PROTECTION AGAINST LOSS OF UTILITY GRID SUPPLY FOR A DISPERSED STORAGE AND GENERATION UNIT

The installation of small or medium sized dispersed storage and genera tion units operating in parallel with the utility supply presents seve ral technical complications for the protection and control of the syst em. Amongst these is the need to protect the system from islanding cau sed by the loss of the utility grid supply and the possible subsequent out-of-synchronism reconnection of that supply. This paper examines t he requirements of an islanding, or 'loss of grid', protection and out lines the principal methods used for this type of relaying. A new prot ection 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 oper ating conditions to demonstrate the operation of this type of protecti on. The new protection algorithm is shown to trip for loss of grid, fo r load fluctuations while the dispersed storage and generation unit is operating independently of the utility supply following a loss of gri d, 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 c onnected to the dispersed generator system.