All diesel generator engines are rated according to 3 power output criteria’s:

1) Prime power rating:

The prime power rating is the maximum power accessible at the variable load for an unlimited number of hours per year in a variable load setting. It is not advisable that the variable load exceed 70% average of the prime power rating during any operational period of 250 hours. If the engine is running at 100% prime power, yearly hours should not exceed 500. Overload situations should be avoided however a 10% overload capability is available for a 1 hour period within a 12 hour cycle of operation.

2) Continuous power rating:

Continuous power rating is used in applications where supplying power is at a constant 100% load for an unlimited number of hours each year. Continuous power rated units are most widely used in applications where the power grid is unreachable. Such applications include mining, agriculture or military operations.

3) Standby power rating:

Standby power rating is rated at 110% of the prime power rating, thereby giving an extra 10% output. This is however limited to 1 hour in every 12 hour working cycle and cannot be exceeded without damaging the engine.
The above ratings are affected by the following site conditions:

• Altitude above sea level.
• Ambient air temperature.
• Quality of the fuel being used.

Unit under-rated for site requirements.

A generator under-rated for site demand will suffer from overheating problems which will inevitably shorten its duty cycle and longevity. Damage to alternator windings can also be caused by overloading.

Unit over-rated for site requirements

• A generator too large for site requirements also comes with possible serious complications. The main complication being engine bore glazing.
• Engine bore glazing can cause major problems in generator sets when they’re too lightly loaded.
• Operating diesel engines at light loads for extended periods of time can cause glazing to the cylinder bores.
• This occurs because poor combustion can lead to soot formation and unburnt fuel residue (slobber) which clogs & gums up piston rings, which in turn causes the piston rings to not seal as well.
• Glazing of the cylinder bores occurs when hot combustion gases blow past the now poorly sealing piston rings and creates a glaze on the cylinder bore & increased oil consumption.
• This can lead to oil entering the exhaust, heavy carbon build up on pistons, piston rings & ring grooves, valves & valve guides, cylinder head ports & exhaust pipes, and turbocharger seals on turbo charged engines.
• Heavy smoke, high oil and fuel consumption and “slobber” from the exhaust manifold/turbocharger or exhaust exit point are signs that the engine is glazing.
• If this is caught in early stages, increasing the load on the generator can burn the residues away, however if it is left unchecked, the engine can be damaged to point where it has to be rebuilt or replaced.

Imbalances in phase loads

A 3-phase alternator needs to run at a load with a maximum variation of 10% between its 3 phases. Any phase imbalances over and above 10% will cause possible damage to the alternator windings. It is important for the installing electrical contractor to make sure that phases balancing is done to prevent such damage.

Site power factor

It needs to be noted that all generators are rated at a power factor (pf) of 0.8. One needs to ascertain the power factor being utilised by equipment on the load side as well as the possibility that the site utilizes a power factor correction system. Certain equipment like heating elements, ovens, furnaces as well as cooling equipment work at a power factor of 1.0.

It is thus vital to size a generator very carefully, and keeping in mind that a diesel generator engine’s optimal duty cycle is between 60% and 80% of the prime power rating of the unit.