A Study of Islanding in Utility-Connected Residential Photovoltaic Systems Part II--case Studies

The phenomenon wherein a group of residential, utility-connected, photovoltaic inverters continue to operate even after loss of the utility source is known as ´islanding.´ Islanded operation is undesirable since it can jeopardize the safety of utility personnel as well as cause damage to connected equipment. Analytical methods for the study of islanding have been presented in the companion paper [1]. It has been shown therein that a power-flow analysis type of method can be effectively used to identify the electrical frequency at which a power balance can be achieved in an islanded system. If such a frequency exists then islanding is possible. At this predicted frequency, largesignal Describing Function analysis and small-signal stability analysis can be used to determine if islanding will actually occur. In the present paper the methods are extended to the analysis of systems containing multiple power conditioning systems (PCS). Beginning with a general state-variable format, models are defined for power-flow analysis, describing functions analysis, and small-signal stability analysis. The methods above are used to study islanding in several test systems. Three types of PCSs are studied-the Gemini manufactured by Windworks, Inc., the Teslaco manufactured by Teslaco, Inc., and