The high penetration of utility interconnected photovoltaic systems is is leading to a need for inverters to include grid support functions, to minimize the negative impact these variable distributed energy resources may have on system voltage and frequency. Unfortunately, grid support functions may interfere with island detection algorithms; specifically, it may be difficult for an island detection scheme to detect voltage and frequency deviations if that converter and other converters on the same bus actively modulate their real and reactive power outputs in response to voltage and frequency deviations while also tolerating greater deviation. This report provides analysis and simulation evidence to investigate the effect of advanced inverter functions on the performance of island detection schemes. A mitigation scheme is also presented and shown to be effective in simulation.
Photovoltaic (PV) distributed energy resources (DER) have reached approximately 27 GW in the U.S., and the solar penetration rate continues to increase. This growth is expected to continue, causing challenges for grid operators who must maintain grid stability, reliability, and resiliency. To minimize adverse effects on the performance of electrical power system (EPS) with increasing levels of variable renewable generation, photovoltaic inverters must implement grid-support capabilities, allowing the DER to actively participate in grid support operations and remain connected during short-term voltage and frequency anomalies. These functions include voltage and frequency regulation features that adjust DER active and reactive power at the point of common coupling. To evaluate the risk of these functions conflicting with traditional distribution system voltage regulation equipment, researchers used several methods to quantify EPS-support function response times for autonomous voltage regulation functions (volt-var function). Based on this study, no adverse interactions between PV inverters with volt-var functions and load tap changing transformers or capacitor banks were discovered.
Increasing the penetration of distributed renewable sources, including photovoltaic (PV) sources, poses technical challenges for grid management. The grid has been optimized over decades to rely upon large centralized power plants with well-established feedback controls, but now non-dispatchable, renewable sources are displacing these controllable generators. By programming autonomous functionality into distributed energy resources-in particular, PV inverters-the aggregated PV resources can act collectively to mitigate grid disturbances. This paper focuses on the problem of frequency regulation. Specifically, the use of existing IEC 61850-90-7 grid support functions to improve grid frequency response using a frequency-watt function was investigated. The proposed approach dampens frequency disturbances associated with variable irradiance conditions as well as contingency events without incorporating expensive energy storage systems or supplemental generation, but it does require some curtailment of power to enable headroom for control action. Thus, this study includes a determination of the trade-offs between reduced energy delivery and dynamic performance. This paper includes simulation results for an island grid and hardware results for a testbed that includes a load, a 225 kW diesel generator, and a 24 kW inverter.
Presently, approximately 20 GW or 2% of the nation's generating capacity comes from solar, and solar penetration is increasing. However, for this trend to continue without adversely affecting electrical power system (EPS) performance, the photovoltaic inverters must participate in voltage- and frequency-regulation requirements. EPS support capabilities under development are the low-/high-voltage and low/high-frequency ride through, volt-VAr, frequency-watt, watt-power factor, commanded power factor, commanded power functions, and others. Each of the functions have parameter set points, and most have ramp rates for implementation of the functions as defined in the International Electrotechnical Commission Technical Report 61850-90-7. This paper focuses on methods to quantify EPS support functions for DER certification. Sandia National Laboratories and Underwriters Laboratories, in collaboration with industry stakeholders, have developed a draft test protocol that efficiently and effectively evaluates support-function capabilities. This paper describes the functions, their intended use, and results of EPS support functions in a controlled laboratory environment.
Cost effective integration of solar photovoltaic (PV) systems requires increased reliability. This can be achieved with a robust fault detection and diagnostic (FDD) tool that automatically discovers faults. This paper introduces the Laterally Primed Adaptive Resonance Theory (LAPART) artificial neural network to perform this task. The present work tested the algorithm on actual and synthetic data to assess its potential for wide spread implementation. The tests were conducted on a PV system located in Albuquerque, New Mexico. The system was composed of 14 modules arranged in a configuration that produced a maximum power of 3.7kW. The LAPART algorithm learned system behavior quickly, and detected module level faults with minimal error.
In order to identify reliability issues associated with advanced inverter operation and array states (e.g. volt-VAR control, high DC/AC ratios), we have collected system and component-level electro-thermal information in a controlled laboratory environment under both nominal and advanced functionality operating conditions. The results of advanced functionality operation indicated increased thermal and electrical stress on components, which will have a negative effect on inverter reliability as these functionalities are exercised more frequently in the future.
The continued exponential growth of photovoltaic technologies paves a path to a solar-powered world, but requires continued progress toward low-cost, high-reliability, high-performance photovoltaic (PV) systems. High reliability is an essential element in achieving low-cost solar electricity by reducing operation and maintenance (O&M) costs and extending system lifetime and availability, but these attributes are difficult to verify at the time of installation. Utilities, financiers, homeowners, and planners are demanding this information in order to evaluate their financial risk as a prerequisite to large investments. Reliability research and development (R&D) is needed to build market confidence by improving product reliability and by improving predictions of system availability, O&M cost, and lifetime. This project is focused on understanding, predicting, and improving the reliability of PV systems. The two areas being pursued include PV arc-fault and ground fault issues, and inverter reliability.
The Characterizing Emerging Technologies project focuses on developing, improving and validating characterization methods for PV modules, inverters and embedded power electronics. Characterization methods and associated analysis techniques are at the heart of technology assessments and accurate component and system modeling. Outputs of the project include measurement and analysis procedures that industry can use to accurately model performance of PV system components, in order to better distinguish and understand the performance differences between competing products (module and inverters) and new component designs and technologies (e.g., new PV cell designs, inverter topologies, etc.).
Increasing the penetration of distributed renewable sources, including photovoltaic (PV) sources, poses technical challenges for grid management. The grid has been optimized over decades to rely upon large centralized power plants with well-established feedback controls, but now non-dispatchable, renewable sources are displacing these controllable generators. This one-year study was funded by the Department of Energy (DOE) SunShot program and is intended to better utilize those variable resources by providing electric utilities with the tools to implement frequency regulation and primary frequency reserves using aggregated renewable resources, known as a virtual power plant. The goal is to eventually enable the integration of 100s of Gigawatts into US power systems.
Increasing the penetration of distributed renewable sources, including photovoltaic (PV) generators, poses technical challenges for grid management. The grid has been optimized over decades to rely on large centralized power plants with well-established feedback controls. Conventional generators provide relatively constant dispatchable power and help to regulate both voltage and frequency. In contrast, photovoltaic (PV) power is variable, is only as predictable as the weather, and provides no control action. Thus, as conventional generation is displaced by PV power, utility operation stake holders are concerned about managing fluctuations in grid voltage and frequency. Furthermore, since the operation of these distributed resources are bound by certain rules that require they stop delivering power when measured voltage or frequency deviate from the nominal operating point, there are also concerns that a single grid event may cause a large fraction of generation to turn off, triggering a black out or break-up of an electric power system.
The anticipated high penetration of distributed photovoltaic (PV) energy sources is expected to lead to significant changes in utility interconnection requirements for PV systems. These changes will include provisions for voltage and frequency regulation capability, as well as better voltage and frequency ride through requirements. For distributed energy resources (DER), in particular PV, to provide grid support, it must participate in frequency and voltage regulation. Frequency and voltage ride through allows inverters to remain connected to ensure robust recovery in the event of voltage and frequency disturbance. Implementing these advanced capabilities is essential to mitigating the negative impacts of high penetration PV, but their integration into a typical distribution system presents significant technical challenges, one of which is the increased risk of unintentional islanding. In this paper, an island detection method is presented that relies on a continuous subharmonic signal, a power line carrier permissive (PLCP), that is injected at the transmission level or at the substation and detected by any type of DERs in any combination. Absence of the signal indicates loss of utility and possible island condition. Laboratory and simulation experiments were done to investigate feasibility of the method. The PLC system discussed herein is novel in that it utilizes a power electronics based series voltage injection method. Advantages include the ability to use a smaller and less expensive transformer and enhanced flexibility in the amplitude, waveform and frequency of the injected signal.