The recently introduced proportional-resonant (PR) controllers and filters, and their suitability for current/voltage control of grid-connected converters, are described. Using the PR controllers, the converter reference tracking performance can be enhanced and previously known shortcomings associated with conventional PI controllers can be alleviated. These shortcomings include steady-state errors in single-phase systems and the need for synchronous d-q transformation in three-phase systems. Based on similar control theory, PR filters can also be used for generating the harmonic command reference precisely in an active power filter, especially for single-phase systems, where d-q transformation theory is not directly applicable. Another advantage associated with the PR controllers and filters is the possibility of implementing selective harmonic compensation without requiring excessive computational resources. Given these advantages and the belief that PR control will find wide-ranging applications in grid-interfaced converters, PR control theory is revised in detail with a number of practical cases that have been implemented previously, described clearly to give a comprehensive reference on PR control and filtering.
Multiport DC-DC converters are of potential interest in applications such as generation systems utilising multiple sustainable energy sources. A family of multiport bidirectional DC-DC converters derived from a general topology is presented. The topology shows a combination of DC-link and magnetic coupling. This structure makes use of both methods to interconnect multiple sources without the penalty of extra conversion or additional switches. The resulting converters have the advantage of being simple in topology and have a minimum number of power devices. The proposed general topology and basic cells show several possibilities to construct a multiport converter for particular applications and provide a solution to integrate diverse sources owing to their flexibility in structure. The system features a minimal number of conversion steps, low cost and compact packaging. In addition, the control and power management of the converter by a single digital processor is possible. The centralised control eliminates complicated communication structures that would be necessary in the conventional structure based on separate conversion stages. A control strategy based on classical control theory is proposed, showing a multiple PID-loop structure. The general topology and a set of three-port embodiments are detailed.
The performance of the brushless doubly-fed machine (BDFM) is analysed using a per-phase equivalent circuit. An expression for the rating of the machine as a function of magnetic and electric loadings is developed, and the rating is compared to those of the doubly-fed induction machine and cascaded induction machines. As the magnetic field in a BDFM is complex, the magnetic loading is considered in detail and a new generalised loading is derived. The BDFM suffers a reduction rating of about one-quarter in comparison to comparable conventional machines, arising from penalties in magnetic and electric loadings consequent on the presence of two stator to rotor couplings. The handling of reactive power has an important effect on the machine performance and this point is illustrated with experimental results from a frame size 180 BDFM. The tests were carried out at modest flux densities to avoid the effects of saturation, but the implications of saturation are considered.
Magnetic gears offer significant potential advantages compared with mechanical gears, such as reduced maintenance and improved reliability, inherent overload protection, and physical isolation between the input and output shafts. However, to date they have received relatively little attention, probably due to the relatively poor torque transmission capability of the magnetic circuit topologies which have been proposed. A new magnetic gear topology which combines a highly competitive torque transmission capability and a very high efficiency is described.
As a result of the equivalent series resistor of the boost inductor, conventional boost converters are not able to provide high voltage gain. A high-efficiency high step-up converter is proposed, with low voltage stress on power switch, power diodes and output capacitors. The circuit topology of the proposed converter consists of an energy clamp circuit and a voltage boost cell. The boost converter functions as an active clamp circuit to suppress the voltage spike on power switch during the turn-off transient period. The boost converter output terminal and flyback converter output terminal are serially connected to increase the output voltage gain with the coupled inductor. By serially connecting the secondary windings of the boost inductor, a high voltage gain is achieved with less voltage stress on the power devices, such as power MOSFETs and power diodes. The operational principle and steady-state analysis are described. A 35W converter with simulation and experimental results is presented to demonstrate the performance. It shows that the efficiency of the proposed converter is very high (nearly 93%) with four times the voltage output.
A simple analytical expression for the current stress on the DC-link capacitor caused by the load-side inverter of a voltage DC-link-converter system is derived. The DC-link capacitor-current RMS value is determined from the modulation depth and by the amplitude and the phase angle of the inverter output current assuming a sinusoidal inverter output current and a constant DC-link voltage. Despite neglecting the output-current ripple, the results of the analytical calculation are within 8% of measurements made from digital simulation and an experimental system, even if the output-current ripple is relatively high as in the case of low-frequency IGBT inverter systems. The simple analytical expression provides significant advantages over simulation methods for designing the DC-link capacitor of PWM converter systems.
Control of the brushless doubly fed machine (BDFM) based on traditional multiple reference frames is complex. To simplify the control scheme, a new and simpler derivation of the dq model of the BDFM is proposed, leading to a unified-reference-frame model. This way, a simple dq model can be established, which could be an interesting tool for control-synthesis tasks. In order to determine the unified reference dq model, restrictions related to BDFM operation, as well as the exact rotor-cage configuration, have been considered. The proposed model has been validated by several experimental results. The work could facilitate future research on improved BDFM field-oriented control strategies.
Nowadays fractional-slot windings are proposed for synchronous motors for different purposes: reduction of end-winding losses, reduction of torque ripple, reduction of mutual coupling among the phases, fault-tolerant applications, and so on. The design of the single-layer fractional-slot synchronous motors is dealt with, in which each slot contains only one side coil. The star of slots, introduced some time ago, proves itself to be again appropriate. After a brief review of the classical theory of the star of slots, its application is extended to the design of these unconventional windings. Thanks to its graphical representation, a simple analytical formulation is carried out describing the harmonic contents of the winding distribution. Finally, the star of slots allows simple rules for the design of fractional-slot single-layer windings to be determined. As a special case, it is used to determine the synchronous motor winding solutions suitable for critical fault-tolerant applications.
The simulation and experimental study of a fuzzy logic controlled, three-phase shunt active power filter to improve power quality by compensating harmonics, and reactive power required by a nonlinear load is presented. The advantage of fuzzy control is that it is based on a linguistic description and does not require a mathematical model of the system. The fuzzy control scheme is realised on an inexpensive dedicated micro-controller (INTEL 8031) based system. The compensation process is based on sensing line currents only, an approach different from conventional methods, which require harmonics or reactive volt-ampere requirement of the load. The performance of the fuzzy logic controller is compared with a conventional PI controller. The dynamic behavior of the fuzzy controller is found to be better than the conventional PI controller. PWM pattern generation is based on carrierless hysteresis based current control to obtain the switching signals. Various simulation and experimental results are presented under steady state and transient conditions.
To be able to develop a complete solar photovoltaic power electronic conversion system in simulation, it is necessary to define a circuit-based simulation model for a PV cell in order to allow the interaction between a proposed converter (with its associated control arrangement) and the PV array to be studied. To do this it is necessary to approach the modelling process from the perspective of power electronics; that is to define the desired overall model in terms of the manner in which the electrical behaviour of the cell changes with respect to the environmental parameters of temperature and irradiance. The authors cover the development of a general model which can be implemented on simulation platforms such as PSPICE or SABER and is designed to be of use to power electronics specialists. The model accepts irradiance and temperature as variable parameters and outputs the I/V characteristic for that particular cell for the above conditions.
Permanent magnet brushless machines employing multipole Halbach magnetised rotors are being developed for various applications, since they offer a number of attractive features. Alternative Halbach machine topologies are reviewed, and the realisation of Halbach magnetised magnets from pre-magnetised sintered rare earth magnet segments (which approximate the Halbach magnetisation distribution and thereby compromise their performance), and as bonded isotropic and anisotropic NdFeB ring magnets (which are subsequently impulse magnetised with a Halbach field distribution), is considered. Radial- and axial-field, slotted and slotless, rotary and linear (tubular and planar), and spherical Halbach magnetised brushless machines are described, and potential applications, including a motor/generator for a high-speed fly-wheel peak power buffer, high-performance linear and rotary servo motors, and passive magnetic bearings are considered.
An analytical and experimental comparison. of the performance of multiphase induction machines as a function of the number of phases is provided. The method of generalised complex harmonic analysis is used to model the multiphase machine, including the calculation of pulsating torques. This general model is validated with experimental results. The test machine used a standard motor frame, which had been modified to bring every coil out to a patch panel. With 12 coils per pole, the machine was tested in 3, 4, 6 and 12-phase operation. Results show moderate reductions in stator losses and significant improvements in pulsating torques.
Utility-voltage information, such as the frequency, phase angle and amplitude, is very important in many industrial systems. In a three-phase system, the utility-voltage information can easily be obtained using a utility-voltage vector, as the magnitude and angle of the voltage vector indicate the amplitude and angle of the utility voltage, respectively. However, for a single-phase system, the utility-voltage information is much more difficult to acquire. Conventionally, the frequency and phase angle of a single-phase voltage are obtained by detecting the zero-cross point. Yet, this method cannot provide the utility-voltage information instantaneously and is very sensitive to noise. Accordingly, the paper presents a novel digital phase-locked loop (PLL) algorithm for single-phase photovoltaic systems. The algorithm uses two virtual phases, and its performance is demonstrated under various utility conditions to show the effectiveness of the proposed algorithm.
A new topology for a hybrid multilevel inverter will be presented, which significantly increases the level number of the output waveform and thereby dramatically reduces the low-order harmonics and total harmonic distortion. To the best of the authors' knowledge, the presented topology has the greatest level number for a given number of stages. Moreover, the stage with higher DC link voltage has lower switching frequency; and thereby reduces the switching losses. Comparison of the results of various multilevel inverters will be investigated to reflect the merits of the presented topology. The details of the PWM control using the harmonic elimination technique for the hybrid inverter will be presented and confirmed by both simulation and experimental results.
Minimisation of the loss in the induction motor is directly related to the choice of the flux level. The higher the flux level, the larger the iron loss. But extreme minimisation causes a high copper loss. There is an optimal flux level that guarantees loss minimisation, and the loss-minimisation algorithm (LMA) was derived by many researchers. However, a technical problem in deriving the LMA lies in the loss model simplification. A loss model simplification is made with a voltage dependent source and loss resistance. The optimal LMA method extends to the field-weakening region where the voltage and current constraints constitute boundaries. A complete loss-minimising control algorithm is proposed and its validity is demonstrated by simulation and experimental results.
The paper describes the engineering and design of a doubly fed induction generator (DFIG), using back-to-back PWM voltage-source converters in the rotor circuit. A vector-control scheme for the supply-side PWM converter results in independent control of active reactive power drawn the supply, from ensuring sinusoidal supply currents. Vector control of the rotor-connected converter provides for wide speed-range operation; the vector scheme is embedded in control loops which enable optimal speed tracking maximum energy for capture from the wind. An experimental rig, which represents a 7.5kW variable speed wind-energy generation system is described, and experimental results are given that illustrate the excellent performance characteristics of the system. The paper considers a grid-connected system; a further paper will describe a stand-alone system.
The detection of broken-rotor-bar faults based on common steady-state-analysis techniques, such as the fast Fourier transform (FFT), is known to be significantly dependent on the loading conditions of the induction motor. At light load, it is difficult to distinguish between healthy and faulty rotors because the characteristic broken-rotor-bar fault frequencies in the stator current are very close to the fundamental-frequency component and their amplitudes are small in comparison. As a result, detection of broken bar faults and classification of the fault severity under light load is very difficult. To overcome this problem, the analysis of the envelope of the transient starting-current waveform using the wavelet-transform has been investigated. The envelope extraction is used to remove the strong fundamental-frequency component, which would otherwise overshadow the characteristic differences between a healthy motor and a faulty motor with broken rotor bars. The wavelet-transform results are processed to develop a normalised parameter called the 'wavelet indicator' which is sensitive to the presence of broken-rotor-bar faults. The results are verified using tests on machines with a varying number of broken bars, as well as partially broken rotor bars, over a wide range of loading conditions. The effects of initial rotor position and supply imbalance are also investigated.
Vector control of a dual three-phase induction machine, with two sets of three-phase stator windings spatially shifted by 30 electrical degrees, is elaborated in the paper. The stator windings are fed from a current-controlled PWM six-phase IGBT voltage source inverter (VSI). The main problem in realisation of such a drive is an adequate current control algorithm, which has to cope with the inherent asymmetries of the drive. Furthermore, the digital implementation has to ensure zero steady state current error for the whole frequency range of the drive. After reviewing the existing Solutions, the paper proposes a current control scheme that is based on four digital current controllers in the stationary reference frame. A design procedure for the current controllers is presented and current controllers are implemented in a laboratory test rig, in conjunction with a previously developed PWM scheme that provides operation with low values of low-order output voltage harmonics. Experimental tests are conducted and the results are given for a direct rotor field oriented control (DRFOC) of a 10 kW dual three-phase induction motor drive prototype. The results confirm the validity of the control scheme.
A novel circulating zero-sequence current-control method for common DC-source parallel inverter systems is proposed and implemented in FPGA. The follower inverter can closely track the three-phase discontinuous PWM modulation waveforms of the master inverter and the circulating current caused by asynchronous modulation waveforms can be reduced. The controller is implemented in FPGA, and the requirement of high-speed control and measurement can be satisfied without any extra burden added to the processor of the drive system. The theoretical analysis of the experimental results verifies the performance of the proposed circulating-current-control method.