SELECTING CURRENT TRANSFORMER

IEC Guidelines IEC 44-6 presents the requirements for CT transient

performance in the form of a minimum saturation voltage as

the following:

Eal KsscKtd Rct Rb Isn where:

Eal is the CT secondary excitation voltage, which

corresponds to a magnetizing current that produces the

maximum permissible error.

Kssc is the ratio of fault current to nominal current.

Ktd is the transient dimensioning factor.

Rct is the CT secondary resistance.

Rb is the CT burden.

Isn is the nominal secondary current

To avoid saturation, the CT shall develop adequate voltage such that

VX

> If (R

CT+R

L+R

B)- - - - - - - (1)

where,

If = Fault current on CT secondary (Amps)

RCT

= CT Secondary resistance (Ohms)

RL

= CT Secondary total lead resistance (Ohms)

and RB

=CT secondary connected burden (Ohms

Example:

What can be the maximum lead resistance for the application of a distance relay with the

following data?

Primary fault current =10kA (from system study)

System X/R =15 (from system study)

CT ratio = 1000/5A

CT rating =C400 (CT existing)

Design burden = 4 Ohms (as per standards for C400 CT)

CT resistance = 0.25 Ohms (Data from manufacturer)

Relay burden = Negligible (numerical relay)

Calculations:

Secondary fault current

= 10000/ (1000/5)

=50A

Ni = 100/50 =2

Nr = (RCT

+4) / (RCT

+RL+R

B)

= (0.25+4) / (0.25+ RL)

= 4.25/ (0.25+ RL)