GE 845 Instruction Manual page 195

CHAPTER 4: SETPOINTS
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
PHASE RELATIONSHIPS OF 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 terminal labels include the characters 1, 2, 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.
Magnitude Compensation
The percent differential protection from the 845 relay uses the phase-phase voltage and
the CT primary setting for Winding 1 as a reference, to perform magnitude compensation
for (bring to common base) the currents measured from the other windings: Winding 2 and
Winding 3.
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
) – magnitude compensation factor for winding 2 currents
) – magnitude compensation factor for winding 3 currents
SYSTEM
4–91