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Unit Trafo - Motor with PFC capacitor trips on Differential protection

raghun (Electrical) (OP)
17 Jan 08 6:05
3.75MW, 11kV Induction motor is connected in 33kV system through a Unit Trafo (33/11kV). There is a capacitor across motor terminals and compensates about 90% of the motor magnetising current. The feeder is tripping on Differential protection. The differential protection covers motor and transformer in its zone but excludes the capacitor. The differential protection is bias type (KBCH130 of AREVA).
Apparently, there is no problem with either the motor or transformer.

Any suggestions!

dpc (Electrical)
17 Jan 08 12:21
This occurs at instant of energization?  
slavag (Electrical)
17 Jan 08 12:42
Hi Raghun.
1. Are you back unit to operation?
2. What is a setting of diff protection?
3. Are you check stabilty of protection again?
4. Are you have DR graph?
5. What is a vector group of XFR?
Please send more information.
raghun (Electrical) (OP)
18 Jan 08 22:50
The info from site is it trips when it is already in service. Do you think if it occurs during starting, it is more readily explainable!

Yes, unit was back in operation.
Differential protection setting, stability, vector group (Dyn1)have been checked and since the equipment is unit transformer connected motor, there is nothing like through fault here.

Thanks to both of you for the responses.
davidbeach (Electrical)
19 Jan 08 0:22
I'm have a bit of difficulty picturing the arrangement.  Are there two, or three sets of CTs?  If only two sets, how do you exclude the capacitors?  If the capacitors are truly excluded, capacitor inrush, or outrush, could be seen similar to through fault.  If the motor is already running when this happens, outrush is more likely than inrush.  Is there another such combination near by?  If so, does the trip happen when that unit is being energized and the capacitors on both units are ringing against each other?
slavag (Electrical)
19 Jan 08 1:13
Raghun, I would like back to my Q:
what is a setting of diff protection.
Not once, XFR diff protection was tripped w/o any reason
after several years of operation. And reson was setting of
I2f blocking and Id> first setting. And problem was not only in energazing of XFR, also in AR on the grid.
I attached this files to Gunnar's thread, but would like attached here too.

raghun (Electrical) (OP)
19 Jan 08 1:22

Let me try to clear - the capacitor branch has separate CT which is wired into the differential protection scheme and thus excludes the capacitors from its zone. The motor branch CT is in the motor star point side, where as the capacitor branch CT is at the tap-off.

slavag (Electrical)
19 Jan 08 1:36
Raghun. In additional to David's Q and your's answer:
Capacitor's set CT's are connected  directly to the protection relay ( as 3 win. XFR) or are paralleling wiring with motor's set CT's?
raghun (Electrical) (OP)
19 Jan 08 4:45
You are right, Slavag. The motor and Capacitor CTs are paralleled externally and connected to the differential relay (Relay type is KBCH120 that is with two bias inputs and not KBCH130). Thanks.
slavag (Electrical)
19 Jan 08 4:59
I'm hope, guys installed same set of Ct's on capacitors and motor.
dpc (Electrical)
19 Jan 08 12:58
Was something nearby being energized when this happened?  Another motor, transformer, cap bank?  I would suspect some type of transient event.  

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