GE 845 Instruction Manual page 454

TRANSFORMER
Transformer
6–2
The relay measures all RMS currents and voltages, frequency, and all auxiliary analog
inputs. Other values like neutral current, phasor symmetrical components, power factor,
power (real, reactive, apparent), are derived. All quantities are recalculated every
protection pass and perform protection and monitoring functions. Displayed metered
quantities are updated approximately three (3) times a second for readability. All phasors
and symmetrical components are referenced to the A-N voltage phasor for wye-
connected VTs; to the A-B voltage phasor for delta-connected VTs; or to the phase A
current phasor when no voltage signals are present.
Figure 6-2: Current Metering Screen (EnerVista 8 Series)
The complete list of actual values available in Metering menu is shown.
INTRODUCTION
The percent (biased) differential protection is the main protection for power transformers
with regards to detecting all types of transformer internal fault. 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 transformer 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
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: METERING