STEPS TO CHOOSE D RIGHT GENERATOR FOR YOUR BUILDING
When there is a necessity to
ensure a continuous energy supply
because of a mains failure at our
home or at our work place, or to
reduce our spending on energy
during peaks, then follow our
handy guide to choosing the right
power generator.
This is not a substitute for a
certified electrician, which we
always suggest talking to before
buying, but it should provide you
with enough information to get a
solid understanding of some of the
key things that are involved.
What do you need in
order to keep working in
case of a sudden or
programmed grid failure?
Firstly we need to know the
starting power and normal running
power of all of the items that need
to be powered by the generator.
All of your items should tell you
the nominal running power,
however they might not tell you
the start-up power required. This
depends on the type of mains
connection. If it is star or delta,
then the start-up power required
should be calculated at 3 times the
normal running power. If it’s a
DOL rating then the start-up
power will be equal to 6 times the
normal running power of the item.
For example in the case of
Elevators/Lifts, they may contain a
a soft starter of variable frequency
drive (VFD) and the starting
current can be estimated at 2.5
times the rated current.
In summary as long as we now
the kilowatts (kW) of the items we
need to keep running in the event
of a mains failure, we can then
calculate the overall start up
power; the final figure being
dependent on the type of mains
system in use – star, delta or DOL.
Which areas of your
installation must have
basic lighting?
You need to calculate the total
amount of light sources, for that
you need to add the consumption
of each light expressed in watts
(W) then dividing the total by
1.000 to obtain the kilowatts of
power consumption from your
lighting needs.
By adding the figures from the
previous question to the figures
obtained for lighting, you will
know the approximate power of
the generator you need.
Who is in charge of
calculating the power
needed for the genset?
The decision of the size and power
of the genset and its electrical
configuration are the responsibility
of the electrical, mechanical or
engineering department, depending
on the kind of project. This
department will define:
Power needed to cover present
and future necessities.
The operational requirements
to be incorporated in the
equipment: manual or
automatic start, redundancy
with other units or with the
mains, soundproofing, auxiliary
fuel tanks, heating system etc
Legal regulations and local
laws (electric, noise, pollution).
Selection of the right place to
install the genset or gensets
(outdoors or indoors, elevation
above sea level, dusty
environments etc)
Example: How to estimate
the power of a generator?
To end this article, we will
estimate the power of the genset
needed for a 12 floor apartment
building, with the following power
necessities:
1 elevator with a 15 kW engine:
starting with a VFD, 380V.
2 water pumps: 10 HP per unit,
star-delta starting (S-T), 380V.
1 Boiler pump: 5 HP, DOL
starting, 380V
1 pressurized fire ladder: 15
HP, DOL starting, 380V.
Corridors lighting: 5,5 kW (55
100W units), 220V.
Entrance hall lighting: 1,5 kW
(15 100W units), 220V.
Garage door: 0,5 kW, DOL
starting, 220V.
Next we design a load table and
define the starting power for each
item. For this example, we will
assume a scenario with the
biggest potential demand, one with
the elevator and the fire ladder
included, although in reality both
cannot work at the same time.
Load table
Equipment
Rated
power
(kW)
Voltage
(V)
Equivalent
power (kW) 380 V Start Starting
factor
Starting
power
(kW)
Elevator 15 380 15 VF 2,5 37,5
Water pump 7,5 380 7,5 E-T 3 22,5
Water pump 7,5 380 7,5 E-T 3 22,5
Boiler pump 3,7 380 3,7 DOL 6 22,2
Fire ladder 11,2 380 11,2 E-T 3 33,9
Corridor lighting 5,5 220 1,9 - 1 1,9
Entrance hall
lighting 1,5 220 0,5 - 1 0,5
Garage door 0,5 220 0,5 DOL 6 3,0
Total with elevator TOTAL 36,6
Total with ladder TOTAL 32,8
Total starting power with elevator (kW) 110,1
Total starting power with ladder (kW) 106,2
We must remember that these
figures are an estimate, the
definitive power of the genset
won’t be bigger than this, but it
could be lower. For more exact
calculations we must consider
other factors as the type of load
(lineal or non-lineal), usage
factors, simultaneity of start ups,
etc.
Worst case scenario is with the
elevator working, 110.1 kW, this
power is the maximum transient
power that could be demanded
eventually to the genset, which has
a power factor of 0.8 (standard for
AC alternators). Now we can
calculate the apparent power:
kVA=kW/0.8, so the apparent
power in our example will be
110.1/0.8= 137.6 kVA, maximum
transient power demanded from
the genset, based on all the
equipment start at the same time.
However, the power needed by this
equipment when running normally
will be 36.6/0.8=45.8 kVA.
Simultaneous start or
sequential start?
To define the capacity of the
genset we need to consider other
factors, like simultaneity and
usage factors, applied by the
electrical contractors. In our
example, we can apply a 0.8 factor
to the maximum power calculated,
137.6×0.8=110 kVA, which will be
the stand-by power or emergency
power that the genset needs to
provide. However, if a sequenced
start could be programmed, the
most critical scenario will be when
the elevator starts with all the
other equipment running, with this
power demand, (36.6 kW – 15 kW)
- 37.5 = 59.1 kW. In this scenario,
the apparent power will be
59.1/0.8=73.9 kVA. You will notice
the difference between both
scenarios, 110 kVA vs 73.9 kVA,
equivalent to 33% less with the
sequential start.
With this information, you are now
able to do a preliminary evaluation
of your needs of back-up power.
To summarize, to choose the right
genset you must consider:
- Define which equipment/items
will be connected to the
genset. - Select the power of the genset
based on the equipment that
will be connected.