The message <
[email protected]>
from Dale E <
[email protected]> contains these words:
I'm not the O.P. but I think he will find your reply as informative as
I did. Thanks.
Well, Dale, it's nice of you to say so but, in view of Bruce's comments
re 'The Magic Smoke'(tm), I feel obliged to point out that the
generator's circuit breakers should take care of that little problem.
Ideally, both generator sets should be free running within a fraction
of a percent of each other's speed to avoid load hogging by the faster
set. Even so, provided this speed difference is sufficiently small, the
load sharing imbalance will also be sufficiently low enough not to
matter.
If you're running two sets to provide for intermittent peak demand
loads then even in the extreme case of one set running out of fuel
during a period of low electrical demand, the fuel starved generator
will keep right on running (the generator now acting as a synchronous
motor) provided the total load on the remaining set is still within its
rating.
Spinning a fuel starved generator set via the electrical link from the
remaining set can present a considerable load (ICE pumping losses at
1500/1800 rpm if the throttle is left wide open on a petroleum or gas
powered 4 stroke engine - not so bad if the throttle is closed - a
diesel engine doesn't normally incorporate any induction throttling).
Generator sets designed to be 'ganged' in this fashion may well
incorporate a 'freewheel' drive between the prime motive power source
and the alternator to avoid imposing the extra mechanical loading under
such conditions (although a solenoid operated contactor driven by
suitable monitoring/control electronics can achieve the same effect,
only better - eliminating the windage losses in the alternator itself as
well).
Whilst the concept of combining the power output of two or more
modestly rated generators onto a single higher power bus (a sort of
'Redundent Array of Inexpensive Generator Sets' as it were) to emulate a
'mini national grid' might seem appealing, it does have a serious
shortcoming compared to the model it has been scaled down from.
The variations of demand on the national grid (in percentage terms) are
a hell of a lot smoother than those imposed on a home generator system
designed to substitute the mains supply and satisfy the normal peak
demands. The PSU might see a 30% rise in demand over a twenty second
period as TV viewers take advantage of an advert break to boil a kettle
of water for a hot beverage.
If the PSU has access to pumped storage backup, the ten seconds run up
of the 'hot standby' genset backed up by the 60 second runup from
standstill gensets will easily cover the peak demand. Even if there is a
shortfall, the grid voltage will simply sag a little until the extra
capacity comes on line to match the demand.
A home setup has no such resiliency. the demand can go from, say, 1.5KW
to 3.5KW in the blink of an eye and can then jump to a 5.5KW loading
within a second or two of that first increase. If you have installed a
pair of 3.6KW gensets which can be ganged onto the main house supply bus
and are running just the one to minimise fuel consumption, the second
loading event will have tripped out the generator before you could even
start the other genset, let alone bring it up to speed and phase to
allow its connection onto the house bus.
If you are going to run two or more ganged gensets, you will need to
keep them all running during the outage unless you are prepared to
follow a strict plan on electrical usage - effectively booking the extra
capacity ahead of the actual demand.
The resulting fuel consumption under light loading of say a pair of
3.6KW rated gensets is likely to be higher than for a single 8KW genset
under the same load. IOW, the running costs will be very nearly the same
between the two options (One large generator versus two or more smaller
generators).
On balance, the use of two or more small gensets as a substitute for a
single larger capacity unit is best served by having split power buses
rather than trying to combine the output onto a single bus. You have the
benefit of redundency and a foolproof system of prebooking capacity for
the extra transient loads.
There is a way to combine two or three smaller gensets economically
onto a single bus without the risk of overloading the single running
genset by the switching of additional loads. Essentially this makes use
of a battery powered inverter capable of making up the shortfall during
the runup time required to bring the extra genset on line. The inverter
needs to be of the type designed to allow export of surplus power back
to the grid. This not a cheap solution considering that we might need
some 3 to 6 KW of inverter capacity and involves a relatively expensive
consumable in the form of a heavy duty rechargable battery pack.
If you're looking at a strategy to minimise the effects of extended
mains supply outages (hours to days worth), the two differently sized
generator sets on seperate buses supplemented by suitable UPSes to
protect PCs and similarly microprocessor controlled devices is going to
be the most cost effective and simplest solution to implement.
Such a system will require you to manage your electrical demands to
avoid overloading your generator capacity, but this will be a relatively
minor inconvenience compared to the alternative of having no electrical
loads at all to be managed.
.
