Saturday, March 14, 2009

Proper Sizing of Your Generator for your Data Center

Written by Rakesh Dogra

Data centers are popping up around the world. These facilities that we rely on to process and store our information are becoming more and more critical every day. The high criticality of these facilities is creating a spike in data center availability requirements.

Of course there are several factors which could lead to a disruption in the services provided by a data center but one main factor is the grid power failure which could bring the entire system to a halt, unless necessary provision is made for back up.

Such a provision exists in the form of flywheels, UPS and battery back up but these are only sufficient for a relatively short duration of time ranging from few seconds to few minutes at the most. Moreover these systems only provide power to the critical IT equipment and not to the secondary systems including cooling. Back up or standby generators are a must if a data center has to ensure long term reliability and provide sufficient back up power which could last a few hours or even a couple of days if circumstances so require.

Selecting the back up generator of the proper size and power rating is of utmost importance to ensure that the generator is able to cope up with the demand when it is actually required from it.

The calculation of the total power required for a data center is basically a simple procedure and involves adding up the power rating of all the equipment which consumes electrical energy. This includes IT and cooling equipment. Of course all the loads may not be working simultaneously during actual operation at all times, but it is always advisable to have a provision to handle peak loads with the generator since it represents the worst case scenario and takes care of the maximum load situation at any given time in case of grid power failure.

It must also be remembered that though the load requirements for IT related equipment can be found from simple addition of the power ratings of the different equipment, the same is not true about machineries such as electric motors. An electric motor draws a much higher current during the initial starting phase and finally settles down to its normal rated value after it has attained sufficient speed. Hence the total number of motors and their power rating plays an important part in determining generator size.

Provision must be kept for a situation wherein all motors are started simultaneously and hence consume several times more power than their combined rating for their starting period and this is the load which the back up generator should be able to handle without much fuss.

Moreover data centers normally tend to grow in their capacity over a period of time with the growth of the company. This in turn means a rise in the power and cooling requirements of the data center. Of course there may not be a magic formula for calculating the given power requirements for a certain time in future, a rough estimate should be available regarding future expansion based on company plans and industry trends. The generator should be able to cope up with this rise in demand in the future and hence its rating should be somewhat higher than the maximum peak load calculated previously.

Another factor to be kept in mind is that generators consume fuel and the bigger the size, the more that fuel is consumed. Hence an optimum balance also needs to be struck between the generator size requirements and fuel efficiency. For example let us take a hypothetical example in which the power requirement is estimated at 50 KW but the normal load is around say 15 KW. This means that the generator would be running at a much lower load than its rated power which has two disadvantages.

Firstly since most generators are diesel engine operated, their efficiency is quite low at low loads and secondly large amount of fuel will go to waste for the relatively lesser amount of power that is required. For these reasons in actual practice in large modern day data centers, a single generator is not feasible to handle all the power requirements; hence companies use an array of generators which can provide the necessary power. For example Google has installed more than three dozen generators in their Iowa Data Center.

Apart from choosing generators of the right size and capacity it is also important that the generators are kept well maintained and serviced at the appropriate intervals. This interval is either in the form of calendar time or running hours as specified by the manufacturer and this schedule should be strictly adhered to. Routine operation of standby generators is necessary to ensure that they start without problem during an actual emergency and when running, they should be properly monitored for their parameters to get any indication of a possible fault.

Testing of the back up generator also has different levels and though some data centers might be happy with starting them once a week, running for some time and shutting them down, other companies might recommend drills which help to ensure and monitor the real availability of these generators during times of need. The grid power in such a case is deliberately cordoned off so that the reaction of the generators and automatic transfer switches (if present) can be seen. But in actual practice data center managers do not like to follow this practice as they shudder at the thought of a possible loss of availability of their data center.

Hence we have seen that the back up generator is a necessary piece of machinery which every data center should have of the right size and ratings. Their installation helps to ensure that the various services provided to clients are continued interrupted despite any failure in grid power for whatsoever reasons.

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