Power system short circuit planning for superconducting power transmission

Short circuit capacity of large power generation facilities is limited by the internal reactance of the generator and the impedance of connecting bus work and transformers. In the event of a fault on a generator get away circuit, d-c offset could double the short circuit current on the cable for a period of from 3 to 5 cycles after the fault initiation. High performance circuit breakers (PCBs) can shorten this fault duty time to 1 cycle (17 msec). If a superconducting cable is arbitrarily designed for 10X rated current fault withstand, its capital and operating cost may be dramatically increased. What can be done? First, a careful look at generator step up transformers and circuit breaker capability shows a marked tendency for this equipment to have very high costs if the generator bus connection scheme is planned to allow very large fault currents. However, the power system designer can set up the circuit arrangement so that massive fault currents are avoided. Second, the development of fault current limiting devices promises the possibility of holding fault current levels below 2X or 3X rated current. DC superconducting cables do not require this extra fault duty consideration, because the rectifier inverter system protects them. Usual fault conditions on d-c cable are such that less than 2X rated current will occur on the cable during a fault. Because a-c superconducting cables include very high fault current capability in their design, a careful trade-off study with current limiters, generator bus layout alternatives and possible d-c cable application should be done to assure that the final design may more readily achieve reasonable economics.