CHAPTER 15: THEORY OF OPERATION PHASE DISTANCE APPLIED TO POWER TRANSFORMERS
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL 645
Equations from the current transformation and voltage transformation columns are used to
derive inputs to the three (AB, BC, and CA) phase distance elements. For example, if the CTs
are located at the delta side of the delta-wye 11 (Dy11) transformer, and a given zone is set
to look through the transformer into the system connected to the wye winding, the CT
location setting for that zone is to be set to “Dy11” and the relay uses –√3I
B
instead of a
traditional I
A
– I
B
for the AB phase distance element.
The current supervision pickup setting applies to the currents specified in the current
transformation columns.
A distance zone originates at the location of the VTs (regardless of the location of the CTs).
Example of system with power transformers
Consider the system shown. To respond to the indicated fault, a distance relay is applied at
the relay point X.
Yd5 AB
BC
CA
Yd7 AB
BC
CA
Yd9 AB
BC
CA
Yd11 AB
BC
CA
Transformer
connection
Loop Current transformation Voltage transformation
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