Generator ratings and the implications for data centers


Figure 3: A continuous rating, represented in this load profile, is intended for peak shaving programs where a designated block of power is sold back to the utility or used on-site, but is not intended for applications where the load varies, like a data center. Courtesy: Kohler Power SystemsGenerator rating basics

Let’s return to the rating of generators and look at the three major components that impact their rating and performance: engine, alternator and cooling System.

Engine. The component that impacts the generator rating most is the engine, because it is the actual power source. Every engine has a horsepower rating at a defined speed. Engine ratings are determined based on the application’s load factor, site altitude, and maximum ambient temperature during operation. The load factor is the percentage of engine power used over a given time period.

Altitude and ambient temperature extremes at the installation site can also affect engine performance. Each engine manufacturer will specify altitude and ambient temperature derates. A larger engine may be required for a high-altitude, high-ambient-temperature site (e.g., New Mexico) than for one at sea level (e.g., Wisconsin). The generator manufacturer and its local representative will have engine-rating information for load factor and altitude and temperature derates.

Alternator. The alternator converts the engine’s mechanical power into electrical power. Therefore, the electrical power from the alternator is always a lower value than the engine’s mechanical power; this is due to losses that occur in the conversion of mechanical power to electrical power. The level of loss is determined by the alternator efficiency. Alternator efficiency varies from manufacturer to manufacturer and from alternator to alternator.

Alternator design and construction is an important factor to consider for reliability. The durability of an alternator can be determined by looking at a few key design parameters governed by the National Electrical Manufacturers Association’s Motors and Generators (NEMA MG 1) standard for alternators. The main attributes to look at are the alternator insulation class and temperature rise capabilities.

Cooling system. Another important factor to consider in the sizing process is the generator cooling system. Most generator manufacturers offer 40 C (104 F) and/or 50 C (122 F) cooling systems. The peak temperatures the generator may experience in a year are a factor in selecting the cooling system. Even if you properly size the generator, failing to specify the right cooling system can lead to performance and durability problems.

Impact of EPA emission standards

With the introduction of emission standards by the U.S. Environmental Protection Agency (EPA) for new stationary and nonroad engines, the application of equipment must be considered when choosing a rating for generator sets installed in the United States. We must understand the EPA’s newly introduced terminology to properly choose the generator rating.

A stationary “emergency” application is where the generator set remains in one location for 12 months or longer, it is the secondary power source when the utility (primary power source) fails, and annual maintenance and readiness testing is less than 100 hours. The term “emergency” refers to the use of the engine when an emergency occurs (utility fails). In virtually all cases, these applications will have a “standby” generator rating. Remember that standby-rated generators can run an unlimited number of hours with varying load during the utility outage. Stationary emergency applications in the U.S. require diesel engines that are EPA-certified to Tier 3 if their power output is between 50 bhp (40 kW) and 560 bhp (500 kW), or certified to Tier 2 if their power output is above 560 bhp (500 kW).

A stationary “non-emergency” application is where the generator set is either the primary power source or a secondary power source connected to an unreliable utility with planned high hours of annual usage. Additionally, using it for peak shaving, interruptible rate, or any financial arrangement with a utility qualifies it as a non-emergency application. Non-emergency applications may use prime, limited running time or continuous ratings. Non-emergency applications in the U.S. require EPA-certified Tier 4 diesel engines, which are designed for lower emissions and are much more expensive than their stationary emergency counterparts at the Tier 2 and Tier 3 levels.

A nonroad application is where the generator set is in more than one location within a 12-month period. Typical applications are mobile, rental, or containerized generator sets. Nonroad applications may use prime, limited running time or continuous ratings. These applications require EPA-certified Tier 4 diesel engines.

There are many variables to consider when properly sizing generators, particularly when specifying for a data center. Choosing the proper generator requires an understanding of how the generator will be applied and what the site conditions will be. It is important to work with local or factory-based manufacturing representatives to properly size the generator for each application. Considering all these factors will provide years of reliable power.

Todd Matte is the manager of the corporate accounts project management group at Kohler Power Systems, located in Kohler, Wis. Matte joined Kohler in 1995 and has held multiple positions with the company including application engineering and product marketing roles. He has a bachelor’s degree in electrical engineering from the University of Wisconsin-Milwaukee.

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Anonymous , 04/09/14 03:08 PM:

I know a manufacture who recommended 1500 hours at standby.
Anonymous , 02/10/16 10:03 AM:

We have even considered the highest available rating for a generator i.e. Standby Rating as the main rating for a data centre design using large number of 2.5 MVA generators. We were sure that utility failures in future are likely to be not more than few hours in a year based on last 30 years of statistics. If we had gone by the continuous rating as per ISO, the rating would have been close to 3.5 MVA resulting huge additional dead investment which would have never been put to use.
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