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# Protection Riddle No.57 – Protection of two power transformer with one relay

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• #403
Hamid

A protection design group which their company had fronted to budget problems, for pecuniary saving, decided to use of one set differential relay with three input current instead of two differential relay for protection of two same power transformers as indicated in below figure.
A protection design group which their company had fronted to budget problems, for pecuniary saving, decided to use of one set differential relay with three input current instead of two differential relay for protection of two same power transformers as indicated in below figure.

All differential protection requirements according to relay manufacturer were considered by protection system designers. But in practice they face to one important problem in some case. Sometimes they fronted to maloperation of protection system when one of two power transformer was energized while another transformer was loaded past. They checked all protection function of relay such as its second harmonic restrain function. All things were very good and they could not analyze their problem.

How can you help them?

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• #1756
AMIR KHASHAYAR

During the test , the relay and restraint function have operated properly . Because the ratio of second harmonics to fundamental component has been measured correctly , and reley has distiguished the inrush current nature . But , during real case , it’s because the other relay has been under load and steady state current , the fundamental component is large and the ratio of second harmonics to fundamental component is low or lower than setting . Thus , the restraint function is not active

#1799
Hamid

Since inrush current appears as operating current to a differential relay, the relay must have sufficient delay and insensitivity to the distorted wave or take advantage of the inrush

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