GE T35 Instruction Manual page 356

SYSTEM SETUP
In our example, the magnitude compensation factors are calculated as follows:
The maximum allowed magnitude compensation factor (and hence the maximum allowed CT ratio mismatch) is 32.
5.5.4.6 Phase and zero sequence compensation
Power transformers can be connected to provide phase shift, such as the common Δ-Y connection with its 30° phase shift.
Historically, CT connections were arranged to compensate for this phase error so that the relaying operated correctly.
In our example, the transformer has the Δ-Y connection. Traditionally, CTs on the Wye connected transformer winding
(winding 2) are connected in a delta arrangement, which compensates for the phase angle lag introduced in the Delta
connected winding (winding 1), 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 is a grounding bank on the Delta winding of the power transformer within the zone of protection, a ground fault
results in differential (zero sequence) current and false trips. In such a case, it is 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.
5
The T35 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
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
winding, is calculated by the T35 for each winding as follows:
In our example, the phase reference winding is winding 1, the first delta winding (that is, w
angle for each winding is then calculated as follows (assuming Rotation = "ABC"):
The following table shows the linear combination of phases of a transformer winding that achieves the phase shift and
zero sequence removal for typical values of Φ
where
I [w] = uncompensated winding 'w' phase A current
A
p
I [w] = phase and zero sequence compensated winding 'w' phase A current
A
5-154
) is the winding to have a phase shift of 0° applied to it. The phase reference winding is
f
), the angle by which a winding current is shifted to refer it to the phase reference
comp
Φ [w] = | Φ[w ] – Φ[w] | where Rotation = "ABC"
comp f
Φ [w] = | Φ[w] – Φ[w
] | where Rotation = "ACB"
comp f
Φ [1] = 0° – 0° = 0°
comp
Φ [2] = 0° – (–30°) = 30° = 330° lag
comp
comp
= 1). The phase compensation
f
T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
Eq. 5-13
Eq. 5-14
Eq. 5-15
Eq. 5-16
Eq. 5-17
Eq. 5-18