CN112072619A - Self-adaptive reclosing method for single-phase earth fault in bipolar direct current website - Google Patents
Self-adaptive reclosing method for single-phase earth fault in bipolar direct current website Download PDFInfo
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- CN112072619A CN112072619A CN202010849753.5A CN202010849753A CN112072619A CN 112072619 A CN112072619 A CN 112072619A CN 202010849753 A CN202010849753 A CN 202010849753A CN 112072619 A CN112072619 A CN 112072619A
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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
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
- H02H3/02—Details
- H02H3/06—Details with automatic reconnection
- H02H3/066—Reconnection being a consequence of eliminating the fault which caused disconnection
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Abstract
The invention relates to a self-adaptive reclosing method for a single-phase earth fault in a bipolar direct-current power supply website, which comprises the following steps: installing a direct current breaker, namely a metallic loop breaker on a metallic loop, and clearing fault current by opening a direct current breaker DCCB and a metallic loop breaker MRCB after the fault MMC is subjected to overcurrent locking; secondly, a self-adaptive reclosing method comprises the following steps: based on the characteristics of the neutral point potential at the valve side, namely the delta side, of the transformer, the property of the single-phase earth fault in the station is distinguished, so that self-adaptive reclosing is realized.
Description
Technical Field
The invention relates to the field of relay protection and automation of a power system, in particular to a strategy for clearing single-phase earth faults in an MMC-HVDC electric website with a symmetrical bipolar topology and a self-adaptive reclosing method.
Background
The flexible direct-current power transmission technology based on the Modular Multilevel Converter (MMC) has good application prospect in the fields of multi-terminal direct-current power grid construction, new energy access and consumption, asynchronous interconnection of power grids, island power supply and the like due to the advantages of high output current waveform quality, low switching loss, high modularization degree, easiness in expansion and the like. The flexible direct current system adopting the symmetrical bipolar connection mode has a more flexible operation mode and a lower overvoltage level, accords with the development trend of high voltage, large capacity and multi-terminal of a direct current power grid, and gradually becomes a mainstream topology of flexible direct current power transmission.
The transient characteristic research and the establishment of a protection scheme aiming at the direct current fault and the alternating current system fault of the bipolar MMC-HVDC power grid tend to be perfected step by step, but the research on the faults in the station is still not mature. The in-station fault refers to a fault on a valve side connecting line between a transformer and a converter, the environment in the converter station is good, the fault probability is low, and the damage of the in-station fault is possibly more serious than that of a direct current fault due to the fact that the in-station fault is closer to the converter. After single-phase earth fault takes place in the station, MMC submodule piece electric capacity can discharge fast, leads to MMC to be because of overflowing and shutting very fast. The traditional fault clearing method after locking helps the alternating current circuit breaker to be reliably disconnected by installing auxiliary devices, the scheme is long in time consumption, the MMC is threatened to bear large current and overvoltage for a long time, and some auxiliary devices can influence the normal operation of the converter with the sound pole.
The Zhang Bei flexible direct current engineering is used as the first flexible direct current power grid engineering in the world, and an overhead line is used as an in-station power transmission line, so that the probability of instantaneous faults is greatly increased. In addition, when the surface of the wall casing is slightly polluted, the surface of the wall casing is easily subjected to flashover caused by moisture, rain and thunder, which is also a common transient fault. If adopt automatic reclosing to handle single-phase earth fault in the station, when reclosing in permanent trouble, can lead to MMC to suffer secondary damage: not only does the MMC lower bridge arm generate large alternating short-circuit current again, but also the direct current line can generate secondary overcurrent.
In conclusion, a fault clearing strategy with better quick action performance is formulated, and an adaptive reclosing method capable of distinguishing the property of the single-phase earth fault in the station is designed, so that the method has important significance for protecting the safety of a power system and improving the transmission efficiency of direct current power.
Disclosure of Invention
The invention provides a fault clearing and self-adaptive reclosing method which can clear faults more quickly and effectively and reduce the damage to a converter as much as possible aiming at single-phase earth faults in a bipolar MMC-HVDC electric website. After the fault current is eliminated, the property of the single-phase earth fault in the station can be reliably identified, the MMC is unlocked immediately and the circuit breaker is closed immediately when the fault occurs instantaneously, and the uninterrupted transmission of the direct current power is realized; and the system is not restarted in case of permanent fault, so that secondary impact on the converter is avoided. The technical scheme is as follows:
a method for eliminating single-phase earth fault and self-adaptive reclosing in a bipolar direct current website comprises
(ii) Fault clearing strategy
Installing a direct current breaker, namely a metallic loop breaker on a metallic loop, and clearing fault current by opening a direct current breaker DCCB and a metallic loop breaker MRCB after the fault MMC is subjected to overcurrent locking;
adaptive reclosing method
Based on the characteristic of neutral point potential at the valve side, namely the delta side, of the transformer, the property of the single-phase earth fault in the station is distinguished so as to realize self-adaptive reclosing, and the identification criterion of the instantaneous single-phase earth fault in the station is as follows:
Uo≤Kset (1)
in the formula (1), UoIs the neutral point potential u on the delta side of the transformeroAmplitude of (A), KsetSetting a threshold value; u. ofoIs obtained by measuring the zero sequence voltage at the delta side, i.e. u is shown as formula (2)A、uB、uCMeasuring three-phase alternating voltage instantaneous values at a delta side; ksetThe reliability of the criterion is ensured by the determination of the formula (3), wherein UlThe amplitude of the AC line voltage on the delta side is the amplitude of the AC line voltage when the system is in normal operation.
Compared with the prior art, the invention has the beneficial effects that:
1. the fault clearing strategy can completely clear fault current only in 4ms, the traditional method based on the disconnection of the alternating current circuit breaker generally needs a plurality of cycles to clear the fault, and the strategy is far superior to the traditional method in terms of speed and mobility, so that the threat of the fault to the MMC is greatly reduced.
2. The self-adaptive reclosing method only needs to measure the instantaneous value of the three-phase alternating-current voltage, the principle is simple, the calculated amount is small, and the algorithm is easy to realize, so that the design difficulty and the cost of a microcomputer protection device in actual engineering are reduced.
3. The discrimination of the observed quantity is high when the faults with different properties occur, which shows that the method has good reliability; theoretically, the method is not influenced by the transition resistance and has strong resistance to the transition resistance.
4. Compared with the traditional automatic reclosing method, the method avoids the serious consequence that the converter suffers from secondary impact under permanent fault, and better ensures the safety of the converter and other power electronic equipment.
Drawings
FIG. 1 is a schematic diagram of a four-terminal bipolar MMC-HVDC power grid topology;
FIG. 2 is a fault current path after locking of a faulty MMC;
FIG. 3 is a potential vector diagram of primary and secondary side phases of a transformer during normal operation;
FIG. 4 is an ungrounded system after the DCCB and the MRCB have been disconnected;
FIG. 5 is a graph of primary and secondary phase potential vectors of a transformer during a permanent fault;
FIG. 6 is a graph of primary and secondary phase potential vectors of a transformer during transient faults;
FIG. 7 is a simulation result of the method in case of permanent failure;
fig. 8 is a simulation result of the method in case of transient fault.
Detailed Description
The technical scheme of the invention is as follows:
(ii) Fault clearing strategy
The existing clearing method based on the cut-off alternating current circuit breaker has poor speed, so that the current converter is damaged by over current and over voltage for a long time. The invention installs a direct current breaker on a metal return wire, namely the metal return wire breaker. After the fault MMC is subjected to overcurrent locking, fault current can be completely cleared within 4ms by opening and closing a direct current breaker (MRCB) and a metal loop breaker (MRCB), so that the time of the converter suffering from overcurrent and overvoltage is greatly shortened, and expensive power electronic devices in the converter are well protected.
Adaptive reclosing method
After the fault clearing strategy is adopted, the property of the single-phase earth fault in the station can be accurately distinguished based on the characteristic of the neutral point potential of the transformer valve side (delta side) so as to realize self-adaptive reclosing, and the specific technical scheme is as follows:
the identification criteria for a transient intra-station single-phase earth fault are as follows.
Uo≤Kset (1)
In the formula (1), UoIs the neutral point potential u on the delta side of the transformeroAmplitude of (A), KsetIs a setting threshold. u. ofoCan be obtained by measuring the zero sequence voltage at the delta side, i.e. u is shown as formula (2)A、uB、uCMeasuring three-phase alternating voltage instantaneous values at a delta side; ksetThe reliability of the criterion is ensured by the determination of the formula (3), wherein UlThe amplitude of the AC line voltage on the delta side is the amplitude of the AC line voltage when the system is in normal operation.
The protection criterion generally detects the single-phase earth fault in the station within 1ms after the fault occurs, and then locks the fault MMC and disconnects the DCCB and the MRCB. After a deionization time of 150-200ms, the observed quantity U is measuredoMultiple measurements are made in succession. When the three continuous results satisfy the formula (1), the instantaneous single-phase earth fault in the station can be determined, the MMC is unlocked, the DCCB and the MRCB are combined, and the direct-current power grid is gradually recovered under the regulation of the control systemNormal stable operation state; if the measurement result does not satisfy the formula (1) all the time, the single-phase earth fault in the station is permanent, the MMC cannot be unlocked, the DCCB and the MRCB cannot be reclosed, and the AC circuit breaker on the network side of the transformer is disconnected to enable the fault MMC to be completely separated from the DC power network for maintenance. At this time, the positive and negative pole converters of other converter stations can still transmit power, and the non-fault pole MMC in the fault converter station can also normally operate through the metallic return wire.
The invention is described in further detail below with reference to the figures and examples.
Fig. 1 is a schematic diagram of a four-terminal bipolar MMC-HVDC grid topology, in which a fault F indicates that an intra-station single-phase ground fault occurs in the positive converter of the converter station S1.
The principle of the fault clearing strategy is first introduced. When a fault occurs and the MMC is overcurrent locked, the fault current path is as shown in fig. 2, and there are 3 parts: the first is the current fed to the fault converter by other converters through the DC line, i.e. ifsm1(ii) a But rather a slowly decaying freewheeling current, i.e. i, of the lower arm of the faulted phasefsm2(ii) a Third, the AC short-circuit current on the lower bridge arm of the non-fault phase, ifac. The MRCB is mounted in the position shown in fig. 2 at the junction of the MMC negative outlet and the metallic return. At the moment, the DCCB and the MRCB are disconnected, all three fault current paths are cut off, and the fault is cleared. Because the DCCB and the MRCB are both hybrid direct-current circuit breakers, current can be switched in 3ms, and the time of 1ms for fault identification is added, the method can realize the fault clearing in 4ms of fault occurrence, and has good speed.
The method of identifying the nature of the fault is described next. When the symmetrical bipolar MMC-HVDC system operates normally, the transformer generally adopts Y0/. DELTA.connection set. Taking Yd11 connection group number as an example, the primary and secondary phase potential vectors are shown in FIG. 3. In the figure, ABC is the head end of the Y-side winding, and O is a zero potential neutral point; abc is the head end of the winding on the delta side, and o is the neutral point on the delta side. It should be noted that in bipolar system, the o-point potential is not zero because the MMC cathode is grounded via the metal loop, so that the o-point potential has
The potential of the neutral point on the delta side is represented by the following formula (4) in normal operation.
Taking the phase-a ground fault as an example, if the fault is a permanent fault, the fault point exists after the DCCB and the MRCB are disconnected. The ac system and the inverter now form a non-grounded system, as shown in fig. 4. The A-phase voltage at the delta side of the transformer consists of two parts of line voltage drop and transition resistance voltage drop. Due to the ungrounded system, only capacitance-to-ground current flows through the transition resistor. And because the power transmission line in the station is short and only tens of meters long, the capacitance-to-ground current and the line voltage drop can be ignored. Therefore, it can be said that the Δ -side a-phase voltage is about zero regardless of the transition resistance value. So far, a vector diagram of the primary and secondary side phase potentials when the A phase grounding in the permanent station fails is shown in FIG. 5,
indicates a potential u of a neutral point on the delta sideo. As can be seen from the graph, the neutral point potential u on the delta side at the time of permanent failureoOf amplitude of delta-side line voltage
Times as high as several hundred kilovolts.
If the fault is a transient fault, the fault point disappears at a certain moment after the DCCB and the MRCB are disconnected, and the fault point and the grounding point of the metal return wire do not exist at the moment. Therefore, compared to normal operation, the dc bias of the Δ -side neutral point potential disappears, i.e., the o point becomes a zero potential point. In this case, as shown in FIG. 6, the vector diagram of the primary and secondary side phase potentials shows, at the time of transient failure, the Δ -side neutral point potential uo＝0。
Based on the above analysis, if the fault is a permanent fault, the neutral point potential u on the delta sideoTo amplitude being line voltage
A multiplied AC waveform; if the fault is instantaneous fault, the neutral point potential u on delta side o0. Therefore u can be utilizedoThe identification of the fault property is realized by the difference of the amplitudes, so that the identification criterion of the single-phase earth fault in the transient station shown in the formula (1) can be obtained.
In summary, the overall implementation steps of the present invention are as follows:
after the protection criterion identifies the single-phase earth fault in the station and locks the MMC, a command is sent to a control system, so that the DCCB and the MRCB are disconnected after 3ms, dissociation is removed after about 150ms, and the three-phase alternating voltage on the delta side is sampled to calculate the amplitude U of the neutral point potentialo. Only when the three continuous measurement results satisfy the formula (1), the fault can be judged to be a transient fault, the MMC is unlocked, and the DCCB and the MRCB are combined; if the measurement result does not satisfy the formula (1) all the time, the fault is permanent, and then the alternating current circuit breaker is directly disconnected, so that the MMC is overhauled off the network. The simulation results of the method under the two-property fault are shown in fig. 7 and fig. 8. As can be seen from the figure, the method can reliably distinguish the single-phase earth fault in the station which is transient and permanent, and has certain engineering significance.
Claims (1)
1. A method for eliminating single-phase earth fault and self-adaptive reclosing in a bipolar direct current website comprises
(ii) Fault clearing strategy
And a direct current breaker, namely the metallic loop breaker, is arranged on the metallic loop, and when the fault MMC is subjected to overcurrent locking, fault current is removed by opening the direct current breaker DCCB and the metallic loop breaker MRCB.
Adaptive reclosing method
Based on the characteristic of neutral point potential at the valve side, namely the delta side, of the transformer, the property of the single-phase earth fault in the station is distinguished so as to realize self-adaptive reclosing, and the identification criterion of the instantaneous single-phase earth fault in the station is as follows:
Uo≤Kset (1)
in the formula (1), UoIs the neutral point potential u on the delta side of the transformeroAmplitude of (A), KsetSetting a threshold value; u. ofoIs obtained by measuring the zero sequence voltage at the delta side, i.e. u is shown as formula (2)A、uB、uCMeasuring three-phase alternating voltage instantaneous values at a delta side; ksetThe reliability of the criterion is ensured by the determination of the formula (3), wherein UlThe amplitude of the AC line voltage on the delta side is the amplitude of the AC line voltage when the system is in normal operation.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN117706943A (en) * | 2024-02-06 | 2024-03-15 | 南京中鑫智电科技有限公司 | Self-adaptive control method and system for converter transformer valve side sleeve end screen voltage divider |
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Cited By (2)
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