While Uptime Institute references the ISO8528-1 definitions for generator ratings in their publication Tier Standard: Topology, they do not require the use of these definitions for generators to meet the Tier III and Tier IV requirements, as described in the same publication. A more cost-effective and reliable generator rating that meets the Tier III and Tier IV requirements can be achieved when the generator manufacturer develops ratings specifically for data centre applications.
Diesel generators in a Tier III or Tier IV system
In Tier III and Tier IV systems, Uptime Institute defines the diesel generators as the primary source of power and the utility as an economic alternative. This definition puts two important requirements on the diesel generators. First, they must be large enough to carry the entire data centre load. Second, there can be no limit on the number of hours the diesel generators can run.
ISO8528-1
In their Tier Standard: Topology publication, Uptime Institute references the Continuous (COP), Prime (PRP) and Standby (ESP) generator rating definitions of ISO8528-1. Of these ISO definitions, the only rating that meets Uptime Tier III or Tier IV at 100 percent of the generator rating is Continuous. The ISO Prime rating definition states that the generator can run continuously if the average load does not exceed 70 percent of the full generator prime rating in any 24-hour period. The ISO Standby definition limits the run-time to no more than 200 hours per year, plus it limits the generator to no more than 70 percent average power during any 24-hour period. See Figure 2 for a comparison table of the ISO8528-1 ratings.
In 2018 ISO added a new rating called Data Centre Power, to the standard. Although it is intended for use in the data centre market this rating is not appropriate for Uptime Institute Tier III or Tier IV certification. The rating definition makes reference to availability of a reliable utility and prohibits prolonged operation in parallel with a utility. A core premise set forth by Uptime Institute is that the only reliable source of power is that generated on site. For this reason, any limitation on operation of the generator set or dependence on a utility is not acceptable for Tier III or Tier IV certification.
The Continuous definition describes a constant load with no variance. In Tier III and Tier IV data centres, the generators need to be sized for the maximum load (they are the primary source of power), but this maximum load will not be the constant load of the facility. Therefore, if the COP rating is used to specify the size of the data centre generators, these generators are most likely oversized for the application.
It is important to note that Uptime Institute references the ISO definitions, but they do not require Tier III and Tier IV generator ratings to be defined by ISO.
No limit on run-time
The Tier Standard: Topology publication also states that the generator manufacturer’s certification of capacity at an unlimited duration will be used to determine compliance with the applicable Tier requirements. This statement allows a generator manufacturer to design and rate a generator that more closely fits the needs of a data centre and still meet the Tier III and Tier IV requirements. The Uptime Institute will also accept a site specific rating for a generator set. In this scenario, the manufacturer submits a letter stating that there is no limit to run time for the generator set at a particular rating at a particular site. See a typical load profile of a data centre in Figure 3. When the generator manufacturer provides a generator with no run-time limit that is sized closer to the site load and can still supply the maximum load spikes, the data centre gets the most efficient use of their generator system.
Some generator manufacturers start with the ISO Prime rating and reduce this rating by a certain percentage to achieve the no run-time limit requirement. Some manufacturers will provide a non-ISO defined rating that guarantees no run-time limit at 100 percent of that rating. These methods are discussed in section 3.3 of Uptime
Institute’s Tier Standard: Topology publication. These unlimited duration ratings from the generator manufacturer will be used to determine compliance with Tier III and Tier IV requirements. Cummins has a complete line of high-horsepower generators with a Data Centre Continuous (DCC) rating meeting these requirements. Detailed specification sheets are available for these generators in Cummins Power Generation’s online resource, Power Suite (powersuite.cummins.com).
