Digital programmable time-parameter relay offers versatility and accuracy

The authors have developed, implemented, and tested an improved time-parameter relay. The relay principles are suitable for all time-overcurrent applications, and are also suitable for any time-parmeter function. Specific advantages include: protection of multiple phase; high accuracy, especially for long operating times; inclusion of a reclosing relay function; any time-parameter curve characteristics; and straightforward implementation of input parameter metering. The relay hardware is essentially an analog-to-digital conversion scheme followed by a dedicated microprocessor which processes samples of the input quantity in accordance with the principles and algorithms described in this paper. The magnitude of the input quantity, which is assumed to be an AC voltage or current, is determined from four samples per cycle. DC components of the input quantity are ignored. However, the relay could be adapted to DC applications by simple modification of the magnitude estimating functions. The accurate long-time characteristics and the selectable reset characteristics suggest this relay is especially applicable to the protection via thermal characteristic emulation of electrical equipment, such as motors or cables, from overheating.