ELECTRICAL INFRASTRUCTURE
Durham comprised two ring main units connected by a cable, with the DNO routinely using this as the open point on its interconnector between two primary substations, thus providing greater resilience (see Figure 1). The two switched alternate supplies are connected to
the new hospital intake switchboard, from which a ring main feeds three sub-stations that serve the main clinical buildings. Each sub-station has a five-panel switchboard equipped with unit protection to enable the ring to operate closed, and any single fault in the ring to be automatically switched out without loss of supply. This feeds two 100% transformers, each feeding an LV switchboard, equipped with automatic changeover to the secondary supply and a manual interconnector. The two streams are labelled ‘Red’ and ‘Green’, with clear colour coding to provide an intuitive awareness for operators that is quite distinct from the previous Essential and Non-Essential systems that it replaces. Another project to upgrade the high voltage
Above. Figure 2: The North Tees Hospital intake switchboard.
Above right. Figure 3: The standby switchboard ‘red section’ at North Tees Hospital during Factory Acceptance Testing.
Undertaking risk assessments will determine the
appropriate risk grades for each department, forming a firm basis for designing the electrical infrastructure. A practical overview of each building and the entire site is necessary to establish a distribution strategy, considering potential future changes in use. For instance, providing a lower-risk supply to only one or two departments might be impractical if these departments may be converted for higher-risk use later. By applying this process rigorously, it can be determined which buildings on a site will receive 100% secondary supplies.
2: How can you ensure resilience in times of disruption to the primary supply?
Figure 4: An aerial view of the Royal Hospitals site in Belfast, the largest hospital campus in Europe at the time.
The primary supply to a hospital, often referred to as ‘grid’ or ‘mains’, is generally a high or low voltage connection from the Distribution Network Operator (DNO). Typically for a larger hospital site, the supply will be at high voltage, and a ‘switched alternate’ supply comprising two cables entering the site will be requested to improve resilience. In the event of local network failure, the DNO can switch the supply to another part of its network; typically, the DNO will aim to do this within three hours of the failure. It is well worth the effort to build relationships with the DNO and gain sight of the single line diagrams for their network to fully understand how the network is configured upstream of the connection. If there is a power failure, there is a greater opportunity for resilience if the connection is on an interconnector between two separate sub-stations. One robust solution agreed upon for the University Hospital of North Tees in Stockton-on-Tees in County
infrastructure at the Royal Hospitals in Belfast, the largest hospital campus in Europe at the time, was conceived out of several years’ liaison with the DNO, which had a long-term aim of replacing the historic 6.6 kV distribution system in the area with an 11 kV system. Ultimately, the catalyst for this happening was the Royal Victoria Hospital Belfast’s (Figure 4) requirement to increase capacity, which led to the DNO establishing a new primary sub-station (Figure 5) on land leased from the hospital to provide the new 11 kV supply. The Trust had the foresight to specify that each new transformer installed on the site during redevelopment projects was dual wound, with primary windings for both 6.6 and 11 kV in readiness for the expected project implementation. This reduced the number of transformers and length of cable to be replaced once the project proceeded. Condition assessment of remaining cable using partial discharge mapping and Tan Delta testing was adopted to inform further cable replacement. Only once the new 11 kV supply was established and proven did the final phase of voltage changeover proceed. For each ring, the open point was moved to allow each substation to be changed over to the new 11 kV supply, and the transformer winding changed.
Changeover on a Sunday To suit the criticality of the hospital services fed, only one substation could be changed over each Sunday. So, once started, the process was irreversible, and lasted several weeks, during which reduced resilience was mitigated by provision of temporary generators and additional on-call staff to ensure that supplies could be quickly restored. The changeovers went smoothly, with no unplanned interruptions to supply. Resilience in the secondary supply is also needed to account for breakdown and maintenance. The mainstay of secondary supply remains the standby generator; while alternative fuels are being developed for internal combustion engine generators, diesel remains the most readily available and reliable way of assuring this. It is important to consider the design of standby systems – whether to have several to serve single buildings or parts of a site, or a single system to serve the whole site. At North Tees, two 2.5 MVA generators were provided
in the new Energy Centre to provide N+1 support to the whole site. Both primary and secondary distribution systems were duplicated, reducing the risk of calling on the generators to start for an internal distribution failure. The arrangement also allowed paralleling for mains restoration and online load testing of each generator in turn. To assure resilience, CHP and solar PV arrays are
56 Health Estate Journal October 2024
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