Transformer - GE 889 Instruction Manual

CHAPTER 4: SETPOINTS

Transformer

NOTE:
NOTE
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
The Transformer setup is only available for the 889 with Overall Differential Protection
order code. Two-winding Transformer setup is used to compensate for transformer
winding connection in overall differential protection.
INTRODUCTION
An 889 unit ordered with the overall differential protection utilizes a two-winding
transformer setup for GSU winding compensation. This protection is based on Kirchoff's
law, where the sum of all currents flowing in and out of the protected equipment equals
zero. However, when applying this law to the overall differential protection, one must keep
in mind that the direct summation of the measured currents per-phase, does not
automatically result into zero differential current. This is because:
1. The transformer voltage ratio defines different winding nominal currents
2. The winding CTs are not rated to the exact match of the winding nominal currents
3. Physically, the transformer windings are connected in Delta or Wye configuration,
and they introduce a phase shift.
For the correct performance of the overall percent differential protection, it is necessary to
correct 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 corrected by connecting the CTs from the Wye winding in
Delta connection, and the CTs from the Delta winding in Wye connection. In the past, the
magnitude correction has been accomplished using interposing CTs, or tapped relay
windings. This however is not required any more when installing the 889 relay.
The 889 relay simplifies the process by performing the magnitude and phase shift
compensations automatically (internally). Upon entering settings for the protected
transformer and winding CT ratings, the relay automatically calculates and applies the
correct magnitude scaling to the winding currents as well as applying the correct phase
shift in order to prepare the currents for summation. To perform the correct currents
compensation, all winding CTs need to be connected in Wye (polarity markings pointing
away from the transformer).
SYSTEM
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