AC motors - Failure Analysis

Single-phase ground fault definition:

Three-phase power system, the insulation between a phase conductor and appear only destruction.

Broadly speaking, single-phase ground faults include single-phase ground short circuit, single-phase ground short circuit is a single-phase ground fault in a special phenomenon, only transformer neutral grounding system will have.

In the narrow sense, single-phase fault and single phase short circuit the two concepts. It is a fault, an accident.

Power system failure and accident is narrow distinction.

Single-phase ground short circuit means FireWire (any one phase) without direct load and the ground is turned on, the power supply system in 380/220 or other grounding system, since the neutral transformer is grounded, and the grounding resistance is small , FireWire (any phase) without load and the ground directly connected, instantly generate a large current, blown wire, power distribution equipment or tripping, so a single ground fault is a serious accident, electricity system as much as possible to avoid.

In the power system, there are many places with ungrounded power supply systems, such as in dusty, explosive and other special venues, our country all 6-10 kV, 35 kV power supply system, are used in neutral point is not isolated or high resistance grounded power supply system, such a system after a single-phase, since no loops or did not produce a large current circuit, no ground current or no large ground currents, it does not affect the normal operation of the system. While not affecting the normal operation of the system, but is a single-phase ground fault, in order to prepare the ground for the two-phase expansion or cause further damage and other ground equipment, rules of the single-phase fault occurs after two hours to check in explain the reasons to deal with, the cause could not be processed within two hours or, in order to supply power after treatment.

Single-phase ground fault finding Daquan

For small current grounding system, how to quickly find the single-phase fault, I'll give you some simple method.

1, manual search method

If the grounding line selection device is not installed in the substation, nor install or short-circuit ground fault indicator fault indicator combo on line, there is no good ground fault detector, it had to use the artificial look awkward. Find the following steps:

⑴ by hand (or scheduling, the same below) in order to wind up, you can know which of substation grounding outlet, scheduling to know which phase ground.

⑵ Then there are two ways to find fault points: First, the line sequential segmentation, or will often faulty line opened, relative insulation 2.5kV shake table testing ground, that part of the insulation resistance is small fault section, in order to restrict the search (of course, in the substation outlet side must do hang ground wire and other security measures); the second is the line as far as possible segment, then progressively close again transmission, interactive and scheduling coordination, there is The segment is residual voltage fault zone alarm.

Artificial method of operation is very troublesome to find, if the line is long, multi-branch, switch segment was small and it is difficult to operate, plus weather and bad weather, it is more a bad deal. Recommendations or adopt some technological means less investment in equipment to match the artificial look, you can achieve a multiplier effect, providing both power supply reliability and social benefits, but also creates economic benefits.

2, the use of grounding line selection device and fault indicators to find

Substations are generally installed a grounding line selection device, although sometimes allowed, but can provide technical reference for artificial wind up. Then the line install some fault indicator (or short-circuit ground fault indicator combo), this indicates a ground fault approach. More reliable ground fault detection method is the use of signal source method, more sensitive ground fault detection method is to use the first half-wave method or the DC transient analysis. We recommend using two fault indicator combination of methods to find better ground fault in the signal source method mainly to the first half-wave method or the DC transient analysis supplement.

3, the use of feeder automation methods to find

If you have money, it is recommended to use the feeder automation methods to locate ground faults. The method uses intelligent switching (electric load switch, segmentation, circuit breakers, reclosers + FTU) logic functions to replace the traditional manual search method, and can automatically implement fault isolation, recovery and transfer of power. Suppose one pair of power supply line hand in hand into six sections, namely a 5-intelligent switch (bilateral with the three-phase five-column capacitive signal PT or PT), the middle station do contact, and a ground fault in the first and second smart switch between, for example, concrete steps are as follows:

⑴ grounding outlet wind up power substation, intelligent switches each segment on the line automatic gates.

⑵ substation switch on power transmission, power zero-sequence voltage is not alarm, the zone restore power success; first intelligent switch FTU line voltage is detected, but did not detect zero-sequence voltage, the delay automatic closing after a period of time , because close to the ground fault is detected zero-sequence voltage, it is immediately opening and "locking"; second intelligent switch also detects zero-sequence voltage, the switch does not move and "blocking" Cancel "energized delay automatic Closing "function.

⑶ intermediate contact switch detects unilateral power failure, automatic closing delay for some time, because the fault is not in the zone, there is no zero-sequence voltage is detected, the transfer of power sector success.

⑷ contact switch after transmission, the second smart switch detects the line voltage, zero-sequence voltage is not detected, but there are "blocked" in the former, it canceled "energized delay automatic switch" function switch in the quintile .

⑸ At this point, the ground fault zone has been separated from other non-fault section have been restored or transfer of power.

4, changing the way to find a neutral ground

Distribution system using ungrounded or via Petersen coil grounding method has advantages and disadvantages. For troubleshooting difficult "malpractice" and some of the personal and property safety of the resulting problems, the users themselves are further thinking, thinking out of the scheme are two:

⑴ will be changed to neutral grounding via low resistance. After the transformation, the use of export breaker two zero-sequence protection function and fault indicator, basically to get rid of about 70% of the ground fault find the problem, but there are about 30% of the medium-resistance and high resistance ground fault bad Find, may also exist with the line fuse protection coordination problems. Measurement data for such a system, a better solution is to use the digital fault indicator, the zero-sequence current (cable), total current line (overhead), the insulation voltage (overhead), and other indicators through wireless communication Send to network scheduling system, comprehensive analysis of substation real-time and historical information, determine ground position.

⑵ neutral changed little resistance + resistance + high-voltage circuit breaker or the contactor mode. High-voltage circuit breaker or contactor usually in quintile, only when the system detects zero after raising only delay closing sequence voltage, short-term changes in resistance or low resistance grounded, then the method to detect small resistance - grounded to find fault. In addition, since the neutral point resistance-off can be flexibly controlled, you can ring action after Petersen, then at a certain timing to control the resistance of combined points-off, so that the protective devices or make a ground fault indicator identifies the signal and indicates the ground current channels.