Solid-state switch-mode rectification converters have reached a matured level for improving power quality in terms of power-factor correction (PFC), reduced total harmonic distortion at input AC mains and precisely regulated DC output in buck, boost, buck-boost and multilevel modes with unidirectional and bidirectional power flow. This paper deals with a comprehensive review of improved power quality converters (IPQCs) configurations, control approaches, design features, selection of components, other related considerations, and their suitability and selection for specific applications. It is targeted to provide a wide spectrum on the status of IPQC technology to researchers, designers and application engineers working on switched-mode AC-DC converters. A classified list of more than 450 research publications on the state of art of IPQC is also given for a quick reference.

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A review of single-phase improved power quality AC-DC converters

This paper presents a comprehensive review on the unified power quality conditioner (UPQC) to enhance the electric power quality at distribution levels. This is intended to present a broad overview on the different possible UPQC system configurations for single-phase (two-wire) and three-phase (three-wire and four-wire) networks, different compensation approaches, and recent developments in the field. It is noticed that several researchers have used different names for the UPQC based on the unique function, task, application, or topology under consideration. Therefore, an acronymic list is developed and presented to highlight the distinguishing feature offered by a particular UPQC. In all 12 acronyms are listed, namely, UPQC-D, UPQC-DG, UPQC-I, UPQC-L, UPQC-MC, UPQC-MD, UPQC-ML, UPQC-P, UPQC-Q, UPQC-R, UPQC-S, and UPQC-VA. More than 150 papers on the topic are rigorously studied and meticulously classified to form these acronyms and are discussed in the paper.

Enhancing Electric Power Quality Using UPQC: A Comprehensive Overview

This paper proposes predictive direct power control (P-DPC), a new control approach where the well-known direct power control is combined with predictive selection of a voltage-vectors' sequence, obtaining both high transient dynamics and a constant-switching frequency. The developed P-DPC version is based on an optimal application of three voltage vectors in a symmetrical way, which is the so-called symmetrical 3 + 3 vectors' sequence. The simulation and experimental results of the P-DPC are compared to standard voltage-oriented control (VOC) strategies in a grid-connected three-phase voltage-source inverter under 400-V 15-kVA operation conditions. The P-DPC improves the transient response and keeps the steady-state harmonic spectrum at the same level as the VOC strategies. Due to its high transient capability and its constant-switching behavior, the P-DPC could become an interesting alternative to standard VOC techniques for grid-connected converters

Predictive Control Strategy for DC/AC Converters Based on Direct Power Control

Dynamic voltage restorers (DVRs) are used to protect sensitive loads from the effects of voltage sags on the distribution feeder. This paper presents and verifies a novel voltage sag detection technique for use in conjunction with the main control system of a DVR. In all cases it is necessary for the DVR control system to not only detect the start and end of a voltage sag but also to determine the sag depth and any associated phase shift. The DVR, which is placed in series with a sensitive load, must be able to respond quickly to a voltage sag if end users of sensitive equipment are to experience no voltage sags. A problem arises when fast evaluation of the sag depth and phase shift is required, as this information is normally embedded within the core of a main DVR control scheme and is not readily available to either users monitoring the state of the grid or parallel controllers. Previous research presented an additional controller, which required phase and sag depth information to manipulate the injection voltage vector returned by the main controller in order to prevent the DVR injection transformers from saturating. Typical standard information tracking or detection methods such as the Fourier transform or phase-locked loop (PLL) are too slow in returning this information, when either applied to the injection voltage vector, or to the supply voltages directly. As a result of this the voltage sag detection method in this paper proposes a new matrix method, which is able to compute the phase shift and voltage reduction of the supply voltage much quicker than the Fourier transform or a PLL. The paper also illustrates that the matrix method returns results that can be directly interpreted, whereas other methods such as the wavelet transform return results that can be difficult to interpret.

Voltage sag detection technique for a dynamic voltage restorer

This paper proposes an improved model predictive direct power control (MPDPC) for a pulse width modulation (PWM) rectifier by using a duty cycle control. The conventional MPDPC achieves good steady-state performance and quick dynamic response by selecting the best voltage vector, which minimizes the errors between the reference power and the real power. However, due to the limited number of voltage vectors in a two-level converter, the sampling frequency has to be high to achieve satisfactory performance. This paper introduces the concept of a duty cycle control in the MPDPC by allocating a fraction of control period for a nonzero voltage vector and the rest time for a zero vector. The nonzero vector is selected by evaluating the effects of each nonzero vector and its duration is obtained based on the principle of power errors minimization. Simulation and experimental results prove that, compared to the conventional MPDPC, the proposed MPDPC with duty cycle achieves further steady-state performance improvement without affecting the dynamic response at a small cost of control complexity increase.

Model Predictive Direct Power Control of a PWM Rectifier With Duty Cycle Optimization

This paper develops a direct power control structure to improve the performance of active filters. The PWM switching functions are defined based on the instantaneous power terms and the bandwidths of the two hysteresis comparators are dynamically adjusted to limit the averaged switching frequency. The controller directly uses the instantaneous power as control variables, to replace the current and voltage variables used in multi-loop control systems. It demonstrates that full control of the active filter, including the line current and the dc-bus voltage, can be realized within an integrated control loop. The advantages of the proposed control strategy are verified by simulation and experimental results on a 2 kVA laboratory prototype.

Direct power control of active filters with averaged switching frequency regulation

The growing concerns regarding electric power quality and availability have lead to the investigation of solutions to eliminate or mitigate the problems created on critical loads by faults in distribution systems. Series and shunt active power conditioners have been proposed and used for this purpose. This paper discusses and compares the potential of D-STATCOMs and DVRs to provide these functions. It introduces a compensator rating factor which defines the ability of the compensator to support the load voltage in the presence of single and three phase faults. The algorithms required to carry out voltage support are derived and alternatives discussed, including the use of sequence components and direct voltage regulation. New approaches are proposed. Similarities and differences between shunt and series compensators are determined. It is shown that the series compensator is more effective in handling symmetrical and asymmetrical faults. Simulation and experimental results are provided to illustrate the transient performance of the compensator and their ability to support load voltages.

Series and shunt active power conditioners for compensating distribution system faults

Static shunt compensation, based on PWM synchronous voltage sources, is becoming a viable solution for improving power quality in distribution systems, including voltage regulation at the load bus. This paper describes the operating principles and control techniques of PWM STATCOMs under nonideal conditions including faults. Negative and zero sequence components are taken into account, and compensator response time is addressed. Two voltage regulation schemes are proposed and evaluated: the first is the direct voltage control scheme, with a parallel current limiting loop; the second scheme is based on the current controlled voltage source inverter concept, which decouples the voltage and current control functions. Steady state and transient performance obtained on an experimental STATCOM prototype are given for balanced, unbalanced and fault conditions. It is demonstrated that both the proposed schemes can satisfactorily operate under most operating conditions found in distribution systems. The inner current loop scheme enhances system performance, however, the system dynamic response may exhibit instability under certain conditions.

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Dynamic performance of PWM STATCOMs operating under unbalance and fault conditions in distribution systems

Distribution static synchronous compensators (DSTATCOMs) are a viable solution for improving power quality in distribution systems, including mitigation for temporary interruptions, voltage dips, harmonics and voltage flicker. Conventional DSTATCOMs are implemented using two cascaded loops, an AC current control loop and a DC-bus voltage loop. This paper proposes an alternative control structure based on the simultaneous control of line current and DC-link voltage by means of instantaneous power control. The instantaneous real and imaginary power calculation is derived and explained. Requirements for flicker and harmonic mitigation are identified. The controller is implemented using two hysteresis comparators and an optimum switching table. The features of the proposed technique are illustrated by means of simulation and experimental results. Advantages include a simple and robust control structure, and better tracking of load fluctuations and/or harmonics.

Direct power control of DSTATCOMs for voltage flicker mitigation

Power quality and availability are becoming important issues for critical and sensitive loads. This paper presents a viable alternative to the use of uninterruptible power supplies solutions for three-phase load voltage support. This support is provided by a series compensator structure with a rating equal to a fraction of the load. The paper discusses control issues and proposes a control algorithm that results in a fast dynamic response and is insensitive to the type of fault and to disturbances and harmonics present on the power network. The technique is applied to a faulted distribution system to compensate for voltage sags. The steady state compensation capability of the scheme is derived. The dynamic performance is analyzed and verified through experimental tests on a 2 kVA prototype power conditioner. Practical issues, such as power and control circuit design and converter ratings are considered.

Control algorithms for series static voltage regulators in faulted distribution systems

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