Application - ABB RET670 Applications Manual

Section 3
IED application
3.5.1.1
98
Function description
Transformer differential protection, two-
winding
Transformer differential protection,
three-winding

Application

The transformer differential protection is a unit protection. It serves as the main
protection of transformers in case of winding failure. The protective zone of a
differential protection includes the transformer itself, the bus-work or cables between
the current transformers and the power transformer. When bushing current
transformers are used for the differential IED, the protective zone does not include the
bus-work or cables between the circuit breaker and the power transformer.
In some substations there is a current differential protection relay for the busbar. Such
a busbar protection will include the bus-work or cables between the circuit breaker and
the power transformer. Internal electrical faults are very serious and will cause
immediate damage. Short circuits and ground faults in windings and terminals will
normally be detected by the differential protection. Interturn faults, which are
flashovers between conductors within the same physical winding, is also possible to
detect if a large enough number of turns are short-circuited. Interturn faults are the
most difficult transformer winding fault to detect with electrical protections. A small
interturn fault including just a few turns will result in an undetectable amount of
current until it develops into an ground or phase fault. For this reason it is important
that the differential protection has a high level of sensitivity and that it is possible to
use a sensitive setting without causing unwanted operations during external faults.
It is important that the faulty transformer be disconnected as fast as possible. As the
differential protection is a unit protection it can be designed for fast tripping, thus
providing selective disconnection of the faulty transformer. The differential protection
should never operate on faults outside the protective zone.
A transformer differential protection compares the current flowing into the transformer
with the current leaving the transformer. A correct analysis of fault conditions by the
differential protection must take into consideration changes due to the voltage, current
and phase angle changes caused by the protected transformer. Traditional transformer
differential protection functions required auxiliary transformers for correction of the
phase shift and ratio. The numerical microprocessor based differential algorithm as
IEC 61850 IEC 60617
identification identification
T2WPDIF
3Id/I
SYMBOL-BB V1 EN
T3WPDIF
3Id/I
SYMBOL-BB V1 EN
1MRK504116-UUS C
ANSI/IEEE C37.2
device number
87T
87T
Application manual