This thesis is to investigate the voltage sag, transient stability and operation feasibility of power islanding with different types of dispersed generation in distribution systems. One radial distribution feeder of Taiwan Power Company (TPC) is selected for computer simulation. The mathematical models of dispersed generations including exciters, governors of steam turbine and frequency controller of wind-driven induction generator are used in the simulation program of Matlab/Simulink. Applying the dispersed generation system with synchronous and induction generators at different locations of the test feeder by executing the short circuit and motor starting analysis to find the discrepancy in the voltage sag and the relation between the motor voltage with the motor power. Finally the simulation analysis of transient stability is executed for unbalance distribution systems with dispersed generations of steam turbines and wind-driven turbines by considering two different operation scenarios after the distribution system has been disconnected from TPC system. Different load and output power control of dispersed generation are applied to maintain the stable operation of the islanding power system. Based on the transient stability analysis, it is suggested that the service reliability of power system with critical loads can be enhanced by the dispersed power generation with proper design of load shedding and output power control.

This research suggests a novel voltage control scheme that can compensate for voltage sag and swell conditions in three-phase power systems. Faults occurring in power distribution systems or facilities in plants cause the voltage sag or swell. For sensitive loads, even voltage sags of short duration can cause serious problems in the entire system. In order to mitigate power interruptions, this research proposes a scheme called Voltage Sag Supporter utilizing a PWM (Pulse Width Modulation)-Switched Autotransformer. The proposed scheme is able to quickly recognize the voltage sag or swell condition, and it can correct the voltage by either boosting the input voltage during voltage sag events or reducing the input voltage during voltage swell events. Among existing methods, the scheme based on the inverter system such as dynamic voltage restorers (DVR) require an inverter, a rectifier, and a step-up down transformer, which makes the system expensive. AC converters can be used for the purpose of the research. However, they consist of two solid-state switches per one phase and include energy storage devices such as reactors and capacitors. The switching device for the high voltage application is relatively expensive so that this research suggests a scheme utilizing only one switch for the output voltage control, which makes the system more stable and cost effective. The proposed scheme can be applied at any voltage and provides cost and size advantages over existing methods due to the reduced number of switching components and no need of energy storage devices. Simulations and experiments have been carried out to verify the validity of the proposed scheme, and prototype experiments are being done to confirm the control scheme.

Abstract This thesis summarizes some of the voltage sag indices that are currently used by the industry. Power quality monitoring results obtained at two science based industrial parks are used to estimate the voltage quality of the electricity service and make comparison with data of a foreign utility company that are available in the literature. Fault position method is used in this study to estimate the distribution of voltage sag vulnerability areas due to the network topology ok in 2005. Comparison of the exposed area is presented. An assessment is conducted for the application of fault position method to estimate the fault location by using fault simulations results and power quality monitoring data.

Voltage sags are short duration of voltage reduction caused by system faults, overloads and starting of large motors. Voltage sags are the main causes of trips of various sensitive equipment. In order to understand the voltage-tolerance performance of some process control equipment, this thesis presents test results of some sensitive equipment (such as computers, AC contactors, high intensity discharge lamps and programmable logic controller) and provides their voltage tolerance curves. A number of magnitudes and durations recommended by IEC 61000- 4- 11 are used to perform the tests. With the performance information in hands, power quality requirements of different types of equipment and customer, and area of vulnerability for sensitive loads could be estimated.

Adjustable-speed drives (ASDs) are often used in commercial and industrial facilities to improve process control and save energy. However, ASDs are the sensitive load equipment as regards to voltage sag. During the sag, the dc-bus capacitor of a typical ASD will discharge depending on the loading condition. The voltage of the dc bus decreases and could lead to a trip of the device. In order to understand the voltage sag tolerance capability of ASDs. Based on IEC 61800-3 and IEC 61000-4-11, this research presents the test results of ASD subjected to voltage sags under different operating conditions. The tests focus on different types of voltage sags, operating situations and designs of ASDs.

Power quality related problems cause large financial losses in the order of billions worldwide. The evaluation process aimed at determining effective remedial actions starts with the correct identification and characterization of power quality disturbances. Measurements performed in the electrical power network and the corresponding collection and process of data are the primary method of characterization of the phenomena. The ideal deployment of monitoring devices would entail a monitor installed at each node of the network so that the power quality throughout the system could be directly assessed. In reality, however, technical and mostly economical constraints limit the number of monitors a network operator can install in the system. Power quality at non-monitored sites, therefore, has to be estimated by extrapolating the data from monitored sites. Consequently, it is crucial to identify the sites that provide the most accurate picture of the system’s overall power quality. Unfortunately, no recommended practices or guidelines for determining the minimum number and the best locations for optimal power quality monitoring have been prescribed in standards or reports. This thesis investigates voltage sag monitoring as part of a larger power quality monitoring scheme. The aim is to develop a methodology for optimal monitor placement for fault location and sag estimation. The thesis, divided in four main parts, focuses on network sag performance estimation and optimal monitor placement for fault localization and sag estimation. The introductory part of the thesis gives an overview of power quality surveys conducted around the world in recent years with special emphasis on the monitor placement criteria used. It also summarizes the main methods for network sag performance estimation proposed to date. The main part of the thesis firstly reviews the most referred optimal monitor placement method for sag estimation proposed in academia, highlighting its limitations. Then a robust fault location algorithm is proposed to enhance this method and overcome the identified limitations. The enhanced method is thereafter used as the basis for the generalization of one of the leading methods for optimal monitor placement for fault location in the second part of the thesis. The formulation of its optimization problem is extended for application in large power networks by adapting the modeling approach for the sag monitor placement problem. To reduce the high computational and memory burden associated with finding the optimal fault location monitor program, the thesis introduces a less memory intensive heuristic search algorithm in the third part of the thesis. A series of custom objective functions are proposed to be used with this algorithm to find optimal fault location and sag monitoring programs aimed at estimating the most critical events for customers. In the final part of the thesis, the main concepts and techniques introduced in the first three sections are combined to develop a synergistic approach to optimal monitor placement for sag characterization based on fault location. The suitability of the new method for techno-economic assessment of voltage sags using strategically or conventionally deployed monitors is established.

With the increase of sensitive loads in industrial and commercial customers, more attention has been paid to the power quality problems. Voltage sag is among the major power quality problems that cause interruptions in production lines that result in significant losses. Power quality mitigation devices, that provide different protection levels, can be adopted to match individual customer¡¦s special needs. This thesis presents a cost-benefit assessment of various voltage sag mitigation options at locations with different types of distribution feeders and customers. Simulation results have revealed that using the assumed power quality cost data, the installation of small capacity and low cost UPS has the highest benefit/cost ratio in all the options tested.

An implementation of a series interconnection scheme for small distributed generation systems is presented in this thesis. The concept uses one set of voltage source converter to control the injected voltage magnitude and phase angle for power injection and voltage sag mitigation. Because of the use of series interconnection scheme, the energy storage device outputs vary concurrently with the line loading and provides loading leveling functions. Under voltage sag situations, it provides missing voltage to effectively deal with power quality problems. The control algorithm uses the reference voltage tracking concept. A signal phase 484 VA prototype of the proposed system is implemented. The performance of the proposed method is simulated and verified by using Matlab-simulink package and the implemented module.

Current practices of power quality mitigation in the industry are characterized by sub-optimal investment decisions where over compensation is often the norm such causing huge wastage in financial resources. Providing power quality management services to industrial customers in the form of power quality contracts could yield substantial return for the network operator. With better understanding of network parameters, and the option of installing network level mitigation devices, network operators could employ wider range of cost effective mitigation solutions. Tapping into the market however, entails bearing the risks for the customers which network operators are not always willing or encouraged to do. With potentially millions at stake, extensive risk assessments are crucial for any proposed power quality management scheme. This thesis investigates the voltage sag aspect of the problem as part of a larger power quality management scheme. The aim is to develop general framework for technical and financial assessments of voltage sags prior to the introduction of power quality management service. The thesis focuses on five major aspects of voltage sag assessment: identification of customer requirement, financial loss assessment, network sag performance estimation, sag mitigation, and financial appraisal of mitigating solutions. The first part of the thesis gives a comprehensive overview of current power quality problems faced by industrial customers and provides ranges of typical financial losses incurred by different types of industries around the world. It then proposes robust methodology for assessment of typical financial loss, i.e., customized customer damage function (CCDF), for a given industry based on available survey data and taking into account characteristics of the assessed customer plant. For failure and financial risk assessments, the thesis introduces new customer models employing probabilistic methods to quantify risks induced by voltage sags and proposes generic models that incorporate full flexibility in failure risk assessment, taking into account the effect of unbalanced sags on equipment behavior. It further quantifies the error introduced by sag performance estimation using limited monitoring data with a case study on actual sag profile. It demonstrates how different estimation methods and different durations of monitoring period affect accuracy of estimation of voltage sag profile and associated risk of industrial process failure. Following this, the thesis presents new models for plant and network level sag mitigation devices. They include power injecting mitigation devices, devices that reduce number of faults in the network and devices that reduce the severity of faults. Developed models are then used to investigate the cost-effectiveness of sag mitigation at different levels. Finally, the thesis presents Genetic Algorithm based methodology for deciding on optimal investment scheme in voltage sag mitigation in the network. The sensitivity of the solution to various influential parameters, including plant type and size, sensitive equipment type, process characteristics, financial loss resulting from process interruption, cost and effectiveness of mitigating solution and network fault rates is also established.

Voltage sag (dip), a sudden reduction of the voltage magnitude within a short duration in power system, is one of major concerns of power quality problems. The main reason of the increased concerns for voltage sag problems is that the losses caused by voltage sag events are high and not negligible. Reliability indices have been used for many years to quantify the effect of sustained interruptions on the electric power system. Power quality indices reflecting the severity of various power quality problems, such as flicker, harmonics, voltage swell and sag conditions, power factor, losses, electromagnetic interference, and other phenomena, are still under development. The representation and classification of voltage sags have been studied recently by standard-setting organizations. In order to find compatibility between service quality and the equipment adopted and a least cost solution for possible power quality problems, the concept of system disturbance level and equipment immunity level was proposed in IEC 61000-3-7 but without clear definitions. A novel voltage sag index based on fuzzy logic technique to quantify system disturbance and equipment immunity levels is proposed in this dissertation. This approach takes network vulnerability, equipment sensitivity and uncertainties in measuring voltage sags into account, thereby, providing meaningful information for both the utility and customers. Using the proposed method, the probabilistic distribution of system disturbances can be obtained from the single event indices of all events recorded and the probabilistic distribution of equipment sag immunity capability can be evaluated based on the device voltage sag tolerance curve. This dissertation also presents a novel framework for predicting the number of equipment disruptions due to voltage sags in a unit of time by using the disturbance and immunity levels concepts. In the proposed approach, the number of disruptions is computed by using the unreliability concept. The area of overlapping between the distributions of site disturbance and equipment immunity levels, which indicates the number of possible disruptions, is calculated based on interference theory and reliability computations. The presented methodology can be used as a planning tool to quantify the system disturbances and equipment sensitivity. It can also be used to perform cost analysis of the compatibility of equipment with an electric power system. To minimize the costs due to voltage sags, it is always a good strategy to maintain a minimum overlap between the equipment immunity level and site disturbance level to have satisfactory operation of the equipment. The tool achieved in this work can be used to perform such analyses.