GE Multilin 345 - Transformer setup phase & magnitude compensations

CHAPTER 6: SETPOINTS S2 SYSTEM SETUP
345 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 6–23
W1 GROUNDING
Range: Not Within Zone, Within Zone
Default: Not Within Zone
Select the setting “Within Zone”, if there is a grounding path at Winding 1 side of the
transformer such as grounded neutral, grounding transformer, or grounded corner of a
delta winding. Select “Not Within Zone”, if there is no grounding path for Winding 1 in the
zone.
W2 NOM VOLTAGE
Range: 0.01 to 250.00 KV in steps of 0.01
Default: 4.16 KV
Enter the nominal phase-to phase-voltage rating of Winding 2 of the transformer.
W2 GROUNDING
Range: Not Within Zone, Within Zone
Default: Not Within Zone
Select the setting “Within Zone”, if there is a grounding path at the Winding 2 side of the
transformer, such as grounded neutral, grounding transformer, or grounded corner of a
delta winding. Select “Not Within Zone”, if there is no grounding path for Winding 2 in the
zone.
The Delta connection of CTs inherently has the effect of removing the zero sequence
components of the phase currents. If there were a grounding bank on the Delta winding
of the power transformer within the zone of protection, a ground fault would result in
differential (zero 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.
Transformer setup phase & magnitude compensations
Percent (biased) differential protection is the main type of protection for power
transformers with respect to detecting all types of transformer internal faults. This
protection is based on Kirchoff’s Law where the algebraic sum of all currents flowing in
and out of the protected equipment is equal to zero. However, when applying this law to
transformer protection, one should keep in mind that direct summation of the measured
per-phase transformer currents does not automatically result in a zero differential current.
This is because:
1. The transformer voltage ratio results in different winding nominal currents.
2. The winding CTs are not rated to exactly match the winding nominal currents.
3. Physically, the transformer windings are connected in Delta, Wye, or Zig-Zag
configuration, and each introduces a phase shift.
For correct performance of percent differential protection, it is necessary to adjust the
magnitude and phase relationships of the CT secondary currents for each winding, in order
to obtain near zero differential currents under normal transformer operating conditions.
Traditionally, the phase shift between the currents from the transformer windings has
been adjusted by connecting the CTs from the Wye winding in Delta connection, and the
CTs from the Delta winding in Wye connection. The magnitude of this adjustment is
accomplished using interposing CTs, or tapped relay windings. This, however, is not
required when the 345 relay is installed.