CHAPTER 10: THEORY OF OPERATION PHASE DISTANCE APPLIED TO POWER TRANSFORMERS
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 10-31
10
• Right blinder difference angle = | –93.0° – (–109.2°) | = 16.2° < 90°
• Left blinder difference angle = | 82.9° – 108.7° | = 25.8° < 90°
• Fault-type comparator difference angle = | 19.8° – 19.8° | = 0.0° < 50°
All six comparators and the overcurrent supervision are satisfied, so the Quad Phase A ground element operates for this
fault.
10.4 Phase distance applied to power transformers
10.4.1 Description
As a Wye-Delta transformer introduces discontinuity for the zero-sequence circuit, the ground distance protection cannot
be applied, except special circumstances, to respond to faults behind the transformer.
The phase distance elements, however, can be developed so that both accurate reach and correct fault phase
identification is retained for faults behind the power transformer as seen from the relaying point. Without appropriate
compensation, the reach of the relay depends on a type of fault, creating considerable difficulties in applying the relay.
The L90 provides for any location of the VTs and CTs with respect to the involved power transformer and the direction of
any given zone.
In the following equations, the VT and CT locations are referenced as “None” if the transformer is not present between the
CT/VT and the intended reach point. Otherwise, the location is to be selected as a type of a transformer as seen from the
VT/CT position towards the intended reach point. The following figure explains the adopted rules.
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