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I have the fortune to be a Project Manager for a unique and global, cross-business unit team called, “Project Cumulus”.  Our team combines some of the best Engineers in the areas of HV, MV, and LV electrical distribution networks and controls.  Our mission is to develop optimized electrical distribution architectures for large data centers (i.e., multi-megawatt).  We document and publish these architectures and all of their related technical studies in reference designs and white papers.  Cumulus serves both as an arena for internal systems-level development and innovation, as well as for being a platform for technical discussion and collaboration with our customers’ engineering teams.  The number of people in the world who understand medium voltage systems and connecting to the utility grid are VERY few and so customers often look to us to advise and assist in early design.  This expertise is a strength of Schneider Electric’s and the Cumulus team, in particular.

A key takeaway from our efforts has been the importance and value of designing the whole system together holistically…and to do so early in the process BEFORE connection to the utility has been specified.  Too often, this is not how things are done…systems are instead designed and spec’d in isolation at different times and by different engineering teams. This means that each successive design has to react to the previous sub-system’s specification.  This, of course, results in sub-optimal choices and performance levels overall.  For example, we have seen in many cases the HV/MV transformer, MV switchgear, and the cabling all specified and ordered separately.  The short-circuit impedance value specified often ends up resulting in a MV short circuit current level that is just above 25 kA, which forces the use of 31.5 kA rated switchgear (instead of 25 kA rated gear).  This lack of coordination in design just cost a ton of money and will use up significantly more space.  Customers are often unaware that significant savings could’ve been achieved just by marginally increasing the specified HV/MV transformer impedance value.   The figure below illustrates the cost of MV switchgear as a function of the short-circuit current rating of the HV/MV transformer.

Source: Schneider Electric White Paper 258, “Specifying HV/MV Transformers at Large Sites for an Optimized MV Electrical Network”

This example should make clear that it is important for the entire electrical network to be considered when doing the design in order to optimize the cost, efficiency, footprint, and to meet or surpass other key project parameters such as load density and redundancy requirements.  Electrical network operational practices also must be considered as they can impact the short-circuit current levels.  For example, if the network control system ensures both transformers are never operated in parallel, then the short circuit current rating of the MV network could be sized just on the basis of one transformer.  If, on the other hand the transformers work in parallel (i.e., the bus section circuit breaker is normally closed) …even if it’s only temporary, then the short circuit current rating will have to be much higher accounting for both transformers together.  This latter practice is common in the U.S. particularly in industrial applications.

Especially for large multi-megawatt data centers it is also important that this holistic design work occur as early as possible BEFORE the connection with the utility has been specified and contracted.  While smaller sites are connected to utility distribution circuits at medium voltage (15 – 35 kV), multi-megawatt sites would be connected either to multiple utility distribution circuits or directly to the utility’s HV transmission grid.  The latter offers several benefits since you would have a great deal of control over key design considerations that ultimately influence the architecture and operation of the facility.  Even if the utility ends up building and operating the equipment, the end user should still have a lot of say and control in what gets specified and installed.  This means, for example, that the owner can select or specify MV distribution equipment and load blocks that allow the leveraging of 25 kA rated MV switchgear that saves space, money, and improves safety.  Connecting at the transmission level can also improve reliability of the utility grid (from the owner’s perspective) while also positively influencing construction schedules, energy rates, and location of the data center.

White Paper 253, “Benefits of Limiting MV Short-Circuit Current in Large Data Centers”, explains this in much more detail.  And technical White Paper 258, “Specifying HV/MV Transformers at Large Sites for an Optimized MV Electrical Network”, is aimed at engineers who are specifying transformers for large data centers.  It introduces the factors to consider while raising awareness of short-circuit impedance on the cost of the HV/MV transformer and the MV electrical distribution installation (i.e., switchgear and cabling).

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