Current Transformers Selection Guide

Electrical characteristics – primary circuits

The primary circuits of the current transformer must withstand the constraints related to the medium voltage network to which it is connected.

Rated frequency

This is the frequency of the installation. A CT defined for 50 Hz can be installed on a 60 Hz network with the same level of accuracy. However, the opposite is not true.

For a non-referenced unit, it is vital to indicate the rated frequency on the order from.

Rated voltage of the primary circuit (Upn)

General case:

Insulation level continuity for the whole installation will be ensured if the rated voltage of the CT used is the rated voltage of the installation. The rated voltage determines the insulation level of the equipment. Generally we choose the rated voltage based on the duty voltage, Us, according to the following table:

Specific case:
If the CT is installed on a bushing or a cable providing insulation, the CT can be LV ring type.

Primary service current (Ips)

Knowledge about the primary service current will enable us to determine the rated primary current for the CT taking into account any eventual derating.

If:
S = apparent power in VA
Ups = primary service voltage in V
P = active power of the motor in W
Q = reactive power of the capacitors in VAR
Ips = primary service current in Amp

We will have :

Incoming cubicle:

Generator incomer:

Transformer feeder:

Motor feeder:

η = efficiency of the motor

If you do not know exact values for j and h as a first approximation, you can assume that: cos j = 0.8 ; h = 0.8

Capacitor feeder:

1.3 is a de-rating factor of 30% which compensates for heat-up due to harmonics in the capacitors.

Bus tie:

The Ips current in the CT is the highest permanent current that can circulate in the connection.