M.M. Prats

Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey

The use of distributed energy resources is increasingly being pursued as a supplement and an alternative to large conventional central power stations. The specification of a power-electronic interface is subject to requirements related not only to the renewable energy source itself but also to its effects on the power-system operation, especially where the intermittent energy source constitutes a significant part of the total system capacity. In this paper, new trends in power electronics for the integration of wind and photovoltaic (PV) power generators are presented. A review of the appropriate storage-system technology used for the integration of intermittent renewable energy sources is also introduced. Discussions about common and future trends in renewable energy systems based on reliability and maturity of each technology are presented

The age of multilevel converters arrives

This work is devoted to review and analyze the most relevant characteristics of multilevel converters, to motivate possible solutions, and to show that we are in a decisive instant in which energy companies have to bet on these converters as a good solution compared with classic two-level converters. This article presents a brief overview of the actual applications of multilevel converters and provides an introduction of the modeling techniques and the most common modulation strategies. It also addresses the operational and technological issues.

Three-dimensional space vector modulation in abc coordinates for four-leg voltage source converters

Four-leg inverters have been selected as one of the preferred power converter topologies for applications that require a precise control of neutral current, like active filters. The main advantage of this topology lies in an extended range for the zero sequence voltages and currents. However, the addition of a fourth leg extends the space vectors from two to three dimensions, making the selection of the modulation vectors more complex. Most of the algorithms that deal with this problem require an /spl alpha//spl beta//spl gamma/ transformation. This paper presents a new space vector modulation algorithm using abc coordinates (the phase voltages) avoiding the /spl alpha//spl beta//spl gamma/ transformation. Thanks to the use of abc coordinates, the algorithm is much simpler and more intuitive than in /spl alpha//spl beta//spl gamma/ representation, drastically reducing the complexity of modulation algorithm and the computational load associated to it.

A 3-D space vector modulation generalized algorithm for multilevel converters

A three-dimensional (3-D) space vector algorithm of multilevel converters for compensating harmonics and zero sequence in three-phase four-wire systems with neutral is presented. The low computational cost of the proposed method is always the same and it is independent of the number of levels of the converter. The conventional two-dimensional (2-D) space vector algorithms are particular cases of the proposed generalized modulation algorithm. In general, the presented algorithm is useful in systems with or without neutral, unbalanced load, triple harmonics and for generating 3-D control vectors.

Improving transition between power optimization and power limitation of variable speed, variable pitch wind turbines using fuzzy control techniques

This paper presents an application of fuzzy controllers for improving wind energy capture for variable speed wind turbines. In recent works, it has been shown that variable speed wind turbines with torque and blade-pitch linear control provide an excellent performance of the closed loop system. However, this performance can be enhanced by using fuzzy control. In the present work, a computational fuzzy model is presented for a variable speed, variable pitch wind turbine. Simulation results show an excellent performance improving transition between power optimization and power limitation of the wind turbine, specially in the rated wind speed working conditions.

Model Based Adaptive Direct Power Control for Three-Level NPC Converters

In this work, a Model Based Adaptive Direct Power Control (MB-ADPC) with constant switching frequency for Three-Phase Three-Level Neutral Point Clamped (3L-NPC) converters is proposed. The rectifier and inverter operation mode are used to illustrate the flexibility of the proposed MB-ADPC controller. The control design process is based on the continuous averaged model of the system. Depending on the operation mode different control objectives have to be guaranteed. The proposed controller ensures the voltage regulation of the dc-link capacitors for the rectifier operation mode and to achieve voltage balance in the dc-link capacitors and the active and reactive power tracking for the rectifier and inverter operation modes. In addition, adaptive techniques are used to avoid system parameters uncertainties as smoothing inductors and grid frequency values. This work shows that the application of advanced control strategies based on the system model allows enhancing the performance of the overall system. The details of the controllers design process and the experimental results using a 50 kVA Three-Phase Three-Level NPC prototype are presented in this paper validating the proposed controllers.

New fast space-vector modulation for multilevel converters based on geometrical considerations

This paper presents an fast and simple modulation algorithm for voltage source multilevel converters for calculating the switching times and the space vectors using simple geometrical considerations. This method provides an online computation of the nearest switching vectors sequence to the reference vector and the on-state durations of the respective switching state vectors without trigonometric function calculations, look-up tables or coordinate system transformations which increases the computational load corresponding to the modulation of multilevel converter. The low computational cost of the proposed method is always the same and it is independent of the number of levels of the converter.

A SVM-3D generalized algorithm for multilevel converters

A novel three dimensional space vector algorithm of multilevel converters for compensating harmonics and homopolar component in system with neutral is presented. This generalized method provides an online computation of the nearest switching vectors sequence to the reference vector and calculates the on-state durations of the respective switching state vectors without involving trigonometric functions, look-up tables or coordinate system transformations which increase the computational load corresponding to the modulation of multilevel converters. The low computational cost of the proposed method is always the same and it is independent of the number of levels of the converter. The conventional 2D space vector algorithms are particular cases of the proposed generalized modulation algorithm. The algorithm provides the switching sequence that minimizes the total harmonic distortion and the commutation number of the semiconductor devices.

DC-link capacitors voltage balancing in multilevel four-leg diode-clamped converters

Multilevel four-leg four-wire converters are creating increasing interest nowadays in applications as active filters. They present several advantages compared to the three-leg converter topologies. Recently the control complexity has been reduced thanks to the use of new and powerful microprocessor systems, hence the balance of the DC-link voltage is the major drawback of this kind of topologies. In this paper, the first control strategy to carry out the balance of the DC-link voltage in the multilevel four-leg four-wire diode-clamped converter is presented. This strategy uses the known technique based on choosing the most suitable redundant vector in the space vector modulation algorithm in order to reduce the unbalance. It is important to notice that the proposed method is independent of the load and independent of the number of levels of the converter. Some simulation results show the obtained balance using the proposed technique.

A new fuzzy logic controller to improve the captured wind energy in a real 800 kW variable speed-variable pitch wind turbine

This paper presents an application of fuzzy controllers for improving the captured wind energy and the performance of variable speed wind turbines using fuzzy controllers. The simulated fuzzy model has been satisfactorily implemented using a programmable logic controller as the designed fuzzy controller. Transition between power optimization and power limitation of the wind turbine is greatly improved especially in the rated wind speed working conditions. Fuzzy controller also permits to increase captured wind energy in low and high wind speed.