Ted Brekken

Optimal Energy Storage Sizing and Control for Wind Power Applications

The variable output of a large wind farm presents many integration challenges, especially at high levels of penetration. The uncertainty in the output of a large wind plant can be covered by using fast-acting dispatchable sources, such as natural gas turbines or hydro generators. However, using dispatchable sources on short notice to smooth the variability of wind power can increase the cost of large-scale wind power integration. To remedy this, the inclusion of large-scale energy storage at the wind farm output can be used to improve the predictability of wind power and reduce the need for load following and regulation hydro or fossil-fuel reserve generation. This paper presents sizing and control methodologies for a zinc-bromine flow battery-based energy storage system. The results show that the power flow control strategy does have a significant impact on proper sizing of the rated power and energy of the system. In particular, artificial neural network control strategies resulted in significantly lower cost energy storage systems than simplified controllers. The results show that through more effective control and coordination of energy storage systems, the predictability of wind plant outputs can be increased and the cost of integration associated with reserve requirements can be decreased.

Control of a Doubly Fed Induction Wind Generator Under Unbalanced Grid Voltage Conditions

Wind energy is often installed in rural, remote areas characterized by weak, unbalanced power transmission grids. In induction wind generators, unbalanced three-phase stator voltages cause a number of problems, such as overcurrent, unbalanced currents, reactive power pulsations, and stress on the mechanical components from torque pulsations. Therefore, beyond a certain amount of unbalance, induction wind generators are switched out of the network. This can further weaken the grid. In doubly fed induction generators (DFIGs), control of the rotor currents allows for adjustable speed operation and reactive power control. This paper presents a DFIG control strategy that enhances the standard speed and reactive power control with controllers that can compensate for the problems caused by an unbalanced grid by balancing the stator currents and eliminating torque and reactive power pulsations

Reserve Requirement Impacts of Large-Scale Integration of Wind, Solar, and Ocean Wave Power Generation

Many sources of renewable energy, including solar, wind, and ocean wave, offer significant advantages such as no fuel costs and no emissions from generation. However, in most cases these renewable power sources are variable and non-dispatchable. The utility grid is already able to accommodate the variability of the load and some additional variability introduced by sources such as wind. However, at high penetration levels, the variability of renewable power sources can severely impact the utility reserve requirements. This paper presents an analysis of the interaction between the variability characteristics of the utility load, wind power generation, solar power generation, and ocean wave power generation. The results show that a diversified variable renewable energy mix can reduce the utility reserve requirement and help reduce the effects of variability.

A Permanent-Magnet Tubular Linear Generator for Ocean Wave Energy Conversion

This paper presents a novel permanent-magnet tubular linear generator (PMTLG) buoy system designed to convert the linear motion of ocean waves into electrical energy. The design incorporates no working seals and a saltwater air-gap bearing surface integration between the PMTLG buoy components. The internal generator design will be discussed, in addition to the system integration with the buoy structure. The simulation and hardware results of the generator are presented.

Control of a doubly-fed induction wind generator under unbalanced grid voltage conditions

Wind energy is often installed in rural, remote areas characterized by weak, unbalanced power transmission grids. In induction wind generators, unbalanced three-phase stator voltages cause a number of problems, such as overheating, over-current, and stress on the mechanical components from torque pulsations. Therefore, beyond a certain amount of unbalance, induction wind generators are switched out of the network. This can further weaken the grid. In doubly-fed induction generators, control of rotor currents allows for adjustable speed operation and reactive power control. In addition, it is possible to control the rotor currents to correct for the problems caused by unbalanced stator voltages, including torque pulsations and unbalanced stator currents. This paper presents a novel voltage mode controller design for a doubly-fed induction generator that provides variable speed, reactive power control. Also, under stator voltage unbalance conditions, the proposed control eliminates torque pulsations and draws more balanced currents from the utility

Utility-connected power converter for maximizing power transfer from a photovoltaic source while drawing ripple-free current

This paper presents a power converter for coupling photovoltaic arrays to the utility grid. The converter draws a programmable, ripple-free DC current from the photovoltaic array and injects power into the grid at unity power factor. The programmable input current feature makes this converter ideal for use with maximum power point tracking technology. The proposed converter has an additional unique feature in that the internal dc link carries a large (approximately 25%) ripple voltage. Allowing a large ripple on the DC link reduces the required size of the link capacitor. This paper includes basic system information, analysis of filter requirements, controller design and preliminary hardware results.

Nonlinear Model Predictive Control of a Point Absorber Wave Energy Converter

This paper presents the application of nonlinear model predictive control (NMPC) to a point absorber wave energy converter (WEC). Model predictive control (MPC) is generally a promising approach for WECs, since system constraints and actuator limits can be taken into account. Moreover, it provides a framework for defining optimal energy capture and it can benefit from predictions. Due to possible nonlinear effects, such as the mooring forces, an NMPC is proposed in this paper, whose performance is compared to that of a linear MPC. Both controllers are supposed to control a nonlinear point absorber model. Computer simulations show that the proposed NMPC is able to optimize the energy capture while satisfying system limits.

On Model Predictive Control for a point absorber Wave Energy Converter

This paper presents the development of Model Predictive Control (MPC) for ocean wave energy applications; specifically for a point absorber Wave Energy Converter (WEC). MPC has several features that make it very attractive for ocean wave energy, in particular the ability to consider constraints on the system forces and limitations in system velocity and position in the context of current future desired velocity or position. The results show that MPC coupled with forecasting can allow the system to optimize energy capture while respecting system limits.

Ocean wave energy overview and research at Oregon State University

The solutions to todays energy challenges need to be explored through alternative, renewable and clean energy sources to enable a diverse national energy resource plan. An extremely abundant and promising source of energy exists in the worlds oceans. Ocean energy exists in the forms of wave, tidal, marine currents, thermal (temperature gradient) and salinity. Among these forms, significant opportunities and benefits have been identified in the area of ocean wave energy extraction, i.e., harnessing the motion of the ocean waves, and converting that motion into electrical energy. This paper presents the fundamentals of ocean wave energy, and also a summary of the wave energy research being conducted at Oregon State University. This paper is intended to serve as an introduction to wave energy for scientists and engineers, particularly those with a wind energy background.

Reserve requirement impacts of large-scale integration of wind, solar, and ocean wave power generation

Many sources of renewable energy, including solar, wind, and ocean wave, offer significant advantages such as no fuel costs and no emissions from generation. However, in most cases these renewable power sources are variable and nondispatchable. The utility grid is already able to accommodate the variability of the load and some additional variability introduced by sources such as wind. However, at high penetration levels, the variability of renewable power sources can severely impact the utility reserve requirements. This paper presents an analysis of the interaction between the variability characteristics of the utility load, wind power generation, solar power generation, and ocean wave power generation. The results show that a diversified variable renewable energy mix can reduce the utility reserve requirement and help reduce the effects of variability.