For a long time, most of my country's 6-35kV (including 66kV) power grids have adopted an ungrounded neutral point operation mode. In this type of grid, when a single-phase ground fault occurs, the voltage to ground of the faulted phase drops to zero, while the voltage to ground of the non-faulted phases rises to the line voltage (UL), but the system's line voltage remains unchanged. Therefore, national standards stipulate that such grids are allowed to operate with the fault for a short period (2 hours) after a single-phase ground fault, thus greatly improving the reliability of power supply in this type of grid. Existing operating procedures stipulate that "after a single-phase ground fault occurs in a neutral point non-effectively grounded system, operation is allowed for two hours," but the procedures do not clearly define the concept of "single-phase ground fault." If the single-phase ground fault is a metallic ground fault, the voltage of the faulted phase drops to zero, and the voltage to ground of the other two healthy phases rises to the line voltage. Electrical equipment in this type of grid should be able to withstand this overvoltage without damage under normal circumstances. However, if a single-phase ground fault is an arcing ground fault, it will generate an overvoltage in the system with a maximum value of 3.5 times the phase voltage. If such a high overvoltage acts on the power grid for several hours, it will inevitably cause cumulative damage to the insulation of electrical equipment. If it causes insulation breakdown to ground at a weak point in the insulation of a healthy phase, it will trigger a major accident involving a phase-to-phase short circuit. I. Hazards of Phase-to-Ground Capacitive Current In a high-voltage power grid with an ungrounded neutral point, the hazards of single-phase grounding capacitive current are mainly manifested in the following four aspects: 1. Hazards of Arcing Ground Fault Overvoltage: When the capacitive current is too large, the arc at the grounding point cannot extinguish itself. When intermittent arcing ground faults occur, an arcing ground fault overvoltage is generated. This overvoltage can reach 3 to 5 times or higher than the phase voltage. It is distributed throughout the entire power grid and lasts for a long time, up to several hours. It not only breaks down weak points in the insulation of the power grid but also poses a great threat to the insulation of the entire power grid. 2. The excessive single-phase grounding capacitive current can cause thermal damage to the grounding point and increase the voltage of the grounding grid. This increases the thermal effect at the grounding point, causing thermal damage to cables and other equipment. After flowing into the ground, this current increases the voltage of the entire grounding grid due to the grounding resistance, endangering personal safety. 3. The harmful effects of stray AC currents: After the capacitive current flows into the ground, it forms a stray current in the ground. This current may generate sparks, igniting gas explosions, potentially causing premature detonation of detonators, and corroding water pipes and gas pipes. 4. Grounding arcs can cause gas and coal dust explosions. II. The Role of Arc Suppression Coils After the arc suppression coil is installed in the power grid, when a single-phase grounding occurs, the arc suppression coil generates an inductive current. This inductive current compensates for the capacitive current formed by the single-phase grounding, reducing the grounding current and slowing down the recovery speed of the fault phase voltage, thus mitigating the hazards caused by excessive capacitive current. Simultaneously, due to the clamping effect of the arc suppression coil, it can effectively prevent the occurrence of ferroresonant overvoltage. III. Some Problems with Arc Suppression Coil Grounding Methods: 1. 1. During a single-phase ground fault, the voltage between the non-faulty phases and ground rises above the three-phase voltage. This prolonged rise can affect all system equipment, potentially causing a second insulation breakdown and escalating the fault. 2. Arc suppression coils cannot compensate for harmonic currents. In some urban power grids, harmonic currents account for 5%-15% of the total current, which can be far greater than 10A, still potentially leading to arcing ground fault overvoltage. 3. For distribution networks with large capacitive currents, to ensure that the single-phase ground fault current Ijd < 10A through compensation, the system must maintain a low degree of detuning. Insufficient detuning will amplify the neutral point displacement voltage caused by three-phase capacitor asymmetry, causing the neutral point voltage deviation to exceed the allowable value (<15%Un), triggering a ground fault signal from the protection system. Furthermore, if the detuning degree is too small, the system will operate close to the resonant compensation state, posing a significant potential danger to the system (resonant overvoltage). To ensure that the neutral point displacement voltage does not exceed the allowable value specified in the regulations, the detuning degree must be increased. However, excessive detuning will lead to an excessively large residual grounding current (Ijd > 10A), which may cause intermittent arcing grounding overvoltage. It is difficult to guarantee both the residual grounding current Ijd < 10A and the neutral point displacement voltage not exceeding the allowable value specified in the regulations—two mutually restrictive conditions. 4. The adjustment range of the arc suppression coil is limited by the adjustment capacity. The ratio of the adjustment capacity to the rated capacity is generally 1/2. If selected according to the final requirements, the system capacitive current is small in the early stages of the project, and the minimum compensation current of the arc suppression coil is too large, which may prevent it from being put into operation. If selected according to the requirements of the early stages of the project, the system capacitive current is large in the final stages of the project, and the maximum compensation current of the arc suppression coil is too small, which also cannot meet the requirements for reasonable compensation. 5. During operation, there will be a large error between the nominal current and the actual current of each tap of the arc suppression coil. During operation, resonance has occurred due to the large error between the actual current and the nominal current. 6. Due to frequent changes in system operation and voltage, the system's capacitive current also fluctuates, making tracking and compensation difficult. Current automatic tracking compensation devices are diverse, but their actual operational testing time is short, and their performance is not ideal. Furthermore, they are expensive, structurally complex, and require significant maintenance, making them unsuitable for unmanned substations. 7. For the above reasons, neutral point grounding via an arc suppression coil can only reduce the probability of arc grounding overvoltage; it cannot eliminate it or reduce its amplitude, and the arc grounding overvoltage multiple is also very high. 8. Locating single-phase grounding fault lines is difficult. Many current low-current grounding fault location methods have unsatisfactory success rates, often requiring the use of trial pulls. 9. When using trial pulls, both short-term power outages of non-faulty lines and operational overvoltages can occur. 10. The system has high resonant overvoltages, which last for a long time and affect all system equipment, often causing PT burnout or PT fuse blowout. During tests at the Quzhuang substation, the Wuhan High Voltage Apparatus Research Institute and the Guangzhou Power Supply Bureau measured a 1/2 frequency resonant overvoltage of 2PU, a 3-frequency resonant overvoltage induced by the closing operation of 4PU, and a resonant overvoltage of 3.8PU when the A-phase conductor was broken and grounded to the load side. 11. When a single-phase ground fault occurs in a cable duct or cable tunnel, failure to promptly disconnect the faulty line may cause a fire. A major accident occurred in Shanghai's 35kV system where a fire broke out an hour after a single-phase ground fault, destroying more than 40 cables in the cable tunnel. 12. Prolonged search for the faulty line, especially during operation with a ground fault, increases the risk of electric shock. 13. During a single-phase ground fault, the voltage of the non-faulty phase rises to the line voltage or higher. If the fault cannot be detected promptly, gapless metal oxide (MOA) surge arresters operating at line voltage for extended periods are prone to damage or even explosion. Arc grounding overvoltages and resonant overvoltages have high amplitudes and long durations. Due to operating load issues, MOAs generally do not require overvoltage protection within WGMOA systems, thus failing to effectively utilize the excellent characteristics of MOAs and hindering their widespread use in distribution networks. IV. Characteristics of Distribution Networks Primarily Based on Cable Lines: 1. The capacitive current per unit length of cable lines is more than ten times greater than that of overhead lines, resulting in a large ground capacitive current in urban power grids primarily composed of cables. 2. Cable lines are less affected by external environmental conditions (lightning, external forces, trees, strong winds, etc.), resulting in few transient grounding faults; grounding faults are generally permanent. 3. When a grounding fault occurs in a cable line, the grounding arc is a closed arc, which is not easily extinguished on its own. If the circuit breaker is not tripped in time, it can easily cause phase-to-phase short circuits, escalating the accident. 4. Cables are weakly insulated equipment. For example, the one-minute power frequency withstand voltage of a 10kV cross-linked polyethylene cable is 28kV, while the insulation level of typical 10kV distribution equipment is 42kV. In the arc suppression coil grounding system, due to the long time to find the fault point, the cable is subjected to power frequency or transient overvoltage for a long time, which is easy to develop into phase-to-phase fault, causing one or more lines to trip. 5. In the cable line, the high frequency oscillation current has a large amplitude and slow decay. The high frequency oscillation current is much larger than the power frequency current. When the power frequency current crosses zero, the high frequency oscillation current still has a large amplitude. Maintaining the arc depends on the decay rate of the high frequency oscillation current and the power frequency current. The arc suppression coil cannot compensate for the high frequency oscillation current. Also, because the residual current after the arc suppression coil compensation is large in the cable line, the arc suppression coil cannot extinguish the arc in the cable line. V. PT Resonance 1. PT Resonance For yo/yo electromagnetic PT, under normal circumstances, a single phase grounding of the line will not cause ferroresonant overvoltage. However, under the following conditions, ferroresonant resonance may be triggered. (1) For a neutral point ungrounded system, when a single phase grounding occurs in the system, the fault point flows through the capacitive current, and the phase voltage of the two ungrounded phases increases by 3 times. However, once the ground fault point is eliminated, the line voltage charge that the ungrounded phase has been charged during the ground fault can only flow into the ground through the PT high-voltage coil and its own grounding point. During this instantaneous voltage change, the excitation current of the two ungrounded phases of the PT high-voltage coil will suddenly increase or even saturate, thus forming a phase-to-phase series resonance. (2) The system experiences ferroresonance. In recent years, due to the increase in the number of PTs, electronically controlled welding machines, speed-regulating motors, etc. of distribution line users, the electrical parameters of the 10kV distribution system have changed significantly, leading to frequent resonance. When the system resonates, the PT will generate overvoltage, causing the current to surge. In addition to causing the primary side fuse to blow, it will also cause the PT to burn out. In some cases, it may also cause flashover or explosion of the bushings of surge arresters, transformers, and circuit breakers. (3) During line maintenance, without applying to the dispatching department for power outage procedures in advance, the branch line isolating switch or the high-voltage drop switch of the distribution transformer is opened under load, causing arc short circuit between the switches and triggering resonance. (4) When a single-phase ground fault occurs inside the distribution transformer, the fault current will discharge to the ground through the insulating oil with strong anti-electricity capability, which will also generate an unstable electric arc to excite the grid resonance. (5) If the power supply operation procedure of the operator is incorrect, and the PT is directly energized to the empty bus without opening the high voltage side disconnect switch, it will cause the PT to have ferroresonance. VI. New type of arc suppression, harmonic suppression and overvoltage protection device Here is an introduction to a new type of arc suppression, harmonic suppression and overvoltage protection device SXH (patented product of Anhui Saipu Power Protection Co., Ltd.) 1. The principle of this device is as follows: (1) If the grounding is a stable metallic grounding, a stable resistance grounding or a TV disconnection fault, the microcomputer controller will issue an indication and alarm signal, waiting for the on-duty personnel or the microcomputer line selection device to handle it. (2) If the ground fault is an unstable intermittent arc grounding, the microcomputer controller determines the phase of the grounding and simultaneously issues a command to close the vacuum contactor of the fault phase, engage the high-energy voltage limiter, limit the arc recovery voltage of the fault phase, absorb the electromagnetic energy caused by the grounding, slow down the system oscillation, and make the dielectric recovery dielectric strength Ujf of the arc path greater than the arc recovery voltage Uhf. This prevents the recovery voltage from breaking down the fault point again, thus completing the arc suppression. After a few seconds, the high-voltage vacuum contactor of the fault phase is opened. If it is a brief arc grounding, the system returns to normal operation; if the ground fault is a stable arc grounding, the controller orders the high-voltage vacuum contactor KD of the fault phase and the grounding to close simultaneously, so that the system quickly changes from a stable arc grounding fault to a stable metallic grounding. The device recognizes this fault as a permanent arc grounding fault and waits for the on-duty personnel or the microcomputer line selection device to handle it. 2. When the system resonates, the microcomputer controller performs different harmonic suppression according to the type of resonance: If the resonance of the system is a frequency division resonance, the microcomputer controller instantly short-circuits the open delta winding of the PT to destroy the resonance parameters and eliminate the resonance. If the resonance occurring in the system is at power frequency or high frequency, the microcomputer controller connects a high-power harmonic suppression resistor to the open delta winding of the PT for harmonic suppression. VII. Advantages of this device Installing this device in a neutral-point non-effectively grounded power grid can prevent insulation faults in electrical equipment and has the following advantages: 1. It can limit various overvoltages to lower voltage levels, greatly reducing insulation accidents caused by overvoltages. 2. Its arc suppression and overvoltage limiting mechanism is independent of the magnitude of the grid's capacitive current; therefore, its protective performance is not affected by changes in the grid's operating mode or grid expansion. 3. It can effectively prevent the internal insulation of electrical equipment from being reduced due to the cumulative effect of repeated or prolonged overvoltages. In 3-35KV power systems, most electrical equipment consists of rotating motors. If a single-phase direct grounding of the contactor is directly grounded to a metallic ground without distinguishing between stable arc grounding faults and transient grounding faults, it will cause the protection differential to trip, resulting in downtime and hindering production. The SXH device can distinguish between transient arc grounding faults and long-term stable arc grounding faults, avoiding unnecessary trouble.