Marta Molinas

A Study of Efficiency in a Reduced Matrix Converter for Offshore Wind Farms

Reduced matrix converter (RMC) is a convenient topology for offshore wind farm due to its potential to reduce the size and weight of the converter, to improve the reliability by removing the electrolytic capacitor, and to increase the efficiency inherent to less stages of conversion. Moreover, it is a very flexible topology which permits different types of operation with a simpler modulation compared with conventional three-phase matrix converter. This paper investigates different modulation strategies applied to RMC for offshore wind farms, focused on efficiency improvement of the entire convention system. Simulation results using a detailed loss model for high-power level are presented. Four cases are investigated according to the modulation strategies (space vector modulation and carrier-based modulation) and the operation principle (current source converter or voltage source converter). Losses in the clamp circuit are also calculated. Different wind velocities are considered in the simulations. Results show that current source operation with space vector modulation presents minimum losses at nominal wind velocity. This operation is suitable for series connection of offshore wind farms which has been reported as the most efficient alternative from the grid losses point of view.

A Generalized Power Control Approach in ABC Frame for Modular Multilevel Converter HVDC Links Based on Mathematical Optimization

This paper presents a generalized and versatile control approach using Lagrange multipliers in the ABC frame for a modular multilevel converter-based HVDC system. The methodology is capable of analytically obtaining desired operative conditions by calculating the differential current references previously established by the constraints in the optimization formulation, while obtaining the result with minimum: 1) differential current oscillations (Δi_diffk) or 2) capacitive phase-energy oscillations (Δω_Σk). Furthermore, the energy distribution inside the MMC (i.e., the capacitive phase average energy sum (ω̅_Σk) and difference (ω̅_Δk)) is being regulated by means of the constraint definitions. The optimization yields a differential current reference in “abc” coordinates with a similar structure to instantaneous power theories: as the addition of the product between varying conductances and the MMC internal dynamics input voltages (i.e., the dc bus voltage (v_dc) and the MMC load electromotice force (emf) (e_vk) on the one hand; and a contribution proportional to the ac load power (e_vki_vk) on the other. Both the objective function minimization and the energy constraints are achieved with one single current reference resulting from the optimization process, without the application of linear superposition techniques.

Integration of Offshore Wind Farm Using a Hybrid HVDC Transmission Composed by the PWM Current-Source Converter and Line-Commutated Converter

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.

High frequency wind energy conversion from the ocean

This paper presents a concept of high frequency link for offshore wind applications based on a reduced matrix converter oriented to reduce the size and volume of the elements in the nacelle. A modular approach aimed at increasing reliability of the conversion system, which can be used in series or parallel connection of the off shore grid is also presented. Features of the proposed conversion concept related to ride through capability, protection strategies and parameters of the high frequency transformer are investigated. A modified space vector modulation is presented which reduces the losses in the conversion system.

Reduced matrix converter operated as current source for off-shore wind farms

Converter topologies for offshore wind farms must be designed taking into account reliability and size of the converter. Reduced matrix converter fulfill this two requirements and permits to built a series connected grid. The modulation of this converter can be done by space vector or carrier based modulation. Both types of modulations must be modified to accomplish the requirements of the three phase to single phase conversion. This paper analyzes possible modified modulations from the losses point of view. The converter is operated as current source converter. Simulations were carried out using PSIM and a dynamic link library programed in c++ for the losses calculation.

Impact of operation principle on the losses of a reduced matrix converter for offshore wind parks

Reduced matrix converter (RMC) could be used in series connection of offshore wind farms as part of the conversion system in each turbine. The converter can be operated as voltage source or current source converter. This paper investigates the influence that voltage source and current source operation principle will have on the losses of the RMC. The modulation of the converter is done by space vector modulation for current source operation. Losses are analyzed by means of simulation in PSIM and a dynamic link library programmed in c++ that calculates the losses based on an experimental behavioral model. The effect of the high frequency transformer is analized. Results indicates that voltage source operation improves the total efficiency compared with current source converter operation.

Operation features of a reduced matrix converter for offshore wind power

The reduced matrix converter (RMC) implemented in offshore wind power conversion system is investigated in this paper. The RMC is built with 6 bi-directional switches, and the losses produced by these switches are compared under different conditions. Each bi-directional switch is made up by two RB-IGBTs in antiparallel, and it is studied how the special structure of the RB-IGBT will affect the total losses of the switch. The entire offshore wind power conversion system is modeled in PSIM. The first conversion stage after the wind turbine is direct ac-ac conversion by the reduced matrix converter. Thereafter a high frequency square wave voltage is fed into a transformer. Due to the high frequency, the transformer will have reduced weight, which is an important aspect of the lightweight and compact component requirements for offshore wind turbine installations. After the transformer the second conversion stage is carried out. This is done by a full-bridge converter where the output is dc voltage, for enabling series connection of wind turbines in dc. The study is aimed at the achievement of an energy efficient converter, with high power density and reliability.

A generalized power control approach in ABC frame for modular multilevel converters based on Lagrange multipliers

This work presents a generalized and versatile control approach for Modular Multilevel Converters using Lagrange Multipliers in the ABC frame. The methodology is capable of analytically obtaining desired operative conditions by calculating the differential current references previously established by the constraints in the optimization formulation, whilst obtaining the result with minimum I) differential current oscillations (Δi_diffk), or II) capacitive phase energy oscillations (Δw_Σk). Furthermore, the energy distribution inside the MMC; i.e., the capacitive phase average energy sum (w_Σk) and difference (w_Δk), is being regulated by means of the constraint definitions.

Optimal control for an HVDC system with series connected offshore wind turbines

This paper studies an HVDC system based on pulse-width-modulated current source converters (PWM-CSC) which is suitable for series connection of offshore wind turbines. A high-frequency link based on reduced matrix converter is proposed as energy conversion system in each wind turbine. An optimal linear quadratic regulator is designed in order to reduce the complexity of the inner-loop and to improve its dynamic performance. Operation features of this control are tested in stationary state and transient conditions.

Integration of Offshore Wind Farm Using a Hybrid HVDC Transmission Composed by PWM Current-Source Converter and Line-Commutated Converter

This article investigates the feasibility of using a hybrid HVDC transmission system for the integration of offshore wind farms (OWF). The proposed hybrid HVDC consists of a pulse width modulation current- source converter (PWM-CSC)and a line-commutated converter (LCC). The PWM-CSC is placed at offshore, while LCC is located at onshore. The PWMCSC may offer similar advantages that a voltage source converter (VSC) for the integration of OWF, e.g the PWM-CSC can supply an island or passive grids without a external commutation voltage and the stations have a relative small footprint, among others benefits. Moreover, both the PWM-CSC and the LCC are current-sourced converter which facilitate the coupling. To date, there are not studies that report the behavior of this type of hybrid topology, so this work can provide a basis for further research. The control design for the entire system is presented and verified in this paper. Simulations are performed using PSCAD/EMTDC under various conditions including AC and DC faults.