The following settings are provided to define this bias characteristic:
● IREF> Is1: sets the minimum trip thr
eshold
● IREF> Is2: sets the bias current kneepoint whereby the required trip current starts increasing
● IREF> k1: defines the first slope (often set to 0%)
● IREF> k2: defines the second slope
Note:
Is1 and Is2 are relative to the line CT, which is always the reference CT.
3.2.2 DELAYED BIAS
The bias quantity used is actually delayed by one cycle. It is the maximum value of the mean bias quantities
calculated within the pr
evious cycle, where the mean bias is the fundamental bias current. This means the bias
level, and thus through-fault stability is maintained after an external fault has been cleared.
The algorithm, shown below, is executed eight times per cycle.
I
bias
= Maximum [I
bias
(n), I
bias
(n-1), …I
bias
, (n – (K-1))]
It is this delayed bias that is used to calculate the operating current.
3.3 HIGH IMPEDANCE REF
The device provides a high impedance restricted earth fault protection function. An external resistor is required to
pr
ovide stability in the presence of saturated line current transformers. Current transformer supervision signals do
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 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.
P24xM Chapter 7 - Restricted Earth Fault Protection
P24xM-TM-EN-2.1 173
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