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The condition assessment of the 190 MVA, OFWF, step-up generator transformer was performed by the mapping of degree of polymerization (DP). The transformer was taken out of service after 36 years and the active part, (transformer core and windings), was removed from the tank for close examination due to the major thermal fault at the transformer core. The transformer insulation consisted of two different materials, i.e. winding insulation was the thermally upgraded paper and spacer-pressboard system was Kraft paper. The temperature mapping is also shown, where the temperature estimation is based on the paper aging kinetics, transformer loading and insulation operating history. New equation for the relative aging rate is given considering all insulation conditions providing possibility of counting transformer loss-of-life more accurately. Copyright © 2010 John Wiley & Sons, Ltd.

Closed form solutions for the evaluation of the wave and natural parameters of inhomogeneous cable configurations where skin- and proximity-effects are present rarely exist. In most cases such parameters can only be retrieved from measurement experiments or by resorting to numerical methods. In this short paper we present the theoretical foundations of a novel measurement principle, as well as specific formulae for the determination of the parameters of a shielded bifilar line, avoiding the end-effects that usually arise when cables are tested using short- and open-circuit terminations. Copyright © 2010 John Wiley & Sons, Ltd.

SUMMARY This paper proposes an algorithm for transformer differential protection. The paper presents the development of two indices based on discrete wavelet transform to discriminate between internal faults and inrush currents. Each of the proposed indices, that is, the retained energy and the number of zeros can be used individually for the suggested algorithm. The proposed technique consists of decomposition of differential current signals up to a specific level and compression by level thresholding. Various simulations and analysis have been performed to determine the best compression ratio to distinguish between transformer inrush and fault currents. The performance of these indices is demonstrated by simulation of different faults and switching conditions on a modeled power system. Obtained results show that the proposed scheme is robust and reliable. Copyright © 2011 John Wiley & Sons, Ltd.

Reportedly, the switching operation in a gas-insulated substation (GIS) can lead to deformation of transformer winding. Also, electromagnetic fields radiated because of switching operations get coupled to the control equipment or data cables present in the GIS. These problems during switching operations in the GIS depend on the wave shape of the very fast transient overvoltages (VFTOs) and the associated very fast transient currents (VFTCs). Available methods are utilizing ferromagnetic rings and resistors to suppress the stresses created by very fast transient (VFT) from the source side. These equipment are mounted on the conductors linked to the disconnector. However, current methods are only applicable before installation period and during the substation design stage. The aim of this paper is to investigate feasible methods for mitigation of the overvoltage magnitude of VFT and suppression of the resonant frequencies amplitude at the power transformer. The proposed method utilizes terminal and capacitance components to suppress VFTs in GIS. Its advantages are their simplicity and low cost of implementation along with producing minimal changes in the installed GIS. The effectiveness of the proposed method to mitigate overvoltage magnitude and resonant frequencies amplitude of VFT is presented via simulation. In addition, the amplitude and frequency spectrum of the VFTO and VFTC at the various locations have been analyzed in this paper.

SUMMARY The Tokyo Electric Power Company, Inc (TEPCO) plans 1100 kV power network system in Japan in order to meet the steady increasing demand for electricity. Four-hundred kilometer 1100-kV-designed double-circuit transmission lines have already been constructed and are now operated at 550 kV. 1100-kV substation equipment has also been developed. To realize the best design technically and economically throughout the transmission line and the substation, important technical solutions for network problems peculiar to the ultra high voltage (UHV) system have been introduced. They include insulation coordination, overvoltage countermeasures, and fast multi-phase reclosing systems with high speed earthing switches (HSESs). The overvoltage countermeasures are realized by high performance metal oxide surge arresters (MOSAs), gas circuit breakers (GCBs) with closing/opening resistor, and disconnectors (DSs) with resistor. These sophisticated technologies realize highly reliable and economical UHV substations and transmission lines. This paper describes important, fundamental, and specific requirements for UHV substations such as their compactness, economical view, and environmental harmony. There are many technical/economical aspects to be considered on UHV substation and based on those considerations: (a) compactness, (b) less impacts to the environment, (c) optimizing cost, could be fulfilled. This paper also describes the technologies addressed to UHV class components such as the introduction of new concept “on-site assembly transformer,” VFTO reduction countermeasure and the necessity of full-scale seismic qualification test. Copyright © 2011 John Wiley & Sons, Ltd.

This paper presents a study on the characterization of aging of transformer oil–pressboard insulation. Traditional and modern diagnostic tools and techniques were used to investigate aging of oil–paper insulation under controlled laboratory conditions. Insulation condition testing includes common chemical diagnostics such as acidity tests and new chemical techniques: determination of relative content of dissolved decay products by spectrophotometry, solid suspension by turbidimetry, Fourier Transform Infrared (FTIR) spectroscopy and electrical diagnostic techniques that have gained exceptional importance for utility professionals: polarisation/depolarisation current, recovery voltage measurement and frequency domain dielectric spectroscopy measurements. From the obtained results it is found that chemical tests depicted more sensitivity to insulation degradation. Also, different oil–pressboard insulation ratios were investigated to assess the influence of the relative amount of cellulosic materials on aged mineral oil decay products and vice versa. Emphasize is laid on the potentialities/avenues offered by FTIR to assess oil insulation condition. Notable experimental results have been reported, which could be valuable to researchers and practicing engineers in this field. Copyright © 2010 John Wiley & Sons, Ltd.

This paper presents an online dynamic event detection and location method that uses wide-area generator rotor frequency measurements. It combines a variance-based approach to event detection with a clustering-based approach to event location. In the offline phase, the method involves partitioning the buses in a power network into coherent groups and selecting a single generator to represent each group. In the online phase, the method involves calculating the variance of each representative's rotor frequency over sliding window. This paper provides explanation and evidence that different detection thresholds should be used for variance-based triggers that have different window lengths. If the threshold is varied linearly with the window length then the variance-based criterion resembles the long-window derivative-based criterion. This paper provides guidelines for adjusting the window length and the detection threshold in a coordinated manner to balance the tradeoff between the detection delay and the prevalence of false detections. Copyright © 2010 John Wiley & Sons, Ltd.

Transients in capacitive voltage transformers (CVTs) are known to cause measurement problems in the relays. These switching transients can take several cycles to die out if the burden imposed by the secondary connected devices is a fraction of the CVT rated burden. In this paper, the transient response of the CVTs connected to low burden numerical relays is studied in detail and than the methods to improve the same are discussed. The methods discussed vary from simple modifications in existing CVT design to some major changes in the CVT construction. The paper gives a comprehensive coverage of design issues for CVTs intended to be used with numerical relays. Copyright © 2010 John Wiley & Sons, Ltd.

This paper focuses the performance and robustness of an algorithm recently proposed in the literature for real-time voltage instability risk indication. Based on local phasor measurements, the real-time identification of the proximity of the equivalent Thevenin impedance seen from an EHV bus and its load impedance is the key to recognize the voltage instability risk at the time it is approaching. The paper highlights the impact of the dynamics of on-load tap changing transformers and generators over-excitation limiters on the Thevenin equivalent identification (and consequently on the timely voltage instability indication). The paper also shows the high reliability of the algorithm with respect to load characteristics and to critical load profiles corresponding to very large perturbations. The paper presents simulation results obtained from a detailed dynamic model of an equivalent single-generator system, as well as from the actual Italian EHV network. Copyright © 2010 John Wiley & Sons, Ltd.

Distributed generation (DG) is playing an important role in enhancing the social sustainability. Because DGs connected to distribution network will impact power flow, it is necessary to develop a new approach of power flow analysis. In this paper, the interfaces between DGs and distribution network are introduced, the operating principles and control schemes of typical DGs, including asynchronous generators and solar batteries, are studied. A DC–AC sequential iteration approach is applied for calculating power flow with DGs. In addition, a practical sensitivity based contingency analysis approach is proposed in this paper. A 15-bus distribution network with wind power generation and solar energy is analyzed to exhibit the effectiveness of the proposed approaches. Copyright © 2008 John Wiley & Sons, Ltd.

Since large power transformers belong to the most valuable assets in electrical power networks, it is suitable to pay higher attention to these operating resources. Thermal impact leads not only to long-term oil/paper-insulation degradation; it is also a limiting factor for the transformer operation. Therefore, the knowledge of the temperature, especially the hot-spot temperature (HST), is of high interest. The calculation of current thermal stress helps to avoid unexpected outages. This paper presents the temperature distribution in windings of the oil power transformers. In this paper, energy (thermal) equation is solved until temperature is obtained. Oil in the transformer is assumed nearly incompressible and oil parameters such as thermal conductivity, special heat, viscosity, and density vary with temperature. For numerical solution of above equation, finite element method is used. The selected models for simulation are 300 KVA and 22.5 MVA ONAN transformers. Copyright © 2008 John Wiley & Sons, Ltd.

This paper presents a method to predict failure probability related to the aging of generating units in power systems. To calculate aging failure probability, Weibull distribution is usually used due to age-related reliability. The shape and scale parameters of Weibull distribution can be estimated by using hybrid method using both normal and Weibull distributions. In this paper, data analytic method applied to type II censoring is proposed which is relatively simpler and faster than the hybrid method for the estimation of parameters. The real historical data of combustion turbine generating units in Korean power systems are used in the case study to show how to calculate the mean life and its standard deviation by the proposed method. Copyright © 2008 John Wiley & Sons, Ltd.

This paper offers a new perspective over the traditional problem of the multiobjective optimal reconfiguration of electrical distribution systems in regular working state. The issue is indeed here formulated including also safety issues. Indeed, dimensioning the earth electrodes of their own secondary substations, distribution companies take into account the probable future configurations of the network due to transformations of overhead lines into cable lines or realization of new lines. On the contrary, they do not consider that, during normal working conditions. the structure of the network can be modified for long periods as a consequence of reconfiguration manoeuvres, with differences between the design current of the earthing systems and the fault current in certain substations significant. As a consequence, often distribution companies limit the implementation of the optimal reconfiguration layouts because they are unable to suitably evaluate the safety issue. In the paper, the problem is formulated including a further objective in order to account for the safety. A suitable constrained multiobjective formulation of the reconfiguration problem is therefore used aiming at: the minimal power losses operation, the verification of safety at distribution substations, the load balancing among the HV/MV transformers while keeping the voltage profile regular. The application carried out uses an NSGA-II algorithm whose performance is compared to that of a fuzzy logic-based multiobjective evolutionary algorithm. In the considered automated network, the remote control of tie-switches is possible and their layout is the optimization variable. After a brief description of the optimal reconfiguration problem for automated distribution networks, the most recent papers on the topic are reported and commented. Then the problem formulation and the solution algorithm are described in detail. Finally, test results on a large MV distribution network are reported and discussed. Copyright (C) 2008 John Wiley & Sons, Ltd.

Power grids have traditionally been designed to withstand without resulting in cascading events transmission outage contingencies of the N-1 or N-2 kind. However, in actual operation, power grids are potentially vulnerable to cascading outages. Reliable operations therefore require a risk-based approach to monitoring and managing the probability and impact of potential cascading outages. This paper defines and quantifies a measure that relates to the vulnerability of the power grid to cascading outages. The approach is to simulate and identify potential cascading modes (PCM) and to compute the probability and impact of each stage of the cascade and display the risk of each potential cascade stage on the two-dimensional display of likelihood and consequences. In this display, the traditional N-1 or N-2 contingencies will also be plotted for comparison. This approach enables grid operators to be fully aware of not only the contingencies for which they are required to assess and mitigate but also the potential cascading modes. This places both types of potential adverse events on the same basis for comparison. When the risk level of a potential cascading mode approaches or exceeds the risk level of all the N-1 contingencies (or N-2, if it is the reliability standard), then the operators will be well advised to take corrective actions to manage the overall risk of blackouts. Computation examples of applying this method are illustrated in this paper. Copyright © 2008 John Wiley & Sons, Ltd.

A reliable electricity supply infrastructure is fundamental to modern living. An interruption to electricity services has such far reaching impact to our everyday lives to various industries compared to other service interruptions, thus to build in significant redundancy and various preventive measures to electric power system operation and planning is needed. Just as important as to incorporate redundancy and preventive measures however, an effective restoration plan needs to be in place since the possibility of electricity service interruption cannot be eliminated completely. In this paper, a novel power system restoration plan that utilizes the characteristic of so-called scale-free networks is proposed. For a scale-free network, the importance of each node is determined based on the number of connections made to other nodes in that the nodes with many connections, thus important, are called “hubs.” A scale-free network is a special complex network which follows a power law distribution, that is few hubs and many nodes with few connections at various system sizes. In the proposed plan the hubs are restored before other insignificant nodes in the system. It is shown that by doing so, the total restoration time can be reduced considerably, and notable improvements can be achieved with respect to the objective function in the mathematical formulation of electric power system restoration problem with a few constraints introduced in this paper. The effectiveness of proposed plan is tested with one of the planning problems such as valuation of black-start capable generators using IEEE-30 bus system. Copyright © 2008 John Wiley & Sons, Ltd.

This paper deals with the problem of transmission loss allocation in very large networks with multiple interconnected regions or countries. The main objective is to propose a method in order to identify the amount of losses that each participant in the market causes on all system areas. Transmission pricing of cross-border trades is often difficult since individual countries may use incompatible internal transmission pricing regimes, and they are usually unwilling to disclose any sensitive information about their own systems. This paper presents a new methodology based on the loss formula concept for allocating electric losses to generators and loads, participating in multiple interconnected energy markets. The only data required are the power flows and characteristics of tie-lines and PV Ward equivalent model of area networks from border nodes point of view. No information is required about individual loads, generations, or detailed internal networks. The methodology is illustrated on the IEEE 118 node network which is divided into three areas, each with a different internal transmission pricing methodology. The proposed methodology is shown to be simple, transparent and very fast, and it can deal effectively with multiple pricing regimes. Copyright © 2008 John Wiley & Sons, Ltd.

Insulation coordination models are an essential part of power system studies and are used to determine the risk of damage to plant during abnormal conditions such as those experienced when lightning strikes the power system. One model used to predict the performance of an overhead line when hit by lightning examines the likelihood of a backflashover; the breakdown of an insulator coordination gap due to a rise in tower potential in comparison to a phase conductor. A backflashover model has many components that must be accurately modelled including the lightning strike, the tower, the shield wire/phase conductors, the insulator coordination gaps and the tower earthing system. Of all of these, the only two significant challenges are the accurate modelling of the insulator coordination gaps and the tower earthing system (the soil ionisation mechanism in particular). This paper focuses on the simulation of the insulator coordination gap for which available models range from a simple voltage controlled switch to the more complex leader progression model (LPM). The paper details the use of experimental data to produce a modified LPM in which certain model parameters are randomly varied. The development of this model using laboratory data is detailed, as is the effect that using this model has on the results of two insulation coordination studies that relate to backflashovers. Copyright © 2007 John Wiley & Sons, Ltd.

In the conventional excitation control concept, the power angle and frequency of a generator are driven to a pre-designed operation point after the fault occurs. It is named as Constant Point Stabilization (CPS) concept in this paper. A novel concept, called dynamic Center of Inertia (COI)-tracking concept is proposed in this paper. In the concept, the power angle and frequency of each generator track the dynamic COI of the power system. Compared to CPS concept, a salient feature the suggested dynamic COI-tracking concept has is that the generators are not restricted to constant angle point or frequency any longer but track the dynamic COI trajectory of the system to keep synchronous in rotor angle and frequency. Wide area measurement system (WAMS) will be used to transform COI signals to each generator. The time delay within a certain limit of WAMS signals is permitted. To make comparison between the two concepts, the control system models based on the two concepts are first established. Then, using the back-stepping method, two robust controllers are designed to achieve the control objectives of the two concepts. At last, dynamic simulations are carried out based on a 2-area-4-machine test power system, and the control effects of the two controllers, together with that of the conventional AVR + PSS excitation system, are compared. Copyright © 2007 John Wiley & Sons, Ltd.

In a deregulated power market scenario, the Independent System Operator (ISO) main problem is how to minimize the power system insecurity risk. To face this problem, in previous papers, assuming the insecurity risk as a part of the global cost, a model named Economically correct Secure Economic Dispatch (EcSED) has been proposed by the authors. Further, the EcSED has been improved considering the presence of an Unified Power Flow Controller (UPFC) device in the transmission network. In this paper, by means of an enhanced formulation of the improved EcSED, the size of an UPFC is determined, by including the UPFC investment cost within the objective function. Indeed, an UPFC installed in the transmission network may minimize the insecurity risk, hence the global cost, although its usage implies an expense, function of the size of the device itself. Some numerical experiments on a 5-bus test system are also reported, to prove the effectiveness of the enhanced EcSED model formulation. Copyright © 2007 John Wiley & Sons, Ltd.

A lossy electric power system including normal and limited energy supply thermal units and hydraulic units are considered in this paper. The total thermal cost for an operation period of the system is minimized under some possible electric, hydraulic, and fuel constraints. The minimization of the total thermal cost is achieved by determining active generations of all units in all subintervals by means of a spot price of electricity algorithm proposed by our research group. Application of the algorithm is demonstrated on a lossy electric power system that contains 16 buses, three normal thermal, two limited energy supply thermal and 4 hydraulic units (coupled). Obtained results are discussed in the example section of the paper. Copyright © 2007 John Wiley & Sons, Ltd.

This paper presents a new neural network-based transmission line distance protection module. The proposed module uses samples of voltage and current signals to learn the hidden relationship existing in the input patterns. Using a power system model, simulation studies are preformed and influence of changing system parameters such as fault resistance and power flow direction is studied. The proposed neural network has also been implemented on a digital signal processor (DSP) board and its behavior is investigated using suitable developed hardware. Details of the implementation and experimental studies are given in the paper. Performance studies results show that the proposed algorithm is able to distinguish various transmission line faults rapidly and correctly. It shows that the proposed network is fast, reliable, and accurate. Copyright © 2007 John Wiley & Sons, Ltd.

This paper is the first part of a work dedicated to the analysis and computation of the electric and magnetic behavior of inductor windings (e.g., power transformer windings) operating at high-frequency regimes; a recognized key issue for very fast transient overvoltage studies. Here, the inductor winding, containing a total number of N turns, is modeled as a system of N-coupled parallel conductors resembling a uniform inhomogeneous multiconductor transmission-line structure (MTL) where proximity effects cannot be neglected. The characterization of the winding's equivalent MTL structure requires the specification and evaluation of its constitutive parameters, namely, the per-unit-length (PUL) capacitance and inductance matrices, and also the PUL resistance and conductance matrices, the latter two associated with losses. Close proximity between inductor turns together with the consideration of dielectric-coated turns prevents the utilization of closed-form expressions for the evaluation of most of the inductor constitutive parameters. Here we employ a harmonic expansion method (HEM) primarily aimed at the construction of the capacitance matrix from where the inductance and conductance matrices can be derived. A new theory based on Dubanton's approach but taking into account proximity effects related to eddy currents in the inductor core is also proposed and developed. This research paper provides, in a tutorial fashion, a comprehensive explanation of the details and approximations involved in the evaluation of the constitutive parameters of the MTL structure under analysis––an aspect that unfortunately is lightly tackled or even ill-treated in the literature of winding models. Copyright © 2007 John Wiley & Sons, Ltd.

This paper describes dynamic performance analysis of Unified Power Flow Controller (UPFC) composed of cascaded H-bridge and single-phase multi-winding transformers for isolation. The operational characteristic was analyzed through simulations with PSCAD/EMTDC, and experimental works with a scaled model. The UPFC proposed in this paper can be directly connected to the transmission line without series injection transformers. It has flexibility to expand the operation voltage by increasing the number of H-bridges. Copyright © 2007 John Wiley & Sons, Ltd.

This paper presents the dynamic simulation of voltage collapse phenomenon in a flexible manner concerning modelling complexity. In that respect a special attention has been paid to modelling aspects of induction motor as a representative of variety of loads. A complete mathematical model used in this paper for the study of dynamic behaviour of an induction motor consists of five differential equations. This set of equations is solved by using the Runge-Kutta method. As a parallel model a simplified approach is considered (without differential equations) reduced to algebraic network equations and the equation of motion. The effect of transformers equipped with automatic under load tap changers is also simulated. The complete model is rigorous enough in order to calculate the precise voltage-time and P-U curves during the induction motor start-up procedure, while the simplified model can be used for calculation of P-U curves only. Results from this study can be used for real-time system control actions and for induction motor relay settings.

This paper presents a new generalized method of modelling of any kind of shunt and series faults as single or simultaneous faults in arbitrary combination. The proposed approach can be used for conventional fault studies for the steady-state model based on the nodal admittance representation as well as for fault simulations under transient conditions by means of the EMTP/ATP program. This method has been developed for phase coordinates but it is also suitable for any modal coordinates. The purpose of this paper is to show the incorporation of these fault conditions in the EMTP/ATP algorithm.

Subsynchronous resonance (SSR) is a condition that can exist in a power system, especially for long-distance transmission systems with series compensated line. It can cause shaft fatigue and possible damage or failure of the generator involved. The analysis of the SSR phenomenon can be performed usually by using three individual analytical methods -frequency scanning, eigenvalue analysis in the frequency domain and transient torque analysis in the time domain. In this paper the comprehensive simulation program NETOMAC with a complete solution for analysing SSR is presented. Based on a common data set of the system concerned, both frequency and transient torque analysis can be performed, taking interactions among the spring-mass system of the turbine-generator units and the electrical network into consideration. Two case studies are demonstrated in the paper. The results achieved validate the functionality of the program for SSR analysis.

Field orientation provides the possibility to control torque and flux of the induction machine independently. However, the widely used control strategy for field orientation is based on constant rotor flux. This strategy does not utilize all advantages by using field-orientation method. This paper reveals a degree of freedom for controlling an induction machine, by controlling both field- and torque components. It presents a control strategy based on varying the flux level to provide optimum torque operation irrespective of speed ranges. The freedom for controlling both field- and torque components of the stator winding currents can make the inverter induction motor drive superior to DC drives, where proper resolving for the stator current components can provide maximum electro-magnetic torque per ampere, optimal efficiency and higher short duty system capability. In this paper, the rotor-field oriented saturated model of the induction machine is presented. The analysis of the electro-magnetic torque, in terms of stator current components, is used to find the optimal resolving ratio for maximum torque. This optimal ratio depends on the machine parameters and the saturation level. Investigation and comparison for widely used and the proposed control strategies are given for different cases, namely: neglecting saturation effect case, then taking saturation into account for low- (0.75 kW) and high power rating (90 kW) machines. Also, the proposed optimal control strategy and the microprocessor implementation method are presented.

This paper deals with mutually connected problems of reactive power compensation and voltage control in radial distribution systems. The paper presents a simple iterative procedure for optimal coordination of actions of main means for reactive compensation (shunt capacitors) and voltage control (variable turn ratio transformers), within the decoupled solution of these two subproblems. The procedure is based on the static optimization, specified as a non-linear programming-type problem. The paper presents the derivation of a method for the solution of specified control problems and gives an illustrative application of this method on the test example of a one-source radial distribution system, consisting of 53 nodes and 52 branches.

In this paper the results of testing and calibration of measuring systems for very fast transients (VFT) in gas-insulated substation (GIS) are presented. The general principles for the construction of measuring systems for recording and transmission of VFT are also discussed. The paper describes in detail the constructions offour measuring probe types adjusted for VFT monitoring in 123 kV SF 6 -insulated substation. The primary construction requirement was the ability to monitor VFT in 123kV SF6-insulated substation containing frequencies ≥ 200 MHz without distortion in both frequency and voltage domains. For the purpose of the measuring system testing and calibration an experimental facility was developed based on the computer experimental modelling technique. The verified experimental facility was then used for testing and calibration of measuring probes type. During the test, various effects on the measuring system response quality were noted with respect to the applied constructional solutions, with different kinds ofdielectric material used for the low-voltage capacitor and different diameters of the sensor probe. The obtained results are explained and the possibilities offurther improvement of SF 6 VFT measuring system characteristics suggested.

This paper investigates the multiconductor busbar behaviour under AC conditions with the aim of comparing the predictions of traditional circuit models with a more accurate analysis based on the electromagnetic field computation. This analysis employs a two-dimensional finite-element technique including eddy-current effects. In linear systems this approach leads to a circuit model described by an impedance matrix, while, in presence of saturable media, field and circuit equations are simultaneously solved by a voltage-driven field formulation. The paper analyses the behaviour of two different busbar systems when a short-circuit current flows through them ; the results obtained by the proposed approach are compared with those deduced according to the methods suggested by the IEC Standards and by the symmetrical components.

This paper deals with improving power quality in a situation where this quality is degraded by a low-quality load that may cause voltage asymmetry, harmonic distortion and variation of the voltage RMS value. The paper discusses a decomposition of the useless component of the supply current into a component compensated with an adaptive passive circuit, of parameters updated to varying load conditions once a cycle, and a component compensated with a continuously controlled current source. Controlling such a hybrid compensator requires that some parameters of the load, defined in the frequency domain, are measured along with instantaneous values of voltages and currents

This paper describes a methodology for the reliability evaluation of bulk power systems. The method encompasses the systematic enumeration of contingencies and the evaluation of their effects on the system over a range of system load levels. Contingencies and electric load levels are described with Markov models. Reliability indices are computed using these models. Both an adequacy approach and a simplified security approach are considered in the effects analysis. The overall framework of the proposed methodology is capable of considering the full security approach but it is not addressed in this paper. In the adequacy approach, an improved system simulation method is used based on the single-phase quadratized power flow (QPF). Remedial actions (RAs) are also applied for the purpose of alleviating system abnormal operating conditions. The adequacy approach determines whether the system is adequate for supplying the electric load without operating constraint violations. In the simplified security approach, the immediate response of the system is of interest. The simulation approach consists of an inertial re-dispatch and operating conditions immediately after a contingency before any controls take effect. The objective is to determine whether cascading failures occur. The proposed methodology has been applied to two reliability test systems.

In today's economic climate, it is important to know the condition, by means of suitable diagnostic tests, of the oil impregnated paper usable as primary insulation in equipment such as transformers, switchgear, bushings, cables, and their accessories. The aim of this paper is regarded as a main task to study the parameters that mostly influence the ageing process of oil/paper insulation used in transformers with preset moisture levels. A comparison is made between the performances of cellulose and Aramid papers. It is shown that Aramid paper is much less sensitive to water than cellulose paper. However, the addition of air (oxygen), via acid formation and oxidation in the oil, has a direct influence on the increase of the loss factor for both papers. The catalysts, that represent the metallic components in the transformer, accelerate the ageing process of the cellulose papers, while no influence on the ageing process of Aramid was observed. Oil ageing without a solid partner is insignificantly influenced by water, but accelerated by air-oxygen, via the moisture and acid formation, and oxidation processes. A direct influence on the increase of the loss factor and the decrease of electric strength particularly at low temperatures is to be noted. Copyright © 2006 John Wiley & Sons, Ltd.

The purpose of this paper is to present and assess a new methodology for obtaining the parameters of static load models, which represent the aggregated load behaviour at the distribution level. This methodology is divided into two stages: the first one obtains the load composition at the point of measurement using a non-intrusive method, while the second one generates the final aggregated static load models. Its main advantage, in contrast to previous parameter estimations, is that it obviates the need to apply artificial voltage steps at the distribution network—generated by load tap changers or capacitor switching, placed at the substations. In this way, the methodology can be used to obtain the static load parameters of the aggregated load in real time. To assess the suitability of the method, several measurements were taken at a distribution transformer—in a university environment—and the obtained results have also been included in this paper. Copyright © 2006 John Wiley & Sons, Ltd.

This paper presents an approach for the combined optimization of energy systems including multiple energy carriers such as electricity, natural gas, and district heat. Power flow and conversion between the different energy infrastructures are described as multi-input multi-output coupling, what enables simple analysis and optimization of the flows. While previous work deals with operational optimization (multi-carrier optimal dispatch and power flow), this paper focuses on optimization of the couplings between the different networks, i.e. the structure of the system.

This paper introduces the structure and operation modes of a doubly-fed brushless machine (DFBM) with axially laminated anisotropy (ALA) rotor. The air gap magnetic field of the DFBM is analysed by means of the field modulation theory. The coupling parameter between the stator main and auxiliary winding is tested and discussed. The adjusting speed performance of the DFBM is also presented in this paper. It is indicated that the DFBM adjustable speed system needs only a much smaller power inverter in comparison with the adjustable speed system using a conventional AC motor. Copyright © 2006 John Wiley & Sons, Ltd.

In this paper a simple and robust new relaying technique for protecting transformers from internal winding faults is proposed. Based on the measurement of zero sequence current inside a delta winding, the technique greatly simplifies the conventional differential relaying arrangement when a delta winding is available. Despite the number of windings of the transformer and the location of the fault, only measurement of induced zero sequence current within the delta winding is needed. Since the proposed technique has been shown to be prone to generate false pick-up signals during inrush, a simple restraining criterion is proposed and analysed. Additionally, use of projection techniques based on self-organizing maps (SOM) is proposed in this paper as a valuable tool for analysing multivariable data which are generated from the huge number of possible combinations existing between switching instant and fault location. Finite element simulations and laboratory tests have been combined into SOM to validate the proposed relaying technique and the restraining criterion. Copyright © 2005 John Wiley & Sons, Ltd.

This paper deals with an important problem related to power quality that has been tackled in the literature quite often. The three-phase distribution transformer in electrical power systems is generally installed for supplying loads with rated frequency. Non-linear loads introduce harmonics to the supply and result in a different performance compared with that of the designed transformer. In this paper a three-phase 50 kVA distribution transformer is modeled using the finite element (FE) method in order to evaluate its losses under non-linear loads. Comparison of the FE computation results with the analytical standard method results shows that the latter method leads to a more conservative estimate of the transformer derating. Copyright © 2005 John Wiley & Sons, Ltd.

SUMMARY This paper presents a design procedure of a flux–switch synchronous machine for an aircraft oil breather application. This work is part of a European project called ‘Power Optimised Aircraft’. The aim of this project is to replace some mechanical equipment by electromechanical devices to improve performance and reduce power consumption. The structure under study was developed at SATIE laboratory and it is based on the flux-switch principle. The permanent magnets are located in the stator, and the rotor is entirely passive. The study described in this paper is limited to the electromagnetic design. Copyright # 2005 John Wiley & Sons, Ltd.

The operation of distribution systems requires a rather high number of planned or forced switching operations. In order to prepare them, it is necessary to estimate loads at different levels in the system. This paper proposes a load estimation method for distribution systems with minimum information available. The purpose of the load estimation method presented in this paper is the economic and efficient use of the available remote measurements in the distribution system considering typical measurements from the past and knowledge of load composition and load behaviour at the distribution transformers' level. The method is load-flow based, and its capability, efficiency and accuracy are demonstrated by application on a practical distribution system. Copyright © 2004 John Wiley & Sons, Ltd.

Complete paper : http://www3.interscience.wiley.com/cgi-bin/fulltext/108061601/PDFSTART; International audience; The numerous qualities of asynchronous machines enhance their use in industry. However, various faults can occur, such as stator short-circuit and rotor failures. We focus our study on the latter since the unknown evolution of currents in the squirrel cage makes the detection of the rotor's failures very difficult. In this paper we first present the effects of the rotor failures by means of a finite element model. In the second part we, present our strategy of monitoring for this kind of fault. We synthesize a so-called 'reduced-order extended high-gain observer' which follows the on-line evolution of an equivalent rotor resistance using a Park model. It provides an on-line alarm. However, this parameter is not enough to indicate the presence of an actual fault, thus we synthesize an extended Kalman filter to survey the three rotor resistances of a three-phase model. This provides an off-line analysis by verifying the balanced state of the rotor resistances.

This paper investigates the switching impulse and temporary overvoltage (TOV) duty for 400 kV underground power cables using ATP. This work together with the work reported in [1], investigates the full range of transient overvoltages due to lightning, switching and other temporary overvoltages (TOVs) using ATP. [1] re-assesses the basic insulation level (BIL) for 400 kV oil-filled cables. The objective of this work was to check the impact of switching surges on the choice of BIL and to re-assess the switching impulse insulation level (SIL) and TOV withstand level for 400 kV cables. A comprehensive study of the transient overvoltages experienced by cables within National Grid's transmission system has been performed. This paper provides an argument, for reducing the BIL and SIL for 400 kV cables.

For the planned, great offshore-wind farms with extended medium-voltage cable networks and distances to the onshore grid connection point of 100 km or more, the costs of the cables and their losses play an important role. In this paper basic equations are developed to determine the losses of long three-phase AC cables, taking into account the longitudinal distributions of current and temperature. It is shown, that the determination of cable losses by means of the line equations based on the nominal ohmic resistance, i.e. in neglecting the temperature-dependence, may lead to essential errors, with the risk of counterfeited valuations of the installation devices under technical and economical aspects. The relations derived in this paper allow a fast determination of the load- and temperature-dependent cable losses by means of a few, only cable-specific parameters.

This paper presents a method based on genetic algorithm for the automatic generation control of power systems. The technique is applied to control a system, which includes two areas tied together through a power line. As a consequence of continuously load variation, the frequency of the power system changes with time. In conventional studies, frequency transients are minimized by using integral controllers and thus zero steady-state error is obtained. In this paper, integral controller gains and frequency bias factors are determined by using genetic algorithm. The results of simulation reveal the application of the genetic algorithm having easy implementation to find the global optimum values of the control parameters.

A frequently applied method of speed-sensorless rotor flux oriented control of induction machines relies on utilisation of model reference adaptive system (MRAS) based speed estimation, where the outputs of the reference and the adjustable model are selected as rotor flux space phasors. Accuracy of the method heavily depends on correct setting of the machine parameters and adjustment of the filter and PI controller parameters within the estimator. The paper at first describes tuning of various parameters of the estimator, using purely experimental data. The speed estimator is operated in parallel with a commercially available rotor flux oriented induction motor drive with speed sensor and sampled stator voltages and currents are used to tune induction motor parameters, various filters and the PI controller within the estimator. The procedure is described and illustrated using a comparison between the measured actual speed response during acceleration transients and the corresponding speed estimate obtained from the speed estimator. In the second part of the paper, speed estimation error that will take place in the base speed region due to incorrect setting and/or variation of the parameters of the machine (stator resistance, rotor resistance and magnetising inductance) within the speed estimator is assessed using experimentally recorded data. The experimental results are found to be in very good agreement with previously published theoretical results.

Before power electronics were developed, non-periodic currents used to occur in distribution systems, apart from arc furnaces supply, mainly during switching and faults. Now, such currents are produced at normal operation of some power electronics equipment. Power electronics enables very fast control of processes and energy flow. Non-periodic currents are by-products of such a fast control. Non-periodic currents in electrical power systems occur as an effect of deformation or conversion of periodic, usually sinusoidal voltage and the periodic component of the supply current is crucial for permanent transfer of energy from generators to loads. Such non-periodic current, with a periodic or quasi-periodic components are referred to as semi-periodic currents in this paper. The paper provides fundamentals of semi-periodic current compensation and discusses a control algorithm of such a compensator.

The paper investigates detuning effects, caused by parameter variations, in an indirect current-fed rotor-flux oriented induction motor drive aimed at operation in both the base speed region and the field-weakening region. Rotor resistance variation and magnetising inductance variation are elaborated. In contrast to the existing studies, where these detuning effects are analysed asing appropriate induction machine models, the approach undertaken in this paper is purely experimental. Detuning is evaluated based on measured speed response in transient operation and measured stator current fundamental component in steady-state operation. Detuning during steady-state operation in the field-weakening region is additionally characterised with measured stator voltage fundamental component.

In island grids and weakly interconnected power systems, the loss of a large proportion of generation will cause the system frequency to fall dramatically. In order to ensure a stable operation with the lowest impact to the system, the disturbed power balance must be equalized within a short specified time by activating the second-reserve of on-line units or by load shedding or both. Unit commitment procedures shall consider these factors to ensure a reliable power supply while minimizing the fuel costs. This paper presents a unit commitment procedure taking the frequency security condition of systems into account. The procedure commits and optimizes units, calculates necessary second-reserve capability, and allocates them among the available on-line units. Parallel to the minimization of daily fuel costs, a specified frequency minimum following the loss of generation is employed as a criterion for maintaining system security. A case study on typical island systems with a large number of different units is demonstrated using the proposed procedure. Results from the study validated robust performance of the proposed procedure that minimizes fuel costs while maintaining frequency security condition. This paper considers only the frequency security condition to be handled. However, it can also be extended with other criteria such transmission capability during transient conditions of interconnected systems.

This paper presents two study cases of interaction analysis of multiple-function FACTS (flexible AC transmission systems) controllers: - the contradiction between the voltage and the damping control of a SVC (static VAr compensator); - the interaction between the voltage control of the DC link capacitor and the damping control of a UPFC (unified power-flow controller). In the paper, an algorithm is proposed for the co-ordinated design of these two multiple-function FACTS controllers. Examples are presented to confirm the analytical conclusions obtained and to demonstrate the effectiveness of the algorithm of the co-ordination proposed.

Damping torque analysis (DTA) is a well-developed technique for understanding and studying power system oscillations. It is established on the basis of physical explanation of damping torque contribution of power system control to the rotor motion of generator. Hence when it is applied in a multi-machine power system, the DTA becomes complicated due to the involvement of large number of generators. In this paper, a graphical explanation of the DTA method is presented by using the small-signal equal-area criterion of power system stability analysis. The graphical explanation presented turns out to be completely independent of the physical concept of damping torque, indicating the significant potential to develop the graphical explanation into a general and simple approach for understanding and studying multi-machine power system oscillations. In the paper, an example of a multi-machine power system installed with a STATCOM (static synchronous compensator) is given to demonstrate how the potential of applying graphical explanation can be explored. Based on the conclusion drawn from the graphical explanation, it is shown why the STATCOM voltage control contributes little damping to power system oscillations and a simple method of designing a STATCOM stabilizer is proposed. Confirmation from simulation results of an example four-machine power system is presented in the paper. Copyright © 2009 John Wiley & Sons, Ltd.