ABB RED615 Technical Manual page 404

Section 4
Protection functions
398
protection includes the transformer, the bus-work or the cables between the current
transformer 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 for the busbar. The
busbar protection includes bus work or cables between the circuit breaker and the
power transformer. Internal electrical faults are very serious and cause immediate
damage. Short circuits and earth faults in windings and terminals are normally
detected by the differential protection. If enough turns are short-circuited, the
interturn faults, which are flashovers between the conductors within the same
physical winding, are also detected. The interturn faults are the most difficult
transformer-winding faults to detect with electrical protections. A small interturn
fault including a few turns results in an undetectable amount of current until the
fault develops into an earth fault. Therefore, 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 for external faults.
It is important that the faulty transformer is disconnected as fast as possible. As
TR2PTDF is a unit protection function, it can be designed for fast tripping, thus
providing a selective disconnection of the faulty transformer. TR2PTDF should
never operate to faults outside the protective zone.
TR2PTDF compares the current flowing into the transformer to the current leaving
the transformer. A correct analysis of fault conditions by TR2PTDF must consider
the changes to voltages, currents and phase angles. The traditional transformer
differential protection functions required auxiliary transformers for the correction
of the phase shift and turns ratio. The numerical microprocessor based differential
algorithm implemented in TR2PTDF compensates for both the turns ratio and the
phase shift internally in the software.
The differential current should theoretically be zero during normal load or external
faults if the turns ratio and the phase shift are correctly compensated. However,
there are several different phenomena other than internal faults that cause
unwanted and false differential currents. The main reasons for unwanted
differential currents are:
• Mismatch due to varying tap changer positions
• Different characteristics, loads and operating conditions of the current
transformers
• Zero sequence currents that only flow on one side of the power transformer
• Normal magnetizing currents
• Magnetizing inrush currents
• Overexcitation magnetizing currents.
TR2PTDF is designed mainly for the protection of two-winding transformers.
TR2PTDF can also be utilized for the protection of generator-transformer blocks as
well as short cables and overhead lines. If the distance between the measuring
1YHT530004D05 D
615 series
Technical Manual