Abraham Ellis

Effect of Variable Speed Wind Turbine Generator on Stability of a Weak Grid

In this paper, we illustrate the effect of adding a hypothetical 100-MW doubly fed induction generator (DFIG) wind power plant to a weak transmission system. The effects of various wind plant load factors (100, 60 and 25% of nameplate rating) are investigated. System performance is compared to a 100-MW conventional synchronous generator interconnected at the same location. The conventional generator is installed some distance away. The simulations demonstrated that DFIG generators provide a good damping performance under these conditions. These results support the conclusion that modern wind power plants, equipped with power electronics and low-voltage ride-through capability, can be interconnected to weak power grids without reducing stability. To conduct the studies, we selected an area of the Western Electricity Coordinating Council power system that is electrically far from major generation centers and is weakly connected to the bulk transmission system. The area contains large motor loads. We observed the dynamic response of large motors in the vicinity, especially their ability to ride through fault events. The studies were conducted using positive sequence phasor time-domain analysis

Making connections [wind generation facilities]

Large-scale wind generation facilities have become a very visible component of the interconnected power grid in many options of the United States. Only a decade ago, wind generation facilities were viewed by most power engineers as a novelty, and by simple engineering judgement, it could be safely concluded that the effects of these unique but smaller facilities on system reliability would be negligible. Now, with individual wind generation facilities approaching the output rating of conventional power plants, a deeper understanding of their potential impacts on the interaction with the bulk electric power system is needed.

Dark Shadows: Understanding Variability and Uncertainty of Photovoltaics for Integration with the Electric Power System

The U.S. Department of Energys National Renewable Energy Laboratory, Sandia National Laboratories, the Solar Electric Power Association, the Utility Wind Integration Group, and the U.S. Department of Energy hosted a day-long public workshop on the variability of photovoltaic (PV) plants. The workshop brought together utilities, PV system developers, power system operators, and several experts to discuss the potential impacts of PV variability and uncertainty on power system operations.

Lanai High-Density Irradiance Sensor Network for characterizing solar resource variability of MW-scale PV system

Sandia National Laboratories (Sandia) and SunPower Corporation (SunPower) have completed design and deployment of an autonomous irradiance monitoring system based on wireless mesh communications and a battery operated data acquisition system. The Lanai High-Density Irradiance Sensor Network is comprised of 24 LI-COR® irradiance sensors (silicon pyranometers) polled by 19 RF Radios. The system was implemented with commercially available hardware and custom developed LabVIEW applications. The network of solar irradiance sensors was installed in January 2010 around the periphery and within the 1.2 MW ac La Ola PV plant on the island of Lanai, Hawaii. Data acquired at 1 second intervals is transmitted over wireless links to be time-stamped and recorded on SunPower data servers at the site for later analysis. The intent is to study power and solar resource data sets to correlate the movement of cloud shadows across the PV array and its effect on power output of the PV plant. The irradiance data sets recorded will be used to study the shape, size and velocity of cloud shadows. This data, along with time-correlated PV array output data, will support the development and validation of a PV performance model that can predict the short-term output characteristics (ramp rates) of PV systems of different sizes and designs. This analysis could also be used by the La Ola system operator to predict power ramp events and support the function of the future battery system. This experience could be used to validate short-term output forecasting methodologies.

Reactive power performance requirements for wind and solar plants

A current challenge faced by the electric utility industry is to determine how variable generation plants (wind and solar) should contribute to the reliable operation of the electric grid, especially as penetration of these resources continues its upward trend. Traditionally, bulk system voltage regulation has predominately been supplied by synchronous generators, and this is reflected in the language of industry requirements. Where variable generation is concerned the requirements are vague and unclear. The technology used in variable generation plants are capable of providing voltage support, but will require a shift from how these plants are traditionally operated. This paper discusses the capability of wind and solar plants to provide voltage regulation. It also examines the deficiencies in existing standards and provides recommendations to improve upon existing requirements in order to clearly define the role of variable generation in providing voltage support to the bulk electric grid.

Generic models for simulation of wind power plants in bulk system planning studies

The need for generic, standard, non-proprietary models for wind power plants continues to be the subject of much discussion and debate. From a technical point of view, the representation of the often complex dynamic behavior of modern wind power plants is not trivial. However, system planners and compliance organizations continue to struggle with the process deficiencies associated with the black-box and proprietary nature of manufacturer-specific models. For several years, the Western Electricity Coordinating Council (WECC) has championed the development of generic models for wind power plant models, and the progress to date is reported in this document. Recently, other organizations including the International Electromechanical Commission (IEC), manufacturers, software developers, and even utilities have been pursuing similar technical goals. It is anticipated that, through the collective efforts of these stakeholders, generic models will fulfill a much needed gap. This paper reports on the progress made to-date within the Western Electricity Coordinating Council (WECC) regarding the development of generic models suitable for representing wind power plants in typical transmission planning studies. The manuscript address technical issues associated with the representation of wind turbine generators for load flow and transient stability analyses. Current capabilities and envisioned enhancements to existing models are also discussed.

PV output smoothing with energy storage

This paper describes a simple algorithm designed to reduce the variability of photovoltaic (PV) power output by using an energy storage device. A full-scale implementation was deployed in an actual PV-Energy demonstration project, in partnership with a utility and a battery manufacturer. The paper describes simulation tests as well as field results. In addition to demonstrating implementation of smoothing controls, this work also served to verify the models, identify best parameter sets for utility operations, and study the operation of an advanced energy storage system under partial state of charge and rapid, irregular charge/discharge cycling.

Simulation of one-minute power output from utility-scale photovoltaic generation systems.

We present an approach to simulate time-synchronized, one-minute power output from large photovoltaic (PV) generation plants in locations where only hourly irradiance estimates are available from satellite sources. The approach uses one-minute irradiance measurements from ground sensors in a climatically and geographically similar area. Irradiance is translated to power using the Sandia Array Performance Model. Power output is generated for 2007 in southern Nevada are being used for a Solar PV Grid Integration Study to estimate the integration costs associated with various utility-scale PV generation levels. Plant designs considered include both fixed-tilt thin-film, and single-axis-tracked polycrystalline Si systems ranging in size from 5 to 300 MW{sub AC}. Simulated power output profiles at one-minute intervals were generated for five scenarios defined by total PV capacity (149.5 MW, 222 WM, 292 MW, 492 MW, and 892 MW) each comprising as many as 10 geographically separated PV plants.

Statistical criteria for characterizing irradiance time series.

We propose and examine several statistical criteria for characterizing time series of solar irradiance. Time series of irradiance are used in analyses that seek to quantify the performance of photovoltaic (PV) power systems over time. Time series of irradiance are either measured or are simulated using models. Simulations of irradiance are often calibrated to or generated from statistics for observed irradiance and simulations are validated by comparing the simulation output to the observed irradiance. Criteria used in this comparison should derive from the context of the analyses in which the simulated irradiance is to be used. We examine three statistics that characterize time series and their use as criteria for comparing time series. We demonstrate these statistics using observed irradiance data recorded in August 2007 in Las Vegas, Nevada, and in June 2009 in Albuquerque, New Mexico.

Code Shift: Grid Specifications and Dynamic Wind Turbine Models

Grid codes (GCs) and dynamic wind turbine (WT) models are key tools to allow increasing renewable energy penetration without challenging security of supply. In this article, the state of the art and the further development of both tools are discussed, focusing on the European and North American experiences.