MM30-DK
Doc. N° MO-0050-ING
Rev.
2
Pag.
6
of
36
Copyright - 2010 - Microener
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.
Similarly, the Positive Sequence Current Component “
Id
“ and Negative Sequence component
“
Is “
, with no Earth Fault, are computed according to the normal equations of the system
symmetrical components, using two currents only:
In case of Earth Fault the Earth Fault Element trips before tripping of the unbalance element.
- 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).
2.2.2.3
– Earth Fault Current and Voltage Inputs
The real-time measurement of the r.m.s. Earth Fault Current is displayed as percent of the rated
current of the Zero sequence Current measuring C.T. (Io= xxxx %On).
The input of the Earth Fault element is supplied by a separated Core Balance C.T., with 1A rated
Secondary Current.
The Earth Fault element can operate as a normal Zero Sequence O/C element or as a Directional
Zero Sequence O/C element.
In this case to discriminate the direction the relays uses as polarizing quantity the Zero Sequence
Residual voltage supplied by the Open-Delta windings of the system P.Ts.
The display directly
gives the measurement “ Uo “ of the Secondary Zero sequence voltage and
the measurement “
o ” of the phase displacement of the Zero Sequence Current Fasor from the
Zero Sequence Voltage Fasor.
120
A
C
120
A
C
2
A
2
C
2
A
C
2
C
A
I
I
3
Id
I
I
3
Is
Id
I
I
Is
I
I
Is
Id
I
Is
Id
I
j
j
e
e
α
α
α
α
α
α
α
α
