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% 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% 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% average power during any 24-hour period.
The continuous definition describes a constant load with no variance. In Tier III and Tier IV data centre, 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 center 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.
Fig. 1: ISO definitions for continuous, prime and standby power.
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. See a typical load profile of a data centre in Fig. 2. 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 center gets the most efficient use of their generator system.
Fig. 2: Typical data centre load profile.
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 Power Generation 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 Suite1.
The graph in Fig. 3 compares a system with 2500 kW DCC rated generators to a system with 2500 kW ISO Prime (PRP) rated generators. This graph shows that this system requires four 2500 kW DCC-rated generators to meet demand, whereas five 2500 kW ISO prime rated generators would be required to meet the same demand with no run-time limit.
Fig. 3: Data centre continuous (DCC) system vs. ISO Prime (PRP) system.
Oversized generators
As stated before, an ISO Continuous (COP) rated generator does meet the no run-time limit for Uptime Tier III and Tier IV. But a COP rated generator may not be the best fit for a typical data centre load profile, nor does Uptime Institute require generators to be ISO COP rated.
There are optimum load levels for running diesel generators. One rule of thumb for optimum engine life is that the generator engine should be run greater than 30% of its maximum horsepower rating. This ensures that the exhaust temperature will be high enough to burn up the majority of particulate in the exhaust. Incomplete combustion at low loads can cause increased valve wear and degradation in the turbochargers, both of which increase maintenance needs and decrease reliability. In Fig. 4, we compare a system utilising five 1,6 MW COP rated generators to a system utilising five 1,6 MW DCC rated generators. The load shown is about 36 to 39% of the generator system’s 100% rating. This light load is very common in real-world data centres. As the graph shows, the COP rated generators are running below 30% of their maximum horsepower, while the DCC rated generators are running above 30% of their maximum horsepower. The DCC rated generator system is the more reliable choice in this example, and it will operate closer to the optimum horsepower of the engines, reducing the risk of low load failures.
Fig. 4: Data Center Continuous (DCC) system vs. ISO continuous (COP) system.
Choose the most efficient generator for your data centre
To summarise, it is best to choose generators that most closely fit the load profile of your data centre and still meet the Uptime Tier III and Tier IV standards. This will provide the most value. Plus, the generators will run closer to their optimum load level, which will increase the life and reliability of the generators.
Contact Kobus Coetzer, Cummins South Africa, Tel 011 321-8700, kobus.coetzer@cummins.com
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