Specific Features of Digital Current and Voltage Transformers for Relay Protection, Automation and Commercial Electric Power Metering

Abstract

At present, active work is under way to develop and introduce digital substations based on modern micro-processing control and relay modules with digital current and voltage instrument (optical) transformers [1]–[3]. Leading relay protection manufacturers have already successfully developed and launched production of microprocessor relay protection modules adapted to work with digital flows, using process buses according to Standard 61850-9-2LE. A somewhat more complicated task is the implementation of current and voltage instrument transformers. The simplest form is to use standard current and voltage transformers provided with measuring merging devices (Stand Alone Merging Units). This approach, however, does not fully resolve the problems of standard current and voltage transformers, such as core saturation. Because of their high price, the use of optical transformers is warranted only at voltages exceeding 110–220 kV. Using digital (unconventional) transformers opens the possibility of operating them in a wide voltage range, from 6 to 220 kV.