Donald Grahame Holmes

Stationary frame current regulation of PWM inverters with zero steady-state error

Current regulators for AC inverters are commonly categorized as hysteresis, linear PI, or deadbeat predictive regulators, with a further sub-classification into stationary ABC frame and synchronous d-q frame implementations. Synchronous frame regulators are generally accepted to have a better performance than stationary frame regulators, as they operate on DC quantities and hence can eliminate steady-state errors. This paper establishes a theoretical connection between these two classes of regulators and proposes a new type of stationary frame regulator, the P+Resonant regulator, which achieves the same transient and steady-state performance as a synchronous frame PI regulator. The new regulator is applicable to both single-phase and three phase inverters.

Multicarrier PWM strategies for multilevel inverters

Analytical solutions of pulsewidth-modulation (PWM) strategies for multilevel inverters are used to identify that alternative phase opposition disposition PWM for diode-clamped inverters produces the same harmonic performance as phase-shifted carrier PWM for cascaded inverters, and hybrid PWM for hybrid inverters, when the carrier frequencies are set to achieve the same number of inverter switch transitions over each fundamental cycle. Using this understanding, a PWM method is then developed for cascaded and hybrid inverters to achieve the same harmonic gains as phase disposition PWM achieves for diode-clamped inverters. Theoretical and experimental results are presented in the paper.

Grid current regulation of a three-phase voltage source inverter with an LCL input filter

Many grid connected power electronic systems, such as STATCOMs, UPFCs, and distributed generation system interfaces, use a voltage source inverter (VSI) connected to the supply network through a filter. This filter, typically a series inductance, acts to reduce the switching harmonics entering the distribution network. An alternative filter is a LCL network, which can achieve reduced levels of harmonic distortion at lower switching frequencies and with less inductance, and therefore has potential benefits for higher power applications. However, systems incorporating LCL filters require more complex control strategies and are not commonly presented in literature. This paper proposes a robust strategy for regulating the grid current entering a distribution network from a three-phase VSI system connected via a LCL filter. The strategy integrates an outer loop grid current regulator with inner capacitor current regulation to stabilize the system. A synchronous frame PI current regulation strategy is used for the outer grid current control loop. Linear analysis, simulation, and experimental results are used to verify the stability of the control algorithm across a range of operating conditions. Finally, expressions for harmonic impedance of the system are derived to study the effects of supply voltage distortion on the harmonic performance of the system.

Optimized Design of Stationary Frame Three Phase AC Current Regulators

Current regulation plays an important role in modern power electronic AC conversion systems. The most direct strategy to regulate such currents is to use a simple closed loop proportional-integral (PI) regulator, which has no theoretical stability limits as the proportional and integral gains are increased, since it is only a second order system. However, pulsewidth modulation (PWM) transport and controller sampling delays limit the gain values that can be achieved in practical systems. Taking these limitations into account, this paper presents an analytical method to determine the best possible gains that can be achieved for any class of practical linear AC current controller. The analysis shows that the maximum possible proportional gain is determined by the plant series inductance, the DC bus voltage and the transport and sampling delays, while the maximum possible integral gain is determined primarily by the transport and sampling delays. The work is applicable to stationary frame PI regulators, stationary frame controllers with back electromotive force compensation, stationary frame P+ resonant (PR) controllers, and synchronous d- q frame controllers, since they all have identical proportional and integral gains that must be optimized for any particular application.

Implementation of a direct digital predictive current controller for single and three phase voltage source inverters

The three major approaches for regulating current in hard switched inverters are ramp comparison, hysteresis control and predictive current control. Of these three, predictive current control offers the potential for achieving more precise current control with minimum distortion and harmonic noise, but is generally more difficult to implement and usually must be matched to a specific load. Also, errors caused by computational delays create further problems in a real system. This paper presents algorithms for directly implementing a predictive current controller in a microprocessor for load situations where the back-EMF is both known and unknown. The algorithms fully compensate for sampling delays and discretisation errors, and are simple enough to be readily implemented in a practical system. Both simulation and experimental results are presented as confirmation of the approach presented.

Implementation of a controlled rectifier using AC-AC matrix converter theory

It is well known that a PWM-controlled rectifier can offer advantages of reduced low-order harmonics and unity input power factor when compared to a conventional thyristor converter. However, theoretically optimum PWM strategies are often difficult to implement physically or are not easily extended to regenerative operation. The authors propose an alternative PWM strategy based on AC-AC matrix converter theory, which generates only high-order switching harmonics, presents a unity power factor load to the supply, implicitly extends to regeneration (and operation with a center tapped DC output), and is feasible to physically implement for real-time output voltage control. Both the theory and physical simulation results are presented. >

Stationary frame harmonic reference generation for active filter systems

Harmonic reference generators for active filter systems are commonly implemented in the synchronous reference frame, so that simple low- or high-pass filter functions can be used depending on the requirements of a particular application. High-pass filters in particular are used when the reference generator is required to produce an everything but the fundamental target waveform. This paper presents a stationary frame notch filter equivalent to a high-pass synchronous frame harmonic reference generator for such systems. The use of the stationary frame allows for quicker calculation of the reference generation in a discrete digital implementation, and also allows classical control techniques to be used to analyze the active filter system without requiring synchronous frame transformations of the outside system model. Both simulation (continuous and discrete) and experimental results showing the equivalency of the stationary and synchronous frame reference generation process are presented using both shift and delta-operator-based infinite-impulse response digital filters.

A simple, novel method for variable-hysteresis-band current control of a three phase inverter with constant switching frequency

A novel method for implementing a variable hysteresis band current controller is described which achieves constant switching frequency without requiring a precise knowledge of the motor parameters. The controller works by using feedback and feedback variables to create a variable hysteresis band envelope, and then compensating for the interaction between phase back-EMFs that occurs when the neutral of a three-phase motor is left floating. The controller has good dynamic and steady-state response, and its performance is substantially immune to variations in the inverter DC supply voltage and motor parameters. It can be readily implemented in hardware, and only requires a few additional components compared to a conventional hysteresis current controller. Analytical, hardware implementation, simulation, FFT (fast Fourier transform) analysis, and experimental results are presented.<<ETX>>

Natural Capacitor Voltage Balancing for a Flying Capacitor Converter Induction Motor Drive

This paper presents an analysis of the natural voltage balancing dynamics of a three phase flying capacitor converter when supplying an induction motor. The approach substitutes Double Fourier harmonic series for the PWM switching waveforms and the frequency response of the motor, to create a linear state space model of this type of load. The model requires the mid-frequency response (500 Hz - 20 kHz) of the induction motor impedance to be identified, and takes skin and proximity effects into account by adding parallel R-L networks to a standard motor model. Model parameters were measured by applying FFT analysis to variable frequency square waves injected into the motor terminals, and fitting parameter values to these measurements using a least squares minimisation method. From the analysis, it was found that the converter voltage balancing behavior degrades substantially at low motor speeds, and that a balance booster filter, as previously proposed, considerably improves the dynamic response. Experimental verification results using a scaled-down flying capacitor converter drive are included in the paper.

A general analytical method for determining the theoretical harmonic components of carrier based PWM strategies

The analysis of pulse width modulation schemes for switched power converters has been a major research area for several decades, and considerable effort has gone into attempting to develop analysis techniques which allow one scheme to be evaluated against another for various fundamental and carrier frequencies and at different modulation depths. Except for a limited number of analytical solutions developed for specific modulation strategies, this analysis is generally done by digital simulation of the switched waveform and subsequent FFT or performance index calculation. This approach can require substantial computing capacity, and also has a significant potential for inaccuracies caused by subtle programming errors, which may cause erroneous comparisons to be made between different PWM schemes under particular operating conditions when the performance differences are slight. This paper presents a general method for determining the theoretical harmonic components of all major known variations of PWM. The method identifies appropriate inner and outer integral limits of the double Fourier integral solution of the switched waveform to suit each modulation strategy, and then solves this double integral using Jacobi-Anger expansions to establish closed form solutions. The method is applicable irrespective of the pulse ratio between the carrier and the fundamental, and the computational requirements are essentially constant irrespective of the absolute value of the carrier frequency.