How should I size a diesel generator unit?

Diesel generator ratings are defined in ISO 8528-1 as being either continuous, prime or standby. Continuous rating is used where the generator is expected to be operating 24 hours a day, 365 days a year at a constant load.  Prime rating allows some fluctuation in the load supplied, and standby rating is suitable for back up supply applications where running time is up to 200 hours per year.

The generator rating should be calculated to meet the maximum demand of electrical loads that require back-up power. The load factor is a ratio of the average load demand to the rated generator power. For standby purposes the load factor is recommended not to exceed 70%. 

Generator ratings can be specified as either kW or kVA, (MW or MVA for larger systems). Where the value is stated in kW or MW there is assumption of a 0.8 power factor, and therefore these values are lower than the kVA or MVA values. It is important that the power factor of the load is understood when specifying a generator.

When starting large motors, the voltage dips due to high inrush currents at start up might mean that a larger generator is required to support this.  Similarly, if the system is known to have high harmonic currents, then these may mean that a larger generator is required. Typically 30% non-linear loads will result in a 10% de-rating, but if powering 100% non-linear loads such as a UPS, it may require the generator rating to be doubled.

There are four performance classes which define the quality of the diesel generator output in terms of stability and level of the frequency, voltage and waveform characteristic. The higher the class the tighter the specification in terms of both the steady state and dynamic condition:

  • Class G1 is for general purpose loads such as lighting.
  • Class G2 has a similar requirement to the public electricity supply, and may include loads such as pumps, fans and hoists.
  • Class G3 is for telecommunications and thyristor controlled loads.
  • Class G4 is for data processing or computer systems where the demands on the quality of the supply may be particularly severe.

Generating sets are also defined in terms of their start-up arrangements:

  • Manual starting for generators with no specified start-up time.
  • Automatic start-up - Long-break; Generator started from stationary on grid failure.
  • Automatic start-up - Short-break;  Power interrupted for only a few milliseconds while the electrical transfer takes place and a source of stored mechanical energy such as a flywheel is used to supply power to the generator for a short period whilst the engine is brought up to speed.
  • Automatic start-up – No-break; Similar to the short-break system but with a continuously running electrical machine providing an uninterrupted supply in the event of grid failure.

Operating for long periods at low load (<30%) can lead to damage of internal components. Consideration to the use of smaller modular units may be necessary if demand variation means that a larger model would regularly be running at low load.

Once the power rating has been established, consideration should then be given to the required duration of the generator operation, and hence fuel consumption. Many generators are provided with a day tank that will typically last for 8 hours at the generator’s rated output. If outages are likely to be longer than this, then either a larger tank, an additional bulk supply tank, or other arrangements for re-fuelling will need to be made.

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