Mitigation of background harmonic amplification caused by offshore wind farm long AC cable integration

The diode rectifier unit (DRU) can improve the reliability and economy of offshore wind farm integration. Since the DRU is a passive unit and its power transmission is unidirectional, neither the DRU nor the onshore grid can start the offshore wind farm. In this circumstance, the grid-forming wind turbine (WT) becomes the black start source for the DRU based offshore wind farm. This paper proposes the black start strategy of the grid-forming offshore wind farm. First, the topology of the DRU based offshore wind farm integration system is introduced, and the control strategy of the grid-forming WT is elaborated. On this basis, the black start strategy of the grid-forming offshore wind farm is proposed. According to the proposed strategy, the black start process of the DRU based offshore wind farm is simulated in PACAD/EMTDC. The simulation results verify the effectiveness of the proposed black start strategy

Summary A reliable protection system is needed for transmitting large amount of power over long distances. An alternating current–direct current mix network in offshore wind farm (OWF) interconnection with onshore grid offers a major challenge for faulty section identification. The transient faults in OWF system are simulated in MATLAB 7.10.0(R2010a). The fault signals, voltage and current, are retrieved at common collector platform and at voltage-source converter–high-voltage direct current sides of network in the OWF system. The detection capability of signal-processing techniques, Stockwell transform, empirical mode decomposition, and Hilbert transform, is explored for localization of different types of faults. In addition, fault events at onshore grid and those developed at the time of change in operating parameters are also simulated and analyzed. Further, the quantitative analysis for detection of fault events with the computation of maximum and mean indices and instantaneous time indices of the fault signal is presented in the study. Coordination of overcurrent relays is another challenging task in OWF interconnection with onshore grid system. The objective function of relay coordination problem is to minimize the operation time of associated relays. Time multiplier setting is used as the test variable in the objective function. An optimal coordination of overcurrent relays by using genetic algorithm, particle swarm optimization, and gravitational search algorithm is discussed in this paper. Further, these techniques are compared with hybrid meta-heuristic optimization method, a combination of particle swarm optimization and gravitational search algorithm to achieve relay coordination more efficiently. Copyright © 2015 John Wiley & Sons, Ltd.

This paper investigates the feasibility of the application of a hybrid HVDC transmission system for the grid integration of offshore wind farms. The proposed hybrid HVDC consists of a pulse width modulated current source converter (PWM-CSC) and a line-commutated converter (LCC). The PWM-CSC is connected to the offshore wind farm and the LCC connects the onshore grid. The hybrid topology takes advantages from self-commutated converters as well as LCCs. On the one hand, LCC-based HVdc is the most mature technology with the lowest power losses and lowest cost. On the other hand, PWM-CSC has the same features that a voltage source converter for offshore applications, i.e., the ability to operate without an external commutation voltage, reactive power control capability, and a relative small footprint. Moreover, both the PWM-CSC and the LCC are current source converters and hence the coupling can be effortlessly done. The control design for the entire system is presented and verified using numerical simulations. Simulations are performed using PSCAD/EMTDC under different conditions including changes in the wind speed and ac and dc faults.

Long-distance offshore wind power transmission systems utilize multi-terminal high voltage direct current (MT-HVDC) connections based on voltage source converters (VSCs). In addition to having the potential to work around restrictions, the VSC-based MT-HVDC transmission system has significant technical and economic merits over the HVAC transmission system. Offshore wind farms (OWFs) will inevitably grow because of their outstanding resistance to climate change and ability to provide sustainable energy without producing hazardous waste. Due to stronger and more persistent sea winds, the OWF often has a higher generation capacity with less negative climate effects. The majority of modern installations are distant from the shore and produce more power than the early OWF sites, which are situated close to the shore. This paradigm shift has compelled industry and professional researchers to examine transmission choices more closely, specifically HVAC and HVDC transmission. This article conducts a thorough analysis of grid connection technologies for massive OWF integration. In comparison to earlier assessments, a more detailed discussion of HVDC and HVAC topologies, including HVDC based on VSCs and line-commutated converters (LCCs), and all DC transmission systems, is offered. Finally, a selection criterion for HVDC transmission is advised, and its use is argued to be growing.

The Voltage Source Converter (VSC)-based MultiTerminal High Voltage DC (MT-HVDC) system is now considered the most effective and reliable method for integrating Offshore Wind Farms (OWF) with onshore grids. This paper focuses on the methods by which wind-generated power can be shared among individual VSCs during contingencies without violating system-wide voltage constraints. A four terminal VSCHVDC system is modeled in PSCAD/EMTDC that connects OWFs with onshore grids via VSCs which share the available power based on the voltage droop. The power sharing and AC/DC voltage control responsibility is distributed among the converters. In case of a converter loss, sharing the desired power while maintaining AC/DC voltage stability is a challenge that this paper aims to address. A droop control strategy is presented to minimize voltage fluctuations in the absence of the converter station controlling the DC voltage. Simulation results are presented for various operating case scenarios.

This paper presents the stability simulation results of an integration of an offshore wind farm (OWF) based on doubly-fed induction generator (DFIG) and an OWF based on permanent-magnet synchronous generator (PMSG) connected to a large power grid through a high-voltage direct-current (HVDC) link based on voltage-source converter (VSC). These two OWFs are connected to the same AC bus through individual step-up transformers and transmission lines. A damping controller located at the converter station of the proposed VSC-HVDC link is designed by using pole-assignment approach to contribute adequate damping to the studied two OWFs under various operating conditions. A systematic approach using a frequency-domain method and a time-domain scheme is performed to examine the effectiveness of the proposed control scheme. It can be concluded from the simulation results that the proposed VSC-HVDC link joined with the designed damping controller located at the converter station can not only render appropriate damping to the modes of the studied OWFs under various operating conditions but also effectively mitigate inherent fluctuations of the power injected to the power grid.

This paper presents an analysis and new fault mitigation methods of grid integration of offshore wind farms and oil & gas installations using Voltage Source Converter (VSC) HVDC transmission system. On the fault occurrence in the onshore AC grid, the proposed DC link voltage controller initiates fast output power drop of the wind farm, in order to avoid the overvoltage in the DC transmission link. On the faults occurrence in the oil & gas installation, the proposed control employs a voltage-dependent limiter, to limit the maximum amount of active power from the offshore VSC. Simulation results in PSCAD / EMTDC show the satisfied performance of the proposed control strategy for the case of one 300 MW VSC transmission system connecting an offshore wind farm and oil & gas installations to the grid.

Offshore windfarms is an emerging technology in the wind energy conversion system. For onshore wind farms high voltage overhead lines are used for transmitting power from the wind farm to the grid. However this option is not possible in the case of an offshore windfarm as large part of the distance to connection point must be covered by submarine cables. The application of high voltage dc (HVDC) transmission for integrating large scale and/or off-shore wind generation systems with the electric grid is attractive in comparison to extra high voltage (EHV) ac transmission due to a variety of reasons like efficiency, economics etc. In the case of HVDC transmission a converter is connected between the wind farm and the utility grid. Even though CSC's and VSC's are widely used for the realization of large HVDC systems, as an alternative approach Bridge of Bridge converter (BoBC) is introduced recently. In this work the performance of the VSC and BoBC are compared and analyzed. The simulation is done using PSCAD/EMTDC software. Compared with the onshore wind farms, the offshore wind farms have access to significantly better wind energy resources and hence offer larger energy generating capability. Therefore offshore wind farms are gaining importance and the BoBC proves to be better than the VSC for similar applications.

High Voltage Alternating Current (HVAC) submarine cables are used to connect large offshore facilities to the onshore power grid leading to the removal of old, inefficient and polluting local power generators. Several issues have to be identified and addressed in the HVAC system design. These issues include reactive power compensation, switching equipment limits, and insulation coordination. The paper present example of 100km, 115kV HVAC submarine cable that used to transfer 90MW from onshore power grid to offshore Gas Oil separation Plant (GOSP) and the decommission of old Combustible Gas Turbine Generators (CGTGs).

Offshore wind power (OWP) is an important green technology owing to its stability and sites for OWP projects (wind farms) are easily obtainable. The authors reviewed 2,217 papers collected from Scopus and used main path analysis (a citation analysis method) to determine the trends in OWP studies. Based on their analysis, they categorized some OWP concepts, including wind farm model, wind farm monitoring, wind speed measurement and analysis, cost and economic analysis, and floating OWP. Furthermore, they found that OWP can be combined with a wave energy unit to generate stable electric power. It is important to monitor the wind turbines of large OWP plants, and OWP plants should be located far from the shore. In this case, floating OWP is more economical than fixed OWP. In addition, high-voltage direct current (HVDC) is more feasible then high-voltage alternating current (HVAC) in terms of cost and stability. However, the magnetic effect of OWP on fishes, plants, and fishermen is uncertain. The European Union plans to install OWP capacity of 40 GW by 2020 and 150 GW by 2030. The results of this study can guide policy making and investment decisions.

Layout of offshore wind farm (OSWF) plays a major role in capital cost of its installation. The cost of OSWF depends on the cost of electrical collector system which includes, submarine cables, number of wind turbines, offshore platforms etc. The optimal design of OSWF will give better economical outcome. This paper explains the methodology for optimal design of OSWF electrical collector system and cost model of collector system. The methodology is based on the combination of Ant Colony optimization and Travelling Salesman problem. The objective is to minimize the length of submarine cable connected between wind turbines in wind farm. It will reduce the cost of cable and cable power loss. With the help of North Hoyle OSWF connected with 30 wind turbines, the optimization approach designs optimal OSWF based on minimum length of cable for radial and ring topologies.

In order to improve the reliability of grid-connected offshore wind turbines (GCOWT), a grid adaptability testing of it should be performed. After analyzing the difference between onshore and offshore wind turbine testing and modeling the system, detailed spectral analysis based on AC cables is made to derive how cable parameters affect the resonances. A modification method based on resonance frequency shifting (RFS) by changing offshore compensation device parameters is proposed. Finally, simulation results demonstrate that the harmonics distortion at high voltage side of GCOWT transformer satisfies the testing standards when applying RFS.

In the offshore wind power transmission system(WPTS) through the AC submarine cable(SC), as the scale of the offshore wind farm(WF) gradually increases, the transmission distance of the AC SC continues to increase. Due to the large charging power of the SC, there are many problems in the actual offshore WF connection project, such as excessive reactive power injection at the point of common connection(PCC) of offshore WFs, over voltage and over current carrying capacity along the SC, and over voltage on the high voltage side of the offshore substation. In response to the above problems, this paper establishes a mathematical model for the WPTS by the 220kV AC SCs, and analyses the WPTS based on the principle of layered division of reactive power compensation. Under the condition of voltage and current limitation along the cable, the offshore high voltage reactor(HVR) value of compensation is determined. Onshore high voltage reactor and SVG are installed on land to make the reactive power exchange between the WPTS and the land AC network zero. When the SVG capacity is minimum, onshore HVR value connected to the system is given. Combined with a specific project in China, the recommended reactive power configuration plan is calculated.

Offshore wind power plants (WPPs) using AC submarine cables connected to the power grid may experience harmonic resonance and harmonic amplification that are not common in onshore wind farms under the action of the distributed capacitance of submarine cables, which seriously threatens the safe and reliable operation of offshore WPPs. To investigate the influencing factors of harmonic amplification in offshore WPPs, a scalable state space-based modeling method for large-scale offshore WPPs were established. Moreover, the root locus method was used to study how the factors including that grid short-circuit capacities, cable parameters and the number of wind turbine generators (WTGs) affect the resonance modes. Finally, an offshore WPP in Jiangsu was built on the MATLAB/Simulink simulation platform, and the analysis results verify the correctness of the theoretical analysis.

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).