CN110995038A - MMC (modular multilevel converter) and DC fault isolation method and system based on MMC - Google Patents
MMC (modular multilevel converter) and DC fault isolation method and system based on MMC Download PDFInfo
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- CN110995038A CN110995038A CN201911148458.0A CN201911148458A CN110995038A CN 110995038 A CN110995038 A CN 110995038A CN 201911148458 A CN201911148458 A CN 201911148458A CN 110995038 A CN110995038 A CN 110995038A
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- 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
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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
- H02H7/10—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 for converters; for rectifiers
- H02H7/12—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 for converters; for rectifiers for static converters or rectifiers
- H02H7/122—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 for converters; for rectifiers for static converters or rectifiers for inverters, i.e. dc/ac converters
- H02H7/1222—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 for converters; for rectifiers for static converters or rectifiers for inverters, i.e. dc/ac converters responsive to abnormalities in the input circuit, e.g. transients in the DC input
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- 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
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Abstract
The invention provides a Modular Multilevel Converter (MMC) and a method and a system for carrying out direct current fault isolation based on the MMC, wherein the MMC is based on a half-bridge submodule and is connected with a structure that a plurality of diodes and thyristors are reversely connected in parallel in an upper bridge arm and a lower bridge arm of each phase unit, and the direct current fault isolation system and the method based on the MMC can realize smooth judgment of the bridge arms of the MMC by controlling trigger signals of the thyristors when the current of the bridge arms passes through zero after the direct current side fails and simultaneously block the current feed of the alternating current side. Compared with the traditional MMC based on the half-bridge sub-module, the modular multilevel converter based on the half-bridge sub-module has the advantages that the added investment cost and the loss are limited, the MMC can realize the quick isolation of the direct current side fault of the direct current power distribution network within 20ms, the isolation speed is high, in addition, during the fault current clearing period, the alternating current side can continuously feed current to the fault point, and therefore time is provided for fault positioning and ranging.
Description
Technical Field
The invention relates to the field of relay protection, in particular to a Modular Multilevel Converter (MMC) and a direct-current fault isolation method and system based on the MMC.
Background
Compared with the traditional alternating current distribution, the direct current distribution network has attracted extensive attention due to the advantages of large transmission capacity, high electric energy quality, good system stability, convenience for distributed energy access and the like. As a core device of the direct-current power distribution network, the performance of the converter directly influences the popularization and development of the direct-current power distribution network. The traditional two-level/three-level VSC has the defects of over-high switching frequency, large harmonic content and the like, and the adaptive voltage level is low. The Modular Multilevel Converter (MMC) has a good application prospect in the field of medium-voltage direct-current power distribution due to superior performances such as good electric energy quality and high reliability. However, in the case of a two-level/three-level VSC or MMC, when a fault occurs on the dc side (especially, a two-pole short-circuit fault), the capacitor will rapidly discharge to the fault point, resulting in a large dc fault current, rapid fault development, and a wide fault influence range. Therefore, fault isolation of the direct current system becomes a technical difficulty for popularization and application of the direct current distribution network. To solve the problem, scholars at home and abroad propose various solutions, which can be classified into the following three categories according to a direct current fault isolation mode:
1) the method for isolating fault points by using an alternating current side system is mainly divided into a single-transistor method and a double-transistor method. The adoption of the single crystal thyristor method usually needs hundreds of milliseconds to isolate a fault point, during the period, an alternating current system can continuously feed current to the fault point, and after the fault point is isolated, the fault current can still be cleared in hundreds of milliseconds; the adoption of the double-thyristor method can prevent the alternating current power supply from feeding immediately after the fault is detected, but still tens of milliseconds or even hundreds of milliseconds are needed after the fault current naturally attenuates to zero. . The method has the advantages of low cost, no additional loss and wide application in engineering practice, but fault current clearing depends on natural attenuation of a fault loop, and the fault clearing time is long;
2) the fault point is isolated by a direct current breaker. At present, the direct-current fault can be separated in 5ms by adopting a hybrid direct-current circuit breaker at the fastest speed, and the direct-current fault can be separated in 1ms by adopting an all-solid-state direct-current circuit breaker at the fastest speed. However, the development of the current direct current circuit breaker is still immature, the cost and the loss of the direct current circuit breaker with high capacity and high speed are too high, current limiting equipment is required to be used in a matched mode, the direct current circuit breaker is applied to a direct current power distribution network, the number of the circuit breakers is large, and the investment is too high;
3) an inverter with fault self clearing capability is utilized. Due to the immature development of the direct current circuit breaker, domestic and foreign scholars throw the eyes on the current converter with the fault self-clearing capability. After self-full-bridge sub-module (HBSM), clamped dual sub-module (CDSM), a variety of sub-module topologies with fault isolation capability have been proposed in succession. The mechanism of the MMC fault isolation is that after a fault, the IGBT is locked, so that the bridge arm capacitor is reversely connected in series to a fault loop, the fault is rapidly isolated, and the fault current is eliminated. However, sub-modules with fault isolation capability are costly and have large operating losses;
generally speaking, the existing dc power transmission and distribution system has a contradiction between the fault isolation speed and the investment cost and the operation loss in the fault processing manner.
Disclosure of Invention
In order to solve the technical problem that contradiction exists between the fault isolation speed, the investment cost and the operation loss of a direct current transmission and distribution system in the prior art in a fault processing mode, the invention provides a Modular Multilevel Converter (MMC), which is formed by connecting three phase units in parallel, wherein each phase unit is connected with one phase in a three-phase circuit of a power grid, and comprises the following components in parts by weight:
the upper bridge arm and the lower bridge arm respectively comprise a plurality of cascaded half-bridge sub-modules and a series reactor;
the switch unit comprises a plurality of first structures which are connected in series, wherein each first structure consists of a diode and a thyristor which are connected in parallel in an opposite direction, the cathode of each diode is connected with the anode of each thyristor, and the anode of each diode is connected with the cathode of each thyristor;
and the switch unit is respectively connected with the upper bridge arm and the lower bridge arm.
Furthermore, a half-bridge submodule in the converter is formed by connecting an insulated gate bipolar transistor IGBT and a capacitor in parallel.
Furthermore, one end of the switch unit is connected with the half-bridge sub-modules of the upper bridge arm and the lower bridge arm, and the other end of the switch unit is connected with the reactors of the upper bridge arm and the lower bridge arm, or one end of the switch unit is connected with the reactors of the upper bridge arm and the lower bridge arm, and the other end of the switch unit is connected with one phase of a three-phase circuit of a power grid.
Furthermore, the voltage withstanding capability of the converter switch unit is not less than the maximum value of interphase voltage between any two phases in the three-phase circuit of the power grid.
According to another aspect of the present invention, there is provided a method for dc fault isolation based on any one of the converters described in the present invention, the method including:
collecting a power grid operation signal;
determining a power grid operation state according to the power grid operation signal, wherein the operation state comprises a normal operation state and a fault state;
generating a control signal according to the running state of the power grid, wherein when the power grid is in a normal running state, a conducting signal is continuously sent to the thyristor; when the power grid is in a fault state, a locking signal is sent to the IGBTs of all the MMC bridge arms, and a closing and conducting signal is sent to the thyristor;
when the control signal is a conducting signal, all thyristors of the MMC are continuously conducted; when the control signal is a cancel conducting signal, when the current of any bridge arm is larger than zero, the thyristor in the switch unit connected with the bridge arm in series is turned off, when the thyristor is turned off, the bridge arm directly connected with the thyristor in series is turned off, and when all bridge arms in the MMC are turned off, the direct-current fault isolation of the power grid is completed.
Further, the collecting of the power grid operation signal includes collecting current of a bridge arm in the MMC and collecting a signal for judging a power grid operation fault required by a power grid protection system, and the signal includes a line outlet voltage traveling wave and a current traveling wave.
Further, the determining the grid operating state according to the grid operating signal includes:
determining whether the MMC bridge arms are in overcurrent or not according to the current of the MMC bridge arms, and determining that a power grid is in a fault state when any one bridge arm is in the overcurrent state; and the protection system determines whether the power grid is in a fault state according to the acquired signal, wherein for the traveling wave protection, whether a fault occurs is judged by using the amplitude or the change rate of the high-frequency component of the voltage traveling wave, the fault direction is identified by using the change rate of the current traveling wave, for the single-end quantity protection, the high-frequency energy of the single-side current traveling wave is extracted, and the fault identification is carried out by using the fault transient energy amplitude.
According to another aspect of the present invention, there is provided a system for dc fault isolation using any one of the converters of the present invention, the system comprising:
the signal acquisition unit is used for acquiring a power grid operation signal and transmitting the power grid operation signal to the fault detection unit;
the fault detection unit is used for determining the power grid operation state according to the power grid operation signal, and the operation state comprises a normal operation state and a fault state;
the control unit is used for sending a control signal to the MMC according to the running state of the power grid, wherein when the power grid is in a normal running state, a conduction signal is continuously sent to the thyristor; when the power grid is in a fault state, a locking signal is sent to the IGBTs of all the MMC bridge arms, and a closing and conducting signal is sent to the thyristor;
the MMC is used for executing the conduction or the disconnection of the thyristors according to the control signals of the control unit, wherein when the control signals are conduction signals, all the thyristors are continuously conducted; when the control signal is a cancel conducting signal, when the current of any bridge arm is larger than zero, the thyristor in the switch unit connected with the bridge arm in series is turned off, when the thyristor is turned off, the bridge arm directly connected with the thyristor in series is turned off, and when all bridge arms in the MMC are turned off, the direct-current fault isolation of the power grid is completed.
Further, the signal acquisition unit includes:
the first signal unit is used for collecting the current of a bridge arm in the MMC;
and the second signal unit is used for acquiring signals for judging the power grid operation fault required by the power grid protection system, and the signals comprise a line outlet voltage traveling wave and a current traveling wave.
Further, the fault detection unit includes:
the first detection unit is used for determining whether the MMC bridge arm is in an overcurrent state or not according to the current signal of the first signal unit, and determining that a power grid is in a fault state when any bridge arm is in the overcurrent state;
and the second detection unit is used for determining whether the power grid is in fault operation according to the signal of the second signal unit, wherein for the traveling wave protection, the fault operation is judged by using the amplitude or the change rate of the voltage traveling wave high-frequency component, the fault direction is identified by using the change rate of the current traveling wave, and for the single-end quantity protection, the single-side current traveling wave high-frequency energy is extracted, and the fault transient energy amplitude is used for fault identification.
The modular multilevel converter provided by the technical scheme of the invention is based on a half-bridge submodule, and is connected with a structure that a plurality of diodes and thyristors are reversely connected in parallel in upper and lower bridge arms of each phase unit of an MMC in series. When the protection system of the power grid detects a fault, the control system immediately sends out blocking signals to all the IGBTs, cuts off the capacitor discharge path of the sub-module, and simultaneously immediately cancels the conduction signals of the thyristors. At the moment, as the thyristor does not have the automatic turn-off capability, the fault current continues flowing through the bridge arm thyristor, and meanwhile, the current is fed to a fault point by the alternating current system and is rapidly increased. When the alternating current side feed current of any bridge arm and the direct current fault current are superposed and pass zero, the thyristor is turned off, and the corresponding MMC bridge arm is turned off. Due to the effect of current feeding at the alternating current side, the MMC bridge arm can be sequentially turned off until the direct current fault current is eliminated. Compared with the traditional MMC based on the half-bridge sub-module, the modular multilevel converter based on the half-bridge sub-module has the advantages that the added investment cost and the loss are limited, the MMC can realize the quick isolation of the direct current side fault of the direct current power distribution network within 20ms, the isolation speed is high, in addition, during the fault current clearing period, the alternating current side can continuously feed current to the fault point, and therefore time is provided for fault positioning and ranging.
Drawings
A more complete understanding of exemplary embodiments of the present invention may be had by reference to the following drawings in which:
fig. 1 is a schematic view of a topology of a modular multilevel converter according to a preferred embodiment of the present invention.
Fig. 2 is a flow chart of a method for dc fault isolation of a modular multilevel converter according to a preferred embodiment of the present invention;
fig. 3(a) is a schematic diagram of a stage of fault isolation when a two-pole short-circuit fault occurs on a dc side of a modular multilevel converter according to a preferred embodiment of the present invention;
fig. 3(b) is a schematic diagram of bridge arm currents in each time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention;
fig. 3(c) is a schematic diagram of bridge arm ac power supply voltages in each time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention;
fig. 4(a) is a schematic diagram of a fault current path in a first time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of a modular multilevel converter according to a preferred embodiment of the present invention;
fig. 4(b) is a schematic diagram of a fault current path in a second time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention;
fig. 4(c) is a schematic diagram of a fault current path in a third time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention;
fig. 4(d) is a schematic diagram of a fault current path in a fourth time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention;
fig. 4(e) is a schematic diagram of a fault current path in a fifth time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention;
fig. 5 is a schematic structural diagram of a system for dc fault isolation of a modular multilevel converter according to a preferred embodiment of the present invention.
Detailed Description
The exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, however, the present invention may be embodied in many different forms and is not limited to the embodiments described herein, which are provided for complete and complete disclosure of the present invention and to fully convey the scope of the present invention to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, the same units/elements are denoted by the same reference numerals.
Unless otherwise defined, terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Further, it will be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense.
Fig. 1 is a schematic view of a topology of a modular multilevel converter according to a preferred embodiment of the present invention. As shown in fig. 1, the modular multilevel converter MMC100 according to the preferred embodiment is composed of three phase units 101 connected in parallel, where each phase unit 101 is connected to one phase of a three-phase circuit of a power grid, and each phase unit 101 includes:
an upper bridge arm 111 and a lower bridge arm 112, which respectively comprise a plurality of cascaded half-bridge sub-modules 1111 and a series reactor 1112;
the switching unit 113 comprises a plurality of first structures 1131 connected in series, each first structure is composed of a diode and a thyristor which are connected in parallel in an opposite direction, wherein the cathode of the diode is connected with the anode of the thyristor, and the anode of the diode is connected with the cathode of the thyristor;
switching unit 113 is connected to upper arm 111 and lower arm 112, respectively.
The thyristor has the advantages of high reliability, good pressure resistance, inrush current resistance, low price and the like. In the early stage of flexible dc transmission engineering, in order to clear a dc-side fault by using an ac circuit breaker, a thyristor is generally connected in parallel with a submodule to shunt a fault current, and this method is referred to as a single thyristor method. However, due to the existence of the freewheeling diode, the conventional MMC cannot effectively isolate the ac side feed current in time, and thus the dc fault current needs tens of ms or even hundreds of ms to decay to zero. In the preferred embodiment, the structure of the modular multilevel converter based on the half-bridge submodule is improved, after a plurality of thyristors and diode anti-parallel modules are connected in series on each bridge arm branch, when a direct current side fails, smooth turn-off of an MMC bridge arm can be realized by controlling a trigger signal of the thyristors when bridge arm current passes through zero, and meanwhile, current feed of a power supply at the alternating current side is blocked. Under a normal working state, the MMC control system continuously sends a conducting signal to the thyristor, at this time, the thyristor in the first structure 1131 of the switch unit 113 may be equivalent to a diode, and the series structure may be equivalent to a resistor module with a constant resistance value, so that the operation principle and characteristics of the converter based on the half-bridge sub-module according to the preferred embodiment are the same as those of the conventional half-bridge MMC.
Preferably, the half-bridge sub-module 1111 of the inverter 100 is formed by connecting an insulated gate bipolar transistor IGBT and a capacitor in parallel.
Preferably, one end of switch unit 113 is connected to half-bridge sub-module 1111 of upper arm 111 and lower arm 112, and the other end is connected to reactor 1112 of upper arm 111 and lower arm 112, or one end of switch unit 113 is connected to reactor 1112 of upper arm 111 and lower arm 112, and the other end is connected to one phase of the three-phase circuit of the power grid.
Preferably, the voltage withstanding capability of the converter switching unit 113 is not less than the maximum value of the interphase voltage between any two phases in the three-phase circuit of the power grid.
Fig. 2 is a flow chart of a method for dc fault isolation of a modular multilevel converter according to a preferred embodiment of the present invention. As shown in fig. 2, a method 200 for dc fault isolation of any converter according to the preferred embodiment starts with step 201.
In step 201, a grid operating signal is collected.
In step 202, a grid operating state is determined according to the grid operating signal, wherein the operating state comprises a normal operating state and a fault state.
In step 203, generating a control signal according to the power grid operation state, wherein when the power grid is in a normal operation state, a conduction signal is continuously sent to the thyristor; when the power grid is in a fault state, a locking signal is sent to the IGBTs of all the MMC bridge arms, and a closing and conducting signal is sent to the thyristor;
in step 204, when the control signal is a conducting signal, all thyristors of the MMC are continuously conducted; when the control signal is a cancel conducting signal, when the current of any bridge arm is larger than zero, the thyristor in the switch unit connected with the bridge arm in series is turned off, when the thyristor is turned off, the bridge arm directly connected with the thyristor in series is turned off, and when all bridge arms in the MMC are turned off, the direct-current fault isolation of the power grid is completed.
Preferably, the acquiring of the power grid operation signal includes acquiring a current of a bridge arm in the MMC and acquiring a signal for judging a power grid operation fault required by a power grid protection system, and the signal includes a line outlet voltage traveling wave and a current traveling wave.
Preferably, the determining the grid operating state according to the grid operating signal includes:
determining whether the MMC bridge arms are in overcurrent or not according to the current of the MMC bridge arms, and determining that a power grid is in a fault state when any one bridge arm is in the overcurrent state; and the protection system determines whether the power grid is in a fault state according to the acquired signal, wherein for the traveling wave protection, whether a fault occurs is judged by using the amplitude or the change rate of the high-frequency component of the voltage traveling wave, the fault direction is identified by using the change rate of the current traveling wave, for the single-end quantity protection, the high-frequency energy of the single-side current traveling wave is extracted, and the fault identification is carried out by using the fault transient energy amplitude.
Fig. 3(a) is a schematic diagram of a stage of fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention. The direct-current side two-pole short-circuit fault is the most serious fault of a direct-current system, and therefore, a fault isolation process is explained by taking the two-pole short-circuit fault as an example. In the following analysis, the upper and lower arms are denoted by letters p and n, respectively. The pa bridge arm represents the a-phase upper bridge arm, ipa represents the fault current of the a-phase upper bridge arm, and so on. Suppose that the fault occurs at time t0According to the operating state of the MMC bridge arm, the fault isolation process can be divided into three stages, as shown in fig. 3 (a).
Stage 1: t is t0To t1And a bridge arm submodule capacitor discharging stage. Once the direct current side has two poles short circuit fault, the impedance between the two poles of the direct current side is suddenly reduced, so that the voltage of the direct current side is suddenly changed. Therefore, the capacitor in the on state is immediately discharged to the fault point, and the dc fault current rapidly increases, as shown in fig. 3 (a). This processIn the method, although the current feed of the alternating current side fault point is increased at the same time, the current feed is limited, and the rising speed of the current feed is not as fast as the discharging speed of the capacitor, so that the main source of the fault current is the discharging of the bridge arm submodule. And when any bridge arm IGBT is in overcurrent or the protection system detects a fault, immediately sending a fault signal to the MMC control system. And after receiving the fault signal, the control system immediately sends a locking signal to the bridge arm submodule and cancels the conduction control signal of the thyristor after delaying to leave the dead zone. After receiving the locking signal, the MMC bridge arm IGBT is at t1Latch-up occurs at all times.
And (2) stage: t1 to t5, the MMC bridge arm alternates closing phases. And after the IGBT is locked, the sub-module capacitor discharge loop is cut off. At this time, because the direct current component of the fault current is greater than the alternating current side feed current, the thyristor does not have the self-turn-off capability, and the six bridge arms of the MMC are in a conducting state. Fig. 4(a) is a schematic diagram of a fault current path in a first time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of a modular multilevel converter according to a preferred embodiment of the present invention, as shown in fig. 4(a), t0To t1Meanwhile, according to the superposition principle, the bridge arm current in the section can be equivalent to the superposition of the alternating current side current feedback and the bridge arm fault current component. Taking pa bridge arm as an example, the fault current can be expressed as
ipa＝ipa1+ifa2
In the formula ipa1Representing the feed component, i, of the AC side supplyfa2Indicating the shunting of the dc-side fault current in phase a. Because of no clamping effect of the bridge arm capacitor voltage, the alternating current side current feed starts to increase gradually. When the bridge arm current passes through zero, the thyristor on the bridge arm is locked, and the bridge arm is turned off.
Fig. 3(b) is a schematic diagram of bridge arm currents in each time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention. As shown in fig. 3(b), from t0To t6In the time period, the nc bridge arm, the pb bridge arm, the na bridge arm, the pc bridge arm, the pa bridge arm and the nb bridge arm sequentially have zero current, and the current becomes zero again along with judgment of the thyristor.
Fig. 3(c) is a schematic diagram of bridge arm ac power supply voltages in each time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention. Such as
Shown in FIG. 3(c), t1The amplitude of the phase voltage of the power supply c on the AC side is larger at the moment, so that the current feed is rapidly increased, and the nc bridge arm is caused to be at t2And the time is quickly cut off.
Fig. 4(b) is a schematic diagram of a fault current path in a second time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention. Thus, t1To t2The fault current path during this time is shown in fig. 4 (b). Fig. 4(c) is a schematic diagram of a fault current path in a third time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention. At t2To t3During this time, pb leg is turned off at time t3, and the fault current path is shown in fig. 4 (c).
Then, the phase a feeds to the phase b through the arm na and the arm nb, and the phase c feeds to the phase a through the arm pc and the arm pa, resulting in that the phase ipaAnd inaAre all reduced. Meanwhile, phase c feeds a current to phase b through a fault point, resulting in an increase in the dc-side fault current, which varies as shown in fig. 3 (a). i.e. inaAt t4The moment is zero-crossing first, and the na bridge arm is switched off. Fig. 4(d) is a schematic diagram of a fault current path in a fourth time period for fault isolation when a two-pole short-circuit fault occurs on the dc side of the modular multilevel converter according to the preferred embodiment of the present invention. At t3To t4Meanwhile, when the na arm is turned off, the fault current path is as shown in fig. 4 (d). When the phase a is the same as the phase c, the current is fed to the phase b through a fault point, and simultaneously, the voltage difference between the phase a and the phase c is reduced. Therefore, the a-phase upper arm current and the fault current rapidly increase. When u isa>ucWhen u is turned onacForce ipcDecrease rapidly at t5And when ipc crosses zero, the pc bridge arm is switched off. So far, the nc, pb, na and pc bridge arms are all turned off.
And (3) stage: t5 to t6, dc side fault current clearing phase. FIG. 4(e) is a schematic view according to the present inventionThe fault current path schematic diagram of the modular multilevel converter according to the preferred embodiment of the invention is used for fault isolation in the fifth time period when a two-pole short-circuit fault occurs on the direct-current side of the modular multilevel converter. At t4To t5During this time, the fault current path is as shown in fig. 4 (e). As shown in FIG. 3(c), t5Time uab>0, therefore, phase a feeds phase b through the pa leg, the fault point, and the nb leg. Resulting in a continued increase in the dc side fault current. When u isab<At 0, the fault current is rapidly reduced to zero due to the reversal of the fault current with the phase-to-phase voltage. And when the fault current on the direct current side passes through zero, the pa bridge arm and the nb bridge arm are turned off. To this end, the MMC achieves self-clearing of the dc side fault current.
According to the fault isolation process and principle analysis, the diode and thyristor anti-parallel structure additionally arranged on the MMC bridge arm needs to bear forward or reverse voltage drop during fault isolation and after fault isolation.
In the fault isolation process, as shown in fig. 4(e), the nc bridge arm is taken as an example, and at t5To t6In the meantime, the interphase voltages of B, C two phases are all applied to the nc bridge arm except for the limited voltage division in the through-current branch, and the conditions of the rest bridge arms are similar. Considering that the voltage division of the current branch is limited, especially when the fault loop impedance is large, when the anti-parallel structure is configured, the voltage resistance of the anti-parallel structure needs to be configured according to the maximum value of the inter-phase voltage. After fault isolation is finished, the interphase voltage of the converter is jointly borne by the thyristor and the sub-module IGBT, and the voltage withstand requirement is smaller than that of the fault isolation process.
It can be seen from the MMC working principle of the preferred embodiment that the thyristor of the MMC of the present invention is in a continuous conduction state in a normal working state, and when a fault is detected, the conduction signal is cancelled, and the bridge arm current is naturally turned off when passing through zero. Therefore, the switching frequency of the thyristor is lower and is the power frequency in the normal working state; the method is applied to fault isolation and has low requirement on conduction speed. Therefore, the selection of the thyristor is based on the principle of optimizing economy and loss on the premise of meeting the current endurance of withstand voltage.
Fig. 5 is a schematic structural diagram of a system for dc fault isolation of a modular multilevel converter according to a preferred embodiment of the present invention. As shown in fig. 5, a system 500 for dc fault isolation according to the preferred embodiment includes:
the signal acquisition unit 501 is used for acquiring a power grid operation signal and transmitting the power grid operation signal to the fault detection unit;
a fault detection unit 502, configured to determine a power grid operation state according to the power grid operation signal, where the operation state includes a normal operation state and a fault state;
the control unit 503 is configured to send a control signal to the MMC according to a power grid operating state, where when the power grid is in a normal operating state, a conduction signal is continuously sent to the thyristor; when the power grid is in a fault state, a locking signal is sent to the IGBTs of all the MMC bridge arms, and a closing and conducting signal is sent to the thyristor;
an MMC504 for performing turn-on or turn-off of the thyristors according to a control signal of the control unit, wherein when the control signal is a turn-on signal, all the thyristors are continuously turned on; when the control signal is a cancel conducting signal, when the current of any bridge arm is larger than zero, the thyristor in the switch unit connected with the bridge arm in series is turned off, when the thyristor is turned off, the bridge arm directly connected with the thyristor in series is turned off, and when all bridge arms in the MMC are turned off, the direct-current fault isolation of the power grid is completed.
Preferably, the signal acquisition unit 501 includes:
the first signal unit 511 is used for collecting the current of a bridge arm in the MMC;
and the second signal unit 512 is configured to acquire a signal for judging a power grid operation fault, which is required by the power grid protection system, where the signal includes a line outlet voltage traveling wave and a current traveling wave.
Preferably, the fault detection unit 502 includes:
the first detection unit 521 is configured to determine whether the MMC bridge arm is in an overcurrent state according to a current signal of the first signal unit, and determine that the power grid is in a fault state when any one of the bridge arms is in the overcurrent state;
and a second detection unit 522, configured to determine whether a fault occurs in operation of the power grid according to a signal of the second signal unit, where for the traveling wave protection, whether a fault occurs is determined by using an amplitude or a change rate of a voltage traveling wave high-frequency component, a fault direction is identified by using a change rate of a current traveling wave, and for single-ended quantity protection, single-side current traveling wave high-frequency energy is extracted, and fault identification is performed by using a fault transient energy amplitude.
The method for fault isolation of the system for DC fault isolation based on the modular multilevel converter is the same as the method for DC fault isolation based on the modular multilevel converter, the steps are the same, the technical effect is the same, and the details are not repeated.
The invention has been described with reference to a few embodiments. However, other embodiments of the invention than the one disclosed above are equally possible within the scope of the invention, as would be apparent to a person skilled in the art from the appended patent claims.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [ device, component, etc ]" are to be interpreted openly as referring to at least one instance of said device, component, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is to be covered by the claims.
Claims (10)
1. The utility model provides a many level of modularization transverter MMC which characterized in that, MMC comprises three looks unit is parallelly connected, and every looks unit is connected with a looks in the electric wire netting three-phase circuit, and wherein, every looks unit includes:
the upper bridge arm and the lower bridge arm respectively comprise a plurality of cascaded half-bridge sub-modules and a series reactor;
the switch unit comprises a plurality of first structures which are connected in series, wherein each first structure consists of a diode and a thyristor which are connected in parallel in an opposite direction, the cathode of each diode is connected with the anode of each thyristor, and the anode of each diode is connected with the cathode of each thyristor;
and the switch unit is respectively connected with the upper bridge arm and the lower bridge arm.
2. The converter according to claim 1, wherein the half-bridge sub-modules in the converter are composed of Insulated Gate Bipolar Transistors (IGBTs) and capacitors connected in parallel.
3. The converter according to claim 1, wherein one end of the switching unit is connected to the half-bridge sub-modules of the upper and lower bridge arms and the other end is connected to the reactors of the upper and lower bridge arms, or one end of the switching unit is connected to the reactors of the upper and lower bridge arms and the other end is connected to one phase of a three-phase circuit of a power grid.
4. A converter according to claim 1, characterized in that the withstand voltage of the converter switching unit is not less than the maximum value of the interphase voltage between any two phases in the three-phase circuit of the power grid.
5. A method for DC fault isolation of a converter according to any of claims 1-4, wherein the method comprises:
collecting a power grid operation signal;
determining a power grid operation state according to the power grid operation signal, wherein the operation state comprises a normal operation state and a fault state;
generating a control signal according to the running state of the power grid, wherein when the power grid is in a normal running state, a conducting signal is continuously sent to the thyristor; when the power grid is in a fault state, a locking signal is sent to the IGBTs of all the MMC bridge arms, and a closing and conducting signal is sent to the thyristor;
when the control signal is a conducting signal, all thyristors of the MMC are continuously conducted; when the control signal is a cancel conducting signal, when the current of any bridge arm is larger than zero, the thyristor in the switch unit connected with the bridge arm in series is turned off, when the thyristor is turned off, the bridge arm directly connected with the thyristor in series is turned off, and when all bridge arms in the MMC are turned off, the direct-current fault isolation of the power grid is completed.
6. The method according to claim 5, wherein the collecting the power grid operation signals comprises collecting currents of bridge arms in the MMC and collecting signals for judging power grid operation faults required by a power grid protection system, wherein the signals comprise line outlet voltage traveling waves and current traveling waves.
7. The method of claim 6, wherein determining the grid operating state from the grid operating signal comprises:
determining whether the MMC bridge arms are in overcurrent or not according to the current of the MMC bridge arms, and determining that a power grid is in a fault state when any one bridge arm is in the overcurrent state; and
the protection system determines whether the power grid is in a fault state according to the acquired signals, wherein for traveling wave protection, whether a fault occurs is judged by using the amplitude or the change rate of the high-frequency component of the voltage traveling wave, the fault direction is identified by using the change rate of the current traveling wave, for single-end quantity protection, single-side current traveling wave high-frequency energy is extracted, and fault identification is carried out by using the fault transient energy amplitude.
8. A system for DC fault isolation of a converter according to any of claims 1 to 4, said system comprising:
the signal acquisition unit is used for acquiring a power grid operation signal and transmitting the power grid operation signal to the fault detection unit;
the fault detection unit is used for determining the power grid operation state according to the power grid operation signal, and the operation state comprises a normal operation state and a fault state;
the control unit is used for sending a control signal to the MMC according to the running state of the power grid, wherein when the power grid is in a normal running state, a conduction signal is continuously sent to the thyristor; when the power grid is in a fault state, a locking signal is sent to the IGBTs of all the MMC bridge arms, and a closing and conducting signal is sent to the thyristor;
the MMC is used for executing the conduction or the disconnection of the thyristors according to the control signals of the control unit, wherein when the control signals are conduction signals, all the thyristors are continuously conducted; when the control signal is a cancel conducting signal, when the current of any bridge arm is larger than zero, the thyristor in the switch unit connected with the bridge arm in series is turned off, when the thyristor is turned off, the bridge arm directly connected with the thyristor in series is turned off, and when all bridge arms in the MMC are turned off, the direct-current fault isolation of the power grid is completed.
9. The system of claim 8, wherein the signal acquisition unit comprises:
the first signal unit is used for collecting the current of a bridge arm in the MMC;
and the second signal unit is used for acquiring signals for judging the power grid operation fault required by the power grid protection system, wherein the signals comprise a line outlet voltage traveling wave and a current traveling wave, and the signals comprise the line outlet voltage traveling wave and the current traveling wave.
10. The system of claim 9, wherein the fault detection unit comprises:
the first detection unit is used for determining whether the MMC bridge arm is in an overcurrent state or not according to the current signal of the first signal unit, and determining that a power grid is in a fault state when any bridge arm is in the overcurrent state;
and the second detection unit is used for determining whether the power grid is in fault operation according to the signal of the second signal unit, wherein for the traveling wave protection, the fault operation is judged by using the amplitude or the change rate of the voltage traveling wave high-frequency component, the fault direction is identified by using the change rate of the current traveling wave, and for the single-end quantity protection, the single-side current traveling wave high-frequency energy is extracted, and the fault transient energy amplitude is used for fault identification.
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