conversion station of each described port and some circuits, circuit two ends described in every bar comprise a circuit breaker, during normal work, one of them current conversion station is used for determining direct voltage, all the other current conversion stations are used for determining active power, described current conversion station comprises A, B, C three-phase, every by upper, lower two brachium pontis composition, it is characterized in that, the method comprises the following steps:

(1) each current conversion station detects and judges that whether DC side is short-circuited fault, is that order performs step (2), otherwise proceeds detection;

(2) electric parameters that described each current conversion station DC port detects carries out fault location;

(3) in described flexible direct current network, described each current conversion station is switched to and determines active power controller, active power instruction is decided to be zero, reactive power instruction remains unchanged when relatively normally working, and obtains the every required alternating voltage reference value e exported mutually of MMC between age at failure according to meritorious and reactive power command calculations _vij;

By regulating described upper and lower brachium pontis output voltage, making direct-current short circuit fault current controlled and being reduced to zero;

(4) all described mechanical DC circuit breakers are by detecting the fault current obtained in circuit, if decay to zero, order performs step (5), otherwise proceed to detect;

(5) the mechanical DC circuit breaker at tripping zero failure current circuit two ends;

(6) wait for a period of time after described each current conversion station carries out cooperation control, the current conversion station that non-fault line is corresponding is switched to normal control mode and completes and pass through.

In general, according to above-mentioned technical conceive of the present invention compared with prior art, following technological merit is mainly possessed:

1, exported by the negative level of converter self, make direct fault current controlled flexibly, utilize mechanical DC circuit breaker namely can meet the demand of isolated DC fault, ensure that non-faulty line runs continuously between age at failure;

2, during cooperation control, converter is non-latching, and can continue provides reactive power support to AC system, reduces the impact on AC system.

Accompanying drawing explanation

Fig. 1 is flexible direct current schematic network structure of the present invention;

Fig. 2 is full-bridge MMC structural representation;

Fig. 3 is direct fault current controller closed-loop structure block diagram of the present invention;

Fig. 4 is any mutually upper and lower brachium pontis output voltage reference value of MMC1 with MMC2 between age at failure;

Fig. 5 is the flow chart controlled based on the MMC and mechanical DC circuit breaker cooperation that possess negative level fan-out capability of the present invention;

Fig. 6 is the overall control block diagram of current conversion station MMC1 and MMC2 between age at failure;

Fig. 7 is three end DC transmission system embodiment schematic diagrames of the present invention;

Fig. 8 is Case Simulation figure of the present invention.Wherein, (a) flows through CB ₃fault current I _faultscheme over time, b () converter MMC1 port voltage is schemed over time, c () converter MMC2 port voltage is schemed over time, d () converter MMC3 port voltage is schemed over time, e meritorious and reactive power variation diagram in time that () converter MMC1 transmits, f meritorious and reactive power variation diagram in time that () converter MMC2 transmits, the meritorious and reactive power variation diagram in time that (g) converter MMC3 transmits.

Embodiment

In order to make object of the present invention, technical scheme and advantage clearly understand, below in conjunction with drawings and Examples, the present invention is further elaborated.Should be appreciated that specific embodiment described herein only in order to explain the present invention, be not intended to limit the present invention.In addition, if below in described each execution mode of the present invention involved technical characteristic do not form conflict each other and just can mutually combine.

The MMC possessing negative level fan-out capability not only comprises the MMC (as full-bridge MMC) be made up of a kind of submodule possessing negative level fan-out capability, also comprises and mixes by the submodule and all the other type submodules that possess negative level fan-out capability the MMC (as half-bridge and full-bridge mixed type MMC) formed.Machinery DC circuit breaker technology is comparatively ripe, but there is cutout ability (self-oscillation ≈ 30 ~ 40ms in this circuit breaker, precharge type ≈ 10ms), the slow (self-oscillation≤4kA of response speed, precharge type≤8kA) deficiency, be difficult to the demand directly meeting flexible direct current power transmission system isolated DC fault.

In the MMC possessing negative level fan-out capability of the embodiment of the present invention and the scheme of mechanical DC circuit breaker cooperation, each port corresponding current conversion station, a DC power transmission line and mechanical DC circuit breaker respectively of described radial n port flexible direct current network, as shown in Figure 1.During normal work, assuming that current conversion station 1 constant DC voltage control, all the other changes of current are stood firm active power controller, and the method comprises the steps:

(1) each current conversion station detects and judges whether DC side is short-circuited fault, is sequentially perform step (2); Otherwise proceed to detect;

(2) electric parameters detected according to current conversion station DC port carries out fault location;

(3) in flexible direct current network described in example, switched to by all current conversion stations and determine active power controller, active power instruction is decided to be zero, remains unchanged when reactive power instruction relatively normally works.The every required alternating voltage reference value e exported mutually of MMC between age at failure is obtained according to meritorious and reactive power command calculations _vij;

As shown in Figure 3, all current conversion stations except the corresponding current conversion station of fault location place circuit, according to the reference instruction value of direct fault current the detected value direct fault current i that described current conversion station DC port is checked through _dciwith DC port voltage U ' _dcicalculate the required negative level voltage U exported of DC port of the corresponding current conversion station of non-fault line _zi, and the upper and lower brachium pontis reference voltage u of the corresponding current conversion station of the described non-fault line that is added to _pij, u _nijin common mode operation part, as shown in Figure 4, add rear reference voltage u through the stack _pij, u _nijmodulation obtain upper and lower brachium pontis submodule triggering and conducting signal, the current conversion station that faulty line is corresponding be Static Var Compensator STATCOM run, reduce the impact on connected AC network; The fault current in faulty line is finally made to decay to zero, wherein, subscript i=1,2..., n, represent current conversion station 1,2..., n in described flexible direct current network respectively, subscript j=a, b, c, represent a, b, c three-phase respectively;

(4) all described mechanical DC circuit breakers are by detecting the fault current obtained in circuit, if decay to zero, order performs step (5), otherwise proceed to detect;

(5) the mechanical DC circuit breaker that tripping zero failure current circuit is corresponding;

(6) current conversion station waits for a period of time after carrying out cooperation control, and the current conversion station that non-fault line is corresponding is switched to normal control mode and completes and pass through.

In embodiments of the invention,

$\left\{\begin{array}{c}\begin{array}{cc}{i}_{dci}^{*}=i_{dci\_rated}& (i\≠1)\end{array}\\ i_{dc1}^{*}=\underset{i=2}{\overset{k-1}{\Σ}}{i}_{dci\_rated}+\underset{i=k+1}{\overset{n}{\Σ}}{i}_{dci\_rated}\end{array}\right.$

$U_{zi}=U_{dci}^{\′}-\[K_p(i_{dci}^{*}-i_{dci})+K_I\∫(i_{dci}^{*}-i_{dci})dt\]$

$\left\{\begin{array}{c}{u}_{pij}=\frac{1}{2}{U}_{dci}^{\′}-\frac{1}{2}{U}_{zi}-e_{vij}\\ u_{nij}=\frac{1}{2}{U}_{dci}^{\′}-\frac{1}{2}{U}_{zi}+e_{vij}\end{array}\right.$

Wherein, assuming that fault occurs in kth bar circuit.I _{dci_rated}for rated current when non-faulty line i normally works, represent described for determining the reference instruction value of the direct fault current of the current conversion station of direct voltage, U' _dcifor current conversion station DC port voltage detecting value, K _p, K _ibe respectively the PI parameter of fault current controllers, e _vijfor the alternating voltage reference value calculated in step 3.Secondly, it should be noted that, due to the randomness of fault point, so a value uncertain (changing to 1 from 0) in figure 3, therefore when controller PI parameter designing, make a=0.

For making those skilled in the art understand the present invention better, below in conjunction with specific embodiment, the scheme of the MMC and mechanical DC circuit breaker cooperation that possess negative level fan-out capability of the present invention is described in detail.

In following example, possesses the MMC of negative level fan-out capability for full-bridge MMC, as shown in Figure 2.Three end flexible DC power transmission network configurations as shown in Figure 7.AC phase voltage effective value 10kV, DC bus-bar voltage is 20kV, and each brachium pontis comprises 8 full-bridge submodules.Submodule electric capacity 15mF, brachium pontis inductance 5.5mH, brachium pontis inductance equivalent resistance 0.2 Ω.During normal work, MMC1 constant DC voltage control, MMC2, MMC3 determine active power controller, and meritorious, the reactive power that each current conversion station transmits and trend positive direction are all indicated in the drawings.

As shown in Figure 7, assumed fault occurs in F point (transmission line that MMC3 is corresponding), and direct voltage is 0kV.After converter detects fault, be all switched to active power controller, and active power instruction is zero, remains unchanged when Reactive Power Control is relatively normal; MMC1 and MMC2 carries out direct fault current control, and its instruction is the output U of direct fault current controller _zi, be added in the common mode component of upper and lower brachium pontis reference signal; MMC3 is STATCOM and runs.As direct-current isolating switch CB ₃fault current I detected _faultwhen being zero, performing and cut-off operation.MMC1 and MMC2 is switched to normal mode of operation after waiting for 30ms.

During cooperation control, flow through CB ₃fault current change in time as shown in Fig. 8 (a), result shows that fault current is controlled, and within 15ms rapid decay to zero, CB ₃zero failure electric current is detected, tripping machinery DC circuit breaker, isolated DC fault after general 20ms occurs fault; The DC port voltage of converter MMC1 changes in time as shown in Fig. 8 (b); result shows; after current conversion station detects DC Line Fault, drop into negative level immediately, direct fault current is declined rapidly, after waiting for 30ms, (guarantee mechanical DC circuit breaker CB ₃reliable isolated DC fault) MMC1 recovers normal mode of operation, and DC port voltage resume is normal; The DC port voltage of converter MMC2 changes in time as shown in Fig. 8 (c), result shows, also negative level is dropped into immediately after current conversion station detects DC Line Fault, direct fault current is declined rapidly, after waiting for 30ms, MMC2 recovers normal mode of operation, and DC port voltage resume is normal; The DC port voltage of converter MMC3 changes in time as shown in Fig. 8 (d), and result shows, after current conversion station detects DC Line Fault, drag down DC port voltage immediately, is STATCOM and runs; Meritorious and the reactive power that converter MMC1 transmits changes in time as shown in figure (e), result shows, cooperation control period MMC1 can continue to provide reactive power support to AC system, and after DC Line Fault is isolated, the fast quick-recovery of active power transfer; Meritorious and the reactive power that converter MMC2 transmits changes in time as shown in figure (f), result shows, cooperation control period MMC2 still can continue to provide reactive power support to AC system, and after DC Line Fault is isolated, the fast quick-recovery of active power transfer is normal; Meritorious and the reactive power that converter MMC3 transmits changes in time as shown in figure (g), and result shows, MMC3 is STATCOM and runs, and can continue to provide reactive power support to AC system, active power maintains zero.

Described cooperation scheme is all applicable in high, medium and low voltage flexible direct-current transmission field.

Those skilled in the art will readily understand; the foregoing is only preferred embodiment of the present invention; not in order to limit the present invention, all any amendments done within the spirit and principles in the present invention, equivalent replacement and improvement etc., all should be included within protection scope of the present invention.

Claims (4)

1. a radial flexible direct current network DC short trouble traversing method, described flexible direct current network comprises n current conversion station port, wherein each port corresponding current conversion station, a DC power transmission line and mechanical DC circuit breaker respectively, during normal work, one of them current conversion station is used for determining direct voltage, and all the other current conversion stations are used for determining active power, described current conversion station comprises A, B, C three-phase, often be made up of upper and lower two brachium pontis, it is characterized in that, the method comprises the following steps:

(1) each current conversion station detects and judges that whether DC side is short-circuited fault, is that order performs step (2), otherwise proceeds detection;

(2) electric parameters detected according to described each current conversion station DC port carries out fault location;

(3) in described flexible direct current network, described each current conversion station is switched to and determines active power controller, active power instruction is decided to be zero, reactive power instruction remains unchanged when relatively normally working, and obtains the every required alternating voltage reference value e exported mutually of MMC between age at failure according to meritorious and reactive power command calculations _vij;

All the other all current conversion stations except the corresponding current conversion station of fault location place circuit, according to the reference instruction value of direct fault current the detected value direct fault current i that described current conversion station DC port is checked through _dciwith DC port voltage U ' _dcicalculate the required negative level voltage U exported of DC port of the corresponding current conversion station of non-fault line _zi, and the upper and lower brachium pontis reference voltage u of the corresponding current conversion station of the described non-fault line that is added to _pij, u _nijin common mode operation part, add rear reference voltage u through the stack _pij, u _nijmodulation obtain upper and lower brachium pontis submodule triggering and conducting signal; The current conversion station that faulty line is corresponding is Static Var Compensator STATCOM and runs, and reduces the impact on connected AC network; The fault current in faulty line is finally made to decay to zero, wherein, i=1,2..., n, represent current conversion station 1,2..., n in described flexible direct current network respectively, subscript j=a, b, c, represent a, b, c three-phase respectively;

Described upper and lower brachium pontis reference voltage u _pij, u _nijobtain according to following formula

$\left\{\begin{array}{c}{u}_{pij}=\frac{1}{2}{U}_{dci}^{\′}-\frac{1}{2}{U}_{zi}-e_{vij}\\ u_{nij}=\frac{1}{2}{U}_{dci}^{\′}-\frac{1}{2}{U}_{zi}+e_{vij}\end{array}\right.$

Wherein, U' _dcifor current conversion station DC port voltage detecting value, e _vijfor the every required alternating voltage reference value exported mutually of MMC, U _zifor the negative level voltage that all current conversion station DC port except the corresponding current conversion station of fault location place circuit export;

(4) all described mechanical DC circuit breakers are by detecting the fault current obtained in circuit, if decay to zero, order performs step (5), otherwise proceed to detect;

(5) the mechanical DC circuit breaker that tripping zero failure current circuit is corresponding;

(6) wait for a period of time after all the other all current conversion stations described carry out cooperation control, the current conversion station that non-fault line is corresponding is switched to normal control mode and completes and pass through.

2. the method for claim 1, is characterized in that, in described step (3), and the reference instruction value of described direct fault current obtain according to following formula

$\left\{\begin{array}{c}\begin{array}{cc}{i}_{dci}^{*}=i_{dci\_rated}& (i\≠1)\end{array}\\ i_{dc1}^{*}=\underset{i=2}{\overset{k-1}{\Σ}}{i}_{dci\_rated}+\underset{i=k+1}{\overset{n}{\Σ}}{i}_{dci\_rated}\end{array}\right.$

Wherein, assuming that fault occurs in kth bar circuit, i _{dci_rated}for rated current when non-faulty line i normally works, wherein represent described for determining the reference instruction value of the direct fault current of the current conversion station of direct voltage.

3. method as claimed in claim 1 or 2, is characterized in that, in described step (3), and the negative level voltage U of the DC port output of all the other all current conversion stations described _ziobtain according to following formula

$U_{zi}=U_{dci}^{\′}-\[K_p(i_{dci}^{*}-i_{dci})+K_I\∫(i_{dci}^{*}-i_{dci})dt\]$

Wherein, U' _dcifor current conversion station DC port voltage detecting value, K _p, K _ibe respectively the PI parameter of fault current controllers.

4. a looped network shape flexible direct current network DC short trouble traversing method, described flexible direct current network comprises several current conversion station ports and several mechanical choppers, the wherein corresponding current conversion station of each described port and some circuits, circuit two ends described in every bar comprise a circuit breaker, during normal work, one of them current conversion station is used for determining direct voltage, all the other current conversion stations are used for determining active power, described current conversion station comprises A, B, C three-phase, often to be made up of upper and lower two brachium pontis, it is characterized in that, the method comprises the following steps:

(1) each current conversion station detects and judges that whether DC side is short-circuited fault, is that order performs step (2), otherwise proceeds detection;

(2) electric parameters that described each current conversion station DC port detects carries out fault location;

(3) in described flexible direct current network, described each current conversion station is switched to and determines active power controller, active power instruction is decided to be zero, reactive power instruction remains unchanged when relatively normally working, and obtains the every required alternating voltage reference value e exported mutually of MMC between age at failure according to meritorious and reactive power command calculations _vij;

By regulating described upper and lower brachium pontis output voltage, making direct-current short circuit fault current controlled and being reduced to zero;

(4) all described mechanical DC circuit breakers are by detecting the fault current obtained in circuit, if decay to zero, order performs step (5), otherwise proceed to detect;

(5) the mechanical DC circuit breaker at tripping zero failure current circuit two ends;

(6) wait for a period of time after described each current conversion station carries out cooperation control, the current conversion station that non-fault line is corresponding is switched to normal control mode and completes and pass through.