Transformer Standby Losses

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PWDickerson

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Location
Clinton, WA
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Solar Contractor
Does anyone ever design a means of disconnecting a PV system at night to avoid transformer standby losses? The question came up on a recent project where we have multiple single phase 240V inverters feeding a 277/480V service. There is a 75 kVA step-up transformer in the system that consumes power all night long. It would have been nice to install a few SMA Tripower inverters and eliminate the need for the transformer, but due to an incentive structure that favors locally made equipment, we were limited in our inverter choices. Have others run into this issue?
 

Besoeker

Senior Member
Location
UK
Does anyone ever design a means of disconnecting a PV system at night to avoid transformer standby losses? The question came up on a recent project where we have multiple single phase 240V inverters feeding a 277/480V service. There is a 75 kVA step-up transformer in the system that consumes power all night long. It would have been nice to install a few SMA Tripower inverters and eliminate the need for the transformer, but due to an incentive structure that favors locally made equipment, we were limited in our inverter choices. Have others run into this issue?
Losses are likely to be in the order of 1% or lower of the transformer rating.
 
In my experience on larger and higher voltage systems, you never see return on investment for the system to manage the disconnecting. Often someone gets it in their head that those losses need to be minimized, but overlooks the associated cost. I'm involved in a job right now where a customer engineer wanted to modify the high voltage transmission design to eliminate the need for a medium voltage cap bank. $2 mil. for the t-line mods to save $150k on the cap bank.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
In my experience on larger and higher voltage systems, you never see return on investment for the system to manage the disconnecting. Often someone gets it in their head that those losses need to be minimized, but overlooks the associated cost. I'm involved in a job right now where a customer engineer wanted to modify the high voltage transmission design to eliminate the need for a medium voltage cap bank. $2 mil. for the t-line mods to save $150k on the cap bank.

This is where the engineer is supposed to do a cost/benefit calculation and go with the one that represents the lowest cost to his customer (whether internal or external), all other things being equal. Sometimes you can't afford the "elegant" solution.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Are you saying that a 100kVA PV system transformer will consume a kWh per hour at night?

Off load, yes thereabouts. I usually work on 0.7% for efficiency calculations.


I guess we are talking about roughly $6000-10000 of electricity over 25 years. Maybe more if I'm underestimating rates.
So if a 75kVA contactor on a timer costs a reasonable fraction of that, and will last the same 25 years, it's worth it?

I have no idea what the cost/benefit is likely to be, but I'd say the contactor should be a normally closed type in case the control circuit fails. And the timer better have an internal battery. And be set to be off for the minimum hours of night, i.e. summer.

A poorly engineered solution could easily cost more than it saves, depending on how if fails.
 
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Sahib

Senior Member
Location
India
Switching off the transformer has one issue: moisture condensation. So the industrial practice is not to switch off the transformer but to keep it on.
 
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