On the use of voltage stability index to prevent undesirable distance relay operation during voltage instability

Distance relays may operate undesirably not because of faults but because of overloads and its operations have contributed to voltage collapses worldwide. To overcome this problem, the voltage stability index (VSI) is used as a criteria to differentiate between faults and voltage collapse. The VSI is developed based on the change of apparent power and it uses some local information such as bus voltage magnitude and load current magnitude. To evaluate the effectiveness of the VSI, fault and the proximity voltage collapse simulations were carried out on the IEEE 9 bus system. Results proved that the VSI is better than the SSI in terms of differentiating between a fault and a voltage instability condition. Therefore, the VSI can be applied in adaptive distance protection relaying for discriminating between a fault and a voltage collapse so as to prevent undesirable zone 3 distance relay operations.