Understanding Generator Set Ratings

Introduction

Generator set ratings may seem complex, but their basic purpose is simple: to fit the application needs at the optimum reliability, performance, and cost. An improper rating means either buying more capacity than needed or risking shorter life to overhaul, more repairs, and more downtime. The key to choosing the right rating is to understand the application in detail—not only the type of duty but also the average load factor, maximum required load, running hours per year, and whether the generator will operate isolated from or in parallel with the utility.

Caterpillar has developed a comprehensive framework for generator set ratings that both aligns with and extends beyond the ISO 8528-1 standard. This article summarises the key concepts from Caterpillar's white paper on understanding generator set ratings, providing practical guidance for specifiers and operators.

The Purpose of Generator Set Ratings

Generator set ratings help ensure that customers' power needs are met and that generating equipment is protected from premature wear. The ratings are important because they directly impact the efficiency and effectiveness of the selected generator set based on how it's going to be used.

Modern generator sets offer more flexibility than ever before. Sophisticated switchgear can be integrated with generator sets, allowing systems that closely match specific installation requirements. However, this flexibility also means that understanding the nuances of different ratings is essential for making informed decisions.

The ISO 8528-1 Standard

The International Standards Organization has developed guidelines for common rating definitions through ISO 8528-1. This standard defines four primary ratings:

• Emergency Standby Power (ESP)
• Prime Power (PRP)
• Limited-Time Running Power (LTP)
• Continuous Power (COP)

While Caterpillar uses ISO 8528-1 as the basis for its generator set power rating classifications, it has developed additional ratings to more closely match rating selection to typical applications. Cat generator set ratings differ in certain respects from those defined by ISO 8528-1, but will always meet the minimum criteria set forth by the standard.

Caterpillar Generator Set Ratings

Caterpillar defines six basic generator set ratings: Emergency Standby Power (ESP), Standby, Mission Critical, Prime, Prime-DCP and Continuous. Each rating is designed for specific applications with distinct load profiles and operational requirements.

Standby Power

In this application, the generator set 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 3 MW 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 2.1 MW.

Emergency Standby Power (ESP)

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. Typical operation is 50 hours per year.

ESP is the ISO 8528-1 equivalent rating, and it represents the maximum power available during a variable electrical power sequence in the event of a utility power outage. No overload is available for either ESP or Standby ratings.

Mission Critical Standby

This is a Caterpillar-specific rating with no direct ISO equivalent. In this application, the generator set 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 rated generator set should be no more than 85% of the nameplate rating with varying loads.

A mission critical generator set typically runs about 200 hours a year, with a maximum of 500 hours per year. No overload is available. Typical peak demand is 100% of the rating for a maximum of 5% of the operating time. The mission critical rating is not for use in utility paralleling applications.

Figure 2: Example Load Profile – 3 MW Mission Critical Standby Rating. This figure shows a load profile where the average load factor during an outage is near 85%, typical of mission-critical facilities.

For example, a 3 MW mission critical standby-rated generator set will provide power for the duration of an outage. It could be run for up to 500 hours per year and have an average load factor of up to 2.55 MW.

Prime Power

In this application, the generator set is capable of providing power to a varying load for an unlimited number of hours per year. A Prime rated generator set 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 1 hour in 12 hours, and for no more than 25 hours per year. Time spent during operation above 70% load may affect the life to overhaul of the generator set. The standard prime rating is for use in either utility paralleling or isolated applications.

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

Prime-DCP (Data Center Power)

This rating is specifically for data center applications. Prime-DCP power output is available with varying load for unlimited time. The average power output is not to exceed 100% of Prime-DCP rated ekW. Typical peak demand is 100% of the prime-DCP rated ekW with 10% overload capability for emergency use for a maximum of 1 hour in 12. Overload operation cannot exceed 25 hours per year.

Load Management Guidelines (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 set 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 2.7 MW unit (same nameplate rating as the Prime rated unit) can be run at 2.7 MW for a maximum of 500 hours.

Continuous Power

In this application, the generator set 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 set is 70-100% 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 co-generation operations.

Understanding the Engineering Behind Ratings

Engine Design Factors

The development of engine ratings involves balancing structural, thermal, and wear factors. Several primary considerations drive the limits of engine ratings:

Peak Cylinder Pressure – Increased peak cylinder pressure impacts structural integrity through increased vibration and effects on main caps, head/block joints, and wrist pins; causes thermal fatigue in components such as the cylinder, block, heads, and pistons; and increases wear in valve seats, piston rings and main bearings.

In-Cylinder Temperature – High temperatures cause thermal fatigue due to increased heat on the bottom head deck, piston crater rim, and exhaust ports; accelerate wear of valves and valve seats; and require careful management of piston cooling.

Exhaust Temperature and Pressure – These impact structural integrity through vibration on the manifold head gasket joint, turbo gasket joint, and slip joint; cause thermal fatigue and oxidation of the manifold, valves and guides, turbo and manifold studs; and increase wear and erosion of the exhaust manifold.

Maximum Injection Pressure – High pressures create mechanical fatigue in the rocker arms/shaft, studs, and injectors, and generate additional heat in the injectors.

Alternator Design Factors

Alternators are thermally limited by the amount of internal heat created and the ability to dissipate that heat. A critical variable is the rise in winding temperature above the ambient temperature, which is due to the flow of current in the windings and internal losses. Caterpillar follows the standard by specifying temperature rise capabilities based on continuous-duty operation at 40°C ambient conditions. For alternators with a standby rating, an additional allowance is made over the maximum temperature rise for continuous-duty operation. However, operation at standby temperature rise values causes the alternator insulation to age thermally at about four to eight times the rate that occurs at continuous-duty temperature rise values. Given that typical usage of standby units is less than 500 hours a year (compared to approximately 8000 hours for continuous duty), this does not lead to premature insulation aging if application guidelines are followed.

Rating Selection Considerations

Rating Trade-offs

The selection of the right rating results from making the proper tradeoffs between run hours, peak load, and average load. An engine de-rates through the various definitions, for example (nameplate kW at Standby / Mission Critical / Prime / Continuous):

• C175-20: 4000 ekW / 4000 ekW / 3600 ekW / 3250 ekW
• C175-16: 3000 ekW / 3000 ekW / 2725 ekW / 2500 ekW
• 3516C-HD: 2500 ekW / 2500 ekW / 2250 ekW / 2050 ekW
• 3516C: 2000 ekW / 2000 ekW / 1825 ekW / 1650 ekW

The Software Factor

In the modern world of electronically controlled engines, the software flashed into the electronic control system determines if the engine operates as a continuous, prime or standby unit. In some cases, equipment may be marked with dual ratings, such as Prime and Standby ratings. This may be puzzling as load factors are limited to 70%, 85%, or even 100% depending on which rating is being applied. The key lies in how the equipment is used.

For example, a standby unit would have the capability of operating at about 90% load factor if the time above that load is minimal. The Mission Critical Standby rating defines a rating that is an aggregation of several others, intended for specific applications.

Environmental and Site Conditions

Careful consideration must be given to the environment and site conditions (altitude, temperature, air flow restrictions, etc.) in which the generator set will be operating. If these conditions are outside the norm, they could prevent the generator set from meeting performance expectations.

Performing Dual Duty

Intelligent use of ratings can also help customers use power systems for the added purpose of load management. In some applications there may be a desire 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.

Conclusion

Generator set ratings may seem complex, but their basic purpose is simple: fit the application needs at the optimum reliability, performance, and cost. An improper rating means either buying more capacity than needed or risking shorter life to overhaul, more repairs, and more downtime.

Caterpillar offers a portfolio of generator set ratings designed to provide coverage from general to special purpose applications while maximizing owner value. By understanding the application in detail—including average load factor, maximum required load, running hours per year, and utility paralleling requirements—specifiers can select the appropriate rating to optimize performance, durability, and cost.

The key to making the right choice lies in understanding the tradeoffs between run hours, peak load, and average load, and working with knowledgeable dealers who can assist with the selection process. Regardless of the application, generator set ratings help ensure that your power needs are met and that the generator set is protected from premature wear.

This article summarises key concepts from Caterpillar's white paper 'Understanding Generator Set Ratings' and related technical publications.

Download the full free white paper here: Understanding Generator Set Ratings – Caterpillar White Paper

References: Caterpillar Inc., Understanding Generator Set Ratings, Electric Power Division White Paper. Caterpillar Inc., A Holistic View on Generator Set Ratings, Electric Power Division White Paper (2014). ISO Standard 8528-1 (2005), Reciprocating Internal Combustion Engine Driven Alternating Current Generating Sets – Part 1: Application, Ratings and Performance. Caterpillar Inc., Application and Installation Guide.

Disclaimer: This article provides a summary of technical information from Caterpillar publications. While efforts have been made to accurately represent the content, please refer to the official documents for complete technical details. Final design, specification, and implementation decisions should be made by qualified professionals in accordance with all applicable regulations and standards.