Supervision
by a ring core type current transformer.
The measuring principles of the circuit supervision are based on comparing the measured and the calculated
residual currents. In an ideal case these are:
IL1
IL2
IL3
KI
∗
IG
=
3
∗
I
0
KI
∗
IG
=
0
KI represents a correction factor which takes the different transformation ratio of the phase- and earth current
transformers into account. The device automatically calculates this factor from the rated field parameters, i.e.
the relation between the rated primary and secondary current values of the phase- and earth current
transformers.
For compensating the current proportional ratio error of the measuring circuits, the dynamic correction factor Kd
can be used. As a function of the measured max. current this factor is considering the linear rising measuring
error.
The limiting value of the CT supervision is calculated as follows:
∆
I
= deviation I (rated value)
Kd
= correction factor
Imax
= current maximum
Limiting value =
∆
I + Kd x Imax
Precondition for identifying an error:
3
∗
I
0
KI
∗
IG
Delta I
Kd
∗
Imax
The evaluation method of the circuit supervision by using factor Kd can be graphically represented as follows.
If the current is measured in two phases only (for instant only IL1/IL3) or if
there is no separate earth current measuring (e.g. normally via a cable-type
CT), the supervision function should be deactivated.
EN MRM4 01/11
Page 415
limit value
Imax
I
Kd*Imax
Summary of Contents for MRM4
Page 1: ...MRM4 HighPROTEC Motor Protection Device Manual DOK HB MRM4E...
Page 3: ...Order Code EN MRM4 01 11 Page 3...
Page 18: ...Important Definitions Page 18 EN MRM4 01 11 41 SG Prot ON 42 SG ON Cmd SG Prot ON SG ON Cmd...
Page 115: ...Navigation Operation Navigation Operation EN MRM4 01 11 Page 115 1 2 3 5 7 6 8 10 9...
Page 162: ...Recorder Name Description I 2 TripCmd Signal Trip Command Page 162 EN MRM4 01 11...
