1 arc-absorbing cabinet principle
Arc-absorbing cabinet is essentially a voltage transformer cabinet with arc-absorbing, harmonic elimination and over-voltage protection function (PT cabinet), the arc-absorbing principle of operation see Figure l. The use of arc-absorbing cabinet 6 ~ 35 kV grid using neutral point of the ungrounded mode of operation Grid normal operation, the three sub-portions of the arc-absorbing cabinet, the arc-absorbing cabinet is a very important part of the power grid. Arc extinguishing cabinet in the three split-phase control of high-voltage vacuum contactor (KM) are in the breaking state. The three-phase voltage output from the secondary side of the voltage transformer (PT) is normal. The zero sequence voltage is almost zero. The microcomputer intelligent controller is responsible for real-time monitoring of the zero-sequence voltage and three-phase voltage of the power grid When a single-phase ground fault occurs in the power grid, the voltage of the faulted phase rises and the voltage of the non-faulted phase falls. The zero sequence voltage increases greatly. When the zero sequence voltage reaches a certain value, the controller decides that a single-phase ground fault has occurred in the system. And through the calculation and analysis of each phase voltage, determine the phase of the grounding fault. To the corresponding phase of the high-voltage vacuum contactor to send a closing command. Realize metallic grounding of the faulted phase directly in the device. At the same time, an alarm signal is sent to the central control room. The alarm signal is sent to the central control room so that the operator can be notified to deal with the grid fault in time. Whether the single-phase grounding fault is intermittent arc grounding or stable arc grounding. Because the arc arc-absorbing cabinet directly to the fault phase in the device into a metallic grounding. The fault phase of the ground voltage drops to zero. The original fault point of the arc must be extinguished. Avoid the generation of arc overvoltage, and the other two phases of the ground voltage is limited to the level of line voltage.
2 Arc arcing cabinet selection
At present. The vast majority of arc arc arc elimination cabinet is not set in Figure l switch (SW), let's set the arc arc elimination cabinet without a switch is called A-type cabinet, and set the switch arc elimination cabinet called B-type cabinet. Although the A-type cabinet only has one less diverter switch than the B-type cabinet, this issue will be a major concern for the A-type cabinet. But this problem will be the fatal flaw of the A-type cabinet should be said. For a single busbar system. For single busbar system, only one arc extinguishing cabinet is required. For a single busbar system, the use of only one arc-absorbing cabinet, the choice of A-type cabinet is no problem, but for the need to use more than one arc-absorbing cabinet of the multi-bus system. Selection of A-type cabinet will seriously affect the reliability of power supply l: L ~ n. In the bus system for a single bus segmentation of the dual power source 6 ~ 35 kV power grid. Normally, two busbars are operated in parallel (i.e., the busbar switch is closed). If an A-type cabinet is installed on each of the two busbars, a single-phase ground fault occurs in the network. When a single-phase ground fault occurs in the grid. Both A-type arc-absorbing cabinets will be operated, so that the same phase of the two sections of the busbar are separately grounded. However. When the grid single-phase ground fault eliminated. The system still exists in two separate arc-absorbing cabinet caused by the two grounding points. This will make the two arc extinguishing cabinet interlocking and can not be restored. Must have a serious impact on the reliability of the power supply to the power grid B-type arc elimination cabinet. For type B arc eliminating cabinet, if the switching switch in the device is closed, its function is identical to that of type A arc eliminating cabinet If the switching switch is opened, when the controller detects a single-phase ground fault, it will only send out an alarm signal and will no longer make the high-voltage vacuum contactor act to close the circuit breaker. At this point, the B-type arc arc arc elimination cabinet no longer has the function of arc elimination. B-type arc-absorbing cabinet can be flexibly applied to all 6 ~ 35 kV power grid As long as the proper use. As long as it is used properly, it will not be like the type A arc elimination cabinet, the arc elimination cabinet interlocked and can not be restored to the situation. Still take the bus system for a single bus segmented double power 6 ~ 35 kV power grid, for example. Two sections of the bus each installed on a B-type cabinet two sections of the bus running in parallel. One of the two busbars can be used as an arc-absorbing device (arc-absorbing cabinet switching switch closed). And use the other one as intelligent PT cabinet (switching switch of arc-absorbing cabinet is open). When a single-phase ground fault occurs in the power grid, the B-type arcing device is used as an arcing eliminator. The B-type cabinet used as an arc-absorbing device operates. Ground the fault phase of a section of busbar. The B-type cabinet used as an intelligent cabinet only sends an alarm signal when the grid single-phase grounding fault is eliminated. When a single-phase ground fault is eliminated, there is only one grounding point in the system. The arc-absorbing cabinet can realize the reversion of the faulty phase. If the busbar switch is open, both B-type cabinets can be used as arc-absorbing devices. The two busbars represent two separate systems. The two busbars represent two separate systems, and there is no interlocking of the two arc-absorbing cabinets. In short, in the design of 6-35 kV power grid. In conclusion, when designing the 6-35 kV power grid, if the arc arcing cabinet is used to suppress the arc overvoltage. The correct selection of arc-absorbing cabinet is very important. Improper selection will seriously jeopardize the safe operation of the power grid and power supply reliability in the need to use more than one arc-absorbing cabinet multi-bus system. Can not choose A-type cabinet ． And should be selected B-type cabinet.
3 arc arc-canceling cabinet configuration
3.1 single bus system for the main connection for the main power supply, single bus system of the grid, arc-canceling cabinet can refer to Figure 2 for configuration. Should be installed in the substation bus at a B type arc extinguishing cabinet (B). If the grid also exists in the distribution (dotted line box part). Should be in one of the distribution bus at the configuration of another B-type arc extinguishing cabinet. Other distribution only need to install intelligent Prr cabinet (P) that can The arc arc extinguishing cabinet at the substation should be used as the main arc extinguishing equipment, while the arc extinguishing cabinet at the distribution as a backup arc extinguishing equipment. Usually. Only the main arc-canceling equipment is used as an arc-canceling device, while the backup arc-canceling equipment is used only as a smart cabinet. Only in the case of failure of the main arc-canceling equipment, the backup arc-canceling equipment can be used as arc-canceling device. Can be used as a backup arc elimination equipment arc elimination device.
3.2 single-bus segmented main connection for the main connection for the dual power supply, single bus segmented system of the grid, arc-canceling cabinet can refer to Figure 3 configuration should be installed in the substation of each section of the bus at each type B arc-canceling cabinet (B). Two arc-quenching cabinet for each other as a backup. If the two busbars operate in parallel in grid operation (busbar switch M closed), one of the arc-absorbing cabinets can be used as a back-up for the other. If the two busbars operate in parallel (busbar switch M closed), one of the arc-absorbing cabinets is used as an arc-absorbing device and the other arc-absorbing cabinet is used as an intelligent cabinet. If the two buses are run independently (busbar switch M open), the two arc-absorbing cabinets at the same time used as arc-absorbing equipment The power distribution network does not need to choose the arc-absorbing cabinet and only need to be equipped with an intelligent Frr cabinet can be (not pictured in the figure).
3.3 Arc extinguishing cabinet configuration principles In the neutral point is not grounded 6 ~ 35 kV power grid. To use arc-absorbing cabinet to inhibit arc overvoltage. It is necessary to ensure the correct configuration of the arc arc-quenching cabinet. Only the correct configuration of the arc extinguishing cabinet. To effectively inhibit arc overvoltage, to ensure the safe operation of the power grid and power supply reliability. At the same time, but also to save equipment investment in the 6 "" 35 kV power grid design. Arc extinguishing cabinet selection should not be too much, otherwise not only increase equipment investment. Will also bring unnecessary trouble to the grid operation Usually each system can be set up two B-type arc-absorbing cabinet, distribution bus can be selected intelligent cabinet. Two arc-absorbing cabinet for each other as a backup. In grid operation at the same time can only be used as an arc-absorbing equipment, the other can be used as intelligent Prr cabinet.
4 Intelligent PT cabinet
Intelligent Pr cabinet is a new type of voltage transformer cabinet with microcomputer voltage measurement, insulation monitoring, harmonic elimination and over-voltage protection, which can be used as a complementary arc elimination cabinet and installed on the bus of the distribution substation, but also can be used separately instead of the traditional Prr cabinet. The function of intelligent cabinet is as follows: ① Basic function: the function of ordinary Frr cabinet: ⑦ Voltage measurement function: real-time monitoring and display of three-phase voltage and zero sequence voltage: ③ Insulation monitoring function: automatic alarm when single-phase grounding of the power grid. Automatic display of grounding phase, automatic recording of voltage parameters during grounding fault for query. When a small current grounding selector is selected, it will automatically indicate the grounding circuit: ④ Pr disconnection detection: automatically detect disconnection and alarm, and automatically indicate the phase of disconnection; ⑤ Harmonic elimination function: automatically eliminate ferromagnetic resonance caused by saturation of the voltage transformer; ⑥ Over-voltage protection function: not only to prevent atmospheric over-voltage, but also to limit the internal over-voltage; ⑦ Other functions: Remote monitoring and computer networking functions.
5 Conclusion
To make the arc eliminating cabinet normal function. Must ensure that the arc-canceling cabinet of the correct selection, configuration and use: ① in the arc-canceling cabinet design selection, should avoid the use of A-type arc-canceling cabinet, and should be selected B-type arc-canceling cabinet; ② in the configuration of the arc-canceling cabinet. For most of the small current grounding system can be set up two B-type arc extinguishing cabinet. As a complement to the arc-absorbing cabinet, power distribution bus can be selected intelligent cabinet: ⑧ in the arc-absorbing cabinet when used. Must ensure that at the same time there can only be a B-type arc-absorbing cabinet used as arc-canceling equipment. And another B-type arc-absorbing cabinet is used as a smart cabinet. The two B-type arc-absorbing cabinets are backed up by each other. At present, the arc-absorbing cabinet has begun to be used in large quantities in the 6-35 kV power grid. To solve the problem of arc grounding overvoltage. Actual operation shows that the correct selection, configuration and use of arc-absorbing cabinet is the premise of effective suppression of arc overvoltage. In order to ensure that the selection, configuration and use of the correct conditions. Arc extinguishing cabinet can effectively inhibit arc overvoltage. And improper selection, configuration and use will seriously jeopardize the safe operation of the power grid. Affect the reliability of the power supply to the grid.
China's power system neutral point of operation are: neutral point is not grounded, neutral point by the arc-canceling coil grounding and neutral point directly grounded three kinds of the first two grounding system called "small current grounding system". In the small current grounding system, single-phase grounding fault is the most common, accounting for more than 80% of the faults in the distribution network. Single-phase grounding, due to the small fault current, making fault routing more difficult. Conventional substations rely on insulation monitoring devices to signal the operator. Then the operator by connecting to the voltage transformer secondary phase voltage in the table value to determine the fault point. Because the insulation monitoring device can only judge a certain voltage level system has grounded, but can not point out the fault point where the line, so in order to find the fault point, must be in order to disconnect the line switch, and then automatically reclosing to restore power supply. This seriously affects the reliability of power supply.
In recent years, with the application of integrated automation equipment in the power supply system, the small current grounding line has been able to do: single-phase grounding can be directly determined where the fault line. This provides a reliable guarantee for us to quickly find the fault point. Correct application of integrated automation equipment in small current grounding function, is a worthy of study and attention to the issue of ?
1 single-phase grounding neutral ungrounded system characteristics
Neutral ungrounded system normal operation, the relative ground voltage is symmetrical, neutral to ground voltage is zero, no zero sequence voltage in the grid.
The following conclusions can be drawn:
a) After single-phase grounding occurs in the neutral ungrounded system, there will be zero-sequence currents and zero-sequence voltages in the grid, and the magnitude of zero-sequence voltages is equal to the phase voltages of the grid in normal operation.
b) fault line and non-fault line zero sequence current, fault line zero sequence current size is equal to the sum of the zero sequence current of all non-grounded lines, capacitive reactive power in the direction of the line to the bus; non-fault line zero sequence current size is equal to the line capacitive current to ground, the direction of its capacitive reactive power for the bus flow to the line.
c) The zero sequence current of the non-fault line exceeds the zero sequence voltage by 90°, the zero sequence current of the fault line lags the zero sequence voltage by 90°, and the zero sequence current of the fault line is in the opposite direction to the zero sequence current of the non-fault line.
d) The magnitude of the current at the ground fault is equal to the sum of the ground capacitance currents of all lines (including faulted and non-faulted lines) and exceeds the zero sequence voltage by 90°.
2 Principle of small-current grounding routing
According to the characteristics of the neutral ungrounded system at single-phase grounding, the current routing device is mainly based on the principle of zero-sequence power direction, the amplitude of the zero-sequence current principle and so on. Comprehensive automation substation equipment (Sifang company equipment as an example), small current grounding line selection function is connected to the bus on the distribution line protection (CSL216B ) device, open delta voltage monitoring point and the main station **** with the completion. When grounding occurs in the system, the zero sequence voltage (3U0) rises, and the device senses a sudden change in voltage with an amplitude of more than 10 V, the centralized measuring (CS12A) device detects and reports to the main station, which then broadcasts to the protective device of the distribution line and calculates the current zero sequence voltage 3U0 and zero sequence current vector. Then according to the direction of the zero sequence current connected to the bus on all lines to determine the grounding of the line, so that the device to determine where the fault is, and respectively, to the on-site monitoring computer and remote control center report, notify the maintenance personnel to deal with the fault point.
At present, there are two ways to counter the zero sequence current: one is to install a zero sequence current filter with three-phase current transformers in the distribution line; the other is to install a special zero sequence current transformer in the distribution line. I think: in the installation of "V" type wiring protection conditions, the use of additional V-phase current transformer method is more suitable (i.e., by the three-phase current transformers constitute the zero sequence current filter way), maintenance test is convenient. It is better to use the same wiring method in the same substation. If in the same substation or the same bus both three-phase current transformer wiring, but also the installation of a special zero sequence current transformer way, then be sure to make the zero sequence current transformer lead polarity is the same, otherwise the grounding selector device is impossible to work correctly.
3 Application of small current grounding selection
When the installation of a dedicated zero sequence current transformer is used in the distribution line, the following points should be noted:
a) Zero sequence current transformer is generally installed below the cable head, the zero sequence current transformer above the cable sheath ground line must be through the zero sequence current transformer grounding. Zero sequence current transformer below the cable sheath ground wire is not required to pass through the zero sequence current transformer, to avoid the formation of a short-circuit loop
b) support zero sequence current transformer iron frame should not form a closed frame.
c) All zero sequence current transformer polarity should be checked correctly. No matter what kind of zero sequence transformer is used, the lead polarity must be unified.
d) Zero sequence current transformer ratio selection should be correct.
It should be noted that the use of integrated automation of secondary equipment, substation primary and secondary equipment to be considered as a whole, otherwise it will cause automation equipment can not work properly.
Integrated automation substation, small current grounding line is an important function, by carefully analyzing the principle of small current grounding line device, and combined with many experiences in the application of engineering, pointing out that the application of small current grounding line need to pay attention to the relevant issues. And emphasizes the substation for new construction or expansion, the primary and secondary equipment should be considered comprehensively. Only after comprehensive consideration of various situations, in order to make the small current grounding line function correctly play a role, to achieve the purpose of the correct line.