Fault Levels in a Typical System
In Figure 7.19, a section of a typical system is shown. At each voltage level the fault level can be ascertained from the reactances given. It should be noted that the short- circuit level will change with network conditions, and there will normally be two extreme values: that with all plant connected and that with the minimum plant normally connected. The short-circuit MVA at 275 kV busbars in Britain is normally
Figure 7.19 Typical transmission system. All reactances on a 100 MVA base
10 000 MVA, but drops to 7000 MVA with minimum plant connected. Maximum short-circuit (three-phase) levels normally experienced in the British system are as follows: 275 kV, 15000 MVA; 132kV, 3500MVA; 33kV, 750/1000MVA; 11 kV, 150/ 250 MVA; 415 V, 30 MVA.
As the transmission voltages increase, the short-circuit currents also increase, and for the 400 kV system, circuit breakers of 35 000 MVA breaking capacity are required. In order to reduce the fault level the number of parallel paths is reduced by sectionalizing. This is usually achieved by opening the circuit breaker connecting two sections of a substation or generating station busbar. One great advantage of direct-current transmission links in parallel with the alternating-current system is that no increase in the short-circuit currents results.