Publications

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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.

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We have examined ground faults in PhotoVoltaic (PV) arrays and the efficacy of fuse, current detection (RCD), current sense monitoring/relays (CSM), isolation/insulation (Riso) monitoring, and Ground Fault Detection and Isolation (GFID) using simulations based on a Simulation Program with Integrated Circuit Emphasis SPICE ground fault circuit model, experimental ground faults installed on real arrays, and theoretical equations.

The increasing pressure for network operators to meet distribution network power quality standards with increasing peak loads, renewable energy targets, and advances in automated distributed power electronics and communications is forcing policy-makers to understand new means to distribute costs and benefits within electricity markets. Discussions surrounding how distributed generation (DG) exhibits active voltage regulation and power factor/reactive power control and other power quality capabilities are complicated by uncertainties of baseline local distribution network power quality and to whom and how costs and benefits of improved electricity infrastructure will be allocated. DG providing ancillary services that dynamically respond to the network characteristics could lead to major network improvements. With proper market structures renewable energy systems could greatly improve power quality on distribution systems with nearly no additional cost to the grid operators. Renewable DG does have variability challenges, though this issue can be overcome with energy storage, forecasting, and advanced inverter functionality. This paper presents real data from a large-scale grid-connected PV array with large-scale storage and explores effective mitigation measures for PV system variability. As a result, we discuss useful inverter technical knowledge for policy-makers to mitigate ongoing inflation of electricity network tariff components by new DG interconnection requirements or electricity markets which value power quality and control.

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.

PV faults have caused rooftop fires in the U.S., Europe, and elsewhere in the world. One prominent cause of past electrical fires was the ground fault detection blind spot in fuse-based protection systems uncovered by the Solar America Board for Codes and Standards (SolarABCs) steering committee in 2011. Fortunately, a number of alternatives to ground fault fuses have been identified, but there has been limited adoption and historical use of these technologies in the United States. This paper investigates the efficacy of one of these devices known as isolation monitoring (or isolation resistance monitoring, Riso) in small (∼3kW) and large (∼700 kW) arrays. Unfaulted and faulted PV arrays were monitored with Riso technology and compared to SPICE simulations to recommend appropriate thresholds to the maximize the range of ground faults which could be detected while minimizing unwanted tripping. Based on analytical and computational models, it is impossible to determine a trip threshold that provides fire safety and negates unwanted tripping issues. This paper mathematically demonstrates that appropriate Riso trip thresholds must be determined on an arrayby- array basis with sufficient leeway by system operators to adjust trip threshold settings for their particular usage cases.

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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.

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.

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