GE G30 Instruction Manual page 310

SYSTEM SETUP
The G30 determines the reference winding as follows:
1. Calculate the rated current (I
Note: enter the self-cooled MVA rating for the P
2. Calculate the CT margin (I
3. Choose the winding with the lowest CT margin.
In our example, the reference winding is chosen as follows.
1. Calculate the rated current for windings 1 and 2:
2. With these rated currents, calculate the CT margin for windings 1 and 2:
3. Since I [2] < I
margin
5
The reference winding is shown in
REFERENCE WINDING
The unit for calculation of the differential and restraint currents and base for the differential restraint settings is the CT
primary associated with the reference winding. In this example, the unit CT is 1500:5 on winding 2.
Magnitude compensation factors (M) are the scaling values by which each winding current is multiplied to refer it to the
reference winding. The G30 calculates magnitude compensation factors for each winding as follows:
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
5-136
) for each winding:
rated
) for each winding:
margin
[1], the reference winding w
margin
ACTUAL VALUES  METERING  TRANSFORMER  DIFFERENTIAL AND RESTRAINT 
.
setting.
rated
is winding 2.
ref
G30 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
Eq. 5-7
Eq. 5-8
Eq. 5-9
Eq. 5-10
Eq. 5-11
Eq. 5-12
Eq. 5-13