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Chapter 8 - Restricted Earth Fault Protection
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 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.
176
IREF
Is
[
]
−
>
I
op
nI
−
e
CT ratio
IREF
Is
nI
(
)
)
[
]
>
+
e
I
R
2
R
(
)
+
F
CT
L
=
IREF
Is
]
[
]
>
P64x
P64x-TM-EN-1.3
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