UHV GAS INSULATED SUBSTATIONS- MITIGATION OF VERY FAST TRANSIENT OVER VOLTAGES (VFTOS)

Over the past three decades Gas Insulated Substations (GIS) have found an extensive range of applications in power transmission and distribution systems due to their high reliability, easy maintenance, and small ground space requirements, etc. Gas Insulated Substations (GIS) have been installed in increasing numbers over the last 25 years, and multiple units are currently in development for reliable power supply and commercial benefits. GIS with a voltage of up to 800 kV have been developed and are being used extensively. High arc interruption capability and strong dielectric strength made the Sulphur hexafluoride gas as an insulation medium in gas insulated sub stations. GIS is sturdy metallic condensed switchgear that includes high-voltage devices including circuit breakers and disconnectors.

Although GISs have been involved in power systems for a long time, it includes a problem of generation and propagation of very fast transient over-voltages (VFTOs) caused by switching operations of disconnectors or circuit breakers. These VFTOs will affect other high voltage components of switchgear and will increase secondary breakdowns.  VFTOs results in failure of insulation of components such as bushings and transformers. The VFTOs stress the adjacent equipment and cause electromagnetic interference to the secondary and control equipment’s. They also increase the dielectric stress in the transformer insulation. With the increase of GIS voltage level, the effect of VFTO should be taken into more consideration. The suppression of VFTOs in GIS has been a long-standing problem.

In this book, the above critical issues have been considered and suppression of very fast transient over voltages in 765KV, 1000KV, 1200KV and 1500KV Gas Insulated Substations were briefed out using four mitigation techniques. The design systems included the equivalent circuits of each equipment by taking consideration of VFTOs during switching process of disconnector switches. Resistor, RC filter, Ferrite rings and Nanocrystalline rings are four mitigation devices used to suppress the VFTOs.

Using Resistor as mitigating device with disconnector switch will decrease the rate of change in magnitude and frequency of refraction and reflection of travelling wave so that VFTOs can be suppressed. By connecting RC filter, high frequency components are attenuated by resistance R, and high frequency oscillations can be filtered by capacitance C. By redirecting very high voltage transients, RC filter with disconnector switch suppresses high frequency VFTOs. 

By connecting ferrite rings around a bus of a disconnecting switch, the magnitude and gradient of VFTOs can be reduced due to the impedance characteristics of ferromagnetic materials used. When Nanocrystalline rings are mounted around the inner conductor of   gas-insulated switchgear the losses occurred will results in voltage transient damping.