Q: Is a generator
fueled with natural gas considered an emergency power plant?
A: There are two
common types of generator systems available to supply emergency power to essential electrical systems. One common type of
generator set utilizes a combustion engine fueled by natural gas. This type of engine is easy to maintain; however, codes
do not recognize the utility company gas supply as an emergency service back-up fuel source unless it can be proved to be
uninterrupted. Natural gas systems normally require a propane system back-up (or must operate entirely on propane). Either
arrangement requires large above-ground tanks, extra maintenance and great care in selecting a storage location.
A second common type of generator set utilizes a diesel oil fired
combustion engine with a fuel tank sized for a minimum of eight hours of full load operations. Depending upon the utility
company's reliability and the number of primary services feeding the site, facilities may require up to twenty four
hours of fuel supply to get them through a utility outage. This type of generator is considered for emergency power being
the required fuel is stored on site and considered uninterrupted.
Q: I
need to replace our existing generator. Can I oversize it being we have a planned expansion in two years?
A: There is a natural tendency to oversize the generator
set for future expansion. However, generator sets operating under lightly loaded conditions experience incomplete fuel combustion.
The unburned fuel accumulates on the engine injectors or in the exhaust muffler and/or exhaust stack. This phenomenon causes
choking, which leads to premature failure and limits the overall system capacity. Latest NFPA 99 guidelines require monthly
testing at 30% of the generator nameplate rating or 50% of the connected load (whichever is greater). Oversizing the units
could lead to a requirement for load banks to make up for a shortfall in actual load available to be applied to the unit during
testing. Often it is better to consider installing two smaller generators operating in parallel instead of one large one.
Both generators start and come on line. If the load does not warrant both units, one will drop off line, leaving the remaining
unit running at optimum load.
Q: What is the temperature rise rating of
a transformer mean?
A: Transformer temperature
rise is the average temperature rise of the windings above the ambient (surrounding) temperature, when the transformer is
loaded at its nameplate rating. Generally more efficient transformers have a lower temperature rise, while less efficient
have a higher temperature rise. Dry type transformers are rated in three standard temperature rise values: 80C, 115C,
and 150C. These values are based on a maximum ambient temperature of 40C. Most dry transformers use the same insulation on
their windings, rated at 220C, regardless of the designed temperature rise of a transformer. For example, a transformer
with a rating of 80C rise unit will operate at winding temperature of 120C when at rated full load. A 80C rise
unit utilizing 220C insulation has a 70C reserve capacity compared to a 150C unit. This translates into an
overload capability of 15-30% without affecting the insulation or transformer life.
Q: How does the rating of temperature rise
of a transformer effect efficiency?
.
A: Actual load and no-load
losses, in watts, should be obtained from the manufacturer. When this data is not available keep in mind that temperature
rise rating is an indicator of efficiency. Typically a unit with a 80C temperature rise rating uses 10-20% less
operating energy than a 150c rise unit. Also higher efficiency units generate less waste heat thus reducing the costs of ventilation
and/or air conditioning.