S. Ceballos

A Comprehensive Study of a Hybrid Modulation Technique for the Neutral-Point-Clamped Converter

This paper presents a hybrid modulation technique for the three-level neutral-point-clamped converter. A modulation strategy, based on two modulation signals per phase, was presented previously. This strategy completely removes the low-frequency voltage oscillations that appear at the neutral point (NP) in some operation conditions. However, it also has a major drawback: it significantly increases the switching losses of the converter. The proposal in this paper combines such a modulation strategy with sinusoidal pulsewidth modulation (SPWM). The main characteristic of this hybrid modulation is the reduction in switching losses at the cost of some low-frequency voltage oscillations at the NP. The amplitude of these oscillations can be controlled by varying the combination of the two strategies. The performance of the hybrid modulation is analyzed and compared with the original strategies. Power losses and oscillation amplitudes on the dc-link capacitors are evaluated. Experimental results show how the hybrid modulation performs by balancing the dc-link capacitors.

New Protection Circuit for High-Speed Switching and Start-Up of a Practical Matrix Converter

The matrix converter (MC) presents a promising topology that needs to overcome certain barriers (protection systems, durability, the development of converters for real applications, etc.) in order to gain a foothold in the market. Taking into consideration that the great majority of efforts are being oriented toward control algorithms and modulation, this paper focuses on MC hardware. In order to improve the switching speed of the MC and thus obtain signals with less harmonic distortion, several different insulated-gate bipolar transistor (IGBT) excitation circuits are being studied. Here, the appropriate topology is selected for the MC, and a recommended configuration is selected, which reduces the excursion range of the drivers, optimizes the switching speed of the IGBTs, and presents high immunity to common-mode voltages in the drivers. Inadequate driver control can lead to the destruction of the MC due to its low ride-through capability. Moreover, this converter is especially sensitive during start-up, as, at that moment, there are high overcurrents and overvoltages. With the aim of finding a solution for starting up the MC, a circuit is presented (separate from the control software), which ensures correct sequencing of supplies, thus avoiding a short circuit between input phases. Moreover, it detects overcurrent, connection/disconnection, and converter supply faults. Faults cause the circuit to protect the MC by switching off all the IGBT drivers without latency. All this operability is guaranteed even when the supply falls below the threshold specified by the manufacturers for the correct operation of the circuits. All these features are demonstrated with experimental results. Lastly, an analysis is made of the interaction that takes place during the start-up of the MC between the input filter, clamp circuit, and the converter. A variation of the clamp circuit and start-up strategy is presented, which minimizes the overcurrents that circulate through the converter. For all these reasons, it can be said that the techniques described in this paper substantially improve the MC start-up cycle, representing a step forward toward the development of reliable MCs for real applications.

Efficiency Optimization in Low Inertia Wells Turbine-Oscillating Water Column Devices

The Wells turbine is a bidirectional air turbine which operates efficiently over a restricted range of air flow. The optimization of its efficiency requires control of rotational velocity in order to maintain the ratio between airflow and tip speed within the high efficiency range. This paper introduces two generator control strategies that optimize the power take-off efficiency for low inertia turbine systems in which instantaneous control of the turbine air flow to tip speed ratio is a realistic goal. The first control strategy requires measurement of turbine rotational speed and air chamber pressure, and the second strategy removes the requirement for air pressure measurement. The implementation issues associated with this level of control are examined and the simulation results are validated in an experimental test rig.

Power losses calculation methodology to evaluate inverter efficiency in electrical vehicles

Nowadays, electrical vehicles (EVs) are of special interest and fuel engines are starting to be substituted by electrical motors. Besides the batteries and the motor, the power electronic devices are important parts in an EV, especially the inverter. This ac/dc power converter drives the electrical motor. Several tools may be needed to achieve an optimal design of the inverter. This paper presents a simplified methodology to estimate power losses in a two-level inverter made up with isolated gate bipolar transistors (IGBTs). This methodology is based on the IGBT manufacturer datasheet; therefore, the knowledge of internal parameters used in other more complex methodologies is not required. The model is implemented in Matlab-Simulink and allows simulation of different power devices, modulation techniques, and operating conditions very easily. Some results are obtained and validated with commercial electronic software and also with the simulation tool Semisel, provided by the manufacturer of the IGBTs.

Switching Frequency Behaviour for a Practical Matrix Converter

There are certain barriers that prevent the commercialisation of matrix converters. In the design of a real converter, the parameters that intervene in the modulation and commutation must be quantified with precision. This paper defines the commutation parameters that intervene in a vector modulation algorithm (SVM). On the other hand, it determines the commutations that occur when the double sided SVM technique is used both with semi-soft commutation, assessing the influence of the modulation frequency and technique on the design of the filter, driver consumption and their power sources. All this facilitates the dimensioning of the clock and the resolution of the variables to be used in the implementation of the algorithm in a FPGA. The paper concludes with a series of experimental results made with a Eupec module, allowing the selection of MC components

Analysis of the technology currently used in Offshore Wind energy systems

This paper gives an overview to the foundations, the Offshore Wind Turbines (OWTs) and the collection and transmission systems of Offshore Wind Farms (OWFs). Although for some of the components of their Electric Systems (ESs) there are different technical possibilities under consideration, under certain conditions of sea depth and distance from shore there are already tested industrial solutions available. However, the technology is not mature yet and decisions adopted today could condition the future expansion of operating OWFs or the development of the already planned Offshore Wind Power Plants (OWPPs). This paper presents the information gathered from about 93% of the currently operating OWFs. Moreover, OWPPs under construction have also been considered to confirm the conclusions presented and to detect current technological trends.

High power high voltage DC/DC converter for MVDC distribution applications

Smart grids and distributed power will play a big role in the future power grid. In the future many DC grids will coexist with the traditional AC grids. This paper presents an efficient and low cost alternative for unidirectional or asymmetric interconnection of DC grids using MVDC transmission lines using High Frequency Transformers.

Market forecasts, feasibility studies and regulatory framework for offshore wind energy integration

This paper gives an overview of several aspects of Offshore Wind Power (OWP) integration. During last year the wind market has exceed forecasts despite of the global crisis, with the offshore market offering very good numbers. Moreover, some projections presented in the paper show that OWP installed during the next five years will probably overcome European Wind Energy Association (EWEA)s forecasts. Logically, investors of OWP projects demand accurate viability plans, which in turn ask for rigorous cost analysis and revenue forecasts considering the whole life cycle of the installation. However, the electricity price in an open market is expected to show certain volatility in the presence of large amount of Renewable Energy Supplies for Electricity (RESE), which makes governmental supporting schemes essential. The European Commission (EC) has established a new regulatory framework called to play a key role in promoting and sustaining RESE and Member States have specified those support schemes in their National Renewable Energy Action Plans (NREAPs).

Matrix Converter Double Sided Space Vector Modulation: a fast way to synthesize via S-Function

Matrix converters present several advantages in some power electronic applications, but their control is a complicated task. Matlab-Simulink is a very useful tool for the developing of control algorithms for power converters. In this paper, an exhaustive analysis of double sided space vector modulation for a matrix converter is presented. The synthesis of the complex space vector modulator is done using a single S-function, thus simplifying and accelerating the implementation and debugging of the controller of a matrix converter. The implementation procedure of the modulation strategy presents several important advantages, mainly, an acceleration of the time dedicated to the implementation of the modulator, a more reliable design, because the debugging is straightforward, and faster simulation times of the entire platform. The simulation method allows an easy way to synthesize the modulation algorithm in a FPGA, in this way, this paper contributes to the development of the IPMs. Finally, the results of applying the present space vector modulator for the control of a doubly fed induction generator (DFIG) are presented, showing that complex control of matrix converter is minimized

Improvement of the Matrix Converter Start-up Process

The behavior of the matrix converter (MC) is usually studied in the stationary condition, however, turning on the MC is a very crucial moment for this device as overcurrents and overvoltages may appear, damaging the converter. In order to improve its ride-through capability, this paper focuses on matrix converter (MC) power-up protection strategies. The input filter and clamp circuit protect the MC and improve its behavior in the stationary condition. Given the importance of the filter, the design criteria to be followed, its mathematical analysis and the procedure to select its constituent parts, are defined in great detail. Following this, an analysis is made of the interaction between the filter and the MC clamp circuit during the initial transitory of the MC. Finally, a new, improved solution for the clamp circuit is proposed. This optimizes the behavior of the MC during power-up. The start-up procedure that mitigates overcurrents around the input filter is also defined. Consequently, this paper represents a step forward in the work being done to increase the reliability of the MC.