Purpose/Aim
GIL is one of the important ways to connect GIS substations through existing overhead lines in urban substations, and GIS is directly connected to the main transformer through oil/SF6 bushings, which is one of the important means to make the substation layout more compact. With the development of substations, it is becoming more and more common for substations to use GIL-GIS-oil/SF6 bushing at the same time. Then the characteristics of VFTO caused by disconnectors or other operations in substations are quite different from those of conventional GIS substations. It is also particularly important to conduct a reasonable assessment of the insulation safety of the equipment such as transformer. Therefore, this paper conducts an in-depth study on the VFTO waveform characteristics of substations using GIL-GIS-oil/SF6 bushing structure by means of simulation, and summarizes its influencing factors, and finally proposes measures that can effectively suppress VFTO.
Experimental/Modeling methods
The research object of this paper is based on a 500kV substation. When VFTO is generated by the operation of the disconnector, the modeling methods of GIL and GIS busduct based on their geometric dimensions to obtain the unit-length-to-ground capacitance and unit-length conductor inductance, together with the travel time and surge impedance. The oil/SF6 bushing can be simulated as a multi-layer equivalent model considering its structure, and other GIS components makes use of electrical equivalent circuits composed by lumped elements (capacitances, inductances and resistances). Finally, the equivalent model of the whole substation for VFTO simulation is established in EMTP, and the VFTO waveform characteristics of the substation are analyzed through different operation modes. Furthermore, the influencing factors of VFTO are analyzed, and the suppression of VFTO waveform is simulated by means of damping resistance and magnetic ring.
Results/discussion
The simulation results show that there are big differences in the amplitude and frequency of the VFTO waveform caused by different operation modes. The waveform propagation path determines the main frequency of the oscillation, and the residual charge voltage and breakdown phase determine the amplitude of the waveform. The maximum VFTO at the transformer is generally caused by the operation of the disconnector switch closest to it.
Conclusions
The VFTO level on the GIS equipment can be reduced by 5~10% compared with the direct connection of the substation connected with the GIL and the GIS to the overhead line, and high frequency components are also significantly increased. Both the damping resistance and the magnetic ring can effectively reduce the amplitude and frequency of VFTO. Considering the economy and long-term reliability, the method of the magnetic ring to suppress VFTO is more suitable in engineering.
 

vfto; GIL; GIS; oil/SF6 bushing; simulation