Transformer Setup Phase & Magnitude Compensations - GE SR345 Instruction Manual

S2 SYSTEM SETUP
Transformer setup phase & magnitude compensations
6–50
Courtesy of NationalSwitchgear.com
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.
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.
The 345 relay simplifies the process by performing the magnitude and phase shift
compensations automatically (internally). Upon receiving entered settings for the
protected transformer and winding CT ratings, the relay automatically calculates and
applies the correct magnitude of scaling to the winding currents, as well as the correct
phase shift, in order to prepare the currents for summation. In order to performing the
appropriate current compensation, all winding CTs must be connected in Wye (polarity
markings pointing away from the transformer).
Magnitude Compensation Reference: Percent differential protection by the 345 relay uses
the user-entered phase-to-phase voltage and the CT primary settings for Winding 1 as a
reference for current magnitude compensation.
Phase Shift Compensation Reference: Percent differential protection by the 345 relay uses
either the Delta or the Zig-Zag currents (depending on the transformer setup) as a phase
shift reference to perform the phase shift compensation. When a Wye/Wye transformer is
selected in the setup, the currents from the first Wye winding are used as a reference for
the phase shift compensation.
EXAMPLE:
This section describes the process of automatic current magnitude and phase shift
compensation using a specific example that shows how CT ratios, transformer voltage
ratios, and transformer phase shifts are used to generate the correction factors. These
correction factors are applied to the current signals to obtain extremely accurate
differential currents. The menu paths in the example are shown graphically in the above
figure.
345 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS