2 CTs on an MV Motor???
2 CTs on an MV Motor???
(OP)
Ive came across an application for a 1200hp 4160V motor, running vertical pumping units. For some reason there are only 2 phase CTs and One zero sequence CT. Does anyone have any feedback or experiance with a 2 CT application. They currently have old Westinghouse protection on the motors.





RE: 2 CTs on an MV Motor???
For new work, you should go with overcurrent protection on all three phases. Since most new relays are three-phase digital relays anyway, that isn't an issue. For retrofit work, the problem will be the missing CT. You should try to add a third CT is possible and go with full three-phase protection.
RE: 2 CTs on an MV Motor???
Thanks!
RE: 2 CTs on an MV Motor???
RE: 2 CTs on an MV Motor???
RE: 2 CTs on an MV Motor???
If customer don't want add CT, possible use standard motor protection with 3-ph current inputs. Not needed relay with option of internal missing phase calculation.
If two-phase connection is used, it is recommended that a current corresponding to the sum of these two phases is connected to the input circuit of the missing phase (in an opposite polarity).
- the unbalance protection can be used
- often the current measurement is more accurate compared to two-phase measurement
- an earth-fault current may affect the unbalance measurement.
Small back to Theory (sorry)
In any network, the sum of three phase current is I1 + I2 + I3 = Io.
In a healthy network the earth-fault current is zero and can be ignored. I1 + I2 + I3 = 0. The missing phase can be calculated from the other two currents. For example I2 = - (I1 + I3).
The minus sign means that the sum of the two phases must be connected to the relay input in an opposite polarity! It's wiring only!!!
We used this applycation many times, it's work, but of course prefer add CT.
Regards.
Slava