offline overcurrent protection

speedtronic · Jan 7, 2009

What is offline overcurrent protection for synchronous generators? How it works?

Guest123 · Jan 7, 2009

Hey.

Ok, First of all a synchronus generator is 1 generator out of a bank of generators connected to provide power to a "grid" for large electrical supplies i.e music concerts, big fairgrounds etc.

You use a system like this due to the large power usage - one on it's own just wouldn't cope and would probably (if you could find one) be the size of a third world country.

Now offline overcurrent protection is only used when you have a "1 in 1 out" bank system e.g 10 generators where 1, 3, 5, 7, + 9 would be supplying power to the grid, and 2, 4, 6, 8, +10 would be idleing say. Now when the "even" geneartors are idleing they are still connected to the system but only the "odd" ones are supplying the power and running at speed. This alternates at pre programmed intervals which basically is more efficient and easier on the generators themselves. Offline overcurrent protection protects the "idleing" or "offline" generators in the event of overloading by preventing the increase in current demand being placed on these "idleing" generators via a series of A.V.R'S and overcurrent relays as this would almost certainly turn the alternators and engines of the offline geneys into molten lumps of steel.

I hope this make sense to you.

Cheers.

speedtronic · Jan 8, 2009

Hello, thanks for your answer. As far as I know ideling means FSNL(full speed no load) which means that Gen is not connected with the system but its excitation system will be on. Then how it can be affected by these overloads, only Gen connected to power grid should be affected by this.
To me offline means that Gen(turbine) is on Turning gear.

Guest123 · Jan 8, 2009

Hi.

I've always used the term "ideling" to mean that a geney is sitting in circuit but only at tick over, not at full speed so if an overcurrent situation arose at the point where the banks were due to changeover but before the excitation sequence had started the offline protection would stop all the loading being "dumped" on the "idleing" geney before it was at full speed and damaging it.

The simplest way to think of it is driving your car - if you "drop" the clutch whilst in gear with no revs i.e engine idleing, then the car will stall. But if you drop the clutch with high revs the car takes off, its the same principal.

Offline protection merely stops the "stalling" aspect happening to the generator, or at least this is my take on it anyway.

Hope this helps mate, these are pretty specialised questions what do you do?

Ben.