AC motor - fault analysis
Single-phase ground fault definition:
Three-phase power system, only in a phase between the wire and the occurrence of insulation damage.
Broadly speaking, single-phase ground fault, including single-phase ground short circuit, single-phase ground short circuit is a single-phase ground fault in a special phenomenon, only the transformer neutral grounding system will have.
Narrow sense, single-phase ground fault and single-phase ground short circuit is two concepts. One is a fault, one is an accident.
The faults and accidents of the power system are narrowly differentiated.
Single-phase grounding short-circuit is the fire line (any phase of the power) is not directly connected to the ground through the load, in the 380/220 power supply system or other grounding system, because the transformer neutral line is grounded, and the grounding resistance is very small , The line of fire (any one phase) is not directly connected to the ground through the load, the moment will produce a lot of current, burn wire, power distribution equipment or trip, so single-phase ground short circuit is a serious accident, The system should be avoided as much as possible.
In the power system, there are many places is not connected to the system power supply, such as dust in a serious, flammable and explosive and other special sites, our country all 6-10 kV, 35 kV power supply system, are used are neutral Point is not grounded or by a large resistance to the ground power supply system, such a system after a single phase ground, because there is no loop or no high current circuit, no ground current or no large ground current, so does not affect the normal operation of the system. Although it does not affect the normal operation of the system, but the single-phase grounding is a failure, in order to prevent ground expansion for the two-phase grounding or further damage to the grounding equipment, the provisions of the provisions of a single-phase ground fault, Cause, to deal with, 2 hours can not find the cause or can not handle, to power off after power supply.
Single - phase earth fault finding method
For small current grounding system, how to quickly find single-phase ground fault, I give you some simple and feasible way.
1, manual search method
If there is no grounding in the substation installation line device, the line is not installed grounding fault indicator or short-circuit ground two-in-one fault indicator, there is no good ground fault detector, it had to use artificial search Benbanfa. Find the steps as follows:
(1) by manual (or scheduling, the following) in turn open, you can know which substation substation ground, through the scheduling know which phase ground.
⑵ There are two ways to find the next point of failure: First, the line step by step, or often the failure of the line opened, with 2.5kV shaking table measured ground insulation, insulation resistance of the small section of the fault (As of course, in the substation side of the substation must be done to mount the ground and other security measures); Second, the line as much as possible sub-section, and then step-by-step trial power transmission, and scheduling interaction with Zero sequence voltage alarm when the paragraph for the fault section.
Manual search method operation is very troublesome, if the line is long, more branches, switch segment and less, it is not good operation, coupled with the weather and bad weather, it is more difficult to deal with the. It is recommended to use some of the equipment to invest less technology to meet the manual search, can achieve a multiplier effect, both to provide power supply reliability and social benefits, but also to create economic benefits.
2, the use of ground line selection device and fault indicator to find
Substation stations are generally equipped with a grounding line selection device, although sometimes not allowed, but can provide technical reference for manual brake. And then install some ground fault indicator (or short-circuit ground two-in-one fault indicator) on the line to indicate the ground fault path. Relatively reliable ground fault detection method is the use of signal source method, the more sensitive ground fault detection method is the first half wave method or DC transient analysis method. It is suggested to use two kinds of grounding fault indicator to find the ground fault is better, mainly to the source method, supplemented by the first half wave method or DC transient analysis method.
3, the use of feeder automation methods to find
If the user is rich, it is recommended to use the feeder automation method to find the ground fault. This method replaces the traditional manual search method with the logic function of intelligent switch (electric load switch, splitter, circuit breaker, recloser + FTU), and can automatically realize fault isolation, recovery and transfer power supply. Assuming a dual power hand-hand line is divided into six sections, that is, there are five intelligent switches (bilateral with three-phase five-column signal PT or capacitive PT), the middle of that station to do contact, and ground fault point in the first and The second intelligent switch between the example, the specific steps are as follows:
⑴ substation will be ground outlet power outage, the line on the intelligent switch automatically sub-gate.
⑵ substation closing power transmission, power station zero sequence voltage does not alarm, the section to resume power transmission success; the first intelligent switch FTU detected line voltage, but did not detect the zero sequence voltage, the delay after a period of time automatic closing , The switch will not move and "lock", cancel the "power delay automatically!" The second intelligent switch also detects the zero sequence voltage, the switch does not move and " Closing "function.
⑶ intermediate contact switch to detect unilateral power failure, delay after a period of time automatically shut down, because the fault point is not in the section, did not detect zero sequence voltage, the transfer of power transfer success.
⑷ contact switch power, the second intelligent switch to detect the line voltage, no detection of zero sequence voltage, but the "lock" in the former, so cancel the "power delay automatic closing" function, the switch remains in the sub-bit The
⑸ At this point, the ground fault point section has been separated, other non-faulty sections have been restored or transferred power supply.
4, change the neutral point of grounding to find
Distribution system using a neutral point or through the arc suppression coil grounding, there are advantages and disadvantages. In view of the trouble of finding the "drawbacks" and the resulting personal and personal safety issues, the user himself is doing further thinking, thinking out of the program there are two:
⑴ to neutral point to a small resistance to ground. After the transformation, the use of export circuit breaker zero sequence protection function and short-circuit fault indicator, basically can solve about 70% of the ground fault to find the problem, but there are about 30% of the resistance and high resistance ground fault is not good Find, there may be problems with the protection of the line fuse. For this system, a better solution is to use the digital fault indicator, the line zero sequence current (cable), the total line current (overhead), ground insulation voltage (overhead) and other indicators of the measurement data through wireless communications Network sent to the scheduling system, a comprehensive analysis of substation real-time and historical information, can determine the location of the ground.
⑵ neutral point to a small resistance + circuit breaker or medium resistance + high voltage contactor mode. Circuit breaker or high-voltage contactor is usually in the sub-bit, only when the system is detected after the zero-sequence voltage elevation after closing, short time into a small resistance or resistance grounding, and then through a small resistance to ground detection method To find the fault. In addition, since the neutral point resistance can be flexibly controlled, it is possible to control the opening and closing of the resistor with a certain completion timing after the arc suppression coil is actuated so that the protection device operates or the ground fault indicator Signal and indicate the ground current path.