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

For understanding the dynamic behavior and assessing the stability of induction generator based wind generation system, the system should be modeled accurately. Power system transient stability is usually assessed in terms of critical clearing time (CCT) of fault. CCT determines the time for which the wind generating systems can be connected to the grid, when the grid is subjected to a fault. In the literature, CCT of induction generator based wind generation system connected to the grid was determined by assuming the mechanical torque output of wind turbine as constant throughout the fault simulation. However, while determining CCT, the mechanical torque output of wind turbine, before and after the fault cannot be considered as a constant as the mechanical torque-speed characteristic of a wind turbine depends on the wind speed and the slip of the induction generator connected to the wind turbine. During fault, the slip of the induction generator changes and hence mechanical torque output of wind turbine also changes accordingly. The results of CCTs obtained by considering nonlinear mechanical torque-speed characteristic of wind turbine are presented and are compared with CCTs obtained with the mechanical torque output of wind turbine assumed constant.

Effect of modeling of induction generator based wind generating systems on CCT

Inter-area oscillations can be identified by analyzing the signals obtained from Phasor Measurement Units (PMUs) dispersed across the power network. In this paper, signals obtained from PMU are decomposed into monocomponent signals by a filter bank based on Zolotarev polynomials. A narrow bandwidth of 0.1 Hz is designed in the range of 0 to 1 Hz using Zolotarev polynomial based filter bank. Modal frequency and damping are estimated from the decomposed monocomponent signals through Eigen Realization Algorithm (ERA). The proposed method is demonstrated on two-area test system and the simulation results are presented.

http://ieeexplore.ieee.org/iel7/7368725/7380859/07380976.pdf

Identification of inter-area oscillations using Zolotarev filter bank with eigen realization algorithm

This paper proposes a control strategy for enabling voltage fault ride-through (VFRT) operation with adaptive dynamic voltage support (DVS) scheme using the energy storage in a microgrid. Firstly, the condition for maximum DVS from the microgrid is presented and its dependency on the parameters of the equivalent network seen by the microgrid is elucidated. Considering the impediments associated with the real-time parameter estimation during the short-term voltage disturbances, reference current signals based on the extreme voltage tracking algorithm are proposed in this paper to achieve an adaptive DVS irrespective of the varying system conditions. Secondly, a control strategy is proposed to enable the energy storage-based VFRT operation of a microgrid. Thirdly, a unique methodology of sizing the energy storage for VFRT operation is presented considering the grid code requirements, equivalent network characteristics, and the maximum current rating of the power electronic interface with the grid. The proposed control strategy is validated through real-time digital simulation case studies for low-voltage, high-voltage, and multiple-fault scenarios. The scalability of the proposed approach is illustrated using the CIGRE distribution test network.

B. Kalyan Kumar

Papers

Effect of modeling of induction generator based wind generating systems on CCT

Identification of inter-area oscillations using Zolotarev filter bank with eigen realization algorithm

Adaptive Dynamic Voltage Support Scheme for Fault Ride-Through Operation of a Microgrid