Practical application of waveform relaxation method for testing remote protective relays

Testing of newly designed protective relay logics is essential prior to the installation of the relay hardware in power systems. Real-time simulators (rts) are widely used to simulate the computer models of the power system in real-time to analyze the closed loop performance of the relay. This kind of simulation is expensive because high performance processing units are required for the rts systems. Also, the closed loop simulation results will be inaccurate if there are communication delays between the rts and the relay hardware. In this paper, a new iterative method for the closed loop performance test of protective relays is presented. The approach uses the iterative method of the Waveform Relaxation (WR) with the use of a specialized interface hardware called the Real-Time Player/Recorder (RTPR). The power system model is simulated in an off-line electromagnetic transient (EMT) simulation program and the relay hardware is connected to the RTPR device. The off-line simulation results (waveforms) are recorded and then played back in real-time to the relay through the RTPR device. The RTPR records the relay response (trip signals) accordingly and makes them available to the EMT simulation. The EMT simulation is once again repeated with the new recorded waveforms and the results are compared with those from the previous iteration. If the waveforms are different, the above process is repeated. The converged results represent the closed loop response of the EMT simulation and the relay hardware. The approach is used to test the reclosing function of a commercially available relay (SEL-421). It is shown that some of the relay settings must be adjusted to facilitate this type of simulation. The relay is also tested by a real-time digital simulator for cross-validation of the proposed approach. Very similar results prove the high accuracy of the proposed method.