Offshore Wind Energy Conversion Systems Research Articles

On Fatigue Damage-Oriented Through-Life Optimization and Control of High-Power IGCT Converters in Wind Energy Systems

Integrated gate commutated thyristors (IGCTs) are critical components in high-voltage, high-current, and high-power conversion systems, particularly in offshore wind energy systems. However, the working environment of offshore wind energy conversion systems is extremely harsh. In this article, we propose an active damage control approach aiming at enhancing the reliability of the conversion system. By employing electro-thermal modeling for the equipment of the offshore wind energy conversion system, the junction temperature and fatigue damage of IGCT are simulated during the operation process. Using the improved model predictive current control (MPCC) method, active damage control effectively regulates the switching frequency of IGCT. IGCTs are symmetrically distributed on each leg of the converter, so the lifespan of the two IGCTs on each leg is also considered to be similar. This method balances the life of the IGCTs on the three legs of the converter and optimizes their utilization to the maximum extent. These measures effectively enhance the reliability of the conversion system and lower the operation and maintenance cost of high-power IGCT converters. The effectiveness of the proposed method is validated by co-simulation results by ANSYS and MATLAB/Simulink.

Torque ripple and mass minimization of a 20-MW direct drive offshore wind generator

This article investigates the structural optimization of a 20-MW offshore wind generator to minimize the torque ripple and mass. The surface-mounted permanent magnet topology will be chosen for the generator. In addition, the double-layer armature winding configuration will be utilized. The direct-drive technology will be adopted to increase the reliability of the offshore wind energy conversion system. The multi-objective optimization of the generator is carried out based on the particle swarm algorithm. Results show that the torque ripple of the optimal model is significantly reduced to as much as 0.41% at full-load conditions making the configuration suitable for the high-power offshore wind generator.

Levelized Cost of Energy Comparison Between Permanent Magnet and Superconducting Wind Generators for Various Nominal Power

The levelized cost of energy (LCOE) of offshore wind energy conversion systems using permanent magnet and fully superconducting generators is compared. Scaling laws are used to calculate the masses and the costs of all the components, to the exception of the generators that are designed and optimized by using the finite element method. The comparisons are done for various turbine’s nominal power, ranging from 2 to 23 MW. The results show that for each generator type, there exists an optimal nominal power that minimizes the LCOE. In addition, the use of a superconducting generator could significantly lower (about 7% at 20 MW) the LCOE compared with using a permanent magnet generator.

A review on development of offshore wind energy conversion system

Wind energy conversion system, aiming to convert mechanical energy of air flow into electrical energy has been widely concerned in recent decades. According to the installation sites, the wind energy conversion system can be divided into land-based wind conversion system and offshore wind energy conversion (OWEC) system. Compared to land-based wind energy technology, although OWEC started later, it has attracted more attentions due to its significant advantages in sufficient wind energy, low wind shear, high power output and low land occupancy rate. In this paper, the principle of wind energy conversion and the development status of offshore wind power in the world are briefly introduced at first. And then, the advantages and disadvantages of several offshore wind energy device (OWED), such as horizontal axis OWED, vertical axis OWED and cross axis OWED are compared. Subsequently, several major constraints, such as complex marine environment, deep-sea power transmission and expensive cost of equipment installation faced by offshore wind conversion technology are presented and comprehensively analysed. Finally, based on the summary and analysis of some emerging technologies and the current situation of offshore wind energy utilization, the development trend of offshore wind power is envisioned. In the future, it is expected to witness multi-energy complementary, key component optimization and intelligent control strategy for smooth energy generation of offshore wind power systems.

Cost and reliability optimization of modular multilevel converter with hybrid submodule for offshore DC wind turbine

The hybrid modular multilevel converter (HMMC) consisting of half- and full-bridge submodules (SMs), with DC fault ride-through capability, have become a promising candidate for offshore wind energy conversion system, especially all-DC output wind turbines. The topology of HMMC needs to be optimized for lower cost and higher reliability with active redundancy. This paper presents an optimization method for HMMC topology with cost and reliability objectives. The reliability is defined to be not only reliable under normal operation, but also able to ride through DC fault even with faulted SMs, which is different from the former model. Firstly, reliability and cost models with power loss, considering active redundancy configuration, are derived. Then, cost and reliability, with different numbers of original and redundant SMs, are quantified by taking a case study of 10-MW and 10-kV HMMC for offshore DC wind turbines. Finally, a multi-objective optimization approach that regards maximum reliability and minimum cost is proposed, and the optimal solution from Pareto-optimal set is selected according to fuzzy membership function, which determines the optimal topology for HMMC. The results show that reliability can be improve with redundant full-bridge SMs (FBSMs) but weakened with excessive half-bridge SMs (HBSMs). Furthermore, the incremental cost with a redundant FBSM is approximately 3 times that of HBSM. The optimal topology, 10 original SMs per arm (5 HBSMs and 5 FBSMs) with 1 redundant HBSM and 4 FBSMs, is validated by comparing with the results of former reliability model and single-objective optimization.

Further Study on a PWM Current-Source-Converter-Based Wind Energy Conversion System Considering the DC-Link Voltage

Previous research on the series-connected pulsewidth modulation current-source-converter-based offshore wind energy conversion system is conducted without considering the effects of the independent control of offshore converters and the cables connected between offshore converters and between offshore and onshore converters. In the present work, a further study on the system is conducted taking these factors into consideration. First, the equivalent circuit of the system is derived, based on which two items are selected to evaluate the performance of the system: the dc-link current and the dc-link voltage. Second, both the dc-link current and the dc-link voltage are derived and analyzed, and the worst case is quantitatively and qualitatively defined. Third, the performance of the system with dc filters is investigated. Finally, both simulation and laboratory-scale experiments are provided.

Reconfigurable Control for Fault-Tolerant of Parallel Converters in PMSG Wind Energy Conversion System

Offshore wind energy conversion system (WECS) usually adopts parallel converters to expand power capacity and improve reliability because of limited power rating of power devices, harsh marine environment, and poor maintenance accessibility. Any faulty converter is cutoff under traditional fault-tolerant control. When all parallel converters are faulty, the wind turbine must be shut down. Although simple, such a fault-tolerant control reduces system availability. Thus, a reconfigurable control for fault-tolerant of parallel wind power converters is proposed in this paper. The current reference of every phase leg in parallel converter system is reconfigured postfault according to their healthy states. The proposed reconfigurable control is realized in software level and without requiring redundant hardware. Compared with traditional fault-tolerant control, the proposed reconfigurable control only cuts off the faulty legs rather than faulty converter, and makes full use of all healthy legs. The reconfigurable control improves the availability of parallel wind power converters and provides multiple-leg fault ride-through capability to WECS, enhancing reliability and electricity generation. The simulation and experimental results validate the feasibility and effectiveness of the proposed reconfigurable control.

Recurrent wavelet-based Elman neural network with modified gravitational search algorithm control for integrated offshore wind and wave power generation systems

A new approach to rotational speed control structures based on an optimized intelligent recurrent wavelet-based Elman neural network (RWENN) controller used for the integration of offshore wind and wave energy conversion systems driven by a doubly fed induction generator. The nodes connecting the weights of the RWENN are trained online using a backpropagation method. A modified gravitational search algorithm (MGSA) is developed to adjust the learning rates and improve learning capability. The proposed control scheme has improved the real power regulation and dynamic performance of a combined wind and ocean wave energy scheme over a wide range of operating conditions. The performance of this control scheme is assessed by comparing it to a traditional proportional-integral based control scheme in a series of case studies representative of maximum power generation. Simulations are carried out using PSCAD/EMTDC software to verify the robustness of the power electronics converters and the efficiency of the proposed controller under steady state and transient conditions.

Bipolar Operation Investigation of Current Source Converter Based Wind Energy Conversion Systems

A series-connected current source converter (CSC) based configuration has recently been proposed for offshore wind energy conversion systems. A big challenge exists for such a system that its maximum insulation level is the full transmission voltage due to its monopolar operation. This introduces significant burden to the system in terms of cost, reliability, and flexibility. To solve this issue, a bipolar operation giving a half insulation requirement is proposed and investigated in this paper. However, a unique challenge exists for the CSC-based system when operating under bipolar mode, that is, the dc-link current control. There are two equivalent paths for the dc-link current, which introduces a concern for proper operation of the bipolar system. Accordingly, an optimized dc-link current control is developed in this study. In summary, the bipolar system with the help of the optimized dc-link current control features lower insulation requirement, higher reliability, higher efficiency, and higher flexibility. Finally, both simulation and experimental results are provided.

Attraction, Challenge and Current Status of Marine Current Energy

Reducing greenhouse gas emissions becomes a top priority in the world with the emergence of global warming and environmental problems. Various renewable energies appear during the last decades. Ocean captures and stores huge amounts of energy, which could satisfy five times of world energy demand. Due to technology limitations and economic considerations, marine current energy appears the most attractive choice compared with the other ocean energy form. Although the existing expertise and technology in offshore wind energy conversion system can be partially transferred to marine current energy conversion system due to the similar structure, there are still many technological challenges to overcome. Meanwhile, the system operates under the water will inevitably have some negative or positive impacts on the surrounding environment. In this paper, it shows the interest and the principle of the marine current energy, and also discusses the advantages and disadvantages. The environmental impacts around the devices, the technological challenges, and the essential support structures are presented as well. The state-of-the-art horizontal axis turbines and their relative technologies and the latest projects are described finally. This review paper gives the useful information about the attraction and challenge of the marine current energy, and the newest development of the technologies and projects.

Power Balancing Investigation of Grid-Side Series-Connected Current Source Inverters in Wind Conversion Systems

In current source inverter (CSI)-based series-connected configurations, recently proposed for offshore wind energy conversion systems, a number of medium-voltage CSIs are connected in series to form a centralized dc/ac inverter to transfer offshore wind power to the grid. In previous studies, the series-connected CSIs are controlled based on assumptions that all CSIs are ideal and identical and the power is evenly distributed among them. However, in practice, such assumptions are invalid as tolerance exists. This study therefore works on the investigation of power distribution among series-connected CSIs considering the tolerance. First, the possible imbalance of power among CSIs is unveiled, analyzed, and quantitatively defined. Second, a combined control scheme is proposed, based on which both conventional control objectives, which are dc-link current control and power factor control, and an equal power distribution among CSIs are ensured simultaneously. Finally, simulations and lab-scaled experiments are provided.

High frequency wind energy conversion system for offshore DC collection grid—Part I: Comparative loss evaluation

Abstract In this paper, the investigation of an offshore wind energy conversion system with a high frequency link is continued. The design of the conversion system aims for a high efficiency and low weight and volume. It is composed of a permanent magnet generator, an AC–AC converter, a high frequency transformer and a diode bridge rectifier. The system is intended for a DC collection grid of turbines connected in series. Three different AC–AC converters are considered: the direct and indirect matrix converter and the back-to-back converter. The design of the entire conversion system is presented with emphasis on the converter and the high frequency transformer. The efficiencies of these two components as well as filters are assessed using a simulation model of the conversion system and device datasheet for the converter and analytical analysis for the transformer and filters. The conversion systems with the three converters show comparable efficiency performance; the system with the matrix converter however displaying the highest efficiency of 96.53 % , the indirect matrix converter 86.4 % and the back-to-back the lowest, 96.36 % or 93.51 % depending on the filter design. This paper is the prequel to a second part where the conversion system is further developed to improve the converter efficiency and the transformer design.

Offshore wind turbine simulation: Multibody drive train. Back-to-back NPC (neutral point clamped) converters. Fractional-order control

This paper is on offshore wind energy conversion systems installed on the deep water and equipped with back-to-back neutral point clamped full-power converter, permanent magnet synchronous generator with an AC link. The model for the drive train is a five-mass model which incorporates the dynamic of the structure and the tower in order to emulate the effect of the moving surface. A three-level converter and a four-level converter are the two options with a fractional-order control strategy considered to equip the conversion system. Simulation studies are carried out to assess the quality of the energy injected into the electric grid. Finally, conclusions are presented.

Three-level Converter in Offshore Wind Energy Systems: New Strategy for Unbalancing in Capacitors Voltage

Abstract This paper is on an offshore wind energy conversion system equipped with full-power three-level converter and permanent magnet synchronous generator. Multi-level converters, namely three-level converters, are limited by unbalance voltage in the direct current link capacitors. A new control strategy for the selection of the output voltage vectors is proposed in order to improve balance of voltage in the capacitors.

High-power Generators for Offshore Wind Turbines

In offshore wind energy conversion system, the dominant technology is the geared drive train with induction generator, and high-power generators are preferred with respect to cost and energy production. However, simply upscaling the conventional design will normally lead to low efficiency, heavy top head mass and expensive machine. The main objective of this paper is to investigate the technological challenges related to the high-power generators for offshore wind turbines. In the first part of this paper, the generator technologies in the global operational offshore wind farms are reviewed; in the second part, the relationships of generator active/supporting mass vs. power are obtained by doing machine optimization with the finite element method and the genetic algorithm. The solutions for high-power generators are reviewed and discussed finally.

State of the Art on the Monitoring Methods of Offshore Wind Energy Conversion System

Along with the rapid economic development, the large power-generating wind energy conversion systems are increasing both in number and in size. Because of the high construction cost, the consequence of any structural failure, especially offshore where replacing a damaged member such as blade which weights tones, is pretty expensive. Consequently particular actions are required to the protection of these structures. Among these many strategies, continuously monitoring of the condition of the system which provides a predication long before the actual failure should be one of the most efficient ways. In the paper, the main problems and their corresponding surveillance schemes of the system including tower, foundation, blade and turbine nacelle are discussed.

State of the Art on the Monitoring Methods of Offshore Wind Energy Conversion System

Along with the rapid economic development, the large power-generating wind energy conversion systems are increasing both in number and in size. Because of the high construction cost, the consequence of any structural failure, especially offshore where replacing a damaged member such as blade which weights tones, is pretty expensive. Consequently particular actions are required to the protection of these structures. Among these many strategies, continuously monitoring of the condition of the system which provides a predication long before the actual failure should be one of the most efficient ways. In the paper, the main problems and their corresponding surveillance schemes of the system including tower, foundation, blade and turbine nacelle are discussed.

FOUNDATION DESIGN METHODS FOR OFFSHORE WIND ENERGY CONVERTION SYSTEMS

It is important to promote the introduction of new energies that impose low loads on the environment. Wind power is viewed as a promising new energy source that is pure, clean, and inexhaustible. In the coastal areas, wind is not disturbed by structures and relatively stakes. These areas therefore, are suitable for constructing wind energy conversion systems. This report describes the design method of the foundation and the execution methodologies for offshore wind energy conversion systems.

Offshore Wind-Energy-Conversion Systems

TRONG prevailing surface winds and relatively shallow water characterize much of the U.S. offshore regions. These properties, plus remoteness from population centers, infer that offshore siting of wind-turbine generators is desirable. However, a number of other factors, some more serious than others, detract from the concept. For example, the wind-turbine-generator plant itself must be marinized or built to withstand the harsh ocean environment including hurricanelike storms. Also, massive support platforms, comparable to those presently used for offshore drilling, are needed to position the wind-turbine generators above the sea. Another consideration is the collection and transmission system needed to deliver the generated energy to shore. These formidable problems and others may overshadow the economic benefits of higher unit energy production. The worth of available and socially acceptable sites is speculative at best. A comprehensive study was completed to access the technical feasibilities and cost of multi-unit wind-energyconversion systems sited in the U.S. off-shore. This paper summerizes the principal findings of the study. Scope of Study Multiunit offshore-wind-energy-conversion systems (OWECS) for delivering bulk electrical power to shore were conceptually designed with cost estimates made for those regions of the U.S. offshore deemed suitable for OWECS. State-of-the-art designs were employed to maximize the credibility of the cost estimates. Platforms for supporting wind-turbine-generator (WTG) plants included bottom-mounted fixed types for shallowwater sites plus moored and dynamically positioned floating types for deep-water sites. No limit was imposed on water depth. Each platform carried only one single-rotor WTG plant. Horizontal-axis and vertical-axis WTG plant types, specifically designed for marine environments, were assessed. Several WTG plant/platform designs were dynamically analyzed to show if low-frequency vibrations could pose design restrictions. Submarine cables, either ac or dc, were the preferred method for delivering the energy produced by the WTG plant