Consequences of Fault Currents Contributed by Distributed Generation

This is a project report from the Power Systems Engineering Research Center (PSERC). PSERC is a multi-university Center conducting research on challenges facing a restructuring electric power industry and educating the next generation of power engineers. Notice Concerning Copyright Material PSERC members are given permission to copy without fee all or part of this publication for internal use if appropriate attribution is given to this document as the source material. This report is available for downloading from the PSERC website. Acknowledgements The Power Systems Engineering Research Center sponsored the research project titled " New Implications of Power System Fault Current Limits. " This project is underway at the time of writing (November, 2004). This report is an intermediate report of progress at Arizona State University. Executive Summary This report concerns fault currents in systems with distributed generation. The main concept described is that fault current throughout power systems is likely to increase when distributed generation is installed. The nature of the increase is described in some detail, mainly using the Z bus method of calculation. IEEE standards are used to apply correction factors. The requirements of IEEE Standard 1547 are applied as well – this is a standard for distributed generation use. Fault calculations in power systems are used to determine the interrupting capability of circuit breakers. The calculation of fault current at the system buses is done by applying the system Z bus matrix. The effects of merchant plants, such as Independent Power Producers, are not taken into consideration in the classical fault current calculation. New developments in deregulation have brought new generation sources to the system. The appearance of DGs is a cause of increasing fault currents that has not previously been envisioned. This report presents a modification of the conventional fault current calculation in the case of addition of DGs. Many consequences arise from increasing of the fault currents, for instance the change of coordination of protective devices, nuisance trips, safety degradation, and the requirement to change recloser and protective relay settings. These consequences result in the cost of equipment upgrades not only at the DG sites but also other system sites. Due to installing new DGs, six main factors that affect the severity of the increase of three phase fault current at each buses are: • the number and size of DGs • the operation of the AVR • the impedance of DGs • the location …