A national laboratory of the U.S. Department of Energy Office of Energy Efficiency & Renewable Energy National Renewable Energy Laboratory Innovation for Our Energy Future Modeling and Testing of Subcontract Report NREL/SR-581-41805 Unbalanced Loading and July 2007 Voltage Regulation Final Report M.W. Davis DTE Energy Detroit, Michigan R. Broadwater Electrical Distribution Design Inc. Blacksburg, Virginia J. Hambrick Virginia Polytechnic Institute and State University Blacksburg, Virginia NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 Modeling and Testing of Subcontract Report NREL/SR-581-41805 Unbalanced Loading and July 2007 Voltage Regulation Final Report M.W. Davis DTE Energy Detroit, Michigan R. Broadwater Electrical Distribution Design Inc. Blacksburg, Virginia J. Hambrick Virginia Polytechnic Institute and State University Blacksburg, Virginia NREL Technical Monitor: Thomas Basso Prepared under Subcontract No. ZAT-5-32616-06 National Renewable Energy Laboratory 1617 Cole Boulevard, Golden, Colorado 80401-3393 303-275-3000 • www.nrel.gov Operated for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy by Midwest Research Institute • Battelle Contract No. DE-AC36-99-GO10337 The National Renewable Energy Laboratory is a national laboratory of the U.S. Department of Energy (DOE) managed by Midwest Research Institute for the U.S. Department of Energy under Contract Number DE-AC36-99GO10337. This report was prepared as an account of work sponsored by the California Energy Commission and pursuant to a M&O Contract with the United States Department of Energy (DOE). 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This report has not been approved or disapproved by Midwest Research Institute, the DOE, or the California Energy Commission, nor has Midwest Research Institute, the DOE, or the California Energy Commission passed upon the accuracy or adequacy of the information in this report. NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available electronically at http://www.osti.gov/bridge Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone: 865.576.8401 fax: 865.576.5728 email: mailto:[email protected] Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone: 800.553.6847 fax: 703.605.6900 email: [email protected] online ordering: http://www.ntis.gov/ordering.htm This publication received minimal editorial review at NREL Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste List of Acronyms ANSI American National Standards Institute BL beginning location CAP capacitor CC constant current CP constant power CT current transformer DG distributed generation/distributed generator DR distributed resource(s) EL end location GMD geometric mean distance HL heavy load HV high voltage LL light load LTC load tap changer/load tap-changing LV low voltage ML midlocation PF power factor VDC voltage-dependent current VR voltage regulator VRR voltage regulating relay VT voltage transformer iii Executive Summary Introduction A distributed generation (DG) penetration limit study (Davis 2003) indicated a range of DG sizes can be interconnected with a distribution circuit. The DG size limit is dependent on system voltage, the location of the DG on the circuit, and system protection, voltage regulation, and other issues related to DG and circuit characteristics. The study showed that the DG could be larger if it was allowed to actively regulate voltage, rather than operate at a fixed unity power factor. Fixed power factor operation of the DG has a minimum effect on the existing traditional voltage regulation controls of the circuit because, as the system voltage changes, the field current of the DG synchronous generator is adjusted to bring the kilovar output typically back to zero and maintain the desired kilowatt setpoint. Therefore, the only effect on the system voltage is the kilowatt injection at that point on the distribution circuit. However, if the DG synchronous machine is allowed to absorb or export volt-amperes reactive, the voltage can be decreased or increased at that point on the circuit, and much larger kilowatt injections—and thus, larger DG—can be installed on the circuit. A number of problems are associated with interconnecting DG with a distribution circuit. Some are related to circuit design and operation, and others are related to the analytical tools used to evaluate DG operation. Distribution circuits are primarily designed for radial, one- way power flow, and distribution line voltage regulators are typically designed to regulate voltage based on a unidirectional flow of power. When DG is interconnected with the circuit, two-way flows can result. In addition, most of the load served on a distribution circuit is single-phase, yet most of the analytical tools used to evaluate circuit performance are based on balanced three-phase loads and balanced three-phase line circuit impedances. When balanced three-phase power flow programs are used to calculate the voltage profile on a distribution circuit and determine if voltage limits are being violated by DG, the accuracy of the service voltages at the individual single-phase loads on the single-phase laterals is a concern because only the three-phase portion of the circuit is modeled. American National Standards Institute Standard C84.1 voltage limits may be satisfied based on three-phase balanced load/impedance analysis, but the voltages at single-phase loads may be violated when the DG operates or shuts down. This is a significant concern for utilities because liability issues arise when customer equipment is damaged because of HV or LV on a circuit. Therefore, it is critical to evaluate the effects of DG on the distribution circuit voltage profile to ensure customers do not receive service voltages (at the customer billing meter) outside Range A or Range B of American National Standards Institute C84.1. This can be accomplished by using modeling and simulation tools that recognize single-phase loads, unsymmetrical distribution transformer connections, and unbalanced line impedances. iv Purpose The purpose of the project is to: • Explain how voltage regulation reduces voltage spread • Define the effects of unbalanced loading and voltage on system protection and DG output ratings • Develop models for an actual distribution circuit, its voltage regulation equipment, and all the DG generator types, recognizing unbalanced loading and unbalanced circuit impedances • Validate these models by comparing power flow simulations and voltage profiles with actual measured circuit data • Determine the optimum generator operating conditions (i.e., P and Q) to provide the greatest improvement in released capacity, reduced energy losses, and voltage regulation • Determine the maximum DG penetration limits with synchronous generator real and reactive power injections. Project Objectives The project objectives are five-fold: • To develop a load model for an actual 13.2-kV distribution circuit that represents how the real and reactive load on the circuit changes when the voltage is raised or lowered with capacitor switching, distribution line voltage regulator step changes, and load tap changer and DG voltage regulation • To develop models for distribution overhead and underground circuit line elements, transformers, shunt capacitors, step voltage regulators, and synchronous, induction, and inverter DG generators • To verify the models by comparing the power flow simulated data