Low-current grounding systems, especially those at 35kV and below, are prone to grounding faults due to their numerous branch lines, complex routing, and low voltage levels. This makes quality control during design and construction difficult to guarantee, resulting in a high probability of grounding faults during operation. To facilitate accurate grounding classification and timely fault handling by power grid operators, ensuring the safe and reliable operation of the power grid and improving power quality for users, this paper analyzes the composition and operating principle of insulation monitoring devices in low-current grounding systems, statistical data on grounding faults over the years, grounding causes, fault identification, and preventative grounding measures, based on the operational practice of the Xingyi City local power grid. This analysis provides valuable insights for operational staff and engineering technicians. 1. Problem Statement Currently, low-current grounding systems, especially those at 35kV and below, are prone to grounding faults due to their numerous branch lines, complex routing, and low voltage levels. Statistical data from the Xingyi City local power grid over the years show that 35kV grids account for 8.2% of grounding faults in low-current grounding systems, while 10kV grids account for 91.8%. This article, based on the author's understanding of power grid operation conditions and summary of operational experience, analyzes several types of grounding faults that are prone to misjudgment in the actual operation of low-current grounding systems. After providing an analysis of the causes, it focuses on elaborating on the methods for identifying grounding faults, handling measures, and countermeasures. It is believed that this will be of some reference value to colleagues. 2. Several Types of Grounding Faults Prone to Misjudgment and Their Causes To facilitate the following discussion, it is necessary to first conduct a simple analysis of the causes of grounding in the power grid. As shown in Figure 1, when the neutral point voltage Uo is not 0 and Uo is greater than the set value of the insulation monitoring system, a grounding signal is issued. Uo reflects the zero-sequence voltage, and its calculation formula is: Uo = (ùa + ùb + ùc) / 3. From the above formula, it can be seen that when the voltages ùa, ùb, and ùc of the power grid are unbalanced, a neutral point voltage Uo is generated. The degree of voltage imbalance in the power grid is the key to whether a grounding signal occurs. The following discussion will focus on a specific analysis of the causes of grounding faults. Based on the operational data of the Xingyi City local power grid over the years, we have compiled the following types of frequent grounding situations: 2.1 Single-phase grounding or incomplete two-phase grounding: In this case, the phase-to-ground voltages (ùa, ùb, ùc) of each phase are unbalanced, and their phasor sum is not zero, resulting in neutral point displacement (as shown in Figure 1). This causes a zero-sequence voltage to appear in the open delta winding of the TV secondary winding, triggering a grounding signal. 2.2 Phase-loss operation on the high-voltage side: Based on operational experience and multiple simulation tests, when the high-voltage side of the system is operating with one or two phases missing, due to the phase-to-ground voltage imbalance (one or two phases being zero), their phasor sum is not zero, and the resulting neutral point displacement causes a zero-sequence voltage to appear in the open delta winding of the TV secondary winding, triggering a grounding signal. 2.3 System resonance: If the excitation reactance XL (equal to ωL) of the voltage transformer TV in the system is too low, or if a phase voltage crosses zero during switching operations, or if the operation method is incorrect, or if the system grounding operation time is too long, ferroresonance may occur in the system. At this time, the three-phase voltage of the system is unbalanced, and the resulting neutral point displacement will also cause the protection to operate and issue a grounding signal. This is the most common type of false grounding fault that leads to false grounding signals in actual operation. In addition, testing the insulation of high-voltage equipment or lightning strikes may also cause false grounding signals. Due to the above reasons, it objectively brings certain difficulties to the operators in identifying grounding faults. 3. Grounding Fault Identification According to the wiring characteristics of TV, there is a zero-sequence magnetic flux path in its iron core, so the secondary induced voltage causes the protection to operate and issue a grounding signal. It is necessary to summarize the general characteristics of various grounding types to help power grid operators accurately identify fault conditions and take the correct methods to eliminate faults in a timely manner. 3.1 One phase of the system is grounded or two phases are not completely grounded. At this time, the voltage of the corresponding phase to ground decreases, the voltage of the ungrounded phase increases, and the voltmeter reading varies depending on the situation. When one phase is completely grounded, the voltage of the faulty phase to ground is zero, the neutral point displacement voltage is the phase voltage, and the voltage of the ungrounded phase to ground increases by a factor of two, becoming the system line voltage. If the faulty phase is not fully grounded, the voltage between the faulty phase and ground is greater than zero but less than the phase voltage, while the voltage between the non-faulty phase and ground is greater than the phase voltage but less than the line voltage. The grounding current is smaller than when fully grounded. Therefore, we can determine the system grounding status based on the readings of various meters connected to the insulation monitoring system shown in Figure 2. 3.2 When the high-voltage side of the system is operating with a phase loss, if one (or two) phases of the high-voltage side of the system are disconnected or the high-voltage fuse of one (or two) phases of the bus voltage transformer blows, the following specific situations occur: 3.2.1 If the insulation monitoring system (Figure 2) uses a Y0/Y0 connection composed of single-phase voltage transformers, assuming that the primary A phase of TV blows, causing a phase loss operation, the secondary a phase has no induced voltage. Logically, Va in Figure 2 should have no indication. However, since the Vab voltmeter is connected in series with the b phase, the voltmeters Vab and Va form a series voltage divider circuit, resulting in a certain indication for the Va meter, the value of which is proportional to the internal resistance of the meter. 3.2.2 If the insulation monitoring system uses a three-phase five-limb voltage transformer, due to the interconnection of the magnetic circuit system, when the fuse of phase A on the high-voltage side blows, causing a phase loss, the secondary phase a can induce voltage, and Va and Vab are higher than the analysis results in 3.2.1 above. The analysis of two phase losses is similar to that of one phase loss. In summary, when the system experiences a phase loss, the meters on the faulty phase will show a certain indication, while the meters on the non-faulty phases will remain unchanged. 3.3 When the system resonates, a significant characteristic of ferroresonance is the generation of overvoltage. We can observe the resonance situation from the changes in the meters. 3.3.1 The meter readings of one phase (or two phases) decrease (are not zero), while the meter readings of the other phases increase, exceeding the system voltage; or the voltmeter readings are excessive. From the XJJ or YJ coil voltage measured in Figure 2 or Figure 3, it can be seen that the neutral point voltage has shifted outside the voltage triangle. 3.3.2 The three-phase meter readings increase sequentially according to the phase sequence, and oscillate at a low frequency between 1.2 and 1.4 times the phase voltage, approximately once per second. 3.3.3 In Figure 2, the readings of the three-phase meters Va, Vb, and Vc are far higher than the line voltage. 3.3.4 In Figure 2, the readings of the meters Va, Vb, Vc, Vab, Vbc, and Vca simultaneously greatly exceed their rated values. In summary, a significant characteristic of ferroresonance is the generation of overvoltage, which can be judged based on the data collected by the system. As for the situation of false grounding signals during insulation testing of high-voltage equipment or lightning strikes, the power grid duty personnel can perform simple judgment and handling based on the actual situation at the time; this article will not provide a specific analysis. 4. Handling Measures During Grounding Faults Based on the author's operational experience, the following handling measures are taken when a grounding fault occurs in the power grid, depending on the situation: 4.1 When the three-phase meter readings are balanced but a grounding signal is issued, the first consideration should be whether the voltage transformer polarity has been reversed after maintenance. 4.2 In cases where a ground fault is confirmed to be genuine and occurs frequently or for extended periods, reasonable power grid rectification measures should be developed. Ground faults can jeopardize equipment safety, cause significant reactive power losses, and consequently reduce voltage quality. In practice, ground faults can be eliminated by correcting or replacing poles, crossarms, and insulators along the line, as well as pruning tree branches along the network. Furthermore, it is crucial to strengthen the operation and maintenance management of the lines. 4.3 Overvoltages generated by ferroresonance can cause insulation breakdown, surge arrester failure, and even the explosion or burnout of bus voltage transformers (TVs). Therefore, we must strive to avoid resonance. Fundamentally, during installation, the electromagnetic characteristics of the TV should be improved (replacing electromagnetic voltage transformers with poor volt-ampere characteristics to increase excitation reactance); or, in 35kV and 10kV systems, anti-harmonic resistors, anti-harmonic lamps, or breakdown fuses should be installed on the secondary side of the bus voltage transformer (TV) to prevent parallel resonance. For operation personnel, the main approach should be to change the operating mode and procedures, taking the following measures: 4.3.1 When changing the operating mode, Xco/XL should be ≤0.01 or Xco/XL ≥3 (Xco is the system capacitive reactance, XL is the system inductive reactance, mainly the TV excitation reactance), keeping the system away from the resonance region. 4.3.2 During operation, the arc suppression coil of a low-current system should be kept out of operation as much as possible; for high-current systems of 110KV and above, if calculations show that neutral grounding is not required during operation, the neutral grounding switch of the power transformer should be closed first, and then the neutral grounding switch should be taken out of operation after the operation is completed. 4.3.3 When powering off or on, pay attention to the operating sequence: When de-energizing the busbar, first disconnect the busbar voltage transformer (TV) to disconnect the inductor (L), then disconnect the bus tie circuit breaker. The process is reversed when energizing. When the circuit breaker outlet is equipped with a voltage equalization capacitor, the circuit breaker should be closed first, then the voltage increased. If power needs to be de-energized after increasing the voltage, the voltage should be reduced to zero before disconnecting the circuit breaker. For dual important busbars equipped with bus differential protection, when the bus differential protection causes one busbar to de-energize, the disconnecting switch of the bus tie circuit breaker or the disconnecting switch of the busbar voltage transformer should be opened promptly to disconnect the LC circuit experiencing series resonance. 4.3.4 Based on operational experience, if newly installed or overhauled 10kV and 35kV systems frequently experience resonance, and the resonance cannot be eliminated by the above-mentioned methods, we should consider whether the installed resonance elimination device is damaged. If no resonance elimination device is installed, a 100W light bulb can be connected to the open delta side of the TV for temporary use. 4.4 If a ground fault is caused by a single-phase operation, the line quality should be inspected, and some substandard line types should be replaced through recalculation. If the single-phase operation is caused by the blown busbar TV high-voltage fuse, and there are no technical or quality problems with the wiring and equipment in the power plant or substation, it should be considered whether it is necessary to recalculate the TV high-voltage fuse capacity. 4.5 If ground faults occur frequently during lightning strikes, the grounding of the incoming line surge arresters of the power plant or substation should be retested to ensure it is good, whether the internal grounding network of the power plant or substation meets the technical requirements, and whether the protection range of the lightning rods is sufficient. If necessary, measures such as adding chemical resistance reducing agents, burying grounding electrodes, and recalculating the protection range of the lightning rods should be taken to eliminate the fault. 4.6 If a ground fault signal is detected when testing the insulation of high-voltage equipment, the possibility of a ground fault point in the primary circuit should be considered. For example, whether the grounding wire has been removed after maintenance, whether the grounding switch has been opened, and whether the working grounding point (such as the TV primary grounding) or protective grounding point of the tested equipment has been disconnected. 5. Conclusion The specific causes of grounding in a system are very complex. With the development of microcomputer-based integrated automation technology in power systems, grounding fault identification has also shown some new characteristics. Due to space limitations, this article will not analyze these in detail. In practical work, we should accurately distinguish between true and false grounding based on the actual situation at the time and place, and then use the correct method to handle it according to the identified grounding category. The above is just some experience and insights from the author's work; any inaccuracies are welcome to be pointed out by colleagues.