Motor Differential CT Ratio Selection
Motor Differential CT Ratio Selection
(OP)
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,
Thanks,





RE: Motor Differential CT Ratio Selection
RE: Motor Differential CT Ratio Selection
RE: Motor Differential CT Ratio Selection
I must not have searched well enough because here is another thread on the exact same topic.Link
RE: Motor Differential CT Ratio Selection
RE: Motor Differential CT Ratio Selection
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
RE: Motor Differential CT Ratio Selection
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(2B)+(2B)' ?
RE: Motor Differential CT Ratio Selection
RE: Motor Differential CT Ratio Selection
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.
RE: Motor Differential CT Ratio Selection
RE: Motor Differential CT Ratio Selection
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.
RE: Motor Differential CT Ratio Selection
RE: Motor Differential CT Ratio Selection
RE: Motor Differential CT Ratio Selection
Also, 600:5A is what the motor manufacturer recommended.
RE: Motor Differential CT Ratio Selection
RE: Motor Differential CT Ratio Selection
RE: Motor Differential CT Ratio Selection
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
RE: Motor Differential CT Ratio Selection
"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.