Two emergency generators 3

[Mbrooke]

No clue where to even begin.


Do I start both generators upon loss of utility? Or let gen #1 start and if it fails to do so start gen #2 after 10 seconds? How do I prevent the ATSs from "fighting" one another?

 

[Mbrooke]

Bigger single line:

 

[waross]

I would consider a few seconds time delay on ATS 2-A, ATS 2B and ATS 2-LS on the initial power failure to avoid an ATS race.
This would be on initial start only.
On a later failure of Gen #1, transfer could be immediate.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

But still start both gensets, correct?

 

[davidbeach]

Oh my gosh, that’s lots and lots of complexity. I’d have the two generators connected to a paralleling bus. Start both on the outage. First speed and voltage gets connected to the bus and the LS ATS closes in; Critical also if capacity exists. Then the second unit synchronizes with the first and the rest of the ATSes close. Could the be a whole lot fewer of the ATSes.

 

[Mbrooke]

What are the benefits of paralleled gen sets? Seems more complex and more that can go wrong to me.


That and the fact most paralleling applications parallel into a single bus presenting a single failure point. Also having multiple ATSs is of benefit in case on sticks.


Of course I am open to anything that will change my mind.

 

[itsmoked]

I agree pretty complicated but also more robust. Parts and pieces can be dropped as needed. With more flexibility you have more options if circumstances require it.

Does this installation actually exist or is it a future thing? It does have cancelled stamped on it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Mbrooke]

Its being considered for a new project. Similar versions are out there and do exist. Ignore the cancelled overlay, thats just one version of it LOL.

 

[davidbeach]

Weakest link in most emergency/standby systems is the ATS. The likelihood of a generator bus being unavailable while the normal source is out is far lower than the probability of an ATS failing.

 

[waross]

Yes. Allow both sets to start normally and to service their normal loads.
Speculation on the development of the control scheme:
1. The simplest solution, one generator and one transfer switch.
2. It was desired to service the critical loads and the life safety load with a dedicated generator. This may well be a regulatory requirement.
Solution: Two generators and two transfer switches.
3. While the generators had the capacity to service the loads, they would not support block loading of the entire loads.
Solution: Multiple transfer switches with staggered closing.
4. Someone in authority desired more redundancy on the critical and life safety circuits.
Solution: Install transfer switches to connect the critical and life safety load to the second generator in the event of the first generator failing. Note, these circuits have already been split to avoid 100% block loading. Hence three transfer switches are needed. This arrangement has the advantage that block loading of the second generator may be reduced by staggering the switch closures.
5. Issue. The combined loads now exceed the capacity of the second generator.
Solution: Install a load shedding panel. There are already multiple transfer switches on the second generator and these may be used to selectively drop feeders to free up capacity for the critical and life safety circuits.
And the last issue.
6. There may be an issue if generator #2 starts faster than generator #1.
Solution: Add a short time delay to ensure that generator one has enough time to come online before the transfer to Generator #2 is enabled.

With respect for David's suggestion.
That is a good solution for many industrial loads.
However I suspect that regulations in regards to critical and life safety circuits may have required the present scheme.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Weakest link in most emergency/standby systems is the ATS. The likelihood of a generator bus being unavailable while the normal source is out is far lower than the probability of an ATS failing.

Yup, hence the idea to have two in series, along with replicating the critical system twice (so in essence 4 ATS would need to fail). Followed by an MCU in parallel application IMHO.

Yes. Allow both sets to start normally and to service their normal loads.
Speculation on the development of the control scheme:
1. The simplest solution, one generator and one transfer switch.
2. It was desired to service the critical loads and the life safety load with a dedicated generator. This may well be a regulatory requirement.
Solution: Two generators and two transfer switches.
3. While the generators had the capacity to service the loads, they would not support block loading of the entire loads.
Solution: Multiple transfer switches with staggered closing.
4. Someone in authority desired more redundancy on the critical and life safety circuits.
Solution: Install transfer switches to connect the critical and life safety load to the second generator in the event of the first generator failing. Note, these circuits have already been split to avoid 100% block loading. Hence three transfer switches are needed. This arrangement has the advantage that block loading of the second generator may be reduced by staggering the switch closures.
5. Issue. The combined loads now exceed the capacity of the second generator.
Solution: Install a load shedding panel. There are already multiple transfer switches on the second generator and these may be used to selectively drop feeders to free up capacity for the critical and life safety circuits.
And the last issue.

Yup- all to the above (except regulations, they can be satisfied with one gen, 3 ATSs and 3 systems (life safety, critical, equipment). In addition to that both gens can be placed in different parts of the building, ie Gen 2 in the basement gen 1 on the 1st floor or even penthouse.

FWIW the actual application may end up ditching the load shedding panel and go to a 100% rated #2 like #1.

6. There may be an issue if generator #2 starts faster than generator #1.
Solution: Add a short time delay to ensure that generator one has enough time to come online before the transfer to Generator #2 is enabled.

This is where I stumble. A delay would work, BUT- it may violate the 10 second rule. Either that live with multiple blinks- ie load transfers to gen 2, then to gen 1 after the first outage.

With respect for David's suggestion.
That is a good solution for many industrial loads.
However I suspect that regulations in regards to critical and life safety circuits may have required the present scheme.

Indeed, and even if the regs don't call for it its stil a good idea. Consider some data center do 3x replication of the emergency system. Why not take the approach to other applications where the end cost is incalculable?

 

[davidbeach]

The data center I'm most familiar with, and not my design, starts four generators to carry a load normally supportable by two generators. All parallel into the load using the normal in-house distribution system. The design obviously assumed the probable need for the generation was loss of utility service. Once all four are up to temperature with no issues the lowest priority unit is shut down; now three carrying the load that two could carry. If one of the three fails the fourth is restarted to replace the failed unit. If the load ever gets down low enough that only one would be sufficient to carry it the lowest priority unit of the three running is shut down leaving two.

If you want to account for the possibility of internal cable failures, rather than just utility loss of supply events, you need to push the supply redundancy as close to the load as possible. Using ATSes that could mean a whole lot more than what you've shown. More ATSes, more single points of failure; fewer ATSes, a different set of more single points of failure.

Does loss of supply to a single ATS represent a failure of the system between a common point and the ATS or does it represent a fault downstream of the ATS that will also trip the standby source once it connects?

There may not be a single "right" answer. For a single building facility, loss of utility supply is probably more likely than loss of supply to a single ATS. For a multi building campus the odds of loss of supply to a single ATS increase. Approaches for each will differ.

 

[Mbrooke]

I will respond to the above, but page 31 forward in the slide is how I'm viewing it:

https://www.slideshare.net/michaeljmack/standby-power-systems-for-hospitals

 

[Mbrooke]

The data center I'm most familiar with, and not my design, starts four generators to carry a load normally supportable by two generators. All parallel into the load using the normal in-house distribution system. The design obviously assumed the probable need for the generation was loss of utility service. Once all four are up to temperature with no issues the lowest priority unit is shut down; now three carrying the load that two could carry. If one of the three fails the fourth is restarted to replace the failed unit. If the load ever gets down low enough that only one would be sufficient to carry it the lowest priority unit of the three running is shut down leaving two.

I've seen and heard of data centers that will actually replicate the backup system 2 or even 3x. To me what you describe is still one system with multiple single failure points and many common mode failure points.

Also, are these generators separated from one another? What if one caught fire or exploded? Will the others still work from the debris getting in the fans/belt/ect?

If you want to account for the possibility of internal cable failures, rather than just utility loss of supply events, you need to push the supply redundancy as close to the load as possible. Using ATSes that could mean a whole lot more than what you've shown. More ATSes, more single points of failure; fewer ATSes, a different set of more single points of failure.

Selective coordination is a must, always.

Its not uncommon for large buildings to have several dozen ATSs scattered through out the electrical closets on each floor which economically works out well.

When its a 3 source ATS type deal, its possible to route each emergency riser in different parts of the building's core converging only at the ATS location. Each emergency riser coming from a different gear and gen room (assuming 1 and 2 are located in different locations as my desire).

Does loss of supply to a single ATS represent a failure of the system between a common point and the ATS or does it represent a fault downstream of the ATS that will also trip the standby source once it connects?

There may not be a single "right" answer. For a single building facility, loss of utility supply is probably more likely than loss of supply to a single ATS. For a multi building campus the odds of loss of supply to a single ATS increase. Approaches for each will differ.

I'm a bit confused here. I think I know what you mean, but I want to make sure I have it right.

 

[waross]

Clarification. Each generator starts normally in the event of a supply failure.
Only the transfer on the failure of generator #1 is inhibited.
A failure to start of generator #1 may be considered a second failure.
If 10 seconds is allowed to restore power after a failure I would argue that 15 or 20 seconds delay is appropriate to restore power after two consecutive failures.

The quality of the transfer switch is important. Not all are created equal.

There was a brand of manual transfer switch around years ago that had a complicated, sealed, internal linkage that was very badly designed and prone to early failure by jamming.
I replaced three of them.
I saw a couple more on scrap heaps that had been replaced by others.
All of them were less than six months old.
The only one that I didn't replace was brand new in the box.
When I opened the box, I tried the operation and it was jammed, new from the factory.
I refused to install it.
We used to see a lot of that in Central America.
When a product was not salable in North America for any reason it was often sold to distributors in Central America.

Just saying;
Quality matters.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Which ATS would you recommend? My money is on ASCO, but admit my experience is thin.

 

[waross]

What size are your generators?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iceworm]

I asked that too:
400KW on the Crit/LS
1200KW Equip feeds

Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

I haven't worked on transfer switches for a decade but from what I remember:
1. The most common and the most dependable were the transfer switches based an reversing motor starters.
Motor starting contactors are very dependable and are designed to take motor starting surges and transients.
This makes them very suitable for the block loading often associated with standby generator service.
Failures:
1. I saw a couple of sets where a "not quite ready for prime time" contractor installed a generator and transfer switch for a home with a lot of DOL A/C. By DOL in this case I mean simple thermostat control. No starting time delay. When the power was applied, the A/Cs all immediately started, or tried to.
This overloaded the gen-set and the voltage dropped to the point that the contactor dropped out. The engine regained lost RPM and the voltage recovered. The A/C starting load again pulled the voltage down and the contactor dropped out.
Repeat.
I saw two installations where the transfer switch contacts were burned to the point of no contact within a few minutes of the first power failure.
This is not an issue here. I've followed your posts for a long time and I know that you know better than that. If you are using load shedding you are aware of possible overloads. You may want to give your load shedding scheme one more review.
As I type this I am thinking;
"You've probably already done that."
Second failure: Chronic low line voltage and contactor coil burn out. These were very basic transfer switches.
A good quality transfer switch will have voltage monitoring for each phase and will transfer to back-up if any phase voltage drops that far.
If large motors are being fed through the switch it is well to introduce an open delay on re-transfer to avoid motor transfer from out of phase re-closure.
All in all, I have been happy with contactor based transfer switches.

2. Some transfer switches are based on switches or circuit breakers operated by one or two electric motors.
These are as dependable as the components. Quality matters.
They are slow acting compared to other types.
That may be an advantage as it inherently allows enough time for motor residual magnetism and back EMF to drop to safe levels.
Another advantage is that many of these transfer switches may be operated manually in the event of a failure of the operating mechanism.

3. ASCO. As I understand the ASCO mechanism, a solenoid gives a mechanical impulse to the mechanism and inertia carries the mechanism through to the other position.
These switches are very fast. When transferring from a standby back to grid power they must synchronize before operating.
An unsynchronized transfer may severely damage large motors.
I don't believe that they have an off position where both normal and standby sides are open other than for less than 100 milli-seconds during the actual transfer.

ASCO offers a closed transition ATS.
You should consult with the supply utility before considering the closed transition model.

If the standby set is running very close to 60 Hz it may take a long time to drift into sync before re-transferring from standby to grid power.

I don't know, but in your position I may be moved to ask the ASCO rep, in writing, to be answered in writing;
How does the transfer switch perform in the event of the loss of one or two grid phases.
Given the very small clearances I am concerned about possible flashover in the event of an out of phase transfer when one pole of the switch is still hot on both sides.

All in all, though, the ASCO switch seems to be a good switch.
If anyone out there has a different experience with the ASCO ATS. please share with us.
I have not seen a lot of ASCOs.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

I know contactor based ATS are common in the IEC world, but are the legal in North America?