A unidirectional hybrid HVDC transmission system based on diode rectifier and full-bridge MMC

To reduce the cost of bulk power transmission using voltage source converter HVDC technology, a unidirectional hybrid converter is proposed, where a diode rectifier and a modular multilevel converter (MMC) based on full-bridge (FB) submodules are connected in series on DC side. The FB-MMC controls its DC voltage to regulate the transmitted power. The majority of the power transmission is via the diode rectifier considering its cost and efficiency superiority and only low power rating FB-MMC is required. A thyristor valve is equipped at the DC side of the FB-MMC to prevent potential overcharge of the FB submodules during DC faults. Compared to conventional MMCs, losses can potentially be reduced by around 20%. An active power controller is proposed to regulate the DC voltage of the FB-MMC so as to control the transmitted power. With the inverter station controlling its DC terminal voltage constant, the FB-MMC increases the output DC voltage to increase the transmitted power and, vice versa. To alleviate overvoltage of the HVDC link during AC grid faults of the inverter station, a dynamic DC voltage limiter is designed to actively reduce the DC output voltage of the FB-MMC. Simulation results confirm the proposed converter operation and control

LCC-HVDC Connection of Offshore Wind Farms With Reduced Filter Banks

Despite being more efficient, line commutated converter-HVDC links for the connection of large offshore wind farms have ac-filter bank size as one of their main drawbacks. This paper shows how the HVDC rectifier filter banks can be substantially reduced by taking advantage of the additional control possibilities offered by the use of wind turbines with fully rated converters. PSCAD simulations validate wind farm and diode rectifier HVDC link operation with a capacitor and filter bank five times smaller than its usual value. The proposed control algorithm allows for good harmonic and reactive power sharing between the different wind turbines. As the reduced capacitor bank operation leads to a redistribution of harmonic and reactive currents, an efficiency study has been carried out to evaluate the new power loss distribution with the reduced filter banks

A critical survey of technologies of large offshore wind farm integration : summary, advances, and perspectives

Offshore wind farms (OWFs) have received widespread attention for their abundant unexploited wind energy potential and convenient locations conditions. They are rapidly developing towards having large capacity and being located further away from shore. It is thus necessary to explore effective power transmission technologies to connect large OWFs to onshore grids. At present, three types of power transmission technologies have been proposed for large OWF integration. They are: high voltage alternating current (HVAC) transmission, high voltage direct current (HVDC) transmission, and low-frequency alternating current (LFAC) or fractional frequency alternating current transmission. This work undertakes a comprehensive review of grid connection technologies for large OWF integration. Compared with previous reviews, a more exhaustive summary is provided to elaborate HVAC, LFAC, and five HVDC topologies, consisting of line-commutated converter HVDC, voltage source converter HVDC, hybrid-HVDC, diode rectifier-based HVDC, and all DC transmission systems. The fault ride-through technologies of the grid connection schemes are also presented in detail to provide research references and guidelines for researchers. In addition, a comprehensive evaluation of the seven grid connection technologies for large OWFs is proposed based on eight specific indicators. Finally, eight conclusions and six perspectives are outlined for future research in integrating large OWFs

Coordinated control of parallel DR-HVDC and MMC-HVDC systems for offshore wind energy transmission

Parallel operation of diode rectifier based high-voltage direct current (DR-HVDC) and modular multilevel converter (MMC) based HVDC (MMC-HVDC) for transmitting offshore wind energy is investigated in this paper. An enhanced active power control scheme of the offshore MMC station is proposed to improve the power flow distribution between the MMC-HVDC and DR-HVDC links which are both connected to the offshore wind farm AC network. By regulating the offshore voltage, all the wind powers are transmitted via the DR-HVDC link in low wind conditions while the offshore MMC power is controlled around zero to reduce transmission losses, considering the efficiency superiority of DR-HVDC over its MMC counterpart. When the DR-HVDC is out of service, wind energy is transferred via the MMC-HVDC and the wind turbine generated power is automatically limited by slightly increasing the offshore AC voltage to avoid potential MMC-HVDC overload. A power curtailment control is also proposed which slightly increases the DC voltage of the DR-HVDC to enable autonomous reduction of the generated wind power so as to avoid DR-HVDC overload during MMC-HVDC outage. The proposed coordinated control only uses local measurements and, without the need for communication, can seamlessly handle transitions including various faults. The proposed scheme enables fault ride-through operation and provides a high efficient solution with flexible operation for integrating large offshore wind farms. Simulation results confirm the proposed control strategy

Offshore AC fault protection of diode rectifier unit based HVDC system for wind energy transmission

Offshore AC fault protection of wind turbines (WTs) connecting with diode rectifier unit based HVDC (DRU-HVDC) system is investigated in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during offshore AC fault transients and quickly provide fault current for fault detection. Considering different fault locations, the fault characteristics during symmetrical and asymmetrical faults are presented and the requirements for fault detection are addressed. A simple and effective offshore AC fault protection solution, combining both overcurrent protection and differential protection, is proposed by utilizing the developed fast fault current providing control. To improve system availability, reduced DC voltage of the DRU-HVDC system is investigated, where one of the series-connected DRUs is disconnected and the onshore modular multilevel converter (MMC) actively reduces DC voltage to resume wind power transmission. The proposed scheme is robust to various offshore AC faults and can automatically restore normal operation. Simulation results confirm the proposed fault protection strategy

Overvoltage Limitation Method of an Offshore Wind Farm with DC Series Parallel Collection Grid

This paper describes the characteristics of a series parallel wind farm (SPWF) topology and investigates the control strategy to ensure its safe operation. The SPWF was found to have advantages over other pure dc wind farm architectures in terms of lower construction cost and lower power losses in the collection system. However, unbalance power productions among the wind turbines cause the variations of their output voltages, which may endanger the safe operation of the entire wind farm. This paper proposes a global control strategy that prevents wind turbines from operating above their overvoltage capabilities.With an active participation of the onshore converter, the proposed strategy allows maximum power point tracking (MPPT) of the wind turbines. The practical limitations of this strategy are discussed and improvements are given. The feasibility of the proposed method is validated in a simulation of 300 MW wind farm developed in EMTP-RV

Developing a new SVPWM control strategy for open-winding brushless doubly fed reluctance generators

In this paper, a new open-winding control strategy is proposed for a brushless doubly fed reluctance generator (BDFRG) used for stand-alone wind turbine or ship generators. The BDFRG is characterized with two windings on the stator: a power winding and a control winding. The control winding is fed with dual two-level three-phase converters, and a vector control scheme based on space vector pulsewidth modulation is designed. Compared with traditional three-level inverter systems, the dc-link voltage and the voltage rating of power devices in the proposed system are reduced by 50% while still greatly improving the reliability, redundancy, and fault tolerance of the proposed system by increasing the switching modes. Its performance is evaluated by simulation in MATLAB/Simulink and an experimental study on a 42-kW prototype machine

A review on power electronics technologies for power quality improvement

Nowadays, new challenges arise relating to the compensation of power quality problems, where the introduction of innovative solutions based on power electronics is of paramount importance. The evolution from conventional electrical power grids to smart grids requires the use of a large number of power electronics converters, indispensable for the integration of key technologies, such as renewable energies, electric mobility and energy storage systems, which adds importance to power quality issues. Addressing these topics, this paper presents an extensive review on power electronics technologies applied to power quality improvement, highlighting, and explaining the main phenomena associated with the occurrence of power quality problems in smart grids, their cause and effects for different activity sectors, and the main power electronics topologies for each technological solution. More specifically, the paper presents a review and classification of the main power quality problems and the respective context with the standards, a review of power quality problems related to the power production from renewables, the contextualization with solid-state transformers, electric mobility and electrical railway systems, a review of power electronics solutions to compensate the main power quality problems, as well as power electronics solutions to guarantee high levels of power quality. Relevant experimental results and exemplificative developed power electronics prototypes are also presented throughout the paper.This work has been supported by FCT-Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020. This work has been supported by the FCT Project DAIPESEV PTDC/EEI-EEE/30382/2017 and by the FCT Project newERA4GRIDs PTDC/EEIEEE/30283/2017

Detecting and Mitigating Wind Turbine Clutter for Airspace Radar Systems

It is well recognized that a wind turbine has a large radar cross-section (RCS) and, due to the movement of the blades, the wind turbine will generate a Doppler frequency shift. This scattering behavior may cause severe interferences on existing radar systems including static ground-based radars and spaceborne or airborne radars. To resolve this problem, efficient techniques or algorithms should be developed to mitigate the effects of wind farms on radars. Herein, one transponder-based mitigation technique is presented. The transponder is not a new concept, which has been proposed for calibrating high-resolution imaging radars. It modulates the radar signal in a manner that the retransmitted signals can be separated from the scene echoes. As wind farms often occupy only a small area, mitigation processing in the whole radar operation will be redundant and cost inefficient. Hence, this paper uses a transponder to determine whether the radar is impacted by the wind farms. If so, the effects of wind farms are then mitigated with subsequent Kalman filtering or plot target extraction algorithms. Taking airborne synthetic aperture radar (SAR) and pulse Doppler radar as the examples, this paper provides the corresponding system configuration and processing algorithms. The effectiveness of the mitigation technique is validated by numerical simulation results