Fault characteristics of single phase grounding faults, what are the causes of single phase groundin

Single-phase grounding is the single-phase grounding of 10kV (35kV) low-current grounding system, and single-phase grounding fault is the most common fault in the power distribution system, which mostly occurs in humid and rainy weather. It is caused by many factors such as tree barriers, single-phase breakdown of insulators on distribution lines, single-phase disconnection, and small animal hazards.

Single-phase grounding not only affects the normal power supply of users, but also may produce overvoltage, burn out equipment, and even cause interphase short circuit and expand the accident.

1) When a phase (such as phase A) is not completely grounded, that is, grounded by high resistance or arc, then the voltage of the faulty phase decreases, and the voltage of the non-faulty phase increases, they are greater than the phase voltage, but cannot reach the line voltage. When the voltage at the opening triangle of the voltage transformer reaches the setting value, the voltage relay acts and sends out a grounding signal.

2) If phase A is completely grounded, the voltage of the faulty phase drops to zero, and the voltage of the non-faulty phase rises to the line voltage. At this time, three times the original phase voltage appears in the opening triangle of the voltage transformer, and the voltage relay acts and sends out a grounding signal.

3) A phase (phase A) disconnection or fuse is blown on the high-voltage side of the voltage transformer, and the indication of the fault phase is not zero, which is due to the fact that the phase voltmeter forms a series circuit through the transformer coil and other two-phase voltmeters in the secondary circuit, and there is a relatively small voltage indication, but it is not the actual voltage of the phase, and the non-fault phase is still the phase voltage. A voltage value of about 35V will appear in the triangle of the transformer opening, and the relay will be started to send out a grounding signal.

4) Due to the presence of capacitive and inductive parameters in the system, especially the ferromagnetic inductive element with an iron core, it will cause ferromagnetic resonance when the parameter combination does not match, and the relay will act, sending out a grounding signal.

5) False grounding of no-load bus. When the bus is running at no load, there may also be a three-phase voltage imbalance and a grounding signal will be issued. However, when a line is sent, the grounding phenomenon will disappear on its own.

Neutral point noIf a single-phase grounding fault occurs in the grounding system, the hand is crackedLine voltageThe constant rather than faulty phase potato mountain to ground voltage rises to the originalPhase voltage, that is, rises to the value of the line voltage, at which point is groundedShort-circuit currentIt is 3 times the capacitance current of a single counter-ground in normal operation.

Take the ground as the reference point, and the ground is zero potential.

The neutral point is grounded.

In the south, the neutral point is zero potential; In the case where the neutral point is not grounded, which point is grounded is the zero potential. If the X phase is grounded, the X phase is zero potential; That is to say, the whole system uses the X phase as the ground (zero) potential as the reference point, so the neutral point potential is "raised".

Neutral point effective grounding:

China's 110kV and above power grids generally adopt high-current grounding mode, that is, the neutral point effective grounding mode (in actual operation, in order to reduce the single-phase grounding current, some transformers can be made to use the non-grounding mode), so that the neutral point potential is fixed to the ground potential, and when a single-phase grounding fault occurs, the voltage of the non-fault phase will not exceed the voltage of the operating phase.

Transient overvoltage levels are also lower; The fault current is very large, and the leakage protection can act quickly to trip, remove the fault, and the system equipment.

The overvoltage period is short. Therefore, the high-current grounding system can reduce the level of equipment insulation requirements of the whole system, thereby greatly reducing the cost.

The neutral point is not effectively grounded.

6 35kV distribution network.

Generally, a small current grounding method is adopted, that is, the neutral point is not effectively grounded. In recent years, the transformation of the two networks, so that small and medium-sized cities 6 35kV distribution network capacitance current has increased greatly, if no effective measures are taken, the neutral point non-effective grounding mode that endangers the safe operation of the distribution network can be mainly divided into the following three: non-grounding, through the arc suppression coil.

Grounding and resistive grounding.

In the event of a single-phase grounding fault in the neutral ungrounded systemLine voltageThe constant and non-fault relative ground voltage rises to a multiple of the original phase voltage, that is, rises to the line voltage value, and the grounding point is at this timeShort-circuit currentIt is a single relative to the normal operationCapacitive current3 times.

Hazards to substation equipment.

In actual operation, our institute recently happened that the plastic cloth of the brick factory fell to the wire due to strong winds, which made the substation voltage transformer.

Burning, causing equipment damage and large-scale power outages. After the occurrence of single-phase grounding fault, it may also produce resonant overvoltage, which is several times the normal voltage, endangering the insulation of substation equipment, and in serious cases, the insulation of substation equipment will break down, causing greater accidents.

Hazards to power distribution equipment. Carry socks.

After the occurrence of a single-phase grounding fault, intermittent arc grounding may occur, resulting in resonant overvoltage, which will produce several times the normal voltage and the overvoltage will further make the insulator on the line.

Insulation breakdown, causing a serious short-circuit accident, while may burn out part of the distribution transformer, so that the lightning arrester on the line.

Fuse insulation breakdown, burning, electrical fire may also occur.

Categories: Science & Engineering >> Engineering & Technology Sciences.

Analysis: There are 4 types of short-circuits in the power grid. The most serious is a 3-phase short circuit.

A single-phase short-to-ground circuit is the most common short-circuit fault. It accounts for 70% of short-circuit faults.

There are many reasons for this: for example, an object is hung on a wire, the object is relatively long, and it is hung on another conductor (e.g. earth).

It is very common for a tree branch to hit a power line when it rains and when the wind blows, which can cause a single-phase short circuit. Fortunately, those switchgear can tell if it's a transient fault or a really serious fault.

Therefore, there should be no buildings, trees, etc. around the wires. Set aside a "transmission corridor".

In the event of a single-phase ground fault, there will be some variation in voltage and current. The following are the changes in voltage and current in general:

1.Voltage variation: When a single-phase ground fault occurs, the voltage of the faulty phase (ground phase) drops instantaneously and may be close to zero.

2.Current Variation: In terms of the path of the current, the current in the fault phase increases instantaneously because the fault path provides a low-impedance path. At the same time, the current in other normal phases may decrease slightly, but the change will not be noticeable.

When the fault of single-phase connection occurs in the spring worm land, a high fault current will be generated, which may cause damage to system equipment and personal safety. Therefore, it is very important to detect and deal with single-phase grounding faults in time to avoid unexpected failures and accidents in the power system. When a fault occurs, corresponding protective measures should be taken, such as quickly cutting off the faulty circuit and overhauling the equipment, so as to restore the normal operation of the power system.

Please note that when it comes to power systems, different situations and special conditions can lead to some subtle differences, so in the practical application of the object circle, accurate measurements and analyses are also required according to the requirements of the specific system and equipment. <>