Probabilistic estimation of risk of failure of a 1200 kv UHVAC transmission line
For high capacity power transfer, with lesser right of way (ROW) and reduced transmission losses between the generation pockets in the Eastern parts to the demand pockets in the West and Northern parts of India, a 765 kV Ultra High Voltage (UHV) AC system up to Wardha and then a hybrid 765 kV and 1200 kV line between Wardha-Aurangabad (405km) is planned. This paper proposes a method to determine the probabilistic risk of failure of 1200kV line against changes in operating dynamic conditions. Switching surge overvoltages determine the insulation design rather than lightning overvoltages with increasing operating voltage of transmission systems. So, an accurate estimate of the switching overvoltages under various conditions of operation is one of the important factors for the design of an UHV transmission system. The switching overvoltage level of the UHV transmission line is mainly associated with the Line Energization, Line Re-energization, Fault Initiation and Fault Clearing events. The magnitude of the overvoltages produced by the other events can be attenuated by employing suitable and approximate protective schemes. Line energization and re-energization generally produce larger overvoltages if restrike-free breakers are used. The literature discusses various means of controlling switching surges caused by energization and re-energization of lines. Effective switching surge control can be gained by synchronous closing of the circuit breaker, by controlling the timing of breaker contact closing, by inserting one or more resistors across the breaker contacts, or by a combination of these methods. When the overvoltages due to line energization are controlled to values below 2 p.u., fault initiation and fault clearing overvoltages require careful examination if they are not controlled to the same degree, consequently this paper focuses on these three events. Statistical distributions of slow-front overvoltages have played an important role in determining insulation levels and considering insulation coordination of UHV systems. Design studies for high voltage systems frequently include the determination of the statistical distribution of overvoltages rather than values corresponding to single known operating times for each breaker pole. This paper proposes the statistical distributions of energization, fault initiation and fault clearing overvoltages of the 1200kV system. The statistical distribution of switching overvoltages is obtained from the simulated results of the circuits modelled in PSCAD. The statistical distribution of switching overvoltage is estimated using randomness of the instants of closing with 200 operations for line energization and randomness of fault location and fault times with 8 fault locations and 21 fault timings, 168 simulations for fault initiation and fault clearing are considered. Based on the test results of switching impulse flash overvoltages, a practical method to calculate the risk of failure is presented.
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