Simulation studies of OLTC transformer AVC relays operating in series using a RTDS, real time digital simulator

The most effective method for demonstrating the performance of protection relay schemes is to test them using a power supply network. If this is not possible, the operation of real protection relays can be demonstrated using a specialist real time power system simulator. This paper describes a series of tests conducted to demonstrate the operation of new tap changer control strategies using a commercial real time digital simulator (RTDS) and commercially available automatic voltage control (AVC) relays. The demonstration tests used AVC relays operating at different voltage levels in the network. Each relay was provided with the required voltage and current outputs from the RTDS as derived from the appropriate locations on the power system network. Different scenarios were then simulated and the performance of the different AVC voltage regulation strategies was investigated. Since the relays were effectively connected in series, the operation of any one of the relays influenced the other AVC relays in the network. The response of the AVC relays with a selection of control strategies to different power system scenarios was demonstrated. The use of the RTDS enabled the real time adaptive features of the different control strategies to be investigated. This was achieved by interfacing the relay inputs and outputs through the RTDS system with control logic developed in PSCAD. These techniques and the results of the tests are described in this paper.