GE Multilin DTP-B, Instruction Manual

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2. APPLICATION
2-2 DTP-B Digital Transformer Protection GEK-106243E
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In some applications, one or more of the power transformer’s windings may be connected to the power
system via two breakers. An example is a ring bus. In this case, the CT ratios must be selected so that the
secondary windings will not be thermally overload on load current flowing around the ring bus in addition to
the transformer load current. In such cases, it is recommended that each of the two CTs be connected to
separate relay restraint windings to assure adequate restraint for heavy through-fault current flowing around
the ring bus.
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Protecting two parallel transformer banks with one DTP relay is not recommended since the sensitivity will be
reduced. Additionally, if the two banks can be switched independently, there is a possibility of false operation
when the inrush current of one transformer bank provokes a “sympathetic inrush current” into the bank
already energized. In this case, the harmonics tend to flow between the two banks with the possibility that
there will be insufficient harmonics in the relay current to restrain the relay. Should one DTP relay be used to
protect two independently switched parallel transformers, the DTP relay’s harmonic restraint (adjustable from
12% to 50%) may be increased to preclude a misoperation.