MC20-CEI
Doc. N°
MO-0431-ING
Microelettrica Scientifica S.p.A. - Copyright 2009
FW 1080.36.03.X
Date 18.11.2011
Rev.
3
Pag.
6
of 35
2.2 - Operation and Algorithms
2.2.1 - Reference Input Values
Display
Description
Setting Range
Step
Unit
In
300
A
Reference primary current of the relay
1 - 9999
1
A
I1
300
A
Rated Primary current of phase C.T.
1 - 9999
1
A
I2
5
A
Rated Secondary current of phase C.T.
1 -
5
1/5
A
Io1
100
A
Rated Primary current of the tore C.T. detecting earth fault
current.
1 - 9999
1
A
Io2
1
A
Rated secondary current of the tore C.T. detecting earth fault
current.
1 -
1
-
A
Vo1
10
kV
Rated primary voltage of VT phase to neutral
0.05 -
500
0.01
kV
Vo2
100
V
Rated secondary voltage of VT phase to neutral
50 -
115
0.01
V
Freq
50
Hz
System rated frequency
50 -
60
10
Hz
2.2.2 – Input quantities
2.2.2.1 - Mains Frequency (Freq)
The relay can operate either in 50Hz or 60Hz systems.
The rated Mains Frequency “
Freq
“ must be set accordingly.
2.2.2.2 – Phase Current inputs (I1)
The relay directly displays the r.m.s. value of the Phase Currents “
IA
“, “
IB
”, “
IC
” flowing in the Primary of
the input Current Transformers and refers all its measurements to that value.
To make the relay properly working with any C.T., when programming the relay settings, input the value “
I1
“
of the primary current of the phase C.Ts
Only phase A and C currents are measured, whereas the current of the phase B is computed as vector
summation of the currents of the other two phases.
The algorithm is based on the following considerations coming from well-known vector relations among the
three-phase currents and the zero sequence current.
-
In any circumstance – currents balanced or not, sinusoidal or not – it is always true that:
( )
0
I
I
I
I
1
0
C
B
A
=
+
+
+
-
When no Earth Fault exists (I
0
= 0)
( )
(
)
C
A
B
C
B
A
I
I
I
0
I
I
I
2
+
=
⇒
=
+
+
The earth fault protection element is independently supplied by the residual current coming either from the
residual connection of 3 system C.Ts. or from the core balance C.T.
If any Earth Fault is experienced (I
0
≠
0) the Earth Fault Protection Element trips independently from the
phase current measuring elements.
If no Earth Fault is present (I
0
= 0), the equation (2) is valid, no matter if currents are balanced or not,
sinusoidal or not.
The third phase current is calculated, in real time, as vector summation of the other two-phase currents.
During Faults:
A)
Single phase to earth Fault
Trip of the earth fault element directly measuring the Residual Current.
B)
Two Phase Fault
In any case one of the currents directly measured is involved, so the
relay trips correctly.
C)
Two Phase to Earth Fault
Same as A + B
D)
Three Phase Fault
All the three currents are correctly measured (in any case two directly).
