Protection against loss of utility grid supply for a dispersed storage and generation unit

The installation of small or medium sized dispersed storage and generation units operating in parallel with the utility supply presents several technical complications for the protection and control of the system. Amongst these is the need to protect the system from islanding caused by the loss of the utility grid supply and the possible subsequent out-of-synchronism reconnection of that supply. This paper examines the requirements of an islanding, or ‘loss of grid’, protection and outlines the principal methods used for this type of relaying. A new protection algorithm is introduced which is based on the rate of change of power as measured at the generator's terminals. The responses of the different measurands are examined for a selection of power system operating conditions to demonstrate the operation of this type of protection. The new protection algorithm is shown to trip for loss of grid, for load fluctuations while the dispersed storage and generation unit is operating independently of the utility supply following a loss of grid, and for an out-of-synchronism reconnection of the utility supply to the dispersed storage and generation unit. It is also shown to remain stable for major load fluctuations while the utility supply remains connected to the dispersed generator system.