Search

This paper jointly addresses two major challenges in power system operations: 1) dealing with non-convexity in the power flow equations, and 2) systematically capturing uncertainty in renewable power availability and in active and reactive power consumption at load buses. To overcome these challenges, this paper proposes a two-stage adaptive robust optimization model for the multi-period AC optimal power flow problem (AC-OPF) with detailed modeling considerations, such as reactive capability curves of conventional and renewable generators and transmission constraints. This paper then applies strong second-order cone programming (SOCP)-based convex relaxations of AC-OPF combined with the use of an alternating direction method to identify worst-case uncertainty realizations, and also presents a speed-up technique based on screening transmission line constraints. Extensive computational experiments show that the solution method is efficient and that the robust AC OPF model has significant advantages both from the economic and reliability standpoints as compared to a deterministic AC-OPF model.

Load modelling plays a key role in assessing peak load reduction and has attracted renewed interest recently due to increasing diversity and uncertainty in load characteristic resulted by emergence of new types of loads and more penetrations of distributed renewable generations onto electricity networks. This paper presents a methodology for the development of a static load model based load characteristic profile in medium voltage distribution networks from year-long field measurements. High-resolution load monitoring devices with the 1 s sampling rate installed at 60 primary substations in the U.K. distribution network are used to collect the data for the purpose of load modelling. Load models at these 60 substations comprising domestic, commercial/industrial, and mixed-type load demand are presented in the paper together with relevant model parameters. Field trials (which involve triggering transformer taps to initiate voltage changes) were performed throughout the year on 15 selected substations aimed at load profile validation. The developed load profiles take into account seasonal, weekly, and daily variations of P–V and Q–V characteristics. Representative 24 h (0.5 h) load matrices are developed for all 60 substations and provide an insight into the operational flexibility and network resilience to voltage variations.

This paper derives the index of dynamic interactions (IDI), which is the sensitivity of a power system electromechanical oscillation mode (EOM) of concern to the dynamic interactions introduced by a grid-connected double-fed induction generator (DFIG). Based on the IDI, analysis is carried out with the following generally-applicable conclusions obtained: 1) Impact of dynamic interactions introduced by the DFIG increases when wind power penetration increases; 2) as long as the frequency response of an equivalent single DFIG model approximately matches the frequency response of a wind farm around the frequency of electromechanical oscillations of concern, this equivalent dynamic model of single DFIG can be used to represent the wind farm to examine the impact of wind farm on EOMs. Furthermore, an indicator of DFIG control (IDC) is derived for comparing the impact of terminal voltage control and reactive power control implemented by the DFIG. Main merit of the IDC is the computational simplicity without need to know dynamic model of the DFIG to indicate that either the terminal voltage control or the reactive power control is more beneficial to power system stability. An example multi-machine power system with grid-connected DFIGs is presented to demonstrate and validate the analysis and conclusions made in the paper.

This paper establishes a closed-loop interconnected dynamic model of a power system with a doubly fed induction generator (DFIG) for wind power generation, wherein, the DFIG and power system are modeled as two open-loop interconnected subsystems. The established model indicates that the impact of the dynamic interactions introduced by the DFIG on the power system's small-signal angular stability is the difference between the closed-loop and open-loop electromechanical oscillation modes (EOMs). The analysis conducted in this study reveals that when an open-loop DFIG oscillation mode (DOM) is close to an open-loop EOM on the complex plane, the modal coupling of the open-loop EOM and DOM can cause strong dynamic interactions between the DFIG and the power system; thus, the DFIG has a significant impact on the power system's small-signal angular stability. An example multimachine power system with grid-connected DFIGs is presented to demonstrate and validate the analysis and conclusions made in the paper.

This paper applies a phasor-only state estimator (PSE) to a large power system consisting of two control regions, using synchrophasor data from 56 765/345/230 kV substations. Although the technique has been previously developed, this paper describes a topology processor to determine the redundant clusters of connected buses such that the PSE can be used in real time. This PSE allows corrections for phase biases, transformer taps, and current magnitude scaling. The ability to simultaneously solve the PSE for the two control regions is due to a PMU monitoring a tie-line between these two regions. The results show that with this method, the total vector errors of the measured voltage phasor data in these two control regions average to less than 0.5% under ambient conditions. The PSE also computes virtual phasor measurements on 70 buses that do not have PMUs, including large steam generator and wind-turbine generator substations.

This paper summarizes simulation based technical feasibility and cost benefit analyses of a range of off-the-shelf electrical losses reduction intervention solutions in the Isle of Wight (IoW) 11 kV distribution network in the United Kingdom (UK), assessed over a period of 45 years. Based on presented results, the paper provides a brief commentary on the merits, drawbacks, risks and recommendations associated with each considered network intervention. Among considered network interventions, the transformer auto stop-start scheme, where one of the transformers among a pair at a primary substation would be switched depending on the substation's varying load, was found to be the optimal solution, i.e., in reducing technical losses significantly and may also have a reasonable Net Present Value (NPV) with a good rate of Return on Investment (RoI) when fully implemented.

An analytical approach and a control strategy are proposed in Part I of this two-part paper for leveraging the aggregate demand of a population of thermostatically-controlled loads (TCLs) to deliver contingency-type reserve capacity. Further, a metric is introduced to quantify the TCL users' comfort satisfaction in response to a control signal. In Part II, we address the validation of the concepts put forth in the first part of the paper. To do so, we develop an extensive set of case studies based upon a population of 10 $\thinspace$ 000 electric water heaters (EWH). Here, a physically-based model is first derived for the calculation of EWHs on-time statistics. The model is then extended to calculate the probability of EWH users' comfort satisfaction. The analytical approach and control strategy presented in Part I are validated using the EWH model against Monte Carlo simulations. Further, the trade-off between demand response capacity and EWH users' comfort satisfaction is investigated.

The paper proposes a framework for the selection of optimal configuration of line compensation devices, thyristor controlled series capacitors (TCSC) and conventional fixed series capacitors, for reducing the risk of subsynchronous resonance (SSR) in the network while maintaining required power transfer. The methodology developed in the paper is based on the robust risk evaluation of SSR that takes into consideration the severity of subsynchronous resonance and probability of its occurrence. The subsynchronous resonance risk index developed previously is used to assess the severity of subsynchronous resonance. The line outage model is employed to determine the probability of contingencies potentially leading to subsynchronous resonance. It is demonstrated that with relatively small participation of TCSC, even with the most basic control, in series compensation of lines, the risk of SSR can be successfully managed.

A study of small signal behavior of power systems including wind generation units and energy capacitor systems (ECSs) is presented in this paper. Modeling of wind units with squirrel-cage induction generators connected to power system through a full-scale AC/AC converter and ECS unit are explained thoroughly. The ECS unit (considered in this paper) consists of electric double-layer capacitors (EDLC) and DC/AC converter. Small signal behavior of three test systems are studied and compared with one another using digital computer simulations. The three systems are: S1) WSCC 9-bus system with three conventional synchronous generators (SGs), S2) the WSCC system with one of the conventional SGs replaced by a wind energy system, and S3) the same as S2) but with the addition of an ECS unit. The study includes comparing of dominant eigenvalues, the participant state variables, and the related participation factors for the three systems. In addition, the effects of loading on the unstable modes of the system with conventional SG, wind and ECS units (system S3) are studied using the trajectory of these modes as loads are increased.

This paper presents the details of a new real-time stability assessment method. The method assesses a particular mechanism of stability: each generator’s capability to generate sufficient steady state electromechanical torque. The lack of sufficient steady state torque causes aperiodic increase in rotor angleand a loss of synchronism, referred to as aperiodic small signal instability. The paper provides the theoretical background of the method and an analytical assessment criterion. Furthermore, a mathematical mapping of the generators’ operating points that enables informative visualization of multiple operating points is derived in the paper. Finally, results from time-domain simulation of instability scenarios in the Nordic32 test system are presentedand results used for testing the assessment method. The results illustrate the method’s capability to efficiently identify the location of the emerging problem and to quantify margins to stability boundary.

The paper presents a new model of the STATCOM aimed at power flow solutions using the Newton-Raphson method. The STATCOM is made up of the series connection of a voltage-source converter (VSC) and its connecting transformer. The VSC is represented in this paper by a complex tap-changing transformer whose primary and secondary windings correspond, notionally speaking, to the VSC's ac and dc buses, respectively. The magnitude and phase angle of the complex tap changer are said to be the amplitude modulation index and the phase shift that would exist in a PWM inverter to enable either reactive power generation or absorption purely by electronic processing of the voltage and current waveforms within the VSC. The new STATCOM model allows for a comprehensive representation of its ac and dc circuits-this is in contrast to current practice where the STATCOM is represented by an equivalent variable voltage source, which is not amenable to a proper representation of the STATCOM's dc circuit. One key characteristic of the new VSC model is that no special provisions within a conventional ac power flow solution algorithm is required to represent the dc circuit, since the complex tap-changing transformer of the VSC gives rise to the customary ac circuit and a notional dc circuit. The latter includes the dc capacitor, which in steady-state draws no current, and a current-dependent conductance to represent switching losses. The ensuing STATCOM model possesses unparalleled control capabilities in the operational parameters of both the ac and dc sides of the converter. The prowess of the new STATCOM power flow model is demonstrated by numerical examples where the quadratic convergence characteristics of the Newton-Raphson method are preserved.

Society's increased concern over green house gas emission and the reduced cost of electric vehicle technologies has increased the number of electric vehicles (EV) and plug-in hybrid vehicles on the road. Previous studies into the effects of electric vehicles on the electric system have focused on transmission, generation, and the loss of life of distribution transformers. This paper focuses specifically on identifying distribution transformers that are most susceptible to excessive loading due to the implementation of electric vehicles. The authors use a binomial probability model to calculate the probability that a specific distribution transformer will experience excessive loading. Variables to the function include the existing peak transformer demand, number of customers connected to the transformer, and the most common EV charger demand. Also included in the paper is an optimization approach that utilizes the results from the binomial function to determine the optimal replacement strategy to minimize replacement costs. An extension of the approach is also utilized to explore the effectiveness of EV targeted demand side management programs. The authors apply the described algorithms to 75 000 distributions transformers within a distribution system located in Denver, Colorado, USA.

This paper proposes a new breed of high-voltage dc (HVDC) transmission systems based on a hybrid multilevel voltage source converter (VSC) with ac-side cascaded H-bridge cells. The proposed HVDC system offers the operational flexibility of VSC-based systems in terms of active and reactive power control, black-start capability, in addition to improved ac fault ride-through capability and the unique feature of current-limiting capability during dc side faults. Additionally, it offers features such as smaller footprint and a larger active and reactive power capability curve than existing VSC-based HVDC systems, including those using modular multilevel converters. To illustrate the feasibility of the proposed HVDC system, this paper assesses its dynamic performance during steady-state and network alterations, including its response to ac and dc side faults.

Network reconfiguration and capacitor placement have been widely employed to reduce power losses and maintain voltage profiles within permissible limits in distribution systems. Reconfiguration method proposed in this paper is based on a simple branch exchange method of single loop. In this simple method of branch exchange, loops selection sequence affects the optimal configuration and the network loss. Therefore, this method has been improved by optimizing the sequence of loops selection for minimizing the energy losses in this paper. Also, a joint optimization algorithm is proposed for combining this improved method of reconfiguration and capacitor placement and therefore maximum loss reduction. For more practical application of the proposed method, different load patterns are considered and a fast method of total energy loss calculation is employed for the economic optimization of energy losses during the planning horizon. Discrete genetic algorithm (GA) is used to optimize the location and size of capacitors and the sequence of loops selection. In fact, the capacitor sizes have been considered as discrete variables. Simulated annealing (SA) is also applied to compare the performance of convergence. The proposed algorithm is effectively tested on a real life 77-bus distribution system with four different kinds of load patterns.

Due to the existence and building of an important number of combined cycle plants throughout electric power systems around the world, there exists the growing need to have a more accurate and practical model to represent these type of power plants when solving the unit commitment problem. A commonly used optimization technique implemented to solve the unit commitment problem is dual programming. This work focuses on solving the subproblem of scheduling a combined cycle plant using dynamic programming under a dual optimization framework. The model used to represent the combined cycle plants is component based; this new way of modeling combined cycle plants takes into account such constraints as the feasible transitions between configurations, and the minimum and maximum time that a combined cycle plant must remain on a certain configuration. The advantage of the model presented in this paper is that it accurately models all the physical constraints of combined cycle plants. Also, this paper presents the modeling of hybrid combined cycle plants; these are the ones that use an auxiliary boiler in order to increase the production of steam used by the steam turbine.

This paper is concerned about optimal placement of synchronized phasor measurements that can monitor voltage and current phasors along network branches. Earlier investigations on placement of phasor measurement units (PMUs) have assumed that a PMU could be placed at a bus and would provide bus voltage phasor as well as current phasors along all branches incident to the bus. This study considers those PMUs which are designed to monitor a single branch by measuring the voltage and current phasors at one end of the monitored branch. It then determines the optimal location of such PMUs in order to make the entire network observable. The paper also addresses the reliability of the resulting measurement design by considering loss or failure of PMUs as well as contingencies involving line or transformer outages. Developed placement strategies are illustrated using IEEE test systems.

This paper discusses a novel approach for damping inter-area oscillations in a bulk power network using multiple unified power flow controllers (UPFC) utilizing ultracapacitors, also known more generally as electrochemical capacitors (EC). In this paper, a new control is introduced to mitigate inter-area oscillations by directly controlling the UPFCs' sending and receiving bus voltages which better utilizes the stored energy in the ECs. The results of this controller are compared with and without ECs. The proposed control provides better inter-area oscillation mitigation when applied to multiple UPFCs in the 118 bus IEEE test system.

Laboratory experiences are an important component of the education of engineering students. Rotating machinery laboratories assist with the visualization of the three-dimensional and rotating equipment. Contemporary use of rotating machines in industry often includes adjustable speed drives and digital controllers. Additionally, instrumentation is used to feedback electrical and mechanical variables to the drives and controls. Using an equipment grant from an industrial automation manufacturer, cost-effective laboratory workstations have been developed. Industrial-grade equipment is not designed for student experimentation but was adapted for this use. The workstations described in this paper incorporate modern power electronic-based drives and motors, electrical and mechanical sensors, and a computer-based control system with data acquisition. The advantages of these workstations include cost-effectiveness and the use of actual industrial devices that students might encounter in industry. Additionally, the workstations can be accessed by remote users, enabling off-campus students to also perform experiments on the workstations. The paper includes details of the workstation and its components, the data acquisition and control system, and sample experimental results.

This paper proposes the planning method for capacitor installation in a distribution system to reduce the installation costs and minimize the loss of electrical energy. The expected lifetime of devices with moving parts depends on the total number of operations, which affects the replacement period and replacement decisions for aging equipment under a limited budget. In this paper, the expected device lifetimes are included in the formulation, and the optimal operation status of the devices is determined using a genetic algorithm. The optimal numbers and locations for capacitor installation are determined based on the optimal operation status. Simulation results in a 69-bus distribution system show that the proposed technique performs better than conventional methods.

This paper proposes a new voltage instability risk indicator based on local phasor measurements at fast sampling rate. The effectiveness of the indicator is analyzed at EHV load and ldquotransitrdquo buses. The risk criterion is based on the real-time computation of the Thevenin equivalent impedance of the classic electrical circuit given by an equivalent generator connected to an equivalent load impedance through an equivalent connecting impedance. The main contribution of the paper is the innovating algorithm utilized on the real-time adaptive identification of the Thevenin voltage and impedance equivalents. The algorithm performance is shown through a detailed sensitivity analysis. The paper presents important numerical results from the actual Italian EHV network.

This paper deals with modeling of the doubly-fed induction generator (DFIG) and the corresponding converter for stability studies. To enable efficient computation, a reduced-order DFIG model is developed that restricts the calculation to the fundamental frequency component. However, the model enhancement introduced in this paper allows the consideration of the alternating components of the rotor current as well, which is necessary for triggering the crowbar operation. Suitable models are presented for the rotor and grid side converters as well as the dc-link, taking into account all four possible operating modes. The proposed model for speed and pitch angle control can be used when wind and rotor speed variations are significant. Simulation results are presented for model verification purposes and also for demonstrating the dynamic behavior of a large offshore wind farm connected through a long undersea cable to the high voltage grid.

The creation of competitive electricity markets has increased the complexity of the economics of system operation. Independent system operators (ISOs) perform optimizations based on a market-driven objective function. However, there may be other objectives that the ISO wishes to consider. These objectives are called secondary objectives, as they are dominated by the economic objective. In this paper, a hierarchical multiobjective (HMO) optimization is developed that, through coordinated control of network devices such as phase shifting transformers and series flexible ac transmission systems (FACTS), allows the market objective to be optimized in a global sense, while the secondary objectives are locally optimized. The method is applied to a modified IEEE 30-bus system incorporating a wide-area impact index-minimizing secondary objective. Formulated in this paper, the wide-area impact index accurately measures the effects of parallel flow over multiple lines in a region. Comparisons are made between the HMO's performance with other single and multiple objective optimizations and between the wide-area impact index and traditional parallel flow calculations

This paper introduces PJM Interconnection's new method to model and predict transformers' service lives. A simplified Perks' equation is used to model the hazard rates, which will be used synonymously with retirement rates in this paper, of transmission transformers. A Bayesian Method is used to model the uncertainty in the three parameters of Perks' equation. This method is different from the traditional Iowa Curve method in that: 1) it gives a continuous family of transformer life distributions as well as the extinction likelihoods, rather than a limited number of distribution patterns and 2) a closed-form distribution can also be deduced with only three parameters, which can assume all the three distribution patterns of Iowa Curves: right, left, and symmetrically modal. The method has been successfully applied to the statistics on hundreds of large electric transformers in PJM. Example problems are provided and solved with this new method

This paper presents genetic algorithm-based post-outage voltage and reactive power calculations for transmission line and transformer outages. Deficiencies of the traditional sensitivity-based (linear) methods are overcome by formulation of a line outage as nonlinear constrained optimization problem for a bounded network. The proposed optimization problem is later solved for bounded network variables by genetic algorithms. The accuracy of the method is tested on IEEE 14-bus and IEEE 57-bus test systems. Simulation results are compared both with the results of a full power flow study and with the results of the papers presented in the literature. The results have shown that the proposed method improves the accuracy of post-outage voltage and reactive power values with less computational effort.

Combined cycle units represent the majority of new generating unit installations across the globe. Combined cycle units feature distinct advantages for power generation such as high efficiency, fast response, shorter installation time, abundance of gas, and environmental friendliness. The short-term scheduling of combined cycle units could represent a complicated optimization problem because combined cycle units could have multiple operating configurations based on the number and the status of combustion turbine and steam turbine. In this paper, we present a method for establishing the state space diagram of combined cycle units for applying dynamic programming and Lagrangian relaxation to the security constrained short-term scheduling problem. Although the proposed method is applied in this paper to analyze combined cycle units, it is suitable for other types of generating units with multiple configurations including fuel switching/blending units, constant/variable pressure units, and dual boiler units. The paper presents a few case studies to verify the advantages of combined cycle units in competitive electricity markets.

The aim of this paper is to illustrate that wide area generator speed measurements combined with Fourier analysis can be used to determine coherent generator groups. Studies are based on simulations in a 68-bus test system. Three test cases are investigated where the new method is compared to conventional methods based on the generator speed, modal analysis, and phasor angle measurement. The method introduced in this paper has proven to be an interesting alternative for generator coherency determination. It gives almost identical results to the offline methods based on the generator speed or modal analysis. Based on their trustworthiness the test cases indicate that the method reflects a more accurate coherency than the conventional online method based on phasor angle measurement. Moreover, compared with online applications using modal analysis the method supports real-time applications better according to its less extensive computational requirements.

Dynamic equivalents have been widely used to reduce computational efforts when a large number of scenarios have to be studied to evaluate the stability of interconnected power systems. Most coherency methods developed for model reduction do not consider the effect of voltage variations on the coherency of generators due to weak coupling between real power and voltage. In reality, however, the coherency of generators can be influenced by voltage related factors such as location and speed of AVR controls, generator terminal voltage variation, and load bus voltage variation. The reduced dynamic equivalent is not accurate if the voltage characteristics of the original system are not included in the model. This paper presents coherency identification techniques incorporating rotor and voltage dynamics in order to improve the accuracy of dynamic equivalents. This paper also presents a new method to match the power flow conditions for generator aggregation. This new technique is shown to be critical for the accuracy of aggregated dynamic models. Numerical results based on the model of an actual power system in Asia are presented to demonstrate the performance of the proposed method.

This is a follow-on paper to "A New Thermal Governor Modeling Approach in the WECC" (Western Electricity Coordinating Council (WECC) was formerly the Western Systems Coordinating Council (WSCC). Thermal plants embraces conventional fired steam, nuclear steam, simple cycle gas turbine, and combined cycle gas turbine plants. The paper describes the steps being taken to obtain validated data for the new governor models. It was recognized that the developmental database that was created to prove the new thermal governor model should not be used to perform operation or planning studies. Invalid governor data for real-time operation simulations gives rise to risks and liabilities in the determination of intertie limits. Due to the large number of governors involved (about 1100 units) and the short timeline for obtaining data in critical 2003 summer operation studies, a concerted effort is being made to assist generator owners to select and validate the appropriate governor model codes for their units. This effort includes a workshop and the issue of detailed guidelines and validation methodologies for governor modeling.

This paper proposes a novel approach to identify the deviations of power flows which are controlled by multiple phase-shifting transformers (PSTs). In order to control the power flows of each loop independently, at the first, the minimum need of PST sets is determined as the number of co-trees to the entire network. For the reason that the co-tree patterns are not unique, there are many PST allocations that exist and can be found as the co-tree patterns. In this paper, since the correlation between power-flow deviations to the control angles of PSTs allocating in one co-tree pattern can be characterized into one matrix, the features of the controllable area of power flows corresponding to all PSTs allocation candidates can be extracted by the eigenanalysis of those matrices. By which, the features of the controllable area of power flows corresponding to all of the PST's allocation candidates can be extracted by the eigenanalysis of the matrix. Finally, judging from the power-flow profiles, the optimal allocation to the specific control target can be found. From the viewpoint of the flexible power-flow control in an open-access network, the proposal should contribute to find out the optimal PST's allocation for power-flow control. The effectiveness of the proposed method is confirmed by the simulation studies of two model systems.

Reactive power optimization with time-varying load demand in distribution systems is investigated in this paper. The objective of this paper is to determine the proper setting values of capacitor banks and transformer taps for the 24 hours in the next day. A heuristic and algorithmic combined approach is proposed in this paper. The approach simplifies the mathematical model of the daily setting values of reactive power/voltage control devices, solves the temporal optimization of each control device by heuristic rules, and then converts the optimization model with time-varying load into the same one as conventional optimization model with constant load. Therefore, the algorithms applied to the conventional optimization model can be easily used to solve the optimization model with time-varying load demand. Results from numerical examples show that a proper dispatch schedule for capacitor banks and transformer taps can be reached by this approach efficiently.

Summary form only given as follows. This paper uses the eigenvalue analysis approach to observe the impact of load models on an unstable low frequency oscillation damping, which recently occurred in the Taiwan power system. In order to enhance oscillation damping, the effect of load models on power system stabilizers is also investigated. Three kinds of the derived load models (a dynamic model, a composite model, and an exponential model) based on the component approach with the measured characteristics of power system loads are used in this paper. Time simulation results are extensively used to verify the eigenvalue analysis results with and without power system stabilizers. One set of field test data obtained from the Taiwan power nuclear plant with power system stabilizers is used to determine which load model provides a more accurate result in enhancement of oscillation damping. Depending on the results, the effect of load models on electromechanical oscillation damping with and without power system stabilizers is verified.

Summary form only given as follows. The accuracy of generator dynamic parameters is one of the crucial factors in correctly simulating the power system transient phenomena. This paper proposes a general procedure for estimating the generator dynamic parameters from measurements. The proposed procedure employs a gradient based simulation optimization technique to estimate the parameters. Effectiveness of the simulation optimization method is shown in some examples for estimating the exciter and governor model parameters. Impacts of noise and parameter uncertainties are briefly discussed in the paper.

This paper illustrates the application of value-based reliability planning (VBRP) methods to the problem of distribution substation capacity enhancement. The traditional approach is to consider relative investment and operating costs of various alternatives. VBRP enhances the traditional approach with the addition of reliability valuation. This includes a detailed representation of performance during transformer failures and the accelerated aging from increased stress on remaining transformers, as well as the cost of customer interruptions. The addition of a transformer is compared with various distributed resource options, and also the option of doing nothing and incurring more frequent interruptions and greater stress. This paper illustrates the usefulness of VBRP techniques for the planner who must consider customer value in planning decisions.

This paper examines the impact of a unified power flow controller (UPFC) on power system reliability. The UPFC is employed in the system to adjust the natural power sharing of two different parallel transmission lines and therefore enable the maximum transmission capacity to be utilized. The results of the investigations show a significant improvement in the system reliability by utilizing the UPFC. The improvement is measured using three reliability risk indices, namely, the loss of load expectation (LOLE), the loss of energy expectation (LOEE), and the system minutes (SM). The paper also presents a comparison between the effects of the UPFC and a thyristor controlled series capacitor on the system reliability.

Deterministic methods are widely used in operating reserve assessment and in small isolated power system capacity planning. These approaches do not normally include any explicit recognition of system risk and do not provide comparable risks for systems of different size or composition. Many large power systems, therefore, use probabilistic methods for generation adequacy evaluation. The reluctance to utilize the existing probabilistic techniques in both large and small systems dictates a need to create a bridge between the two different approaches. This paper presents a new probabilistic method designated as 'system well-being analysis', which incorporates the accepted deterministic criteria in the definition of 'healthy' and 'marginal' power system states. This paper illustrates methodologies to evaluate the system well-being indices of practical systems and describes how they can be used in the assessment of power system capacity reserves.

The analysis of railway power supplies (RPS) presents a unique type of load flow problem which is characterized by its constant moving loads, dynamically changing load types (constant current/constant power), and its specialized network structure. When multiple trains and the capacitors for power factor correction are also involved, the analysis of this special kind of power system can be very complicated and time consuming which will prevent on-line state analysis from becoming possible. This paper presents a novel and extremely efficient algorithm to solve this special kind of power flow problem with high precision. By first decoupling the autotransformer (AT) current into 2 equal phasor components and then applying an iteration-based circuit-analysis approach, the RPS power flow problem can be solved in an accurate but extremely efficient manner. An on-line railway power-supply state analyzer (RPSA) based on this approach is presented in this paper while the extension of using this approach to perform power factor corrector (PFC) simulation is also presented.

A hybrid genetic rule-based power system stabilizer (GRBPSS) is presented in this paper. The proposed approach uses genetic algorithms (GA) to search for optimal settings of rule-based power system stabilizer (RBPSS) parameters. Incorporation of GA in RBPSSs design will add an intelligent dimension to these stabilizers and significantly reduce the time consumed in the design process. It is shown in this paper that the performance of RBPSS can be improved significantly by incorporating a genetic-based learning mechanism. The performance of the proposed GRBPSS under different disturbances and loading conditions is investigated for a single machine infinite bus system and two multimachine power systems. The results show the superiority of the proposed GRBPSS as compared to both conventional lead-lag PSS (CPSS) and classical RBPSS. The capability of the proposed GRBPSS to damp out the local as well as the interarea modes of oscillations is also demonstrated.

This paper reviews several publications on the use of switching operations as a control means to relieve overload in transmission lines/transformers or to solve voltage problems. Previous research efforts dealing with the subject are analysed in relation to the objective of the manoeuvre, priority of this kind of action in relation to the others available and the search procedure employed, besides other implementation details. Some practical issues related to switching as a control option are also addressed in this paper. The purpose is to investigate the reasons why such procedures are adopted only in a few systems, and in most of them rarely employed. Distribution feeder reconfiguration for loss reduction or load balancing is not dealt here, neither are switching sequences for network restorative control.

This paper proposes new methods to improve the performance of the current volt/VAr control schemes at distribution substations. The approach makes use of the new measurement and computer resources that are now available at the substations. The paper shows that new supervisory type control schemes can be developed to improve the performance of both the voltage regulator controllers and also the substation capacitor controllers that are used at the substations for volt/VAr control. The new voltage regulator controller makes use of the voltage drop characteristics of the feeders it regulates. The capacitor controller uses local real and reactive power measurements for power factor control at the substation.

A new technique for artificial neural network (ANN) based short-term load forecasting (STLF) is presented in this paper. The technique implemented active selection of training data, employing the k-nearest neighbors concept. A novel concept of pilot simulation was used to determine the number of hidden units for the ANNs. The ensemble of local ANN predictors was used to produce the final forecast, whereby the iterative forecasting procedure used a simple average of ensemble ANNs. Results obtained using data from two US utilities showed forecasting accuracy comparable to those using similar techniques. Excellent forecasts for one-hour-ahead and five-days-ahead forecasting, robust behavior for sudden and large weather changes, low maximum errors and accurate peak-load predictions are some of the findings discussed in the paper.

The transfer of real power cannot be realized without reactive power/voltage support. However, compared with real power, the idea of reactive power costing and pricing is still not well defined. In this paper, the economic cost of reactive power is first analyzed. This cost includes the explicit and opportunity cost from various generation sources, such as generators, and the explicit cost from various transmission sources, such as reactive compensators, tap-changing transformers and the cost of transmission losses. Different from previous objectives functions, in this paper a cost-based reactive power dispatch is presented, which minimizes the total cost of reactive power support and determines the resultant reactive profile. This cost-based reactive power dispatch methodology provides information on the costing of reactive power and the information may be further expanded to include the pricing of reactive power.

This paper is one of a series presented on behalf of the Transmission Operation Subcommittee with the intent of focusing attention on EHV asymmetry issues during restoration. Many existing EHV lines have unsymmetrical (horizontal) conductor spacing without being transposed. These characteristics generate unacceptable negative sequence currents (NSC). Under light-load condition and during restoration, NSC has caused cascade tripping of a number of generators resulting in an area wide blackouts, has prevented synchronization of incoming generators, and has blocked remote cranking of thermal units. The paper describes a simple and approximate method for assessing the extent of NSC in EHV lines and then determining corrective measures. It recommends that when attempting to provide an offsite source to a thermal power plant the extent of NSC be determined and if unacceptable, either the operation be deferred to after the initial restoration phase, or the appropriate "underlying load" be determined for connection to the receiving end of the EHV lines.

Existing composite system reliability evaluation models typically calculate the reliability indices based on the events that lead to load curtailment. These indices do not reflect whether or not a system is on the edge of being in trouble. A power system, however, operates under different degrees of stress and strain throughout a period. This paper presents an approach to quantify a power network into several operating states in terms of the degree to which adequacy and security constraints are satisfied. The probabilistic indices calculated in this paper to quantify the power network at bulk load points and at system level reflect the actual operating behaviour of a power system. The algorithm proposed can be easily incorporated in any composite system reliability model that uses enumeration techniques for reliability evaluation.

This paper explores the influence of load models on decisions of undervoltage load shedding in power systems. A controlled load rejection can be used as an emergency countermeasure to avoid widespread blackout when system voltages are unstable. In this paper, dynamic simulations of a small power system using both static and dynamic load models are presented. When using a static load model, the system includes an explicit model of a transformer with load tap changer. The aim is to demonstrate how different load models influence the analysis and calculation of the amount of load needed to be shed to stabilise the system voltage.

This paper addresses the analysis and design issues in load frequency control (LFC) for a power system entity that participates in interconnection. The basic physical control mechanisms are the same whether the entity is a vertically integrated utility, an independent system operator or a virtual control area in a fully de-regulated market. Goals of this paper are twofold: to examine a rationale for known heuristic design guidelines; and to suggest an orderly methodology for control synthesis. Motivated by the large uncertainty in dynamic models of power system components and their interconnections, this paper proposes a physically motivated passivity objective as a means to achieve effective closed-loop control. Quantitative feedback theory provides convenient graphical tools for suggested design and analysis methods. A simulation model of a two-system interconnection is used as an illustration.

This paper presents the implementation of an efficient method for computing low order linear system models of power systems from time domain simulations. The method is the identification algorithm initially introduced by K. Steiglitz and L.E. McBride for calculating the transfer function of a linear system from samples of its input and output. The paper describes the original algorithm, added extensions, and issues addressed for its practical implementation. The applicability of the method is demonstrated by computing a low-order equivalent linear system of a detailed three-phase power system, modeled on an Electromagnetic Transients Program (EMTP). The paper discusses practical implementation issues.

This is part II of a two part paper dealing with control design in power systems using the normal forms technique. The companion paper (part I) describes the general theory and control procedure. Also, the proposed procedure was applied to the Base Case of the 50-generator IEEE Test System. This paper depicts the case studies to show the validation of the proposed method and to observe the effects of controller setting changes. In addition the results dealing with the design of the controller are given and verification of the controller design using nonlinear time simulation is provided.

Several models are being considered by the power industry for competition under open access transmission. Each alternative offers a radically different solution which must be evaluated, not only in terms of economic benefits, but also in terms of impact on power system security. This paper therefore presents a general mathematical framework for the analysis and management of power transactions under open access subject to system security constraints. The framework introduces the notions of a virtual network of transactions and the transactions matrix, both describing virtual power flows among financial entities. Finally, this paper shows how the transactions selected by the market forces are influenced by the security requirements of the physical network.

The European power systems are in the process of a new orientation which will facilitate and promote exchange of electric power between Eastern and Western European nations. One such example is the 4 GW HVDC link from Russia to Germany where a 1 GW power converter substation has been proposed for Central Poland. This paper considers the impact of the proposed 4 GW Russia-Germany HVDC link on the Polish grid network and possible excitation of torsional vibrations in shafts of steam turbine-generator exciter units connected to the system by noncharacteristic currents due to the link. Excitation of torsional vibrations in machine shafts due to modulating currents and the level of risk of damage to machine shafts due to subsynchronous currents injected into the network by the inverter is assessed. Dynamic parameters of system loads and the power factor of loads which influence system scaling factors are also discussed. The paper also examines the most probable location of the proposed 1000 MW converter on the Polish grid network. System scaling factors are evaluated for different system configurations for each generator at risk and generators most at risk are identified. The paper then investigates transient behaviour of the Polish power system following loss of the proposed link at full import. Transient stability performance is also evaluated for a range of system faults of different fault durations, distances and locations from power plants.

In this paper, a Petri net (PN) algorithm combined with a new power system restoration approach is implemented for multiple distribution contingencies. This approach is performed by releasing some loads required to maintain the rest of the system in safe operation in overload cases and opening all of the switches in the out-of-service areas for fault cases, and then closing proper switches to restore the isolated areas. PN switching models and the construction of PNs for multiple contingency operation are proposed to implement the restoration algorithm. Problem solving is efficiently performed by parallel-like reasoning in the PN for all the multiple events simultaneously. Heuristic rules and evaluation functions are presented for the best first search in the PN. Multiple contingencies studied in this paper include problems of feeder fault/overload/over-unbalance and main transformer fault/overload. A practical Taiwan power (Taipower) distribution system is simulated to demonstrate the effectiveness of the proposed method. It is concluded that multiple contingencies of distribution systems can be solved by the proposed method effectively.