Application - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.3.3.7
644
Wnd 1, CT ratio Cor Wnd 2 and CT ratio Cor 3 must be used to correct the
transformation ratios. CT ratio Cor Wnd 1 and CT ratio Cor Wnd 2 are used to correct
the ratios on winding 1 and 2 while the use of CT ratio Cor 3 depends on the setting
Current group 3 type. If Current group 3 type is "Winding 3", the CT ratio Cor 3
setting is used to correct the ratios on winding 3. If Current group 3 type is "Wnd 2
restraint", the ratio correction is made to the ratios of the second restraint on winding
2. If Current group 3 type is "Wnd 1 restraint", the ratio correction is made to the ratios
of the second restraint on winding 1.

Application

TR3PTDF is a unit protection. It acts as the main protection of transformers in case of
winding failure. The protective zone of a differential protection includes the
transformer itself and the bus work or cables between the current transformer and the
power transformer. When bushing current transformers are used for the differential
protection relay, 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 for the busbar. Such a
busbar protection includes the bus work or cables between the circuit breaker and the
power transformer. The internal electrical faults are serious and cause immediate
damage. The short circuits and earth faults in the windings and terminals are normally
detected by the differential protection. Inter-turn faults, which are flashovers between
conductors within the same physical winding, can also be detected if a large enough
number of turns are short-circuited. Inter-turn faults are the most difficult transformer
winding faults to detect with electrical protections. A small inter-turn fault including
just a few turns results in an undetectable amount of current until it develops into an
earth fault. Therefore the differential protection must have a high level of sensitivity
and it must be possible to use a sensitive setting without causing unwanted operations
for external faults.
The faulty transformer must be disconnected as fast as possible. As the differential
protection is a unit protection, it can be designed for fast tripping, thus providing a
selective disconnection of the faulty transformer. The differential protection should
not operate because of faults outside the protective zone.
A transformer differential protection compares the current flowing into the
transformer to the current leaving the transformer. A correct analysis of fault
conditions by the differential protection must consider changes to voltages, currents
and phase angles. The traditional transformer differential protection functions
required auxiliary transformers for the correction of the phase shift and ratio. The
numerical microprocessor-based differential algorithm implemented in TR3PTDF
compensates for both the turns ratio and the phase shift internally in the software.
The differential current should theoretically be zero during a normal load or external
faults if the turns ratio and phase shift are correctly compensated. However, there are
several different phenomena other than internal faults that cause unwanted and false
differential currents. There can be several main reasons for unwanted differential
currents.
1MRS759142 C
REX640
Technical Manual