High Impedance Ref Calculation Principles - GE MiCOM P40 Agile Technical Manual

P14x
not block the high impedance REF protection. The appropriate logic must be configured in PSL to block the high
impedance REF when any of the above signals is asserted.
3.3.1

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 current 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
1
<
op

I
e

n CT ratio
2. The maximum current setting to achieve a specific primary operating current with a given current
transformer magnetizing current:

[ ]
> <

IREF Is

3. The protection primary operating current for a particular operating current with a particular level of
magnetizing current:
I CT ratio
(
=
op
To achieve the required primary operating current with the current transformers that are used, you must select a
current setting for the high impedance element, as shown in item 2 above. You can calculate the value of the
stabilising resistor (R ) in the following manner.
ST
Vs
R
=
st
IREF Is
[
>
where:
R = the resistance of the CT winding
●
CT
● R = the resistance of the lead from the CT to the IED.
L
Note:
The above formula assumes negligible relay burden.
We recommend a stabilizing resistor, which is continuously adjustable up to its maximum declared resistance.
P14xEd1-TM-EN-1

[ ]
− >

IREF Is


I
−
op

nI
e

CT ratio
IREF Is nI
( )
) [ ]
> +
e
I R 2 R
( )
+
F CT L
=
IREF Is
] [ ]
>
Chapter 7 - Restricted Earth Fault Protection
173