IN THE PROCEEDING OF THE FORTH POWER CONVERSION CONFERENCE (PCC2007) NAGOYA-JAPAN, APRIL 2007 1 Design of an Electronically-Interfaced Dispatchable Power Generation Source to Facilitate Islanding and Autonomous Operation of a Distribution Network Farid Katiraei, Member, IEEE, and Chad Abbey, Student Member, IEEE CANMET Energy Technology Centre - Varennes, Natural Resources Canada Abstract— Planned islanding and autonomous operation of a medium voltage distribution system with high-penetration of renewable energy resources are examined in this paper. The intermittent nature and highly variable characteristics of renewable sources identify them as non-dispatchable sources that require a stiff grid and/or electric energy storages for continuous supply of electricity and reliable operation of a power distribution network. A design methodology based on characterization of the frequency-power relationship of all sources in the planned island is introduced and applied to the development of a fast- acting, dispatchable distributed energy resource (DER). The dispatchable DER is employed to quickly maintain power balance of a distribution system upon disconnection from the grid and stabilize the system operation following the islanding transients. The proposed DER is electronically-interfaced and composed of a slow-response primary source and a short-term energy storage. Design criteria aim to minimize the size of the storage while providing adequate power to compensate fluctuations of variable sources during an autonomous operation. Case studies based on the islanding of the distribution network with and without the dispatchable source are performed. The study results illustrate operating limits and power mismatch levels for which the islanded system can be sustained given specified acceptable voltage and frequency ranges. Index Terms— Dispatchable source, electric energy storage, renewable energy sources, planned islanding, Microgrid. I. I NTRODUCTION Planned islanding and autonomous operation of part of a distribution network supplied by local distributed energy resources (DER) has recently attracted major utilities’ interest worldwide, [1], [2]. The concept can potentially improve reliability and supply security of the distribution network by reducing system downtime. It also allows the utility company to perform maintenance on upstream medium/high voltage feeders without supply interruption of the low voltage cus- tomers. As the penetration depth of DERs based on alternative energy sources - particularly renewable energy technologies - increases, DER owners may demand islanding capability during the interval of a sustained grid failure and, in some cases, to enhance power quality for disturbances initiated on the main grid. Current planned-islanding practices are based on pre- scheduling of controllable DERs to balance the power gen- eration and consumption locally prior to disconnection from ——————————————————— Authors are with Natural Resources Canada, CANMET Energy Technol- ogy Centre-Varennes, 1615 Lionet-Boulet Blvd., Quebec, Canada, J3X 1S6, (emails: farid.katiraei@nrcan.gc.ca, chad.abbey@nrcan.gc.ca) the utility grid, [3]. This operating strategy requires close control and supervision of generation sources that limits the utility applications to specific systems with no or very low- penetration of intermittent and renewable energy sources. One solution to expand the application is to utilize electric energy storage; however, the high cost of energy storage requires optimal sizing and selection of appropriate storage technolo- gies. Implication of advanced control strategies may also help enhance the dynamic behavior of the system, enabling the reduction of the storage size. This paper presents the design methodologies and perfor- mance analyses of an electronically-interfaced dispatchable distributed energy resource (EID-DER) to manage the instan- taneous power fluctuations and overall energy balance of a dis- tribution network during islanding and autonomous operation. The fast-acting EID-DER is composed of a gas turbine, the primary source, and an ultracapacitor energy storage bank as the secondary source of energy generation. The ultracapacitor supplies the short-term energy requirements during start-up and/or acceleration/deceleration intervals of the comparably slow-response conventional source, i.e. gas turbine. Ultracapacitor-based energy storage has been primarily used for hybrid electric vehicle applications, [4], to provide in- stantaneous energy during engine acceleration; also in dc traction systems to operate as a shock absorber to reduce voltage sags on the dc bus [5]. Recent demonstration projects, e.g. ”capacitor-stabilized soft-transfer interface system” con- ducted by EPRI [6], and ”Ultracapacitor EnergyBridge TM ” technology introduced by Northern Power Systems, [7], have investigated ultracapactor-based energy storage applications for distribution systems. However, the sizing issue and design of an efficient energy storage medium for these applications imply that in-depth analysis of the system characteristics is required. The main contribution of this paper is the development of a design approach and an analytical method for sizing and control system design of an ultracapacitor-based dispatchable source. The proposed method is based on the investigation of the system stiffness and rate of voltage/frequency changes to determine the instantaneous real and reactive power require- ments. The paper also presents dynamic performance analyses of the distribution network, during and subsequent to planned and unplanned (accidental) islanding, with and without the EID-DER. The following topics are treated in turn. First, planned islanding control strategies based on a typical utility practice