Getting the most out of your generator set is critical. The place to start is with a solid understanding of the genset ratings definitions. Bryan Snyder gives us a great explanation of generator set ratings. Be sure to like and subscribe to the show wherever you listen. Send feedback to [email protected]
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Lou: One topic that continues to confuse genset users are the ratings. A) they seem to mean different things from different manufacturers and B) who has time to keep up with them? Well today, we are going to attempt to shed some light on a few of the ratings we see a lot of at Caterpillar.
Lou: Joining me today via the Cat Electric Power Hotline is Bryan Snyder.
Lou: Thanks for joining us
Lou: Can you start us out with a brief definition for generator ratings and why we have them?
Bryan: Certainly, generator ratings are made up of two components. First there is the numeric rating value that describes the amount of power a genset can produce, and second is the named rating category which describes the operating conditions. These two components combine to define how the operation of the genset impacts the life expectancy of the genset.
Lou: That is interesting, are there any standards organizations that helps to govern the genset rating categories?
Bryan: Yes, the International Standards Organization (or ISO) has the most commonly referenced standard for design and specification of reciprocating engines with their standard ISO 8528. There are several parts to this standard with part 1 defining Application, ratings and performance. In the 2018 version you would find the information on Power Ratings in section 14 of Part 1.
Lou: Can you provide us with a brief overview of the rating categories that are defined by ISO?
Guest: Absolutely. The first category is Continuous Power (COP) which generally references a consistent electrical load of 100% load factor for unlimited hours.
Next is Prime Power (PRP) which references a variable load of 70% load factor for unlimited hours with an allowance of 10% for overload conditions. It is important to note there are several more complex stipulations for these categories that should be referenced in the standard, but this is just a short overview.
Moving on, there is Limited-Time Running Power (LTP) which references a 100% load factor for a limit of 500 hours per year.
The next category is Emergency Standby Power (ESP) which references a variable electrical load of 70% for a limit of 200 hours per year.
The final rating category is the newest and was just recently added. This is known as the Data Center Power (DCP) for this rating the power could be consistent or variable but is considered a 100% load factor for unlimited hours. The one major difference between this rating and the others is that it references that it is dependent on the site to supply and the availability of a reliable utility.
Lou: So as you said, there is more details to the definitions that our listeners could reference in the standard, but do you see any significant deviations from the standards definitions in the industry usage?
Guest: Actually yes, the standard has been the baseline, but each manufacturer may have some deviations for their specific offerings. For example Caterpillar's standby rating allows users up to 500 hours per year of operation instead of the 200 hours per year allowed for in the ESP rating from ISO. Also, several years ago another manufacturer extended the load factor for their standby rating to 85%. Caterpillar has since developed the Mission Critical rating which provides 85% load factor for up to 500 hours per year. It is important to work with your manufacturer to understand all of their ratings, the limitations and the expected maintenance intervals.
Lou: This all seems a bit confusing, so let's boil this down to practical terms on how this impacts our listeners. Can you walk us through how the 'time' elements of the different definitions impact power output?
Guest: Certainly, let us think of it this way. If we have a specific engine configuration, the physical properties of the engine are limited by temperature and time. If you run the engine cooler it can last longer. If you run it hotter you can't run it as long before you need to do maintenance. So if we take into consideration a standby genset rated for 2500kVA. That generator should be operating at an average load factor 1750kVA for less than 500 hours a year per Caterpillar's requirements. Just to be clear, it can produce the full 2500 kVA if required, but this assumes there are variabilities in your load, like cooling loads or day to night time cyclical loads that delivers an average load of 70% over 24 hour period.
Now, If we were to run that same genset configuration in a Prime (or PRP) application which has unlimited hours of operation at 70% average load factor, we would reduce the 100% rating of that genset to 2250kVA. The 70% load factor would then be 1575kVA.
Finally, if we were to rate that same engine configuration at a Continuous rating the unit would be rated at 2000kVA. It would then be expected to be capable of 100% load factor for unlimited hours per year.
Lou: So, it seems like you must start with the user expectations of load and runtime then move to the ratings to assure the expectations are met?
Guest: Yes, the ratings are intended to help users understand how much power they can get from a genset for how long. One of the cases we see where users miss apply ratings is when they may choose a continuous rating because they don't want limits on the power or the runtime. This often means they will end up with a genset that is way oversized for the application because the engine configuration has to be increased based on the assumption they are actually going to run at high load factors continuously. This can cause real problems when the application doesn't meet these standards because the gensets are severely underloaded which can cause long term maintenance issues.
One important thing I would like to point out with regards to Caterpillar gensets is the hour limitations we spoke of are for guidance related to the operation and maintenance of the genset. We also include a stipulation in our ratings which states they are capable of providing power for the duration of an outage. This means if you are at 490 hours of operation in a year and you have a storm that takes out your power, the genset will not automatically shutdown at 500 hours. It would be recommended to be in touch with your Cat dealer to discuss maintenance and reviewing the unit at the next most convenient time, but Caterpillar will continue to support you through the outage.
Lou: That's great information and I think it is a bit more clear on how this impacts the user. There is one rating that I would like to go back to that is getting a lot of press. It is the new Data Center Power rating you referenced. What can you tell us about this rating and maybe the background behind it?
Guest: Yes, the DCP rating came about based on a culmination of several factors.
First the Data Center market has been booming for several years now and the data center customers have driven some fairly specific requirements. The most basic of these are that the Data Center customers do not want limits on their operation of the gensets when they need them. Data Centers have become critical to our way of life and if the utility power is lost they must be able to rely on their gensets to operate.
Along with that requirement from the customer's themselves the Uptime Institute, which has been one of the leaders in developing standards and certifications for data centers, provides specific requirements which also require the removal of limitation on the operation of the genset. As this market driver was pushing the market requirements one of the manufacturers came forward and developed their own data center rating. To keep up with this trend and in a hope to standardize the industry the ISO standard was updated to include the DCP rating. There are some factors associated with this rating from ISO that the market doesn't accept completely at this point Caterpillar is investigating the rating further.
Based on our understanding of the actual operation of data centers, our current path is to provide our Mission Critical rating. If a customer requires a letter to meet a certification requirement from the Uptime Institute or something similar, they can work with their Cat Dealer and the Cat factory to meet their needs and verify they have chosen the best rating category for their specific application.
Lou: So Bryan, I've been working and taking some notes today and it seems the takeaway really comes down to this.
1) Ratings are a baseline for understanding a gensets capabilities based on a specific life expectancy.
2) For the most part it's an evaluation of temperature over time. If you need more hours, you're going to get a lower mean power level which in turn, delivers a lower temperature. The Fewer hours required, basically, the more power you can pull from the generator knowing it's not going to run for long.
With the notable factor being that Cat Gensets are rated capable of providing power for the duration of the outage.
Did I get that right?
Bryan (response) followed by the remark below.
We have much more in-depth presentations and white papers on this subject. If there are listeners that would like to have further discussions on this material I would ask that they reach out to their Cat Dealer and request some further support. We would be happy to work with them on their specific application.
Lou: There you have it folks. I'd like to thank "Bryan" for sharing his insights with us today… and thank you, our listeners for listening to Power Bytes.
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