Transient behavior of a distribution network incorporating decentralized generation

Dispersed or decentralized storage and generation of electric energy (noted DSG) finds its source in cogeneration using either clean fuels or renewable resources, such as solar and wind. Typically these small energy sources are grafted to existing networks through weak utilities ties. This paper discusses a particular operating problem that may be faced by the electrical utility wishing to integrate DSGs. The latter are especially at risk of losing their utility ties following some electrical disturbances such as short circuits, line outages, etc. and consequently risk losing synchronism or being separated from the main network. Hence the anticipated penetration of DSGs in distribution networks will be accompanied by an increase in system vulnerability. Here we describe transient stability simulations of distribution networks in which DSGs are integrated, and in particular we investigate their needs in terms of frequency regulation. In our simulations the distribution network is fed simultaneously by the main power system and by one or two DSGs; this allows us to study the interactions between all of the components. All simulation scenarios include a small three-phase turbine-alternator powered on gas (TAG) synchronously connected to the distribution network. The second DSG is either a directly-connected TAG, or photovoltaic cells (PV) interfaced to the distribution network through a three-phase converter and accumulators. Equipped with a continuous frequency measurement capability, real time transient stability assessment of DSG distribution/main network interconnections is now possible. An algorithm, which achieves that goal, has been developed and tested on a power system model subjected to various contingencies.