Ratings Overview

Caterpillar defines five basic generator set ratings:

Cat generator ratings differ in certain respects from those defined by the industry standard ISO 8528-1. Basic descriptions of the Caterpillar generator ratings are offered below.

STANDBY RATING

In this application the generator is capable of providing emergency backup power at the nameplate rating for the duration of an outage.

The average load factor of a Standby rated generator set should be no more than 70% of the nameplate rating and applied to varying loads. A Standby generator set can run for a maximum of 500 hours per year. The normal standby rating is not for use in utility paralleling applications. For example, a 3MW standby rated generator set will provide power for the duration of an outage. It should be run for up to 500 hours per year and have an average load factor of 2.1MW.

EMERGENCY STANDBY POWER (ESP) RATING

The ESP rating differs from the Standby rating only in the number of running hours allowed per year. ESP ratings allow a maximum running time of 200 hours per year at a 70% average load factor with varying load.

MISSION CRITICAL STANDBY RATING

In this application the generator is capable of providing emergency backup power at the nameplate rating for the duration of an outage. The average load factor of a mission critical standby rated generator set should be no more than 85% of the nameplate rating with varying loads. A mission critical standby generator set can run for a maximum of 500 hours per year.

Typical peak demand is 100% of the rating for a maximum of 5% of the operating time. The mission critical standby rating is not for use in utility paralleling applications. For example, a 3MW mission critical standby rated generator set will provide power for the duration of an outage. It should be run for up to 500 hours per year and have an average load factor of up to 2.55MW.

PRIME RATING

In this application the generator is capable of providing power to a varying load for an unlimited number of hours per year. A Prime rated generator is capable of providing full nameplate rating for a period of time, but must have an average load factor of no more than 70% of the Prime rating. Ten percent overload is allowed for emergencies for a maximum of one hour in 12 and for no more than 25 hours per year. The standard prime rating is for use in either utility paralleling or isolated applications.

For example, a 2.7MW rated unit may provide the full nameplate rating for a short duration, but should have a maximum average load of 1.89MW (not including generator non-running time per ISO8528-1). The generator set can also provide 3MW of power in emergencies as defined above.

LOAD MANAGEMENT RATING

The Load Management rating is a special application of the Prime rating. A Prime rated generator applied under Load Management guidelines allows for a Prime rated generator set to be used in parallel with the utility.

A Prime rated generator set under Load Management guidelines can run for a maximum of 500 hours per year. This generator has the same nameplate rating as a Prime rated unit, but allows for an average load factor of up to 100%. The Prime rating with Load Management guidelines does not allow for a 10% overload capability. For example, these guidelines state that a 2.7MW unit (same nameplate rating as the Prime rated unit) can be run at 2.7MW for a maximum of 500 hours.

There are two basic load management practices: base loading and peak shaving. In base loading the generator operates at a fixed kW output and the utility provides power for any peaks above that level. In this scenario the end user may export power to the grid if more power is being generated than is required by the facility loads and the appropriate agreements with the utility are in place.

CONTINUOUS RATING

In this application the generator is able to provide power to a non-varying load for an unlimited number of hours per year. The average power output of the generator is 70% to 100% of the rating. The rating is designed to provide 100% of the rating for 100% of the operating hours.

Typical Continuous rating applications include base loading in parallel with the utility and cogeneration operations.

PERFORMING DUAL DUTY

Intelligent use of ratings can help you to use power systems for the added purpose of load management. Here, advanced switchgear is part of the equation.

Customers often want to use backup generators for load management to produce an additional return on their investment. However, standby rated generator sets are not intended for operation in parallel with the utility. To perform utility paralleling applications a prime rating with load management guidelines or a continuous rated unit are appropriate.

Paralleling switchgear combined with the correct prime rated units is required when operating under load management guidelines. The switchgear controls provide increased system flexibility allowing generator sets to operate in parallel with the utility. The switchgear is able to control the load on the generator sets and monitor the power supplied from the utility. This ensures that the generator sets are providing the proper load and are not operating outside of their rating guidelines.

DATA CENTRE CONTINUOUS

This is a project specific rating to provide compliance with the Uptime institute Tier 3 & 4  categories where a data centre has a specific requirement for a 'continuous' rating.

DO YOU NEED MORE INFORMATION?

Regardless of the application, generator ratings help to ensure that your power needs are met and that generating equipment is protected from premature wear. Choosing the right rating means making the proper tradeoffs between run hours, peak load and average load. The proper rating delivers the optimum combination of installed cost and long-term cost of ownership.

Energy Power Systems sales staff understand these generator ratings and are well positioned to assist with rating choices and act as a safe source of information for supporting the entire power system.

Contact your Energy Power Systems Sales Representative for more information on choosing the best rating for your project.

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