7SR242 Duobias Commissioning & Maintenance Guide
©2010 Siemens Protection Devices Limited Chapter 6 Page 51 of 57
A compromise test can be made by the use of a diode to generate a half-wave rectified waveform from a
sinusoidal source. The half-wave rectified current will contain a combination of fundamental and harmonic
currents. The rectified waveform contains even harmonics higher than 2
nd
but the relationship between the 2
nd
harmonic current content, the fundamental component and the total RMS current is as shown below. Note that
some protection elements can be set to operate on the RMS current or the Fundamental current and the applied
values are different when non-sinusoidal waveforms are applied. The Inrush Detector setting is based on the ratio
of 2
nd
harmonic to fundamental. This method is not suitable for use with constant current generating test sets such
as modern digital equipment.
Full Sine
RMS
Rectified RMS
Current
Fundamental
component
2
nd
Harmonic
component
2
nd
/Fundamental
Half Wave Rectified 1.0 0.5 0.5 0.212 0.424
Assuming that the Gn 81HBL2 Setting is less than 40%, inject half-wave rectified current at an RMS or
Fundamental component level above the element setting to prove that the block is applied and the element is
stable. Care should be taken that the thermal limits of the relay are not exceeded during these tests.
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