Rotary UPS really make most sense if you want 'cost no option' uninterrupted power, as even when not used, there is a lot of energy and maintenance require to keep tons of metal spinning on the off chance there is a power cut.

traditional gensets take time (seconds to tens of seconds) to start, perhaps longer if they need to be synchronized with other sources, and if not used for a long time occasionally do not start, so while less effort do need a regime of periodic testing, including things like is the fuel still OK, is the cranking battery holding charge etc.

Static UPS (a big inverter) can give glitch free handover,  and may be undersized a bit if used either just to cover genset spin-up time, or larger if to be used for longer periods with very large batteries.  Attractive though htis sounds, batteries do not last for ever and are considerable capex and space.

It is important to know how much and what loads are to be shed totally (try and maximise this), interrupted briefly (many things), or maintained without glitch (try and minimise this). This load distribution drives exactly what level of supply protection is needed. It is fair to say it is sometimes not done quite right...

Mike.

Thanks Mike, I appreciate your response. Regarding the issue of the generator failing to start after being idle for a long time, we conduct annual load banking tests, but they incur a high expense as we have to hire a backup generator in the event of a power failure during the 2-3 hour testing period. Furthermore, there is a lot of planning involved in obtaining the permits for the work to be carried out, etc. I am not familiar with the rotary system, either. Does it also require annual load banking?

Well gensets do not require annual load bank tests. I assume your organization, or rather their insurers, have decided that is required.

We have gensets and UPS, and the gensets are run (by effectively removing power to the site, or at least parts of it) on a regular basis. I think we now also have a fuel polishing loop, where the tanked fuel is pumped out filtered and put back, less any water and rust, but I'm none too sure of the detail

A DRUPS has a lot of the same elements, except that the maintained load is effectively on genset all the time, as the generating part spins non-stop - being in effect spun by motor from the external supply. The key point is that there is nothing to spin-up, in fact energy in the already flywheel starts the diesel.

However that is a heavyweight solution, being a motor and gensst and diesel, so probably needing its own hanger for all but the smallest installation. I'm not sure of the smallest rotary UPS, but I do not think I have ever seen one below 1MW - containerised ones are available - I have never had anything to do with one, though these chaps claim 100kW is possible

But all the same sort of regular checks of the diesel part are needed, and it is further complicated by the fact that you cannot stop it turning without losing your UPS,

Mike.

Surely a simpler way to test the system is to start the generator, synchronise to line, then increase the excitation voltage until the generator is nearly fully loaded. Quick, functionally realistic, value for money, and importantly is not a waste of our precious carbon resources. 

Surely a simpler way to test the system is to start the generator, synchronise to line, then increase the excitation voltage until the generator is nearly fully loaded. Quick, functionally realistic, value for money, and importantly is not a waste of our precious carbon resources. 

Many (maybe most) standby generators are specifically not allowed to synchronise, there are interlocks etc. But this may allow them to be run off line, not interrupting normal operations. Also very few installations will allow a full load test (or anything like it), it has to simulated with a load bank. When it's important enough the load bank will be permanently installed and switched in as required (e.g. data centres where big money is involved).

On ships, drilling rigs and many other applications emergency generators can automatically be synchronised to line and loaded up. Automatic load shedding or power limiting (e.g. - variable speed drives) systems then matched the load to the generator power available. This was common practice in the back in 1970. The emergency generator can then be shutdown by shedding load to the main supply bus.It is virtually glitch free and if there is large motor load or capacitor bank that can regenerate onto the bus while other loads can automatically ride through several seconds until the power recovers ( though not long enough for the generator to start and sync).

The problem is very often power recovery to the main bus may be shortly be followed by another outage, so the generator can be programmed to load shed but only disconnect a few minutes later.

 The UPS can be used to maintain power during initial outage, but clearly only if the load shed system is programmed to rapidly shed loads to match the UPS capacity. This is not truly glitch free unless you have an oversized UPS. The downside is that the system requires large batteries which continually have to be charged and ideally cycled on a regular basis.

The best solution is a combinations of automatic load shedding / load recovery,  automatic synchronisation, emergency generator and UPS ( to maintain essential power until the generator syncs to line) . Some of the systems use a DC bus configuration, which is almost glitch free.

As stated previously emergency generators can easily be tested by synchronisation to line. You do not have to shutdown the main propulsion to test a generator.

As stated previously emergency generators can easily be tested by synchronisation to line. You do not have to shutdown the main propulsion to test a generator.

That does have complications where the main line is fed from the public supply - DNOs (understandably) aren't keen on the possibility of local generators back feeding onto a grid circuit they've just isolated (or been subject to ADS). It is doable, generally you somehow need to ensure that your generators are disconnected from the grid if the grid supply fails for any reason - and the rules get even more complicated if generators are rated at over 16A/phase (regardless if you're system is configured to export that much or not).

   - Andy.

Some UK customers may opt to follow NFPA 110 for their emergency and standby power systems depending on their needs and preferences.