| 摘要 |
Even when the terminations of the high voltage winding or windings 2 of a power distribution transformer 1 are protected by surge arresters or diverters 7, 8 connected to ground via the transformer casing 5 and casing earth connection, a lightning strike may cause transformer failure due to the surge current (typically 10KA) passing through the surge diverter 7 or 8 and the casing ground resistance Rc-e (typically 10 ohms) giving rise to a transient voltage (typically 100KV) on the casing 5. This transient voltage appears between the casing 5 and the grounded neutral termination of the low voltage winding 3 of the transformer and exceeds the withstand voltage of the low voltage winding. To overcome this problem, a high voltage surge arrester 10 is connected between the casing 5 and the neutral termination of the low voltage winding 3. The surge arrester 10 is chosen to clamp the transformer casing 5 to a voltage (eg. no greater than 20KV) which is below the withstand voltage (eg. 25KV) of the low voltage winding 3. When used in a distribution system having auto-reclosing type circuit breakers on the high voltage side, the arrester 10 must also be able to withstand, for the re-make period of the circuit breaker, a fault voltage to which it will be subjected when a ground fault exists on the high voltage side. In an 11KV distribution system, arrester 10 must withstand 7KV for 10 seconds under such ground fault conditions. The transformer 1 may be pole-mounted, and additional protection may be provided by connecting the low voltage lines to ground by means of low voltage surge arresters located at the next pole downstream of the low voltage side of the transformer. |
