Project Scope

The primary goal for the first phase of this project was the development of a set of tools to aid utility distribution and planning engineers in analyzing wind generation at the distribution system level. The tools consist of a set of engineering software application tools. These tools, or applets, aid users in analyzing the various impacts of wind turbines on utility distribution feeders. The tools are intended for use by technical personnel who might not have specialized expertise with wind turbines or generation, but need to determine the impact wind turbines might have on voltage regulation, power quality, feeder and equipment protection, and system safety.

These applets are useful as screening tools, where only cursory information about the distribution system and the prospective distributed wind project would need to be input. The tools permit more detailed studies involving specific characteristics of wind turbines, wind resource, and distribution feeders. The applets consider certain economic issues related to distributed wind turbines, as well as primarily technical concerns.

The project deliverables are available on this Web site, along with application guides and information resources that can help utility personnel and others with analysis of distributed wind applications. This analysis focuses on issues of most concern with wind, including grid interconnection requirements, voltage regulation and flicker problems, and preliminary economic screening.

The applets run over the Web and are accessed using this project web site. The simulation-based applets use a customized version of Electrotek Concepts' Distribution System Simulator and utilize an electrical model of a single feeder, with one or more wind generation sources. The feeder electrical model is presented in a one-line diagram with symbols for wind turbines, regulating transformers, shunt capacitors, and overcurrent protective devices.

The project covers four separate focus areas with information tools and associated software applications. These include

A specific description of each area follows.

General Interconnection Application Guide

IEEE 1547 is the industry recognized DG interconnection standard. It establishes criteria and requirements for interconnection of DG to the electric grid. A set of guidelines for applying this standard specifically to distributed wind generation has been developed. Included are practical recommendations for meeting guidelines concerning voltage regulation, harmonic specifications, and protection/disconnect requirements.

A Web-based system overcurrent protection evaluator was developed. This application uses a single feeder and wind generation electrical data to estimate the new fault current levels, and flag possible device coordination problems. The input factors are

  • Wind generator and interface electrical characteristics.
  • Service transformer impedance and winding connections.
  • Utility source impedance and feeder electrical data.
  • Utility protective devices.

If detailed information on protective devices is not available, the user may select generic characteristics for fuse links, reclosers and relays The application simulates the four basic fault types - SLGF, DLGF, DLF, and 3LGF - on both the service transformer primary and secondary. Faults at the downstream terminal of each utility protective device on the feeder are also simulated.

The output focuses on comparing performance with and without the wind generator operating. This provides a convenient filter for more detailed feeder analysis of potential problems. The application output includes

  • Fault current levels at the service transformer secondary.
  • Fault current levels at the interconnection point, with and without the wind generation.
  • Identify fault events and clearing sequences where the wind generation causes an increase in the interrupted load, or an increase in the duration of an interruption for some of the load.

Detailed Flicker Application Guide

The most prominent power quality concern when interconnecting wind turbines to distribution systems is whether the resulting variations in power flow along the feeder will result in objectionable levels of flicker. Using present techniques for predicting flicker caused by varying loads to predict the possible flicker caused by wind turbines can be very difficult to do accurately. Although flicker is addressed in IEEE 1547, it is such a concern when wind is utilized as a distributed resource that a focused application guide is needed. An application guide was developed detailing techniques used by many European countries; incorporating the widely accepted IEC methodologies adopted by IEEE for measuring flicker. This application guide describes in detail the phenomena of flicker, what causes it, how it can accurately be measured and predicted, how to prevent it, and how to mitigate it once it is present.

This application area includes two software applets, one analyzing possible voltage regulation problems and the other analyzing potential flicker problems.

Voltage Regulation Applet

A Web-based voltage profile simulator was developed. This application simulates the voltage profile along the feeder as the level of generated wind power varies. The user can select from a library of typical power profiles, scaled by the size of installed wind generation at the site. The model consists of a single feeder requiring the following input data

  • Feeder electrical characteristics.
  • General turbine characteristics.
  • Load characteristics and distribution (time variation of load is not simulated).
  • Shunt capacitor banks and control strategies.
  • Regulating transformers and settings.
  • A and B voltage range limits, either from ANSI C84 or as modified by the state PUC.

For a selected feeder location, the outputs will include trend plots of

  • Individual phase voltage magnitude.
  • phase voltage unbalance.
  • negative sequence voltage magnitude.

Other outputs include

  • percentage of time outside the A and B range limits.
  • projected number of regulator tap changes and capacitor switching operations.

Wind Turbine Flicker Calculator Applet

The wind turbine flicker calculator estimates the levels of flicker that would be found on the distribution feeder as a result of interconnecting a single or multiple wind turbines to the distribution system. This calculator takes distribution feeder parameters from the user and calculates estimated flicker levels along the feeder for various wind turbine sizes, makes, and models.

The wind turbine flicker calculator can be launched from within the voltage regulation applet, or as a stand-alone Web applet. If launched from the voltage profile applet, the flicker calculator uses the distribution feeder parameters previously entered to calculate expected flicker levels along the distribution feeder. As a stand-alone applet, the flicker calculator requires the user to input the short-circuit impedance and angle at the wind turbine PCC, along with distance from the substation to the turbine PCC.

Using data collected from turbine manufacturers, published papers and reports, a database of wind turbine flicker coefficients is being developed. This database will allow the user the option of selecting a specific wind turbine size, make, and model to interconnect with the feeder model.

Distributed Wind Economic Screening Applet

An economics screener was developed for use through a Web browser. This application uses utility economic parameters to estimate the economic benefits of a proposed wind generator application. The analysis includes wind turbine characteristics and site wind resource profiles selected from a library of typical characteristics. Users are able to modify the typical data. The factors included in the economic analysis are

  • Utility installation costs, discount rates, planning horizon, and escalation factors.
  • Taxes and special credits.
  • Selling and generating price of electricity at the peak and off-peak periods.
  • Wind turbine, generator, and interface characteristics.
  • Wind profile at a site.
  • Capital cost and O&M cost of the wind-related equipment.
  • Net present worth, cost-benefit, and payback period outputs.

The analysis combines the site profile and wind turbine characteristics to estimate the total energy production. Interactions between turbines are not included, except as encapsulated within a single selection of typical data. Detailed electrical simulation is also not included.

Existing Distributed Wind Case Studies

A set of case studies was developed in cooperation with UWIG members. Working closely with each selected member, each installation was analyzed for technical and economic merits using monitored data and simulations. The case studies include the following elements

  • Site information.
  • Feeder electrical characteristics, loads, and wind generator penetration level.
  • Wind turbine, generator, and interface characteristics.
  • Economic analysis of wind generation.
  • Voltage regulation and flicker analysis.
  • Summary of impact on utility operations, including reliability, overcurrent protection, and harmonics.
  • Expected vs. actual performance.

Monitoring of Distributed Wind Turbines

The analytical capabilities yielded by this project were supplemented with monitoring of the performance of wind turbines operating in the field under actual operating conditions. This measurement data can be used to quantify characteristics for assessing distribution system impacts. Monitoring can also capture details of turbine operation during disturbances on the distribution feeder, providing more important information for characterizing dynamic behavior and understanding impacts on distribution feeder protective schemes.

Other turbine performance attributes, such as higher-order harmonics or switching noise from advanced power conversion equipment, can be characterized with data provided by monitoring.

In this task, three distributed wind turbine installations were selected from UWIG member sites for monitoring. Collected data is made available to UWIG members through the project Web site, with views of real-time and historical data and events.

Data collected from each distributed wind generation installation includes:

  • High-resolution (1 sec.) samples of interconnect voltage, current, and complex power.
  • Waveform and rms data (voltage and current) for distribution feeder disturbances, including switching transients, faults, voltage sags, and outages.
  • At additional cost, meteorological data (wind speed and direction, temperature, pressure) can also be obtained from any or all of the sites.

Moving Ahead

The first phase of this project resulted in a set of useful tools for analysis of distributed wind applications, but a day of training is helpful for most utility engineers to use the tools. UWIG decided that the next step should be a tool suite that can be used on-line with no formal training at all. To that end, UWIG has assembled funding from APPA’s DEED program as well as NRECA in partnership with CEATI International to undertake an effort that will yield the following deliverables:

  • Wind Energy FAQ Document – will help utilities and their customers decide whether wind power is a viable strategy or a good policy for their situation.
  • Distributed Wind Interconnection Guide – will help utilities specify wind turbine interconnection requirements, and identify areas of technical concern for safe and reliable operation.
  • Distributed Wind Modeling Guide – providing technical background and typical data on wind turbine dynamics, flicker, and power electronic interfaces for more detailed study by utilities or their consultants, using the UWIG Feeder Simulator.
  • Interconnection Checklist – a concise, time-saving guide for handling wind turbine interconnection requests, with links to appropriate software tools and application guide sections for each item in the checklist.
  • Updated Screening Tool – quickly evaluates a proposed wind turbine interconnection with respect to the FERC fast-track acceptance criteria, flicker, voltage control, overcurrent protection, and temporary overvoltage.
  • Turbine Monitoring Report – providing the flicker performance and capacity factor for two turbine types at three sites. This will provide a template for possible future monitoring of other turbine types.

 

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