Removing Barriers to Utility Interconnected Photovoltaic Inverters Sigifredo Gonzalez, Russell Bonn, Jerry Ginn, Sandia National Laboratories Albuquerque, NM 87185-0753 ABSTRACT The Million Solar Roofs Initiative has motivated a renewed interest in the development of utility interconnected photovoltaic (UIPV) inverters. Government-sponsored programs (PVMaT, PV:BONUS) and competition among utility interconnected inverter manufacturers have stimulated innovations and improved the performance of existing technologies. With this resurgence, Sandia National Laboratories (SNL) has developed a program to assist industry initiatives to overcome barriers to UIPV inverters. In accordance with newly adopted IEEE 929-2000, the utility interconnected PV inverters are required to cease energizing the utility grid when either a significant disturbance occurs or the utility experiences an interruption in service [5]. Compliance with IEEE 929-2000 is being widely adopted by utilities as a minimum requirement for utility interconnection. This report summarizes work done at the SNL balance-of-systems laboratory to support the development of IEEE 929-2000 and to assist manufacturers in meeting its requirements. INTRODUCTION Utility compatibility is a major issue with the implementation of UIPV static inverters. The focus of the concern has been primarily on the inverters’ ability to detect an abnormal condition on the utility grid and take appropriate actions to ensure the safety of personnel and to maintain the high reliability of utility equipment. The abnormal conditions are defined in detail in the newly adopted IEEE 929-2000 IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems. Presently SNL is assisting UIPV static inverter manufacturers to meet the interconnection issues that are addressed in IEEE 929-2000. SNL has conducted extensive anti-islanding testing to support the development of IEEE 929-2000 and to monitor the performance of grid-tied inverters [2]. SNL has contracted with UIPV static inverter manufacturers to develop and implement solid-state circuitry that both meets the IEEE 929-2000 requirements and increases the reliability and performance of the UIPV static inverters. To analyze the validity of the anti-islanding techniques, a standardized anti-islanding test procedure was developed at SNL and incorporated into IEEE 929-2000 Annex A. This recommended test procedure specifies the methods by which the tests for the RLC 60-Hz resonant circuit are conducted. ISLANDING The condition referred to as an island occurs when a residential distributed generation source continues to energize a portion of the utility grid after the utility experiences an interruption in service. The concern is that the utility no longer controls this isolated portion of the distribution system, consisting of a PV generation source and local loads. Therefore, an islanding occurrence may compromise safety, restoration of service, and equipment reliability. For an island condition to occur, the situation must be such that the inverter does not recognize an interruption in utility service. If the loads that remain on the isolated portion of the grid are closely matched to the output of the inverter, it is possible for voltage and frequency to remain relatively constant after the interruption in utility power. Anti-islanding schemes that depend on only monitoring the voltage and frequency may not detect this condition, and continue energizing the utility thus creating an island. Significant prior work on PV-inverter islanding was done at SNL in the 1980s [1]. Since that time, inverter technology has evolved significantly, and a new generation of products has appeared. Renewed interest (spurred partially by the Million Solar Roofs Initiative) made it necessary to evaluate the performance of the present generation of grid-tied inverters with particular emphasis on the islanding issue. Most of the new generation of utility interconnect photovoltaic inverters are able to identify when the utility has experienced an interruption in service when connected to a dedicated utility feeder; however, what remained unclear was the UIPV inverter interactions with several inverters connected to the same feeder. Multi-Inverter Test To address this concern, a test bed was created to investigate the operation of multiple inverters on the same ac circuit. The intent was to simulate a residential neighborhood having a high penetration of UIPV inverters. Figure 1 shows a one-line diagram of the multi-inverter test set-up. Up to five inverters from different manufacturers and of different types were placed on the same feeder. A variable resistive load was connected onto the same feeder and adjusted to closely match the total output power being generated by the inverters. The next step was to interrupt utility service. A motor start contactor was utilized to remove the utility and initiate the islanding test. A current transformer on each of the UIPV inverters’ output was used to identify when the inverter ceased to invert and to monitor its pre-interruption operating condition. An oscilloscope waveform capturesthe output current from each of the UIPV inverters as well as the feeder voltage. This acquisition is triggered by a signal from the contactors’ control contacts. The run-on time was calculated from the opening of the contactor until the current output of each inverter decreased to approximately 0.1 amp peak. This test represents a no-fault disconnect since the voltage and frequency are within the operating ranges of the inverters prior to removal of the grid. For a no-fault condition and matched loads, IEEE 929-2000 allows up to 2 seconds for the UIPV inverter to identify the loss of utility and take appropriate action. [Note that under fault conditions, the voltage and/or frequency will vary from the nominal state and the allowable disconnect duration varies from 2 cycles to 10 cycles depending on the severity of the disturbance. A fault-induced condition requires the UIPV inverter to disconnect from the utility much faster primarily so that the distributed generation will not interfere with utility hardware (automatic reclosers). Utility engineers involved in the development of IEEE 929-2000 have stated that run-on times greater than ten cycles (0.167 seconds) constitute a potential problem on systems equipped with automatic fault-clearing
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