ABB 615 Series ANSI, Техническое руководство

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Section 4
1MAC050144-MB C
Protection functions
352 615 series ANSI
Technical Manual
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 ground 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 inter-turn 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 a ground 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 87T is a
unit protection function, it can be designed for fast tripping, thus providing a selective
disconnection of the faulty transformer. 87T should never trip to faults outside the
protective zone.
87T compares the current flowing into the transformer to the current leaving the
transformer. A correct analysis of fault conditions by 87T 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 87T
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.
87T is designed mainly for the protection of two-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 can be required to reduce the burden of the CTs.