FUNCTION CHARACTERISTICS
187
NVA100X-D - Manual - 02 - 2016
Low impedance restricted ground fault- 64REF
Preface
The restricted earth fault protection is used to detect the ground faults in a three-phase winding
with star point connected to earth in the area between the CTs on side H and the CT on the neutral
earthing.
Operation and settings
In order to avoid nuisance tripping caused by rise of differential current (CT measurement errors in
the presence of high through current, eg, external faults), the tripping is a percentage characteris-
tic.
When there is no earth fault, no starpoint current
I
E1
fl ows and the computed residual current
(
I
ECH
=
I
L1H
+
I
L2H
+
I
L3H
) is also zero.
I
E1
and
I
ECL
are conventionally regarded as positive if coming into their respective amperometric
clamp reference input (A7 per
i
E 1
, C1-C3-C5 per
i
E CL
), negative if outgoing.
For internal fault to the protected area (
I
L2(H)
in the following example),
i
E
and
i
E(H)
are thus both
positive, ie the phasors
I
E
and
I
E(H)
are in phase (assuming the same system of grounding of the star
point and network).
When the fault is outside the protected area
I
E1
is positive and
I
ECH
is negative ie phasors
I
E1
e
I
ECH
are opposite in phase.
The residual fundamental frequency current, acquired by means of a current transformer installed in
the starpoint connection to ground - side H, is compared with the stabilization current defi ned as:
I
ES
= 4 · [|pE·
I
E1
- ME·
I
ECH
| - |pE·
I
E1
+ ME·
I
ECH
|] if [|pE·
I
E1
- ME·
I
ECH
| - |pE·
I
E1
+ ME·
I
ECH
|] > 0
or
I
ES
= 0 if [|pE·
I
E1
- ME·
I
ECH
| - |pE·
I
E1
+ ME·
I
ECH
|] ≤ 0
I
E1
is the residual current phasor - side 1,
I
ECH
is the calculated residual current (fundamental wave of the phase currents sum) side H,
p
E
is the amperometric polarity of the residual current so that it can be corrected any possible re-
versal of polarity; if the amperometric polarity is according the schematic connection diagram the
corresponding setting is
C7-C8 POL = NORMAL (
pE1 = +1), otherwise is
INVERSE
(pE1 = -1).
The
C7-C8 POL
parameter is available inside the
Set \ Polarity
menu.
ME is an amplitude compensating factor of residual current calculated by the relay as follows from
the
I
npH
,
I
nH
,
I
En1p
,
I
En1n
, parameters setting inside the
Set \ Base
menu:
ME = (
I
npH
/
I
nH
) / (
I
En1p
/
I
En1
)
The ME factor expresses the value that is multiplied by the residual current calculated by the cur-
rent of phase CTs in order to obtain an amplitude equal to that of the residual current measured by
CT on grounding the neutral in case of earth fault outside the protected area. The relay also checks
that the ME value is ≤ 200 and if this condition is not met, the relay displays a warning and requires
the programming of parameters
I
np(H)
,
I
n(H)
,
I
Enp
,
I
En
. Being compensated in amplitude the calculated
residual current, the threshold setting of the element is always referred to the rated current of the CT
on grounding connection of neutral (
I
E1
).
The current
i
E
and
i
E(H)
are conventionally regarded as positive if entrants in their respective ref-
erence terminal of the current input, whether negative terminals. Internal fault in the protected
area,
i
E
and
i
E(H)
are thus both positive, and that the phasors are in phase (assuming the same sys-
tem of earthing of the star and the network), while a fault outside the protected area
i
E
is positive
and
i
E(H)
negative and that the phasors are in phase opposition.
To understand how the 64REF element does work, to simplify, consider PE1 = +1 and ME(H)=1, , accord-
ing to the connection diagram and use of CTs with the same ratio on the grounding neutral-side H.
The trip current is the residual current measured directly by the CT on the starpoint grounding con-
nection.
The stabilization current is computed by the relay using the sum and difference method of the cur-
rents on the grounding path (direct measurement) and line currents (calculated as sum of phase
currents).
—
•
•
•
•
I
ECH
I
E1
L1
L2
L3
I
E1
I
ECH
I
ECH
=
I
L1H
+
I
L2H
+
I
L3H
I
ECH
L1
L2
L3
I
E1
I
E1
I
ECH
I
ECH
=
I
L1H
+
I
L2H
+
I
L3H
