D-2 MM2 Motor Manager 2
GE Power Management
D.1 ASYMMETRICAL CURRENT APPENDIX D
D
Where
I
RMS
is the current when voltage is applied at a maximum – or the symmetrical current.
A motor or a transformer is never a perfect inductor, therefore, the value of 1.73 will never be
reached. The DC offset will die away as a function of the X/R ratio (typically a few cycles). The fol-
lowing figure represents an exaggeration of the three phase current of a motor starting.
Figure D–3: MOTOR START THREE PHASE CURRENT
When is this 'asymmetrical current' a concern?
When setting instantaneous relays, care must be taken to ensure that the instantaneous element
does not operate during normal operating conditions such as a motor start. Symptoms of an instanta-
neous element that is set too sensitive are nuisance or intermittent tripping of the relay during ener-
gizing of the system.
Furthermore, CTs do not react predictably when a DC current is applied. The waveform that is shown
above is not necessarily the waveform that each of three phase CTs would output. If there is a resid-
ual connection for ground fault detection, that element could operate when asymmetrical currents
are present.
To Next Page
Loading...
Need help?
General
