x/EN AP/Na7
-20 MiCOM P74
3. CURRENT TRANSFORMERS
It is important that the CT settings are entered in full as these are required to calculate
additional parameters for use in the saturation detection algorithms that run within the
peripheral units.
A P74x scheme can accommodate different CT ratios throughout the protected zone, the
maximum difference being 40. In other words, the maximum ratio between the smallest
primary CT winding and the biggest primary CT winding is 40. This mix must, therefore, be
accounted for by the scheme and this is achieved by using the primary currents sent by the
Peripheral Units to the central unit that undertakes scheme calculations.
In the P741, a common virtual current transformer of 1000/1 A is used to convert to
secondary values.
3.1 Current Transformers Supervision
3.1.1 “Low voltage” Current Transformers Supervision
The “Low voltage” CTs in the PU are supervised by the 3I0 supervision algorithm.
If the difference between 3I0 and the measured In is higher than set, after a set timer, the
“PU CT Fail IN” alarm appears in the PU.
This supervision does impact the 87BB protection.
3.1.2 ““High Voltage” Current Transformers Supervision
The “High voltage” CTs are supervised by the CT supervision algorithm:
If there is at least 10% of In current in the CT, the difference between the magnitude of the
current measured by 2 phases should not exceed 50%.
If that happens, after a set timer, the “CT Fail” alarm appears in the PU and in the dynamic
synoptic (if connected).
This supervision does not impact directly the 87BB protection, there is no blocking request
sent to the CU.
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