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This paper presents a transient analysis of a unified power flow controller (UPFC), and design of capacitance of the dc-link capacitor. Active power flowing out of the series device in transient states is theoretically discussed to derive what amount of electric energy the dc link capacitor absorbs or releases through the series device. As a result, it is clarified that the active power flowing out of the series device is stored in the line inductance as magnetic energy during transient states. Design of capacitance of the dc-link capacitor is also presented in this paper, based on the theoretical analysis. Experimental results obtained from a 10-kVA laboratory setup are shown to verify the analytical results. Index Terms--DC-link capacitors, dc-voltage regulation, transient analysis, unified power flow controllers.

This paper presents a simple method for reduction of switching and snubbing losses in a large-capacity static var compensator (SVC) consisting of multiple three-phase voltage-source square-wave inverters. The proposed method is characterized by a 'commutation capacitor' connected in parallel with each switching device. The commutation capacitor allows the SVC to perform zero-voltage switching, and to reduce switching losses. The electric charge stored in the commutation capacitor is not dissipated, but regenerated to the dc-link capacitor. Moreover, a soft-starting method for the SVC is also presented to avoid forming a short circuit across the commutation capacitor during startup. Experimental results obtained from a 10-kvar laboratory setup are shown to verify the viability of the operating principle of the commutation capacitor. Index Terms--Reactive power, soft switching, static var compensators, zero-voltage switching.

This paper proposes a hybrid active filter for damping of harmonic resonance in industrial power systems. The hybrid filter consists of a small-rated active filter and a 5th-tuned passive filter. The active filter is characterized by detecting the 5th-harmonic current flowing into the passive filter. It is controlled in such a way as to behave as a negative or positive resistor by adjusting a feedback gain from a negative to positive value, and vice versa. The negative resistor presented by the active filter cancels a positive resistor inherent in the passive filter, so that the hybrid filter acts as an ideal passive filter with infinite quality factor. This significantly improves damping the harmonic resonance, compared with the passive filter used alone. Moreover, the active filter acts as a positive resistor to prevent an excessive harmonic current from flowing into the passive filter. Experimental results obtained from a 20-kW laboratory model verify the viability and effectiveness of the hybrid active filter proposed in this paper. Index Terms--Active filters, harmonic resonance, power quality, power systems, PWM inverters, voltage harmonics.

This paper presents a control scheme and comprehensive analysis for a unified power flow controller (UPFC) on the basis of theory, computer simulation, and experiment. This developed theoretical analysis reveals that a conventional power-feedback control scheme makes the UPFC induce power fluctuation in transient states. The conventional control scheme cannot attenuate the power fluctuation, and so the time constant of damping is independent of active- and reactive-power feedback gains integrated in its control circuit. This paper proposes an advanced control scheme which has the function of successfully damping out the power fluctuation. A UPFC rated at 10 kVA is designed and constructed, which is a combination of a series device consisting of three single-phase pulsewidth modulation (PWM) converters and a shunt device consisting of a three-phase diode rectifier. Although the dynamics of the shunt device are not included, it is possible to confirm and demonstrate the performance of the series device. Experimental results agree well with both analytical and simulated results and show viability and effectiveness of the proposed control scheme. Index Terms--High-power PWM converters, power feedback control, power fluctuation, unified power flow controllers.