Current Transformer Supervision - GE MiCOM P40 Agile Technical Manual

P543i/P545i
4

CURRENT TRANSFORMER SUPERVISION

The Current Transformer Supervision function (CTS) is used to detect failure of the AC current inputs to the
protection. This may be caused by internal current transformer faults, overloading, or faults on the wiring. If there
is a failure of the AC current input, the protection could misinterpret this as a failure of the actual phase currents
on the power system, which could result in maloperation. Also, interruption in the AC current circuits can cause
dangerous CT secondary voltages to be generated.
The IED has two methods of providing Current Transformer Supervision (CTS); differential and standard. The
differential method uses the ratio between positive and negative sequence currents to determine CT failure. This
method is not voltage dependant and relies on channel communications to declare a CTS condition.
The standard method relies on local measurements of zero sequence currents and voltages. You select the
method according to the application. Both methods can be applied individually or in parallel.
The CTS Mode setting in the SUPERVISION column is used to set the method. The options are:
Disabled
●
Standard
●
Idiff
●
Idiff + Std
●
The CTS Reset Mode setting determines whether the CTS will reset automatically or will need manual intervention.
4.1 DIFFERENTIAL CTS
Differential CTS does not need any local voltage measurements to determine a CTS condition. It is based on
measurement of the ratio of negative sequence current to positive sequence current (I2/I1) at all line ends. When
this ratio is small (theoretically zero), one of four possible conditions apply:
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, but if this is the case it 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 unbalanced fault (both I2 and I1 are non-zero)
There is a 1 or 2 phase CT problem (both I2 and I1 are non-zero)
●
Measurement at a single end cannot provide any more information than this. However, if the ratio is calculated at
all ends and compared, the device can make a decision based on the following criteria:
● 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 criterion looks to see whether the differential system is loaded or not. For this purpose the device looks
at the positive sequence current I1. If load current is detected at one-end only, the device assumes that this is an
internal fault condition and prevents CTS operation. However, if load current is detected at two or more ends, this
indicates CT failure, so CTS operation is allowed.
There are two modes of operation, Indication and Restrain. You determine the mode of operation with the
CTS Status setting. In Indication mode, a CTS alarm is raised but there is no effect on tripping. In Restrain
mode, the differential protection is blocked for 20 ms after CT failure detection, then the Current Differential
threshold setting is raised above the load current.
For correct operation of the scheme, Differential CTS must be enabled at each end of the protected zone.
P54x1i-TM-EN-1
Chapter 18 - Supervision
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