Mohit Singh

Understanding inertial and frequency response of wind power plants

The objective of this paper is to analyze and quantify the inertia and frequency responses of wind power plants with different wind turbine technologies (particularly those of fixed speed, variable slip with rotor-resistance controls, and variable speed with vector controls). The fundamental theory, the operating range, and the modifications needed for the wind turbine to contribute to the inertial and primary frequency response during the frequency drop will be presented in this paper. We will demonstrate practical approaches to allow variable slip and speed wind turbines to contribute inertia to the host power system grid. The approaches are based on the inclusion of frequency error and the rate of change of frequency signals in the torque control loop and pitch control actions for wind speeds below and above its rated value. Detailed simulation models in the time domain will be conducted to demonstrate the efficacy of the approaches.

Dynamic Models for Wind Turbines and Wind Power Plants

The primary objective of this report was to develop universal manufacturer-independent wind turbine and wind power plant models that can be shared, used, and improved without any restrictions by project developers, manufacturers, and engineers. Manufacturer-specific models of wind turbines are favored for use in wind power interconnection studies. While they are detailed and accurate, their usages are limited to the terms of the non-disclosure agreement, thus stifling model sharing. The primary objective of the work proposed is to develop universal manufacturer-independent wind power plant models that can be shared, used, and improved without any restrictions by project developers, manufacturers, and engineers. Each of these models includes representations of general turbine aerodynamics, the mechanical drive-train, and the electrical characteristics of the generator and converter, as well as the control systems typically used. To determine how realistic model performance is, the performance of one of the models (doubly-fed induction generator model) has been validated using real-world wind power plant data. This work also documents selected applications of these models.

Interarea Oscillation Damping Controls for Wind Power Plants

This paper investigates the potential for wind power plants (WPPs) to damp interarea modes. Interarea modes may be the result of a single or a group of generators oscillating against another group of generators across a weak transmission link. If poorly damped, these power system oscillations can cause system instability and potentially lead to blackouts. Power conversion devices, particularly, megawatt-scale converters that connect wind turbines and photovoltaic power plants to the grid, could be used to damp these oscillations by injecting power into the system out of phase with the potentially unstable mode. In our model, this power may be provided by a WPP. Over time, the net energy injection is near zero; therefore, providing this static damping capability is not expected to affect the energy production of a WPP. This is a measurement-based investigation that employs simulated measurement data. It is not a traditional small-signal stability analysis based on Eigenvalues and knowledge of the power system network and its components. Kundurs well-known two-area, four-generator system and a doubly fed induction generator (DFIG)-based WPP are modeled in PSCAD/EMTDC. The WPP model is based on the Western Electricity Coordination Council (WECC) standard model. A controller to damp interarea oscillations is added to the WECC DFIG model, and its effects are studied. Analysis is performed on the data generated by the simulations. The sampling frequency is set to resemble the sampling frequency at which data are available from phasor measurement units in the real world. The YuleWalker algorithm is used to estimate the power spectral density of these signals.

Modeling and Control to Mitigate Resonant Load in Variable-Speed Wind Turbine Drivetrain

Failure of the drivetrain components is currently listed among the most problematic failures during the operational lifetime of a wind turbine. Guaranteeing robust and reliable drivetrain designs is important to minimize the wind turbine downtime as well as to meet demand in both power quantity and quality. While aeroelastic codes are often used in the design of wind turbine controllers, the drivetrain model in such codes is limited to a few (mostly two) degrees of freedom, resulting in a restricted detail in describing its dynamic behavior and assessing the effectiveness of controllers on attenuating the drivetrain load. In the previous work, the capability of the well-known FAST aeroelastic tool for wind turbine has been enhanced through integration of a dynamic model of a drivetrain. The drivetrain model, built using the Simscape in the MATLAB/Simulink environment, is applied in this paper. The model is used to develop a power-electronics-based controller to prevent excessive drivetrain load. The controller temporarily shifts the closed-loop eigenfrequency of the drivetrain through the addition of virtual inertia, thus avoiding the resonance. Simulation results demonstrating the fidelity of the expanded drivetrain model as well as the effectiveness of the virtual inertia controller are presented.

Doubly Fed Induction Generator in an Offshore Wind Power Plant Operated at Rated V/Hz

This paper introduces the concept of constant volt/hertz operation of offshore wind power plants (WPPs). The deployment of offshore WPPs requires power transmission from the plant to the load center inland. Because this power transmission requires submarine cables, there is a need to use high-voltage direct current (HVDC) transmission, which is economical for distances greater than 50 km. In the concept presented here, the onshore substation was operated at 60 Hz synced with the grid, and the offshore substation was operated at variable frequency and voltage, allowing the WPP to be operated at constant volt/hertz. In this paper, a variable frequency at rated volt/hertz operation was applied to a Type 3 doubly fed induction generator (DFIG) wind turbine generator. The size of the power converter at the turbine can be significantly reduced from 30 % of the rated power output in a conventional Type 3 turbine to 5 % of the rated power. The DFIG allows each turbine to vary its operating speed with respect to the other turbines. Thus, small wind diversity within the WPP can be accommodated by the DFIG, and the collector system frequency can be controlled by HVDC to follow large variations in average wind speed.

Gearbox and drivetrain models to study dynamic effects of modern wind turbines

Wind turbine drivetrains consist of components that directly convert kinetic energy from the wind to electricalenergy. Therefore, guaranteeing robust and reliable drivetrain designs is important to prevent turbine downtime. Currentdrivetrain models often lack the ability to model both the impacts of electrical transients as well as wind turbulenceand shear in one package. In this paper, the capability of the FAST wind turbine computer-aided-engineering tool,developed by the National Renewable Energy Laboratory, is enhanced through the integration of a dynamic model of thedrivetrain. The dynamic drivetrain model is built using Simscape in the MATLAB/Simulink environment and incorporatesdetailed electrical generator models. This model can be used to evaluate internal drivetrain loads due to excitationsfrom both the wind and generator.

Using generic wind turbine models to compare inertial response of wind turbine technologies

This paper describes the use of generic wind turbine models to study the comparative inertial response of wind turbine technologies, that is, their response to a frequency event on the grid such as loss of generation. The dynamic models are manufacturer-independent and portable to any dynamic modeling software. Four dominant types of wind turbine technology are modeled; the basic fixed-speed model is used as a platform to develop dynamic rotor resistance, DFIG and full-converter models. All the models share the same aerodynamic and mechanical characteristics as well as the same induction generator unit. These models are particularly suited for studying inertial response since they employ detailed representations of the wind turbine aerodynamics, drive-train, electrical machine, power converters (when present), and controls. The results showed that converter-based turbines provide poorer inertial response than turbines which do not employ converters.

Wind Power Plant Model Validation Using Synchrophasor Measurements at the Point of Interconnection

A wind power plant (WPP) is different from a conventional power plant in the sense that a WPP may consist of hundreds of small (e.g., 1.5-MW) wind turbine generators (WTGs), whereas a conventional power plant may consist of one or several large generators. Common practice in power system planning to simulate a WPP is to use a single-turbine representation. However, it is important to realize that the response of a single-turbine representation is not the response of an individual turbine; instead, it represents the collective behavior of a WPP. In this paper, we present our experience in validating WPP from available measured data. We investigate the discrepancies between the simulation results and the actual measurement, and we examine the probable causes of these discrepancies. Finally, we offer methods to validate WPP dynamic model to better match the simulation result to the measured data. Understanding the nature of a WPP and the meaning of WPP equivalency is very important to determine the representation of a WPP.

Validation and analysis of wind power plant models using short-circuit field measurement data

This paper describes the use of field measurements to validate a wind power plant model. A three-phase time-domain wind power plant model, based on the Western Electricity Coordinating Council generic model, has been developed with this validation in mind. The available fault data consists of the three-phase voltages and currents measured at the point of interconnection (POI) of the wind plant with the grid. For validation, the measured voltage data is injected into the simulation model and the current response from the model is compared to the measured current data from the real-world wind plant. Real power and reactive power flows calculated from the measured data and the model output are compared. The advantage of this validation method, namely use of currents in addition to real and reactive power flows, is also discussed. Comparison with a positive-sequence model is also provided.

Fixed-speed and variable-slip wind turbines providing spinning reserves to the grid

As the level of wind penetration increases, wind turbine technology must move from merely generating power from wind to taking a role in supporting the bulk power system. Wind turbines should have the capability to provide inertial response and primary frequency (governor) response. Wind turbine generators with this capability can support the frequency stability of the grid. To provide governor response, wind turbines should be able to generate less power than the available wind power and hold the rest in reserve, ready to be accessed as needed. In this paper, we explore several ways to control wind turbine output to enable reserve-holding capability. The focus of this paper is on fixed-speed (also known as Type 1) and variable-slip (also known as Type 2) wind turbines.