Answer by Dale Blundell (Atkins)
Should there be a blackout, i.e. loss of supply from the grid, CHP systems can be set up to operate in a backup or ‘islanding mode’. But since the CHP unit is usually only designed to meet part of the site demand, then the system would be designed to feed only essential loads. This may involve segregating these essential loads, and installing equipment to control the CHP and switch over its supply to the dedicated essentials board. So, should loss of supply to the site occur, the CHP would disconnect so that it can resynchronise after transient events. For longer outages it would switch over to the essential supplies board. There is also the possibility for CHP systems to be set up to ‘load follow’ i.e. match the site demand as it fluctuates with time. While the additional cost of controls and monitoring equipment may not be justifiable as a stand-alone proposal, other factors such as security of supply might help to make this a worthwhile investment.
As the generator would normally be earthed in common with the grid connection, a separate earthing point with a changeover switch and a neutral earthing resistor will be required if island mode operation is required.
When the CHP is operating in islanding mode, back-synchronisation equipment will be required to seamlessly re-synchronise it to the network when the grid becomes available.