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4–76
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
SYSTEM
CHAPTER 4: SETPOINTS
sequence) current and false trips. In such case, it would be necessary to insert a zero
sequence current trap with the wye connected CTs on the Delta winding of the
transformer.
In general, zero sequence removal is necessary if zero sequence can flow into and out of
one transformer winding but not the other winding. Transformer windings that are
grounded inside zone protection allow zero sequence current flow in that winding, and
therefore it is from these windings that zero sequence removal is necessary.
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 889 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 Winding 2.
To compute differential and restraint currents, the 889 relay uses Winding 1 phase-phase
voltage and the primary CT rating from the Signal input used for Winding 1. The 889 relay
computes magnitude compensation factors for Winding 2 currents as shown below:
V
W1
, V
W2
- Phase-to-phase voltages (Setpoint) programmed for Winding 1 and Winding 2.
CT
W1
, CT
W2
– Current transformer rated primary current (Setpoint) programmed in Current
Sensing menu and used as Signal Inputs to represent the currents from Winding 1 and
Winding 2.
Magnitude Compensation is given by:
M
W1
= 1 – magnitude compensation factor for winding 1 - REFERENCE
M
W2
= (CT
W2
.V
W2
)/(CT
W1
.V
W1
) – magnitude compensation factor for winding 2 currents
NOTE
NOTE:
The maximum allowed magnitude compensation factor (and hence the maximum allowed
CT ratio mismatch) is 20.
Содержание Multilin 889
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