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11kV Utility supplies for Data Centres

The amount of electrical power needed to supply data centres rises inexorably. This is despite seemingly never ending gains in IT computing power efficiency which only results in greater density, to the extent that the only viable option is to take electrical power from the utility grid at 11kV. This trend presents new challenges to the electrical engineer designing data centre power systems.

In the UK and Europe the standard LV system is 400/230V TN-S. The transformers needed to convert from the HV system (11kV in the UK) to LV should result in a standard LV system of this type. The designation TN-S describes the fact that the LV system neutral is separate from the earth system throughout the installation, with one neutral-earth connection at the transformer. There are various other configurations – TN-C-S, IT,TT etc but these are not suitable for data centres for various reasons, this is the subject of a separate blog.

The overriding concern for the designer is safety – life safety for personnel in the facility and the safety of the critical IT equipment. These are intimately linked. The danger to life of direct contact with the HV is prevented by insulation and secure containment and can be considered the easy part of the design. The difficult part is fault protection – ensuring that an earth fault on the HV system does not result in a voltage rise on the LV system that is dangerous to life or property.

The applicable safety standard is BS EN 50522:2010 “Earthing of Power Systems exceeding 1kV a.c”. Other information about the interface between the HV and LV systems is contained in BS 7040:2011+A1 2015 “Code of practice for the protective earthing of electrical installations”.

The permissible touch voltage is a function of time – nearly 800V for a duration of 10mS, falling to about 80V for durations of 10 seconds or more. Two basic methods exist for achieving this, separation of the HV and LV earth systems, and deliberately joining the two systems. Separation is difficult to achieve and is only used where site conditions mean that joining the two systems cannot be used.

Joining the two systems is only allowed where the resulting touch voltage under HV earth fault is less than the graph in figure 4 above. To achieve this a very low impedance between the combined earth system and the bulk earth must be achieved, meaning that an extensive system of LV earth electrodes is required. Historically a figure of 1 ohm was used, above this figure separation is required, below it the two earth systems could be joined. This is no longer recognised in BS 50522, and the designer must demonstrate by calculation, software modelling or the use of standard arrangements that safety has been achieved. The design process is illustrated in BS 50522 as a flow chart (below).

The calculations for HV prospective fault current through the mass of earth are complex and require a knowledge of the utility infrastructure that is not commonly available. The designer often has to fall back on standard designs, “Recognised specific measures” and “Additional measures” in the flow chart. Both Standards give information on the design of the LV earth electrodes including likely resistivities of various types of soil in the UK. Earth fault current levels on both the HV and LV systems must be calculated as part of the design process. To achieve the necessary low impedance a substantial amount of external land area is required. If it is a new build the concrete piles can be utilised as earth electrodes, but this is a highly-specialised subject usually designed using software modelling.

If the transformers are located outside of the main data centre building they can be treated as a typical utility substation as in the standard designs of BS7430. However, if the designer has to locate them within the main building, possibly using cast-resign transformers, there is little choice but to use the recognised strategy of a “Global Earth System” from BS50522.

A global earth system requires that all earth systems in the buildings within the area under consideration are solidly bonded together by metallic bonds to minimise the earth potential rise under HV fault conditions.
These systems were originally intended for crowded city centres such as Canary Wharf in London, and for large industrial sites such as oil refineries. These days large data centres are approaching the power usage of these sites so a similar strategy has to be used.

Future-tech Conclusion

Increasing power demands of data centres is driving the use of HV Utility intakes, requiring the designer to use HV/LV transformers. These substations have to be designed to similar standards to Utility substations to ensure safety for the personnel and equipment in the data centre. An extensive LV earth electrode system is required that needs substantial external land area. Where the transformers are located within the data centre building a Global Earth System should be considered.

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