POWERGRID Experience on Operation of 765 kV Transmission System

Indian Economy is growing in the range of 5%-8% per annum. Electricity is one of the key inputs for such sustained growth. The demand of electricity in India is expected to increase to more than 437,000 MW by 2027 in comparison to the present demand of about 125,000MW. The primary energy resources for such massive requirement shall continue to be Coal and Hydro. Coal and Hydro resources are mainly located in Central/Eastern part and Northern/North Eastern part of India respectively whereas the loads are concentrated in the Western and Southern part of the country. This necessitates strong, reliable and long transmission network to transfer electricity to distant geographically spread load centers. Considering the above requirement, POWERGRID has taken up massive transmission network expansion plan by developing 765kV Transmission system apart from +500kV & 800kV HVDC system. In last 5 years, 19 Nos of 765kV grid Sub-stations with 67000 MVA, 765/400kV Transformers and 7300 ckms Transmission lines have been commissioned and are in operation. Apart from this, number of 765kV Sub-Stations and Transmission lines are under construction and are being commissioned progressively.

765kV Transformers capacity is generally large viz., 1500MVA which requires huge magnetizing currents if charged randomly, without controlled switching. In number of cases, Magnetizing currents were very high and even huge humming sounds were observed for about 10–15 minutes. This required careful study of the problem and field trials were carried out for optimum point for Circuit Breaker closing so that magnetizing current was reduced to acceptable levels. Switching of 765kV Transformers and Reactors is through controlled switching devices provided in Circuit Breakers for switching in/Switching out of Transformers/Reactors. The correct instance of Circuit Breaker closing and tripping operation is very important for minimization of electrical stresses on various equipments. This also depends on make and type of Circuit Breaker and hence, it required lot of interactions with manufacturers for finding the optimum solution. This paper covers details of investigations carried out and settings required for point on wave controlled switching of Circuit Breakers so that, electrical stresses are reduced to acceptable levels.

At two nos. of the 765kV Sub stations, failures of Surge Arrestors (provided for protection of 145kV class Neutral Grounding Reactor) have taken place during tripping operation of 765kV lines after opening from both the sides. The problem was investigated thoroughly and computer studies were also carried out to simulate the problem. It was observed that high voltage of 30Hz oscillating frequency is generated after opening of Circuit Breaker from both the ends. The magnitude of the oscillating voltage was also varying leading to very high voltage across reactor neutral. In this paper, various problems encountered in 765kV System have been discussed in detail and preventive and corrective measures taken by POWERGRID are also presented.