The author comments on the paper by V.T. Morgan on mixed convection effects on thermal resistance of multicore bundled cables in air (see IEE Proc. C, Gener. Transm. Distrib., vol.139, no.2, p.109-16, 1992). The author gives a qualitative approach to determining the thermal resistances due to both natural and forced convection respectively. The original author replies to the comments
A new decomposition of sinusoidal 3-phase currents is proposed. The main feature is that it leads to the compensation of nonactive currents partly by elements without energy storage and partly by means of linear elements with energy storage (inductors and capacitors). The proposed decomposition is compared with existing procedures, such as those developed by Akagi-Nabae and by Czarnecki.
The alleviation of emergency transmission line overload is a critical problem in power system operation. A method is presented for generation rescheduling and load shedding to elleviate line overloads, with the help of a local optimisation concept. The method developed is superior to existing methods, as the new secure operating point is obtained for all line overload cases efficiently, and a significantly smaller amount of load shedding is required. It is an aid to the load dispatcher and will find extensive applications in operational planning, security studies and the reliability evaluation of power systems. Test results for the 14-bus, 118-bus IEEE test systems, 6-bus and 26-bus SPC systems, and the 64-bus model of the North-West power system of India are presented to show the effectiveness of the method.
In this paper a new transformer model, named ‘geometrical’, is developed, based on the circuit analysis of its magnetic core. The method is general and can be used for any type of multiphase multiwinding transformer. The model simulates the transformer as a set of coupled branches and can be incorporated into the widely known electromagnetic transients program (EMTP). A basic advantage of the model is that it can represent any asymmetry of the transformer magnetic circuit. As a study case, the model of an actual 280 MVA autotransformer is derived and used for the simulation of various steady state and transient conditions.
An application of artificial neural networks in short-term load forecasting is described. An algorithm using an unsupervised/supervised learning concept and historical relationship between the load and temperature for a given season, day type and hour of the day to forecast hourly electric load with a lead time of 24 hours is proposed. An additional approach using functional link net, temperature variables, average load and last one-hour load of previous day is introduced and compared with the ANN model with one hidden layer load forecast. In spite of limited available weather variables (maximum, minimum and average temperature for the day) quite acceptable results have been achieved. The 24-hour-ahead forecast errors (absolute average) ranged from 2.78% for Saturdays and 3.12% for working days to 3.54% for Sundays.
A Fredholm integral equation of the first kind is developed for finding the charge distribution on the surface of a rectangular plate buried in a homogeneous earth. The method of moments is used to solve the functional equations. Charge accumulations at the edges and the corners are represented by expansion functions with an appropriate singularity. Owing to edge and comer effects, the surge current is dispersed into earth in a relatively small region near the edge, especially the corners. Thus, the capacitance of an earthing plate is not directly proportional to its area. Based on the charge distribution, other characteristics of the plate are derived. The earthing resistance of a rectangular earthing plate is lower than that of a square plate of the same size, and the resistance of the latter is lower than that of a circular disk. The potential profile of a square plate on the earth surface is much smoother compared with that of a rectangular plate, and therefore it exhibits lower step and touch potentials.
A method of increasing the pulse number of an HVDC convertor based on the reinjection of the DC ripple is applied to a unit-connected generator-convertor scheme operating at variable speeds. Theoretical and experimental results are provided which demonstrate that, with the proposed modification, a single-bridge configuration can be made to operate as an 18-pulse convertor for a wide range of generator frequencies.
A new technique for ranking outages in power systems is presented. Traditional methods for ranking are limited to cases where the system state under the respective contingency condition can be simulated. Cases where a load-flow solution is unobtainable are unexplained. The new method presented in the paper can similarly rank all cases of contingencies, and cases that can cause voltage instability and system collapse are revealed. The information given by this new method is important because it can then attract the attention of the planning engineer to the problem of voltage instability that may exist in a power system. Thus power systems can now be designed to take into account the voltage instability problem as well as the normal security requirements.
A two-stage power system distribution planning algorithm is developed for solving sizing, location and timing problems of distribution substations and primary feeders. This comprehensive algorithm involves an accurate small-area electric load forecasting procedure, and provides the flexibility for both static- and dynamic-planning application modes. Also, the algorithm includes a new nondiscrete planning model which accurately simulates the different cost functions of substations and primary feeders. Moreover, constaints on voltage drop, equipment capacity and power-conservation constraints are included in the planning problem formulation. Finally, the proposed algorithm is illustrated using a detailed numerical example, and subsequently tested for both efficiency and accuracy.
This paper presents simple and efficient algorithms for the alleviation of line overloads and voltage violations by corrective rescheduling. The proposed approach utilises the decoupling of real and reactive power and the decomposition between optimisation without security constraints and optimisation to satisfy security constraints. Highlights of the proposed approach are: (i) a choice of performance index which ensures that alleviation of some of the existing violations does not create any fresh violations, thus avoiding the need for cycling in optimisation and, (ii) the use of a classical optimisation technique for faster solutions. Results for two sample test systems have been presented to validate the proposed algorithms.
The current electricity supply position of Poland, Czechoslovakia and Hungary is reviewed, the political and economic background examined, and details of primary energy sources for the three states presented. Generation capacity, production and consumption are detailed before an examination of some of the environmental consequences of past policy is made. Conclusions are made on the present problems, and the possible future direction of the electricity supply industry in these states is examined
The paper presents a numerical study of the behaviour of a transformer winding with one grounded end, when stressed by standard impulse voltage waves chopped at different instants on the front as well as on the tail of the wave. This is basically an extension of the investigation undertaken by the authors on surge performance of a transformer winding. The mathematical model considered takes into account the series and shunt capacitances and inductance of the winding elements, including the effect of their mutual inductances. Conductor and dielectric losses are neglected for the sake of simplicity and to highlight the most critical potential swing and mounting of electrical stresses in winding discs that are likely to be promoted along the winding during the course of free oscillations going undamped in the absence of a damping factor. A defined chopped impulse voltage is judiciously split into its component impulse waves, and superimposition of their effects is recognised to be the response to the chopped wave. It is observed by the authors that for a severe stress concentration, the instant of chopping plays a vital role and hence becomes highly critical for transformer testing. The paper aims at suggesting the propriety of finding the critical instant of chopping determined with regard to the design and rating of the transformer under test. It also confirms much lesser stress concentration in midwinding discs than in those at the ends.
A method is proposed for identifying the critical cluster of machines, i.e. the machines responsible for loss of synchronism in a power system following a large disturbance. it is based on the conjecture that the loss-of-synchronism condition can be recognised by, first, considering 'near-critically cleared trajectories' of the machines and, secondly, observing how they are organised near the system's unstable equilibrium point. The term 'near-critically cleared trajectories' is meant to imply the swing curves of the system in the postfault-phase and is computed for a fault-clearing time that is slightly larger than the actual critical clearing time. To realise the above, the method uses the extended equal-area criterion. This direct criterion makes it possible to assess a convenient clearing time for computing the swing curves; to determine the system's unstable equilibrium point with great ease; and to select the critical machines by observing them at the time corresponding to this unstable equilibrium point. It also makes the speed of critical-cluster identification compatible with real-time requirements. Examples are given using the IEEE test system to illustrate the essential features: reliability in correctly identifying the critical clusters, robustness with respect to its capacity to do so under very stringent conditions, and effectiveness concerning its ability automatically to identify critical clusters of any size, i.e. irrespective of the number of machines that they contain.
The advanced static VAR compensator (now widely known as the static condenser or STATCON) uses a high power self-commutating inverter to draw reactive current from a transmission line. Two fundamentally different types of invertor can be used for this purpose, one providing control of output voltage magnitude and phase angle, and the other having only phase angle control. For each of these types, the governing equations are derived, and frequency domain analysis is used to obtain the relevant transfer functions for control system synthesis. Further analysis is provided to determine the response of the STATCON to negative sequence and harmonic voltage components on the transmission line. The results are illustrated with measured waveforms obtained from a scaled analogue model of an 80 MVAR STATCON.
The objective of the paper is to present a simple method of finding the harmonic coefficient of a trigonometric Fourier series representation for signals which are defined over a finite range. This method can be used selectively to track any harmonic component or range of harmonic components of the signal. A matrix method has been developed which uses the least square (LSQ) criterion to approximate the given signal with that of the assumed Fourier series representation. The method can be used, in general, as signal processing tool. For the purpose of illustration, a digital computer program is developed for selective harmonic tracking (SHT) of waveforms generated in multiple pulse converter systems connected to an HV AC system.
A new accurate model representing Phase-shifting transformers for both load flow and fault analysis is introduced. The model is simple, efficient and numerically stable. The theoretical basis of the model and the practical procedures of implementation for the fast-decoupled load flow method and short-circuit calculation are presented. For load flow implementation the developed model requires no additional storage except for data; the number of iterations and the time per iteration is unchanged. The phase-shifting transformers have been used for real power control in parallel circuits and the relative analyses and test results are presented. For short-circuit analysis, the asymmetry in the admittance matrix introduced by the phase-shifting transformers is dealt with in two stages. The network is first solved using symmetrical admittance matrix, fully exploiting sparsity, and subsequently the effect of the asymmetry is superposed using the diakoptical approach and effective programming, which optimises the calculation time and storage. To illustrate the process involved a small numerical example is given in Appendix 9. 1. The model described was implemented in the Interactive Power System Analysis program [12], and the results of verification for various transformer arrangements in typical practical networks are presented.
In a number of applications of generating and motoring, a low shaft speed is required. The usual way of providing this low speed is either through large diameter multipole machines or through high-speed machines in conjunction with a gearbox. A novel machine has been designed which can interface the mains to a low-speed shaft without the necessity for a large number of poles. The principle of operation is based on a stationary DC magnetic field, a short-circuited rotor winding and a stator winding connected to the supply through a set of filters. The rotation of the rotor causes circulation of harmonic currents in the stator. The filters are designed to allow these circulations, but to divert the 50 Hz current into the mains. This effect provides a speed gearing, the level of which depends on the number of harmonics permitted to circulate. The principle can be applied to a single or 3-phase design. The operational characteristics are those of a synchronous machine possessing a distinct number of synchronous speeds.
A new algorithm for constraint relaxation to deal with infeasible problems in LP-based optimal power flow is presented, together with results from tests of the algorithm. This alsorithm follows an analytical approach. In contrast with the traditional iterative methods, the new approach can spread the violations more evenly among all the bottleneck constraints. The computational requirements of the proposed algorithm are modest, and the number of LP iterations in the infeasible case is greatly reduced, enabling constraint relaxation to be applied for all constraints with the ability to use different engineering weightings or priorities.
In a Monte Carlo simulation model and procedure for reliability assessment of multiarea generation systems a chronological simulation scheme is described which is capable of recognising different unit failure and repair distributions, different unit types, load forecast uncertainty and tie-line directional transfer capabilities and capacity distributions. Distributions of reliability indices can be obtained by taking advantage of a wide range of output data provided by the chronological Monte Carlo method. The proposed method is illustrated with simulation results based on two multiarea configurations created by connecting several IEEE Reliability Test Systems together. The results show that the proposed model can be an effective tool in multiarea generating system adequacy evaluation
The paper presents the application of a new impedance relaying principle to a Teed feeder. The relay design is immune from false tripping owing to power swings or prefault load conditions. This has been achieved by extracting and using prefault and superimposed components of relaying voltages and currents. Results are presented for earth faults on a Teed feeder configuration. Relay trip time is typically in the range 11 to 20 ms and is plotted as a surface against pre-fault power angle and fault point on wave angle.
