Essential Load

Essential Loads

9The power for these services is normally provided from a second separate MV/LV transformer via its own LV switchboard. Non essential loads need the highest level of reliability of power supply in order that the computers of the data centre can run in the event of a complete mains failure whatever its duration. Clearly even the shortest of mains failure (some milli-seconds) can result in a computer disturbance and loss of its data.
For these services the diesel generator system is also required, however even its start up time cannot be tolerated by the computer load. To overcome these issues a further back-up system needs to be interposed between the incoming MV/LV transformer or diesel generator system and the computer load in order to improve the stability/availability of the power supply. This second back-up system is named an “Uninterruptible Power Supply” or UPS and consists of a converter system (in these days mainly of the static converter type). Within this UPS system a large battery is incorporated designed to bridge the start up time of the diesel generator and provide a safe computer orderly shutdown should the diesel generator not start.
The UPS output power is then fed to a UPS Output Switcboard and in turn to the computer racks via Power Distribution Unit(PDU switchboard) located in the data hall.

System Rating
A normal sized data room is in the order of 500m2 of useable data space with 1.5 to 2.0 kW per m2 computer rack power density. This would lead to an “essential load” of 1MW. With air-conditioning “non essential” loads can be of the same order giving site loads of the multi megawatt level.
This in turn results very often in currents of some thousands of amps at the LV level. Careful consideration of the design of switchgear and the interconnecting cables or bus-bars is needed.

Increasing Power Rating and Reliability
For major data centres clearly it may be necessary to go further in the design to ensure that higher power levels can be accommodated or that higher reliabilities can be achieved. In order to do this a number of diesel generators may have to be operated in parallel. To do this synchronising switchgear will be required together with a control system to connect the right number of generators for the specific connected load. This control system is normally automatic and allows the number of generators to be optimised to supply the connected load.
For increased reliability the number of generators may also be increased to give an N+1 capability. This means that the number of diesel generators connected load will be such that should any generator fail to operate then the remaining generators will have sufficient capacity to meet the load requirement.
Similar principles can be applied to the UPS system in that increasing numbers of modules can be connected in parallel to meet a greater load requirement and or provide an N+1 redundancy for the UPS system. Normally the UPS system has a parallel operation control system whereby the load is constantly supplied via the UPS and is shared between the connected UPS modules.
In order to operate the power system of a data centre it is necessary to provide switchgear and correct protection of the entire critical power and air handling systems. The detail design of such switchgear is quite specialised in order to deal with the correct operation and discrimination of the switchgear during all overload and fault conditions on the system. Because of the unique protection of UPS systems in combination with diesel generators, care has to be taken to ensure the protective device nearest to the source of a any fault operates first without causing multiple tripping of other switchgear devices.
Another problem is often the fault level on the power system. With large low impedance electrical networks then the very high currents can fall during a system fault condition. Care needs to be taken in the design to limit the potential for current and to ensure that any switchgear devices and bus-bars are rated for such fault levels. 

Operation and Maintenance
Clearly, the correct design and specification of the electrical system goes a long way to providing a high integrity, reliable electrical power system for a data centre. The design needs to go further in order to ensure correct operation and maintenance of the system is possible. An additional consideration needs to be that of expansion as all data centres grow in load all the time.
Often the data centre is largely unmanned and so its electrical system needs to monitored carefully and alarmed remotely to off-site personnel. The site personnel often have not had reason to operate the electrical system on a regular basis and this can lead to confusion and misinterpretation of the electrical situation during fault conditions. For these reasons it is important to regularly test systems and to maintain training of personnel on a regular basis during the lifetime of the data centre.
Another key issue in regard to satisfactory operation of the data centre electrical system is to ensure that regular scheduled maintenance of all system components is carried out. It is not satisfactory to neglect such maintenance because of the difficulty in provisioning time to do maintenance or due to the cost of maintenance. Another issue is the provision of spare parts either on-site or with the maintenance subcontractors. Spare parts should be available for the specific data centre should be identified and recorded on an inventory on a regular basis.