Showing papers presented at "International Power Electronics and Motion Control Conference in 2006"

Overview of Anti-Islanding Algorithms for PV Systems. Part I: Passive Methods

Abstract: For pt.I see ibid., p.420-5 (2006).This paper offers an overview of the active methods used for islanding detection. The inverter control algorithm is based on the use of harmonic compensators in order to create ideal working conditions (i.e. the PV-system produces a sinusoidal current and reject grid harmonics). This condition is the indispensable starting point to implement active islanding detection methods. In fact these methods are based on the perturbation of the ideal conditions (e.g. the injection of active/reactive power, the injection of current harmonics, or the insertion of an external capacitor). All the reported algorithms are carefully analysed in their advantage and drawbacks and the considerations are validated by tests results obtained in accordance with the IEEE Std. 929-2000 procedure

Improved MPPT Algorithms for Rapidly Changing Environmental Conditions

Abstract: The first part of this paper intends to give an overview of the Maximum Power Point Tracking methods for Photovoltaic (PV) inverters presently reported in the literature. The most well-known and popular methods, like the Perturb and Observe (P&O), the Incremental Conductance (INC) and the Constant Voltage (CV), are presented. These methods, especially the P&O, have been treated by many works, which aim to overcome their shortcomings, either by optimizing the methods, or by combining them. In the second part of the paper an improvement for the P&O and INC method is proposed, which prevents these algorithms to get confused during rapidly changing irradiation conditions, and it considerably increases the efficiency of the MPPT.

Advanced Incremental Conductance MPPT Algorithm with a Variable Step Size

Abstract: This paper proposes an advanced incremental conductance MPPT algorithm with a variable step size for a solar array regulator (SAR) system. This approach adjusts automatically a step size to the solar array operating point, thus improving the MPPT (maximum peak power tracking) speed and accuracy compared with the conventional method with a fixed step size. For the analysis of stability of the proposed algorithm near the maximum power point, the small signal modeling is carried out. Also, it is verified by experiment using a 180 W parallel connected prototype hardware

Power Electronics in Wind Turbine Systems

Abstract: The global electrical energy consumption is still rising and there is a steady demand to increase the power capacity. The production, distribution and the use of the energy should be as technological efficient as possible and incentives to save energy at the end-user should be set up. The deregulation of energy has lowered the investment in larger power plants, which means the need for new electrical power sources may be very high in the near future. Two major technologies will play important roles to solve the future problems. One is to change the electrical power production sources from the conventional, fossil (and short term) based energy sources to renewable energy resources. The other is to use high efficient power electronics in power systems, power production and end-user application. This paper discuss the most emerging renewable energy source, wind energy, which by means of power electronics is changing from being a minor energy source to be acting as an important power source in the energy system. By that wind power is also getting an added value in the power system operation.

Power Electronics in Renewable Energy Systems

Abstract: The global electrical energy consumption is still rising and there is a steady demand to increase the power capacity. It is expected that it has to be doubled within 20 years. The production, distribution and use of the energy should be as technological efficient as possible and incentives to save energy at the end-user should also be set up. Deregulation of energy has lowered the investment in larger power plants, which means the need for new electrical power sources may be very high in the near future. Two major technologies will play important roles to solve the future problems. One is to change the electrical power production sources from the conventional, fossil (and short term) based energy sources to renewable energy resources. An other is to use high efficient power electronics in power generation, power transmission/distribution and end-user application. This paper discuss some of the most emerging renewable energy sources, wind energy and photovoltaic, which by means of power electronics are changing from being minor energy sources to be acting as important power sources in the energy system.

Predictive Current Control Strategy with Imposed Load Current Spectrum

Abstract: A modified predictive current control strategy is presented in order to have control over the spectrum of the load current. The proposed strategy uses a model of the system to predict the behavior of the current for each possible voltage vector generated by the inverter. For that purpose, at each sampling interval, signal predictions are evaluated using a cost function that quantifies the desired system behavior. The cost function used in this work evaluates the filtered error of the load currents. The inclusion of a filter for the load error allows one to manipulate current spectra. Thus, by designing this filter appropriately, the load spectrum can be shaped.

Topological and Modulation Design of Three-Level Z-Source Inverters

Abstract: This paper presents the development of two three-level cascaded Z-source inverters, whose output voltage can be stepped down or up unlike a traditional buck three-level inverter. The proposed inverters are designed using two three-phase voltage-source inverter bridges, supplied by two uniquely designed Z-source impedance networks. These three-phase bridges can either be cascaded at their dc sides to form a dc-link-cascaded Z-source inverter or at their ac outputs using single-phase transformers to form a dual Z-source inverter. The dc-link-cascaded inverter has the advantages of not using any clamping diodes and transformers, but does not have redundant switching states within a phase leg for equalizing switching losses among the power devices. This constraint limits the modulation options for the dc-link-cascaded inverter, and indeed, it can only be controlled using the modified carrier disposition technique with appropriate ldquoZ-source shoot-throughrdquo states inserted for achieving balanced voltage boosting and optimal ldquonearest-three-vectorsrdquo switching. On the other hand, the dual Z-source inverter with transformer isolation can be controlled using different modulation approaches due to the presence of redundant switching states within a phase leg. Particularly, using a modified phase-shifted-carrier (PSC) scheme with shoot-through states inserted, it is shown that the dual inverter can be implemented using only a single Z-source network, while still achieving the correct volt-sec average and switching loss equalization. This represents a significant reduction in cost, and can more than compensate for the slightly degraded spectral characteristics of the PSC scheme. To verify the theoretical concepts discussed, experimental testing has been performed with the captured results presented in a later section of the paper.

The BDFM as a Generator in Wind Turbines

Abstract: The Brushless Doubly-Fed Machine (BDFM) is attractive for use in wind turbines, especially offshore, as it offers high reliability by virtue of the absence of brushgear. Critical issues in the use of the BDFM in this role at a system level include the appropriate mode of operation, the sizing of associated converter and the control of the machine. At a machine level, the design of the machine and the determination of its ratings are important. Both system and machine issues are reviewed in the light of recent advances in the study of the BDFM, and preliminary comparisons are made with the well-established doubly fed wound rotor induction generator.

A Stand-Alone Hybrid Generation System Combining Solar Photovoltaic and Wind Turbine with Simple Maximum Power Point Tracking Control

Abstract: This paper proposes a hybrid energy system combing solar photovoltaic and wind turbine as a small-scale alternative source of electrical energy where conventional generation is not practical. A simple and cost effective control technique has been proposed for maximum power point tracking from the photovoltaic array and wind turbine under varying climatic conditions without measuring the irradiance of the photovoltaic or the wind speed. The proposed system is attractive owing to its simplicity, ease of control and low cost. A complete description of the proposed hybrid system along with detailed simulation results which ascertain its feasibility are given to demonstrate the availability of the proposed system in this paper. Simulation of the hybrid system under investigation was carried out using PSIM software.

Optimum Control for Interior Permanent Magnet Synchronous Motors (IPMSM) in Constant Torque and Flux Weakening Range

Abstract: Interior Permanent Magnet Synchronous Motors (IPMSM) gain importance due to their high torque per volume ratio particularly for hybrid electrical vehicles. However, unlike to standard control theory, the torque control strategy for these motors is not apparently due to their reluctance torque, which is typical with interior magnet design. In this contribution, a control strategy is presented, which enables optimal torque control both in the lower speed range as well in the full flux weakening range. Operation during flux weakening, however, causes stress to the magnets of the motor with the risk of permanent demagnetization. The relations between the crucial design parameters are shown.

An Improved Sliding Mode Observer for Speed Sensorless Vector Control Drive of PMSM

Abstract: This paper proposes a speed sensorless control scheme for permanent magnet synchronous motor (PMSM) drive using an improved sliding mode observer (SMO) A variable frequency or cutoff frequency in low pass filter is not essential to use in the improved SMO Since the product of the improved SMO gain and the control action of sigmoid function, which replaces the Bang-Bang control or discontinuous control, commonly found in the conventional SMO, can determine the equivalent back emfs The estimated rotor position and speed are obtained directly from them Therefore, the low pass filter can be eliminated and the improved SMO could simplify the conventional SMO Also, the cutoff frequency tuning is not necessary A DSP based digital controller using the TMS320F2812 from the Texas Instruments has been employed to realize the proposed sensorless control scheme Experimental results show that the proposed control scheme can achieve robust sensorless requirement

Voltage Fed and Current Fed Full Bridge Converter for the Use in Three Phase Grid Connected Fuel Cell Systems

Abstract: Fuel cells for low or medium power deliver comparatively low voltages compared to the mains voltage at high currents. A high dc-link voltage is needed to feed in electrical energy from fuel cells to the mains via a voltage source inverter. Several well known dc/dc converters can be used to transform the varying fuel cell voltage to the requested amplitude of the dc-link-voltage. Besides transformerless converters like the boost converter, converters with high frequency transformers can be used. In the considered higher power range, the full-bridge converter circuit is the appropriate solution. Depending on their input circuit, the converters are classified between voltage fed and current fed full bridge converter. In this paper, full bridge converters of the voltage type and of the current type as active front end for fuel cell inverters in the power range of 20 kW and higher are analysed and compared to each other. The focus is set on the operating behavior, the system complexity and the efficiency of the different converters.

General Analysis of Multi-Phase Systems Based on Space Vector Approach

Abstract: In this paper the space vector approach is applied to the analysis of multi-phase electric systems. This approach builds on an existing but non systematic knowledge base, partially available in literature. In particular, power and RMS are expressed in terms of space vectors, and a comparison is carried out with respect to the Fortescue's symmetrical components in the case of n-phase circuits with sinusoidal waveforms. The use of space vectors allows real time analysis and regulation of both multi-phase converters and multi-phase machines with an elegant and effective vectorial approach.

A New BLDC Motor Drives Method Based on BUCK Converter for Torque Ripple Reduction

Abstract: This paper presents a comprehensive analysis on torque ripples of brushless dc motor drives in conduction region and commutation region. A novel method for reducing the torque ripple in brushless dc motors with a single current sensor has been proposed by adding BUCK converter in the front of 3-phase inverter. In such drives, torque ripple suppression technique is theoretically effective in commutation region as well as conduction region. Effectiveness and feasibility of the proposed control method is verified through experiments.

Integrating Large Wind Farms into Weak Power Grids with Long Transmission Lines

Abstract: In China, as in other parts of the world, many of the best resources for wind generation are located far away from load centers. Large generating facilities connected to distant load centers by long ac transmission lines face numerous technical challenges, regardless of the type of generating facility. This paper addresses some of the most significant challenges for wind generation facilities, including voltage control, reactive power management, dynamic power-swing stability, and behavior following disturbances in the power grid. Wind generation technology has evolved significantly over the past several years, and proven solutions to these technical challenges now exist. Controls integrated into the power electronics and mechanical controls of individual wind-turbinegenerators, combined with integrated wind-farm control systems, have the capability of controlling numerous wind turbines so that they act as one unified generating plant at the point of interconnection with the power grid. This advanced hierarchical control of both real and reactive power output can provide dynamic performance that is, in many cases, superior to that achievable with modern conventional synchronous generation. This paper describes: a. Wind farm control functions, including performance for controlling grid voltage in quasi-steady-state and dynamic conditions. b. Low-voltage ride-through characteristics, including performance following severe system disturbances c. Dynamic power control functions within wind turbinegenerators, including transient and dynamic performance for power swings.

Active and Reactive Power Control of DFIG for Wind Energy Conversion under Unbalanced Grid Voltage

Abstract: In this paper, a novel control method for smoothening the stator active or reactive power ripple components under unbalanced grid voltage for wind power generation using doubly-fed induction generators (DFIG) is proposed The negative sequence component of the rotor current is used to reduce the active and reactive power ripples, which is extracted by band pass filters It is shown that reducing the reactive power ripples results in the torque ripple reduction Simulation results using PSCAD show the proposed control algorithm can reduce the active or reactive power ripple components as well as reducing the torque and speed ripples

Analytically Computing Winding Currents to Generate Torque and Levitation Force of a New Bearingless Switched Reluctance Motor

Abstract: The paper has developed a new bearingless switched reluctance motor (SRM) with the same winding structure of a common SRM. In order to generate the suspending force and torque at the same time, more than two windings are used. One developed analytical model for the flux linked by a phase of the SRM, which includes the iron saturation and the fringing flux between teeth of the stator and rotor, is applied, in order to fulfill the calculation of currents. First it is used to obtain an analytical expression for the SRM' co-energy. Then equations of torque and radial forces are gotten by derivation of the co-energy for turn angle and air gap. Coupling inductances of windings are analysed in the calcualtion. Finally one finite-element-method (FEM) calculation is used to verify the effective theoretical calculation and analysis.

High-Efficiency Transformerless Single-phase Photovoltaic Inverter

Abstract: Grid-connected photovoltaic (PV) systems usually include a line transformer in their power conversion stage. This transformer guarantees galvanic isolation between the grid and the PV system, thus providing personal protection and avoiding leakage currents between the PV system and the ground. Furthermore, it also ensures that no continuous current is injected into the grid. However, because of its low frequency (50 Hz), the transformer is big, heavy and expensive. As an alternative to the grid-connected PV systems with line transformer, new transformerless inverter topologies are being studied. If no transformer is used, a galvanic connection between the grid and the PV array exists. In these conditions, leakage currents due to the capacitance between the photovoltaic array and earth could occur and increase the electromagnetic emissions. To avoid these leakage currents, it is necessary to use inverter topologies that avoid commonmode voltages. In this paper, a new transformerless singlephase PV inverter with six IGBTs and one diode is proposed. This topology generates no common-mode voltage and has a higher efficiency than the existing topologies. The topology has been validated by simulation.

Status and Opportunities of Photovoltaic Inverters in Grid-Tied and Micro-Grid Systems

Abstract: This paper reviews the status in industry and academia regarding configurations, topologies, controls, and grid connections in grid-tied and micro-grid PV inverter applications. The paper will discuss the major technical needs to address problems in bringing cost down, increasing efficiency and improving reliability/availability. The paper foresees that new grid interconnection features will have to be integrated more into the inverters, along with the wide-spreading use of distributed generations.

Design and Performance Evaluation of a 10-kW Interleaved Boost Converter for a Fuel Cell Electric Vehicle

Abstract: A step-up converter is described to interface the low, poorly regulated fuel cell output voltage (70-120 V) with the higher voltage supercapacitor bank and traction drive system on a small fuel cell powered electric vehicle. The paper focuses on the design of an interleaved boost converter, in particular comparing a two inductor circuit with an alternative topology comprising a single inductor plus interphase transformer (IPT). Whilst the two circuits have identical inductive stored energies, the lower AC excitation of the inductor in the IPT topology offers performance benefits.

The Chattering Analysis

Abstract: This paper presents analysis of the chattering and a new approach of the chattering suppression. The chattering caused by the unmodeled dynamics is discussed and analyzed by the Lyapunov method and the describing function approach. It is shown that the amplitude of chattering depends on the magnitude of discontinuous control. To reduce the chattering, new sliding mode control design is proposed using the methods of changing switching gain along the system states or the magnitude of equivalent control. To support the methods, simulation results are provided.

Grid Connection of Doubly-Fed Induction Generators in Wind Energy Conversion System

Abstract: This paper presents a new synchronization algorithm for grid connection of a doubly fed induction generator (DFIG) in wind generation system. A stator flux-oriented vector control is used for the variable speed DFIG operation. By controlling the generator excitation current the amplitude of the stator EMF is adjusted equal to the amplitude of the grid voltage. To set the generator frequency equal to the grid one, the turbine pitch angle controller accelerates the turbine/generator until it reaches the synchronous speed. A slight difference of stator and grid frequencies may cause the large phase difference between the two voltages. To compensate for this phase difference, a PLL algorithm is used. After the synchronization is achieved, the generator is connected to the grid and is controlled to extract the maximum power. The effectiveness of the proposed synchronization algorithm is verified by simulation results using PSCAD.

Design and Development of a 50kW Z-Source Inverter for Fuel Cell Vehicles

Abstract: A detailed design process of the Z-source inverter is presented in this paper. A dc rail clamp circuit is used to reduce the overshoot of the device during turn off. The thermal and 3-D design process is gone through, and the loss calculation of the inverter is discussed, which is different from traditional PWM inverters. A 50 kW inverter for fuel cell vehicle is developed to demonstrate the validity of the design process. Experimental results confirmed the design process and demonstrated the high efficiency characteristic of the Z-source inverter.

Optimal Power Control Strategy of Maximizing Wind Energy Tracking and Conversion for VSCF Doubly Fed Induction Generator System

Abstract: This paper focuses on the development of maximum wind power extraction strategies for variable speed constant frequency (VSCF) grid-connected wind power generation systems with a doubly fed induction generator (DFIG) A new optimal control method is proposed by controlling the generator stator active and reactive power, which is based on the condition of the system operation for not only the extracted maximum power of the wind turbine below the rated wind speed but also the higher generator efficiency Based on the DFIG mathematical models, the optimal stator reactive power value is derived for minimal machine copper losses According to wind turbine power characteristics and generator power flow equations, the optimal stator active power reference value is also obtained for capturing maximal output power from wind turbines A dual-passage excitation control strategy is applied to control the active and reactive power independently Detailed simulation results have confirmed the feasibility and performance of the optimal control strategy

Design Challenges For Distributed Power Systems

Abstract: Remarkable progresses have been made over the past decade in power conversion technologies, including advanced power semiconductor devices, power management ICs, innovative circuit topologies, and packaging and integrated system solutions. These technological advancements have been manifested in a wide range of products and applications with ever increasing performances, efficiency, and power density. This paper highlights some of the challenges and opportunities of power conversion technologies in the Distributed Power System (DPS) for computer, telecommunication and network products. Topics discussed in this paper include improved EMI filter design techniques to mitigate the detrimental effects of filter/converter parasitics; impacts of the operating frequency of PFC to the size and weight of EMI filter; power conversion architecture and potential simplification; high-frequency high-density AC/DC and DC/DC topologies and designs; bus converters; as well as non-isolated point- of -load converters

Matrix Converter Induction Motor Drive

Abstract: The presented paper deals with the matrix converter induction motor drive with emphasis on the PWM modulation strategies design. Matrix converters provide an all-silicon solution to the problem of converting AC power from one frequency to another, offering almost all the features required of an ideal static frequency changer. They possess many advantages compared to the conventional voltage or current source inverters. A matrix converter does not require energy storage components as a bulky capacitor or an inductance in the DC-link, and enables the bidirectional power flow between the power supply and load. The most of the contemporary modulation strategies are able to provide practically sinusoidal waveforms of the input and output currents with negligible low order harmonics, and to control the input displacement factor. Sophisticated pulse width modulation strategies in terms of various optimization criteria are known in case of indirect frequency converters, whereas both the inverter and the rectifier can be operated with pulse width modulation. The instantaneous state of both the output and the input converter waveforms depend at any time on the switch state of the converter power switches. Suitable switch states sequence of the nine matrix converter switches can be indirectly derived from the given switch states sequence of the twelve switchers of the indirect frequency converter.

A Multi-Directional Power Converter for a Hybrid Renewable Energy Distributed Generation System with Battery Storage

Abstract: Power electronics is the key technology for enabling renewable power generation and dispatching Distributed Generation (DG) with improved load-side efficiency This paper presents a multi-directional power converter with a high-frequency isolation transformer for a stand-alone hybrid renewable energy DG system with battery storage Full-bridge power converters are employed to realize bi-directional power flow among renewable generator, battery storage and load Detailed analysis is provided for a four-directional power converter in a wind-photovoltaic distributed generation system In addition, a two loop PI control scheme is proposed to achieve constant output voltage and exact power flow management Theoretical analyses are verified by simulations in PSPICE and MATLAB

Encoderless Control of Industrial Servo Drives

Abstract: High resolution optical encoders are used in industrial servo drives with position and/or speed control for providing the respective feedback signals. In applications with lower accuracy requirements resolvers are used as feedback sensors with respect to lower cost. As a resolver is nothing else than an electrical AC machine of special design, there is the basic idea to operate the servo motor itself like a resolver ? in parallel to its original drive operation, of course. High frequency resolver signals are injected to the servo motor without disturbing its original purpose (i.e. the production of torque). The reaction of the servo motor on the high frequency signals can be detected by the current sensors, which are available with respect to the current control loop anyway. A lot of schemes and concepts for encoderless AC drive control providing good operation during standstill are already published. Acceptance by industry, however, is not very extended. The main reasons for hesitations in industry are ? additional processing performance being necessary for the controller or signal processor ? additional sensors or hardware ? parameters to be adjusted for encoderless control The concept for encoderless control presented in this paper has no restrictions with respect to a minimal speed or frequency. Furthermore it is possible to implement it in industrial servo drives without additional hardware requirements and without parameter adjustments.