This paper discusses and compares phase-shifted pulse-width modulation (PS-PWM) and level-shifted PWM (LS-PWM) in a modular multilevel single-delta bridge-cell (SDBC) inverter for utility-scale grid-tied photovoltaic applications. This three-phase SDBC inverter provides the capability of operating even under power-distribution imbalances not only among three clusters, but also among cascaded bridge cells in each cluster. The main focus of this research lies in analyzing the capability of operation under severe power-distribution imbalances and presenting power-balancing methods based on zero-sequence current injection, which result in an extended power-balancing capability. This paper highlights that LS-PWM is superior to PS-PWM in both power-balancing capability and harmonic performance at the ac side of the SDBC inverter when operating under power-distribution imbalances. Experimental results obtained from a three-phase 10-kW downscaled system show good agreement with those from the theoretical analysis.
A control scheme is presented for compensating the torque transfer function of a self-controlled synchronous motor without damper windings. In effect, the armature current phase and amplitude are altered during transients to compensate for the slow response of the field current. The system also is arranged to come to unity power factor in the steady state which requires that the field current be adjusted to each specific load point by the controller. The validity of the control scheme is verified by the experiments.
Trends and future prospects in active power line conditioners using pulse-width-modulation inverters are presented with the focus on practical applications. Attention is given to shunt active power line conditioners and to two combined systems of active power line conditioners and shunt passive filters, which are classified into a combined system with shunt active conditioners, and with series active conditioners.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
This paper presents the modular multilevel cascade converter based on double-star chopper-cells, which is intended for grid connection to medium-voltage power systems without using line-frequency transformers. The converter is characterized by a modular arm structure consisting of cascade connection of multiple bidirectional pulsewidth modulation chopper-cells with floating dc capacitors. This arm structure requires voltage-balancing control of all the dc capacitors. However, the voltage control combining an averaging control with an individual-balancing control imposes certain limitations on operating conditions. This paper proposes an arm-balancing control to achieve voltage balancing under all the operating conditions. The validity of the arm-balancing control as well as the theory developed in this paper is confirmed by computer simulation and experiment.
This paper deals with a curious phenomenon referred to as the "whack-a-mole" that may occur in a long-distance distribution feeder having many capacitors for power-factor correction. The idea of whack-a-mole is that installation of an active or passive filter on the feeder makes voltage harmonics increase on some buses, whereas it makes voltage harmonics decrease on other buses, especially at the point of installation. The distributed-parameter representation is applied to a simplified feeder, thus making it possible to perform analysis of the whack-a-mole. As a result, this analysis yields such a basic way as to avoid the whack-a-mole. Moreover, both theory and experiment clarify that installation of the active filter acting as a harmonic terminator on the end bus of the feeder can damp out harmonic propagation throughout the feeder without causing any whack-a-mole.
No abstract is provided for this article.
A control scheme of a high-frequency high-power current source inverters using static induction transistors is described which aims at the suppression of the surge voltage and reduction of the switching loss during the commutation of current. The inverter is operated at a leading power factor, which requires the phase angle of the output current to be adjusted to each specific load point by the controller. The stable operation is verified by the experiments under the commutation inductance 1.8 mu H, i.e., 18% reactance (130 kHz, 250 V, 30 A, base). As a result, the inverter efficiency is estimated to be 97%, excluding the loss of the drive circuit and the control circuit.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
This paper deals with a 50-MW self-commutated Back-To-Back (BTB) system intended for power-flow control between transmission networks. It focuses on the dynamic behavior of the BTB system during single-line-to-ground (SLG) faults that often result in voltage sags. A non-negligible amount of dc magnetic flux deviation appears in the converter-transformers of the BTB system just after the occurrence and restoration of an SLG fault. The dc deviation may bring magnetic flux saturation and an excessive magnetizing current to the transformers. This paper derives theoretical equations related to the dc deviation with reasonable approximation. The theoretical results are confirmed by computer simulation.
This paper deals with a hybrid shunt active filter for harmonic compensation of a three-phase diode rectifier in a 480-V adjustable-speed motor drive system. The hybrid filter is formed by a three-phase LC filter tuned around the 7-th harmonic frequency and a small-rated three-phase voltage-source PWM inverter. The LC filter and PWM inverter are directly connected in series. As a result, the dc capacitor voltage of the PWM inverter in the hybrid filter is much lower than that of a conventional pure shunt active filter. This results in higher efficiency, less switching ripple and less EMI emission. Computer simulation is carried out to compare the hybrid filter with the pure filter in terms of circuit configuration and filtering performance. Simulation results indicate that the hybrid filter is superior in effectiveness and viability to the pure filter. In addition, theoretical analysis based on vector loci confirms the validity of the simulation results.
This paper describes transient characteristics of a UPFC and its performance of suppressing power flow variation in a power transmission system having two parallel lines. The UPFC is installed in series with the two parallel lines and it controls the total power flow of the lines. Dynamic performance caused by a single line trip is theoretically and experimentally discussed. Experimental results tells us that a conventional control may produce an excessive power flow variation at the instant of the trip. On the other hand, fast control scheme which has been proposed by the authors shows good performance of power flow control without any variation not only in steady state but also in transient state.
No abstract is provided for this article.
The emergence of power semiconductor devices such as insulated-gate bipolar transistors (IGBTs) or injection-enhanced gate transistors (IEGTs) and gate-commutated turn-off (GCT) thyristors or integrated gate-commutated thyristors (IGCTs) enables power conversion systems to expand into utility and industry applications. Much attention has been paid to power switching devices based on wide-bandgap semiconductors such as silicon carbide(SiC). These devices are expected to have the following advantages over silicon devices; low switching and conduction losses, especially in power conversion systems operating as high as, or exceeding 600 V. This paper describes trends in power electronics and motor drives, including the personal views and expectations of the author.
The conventional reactive power in single-phase or three- phase circuits has been defined on the basis of the average value concept for sinusoidal voltage and current waveforms in steady states. The instantaneous reactive power in three-phase circuits is defined on the basis of the instantaneous value concept for arbitrary voltage and current waveforms, including transient states. A new instantaneous reactive power compensator comprising switching devices is proposed which requires practically no energy storage components.
