Showing papers in "IEEE Power & Energy Magazine in 1989"

Network reconfiguration in distribution systems for loss reduction and load balancing

Abstract: A general formulation of the feeder reconfiguration problem for loss reduction and load balancing is given, and a novel solution method is presented. The solution uses a search over different radial configurations created by considering switchings of the branch exchange type. To guide the search, two different power flow approximation methods with varying degrees of accuracy have been developed and tested. The methods are used to calculate the new power flow in the system after a branch exchange and they make use of the power flow equations developed for radial distribution systems. Both accuracy analysis and the test results show that estimation methods can be used in searches to reconfigure a given system even if the system is not well compensated and reconfiguring involves load transfer between different substations. For load balancing, a load balance index is defined and it is shown that the search and power flow estimation methods developed for power loss reduction can also be used for load balancing since the two problems are similar. >

Heuristic search approach to distribution system restoration

Abstract: Service restoration, service reconfiguration, and other related problems are formulated and solved by heuristic search, which is a search strategy (e.g. depth-first search) armed with practical rules (e.g. based on operator experience) to guide the search. The method is based on operator procedures and usually generates the same solutions as operators would do. However, it is also possible to investigate alternatives that normally would not be considered by system operators, which can be very helpful under certain critical operating conditions. Moreover, the proposed framework makes it possible to investigate the effect of practical rules on the optimality of the final solution and so can be a useful tool in designing new algorithms. Test results are presented, and an illustrative example is given. The preliminary results indicate that the proposed technique is very promising. >

Detection of Topology Errors by State Estimation

Abstract: In modern energy management systems (EMS), there are two types of measurement data collected by the SCADA (supervisory control and data acquisition) system, namely, status data of breakers and switches, and analog data of real and reactive power flows, injections, and bus voltages. The status data are used to determine real-time topology of the network. The analog data are used to determine line and transformer loading and voltage profile. These data are noisy due to measurement errors, communication noise, missing data, etc. In addition to simple checking of the analog data locally, in most modern EMS, state estimation is used to process these data globally to correct errors in the raw analog measurement data. In this paper, the use of global information in state estimation to correct errors in the direct measurements of the status data is proposed and conditions for detectability of errors are analyzed. The telemetered data of breaker and switch status are processed in the EMS computer to determine the present network topology of the system, and this function is called network topology processor. Errors in status data will show up as errors in the network topology. Sasson et al and Dy Liacco et al used a tree search algorithm for the network topology processor. The method is widely used in practice. Bonanomi et al proposed a sequential search method through the network graph. Recently Lugtu et al suggested the approach of using state estimation results for topology error detection.

Loss minimization of distribution feeders: optimality and algorithms

Abstract: Determining open switch positions for the minimum-loss configuration of a radial distribution system is a discrete optimization problem. The authors present a global optimality condition of the problem and two algorithms: one is based on the uniformly distributed load model and the other on the concentrated current (or power) demand model. The derivation of the optimality condition relies on a concept of 'basic' current profiles of feeder sections and a transformation of the optimization problem. In the first algorithm, a closed form of the minimum-loss open point of a feeder pair can be found since the loss function is piecewise-parabolic. The algorithm obtains the optimal solution when the minimum is obtained for every feeder pair. In the second algorithm, a similar procedure is performed by moving open points, one at a time, from an actual switch position to another until no further loss reduction can be achieved. The proposed algorithm can be utilized as an online aid to distribution system operators. >

A new algorithm for service restoration in distribution systems

Abstract: This paper proposes a new algorithm to quickly. restore the deenergized loads in a distribution system by using the sectionalizing switches. The computational burden and the solution accuracy of the algorithm is improved by using the concept of the dual effective gradient method. Test results on practical system are given to demonstrate that the algorithm can be used in actual, large scale urban distribution system applications.

Off-nominal frequency measurements in electric power systems

Abstract: Some power system protection applications require accurate estimates of voltage magnitude and frequency over a wide measurement range. Most digital techniques for measuring frequency have acceptable accuracy over a small range in the neighborhood of the nominal value. This paper presents a method for extending the measurement range of the Least Error Squares (LES) technique. The paper also examines the effectiveness of the technique and presents some results from simulated studies and laboratory experiments. It is essential to maintain the frequency of a power system close to its nominal value. Usually, frequency deviations in the range of two to three percent only are allowed for short durations of time. In this range, the Least Error Squares algorithm (LES) provides accurate estimates and is, therefore, suitable for implementing in most frequency relays. However, the measurement errors increase as the frequency deviation from the nominal value increases. During start-up and shut-down of a generator, its frequency varies over a wide range. If the generator is connected to a transformer, both the generator and the transformer can experience overexcitation. Excessive volts-perhertz (V/Hz) indicates that overexcitation has occurred. One method of estimating this parameter is to measure the voltage magnitude and frequency and then calculate volts-per-hertz. Voltsper-hertz relays that provide accurate measurements over 10 to 70 Hz range would be desirable for use on steam turbine driven generators. The frequency of the voltage of a thermal generator is low during start-up and shut-down procedures.

A new fault location algorithm for radial transmission lines with loads

Abstract: Conventional fault location schemes do not take loads and their valuable impedance behavior into account. This leads to unacceptable errors in the case of radial transmission lines with loads, commonly found at the 120 Kv and lower levels. A new method for such lines is proposed in this paper. The fault distance is obtained by the solving an implicit equation. Single phase to ground, phase to phase and three phase to ground faults are treated. The effect of the new scheme is illustrated by simulated faults.

Suppression of Numerical Oscillations in the EMTP

Abstract: When compared to other possible integration schemes for the EMTP, the trapezoidal rule of integration presents very good overall characteristics in terms of accuracy, efficiency, and run-off stability. During certain system conditions, however, the solution with trapezoidal may present sustained numerical oscillations (Fig. 1(a)). These oscillations are related to the behaviour of trapezoidal as a differentiator after a discontinuity is encountered (e.g., after a switching operation). To solve this problem, two kinds of approaches have been proposed in the past. One approach has been to add artificial damping to the system either through the integration rule itself, using for instance backward Euler instead of trapezoidal, or by adding external resistances to provide damping. The main drawback of this approach is that distortion through damping is introduced not only on the discontinuity but also on the rest of the simulation. The other approach has been to use interpolation to correct the initial conditions after the disturbance. These type of techniques, however, are relatively complicated to implement. The technique presented in this paper solves the problem of trapezoidal as a differentiator by neutralizing the overshoot at the discontinuity (for instance, in the voltage ? = L di/dt after switching) within one time step.

Analysis of Electrical Interference from Power Lines to Gas Pipelines Part I: Computation Methods

Abstract: Analysis of electrical interference effects of transmission lines upon nearby pipelines has been a topic of growing interest due to the proliferation of rights-of-way which must be shared by transmission lines and pipelines. This paper describes the results of a recent joint EPRI/A.G.A. research project whose objectives were to develop an effective tool for simulating complex realistic right-ofway problems accurately and to investigate the effects of various system parameters. The ECCAPP computer program, which resulted from the EPRI/A.G.A. research program, combines a powerful input data preprocessor with a computation algorithm which accurately evaluates the effects of both conductive and inductive interference for arbitrarily positioned above-ground and buried conductors which could occur in typical rights-of-way. This paper discusses, in a detailed manner, the computation methods used by ECCAPP. A companion paper summarizes some of the results of an extensive parametric analysis which was conducted using ECCAPP and which illustrates some of the capabilities of the computer program and provides insight on how to control both conductive and inductive interference effects. Computer Program ECCAPP analyzes the effects of power transmission lines on neighbouring gas pipelines and determines the influence of mitigation measures, if any, on interference levels. A distinguishing feature of this computer program is its ability to analyze the combined effects of inductive (electromagnetic) and conductive (galvanic, through earth) coupling. These effects may develop simultaneously during power faults at transmission lines structures which are near gas pipelines, or during normal conditions.

Cloud effects on distributed photovoltaic generation: slow transients at the Gardner, Massachusetts photovoltaic experiment

Abstract: The Photovoltaic Generation Effects project was undertaken to verify that no serious consequences arise from the interconnection of distributed photovoltaic generation on electric power distribution systems. Here, slow transient responses at frequencies corresponding to fluctuations of photovoltaic generation resulting from the passage of clouds over the experiment site in Gardner, Massachusetts, are described. A statistical basis is developed extending the measured results to the entire area served by the distribution circuit in Gardner, as it might be both loaded and equipped with photovoltaic generation in the twenty-first century. Observations and analyses of the solar irradiance and photovoltaic system performance at a single point indicate that there are insufficient step changes in load levels to cause voltage flicker problems. Numerical techniques developed for interpolation of irradiance conditions over the experiment site are adequate for estimating the irradiance at intermediate points and times. The technique appears to be adaptable to extrapolation over larger areas. >

Reactive-Power Dispatch by Successive Quadratic Programming

Abstract: The reactive-power dispatch is formulated as the minimization of real-power losses in the system, utilizing a full set of control variables: generator voltages, switchable shunt susceptances, and transformer taps. The solution of the loss problem is obtained by successively solving quadratic programming problems. First- and second-order loss sensitivity coefficients are derived for the quadratic problem formulation. The derivations are based on the Jacobian method for sensitivity calculations. Sensitivity relations for the dependent constraints are based on the complete reactive-power model of the fast decoupled load flow method. The active-set projection method for quadratic programming is described and utilized as the solution algorithm for the quadratic reactive-power dispatch problems. Tests are conducted on the IEEE 30-bus and Mexican 253-bus systems. The computer results are discussed. >

Transient performance of grounding grids

Abstract: Understanding the behaviour of grounding systems to transient currents is important to establish the efficacy of protection offered by these systems to personnel and to the electrical installations against lightning. In this paper, an analytical method to determine the transient performance of grounding grids is presented. The validity of the proposed methodology has been verified by extensive field test results. Most published studies have been aimed at improving the knowledge of transient behaviour of ground electrodes of various forms. Some researchers have proposed methods to determine the impulse impedance of grounding grids also, by considering square grounding grids and by assuming the grid to be an equivalent circular plate. In actual practice however, the station grounding grids can have complex configurations. In this paper, a generalised model of grounding grid is considered for analysis under transient conditions. The two factors of importance in such an analysis are 1) the ground resistance (or conductance) and 2) self and mutual inductances of each element of the grid. In the proposed model, the distributed nature of ground conductance of each element has been lumped and represented as a nominal-pi. In this way, the leakage current is better represented than by considering the grid as an equivalent circular plate. In addition, the model proposes formation of susceptance matrix for the grid in a similar manner as the bus-admittance matrix for the load flow solution.

A hybrid expert system for faulted section identification, fault type classification and selection of fault location algorithms

Abstract: This paper presents an expert system developed in turbo prolog to identify faulted sections and interpret protective apparatus operation in large interconnected power systems. The expert system presented here is capable of identifying bus faults, line fault sections, and fault sections in the common area of a specific bus and line. Also, the expert system identifies relays or breakers malfunctions. The expert system is then expanded to include real-time measurements of current and voltage phasors to classify the type of fault that the faulted section has experienced. Furthermore, when the faulted section is a transmission line, the expert system selects an appropriate fault location algorithm to compute the fault location in miles. This paper shows that the combination of numeric and data base algorithm is essential to many developments in expert system application in power systems. Evaluating the expert systems reported so far for fault diagnosis reveals that all of these schemes utilize only the data received from breaker and relay status. Consider the recent trend in digital protection, real-time phasor measurements would be available. To combine real-time phasor measurements with relay and breaker status, a hybrid expert system is required. A hybrid expert system combines numeric algorithms with data base algorithms in one scheme. This paper recognizes this feature in the expert system developed here. The expert system reported in this paper includes four stages. The first stage determines the faulted section of the power system and reports correct and incorrect breaker and relay operation.

Mechanism and effect of DC charge accumulation on SF/sub 6/ gas insulated spacers

Abstract: Mechanism and effect of DC charge accumulation on the surface of a solid insulating support (spacer) have been studied in compressed SF/sub 6/ gas using various cylindrical model spacers. The distribution of surface charged is closely related to the normal component (gas side) E/sub n/ of electric field on the spacer surface. The ,maximum charge density can be estimated from the condition of E/sub n/=0. When voltage is applied in a polarity opposite to prestressed DC, surface charge increases the maximum field strength in the arrangement, resulting in the reduction of the insulating ability. It is possible to estimate the lowest flashover voltage due to surface charges only from numerical fields calculations. An anticharging spacer shaped along electric lines of force is proposed and studied. >

Derivation of Induction Motor Models from Standstill Frequency Response Tests

Abstract: Standstill frequency response (SSFR) tests were conducted on four induction motors with ratings of 300, 400, 1000, and 1750 horsepower. Using the SSFR test data, an equivalent circuit was derived for each motor. The speed-torque and speed-current curves calculated from the equivalent circuit were compared with those reported by the manufacturer. The comparison showed good agreement of the SSFR-based model with manufacturer's performance curves in the case of the 300 hp motor. Including a simple approximation for saturation effects provided reasonable agreement for the 1000 hp and 1750 hp motors. Results for the 400 hp motor were at considerable variance with manufacturer's curves. >

Switching impulse strength of air insulation: leader inception criterion

Abstract: The author introduces a general criterion for leader inception of phase-to-ground air insulation under positive critical switching impulses. This criterion established the correspondence between continuous leader initiation in the vicinity of the highly stressed electrode and conditions prevailing later at the instant of the final jump. The criterion is applied to several complex electrode configurations, including rod-rod, conductor-rod, and conductor-structure gaps, where both analytic and digital techniques are used to compute the leader inception voltage, 50% breakdown voltage, and gap factor. The theory has been systematically tested against available experimental results with quite satisfactory agreement. >

An adaptive sampling-interval generator for digital relaying

Abstract: A method for measuring frequency at a fixed sampling rate is described, and a variable-rate method is introduced. An algorithm based on a digital Fourier transform, which locks the sampling frequency in a microprocessor-based system to the fundamental of an incoming voltage waveform, is devised. The algorithm also intrinsically provides a measurement of the frequency. The algorithm is applied to the case of a volt/hertz relay. >

Experimental study of response of power distribution lines to direct lightning hits

Abstract: In order to clarify the response of overhead distribution lines to direct lightning hits, experimental facilities were installed at the UHV Shiobara Testing Laboratory of CRIEPI. A lightning impulse current with a crest value of 18 kA and a waveform of (2/11) mu s was generated through the impulse current generator with a high internal resistance of 400 Omega . This impulse current is not easily affected by the conditions of a distribution line side. Protective effects of surge arresters and an overhead ground wire against direct strokes to the pole of a distribution line were clarified experimentally. Experimental results showed a better protective effect than earlier calculated results. Overvoltages between a line conductor and the earth are inclined to have a short wavetail. The 50% flashover voltage of an insulator against overvoltages with a short wavetail is higher than that against overvoltages with a standard wavetail. >

Performance of Large Air Gaps under Lightning Overvoltages: Experimental Study and Analysis of Accuracy of Predetermination Methods

Abstract: The paper summarizes the results of the research recently carried out at CESI, with lightning overvoltages. Tests were performed on various gap configurations and with several impulse shapes of both polarities to get information about macroscopic parameters and basic processes associated with discharge development. The basic aim was to have a consistent amount of data to carry out a validation and comparison of the existing models, while providing the required information to support the refinement of those based on physical approach. Breakdown voltages versus time to breakdown characteristics, (Volt-time curves), were determined. The curves give an indication of the importance of the impulse-shape and configuration, thus underlying the necessity of calculations methods. Discharge parameters have been presented and discussed to put into evidence the relative importance of the impulseshape characteristics. Data about the discharge process have been compared with other available data and with evaluations according to methods suggested in the literature. A refinement of the available formulas with reference to leader velocity has also been proposed, as in the following: v1 = 170 * d * e(1.5* 10-3* U/d) * (U/x-Eo) where ?1 is the velocity (m/s), U is the applied voltage (kV), d the gap clearance (m), x the part of the gap unbridged by the leader (m) and Eo the average gradient in the gap of the configuration examined at 50% breakdown voltage with standard lightning impulse (kV/m). The equation is of general type, applicable for all the configurations and for impulses of different shape and polarity.

Nonceramic insulators for transmission lines

Abstract: Progress in the development of nonceramic transmission line insulator technology is reviewed. The results of a survey of utilities in the United States are presented to indicate the range of applications, distribution by voltage class, appraisal of performance, and failure modes. It was found that the tracking, erosion, and mechanical problems of the first generation of nonceramic insulators have been reduced. Typical problems with nonceramic insulators in the past were discoloring, crazing, chalking, corona cutting and deterioration, water penetration and subsequent electrical failure, interface flashover, loosened end-fittings, and mechanical failures. Utilities have in general utilized these insulators in short lines, in trouble spots, and for data gathering. A number of utilities have valuable test facilities to verify both short and long-term insulator behavior. The major problem with nonceramic insulators is deterioration caused by contamination-induced discharges and poor electric-field distribution. Concerns expressed by utility representatives at a recent workshop are described, and future research standardization needs are presented. >

A computer model for calculating steep-fronted surge distribution in machine windings

Abstract: A computer model for predicting the distribution of steep-fronted surges in the line-end coils of machine windings is described. The model uses certain aspects of previous models combined to achieve simplification of solution while retaining accuracy. The solution technique is based on multiconductor transmission line theory and modal analysis. Parameter evaluation is based on the relationship between capacitance, inductance and wave velocity. The theory is extended to include a number of series-connected coils. Experimental and computational results are presented to illustrate the application and accuracy of the model. >

Modeling and Dynamic Performance of a Line-Commutated Photovoltaic Inverter System

Abstract: Interest in utility-interactive photovoltaic (PV) inverter systems has increased over the past decade and numerous central-station PV systems have been installed. It is anticipated that as PV system costs decrease, residential systems will be installed in increased numbers. Although a substantial amount of literature is available concerning the design, protection, safety, economics, and operating experience of residential and central-station PV systems, little information is available regarding their dynamic electrical characteristics and the computer modeling of these systems. Moreover, most of the available literature concerning modeling and/or dynamic performance focuses either upon the long-term dynamic behavior as it affects power system scheduling or upon the steady-state harmonic characteristics. In recent work, highly detailed computer models of a representative set of PV systems have been developed and several of these models have been verified by comparison with system measurements [1, 2]. However, the models described in [1, 2] are more complex than necessary for large-scale power system studies in which the fast switching transients associated with the dc-to-ac inverter are of little concern and only the slower cycle-to-cycle behavior of the PV system is of interest. In fact, it is not possible to incorporate these detailed models into conventional transient stability programs due, in part, to the very small time-step requirements associated with these models. In this paper, a three-phase self-commutated utility-interactive photovoltaic inverter system is described including its associated control system. A schematic diagram of the selected PV inverter system is depicted in Fig. 1.

Lightning surge response of ground electrodes

Abstract: Experimental and analytical evidence shows that lighning surge currents ionize the air in the soil. This ionization process tends to increase the contact area of the electrode with surrounding material. The larger area results in lower resistance and less voltage stress per unit current. Most realistic models for non-linear changes in footing resistance are difficult to derive. Recently, a similarity approach to footing modeling was proposed [1]. We describe the approach and derive a new model for low-current response, based on electrode size and surface area. The simple model agrees well with formulas derived by Rudenberg [2] for a wide range of electrode shapes. When models for impulse footing impedance are used in the prediction of lightning outage rates, the currents computed for backflashovers on EHV transmission lines are often over 400 kA. These currents are well beyond the values estimated from observations of lightning radiated fields for negative flashes. In spite of that, lightning flashovers are observed on 500-kV lines. To resolve the discrepancy, in addition to the work on surge-reduced footing resistance, the surge response of a conducting ground plane was studied. Any ground electrode, even a perfectly conducting plane, will exhibit a transient surge impedance as the front of the lightning current spreads out. This impedance is fairly high (60 ?) at the base of the tower but quickly falls off to low values. In effect, the ground plane near the tower looks like an extension of the tower, bent outward.

Frequency domain techniques for modeling distribution or transmission networks using capacitor switching induced transients

Abstract: This paper presents two frequency domain methods to model and identify the equivalent impedance transfer function of a distribution or transmission system using recorded voltage and current transients due to capacitor switching. The first frequency domain method discussed was the fast Fourier transform method of identifying the frequency components of the voltage and current, then using the voltage and current spectra to find the impedance transfer function. The second method first obtains estimates of the sample auto and cross-correlation functions. These functions are then used, via the FFT, to find the respective power spectra of the signals, and finally to find the impedance transfer function. The FFT method was found to produce accurate results when the frequency components signal to noise ratios are large. The power spectral identification method and the coherence function added a measure to reject the frequency components that are contaminated with noise, but this method suffers from spurious cross correlations between the voltage and current. Once the impedance transfer function has been identified, several techniques were investigated that fit a parametric model to the function. Two polynomial transfer functions were fit to the real and imaginary parts of the impedance. These polynomials resulted in accurate representations of the impedance function of the system. The second parametric method discussed used the poles and zeroes identified from the impedance transfer function to develop an equivalent circuit representation of the system. This equivalent circuit did not fit the estimated impedance/frequency characteristics as well as the polynomial transfer function.

Voltage Compensation of an Induction Generator with Long-Shunt Connection

Abstract: Preliminary studies of voltage vs. speed relationships, stability of generated voltage, and voltage compensation of the self-excited induction generator are presented. In order to improve the voltage regulation, the long-shunt connection of the capacitors to the generator is used. The magnetizing inductance of the machine is used as a saturable reactor. The values of the capacitors for series and parallel connections are obtained from the saturation characteristics of the machine. Experimental results are reported. The maximum voltage drop is about 4%. >

Core loss in buried magnet permanent magnet synchronous motors

Abstract: The steady-state core-loss characteristics of buried-magnet synchronous motors operating from a sinusoidal constant frequency voltage supply are investigated. Measured and calculated core loss, with constant shaft load, is shown to increase with decreasing terminal voltage due to an increase in armature reaction-induced stator flux-density time harmonics. Finite-element modeling is used to show that the additional loss due to the time-harmonic fields can increase core loss by a factor of six over the loss associated with only the fundamental component field at low motor flux levels. A simple air-gap model of motor flux components shows that this increased loss is due to localized rotor saturation. Thus, stator-core harmonic fields should be expected for all buried-magnet rotor synchronous motors (with or without a cage) operating at low flux levels. This factor becomes increasingly important when the motors are operated in the high-speed low-flux mode in conjunction with a variable-speed drive. >

Steady State Analysis of a Doubly-Fed Induction Generator under Synchronous Operation

Abstract: A detailed analysis of the performance of the doubly-fed induction generator (DFG) under synchronous operating condition in the steady state is presented. The rotor is excited by a voltage phasor, the magnitude and the frequency of which can vary independently. The frequency supplied always has such a value that, if superimposed on the rotor speed, a synchronous rotating field results. This produces variable-speed constant-frequency characteristics of the DFG over a wide speed range. By varying the excitation voltage magnitude the stator and rotor power factor can be controlled so that maximum efficiency can be achieved. By varying the angle between the stator and the rotor applied voltages, the DFG active power delivered can be controlled. The wide range of variable-speed constant-frequency operation and the capability of load variation and power factor control makes the DFG attractive for wind power conversion systems as well as variable-speed hydroelectric generators. >

DC electric field effects during measurements of monopolar charge density and net space charge density near HVDC power lines

Abstract: The influence of a DC electric field on the measurement of monopolar charge densities using an aspirator-type ion counter and the measurement of net space-charge density using a Faraday cage or filter is examined. Optimum configurations which minimize the effect of the electrical field are identified for each type of instrumentation. It is shown that the influence of a DC electric field on measurement of monopolar space-charge densities with an ion counter can be made negligible when the measurements are performed in the ground plane and oriented vertically or horizontally above the ground plane with an attractive potential applied to the ion counter housing. Measurements of net space-charge density with a Faraday cage appear to be significantly influenced by a large field except when the space charge is predominantly monopolar or when composed of large ions (aerosols) with small mobilities. The optimum configuration for the filter is an above-ground horizontal orientation with the filter housing at ambient space potential. >

IEEE Power Engineering Society

Abstract: The authors review solar phenomena and describe auroral effects arising from solar disturbances. They discuss geomagnetically induced currents (GICs) that arise as a result of magnetic-field transients due to solar activity. The authors point out that understanding GICs is particularly important since solar activity is rapidly increasing to a peak forecast to be in 1990 followed by several years of generally high sunspot levels. They discuss GIC impact on systems and equipment, GICs in power transformers, equipment and system vulnerability, and modeling and simulation results. >