Edwin Nowicki

Employing a very sparse matrix converter for improved dynamics of grid-connected variable speed small wind turbines

The amount of energy obtained from a wind turbine depends not only on the wind regime but also on the control technique used to govern the turbine. When wind speed changes, the turbine moves away from its optimal tip speed ratio until maximum power point tracking forces the turbine to operate at its optimal speed. For rapid wind speed changes, the maximum power point tracking controller can take longer than the necessary time to return the turbine to its maximum power coefficient which reduces the energy harvest, especially in gusty conditions. Proposed in this paper is a motoring-generating control technique for grid-connected small wind turbines in order to accelerate the turbine to its optimal operating speed in the case of increasing wind speed. In the case of decreasing wind speed, the turbine decelerates by loading the generator at its maximum torque. The generator torque is controlled through the backward bidirectional very sparse matrix converter where the grid is connected to the rectifier stage and the generator is connected to the inverter stage. The generator torque is regulated by controlling the generator current based on the principle of field oriented control where a decoupling between the field and torque current components is achieved. The direct axis current is set to zero while the quadrature axis current is used to control the generator torque and speed. Then the generator reference voltage and the converter switching signals are synthesized using the principle of space vector modulation. In order to show the feasibility of the proposed control technique, a 5.7kW grid-connected variable speed wind turbine is simulated using Matlab/Simulink.

A simple and cost effective maximum power point tracker for PV arrays employing a novel constant voltage technique

Photovoltaic generation is the direct conversion of solar energy into direct current electricity. For the past several years the use of the photovoltaic (PV) panels for power generation is increasing in popularity since PV electricity is considered to be a sustainable source of energy which is rapidly becoming more economical. Extensive research is being conducted on the photovoltaic cells and on controllers to cost-effectively increase system efficiency to encourage PV power as a reliable power source for stand-alone and grid-connected applications. This paper presents the modeling and simulation of a very simple maximum power point tracker (MPPT) used in conjunction with a buck converter for a polycrystalline PV array. The proposed MPPT controller is very simple with no PI controller.

Accelerated starting by motoring a grid-connected small wind turbine generator

In recent years, an increasing proportion of small wind turbines have been connected to the electrical grid. With appropriate switchgear and control, it should be possible to use grid power to operate the generator as a motor for faster starting of the turbine. The artificial case of a step increase in wind speed from zero is considered. It is shown that motoring yields an energy gain for all wind speeds typical of the operating range of small turbines. This paper proposes short-period motoring of a permanent magnet generator in a 5kW wind turbine system connected to the grid with a backward very sparse matrix converter which has bidirectional power flow capability. Motoring produces torque in the same direction as the aerodynamic torque on the turbine which in turn accelerates the turbine more quickly towards the maximum power point. Interestingly, the overall result is that more energy is delivered to the grid by short-term motoring than in the case where the turbine is accelerated aerodynamically without motoring. In the proposed system, the very sparse matrix converter controls the generator torque through field orientation where d-axis current is set to zero while the q-axis current is used to control the generator torque and thus control the turbine speed in both motoring and generating modes. The switching signals of the converter are formed based on the principle of space vector modulation. Simulation results, with and without an electromagnetic starting torque, indicate that the proposed technique increases the overall energy efficiency of small wind turbine systems while providing fast starting.

Improving inverter efficiency at low power by reducing switching frequency

The inverter is a major component of a renewable energy system and its performance affects the overall performance of the system. For typical household applications in remote areas, often there is need to operate at low power conditions where inverter efficiency can drop dramatically. Efficient operation at low power is important especially for stand-alone applications in developing countries where system cost must be kept low. In this paper, we investigated the impact of switching frequency upon switching loss for a single-phase Sinusoidal Pulse Width Modulation (SPWM) inverter. Results show that reducing the switching frequency reduces switching loss at low power levels thus improving inverter efficiency. This may result in a reduced PV module size requirement and thus lower system cost. In addition, efficient low power inverters would allow the continued operation of essential lighting when the battery bank is nearly discharged. A mathematical model is given to calculate the major loss components (i.e. switching loss, conduction loss) in the system. The inverter is operated in the range of 200W rated power to 9W (4.5 % of the rated power) along with the change in frequency from 20 kHz to 200 Hz.

Modified nearest three virtual space-vector modulation method for improved dc-capacitor voltage control in N-level diode clamped inverters

A major disadvantage of diode clamped inverters (DCI) in practical implementations is capacitor dc voltage fluctuation. A new and very fast space vector modulation scheme for controlling the capacitor dc voltage fluctuation is introduced in this paper. The proposed algorithm accelerates the computation of the nearest three vectors and calculation of corresponding duty cycles without an increase in computational requirements when extended to DCIs with more levels. This new algorithm is appropriate for real time applications with DSP controllers used in industry. Performance of the proposed method has been simulated in MATLAB Simulink for both 3-level and 4-level three-phase DCIs, with experimental verification presented using a TMS320F2812 DSP. Experimental implementations demonstrate that the proposed method has the ability to effectively control dc-capacitor voltage in an N-level DCI where N is the number of levels (including zero volts) in one half cycle of the inverter line-to-line output voltage.

The use of geometric algebra in circuit analysis and its impact on the definition of power

Geometric algebras of the Euclidean 2-dimensional and 3-dimensional spaces have been used to analyze electric circuits with linear and harmonic generating loads (HGLs). It is shown in this paper that with both loads, time domain signals may be transformed to the Gn domain such that the resulting multivectors permit circuit analysis through rotations and dilation-contractions of the excitation signal. The power equation, in particular, is created by applying the geometric product of the voltage and current multivectors and is suitable for linear and nonlinear loads. This is in contrast to commonly used frequency analysis methods where the reactive power cannot be obtained in correspondence with its definition in the time domain and it fails at providing a unified power equation for linear and HGLs. We interpret the proposed power equation in the frequency and time domains and introduce an analogous quantity to the reactive power Q, the CN-power, for circuits with nonlinear loads. A power factor equation applicable to both loads is also presented. The resulting one-to-one correspondence between the time domain and the Gn domain avoids eliminating any component from the power equation in the time domain. Single-phase circuit examples are provided.

A computational small-signal modeling technique for switch mode power converter

A computationally efficient method for obtaining the small-signal model of a switch mode power converter operating in the continuous conduction mode is presented. To apply this proposed technique, the only analytical work involved is the derivation of the two switched circuits of a given switch mode power converter in state-space representation form, and the developed computer simulation program provides a good approximated small-signal model in the form of open-loop Bode plots. The proposed modeling technique is applied to the buck converter and the derived model is compared with the small-signal model obtained using the statespace averaging technique. To further verify the validity of the proposed modeling technique, a 24W buck converter was built to demonstrate the effectiveness of the proposed modeling technique.

An experimental investigation of the Distributed Electronic Load Controller: A new concept for voltage regulation in microhydro systems with transfer of excess power to household water heaters

Constant voltage and frequency can be generated by a stand-alone Self-Excited Induction Generator (SEIG) driven with a fixed-speed low-head hydro-turbine when the electrical load is maintained constant by an Electronic Load Controller (ELC). However for a Conventional-ELC (C-ELC) most of the generated electrical energy can be dissipated in the dump load if the village load is low. So, the objective of this research is designing a simplified ELC for each household to transfer the excess power for domestic consumption in addition to providing voltage regulation for the generator. A regular ELC, possibly of reduced rated power, should still be installed at the generator site. At the same time, a simplified and inexpensive ELC is installed at each household. The proposed controller, installed in each household, is referred to as the Distributed Electronic Load Controller (DELC). MATLAB simulation results verify the feasibility of the proposed approach. Experimental results using the MSP-430 LaunchPad microcontroller are also presented to verify the validity of the proposed DELC approach.

A novel adaptive controller using radial basis function neural network for the wind energy conversion system

For a safe and efficient operation of large variable-pitch wind turbines, the controller plays a vital role. Below the rated wind speed, there is a power-optimal rotor speed for a given wind speed, achieved by designing a generator torque control. Pitch control is appropriate above the rated wind speed, to keep the rotor speed constant. In order to deal with nonlinearities and uncertainties, in this paper an approximate-adaptive control based on radial basis function networks is proposed, developed within a Lyapunov framework. Simulation results demonstrate that the proposed controller has ability to deal with system nonlinearities and uncertainties.

Recent Advances in Converters and Control Systems for Grid- Connected Small Wind Turbines

In response to the introduction of feed-in tariffs around the world, an increasing number of small wind turbines are being grid connected [1]. Variable speed wind turbines, though ini‐ tially more costly, have several advantages over fixed speed systems: (i) average power pro‐ duction is typically 10% higher since the turbine operates more frequently near its ideal tipspeed-ratio, (ii) the turbine and mechanical transmission operate with reduced stresses, (iii) turbine and generator torque pulsations are reduced, and (iv) noise is reduced [2-4]. Varia‐ ble speed operation in general has only become possible over the last twenty years or so be‐ cause of major developments in power electronics and associated cost reductions [5]. This reference indicates that power electronic devices are reducing in cost at about 1-5% per year. This chapter will discuss the impact of these developments on small turbine design and op‐ eration along with some important aerodynamics issues related to turbine starting.