Mikel Alberdi

Modeling and Simulation of Wave Energy Generation Plants: Output Power Control

The control and simulation of the power delivered to the grid are becoming an important topic, particularly when the number of distributed power generation systems increases. In this paper, two different control schemes for an oscillating-water-column Wells-turbine-generator module are simulated, implemented, and compared. In the first method, the control system does appropriately adapt the slip of the induction generator according to the pressure drop entry in order to maximize the generated power, while in the second method, a traditional proportional-integral-derivative-based control is implemented in order to deal with the desired power-reference-tracking problem. It will be shown how the controllers avoid the stalling behavior and that the average power of the generator fed into the grid is significantly higher in the controlled cases than in the uncontrolled one while providing the desired output power.

Sliding-Mode Control of Wave Power Generation Plants

The worldwide demand for energy requires alternatives to fossil fuels and nuclear fission, so renewable resources, particularly ocean energy, are called to play a relevant role in a near future. In particular, the oscillating water column (OWC) is one of the most promising devices to harness energy from the sea, as it is the case of the Nereida project plant, located in the Basque coast of Mutriku. This kind of devices consists of a particular type of turbine and a doubly fed induction generator. The turbogenerator module is usually controlled using a traditional proportional-integral (PI)-based vector control scheme, which requires an accurate knowledge of the system parameters and lacks of robustness, limiting, in some cases, the power extraction. To avoid these drawbacks, a novel sliding-mode-control-based vector control scheme for the OWC plant is presented in this paper. This variable-structure control is intrinsically robust under parameter uncertainties, which always appear in real systems, and presents a convenient disturbance rejection. The stability of the proposed controller is analyzed using the Lyapunov theory. The performance of the control scheme presented is proved by comparing it to the traditional PI-based vector control scheme in a series of representative maximum power generation case studies. Both numerical simulations and experimental results show that the proposed solution provides high-performance dynamic characteristics, improving the power extraction in spite of parameter uncertainties and system disturbances.

Complementary Control of Oscillating Water Column-Based Wave Energy Conversion Plants to Improve the Instantaneous Power Output

In this paper, an oscillating water column-based wave energy conversion plant is modeled and controlled by means of two complementary control strategies in order to improve the conversion of wave energy into electric power. This wave power generation system consists of a capture chamber, a Wells turbine, and an induction generator. The improvement relays on the implementation of a control scheme that combines two different control methods: a rotational speed control and an airflow control implemented by means of a throttle valve in series with the turbine. The use of rotational speed control provides a fast way to react to the abrupt and short changes in the turbine speed, ensuring that the average power of the generator is adequately adjusted according to the incident wave power level. Besides, a throttle-valve is used to control the flow through the turbine so as to increase the amount of energy produced, particularly at the higher incident wave power levels. These two control strategies complement each other, maximizing and improving the quality of supply by controlling and smoothing the generated power for different scenarios of wave oscillations, variations of wave grouping, and changes in the sea state.

Fault-Ride-Through Capability of Oscillating-Water-Column-Based Wave-Power-Generation Plants Equipped With Doubly Fed Induction Generator and Airflow Control

The increasing use of distributed power-generation systems, as with the case of wave-power-generation plants, requires a reliable fault-ride-through capability. The effects of grid fault include uncontrolled turbogenerator acceleration, dangerous rotor peak currents, and high reactive-power consumption so that the plant may contribute to the voltage dip. A simple solution is automatic disconnection from the grid, but this policy could lead to a massive power-network failure. This is why new Grid Codes oblige these systems to remain connected to the grid. In this paper, an oscillating-water-column-based wave-power-generation plant equipped with a doubly fed induction generator is modeled and controlled to overcome balanced grid faults. The improvement relies on the implementation of a control scheme that suitably coordinates the airflow control, the active crowbar, and the rotor- and grid-side converters to allow the plant to remain in service during grid fault, contributing to its attenuation by supplying reactive power to the network and complying with new Grid Code requirements. The simulated results show that it obtains a great reduction of the rotor currents, improving the transients and avoiding rotor acceleration. Similar results are obtained from the experimental implementation.

Performance of an Ocean Energy Conversion System with DFIG Sensorless Control

The 2009/28/EC Directive requires Member States of the European Union to adopt a National Action Plan for Renewable Energy. In this context, the Basque Energy Board, EVE, is committed to research activities such as the Mutriku Oscillating Water Column plant, OWC. This is an experimental facility whose concept consists of a turbine located in a pneumatic energy collection chamber and a doubly fed induction generator that converts energy extracted by the turbine into a form that can be returned to the network. The turbo-generator control requires a precise knowledge of system parameters and of the rotor angular velocity in particular. Thus, to remove the rotor speed sensor implies a simplification of the hardware that is always convenient in rough working conditions. In this particular case, a Luenberger based observer is considered and the effectiveness of the proposed control is shown by numerical simulations. Comparing these results with those obtained using a traditional speed sensor, it is shown that the proposed solution provides better performance since it increases power extraction in the sense that it allows a more reliable and robust performance of the plant, which is even more relevant in a hostile environment as the ocean.

Neural rotational speed control for wave energy converters

Among the benefits arising from an increasing use of renewable energy are: enhanced security of energy supply, stimulation of economic growth, job creation and protection of the environment. In this context, this study analyses the performance of an oscillating water column device for wave energy conversion in function of the stalling behaviour in Wells turbines, one of the most widely used turbines in wave energy plants. For this purpose, a model of neural rotational speed control system is presented, simulated and implemented. This scheme is employed to appropriately adapt the speed of the doubly-fed induction generator coupled to the turbine according to the pressure drop entry, so as to avoid the undesired stalling behaviour. It is demonstrated that the proposed neural rotational speed control design adequately matches the desired relationship between the slip of the doubly-fed induction generator and the pressure drop input, improving the power generated by the turbine generator module.

Stalling behaviour improvement by appropriately choosing the rotor resistance value in wave power generation plants

In this paper the performance of the Wells turbine has been improved, studying its stalling behaviour when the flow coefficient reaches a specific characteristic value. The generator increases its velocity when the flow coefficient of the turbine is approaching the critical point at which the turbine losses power.

Control strategies for OWC wave power plants

The present work deals with the improvement of OWC-Wells turbine-generator systems by adequately choosing the applied control scheme. For this purpose, two different control strategies are presented and compared. In the first one, the control system does appropriately adapt the slip of the induction generator according to the pressure drop entry. The second control strategy consists of a traditional control of the OWC air valve. It is demonstrated that the proposed rotational speed control design adequately matches the desired relationship between the slip of the induction generator and the pressure drop input, whilst the valve control using a traditional PID controller successfully governs the flow that modulates the pressure drop across the turbine.

Sensor control for an Oscillating Water Column plant

The Oscillating Water Column concept consists of two key elements, a turbine located in a pneumatic energy collection chamber and a doubly fed induction generator that converts energy extracted by the turbine into a form that can be returned to the network. Since the turbo-generator control requires the rotor angular velocity, removing the rotor speed sensor simplifies the hardware, with the consequent reduction in installation and maintenance costs which is even more relevant in the rough working sea conditions. In this particular case, a Luenberger based observer is considered and the effectiveness of the proposed control is shown by numerical simulations. Comparing these results with those obtained using a traditional speed sensor, it is shown that the proposed solution provides better performance since it allows a more reliable and robust performance of the plant.

Linear models for plasma current control in tokamak reactors

The control of plasma in nuclear fusion has been revealed as a promising application of Control Engineering, with increasing interest in the control community during last years. In this paper it is developed a control-oriented linear model for the control of plasma current. For this purpose, it is provided a summary of the background necessary to deal with control problems in tokamak-based nuclear fusion reactors as it is the case of the future ITER tokamak. Besides, it is also given a review of the most used simulators and plasma models, with the aim of providing an adequate background for control engineers to derive their own control-oriented model or to choose the appropriate existing one. Finally, the proposed plasma model performance is proven in a current drive profile trajectory tracking problem using a modified anti-windup PID-based control scheme.