Use of local measurements to estimate voltage-stability margin

Estimating the proximity of power systems to voltage collapse in real time still faces difficulties. Beside the data-management and computational issu es, any central-control method is subject to the reliability of long-distan ce data communications. In the paper, we describe a new data-processing method to estimate the prox imity to voltage collapse. The method (code-named SMARTDevice, for Stabilit y Monitoring And Reference Tuning Device) employs only local measurements-b us voltage and load current-and calculates the strength of the transmission system relative to the bus. The collapse occurs when the local load approa ches this value. The method is simple enough so that it can be implemented in a numerical relay. The performance of SMARTDevice is compared against the conventional undervo ltage relay. It is shown that the latter can misoperate while the new devic e does not. SMARTDevice is in fact a new breed of voltage relays whose setp oint is automatically tuned to the system condition.