CN111682510A - First-removal and then-isolation protection method for ground fault of symmetrical unipolar direct-current microgrid - Google Patents
First-removal and then-isolation protection method for ground fault of symmetrical unipolar direct-current microgrid Download PDFInfo
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- CN111682510A CN111682510A CN202010422062.7A CN202010422062A CN111682510A CN 111682510 A CN111682510 A CN 111682510A CN 202010422062 A CN202010422062 A CN 202010422062A CN 111682510 A CN111682510 A CN 111682510A
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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
- H02H3/16—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 responsive to fault current to earth, frame or mass
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Abstract
The invention relates to a first-removal and second-isolation protection method for a ground fault of a symmetrical unipolar direct-current microgrid, which is characterized in that the fault type is judged through an electric quantity signal at the outlet of a VSC (voltage source converter), fault selection is carried out, the fault circuit is removed by using an anti-parallel IGBT (insulated gate bipolar transistor) connected in series on a neutral line, fault circuit partitioning is realized through current direction information and differential current information, finally a fault circuit is isolated through a direct-current circuit breaker, and fault partitioning is carried out through current direction information or differential current information before the ground fault circuit is removed according to different microgrid topologies and ground fault types.
Description
Technical Field
The invention belongs to the field of relay protection of power systems, and particularly relates to a protection method for a fault line isolated in a partition mode after a ground fault loop is cut off aiming at a ground fault of a directly-grounded symmetrical unipolar direct-current microgrid system.
Background
With the continuous exhaustion of fossil energy, power generation technologies based on renewable energy become research hotspots, and distributed power generation technologies based on wind power, photovoltaic and the like are gradually developed and bear the power utilization requirements of part of users. However, the distributed power generation technology is mostly direct current output, and in order to reduce the cost and the electric energy loss of a current conversion link, a direct current microgrid using VSC based on the PWM modulation technology as a main current conversion unit is subjected to a great deal of research with excellent performance and has been applied at home and abroad.
At present, the research on a protection scheme for ground faults after a direct-current microgrid grounding mode and a grounding mode are determined is insufficient. The grounding mode of the dc microgrid needs to consider the influence of many factors, including the normal operation performance of the dc microgrid, personal safety, cost for equipment configuration, insulation requirements, corrosion and noise caused by leakage current, and the like. The grounding mode which completely meets the requirements and is cost-effective is difficult to realize, and an optimal scheme is sought in the selectable grounding modes according to different application occasions to become a main method for solving the direct-current microgrid grounding problem. In commercial and civil direct-current microgrid systems, symmetrical unipolar direct-current microgrids with neutral points directly grounded are widely researched.
Most of the existing protection schemes for the direct-current microgrid after a fault occurs need a direct-current circuit breaker with high communication speed, high reliability, high absolute sampling synchronism and high interruption capacity, and the direct-current circuit breaker and the direct-current microgrid need high cost, so that the economy of the direct-current microgrid is reduced. Moreover, immediately after a large transition resistance ground fault (larger than the load resistance, here set to 20 Ω or more) occurs, the voltage current change in the network is small, and fault detection and selection cannot be realized. Therefore, it is necessary to provide a fault detection and selection scheme for a large-transition-resistance ground fault of a symmetric unipolar dc microgrid, and it is necessary to provide a protection scheme for a dc microgrid ground fault in the grounding mode, in which a ground fault circuit is cut off first and then a fault line is isolated in a partitioned manner, which can improve the economy.
Disclosure of Invention
The invention provides a protection method for a partitioned isolated fault line aiming at a symmetrical unipolar direct-current micro-grid which adopts a VSC converter as direct ground and is connected with an alternating-current power grid. The method can effectively overcome the defect of high cost of the existing protection method, and can improve the power supply reliability after the ground fault to a greater extent. The technical scheme is as follows:
the utility model provides a first excision isolation protection method after for symmetrical monopole direct current microgrid ground fault, judges the fault type and carries out the trouble selection through VSC exit electric quantity signal, uses the anti-parallel connection IGBT who establishes ties on the neutral conductor to realize the trouble return circuit excision, rethread current direction information and differential current information realize the fault line subregion, keeps apart the fault line through direct current circuit breaker at last, and the step is as follows:
(1) a pair of anti-parallel IGBTs are connected in series to a grounding wire at the VSC outlet of the direct-current microgrid, and a conducting signal is given during normal operation so as to flow leakage current and realize effective grounding.
(2) After the micro-grid has a small transition resistance ground fault of less than 10 omega, performing rapid fault detection by using an electric quantity differential method; when the microgrid has a large transition resistance earth fault of more than 20 omega, fault detection is carried out through the characteristic of voltage unbalance of series capacitors on the direct current side of the VSC, and when the transition resistance earth fault is between 10 omega and 20 omega, if the current differential cannot exceed the threshold value, the fault is detected by utilizing the characteristic of voltage unbalance.
(3) And after the occurrence of the ground fault is detected, blocking the conducting signals of the anti-parallel IGBTs on the neutral line, and cutting off a fault loop which must pass through the neutral line.
(4) And after the fault loop is cut off, fault partitioning is carried out by cutting off the current direction information or differential current information before the ground fault loop according to different microgrid topologies and ground fault types.
(5) And after the fault partition is completed, the fault is removed through the load switch.
(6) And re-connecting the signal to each converter and the inverse parallel IGBT of the neutral line in the microgrid to restore normal operation.
Preferably, the load switch is a dc breaker with breaking capacity of normal load current.
Compared with the prior art, the invention has the following advantages:
1. the detection of the large-transition-resistance ground fault (>20 omega) can be realized without using sensitive measuring elements and complex algorithms.
2. Aiming at the ground fault, the power supply reliability of the conventional method of firstly carrying out fault partition and then isolating the fault line is improved, and the preparation time for restoring the normal operation of the converter after the protection is finished is shortened.
3. And a direct-current breaker with large breaking capacity is not needed, so that the cost is reduced.
4. The special fast communication line does not need to be configured for fast protection, the original communication mode for controlling the operation of the direct-current micro-grid is used, and the cost is reduced.
5. And a mutual inductor with high sampling synchronism based on a GPS is not required to be configured, so that the cost is reduced.
Drawings
Fig. 1 is a schematic diagram of a symmetrical monopole radial direct current microgrid with an anti-parallel IGBT connected in series on a ground wire.
Fig. 2 is a schematic diagram of a symmetrical unipolar ring-shaped direct-current microgrid with anti-parallel IGBTs connected in series on a ground wire.
Fig. 3 is a VSC dc side capacitance-voltage curve when a 50 Ω large transition resistance ground fault occurs in the dc microgrid positive electrode line.
Detailed description of the invention
The invention will be described in further detail with reference to the accompanying drawings
Aiming at a symmetrical unipolar direct-current microgrid which adopts a VSC converter as direct ground and is connected with an alternating-current power grid, a method for fault detection and fault pole selection after a large transition resistance ground fault occurs is designed, and then a protection method for isolating a fault line in a subarea mode after a ground fault loop is cut off is designed respectively aiming at the characteristics of a small transition resistance ground fault (less than 10 omega) and a large transition resistance ground fault (more than 20 omega) and the fault detection result.
Fig. 1 and fig. 2 are schematic diagrams of symmetrical monopole radial and annular direct-current micro-grids with anti-parallel IGBTs connected in series on a ground wire respectively. Only one ground point is generally established to the direct current microgrid, and the ground point sets up in VSC department. According to the scheme, a pair of anti-parallel IGBTs with rated current values far smaller than that of IGBTs in the converter are connected in series on a grounding wire of a VSC neutral point, and a signal is conducted to flow leakage current during normal operation.
When any line of the direct-current microgrid has a small-transition-resistance ground fault, capacitors at all positions of the system discharge to the ground point, the superposed fault current needs to flow through the ground line to form a fault loop, the current value of the ground line is the largest, the change rate of the current value of the ground line is large, the occurrence of the small-transition-resistance ground fault can be judged through the current signal at the ground line, and the fault pole selection can be carried out through the change situation of the current values of the positive and negative lines. When a large-transition-resistance ground fault occurs on any one line of the direct-current microgrid, within tens of milliseconds, the electrical quantity in the system has no obvious change, so that the fault cannot be detected and the fault pole selection is performed, but as the time is prolonged, the voltage unbalance occurs on the two series capacitors on the direct-current side of the VSC, and the voltage unbalance is used for detecting the large-transition-resistance ground fault and performing the fault pole selection.
Large-transition resistance fault (>20 omega) detection and pole selection method: when no obvious overcurrent exists on the direct current side of the VSC, the voltage of the series capacitors shifts simultaneously, the voltage value of one capacitor increases, the voltage value of the other capacitor decreases, it can be determined that a large-transition-resistance ground fault occurs in the network, and a line connected with the capacitor with the decreased voltage value is a fault pole. This scheme takes about 1-2s to detect a large transition resistance ground fault. Fig. 3 is a VSC dc side capacitance-voltage curve when a 50 Ω large transition resistance ground fault occurs in the dc microgrid positive electrode line.
And after the fault detection is finished, the anti-parallel IGBT connected in series on the grounding wire is locked, and the grounding fault loop is cut off.
After the ground fault loop is cut off, fault partitioning is carried out according to the size of the ground fault resistor and the direct-current microgrid topology, and the specific method is as shown in the following table.
After the fault line is cut off, the IGBT is connected in anti-parallel on the grounding line again to set a conducting signal, and the system gradually recovers to normal operation.
Claims (2)
1. The utility model provides a first excision isolation protection method after for symmetrical monopole direct current microgrid ground fault, judges the fault type and carries out the trouble selection through VSC exit electric quantity signal, uses the anti-parallel connection IGBT who establishes ties on the neutral conductor to realize the trouble return circuit excision, rethread current direction information and differential current information realize the fault line subregion, keeps apart the fault line through direct current circuit breaker at last, and the step is as follows:
(1) a pair of anti-parallel IGBTs are connected in series to a grounding wire at the VSC outlet of the direct-current microgrid, and a conducting signal is given during normal operation so as to flow leakage current and realize effective grounding.
(2) After the micro-grid has a small transition resistance ground fault of less than 10 omega, performing rapid fault detection by using an electric quantity differential method; when the large transition resistance earth fault of more than 20 omega occurs in the microgrid, fault detection is carried out through the characteristic of voltage unbalance of series capacitors on the direct current side of the VSC, and when the transition resistance earth fault is between 10 omega and 20 omega, if the current differential cannot exceed the threshold value, the fault occurrence is detected through the characteristic of voltage unbalance;
(3) after the occurrence of the ground fault is detected, closing a conducting signal of the anti-parallel IGBT on the neutral line, and cutting off a fault loop which must pass through the neutral line;
(4) after the fault loop is cut off, fault partitioning is carried out through current direction information or differential current information before the ground fault loop is cut off according to different microgrid topologies and ground fault types;
(5) after fault partition is completed, the fault is removed through a load switch;
(6) and re-connecting the signal to each converter and the inverse parallel IGBT of the neutral line in the microgrid to restore normal operation.
2. The method of claim 1, wherein the load switch is a dc circuit breaker having a breaking capacity for normal load current.
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| Application Number | Priority Date | Filing Date | Title |
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| CN202010422062.7A CN111682510A (en) | 2020-05-18 | 2020-05-18 | First-removal and then-isolation protection method for ground fault of symmetrical unipolar direct-current microgrid |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2022174623A1 (en) * | 2021-02-20 | 2022-08-25 | 阳光电源股份有限公司 | Series-type inverter system and protection method therefor |
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| CN104953568A (en) * | 2015-07-17 | 2015-09-30 | 河南行知专利服务有限公司 | Fault protection method for flexible DC power transmission system |
| CN205246812U (en) * | 2015-12-22 | 2016-05-18 | 南京南瑞继保电气有限公司 | Fault detection device of flexible direct current circuit |
| CN108923398A (en) * | 2018-07-23 | 2018-11-30 | 国网浙江省电力有限公司电力科学研究院 | A kind of DC distribution network protection method based on voltage characteristic traveling wave Similar measure |
| CN109494695A (en) * | 2019-01-07 | 2019-03-19 | 南京南瑞继保电气有限公司 | A kind of method and system of middle straightening stream distribution network failure isolation and fault recovery |
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- 2020-05-18 CN CN202010422062.7A patent/CN111682510A/en active Pending
Patent Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2011157305A1 (en) * | 2010-06-14 | 2011-12-22 | Abb Research Ltd | Fault protection of hvdc transmission lines |
| CN104953568A (en) * | 2015-07-17 | 2015-09-30 | 河南行知专利服务有限公司 | Fault protection method for flexible DC power transmission system |
| CN205246812U (en) * | 2015-12-22 | 2016-05-18 | 南京南瑞继保电气有限公司 | Fault detection device of flexible direct current circuit |
| CN108923398A (en) * | 2018-07-23 | 2018-11-30 | 国网浙江省电力有限公司电力科学研究院 | A kind of DC distribution network protection method based on voltage characteristic traveling wave Similar measure |
| CN109494695A (en) * | 2019-01-07 | 2019-03-19 | 南京南瑞继保电气有限公司 | A kind of method and system of middle straightening stream distribution network failure isolation and fault recovery |
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| WO2022174623A1 (en) * | 2021-02-20 | 2022-08-25 | 阳光电源股份有限公司 | Series-type inverter system and protection method therefor |
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Application publication date: 20200918 |