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To study the stability of a nominal cyclic steady state in power electronic converters, it is necessary to obtain a linearization around the periodic orbit. In many past studies, this was achieved by explicitly deriving the Poincare map that describes the evolution of the state from one clock instant to the next and then locally linearizing the map at the fixed point. However, in many converters, the map cannot be derived in closed form, and therefore this approach cannot directly be applied. Alternatively, the orbital stability can be worked out by studying the evolution of perturbations about a nominal periodic orbit, and some studies along this line have also been reported. In this paper, we show that Filippov's method--which has commonly been applied to mechanical switching systems--can be used fruitfully in power electronic circuits to achieve the same end by describing the behavior of the system during the switchings. By combining this and the Floquet theory, it is possible to describe the stability of power electronic converters. We demonstrate the method using the example of a voltage-mode-controlled buck converter operating in continuous conduction mode. We find that the stability of a converter is strongly dependent upon the so-called saltation matrix--the state transition matrix relating the state just after the switching to that just before. We show that the Filippov approach, especially the structure of the saltation matrix, offers some additional insights on issues related to the stability of the orbit, like the recent observation that coupling with spurious signals coming from the environment causes intermittent subharmonic windows. Based on this approach, we also propose a new controller that can significantly extend the parameter range for nominal period-1 operation. Index Terms--Bifurcation, buck converter, differential inclusions, discontinuous systems, Filippov systems, power electronics.

This brief proposes a novel controller which greatly enhances the performance of a power-factor correction converter. This controller is optimally tuned to place the eigenvalues of the system well inside the unit circle and hence it guarantees stable operation over a wide range of input voltages. The design of the controller is based on the stability analysis of the system using the state transition matrix over a clock cycle. It is shown that the transition matrix across the switching manifold greatly influences the system's performance, allowing the system to be stabilized by periodically altering the manifold. The results are validated by analytical and numerical studies. Index Terms--Bifurcation, control of power electronics, powerfactor correction (PFC).

A current sharing controller is proposed in this paper for parallel-connected converters. The proposed controller is based on the calculation of the magnitudes of system current space vectors. Good current distribution between parallel converters is achieved with only one Proportional-Integral (PI) compensator. The proposed controller is analyzed and the circulating current impedance is derived for paralleled systems. The performance of the new control strategy is experimentally verified using two parallel connected converters employing Space Vector Pulse Width Modulation (SVPWM) feeding a passive RL load and a 2.2 kW three-phase induction motor load. The obtained test results show a reduction in the current imbalance ratio between the converters in the experimental setup from 53.9% to only 0.2% with the induction motor load. [ABSTRACT FROM AUTHOR]

Grid operating conditions have a significant effect on the harmonic and resonant performance of grid-connected photovoltaic (PV) inverters and changes in grid impedance can cause a notable change in the resonant excitation between the PV inverter and the grid. This study proposes an adaptive control algorithm for grid-connected PV inverters to suppress the resonance condition excited by grid inductance variation, resulting from the dynamic changes in the operating conditions of the distribution network. The causes of resonance between grid-connected PV inverters and the distribution grid are discussed and the design of an active band-pass filter for capturing resonance is described. The proportional gain within the proportional- integral controller is then adaptively controlled in real time to compensate for changes in the grid impedance and suppress resonant excitation while maintaining excellent low-order harmonic performance compared with alternative fixed gain controller techniques, particularly for systems with high values of grid inductance. The performance of the proposed controller is experimentally verified using a 240 V, 2 kVA single-phase grid-connected inverter. [ABSTRACT FROM AUTHOR]

The chaotic response of a ferroresonant 275-kV electromagnetic voltage transformer circuit is quantified using fractal mathematics and geometry. Three fractal dimensions methods, capacity dimension, information dimension, and correlation dimension, were used to confirm the existence of chaotic ferroresonance, with all three methods giving noninteger dimension values. Index Terms--Chaos, ferroresonance, transformers.

A new type of GTO thyristor assisted tap changer is outlined which addresses the problems of traditional schemes. The scheme minimizes conduction losses in the solid state devices and also permits a fast response speed through the adoption of fast actuator driven vacuum switches. This paper describes a prototype tap changer for a low voltage 300 kVA transformer using the new method of tap changing. The prototype operates at current levels similar to those at which a full scale tap changer would work. Test results are presented. Index Terms--Tap changer, vacuum switches.