2 CTs on an MV Motor???

[RodVito]

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.

 

[dpc]

In olden times, it was common to only provide two phase overcurrent relays along with a ground relay. As long as all the relays work as intended, this provides adequate protection against all types of faults. Obviously three-phase and line-line faults are covered. And a line-to-ground fault in the phase without a phase relay will be detected by the ground relay.

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.

 

[RodVito]

Thanks for the insight dpc. I found out that by installing the Multilin 369 (or any such Microprocessor driven relay) we can use a calculated phase B, using the inputs of A and C. Seems somewhat crazy to me, purchase one CT and get full protection, but they want to go that way, so be it.

Thanks!

 

[dpc]

With two CTs, overcurrent and short circuit protection will be OK and probably the motor thermal model will still be reasonably accurate. But I wouldn't count on the calculated B phase giving great results for negative sequence current and phase imbalance. In theory it should work, but in theory the motor shouldn't fail either.

 

[jraef]

It should be noted that if you are in the US, the code requires 3 leg overload protection now, at least on LV motors. I don't have my copy with me to see if they said the same for MV motors.

 

[slavag]

Hi
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