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

Generation of electricity using wind power has received considerable attention worldwide in recent years. In order to investigate the impacts of the integration of wind farm into utilities' network various windmill models have been developed. One such impact is related to the critical clearing time (CCT) of the wind power based embedded generators (WPBEG). The work in this paper has shown that oversimplification of the modeling of windmill mechanical drive train could introduce significant error in the value of the CCT that defines the stability limit of an integrated wind farm. This paper also reports investigation into the factors that influence the dynamic behavior of the WPBEGs following network fault conditions. It is shown that wind farm CCT can be affected by various factors contributed by the host network. Results obtained from several case studies are presented and discussed. This investigation is conducted on a simulated grid-connected wind farm using EMTP.

Windmill Modeling Consideration and Factors Influencing the Stability of a Grid-Connected Wind Power Based Embedded Generator

Transient stability of wind generation system is evaluated in terms of critical clearing time (CCT) of fault. CCT of a wind generation system connected to grid is calculated based on the maximum amount of time that the wind generation system remains connected to grid, without becoming unstable, when the grid is subjected to a fault. In the literature, the wind turbine model has been used for steady state analysis alone, that is, it is used for finding the initial operating mechanical torque output of the turbine. The turbine model is then completely neglected in case of transient stability analysis by assuming constant mechanical torque output for a particular wind speed. This may not be true, as the mechanical torque-speed characteristic of a wind turbine depends not only on the wind speed but also on the slip of the induction generator connected to the wind turbine. This paper explains the necessity of considering the wind turbine model for the transient stability and steady state analysis of grid connected induction generator based wind generation system. A detailed analysis has been done on the effect of various electrical and mechanical factors on CCT determination and the simulation results are presented.

Effect of Modeling of Induction Generator Based Wind Generating Systems on Determining CCT

This paper discusses the utilization of complex continuous wavelet transform based on B-spline function for reference voltages and currents extraction which are used for compensating power quality disturbances by using unified power quality conditioner. The major advantages of choosing B-spline function are the reduction of computation time and ease of online implementation. The performance of the proposed wavelet transform is compared with the discrete Meyer wavelet and complex Morlet wavelet transforms. Detailed simulation studies are carried out using Matlab/Simulink to prove the ability of the proposed method. It is inferred from the simulation results that the proposed B-spline complex wavelet transform based reference signal extraction is as accurate as complex Morlet wavelet transform but has less computational burden.

Complex wavelet based control strategy for UPQC

In this paper, a new methodology is proposed to mitigate unbalanced voltage sag with phase jumps by Unified Power Quality Conditioner (UPQC) with minimum real power injection. Particle Swarm Optimization (PSO) has been used to find the solution of the objective function derived for minimizing real power injection of UPQC along with the specified constraints. The proposed method is validated through detailed simulation studies.

Power quality improvement by UPQC with minimum real power injection

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

Multilevel neutral point clamped inverter systems are increasingly used in load compensation applications. But, the most significant problem associated with these compensators is the capacitor voltage imbalances and drift due to dc components in the zero sequence current, resulting in degradation of tracking performance of the voltage source inverter. This paper proposes a carrier based PWM control for an inverter-chopper circuit in order to regulate the capacitor voltages to their reference values. To demonstrate the simplicity and effectiveness of the above control scheme, a three-phase four-wire, three-level neutral point clamped compensator system is taken as an example. Detailed simulation has been carried out in PSCAD 4.2.1. Experiments are conducted to validate the proposed control scheme.

B. Kalyan Kumar

Papers

Effect of Modeling of Induction Generator Based Wind Generating Systems on Determining CCT

Complex wavelet based control strategy for UPQC

Power quality improvement by UPQC with minimum real power injection

A robust controller for DSTATCOM

Capacitor voltage balancing in neutral clamped inverters for DSTATCOM application