Neutral point grounding method selection - Solutions - Huaqiang Electronic Network

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The electrical connection between the neutral point of the three-phase AC power system and the earth is called the neutral point grounding method of the power grid. The neutral grounding method involves the safety, reliability and economy of the power grid; at the same time, it directly affects the selection of the insulation level of the system equipment, the overvoltage level, the relay protection mode, and the communication interference. In general, the neutral point grounding method of the power grid is also the neutral point grounding method of the voltage level of the transformer in the substation. Therefore, in the planning and design of the substation, the transformer neutral point grounding method should be selected for specific analysis and comprehensive consideration.
China's 110kV and above power grids generally adopt high-current grounding mode, that is, neutral grounding effective grounding mode (in actual operation, in order to reduce single-phase grounding current, some transformers can be grounded ungrounded), so that the neutral point potential is fixed to ground. Potential, when a single-phase ground fault occurs, the non-fault phase voltage rise will not exceed 1.4 times the operating phase voltage; the transient over-voltage level is also low; the fault current is large, the relay protection can quickly act on the trip, and the fault is removed. System equipment is subject to a short period of overvoltage. Therefore, the high current grounding system can reduce the insulation level of the entire system equipment, thereby greatly reducing the cost.
The 6~35kV distribution network generally adopts a small current grounding method, that is, a neutral point non-effective grounding method. In recent years, the transformation of the two networks has greatly increased the capacitive current of the 6~35kV distribution network in small and medium-sized cities. If no effective measures are taken, the safe operation of the distribution network will be jeopardized.
Neutral point non-effective grounding methods can be mainly divided into the following three types: ungrounded, grounded via arc suppression coil and grounded via resistor.
1 Neutral point ungrounded mode is suitable for single-phase ground fault capacitor current IC < 10A, mainly for overhead lines, especially for rural 10kV distribution network. This type of power grid instantaneous single-phase ground fault rate accounts for 60% to 70%, and it is hoped that the instantaneous ground fault will not act on the trip.
Its characteristics are:
· Single-phase ground fault capacitor current IC < 10A, the arc at the fault point can self-extinguish, and the insulation at the fault point recovers automatically after the arc is extinguished;
· Single-phase grounding does not damage the symmetry of the system, and can be operated with fault for a period of time to ensure continuity of power supply;
·Communication interference is small;
· When single-phase ground fault occurs, the non-fault relative power frequency voltage rises by 31/2UC. The insulation requirements of electrical equipment in this system are designed according to the line voltage.
·When IC > 10A, the grounding point arc is difficult to self-extinguish, and intermittent arcing grounding overvoltage with relatively high overvoltage level may occur, and the duration is long, which jeopardizes weak insulation equipment inside the network, and then causes two-phase grounding fault. Causing a power outage;
• The resonance overvoltage in the system causes the voltage transformer fuse to blow, burns the TV, and even burns the main equipment.
2 The neutral point is grounded by the arc suppression coil for single-phase ground fault capacitor current IC > 10A, and the overhead line with more transient single-phase ground faults is the main distribution network.
Its characteristics are:
·Using the inductive current of the arc suppression coil to compensate the grid capacitive current flowing through the grounding point, so that the fault current is <10A, the arc self-extinguish, and the insulation of the fault point is self-recovery after the arc extinction;
· Reduce the probability of system arc grounding overvoltage;
· The system can be operated with a fault for a period of time;
· Reduced ground power frequency current (ie residual current) and ground potential rise, reducing the step voltage and ground potential difference, reducing the counterattack of low-voltage equipment and interference to the information system.
At present, the arc-suppression coils running in China are divided into two types: manual adjustment and automatic tracking compensation. In the former manual adjustment, the arc suppression coil needs to be taken out of operation, and the artificially estimated capacitance current value is large, and the error is now used less; The type can automatically measure the capacitance current and automatically adjust the arc suppression coil to adapt the compensation current to the change of the system. Generally, the arc suppression coil is selected.
The automatic tracking compensation arc suppression coil is divided into 匝 type, adjustable air gap type, DC magnetic assist type and adjustable volume type. According to the operation of the substation of our bureau, the price of the tuning is lower, but the adjustment level is less, and it cannot fully adapt to the system change. The air-gap compensation linearity is better, but the vibration noise is extremely high, and the operators have a strong response and need to be improved. The accommodating response is quick and reliable, the operation is quiet and stable, and the operating personnel report better.
3 Neutral point via resistor grounded neutral point via resistor grounding is suitable for power cable lines with less transient single-phase ground faults.
The characteristics of the neutral point through the resistance grounding operation mode:
· Reduce operating overvoltage. When a single-phase ground fault occurs in the distribution network where the neutral point is grounded by the resistor, the zero-sequence protection action can accurately judge and quickly cut off the fault line;
· It can effectively reduce the power frequency overvoltage. When the single phase ground fault occurs, the non-fault phase voltage is 31/2UC, and the duration is short;
The neutral point resistance is an energy consuming element and also a damping element (the arc suppression coil is a resonant element);
· Effectively limit the arc grounding overvoltage. When the arc is extinguished, the residual charge in the system's capacitance to ground will be discharged through the grounding resistance. When the next arc is reignited, no overvoltage will be formed.
· It can effectively eliminate the resonant overvoltage in the system. The neutral point resistance grounding is equivalent to the parallel connection of the damping resistor in the resonant circuit. The test shows that as long as the neutral point resistance is <1500Ω, various resonant overvoltages can be eliminated, and the smaller the resistance, The better the effect of eliminating resonance;
· The application range of capacitance current is large, simple, reliable and economical.
Neutral point grounding resistance selection:
·Reducing the impact angle of the short-circuit current on the equipment and from the safety point of view, reducing the ground current of the fault point, reducing the step voltage and the contact voltage, the smaller the I value, the better, that is, the greater the neutral point grounding resistance should be;
· In order to limit the arc grounding overvoltage to less than 2 times, the grounding resistance is generally selected according to the IR = (1~4) IC requirement;
· The neutral point resistance grounding system judges and cuts the fault line through the zero sequence protection of each line. When selecting Rn, it is necessary to ensure the zero sequence protection sensitivity requirement of each line.
The neutral grounding resistance must be selected according to the specific conditions of the power grid, such as limiting arc grounding overvoltage, relay protection sensitivity, communication interference, and safety. At present, the neutral points of substations in Shenzhen are 15W, and substations in Beijing and Guangzhou use 9.9W small resistance grounding.
4 6~35kV distribution network grounding mode selects urban and rural distribution network based on overhead lines, 70% of grounding faults in overhead lines are instantaneous faults; only need to determine whether the system capacitor current is greater than 10A according to the requirements of the regulations. The ground point is not grounded or the tracking arcing coil is grounded automatically.
The urban and rural distribution network mainly consists of cable lines. The substation has a large coverage area, and there are many outlet lines and generally cable lines. The system capacitor current is also large. According to relevant literature and operational practices, the ground fault of the cable line is about 50% instantaneous. malfunction. However, due to the particularity of the cable line, a small resistance grounding method can be generally used to sacrifice some power supply reliability to prevent an accident.
The urban and rural distribution network, which mainly consists of overhead and cable hybrid lines, takes into account the characteristics of overhead and cable lines, making it difficult to select the grounding mode of the distribution network in two ways: automatic tracking arc suppression coil and small resistance.
In the case of single-phase earth fault, the non-fault relative power frequency voltage rises by 31/2 UC and lasts for a long time, which may cause multi-point insulation breakdown and the accident expands.
The arc suppression coil cannot compensate for harmonic current, and the proportion of harmonic current in some cities or factories is 5%~15%. Only the harmonic current is enough to support the stable combustion of the arc.
It is difficult to find a single-phase ground fault line. At present, the operation rate of many small current grounding line selections is still not ideal, and the trial pulling method is still used.
When the cable in the cable trench or cable duct is single-phase grounded, it takes a long time to find the faulty line. During the operation with ground fault, it is easy to cause electric shock. On the other hand, the use of a small resistance grounding method may incorrectly remove the instantaneous fault line, causing power interruption to the user, reducing power supply reliability and reducing power supply.
5 Comments For this type of hybrid system, the rated voltage of the cable application is 8.7/10 or 12/15kV to strengthen the insulation. On this basis, the automatic tracking arc-suppression coil grounding method is adopted, and a small current line selection device is installed. When single-phase ground fault occurs, the ground line should be found and removed as soon as possible.
It is worth noting that an automatic tracking arc suppression coil is connected in parallel with a small resistance. The grounding device that uses the microprocessor to control the parallel small resistance switching is about to come out. The principle is that when the instantaneous ground fault occurs, the arc suppression coil is automatically tracked, after a certain delay. When the ground fault does not disappear, the microprocessor can automatically determine that it is a permanent ground fault, put in parallel small resistance, make the protection action, cut off the ground line, and better solve the grounding problem of the hybrid line.
In summary, several neutral grounding methods have their own advantages and disadvantages. The choice should be based on the actual situation of the network, weigh the pros and cons, choose according to local conditions, and should not be "one size fits all" according to the voltage level.