7.1.3.9 Open CT detection
The Generator differential function has a built-in, advanced open CT detection
feature. This feature can block the unexpected operation created by the Generator
differential function in case of open CT secondary circuit under normal load
condition. An alarm signal can also be issued to station operational personnel to make
remedy action once the open CT condition is detected.
The following setting parameters are related to this feature:
• Setting parameter OpenCTEnable enables/disables this feature
• Setting parameter tOCTAlarmDelay defines the time delay after which the alarm
signal will be given
• Setting parameter tOCTReset defines the time delay after which the open CT
condition will reset once the defective CT circuits have been rectified
• Once the open CT condition has been detected, then all the differential protection
functions are blocked except the unrestraint (instantaneous) differential
protection
The outputs of open CT condition related parameters are listed below:
• OpenCT: Open CT detected
• OpenCTAlarm: Alarm issued after the setting delay
• OpenCTIN: Open CT in CT group inputs (1 for input 1 and 2 for input 2)
• OpenCTPH: Open CT with phase information (1 for phase L1, 2 for phase L2, 3
for phase L3)
7.1.3.10 Switch onto fault feature
The Transformer differential function in the IED has a built-in, advanced switch onto
fault feature. This feature can be enabled or disabled by the setting parameter
SOTFMode. When SOTFMode = On this feature is enabled. It shall be noted that when
this feature is enabled it is not possible to test the 2
nd
harmonic blocking feature by
simply injecting one current with superimposed second harmonic. In that case the
switch on to fault feature will operate and the differential protection will trip. However
for a real inrush case the differential protection function will properly restrain from
operation.
For more information about the operating principles of the switch onto fault feature
please read the Technical Manual.
7.1.4 Setting example
7.1.4.1 Introduction
Differential protection for power transformers has been used for decades. In order to
correctly apply transformer differential protection proper compensation is needed for:
Section 7 1MRK 502 071-UEN -
Differential protection
136 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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