Control of fluctuating renewable energy sources: energy quality & energy filters

This doctoral study discusses how to control fluctuating renewable energy sources at converter, unit, and system layers to deliver smoothed power output to the grid. This is particularly relevant to renewable power generation since the output power of many kinds of renewable energy sources have huge fluctuations (e.g. solar, wind and wave) that needs to be properly treated for grid integration. In this research, the energy quality is developed to describe the friendliness and compatibility of power flows/waveforms to the grid, by contrast with the well-known concept of power quality which is used to assess the voltage and current waveforms. In Chapter 1 & 2, a background introduction and a literature review of studied subjects are presented, respectively. In Chapter 3, the problem of determining the PI parameters in dq decoupling control of voltage source converter (VSC) is studied based on a state-space model. The problems of the conventional method when there is insufficient interface resistance are addressed. New methods are proposed to overcome these drawbacks. In Chapter 4 & 5, energy quality and the energy filters (EFs) are proposed as tools to assess and manage power fluctuations of renewable energy sources. The proposed EFs are energy storage control systems that could be implemented on a variety of energy storage hardware. EFs behave like low-pass filters to the power flows. Finally, in Chapter 6, as an application example of renewable power plant with energy filter control and smoothed power output, a master-slave wave farm system is proposed. The wave farm system uses enlarged rotor inertia of electric machines as self-energy storage devices

Power smoothing and oscillations suppression by controlling inertial energy of wind energy systems

In the first part, an economic scheme to smooth short periodic and heavily fluctuating wave power is proposed by controlling the inherent large amount of inertial energy of nearby offshore wind turbine systems (WTSs). The smoothing principle is that these WTSs are controlled to absorb the fluctuations of the wave power or release power opposite to them. The control challenge is that two objectives have to be achieved simultaneously: the rotor speed of a WTS has to be controlled against smoothing requirement whilst controlled to follow changes of wind speed to achieve wind power capture close to the maximum. To resolve this issue, Integrated Compensation Control is developed by adding two supplementary terms into the original maximum power point tracking (MPPT) control. In the second part, a method for short-term wind power smoothing is proposed by controlling wind turbines inertial energy. To achieve this, the designed power reference of a WTS includes two components: one component can approximately recover the original power trajectory of the MPPT control and the other can compensate the fluctuations of the former. In the third part, a new scheme to isolate and suppress forced oscillations is proposed. It controls the inertial energy of wind farms to timely release or absorb power opposite to the forced oscillating power from perturbation areas. Thus, the forced oscillations are prevented from propagating to the rest of power grid - isolated and the oscillating power in the disturbed areas is also reduced - suppressed

Chapter Renewable power sources in coastal areas. A viability assessment in the scope of needs and regulations

The work deals with renewable energy project, in the context of the deregulated energy market. Special attention is focused on renewables and on the situation in Italy from the standards point of view. The set up of a wind farm and a PV plant in coastal Sardinian area for both electricity and desalinated water production is studied. The convenience of fuelling desalination plants through renewables is investigated by taking into account additional on-side trading instruments. A model to simulate the operation wind and PV systems is applied both to calculate the produced energy and to assess the performance of a desalination plant, namely a reverse osmosis plant driven with PV and wind sources that works in a small island site

Master-slave wave farm systems based on energy filter with smoothed power output

Wave energy is an important renewable energy source. Previous studies of wave energy conversion (WEC) have focused on the maximum power take-off (PTO) techniques of a single machine. However, there is a lack of research on the energy and power quality of wave farm systems. Owing to the pulsating nature of ocean waves and popular PTO devices, the generated electrical power suffers from severe fluctuations. Existing solutions require extra energy storage and overrated power converters for wave power integration. In this study, we developed a master-slave wave farm system with rotor inertia energy storage; this system delivers self-smoothed power output to the grid and reduces the number of converters. Two control methods based on the moving average filter (MAF) and energy filter (EF) are proposed to smooth the output power of wave farms. RTDS simulations show that the proposed systems and control methods facilitate simple and smooth grid integration of wave energy. Keywords: Wave farm, Energy storage, Power smoothing, Power quality, Energy qualit

A Review on Grid Integration Challenges of Wind Energy Systems

In recent decades, the strengthening of electric energy security and the reduction of greenhouse gas emissions acquired great traction. The integration of large-scale intermittent renewable energy resources (RER) such as wind energy into existing electrical systems has risen dramatically in recent years. In the last few years, however, this integration creates several operational and control issues that impede the process. Grid functioning must be reliable and stable. This article will look at the problems that have been documented as a result of the recommended solutions techniques and the integration of wind energy Among the many difficulties, Generation uncertainty, power quality difficulties, angular and voltage stability, and reactive power support are all factors to consider. The ability to ride through faults is examined and explored. Aside from that, there are financial, environmental, and political factors to consider

Offshore Electrical Networks and Grid Integration of Wave Energy Converter Arrays - Techno-economic Optimisation of Array Electrical Networks, Power Quality Assessment, and Irish Market Perspectives

Wave energy is an emerging industry and faces many challenges before commercial wave energy converter (WEC) arrays are installed. One of these challenges is the grid integration of WEC arrays. This includes offshore electrical networks, grid compliance, and access to electrical markets. This must be achieved in a technically viable manner and also at an acceptable cost. As electrical networks are expected to make up a large proportion of the overall WEC array CAPEX, perhaps up to 25%, this area is critical to the long term competitiveness of wave energy. The objectives of this thesis are to develop technically and economically acceptable electrical network designs for WEC arrays, evaluate voltage flicker issues for WEC arrays and develop design tools to analyse same, and evaluate the market scale for wave energy in Ireland, considering electrical integration issues in both the domestic and export markets. This thesis presents the optimum design for WEC array electrical networks. By building from the industry state of the art, including offshore wind experience, a comprehensive techno-economic optimisation process is undertaken. This includes optimising the key electrical interfaces between the WEC and the array electrical network, optimising the array network configuration, assessing efficiency of the network, and demonstrating that the network can be achieved at a cost which will allow competitiveness. Some challenges to the economics of WEC array electrical networks and some strategies for improving the economics are presented in this research also. The results provide timely guidance to WEC and WEC array developers. This research also demonstrates the critical link between voltage flicker emissions from WECs and the primary resource, i.e. ocean waves. Some practical assessment tools for the evaluation of this power quality issue are shown to assist in quantifying the problem. Also the full flicker performance of a candidate WEC is assessed helping characterise this link further. In this thesis both the domestic and export markets for Ireland’s wave energy resource are assessed as, although Ireland has an enviable wave energy resource, it is unclear where the market for this resource lies. This analysis shows that the medium term market for wave energy in Ireland is an export market. Also, although technically feasible, there is an additional cost for export transmission which must be considered in evaluating export markets. Some of the critical grid integration issues have been evaluated and addressed in this thesis. Future work is recommended in the areas of weather risk to cable installation at high energy wave sites, evaluating the benefits of shared electrical infrastructure across a range of renewable projects, designing offshore substations for WEC arrays, and quantifying the benefits of the addition of wave energy to the Irish renewable energy mix

Renewable Energy in Marine Environment

The effects of human-caused global warming are obvious, requiring new strategies and approaches. The concept of business-as-usual is now no longer beneficial. Extraction of renewable energy in marine environments represents a viable solution and an important path for the future. These huge renewable energy resources in seas and oceans can be harvested, including wind, tide, and waves. Despite the initial difficulties related mostly to the elevated operational risks in the harsh marine environment, newly developed technologies are economically effective or promising. Simultaneously, many challenges remain to be faced. These are the main issues targeted by the present book, which is associated with the Special Issue of Energies Journal entitled “Renewable Energy in Marine Environment”. Papers on innovative technical developments, reviews, case studies, and analytics, as well as assessments, and papers from different disciplines that are relevant to the topic are included. From this perspective, we hope that the results presented are of interest to for scientists and those in related fields such as energy and marine environments, as well as for a wider audience