What is Continuous Rated Output of Generator
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| Load-Factor-Advantage1: Selecting a generator set can be confusing. By aligning your requirements with the proper generator set ratings, you can ensure your generator set's required performance. Photo courtesy: MTU Onsite Energy |
By Brandon Kraemer, MTU Onsite Energy
Given the array of ratings and standards in the marketplace, selecting a generator set can be confusing. But, by aligning your requirements with the proper generator set ratings, you can ensure your generator set's required performance. The right standards and ratings for your needs, such as total kW output, running time, load factors and emissions regulations, must be defined prior to every generator set installation.
This article will review available industry standards, manufacturers' and federal emissions ratings, and provide a clear guide to specifying the best power system solution.
ISO STANDARDS
ISO-8528-1:2005 is an industry standard for performance parameters in on-site power applications. The ratings outlined in ISO-8528-1:2005 define basic generator set rating categories based on four segments: emergency standby, prime power, limited-time running time and continuous power. In each category, a generator set's rating is determined by its maximum allowable power output in relation to running time and load profile. Misapplication of these ratings can jeopardize the longevity of the generator set, void manufacturer's warranties and put the set at risk for failure.
Emergency Standby Rating
The emergency standby (ESP) rating is one of the most common, and represents the maximum amount of power that a generator set delivers. An ESP generator set is normally used to supply emergency power during a utility outage until power can be restored.
As defined by ISO-8528-1, an ESP-rated generator set must provide power "for the duration of the outage" and "with the maintenance intervals and procedures being carried out as prescribed by the manufacturers." Individual manufacturers may determine whether or not to authorize a higher 24-hour average load factor.
For example, all engines in MTU Onsite Energy's ESP-rated generator sets are approved for an 85 percent 24-hour average load factor. This high load factor increases the 24-hour average available generator capacity by 15 percent over the ISO standard. It can also help to reduce the size or number of generator sets needed to support an application.
Prime Power
Generator sets rated for prime power (PRP) are designed to supply electric power in lieu of commercially purchased power from a utility. This may include supplying power for temporary use, or for remote locations, such as wilderness outposts, remote mining, quarrying or petroleum exploration operations.
All MTU engines in MTU Onsite Energy's PRP-rated generator sets are approved for a 75 percent 24-hour average load factor. ISO-8528-1 sets the 24-hour average load factor to 70 percent of the PRP rating. However, similar to the ESP rating, an individual engine manufacturer can authorize a higher 24-hour average load factor. MTU's standard of operating a higher load factor increases the 24-hour average available generator capacity by five percent over the ISO standard.
Limited-Time Running Prime
Generator sets rated for limited-time running prime (LTP) are typically designed for supplying electric power to a utility as part of a financial arrangement. LTP applications include peak shaving, load curtailment and cogeneration.
Continuous Power
The continuous power (COP) rating is used when the generator set must supply a constant load for an unlimited number of hours annually. These applications, such as remote power stations, typically use multiple generator sets to power this constant load, which is also known as a "baseload power station." The baseload is the minimum amount of power that a utility must make available to meet its customers' demand for power.
Most manufacturers follow the ratings detailed in ISO-8528-1. However, some have slight exceptions to these ratings, which help them meet their customers' requirements, performance capabilities or maintenance schedules.
MANFACTURERS' RATINGS
Because most manufacturers follow the ratings detailed in ISO-8528-1, it is important to understand ISO ratings. However, manufacturers make slight exceptions in order to best serve their customers, so it's also necessary to learn the terminology. As technology advances, terminology is amended and sometimes causes confusion. Five of the most common confusion points are: net power versus gross power output, overload capability, load factor, maximum run time allowed and time between overhaul.
Net versus Gross Power Output
Think of this as you would your pay. Your gross pay is what you make, but your net pay is what you put in the bank after taxes and other deductions. When comparing generator set ratings, it's important to evaluate them based on the complete system power output, and this should include the power draw for the cooling system as it's required for the system to perform.
Overload Capability
In the past, the PRP generator set output was less than the ESP rating, and this would allow for an overload capability. For PRP-rated units, this is commonly advertised as the 10 percent overload capability for a set period of time. For MTU-powered generator sets, the 10 percent overload is available for one hour out of every 12, with a usage recommendation of one percent of the year, or 87 hours per year. Other generator set manufacturers state that this one-hour overload can only be used up to 25 hours per year.
Load Factor
Load factor is commonly misunderstood because historically, generator sets were only known at their maximum application rating, or nameplate rating. As generator set technology progressed, higher demands were placed on the equipment, and as a result generator manufacturers used the average load factor as described by ISO-8528-1 to establish their equipment's expected usage.
When comparing products with different published load factors, it's important to consider some of the advantages of a generator set with a higher published load factor. In the past, a simpler, single-step loading method was often used for motor loads. Because of this method, this often set the highest power requirement for the generator set, and the size of the generator was dictated by this rating. This is known as "starting power requirement."
In comparison, electrical engineers often prefer today's more complex, soft-loading methods because they reduce the starting power requirement, which often reduces the maximum power output required. The result is smaller generator sets, running at a higher average load factor, for a better total cost to the owner.
Maximum Run Time
Although the ISO-8528-1 standard makes a statement for the ESP maximum run time per year under test conditions, it does not state any run time limits in the event of a utility outage. As a result, most manufacturers have declared their own expected maximum annual run time based on typical experience from the field. MTU-powered generator sets have a 500-hour annual recommendation while other manufacturers have a 50 – 200 hour limit.
Time Between Overhaul (TBO)
There is a natural inverse relationship between generator set application and the estimated time before overhaul (TBO). In general, ESP-rated equipment has higher power output than the same equipment with a PRP or COP rating, and as a result the ESP-rated equipment also has the shortest TBO. With the typical usage of a PRP- or COP-rated generator set running many more hours in a year than ESP, the higher TBO is a significant benefit to the users of these applications by extending their maintenance schedules and decreasing product life-cycle costs.
FEDERAL EMISSIONS RATINGS
In addition to ISO, the Environmental Protection Agency (EPA) also influences generator set ratings by the engine usage.
Stationary emergency classified diesel engines are used during utility outages with some exceptions. Stationary non-emergency classified diesel engines can be used without any restrictions, but the emission requirements are stricter (Tier 4).
The EPA's requirements for generator sets also include mobile generator set engines. Mobile generator sets must abide by the same Tier requirements as the stationary non-emergency engines. For diesel units, this means Tier 4 emissions.
However, there is one exception: the Transition Program for Equipment Manufacturers (TPEM). TPEM allows mobile generator set manufacturers to use the previously accepted EPA Tier requirement for new equipment in a limited quantity for a limited time. This is often referred to as the "mobile flexibility" provision, which MTU Onsite Energy is taking part in.
Author
Brandon Kraemer is a senior product manager at Mankato, Minn.-based MTU Onsite Energy Corp. In his current role, Kraemer helps lead market-specific new product development.
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Source: https://www.power-eng.com/emissions/air-pollution-control-equipment-services/understanding-generator-set-ratings-for-maximum-performance-and-reliability/
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