There is increasing interest in interconnecting individual wind turbines in offshore wind farms using DC networks rather than AC networks. As the output from each turbine/generator is normally low (less than 6 kV), DC-DC converters may be required to step-up the voltage to an intermediate level to interconnect the turbine outputs, and another DC-DC converter to step-up this intermediate voltage to a high level for transmission to the shore. One possible structure of the transmission side of the multiterminal system is radial topology with tens or hundreds of submodules, as shown in Figure 1. Each of the submodules is combined with a permanent magnet synchronous generator, a PWM rectifier and a DCDC converter. Filters are required to achieve relatively smooth output DC waveforms at each stage: these filters between the PWM rectifiers and the DC-DC converters play an important role during the operation. This paper will concentrate on the performance of different types of filters and their influences on the sending end of the system, while the most suitable structures of the filter will be proposed.

In this paper, a combinational AC/DC power flow approach is proposed for the solution of the combined AC/DC network. The unified power flow approach is extended to include DC voltage droop control. In the VSC based MTDC grids, DC droop control is regarded as more advantageous in terms of operational flexibility, as more than one VSC station controls the DC link voltage of the MTDC system. This model enables the study of the effects of DC droop control on the power flows of the combined AC/DC system for steady state studies after VSC station outages or transient conditions without needing to use its complete dynamic model. Further, the proposed approach can be extended to include multiple AC and DC grids for combined AC/DC power flow analysis. The algorithm is implemented by modifying the MATPOWER based MATACDC program and the results shows that the algorithm works efficiently.

Offshore wind power is attracting increasing levels of research and investment. The use of HVDC transmission and the development of dc grids are topics with similar high levels of interest that go hand in hand with the development of large scale, far from shore wind farms. Technical challenges result from the interaction between power-electronic dc-dc converters and the cables in a dc transmission network. In particular, the propagation of the ripple current in bipole dc transmission cables, constructed with a lead sheath and steel armor, is examined in detail. The finite-element method is used to predict the currents induced in the outer layers of the cable by the ripple current. These results are used along with wave propagation theory to demonstrate that cable design plays a crucial role in the behavior of the dc system. Applications include the prediction of transmission losses, resonance, and high-voltage filter design.

In an offshore wind HVDC system, one option for connecting the variable speed wind turbine/permanent magnet generators to the DC grid is via passive rectifiers in series with DC/DC converters. This paper focuses on the DC/DC converters in the PMSG-rectifier-DC/DC converter the system. Three DC/DC converter topologies are investigated supplied by MW level PMSG wind generators. The topologies include the Phase Shift Full Bridge Converter, Series Load Resonant (SLR) Full Bridge Converter and Thyristor Based Resonant Converter. Detailed models of the DC/DC converters are built in the PLECS simulator. A 15kW 150V/600V 500Hz Phase Shift full bridge DC/DC converter prototype has been built to validate the simulations.

This paper presents a secondary control for MTDC systems to update the DC voltage and converter power references of the VSC stations operating under droop control, in order to eliminate the large power deviation from their desired reference power. The rated values of the VSC station used as reference does not take into account the power losses in the converter, DC line and the voltage drops in the DC transmission lines. This creates a constant difference between rated DC voltage reference and actual DC voltage value of the VSC station. This difference is multiplied by the droop gain of the VSC station, hence causing power deviations in the MTDC grid system. In this paper a secondary control is implemented for a four-terminal HVDC system modelled in EMTDC/PSCAD, to demonstrate the reduction in power deviation by the proposed secondary control.

This paper compares six DC/DC converter topologies for offshore wind HVDC applications. The topologies include five conventional transformer based topologies and one novel transformerless topology. Detailed models of the DC/DC converters are simulated using PLECS, and the component stress, component count and losses are presented. The overall performance of each topology is evaluated based on the simulated results.

Offshore wind power is attracting increasing levels of research and investment. The use of HVDC transmission and development of DC grids are topics with similar levels of interest that go hand in hand with the development of large scale, far from shore wind farms. In this paper, technical challenges resulting from the interaction between power electronic DC-DC converters and the cables in a DC transmission network are identified. Also the behaviour of a typical ripple current generated by such converters in a small DC network is simulated.

This paper proposes control strategies for arrays of direct drive wave energy converters. Two control strategies taken from the theoretical study of the hydrodynamic properties of oscillating bodies are applied to a wave energy converter (WEC) system including a linear generator (LG) and voltage source converter system. The purpose of this study is to investigate the feasibility of the control strategies on WECs working in arrays. Simulations of an array of direct-drive WECs connected together by DC link were developed in Matlab/Simulink. The study concludes with a discussion on their feasibility in terms of electrical power generated and the deployment suitability in real seas.

The offshore location of wave energy converters demands a highly reliable and fault-tolerant system. Capacitors account for the majority of failures in power converters and should be replaced prior to failure to reduce system downtime. This paper presents a control methodology to reduce the rate of capacitor degradation as a means to improve fault tolerance. Modeling, simulation, and power converter control are systematically investigated using MATLAB/Simulink.

Marine energy devices such as wave energy converters and tidal current turbines are set to play a significant role in contributing to electricity generating by renewable means. The offshore location where these devices operate demands high reliability and robust fault tolerance. Bearing failure is a key issue for renewable energy devices and is a cause of significant downtime in wind turbines. Bearing wear is accelerated due to unbalanced magnetic pull in the rotor of an electrical generator. This paper presents a review and assessment of methods to reduce bearing wear through both machine design and active control through power electronics. (6 pages)

This paper describes causal sub-optimal control of a direct drive linear generator to extract the maximum power from real sea conditions without wave prediction. The reaction force required to control the buoy to move in phase with the ocean waves is provided by the linear generator. After determining the power take-off force based on the model of a machine, the required currents from the generator can be obtained. These currents have varying amplitudes and frequencies thus presenting a challenge to traditional Pulse Width Modulation control. In this paper, a continuously varying average voltage method is presented where the average voltage for each switching cycle is determined by pre-calculated duty ratio to meet the required voltages before coming into the power converter. Using Matlab/Simulink, simulated results of the power converter current control are presented.

It has been shown through modelling and simulation that a linear electrical generator can be effectively controlled to maximise the energy extracted from sea waves. A reaction force control scheme allows the performance of a direct drive wave energy converter to be optimised, which adds to the benefits of low mechanical complexity and high conversion efficiencies in a direct drive system. In this study, reaction force control through experimental verification is presented. The use of a linear generator test rig and electronic hardware to control the phase and amplitude of oscillation are investigated.

Wave energy converters require a power conversion stage to convert the variable output from an electrical generator into a constant voltage and frequency for grid connection. To enable further development in grid integration of wave energy converter systems, this paper looks at the main power conversion considerations in converting wave energy into electrical energy and reviews current power converter topologies. In particular, the paper highlights the trade-off between energy storage, which brings about improved power quality, and improved reliability due to the absence of capacitors.

Direct drive power take-off for wave energy conversion has been proposed as a viable alternative to hydraulic and pneumatic based systems found in conventional wave energy converters. Allowing for further benefits to be realised, this paper presents a reaction force control scheme to maximise energy extraction, and investigates the modelling and simulation of a direct drive wave energy converter. The control scheme is applied to an experimental test rig with a prototype linear machine with results presented and analysed.

A transformer with a separation between the primary and secondary windings can be used to transfer power to artificial organs, pacemakers or robot joints without electrical contacts or wires. This paper examines the impact of the gap fringing field in the operation of the dc-dc converter. As has been implied in previous work the power losses and EMI generated by the fringing field can have a significant impact on the converter operation. This paper presents the mechanism behind the winding power losses using FEA tools and actual measurements. Furthermore the effect of the field vector in electromagnetic emissions of the converter is investigated. A reduction of EMI by means of adjusting the air-gap length is demonstrated and the benefit is quantified using field measurements.

Direct drive wave energy converters have been proposed in view of the disadvantage of mechanical complexity and low conversion efficiencies in conventional wave energy converters. By directly coupling a linear generator to a reciprocating wave energy device, it is suggested that direct drive power take-off could be a viable alternative to hydraulic- and pneumatic-based systems. To further realise the benefits of a direct drive system, a control scheme based on reaction force control to maximise energy extraction is presented. It focuses predominantly on the theoretical analysis of the linear generator reaction force. The modelling, simulation and control of direct drive wave energy conversion are systematically investigated by computer-aided analysis via Matlab/Simulink

In this paper a new single-stage PFC topology, to meet IEC 61000-3-2, based on the forward converter with a low frequency switch is presented. The proposed converter is simple and surfers from lower component stresses than other single-stage PFC converters. Due to this it is expected that the converter will operate with higher conversion efficiency and lower cost. The harmonic content of the input current, efficiency and component stress are investigated.

In high voltage (HV) applications, it is attractive to dispense with the expensive output inductor and operate with a purely capacitive filter. This paper shows that the HV Phase Shifted Bridge with capacitive filter (PSBCF) converter can lose soft switching in two MOSFETs. This can result in a lower efficiency and a limitation of operation to the lower switching frequencies. The LDD auxiliary circuit developed for the standard PS-PWM-ZVS-FB converter is adapted to assist the HV PSBCF with ZVS. The operation of the auxiliary circuit is explained and its performance examined.