Understanding Generator Set Ratings

Mission Critical 

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 can run for a maximum of 500 hours per year. Typical peak demand is 100% of the rating for maximum of 5% of the operating time. The mission critical rating is not for use in utility paralleling applications. An example of the mission critical rating is shown in Figure 2.

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

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, 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 2.7 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. An example of the Prime rating is shown in Figure 3. Prime-DCP differs by allowing up to 100% average load factor in data center applications.

LOAD MANAGEMENT

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.

Load Management Practices

There are two basic load management practices: base loading and peak shaving. In base loading, the generator set 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 then is required by the facility loads, and the appropriate agreements with the utility are in place. An example of base loading is shown in Figure 4.

In peak shaving, the utility provides a fixed amount of kW and the end user deploys the generators to pick up, or shave, demand peaks and any other load requirement over that fixed amount of load. Peak shaving can be used during times of the year when the utility has demand higher than its capacity. In this case, the utility provides incentives to generator set owners to use peak shaving to decrease the load demand from the grid. An example of peak shaving is shown in Figure 5.

Continuous

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. An example of a continuous rated generator set is shown in Figure 6.

Performing Dual Duty

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

CONCLUSION

Regardless of the application, generator ratings help ensure that customers’ 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 means the customer receives the optimum combination of installed cost and long-term cost of ownership.