Generator Protection setting - key points

[NickParker]

What are the key points to be noted while performing Protection coordination for,
a) Single generator operation
b) Parallel generators (Islanded mode)
c) Parallel generators with Grid power (Cogeneration)

Any suggestions are greatly welcomed!

 

[wcaseyharman]

Typically I see differential, Overvoltage, V/Hz over frequency and sync check on most machines above 500kVA or so.
After that it depends on the size. Smaller machines relay on overcurrent, large grid connected would also include loss of field, distance/restrained overcurrent, reverse power, over/under frequency and occasionally out of step.
Other elements would be utilized for specific applications or prime mover types.

 

[wcaseyharman]

Oh, I forgot, typically larger machines also include negative sequence overcurrent and ground overvoltage.

 

[crshears]

In larger machines, depending on winding configuration, there may be split-phase protection used as well.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[FreddyNurk]

Single Set would generally be overcurrent, overload, undervoltage, overvoltage, underfrequency, separate overspeed as a minimum.

Parallel set would extend to reverse power as an absolute minimum. Sync check depends on the system, if there's any possibility of an operator closing out of sync then it gets included. When its integrated into the Set controls, not so much.

Parallel to grid, specifically from the gen point of view doesn't necessarily change much, but interconnect or mains protection is critical, such as ROCOF or Vector Shift to detect when the grid has disappeared and open the grid breaker.

Additional protection such as differential, out of step is often more a matter of cost and importance to protect the machine versus the additional complexity of the protection. I knew of one site out of thirty three islanded generator sites that had differential protection installed, and it was an MV site.

The additional protection required is a general function of both how much the asset is worth (not to mention the safety aspects...) and the increase in required protection the more complex the system becomes.Also consider that the purpose of the protection is not just to protect the machine(s) itself, but also the rest of the installation and wider network from adverse effects of the machine as well.

EDMS Australia

 

[cranky108]

Why does everyone want over current? It just does not coordinate well transmission systems. I have had to tell many consultants that, and they still want an over current.
Apparently they also have other issues, as the generation group seem to use a different consultant on every project.

 

[Sn00ze]

Why does everyone want over current? It just does not coordinate well transmission systems. I have had to tell many consultants that, and they still want an over current.

I've seen O/C element as a back up to guarantee Dead bus for reclosing eliminating the need for synch-check.

 

[dpc]

I would suggest looking at the chapter on Generator Protection in "Protective Relaying" by J. Lewis Blackburn. It's a great reference to have.

 

[wcaseyharman]

We use either 21 or 51V as a backup element to the transmission system. You can’t use a normal overcurrent because either you trip off during field forcing (and violate PRC-25 requirements) or the setting is so high it’s worthless. On small emergency systems though it seems like straight overcurrent protection is preferred.

There’s a really good book on generator protection by Donald Reimert, definitely the best and most thorough text on generator protection I have seen, would highly recommend it. Goes into excellent detail and includes information on excitation coordination.

 

[FreddyNurk]

Why does everyone want over current? It just does not coordinate well transmission systems. I have had to tell many consultants that, and they still want an over current.

Strictly speaking that doesn't really include the first two options asked about, only the third. I've also dealt with plenty of applications where overcurrent is used and coordinated on a per set basis as far as is practicable given the size of the load and the size of the generator. It's not always possible to coordinate but I'd still not consider not including overcurrent in those applications.

For reference, I'd never do a single site with just one generator, it makes it too hard to do servicing and maintenance, or deal with failures without an extended outage. There are probably cases where it's applicable but I don't work on those cases.

If connected to a transmission network, then yes, staying connected during specific grid conditions becomes important and there are alternate methods for ensuring the machine stays on for external faults (within reason) but trips off for internal faults as opposed to just overcurrent. But in a similar fashion, not everything is transmission connected either, and from the context I expect that transmission scale generation was not really within the scope of the question.



EDMS Australia

 

[cranky108]

"I would suggest looking at the chapter on Generator Protection in "Protective Relaying" by J. Lewis Blackburn. It's a great reference to have."
Yes I had my book signed by Blackburn himself. It is a good book, but as a person who must see that generation is coordinated with transmission, I have to tell the consultants that the over current will not work.

For lower voltage generation, over current does apply.