A comparative analysis of linear and nonlinear control of wave energy converter for a force control application

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

An experimental approach on linear synthetic inertia

The interest in renewable energy has significantly increased in the last decades which has led to an increased amount of renewable energy sources in the grid. In the Nordic grid, the major contribution to renewable energy is hydro power and wind power and an increase in the amount of wind power is expected in the future. The increase in wind power and decommissioning of nuclear power is expected to decrease the mechanical inertia in the system which helps to stabilise the electrical grid frequency. The inertia is expected to decrease by a factor of two within 20 years and other solutions for frequency stability must be implemented to assure a stable power system. At Uppsala University several projects are investigating how grid-connected energy storages can increase the frequency stability with a high penetration of intermittent renewable energy sources. In this thesis, a linear synthetic inertia control algorithm is implemented on a national Instruments FPGA for controlling the power flow from a supercapacitor energy storage via a two-level three-phase inverter. The control strategy is evaluated both via simulations and experimental tests in a nano grid. The results of the simulations and experimental work are presented and show that it is possible to calculate the frequency derivative in real time to reduce the frequency ROCOF and nadir. The results of the increased frequency stability are presented

Power electronics and controller interface for a Voltage Source Converter

The purpose of the thesis is to develop a system for a split-rotor drive and evaluatecontrollers and their internal components such as processors, communicationprotocols and execution speed for controlling magnetization currents in a hydropower station.The first part of the thesis builds the theory review and provides an introduction tothe most common processors and controllers available. The processors which wasevaluated were microprocessor, DSP and FPGA which have a high capacity andvariety of implementation possibilities. Two controllers, PLC and PAC whereevaluated, which contain some or several of the processors and have a wide variety ofinputs and outputs and support as well several communication protocols.Three different communication protocols; WLAN 802.11, Ethernet 802.3 andBluetooth 802.15.1. Evaluation was made by comparing BER, throughput, speed and implementationcomplexity. The second part of the thesis was to develop and order an interface card forconnecting power-electronics and measurements circuits for the system, based on thetheory and evaluation of the controller and communication protocols

An experimental approach to energy storage based synthetic inertia and fast frequency regulation for grid balancing

The increasing interest in renewable energy has significantly increased in the last decades. The increasing amount of variable renewable energy resources in the grid, which are connected via power electronics, reduces the total mechanical system inertia. Frequency-regulating resources such as hydropower will become more important in balancing variable renewable energy resources, setting higher requirements on stability and performance to maintain a stable electrical grid. This thesis concerns the decreased mechanical inertia from non-directly electrically coupled generation units. The thesis starts with a description of the grid system inertia situation today and presents two methods for estimating the grid frequency derivative used to provide synthetic inertia and one method used to enhance the mechanical inertia response of a synchronous generator. The synthetic inertia and enhanced inertia methods are tested in a small-scale experimental setup and compared with results from tests in the Nordic grid. A full-scale hybrid energy storage system was designed and built using a split frequency method as a power controller. The results show that a power-frequency derivative controller-based synthetic inertia method achieved an improved grid frequency quality during regular operation in the nano-grid experimental setup. The results are evaluated both via simulations and experimental tests. The results from the hybrid energy storage solution showed the possibility of increasing frequency quality by using a slow run of the river hydroelectric power plants and a battery energy storage system for frequency containment reserve

An experimental approach on linear synthetic inertia

The interest in renewable energy has significantly increased in the last decades which has led to an increased amount of renewable energy sources in the grid. In the Nordic grid, the major contribution to renewable energy is hydro power and wind power and an increase in the amount of wind power is expected in the future. The increase in wind power and decommissioning of nuclear power is expected to decrease the mechanical inertia in the system which helps to stabilise the electrical grid frequency. The inertia is expected to decrease by a factor of two within 20 years and other solutions for frequency stability must be implemented to assure a stable power system. At Uppsala University several projects are investigating how grid-connected energy storages can increase the frequency stability with a high penetration of intermittent renewable energy sources. In this thesis, a linear synthetic inertia control algorithm is implemented on a national Instruments FPGA for controlling the power flow from a supercapacitor energy storage via a two-level three-phase inverter. The control strategy is evaluated both via simulations and experimental tests in a nano grid. The results of the simulations and experimental work are presented and show that it is possible to calculate the frequency derivative in real time to reduce the frequency ROCOF and nadir. The results of the increased frequency stability are presented

An experimental approach to energy storage based synthetic inertia and fast frequency regulation for grid balancing

The increasing interest in renewable energy has significantly increased in the last decades. The increasing amount of variable renewable energy resources in the grid, which are connected via power electronics, reduces the total mechanical system inertia. Frequency-regulating resources such as hydropower will become more important in balancing variable renewable energy resources, setting higher requirements on stability and performance to maintain a stable electrical grid. This thesis concerns the decreased mechanical inertia from non-directly electrically coupled generation units. The thesis starts with a description of the grid system inertia situation today and presents two methods for estimating the grid frequency derivative used to provide synthetic inertia and one method used to enhance the mechanical inertia response of a synchronous generator. The synthetic inertia and enhanced inertia methods are tested in a small-scale experimental setup and compared with results from tests in the Nordic grid. A full-scale hybrid energy storage system was designed and built using a split frequency method as a power controller. The results show that a power-frequency derivative controller-based synthetic inertia method achieved an improved grid frequency quality during regular operation in the nano-grid experimental setup. The results are evaluated both via simulations and experimental tests. The results from the hybrid energy storage solution showed the possibility of increasing frequency quality by using a slow run of the river hydroelectric power plants and a battery energy storage system for frequency containment reserve

Start of a synchronous motor using rotor field polarity inversion and rotor back-emf sensing

Synchronous motors are hard to line start due to torque pulsations at zero rotor speed and low starting torque when started using induced current in a damper squirrel cage. By inverting the rotor pole polarity at appropriate times it is possible to, in principle, achieve uniform torque, albeit pulsating with twice the line frequency at zero initial rotor speed. This has been demonstrated in an earlier work. In this paper we demonstrate that high torque starting using the back-emf in the field winding as triggering signal for the rotor polarity inversion is possible. We further discuss the origin of the rotational energy and active and reactive power pulsations. Finally, we show that it is possible to operate a synchronous motors at continuous asynchronous speed by inverting the polarity of the rotor current and adjusting the field current accordingly, although down rated

Energy management for a grid-connected wave energy park through a hybrid energy storage system

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

Experimental Test of Grid Connected VSC to Improve the Power Quality in a Wave Power System

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

Virtual Synchronous Generator Based Current Synchronous Detection Scheme for a Virtual Inertia Emulation in SmartGrids

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