ABB RELION REX640 Technical Manual page 732

Protection functions
the transformation ratios.
correct the ratios on winding 1 and 2 while the use of
setting
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
to the ratios of the second restraint on winding 1.
4.3.3.7 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.
• Mismatch due to varying tap changer positions.
• Different characteristics, loads and operating conditions of the CTs.
• Zero-sequence currents that flow only on one side of the power transformer.
• Normal magnetizing currents.
• Magnetizing inrush currents.
• Overexcitation of magnetizing currents.
732
CT ratio Cor Wnd 1 and CT ratio Cor Wnd 2 are used to
Current group 3 type . If Current group 3 type is "Winding 3", the CT ratio Cor
Current group 3 type is "Wnd 1 restraint", the ratio correction is made
1MRS759142 F
CT ratio Cor 3 depends on the
Technical Manual
REX640