Portable
diesel generator sets are often used to provide AC power to temporary outdoor public
events, like festivals, promotions and concerts. It is now common to have large HD screen
displays and a host of other electronics being used to provide additional
multimedia.
When
asked about running electronic power supplies on portable generators, we tend
to consider the waveform quality, distortion and high voltage noise
spikes. Upon further research, this may
not be our greatest concern.
I
mention portable generators, rather than the fixed location, back-up generators
that many facilities have in place against power outages. The fixed generator would typically provide
power to a number of different load types, such as heating, cooling, lighting,
machinery and office equipment. These
loads would change, but not significantly during operation, and it would be
reasonably safe to assume that there would be a “base-load” that remains present
at all times.
With
a portable location, that might not be the case, particularly during a break in
the event schedule or at the end of a set when the power draw drops
dramatically. When this occurs, there
could be significant rise in the generator output voltage before it compensates
for the light load.
Generator Voltage with Sudden Load
Change
Until
ISO 8528 was published, generator specifications were governed by local country
standards, with many of the tests only ensuring that the generator could handle
and recover from large load steps. Now, under the governor section of the
standard, in section ISO 8528-1-7, the response regulation states four
performance standards.
Class G1 –
Used for applications where the connected loads only require the basic
parameters to be specified. This
includes general purpose applications like lighting and electrical loads which
can easily withstand the input voltage surges.
Class G2 – Required
for applications where regulation is not that critical and temporary deviations
are acceptable. Lighting systems, pumps,
fans and hoists have some tolerance to frequency and voltage.
Class G3 –
Applications where the equipment demands are moderately severe and includes
telecommunications equipment and thyristor-controlled loads.
Class G4 – Required
for applications where the demands are extremely severe. This typically includes data-processing and
computer equipment.
The limits for these deviations
are shown below.
*Class
G4 systems are usually customer specified
For
different regions around the world this means the following overshoot profiles
are possible:
The
input voltage rating for many off the shelf AC-DC power supplies is 85/90Vac to
264Vac. Recently though, a number of
manufacturers have added a peak voltage rating of 300Vac for five seconds. This is usually found on enclosed type of
product, like TDK-Lambda’s RWS-B series of 50 to 600W industrial power
supplies.
TDK-Lambda’s RWS-B
Looking
at the tables above, the newer generation of power supplies with a peak rating
of 300Vac for 5 seconds may be used on Class G3 generators. Depending on the extent of the anticipated load
changes where there is a base-line of fixed load, can probably be used with
Class G2.
As
open frame (embedded) power supplies tend to be used in ITE equipment, that
surge rating is not usually specified, and a Class G4 generator should be
utilized.
The
concern is that the choice of generator will probably lie with the event
organiser, and they may opt for a lower cost Class G1 or G2 if they are not
familiar with the standards. If an
equipment manufacturer believes that their product may get used with portable
generators, they should consider using an AC-DC power supply with a 300Vac
surge rating. Any product literature
should state the minimum class of generator to be used for reliable operation.
