Offshore Wind Farm Integration Research Articles

Parallel operation of diode‐rectifier based HVDC link and HVAC link for offshore wind power transmission

This paper investigates the integration of large offshore wind farms using parallel HVAC and diode-rectifier based HVDC (DR-HVDC) systems. Three different operation modes, i.e. HVAC operation mode, DR-HVDC operation mode and parallel operation mode are investigated. A wind turbine control scheme including distributed control and centralised control is proposed to ensure the stable operation of the offshore wind farms under different operation modes. The proposed control requires no switching of the distributed control strategy when the operation mode is changed. Moreover, power flow between the DR-HVDC link and HVAC link under parallel operation can be well controlled with the centralised control. Simulation results in PSCAD/EMTDC verify the proposed control during transition among the three operation modes.

Unidirectional DC/DC modular multilevel converter for offshore windfarm with the control strategy based on stationary frame

This paper presents the design and control of an advanced unidirectional DC/DC modular multilevel converter (MMC) that facilitate the integration of off-shore windfarms with the high-voltage direct current (HVDC) transmission system. The proposed converter deploys a single-phase MMC inverter coupled with series-connected rectifier modules through multiple medium frequency transformers. Unlike the conventional dq control method, which involves multiple transformations, this paper also proposes a simple control strategy that directly acts on the AC output of the MMC, under the stationary reference frame.

DC Fault Protection in Multi-terminal VSC-Based HVDC Transmission Systems with Current Limiting Reactors

Multi-terminal VSC-based HVDC transmission system is the recent interest for grid integration of large-scale offshore wind farms. Protection of multi-terminal voltage source converters (VSC)-based HVDC transmission systems against DC faults is challenging. This paper presents a single-ended protection scheme for DC faults in a three-terminal VSC-HVDC transmission system. The under-voltage criterion is used to distinguish the DC faults from the transient and normal conditions. The rate of change of DC voltage and current as well as the variation of transient energy is used to discriminate the internal faults from the external faults. The DC fault current has very high value within a few milliseconds during the transient phases such as the capacitor discharging and diode freewheeling stages. Therefore, current limiting reactors are introduced in series with the DC circuit breaker to maintain the DC fault current within the breaker capacity. The single-ended protection scheme is tested with the three-terminal VSC-HVDC transmission system with current limiting reactors for various DC fault conditions. The DC fault data is generated from PSCAD/EMTDC simulation and the protection scheme is tested in MATLAB environment. Test results show that the proposed protection scheme gives reliable protection for the DC faults in a three-terminal VSC-HVDC transmission system.

Low‐frequency oscillation analysis of AC/DC system with offshore wind farm integration via MMC‐based HVDC

Low‐frequency oscillation analysis of AC/DC system with offshore wind farm integration via MMC‐based HVDC

A New Frequency for Offshore Wind-farm Based on Component Loss Calculation

<span lang="EN-IN">Offshore wind power plants are gaining importance in recent years, as there is adequate space available for its installation, high wind speed, no restriction on the size of turbine blades (no transportation and construction problem) and blades can be allowed to rotate at higher speed without any noise constraint, thereby increasing the rated power. However, the existing offshore wind farms face greater cost related challenges than those of onshore plants. The integration of offshore wind farm with onshore power grid is a complex issue. Feasible solutions for power transmission through cables from offshore wind farms to onshore are HVAC, line commutated HVDC and VSC-HVDC. This paper analyses the various schemes for integration of offshore wind farm with onshore power grid and suggests that LFAC with submarine cable operating at 0.7 Hz is an optimal choice in obtaining better performances. </span>

Fault characteristics and protection adaptability analysis in VSC‐HVDC‐connected offshore wind farm integration system

Due to different fault ride through controls of both the wind farm and the voltage-source converter high-voltage alternating current (VSC-HVDC) converters, the offshore wind farms that are connected to the power system through VSC-HVDCs have unique fault transients. During a fault on AC side transmission lines, the wind farm and the VSC-HVDC together provide faulted current, whose amplitude and phase will be decided by their different control strategies collectively. The fault transients are different from those in the traditional AC grid. Thus, the performance of traditional protection will be affected. In this study, the AC fault current expression is deduced, while the influencing factors on the current's amplitude and phase are analysed in detail. Based on these characteristics, the performance of distance relays under the grid side fault of the VSC-HVDC-connected offshore wind farm integration system is evaluated. Simulations based on PSCAD have verified the theoretical analysis.

A State of Art Review on Offshore Wind Power Transmission Using Low Frequency AC System

Offshore wind power generation, transmission and integration around the world are becoming higher, and it provides technical and economic challenges for future practitioners and industries to make an alternative transmission system to an existing system in reduction of cost. In most of countries, e.g. The German government planned to install 25000MW of offshore wind farms by 2030. The greater part of offshore wind farms are integrated with High Voltage AC (HVAC) transmission to the onshore grid. Offshore wind farms are integrated with High Voltage DC (HVDC) transmission for long distances (>50km) to the grid because of capacitive cable current in HVAC. The major challenge in HVDC transmission is the installation, operation and maintenance (O&M) of the Voltage Sourced Converter (VSC) HVDC substation in the offshore climate. Currently, in case offshore wind farm the research have been fastened to reduce the complexity with increasing reliability and minimizing cost. This paper gives a comprehensive review of integration of offshore wind farm via Low Frequency AC or Fractional Frequency AC (LFAC or FFAC) transmission. LFAC transmission is adopted from HVAC and it is operated at one third of nominal frequency (16.67 Hz). As compared to HVDC the main advantage of LFAC is an absence of the offshore converter station, hence system complexity and cost reduced. In design considerations, especially the offshore transformer is one of the challenges. This paper presents a comprehensive review on components and its design considerations of offshore novel LFAC transmission. The offshore wind turbine considerations, collector network and different types of onshore frequency converters explained in detail.

Fault Ride Through Strategy for Offshore Wind Farm Integration System via MMC-HVDC

When offshore wind power is transmitted to ac grid through MMC-HVDC, the current and voltage will be quite different from those in traditional ac grid during grid side fault. This paper sets up an offshore wind farm integration system via MMC-HVDC and designs control strategies for each unit in the system. The fault ride through strategy of the system is proposed and its effectiveness has been verified. Thus, the AC bus voltage on wind farm side will stay stable during AC side fault. Once the chopper resistance is set properly, the output power and current of the wind farm can basically remain unchanged, which can successfully achieve fault isolation. The simulation results based on PSCAD have verified the theoretical analysis.

Interactive teaching–learning optimiser for parameter tuning of VSC‐HVDC systems with offshore wind farm integration

This study proposes a novel interactive teaching–learning optimiser (ITLO) for voltage source converter based high voltage direct current (VSC-HVDC) systems with offshore wind farm integration. Conventional vector control strategy is adopted, in which the parameters of eight proportional–integral loops are optimally tuned to achieve a reliable and satisfactory control performance under various operation conditions. Multiple classes are employed to realise a wider exploration compared with that of original teaching–learning based optimisation. Moreover, a small world network is incorporated for an interactive learning among the teachers or students from different classes, such that a deeper exploitation can be resulted in. Hence, ITLO can effectively avoid a local optimum due to the appropriate trade-off between explorations and exploitations. Three case studies are carried out, such as active and reactive power tracking, short-circuit fault at power grid, and offshore wind farm connection. Simulation results verify the effectiveness and advantages of ITLO over that of existing meta-heuristic optimisation algorithms.

Grid Integration of offshore wind farms using a Hybrid HVDC composed by an MMC with an LCC-based transmission system

This paper presents a hybrid HVDC-transmission system composed by a Full-Bridge Modular Multilevel Converter (FB-MMC) and a Line-commutated Converter (LCC) from the grid integration of offshore wind farms. The operational characteristics of a three-terminal hybrid-HVDC system, e.i. two LCC stations plus one MMC station, are investigated using PSCAD/EMTDC. This paper mainly focuses on the performance of the system under ac faults at the LCC inverter. This condition is critical for the entire system because LCC is prone to commutation failure, which can be translated into dc faults on the hybrid system. Numerical results show that FB-MMC can help to alleviate ac faults conditions in the LCC, and the system is able to restart after clearance of the fault.

Fuzzy Logic Based Adaptive Droop Control in Multiterminal HVDC for Wind Power Integration

Following a converter outage in a Multiterminal dc (MTDC) for offshore wind farm integration, the conventional droop control shares the power mismatch to minimize the impact on the wind power generation with a fixed droop coefficient; however, it may easily lead to the converter overloading and large dc voltage deviation. The paper proposes a fuzzy logic based adaptive droop controller, which utilizes the available power capacity of the converters to update the droop coefficient. It can make a compromise between the dc voltage deviation and the power sharing. Furthermore, the impact of the droop coefficient change on the stability of the MTDC grid is analyzed, and the effect of the dc-bus capacitor on the system dynamic is identified. The stability region is determined through the eigenvalue analysis. Time-domain simulations are conducted to compare the control strategy proposed with the conventional one. It has been proved that the adaptive control scheme can constantly track dynamic behaviors of the converters, and exhibit desirable response under different outage scenarios.

Coordinated Control Strategies for Offshore Wind Farm Integration via VSC-HVDC for System Frequency Support

Coordinated control strategies to provide system inertia support for main grid from offshore wind farm that is integrated through HVdc transmission is the subject matter of this paper. The strategy that seeks to provide inertia support to the main grid through simultaneous utilization of HVdc capacitors energy, and wind turbines (WTs) inertia without installing the remote communication of two HVdc terminals is introduced in details. Consequently, a novel strategy is proposed to improve system inertia through sequentially exerting dc capacitors energy and then WTs inertia via a cascading control scheme. Both strategies can effectively provide inertia support while the second one minimizes the control impacts on harvesting wind energy with the aid of communication between onshore and offshore ac grids. Case studies of a wind farm connecting with a HVdc system considering sudden load variations have been successfully conducted to compare and demonstrate the effectiveness of the control strategies in DIgSILENT/PowerFactory.

Research and application on multi‐terminal and DC grids based on VSC‐HVDC technology in China

Voltage source converter (VSC)-based high-voltage direct current (HVDC) and multi-terminal (MT)/DC grid technologies are the new HVDC transmission technologies after ultra-high voltage alternative current (UHVAC) and UHVDC transmission technologies which have been widely used in China. The application of the new technologies has resulted in a rapid increase in the number of schemes in construction and planning worldwide. This has been stimulated by the greater level of functionality available from the VSC technology, which makes it suitable for a wide variety of applications. These include the integration of off-shore wind farms, embedded links within AC networks and interconnectors, especially where the AC networks are relatively ‘weak’. VSC technology has renewed interest in MT DC systems, which may ultimately lead to wide area DC grids. This study outlines the research and application on MT and DC grids in China with respect to VSC-HVDC key technologies and DC grid key technologies based on the presentation given in the International Workshop on Next Generation Power Equipment held on 23 September 2016 in Xian, China. The briefing details of the VSC-HVDC projects constructed and to be constructed in China are summarised in this study.

HVDC Converter Stations to Enable Offshore Wind Farm Integration with Power System

HVDC Converter Stations to Enable Offshore Wind Farm Integration with Power System

Impact of DC Breaker Systems on Multiterminal VSC-HVDC Stability

The use of VSC-HVDC grids for offshore wind farm integration will require the use of dc breaker systems and presently they require dc reactors to limit the rate of rise of fault current. The introduction of large dc reactors throughout a VSC-HVDC system can have a significant impact on its stable operation and will require additional control. This paper analyzes this problem and proposes a PSS-like control (DCPSS) to aid dc grid stability and cope with this effect. A generalized analytical model for studies on dc voltage control is presented. Key stability and transient performance issues caused by the use of the dc reactors in a multiterminal system are investigated by analyzing poles, zeros, and frequency responses of open-loop and closed-loop models. Design and location identification methods for the DCPSS are provided. An excellent damping enhancement is achieved by this controller. The analytical studies and time-domain simulations in this paper are performed based on two VSC-HVDC models.

A Novel Topology Design for Integration of Offshore Wind Farm via High-voltage DC Transmission

A novel configuration for offshore wind farms based on variable-speed permanent magnet synchronous generator integration via high-voltage DC transmission is proposed. Each permanent magnet synchronous generator is equipped with a full-bridge diode rectifier and a DC/DC boost chopper, which constitutes a cascaded submodule. Quite a number of submodules are series-parallel connected to form a DC collector system for the offshore wind farm. Any unit that needs maintenance or that undergoes fault in a series-connected branch can be isolated from the system by the natural commutated process without influencing other normal units in the same branch. The proposed submodule has several prominent features, such as low cost, low power loss, and simple control strategy. A maximum power point tracking algorithm of each wind turbine is realized by controlling the current through the inductor of each DC/DC boost circuit in one submodule. The proposed topology and control strategy are verified by simulations in Power Systems Computer Aided Design/Electromagnetic Transients including Direct Current (PSCAD/EMTDC; Manitoba HVDC Research Centre, Manitoba, Canada).

Study of Hard and Soft Countermeasures for Scour Protection of the Jacket-Type Offshore Wind Turbine Foundation

Physical model tests with the scale of 1:36 are carried out in the Near-Shore Wave Basin (NSWB) at Tainan Hydraulics Laboratory (THL) with the jacket-type offshore wind turbine foundation (jacket-type foundation) and the combination of hard or soft scour protection in the test area. Scouring around the jacket-type foundation exposed to wave and current was conducted in the NSWB with a mobile bed experiment. Two locations (a water depth of 12 m and 16 m) of the foundations are separately simulated in this study. Based on the analysis from the former NSWB experimental results, one traditional hard scour protection usually used in Taiwan with four layers around the foundation is proposed for the mitigation of scouring. From the experimental results, a four-layer scour protection is tested and found to be effective in preventing scouring around the jacket-type foundation. Besides the hard scour protection countermeasure, the scour protection effect of an integrated offshore wind turbine and cage net aquaculture facility as a soft countermeasure for scour protection of the jacket-type foundation is further evaluated in this study. Meanwhile, a detailed analysis for stakeholders’ opinions on the integration of offshore wind farms and coastal aquaculture is also considered to obtain important experience and knowledge; and furthermore, to understand the real demand for adjusting the feasibility of this soft countermeasure.

Offshore Wind Farm Integration and Frequency Support Control Utilizing Hybrid Multiterminal HVDC Transmission

This paper proposed a hybrid multiterminal-high-voltage-direct-current system to integrate several distributed offshore wind farms to onshore power grids. A comprehensive frequency support algorithm for the onshore grid was designed based on this system. The configuration and operating principles of a typical four-terminal system are described. System control strategies are proposed for both wind-farm-side voltage-source converters (WFVSCs) and grid-side line-commutated converters (GSLCCs). To obtain smooth operation, the WFVSCs are controlled as a voltage source that automatically absorbs power generated by the wind farm and establish a stable ac voltage for wind turbines. DC voltage-current droop control is designed for GSLCCs to maintain the dc voltage and distribute power between two onshore grids. A utility black start-up process without auxiliary power supply was designed and tested. The simulation results in Power System Computer Aided Design/ Electromagnetic Transients including DC are presented to demonstrate the system performance during black starting-up, converter disconnection, and frequency variation of one onshore power grid.

Key technologies of VSC-HVDC and its application on offshore wind farm in China

In recent years, wind power industry has been flourishing and in China the focus has been gradually shifted from land-based to offshore wind farms. There are many advantages for offshore wind farms, such as abundant wind energy reserves, high utilization of the wind turbine capacity, not taking up land resources and so on. As wind turbine technology improves, offshore wind farms have been expanding quickly and are located further away from the onshore grid. The power transmission problem has become one of the key issues for restricting the development of offshore wind farms. For example, the technology of high voltage direct current transmission based on voltage source converter (VSC-HVDC), suitable for long-distance transmission of offshore wind energy, has become one of the current research focuses. This has resulted in higher requirements on the aspects of converter voltage level, system dynamic performance, and network power quality. The paper first discusses converter topology for offshore wind farm grid integration. Two VSC-HVDC projects for connecting offshore wind farms, which are located in Shanghai and Dalian of China, are presented in detail. Based on the two projects, the structure, control methods and application of modular multi-level converter are presented. The control strategies of VSC-HVDC are then discussed, focusing on double closed-loop vector control, direct power control, deadbeat control and the control methods for unbalanced grid voltage. This can provide good theoretical foundation for the grid integration of large offshore wind farms.

Experimental Verification of a Voltage Droop Control for Grid Integration of Offshore Wind Farms Using Multi-terminal HVDC

This paper presents an experimental verification of a voltage droop control for a multi-terminal HVDC system for the grid integration of offshore wind farms. The laboratory setup is composed by four voltage-source converter terminals which aim to emulate behavior of the future power grid in the North Sea, where Norway, Germany and the UK are interconnected together with an offshore wind farm. Two main scenarios have been performed to test the robustness of the droop-control strategy: variation in wind power, including changes in the parameters of the droop line, and the sudden disconnection of converter terminal during full wind production. In all performed cases, the implemented system was able to ensure that the voltage stays within its steady state limits and to reach a stable operation point.