A digital simulator for testing over-current relay immunity against decaying DC offsets in the current waveform | ||||
| The International Conference on Electrical Engineering | ||||
| Article 126, Volume 6, 6th International Conference on Electrical Engineering ICEENG 2008, May 2008, Page 1-14 PDF (199.95 K) | ||||
| Document Type: Original Article | ||||
| DOI: 10.21608/iceeng.2008.34403 | ||||
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| Authors | ||||
| A. M. El-Hadidy; A. A. Mahfouz | ||||
| Electrical Power and Machines Dept., Faculty of Engineering, Cairo Univ. , Egypt. | ||||
| Abstract | ||||
| Abstract: A major requirement of a protective system is to be secure. Through a proper design of both algorithm and hardware of relay circuits, a secure protection system could be achieved. However, setting of the protective relays is done according to steady state short circuit calculations; although relays are always subject to disturbances that contain transient waveforms of both voltage and current. For over-current protective relays, the decaying DC offset is one of the well known components that appear in the currents seen by the relay. Some types of relays might be affected or show a very bad response towards this DC offset; especially static and electromechanical relays. Other types, like digital relays, are expected to be more secure towards the DC offset. Even relays of the same category (i.e. static) might respond in a different way for the same current. It is important for protection engineers to have a capability to test the performance of protective relays when a DC offset appears in the current waveform. Commercially available testers, with playback function, allow the injection of a current signal with decaying DC offset. However, this injected signal is extracted from other simulation programs such as EMTP or from recorded data. In order to test the relay using the commercially available testers, it would take a huge time and large hard disk space to accomplish the test as this requires to: create the signal, converting it to a COMTRADE format then injecting the signal (using the tester) for each possible time constant and amplitude of the decaying DC offset. These steps are then repeated for each setting of the relay; which is not practical. This paper introduces the design of a simple and cheap relay tester based on hysteresis current control of a half bridge inverter. The tester is capable of performing a complete test of the relay behaviour towards the decaying DC offset by injecting a current signal with a variable amplitude and time constant as well as a variable steady state value of the current. The values of the amplitude and the time constant are changed according to their practical ranges. The testing program is also introduced. The test results are compared with those obtained by commercially available testers and the difference was found to be negligible. The tester can be used also for injecting steady state waveforms of current. It is suitable for usage at laboratories or relay manufacturers testing facilities. | ||||
| Keywords | ||||
| Digital Simulators; Relay Testing; Transient Testing; Half Bridge Inverter; hysteresis control; DC Offset; Decaying DC Component; Relay Characteristics | ||||
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