Coasting Induction Motor Trips when supply CB open 2

[Vegemite]

This induction motor has been running successfully for many years on a 3.3 kV switchboard which has become obsolete. It was re-connected to a new 3.3 kV switchboard and the following observed immediately after completion of a successful bump test. What was different about this bump test is that the motor was run for five seconds to ensure that it reached rated speed.
After the motor was stopped and its supply circuit breaker had been racked out, it tripped on earth fault. The motor was uncoupled from its load so after it was stopped it kept on rotating.
The earth fault protection was supplied by a core balance CT located in the cable box of the starter panel.
The motor specification is as follows:
• UN = 3.3 kV
• PN = 315 kW
• Frequency = 50 Hz
• FLC = 65 A
• Speed = 2973 rpm
• Torque (MN) = 1012 Nm
• Winding = star connected
• Class of Rating = S1.
The motor supply cable specification is:
• 3c. 50 mm², Aluminium conductor
• Heavy duty Copper screens
• XLPE insulation with PVC sheath
• Length = 280 m.
The motor earth fault protection details:
• Relay type = SEL710
• Core balance CT = 50/1 A
• Earth fault element = Neutral overcurrent
• Trip current = 0.1 A secondary (5 A primary)
• Trip delay = 0.1 seconds.
The neutral current was captured by the SEL 710 relay and the attached trace confirms the presence of genuine earth fault current with a magnitude of 5.5 A.

 

[waross]

Check the grounding path of the copper screens. They may be connected differently than before.
Particularly check that the path through the neutral CT is the same as before.
A screened cable that long will have a significant capacitance to ground (screen). The combination of back EMF and capacitance will hod the back EMF up while the motor is spinning after it has been disconnected from the supply.
I suspect that originally the neutral CT did not see this charging current but the cable has been reconnected in such a way that the charging current is now passing through the CT.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

The earth fault operation seems to be spurious.
It is probably due to a common mistake in the power cable termination.
Please check whether the power cable screen/armour is taken through the CBCT or connected to the cable gland plate prior to taking the cable cores through the CBCT. If the armour/screen is taken through the CBCT, it also needs to be returned through CBCT after termination.
If not, stray currents in the armour/screen can cause Earth fault protection maloperation.

Rompicherla Raghunath

 

[electricpete]

the attached trace confirms the presence of genuine earth fault current with a magnitude of 5.5 A.

I don't see any trace attached. It may provide some clues.

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

 

[controlsdude]

could separate the motor from the controls?

by turning the local disconnect off, bypass the disc aux if any. Turn the controller/vfd on see if you have the same result? If you do then its your controller/vfd or wiring or CT as suspected. If not then motor.

 

[ScottyUK]

Is this a conventional vacuum contactor starter, or is there anything unusual about it? A schematic of the starter cubicle would be a starting point, and photographs of the cable end boxes showing where the screens are routed and where the CT's are positioned.

 

[Vegemite]

Thank you for the valuable feedback. For your information it is a DOL starter incorporating an ABB type VM1 vacuum circuit breaker. In the attached zip file are:
1. the trace that was omitted in error from the original post,
2. the AC schematic,
3. a photo of the cable termination.

 

[ScottyUK]

By any chance did the trip coincide with the actions of racking out the contactor and/or applying the circuit earth switch?

 

[Vegemite]

Scotty,

The operators stopped the running motor from the relay panel and the engineer who was present did not notice any flags on the relay. A short time later, after the operators had racked out the CB truck, he noticed the earth fault LED. The cable cores were meggered and found to be in good condition before the motor was put back into service.
I am expecting some SoE records from the SEL relay later today and will post them when I receive them. He confirmed that the earth switch had not been applied.

If an earth leakage test circuit had been installed, my bet would be that someone had accidentally pressed the test push button. At this substation, however, no such circuits exist.

 

[ScottyUK]

Hi Vegemite,

I can't see anything obviously wrong in the photos and diagrams you posted. The waveform looks like the amplitude is starting to roll off quite quickly toward the right hand side. It would be interesting to how long that current is sustained for after the contactor opens.

I assume you've injection tested the relay? My guess is that you've used a secondary set if you have. If you can't reproduce the problem using a secondary set, can you get access to a 3-phase primary set or a mobile generator capable of handling a couple of hundred amps (don't worry about the voltage)?

 

[Vegemite]

Hi Scotty,

Injection tests were conducted several weeks prior to final commissioning. The Blower is running now with the next window of opportunity for testing being 12 months away. I still don't have any SoE from the relay but believe it is coming tomorrow.

Right now, my client is quite happy that the drive is running and is confident that the chances of this phenomenon reoccurring and causing grief are low. He would not consider conducting any tests that interrupt production.

 

[ScottyUK]

The SoE record might give something away, I guess we'll have to wait until then. These faults can be frustrating when you something is not quite right but it is running anyway.

 

[waross]

Something to consider that will waste a lot of trouble shooting time.
I have seen equipment where the indication of a trip was wrong.
The actual reason of the trip was not related to the indicated reason for the trip.
For instance, racking the breaker out may be reported as a ground trip.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Vegemite]

Folks,

Below are the events for the day in question:
FID = SEL-710-R410-V0-Z006004-D20140303 CID = 9A42

# DATE TIME ELEMENT STATE
52 08/02/2018 17:06:18.923 IN407=CB READY (TO RACK) Deasserted
53 08/02/2018 17:01:59.333 50N1T=E/F ELEMENT RESET Deasserted
54 08/02/2018 17:01:58.114 50N1T=E/F ELEMENT PICK UP Asserted
55 08/02/2018 16:59:18.043 IN404=CB TRUCK IN TEST Asserted
56 08/02/2018 16:58:57.713 IN403=CB TRUCK IN SERVICE Deasserted
57 08/02/2018 16:58:57.713 OUT303=STARTER READY Deasserted
58 08/02/2018 16:55:11.082 IN101=FIELD STOP RESET Asserted
59 08/02/2018 16:55:11.082 OUT303=STARTER READY Asserted
60 08/02/2018 16:55:10.867 MOTOR_STOPPED BEGINS
61 08/02/2018 16:55:10.867 MOTOR_RUNNING ENDS
62 08/02/2018 16:55:10.567 IN407=CB READY (TO RACK) Asserted
63 08/02/2018 16:55:10.562 IN401=CB CLOSED Deasserted
64 08/02/2018 16:55:10.547 IN402=CB OPEN Asserted
65 08/02/2018 16:55:10.502 IN101=FIELD STOP RESET Deasserted(=FIELD STOP OPERATED)
66 08/02/2018 16:55:10.502 OUT303=STARTER READY Deasserted
67 08/02/2018 16:54:57.518 MOTOR_RUNNING BEGINS
68 08/02/2018 16:54:57.518 MOTOR_STARTING ENDS
69 08/02/2018 16:54:55.808 IN407=CB READY (TO RACK) Deasserted
70 08/02/2018 16:54:55.803 IN402=CB OPEN Deasserted
71 08/02/2018 16:54:55.798 MOTOR_STOPPED ENDS
72 08/02/2018 16:54:55.798 MOTOR_STARTING BEGINS
73 08/02/2018 16:54:55.788 IN401=CB CLOSED Asserted

Up until the earth fault trip, the events are consistent with the planned combined direction of rotation test and field E Stop operation test.
[ul]They are:-
[li]the motor starting at 16:54:55.788 hrs,[/li]
[li]the field E Stop being tested at 16:55:10.502 hrs,[/li]
[li]the SEL relay declaring the motor stopped at 16:55:10.867,[/li]
[li]the CB truck being racked out between 16:58:57.713 & 16:59:18.043 hrs,[/li]
[li]the earth fault element giving a trip output at 17:01:58.114 hrs, then[/li]
[li]the earth fault element resetting at 17:01:59.333 hrs.[/li]
[/ul]
The total earth fault duration was 1.319 seconds.

This looks like a typical fault duration for rural feeders. This process plant has a 30 MVA, 22 kV tie to a novel utility substation located on its boundary. The utility substation feeds a 22 kV Single Wire Earth Return (SWER) system and to be sure to keep the neutral displacement voltage down for faults near to the substation, it has 4,500 Amp, 22 kV earthing transformers installed. Several years ago it was observed that earth faults in the SWER system were partly returning to the substation via the solidly earth neutrals of the plant's four off 50 MVA-ish generator step up transformers which are very close to the utility substation. In one instance, SWER earth fault current caused the tie line to trip. A decision was made at the time to disable the earth fault on the tie.

I believe that the client should:
1. Check for correct earthing of the CBCT secondary wiring, &
2. Ask the utility if they experienced any earth faults in the local SWER system at the time of the trip.

Please, let me have any thoughts.

 

[panter]

Good suggestion from Waross.
Check how the earth conductor returns, I did not see good at picture , and check it is passed or not through a earth fault transformer.
From position of the transformer on the cable it depends whether the conductor should be returned through the transformer or not.
Greeting !