EFFECTS OF INTERPRETATION OF i2 IN THE CALCULATION OF I2eq2t CAPABILITY OF LARGE TURBOGENERATORS.

The negative sequence current capability of large turbogenerators, commonly specified as I//2**2t capability, is of special interest and of much significance to the designers and users of modern, large turbogenerators. The term is a direct measure of the machine's performance during unbalanced fault conditions. In the negative-sequence current capability calculations of large turbogenerators during transient unbalanced fault operation, the post-fault currents are generally assumed equal to sustained transient currents. The subtransient and dc components are disregarded so that the I//2**2t specification tends to be rather optimistic, rendering the machine vulnerable to serious thermal and mechanical damage during short duration, but relatively severe, unbalanced faults. The limitation arises from the lack of proper interpretation and specification of instantaneous negative-sequence currents which initiate double frequency eddy current heating of the rotor immediately after the occurrence of the fault. This paper describes four modes of interpreting the instantaneous negative-sequence currents in the machine that could be applied in the predetermination of I//2**2t capability of large, directly cooled turbogenerators. The analysis is used for the theoretical study of two 500 MW generators, for different time intervals and switching angles, showing the significant contribution by subtransient and dissymmetrical components of fault currents to the I//2**2t capability of the machines. The effect of these currents cannot be disregarded particularly when the fault duration is sufficiently small, even for high values of switching angles.