Sometimes to cover failures it is necessary to run a transformer above its rated output.
DNO transformers are rated for 125% summer, 150% winter. This is for cyclic loading as loads vary during the day.
A housing estate is liable to have low periods mid morning / mid afternoon / night. The peaks will depend on meal times and the television schedule. (Look out UKPN, Coronation Street is about to end!)
Whereas a business park will be high during the day, low at night.
Industrial the load is pretty constant. I can’t speak for other companies but we tried to keep loading to <70%.
Some plants like Topsy just grew and grew. I got a phone call, “can you check the loading on “C” plants LV feeders?” I knew they were trying to squeeze an extra 500KVA out the existing transformers for an extension to the plant.
Tx. 1 90% peaking at 105% during some operating conditions.
Tx. 2 95% almost continuous.
Tx. 3 50% fluctuating load.
Over the years bits and pieces had been added, uprated, etc, with not a thought to the supply. Now they want another 500KVA?
Tx. 4 was added for the plant extension and it gave us the opportunity to spread the loads about a bit.
Even with rearranging the loads in the event of a transformer failure we couldn’t carry on working. Bad planning and a short sighted outlook to my way of thinking, also the reason engineers never get on with accountants.
A fellow quarry electrician RoB pushed a 2Mva transformer at 130% continuous for 4months with no I'll effects, OK, he did hang a couple of fans on the cooling radiators making it ONAF cooling. The transformer was 45years old from an ex Mod site, apparently it’s still in service now. They don't build them like they used to.
I’ve only run them overloaded for a matter of days while repairs were carried out on an adjacent unit. Mind you one grunted when a 300HP compressor started DOL.
© Tony S