The generator sweet spot

9th December 2020
Source: Finning
Posted By : Lanna Cooper
The generator sweet spot

In cycling, sweet spot training involves placing the body under sustained stress, but not so much stress that it cannot be held for a long duration. It can be defined in relation to a percentage of the cyclist’s functional threshold power and aims to increase the cyclist’s fitness, while getting the most out of their timeHere Clinton Noble, Diesel Generator Sales Manager at Finning UK & Ireland power systems, explains how generators, like cyclists, have a sweet spot that can mean operators get the best out of their investment.

Generator sets are designed to run with load - loading them properly is essential to availability, reliable operation and optimal engine life. The ideal amount of load a genset should be run with will depend both on the application the generator is being used in, and its rating. Choosing an improper rating means buying more capacity than needed, or risking shorter life, increased maintenance and down time.

Under or over loading

The maintenance schedule and predicted component life are dependent on operating the genset within its correct range. Similar to the cyclist trying to sustain too fast a pace, a genset operated with too much load could falter prematurely.

On the other hand, underloading can also lead to negative consequences. Operating a diesel generator set at load levels less than 30% of rated output for a long period of time can lead to negative consequences, such as engine exhaust slobber.

Slobber occurs when reduced heat in the cylinder allows unburned fuel and oil to leak through exhaust slip joints. While it does not harm an engine, it could indicate other underloading effects, which can lead to poor performance, component and power loss, increasing maintenance and down time costs. Choosing an engine with the right rating helps prevent this premature wear and ensure your engine has a long lifespan.

Understanding generator ratings

The amount of load the genset should be run with is in part set by its rating - approved methods of operating the equipment. In many cases, the iron of the machines is identical and it is the software in the electronic control system that determines how the engine will operate. Because more sophisticated switchgear sets are now available, there is more flexibility to specify a generator system that is a close match to the application.

Ratings are based on ISO 8528, which was created to make engine ratings consistent across industries and to improve understanding. Ratings are defined in terms of maximum power available, load factor, annual hours of use, peak demand and typical applications. Differing slightly from the ISO definition, Caterpillar defines five ratings as: emergency standby power, standby, mission critical standby, prime and continuous.

Choosing a generator with the right rating can be confusing, particularly when a piece of equipment is marked with multiple ratings. In these cases, how the equipment will be used will determine the load factor.

In standby ratings, the generator set can provide its nameplate of back-up power for the duration of an outage. The average load factor should be no more than 70%, with annual hours below 500, and it will not typically be connected to the grid. Emergency standby power is similar, but has a maximum running time of 200 hours a year.

Mission critical applications, such as in data centres and healthcare facilities, are cases where reliable power is paramount. For mission critical standby applications, where the running hours of standby generator sets are low, the load factor requirement is relaxed. The average load factor can be up to 85% of the nameplate rating, for up to 500 hours per year. In these applications, alternators are typically over specified and Caterpillar recommends using continuous-duty rated alternators.

Understanding the details of your application will enable you to choose the right rating. Operators can consider factors such as average load, maximum required load, hours per year and if the genset will be run isolated from, or in parallel to, the utility.

An installation for your application

Choosing a generator is a complex task and to benefit from theideal mix of performance, power density and up time, the operator must consider more than just the rating. Ensuring the generator set is correctly rated to operate within the manufacturer’s recommended load levels, while meeting the facility’s requirements, is essential.

One major challenge for UK operators is available space. In many cases, particularly in mission critical applications like the hospitals and data centres we discussed earlier, space is at a premium. Our customers in these industries are looking to generate the maximum amount of power they can in the space available to them. We’ve seen customers implement some very creative solutions to fit more generators into their facility, including stacking megawatt generators four to five high.

One of the major questions we get asked about backup generators is, how can I fit it in? In many cases, operators feel they need to compromise between a smaller generator set with less power, or a larger machine that requires additional building works. That isn’t the case. There are now generators available with a high power density, meaning they produce more power from a smaller footprint.

Other benefits of generators with increased power density is in the reduced costs of transportation, installation, attenuation and bill of parts. It will decrease total cost of ownership, meaning more kW per pound spent.

A power dense solution may be more expensive up front, but will deliver more value for money. It is more efficient to run a generator at its maximum rating, so optimising the generator to meet the required power needs, as well as the requirements of the space, will help achieve a low total cost of ownership.

The generator sweet spot varies for every application, just as sweet spot training will mean a different intensity for every cyclist. Whatever your backup generator needs, Finning has the skills and expertise to engineer a solution that offers reliable power and low total cost of ownership today.


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