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Dynamic voltage restorers (DVRs) are now becoming more established in industry to reduce the impact of voltage sags to sensitive loads. However, DVRs spend most of their time in standby mode, since voltage sags occur very infrequently, and hence their utilization is low. In principle, it would be advantageous if the series-connected inverter of a DVR could also be used to compensate for any steady-state load voltage harmonics, since this would increase the power quality 'value-added' benefits to the grid system. However, before this can be done, consideration must be given to the control of steady-state power through the DVR, the increased losses, and the low modulation depths at which the scheme must operate to achieve acceptable harmonic compensation performance. This paper presents a selective harmonic feedback control strategy that can be easily added to medium-voltage DVR systems to provide voltage harmonic compensation capabilities with minimal effect on the sag compensation performance of the basic DVR. The proposed controller has been experimentally verified on a medium-voltage (10 kV) three-phase DVR prototype under a range of conditions, including distorted supply voltages, nonlinear loads, and operation during distorted voltage sags. Index Terms--Active filter, dynamic voltage restorer, harmonics, medium voltage, series transformer, stationary frame control.

Current regulation techniques for pulsewidth-modulated (PWM) voltage source inverters (VSIs) can be classified as either linear or nonlinear. Linear techniques consist principally of either a proportional-integral (PI) or a predictive current control strategy, while nonlinear schemes are usually based on a hysteresis strategy. Of the two linear strategies, predictive current control offers the advantages of precise current tracking with minimal distortion and can also be fully implemented on a digital platform. However, the conventional implementation of the predictive current regulation (PCR) algorithm is sensitive to noise and errors in the load inductance estimate, particularly when the back EMF is also estimated. This paper presents an improved PCR algorithm that retains all the benefits associated with PCR while achieving significantly increased robustness to load parameter mismatch and reduced zero current clamped oscillation effects. It is also relatively insensitive to noise in the sampled current measurements. The algorithm is equally applicable to variable fundamental frequency applications such as variable speed drives and to fixed fundamental frequency applications such as PWM rectifier systems or active filters. Simulation and experimental results are presented to confirm the improved robustness of the new algorithm. Index Terms--Digital current control, predictive current regulation, pulsewidth-modulated inverters.

The static series compensator (SSC) has shown a significant capability to mitigate voltage dips, which are the most severe problem to sensitive loads. Also, it has been declared that the function of the SSC can be extended to work as a series active filter. This paper proposes a moving-average filter to detect the fundamental component of the measured voltages and currents (needed to control the SSC) while using a double vector control algorithm to improve the transient performance of the SSC. This is made in order to accurately control the fundamental voltage component at the load terminals in the case of distorted grid voltage. Furthermore, a selective harmonic compensation strategy is applied to filter out the grid harmonics. The operation of the SSC under distorted utility conditions and voltage dips is discussed. The validity of the proposed controller is verified by experiments, carried out on a 10-kV SSC laboratory setup. Experimental results have shown the ability of the SSC to mitigate voltage dips and harmonics. It is also shown that the proposed controller has improved the transient performance of the SSC even under distorted utility conditions. Index Terms--Active filters, dynamic voltage restorer, moving average filter, resonant filter, static compensator.

Most of the many reported control algorithms for uninterruptible power supplies (UPSs) use either filter inductor or filter capacitor currents as feedback variables to regulate the output voltage. This paper explores the fundamental performance issues associated with the use of these quantities as feedback variables, with a view to determining their contribution to the transient system response and output harmonic compensation in any particular situation. A proportional plus resonant (P + resonant) compensator is then added into the outer voltage regulation loop to achieve zero steady error, to develop a high performance UPS control algorithm, which is applicable to both single-phase and three-phase systems. Theory, simulation, and experimental results are presented in the paper. Index Terms--Current control, harmonic impedance, PID, transient analysis, UPS, THD, voltage control.

Most infinite impulse response (IIR) digital filter implementations in power electronic inverter applications are based on the time shift operator q and its associated z-transform. But for higher sampling frequencies where the sample period approaches zero for z-transform discrete systems, their dynamic response does not converge smoothly to the continuous counterpart, causing substantial implementation problems. In contrast, the response of filters based on the delta operator does converge to the continuous counterpart for smaller sample periods, and hence they are much better suited for digital control applications where sampling frequencies are much higher than the system poles. This paper describes the basis of the delta operator, its use for IIR digital filter systems, and shows how the technique can be used in power electronic inverter applications to achieve substantial performance benefits compared to equivalent shift-based implementations. A brief review of shift based IIR filtering is presented and the required conversions to the delta form given. The specific examples of an active filter and a P + Resonant current regulator are used to illustrate the improvements that can be expected. The superior performance of the delta operator for digital control of inverter applications has been verified in both simulation and experiment. Index Terms--Delta operator, digital filters, IIR, power electronics, shift operator.

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. Index Terms--Active filter, delta operator, reference frame, signal extraction, stationary, synchronous.

This paper presents an integrated scheme for protecting and controlling series injection voltage-source inverters used with electrical grid systems. The scheme uses a number of strategies to provide a continuous current path for current from the secondary winding of the injection transformer for both transient and longer term fault conditions. These strategies can also be used to provide a current path during normal operation of the external ac system, facilitating startup and shutdown of the series injection system, as well as recovery from a fault. Both simulation and experimental confirmation of the integrated protection scheme are presented. Index Terms--Current transformer, protection, series, varistor, voltage-source inverter.

The potential for high force and low inertia makes the long stator linear induction motor a good candidate for use in aircraft launch. These motors typically have a segmented stator that runs the entire length of travel. A conductive plate is used as the moving shuttle. If high reliability is desired, one approach is to drive shuttle plate using multiple, separately excited stators. Although the stators are separately excited, they are still coupled via mutual inductance and common current paths in the shuttle plate. This paper considers the development of an indirect vector control scheme that accommodates the cross-coupling between stators and allows responsive control of the total force on the shuttle in the presence of faulted stators. [ABSTRACT FROM AUTHOR]

Presents a comparative analysis of multiloop voltage regulation strategies for single and three-phase uninterruptible power supplies (UPS) systems. Alternative control structures for a line interactive UPS system; Mathematical analysis; Simulation investigations; Experimental results.