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Negative Seq Current Relay for a generator

Negative Seq Current Relay for a generator

Negative Seq Current Relay for a generator

(OP)
Is it necessary to provide Neg. Seq Current Relay for protection of all ranges of Generators when it is feeding a 3 phase motor load only.

Can we keep away lower ratings of generators from this protection?

   
  

RE: Negative Seq Current Relay for a generator

It used to be an expensive relay when it was a discrete type, but the feature is usually included in an integrated relay for generator protection along with other generator-specific functions like 51VR or 51VC so I'd probably ask "What do you save by not having it?".

Personally I'd say it is justified on anything larger than a couple of MVA, but others will no doubt have their own opinions. Your motor load should be well balanced and have minimal negative phase sequence components so omitting the function should not seriously compromise the machine protection, and any individual motor drawing a heavy imbalanced current should be taken out by its own protection relay before it becoems a threat to the generator.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Negative Seq Current Relay for a generator

(OP)
Thanks ScottyUK for your nice explanation. Was useful.

RE: Negative Seq Current Relay for a generator

I agree with Scotty's assessment of a break point at a few MW.
The mass to surface are, the diameter to length ratio, and the ability to reject heat, facilitated by the smaller size makes negative sequence protection less important.
I have yet to see negative sequence protection implemented on a diesel set.
In the newer sets with integrated protection relays, negative sequence protection would probably be used, "Because it's there" rather than a real need.
I always enjoy learning the differences between the large machines that Scotty works on and the small machines that I work on. size makes a difference.
What size range of machines are you concerned with, IndianTechEngg1001?

Bill
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"Why not the best?"
Jimmy Carter

RE: Negative Seq Current Relay for a generator

I used to work for a company that had a rather large generator rental fleet.  We applied a multifunction relay to units over 1 MW in many applications, and enabled the negative sequence protections on most of the units. Based on event records and some customer complaints over the years I feel it did protect our machines from some load induced problems.  I don't have positive data, but we did see an overall decline in the number failures for several years.

If it's a dedicated generator to a known load with downstream protections, maybe overkill.  In some applications I think it's beneficial, most of my experience is in the .5 to 10 MW range driven by recip engines.

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