[b]0 Introduction[/b] Many documents have introduced the research on the neutral grounding method of large generator stators to varying degrees [1-8]. Some have conducted qualitative analysis [8], while others have provided quantitative calculations [1, 2, 6]. Regarding model selection, some have adopted lumped parameter circuit models [2], while others have adopted quasi-distributed parameter circuit models [1]. In terms of analysis methods, some have used transient network analyzers (TNAs) [2], symmetrical component methods [3, 4], and PSPICE circuit simulation [1]. Regarding the choice of grounding method, some have favored high-resistance grounding [7], while others have focused on arc suppression coil grounding [4]. Reference [2] used a lumped parameter circuit model and a transient network analyzer (TNA) to study two grounding methods: high resistance and arc suppression coil, and provided curves showing the relationship between overvoltage and frequency in the healthy phase. However, due to the oversimplification of the lumped parameter circuit model, unavoidable errors are introduced. The symmetrical component method, based on steady-state conditions, cannot accurately describe the relationship between transient quantities. Reference [1] considered the existence of the distributed capacitance of the generator stator winding to ground, and used a quasi-distributed parameter circuit model and PSPICE circuit simulation software to conduct simulation studies on the two grounding methods mentioned above. The correctness of the quasi-distributed parameter circuit model was verified by various methods. Due to space limitations, some issues were not addressed. This paper intends to further explore the relevant issues of the high resistance and arc suppression coil grounding methods based on the quasi-distributed parameter circuit model and PSPICE circuit simulation software. The basic contents are: overvoltage and grounding current during arcing in the two grounding methods; voltage and current after considering the influence of the ultra-transient armature reaction inductive reactance of the stator winding during the transient process of arcing; the influence of different unit circuit forms in the quasi-distributed parameter circuit, and the influence of different number of series circuits in the unit circuit; the influence of the existence of arc suppression coil resistance, generator frequency variation, the length of each arcing time, and multiple arcing problems. The results are compared with those of previous studies, the advantages and disadvantages of the two grounding methods are analyzed, and the issues that should be considered in the selection of grounding methods are pointed out. The selection of generator grounding methods is of great significance. [b]1 Research Object, Model, Basic Assumptions[/b] 1.1 Research Object The research object is the same as that in reference [1], namely the hydroelectric generator of Ertan Hydropower Station. The basic data are as follows: rated frequency PN=550 MW, rated voltage UN=18 kV, rated current IN=19630 A, rated power factor cosφN=0.9, excitation current IF0=1587 A at rated no-load voltage, excitation current IFN=2709 A at rated load, number of parallel branches per phase of stator winding a=6, number of series coils per branch of stator N=27, number of pole pairs P=21, capacitance to ground per phase C0=1.686 μF, resistance per phase of stator (at 75 ℃) RS=3.6 mΩ, leakage inductance per phase of stator LS=227.05μH, and rated frequency fN=50 Hz. The recommended value for the neutral point inductive reactance XN of the arc suppression coil grounding is XC0/3, and the recommended value for the neutral point resistance RN of the high-resistance grounding (viewed from the primary side of the distribution transformer) is also XC0/3, where XC0 is the phase-to-ground capacitive reactance of the generator. Therefore, LN = 2.003 2 H and RN = 629.32 Ω. 1.2 Circuit Model Considering the existence of the distributed capacitance of the generator stator winding to ground, a quasi-distributed parameter circuit model composed of series-connected unit circuits is adopted. Figure 1(a) and (b) show two unit circuit forms. Figure 2 shows a quasi-distributed parameter circuit consisting of six (a) unit circuits connected in series. The inductance, resistance, capacitance, and electromotive force are respectively... When the neutral point is grounded with an arc-suppression coil, if there are two arcs in phase A, the simulated voltage waveforms of phases B and C are shown in Figures 5 and 6 (to reflect the rhythmic oscillation of the voltage recovery process after arc clearing, the time is longer and contains more waves, making it appear denser). From the figures, it can be seen that the overvoltage of a single arc is close to that of the high-resistance grounding method. The overvoltage of reignition is larger than that of a single arc, and obviously also larger than the overvoltage of reignition in high-resistance grounding. Furthermore, the recovery voltage after arc clearing forms a rhythmic oscillation and recovers slowly. Figure 7 shows the neutral point voltage waveform, which is equivalent to the transient value of a single-phase voltage. This value deviates significantly from the neutral point voltage to ground during normal operation. Table 1 gives a comparison of the overvoltage and grounding current values ​​for the two grounding methods (the overvoltage is taken as the larger of phases B and C, the same below). Table 1 Voltage and current values ​​for two grounding methods High resistance grounding (single flame) High resistance grounding (re-flame) Arc suppression coil grounding (single flame) Arc suppression coil grounding (re-flame) Overvoltage/kV 37.313 37.427 38.700 50.457 Grounding current/A 33.036 0 33.205 0 0.558 8 2.348 4 2.2 Considering that the inductance in the simulation circuit is the ultra-transient armature reaction inductance, the inductance of the stator winding in the simulation paper [1] is taken as the leakage inductance LS. However, in the transient process of arcing, it is more reasonable to take the ultra-transient armature reaction inductance in the simulation circuit. Considering this factor, the inductance of each phase should be greater than LS. We set the ultra-transient inductance of the stator winding during arcing to 2 times (2LS) and 4 times (4LS) of the leakage inductance, respectively. Using circuit diagram 2 again, the simulation calculations for reignition and LS were performed with the arc suppression coil grounded. The results are shown in Table 2. The comparison in the table shows that when the simulation circuit uses the ultra-transient armature reaction inductance, the overvoltage of the healthy phase during arcing increases, while the grounding current decreases. Clearly, considering the influence of the ultra-transient armature reaction reactance makes the simulation closer to reality. Table 2 Voltage and Current of Different LS LS 2LS 4LS Overvoltage/kV 50.457 51.764 53.795 Grounding Current/A 2.348 4 1.872 1 1.739 0 2.3 Simulation of different unit circuit forms in series Using the unit circuit in Figure 1(b), a distributed parameter circuit model composed of 6 unit circuits in series is formed. The simulation calculation (arc suppression coil grounded) yields an overvoltage of 52.117 kV and a grounding current of 2.784 7 A. Comparing with the second column of Table 2 (results of the circuit in Figure 2), it can be seen that the results are different depending on the form of the unit circuit. In practice, the worst-case scenario should be considered. 2.4 Simulation of Quasi-Distributed Parameter Circuits with Different Numbers of Unit Circuits Connected in Series Using the unit loop in Figure 1(a), quasi-distributed parameter circuits with 3, 6, 10, and 18 units connected in series were constructed respectively. These were compared with the lumped parameter circuit in reference [2] in a re-ignition simulation (arc suppression coil grounded). The results are shown in Table 3. The comparison in the table shows that as the number of unit circuits connected in series increases, the overvoltage of the healthy phase increases, but the rate of increase decreases. This increase is not infinite and must have an upper limit. Clearly, the quasi-distributed parameter circuit composed of 18 unit circuits connected in series is closer to reality. Table 3. Lumped parameters of voltage and current when the number of unit circuits in series varies. 3 unit strings, 6 unit strings, 10 unit strings, 18 unit strings. Overvoltage/kV: 44.586, 47.250, 50.457, 52.300, 52.461. Grounding current/A: 1.666, 0, 0.985, 5, 2.348, 4, 2.443, 6, 1.486, 1, 2.5. Simulation considering arc suppression coil resistance: The actual arc suppression coil has a certain resistance. Considering the resistance, the circuit in Figure 2 was simulated, and the results in Table 4 were obtained. It can be seen that connecting a suitable resistor in series with the arc suppression coil can not only greatly reduce the overvoltage of the healthy phase, approaching the overvoltage value when grounded with high resistance (Table 1), but also make the arc current (taking the larger value for single arcing and reignition) less than or close to the arc current when the arc suppression coil is grounded. Taking into account both overvoltage and arc current, a resistance value between 10 Ω (1.6% of ωLN) and 40 Ω (6.4% of ωLN) connected in series with the arc suppression coil is an ideal choice. Table 4 Voltage and Current Values ​​of Series Resistance of Arc Suppression Coil No Resistance Series Resistance/Ω 5 10 20 Overvoltage/kV 50.457 42.587 39.934 38.858 Grounding Current/A 2.348 4 0.767 6 1.268 5 1.493 2 Series Resistance/Ω 30 40 50 100 Overvoltage/kV 38.418 38.485 38.868 38.574 Grounding Current/A 2.369 0 2.708 4 3.407 2 6.000 9 2.6 Simulation of Generator Frequency Variation Considering the generator starting, stopping, and grounding under sudden overload, its frequency has a fluctuation range. Without considering the change in generator electromotive force amplitude, the simulation results for the frequencies of 40 Hz, 50 Hz, and 60 Hz in Figure 2 are... Simulations were performed at Hz, and the results are shown in Table 5. The table shows that frequency variation has little effect on overvoltage when the high-resistance grounding is used, but both overvoltage and arcing current increase when the arc suppression coil is grounded. Through simulation calculations at multiple frequency points, Figure 8 shows the relationship between the per-unit overvoltage of the healthy phase and frequency during reignition. This figure is flatter than the results obtained by Brown PG et al. in [2], because this paper uses a quasi-distributed parameter circuit and considers the resistance and inductance of the stator winding. Table 5 Voltage and current f/Hz at different frequencies High resistance grounding Arc suppression coil grounding 40 50 60 40 50 60 Overvoltage/kV 38.450 38.731 38.780 55.184 50.457 55.680 Grounding current/A 36.675 35.513 36.695 12.302 2.348 4 12.855 1 is the arc suppression coil grounding method (neutral point is pure inductance); 2 is the high resistance grounding method; 3 is the arc suppression coil grounding method (neutral point inductance has a certain resistance) Figure 8 Relationship curve between overvoltage and frequency 2.7 Relationship between overvoltage and arcing time In the above simulation, the arcing time for the first and second times was selected as 3 power frequency cycles. If the arcing time is taken as a different integer number of cycles (we also performed simulations for 2, 5, 7, and 10 cycles, etc.), the simulation results are not significantly different from those above. 2.8 Regarding the third arcing after the third or more arc reignitions and the third arcing at the maximum amplitude of the A-phase beat oscillation overvoltage, the overvoltage generated in the healthy phase is very similar to that of the second arcing. Using the same method for the fourth, fifth, and subsequent arcings, the results are basically the same. However, in reality, the probability of this is extremely small. [b]3 Conclusions[/b] (1) For the two grounding methods of high resistance and arc suppression coil, the overvoltage of the healthy phase after arcing is divided into three stages: the first is the spike, which is actually a transient process when expanded; the second is the steady-state value of the overvoltage during the arcing period after the spike, which is equal to the line voltage value; the third is the voltage recovery stage after the arc is cut off. The high resistance grounding method recovers faster, which is beneficial to the system, but for the arcing point, the fast recovery speed means that the ionized medium of the arcing point has to withstand a larger voltage impact and a larger probability of reignition; while the recovery process of the arc suppression coil grounding method is a beat oscillation, which is slower, which is unfavorable to the system, but reduces the probability of reignition. (2) The spike at the beginning of the arcing comes from the instantaneous discharge of the capacitor, which forms a transient oscillating decay current with the transient inductance of the winding. Such a current will generate a voltage drop on the resistance and inductance of each phase, which is superimposed on the voltage of phase B and phase C to form a larger transient overvoltage spike. This spike is very unfavorable to the motor insulation. (3) When the arc suppression coil is grounded, the beat oscillation of the voltage after the arc is cut off is due to the energy exchange between the arc suppression coil and the capacitor, forming a slowly decaying oscillating voltage. Its frequency is close to the power supply frequency of 50 Hz. When superimposed with the power supply voltage, it forms a beat oscillation voltage with a larger period. If the inductance of the arc suppression coil is slightly changed or the power supply frequency is changed, the beat period will be greatly shortened. (4) For the arc suppression coil grounding method, if the frequency deviates from the rated frequency during generator start-up, shutdown, or loss of synchronism, arcing will occur, and both overvoltage and grounding current will increase. Once this happens, measures such as generator tripping and demagnetization should be taken to avoid this phenomenon. (5) The overvoltage of the high resistance grounding method is smaller, and the single-phase grounding current is larger. When selecting the high resistance grounding method as the generator stator neutral point protection method, corresponding measures must be taken to prevent the iron core from burning under high current. (6) The overvoltage of the arc suppression coil grounding method is larger, and the single-phase grounding current is smaller. If an arc suppression coil grounding method is used as the generator stator grounding protection, the problems of large overvoltages caused by arc reignition and increased overvoltages and grounding currents during frequency fluctuations should be properly addressed. Connecting a suitable small resistor in series with the arc suppression coil can effectively suppress overvoltages while retaining the advantage of relatively small grounding currents. About the authors: Li Ruliang (1956-), male, professor, works in the Department of Physics, Shangqiu Teachers College, Henan Province, specializing in electrical engineering and control. He is currently a visiting scholar in the Department of Electrical Engineering, Tsinghua University, researching transient processes in motors; Li Yixiang (1974-), male, doctoral student, researching transient processes in motors; Wang Xiangheng (1940-), male, professor, doctoral supervisor, researching motor analysis and control, electrical drives and their automation. [b]References[/b] 〔1〕 Li Yixiang, Wang Xiangheng, Wang Weijian, et al. A new approach to the study of neutral grounding method of large generator stator［J］. Power System Technology, 1997, 21（9） 〔2〕 Brown PG, Johnson IB, Stevenson J R. Generator Neutral Grounding, Some Aspects of Application for Distribution Transformer with Secondary Resistor and Resonant Types［J］. IEEE Transactions on Power Apparatus and Systems. 1978, PAS-97（3）: 683～694 〔3〕 Wang Weijian. Principles and Applications of Relay Protection for Main Electrical Equipment［M］. Beijing: China Electric Power Press, 1996 〔4〕 Wang Weijian, Liu Junhong. The choice of neutral grounding method of large generator［J］. Power System Technology, 1995, 19（6） 〔5〕 Wang Weijian, Liu Junhong, Tang Lianxiang, et al. Analysis of neutral grounding method of generator unit from the perspective of Three Gorges generator unit safety [J]. Electric Power Automation Equipment, 1995, 15(4) [6] Wang Weijian, Lu Huafu. Analysis and calculation of the relationship between neutral grounding method and stator grounding protection sensitivity of large generator [J]. Electric Power Automation Equipment, 1995, 15(3) [7] Wang Zhiying, Rong Jiangang. Research on neutral grounding method of generator [J]. Power Grid Technology, 1994, 18(4) [8] You Gaolin. Discussion on neutral grounding method of unit-connected generator [J]. Electric Power Technology, 1983, 16(12) Editor: He Shiping