GE 845, Instruction Manual

Page: 212 / 552

4–84 845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
SYSTEM CHAPTER 4: SETPOINTS
In general, zero sequence removal is necessary if zero sequence can flow into and out of
one transformer winding but not the other windings. 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 845 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 (w
f
) 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 “wye” winding is chosen.
The phase compensation angle
φ
comp
is the angle by which a winding current is shifted
with reference to the angle of the reference winding, and is calculated by the 845 for each
winding as follows
φ
comp
[w] = |
φ
[w
f
] –
φ
[w] | where Rotation = “ABC”
φ
comp
[w] = |
φ
[w] –
φ
[w
f
] | where Rotation = “ACB”
The following figure: Phase and Zero Sequence Compensation for Typical Values of
φ
comp
,
shows the phase shift compensation equations of transformer winding currents, based on
typical phase compensation angles
φ
comp
, and whether or not the zero sequence current
is removed:
where:
I
A
[w] = uncompensated winding ‘w’ phase A current
I
A
p
[w] = phase and zero sequence compensated winding ‘w’ phase A current