This paper provides a summarized status update ofthe Lysekil wave power project. The Lysekil project is coordinatedby the Div. of Electricity, Uppsala University since 2002, with theobjective to develop full-scale wave power converters (WEC). Theconcept is based on a linear synchronous generator (anchored tothe seabed) driven by a heaving point absorber. This WEC has nogearbox or other mechanical or hydraulic conversion systems,resulting in a simpler and robust power plant. Since 2006, 12 suchWECs have been build and tested at the research site located atthe west coast of Sweden. The last update includes a new andextended project permit, deployment of a new marine substation,tests of several concepts of heaving buoys, grid connection,improved measuring station, improved modelling of wave powerfarms, implementation of remote operated vehicles forunderwater cable connection, and comprehensive environmentalmonitoring studies.

The aim of wave energy converters (WECs) is to harvest the energy from the ocean waves and convert into electricity. Optimizing the generator output is a vital point of research. A WEC behaves as a nonlinear system in real ocean waves and a control that approximates the behaviour of the system is required. In order to predict the behaviour of WEC, a controller is implemented with an aim to track the referenced trajectory for a force control application of the WEC. A neural model is implemented for the system identification and control of the nonlinear process with a neural nonlinear autoregressive moving average exogenous (NARMAX) model. The neural model updates the weights to reduce the error by using the Levenberg-Marquardt back-propagation algorithm for a single-input-single-output (SISO) nonlinear system. The performance of the system under the proposed scheme is compared to the same system under a PI-controller scheme, where the PI gains have been tuned accordingly, to verify the control capacity of the proposed controller. The results show a good tracking of dq (direct-quadrature) axes currents by regulating the stator currents, and hence a force control is achieved at different positions of the translator. The dynamic performance of the control is verified in a time domain analysis for the displacement of the translator.

The concern for climate change and energy consumption has increased the demand for renewable energy production considerably. Marine energy sources attract attention because of their high energy density. Wave energy is an attractive renewable energy source with large potential. Due to the nature of the ocean waves, a linear wave energy converter generates intermittent power. It is therefore crucial to regularize the power before connecting to the grid. Energy storage systems present effective ways to minimize the power fluctuations and deliver a steady power to the grid. In this paper, we present an energy management control system with a dynamic rate limiter. The method is applied to control a hybrid energy storage system, combining battery and supercapacitor, with a fully active topology controlled by the power converters. The results show that the method is able to control the charging and discharging states of the battery and the supercapacitor, and minimize the power fluctuation to the grid. The algorithm ensures low losses by shifting the required power and the stored power smoothly over the energy storage system.

Renewable energy sources, such as photovoltaicwind turbines, and wave power converters, use power converters to connect to the grid which causes a loss in rotational inertia. The attempt to meet the increasing energy demand means that the interest for the integration of renewable energy sources in the existing power system is growing, but such integration poses challenges to the operating stability. Power converters play a major role in the evolution of power system towards SmartGrids, by regulating as virtual synchronous ge-nerators. The concept of virtual synchronous generators requires an energy storage system with power converters to emulate virtual inertia similar to the dynamics of traditional synchronous generators. In this paper, a dynamic droop control for the estimation of fundamental reference sources is imple-mented in the control loop of the converter, including active and reactive power components acting as a mechanical input to the virtual synchronous generator and the virtual excitation controller. An inertia coefficient and a droop coefficient are implemented in the control loop. The proposed con-troller uses a current synchronous detection scheme to emulate a virtual iner-tia from the virtual synchronous generators. In this study, a wave energy converter as the power source is used and a power management of virtual synchronous generators to control the frequency deviation and the terminal voltage is implemented. The dynamic control scheme based on a current synchronous detection scheme is presented in detail with a power manage-ment control. Finally, we carried out numerical simulations and verified the scheme through the experimental results in a microgrid structure.

This study presents a step toward the grid connection of a wave-energy park through an electric power conversion system (EPCS) developed and installed for the wave-energy harvesting in Lysekil, Sweden. The EPCS comprises a rectifier, a DC bus, and an inverter followed by a harmonic filter (HF). The higher- and lower-order harmonics injected by the inverter in a power quality context are investigated. The lower-order voltage harmonics partially distort the voltage-source inverter output grid current. A phase-locked loop-based (PLL) grid-phase tracking is used to attenuate the lower-order harmonics by reflecting the grid harmonics in the inverter output. An expression for the grid-current harmonics as a function of the grid-voltage harmonics has been derived and implemented. A mathematical model is derived to obtain a transfer function for the PLL, and finally, proportional–integral gains are tuned for stable system operation. An HF for mitigating the higher-order harmonics has been implemented. The total harmonic distortion is evaluated experimentally, and the results fulfil the grid-code requirements at various frequencies and harmonic orders.

This paper provides an overview of electric power conversion system installed at the Lysekil research site, located at the west coast of Sweden. The electric power conversion system consists of rectifiers, rectifying the power from the wave energy converters, a DC-link and a grid-tied inverter. The paper focuses on the performance of the inverter and the filter and presents experimental results obtained during the grid integration.

Power fluctuations induced by wave energy converters (WECs) may reflect negative impact on the power quality of the power grid. Assessing their impact is an important step to ensure the grid compliance level of the energy park. The IEC 61000-4-15 standard classifies the allowable disturbances in the grid. This study analysed and assessed the grid impact in terms of flicker, harmonic distortion and voltage variations. The assessments were performed without energy storage and compared when using the energy storage. A single WEC is emulated as an irregular power output of a real WEC using a combined model of power take-off in the Simulink model. Time series based on data obtained in earlier offshore experiments, conducted at the Lysekil research site in Sweden, is used to emulate a wave energy park (WEP) power in a land-based test rig in real-time power hardware-in-the-loop simulations. A total of three and ten WECs are emulated by introducing a time delay in the time series to investigate the grid impact in each layout. Flicker emissions, voltage variations, individual and total harmonics of the voltage at the connection point in each layout are studied and compared with the limits to be grid compliant for layouts of the WEP. In addition, voltage and current harmonics for the single WEC and individual harmonics in each phase of the voltage are measured and analysed to assess the compliance level of the WEP.