Due to the poor breaking performance, the disconnector in gas-insulated metal-enclosed switchgear (GIS) will produce a fast transient overvoltage (VFTO) with a very short rise time during operation, and the characteristics of short rise time and high amplitude will cause serious harm to the insulation of electrical equipment.
Zhang Mingyang, Gao Youhua, Li Qian, Yang Fan, and Yan Maowei from the School of Electrical Engineering of Shenyang University of Technology wrote an article in the 12th issue of Electrical Technology in 2023, using simulation to study the fixed arc resistance model, exponential arc resistance model, hyperbolic arc resistance model, Mayr arc resistance model and re-ignition arc resistance model, to obtain the amplitude and spectrogram of the fast transient overvoltage under each arc model, and to analyze the attenuation, oscillation and rise time of the fast transient overvoltage waveform under each arc model. By comparing the simulation results of each model, it is concluded that the re-ignition arc resistance model is more in line with the actual arc model, and the fast transient overvoltage amplitude, oscillation and rise time obtained by the model can better describe its waveform.
Gas-insulated metal-enclosed switchgear (GIS) is mainly composed of circuit breakers, disconnectors, grounding switches, busbars, bushings and other key equipment packaged together, and then filled with SF6 gas as phase-to-phase and ground insulation. GIS is widely used in power systems because of its small footprint, easy maintenance, good insulation performance and high reliability.
However, the operation of some of the most commonly used switches in GIS (e.g., disconnectors, circuit breakers), as well as the occurrence of single-phase ground faults, can generate fast transient overvoltages (VFTOs) within them. Especially in the operation of the disconnector, when the disconnector opens or closes the empty bus, the contact gap will be re-ignited many times, which causes the traveling waves to be reflected many times in the line, and the overlapping of each traveling wave causes high-frequency oscillation, forming a fast transient overvoltage with a very short rise time.
In addition, the arcing will also generate residual charge on the disconnector and bus, and the residual charge will accumulate on the bus with the extinguishing and re-ignition of the arc, and the residual charge discharge will superimpose with the traveling wave reflex reflection to produce a more serious rapid transient overvoltage. The fast transient overvoltage itself has the characteristics of fast generation, high frequency and high amplitude, which will cause great harm to GIS insulation, so the amplitude and frequency of the fast transient overvoltage must be suppressed to ensure the safe operation of the power system.
Researchers from the School of Electrical Engineering of Shenyang University of Technology used the electromagnetic transient program (EMTP) to build a simulation model of the actual GIS to analyze the generation of fast transient overvoltage and its waveform amplitude and frequency characteristics. At present, the arc model is mainly based on the actual arcing situation, and the arc model is mainly established according to the actual arcing situation, and the influence of different models on the fast transient overvoltage is different, so it is very important to choose the arc model reasonably for the simulation calculation of fast transient overvoltage.
Based on the fast transient overvoltage generated by the closing operation of a 1000kV GIS line disconnector switch, they used the MODE module in the electromagnetic transient program to establish a fixed-value arc resistance model, an exponential arc resistance model, a hyperbolic arc resistance model, a Mayr arc resistance model and a re-ignition arc resistance model, and obtained the amplitude and frequency characteristics of the fast transient overvoltage under different arc resistance models, and analyzed and compared the rise time, amplitude and harmonic components of the fast transient overvoltage waveform under each model.
Fig.1. Arcing process of re-ignition resistance model
The researchers found that the maximum value of fast transient overvoltage appeared at the end of the bus bar, and the minimum value appeared at the inlet capacitance of the transformer under different models. The harmonic components of the fast transient overvoltage under the five models are similar, among which the first, second and twelfth harmonics account for the largest proportion, all of which contain harmonic components of tens of megahertz, and the rise time of the fast transient overvoltage is about 1.45 μs.
Table 1 The rise time of VFTO under different arc resistance models
In addition, the fast transient overvoltage waveform oscillation under the fixed-value arc resistance model is smaller than that of other models, and the attenuation amplitude is larger. Compared with the fixed-value arc resistance model, the fast transient overvoltage waveform in the exponential arc resistance model is similar to the waveform in the first 10 μs, and the oscillation is more obvious after 10 μs. The amplitude, rise time, oscillation and attenuation of the fast transient overvoltage waveform in the hyperbolic arc resistance model and the Mayr arc resistance model are very similar.
They pointed out that compared with the incomplete or single-time description of the arcing process in other models, the re-ignition resistance model reflects the multiple combustion and extinguishing processes of the arc, which is closer to the actual arcing process. The rise time of the fast transient overvoltage under the re-ignition arc resistance model is relatively long, and the amplitude is similar to that of other models and the oscillation is more obvious, which is more consistent with the experimental waveform than other models.
The results of this work were published in the 12th issue of Electrical Technology in 2023, with the title of "The Effect of Arc Resistance Model on Fast Transient Overvoltage in 1000kV GIS", and the authors are Zhang Mingyang, Gao Youhua, et al.