Considering a diesel generator? Is it to be used as the primary unit off grid or standby emergency generator for a hospital or retail center? Your first consideration is the load. Duty categories are prime-continuous or emergency. Operating altitude, temperature and emissions regulations are all important. Current emissions standards are Tier 3 and all new generator units are built to this standard. The electrical load is rated as to single or three phase, kW or KVA frequency (60 Hz) in most US applications surge current loads and harmonic content.
Parallel Operation of Two Generators
Island mode is a generator ‘off’ grid and supplies only local power. Parallel mode is wired into a grid of more than one genset or parallel to the power utility grid to handle local high loads. Gensets must synchronize to match voltage, frequency and phase. Parallel installations share the load across multiple generators. The power utilities tend to use large scale parallel installations to handle high load periods. These ‘substations’ use diesel powered gensets. In many locations these are dual-fuel switching from diesel fuel to natural gas. Diesel provides more power so as the load increases so does the diesel to natural gas ratio.
Generator Sizing & Rating
• Rating – As the genset is expected to provide the anticipated power loads without damage the generating is given multiple rating depending or standby use with only a few hours run per year whereas a prime unit would provide power continuously. Generally, the standby rating is higher than a continuous rating and many will tolerate up to 10% overload for short durations. Manufactures rate their units under internationally agreed guidelines.
• Standby Rating is for emergency power production for the duration of the power outage. No sustained overload capability is available for this rating.
• Prime (continuous, unlimited running time) are applied to construction power applications. Output is available under varying load conditions for an unlimited time. The rating is based on a 100% of prime-rated estimated kW with a 10% overload capability for emergency use for a maximum of 1 hour in 12. Overloads capabilities are only available for a limited time. These overload ratings are not available for all gensets.
• Base Load Rating is based on supplying power continuously to a constant load up to 100% of the rating for an unlimited time. No sustained overload is factored into this rating.
• Sizing Considerations include both altitude and ambient air temperature. For electric motor start ups the genset has to be able to provide 3 times the power of the largest motor driven by the genset. A hundred amp running rating for the motor will require 300 amps to start. Biggest draw is started first.
Diesel Generator Fuels
Fuels were named for the engine, as diesel engines are compression ignition engines. The most common is a diesel #2 or #1 and natural gas combo with diesel used for starting. Diesel produces more power and the electronic controls vary the diesel to natural gas ratio according to load. Load goes up, burn more diesel fuel.
Alternate fuels are alcohols, gasoline, wood gas to biodiesel, straight vegetable oils, animal fats, tallows, glycerin and coal water slurry. Many of the heavy oils like animal fats and vegetable oils need to be heated to maintain the liquid state. Balance heating and the flash temperature of the oils is a balancing act handled well by modern electronic controls.