Phase Distance Applied To Power Transformers - GE D90 Plus Instruction Manual

CHAPTER 15: THEORY OF OPERATION
Other phase and ground distance elements

Phase distance applied to power transformers

PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
The results are shown in the following table.
Table 23: Quadrilateral phase A to ground element analysis
Parameter
Overcurrent supervision
Reactance difference angle
Zero-sequence directional
difference angle
Negative-sequence directional
difference angle
Right blinder difference angle
Left blinder difference angle
Fault type comparator
difference angle
All the comparators and the overcurrent supervision are satisfied. The quadrilateral phase
A ground element will operate for this fault.
Repeating the analysis in the preceding sections for each phase and ground distance
element, it follows that only the ground element in phase A will operate for this fault.
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, could 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 D90
would depend on a type of fault, creating considerable difficulties in applying protection.
Plus
The D90 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.
PHASE DISTANCE APPLIED TO POWER TRANSFORMERS
Calculation Requirement
3 × 1.37 A = 4.09 A > 3 A
| –3.9° – 24.8° | = 28.7° < 75°
| 19.8° – 0.0° | = 19.8° < 75°
| 19.8° – 0.0° | = 19.8° < 75°
| –93.0° – (–109.2°) | = 16.2° < 90°
| 82.9° – 108.7° | = 25.8° < 90°
| 19.8° – 19.8° | = 0.0° < 50°
Condition met
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Plus
reach
553