Plant Design: Motor Protection Parameters

The table below is an example of a working protection parameters of an electrical system. A parameter sheet such as this will enable the designer to tabulate necessary initial values and  calculations.

NO.	Equipment name	Nominal power (kW)	Circuit calculation current(A)	CT trans. ratio	Zero sequence CT trans. ratio	Circuit type
1	Motor Pump No.1	3400	355.6	1000/1 2000/1(Differential)	100/1	Vacuum breaker
2	Motor Pump No.3	1400	146.4	500/1	100/1	Vacuum breaker
3	Motor Pump No.3	1000	104.6	400/1	100/1	Vacuum breaker
4	Motor Pump No.4	400	41.8	200/1	100/1	F-C circuit
5	Motor Pump No.5	900	94.1	300/1	100/1	F-C circuit
6	Motor Pump No.6	710	74.3	200/1	100/1	F-C circuit

First is to work out in calculating the circuit current of a three phase motor circuit:

Circuit Current=kW/(√3∗kV∗pf )

Instrument and protection CTs are governed by standard IEC 60044-1 . The matching of CTs with protection relays calls for a thorough knowledge of CTs. The following section gives a few reminders in determining   the CT ratio, the table below will give the recommended primary CT ratio for various circuit current in the  motor protection  section. See (link) for complete list of CT ratio per application.

CT Primary=kW/(√3∗kV∗pf *η )


If you do not know exact values for ϕ and η as a first approximation, you can assume that: cos ϕ = 0.8 ; η = 0.8. T. The secondary circuits of a CT must be suitable for the constraints related to its application for  or protection purposes.

CT Secondary  
For use in a local situation Isn = 5 A
For use in a remote situation Isn = 1 A

The use of 5 A in a remote situation increase the cross section of the line or the sizes of the transformer (lost in line). IEEE C57.13 Table 8 - Standard multi-ratio current transformer taps gives the standard ratings for instrument transformers. Sequence CT Ratio are constant 100/1 in 4.160 and 6.9kV Circuit.

For the circuit Type in 6.9kV , there are two recommended circuit types. Use  Vacuum Circuit when the Current reached above 100 Amps or Fused Contractor when dealing with currents below 100 Amps.