While energy storage technologies do not represent energy sources, they provide valuable added benefits to improve stability power quality, and reliability of supply. Battery technologies have improved significantly in order to meet the challenges of practical electric vehicles and utility applications. Flywheel technologies are now used in advanced nonpolluting uninterruptible power supplies. Advanced capacitors are being considered as energy storage for power quality applications. Superconducting energy storage systems are still in their prototype stages but receiving attention for utility applications. The latest technology developments, some performance analysis, and cost considerations are addressed. This paper concentrates on the performance benefits of adding energy storage to power electronic compensators for utility applications.

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Energy storage systems for advanced power applications

Microgrids have been proposed as a way of integrating large numbers of distributed renewable energy sources with distribution systems. One problem with microgrid implementation is designing a proper protection scheme. It has been shown that traditional protection schemes will not work successfully. In this paper a protection scheme using digital relays with a communication network is proposed for the protection of the microgrid system. The increased reliability of adding an additional line to form a loop structure is explored. Also a novel method for modeling high impedance faults is demonstrated to show how the protection scheme can protect against them. This protection scheme is simulated on a realistic distribution system containing a high penetration of inverter connected Distributed Generation (DG) sources operating as a microgrid. In all possible cases of operation the primary and secondary relays performed their intended functions including the detection of high impedance faults. This system is simulated using Matlab Simulink's SimPowerSystems toolbox to establish the claims made in this paper.

Microgrid Protection Using Communication-Assisted Digital Relays

The dynamic voltage restorer (DVR) has become popular as a cost effective solution for the protection of sensitive loads from voltage sags. Implementations of the DVR have been proposed at both a low voltage (LV) level, as well as a medium voltage (MV) level; and give an opportunity to protect high power sensitive loads from voltage sags. This paper reports practical test results obtained on a medium voltage (10 kV) level using a DVR at a distribution test facility in Kyndby, Denmark. The DVR was designed to protect a 400-kVA load from a 0.5-p.u. maximum voltage sag. The reported DVR verifies the use of a combined feed-forward and feed-back technique of the controller and it obtains both good transient and steady-state responses. The effect of the DVR on the system is experimentally investigated under both faulted and nonfaulted system states, for a variety of linear and nonlinear loads. Variable duration voltage sags were created using a controllable LV breaker fed by a 630 kVA distribution transformer placed upstream of the sensitive load. The fault currents in excess of 12 kA were designed and created to obtain the required voltage sags. It is concluded the DVR works well in all operating conditions.

Control and testing of a dynamic voltage restorer (DVR) at medium voltage level

This paper addresses the timely issue of modeling and analysis of custom power controllers, a new generation of power electronics-based equipment aimed at enhancing the reliability and quality of power flows in low-voltage, distribution networks [2],[3] The modeling approach adopted in this article is graphical in nature, as opposed to mathematical models embedded in code using a high-level computer language The well-developed graphic facilities available in an industry standard power system package, namely, PSCAD/EMTDC, are used to conduct all aspects of model implementation and to carry out extensive simulation studies Graphics-based models suitable for electromagnetic transient studies are presented for the following three custom power controllers: the distribution static compensator (D-STATCOM), the dynamic voltage restorer (DVR), and the solid-state transfer switch (SSTS) Comprehensive results are presented to assess the performance of each device as a potential custom power solution The paper is written in a tutorial style and aimed at the large PSCAD/EMTDC user base

https://ecourses.dbnet.ntua.gr/fsr/14715/Modeling_and_Analysis_of_Custom_Power_Systems_by_PSCAD-EMTDC.pdf

Modeling and Analysis of Custom Power Systems by PSCAD/EMTDC

In this paper, the closed-loop control of both voltage-source converter (VSC) and current-source converter (CSC) systems with LC filters is investigated, with a focus being put on the damping of LC resonance. First, both single-loop and multiloop control schemes for a voltage-source inverter (VSI) with output LC filter are analyzed, where the design and tuning procedure can also be applied to a current-source inverter (CSI) with output CL filter. It is shown that both VSI and CSI systems are subject to LC resonance, even with multiloop control, while the CSI system is also subject to LC resonant frequency variations. On the other hand, the LC resonant frequencies in both current-source rectifier (CSR) and voltage-source rectifier (VSR) systems with input LC filter circuits are relatively fixed due to the small variation range of source impedance. To further dampen the LC resonance in these converter systems, active damping techniques such as virtual harmonic damper and control-signal-shaping methods are investigated, and their implementation into the converter systems is discussed in detail. Experimental results from both a VSC system and a CSC system are obtained.

Control and Resonance Damping of Voltage-Source and Current-Source Converters With $LC$ Filters

The installation of the world's first dynamic voltage restorer (DVR) on a major US utility system to protect a critical customer plant load from power system voltage disturbances ushers in a new era of power quality problem solution on the utility side of the revenue billing meter. The prototype DVR built by Westinghouse for EPRI was installed in August, 1996 on the Duke Power Company (North Carolina) 12.47 kV system at an automated yarn manufacturing and weaving factory where it provides protection from disturbances coming from the utility distribution system that serves the plant. This paper describes the prototype DVR installation and presents early results from the demonstration project.

Experience with an inverter-based dynamic voltage restorer

For many years, electricity distribution companies have used sustained interruption indices as indicators of the reliability of service provided on their systems. Today, however, many electricity consumers are adversely affected by more subtle voltage disturbances such as sags and swells. Many utilities are well aware of such service quality concerns and are implementing extensive monitoring systems to detect such disturbances and assess service quality in this regard. This paper presents a subset of work completed which provides utilities with tools to make more complete service quality assessments. Indices developed to reflect system service quality with respect to all RMS variations are presented. Example values for the indices are calculated using data from a national distribution power quality data collection project. Finally, an example application of the indices currently being made by a distribution utility is discussed.

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Indices for assessing utility distribution system RMS variation performance

Typically, the modeling of highly varying, nonlinear loads such as electric arc furnaces has involved stochastic techniques. This paper presents the use of chaotic dynamics to describe the operation of nonlinear loads. Included is a discussion of the Lyapunov exponents, a measure of chaotic behavior. The alternate approach is applied to electric arc furnaces. A tuning mode is described to develop the parameters of a chaotic model. This model is trained to have time and frequency responses that are tuned to match the current from the arc furnace under study. The simulated data are compared to actual arc furnace data to validate the model. This model is used to assess the impact of various highly varying nonlinear loads that exhibit chaos in power systems.

Nonlinear deterministic modeling of highly varying loads

A versatile AC-AC converter that can be utilized as a control device for custom power applications is presented. The converter has the ability to regulate bus voltage through voltage sags and overvoltages, and act as a solid state circuit breaker. Performance characteristics under typical conditions are presented along with experimental verification of converter operation. Technical feasibility, protected efficiency and other practical implementation issues are discussed. Operation of the power converter is illustrated using EMTP simulations.

An AC-AC power converter for custom power applications

We use EPRI's distribution power quality project data to characterize and predict voltage sags and momentary interruptions based on site characteristics. The quality of sites is widely dispersed. Rural sites have many more sags and momentary interruptions than suburban and urban sites. The strongest indicators of voltage sags are: 1) circuit exposure, 2) lightning, and 3) a term with transformer impedance and number of feeders. We develop a linear model for predicting sags and another for predicting momentary interruptions based on site characteristics. We also compare transmission-level power quality to distribution power quality.

A. Sundaram

Papers

Experience with an inverter-based dynamic voltage restorer

Indices for assessing utility distribution system RMS variation performance

Nonlinear deterministic modeling of highly varying loads

An AC-AC power converter for custom power applications

Site variation and prediction of power quality