Jovica V. Milanovic

Reactive Power Control Strategies for DFIG-Based Plants

This paper explores and compares the performance of alternative voltage control strategies applied to doubly fed induction generator (DFIG). Different combinations of reactive power control of rotor- and grid-side converters are investigated for voltage-control purposes. Simulations are performed using detailed models built in DIgSILENT PowerFactory in order to illustrate the influence of controllers on transient stability and steady-state operation of the DFIG-based wind plant. This paper also proposes appropriate control strategies for different sets of network operating conditions and topologies. Operational limits, such as current margins and pulse-width modulation limits, are also taken into account.

Dynamic Contribution of DFIG-Based Wind Plants to System Frequency Disturbances

The paper investigates contribution of doubly fed induction generator (DFIG) to system frequency responses. Impact of different governor settings and system inertia are investigated. Three distinct cases are simulated in order to illustrate the influence of DFIG penetration on frequency regulation. Provision of inertial response by DFIG through artificial speed coupling is also presented. The effects of the inertial response on the machine behavior and its significance for frequency regulation are discussed. The influence of converter current limits and auxiliary loop parameters on the inertial response are illustrated and a novel control algorithm is developed for extracting maximum energy from the turbine in a stable manner. The results of the study are illustrated on the example of an isolated power system consisting of a diesel generator and a DFIG.

Overview and Comparative Analysis of Gas Turbine Models for System Stability Studies

Gas turbines have become increasingly popular in the different power systems, due to their lower greenhouse emission as well as the higher efficiency, especially when connected in a combined cycle setup. With increasing installations of gas turbines scheduled in different countries, the dynamics of the gas turbines become increasingly more important. In order to study such dynamics, accurate models of gas turbines are needed. This paper presents a comparative analysis and an overview of various models of gas turbines published in different literature.

Assessing Transient Response of DFIG-Based Wind Plants—The Influence of Model Simplifications and Parameters

There has been a fast growing demand for the application of doubly fed induction generators (DFIG) in wind power plants in recent years. They have in particular dominated the market in last two years. DFIG is an ideal candidate to satisfy the requirements of the recently proposed challenging grid codes. However, many uncertainties still exist or at least there are no published reports regarding validated comprehensive DFIG models. This paper attempts to clarify the existing ambiguities in modelling of DFIG under various operating conditions using the power system analysis package DIgSILENT PowerFactory. This paper uses available DFIG models and investigates/demonstrates the influence of various model parameters and simplifications (for both mechanical and electrical subsystems) on DFIG-based wind plant transient responses. It further explores and compares the performance of different crowbar protection and rotor side converter restart schemes. Realistic operational limits, such as current margins and PWM modulation limits, are taken into account in all simulations.

Sensitivity of personal computers to voltage sags and short interruptions

This paper discusses the sensitivity of personal computers (PCs) to voltage sags and short interruptions on the basis of the extensive test results. Existing standards and previously published works are reviewed, and a description of a used testing procedure is presented. The following tests were performed: sensitivity to rectangular voltage sags with ideal and nonideal supply characteristics, and sensitivity to voltage sags caused by the starting of large motors. The results obtained emphasize the importance of clear definition of the malfunction criteria for this equipment.

Sensitivity of AC coil contactors to voltage sags, short interruptions, and undervoltage transients

This paper discusses the sensitivity of ac coil contactors to voltage sags, short interruptions, and undervoltage transients on the basis of extensive test results. Existing standards and previously published work are reviewed, and a detailed description of the testing procedure used is presented. The following tests were performed: sensitivity to rectangular voltage sags with ideal and nonideal supply voltage, testing with two-stage voltage sags, and voltage sags which occur during starting of large motors, as well as testing against measured voltage sag. The importance of voltage sag characteristics besides magnitude and duration was also investigated. Generic sensitivity curves are proposed.

Probabilistic assessment of equipment trips due to voltage sags

This paper discusses the uncertainty involved in the behavior of sensitive equipment used in various industrial processes and the methodology to incorporate this effect in quantifying the equipment trips due to voltage sags over a specified time period. The exact and more precise counting of equipment trips is essential for the accurate economic assessment of power quality. Existing standards and previously published test results on the sensitivity of this equipment against voltage sags are critically reviewed and possible ranges for variation in equipment sensitivity are investigated. Four different probable behaviors within respective ranges were considered for the stochastic assessment of equipment trips due to voltage sags. This study is performed on a generic power system.

International Industry Practice on Power System Load Modeling

Power system load modeling is a mature and generally well researched area which, as many other in electrical power engineering at the present time, is going through a period of renewed interest in both industry and academia. This interest is fueled by the appearance of new non-conventional types of loads (power electronic-based, or interfaced through power electronics) and requirements to operate modern electric power systems with increased penetration of non-conventional and mostly intermittent types of generation in a safe and secure manner. As a response to this renewed interest, in February 2010 CIGRE established working group C4.605: “Modelling and aggregation of loads in flexible power networks”. One of the first tasks of the working group was to identify current international industry practice on load modeling for static and dynamic power system studies. For that purpose, a questionnaire was developed and distributed during the summer/autumn of 2010 to more than 160 utilities and system operators in over 50 countries on five continents. This paper summarizes some of the key findings from about 100 responses to the questionnaire received by September 2011 and identifies prevalent types of load models used as well as typical values of their parameters.

Global Minimization of Financial Losses Due to Voltage Sags With FACTS Based Devices

FACTS based devices are proven to be an efficient mitigation solution for voltage sag prevention. The high cost of FACTS based devices often prohibits their wider deployment within power networks. This paper presents an approach for comprehensively assessing the financial benefits to the network resulting from their use. The annual financial losses of the entire network due to voltage sags are used as a savings target. The three most widely used FACTS based devices for voltage sag mitigation, namely, static VAr compensator (SVC), static compensator (STATCOM) and dynamic voltage restorer (DVR) are then optimally placed using a Niching genetic algorithm (NGA). The aim is to reduce overall financial losses in the network due to voltage sags. The cost of the individual devices along with their installation costs and annual maintenance are taken into account in the optimization procedure. Since this methodology is largely based on an economic evaluation of the solution, several conventional economic analysis methods are utilized. Simulations are performed on a 295-bus generic distribution network (GDN).

The influence of fault distribution on stochastic prediction of voltage sags

This paper analyzes the influence of modeling of fault distribution along transmission line on the assessment of number and characteristics of voltage sags. The generic distribution network was used in all simulations. Different types of transformer winding connections were modeled and different (symmetrical and asymmetrical) types of faults were simulated. A line was selected from the previously identified area of vulnerability for a given bus and different faults having different distributions along the line were simulated. It was shown that depending on the fault distribution (uniform, normal, exponential) along the line, different numbers and characteristics of voltage sags could be expected at the selected bus.