CN107069679B - A kind of symmetrical bipolar MMC DC side monopolar grounding fault passes through and restoration methods - Google Patents
A kind of symmetrical bipolar MMC DC side monopolar grounding fault passes through and restoration methods Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
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Abstract
It is passed through the invention proposes a kind of symmetrical bipolar MMC DC side monopolar grounding fault and restoration methods.The present invention by the cooperation of dc circuit breaker and inverter, realize to direct fault current cut-off and the recovery of power, protect the safe operation of inverter.Cooperate the active power shortage of converter station during reducing failure by the active power and reactive power that perfect pole and failure pole, while specified reactive power being provided to power grid, reduces impact of the failure to AC system；By the active control to the upper and lower bridge arm reference voltage common mode component of inverter, inverter is avoided because excessive current surge stress caused by unsuccessful reclosing gives system safety operation bring risk.Bipolar short trouble, which occurs, based on symmetrical bipolar MMC DC side can regard a kind of special circumstances that monopolar grounding fault occurs respectively for positive and negative anodes DC bus as, therefore the present invention can also be used in the processing that bipolar short trouble occurs for symmetrical bipolar MMC DC side.
Description
Technical field
The invention belongs to flexible high pressure DC transmission system DC error protection fields, more particularly, to a kind of symmetrical
Bipolar MMC DC side monopolar grounding fault passes through and restoration methods.
Background technique
The problems such as due to energy shortage and environmental protection, is increasingly prominent, the power generation of the novel renewable energies such as wind-force and solar energy
The ratio that amount accounts for total power generation gradually increases.In China, the energy such as wind energy, solar energy have focused largely on remote districts, cause
The location distribution of the energy and load is unbalanced, needs long range, large capacity conveying electric energy.It is being applied to over long distances, great Rong
When amount transmission of electricity, the cost of high-voltage AC transmission system and reactive loss increase sharply with the increase of transmission distance.Therefore, high pressure
HVDC Transmission Technology gradually causes the concern of domestic and foreign scholars, and is rapidly developed.
D.C. high voltage transmission HVDC based on voltage source type converter VSC (Voltage Source Converter)
(High-Voltage Direct-Current) technology includes without reactive compensation relative to Traditional DC technology of transmission of electricity, does not have
Commutation failure risk can be caused domestic and foreign scholars and widely be closed with advantages such as the active and reactive powers of separately adjustable system
Note.Compared to traditional two level and three-level topology structure, modularization multi-level converter MMC (Modular
Multilevel Converter) there is structure height modularization, be easy to extend, without multi-winding isolation transformer and output electricity
The advantages such as harmonic wave is low are pressed, are applied in practical projects, as the Transbay engineering in the U.S., five end of Zhoushan of China are flexible
DC transmission engineering.
Symmetrical bipolar connection plan is one of current main Main Wire Connection Scheme of MMC-HVDC system.Symmetrical bipolar MMC phase
Have many advantages, such as that the method for operation is flexible, transmission capacity is big, high reliablity, bipolar individually controllable than symmetrical monopolar MMC.It is symmetrical bipolar
For MMC in DC side positive and negative anodes neutral earthing, positive and negative anodes are relatively independent.When a pole of system is out of service because of failure, Ling Yiji
It remains to operate normally, and keeps the specified transmission capacity of half.The symmetrical bipolar DC transmission engineering currently to have put into operation has Xiamen
Flexible DC transmission engineering.
However, the protection of symmetrical bipolar MMC direct current side pole fault to ground is still the significant challenge in Practical Project.It is existing
MMC-HVDC system in mostly use that device is few, half-bridge submodule HBSM (Half-Bridge SM) of cost economic.Due to
HBSM does not have DC Line Fault self-cleaning ability, then system can generate excessive fault current.This not only can be to the device of inverter
Part damages, and can also generate biggish impact to AC system, influence the stability of AC system.In the prior art, it handles
Symmetrical bipolar MMC DC side monopolar grounding fault is mainly the following method:
1) using the submodule with DC Line Fault Scavenging activity.After monopolar grounding fault occurs for DC side, submodule
It is latched immediately.Establishing backward voltage by submodule capacitor forces fault current to be decayed rapidly.But the submodule that this method uses
Block needs additional switching device to make construction cost higher compared to half-bridge MMC, and running wastage is larger.During failure, the change of current
Device locking is big to the power rush of AC system.
2) high voltage DC breaker is used.When monopolar grounding fault occurs for DC side, behaviour is cut-off in dc circuit breaker execution
Make to cut-off direct fault current.In this way during failure, since failure is extremely out of service, system is caused to be transmitted
Rated active power has lost half, so that active power shortage is big.Meanwhile when dc circuit breaker attempts reclosing, if weight
It closes a floodgate and fails, then failure pole is equivalent to generation " second short circuit ", and huge surge current stress will endanger inverter safe operation.
In conclusion there are active when handling symmetrical bipolar MMC generation monopolar grounding fault using high voltage direct current short-circuiting device
Power shortage is big, surge current stress big problem when reclosing.
Summary of the invention
Aiming at the above defects or improvement requirements of the prior art, the present invention provides the events over the ground of symmetrical bipolar MMC direct current pole
The traversing method and recovery policy of barrier, it is intended to solve the prior art during direct current side pole fault to ground active power shortage greatly with
And surge current stress big problem when dc circuit breaker reclosing.
To achieve the above object, the present invention proposes that a kind of symmetrical bipolar MMC DC side monopolar grounding fault passes through and restores
Method.The DC transmission system that is applicable in of the present invention include comprising two connection transformers, two MMC and two DC bus,
Symmetrical bipolar MMC is in DC side positive and negative anodes neutral earthing.The MMC includes A, B, C three-phase, and every phase includes upper and lower two bridge arms,
Each bridge arm is concatenated a bridge arm inductance and is constituted by n half-bridge sub-module cascade.The half-bridge submodule is by two IGBT and two
A antiparallel diode connects a capacitor and constitutes, and the half-bridge submodule further includes the brilliant lock for being used for error protection
Pipe.Every DC bus concatenates a smoothing reactor and a dc circuit breaker；It is assumed that in the anode hair
Raw monopolar grounding fault, then claim just that extremely failure pole, cathode are to perfect pole.It is characterized by comprising the following steps:
(1) differentiate whether MMC DC side occurs monopolar grounding fault；
It is then to execute following movement: issues failure pole MMC block signal, make in all half-bridge submodules of failure pole MMC
IGBT is all off, to block the discharge path of the half-bridge submodule capacitor；The bypass thyristor for issuing half-bridge submodule is led
Messenger bypasses antiparallel diode in the half-bridge submodule, to protect anti-paralleled diode；It is disconnected to the direct current of failure pole
Road device issues open command, blocks the short circuit path of AC network；
Otherwise system keeps stable state control model；
The MMC refers to modularization multi-level converter；
(2) after bus direct fault current decays to zero, failure pole MMC unlocking signal is issued, failure pole is transferred to
STATCOM operational mode persistently provides reactive power support to exchange side；
(3) when DC bus goes free process to terminate, that is, restores dielectric level, to the dc circuit breaker on DC bus
Issue reclosing signal；
(4) whether the surge current stress of Judging fault pole DC bus is excessive；
It is to show that temporary fault is not removed, backs off signal, isolated fault to dc circuit breaker sending
Point；Go to step (5)；
Otherwise show that temporary fault is removed, issued to failure pole MMC and gradually restore power transmission signal, make its turn
Enter stable state control model；Turn to terminate；
(5) judge whether the dc circuit breaker is greater than presetting overcurrent number by the number of electric current overrate；
It is that MMC is made to keep STATCOM operational mode, direct-current short circuit device no longer carries out reclosing trial, turns to terminate；Otherwise it goes to step
(1)；Presetting overcurrent number takes 2~3 times.
The traversing method and recovery policy of symmetrical MMC monopolar grounding fault provided by the invention are by inverter and direct current
What breaker cooperation was realized.During failure, it is latched inverter, cut-offs dc circuit breaker to protect the peace of failure pole inverter
Row for the national games.Under the limitation for considering multi-constraint condition, matched by the active power and reactive power coordination that perfect pole and failure pole
It closes, overload is run on when making to perfect extremely short, reduce power shortage of the system during failure.Meanwhile run failure pole
Persistently to provide reactive power support to exchange side and reduce the impact to AC system in STATCOM state.By to MMC inverter
The active control of upper and lower bridge arm reference voltage common mode component, can effectively reduce the wave generated by dc circuit breaker unsuccessful reclosing
Gush current-rising-rate.
Preferably, the step (2) includes following sub-step:
(2-1) press according to converter power transformer capacity, MMC bridge arm through-current capability, half-bridge submodule, submodule capacitor voltage
Fluctuation and DC line overload capacity constraint condition, acquire the active power for perfecting pole MMC and reactive power operation region；
In the active power and reactive power operation region, adjustment perfects pole MMC stable operation operating point, it is made to run on mistake in short-term
It carries, with transmitting active power as much as possible, to make up the power shortage of monopolar grounding fault；
(2-2) transmits twice of rated reactive power signal during being emitted in failure to the failure pole MMC, make described strong
The required rated reactive power transmitted is undertaken full pole MMC by failure pole MMC in the steady state.
Preferably, the restrictive condition of converter power transformer capacity described in the step (2-1) are as follows:
The transmission capacity S of active-power P, reactive power Q and converter power transformer that inverter is transmittedmaxMeet:
P2+Q2< Smax 2；
Wherein, active-power P, reactive power Q are respectively the active and reactive power that MMC is flowed to from PCC, SmaxFor the change of current
The maximum transfer capacity of transformer.
Preferably, the restrictive condition of MMC bridge arm through-current capability described in the step (2-1) are as follows:
Each bridge arm current meets:
Wherein, IlimFor the maximum current capacity of IGBT each in submodule, UdcFor the DC bus-bar voltage of single MMC, Usm
For ac bus voltage magnitude；Rectification direction is positive direction.
Preferably, the restrictive condition that each half-bridge submodule is pressed in the step (2-1) are as follows:
The DC component of each bridge arm current is necessarily less than AC compounent；I.e. active-power P and reactive power Q meet:
Preferably, in each submodule voltage fluctuation of capacitor restrictive condition are as follows:
When MMC steady-state operation, the voltage fluctuation of capacitor of submodule is no more than the permitted maximum range, it may be assumed that
Wherein, N is submodule quantity, and C is submodule capacitance, UcFor submodule voltage rating, k is built-in potential modulation
Than submodule capacitor voltage fluctuates percentageIts maximum value is εmax.Inverter institute
The reactive power Q of transmissioncFor Q and exchange the difference of reactive power consumed by the equivalent inductance L of sideI.e.XLFor the induction reactance X for exchanging side equivalent inductanceL=ω L, ω are angular frequency.
Preferably, DC line overload capacity described in the step (2-1) meets:
DC current no more than DC line the short-time overload capacity limit, i.e.,
Wherein, idcFor DC bus current, idc_maxFor the DC line short-time overload capacity limit.
Preferably, after the step (2-1), step (2-2), following steps are also executed:
(2-3) is superimposed auxiliary voltage on the upper and lower bridge arm reference voltage of MMCMake the upper and lower bridge of MMC
Arm voltage are as follows:The superposition auxiliary voltage reduces the rising of surge current
Rate, to avoid inverter because excessive current surge stress caused by unsuccessful reclosing gives system safety operation bring risk；
Wherein, δ is PCC point built-in potential evRelative to ac bus voltage usPhase difference, idcFor failure DC bus electricity
Stream (rectification direction be positive direction),For DC current reference value, R (R > 0) is damped coefficient, up_upFor bridge arm voltage on MMC,
up_downFor MMC lower bridge arm voltage；If dc circuit breaker reclosing success, due to the presence of the flat wave reactance of direct current, idcIt will not go out
Existing surge current, therefore take DC current reference valueIf dc circuit breaker unsuccessful reclosing, surge current idcOn rapidly
It rises.
In the present invention, when dc circuit breaker unsuccessful reclosing, (i.e. active resistance can be inhibited by the active control of inverter
Buddhist nun control) system generate huge surge current stress.
In general, through the invention it is contemplated above technical scheme is compared with the prior art, have below beneficial to effect
Fruit:
Direct fault current is cut-off 1. being realized by the cooperation of dc circuit breaker and inverter, protects inverter
Safe operation.After Failure elimination, the foundation of DC voltage and the recovery of power are realized.
2. during failure, makes to perfect pole meeting converter power transformer capacity, bridge arm through-current capability, half-bridge submodule and press need
Ask, submodule capacitor voltage fluctuation and the several aspects of DC line overload capacity constraint condition under, run on short-time overload shape
State to make up the power shortage of monopolar grounding fault, while changes the transmission of failure pole with transmitting active power as much as possible
Whole rated reactive powers needed for stream station.Event is reduced by the active power and reactive power cooperation that perfect pole and failure pole
The active power shortage of converter station during barrier, while specified reactive power being provided to power grid, failure is reduced to AC system
Impact.
3., by the active control to the upper and lower bridge arm reference voltage common mode component of MMC, being effectively reduced during failure
The climbing of surge current avoids inverter because excessive current surge stress caused by unsuccessful reclosing gives system safety
Run bring risk.
4. since based on symmetrical bipolar MMC, in DC side positive and negative anodes neutral earthing, what bipolar short circuit occurred for DC side can
Regard a kind of special circumstances that monopolar grounding fault occurs respectively for positive and negative anodes DC bus as.Therefore, base proposed by the invention
It is passed through in the DC side monopolar grounding fault of symmetrical bipolar MMC and recovery policy is equally applicable to symmetrical bipolar MMC DC side
Bipolar short trouble occurs.
Detailed description of the invention
Fig. 1 is both-end symmetrically bipolar MMC flexible direct-current system topology schematic；
Fig. 2 is symmetrical bipolar half-bridge MMC topological structure and monopolar grounding fault schematic diagram；
Fig. 3 DC side monopolar grounding fault passes through and recovery policy flow chart；
Fig. 4 is surge current stress optimization control block diagram；
Active-power P and reactive power Q can range of operation schematic diagrames during Fig. 5 is failure；
Wherein, Fig. 5 (a) considers the active power and reactive power of the limitation of converter power transformer capacity and bridge arm through-current capability
It can range of operation schematic diagram；Fig. 5 (b) considers the active power of the limitation of converter power transformer capacity and submodule capacitor voltage fluctuation
It can range of operation schematic diagram with reactive power；Fig. 5 (c) considers the limitation of converter power transformer capacity and DC line overload capacity
Active power and reactive power can range of operation schematic diagrames；
Fig. 6 is to perfect pole and failure pole active reactive Power operation range schematic diagram under more limiting factors during failure；
Fig. 7 is to be not introduced into the symmetrical bipolar semi-bridge type MMC DC Line Fault that surge current stress optimization controls to pass through emulation knot
Fruit；Wherein, Fig. 7 (A) representing fault pole, Fig. 7 (B) representative perfect pole.
Wherein, Fig. 7 (A1) be failure pole DC bus-bar voltage (kV), Fig. 7 (A2) be failure pole DC bus current (kA),
Fig. 7 (A3) is that active and reactive power (MW/MVar), Fig. 7 (A4) of failure pole PCC point transmission are failure pole ac-side currents
(kA), it is the bridge arm current of failure pole six that Fig. 7 (A5), which is failure pole upper and lower bridge arm submodule capacitor voltage (kV), Fig. 7 (A6),
(kA)；Fig. 7 (B1) be perfect pole DC bus-bar voltage (kV), Fig. 7 (B2) is to perfect pole DC bus current (kA), Fig. 7 (B3)
It is that perfect the active of pole PCC point transmission with reactive power (MW/MVar), Fig. 7 (B4) be to perfect pole ac-side current (kA), Fig. 7
(B5) be perfect pole upper and lower bridge arm submodule capacitor voltage (kV), Fig. 7 (B6) is to perfect the bridge arm current (kA) of pole six；
Fig. 8 is that the symmetrical bipolar semi-bridge type MMC DC Line Fault of introducing surge current stress optimization control strategy passes through emulation
As a result.
Specific embodiment
In order to make the objectives, technical solutions, and advantages of the present invention clearer, with reference to the accompanying drawings and embodiments, right
The present invention is further elaborated.It should be appreciated that described herein, specific examples are only used to explain the present invention, not
For limiting the present invention.As long as in addition, technical characteristic involved in the various embodiments of the present invention described below that
Not constituting conflict between this can be combined with each other.
The present invention describes symmetrical bipolar MMC monopolar grounding fault by taking both-end symmetrically bipolar MMC as an example and passes through and recovery policy.
Fig. 1 is both-end symmetrically bipolar MMC system topology schematic, includes two converter stations.Each converter station is all made of symmetrical bipolar master
The mode of connection, as shown in Figure 2.Symmetrical bipolar MMC includes two connection transformers in DC side positive and negative anodes neutral earthing, and two
MMC and two DC bus.Every DC bus concatenates a smoothing reactor and a dc circuit breaker；Each MMC packet
Containing A, B, C three-phase, every phase includes upper and lower two bridge arms, and each bridge arm concatenates a bridge arm electricity by N number of half-bridge sub-module cascade
Sense is constituted.The topological structure schematic diagram of semi-bridge type MMC, as shown in Figure 3.Half-bridge submodule is antiparallel by two IGBT and two
One diode, connection capacitor are constituted, and the half-bridge submodule further includes the thyristor for being used for error protection.It is assumed that
Monopolar grounding fault occurs for the described anode, then claims just that extremely failure pole, cathode are to perfect pole.
Fault traversing of the present invention and recovery policy are related to cooperation such as Fig. 4 institute of inverter and dc circuit breaker
Show, specifically includes the following steps:
(1) differentiate whether MMC DC side occurs monopolar grounding fault, be, execute following movement: issuing failure pole MMC and close
Lock signal keeps IGBT in all half-bridge submodules of failure pole MMC all off, to block the electric discharge of the half-bridge submodule capacitor
Access；The bypass turn on thyristors signal for issuing half-bridge submodule, bypasses antiparallel diode in the half-bridge submodule, with
Protect anti-paralleled diode；Open command is issued to the dc circuit breaker of failure pole, blocks the short circuit path of AC network；Otherwise
System keeps stable state control model；The MMC refers to modularization multi-level converter；
(2) after bus direct fault current decays to zero, failure pole MMC unlocking signal is issued, failure pole is transferred to
STATCOM operational mode persistently provides reactive power support to exchange side；
(3) when DC bus, which goes free process, terminates (i.e. recovery dielectric level), to the direct current interruption on DC bus
Device issues reclosing signal；
(4) whether the surge current stress of Judging fault pole DC bus is excessive；It is to show that temporary fault is not clear
It removes, backs off signal, isolated fault point to dc circuit breaker sending；Go to step (5)；Otherwise show temporary fault
It is removed, is issued to failure pole MMC and gradually restore power transmission signal, it is made to be transferred to stable state control model；Turn to terminate；
(5) judge whether the dc circuit breaker is greater than presetting overcurrent number by the number of electric current overrate；
It is that MMC is made to keep STATCOM operational mode, direct-current short circuit device no longer carries out reclosing trial, turns to terminate；Otherwise it goes to step
(1)；Presetting overcurrent number takes 2~3 times.
As a kind of prioritization scheme, the step (2) includes following sub-step:
(2-1) press according to converter power transformer capacity, MMC bridge arm through-current capability, half-bridge submodule, submodule capacitor voltage
Fluctuation and DC line overload capacity constraint condition, acquire the active power for perfecting pole MMC and reactive power operation region；
In the active power and reactive power operation region, adjustment perfects pole MMC stable operation operating point, it is made to run on mistake in short-term
It carries, with transmitting active power as much as possible, to make up the power notch of monopolar grounding fault；
(2-2) transmits twice of rated reactive power signal during being emitted in failure to the failure pole MMC, make described strong
The required rated reactive power transmitted is undertaken full pole MMC by failure pole MMC in the steady state.
As a kind of prioritization scheme, the restrictive condition of converter power transformer capacity described in the step (2-1) are as follows:
The transmission capacity S of active-power P, reactive power Q and converter power transformer that inverter is transmittedmaxMeet:
P2+Q2< Smax 2；
Wherein, active-power P, reactive power Q are respectively from points of common connection PCC (Point of Common
Coupling the active and reactive power of MMC, S) are flowed tomaxFor the maximum transfer capacity of converter power transformer.
As a kind of prioritization scheme, the restrictive condition of MMC bridge arm through-current capability described in the step (2-1) are as follows:
Each bridge arm current meets:
Wherein, IlimFor the maximum current capacity of IGBT each in submodule, UdcFor the DC bus-bar voltage of single MMC, Usm
For ac bus voltage magnitude；Rectification direction is positive direction.(// note: power represents direction here, and the rectification direction direction that is positive is retouched
It is more accurate to state, write in this way it will be realized that.//)
As a kind of prioritization scheme, each half-bridge submodule is pressed in the step (2-1) restrictive condition are as follows:
The DC component of each bridge arm current is necessarily less than AC compounent；I.e. active-power P and reactive power Q meet:
As a kind of prioritization scheme, the restrictive condition of voltage fluctuation of capacitor in each submodule are as follows:
When MMC steady-state operation, the capacitance voltage wave rate of submodule is no more than the permitted maximum range, it may be assumed that
Wherein, N is submodule quantity, and C is submodule capacitance, UcFor submodule voltage rating, k is built-in potential modulation
Than submodule capacitor voltage fluctuates percentageIts maximum value is εmax.Inverter institute
The reactive power Q of transmissioncFor Q and exchange the difference of reactive power consumed by the equivalent inductance L of sideI.e.XLFor the induction reactance X for exchanging side equivalent inductanceL=ω L, ω are angular frequency.
As a kind of prioritization scheme, DC line overload capacity described in the step (2-1) meets:
DC current no more than DC line the short-time overload capacity limit, i.e.,
Wherein, idcFor DC bus current, idc_maxFor the DC line short-time overload capacity limit.
As a kind of prioritization scheme, after the step (2-1), step (2-2), following steps are also executed:
(2-3) is superimposed auxiliary voltage on the upper and lower bridge arm reference voltage of MMCMake the upper and lower bridge of MMC
Arm voltage are as follows:The superposition auxiliary voltage reduces the rising of surge current
Rate, to avoid inverter because excessive current surge stress caused by unsuccessful reclosing gives system safety operation bring risk；
Wherein, δ is PCC point built-in potential evRelative to ac bus voltage usPhase difference, idcFor failure DC bus electricity
Stream (rectification direction be positive direction),For DC current reference value, R (R > 0) is damped coefficient, up_upFor bridge arm voltage on MMC,
up_downFor MMC lower bridge arm voltage；If dc circuit breaker reclosing success, due to the presence of the flat wave reactance of direct current, idcIt will not go out
Existing surge current, therefore take DC current reference valueIf dc circuit breaker unsuccessful reclosing, surge current idcOn rapidly
It rises.
The traversing method and recovery policy of symmetrical bipolar MMC monopolar grounding fault provided by the invention are by MMC and straight
Flow what breaker cooperation was realized.During failure, MMC locking cut-offs dc circuit breaker to protect the peace of failure pole inverter
Row for the national games.Under the limitation for considering multi-constraint condition, matched by the active power and reactive power coordination that perfect pole and failure pole
It closes, overload is run on when making to perfect extremely short, reduce power shortage of the system during failure.Meanwhile run failure pole
Persistently to provide reactive power support to exchange side and reduce the impact to AC system in STATCOM state.By to MMC inverter
The active control of upper and lower bridge arm reference voltage common mode component, is effectively reduced the surge generated by dc circuit breaker unsuccessful reclosing
Current-rising-rate.
In order to enable those skilled in the art to better understand the present invention, combined with specific embodiments below to symmetric double of the invention
Pole semi-bridge type MMC monopolar grounding fault is passed through to be described in detail with recovery policy.
This example uses the data of Zhangbei County's flexible DC transmission engineering, and sub-modular structure is all made of the knot of semi-bridge type submodule
Structure, system parameter are as follows: DC bus voltage rating is 500kV, and rated power 1500MW, bridge arm inductance is 80mH, Mei Geqiao
Arm includes 218 submodule numbers, and submodule capacitor is 15mF, and submodule voltage rating is 2.294kV.Transformer uses Y0/ △
Bind mode, net side/valve side no-load voltage ratio (l-l, rms) are 525/260kV, and rated impedance per unit value is 15%, rated capacity
1700MVA.It is assumed that reactive power of each pole inverter to exchange side offer 450MVar, i.e., the stable state of each pole during stable state
Acceptable operating point is P (1500MW, -450MVar).The present invention is by taking failure occurs at positive direct-current bus as an example, such as Fig. 2.
Analyze MMC active power and reactive power operation regional scope: converter power transformer capacity, bridge arm through-current capability, half
Bridge submodule is pressed, submodule capacitor voltage fluctuates, DC line overload capacity is determined by following formula respectively: P2+Q2<
Smax 2、
Using overhead line as the symmetrical bipolar MMC DC power transmission line of both-end.After detecting monopolar grounding fault, failure
Pole MMC inverter is switched to fault traversing control model: failure pole inverter is latched immediately, and the bypass of half-bridge submodule is connected
Thyristor, while dc circuit breaker is immediately performed and cut-offs operation.After direct fault current decays to zero, failure pole inverter
Unlock, is transferred to the operational mode of STATCOM.
During failure, perfect pole short-time overload under conditions of considering more limiting factors, it is whole that converter station is transmitted in failure pole
Rated reactive power.Meanwhile during failure, auxiliary voltage u is superimposed on upper and lower bridge arm reference voltagez,Upper and lower bridge arm voltage are as follows:
When DC power transmission line goes free process to terminate, due to using open-wire transmission line, DC side monopolar grounding fault is more
For non-permanent failure, therefore this example has carried out voltage twice restores to attempt.If there is huge surge current stress, direct current is disconnected
Road device backs off, isolated fault point；If there is not huge surge current stress, failure pole MMC gradually restores power biography
It is defeated, finally it is transferred to stable state control model.Meanwhile judging DC bus current is more than whether DC bus current rated value number is big
In presetting overcurrent number, if so, MMC keeps current state, direct-current short circuit device no longer carries out reclosing trial；Otherwise direct current
Breaker backs off, and repeats above step.
It is pressed by converter power transformer capacity, bridge arm through-current capability, half-bridge submodule, submodule capacitor voltage fluctuation, direct current
The constraint condition of circuit overload ability can obtain the operation area of active power and reactive power, as shown in Figure 5.The result shows that: MMC
Active power and reactive power can range of operation and bridge arm through-current capability, submodule capacitor voltage fluctuation maximum value and AC line
Loading capability is passed by be positively correlated.Take the parameter of multiple limiting factors during failure are as follows: MMC bridge arm is through-flow, and maximum current capacity is
1.1pu, the maximum allowable stability bandwidth of submodule capacitor voltage are 1.1pu, and DC line overload capacity is 1.2pu, then can be obtained such as
Active power and reactive power shown in fig. 6 can operation area ranges (shadow region be can operation area), the results showed that direct current
During failure, perfecting pole be can run on operating point P2 (1700MW, 0MVar), failure pole can run on operating point P1 (0MW ,-
900MVar).MMC can provide the active power of 1700MW and the reactive power of 900MVar during failure.The fault traversing phase
Between, by perfecting the active power of pole and failure pole and the cooperation of reactive power, can not only continue to provide to exchange side
Nominal reactive support, and can overcome the disadvantages that the power shortage of failure pole 11.76%.
Assuming that ground short circuit failure, the fault clearance after 450ms occur in 1s for positive direct-current bus.Fig. 7 is based on symmetrical
The semi-bridge type MMC simulation result of bipolar main electrical scheme, (A) indicate failure pole, and (B) expression perfects pole.Failure pole DC bus current
As shown in Fig. 7 (A2), the results showed that after dc circuit breaker cut-offs DC Line Fault, fault current decays to zero.Perfect pole and failure
Shown in active power and reactive power such as Fig. 7 (A3) and Fig. 7 (B3) of pole, the results showed that inverter can mention during failure
The reactive power of active power and 900MVar for 1700MW.By perfecting pole and failure pole during demonstrating fault traversing
The active cooperation with reactive power can continue to support to exchange side and provide nominal reactive, and can overcome the disadvantages that failure pole 11.76%
The conclusion of power shortage.Perfect pole upper and lower bridge arm submodule capacitor voltage and six bridge arm currents such as Fig. 7 (B5) and Fig. 7 (B6) institute
Show, the results showed that when perfecting pole inverter and running on short-time overload state, it is specified that submodule capacitor voltage, which fluctuates percentage,
1.11 times of value, MMC bridge arm maximum current are 1.104 times of rated value, are coincide substantially with theory analysis.Perfect pole and failure pole
Ac-side current such as Fig. 7 (A4) and Fig. 7 (B4) shown in, the results showed that entire fault traversing and restore control, smoothly, quickly,
Without impact.
It after dc circuit breaker isolated DC failure, goes to dissociate through the 300ms time, then carries out reclosing.Due at this point, straight
Stream failure is not yet removed, therefore can generate huge surge current stress.As a result as shown in Fig. 7 (A).The result shows that: if
System failure unsuccessful reclosing, if being added without surge current Optimal Control Strategy, voltage step and current surge current stress are big；
Meanwhile power and the bridge arm voltage fluctuation of system are also very big, bring risk to the safe operation of system.
After introducing surge current optimal control, for different damped coefficient R, the surge current stress of system is carried out
Contrast simulation.Simulation result is as shown in Figure 8.The result shows that: with the increase of damped coefficient R, dc circuit breaker loses in reclosing
Surge current stress caused by after losing is gradually reduced, this sufficiently demonstrates the validity of proposed optimisation strategy.However, into one
Step it can be found that with damped coefficient R increase, the inhibitory effect of surge current is gradually decreased.This is because semi-bridge type MMC
Bridge arm voltage output area are as follows: 0≤uarm≤Udc.With the increase of damped coefficient R, auxiliary voltage uzIt is gradually increased, so that
The reference voltage of upper and lower bridge arm reaches the limit of fan-out capability, so that the inhibitory effect to surge current reduces.
Dc circuit breaker detects after surge current isolated DC failure again.Again pass by 300ms goes the free time
Afterwards, it attempts to carry out reclosing for second of dc circuit breaker, final system is successfully recovered.
As it will be easily appreciated by one skilled in the art that the foregoing is merely illustrative of the preferred embodiments of the present invention, not to
The limitation present invention, any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should all include
Within protection scope of the present invention.
Claims (8)
1. a kind of symmetrical bipolar MMC DC side monopolar grounding fault passes through and restoration methods, which is characterized in that including walking as follows
It is rapid:
(1) differentiate whether MMC DC side occurs monopolar grounding fault:
It is then to execute following movement: issues failure pole MMC block signal, keep IGBT in all half-bridge submodules of failure pole MMC complete
Portion's shutdown, to block the discharge path of the half-bridge submodule capacitor；The bypass turn on thyristors signal of half-bridge submodule is issued,
Antiparallel diode in the half-bridge submodule is bypassed, to protect anti-paralleled diode；To the dc circuit breaker hair of failure pole
Open command out blocks the short circuit path of AC network；The above movement is in no particular order；The MMC refers to the modular multilevel change of current
One pole of DC side ground fault extremely occurs for device, the failure；Go to step (2)；
Otherwise system keeps stable state control model, goes to step (6)；
(2) after bus direct fault current decays to zero, failure pole MMC unlocking signal is issued, failure pole MMC is made to be transferred to nothing
Function compensates STATCOM (Static Synchronous Compensator) operational mode, persistently provides idle branch to exchange side
Support；
(3) when failure pole, DC bus goes free process to terminate, that is, when restoring dielectric level, to the direct current interruption on DC bus
Device issues reclosing signal；
(4) whether the surge current stress on the DC bus of Judging fault pole is excessive；
It is to show that failure is not removed, the dc circuit breaker sending on the DC bus of the failure pole backs off signal,
Isolated fault point；Go to step (5)；
Otherwise show that temporary fault is removed, issued to failure pole MMC and gradually restore power transmission signal, be transferred to it surely
State control model goes to step (6)；
(5) judge whether the dc circuit breaker is greater than default overcurrent number by the number of electric current overrate；It is to make
MMC keeps STATCOM operational mode, and direct-current short circuit device no longer carries out reclosing trial, goes to step (6)；Otherwise (1) is gone to step；
The presetting overcurrent number takes 2~3 times；
(6) terminate.
2. fault traversing according to claim 1 and restoration methods, which is characterized in that the step (2) includes following son
Step:
(2-1) is pressed according to converter power transformer capacity, MMC bridge arm through-current capability, half-bridge submodule, submodule capacitor voltage fluctuates
With DC line overload capacity constraint condition, active power and the reactive power operation region for perfecting pole MMC are determined；It is active at this
In power and reactive power operation region, adjustment perfects pole MMC stable operation operating point, it is made to run on short-time overload, with to the greatest extent
Transmitting active power more than possible, to make up power shortage caused by monopolar grounding fault；
(2-2) transmits twice of rated reactive power signal during being emitted in failure to the failure pole MMC, make described to perfect pole
The required rated reactive power transmitted is undertaken MMC by failure pole MMC in the steady state.
3. fault traversing according to claim 2 and restoration methods, which is characterized in that changed described in the step (2-1)
The restrictive condition of convertor transformer capacity are as follows:
The transmission capacity S of active-power P, reactive power Q and converter power transformer that inverter is transmittedmaxMeet: P2+Q2<
Smax 2；
Wherein, active-power P, reactive power Q are respectively to flow from points of common connection PCC (Point of Common Coupling)
To the active and reactive power of MMC, SmaxFor the maximum transfer capacity of converter power transformer.
4. fault traversing according to claim 2 and restoration methods, which is characterized in that MMC described in the step (2-1)
The restrictive condition of bridge arm through-current capability are as follows:
Each bridge arm current meets:
Wherein, IlimFor the maximum current capacity of IGBT each in submodule, UdcFor the DC bus-bar voltage of single MMC, UsmFor exchange
Bus voltage amplitude；Rectification direction is positive direction.
5. fault traversing according to claim 2 and restoration methods, which is characterized in that each half-bridge in the step (2-1)
The restrictive condition that submodule is pressed are as follows:
The DC component of each bridge arm current is necessarily less than AC compounent；I.e. active-power P and reactive power Q meet:Wherein, UdcFor the DC bus-bar voltage of single MMC, UsmFor ac bus voltage magnitude；Rectification square
To for positive direction.
6. fault traversing according to claim 2 and restoration methods, which is characterized in that voltage fluctuation of capacitor in each submodule
Restrictive condition are as follows:
When MMC steady-state operation, the voltage fluctuation of capacitor of submodule is no more than the permitted maximum range, it may be assumed that
Wherein, N is submodule quantity, and C is submodule capacitance, UcFor submodule voltage rating, k is built-in potential modulation ratio, submodule
Block voltage fluctuation of capacitor percentageIts maximum value is εmax；The nothing that inverter is transmitted
Function power QcFor Q and exchange the difference of reactive power consumed by the equivalent inductance L of sideI.e.XLFor the induction reactance X for exchanging side equivalent inductanceL=ω L, ω are angular frequency；UsmFor ac bus electricity
Pressure amplitude value；Rectification direction is positive direction.
7. fault traversing according to claim 2 and restoration methods, which is characterized in that straight described in the step (2-1)
Flow Line overload capacity meets:
DC current no more than DC line the short-time overload capacity limit, i.e.,
Wherein, idcFor DC bus current, idc_maxFor the DC line short-time overload capacity limit；UdcIt is female for the direct current of single MMC
Line voltage.
8. fault traversing according to claim 2 and restoration methods, which is characterized in that the step (2-1), step (2-
2) after, following steps are also executed:
(2-3) is superimposed auxiliary voltage on the upper and lower bridge arm reference voltage of MMCMake the upper and lower bridge arm voltage of MMC
Are as follows:The superposition auxiliary voltage reduces the climbing of surge current, with
Inverter is avoided to give system safety operation bring risk because of excessive current surge stress caused by unsuccessful reclosing；
Wherein, UdcFor the DC bus-bar voltage of single MMC, δ is PCC point built-in potential evRelative to ac bus voltage usPhase
Difference, idcFor failure DC bus current (rectification direction be positive direction),For DC current reference value, R (R > 0) is damping system
Number, up_upFor bridge arm voltage on MMC, up_downFor MMC lower bridge arm voltage；If dc circuit breaker reclosing success, due to direct current
The presence of flat wave reactance, idcBe not in surge current, therefore take DC current reference valueIf dc circuit breaker reclosing
Fail, then surge current idcIt rises rapidly.
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