Motor Differential CT Ratio Selection

[Detective89]

Can anyone please explain to me how to select the size for the primary of a three phase motor's core balance (zero sequence) differential current transformers. The secondary will be 5A.

Thanks,

 

[Detective89]

Edit: I am thinking the CT is not suppose to measure any current, so I don't think it would need to be rated for FLA, but I'm not sure. Also, the system is on an HRG, so if there was a ground fault in the motor it should see more than 5A of fault current.

 

[davidbeach]

No load current in the CT, so size for sufficient sensitivity during faults. The HRG makes detection of ground faults difficult, so you probably don't want to go beyond a 50:5 CT.

 

[Detective89]

Thank you for your response.

I must not have searched well enough because here is another thread on the exact same topic.Link

 

[stevenal]

The diagram does not show a core balance CT, it shows three individual phase CTs. These CTs will indicate load current.

 

[oldfieldguy]

I have seen then from 50:5 to 300:5. 50:5 is the most common, especially in modern protective relaying where the relay impedance is very low.

stevenal-

the current through the CT is balanced under normal circumstances, so the Ct sees zero total current as the current comes in through the incoming line end of each winding and goes out through the neutral end of the same winding. In the event of an internal fault, the current comes in through the incoming line end of the winding and part of it goes out through the fault path. Since the current thought the Ct is no longer balanced, the difference shows up in the current output of the CT.

Typical setpoints for motor differential elements using this scheme are normally 0.1xCT, with no delay.

old field guy

 

[electricpete]

stevenal is pointing out the conflict between the graphic (which shows phase ct's) and the text (which talks about core balance).

=====================================
(2B)+(2B)' ?

 

[davidbeach]

Given the OP question I assumed (yeah, I know) that the two phase wires shown were terminal and neutral connections of a six lead motor and that the neutral wye connection is out of the picture to the left. If that is actually the case, the CTs are functioning as core balance CTs and see essentially no load current (because the two conductors don't both pass through the center of the CT opening there may be some difference in the coupling and therefore some standing load component in the secondary current). This allows a sensitive differential, particularly on an HRG system. Doing a differential using phase CTs on both ends of the motor may miss ground faults on an HRG system since the CTs will have a much larger ratio and then there are mismatch issues.

 

[Detective89]

The graphic is correct as to how the motor is installed with the CT's in the terminal box and they are core balance CT's.

After speaking with someone that programs the relays for these motors, he told me the facility uses these CT's for phase to phase faults inside the motor. They use the HRG to detect the ground faults. He gave me a size that they have calculated to work for the application.

Thank you all for the help.

 

[stevenal]

Thanks David and Oldfieldguy. Looked like parallel phase conductors to me. Is it common to use the same color for phase and return?

 

[ScottyUK]

stevenal,

I agree it looks like phase conductors - if a motor is large enough to justify stator differential then it's almost certainly an HV machine. Running six leads for a standard DOL machine is unusual, at least in IEC-land: we would normally run a 3-core cable, or perhaps a trefoil group for a big motor. The neutral would go to an internal star point which is not externally connected, and the stator diff CTs would be at either end of the winding. CTs for overcurrent & E/F would typically be at the switchgear / contactor.

 

[LionelHutz]

So, we go through the whole thread and the final selected CT is "calculated to work". You posted the question so care to share the answer you found?

 

[stevenal]

And the calculation used?

 

[Detective89]

I would share if I knew. The client with the motor has another contractor company do all of their power studies and Schweitzer relay programming and that is who I spoke to. They didn't provide me the details on the calculations they've ran. They just said the calculation show the 600:5A to work for this size motor and it is the ratio that they are having success with currently. Sorry I can't explain any better.

Also, 600:5A is what the motor manufacturer recommended.

 

[LionelHutz]

What you are describing really doesn't make much sense. The protective relay will have phase CTs which can be used to detect phase to phase faults.

 

[Detective89]

That is why we originally figured they were for a ground fault in the motor. We said the same thing about the phase CT's in the starter at the substation, but apparently they use the core balance CTs for that purpose at the motor as well. I am new to this industry so I can't really argue this topic, but in this particular case this is what the client wants to do with those CTs.

 

[oldfieldguy]

If the CT is employed in the core balance function it will not 'see' phase current. The amount of current it might see under fault conditions may be as high as the full phase to ground current, for faults occurring just inside the protective zone but before the motor windings, to some very low value of current for a fault occurring near the neutral point. To gain sensitivity for the second condition, a low ratio CT is called for.

I cannot put my finger on a reference at the moment, but I seem to recall that the CT should be sized to expect a fault 90% into the winding.

old field guy

 

[DTR2011]

This appears to be a GE/Multilin SR469 Relay. From the manual...


"The PHASE DIFFERENTIAL CT PRIMARY must be entered if the differential feature is to be
used. If two CTs are used per phase in a vectorial summation configuration, the CTs should
be chosen to ensure there is no saturation during motor starting. If however, a core
balance CT is used for the differential protection in each phase, a low CT rating of 50 or 100
A allows for very sensitive differential protection."

I have found that the use of the 87-Differential description for this relay a bit misleading, vs. a traditional 87 relay (GE CFD, which is a percentage differential for a machine application). The SR 469 implementation is more like an inst or short time over current element, or a traditional partial differential application using overcurrent relays.