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 introduces a new concept of optimal utilization of a unified power quality conditioner (UPQC). The series inverter of UPQC is controlled to perform simultaneous 1) voltage sag/swell compensation and 2) load reactive power sharing with the shunt inverter. The active power control approach is used to compensate voltage sag/swell and is integrated with theory of power angle control (PAC) of UPQC to coordinate the load reactive power between the two inverters. Since the series inverter simultaneously delivers active and reactive powers, this concept is named as UPQC-S (S for complex power). A detailed mathematical analysis, to extend the PAC approach for UPQC-S, is presented in this paper. MATLAB/SIMULINK-based simulation results are discussed to support the developed concept. Finally, the proposed concept is validated with a digital signal processor-based experimental study.

UPQC-S: A Novel Concept of Simultaneous Voltage Sag/Swell and Load Reactive Power Compensations Utilizing Series Inverter of UPQC

This paper presents a novel philosophy to compensate the load-reactive power demand through a three-phase unified power quality conditioner (UPQC). Most of the UPQC-based applications show the dependency on shunt inverter for load-reactive power compensation, whereas the series inverter is always looked as controlled voltage source to handle all voltage-related problems. This paper proposes a new functionality of UPQC in which both the shunt and series APFs supply the load-reactive power demand. This feature not only helps to share the load-reactive power demand, but also helps to reduce the shunt APF rating, and hence, the overall cost of UPQC. This results in better utilization of the existing series inverter. The theory and complete mathematical analysis termed as ldquopower angle control (PAC)rdquo is presented. The simulation results based on MATLAB/Simulink are discussed in detail to support the concept developed in the paper. The proposed approach is also validated through experimental study.

A New Control Philosophy for a Unified Power Quality Conditioner (UPQC) to Coordinate Load-Reactive Power Demand Between Shunt and Series Inverters

The paper discusses load compensation using a distribution static compensator (DSTATCOM). It is assumed that the DSTATCOM is associated with a load that is remote from the supply. It is shown that the operation of a DSTATCOM assuming that it is connected to a stiff source in such situations will result in distortions in source current and voltage at the point of common coupling. To avoid this, the DSTATCOM is connected in parallel with a filter capacitor that allows the high frequency component of the current to pass. However, this generates control issues in tracking, as standard controls such as a hysteresis control are not suitable in these circumstances. This paper proposes a new switching control scheme and demonstrates its suitability for this problem. It also proposes a scheme in which the fundamental sequence components of a three-phase signal can be computed from its samples. The overall performance of the proposed scheme is verified using digital computer simulation studies.

Load compensating DSTATCOM in weak AC systems

In this paper, the application of a cascaded multilevel inverter as a dynamic voltage restorer (DVR) is investigated. Two discontinuous multilevel space vector modulation (SVM) techniques are implemented for DVR control and are shown to reduce inverter switching losses while maintaining virtually the same harmonic performance as the conventional multilevel SVM at a high number of levels. This paper also presents a mathematical relationship for computing the distortion at the point of common coupling (PCC) as a function of the distortion of the DVR. This enables the selection of the number of levels required for a certain application. An extended sag duration support compared to the two-level DVR is another advantage of the DVR with a cascaded multilevel inverter. The common-mode voltage (CMV) at the PCC has been evaluated for the three SVM techniques (the conventional multilevel SVM and the two discontinuous SVM), presenting a lower CMV for the second discontinuous SVM. A design example is presented for an 11-kV 5-MVA DVR multilevel cascaded inverter for up to 17 levels, employing the conventional multilevel SVM and the two discontinuous SVM techniques.

Evaluation of a Multilevel Cascaded-Type Dynamic Voltage Restorer Employing Discontinuous Space Vector Modulation

The Unified Power Quality Conditioner (UPQC) is a versatile device which could function as series active filter and shunt active filter. UPQC can simultaneously fulfill different objectives like, maintaining a balanced sinusoidal (harmonic free) nominal voltage at the load bus, eliminating harmonics in the source currents, load balancing and power factor correction. Keeping the cost effectiveness of UPQC, it is desirable to have a minimum VA loading of the UPQC, for a given system without compromising compensation capability. Hence, a methodology has been proposed in this work to minimize VA loading of UPQC considering the effects of source voltage and load current distortion. The series active filter of UPQC can be made to inject voltages in such a way that a minimum VA loading of UPQC is possible. The optimal angle at which series voltage has to be injected so as to have minimum VA loading of UPQC is computed by Particle Swarm Optimization (PSO) technique. The proposed method has been validated through detailed simulation studies. The VA loading without optimization and with optimization has been compared.

Minimization of VA loading of Unified Power Quality Conditioner (UPQC)

This paper discusses design and performance issues of three design techniques for DSTATCOM controller with an aim to improve the robustness. The state feedback controller of DSTATCOM has been designed using Linear Quadratic Regulator (LQR), Linear Quadratic Regulator with prescribed degree of stability and Particle Swarm Optimization (PSO). Weak ac source and unbalanced nonlinear load are considered. The performance of the controller designed through the three methodologies has been compared under different operating conditions. Particle Swarm Optimization based controller is more robust and is suitable for a DSTATCOM working under uncertain parametric conditions like feeder impedance and load that are most common in a distribution system. This controller ensures stability under all possible operating conditions and has superior performance over the other two controller design techniques. The performance of the DSTATCOM controller designed through the three different methodologies is verified by detailed simulation studies.

A robust controller for DSTATCOM

Shunt compensating devices like distribution static compensator (DSTATCOM) are extensively used for compensating harmonics/unbalance/voltage sag-swell in a power distribution system. The design and performance of DSTATCOM for load compensation with stiff and non-stiff sources have been well reported in literature. Using linear control strategies like linear quadratic regulator (LQR), feedback controllers have been designed for DSTATCOM to track the reference currents for effective compensation under non-stiff source. There are many uncertain parameters like the feeder impedance, load impedance and harmonic content in load currents etc. Due to the parametric uncertainties, feedback controller designed using LQR may not be robust to these uncertainties. In this paper, a particle swarm optimisation(PSO)-based feedback controller is designed for DSTATCOM for load compensation. This feedback controller has optimal performance in various operating conditions and is robust to parametric uncertainties. The performance of the proposed feedback controller is compared with that of the conventional feedback controller designed with LQR.

P. Harsha Vardhana

Papers

Minimization of VA loading of Unified Power Quality Conditioner (UPQC)

A robust controller for DSTATCOM

PSO-based feedback controller design of DSTATCOM for load compensation with non-stiff sources