GE P645

2.5 ZERO SEQUENCE FILTERING
An earth fault in a three-phase system will always produce a zero sequence current component. With earthed Y-
connected windings, this zer
o sequence current flows through the neutral conductor to earth. With delta-
connected windings, this zero sequence current component just circulates around the delta connected windings
(unless an earthing transformer is used). So in the case of a Y-delta transformer, an external fault will cause zero
sequence current to be measured by the Y-side CTs. However, because this zero sequence current is trapped on
the delta side, it is not measured by the delta-side CTs. This could cause maloperation if not compensated for.
Before the advent of numerical IEDs, this was handled by a configuration involving interposing CTs. Numerical
IEDs, however, can do this by filtering out the zero sequence component in software.
2.6 MAGNETISING INRUSH RESTRAINT
Whenever there is an abrupt change of magnetising voltage (e.g. when a transformer is initially connected to a
sour
ce of AC voltage), there may be a substantial surge of current through the primary winding called inrush
current.
In an ideal transformer, the magnetizing current would rise to approximately twice its normal peak value as well,
generating the necessary MMF to create this higher-than-normal flux. However, most transformers are not
designed with enough of a margin between normal flux peaks and the saturation limits to avoid saturating in a
condition like this, and so the core will almost certainly saturate during this first half-cycle of voltage. During
saturation, disproportionate amounts of MMF are needed to generate magnetic flux. This means that winding
current, which creates the MMF to cause flux in the core, could rise to a value way in excess of its steady state
peak value. Furthermore, if the transformer happens to have some residual magnetism in its core at the moment
of connection to the source, the problem could be further exacerbated.
The following figure shows the magnetizing inrush phenomenon:
Steady state
S
witch on at voltage
zero – No residual flux
+F
m
-
F
m
2F
m
V03123
V = Voltage, F = Flux, Im = magnetising current, Fm = maximum flux
Figure 38: Magnetising inrush phenomenon
P64x Chapter 6 - Transformer Differential Protection
P64x-TM-EN-1.3 105