Ct Requirements; Phase Overcurrent Protection; Directional Elements - GE P24DM Technical Manual

Chapter 9 - Current Transformer Requirements
2

CT REQUIREMENTS

The current transformer requirements are based on a maximum fault current of 50 times the rated current (In) with
the device having an instantaneous overcurrent setting of 25 times the rated current. The current transformer
requirements are designed to provide operation of all protection elements.
Where the criteria for a specific application are in excess of this, or the lead resistance exceeds the limiting lead
resistance shown in the following table, the CT requirements may need to be modified according to the formulae in
the subsequent sections:
Nominal Rating Nominal Output
1A 2.5 VA
5A 7.5 VA
The formula subscripts used in the subsequent sections are as follows:
V = Required CT knee-point voltage (volts)
K

= Maximum through-fault current level (amps) / motor starting current
f

= Rated secondary current (amps)
n
 = Maximum prospective secondary earth fault current or 31 times > setting (whichever is lower) (amps)
cn
 = Maximum prospective secondary phase fault current or 31 times > setting (whichever is lower) (amps)
cp
R = Resistance of current transformer secondary winding (ohms)
CT
R = Resistance of a single lead from relay to current transformer (ohms)
L
R = Value of stabilising resistor for REF applications (ohms)
st

= Current setting of REF elements (amps)
s
V = Required stability voltage
S
2.1

PHASE OVERCURRENT PROTECTION

2.1.1

DIRECTIONAL ELEMENTS

Time-delayed phase overcurrent elements
I
cp
V = ( R +
K CT
2
Instantaneous phase overcurrent elements
I
fp
= ( +
V R
K CT
2
194
Accuracy Class
10P
10P
R + R )
L p
+ )
R R
L p
Accuracy Limited Factor
20
20
P24xM
Limiting Lead
Resistance
1.3 ohms
0.11 ohms
P24xM-TM-EN-2.1