Alejandro Garces

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.

An Adaptive Control Strategy for a Wind Energy Conversion System Based on PWM-CSC and PMSG

This paper proposes a new adaptive control strategy for a wind energy conversion system based on a permanent magnet synchronous generator and a pulse-width modulated current source converter. Most of the studies on wind farms are based on double fed induction technology. Nevertheless, the proposed conversion system is a good alternative due to its high efficiency and reliability. Electrolytic capacitors are not required in this type of converter and the voltage in the DC-link as well as the generated reactive power can be dynamically modified according to the wind velocity, being even negative if required. However, it is challenging from the control and stability standpoint. Capacitive filters placed on the AC side, which are required for safe commutation, can create resonances with the power grid. Reactive power is generated according to the capacity of the converter, the wind velocity and the load profile. The adaptive control strategy uses an adaptive PI which is self-tuned based on a linear approximation of the power system calculated at each sample time. A model reference is also proposed in order to reduce the post-fault voltages. Simulation results demonstrate the advantages of the proposed control.

A Linear Three-Phase Load Flow for Power Distribution Systems

This letter proposes a linear load flow for three-phase power distribution systems. Balanced and unbalanced operation are considered as well as the ZIP models of the loads. The methodology does not require any assumption related to the R/X ratio. Despite its simplicity, it is very accurate compared to the conventional back-forward sweep algorithm.

Identification of a Proton-Exchange Membrane Fuel Cell’s Model Parameters by Means of an Evolution Strategy

This paper presents the parameter identification of an equivalent circuit-based proton-exchange membrane fuel cell model. This model is represented by two electrical circuits, of which one reproduces the fuel cells output voltage characteristic and the other its thermal characteristic. The output voltage model includes activation, concentration, and ohmic losses, which describe the static properties, while the double-layer charging effect, which delays in fuel and oxygen supplies, and other effects provide the models dynamic properties. In addition, a novel thermal model of the studied Ballards 1.2-kW Nexa fuel cell is proposed. The latter includes the thermal effects of the stacks fan, which significantly improve the models accuracy. The parameters of both, the electrical and the thermal, equivalent circuits were estimated on the basis of experimental data using an evolution strategy. The resulting parameters were validated by the measurement data obtained from the Nexa module. The comparison indicates a good agreement between the simulation and the experiment. In addition to simulations, the identified model is also suitable for usage in real-time fuel cell emulators. The emulator presented in this paper additionally proves the accuracy of the obtained model and the effectiveness of using an evolution strategy for identification of the fuel cells parameters.

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.

Environmental Transmission Expansion Planning using non-linear programming and evolutionary techniques

Current energy consumption has led to an increase in the use of fossil fuels to generate electricity, which in turn produces negative impacts on the environment. Studies show that demand will continue growing and new schemes for generating power and managing resources should be developed. In the case of the transmission network, some constraints may lead to use plants with high emission levels, and therefore, appropriate planning is key to minimize environmental impact. This work proposes a methodology for solving the Transmission Expansion Planning Problem (TEPP) when emissions of CO2 are considered. The result is an investment plan that leads to the lowest level of emissions by means of a Chu-Beasley Genetic Algorithm (CBGA). An improvement step is carried in the CBGA in order to minimize also the cost of the plan. Non-linear Interior Point Method (NLIPM) is used to generate the initial population and Linear (LIPM) is used to solve the operative problem in the evolutionary process. The approach is validated using the IEEE-24 bus system in order to show its effectiveness.