There are many causes of data centre outages but loss of power doesn’t need to be one of them and a well-chosen generator set can take this concern away.
If you’re thinking of installing a standby generator into a data centre’s power system, there are several key decisions:
- Data centre generator set selection
- Data centre continuous ratings
- Generator set package integration
- Generator set control systems
- Generator set project delivery
Generator Set Selection
The first and most important decision is choosing the right generator set rating to meet demands from the critical UPS / IT and cooling / chiller loads (also called the N demand). If the data centre is likely to expand, it’s important to consider that this demand may change over time.
The decision on whether the generator set will supply power at high or low voltage depends on the size and overall scale of the data centre, on the tier distribution topology and on the actual space available to accommodate the generator sets. Typically when the N load requirements are above 4 -5 MW, high voltage is the best option.
The critical UPS / IT load can influence generator selection in several ways:
- UPS battery recharge which can be 10-20% of UPS rating. This power may need to be covered by the generator. In some cases, an operator may opt not to recharge when running the generator set. This will affect the design autonomy when transferred back to the mains and presents a risk which the data centre operator must weigh up.
- Modern UPSs present quite low levels of harmonic current distortion (THDi) to the generator set although in situations where the UPSs are in bypass mode, the higher harmonic distorting IT / Server load needs to be supplied by the generator set which may require an oversized alternator to ensure the quality of voltage waveform is not affected.
- Most modern IT loads present a leading power factor by nature. This does not create an issue for UPSs in normal mode, but again if the UPS is in bypass mode, the leading power factor load must be supplied by the generator set directly and this may cause voltage instability issues which may require an oversized alternator.
It’s important to note that the effect of UPSs in bypass mode will depend on the UPS kVA rating in proportion to the generator / generators rating. A supplier will be able to provide detailed support on this and other aspects of generator set selection.
Data Centre Continuous Ratings
The running duty of the generator set in a data centre application is very important, especially when we look at the requirements defined by the Uptime Institute. FG Wilson in conjunction with our sister company Perkins have developed a Data Centre continuous rating which delivers unlimited hours of annual operation with no restriction on the average load factor up to 100% constant load. This rating complies fully with Uptime Institute Tier III and IV continuous operation requirements and is currently available from the P400-1 through to our P2500-1 models.
Generator Set Package Integration
After determining the rating and number of generator sets required to meet site load demands, the physical integration of the units means consideration of ambient, noise, local planning regulations and fuel autonomy. The outcome of these considerations will influence the generator set package installation in terms of noise attenuators, exhaust silencers, cooling systems, package footprint, height and fuel storage. For example if a very low noise level is required this will greatly increase the size of attenuation, exhaust silencers and the overall package footprint, which of course will increase the package costs. It’s vitally important that the criteria outlined are as accurate as possible at the concept stage of the project. A generator set supplier should be able to provide in-depth support during this development phase.
Generator Set Control Systems
This covers a wide range of features and functions, many of them vitally important when delivering key control requirements for the generator set. For example, the ability to load sequence enables N+1 or N+2 functionally for redundancy. The extensive communication interfaces can facilitate interfacing with on-site management systems and enable operators to monitor the generator set remotely, improving 24/7 maintenance support.
Redundant PLC control systems are essential for delivering a high level of availability when managing the primary power of the data centre, essentially the mains and standby primary incomers and distribution.
Generator Set Project Delivery
When choosing a generator set brand it’s important to evaluate a supplier in terms of their ability to support in the definition of the project, the development of a project plan, the manufacturing, testing, site installation to the final commissioning and most importantly after sales maintenance support.
Robert Breadon is an Application Consultant at FG Wilson.