ensures quick differential protection tripping in cases where a transformer is energized with an
internal fault (for example, forgotten grounding on transformer LV side). Operation of this feature
is based on the fact that a current gap (term current gap is explained under waveblock feature
above) will exist within the first power system cycle when healthy power transformer is energized.
If this is not the case the waveblock criterion will reset quickly. This quick reset of the 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
M13039-440 v8
Transformer 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 Transformer 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 transformer 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
(
Disabled
/
Enabled
). When enabled, this feature tries to prevent mal-operation when a loaded main
CT connected to Transformer 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. Transformer 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...110% 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 50% of IdMin, the open CT condition cannot be detected.
Therefore, the Open CT algorithm only detects an open CT if the load on the power transformer is
10...110% of rated load and the differential current is higher than 50% of IdMin on that phase. The
search for an open CT starts 60 seconds (50 seconds in 60 Hz systems) after the transformer is
energized. The Open CT detection feature can also be explicitly deactivated by setting:
OpenCTEnable
= 0 (
Disabled
).
If an open CT is detected and the output OPENCT set to 1, then all the differential functions are
blocked, except the unrestrained (instantaneous) differential. An alarm signal is also produced
after a settable delay (
tOCTAlarmDelay
) to report to operational personnel for quick remedy
actions once the open CT is detected. When the open CT condition is removed (that is, the
previously open CT is reconnected), the functions remain blocked for a specified interval of time,
1MRK 502 066-UUS B Section 6
Differential protection
137
Technical manual
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