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 asser
ted.
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 curr
ent transformer
s 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 current:
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 current:
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
current 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 resistance of the CT winding
● R
L
= the r
esistance 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.
P14x Chapter 7 - Restricted Earth Fault Protection
P14xEd1-TM-EN-1 173
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