Grounding refers to the connection of a part of a power system or electrical installation to a grounding electrode via a grounding wire. Electrical equipment includes power generation, transformation, transmission, and consumption equipment. Electrical equipment can be ungrounded, directly grounded, or grounded through a certain impedance. These grounding methods are called grounding systems. Germany, France, the United Kingdom, the United States, Japan, and Sweden all follow the International Electrotechnical Commission (IEC) regulations, which stipulate five grounding systems for low-voltage systems: TN-S, TN-C, TN-CS, TT, and IT. my country's "Technical Regulations for Electrical Design of Mechanical Plants" stipulates that my country has three grounding systems: TN, TT, and IT. The first letter T indicates that the power source has one grounding point; I indicates that all live parts at the power source are insulated from the ground. The second letter represents the relationship between the exposed parts of the electrical installation and the ground: T indicates that the exposed conductive parts are directly electrically connected to the ground, regardless of any grounding point in the power system; N indicates that the exposed conductive parts are directly electrically connected to the grounding point of the power system. my country's low-voltage generator grounding system generally adopts the TN grounding system. I. Problem Statement The Yujiantan Hydropower Station was put into operation in the early 1980s, with an installed capacity of 4×320kW and one 1600kVA main transformer. The neutral lines of the generators and the main transformer are connected together, sharing a single grounding device. According to China's grounding system, this is a TN system. After the power station went into operation, electric shock phenomena were repeatedly observed in the station's water collection wells and living facilities; sparks were also found in the grounding device during grounding resistance testing. Despite multiple tests showing that the grounding resistance was within acceptable limits, the transformer neutral point current reached as high as 122A, and the neutral point voltage reached as high as 22V. The four generators at the Yujiantan Hydropower Station operate in parallel with the grid, and the three-phase currents are balanced. Where does the neutral point current come from? This has drawn our serious attention. Please visit: Power Transmission and Distribution Equipment Network for more information. II. Cause Analysis Analysis revealed that the problem lies in the generator grounding system, caused by harmonics. According to electrical engineering principles, the generator's potential waveform is determined by the shape of the air gap magnetic flux density distribution along space. In actual motor structures, it is impossible to make the magnetic flux density distribution along space perfectly sinusoidal, so there are higher harmonics in the magnetic flux. The third harmonic is the main component of the higher harmonics, as shown in Figure 1. The characteristics of the third harmonic are that the harmonic E in the three-phase potentials A, B, and C is small, EB3 and EC3 are in phase, and one cycle of the fundamental wave is equivalent to three cycles of the third harmonic. In the electromotive force of each phase of a synchronous generator, the peak value of the third harmonic is the largest after the fundamental wave, and its impact on the stator electromotive force of the generator is also the greatest. Figure 1 shows that generators generally adopt a star connection, as shown in Figure 2. The third harmonic potentials all point to or away from the neutral point at the same time, and do not form a current loop, so the third harmonic current cannot flow. Although there is a third harmonic phase potential, the line potential does not exist because. Figure 2 shows the grounding system of the four generators of the Yujiantan Power Station, which forms a third harmonic current loop, as shown in Figure 3. The formation of harmonic circulating current will inevitably generate a large neutral line current and produce the following adverse consequences: Figure 3 1. Increased internal losses and reduced efficiency of the generator. For generators, harmonics not only degrade the electromotive force waveform but also induce additional losses. Because higher harmonic frequencies are higher than the fundamental frequency, the high-frequency current and its generated magnetic field, acting on the conductors, cause a skin effect; acting on the iron core, it generates losses on the core surface. The additional losses generated by higher harmonic currents in the stator conductors are related to the conductor's location and the current's penetration depth. The penetration depth is inversely proportional to the square root of the harmonic frequency, and the additional losses in the upper stator bars are 6-7 times greater than those in the lower bars. Furthermore, harmonic magnetic fields also generate additional losses on the stator core teeth. All these losses increase the generator's temperature rise, affecting its service life. 2. Additional losses increase the main transformer's temperature rise, accelerating insulation aging and also affecting its service life. 3. They affect metering accuracy. Low-voltage metering generally uses induction-type electricity meters. These meters are designed and manufactured for rated operation under pure sinusoidal AC conditions at the power frequency. Both fundamental and harmonic currents generate eddy currents on the electricity meter dial. The presence of harmonics, especially low-order harmonics, will lead to inaccurate meter readings. III. Measures The grounding system of the generators will be changed from TN to IT, eliminating harmonic paths. This involves disconnecting the neutral wire of each generator, as shown in Figure 2. The generator casing grounding remains unchanged, changing the generator operation from a three-phase four-wire system to a three-phase three-wire system. This prevents the third harmonic component from forming a loop. In the IT system, the generator neutral point is insulated from ground, while the generator casing is grounded. After changing the grounding system, the economic and safety operation of the generators and transformers is improved, achieving the following effects: First, the neutral current is reduced from 122A to 102A, eliminating additional losses; second, the neutral voltage is reduced from 20V to 0V, solving the problem of electric shock; third, the temperature rise is reduced by 4℃, solving the problem of excessive temperature rise during generator and main transformer operation and extending service life. In my opinion, the grounding system of a low-voltage generator should be determined according to the wiring method of the main electrical wiring. Low-voltage generators with direct load supply should adopt a TN system; low-voltage generators directly connected to the main transformer should not adopt a TN system, but should adopt an IT system.