ABB RELION 620 Series Technical Manual page 445

1MAC504801-IB E
620 series ANSI
Technical Manual
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 ground faults in the windings and terminals are normally detected by the
differential protection. Interturn faults, which are flashovers between conductors within
the same physical winding, are also possible to detect if a large enough number of turns are
short-circuited. Interturn faults are the most difficult transformer winding faults to detect
with electrical protections. A small interturn fault including just a few turns results in an
undetectable amount of current until it develops into a ground 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 for external faults.
It is important that the faulty transformer is 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 to 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 take into consideration changes to voltages, currents and
phase angles. The traditional 87T required auxiliary transformers for the correction of the
phase shift and ratio. The numerical microprocessor-based differential algorithm as
implemented in 87T 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 current transformers
• Zero-sequence currents that only flow on one side of the power transformer
• Normal magnetizing currents
• Magnetizing inrush currents
• Overexcitation of magnetizing currents
87T is designed mainly for protection of two-winding or three-winding transformers. 87T
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 points is relatively long
in line protection, interposing CTs might be needed to reduce the burden of the CTs. For
Section 4
Protection functions
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