EMR-5000
IM02602012E
CTS -
Current Transformer Supervision
CTS
Most functions of metering, protection, and control in the relay rely on correct current measurements. It is important to
make sure the CT connections and their operations are correct. The failures (including CT secondary wire broken,
insulation broken down, broken wiring between CT and relay, and mismatched polarities) will cause the incorrect
current measurements. The other CT errors (due to the magnetizing current that is proportional to the primary current,
CT saturation, and measuring circuit and quantization error) can also cause inaccurate current measurements.
The CTS utilizes the Kirchhoff
’
s current law to detect a CT failure and can differentiate the wiring errors from the
measurement errors by adding biases to offset the measurement related errors. The biases include two terms, one of
which is related to the static error that accounts for CT magnetizing characteristic differences and current measurement
circuit calibration errors and other is the dynamic error that is proportional to the measured maximum current due to CT
transformation characteristics. The CTs are assumed to be used in the wye-grounded winding sides. Under normal
conditions, the mismatch between the calculated and the measured zero sequence current should be less than the bias
value. However, if there is a CT wiring error, such relationship will not hold true. If the mismatch exceeds the bias for a
specified time, an alarm will be generated.
The operating principle can be expressed in terms of CT secondary currents as follow:
IL
1
IL
2
IL
3
KI
∗
I
G
=
3
∗
I
0
KI
∗
IG
∆
I
Kd
∗
Imax
KI is the ratio of the ground CT ratio over the phase CT ratio, and it is automatically calculated from the rated system
parameters.
∆
I
=
The static error, a minimum mismatch allowed between the calculated and measured zero
sequence current.
Kd
=
The dynamic error factor, a restrain slope that defines a percentage error generated by a high
current.
Imax
=
maximum phase current.
Total bias value =
∆
I + Kd x Imax.
The current transformer supervision operation can be graphically represented as follows.
www.eaton.com
825
Limit Value
Imax
I
Kd*Imax
