In view of the continuous expansion of the neutral point ungrounded power supply network system and the increase in the medium voltage cable feeder circuit, the single-phase grounding capacitance current is also increasing. It is a key technical problem related to the reliability of the power grid operation to reform the neutral point grounding mode and select a reasonable neutral point grounding mode.

Comparison of different grounding methods for neutral points：

In the power supply system of medium voltage cable, most of the low current grounding system (that is, the neutral point is not grounded or the arc suppression coil or resistance grounding system).

1.Grounding with a small resistance at neutral point:

The grounding with a low resistance at neutral point is suitable for medium voltage cables that are mainly based on medium voltage cable lines, have few transient single-phase grounding faults, have large system capacitance current, have a strong and reasonable network frame, and have high automation level. The characteristics of the neutral low resistance grounding method are as follows:

• It can reduce the power frequency over-voltage, and the non-fault phase voltage is less than 3 times the phase voltage when the single-phase grounding fault occurs, and the duration is very short.
• Effectively limit the arc grounding over-voltage in the distribution network where the neutral point is grounded by resistance. When the grounding arc is extinguished, the residual charge in the system's ground capacitance will be discharged through the neutral point resistance, and the over-voltage amplitude in the next arc burning is the same as when the single-phase grounding fault occurs during normal operation and will not produce a high over-voltage. The smaller the value of neutral resistance, the faster the residual load is discharged. Appropriate selection of the neutral resistance value can limit the over-voltage multiple to 2.8 times the phase voltage.
• The neutral resistance is equivalent to a damping resistance in the resonant circuit, and because of the damping effect of the resistance, it can basically eliminate various resonant over-voltages in the system. The test shows that as long as the neutral resistance is not too large (no more than 1500 ohms), you can eliminate all kinds of resonant over-voltage. The smaller the resistance value, the better the effect of eliminating resonance.
• The neutral point grounding system with low resistance can be simply configured with zero sequence over-current protection. In the event of a single-phase grounding fault, when the fault current reaches the zero sequence protection action value, the zero sequence protection action will jump the circuit breaker on the line.
• When the ground fault occurs in the low resistance grounding system, when the fault current reaches the zero sequence protection action value, it can be operated in a very short time to remove the power supply, which greatly reduces the opportunity for personnel to contact the live fault equipment.
• Help to improve the level of safe and reliable operation of the system. Because the power frequency voltage of the system is increased and the transient over-voltage multiple is lower, the safety, reliability, and service life of equipment using conventional standards are improved.
• The neutral point through the low-resistance grounding system, in the occurrence of a single-phase grounding fault, the fault point of the current flowing far more than the resonant grounding and ungrounded system, the fault point of the high-temperature arc, step voltage, and contact voltage pose a greater threat to people and animals. When the fault current does not reach the zero sequence protection action value, it is more unfavorable to personal safety.
• The neutral point is grounded by a low-resistance system that interferes with communication and electronic equipment, and the comprehensive investment is relatively high.

2.Neutral point grounding through arc suppression coil:

In 1916, the invention of the arc suppression coil, and in 1917, the first in Germany Pleidelshein power plant put into operation so far, has 84 years of history. Operating experience shows that it is widely used in the middle voltage cable, and the world has Germany, China, the former Soviet Union, Sweden, and other countries of the middle voltage cable are long-term users of this way. The safe and economical operation level of medium voltage cable has significantly improved. The characteristics of neutral point grounding through an arc suppression coil are as follows:

• The inductive current of the arc suppression coil is used to overcompensate the ground capacitance current of the power grid, so that the single-phase grounding fault current is limited to 10 amps, which is beneficial to personal safety.
• The instantaneous single-phase grounding fault arc can self-extinguish, and the fault insulation can recover by itself after extinguishing.
• Can reduce the probability of gap arc grounding over-voltage; single-phase grounding does not destroy the symmetry of the system, it can run for a period of time with a fault, in order to find the fault line.
• It can eradicate the saturation over-voltage of the voltage transformer core, and the operating over-voltage can generally be suppressed below 2.8 times the phase voltage.
• Limit the occurrence and expansion of medium voltage cable faults. According to United States statistics, 66% of cable failures are developed from the outside to the inside. The loss of ground insulation ability of the medium voltage cable body is a gradual development process. The automatic tracking arc suppression coil grounding method is very sensitive to the unbalance of three relative ground admittances and can be reflected in the initial fault stage. If handled in time, insulation breakdown can be prevented. In the event of a breakdown, because the residual current at the fault point is very small, it is difficult to form an inter-phase short-circuit accident.
• Small communication interference, operation management is relatively simple.
• When the single-phase grounding fault occurs, the non-fault-phase power frequency voltage rises to a maximum of 3-phase voltage. For a distribution network with a large capacitance current, if the residual current of the single-phase ground fault current is less than 10 amps by compensation, the system must maintain a small detuning degree. If the tuning of the system is too small, the neutral point displacement voltage caused by the three-phase capacitance asymmetry will have a strong amplification effect, and the neutral point voltage deviation will easily exceed the allowable value of the regulation.

From the characteristics of the neutral point through the arc suppression coil grounding mode, it can be seen that when the system is single-phase grounding, the current flowing through the ground point is small, which is characterized by the single-phase grounding of the line. According to the regulations of the power grid, it can run for 2 hours with a single-phase grounding fault. Therefore, the power supply reliability of neutral point grounding through an arc suppression coil is much higher than that of neutral point grounding through small resistance in medium voltage cable.