Description
- The “zero position” terminal and the “wiper” terminal of the tap position circuit areconnected to the positive and negative 745 tap position terminals. Polarity is not
consequential. The following setpoints configure the 745 to determine tap position. In the
S2 SYSTEM SETUP ONLOAD TAP CHANGER setpoint menu, make the following settings:
WINIDNG WITH TAP CHANGER: “Winding 2”
NUMBER OF TAP POSITIONS: “33”
MINIMUM TAP POSITION VOLTAGE: “61.0 kV”
VOLTAGE INCREMENT PER TAP: “0.50 kV”
RESISTANCE INCREMENT PER TAP: “33 Ω”
The maximum value resistance on the top tap is 5 kΩ.
5.2.3 Phase Shifts on Three-phase Transformers
Power transformers that are built in accordance with ANSI and IEC standards are required
to identify winding terminals and phase relationships among the windings of the
transformer.
- ANSI standard C.37.12.70 requires that the labels of the terminals include the characters 1,
2, and 3 to represent the names of the individual phases. The phase relationship among
the windings must be shown as a phasor diagram on the nameplate, with the winding
terminals clearly labeled. This standard specifically states that the phase relationships are
established for a condition where the source phase sequence of 1-2-3 is connected to
transformer windings labeled 1, 2 and 3 respectively.
IEC standard 60076-1 (1993) states that the terminal markings of the three phases follow
national practice. The phase relationship among the windings is shown as a specified
notation on the nameplate, and there may be a phasor diagram. In this standard the
arbitrary labeling of the windings is shown as I, II, and III. This standard specifically states
that the phase relationships are established for a condition where a source phase
sequence of I – II – III is connected to transformer windings labeled I, II and III respectively.
- Note that the delta winding currents leads the wye winding currents by 30°, (which is atype Yd11 in IEC nomenclature and a type Y/d330 in GE Multilin nomenclature) 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.
It may be suggested that for the ACB sequence the phase relationship can be returned to
that shown on the transformer nameplate by connecting source phases A, B and C to
transformer terminals A, C, and B respectively. This will restore the nameplate phase shifts
but will cause 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 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
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