Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-127
OP
This scheme is also able to correctly operate under very low load or even no load conditions,
by the combination of time delayed signals derived from the DDB signals VTS Fast block
and all Poles Dead, to generate the Block Distance DDB.
Note: All non-distance elements are blocked by the “VTS Fast Block” DDB.
Where a miniature circuit breaker (MCB) is used to protect the voltage transformer ac output
circuits, it is common to use MCB auxiliary contacts to indicate a three phase output
disconnection. As previously described, it is possible for the VTS logic to operate correctly
without this input. However, this facility has been provided for compatibility with various
utilities current practices. Energizing an opto-isolated input assigned to DDB: MCB/VTS on
the relay will therefore provide the necessary block.
3.2 Current transformer supervision
The
current transformer supervision feature is used to detect failure of one or more of the ac
phase current inputs to the relay. Failure of a phase CT or an open circuit of the
interconnecting wiring can result in incorrect operation of any current operated element.
Additionally, interruption in the ac current circuits risks dangerous CT secondary voltages
being generated.
MiCOM P54x has two methods to achieve CT supervision feature (CTS). The first method
called differential (I diff) method uses the ratio between positive and negative sequence
currents to determine CT failure. Is non voltage dependant and relies on channel
communications to declare a CTS condition. The second method called standard method
relies on local measurements of zero sequence currents and voltages to declare CTS. The
user should select what method to use according to the application.
Both methods could be applied in parallel.
The setting options per each setting group are:
CTS Disabled/ Idiff CTS/ Standard CTS/ (Idiff+Standard). It should be noted that the ‘CTS
Status’ (Restrain/Indication), ‘CTS Reset Mode’ (Manual/Auto) and ‘CTS Time Delay’ are
common for both algorithms.
3.2.1 Differential CTS (no need of local voltage measurements to declare CTS)
Differential
CT supervision scheme is based upon measurement of the ratio of I2 to I1 at all
line ends. When this ratio is small (theoretically zero), one of four conditions is present:
The system is unloaded - both I2 and I1 are zero
The system is loaded but balanced - I2 is zero
The system has a three phase fault present - I2 is zero
There is a genuine 3 phase CT problem - unlikely, would probably develop from a single
or two phase condition
If the ratio is non-zero, we can assume one of two conditions are present:
The system has an asymmetric fault - both I2 and I1 are non-zero
There is a 1 or 2 phase CT problem - both I2 and I1 are non-zero
Any measurement at a single end doesn’t provide any more information than this, but if the
ratio is calculated at all ends and compared, the MiCOM P54x assumes:
If the ratio is non-zero at more than two ends, it is almost certainly a genuine fault
condition and so the CT supervision is prevented from operating.
If the ratio is non-zero at one end, there is a chance of either a CT problem or a single-
end fed fault condition.
A second criteria looks to see whether the differential system is loaded or not. For this
purpose MiCOM P54x looks at the positive sequence current I1. If load current is detected
at one-end only, MiCOM P54x assumes that this is an internal fault condition and prevents
CTS operation, but if load current is detected at two or more ends, CTS operation is
permitted.
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