CR4® - The Engineer's Place for News and Discussion®


Previous in Forum: Loading Up Power to A Vault   Next in Forum: Double Bus System
Close
Close
Close
9 comments
Power-User

Join Date: Jan 2010
Location: Hyderabad
Posts: 159
Good Answers: 3

6.6kV Vacuum Contactor (Latched) Opening Time

11/03/2017 11:52 PM

We have a problem with 6.6kV Vacuum contactors (VCUs)-latched type.

The contactor closing time is about 60ms and opening time is about 25ms. There is no issue with that.
When the trip command came while motor is starting, the contactor takes ~400ms to open.

This time the trip was from process (O&G industry), But, it could have been from earth fault protection.

As we know the likelihood of faults during inrush (whether it is motor or transformer) is high statistically and it is important that the VCU opens without additional delay when commanded by earth fault protection.
In VCU feeders, short circuit protection is not enabled in feeder protection relay (fuse is expected to clear large current faults) but earth fault protection is enabled. 6.6kV system is resistance earthed with earth fault current limited to 300A.

Kindly share your views/experiences. Thanks in anticipation.

__________________
Raghunath
Register to Reply
Pathfinder Tags: VCU
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Jun 2007
Location: chennai,India
Posts: 579
Good Answers: 20
#1

Re: 6.6kV Vacuum Contactor (Latched) Opening Time

11/04/2017 10:49 PM

Generally contactors in such circuits are not opened during high currents say short circuit currents or starting currents to avoid contactor life.

__________________
Ramesh,Freelance Electrical/automation Consultant
Register to Reply
Power-User

Join Date: Jan 2010
Location: Hyderabad
Posts: 159
Good Answers: 3
#2
In reply to #1

Re: 6.6kV Vacuum Contactor (Latched) Opening Time

11/04/2017 11:32 PM

Thanks Ram Vinod.

Short circuit currents, I agree and it is also our practice not to enable short circuit protection in relays in VCU feeders.

Motor starting currents - The contactor is expected to break currents up to 8-times its rating (i.e. 3200A for a 400A contactor) as per IEC 60470.

As I mentioned in my original post, the earth fault currents in 6.6kV system are limited to 300A. So, the contactor should have no problem in interrupting the circuit on earth fault protection operation.

__________________
Raghunath
Register to Reply
Guru

Join Date: Jun 2007
Location: chennai,India
Posts: 579
Good Answers: 20
#3
In reply to #2

Re: 6.6kV Vacuum Contactor (Latched) Opening Time

11/05/2017 1:29 AM

We have to find out the delay of tripping , is it from the relay?

May be trip time , as per earth fault current and curve , 400 ms.

We have to study this.

I do not think the contactor will have any intelligence in tripping, it has to open when trip signal is given

__________________
Ramesh,Freelance Electrical/automation Consultant
Register to Reply
Guru
New Zealand - Member - Kiwi Popular Science - Weaponology - New Member Engineering Fields - Power Engineering - New Member Engineering Fields - Electrical Engineering - New Member

Join Date: Sep 2006
Location: Auckland, New Zealand
Posts: 8448
Good Answers: 350
#4

Re: 6.6kV Vacuum Contactor (Latched) Opening Time

11/05/2017 2:35 PM

What does the contactor literature show?

Is there a problem with the motor causing a delay?

Is there a problem with the motor supply voltage sagging during motor operation causing an operation delay?

__________________
jack of all trades
Register to Reply
Power-User

Join Date: Jan 2010
Location: Hyderabad
Posts: 159
Good Answers: 3
#5
In reply to #4

Re: 6.6kV Vacuum Contactor (Latched) Opening Time

11/05/2017 10:29 PM

Thanks for trying to help.

Literature just says opening time is ~25ms. When the motor is running steady and the trip command is given to contactor, it is tripping in ~25ms, as expected. The problem appears only when the contactor open command is issued just after the close command.

Manufacturer says the delay of ~400ms is common for all manufacturers if the trip command is received just after closing of contactor. This claim, we are yet to verify with other makes.

__________________
Raghunath
Register to Reply
Guru

Join Date: Jul 2007
Location: Commissariat de Police, Nouvions, occupied France, 1942.
Posts: 2532
Good Answers: 76
#6
In reply to #5

Re: 6.6kV Vacuum Contactor (Latched) Opening Time

11/06/2017 2:47 AM

Then do you, or do you not, have a problem, then, Mildred? What sort of grief is this 400ms causing you? Are you trying to solve a problem that doesn't exist?

__________________
Good moaning!
Register to Reply
Guru

Join Date: Dec 2010
Posts: 1221
Good Answers: 89
#7
In reply to #5

Re: 6.6kV Vacuum Contactor (Latched) Opening Time

11/06/2017 4:46 PM

The contactor could be closing onto a fault. It is important enough time is allowed for the fuse to open the circuit before the contactor opens [contactor rated to make & carry, but not break maximum fault current of circuit].

The contactors will disconnect all phases but the fuse(s) will only open faulted phases - it is possible, for a phase-phase fault, only one fuse blows due to significant minimum to maximum tolerance of fusing current - so it is good that the contactors "follow-on" and isolate all phases.

I just looked at fuse characteristics - max current for 0.4 seconds is typically about 11 times rated current - I suggest you look at the curves for the fuses feeding your breaker.

Register to Reply
Guru

Join Date: Jun 2007
Location: chennai,India
Posts: 579
Good Answers: 20
#8
In reply to #7

Re: 6.6kV Vacuum Contactor (Latched) Opening Time

11/06/2017 8:10 PM

Here the issue is why a contactor is taking 400ms to open?

All your explanation is considered good for fuse, but our analysis is why contactor not opening with in stipulated time of 25 ms?

__________________
Ramesh,Freelance Electrical/automation Consultant
Register to Reply
Guru

Join Date: Dec 2010
Posts: 1221
Good Answers: 89
#9
In reply to #8

Re: 6.6kV Vacuum Contactor (Latched) Opening Time

11/07/2017 7:03 AM

Raghun has access to the logic in the relay and any auxilliary relays.

The contactor obviously opens in 25ms as per original post, so it must be the protection system which delays VCB feeder earth fault operation 400ms immediately after a close.

It was not specified how earth fault is detected on the VCB circuit. Most likely it is the residual of 3 line CTs in the VCB. It is well known that CT saturation, particularly with transfo/motor inrush and starting currents, limits the minimum current setting and speed of the residual method if false tripping is to be avoided.

The system is resistance earthed, limiting earth fault currents. This limits the amount of damage at the earth fault & allows a longer delay before tripping to give reliable time grading of feeder and primary earth fault protection. It also means the VCB can safely break circuit - when it is solely an earth fault.

One has to consider what would happen if the VCB closed onto a severe fault & opened immediately on "earth fault". The VCB would blow-up, the motor would be more damaged, because the back-up protection must be slower than the fuse. There is risk of personnel injury. The whole bus (plant?) would be out of operation until the VCB was repaired.

No information has been given about the fault levels or protection behind the VCB or the motor load. Maybe it is theoretically possible to identify that an earth fault is the sole fault in less than 400ms, despite inrush & 8 x full load starting current of motor, however, the risk detailed in my previous paragraph must be considered & one would have to be sure that VCB earth fault detection was completely reliable. That might be acceptable with a core-balance CT at increased cost & maybe size but I do not think that is present.

Register to Reply Score 1 for Good Answer
Register to Reply 9 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

67model (2); Crabtree (1); jack of all trades (1); raghun (2); ramvinod (3)

Previous in Forum: Loading Up Power to A Vault   Next in Forum: Double Bus System

Advertisement