If I wanted to make a reliable power supply, what is the best way to connect transformers?
I'm just curious, because I've been doing some reading into the subject after a recent power failure at my local hospital. Basically, the automatic transfer switch shorted out during a power failure, screwing up the generator and frying the critical areas UPS.
Anyway, the work involves replacing a single primary feeder and single transformer system with a dual feeder, dual transformer system for N+1 redundancy.
However, while there is no local code as to how these should be connected, there is an official department of health memo on power systems for healthcare facilities which does give some guidance. Of interest, it says "Where multiple transformers and feeders are provided, they should not be connected in parallel or in a spot network configuration. Transformers should not be operated as a "spare". Loads should be evenly distributed among all feeders and transformers."
It goes on in more detail about configuration, and the recommendation is that normally-open bus-ties are available for interconnection of redundant transformer primaries and secondaries.
I'm therefore curious as to why the guidance should explicitly state that interconnected networks should not be used. I know that the local power company uses triple-fully-interconnected N+1 spot-networks as the default pattern, at low, medium and high voltage. In their annual report, they state that they use this expensive topology to ensure maximum service availability in urban areas.
That said, the power company is installing a dedicated dual redundant non-interconnected pair of feeds, from a dedicated dual redundant high-voltage substation with dual HV feeders, so this is a departure from their normal network design.
I'm guessing that it may be due to fault containment - as with an interconnected system, you can get fault propagation to a wider area. Whereas with a segregated system, faults can be contained - and loads transferred onto other circuits.
As another interesting factoid from the memo. A specific warning is given about the use of monolithic automatic transfer switches - "These should be avoided, as they can constitute a single point of failure". Perhaps the electrical contractors that built the place, should have read the memo.
I'm just curious, because I've been doing some reading into the subject after a recent power failure at my local hospital. Basically, the automatic transfer switch shorted out during a power failure, screwing up the generator and frying the critical areas UPS.
Anyway, the work involves replacing a single primary feeder and single transformer system with a dual feeder, dual transformer system for N+1 redundancy.
However, while there is no local code as to how these should be connected, there is an official department of health memo on power systems for healthcare facilities which does give some guidance. Of interest, it says "Where multiple transformers and feeders are provided, they should not be connected in parallel or in a spot network configuration. Transformers should not be operated as a "spare". Loads should be evenly distributed among all feeders and transformers."
It goes on in more detail about configuration, and the recommendation is that normally-open bus-ties are available for interconnection of redundant transformer primaries and secondaries.
I'm therefore curious as to why the guidance should explicitly state that interconnected networks should not be used. I know that the local power company uses triple-fully-interconnected N+1 spot-networks as the default pattern, at low, medium and high voltage. In their annual report, they state that they use this expensive topology to ensure maximum service availability in urban areas.
That said, the power company is installing a dedicated dual redundant non-interconnected pair of feeds, from a dedicated dual redundant high-voltage substation with dual HV feeders, so this is a departure from their normal network design.
I'm guessing that it may be due to fault containment - as with an interconnected system, you can get fault propagation to a wider area. Whereas with a segregated system, faults can be contained - and loads transferred onto other circuits.
As another interesting factoid from the memo. A specific warning is given about the use of monolithic automatic transfer switches - "These should be avoided, as they can constitute a single point of failure". Perhaps the electrical contractors that built the place, should have read the memo.
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