The fine balance of generator selection

HEJ editor Jonathan Baillie reports.

The speaker began an extremely wideranging and in-depth presentation by comparing some of the advantages and disadvantages of centralised electricity distribution on a healthcare estate with those of distributed generation. Deciding which to go for – centralised or distributed – was largely, he explained, determined by the size of the load and the “critical elements”, as well as the availability of space for either choice. In Geoff Halliday’s experience most large healthcare sites opted for the distributed power option. This was largely because this enabled progressive development of the power network around the site in the future, due to limitations in the ability to develop the estate’s existing HV ring, and because of space constraints for any new plant. High voltage (HV) central generation tended to be the chosen option for estates teams with loads of over 4 MVA. Among the advantages, he explained, were straightforward installation – providing, of course, that there was plenty of available plant room space; the fact that dedicated, planned space could be apportioned for the required generating sets, and that facilities could be custom built for noise attenuation and the appropriate fuel systems. Other advantages included the ability to build in N+1 resilience capacity from the start, to add load management more easily, and to augment local resilience as a back-up to the primary power supply. HV switchgear also tended to be “a lot more cost-effective” than that used for LV power generation. Looking next at HV central generation’s disadvantages, Geoff Halliday explained that such systems generally required dual PLC management for load shedding and mains failure changeover control – something of an “operational and maintenance complication”, tended to incur higher capital costs than a comparable LV solution, and had more “specialist” maintenance requirements. HV central generation equipment also needed to incorporate “fail open” valves. Conversely, he explained, LV central generation’s drawbacks, in its turn, included the fact that installing and distributing power at low voltage beyond 6000A / 4MVA tended to become costprohibitive, while other disadvantages included limitations on the switchgear, LV fault level limitations, and potential transmission losses over the hospital network

LV step-up systems