effects of high-impedance isolation transformer on utility billing

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malachi constant

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Minneapolis
I have a situation where multiple chillers arrived on site with a lower SCCR (5kAIC) than specified (65kAIC). Manufacturer/vendor is taking responsibility for it but they need me to engineer a solution that they will pay for. Current-limiting fusing doesn't resolve the situation so looking at an isolation transformer. I'm learning about these for the first time. I am told standard xfmr impedance is 6%, but custom are available up to 10%. Assuming this is the direction we go a custom transformer will work. What effect will this have on a utility bill - I had assumed inefficiency/losses associated the transformer, but will there also be potential power factor issues? Building is a typical mid-sized K-12.

For another option I am trying to find a contractor - one with a UL shop - with capability of taking the control panel back to their shop and rebuilding it as required to list it at the appropriate SCCR. We had a contractor do something like that ten years ago - I was hoping that was the first and only time I would face this issue, but here we are - and think that may be the more palatable solution here. Am I correct in thinking that this is an option? This project is in a different market (Chicago area) than the previous one so am struggling to find the right contractor.

I'm also chasing some leads on "current limiting reactors", which is an unknown thing for me - not expecting that to pan out but I'm desperate enough to be making the calls. For context we're looking at 208V systems with available ISC between 15-35k on 400A-1000A equipment. I'm not looking for someone to engineer it for me, but general thoughts on pros/cons/feasability of solutions are appreciated. Thanks!
 
I've been involved with this issue a number of times. In every case I've had with chillers the manufacture had the higher rating available as an option had it been ordered correctly. Had them come and modify the control panel to what it would have been if correctly specified at order. The had a field evaluation done by UL. I think this is the best way as you don't have all that extra stuff to install a waste power over time.
I could comment on why this happens so often, but let's just say it is a dirty business. Lot's of people to blame.
 
I've been involved with this issue a number of times. In every case I've had with chillers the manufacture had the higher rating available as an option had it been ordered correctly. Had them come and modify the control panel to what it would have been if correctly specified at order. The had a field evaluation done by UL. I think this is the best way as you don't have all that extra stuff to install a waste power over time.
I could comment on why this happens so often, but let's just say it is a dirty business. Lot's of people to blame.
Unfortunately they claim to not have any other SCCR options (at 208V) for this particular unit. This is their excuse for not publishing what their rating is when they submit their shops. We are holding their feet to the fire and they are accepting that. But I hate being the one having to clean up the mess. I've got about 40 hours of meetings and research and calcs in this already and haven't solved it yet.

I may end up having to say "I don't care if you don't offer it as a standard option, looks like you will have to offer it as a custom option with UL field evaluation".
 
Air-core current-limiting reactors won't have a huge impact on energy consumption, but you shouldn't be pursuing half-alkaline add-on kludges.

Why does 5kAIC even exist? It's only marginally adequate for single-family residential work.

Tell them to deliver what you specified. (and presumably, what they accepted when they bid on it) If they say no, remind them that there's more than one chiller manufacturer in the business.
 
Air-core current-limiting reactors won't have a huge impact on energy consumption ...
They would be huge physically, though.

... you shouldn't be pursuing half-alkaline add-on kludges.

Why does 5kAIC even exist? It's only marginally adequate for single-family residential work.

Tell them to deliver what you specified. (and presumably, what they accepted when they bid on it) If they say no, remind them that there's more than one chiller manufacturer in the business.
I totally agree.
 
An isolation transformer is a reasonable solution if you have the footprint available to place it. The 2016 DOE efficiencies of a transformer are pretty good and the effects on the utility bill will be ~1% or less irrespective of the impedance. The impedance and the efficiency are relatively unrelated.
 
What is your role in this project? If you are on the design side, I would be pushing back on the mechanical contractor as well as the manufacturer/vendor to get the issue resolved. I would not accept a solution that adds cost to the owner's operational budget because someone tried to save a couple of dollars by ordering the cheapest chiller they could buy that ended up not meeting the specification. I definitely would not be working for free for the manufacturer to fix their mistake.

If you are on the contracting side, if you ordered the wrong chiller it gets really messy to fix and you probably will be looking at all of the options you mentioned above with some concessions to the owner to cover the increased operational costs. If you tried to order the correct SCCR and nobody from the manufacturer told you that it wasn't available, I would be throwing a huge fit and again, would not be working for free to fix the issue.

An SCCR of 5kA, especially on a chiller, just means that the manufacturer hasn't tested the combination of parts that are in the controls. It might be time for them to get this particular combination tested/listed.
 
... The impedance and the efficiency are relatively unrelated.
Oh, D'Oh! I knew that!

Only the real (resistive) portion of the impedance contributes to efficiency loss, but both the real and the imaginary (inductive) portions contribute to current limiting.

(core hysteresis losses also contribute to the real fraction and the efficiency loss, but those are pretty minor if you're not overloading anything)
 
Now the question on the site is "can the utility do anything to limit available short circuit".

Sometimes you get a utility that is responsive, collaborative and nimble, but I am dealing with a large bureaucratic utility and a local utility designer/engineer that answers every question with "you need to fill out a service request form for that", which takes weeks to push through the bureaucracy. So for all practical purposes the answer here is "no the utility is not going to do anything to limit available fault current".

But theoretically, if one was working with a utility that was more of a team player, is there anything that can be done to help on their end? I have always thought of the available short circuit current as immutable...but to what extent is that true? Anything they could revise that would run under $5K that could help with the matter?
 
I have a situation where multiple chillers arrived on site with a lower SCCR (5kAIC) than specified (65kAIC).

I agree that the best option is to have the control panel brought to a UL shop as mentioned above.
But if you go with an isolation transformer to increase the source impedance, I'd have a transformer for each of your multiple chillers rather than one larger transformer that feeds all of them. Multiple smaller transformers which each provide a sufficiently low fault current to accommodate a 5kA SCCR should have a lower voltage drop, and when one chiller starts it will have less effect on the voltage to the other units.
Even if you could get the utility to provide a high enough impedance for 5kA SCCR units, you could have the same issues with voltage drop as would a single larger isolation transformer on the customer premises
 
Now the question on the site is "can the utility do anything to limit available short circuit".

Sometimes you get a utility that is responsive, collaborative and nimble, but I am dealing with a large bureaucratic utility and a local utility designer/engineer that answers every question with "you need to fill out a service request form for that", which takes weeks to push through the bureaucracy. So for all practical purposes the answer here is "no the utility is not going to do anything to limit available fault current".

But theoretically, if one was working with a utility that was more of a team player, is there anything that can be done to help on their end? I have always thought of the available short circuit current as immutable...but to what extent is that true? Anything they could revise that would run under $5K that could help with the matter?
If the utility gave you an available fault current on the secondary of their transformer that was based on infinite bus on the primary side, they may be able to give you a lower value based on the actual available current on the primary side. I would not count on being able to lower that value enough to fix your issue.

I would bet there is nothing a utility company would do for under $5K to help you. If you have a solution for $5K figured out, you have already spent more than that amount on this issue assuming your 40 hours of work in post #3 is accurate.
 
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