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Functions
2.14 Differential Protection and Its Protected Objects
2.14.3
Protected Object Transformer
Transformers are subject to a number of influences that induce differential currents even during normal opera-
tion:
2.14.3.1 Functional Description
Mismatching of CTs
Differences in the matching of CTs to the transformer rated current are not uncommon. These differences result
in an error that leads to a differential current.
Voltage Control by Tap Changers
Voltage control tap changers (usually in-phase regulators) change the transformation ratio and the rated current
of the transformer. They cause mismatching of the CTs and thus a differential current.
Inrush Current
Transformers may absorb on power-up considerable magnetising currents (inrush currents) that enter the pro-
tected zone but do not exit it. They act therefore like fault currents entering on one side.
The inrush current can amount to a multiple of the rated current and is characterized by a considerable 2nd
harmonic content (double rated frequency) which is practically absent during a short-circuit.
Overexcitation
Where a transformer is operated with an excessive voltage, the non-linear magnetising curve leads to in-
creased magnetising currents, which cause in turn an additional differential current.
Vector Group
Depending on their application, transformers have different vector groups, which cause a shift of the phase
angles between the primary and the secondary side. Without adequate correction, this phase shift would cause
a differential current.
The following paragraphs describe the main functional blocks of the differential protection for managing these
influences.
118
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
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