instantaneous setting of medium voltage motor 2

[SELEC]

I am doing a MV motor protection setting. But the Asymetrical inrush current is overlapped with SR469 Inst. setting, if if shift Inst a bit above, like 0.06s, which is still below Cirucir breaker openning time.Then they would be co-ordinated.Is it reliable to do so?

 

[dpc]

First of all, you need to check your motor relay specs and determine if it has a dc offset filter. If so, the asymmetrical current is not a factor. I believe the SR469 may filter dc out, but you need to to figure this out to your own satisfaction. You also to need to review the SR469 specifications to determine the minimum response time of the unit. If it is doing dc filtering in software, it will need to sample enough points to be able to determine the dc offset. This will unavoidably delay operation of the instantaneous unit.

It is not enough to drag lines around on a TCC. You need to understand how the relay operates.

I don't understand the question about the breaker operating time. The breaker operating time must be added to the relay operate time to determine clearing time.

 

[SELEC]

Actually it is the recommended time by GE Multilin, it might be accurate,

CB opening time: 0.08s
Relay overtravel: 0.10s
Safety factor for CT saturation: 0.22s

 

[dpc]

Digital relays have no overtravel.

I guess do whatever you think is correct.

 

[SELEC]

I just checked SR469 Manual. It doesn't have DC filter so we might have to delay Inst setting in order to avoid tripping while starting. Total clearing time for CB is 0.25s.Asymetrical inrush current lasts only 1 cycle. That should work.

 

[dpc]

Actually, I'm pretty sure that the SR469 has a selectable dc offset (overreach) filter.

A 0.25 sec breaker clearing time is fairly unusual.

There are a lot of good references, freely available on the Web, that cover relay coordination. You obviously have Internet access, so I'd recommend that you spend a little time doing your own research and problem solving. And since you appear to be doing this for a living, maybe invest in some reference books (and then read those too.)

Good luck.

 

[electricpete]

I'm not savvy on the digital relays.

With the electromechanical relays there were two approaches in IEEE standards:

1 - Set instantaneous just above 1.73*LRC as recommended by IEEE. Allows for dc component.

OR

2 - Two different instantaneous relay functions. One is set up above 1.73*LRC... maybe even 2.0*LRC. It has no time delay. The other is set around 1.2*LRC. It has a time delay of 100 milliseconds. (GE High Dropout function)

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[stevenal]

Lots of info at selinc.com. Look for technical articles by Stan Zocholl.