GE T35 Instruction Manual page 355

CHAPTER 5: SETTINGS
5.5.4.5 Magnitude compensation
Transformer protection presents problems in the application of current transformers. CTs should be matched to the
current rating of each transformer winding, so that normal current through the power transformer is equal on the
secondary side of the CT on different windings. However, because only standard CT ratios are available, this matching is
not always exact.
In our example, the transformer has a voltage ratio of 220 kV / 69 kV (that is, about 3.188 to 1) and a compensating CT ratio
is 500 A to 1500 A (that is, 1 to 3). Historically, this would have resulted in a steady state current at the differential relay.
Interposing CTs or tapped relay windings were used to minimize this error.
The T35 automatically corrects for CT mismatch errors. All currents are magnitude compensated to be in units of the CTs of
one winding before the calculation of differential and restraint quantities.
The reference winding (w ) is the winding to which all currents are referred. This means that the differential and restraint
ref
currents are in per unit of nominal of the CTs on the reference winding. This is important to know because the settings of
the operate characteristic of the percent differential element (pickup, breakpoints 1 and 2) are entered in terms of the
same per unit of nominal.
The reference winding is chosen by the relay to be the winding that has the smallest margin of CT primary current with
respect to winding rated current, meaning that the CTs on the reference winding most likely begin to saturate before those
on other windings with heavy through currents. The characteristics of the reference winding CTs determine how the
percent differential element operate characteristic need to be set.
The T35 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 [1], the reference winding w
margin margin
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 T35 calculates magnitude compensation factors for each winding as follows:
T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
) for each winding:
rated
setting.
rated
) for each winding:
margin
is winding 2.
ref
ACTUAL VALUES  METERING  TRANSFORMER  DIFFERENTIAL AND RESTRAINT 
SYSTEM SETUP
5
Eq. 5-8
Eq. 5-9
Eq. 5-10
Eq. 5-11
Eq. 5-12
5-153