A Study on Characteristics of 1000kV GIS Substation UFTO

Abstract: The ultra fast transient overvoltage (UFTO) of GIS and HGIS substation is calculated and analyzed to ensure safe and reliable UHV power transmission project. The results show that on-off short line of GIS substation disconnector may generate up to 3 p.u. or so UFTO, which is likely to impose threat to GIS body, insulation and secondary equipment. The about 500Ω parallel on-off switch resistor is recommended for GIS substation. The main transformer in the GIS substation is connected with GIS through overhead lines and the UFTO of transformer is not high. The UFTO falls within the range, so it is acceptable not to install parallel resistor at HGIS substation disconnector.  

Key words: UHV, power transmission, substation, GIS, UFTO, disconnector, overvoltage   

1. Introduction

The UFTO refers to the transient overvoltage when front time is between 3ns and 100ns. When the disconnector in the GIS operates, UFTO may arise. Therefore, it is called switching overvoltage of disconnector.In the GIS, the line connecting disconnector and breaker is very short and its capacitance is small. The oscillation frequency of transient overvoltage is extremely high when the disconnector cuts the line. The typical waveform of UFTO put forward by IEC 60071-1:1993 and GB 311.1-1997 is shown in Fig.1. The frequency of oscillation wave at the beginning can reach 300kHz - 100MHz.  The pre-breakdown or severe breakdown between contacts will occur many times because moving contacts move slowly. The pre-breakdown may occur if one contact approaches the other during the process of switch on. The first breakdown must arise at the peak of power frequency voltage due to slow operation speed.