GE G30 Instruction Manual page 366

SYSTEM SETUP CHAPTER 5: SETTINGS
Figure 5-71: Phasors for ACB sequence
Note that the delta winding currents leads the wye winding currents by 30°, (which is a type Yd11 in IEC nomenclature and
a type Y/d330 in GE Multilin nomenclature) and which is in disagreement with the transformer nameplate. This is because
the physical connections and hence the equations used to calculate current for the delta winding have not changed. The
transformer nameplate phase relationship information is only correct for a stated phase sequence.
Phase relationship for the ACB sequence can be returned the transformer nameplate values by connecting source phases
A, B, and C to transformer terminals A, C, and B respectively. Although this restores the nameplate phase shifts, it causes
incorrect identification of phases B and C within the relay and is therefore not recommended.
All information presented in this manual is based on connecting the relay phase A, B, and C terminals to the power system
phases A, B, and C respectively. The transformer types and phase relationships presented are for a system phase
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sequence of ABC, in accordance with the standards for power transformers. Users with a system phase sequence of ACB
must determine the transformer type for this sequence.
If a power system with ACB rotation is connected to the Wye winding terminals 1, 2, and 3, respectively, from a Y/d30
transformer, select a Power Rotation setting of ACB in the relay and enter data for the Y/d330 transformer type.
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 G30 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.
5-152 G30 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL