Implementation of a MOV computation method within a protection system simulation

In this paper, we present an iterative MOV calculation method using phase coordinates rather than sequence coordinates. This method also uses the total capacitor/MOV current, Z_EQ of Goldsworthy (1987). However, since each phase is iterated independently of the other phases, the inter-sequence coupling does not affect the computations. That is, both balanced and unbalanced faults can be easily handled. Further, the MOV computation does not need any knowledge of the fault dependent sequence circuits. This means that any type of system unbalance can be treated correctly. The MOV conduction is represented by a series "fault" across the capacitor. Since the capacitors are retained in the computation, recalculation of the system impedance matrices is not required. The method was tested by incorporating it into an existing short-circuit program. The results from testing were compared with Alternative Transients Program (ATP) results to validate the computation method. This paper also describes a protection coordination study with and without the MOV computation. Detailed models of relay comparators, which are part of the protection analysis system containing the short-circuit program, can predict whether or not the relays should operate for a given fault. When the MOV is not included, the relays can misoperate. This is demonstrated by using the short-circuit program