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Old 05-28-2020, 12:31 AM   #1
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Default Negative Sequence Overcurrent

Help me understand negative sequence overcurrent elements.

I've recently taken over an account that uses methane from a landfill to generate power that they sell to the utility. The main breaker that connects this generation to the utility grid is protected with a SEL 351 protection relay.

They had this protection relay replaced a few months before we took over the account by another company. After a recent downstream fault, they were unable to bring one of their generators online as it would cause the negative sequence overcurrent element to assert and shut the whole facility down.

Working with a support engineer at SEL and digging through the manual, I discovered that a delay was recommended with this element to prevent transients from closure events causing nuisance trips. I added a 1.5 cycle delay and got them up and running.

A couple of weeks ago they called me again as they had experienced another trip event. The logs show that the reverse directional element was asserted. They were able to reset and be back in business, but want to eliminate these nuisance trips. SEL has been very helpful and tell me that these elements are set to default values and are working on getting recommendations for setting this up correctly.

I'm sure SEL will be able to help get this running, but this is new territory for me and I'd like to understand it better. Any of you utility or industrial guys able to explain how this works?

I get that it has to do with the voltage to current lead or lag, but I'm unclear on exactly how the values are derived and what typical setups look like. The manual is quite cryptic, any of you guys good at putting this in plain english?

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Old 05-28-2020, 02:47 PM   #2
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First. Welcome to the forum.
I am not familiar with your issue or equipment? However, I'm sure some of the smart fellows on this forum can help or try to help. So, I am bumping this to the top.
Most guys are working and will not be on here until later.
Glad to have you onboard.
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Old 05-28-2020, 06:30 PM   #3
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Thanks! Been an off and on lurker for a while.

Not equipment I typically deal with either, but we seem to be moving towards more of it, so I'm trying to get educated.

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Old 05-28-2020, 06:48 PM   #4
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This may assist you or confuse you.


https://www.myprotectionguide.com/up...ectionaloc.pdf
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Old 05-28-2020, 08:30 PM   #5
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Thanks! That's easier to read than anything else I've dug up so far anyway. I'm gradually getting the picture the more I read.

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Old 05-28-2020, 10:24 PM   #6
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Does it trip when the gen breaker is first closed in? If so, take a look at your synch relaying, if it's not really close to synchronizing, 1.5 cycles is not enough to prevent a trip.

If it's tripping during operation, it's possible that it sees the gen 'motoring' the turbine as negative sequence.

What voltage is the gen operating at?
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Old 05-28-2020, 10:44 PM   #7
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Originally Posted by micromind View Post
Does it trip when the gen breaker is first closed in? If so, take a look at your synch relaying, if it's not really close to synchronizing, 1.5 cycles is not enough to prevent a trip.

If it's tripping during operation, it's possible that it sees the gen 'motoring' the turbine as negative sequence.

What voltage is the gen operating at?
The negative sequence element was asserting when the breaker first closed. The 1.5 cycle delay solved this problem.

The current problem is a reverse directional element asserted during normal operations. I don't doubt SEL will help find an answer, I'm more looking for general information/education on how these protection schemes are implemented.

Operating voltage is 480

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Old 05-29-2020, 08:49 PM   #8
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Reverse current is usually caused by the generator becoming a motor because whatever is driving it is now being driven by it.

I've seen this with methane plants, the energy in the gas isn't consistent and if it's weak, the turbine doesn't produce much torque and the gen will motor it along until the gas begins to burn more or the relay trips out.
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Old 05-29-2020, 09:24 PM   #9
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That makes sense, but in this case there are three generators. They are constantly adjusting gas flow and voltage regulation to keep production levels where they want them.

I do believe they had engine trouble with generator 2 not too long after the trip event, but generators 1 and 3 have been running steadily. This protection relay handles the current for all three generators as they feed into the grid. At the time of the trip event, the logs show that there was approximately 2,000 amps of current.

I had a suggestion from a utility engineer to simply disable the additional features of the protection relay and set it up for straightforward overcurrent and ground fault detection. This makes me wonder, when are the additional functions normally used? For what conditions would want to enable reverse directional elements?

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Old 05-29-2020, 10:39 PM   #10
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This sounds very much like a vendor, or engineering issue.
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Old 05-29-2020, 11:00 PM   #11
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Originally Posted by joab View Post
That makes sense, but in this case there are three generators. They are constantly adjusting gas flow and voltage regulation to keep production levels where they want them.

I do believe they had engine trouble with generator 2 not too long after the trip event, but generators 1 and 3 have been running steadily. This protection relay handles the current for all three generators as they feed into the grid. At the time of the trip event, the logs show that there was approximately 2,000 amps of current.

I had a suggestion from a utility engineer to simply disable the additional features of the protection relay and set it up for straightforward overcurrent and ground fault detection. This makes me wonder, when are the additional functions normally used? For what conditions would want to enable reverse directional elements?

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Some utilities prohibit a grid-tied gen from reverse power flow, no matter how small.

Further, some turbines and especially their gearboxes don't handle reverse torque very well.

Also, if the prime mover (engine or turbine) suffers a catastrophic failure, you'd want to de-energize the gen quickly.....the last thing you'd need is a gen to become a motor and apply torque to a broken engine or gearbox.

Are the prime movers piston engines?
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Old 05-30-2020, 07:05 AM   #12
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Originally Posted by micromind View Post
Some utilities prohibit a grid-tied gen from reverse power flow, no matter how small.



Further, some turbines and especially their gearboxes don't handle reverse torque very well.



Also, if the prime mover (engine or turbine) suffers a catastrophic failure, you'd want to de-energize the gen quickly.....the last thing you'd need is a gen to become a motor and apply torque to a broken engine or gearbox.



Are the prime movers piston engines?
Thanks for the perspective... good information. The reverse power flow from the utility is probably not an issue here. Their protection scheme has never tripped to my knowledge, except for some remote trips/intentional **** downs so they could de-energize the fault for repairs. This relay is entirely controlled by the generation facilty.

Yes, modified Caterpillar piston engines.

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Old 05-30-2020, 07:12 AM   #13
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This sounds very much like a vendor, or engineering issue.
You are correct, it's really more Engineering than anything, but it was the original design engineering firm that screwed this up, plus their not really interested in sending people to this site anymore. They seemed quite happy to share any information at all if I could just make this ten year long headache go away.

So, let's do some engineering, we do a lot more of that than most of us realize anyway.

Here's a couple of pics from my last visit to help explain the general scope of things.

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Old 05-30-2020, 05:10 PM   #14
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Originally Posted by joab View Post
You are correct, it's really more Engineering than anything, but it was the original design engineering firm that screwed this up, plus their not really interested in sending people to this site anymore. They seemed quite happy to share any information at all if I could just make this ten year long headache go away.

So, let's do some engineering, we do a lot more of that than most of us realize anyway.

Here's a couple of pics from my last visit to help explain the general scope of things.

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A PLC wizard could probably help greatly, problem is finding one. I knew one at one time, this guy was so smart he made people feel stupid just looking at them. He retired long ago, and may no longer be with us, that would be a shame.
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Old 05-30-2020, 05:32 PM   #15
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Originally Posted by joebanana View Post
A PLC wizard could probably help greatly, problem is finding one. I knew one at one time, this guy was so smart he made people feel stupid just looking at them. He retired long ago, and may no longer be with us, that would be a shame.
I've done my share of PLC programming, but actually the programming for these is very simple.

The part that I don't have is the concept and math of what needs to happen. If I can find the right guy who understands that, or keep reading until I figure it out, this site will become easy to support and I'll have another little chunk of knowledge in my head to figure out the next thing I run into.

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Old 05-30-2020, 05:54 PM   #16
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Originally Posted by joab View Post
I've done my share of PLC programming, but actually the programming for these is very simple.

The part that I don't have is the concept and math of what needs to happen. If I can find the right guy who understands that, or keep reading until I figure it out, this site will become easy to support and I'll have another little chunk of knowledge in my head to figure out the next thing I run into.

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Yeah, that's where the wizard part comes in. Phase angles, vectors, polar notation, event sequences, synchronization, etc. how, and where to apply that knowledge to the environment involved is pure wizardry. Some people are just blessed that way. Others just beat their head against the wall. Repeatedly.
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