GE 845 Instruction Manual page 208

SYSTEM
4–80
To compute differential and restraint currents, the 845 relay uses Winding 1 phase-phase
voltage and the primary CT rating from the Signal input used for Winding 1. The 845 relay
computes magnitude compensation factors for Winding 2 and Winding 3 currents as
shown below:
V , V , V - Phase-to-phase voltages (Setpoint) programmed for Winding 1, Winding 2,
W1 W2 W3
and Winding 3
CT , CT , CT – Current transformer rated primary current (Setpoint) programmed in
W1 W2 W3
Current Sensing menu and used as Signal Inputs to represent the currents from Winding 1,
Winding 2, and Winding 3.
V – Minimum winding voltage
min
∆V
– Voltage difference between winding voltage at actual tap, and minimum voltage
TLC
(Setpoint)
1. The setpoint Tap Position Detection from Tap Changer setup set to "Disabled", or the
setpoint TC Winding Currents set to "None"
M = 1 – magnitude compensation factor for winding 1 - REFERENCE
W1
M = (CT .V )/(CT .V
W2 W2 W2 W1 W1
M = (CT .V )/(CT .V
W3 W3 W3 W1 W1
2. Tap Position Detection set to "Enabled", and CT bank selected for TC Winding
Currents
The purpose of the tap changer installed on one of the transformer windings is to maintain
transformer output voltage within a certain bandwidth. To do this, the tap changer either
increases or decreases the turns of that winding i.e., changes its phase-phase voltage,
which in return leads to a different transformation ratio. When the transformation ratio is
changed, the nominal currents of the windings change as well, meaning that the
computation for CT mismatch based on the initially entered CT and ph-ph voltage per
winding, will be different, than the actual CT mismatch. This mismatch will appear as
differential current.
To compensate for the differential current, in cases of installed tap changer causing the
winding voltage to either go higher or lower than the nominal winding voltage entered in
the setpoints, the 845 provides dynamic CT mismatch compensation based on detection
of tap position.
The algorithm checks if the Tap Changer function is "Enabled", and on which winding it is
installed. To determine the winding, the algorithm checks which winding has the same
selection of Signal input as the one selected for the tap changer.
If the Signal input selected for the OLTC (Onload Tap Changer) is the same as the one for
the reference winding, the magnitude compensation factors for each non-reference
winding will be computed using the following equation.
a. OLTC installed on Winding 1
M = 1 – magnitude compensation factor for winding 1 - REFERENCE
W1
M = (CT .V )/ (CT
W2 W2 W2
winding 2 currents
M = (CT .V )/(CT
W3 W3 W3
3 currents
b. OLTC installed on Winding 2
M = 1 – magnitude compensation factor for winding 1 - REFERENCE
W1
+∆V
M = (CT .(V
W2 W2 min
winding 2 currents
) – magnitude compensation factor for winding 2 currents
) – magnitude compensation factor for winding 3 currents
+∆V
.(V )) – magnitude compensation factor for
W1 min TLC
+∆V
.(V )) – magnitude compensation factor for winding
W1 min TLC
))/ (CT .V ) – magnitude compensation factor for
TLC W1 W1
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS