Using Rogowski coils inside protective relays

Traditionally, microprocessor-based relays incorporate a secondary current transformer to convert the 5 A or 1 A input current to a lower level for input to an analog-to-digital (A/D) converter as part of the input processing. The limits of this input circuit are well known, and relay designers take them into account when determining operating characteristics. A Rogowski coil produces an output based on the rate of change of the current input, so passing the output of the coil through an integrator produces a signal that is proportional to the current. There are additional characteristics of a Rogowski coil that give it particular advantages for use in a microprocessor-based relay, including the following: An air core. This leads to no saturation, even at very high currents; A flexible shape. With no iron core, the coil can be shaped and sized to fit the application; Immunity to electromagnetic interference. This makes the coil suitable for electrically noisy environments. These advantages, and others, make a Rogowski coil something to consider for the input of microprocessor-based relays, where the integration required to obtain the input current is a relatively easy operation. This paper discusses the practical considerations of using this coil in a relay. Complicating and mitigating factors are discussed, along with the performance impact on the relay and practical experiences in the field. Application impacts of Rogowski coil use are presented, including which types of relays are most benefited by this technology. Future implications of this technology are also presented.