This paper deals with improving the voltage quality of sensitive loads from voltage sags using a dynamic voltage restorer (DVR). The higher active power requirement associated with voltage phase jump compensation has caused a substantial rise in size and cost of the dc link energy storage system of DVR. The existing control strategies either mitigate the phase jump or improve the utilization of dc link energy by the following: 1) reducing the amplitude of the injected voltage or 2) optimizing the dc bus energy support. In this paper, an enhanced sag compensation strategy is proposed, which mitigates the phase jump in the load voltage while improving the overall sag compensation time. An analytical study shows that the proposed method significantly increases the DVR sag support time (more than 50%) compared with the existing phase jump compensation methods. This enhancement can also be seen as a considerable reduction in dc link capacitor size for new installation. The performance of the proposed method is evaluated using simulation study and finally verified experimentally on a scaled laboratory prototype.

An Enhanced Voltage Sag Compensation Scheme for Dynamic Voltage Restorer

Voltage sag distributions caused by faults at different voltage levels and experienced by low-voltage customers were established for four different power system areas. The shares of different fault types at each voltage level and the sag propagation throughout the power system were taken into account. The results show that the origin of sags in urban and rural areas tends to be different. These data are needed when, for example, planning measures for sag mitigation in different parts of the power system.

Voltage sag distributions caused by power system faults

The dynamic voltage restorer (DVR) is one of the modern devices used in distribution systems to protect consumers against sudden changes in voltage amplitude. In this paper, emergency control in distribution systems is discussed by using the proposed multifunctional DVR control strategy. Also, the multiloop controller using the Posicast and P+Resonant controllers is proposed in order to improve the transient response and eliminate the steady-state error in DVR response, respectively. The proposed algorithm is applied to some disturbances in load voltage caused by induction motors starting, and a three-phase short circuit fault. Also, the capability of the proposed DVR has been tested to limit the downstream fault current. The current limitation will restore the point of common coupling (PCC) (the bus to which all feeders under study are connected) voltage and protect the DVR itself. The innovation here is that the DVR acts as a virtual impedance with the main aim of protecting the PCC voltage during downstream fault without any problem in real power injection into the DVR. Simulation results show the capability of the DVR to control the emergency conditions of the distribution systems.

A New Approach to Multifunctional Dynamic Voltage Restorer Implementation for Emergency Control in Distribution Systems

This paper deals with the protection of critical loads from voltage-related power quality issues using a dynamic voltage restorer (DVR). A generalized control algorithm based on instantaneous space phasor and dual $P$ - $Q$ theory has been proposed to generate the instantaneous reference voltages to compensate the load voltages with direct power flow control. The proposed algorithm adapts energy-optimized series voltage compensation, which results in a reduction of energy storage requirement. The proposed DVR control scheme can support the load from voltage-related power quality issues irrespective of the load current profile. Each leg of the three-phase three-leg split capacitor inverter is used to inject series compensation voltage in respective phases of the system. Model-based computer simulation studies and real-time experimental results validate the effectiveness of the proposed control algorithm.

Dual $P$ - $Q$ Theory Based Energy-Optimized Dynamic Voltage Restorer for Power Quality Improvement in a Distribution System

This paper presents a new system configuration for integrating a grid-connected photovoltaic (PV) system together with a self-supported dynamic voltage restorer (DVR). The proposed system termed as a “six-port converter,” consists of nine semiconductor switches in total. The proposed configuration retains all the essential features of normal PV and DVR systems while reducing the overall switch count from twelve to nine. In addition, the dual functionality feature significantly enhances the system robustness against severe symmetrical/asymmetrical grid faults and voltage dips. A detailed study on all the possible operational modes of six-port converter is presented. An appropriate control algorithm is developed and the validity of the proposed configuration is verified through extensive simulation as well as experimental studies under different operating conditions.

Integrated Photovoltaic and Dynamic Voltage Restorer System Configuration

This paper presents a summary of the work performed by the authors on voltage sag analysis using a time-domain tool. The document describes a procedure for stochastic prediction of voltage sags based on the Monte Carlo method. A medium size distribution network is used to analyze the advantages of this approach, the convergence of the Monte Carlo method, the influence of protective devices and the importance of voltage sag indices. The limitations of the present version and the future work are also discussed.

Voltage sag stochastic prediction using an electromagnetic transients program

This paper is the first part of a work aimed at predicting the voltage sag performance of distribution networks using digital simulation and an EMTP-like tool. The paper discusses modeling guidelines for representing power distribution components in voltage sag simulations and details the approaches selected in this work. Different models for representing some components can be used to obtain voltage sag characteristics with similar accuracy. Test results are included to illustrate the performance of the developed models and support the selection of different modeling approaches.

Voltage sag studies in distribution Networks-part I: system modeling

This paper is the second part of a joint paper aimed at predicting the voltage sag performance of distribution networks by estimating voltage sag indices This part explores the characteristics of voltage sags caused by faults in a distribution network considering the effect of the protective devices installed in the network The work is based on a Monte Carlo procedure implemented in a time-domain simulation tool Several scenarios with a different coordination between protective devices have been analyzed As expected, the main conclusions are that the protection system can have a significant influence on voltage sag characteristics and proper coordination between protective devices can reduce voltage sag impact on end-user equipment

Voltage sag studies in distribution networks - part II: voltage sag assessment

Power quality indices have been developed to assess the performance of a single site or an entire system. They can guide utilities to improve their systems and can be used by manufacturers to decide about equipment immunity. This paper is the third part of a joint paper aimed at predicting the voltage sag performance of distribution networks by estimating voltage sag indices. Two types of indices are analyzed in the present paper, they give, respectively, the average number of events and the average interruption duration of each kilovolt ampere served at the low-voltage level over the assessment period. Their values are obtained from a stochastic prediction using a time-domain simulation tool. The methodology used to obtain these indices and the analysis of the influence of the protective devices are the main contributions of this paper. The limitations of the present work are also discussed.

Voltage sag studies in distribution Networks-part III: Voltage sag index calculation

EMTP-type tools are widely used in power quality studies. This paper presents a summary of the work performed by the authors to adapt and apply the ATP version to voltage sag analysis. The document includes an introduction to the capabilities of this tool, a short description of the new modules developed for voltage sag analysis (stochastic prediction and index calculation), and some illustrative examples showing the advantages of the new ATP modules.

J. Martin-Arnedo

Papers

Voltage sag stochastic prediction using an electromagnetic transients program

Voltage sag studies in distribution Networks-part I: system modeling

Voltage sag studies in distribution networks - part II: voltage sag assessment

Voltage sag studies in distribution Networks-part III: Voltage sag index calculation

Voltage sag analysis using an Electromagnetic Transients Program