Showing papers by "Jovica V. Milanovic published in 2010"

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

Abstract: 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).

Modeling of FACTS Devices for Voltage Sag Mitigation Studies in Large Power Systems

Abstract: Flexible ac transmission system (FACTS) devices or their subderivative custom power devices are efficient and often used and recommended power-electronics-based devices for mitigation of voltage sags in electrical power system. With FACTS devices installed, the overall system (and individual bus) sag performance could significantly change depending on the type of the device used. In order to assess this change in sag performance in realistic large power systems, the classical (essentially static) fault calculation procedure should be amended to incorporate the effects of these devices on bus voltages. This paper presents a new approach of quasistatic sag analysis using a system impedance (Z _bus) matrix that incorporates FACTS devices. Three types of FACTS devices which are most often used in practical applications are considered in this study: static compensator, static var compensator, and the dynamic voltage restorer. The case studies based on the 295-bus generic distribution system are used to illustrate the modeling method and the effectiveness of these devices in sag mitigation.

Global Voltage Sag Mitigation With FACTS-Based Devices

Abstract: This paper presents an approach to optimally select and allocate flexible ac transmission (FACTS) devices in a distribution network in order to minimize the number of voltage sags at network buses. The method proposed is based on the optimization of a preselected objective function using simple and niching genetic algorithms (GA). The objective of the optimization is to achieve the improvement in overall system sag performance of the network. Using proposed GA-based optimization, the location, the type and the rating of six (in total) FACTS devices are optimized simultaneously. Three types of FACTS devices are implemented in this study, namely, static var compensator, static compensator, and dynamic voltage restorer. The performance of the proposed algorithm is tested and illustrated on 295-bus generic distribution system.

Comparison of dynamic performance of meshed networks with different types of HVDC lines

Abstract: The paper presents classical, Line Current Commutated (LCC) based and Voltage Source Converter (VSC) based HVDC line models for system dynamic studies The models are developed based on the injection modelling principle and incorporated into a meshed four machine, multi-bus network Small and large disturbance stability studies are performed to identify the sensitivity of electromechanical inter-area oscillatory modes to varying HVDC transmission capacity, converter type and HVDC point of connection The application of LCC and VSC-HVDC resulted in varied system responses depending on the HVDC technology used and point of connection In all cases connection of HVDC line was beneficial to overall system dynamic performance (5 pages)

Probabilistic clustering of wind generators

Abstract: Increasing number of wind farms (WFs) connected to power systems calls for efficient aggregate models so that large farms can be represented by only few equivalent wind turbines for steady state and dynamic system studies. A wind power plant consists of many small generators inside a wind farm. Future wind projects predict even greater number of wind turbines inside a wind farm for increased capacity. If each generator is represented individually this adds considerably towards calculation time for dynamic simulations. For this reason, wind farms are required to be modelled by few equivalent wind turbines which will reduce computation time. In this paper wind turbines (WTs) are clustered based on wind speed they receive using the Support Vector Clustering (SVC) technique. It was found that if set of clusters that reoccur several times during the year, as the best representation of the entire WF, can be obtained probabilistically, the highly frequent set then can be used to represent the wind farm for the entire year. This method can prevent time consuming way of choosing new set of clusters every time the wind speed (WS) and wind direction (WD) varies. The most probable equivalent set is also better than a single turbine equivalent model as the latter is good only at very high wind speeds (if same farm layout and site are considered) and would inaccurately represent the wind farm at other speeds.

Estimating the voltage unbalance factor using distribution system state estimation

Abstract: Unbalance is a damaging and costly phenomenon for both network operators and end users. Network operators are increasingly looking at ways in which they can intelligently monitor their exposure to unbalance and its related problems. This paper presents initial results obtained with a new methodology for estimation of the voltage unbalance factor at any busbar in a distribution network. The proposed methodology utilizes distribution system state estimation to estimate the statistical properties of voltage unbalance factor. Both real and pseudo measurements are used to estimate the voltage unbalance factor and the corresponding error in the estimate. The methodology is validated, at this stage, on a simple generic unevenly loaded power system network using real data from UK distribution networks. It is shown to yield promising results across a series of unbalance simulations.

Economic viability of application of FACTS devices for reducing generating costs

Abstract: the multifunctional capabilities of Flexible AC Transmission System (FACTS) devices paved new paths for controlling and enhancing the power transfer and usable capacity of transmission lines. The ability to enhance power transfers lead to their applications in a multi machine power system for the purpose of an overall reduction in power generation cost, among the others. The study presented in this paper deals with evaluation of economic viability of application of FACTS devices for generation cost reduction. It starts with optimal placement of four types of FACTS devices, namely Static Var compensator (SVC), Thyristor-Controlled Series Capacitor (TCSC), Thyristor-Controlled Voltage Regulator (TCVR), and Thyristor-Controlled Phase Shifting Transformer (TCPST) in a multi machine power system. The allocation methodology accounts for the cost of generated active and reactive powers, load growth, and cost of selected FACTS devices over a wide range of operating conditions. The allocation task is handled by the Optimal Power Flow (OPF) and Genetic Algorithm (GA) based optimization procedures. The economic viability of the solution is assessed using Net Present Value (NPV) calculation. Finally, Tornado diagram is used to assess sensitivity of the solution to various parameters involved in the optimization procedure.

Coordinated Q–V Controller for Multi-machine Steam Power Plant: Design and Validation

Abstract: This paper presents laboratory model of power plant reactive power flow developed for testing and validation of coordinated reactive power voltage (Q-V) control system for steam power plant (SPP) with six generators The real time simulator (RTS) is based on series of experiments performed on the SPP site The proposed Q-V controller performs coordinated reactive power terminal voltage (Q-V) regulation task in the steam power plant This is achieved by adjusting the individual generator reactive power output as fast, and as accurate as possible, following the change in voltage reference value of automatic voltage regulator (AVR) of each generator The Q-V controller inputs are generators' reactive powers (Q) and outputs are up/down commands to AVRs Developed SPP RTS model therefore, uses up/down commands for setting reference values for AVRs as inputs and Qs as outputs The SPP RTS responses are verified through series of additional experiments performed in the laboratory and on the SPP site (6 pages)

Voltage dip immunity of equipment and installations - status and need for further work

Abstract: This paper presents the results from the work of WG C4.110, a joint working group by CIGRE, CIRED and UIE. Its mandate period stretched from early 2006 through early 2009. The group has addressed several aspects of the immunity of, especially, industrial equipment against voltage dips. Compared to the work earlier groups, the equipment performance is not seen as a final aim, but as a step towards the ultimate aim: allowing the process to ride through the voltage dip. Some of the contributions and conclusions from the WG C4.110 are discussed in this paper, ✓ Description of voltage dips, ✓ Equipment and process immunity, ✓ Testing and characterization, ✓ Economics, ✓ Immunity classes and application, ✓ Further work.

Voltage dip immunity aspects of power-electronics equipment — Recommendations from CIGRE/CIRED/UIE JWG C4.110

Abstract: This paper presents some of the results from an international working group on voltage-dip immunity. The working group has made a number of recommendations to reduce the adverse impact of voltage dips. Specific recommendations to researchers and manufacturers of power-electronic equipment are: considering all voltage dip characteristics early in the design of equipment; characterize performance of equipment by means of voltage-dip immunity curves; and made equipment with different immunity available.

A comparison of voltage sag estimation algorithms using optimal monitorring locations

Abstract: The aim of voltage profile sag estimation (VPSE) is to reconstruct the voltage profile at non-monitored buses using measurements only from monitored buses. This paper compares two methods for voltage profile estimation: one which focuses on fault localization and another which reconstructs profiles using voltage thresholds. The two algorithms are compared using a generic 295 bus generic distribution network and 4 sets of monitoring locations. The 4 monitoring locations are selected using a bespoke artificial immune system optimization algorithm. The results of the two methods are compared by analyzing the distribution of error between the two methods and the estimated generalized sag tables of both algorithms. Preliminary results suggest that the two methods are very comparable in performance. Both algorithms are shown to be sensitive to the selected monitoring locations, and perform well even when the number of monitors is much less than the number of buses.

Probabilistic identification of turbines facing high and low wind speeds in a wind farm

Abstract: Due to growing number of wind turbines (WT) within a wind farm (WF) scheduling of maintenance of turbines within the WF becomes challenging issue. Also, in instances when wind energy curtailments are required it can be difficult to decide which turbines should be shut down first. In this paper, a probabilistic methodology is presented to identify WT in the WF that face higher and lower wind speeds during the year. Since this calculation is dependant on WF layout, location of WF and position of WTs inside the WF probabilistic site analysis is performed, along with turbine clustering, after determining wind speed approaching each turbine by using a detailed wake effect model. The approach presented can be applied to a wind farm of any size and layout at any location.

On improvement of accuracy of optimal voltage sag monitoring programmes

Abstract: Selection of monitor locations is a key part of power systems voltage sag characterization utilizing limited monitoring data. In this paper, a technique of determining the optimal locations of monitors is proposed. It calculates the overall sag estimation accuracy to identify the best buses for sag monitoring. The technique can also be implemented to determine the sequence of monitor placement of already configured optimal monitoring programmes and to assess the effectiveness of sub-optimal monitor arrangements. The optimal monitoring scheme for a 295-bus generic distribution system (GDS) is determined using the proposed technique and its performance compared against monitoring schemes obtained with existing methods.

Voltage-dip immunity : statistics and need for further work

Abstract: This paper presents the main results from CIGRE/CIRED/UIE working group JWG C4.110 and an overview of the need for further work identified by this working group. The paper goes into more details of the volt age-dip statistics collected by the working group and on me thods to present the results from voltage-dip surveys.

Correlation between uncertainties in system model parameters and distribution of critical electromechanical modes

Abstract: Numerous studies conducted in the past broadly assume that normally distributed parameters of a power system, load parameter values in particular, within a given range would lead to normally distributed critical eigenvalues of the system, i.e., electromechanical modes. Normal distribution of input parameters however does not necessarily assure a Gaussian distribution of the output parameters due to various nonlinearities inherent to power systems. The uncertainties in input parameters will result in uncertainties in values of elements of reduced system state matrix from which the eigenvalues of the system are calculated. The relationship between model parameters and elements of reduced state matrix is non-linear as well as the relationship between state matrix elements and eigenvalues. This paper first shows that normally distributed system load bears no systematic distribution of electromechanical modes and then carries on to establish correlation, if any, between uncertainties in other parameters and critical modes. (6 pages)

Generator ranking index

Abstract: The paper investigates the possibility of ranking synchronous generators in a power system according to their contribution to system angular and voltage stability The aim of this research is to establish fast semi-analytical methodology, ie, avoiding transient simulations, to determine which synchronous generators can be safely disconnected from the network or replaced by other types of generators, eg, wind generators, that do not provide similar support to the system The ranking methodology proposed here is based on sensitivity analysis of system electromechanical modes and takes into account location of generators, their inertia, active and reactive power outputs and control functions provided by the generators The results obtained so far are promising and largely agree with previously attempted methods to rank generators based on singular value decomposition and P-V curve analysis Various operating conditions are examined and yielded the same ranking order across the different ranking methods at the respective operating points

Contribution of phase shifting transformers to improvement of the security of power transfer from the power plant

Abstract: The paper presents a case study of the use of phase shifting transformer (PST) for improvement of security of power transfer from a power plant. The phase shifting transformer is used for increasing real power flow from the plant to higher voltage level (220 kV), i.e., redistributing the real power flows, and such reducing loading of heavily loaded 110 kV lines. This re-distribution of power flow from the plant results in local transmission network becoming more secure. The local network performance, with and without PST in place, is analysed in both steady state and transient conditions. Interconnection model of Southeast Europe, which consists of transmission network models of Albania, Bosnia and Herzegovina, Bulgaria, Croatia, Greece, Macedonia, Montenegro, Romania, Serbia, Slovenia, Turkey and part of Ukraine is used in simulations. All simulations are made using PSS®E software. (5 pages)

The economic value of advanced governor control

Abstract: Stability, both small and large disturbances, is a critical part of day to day operation of the power system. Unstable system would not able to operate. Though critical, stability has often been overlooked in the markets and thus undervalued. In this paper different approaches used in the past have been critically reviewed and summarised. A simple example is then used to elucidate the physical meaning of optimal power dispatch with a limit due to stability. Finally, it is demonstrated that the system performance could improve with advanced governor control, i.e., using a phase compensated governor, and how this enhanced performance can be linked to economic values in the market.