A digital simulator for testing over-current relay immunity against decaying DC offsets in the current waveform

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.