The Impact of Dispersed Generation Upon the Quality of Electric Power:The Solar Park in the Ft. Davis Distribution System

BALDA, JUAN CARLOS; OLIVA, ALEJANDRO RAUL

1994-09-01/1996-06-01

1-151

Electrónica

Energia-Solar

The Impact of Dispersed Generation Upon the Quality of Electric Power:The Solar Park in the Ft. Davis Distribution System EPRI Report TR-107725Date PublishedApr 1997, Research Ended Jun 1996 DetailsTR-107725 Final Report - Available On-line 151 pages BackgroundDue to economic and environmental reasons, electric utilities are interested in investigating renewable and clean energy sources as alternatives to coal- and gas-fired power stations.Presently, special emphasis is being placed on solar- and wind-based renewable energy sources since it is expected that they will be cost competitive within the next 20 years due to the technical advances presently taking place. To this end, Central and South West Services (CSW), based in Dallas (TX), has developed a five-year program to determinethe best potential technologies for converting sun and wind energies into environmentally clean and affordable electric energy. The Electric Power Research Institute (EPRI) and CSW contracted with the University of Arkansas at Fayetteville to undertake an evaluation of the impact that the sun- and wind-based generating systems installed in the Ft. Davis area (TX) would haveon the quality of electric power. This report deals with two sun-based generating systems at the Solar Park that are interconnected to a 12.47-kV three-phase multi-grounded distribution feeder through a pulsewidth modulated (PWM) inverter, an inductor, a D-D 374-480 V isolation transformer and a D-Y ungrounded 500-kVA480-12,470 V three-phase distribution transformer bank. ObjectiveIt is well known that solid-state inverters inject harmonic currents; this may cause harmonic-sensitive equipment to malfunction if the injection ofharmonic currents into the distribution system are allowed to reach excessive levels. Therefore, the main objective of the research work covered in this report is to establish if the Solar Park (located 10.5 miles from the town of Ft. Davis) might cause any degradation of the quality of electric power. The results of this evaluation should bevaluable when sun-based renewable energy sources becomecost effective to warrant their applications throughout the CSW system. ApproachA complete power quality analysis was performed as recommended by the IEEE 519-1992. This analysis included modeling the Ft. Davis distribution system; selecting the buses to be monitored usinga harmonic simulation tool (i.e.,SuperHarm); monitoring the selected harmonic-sensitive buses including the Ft. Davis substation bus; characterizing the quality of the current and voltage waveforms at the monitored buses; performingthe harmonic penetration studies using SuperHarm; and evaluating all measurements and simulation results. ResultsAfter the monitoring period, recorded data were analyzed and compliance of the IEEE 519-1992 harmonic current and voltage limits was evaluated for the Solar Park being a dispersed generator, generating equipment and a customer. The Solar Park was within the customer IEEE 519-1992 harmonic current limits, except for the even-harmonic current components larger than16. However, the Solar Park was completely out of range when considered as dispersed generation or generating equipment. Regarding the harmonic voltage limits, a voltage THD over the recommended limit of 5% was measured at the end of the feeder. Furthermore, a 5th-harmonic voltage distortion larger than the recommended 3% was found at the Ft. Davis substation and at the end of the feeder. Also, an average background voltage THD of approximately 3% was recorded at the end of the feeder but cannot be attributed to the Solar Park because it was also registered during the night.SuperHarm-based simulations were performed using the recorded current injection at the Solar Park at average peak apparent power of 140 kVA and with the Solar Park as the only source of current injection. Under these circumstances, Solar Park produced a voltage THD between 0.2 and 0.4% along the feeder to which it is connected. Those buses located between the Solar Parkand the end of the feeder exhibited the largest current and voltage harmonic distortions in the simulation results. The simulation did not show any excitation of parallel resonances at the 5th harmonic. A connection/disconnection test was performed at the Solar Park, and the event was recorded by sensors (PQNodes) installed at the substation and the Solar Park. During the test, the Solar Park was generating approximately 70 kVA. No voltage transient was recorded by either PQNode. A comparison of voltage THDs with and without Solar Park input showedthat its contribution was minimal. In summary, the Solar Park, with its PWM inverter, ac choke, and D-D isolation transformer, should not produce any power quality problem in the Ft. Davis distribution system. This analysis is presently supported by the lack of customer complaints. EPRI Perspective The main objective of this project was to analyze the impact of inverter-interfaced renewable energy sources upon the quality of electric power. Results show that Solar Park is not creatingpower quality problems for the Ft. Davis distribution system. However, simulations forecast that a 10-times larger photovoltaic plant located at Solar Park would exceed  IEEE 519 voltage and current limits, thus requiring the implementation of harmonic-suppressing systems at the installation.