diesel generators

Prime/Continuous vs. Standby/Backup Power

Diesel generators to suit a wide variety of applications...

Burtonwood Generator & Switchgear Services Ltd offers a wide range of diesel generator solutions. The choice of which generator unit is suitable will depend on the application and power usage required.

What type of generator do you need - Continuous, Prime or Standby?

The decision to purchase a diesel generator will very much depend on the purpose of the generating set. Sometimes a generator may be required for standby or emergency usage only. However, if power outages occur frequently and/or for long periods of time, it might be worth the added investment to source either prime or continuous rated diesel power generators to ensure uninterrupted supply of backup power for extended periods of time. If you attempt to operate your standby generator for longer periods than the manufacturer’s prescribed number of operating hours in any given period, it will most likely lead to more frequent breakdowns and faults occurring in the unit.

What's the difference between Continuous, Prime or Standby Generators?

Electric power generators can be grouped into one of three different types depending on their style of operation: continuous, prime, or standby.

Continuous and prime power generators are very similar as they act as the main source of power and are designed to operate continuously or for extended periods of time. The main difference between the two is that continuous power generator sets are designed to operate continually with a consistent load while prime generators are designed to operate for long durations at variable load.

The alternative type of generator – standby/emergency – is designed to run only in a backup situation when there is a power outage on the main utility grid or the main source of power.

Continuous and prime generators are typically used in remote locations where there is no access to the grid to supply electric power in applications such as mining or quarrying, oil and gas operations, construction, theme parks, etc., They are also used when there is limited electric power from the grid. In rare cases, it may also be cheaper to generate power via a diesel generator than to buy electric power from a utility.

On the other hand, the need for standby/emergency power arises when there is a temporary disruption in the primary power supply. Like any other machinery running continuously for long hours, continuous or prime generators need to be robust enough to handle the heavy power loads supported by them. Additionally, these units have to be able to handle friction between moving parts for longer periods of time and also handle sustained heat generation. This results in prime generators being more expensive than generators of similar capacities used for standby or backup support.

What features vary in Continuous, Prime or Standby Generators?

There are several features that need to be included in continuous power diesel generators to make them function efficiently over longer periods of time. These are listed below: -

Cooling Systems:

A. Continuous/Prime Generators
(1.) For the diesel engine:

Prime generators typically have a large cooling system. Continuous fuel combustion in the engine causes enormous amounts of heat to be generated. This heat has to be removed by artificial means in order to prevent the inevitable temperature build-up. Circulating cooling water in a jacket around the engine often cools diesel engines. The circulating water absorbs heat from the engine and is in turn cooled by a large radiator fitted with a fan. A cooling system is also required to cool the engine lubricating oil. In very large industrial generators (2250kW and above), a cooling tower or other elaborate cooling systems are often required.

(2.) For the alternator:

In continuous or prime applications, the alternator is also subject to temperature rises due to the continuous flow of current through the alternator windings. As a result, the alternator needs to have heavy-duty windings. The alternator operates continuously to provide uninterrupted power supply. Therefore, there is no specific cooling time that can be allotted for the alternator to cool down naturally. The cooling system usually restricts the temperature rise of an alternator to 105°C.

B. Standby Generator

As standby power generators usually work for shorter periods, such as a few hours at a time. This provides them with two advantages over continuously operated power generators. Standby units have time to cool down naturally when not in operation. In addition maintenance activities on standby units can be performed without disrupting the power supply. Therefore, these units do not require some of the more elaborate and robust features expected in continuous power diesel generators.

(1.) For the engines:

As standby power generators work for only a limited time, they do not heat up as much as prime generators do. Therefore, these power units require smaller cooling systems. On some smaller capacity standby power generators, air-cooling eliminates the need for water circulation.

(2.) For the alternator:

The alternators of diesel generators can withstand high temperatures for small durations but not on a regular continuous basis (without cooling). Therefore alternators of standby power generators are not designed to withstand prolonged exposure to high temperatures. These temperature limit differences require the windings of the alternator to have different insulation for prime or continuous vs. standby units.

Air Cleaner Assemblies:

Combustion of fuel in the engine takes place in the presence of air. Airborne dust or any other particulate matter will tend to clog the engine. Since a prime generator would usually be in continuous operation, it is not possible to clean its internal parts and components intermittently. This means that continuous power generators need heavy-duty air cleaners, air filters, and air cleaner assemblies.

Revolution Speeds:

Continuous power generators usually operate at lower revolution speeds. Usually, diesel generator sets operate at 1,800rpm but larger capacity continuous power generators have been designed to operate at 1,200rpm or even 900rpm. This increases the life of the unit significantly and also reduces maintenance costs. However, these diesel generator sets tend to be more expensive as a result.

Power Output:

Continuously operated generators usually provide 25% to 100% of the rated capacity for an unlimited amount of time. Additionally, these units provide only constant output at the rated capacity. Where loads are entirely dependent on power supply from diesel generators, it is often prudent to have an identical duty & standby generator installed. These would be connected so that if one of the units fails to function the other one would start up automatically.

Standby generators usually provide varying outputs for specific time periods. These generators are connected with an Automatic Transfer Switch (ATS) so they start automatically when there is an outage from the main electrical supply.


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