A Study of Islanding in Utility-Connected Residential Photovoltaic Systems Part I-Models and Analytical Methods

Residential, utility-interactive, photovoltaic systems consist of a photovoltaic (PV) array, and a Power Conditioning System (PCS), or inverter, which converts the dc array energy to an ac voltage or current at the proper frequency for connection to an electric utility distribution feeder. One of the concerns wth such an interconnection is the potential for "islanding." This is a phenomenon in which one or more PCSs continue to operate, and keep the system energized even after the utility supply is disconnected. The loss of utility may be related to a scheduled outage, or the operation of circuit breakers, sectionalizers, or fuses in response to a system disturbance. Since the electric utility company cannot exert direct control on residential photovoltaic systems, islanding is generally considered to be undesirable. Concern is primarily related to the safety of personnel attempting to restore service in a system energized by islanded PCSs. Additionally, there is cause for concern as to the effect of transients and off-nominal frequency and/or voltage on connected load and on the PCS itself. Therefore, an understanding of the islanding problem is extremely important to the commercialization of utility-interactive PV systems. The objective of the work described in this paper and its companion [1] is to develop simple analytical methods to determine if islanding is possible in a system consisting of one or more PCSs connected to a single-phase distribution lateral. Attention is focused on three commercially available PCSs, the \´Gemini\´ manufactured by Windworks, Inc., the \´Teslaco\´ manufactured by Teslaco, Inc.