GE Multilin 345 Instruction Manual page 144

S2 SYSTEM SETUP
6–28
In our example, the transformer has the Y-D connection. Traditionally, CTs on the Wye
connected transformer winding (winding 1) would be connected in a delta arrangement,
which compensates for the phase angle lag introduced in the Delta connected winding
(winding 2), so that line currents from both windings can be compared at the relay. The
Delta connection of CTs, however, inherently has the effect of removing the zero sequence
components of the phase currents. If there were a grounding bank on the Delta winding of
the power transformer within the zone of protection, a ground fault would result in
differential (zero sequence) current and false trips. In such a case, it would be necessary to
insert a zero sequence current trap with the Wye connected CTs on the Delta winding of
the transformer.
In general, zero sequence removal is necessary if zero sequence can flow into and out of
one transformer winding but not the other winding. Transformer windings that are
grounded inside the zone of protection allow zero sequence current flow in that winding,
and therefore it is from these windings that zero sequence removal is necessary.
The 345 performs this phase angle compensation and zero sequence removal
automatically, based on the settings entered for the transformer. All CTs are connected
Wye (polarity markings pointing away from the transformer). All currents are phase and
zero sequence compensated internally before the calculation of differential and restraint
quantities.
The phase reference winding (wf) is the winding which will have a phase shift of 0° applied
to it. The phase reference winding is chosen to be the delta or zigzag (non-wye) winding
with the lowest winding index, if one exists. For a transformer that has no delta or zigzag
windings, the first winding is chosen.
The phase compensation angle Φ?
to refer it to the phase reference winding, and is calculated by the 345 for each winding as
follows:
where Rotation = "ABC"
where Rotation = "ACB"
In our example, the phase reference winding would be winding 2 (i.e. wf = 2). The phase
compensation angle for each winding would then be calculated as follows (assuming
Rotation = "ABC"):
TRANSFORMER TYPES
A complete table of two-winding transformer types is shown below.
is the angle by which a winding current is shifted
comp
345 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS
Eq. 7
Eq. 8
Eq. 9