Your questions answered: Understanding Generator Set Rating Definitions

Nicholas Paolo, PE, regional sales engineer at MTU Onsite Energy tackled unanswered questions from the March 2, 2017, webcast on understanding generator set rating definitions.


Nicholas Paolo, PE, regional sales engineer at MTU Onsite Energy tackled unanswered questions from the March 2, 2017 webcast on understanding generator set rating definitions.Various factors influence a generator set’s ratings, and how familiarity with these factors can contribute to more economical and reliable facility designs. The focus of this webcast includes ISO standards for generator sets rated for use in standby, prime, and continuous power applications, as well as flexibilities that generator set manufacturers use when rating their products—such as net power output versus gross power output—and their implications in terms of generator performance. In addition, various starting methods are compared including across-the-line start and soft-start solutions. EPA exhaust emission requirements are examined, as well as their implications in emergency, non-emergency, and mobile applications. Specific industry ratings, such as data centers, are reviewed and considered, including the implications when specifying generator sets.

Nicholas Paolo, PE, regional sales engineer at MTU Onsite Energy tackled unanswered questions from the March 2, 2017, webcast on understanding generator set rating definitions.

Question: What is the typical time that manufacturers use in their software between steps? We can design steps of delay via time clocks, time delay relays, or software. But how do we know that our time settings between steps match the generator vendor’s software time delay between steps?

Nicholas Paolo: The software assumes full recovery between steps. The report will state the time in seconds to recover from a step, which would represent the minimum delay time.

Q: Are you aware that some VFD manufacturers l recommend that the nonlinear loads should not be more than 20% of the generator capacity to minimize distortion of the generator output voltage waveform? Thus, the genset would have to be oversized, reducing the cost savings noted on your soft-start example. 

Paulo: Yes, MTU OE software has the ability to account for harmonics and oversize the generator. Other manufacturer sizing programs have similar functionality.

Q: What are the pros and cons of using an off-skid cooling system?

Paulo: The pros would be to locate the cooling package separate from the generator set, such as on the roof to be able to discharge heat if the cooling package cannot be located in front of the generator set. The cons could include added complexity, such as additional piping, pumps, valves, and power distribution for the motor on the cooling package.

Q: What is wet stacking?

Paulo: Wet stacking is a condition where hydrocarbons from the diesel fuel are not completely combusted and instead build up in the exhaust. Diesel engine generator sets are recommended to be operated at or above 30% of the generator set nameplate rating.

Q: Do you recommend fractional pitch 2/3, 5/6, etc. for onsite generators?

Paulo: The industry has standardized on 2/3 pitch today for backup generator sets, but custom generator pitch machines are available for applications, if required.

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