Power for Data Centres: Save the (Virtual) Kittens!
Data centres are essentially hubs for storing and processing electronic information; they perform functions ranging from storing financial information (how big is my overdraft today?) to hosting web sites (videos of cute kittens). The servers and IT equipment they contain are dependent upon the power supply, yet much of the power requirements stem from auxiliary systems including temperature and humidity control, security and fire suppression. Proper design of the power system is a key element in the success (or failure) of a data centre; CEE can offer a range of programs to aid designers.  

What are the challenges facing data centre power system designers? 
Put simply: if power is lost, data will be lost. For this reason, many data centres employ back-up power systems in the form of redundant power supplies, Uninterruptible Power Supplies (UPSs) and back-up generators. The Power*Tools for Windows (PTW) power system analysis software sold by CEE can answer some key reliability questions: 

     • How long can I support my system if main power is lost? (How much life support do the kittens have?) 
     • If there is a fault, how much of the system can I isolate? (How many kittens can I save?) 
     • Which parts of the system are most likely to fail? (Where are the kittens in most danger?) 
There are many sources of harmonics in a datacentre, ranging from variable-speed drives (VSDs) in air conditioning plants to thyristor-based rectifiers and inverters in UPS systems. Third-order harmonics are a particular problem in a three-phase system since the third-order currents from each phase add together in the neutral conductor to produce an unusually high neutral current (see later). Delta-connected systems without a neutral do not experience this problem; however datacentres employ many single-phase loads which must therefore be connected in star formation for a three-phase supply.  
Worse still, the need for energy efficiency has led to an increase in the use of solid-state transformers. These devices use thyristors to increase the supply frequency to 10,000-20,000 Hz, then operate a transformer at the higher frequency before using more transistors to drop the output frequency to 50 or 60Hz. Solid-state transformers experience lower conversion losses than traditional ones, but the impact of all the transistors is to inject many high-order harmonics into the transmission system; often up to the 100th harmonic. High-frequency (high-order harmonic) currents cause conductors to overheat and also stray capacitances that can damage switchgear. And hurt kittens.

The PTW program from CEE has a dedicated module for frequency analysis and another for single-phase and unbalanced systems. These tools can be used to help you to calculate where harmonic problems are likely to occur and take steps to mitigate them.
It is well known that a strong earth connection can be useful for detecting electrical faults and for safely dissipating fault current. However, earth connections for datacentres have greater significance since the equipment used often generates high neutral currents (see above). Therefore, the neutral must have a strong earth to ensure that it does not experience a voltage rise. CEE offers the GroundMat software for earth system design, including the effects of different layers of substrate with different resistivities. Since power system earths are used to help reduce harmonic currents (for example using earthed capacitor banks), the earth conductors may themselves be subject to “noise” in the form of high-frequency currents. 
Sensitive electronic components requiring a “clean” (noise-free) earth will therefore require a separate earth connection. It is also important to ensure that a normal (“noisy”) earth conductor is segregated from sensitive data cables to prevent induction of harmonic currents.

Kittens don’t like noise.
Inrush currents:
Generally inrush currents are caused by switching of motor loads or capacitive circuits. In data centres the main motor loads are pumps and fans in the air conditioning plant; often these motor starting currents are limited with soft-starters or variable speed drives so the overall impact is reduced. The Transient Motor Starting (TMS) module of PTW can help you to calculate what these starting currents are likely to be.  

However, inrush currents can be a problem on low-power circuits too. The drive for efficiency in data centres has led to the installation of motion sensors for controlling lights and for non-essential equipment to be powered down when not in use. This in turn means that low-power circuits often experience frequent switching operations (for example, as employees move through the building triggering lights). Lighting circuits are a particular problem because efficient LED light fittings and electronic ballasts for fluorescent tubes are both capacitive devices which have high peak inrush currents, albeit for a fraction of a cycle. If the impedance of a circuit is too high, inrush currents can cause brief voltage dips which may cause light fittings to fail to start. Inrush currents can also cause the premature failure of contactors and other switchgear. Kittens should never be in a rush.  
The electronic equipment in datacentres generates a large amount of heat. Cooling systems are installed to protect server cabinets, but care must be taken that power cables are also protected from higher temperatures, particularly in remote corners where cooling is less effective.

PTW’s Cable Ampacity module can calculate de-rating (reduced current capacity) of cables based on ambient temperatures, poor ventilation or bunching of cables within ducts. Kittens don’t like being overworked.
Do you want to know more about what CEE can offer you? Why not visit us at Data Centre World in London on 21st – 22nd March. Think of the kittens!