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]

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.

Question- what is the theory behind this? Why the flash over?

 

[catserveng]

First off, here is a link to a nice and recent article about transfer switch types,

https://electrical-engineering-portal.com/contactor-circuit-breaker-transfer-switch

From my own experience, and I work on systems with ATS switches, multiple generators and critical systems all the time,
Feeding an ATS from another ATS is always a problem at some point. If the system is only going to be in place a few years you may get lucky, but if the system is really "critical", how much do you want to depend on that?

I would expect for you to get some really good answers first a bunch more questions need to be asked, such as,

Where is this system located, what codes must be followed?

Your drawings shows two utility sources and a bus tie, so does the system have both utilities in service and the tie open, or one utility service as primary, the other in backup and the tie normally closed? Has the timing of utility source transfer been accounted for in the ATS control timing sequences?

Does the "life safety" circuit actually involve life safety apparatus, and is it required to meet a 10 second (or as defined by local regulation) transfer time?

What kinds of loads are the "critical" ATS's feeding?

What kinds of loads are being fed from the three loads called "equipment" and what is their tolerance for not having redundancy in the emergency power supply?

Your drawing shows "Load Shed Controls" for Generator 2 but not for Generator 1 or no apparent interconnect between the two emergency sources controls, is that correct?

Have you accounted for the ability to perform regular maintenance, testing and repairs to the ATS switches, especially the ones connected to your "critical" loads? Are you planning on isolation bypass type ATS's? Or have you considered bypass maintenance circuits that allow you to completely isolate an ATS for service/repair?

Do you require Open or Closed Transition from generators back to utility? Do you require any form of load demand response, or the ability to do a closed transition transfer from utility to generators without load shedding or a system disturbance?

There are a large number of systems in service today with multiple generator emergency busses as David described above (at least in my area), and load shed/load add schemes to deal with starting and adding loads after the initial utility failure to being able to respond with the load shed scheme to drop loads if a generator fails while online.

How will you support regular testing of the generators? Do you plan on adding a spare (or defined load bank) breaker to each emergency bus? Do you plan on generator testing with site loads, or just dropping the generator leads and hooking up a load bank once in a while?

Have you developed a draft Sequence Of Operations for the power and emergency system first?

My 2.5 cents worth, hope that helps, MikeL.

 

[Mbrooke]

First off, here is a link to a nice and recent article about transfer switch types,

https://electrical-engineering-portal.com/contacto...

Thanks for the link

From my own experience, and I work on systems with ATS switches, multiple generators and critical systems all the time,
Feeding an ATS from another ATS is always a problem at some point. If the system is only going to be in place a few years you may get lucky, but if the system is really "critical", how much do you want to depend on that?

I would expect for you to get some really good answers first a bunch more questions need to be asked, such as,

Where is this system located, what codes must be followed?

Hospital, Article 517, rules for level 1 care.

Your drawings shows two utility sources and a bus tie, so does the system have both utilities in service and the tie open, or one utility service as primary, the other in backup and the tie normally closed? Has the timing of utility source transfer been accounted for in the ATS control timing sequences?

Load will be split between each bus, tie normally open.

It will be accounted for if chosen to be automatic.

Does the "life safety" circuit actually involve life safety apparatus, and is it required to meet a 10 second (or as defined by local regulation) transfer time?

It only powers hallway lighting, stair well lighting, elevator cab lighting, automatic door openers, fire alarm system and medical gas indicators.

What kinds of loads are the "critical" ATS's feeding?

Head well receptacles in intensive care units, cardiac intensive care, neonatal intensive care, emergency department beds, isolated power supplies in the ORs, patient room lighting, and select receptacle loads the blood/medication banks, nurse station computers, ect.

What kinds of loads are being fed from the three loads called "equipment" and what is their tolerance for not having redundancy in the emergency power supply?

Elevator motors, AC and AHUs for the: ORs, intensive care and emergency department. Med gas vacuum pumps. Just a few off the top of my head.

Your drawing shows "Load Shed Controls" for Generator 2 but not for Generator 1 or no apparent interconnect between the two emergency sources controls, is that correct?

Yes, and honestly, ignore the load shed. Come real world that might be eliminated.

Have you accounted for the ability to perform regular maintenance, testing and repairs to the ATS switches, especially the ones connected to your "critical" loads? Are you planning on isolation bypass type ATS's? Or have you considered bypass maintenance circuits that allow you to completely isolate an ATS for service/repair?

One reason (perk) behind the dual ATS is that if power goes out during the testing/repair of one ATS the other can still transfer automatically in 10 seconds for loss of utility.

Do you require Open or Closed Transition from generators back to utility? Do you require any form of load demand response, or the ability to do a closed transition transfer from utility to generators without load shedding or a system disturbance?

I'd like to have open transition.

There are a large number of systems in service today with multiple generator emergency busses as David described above (at least in my area), and load shed/load add schemes to deal with starting and adding loads after the initial utility failure to being able to respond with the load shed scheme to drop loads if a generator fails while online.

And what happens if the MCU fails? Breaker failure? Switch-gear down for service?

How will you support regular testing of the generators? Do you plan on adding a spare (or defined load bank) breaker to each emergency bus? Do you plan on generator testing with site loads, or just dropping the generator leads and hooking up a load bank once in a while?

Site loads when there is no surgery and staff are aware in order to exercise the ATSs. Load bank off the emergency bus if site loads are not enough.

Have you developed a draft Sequence Of Operations for the power and emergency system first?

My 2.5 cents worth, hope that helps, MikeL

Working on it. But as of right now is basically evaluating various designs, ie single ATS, multi ATS, paralleling, ect.

 

[waross]

Thanks Mike.
The ASCO is another breed.
The few that I have seen have a load contact that moves between the normal and standby contacts.
The clearances are quite small.
That is no problem when an in phase transfer is done.
The ASCOs that I have seen had a sync check relay on the transfer from standby back to normal power so that is not an issue.
The small clearances have always worried me in the event that an out of phase transfer is allowed.
There may be a possibility that opening under heavy load, that the moving contact may draw an arc across to the other live terminal.
This may happen if a single phase loss causes a transfer with one normal phase still energized.
As I said, I would ask the ASCO rep for his comments.




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

 

[catserveng]

Bill,

ASCO's range of transfer switches has become very broad, especially after being bought by Schneider

https://www.ascopower.com/en-us/products-catalog/products/power-transfer-switches/

They are pretty popular around here, but in most truly critical applications I see more ATS products from Russelectric (Siemens just bought them not too long ago). GE Zenith (recently acquired by ABB)also has a larger presence out here as well.

I used to do a lot of work with Thomson ATS products, made in Canada, nice equipment and well supported, but they also got bought out by a larger company and have not quite been the same since.

If you look recently all of the main players in the ATS marker have been bought out by the big boys in the electrical industry, not sure if that is a good thing or not, guess we'll see.

Most all the newer ATS controllers are very capable, have a lot of features for voltage and frequency monitoring, most have sync check built in and ready to be enabled, most have communications and interface capabilities to tie into switchgear or BMS control systems. Some even have a pretty nice event and datalogging feature. Most of the issues you are concerned about can be taken care of with proper setup of the ATS controls, only real downside is that the new controllers have so many features that most service calls I get on newer ATS's is that they were not properly setup for the installed conditions.

This post hits a fairly raw nerve for me, have a site near me that I have serviced (actually mostly tried to service) for the last 5 years, as the site expanded they ended up with a cascaded ATS system, mostly two but in one circuit three ATS's on the Normal side of the circuit. Have been trying to address some overheating and other issues on some of their most critical ATS switches for quite some time, always same reply, circuits too important to take down for repairs. On Friday afternoon as I was driving back from a job site in the desert I got a panicked phone call that the utility power was out and that their big money UPS was dying, after slowly getting thru traffic I finally got to site, the second ATS in a two ATS string had failed, control power transformer had blown and did some resultant damage. We were able to manually transfer but I needed to isolate the ATS to make a proper repair. Site CEO went off on my like a bomb, screamed, yelled, used a lot of words I probably shouldn't type here, and when he ran out of breath I dragged his ass to the maintenance office and showed him 5 years of job tickets and emails telling him that his ATS switches needed attention. About that time the "Risk Manager" for the site showed up and asked me to leave. I informed him I'd be happy to leave but to be aware that the ATS was "stuck" in the Emergency position, and the utility had been restored, and as of now he was eating up maintenance run time on his emergency generator (and our air district is REALLY stingy with maintenance run time). I was able to find some parts from an ATS that was recently removed and got the system working again, but I was clear that this was a patch job and that a proper repair and service still needed to be completed. At that point the CEO started off again, I left site, will likely not be paid for my work onsite, and this "critical" power system will still be at risk because it is "too important" to be regularly tested and serviced or properly repaired.

To the OP, ATS switches fail, maybe not right away, some types are better than others, but at some point they don't work like they are supposed to, and in the current safety environment, working on a live switch is pretty much impossible to do safely. So please plan for testing and for regular maintenance if you can.

MikeL.

 

[Mbrooke]

Wait, I'm confused. Wouldn't the bypass has let you rack out the ATS portion?

 

[Mbrooke]

And speaking of lots of ATSs- anyone knows whats going on here? It sounds like they have over 20 ATSs (listen to all that banging)

https://www.youtube.com/watch?v=117gLyWQtwU

 

[catserveng]

Bypass functions are not standard in an ATS, they cost a lot more than non-bypass ATS switches, in the total ATS population I deal with less than 20% have a bypass feature of any kind. So unless you specify a bypass/isolation type ATS and convince whoever has the purse strings that it's worth the extra cost, you may have to deal with the issues discussed later.

MikeL.

 

[Mbrooke]

Understandable.


In your experience, is it better to have many smaller ATSs or a few giant ATSs. Half the designs I see have something like a 2000amp ATS in the main electrical rooms, others I see have 200amp ATSs scattered throughout the electrical closets.

 

[catserveng]

Depends on your connected systems, in data centers we see lots of smaller ATS switches feeding dedicated circuits, in more industrial settings you may only have 1 or 2 ATS switches. In a place like a hospital or care facility there will likely be at least three, one for life safety, one for emergency circuits and one for auxiliary equipment.

Some sites don't use any ATS switches, instead use a generator or multiple generators feeding directly to the main switchgear interlocked with the utility service, lots of ways to do it, just need to develop a desired operational sequence and then see what equipment works best for your facility or system.

MikeL.

 

[Mbrooke]

Breakers more reliable then ATS by chance?

 

[waross]

One thing that I saw a lot down south.
Undersized transfer switches.
Remember, the utility side of the transfer switch may have to carry the whole load.
I saw a lot of smaller sets where the owner knew that he could not re-start all of his A/C units on the generator.
I was seldom consulted before a set was bought.
I didn't get involved until the cheap guy had messed up the installation.
One of the issues that I had to deal with was a transfer switch that was big enough for the standby generator, but less than half the capacity needed to carry normal loads fro the grid.
This may be an issue if you are using load shedding.

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

 

[crshears]

Late to the party...

A more mechanical point [sorry, Bill]: don't get drawn into the ugly vortex of underloading Diesel-engine-driven back-up generators for any significant length of time as, depending on design, they may run "cold" and foul up as a result [I mention this because the chronic issue I've encountered has been oversizing the B/U generator].

The transfer schemes I dealt with were mostly manual; the first layer of defense was/is for the B/U generator to auto-start and supply all critical loads, with the intent that staff would be immediately dispatched to the site to monitor the operation of the B/U system on a continuous basis, correct any issues as they arise, perform manual transfers to other sources in the event of contingency, see to ordering in more fuel ad hoc, etc., etc., plus that famous catch-all, ODAR [other duties as required].

CR

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

 

[waross]

Welcome CR
I remember one: It peaked at about 25% load. It often ran for 18 Hrs per day at 10% of rated.
BUT
When that one big motor started, the set blew black smoke, and leaned into the motor mounts.
The lights in the plant dimmed as well.
A standby generator lives a tough life.
Some go from stopped, cold to 150% block loading in under 10 seconds.
Thank goodness for UFRO.

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

 

[Mbrooke]

UFRO?

 

[catserveng]

Under Frequency Roll Off, in most AVR's today called Volts/Hz setting.

Its a setting that uses a "knee frequency" setting that then allows a ramped reduction of voltage based on the actual generator frequency.

So if you have a knee frequency set at 59.5, the AVR will control voltage to a constant level as long as the generator frequency is above the setpoint, if the generator frequency drops below the knee frequency setpoint, the voltage output will ramp proportionally to the drop in frequency. In most AVR's a "single volts/Hz slope is 6 volts per Hz. Newer digital AVR's typically allow you to make a pretty broad range of adjustments to suit your specific site needs.

The purpose of this settings is to allow a faster recovery of a genset during a load transient.

Hope that was clear.

MikeL.

 

[FreddyNurk]

I'm curious, what is the advantage of a 'transfer switch' over standalone breakers and synchronising controls?
I've never had to deal with ATS, all my applications have been prime power systems rather than transfer to and from mains.

EDMS Australia

 

[waross]

Price and interlocks.
Gen set manufacturers typically buy the transfer switch components in bulk at very good prices and pass the savings on as an incentive to buy their gen-set.
If you buy the transfer switch with the gen-set, adding a transfer switch to the order will cost less than you can buy the components.
On the other hand, if you have to order a standalone transfer switch, the pricing is much higher and it is often cheaper to "Roll your own".
I have gone both routes.
I have ordered special transfer switches at very good prices.
One order was for four occupancy and four utility meters on one transfer switch.
Another customer acquired a generator but no transfer switch. It was cheaper to buy the components and build an ATS.

Another advantage of the ATS is the mechanical interlock between the breakers or contactors. Sometimes difficult to add in the field.
Some customers may require an approved assembly rather than a "Roll your own".

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

 

[crshears]

I'd imagine the choice to "roll your own" requires joint agreement...

UFRO: seems to be the corollary of the way VFDs are programmed to maintain a specified v/Hz ratio...

CR

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

 

[Mbrooke]

In other words UFRO prevents saturation of transformers, ballasts and motors during heavy loading? Does it work as the gen is starting?