CN111654011A - Direct-current fault clearing method for MMC asymmetric capacitance clamping submodule - Google Patents

Direct-current fault clearing method for MMC asymmetric capacitance clamping submodule Download PDF

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CN111654011A
CN111654011A CN202010432579.4A CN202010432579A CN111654011A CN 111654011 A CN111654011 A CN 111654011A CN 202010432579 A CN202010432579 A CN 202010432579A CN 111654011 A CN111654011 A CN 111654011A
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mmc
submodule
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CN111654011B (en
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曾琦
王渝红
刘进飞
凌楠
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Sichuan University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
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Abstract

The invention discloses a direct current fault clearing method of an MMC asymmetric capacitance clamping submodule, which comprises the following steps: providing a novel topological structure of an MMC asymmetric capacitance clamping submodule; performing direct-current fault characteristic analysis on the topological structure of the novel MMC asymmetric capacitance clamping submodule; judging the direct-current fault clearing capacity of the topological structure of the novel MMC asymmetric capacitance clamping submodule according to the result of the direct-current fault characteristic analysis; and verifying the direct-current fault clearing capacity of the topological structure of the novel MMC asymmetric capacitance clamping submodule. The invention utilizes the capacitance clamping function of the ACSM submodule and the reverse cut-off characteristic of the diode, and has the capability of clearing direct current faults; the MMC formed by the improved sub-modules can cut off the fault current on the direct current side of the system relatively quickly.

Description

Direct-current fault clearing method for MMC asymmetric capacitance clamping submodule
Technical Field
The invention belongs to the technical field of flexible direct current engineering, and particularly relates to a direct current fault clearing method of an MMC asymmetric capacitance clamping submodule.
Background
High Voltage Direct Current (HVDC) transmission technology using a Voltage Source Converter (VSC) provides reliable guarantee for realizing large-scale renewable energy grid connection, asynchronous ac network interconnection and island power transmission. Compared with a traditional two-level converter and a diode-clamped three-level converter, the Modular Multilevel Converter (MMC) has the advantages of modular design, lower switching frequency, excellent output waveform and expandability, can easily realize a multilevel technology, shows a good development prospect in the application of high power and high voltage directions, and is deeply researched and widely applied in recent years. A sub-module (SM) is an MMC basic component, and a half-bridge sub-module (HBSM) topology is often adopted. The structure is applied to Zhang Bei flexible direct current engineering, flexible direct current engineering at two ends of a building door, flexible direct current engineering at five ends of a boat and a mountain and the like.
However, MMCs based on half-bridge sub-modules do not have dc-side fault self-blocking capability, it must rely on dc breakers to isolate the dc fault, otherwise the converter will be subjected to high fault currents from the coupling system. Even if the converter is locked out, the freewheeling diode can cause the fault current to continue to harm the device components of the MMC. Due to the fact that damping of the flexible direct current system is low, fault current can reach a high peak value in a short time, temporary overcurrent of an alternating current side system is easily caused, and the fault current can bring continuous impact to the system.
Disclosure of Invention
The invention aims to provide a direct-current fault clearing method for an MMC asymmetric capacitance clamping submodule, which is used for solving one of the technical problems in the prior art, such as: in the prior art, MMCs based on half-bridge sub-modules do not have dc-side fault self-blocking capability, and must rely on dc breakers to isolate the dc fault, otherwise the converter will be subjected to high fault currents from the coupling system. Even if the converter is locked out, the freewheeling diode can cause the fault current to continue to harm the device components of the MMC. Due to the fact that damping of the flexible direct current system is low, fault current can reach a high peak value in a short time, temporary overcurrent of an alternating current side system is easily caused, and the fault current can bring continuous impact to the system. An improved novel submodule is provided, and the submodule has the direct-current side fault asymmetric blocking capacity. The topological structure of the submodule is designed and the operation characteristics of the submodule are researched; the direct current bipolar short-circuit fault response characteristic and the device voltage withstanding situation of the submodule are analyzed in detail, and the submodule is compared with other typical improved submodules in terms of economy. Finally, based on a PSCAD/EMTDC simulation platform, the improved sub-module topology provided by the method is verified to be capable of effectively achieving the system direct-current short circuit fault blocking function.
In order to achieve the purpose, the technical scheme of the invention is as follows:
the direct current fault clearing method of the MMC asymmetric capacitance clamping submodule comprises the following steps:
s1: providing a novel topological structure of an MMC asymmetric capacitance clamping submodule;
s2: on the basis of the step S1, performing dc fault characteristic analysis on the topological structure of the novel MMC asymmetric capacitance clamping submodule;
s3: on the basis of the step S2, determining the dc fault clearing capability of the topology structure of the novel MMC asymmetric capacitance clamping submodule according to the result of the dc fault characteristic analysis;
s4: and on the basis of the step S3, verifying the dc fault clearing capability of the topology structure of the novel MMC asymmetric capacitance clamping submodule.
Further, in the above-mentioned case,
in step S1, the topology of the novel MMC asymmetric capacitance clamping submodule is as follows:
the high-voltage power supply comprises a first insulated gate bipolar transistor T1, a second insulated gate bipolar transistor T2, a third insulated gate bipolar transistor T3, a fourth insulated gate bipolar transistor T4, a fifth insulated gate bipolar transistor T5, a first current-limiting diode D1, a second current-limiting diode D2, a third current-limiting diode D3, a fourth current-limiting diode D4, a fifth current-limiting diode D5, a sixth current-limiting diode D6, a first clamping capacitor C1 and a second clamping capacitor C2; wherein, a first insulated gate bipolar transistor T1 is connected with a first current-resisting diode D1 in parallel, a second insulated gate bipolar transistor T2 is connected with a second current-resisting diode D2 in parallel, a third insulated gate bipolar transistor T3 is connected with a third current-resisting diode D3 in parallel, a fourth insulated gate bipolar transistor T4 is connected with a fourth current-resisting diode D4 in parallel, a fifth insulated gate bipolar transistor T5 is connected with a fifth current-resisting diode D5 in parallel, an emitter of the first insulated gate bipolar transistor T1 is connected with a collector of the second insulated gate bipolar transistor T2, a collector of the first insulated gate bipolar transistor T1 is connected with an anode of a first clamping capacitor C1, a cathode of the first clamping capacitor C1 is connected with a collector of the third insulated gate bipolar transistor T3, an anode of a second clamping capacitor C2 and a collector of the fourth insulated gate bipolar transistor T4 respectively, an emitter of the fourth insulated gate bipolar transistor T4 is connected with a collector of the fifth insulated gate bipolar transistor T5, the emitter of the second insulated gate bipolar transistor T2 is connected to the emitter of the third insulated gate bipolar transistor T3, and the emitter of the second insulated gate bipolar transistor T2 is further connected to the cathode of the second clamping capacitor C2 and the emitter of the fifth insulated gate bipolar transistor T5 through a sixth current-blocking diode D6 connected in series in an opposite direction.
Further, the novel MMC asymmetric capacitor clamping submodule topological structure is according to T1、T2、T3、T4、T5Can output 0 and U by alternatively switching on and offCAnd 2UCThree levels; wherein, in steady state operation, if T1And T4Closed, T2、T3And T5Turn-off, submodule voltage USM＝UC(ii) a If T1And T5Closed, T2、T3And T4Turn-off, submodule voltage USM＝2UC(ii) a If T2、T3And T5Closed, T1And T4Turn-off, submodule voltage USM＝UC(ii) a If T2、T3And T4Closed, T1And T5Turn-off, submodule voltage USM＝0；
When sending outAfter the short-circuit fault of the direct current side occurs, the converter is locked, the reverse voltage of the capacitor blocks the fault current fed to the direct current fault point from the alternating current side, and when I is detectedSMWhen > 0, the capacitance C1、C2Connected in series to the fault loop, sub-module voltage USM＝2UC(ii) a When I isSMWhen < 0, the capacitance C2Providing a reverse voltage in the fault loop, the submodule voltage USM＝UC。
Further, the direct current faults of the novel MMC asymmetric capacitance clamping submodule topological structure comprise a bipolar short circuit fault, a unipolar ground fault and a direct current disconnection fault.
Further, when the topological structure of the novel MMC asymmetric capacitance clamping submodule has a direct-current bipolar short-circuit fault, and a current converter is locked;
when bipolar short circuit fault takes place in gentle straight system direct current side overhead line's the twinkling of an eye, the novel MMC asymmetric capacitance clamping submodule piece electric capacity and the system alternating current end that are just investing can be before system shutting simultaneously to short-circuit point feed-in current, wherein the former is far more than the latter, most fault current has been contributed, before the transverter does not block, novel MMC asymmetric capacitance clamping submodule piece still carries out the switching according to normal modulation strategy, electric capacity and fault point form the circuit that discharges, fault current increases rapidly, the single-phase equivalent circuit in fault circuit can be expressed as following differential equation:
Figure BDA0002501096580000031
wherein C is1And C2And 2C/n, R is equivalent resistance of bridge arm resistance, fault point resistance and device on-off resistance, R is equivalent resistanceaIs a bridge arm resistance, LaIs bridge arm inductance, L is current-limiting reactance, RfAs a fault point resistance, C1、C2Equivalent capacitances R of the upper and lower bridge arms, respectivelysIs an AC equivalent resistance, LsIs an alternating current equivalent reactance; if it is a metallic short-circuit fault, R2Much less than 2n (2L)a+ L)/C, the discharge process is a second-order under-damped attenuation process, and the capacitance voltage expression is
Figure BDA0002501096580000041
The short-circuit current direct current quantity is expressed as
Figure BDA0002501096580000042
Wherein, U0For fault transient DC line voltage, I0For direct line current, τ1Is the equivalent loop time constant, omega, before system lock-up0Is the loop natural angular frequency, omegafIs the angular frequency of the loop oscillation, theta is the initial phase angle of the capacitor voltage, β is the initial phase angle of the bridge arm current, as follows
Figure BDA0002501096580000043
Before the converter is locked, the AC side equivalently generates three-phase short circuit and is provided with a phase-a power supply voltage ua＝Usin(ωst)，ωsAt a power frequency angular frequency, the a AC current is
Figure BDA0002501096580000044
Wherein I is U/[ (R)s+R)2+ωs 2(Ls+L+2La)2]1/2,
Figure BDA0002501096580000045
In this stage, the short-circuit current of the bridge arm is the superposition of the DC component and the AC component, and the upper and lower bridge arm currents are respectively
Figure BDA0002501096580000046
Further, when the topological structure of the novel MMC asymmetric capacitance clamping submodule has a direct-current bipolar short-circuit fault, and the current converter is locked;
when a short-circuit fault occurs, the system is locked, a discharge energy feedback path of a novel MMC asymmetric capacitance clamping submodule capacitor is cut off, but a bridge arm inductor can feed energy to a fault point through a freewheeling diode, and at the moment, a short-circuit point current mainly comprises the energy feedback of the bridge arm inductor and an alternating-current side feed-in current; after being locked, the bridge arm of the novel MMC asymmetric capacitance clamping submodule is equivalent to an RLC equivalent loop formed by a diode, a capacitor series, a loop resistor and a loop inductor, wherein the loop charged capacitor provides reverse voltage to enable short-circuit current ifWhen the current of the fault loop is zero, the reverse voltage provided by the capacitor borne by the diode is in a cut-off state, and the fault loop is cut off; when the voltage at the alternating current side is larger than the reverse voltage provided by all the capacitors of the bridge arm, the alternating current source continues to feed energy to the short-circuit point;
Ceqthe equivalent charged capacitor u is formed by connecting an upper bridge arm and a lower bridge arm of a novel MMC asymmetric capacitor clamping submodule in series to an equivalent fault loopeqFor its equivalent reverse blocking voltage, LeqIs a loop equivalent inductor and mainly comprises a bridge arm inductor, a current-limiting reactance and a line reactance ReqThe circuit equivalent resistance is the bridge arm resistance, the short circuit point resistance and the line resistance; let t 00, the initial state of the equivalent circuit and the RLC second order differential equation can be listed as follows
Figure BDA0002501096580000051
Figure BDA0002501096580000052
In the formula of Ueq0、If0Respectively setting the initial value of the equivalent capacitor voltage of the loop and the initial value when the short-circuit current is locked;
due to R in the systemeqUsually much less than 2 (L)eq/Ceq)1/2The differential equation is solved to obtain the short-circuit current and capacitance voltage expression as
Figure BDA0002501096580000053
Wherein, tau2Is the equivalent loop time constant, omega, after system lock-up0Is the natural angular frequency of the loop, omega is the angular frequency of the oscillation of the loop,
Figure BDA0002501096580000054
for the initial phase angle of the short circuit current, η is the initial phase angle of the capacitor voltage, which is as follows
Figure BDA0002501096580000061
The analytical formula (9) can be obtained, after the converter is locked, the short-circuit current is reduced due to the attenuation of the energy storage of the bridge arm inductor, and the back-pressure electromotive force is established for the capacitance charging of the novel MMC asymmetric capacitance clamping submodule on the other hand, so that the rapid attenuation of the fault current is realized after the converter is locked.
Compared with the prior art, the invention has the beneficial effects that:
the invention provides an MMC asymmetric capacitance clamping submodule topological structure capable of blocking direct current fault current, which utilizes the capacitance clamping function of an ACSM submodule and the reverse cut-off characteristic of a diode and has the capability of clearing direct current fault; the MMC formed by the improved sub-modules can cut off the fault current on the direct current side of the system relatively quickly. Meanwhile, the improved sub-module uses fewer fully-controlled power electronic devices, and the control method is simple and reliable and has better expansibility. And finally, the DC interelectrode short-circuit fault is verified on simulation software, and a simulation result shows that the ACSM can rapidly block the fault current on the DC side of the system after the DC fault occurs, so that the action of an AC circuit breaker is avoided, and the safety of the converter equipment is improved.
Drawings
Fig. 1 is a schematic diagram of a topology of a novel MMC asymmetric capacitance clamping submodule according to an embodiment of the present invention.
FIG. 2 is a schematic diagram of the current path of the locked asymmetric capacitor clamping submodule of the MMC in accordance with an embodiment of the present invention.
Fig. 3 is a schematic diagram of a single-phase equivalent circuit after a short-circuit fault according to an embodiment of the present invention.
Fig. 4 is a schematic diagram of a single-phase equivalent circuit after latching according to an embodiment of the present invention.
Fig. 5 is a schematic diagram of simulation waveforms of the ACSM-MMC dc side according to an embodiment of the present invention.
Fig. 6 is a schematic diagram of simulation waveforms of ac side of ACSM-MMC in accordance with an embodiment of the present invention.
FIG. 7 is a schematic flow chart of steps in accordance with an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to fig. 1 to 7 of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example (b):
as shown in fig. 7, the method for clearing the dc fault of the MMC asymmetric capacitor clamping submodule includes the following steps:
s1: providing a novel topological structure of an MMC asymmetric capacitance clamping submodule;
s2: on the basis of the step S1, performing dc fault characteristic analysis on the topological structure of the novel MMC asymmetric capacitance clamping submodule;
s3: on the basis of the step S2, determining the dc fault clearing capability of the topology structure of the novel MMC asymmetric capacitance clamping submodule according to the result of the dc fault characteristic analysis;
s4: and on the basis of the step S3, verifying the dc fault clearing capability of the topology structure of the novel MMC asymmetric capacitance clamping submodule.
Further, in the above-mentioned case,
in step S1, as shown in fig. 1, the topology of the novel MMC asymmetric capacitor clamping submodule is as follows:
the high-voltage power supply comprises a first insulated gate bipolar transistor T1, a second insulated gate bipolar transistor T2, a third insulated gate bipolar transistor T3, a fourth insulated gate bipolar transistor T4, a fifth insulated gate bipolar transistor T5, a first current-limiting diode D1, a second current-limiting diode D2, a third current-limiting diode D3, a fourth current-limiting diode D4, a fifth current-limiting diode D5, a sixth current-limiting diode D6, a first clamping capacitor C1 and a second clamping capacitor C2; wherein, a first insulated gate bipolar transistor T1 is connected with a first current-resisting diode D1 in parallel, a second insulated gate bipolar transistor T2 is connected with a second current-resisting diode D2 in parallel, a third insulated gate bipolar transistor T3 is connected with a third current-resisting diode D3 in parallel, a fourth insulated gate bipolar transistor T4 is connected with a fourth current-resisting diode D4 in parallel, a fifth insulated gate bipolar transistor T5 is connected with a fifth current-resisting diode D5 in parallel, an emitter of the first insulated gate bipolar transistor T1 is connected with a collector of the second insulated gate bipolar transistor T2, a collector of the first insulated gate bipolar transistor T1 is connected with an anode of a first clamping capacitor C1, a cathode of the first clamping capacitor C1 is connected with a collector of the third insulated gate bipolar transistor T3, an anode of a second clamping capacitor C2 and a collector of the fourth insulated gate bipolar transistor T4 respectively, an emitter of the fourth insulated gate bipolar transistor T4 is connected with a collector of the fifth insulated gate bipolar transistor T5, the emitter of the second insulated gate bipolar transistor T2 is connected to the emitter of the third insulated gate bipolar transistor T3, and the emitter of the second insulated gate bipolar transistor T2 is further connected to the cathode of the second clamping capacitor C2 and the emitter of the fifth insulated gate bipolar transistor T5 through a sixth current-blocking diode D6 connected in series in an opposite direction.
Wherein, USMFor the output voltage of the submodule, ISMFor sub-module current, wherein C in ACSM (New MMC asymmetric capacitance clamping sub-module)1And C2The capacitance values are the same and are all C, and the capacitance voltage keeps balance under the control of the valve level. ACSM has 2 operating modes according to the switching state of an Insulated Gate Bipolar Transistor (IGBT): a normal operating mode and a lock-out mode. In normal operation, ACSM is according to T1、T2、T3、T4、T5The sub-modules can output 0 and UCAnd 2UCThree levels ensure the constancy of the direct voltage and the stability of the direct power. The relationship between the ACSM working state and the IGBT on-off condition is shown in table 1, where switches 1 and 0 represent the IGBT on-off, respectively.
TABLE 1 working states of asymmetrical capacitive clamping sub-modules
Table 1Operating status of ACSM
Figure BDA0002501096580000081
Further, the novel MMC asymmetric capacitor clamping submodule topological structure is according to T1、T2、T3、T4、T5Can output 0 and U by alternatively switching on and offCAnd 2UCThree levels; wherein, in steady state operation, if T1And T4Closed, T2、T3And T5Turn-off, submodule voltage USM＝UC(ii) a If T1And T5Closed, T2、T3And T4Turn off, submodule voltage USM 2UC(ii) a If T2、T3And T5Closed, T1And T4Turn-off, submodule voltage USM＝UC(ii) a If T2、T3And T4Closed, T1And T5Turn-off, submodule voltage USM＝0；
When a short-circuit fault occurs on the direct current side, the converter is locked, the reverse voltage of the capacitor blocks the fault current fed to the direct current fault point from the alternating current side, and the current flow path of the ACSM after the system is locked is shown in fig. 2. When I isSMWhen > 0, the capacitance C1、C2Connected in series to the fault loop, sub-module voltage USM＝2UC(ii) a When I isSMWhen < 0, the capacitance C2Providing a reverse voltage in the fault loop, the submodule voltage USM＝UC。
Further, the direct current faults of the novel MMC asymmetric capacitance clamping submodule topological structure comprise a bipolar short circuit fault, a unipolar ground fault and a direct current disconnection fault.
Further, when the topological structure of the novel MMC asymmetric capacitance clamping submodule has a direct-current bipolar short-circuit fault, and a current converter is locked;
when a bipolar short-circuit fault occurs in the moment of a flexible direct-current side overhead line of a system, the capacitance of a novel MMC asymmetric capacitance clamping submodule which is just put into use and the alternating-current end of the system can feed current into a short-circuit point before the system is locked, wherein the capacitance of the novel MMC asymmetric capacitance clamping submodule is much larger than the alternating-current end of the system, most of fault current is contributed, before a current converter is not locked, the novel MMC asymmetric capacitance clamping submodule is still switched according to a normal modulation strategy, the capacitance and the fault point form a discharge circuit, the fault current is rapidly increased, a single-phase equivalent circuit of the fault circuit is shown in figure 3, and R in the figureaIs a bridge arm resistance, LaIs bridge arm inductance, L is current-limiting reactance, RfAs a fault point resistance, C1、C2Equivalent capacitances R of the upper and lower bridge arms, respectivelysIs an AC equivalent resistance, LsIs an ac equivalent reactance. Can be expressed as the following differential equation:
Figure BDA0002501096580000091
wherein C is1And C2And 2C/n, R is equivalent resistance of bridge arm resistance, fault point resistance and device on-off resistance, R is equivalent resistanceaIs a bridge arm resistance, LaIs bridge arm inductance, L is current-limiting reactance, RfAs a fault point resistance, C1、C2Equivalent capacitances R of the upper and lower bridge arms, respectivelysIs an AC equivalent resistance, LsIs an alternating current equivalent reactance; if it is a metallic short-circuit fault, R2Much less than 2n (2L)a+ L)/C, the discharge process is a second-order under-damped attenuation process, and the capacitance voltage expression is
Figure BDA0002501096580000092
The short-circuit current direct current quantity is expressed as
Figure BDA0002501096580000093
Wherein, U0For fault transient DC line voltage, I0For direct line current, τ1Is the equivalent loop time constant, omega, before system lock-up0Is the loop natural angular frequency, omegafIs the angular frequency of the loop oscillation, theta is the initial phase angle of the capacitor voltage, β is the initial phase angle of the bridge arm current, as follows
Figure BDA0002501096580000101
Before the converter is locked, the AC side equivalently generates three-phase short circuit and is provided with a phase-a power supply voltage ua＝Usin(ωst)，ωsAt a power frequency angular frequency, the a AC current is
Figure BDA0002501096580000102
Wherein I is U/[ (R)s+R)2+ωs 2(Ls+L+2La)2]1/2,
Figure BDA0002501096580000103
In this stage, the short-circuit current of the bridge arm is the superposition of the DC component and the AC component, and the upper and lower bridge arm currents are respectively
Figure BDA0002501096580000104
Further, when the topological structure of the novel MMC asymmetric capacitance clamping submodule has a direct-current bipolar short-circuit fault, and the current converter is locked;
when a short-circuit fault occurs, the system is locked, the discharge energy feedback path of the novel MMC asymmetric capacitance clamping submodule capacitor is cut off, but the bridge arm inductor can feed energy to a fault point through a freewheeling diode, and the current of the short-circuit point at the momentThe bridge arm inductor mainly comprises energy feed of a bridge arm inductor and current feed at an alternating current side; after locking, the bridge arm of the novel MMC asymmetric capacitance clamping submodule is equivalent to an RLC equivalent circuit formed by a diode, a capacitor in series connection, a loop resistor and a loop inductor, and the single-phase equivalent circuit is shown in figure 4. The loop charged capacitor provides reverse voltage to make short-circuit current ifWhen the current of the fault loop is zero, the reverse voltage provided by the capacitor borne by the diode is in a cut-off state, and the fault loop is cut off; when the voltage at the alternating current side is larger than the reverse voltage provided by all the capacitors of the bridge arm, the alternating current source continues to feed energy to the short-circuit point;
Ceqthe equivalent charged capacitor u is formed by connecting an upper bridge arm and a lower bridge arm of a novel MMC asymmetric capacitor clamping submodule in series to an equivalent fault loopeqFor its equivalent reverse blocking voltage, LeqIs a loop equivalent inductor and mainly comprises a bridge arm inductor, a current-limiting reactance and a line reactance ReqThe circuit equivalent resistance is the bridge arm resistance, the short circuit point resistance and the line resistance; let t 00, the initial state of the equivalent circuit and the RLC second order differential equation can be listed as follows
Figure BDA0002501096580000111
Figure BDA0002501096580000112
In the formula of Ueq0、If0Respectively setting the initial value of the equivalent capacitor voltage of the loop and the initial value when the short-circuit current is locked;
due to R in the systemeqUsually much less than 2 (L)eq/Ceq)1/2The differential equation is solved to obtain the short-circuit current and capacitance voltage expression as
Figure BDA0002501096580000113
Wherein, tau2Is the equivalent loop time constant, omega, after system lock-up0Is the natural angular frequency of the loop, and omega is the angular frequency of the oscillation of the loop，
Figure BDA0002501096580000114
For the initial phase angle of the short circuit current, η is the initial phase angle of the capacitor voltage, which is as follows
Figure BDA0002501096580000115
The analytical formula (9) can be obtained, after the converter is locked, the short-circuit current is reduced due to the attenuation of the energy storage of the bridge arm inductor, and the back-pressure electromotive force is established for the capacitance charging of the novel MMC asymmetric capacitance clamping submodule on the other hand, so that the rapid attenuation of the fault current is realized after the converter is locked.
Carrying out voltage withstanding analysis on the power electronic device on the topological structure of the novel MMC asymmetric capacitance clamping submodule;
sub-module overvoltage conditions have a critical impact on the device type selection of ACSM. According to FIG. 1, under normal operating conditions, T in the submodule is determined by KVL law1、T2、T4、T5And a diode D1、D2、D4、D5The bearing voltage stress is similar to that of a half-bridge submodule component, and the maximum is a single capacitor voltage U of the submoduleC。D3In and D6、C2In the formed loop, the maximum withstand voltage is UC/2 due to T3And D3Antiparallel to bear the maximum capacitor voltage UC/2。D6Maximum withstand voltage occurrence and D1、D2、C1、C2In the formed loop, the maximum voltage is 2UC/3。
When the direct current line of the system has two-pole short circuit fault, the converter is locked, and the diodes and the IGBTs in the sub-modules bear the voltage stress of the capacitor. In the circuit shown in FIG. 2(a), T4、D4The maximum endured potential is UC。D2、D6Maximum withstand capacitor voltage UC。D3、D6Common tolerance is at most UCTo (c) is detected.
In the circuit shown in FIG. 2(b), T5、D5The maximum endured potential is UC。D3Maximum bearing UCDue to T3And D3Antiparallel to bear the maximum capacitor voltage UC。
In summary, it can be found from the analysis that when ACSM performs device type selection, the type selection of other devices of the improved sub-module can be consistent with that of the half-bridge sub-module, and each device needs to have a U at the maximumCThe capacity voltage withstanding capability of (2) is not the case where a high-strength withstanding device is used.
Carrying out economic analysis on the topological structure of the novel MMC asymmetric capacitance clamping submodule;
generally, the improved sub-module with dc fault self-clearing capability will increase the number of power devices to a different extent than the half-bridge sub-module. The increase in the number of sub-module power devices severely affects the economics of converter operation, and the economics analysis herein considers both the additional cost and the extra loss of the converter.
(n +1) level ACSM-MMC has the comprehensive additional cost C caused by IGBT and diode which are additionally arranged relative to HBSM-MMCcoaCan be represented by the following formula:
Ccoa＝6nkCI(NI+qND)(11)
wherein k is a mixed influence factor, in order to reduce the investment cost of a converter in actual engineering, a sub-module topology with fault blocking capability is often mixed and connected with a half-bridge structure, if the sub-module topology and the half-bridge structure are mixed according to the proportion of 1:1, k is 0.5, and it is worth explaining that the mixed influence factor is corrected to be 1 when the additional cost of the CDSM and the ACSM is calculated because the fault current cannot be effectively eliminated when the CDSM (clamping dual sub-module) and the ACSM and HBSM are mixed and connected according to the proportion of 1: 1; cIThe cost is the IGBT price; n is a radical ofI、NDRespectively representing the number of IGBTs and the number of diodes added by the sub-module unit under the condition of outputting unit level; q represents the diode to IGBT price ratio and is set to 0.2 in this example. The calculation of additional cost parameters for several typical sub-module units for equivalent voltage classes is shown in table 2.
TABLE 2 additional cost parameters for each type of SM
Tab.2 Addition cost parameters of different SM
Figure BDA0002501096580000131
The comprehensive additional cost of the current converter formed by each type of SM is in direct proportion to the number of levels. k (N)I+qND) The number of the IGBTs which are comprehensively used when the MMC outputs unit level. Specifically, when the comprehensive additional cost of various types of SM is calculated, the sub-module IGBT takes the model of FZ600R65KF1 produced by the company England as an example, and the unit price is 0.7 ten thousand yuan/piece. Additional cost C at typical SM output unit levelcouAnd an additional cost C for the converter in outputting the (n +1) levelcoaAs shown in table 3. As can be seen from table 3, the ACSM proposed herein has better economics.
TABLE 3 Total additional costs for various types of SM
Tab.3 Addition cost of different SM
Figure BDA0002501096580000132
The on-state loss of the power device is mainly considered when the extra new loss of the novel converter MMC is evaluated, and the on-state loss of a single IGBT can be determined under the condition that the same voltage level and device parameters are determined. Therefore, the extra loss P of the (n +1) level improved MMC sub-module is larger than that of the traditional half-bridge sub-modulesumCan be calculated by the following equation:
Psum＝k(nT+0.5nI)PT(12)
nTnumber n of additional always-on IGBTs for sub-module output unit levelIFor the additional number of IGBTs to be modulated at the output unit level, the total on-time is half of the total time, PTIs a single IGBT conduction loss. Table 4 compares several exemplary topology excess loss cases, and as can be seen from table 4, ACSM is lower in loss than the other several improved sub-modules in the table.
TABLE 4 excess loss parameters for various types of SM
Tab.4 Addition cost parameters of different SM
Figure BDA0002501096580000133
Figure BDA0002501096580000141
Simulation experiment analysis:
in order to verify the blocking feasibility of the topology of the asymmetric capacitance clamping submodule on the direct current fault of the MMC system, a 9-level single-ended MMC current converter of 40 MW/+/-20 kV is built on the basis of PSCAD/EMTDC and operates at the frequency of 50 Hz. The current converter adopts the nearest level approach modulation, the IGBT is locked for protection after the fault occurs, and the MMC model parameters are shown in the table 5. Each bridge arm of the converter is connected with 4 ACSMs in series.
TABLE 5 Single-ended MMC parameters
Tab.5 Parameters of single-ended MMC
Figure BDA0002501096580000142
Simulation experiments are conducted herein for the most severe dc bipolar short circuit fault in a flexible direct current system. Suppose that the system has a dc line double short circuit fault at t-1.5 s, lasting 50 ms. The converter is locked when t is set to 1.501s in consideration of the time delay caused by the actual system fault detection, and the like, and the simulation results on the dc side and the ac side are shown in fig. 5 and 6.
Fig. 5 is a system dc side simulation graph. Fig. 5(a) shows a dc waveform when the system is locked, and the dc is the sum of three-phase currents of the MMC. Because the sub-module capacitor is continuously discharged, the direct current almost linearly rises within a few ms before locking after the fault occurs, and the maximum value can reach 6kA, so that the direct current is multiplied by the rated current. After the 1.501s converter is locked, the capacitor in the ACSM of the bridge arm establishes a reverse electromotive force in a fault current loop, when the reverse electromotive force in the loop is larger than the voltage of an alternating current lateral line, a loop diode cannot be conducted due to the reverse cut-off characteristic, the fault loop is cut off, and the fault current is cut off to 0 in a short time. DC voltage waveform As shown in FIG. 5(b), the ACSM-MMC DC voltage drops rapidly from 20kV to 0kV before latching due to the DC double-pole short-circuit fault of the flexible direct-current system. Fig. 5(c) shows that after the fault occurs, the dc voltage rapidly drops to 0, and the system dc power transmission also drops to 0. It is seen from the voltage waveform of the capacitor of the sub-module shown in fig. 5(d), after the fault occurs, before the IGBT is locked, since the discharge time is only 1ms, the capacitor voltage is slightly reduced at the moment of the fault, and after the converter is locked, the capacitor voltage of the sub-module is increased due to the capacitor charging effect. As shown in fig. 5(e), before and after the fault occurs, the sum of the voltages of the upper and lower bridge arms of the inverter has the same trend as the change trend of the dc voltage, and the voltage drops from the normal operation voltage to zero. Similarly, in the bridge arm current waveform shown in fig. 5(f), the sub-module capacitor provides a back-pressure blocking effect after the bridge arm current is locked by the inverter, so that the bridge arm current is rapidly reduced, and the direct current is reduced without operating the ac-side circuit breaker.
The simulation waveform on the AC side of the system model is shown in FIG. 6. As seen from the ac voltage in fig. 6(a), since the inverter is latched in time after the occurrence of the fault, the ac voltage is not significantly changed. When the direct current bipolar short circuit occurs, the alternating current side is approximately regarded as a three-phase short circuit, and meanwhile, the ACSM cuts off the short circuit current feed-in path, the alternating current system does not feed energy into the short circuit point any more, and the alternating current is reduced to 0. And through comprehensive simulation result analysis, when the ACSM-MMC generates a direct-current side bipolar short-circuit fault, the function of quickly blocking the fault current can be realized.
And (4) conclusion: the asymmetrical capacitance clamping submodule topological structure has the function of blocking direct current fault current, utilizes the capacitance clamping function of the ACSM submodule and the reverse cut-off characteristic of a diode, and has the capability of clearing direct current faults; the MMC formed by the improved sub-modules can cut off the fault current on the direct current side of the system relatively quickly. Meanwhile, the improved sub-module uses fewer fully-controlled power electronic devices, and the control method is simple and reliable and has better expansibility. And finally, the DC interelectrode short-circuit fault is verified on simulation software, and a simulation result shows that the ACSM can rapidly block the fault current on the DC side of the system after the DC fault occurs, so that the action of an AC circuit breaker is avoided, and the safety of the converter equipment is improved.
It is worth noting that the scheme of the application is funded by a scientific and technological plan of Sichuan province, the project research results and the formed intellectual property rights thereof are owned by project undertaking units, and under specific conditions, the country reserves the right of gratuitous use, development, effective utilization and income acquisition according to the requirements.
The above are preferred embodiments of the present invention, and all changes made according to the technical scheme of the present invention that produce functional effects do not exceed the scope of the technical scheme of the present invention belong to the protection scope of the present invention.

Claims (6)

  1. The direct-current fault clearing method of the MMC asymmetric capacitance clamping submodule is characterized by comprising the following steps of:
    s1: providing a novel topological structure of an MMC asymmetric capacitance clamping submodule;
    s2: on the basis of the step S1, performing dc fault characteristic analysis on the topological structure of the novel MMC asymmetric capacitance clamping submodule;
    s3: on the basis of the step S2, determining the dc fault clearing capability of the topology structure of the novel MMC asymmetric capacitance clamping submodule according to the result of the dc fault characteristic analysis;
    s4: and on the basis of the step S3, verifying the dc fault clearing capability of the topology structure of the novel MMC asymmetric capacitance clamping submodule.
  2. 2. The MMC asymmetric capacitor clamping submodule DC fault clearance method of claim 1,
    in step S1, the topology of the novel MMC asymmetric capacitance clamping submodule is as follows:
    the high-voltage power supply comprises a first insulated gate bipolar transistor T1, a second insulated gate bipolar transistor T2, a third insulated gate bipolar transistor T3, a fourth insulated gate bipolar transistor T4, a fifth insulated gate bipolar transistor T5, a first current-limiting diode D1, a second current-limiting diode D2, a third current-limiting diode D3, a fourth current-limiting diode D4, a fifth current-limiting diode D5, a sixth current-limiting diode D6, a first clamping capacitor C1 and a second clamping capacitor C2; wherein, a first insulated gate bipolar transistor T1 is connected with a first current-resisting diode D1 in parallel, a second insulated gate bipolar transistor T2 is connected with a second current-resisting diode D2 in parallel, a third insulated gate bipolar transistor T3 is connected with a third current-resisting diode D3 in parallel, a fourth insulated gate bipolar transistor T4 is connected with a fourth current-resisting diode D4 in parallel, a fifth insulated gate bipolar transistor T5 is connected with a fifth current-resisting diode D5 in parallel, an emitter of the first insulated gate bipolar transistor T1 is connected with a collector of the second insulated gate bipolar transistor T2, a collector of the first insulated gate bipolar transistor T1 is connected with an anode of a first clamping capacitor C1, a cathode of the first clamping capacitor C1 is connected with a collector of the third insulated gate bipolar transistor T3, an anode of a second clamping capacitor C2 and a collector of the fourth insulated gate bipolar transistor T4 respectively, an emitter of the fourth insulated gate bipolar transistor T4 is connected with a collector of the fifth insulated gate bipolar transistor T5, the emitter of the second insulated gate bipolar transistor T2 is connected to the emitter of the third insulated gate bipolar transistor T3, and the emitter of the second insulated gate bipolar transistor T2 is further connected to the cathode of the second clamping capacitor C2 and the emitter of the fifth insulated gate bipolar transistor T5 through a sixth current-blocking diode D6 connected in series in an opposite direction.
  3. 3. The MMC asymmetric capacitor clamping submodule DC fault clearance method of claim 2, wherein said novel MMC asymmetric capacitor clamping submodule topology is according to T1、T2、T3、T4、T5Can output 0 and U by alternatively switching on and offCAnd 2UCThree levels; wherein, in steady state operation, if T1And T4Closed, T2、T3And T5Turn-off, submodule voltage USM＝UC(ii) a If T1And T5Closed, T2、T3And T4Turn-off, submodule voltage USM＝2UC(ii) a If T2、T3And T5Closed, T1And T4Turn-off, submodule voltage USM＝UC(ii) a If T2、T3And T4Closed, T1And T5Turn-off, submodule voltage USM＝0；
    When short-circuit fault occurs on the DC side, the converter is locked, the reverse voltage of the capacitor blocks the fault current fed to the DC fault point from the AC side, and when I occursSMWhen > 0, the capacitance C1、C2Connected in series to the fault loop, sub-module voltage USM＝2UC(ii) a When I isSMWhen < 0, the capacitance C2Providing a reverse voltage in the fault loop, the submodule voltage USM＝UC。
  4. 4. The MMC asymmetric capacitor clamping sub-module DC fault clearance method of claim 3, wherein the DC faults of the novel MMC asymmetric capacitor clamping sub-module topology comprise bipolar short circuit faults, unipolar ground faults, and DC disconnect faults.
  5. 5. The MMC asymmetric capacitor clamping submodule direct current fault clearing method of claim 4, wherein when a direct current bipolar short circuit fault occurs in the novel MMC asymmetric capacitor clamping submodule topology and before a converter is locked;
    when bipolar short circuit fault takes place in gentle straight system direct current side overhead line's the twinkling of an eye, the novel MMC asymmetric capacitance clamping submodule piece electric capacity and the system alternating current end that are just investing can be before system shutting simultaneously to short-circuit point feed-in current, wherein the former is far more than the latter, most fault current has been contributed, before the transverter does not block, novel MMC asymmetric capacitance clamping submodule piece still carries out the switching according to normal modulation strategy, electric capacity and fault point form the circuit that discharges, fault current increases rapidly, the single-phase equivalent circuit in fault circuit can be expressed as following differential equation:
    Figure FDA0002501096570000021
    wherein C is1And C2And 2C/n, R is bridge arm powerResistance, fault point resistance and equivalent resistance of device on-off resistance, RaIs a bridge arm resistance, LaIs bridge arm inductance, L is current-limiting reactance, RfAs a fault point resistance, C1、C2Equivalent capacitances R of the upper and lower bridge arms, respectivelysIs an AC equivalent resistance, LsIs an alternating current equivalent reactance; if it is a metallic short-circuit fault, R2Much less than 2n (2L)a+ L)/C, the discharge process is a second-order under-damped attenuation process, and the capacitance voltage expression is
    Figure FDA0002501096570000031
    The short-circuit current direct current quantity is expressed as
    Figure FDA0002501096570000032
    Wherein, U0For fault transient DC line voltage, I0For direct line current, τ1Is the equivalent loop time constant, omega, before system lock-up0Is the loop natural angular frequency, omegafIs the angular frequency of the loop oscillation, theta is the initial phase angle of the capacitor voltage, β is the initial phase angle of the bridge arm current, as follows
    Figure FDA0002501096570000033
    Before the converter is locked, the AC side equivalently generates three-phase short circuit and is provided with a phase-a power supply voltage ua＝Usin(ωst)，ωsAt a power frequency angular frequency, the a AC current is
    Figure FDA0002501096570000034
    Wherein I is U/[ (R)s+R)2+ωs 2(Ls+L+2La)2]1/2,
    Figure FDA0002501096570000035
    In this stage, the short-circuit current of the bridge arm is the superposition of the DC component and the AC component, and the upper and lower bridge arm currents are respectively
    Figure FDA0002501096570000036
  6. 6. The MMC asymmetric capacitor clamping submodule direct current fault clearing method of claim 5, wherein when a direct current bipolar short circuit fault occurs in the novel MMC asymmetric capacitor clamping submodule topology and after a converter is locked;
    when a short-circuit fault occurs, the system is locked, a discharge energy feedback path of a novel MMC asymmetric capacitance clamping submodule capacitor is cut off, but a bridge arm inductor can feed energy to a fault point through a freewheeling diode, and at the moment, a short-circuit point current mainly comprises the energy feedback of the bridge arm inductor and an alternating-current side feed-in current; after being locked, the bridge arm of the novel MMC asymmetric capacitance clamping submodule is equivalent to an RLC equivalent loop formed by a diode, a capacitor series, a loop resistor and a loop inductor, wherein the loop charged capacitor provides reverse voltage to enable short-circuit current ifWhen the current of the fault loop is zero, the reverse voltage provided by the capacitor borne by the diode is in a cut-off state, and the fault loop is cut off; when the voltage at the alternating current side is larger than the reverse voltage provided by all the capacitors of the bridge arm, the alternating current source continues to feed energy to the short-circuit point;
    Ceqthe equivalent charged capacitor u is formed by connecting an upper bridge arm and a lower bridge arm of a novel MMC asymmetric capacitor clamping submodule in series to an equivalent fault loopeqFor its equivalent reverse blocking voltage, LeqIs a loop equivalent inductor and mainly comprises a bridge arm inductor, a current-limiting reactance and a line reactance ReqThe circuit equivalent resistance is the bridge arm resistance, the short circuit point resistance and the line resistance; let t00, the initial state of the equivalent circuit and the RLC second order differential equation can be listed as follows
    Figure FDA0002501096570000041
    Figure FDA0002501096570000042
    In the formula of Ueq0、If0Respectively setting the initial value of the equivalent capacitor voltage of the loop and the initial value when the short-circuit current is locked;
    due to R in the systemeqUsually much less than 2 (L)eq/Ceq)1/2The differential equation is solved to obtain the short-circuit current and capacitance voltage expression as
    Figure FDA0002501096570000043
    Wherein, tau2Is the equivalent loop time constant, omega, after system lock-up0Is the natural angular frequency of the loop, omega is the angular frequency of the oscillation of the loop,
    Figure FDA0002501096570000044
    for the initial phase angle of the short circuit current, η is the initial phase angle of the capacitor voltage, which is as follows
    Figure FDA0002501096570000051
    The analytical formula (9) can be obtained, after the converter is locked, the short-circuit current is reduced due to the attenuation of the energy storage of the bridge arm inductor, and the back-pressure electromotive force is established for the capacitance charging of the novel MMC asymmetric capacitance clamping submodule on the other hand, so that the rapid attenuation of the fault current is realized after the converter is locked.
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CN110224623A (en) * 2019-06-12 2019-09-10 四川大学 A kind of modularization multi-level converter and submodule that DC Line Fault blocks
CN110535359A (en) * 2019-08-29 2019-12-03 华北电力大学（保定） A kind of diode clamp mixing MMC circuit with from equal pressure energy power
CN110601570A (en) * 2019-09-04 2019-12-20 清华大学 Many level of modularization current regulator
CN111030060A (en) * 2019-12-06 2020-04-17 华北电力大学 Clamp-on circuit breaker topology suitable for direct current fault removal
CN111181416A (en) * 2020-01-09 2020-05-19 华北电力大学 Modular multilevel converter and direct-current fault clearing method

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* Cited by examiner, † Cited by third party
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JP2015035902A (en) * 2013-08-09 2015-02-19 株式会社明電舎 Multi-level power converting device
CN110224623A (en) * 2019-06-12 2019-09-10 四川大学 A kind of modularization multi-level converter and submodule that DC Line Fault blocks
CN110535359A (en) * 2019-08-29 2019-12-03 华北电力大学（保定） A kind of diode clamp mixing MMC circuit with from equal pressure energy power
CN110601570A (en) * 2019-09-04 2019-12-20 清华大学 Many level of modularization current regulator
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