Automatic Control Circuit for Standby Generator

Dear levels !

<< The control circuit senses the dropout of AC for 10-30 seconds, >>

Is it Seconds or milli-seconds?

So long power outage may not be needed by the system.

Electronic-Transfer-Switches take less than a 1/2 cycle to sense & transfer if the Stand-by Power-source is available.

Leveles, I am sorry but I cannot see this as the best answer, certainly not compared with the other's more detailed information.

The control circuit has a delay that is pre-set; this could be set within the range of 10-30 sec. true, but it could be from 3sec. to 1 hour depending on the requirement. This is not the sensing time as others have pointed out but the delay.

Sad to see such a basic, inaccurate answer rated so highly, I am not putting down the input but wondering who voted for it!

As answered there are several dedicated gen.set control modules on the commercial market, we use DSE [Deep Sea Electronics] where the basic modules have set times e.g. on sensing 10sec loss of power - auto-start engine via a 3 attempt start cycle [10 sec crank, 10 sec. rest x 3 until engine starts; starter motor will disengage on sensing alternator output] Basic units will also have over & under speed protection, engine protection (shutdown on low oil pressure & high engine temp] All explained on suppliers website.

More complex units have adjustable software so times can be adjusted for special circumstances.

You ask the max. delay? If one has a UPS [Uninterruptible Power Supply - a battery bank /inverter supplying the load and a mains or gen.set charging the batteries] There would be no break in supply. It depends on the application weather they can stand time without power, e.g. a hospital against a storage facility. Some places may not be bothered if they loose power for an hour!

What is to be avoided is the gen.set changeover contactors (like a high powered auto-switch) & gen.set being put into operation on a 'brown out' [apposed to a 'black out' this is just a very short loss of volts / power] This is normally seen as <2-3 sec. by the customer, but legally in the UK It was anything over 10sec. is classed as a power loss [monitored by regulators as Customer Minutes Lossed]

In a non critical situation we would start a gen.set after 3-5 sec of loss of power, the start and changeover process takes from 10 -60 sec. depending if the gen.set starts first time. The gen.set would not be stopped unless power returned for >5min. then the gen.set would disengage (via change-over contactor) and run off load for a further 3min to cool down.

But times could be as mentioned above if that's what was required.

As mentioned the switch back to mains is a fraction of a second as the contactor changes over. The contactor is mechanically & electrically interlocked so the mains and gen.set do not come into contact. A mains synchronised gen.set is a different setup completely.

In the USA most standby power systems use a transfer switch, manual or automatic, with the generator. Several codes address this. We define whether the standby system is legally required or an optional standby system.

In the US the transfer timing of legally required systems is further defined by the authority having jurisdiction. Typically a life safety circuit is required to have power restored in 10 seconds MAXIMUM. Typical UV/UF settings are 1sec on dropout to start the gen. These switches are usually required to be tested weekly. Typically the rest of the legally required circuits are required to be restored in 60sec...stuff like stairwell lighting..emergency exits, etc. and are required to be tested weekly or in some places monthly. In an optional system we don't care if you leave power off for 3 minutes or three days.

There are quite a few transfer switches out there on the market ranging from very basic circuit breakers turned into MTS/ATS to very advanced closed transition transfer switches.

A typical control will have the following settings. In parenthesis I'll put the factory setting for a standard ASCO switch.

Normal (utility) source voltage pickup. (90% of nominal)

Normal source voltage dropout...usually set to a percentage of pickup. (94% of pickup)

Emergency (gen) voltage pickup. (90% of nominal)

Emergency freq. pickup. (95% of nominal)

Override momentary outage time delay. (1sec)

Transfer to emergency time delay. (0sec...you can give some time if you are on an optional requirement and you have an older gen needing a bit of warm time)

Retransfer to normal time delay. (30min)

Gen set cool down timer. (5min)

Also be aware that for hot-to-hot switching, without an in-phase monitor (simply a passive sync method) you may need to do some selective load shed/add control or you can trip breakers. If you have heavy motor loads and switch out of phase you get some wicked hits.

I am not sure of the terminology in the States, but a pair of interlocked contactors can be set up as an 'auto changeover switch'. Some older installations will have a manual switch where time is not critical. Several farms I have maintained have installations like this on grain stores. A battery chicken unit is more critical, so will have an auto changeover unit, in some cases the birds are so tightly packed they can overheat & die in less than 20min if the cooling fails.

We are beginning to suffer from 'brown outs' in the UK; gen.sets would be continually starting unnecessarily on a 1sec. low volts setting. If power supply is that critical a UPS in the usual route. Small units to supply power requirements for 5min to cover any sensing delay, gen.set start & changeover, they are readily available commercially off the shelf (COTS).

To meet insurance cover these have regular test & maintenance intervals.

A gen.set will not take 100% load instantaniously, unless it is deliberately over-sized for the application. Approx 65-70% of the standby rating is the usual max in one bite, depends on manufacturer's advice.

We did have a gen.set which was set at the high end of the allowed frequency band (55Hz) to take a bigger load drop in one go, it would bog down to ~47Hz and recover! This was an extreme circumstance restricted by space.

Load circuits can be switched in on in a priority basis to allow gen.set to take full load. Again a UPS would make this simpler and is becoming a more common solution as it is cost effective compared to dedicated circuits on a lower spec installation.

I am interested in possible hangups in backup systems. My son's maintenance team have a problem reverting generator to mains after test running; took 12 hours to switch back last time! They have double UPS and twin generators, yet once up and running I assume you restore power and things should switch back fairly quickly??

My theory is that the generator may be running slow (heavy load) and having difficulty syncing to the incoming 3phase mains. Not being in the industry myself I'm trying to dig out realistic solutions. Once upon a time computer systems had high leakage currents, I don't know if modern computer servers are bad for loading?