Distributed Generation Anti-Islanding
Distributed Generation Anti-Islanding
(OP)
I'm currently involved in evaluating a distributed generation proposal for a utility. It includes 600kW of 1-ph synchronous machines connected to a long 1-ph line at 16kV (p-n). This line is protected by a 1-ph recloser.
The gen owner is proposing the normal set of protection (overcurrent, under/over voltage, under/over frequency).
For anti-islanding, as required by IEEE-1547, nominally the installation should provide transfer trip to the local recloser as the size of the generation is around the same size as the load supplied from the recloser.
The customer is instead proposing to set the generation to run at unity power factor and to use reverse VAr sensing to trip the generation if any VArs are output from the generation to support local loads (normally the VArs would be provided by the distribution system).
Has anyone seen reverse VAr sensing used for anti-islanding?
It seems like a simple approach that avoids the need for transfer trip capabilities. Can you see any drawbacks to this approach?
The gen owner is proposing the normal set of protection (overcurrent, under/over voltage, under/over frequency).
For anti-islanding, as required by IEEE-1547, nominally the installation should provide transfer trip to the local recloser as the size of the generation is around the same size as the load supplied from the recloser.
The customer is instead proposing to set the generation to run at unity power factor and to use reverse VAr sensing to trip the generation if any VArs are output from the generation to support local loads (normally the VArs would be provided by the distribution system).
Has anyone seen reverse VAr sensing used for anti-islanding?
It seems like a simple approach that avoids the need for transfer trip capabilities. Can you see any drawbacks to this approach?






RE: Distributed Generation Anti-Islanding
Others invited to comment here...
regards, rasevskii
RE: Distributed Generation Anti-Islanding
Furthermore, the utility has the final say on the power factor that you operate at. This can change. Should the utility demand that you export VARs as a condition of renewing a contract, you will be dead in the water.
Run the machines in droop mode and use over frequency trips as most others do.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Distributed Generation Anti-Islanding
rasevskii
- the machine protection is supposed to operate before the recloser closes back in (i.e. machine is dropped if the recloser operates)
waross
- in this case the generator is obliged to operate between .9 lagging and .95 leading (the utility doesn't actually have a say in this according to the regulator) so planning on operating at unity PF is an option.
- I was under the impression that over/under frequency protection was inadequate for anti-islanding protection (in cases where the islanded load may be similar to generator output)
- I like your point about VAr transients. I'm guessing it wouldn't be easy to quantify how large these would be.
RE: Distributed Generation Anti-Islanding
RE: Distributed Generation Anti-Islanding
It would be necessary to install voltage sensing at the recloser to block reclosing if the voltage on the load side of the recloser was more than a few percent of nominal.
Undervoltage tripping of the generator will probably be more reliable than "reverse" VAR flow detection (it isn't really reverse) for line faults.
David Castor
www.cvoes.com
RE: Distributed Generation Anti-Islanding
RE: Distributed Generation Anti-Islanding
ROCOF-df/dt
voltage jump
U1< PPS undervoltage.
Of course option of voltage sensing at the recolsere is very good option.
Best Regards.
Slava
RE: Distributed Generation Anti-Islanding
I suppose that these are some sort of biogas units in a rural setting in view of the single phase line. An interesting challenge indeed. The timing on the reclose is usually less than 2 seconds, not time enough for the frequency or VAR protections oo each unit to clear, as said above.
Why is the supplier afraid of supplying VARS? They are almost free, actually. Just have an AVR working on droop as suggested.
Tell us more...
regards, rasevskii
RE: Distributed Generation Anti-Islanding
rasevskii
- good guess! - it's rural biogas
- The one reason to run at unity power factor is that it prevents excessive voltage rise at the generator end of the feeder. If they export VArs it tends to increase the voltage to the point where there is overvoltage under light load conditions or the potential for flicker issues as the generation starts/stops.
davidbeach
- good point. I'm not sure what control mode they have in mind for the generators. I'll find out.
slavag
- those are certainly more conventional options for anti-islanding.
The generator claims they can set the undervoltage/over-frequency/under-frequency to trip in 0.4 - 0.5 seconds so the claim is that this will take care of recloser timing issues. I agree with the comments that this is questionable / potentially unreliable.
I think they threw in the 'reverse-VAr' sensing as anti-islanding so they can try to avoid the cost of a tranfer trip arrangement.
I think I'll go back to the engineer for the generator and have a chat.
RE: Distributed Generation Anti-Islanding
Or eliminate reclosing.
David Castor
www.cvoes.com
RE: Distributed Generation Anti-Islanding
Attached is a useful document on anti islanding and a discussion on the advantages and disadvantages of each type of protection.
Regards
Marmite
RE: Distributed Generation Anti-Islanding
RE: Distributed Generation Anti-Islanding
Perhaps there should be an independent overcurrent/undervoltage/etc electromechanical protection to clear the generator off the line if it gets really ugly, electrically speaking.
Our friend Catserveng hopefully can add some input here...
Has the genset supplier done this before somewhere and what were the results..
rasevskii
RE: Distributed Generation Anti-Islanding
Does the recloser have a sync check function?
Do the machines have to be synchronus? Do they need to be able to island or run isolated? Is there a future requirement for the units to provide voltage support? Can they be induction machines instead?
What kind of governing? Will they have real power control or small units leaned against the grid in droop?
What kind of voltage regulation? Does the regulator have a built in PF/VAR control mode, will it operate in voltage droop, or take an interface from a PF/VAR controller?
I've done a fair number of this type project, never found a one size fits all solution. Unit size, what kind of installed controls and protections, site load requirements, line voltage stability, fuel quality, maintenance practises, utility requirements all had impacts and each site ended up with unique problems. Sorry for all the questions, but I usually ended up on projects like this after they were installed and things didn't work and I was looking at a piece of damaged equipment (and a gang of upset people).
Great discussion, and you're getting lot's of great comments, but without more details on what you're putting on the ground and what you actually expect it to do, it's hard to provide a precise solution.
Marmite, great paper! Wish I'd had that a few years ago.
RE: Distributed Generation Anti-Islanding