GE P642 User Manual

3.4.1 HIGH IMPEDANCE REF CALCULATION PRINCIPLES
The primary operating current (Iop) is a function of the current transformer ratio, the device operate current
(IREF>Is), the number of curr
ent transformers in parallel with a REF element (n) and the magnetizing current of
each current transformer (Ie) at the stability voltage (Vs). This relationship can be expressed in three ways:
1. The maximum current transformer magnetizing current to achieve a specific primary operating current with
a particular operating current:
I
n
I
CT ratio
IREF Is
e
op
< − >
[ ]






1
2. The maximum current setting to achieve a specific primary operating current with a given current
transformer magnetizing curr
ent:
IREF Is
I
CT ratio
nI
op
e
>
[ ]
< −






3. The protection primary operating current for a particular operating current with a particular level of
magnetizing curr
ent:
I CT ratio IREF Is nI
op e
=
( )
>
[ ]
+
( )
To achieve the required primary operating current with the current transformers that are used, you must select a
curr
ent setting for the high impedance element, as shown in item 2 above. You can calculate the value of the
stabilising resistor (R
ST
) in the following manner.
R
Vs
IREF Is
I R R
IREF Is
st
F CT L
=
>
[ ]
=
+
( )
>
[ ]
2
where:
● R
CT
= the r
esistance of the CT winding
● R
L
= the resistance of the lead from the CT to the IED.
Note:
The above formula assumes negligible relay burden.
We recommend a stabilizing resistor, which is continuously adjustable up to its maximum declared resistance.
Chapter 8 - Restricted Earth Fault Protection P64x
176 P64x-TM-EN-1.3

GE P642 Specifications

General

Brand	GE
Model	P642
Category	Relays
Language	English