ABB Relion REG670, Техническое руководство

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waveblock criterion will temporarily disable the second harmonic blocking feature of
the differential protection function. This consequently ensures fast operation of the
transformer differential function for a switch onto a fault condition. It shall be noted
that this feature is only active during initial power transformer energizing, under the
first 50 ms. When the switch onto fault feature is disabled by the setting parameter
SOTFMode , the waveblock and second harmonic blocking features work in parallel
and are completely independent from each other.
Open CT detection feature
differential protection has a built-in, advanced open CT detection feature.
A sudden inadvertently opened CT circuit may cause an unexpected and unwanted
operation of the differential protection under normal load conditions. Damage of
secondary equipment may occur due to high voltage from open CT circuit outputs. It
is always an advantage, from the point of view of security and reliability, to have the
open CT detection function to block the differential protection function in case of an
open CT condition, and produce an alarm signal to the operational personnel to
quickly correct the open CT condition.
The built-in open CT feature can be enabled or disabled by the setting parameter
OpenCTEnable ( Off / On ). When enabled, this feature tries to prevent mal-operation
when a loaded main CT connected to differential protection is by mistake open
circuited on the secondary side. Note that this feature can only detect interruption of
one CT phase current at a time. If two or even all three-phase currents of one set of CTs
are accidentally interrupted at precisely the same time, this feature cannot operate.
differential protection generates a trip signal if the false differential current is
sufficiently high. An open CT circuit is typically detected in 12–14 ms, and if the load
in the protected circuit is relatively high, about the nominal load, the unwanted trip
cannot always be prevented. Still, the information about what was the cause of the
open CT secondary circuit, is vital.
The principle applied to detect an open CT is a simple pattern recognition method,
similar to the waveform check used by the Power Transformer Differential Protection
in order to detect the magnetizing inrush condition. The open CT detection principle
is based on the fact that for an open CT, the current in the phase with the open CT
suddenly drops to zero (that is, as seen by the protection), while the currents of the
other two phases continue as before.
The open CT function is supposed to detect an open CT under normal conditions, that
is, with the protected multi-terminal circuit under normal load (10...120% of the rated
load). If the load currents are very low or zero, the open CT condition cannot be
detected. In addition to load condition requirement, Open CT function also checks the
differential current on faulty phase. If the differential current is lower than 10% of
IBase, the open CT condition cannot be detected. Therefore, the Open CT algorithm
only detects an open CT if the load on the power transformer protected object is
10...120% of rated load and the differential current is higher than 10% of IBase on that
phase. The search for an open CT starts 60 seconds (50 seconds in 60 Hz systems) after
the bias current has entered the 10...120% range. The Open CT detection feature can
also be explicitly deactivated by setting:
OpenCTEnable = 0 ( Off ).
Section 6
1MRK502052-UEN B
Differential protection
134
Technical manual