Showing papers on "AC power published in 1994"

Power System Stability and Control

Abstract: Part I: Characteristics of Modern Power Systems. Introduction to the Power System Stability Problem. Part II: Synchronous Machine Theory and Modelling. Synchronous Machine Parameters. Synchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems. High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems, High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Oscillations. Mid-Term and Long-Term Stability. Methods of Improving System Stability.

Optimal reactive dispatch through interior point methods

Abstract: An implementation of an interior point method to the optimal reactive dispatch problem is described. The interior point method used is based on the primal-dual algorithm and the numerical results in large scale networks (1832 and 3467 bus systems) have shown that this technique can be very effective to some optimal power flow applications. >

Trends in active power line conditioners

Abstract: Active power line conditioners, which are classified into shunt and series ones, have been studied with the focus on their practical installation in industrial power systems. In 1986, a combined system of a shunt active conditioner of rating 900 kVA and a shunt passive filter of rating 6600 kVA was practically installed to suppress the harmonics produced by a large capacity cycloconverter for steel mill drives. More than one hundred shunt active conditioners have been operating properly in Japan. The largest one is 20 MVA, which was developed for flicker compensation for an arc furnace with the help of a shunt passive filter of 20 MVA. In this paper, the term of "active power line conditioners" is used instead of that of "active power filters" because active power line conditioners would cover a wider sense than active power filters. The primary intent of this paper is to present trends in active power line conditioners using PWM inverters, paying attention to practical applications. >

Dynamic compensation of AC transmission lines by solid-state synchronous voltage sources

Abstract: This paper describes a novel approach in which solid-state synchronous voltage sources are employed for the dynamic compensation and real time control of power flow in transmission systems. The synchronous voltage source is implemented by a multi-pulse inverter using gate turn-off (GTO) thyristors. It is capable of generating internally the reactive power necessary for network compensation, and is also able to interface with an appropriate energy storage device to negotiate real power exchange with the AC system. The paper develops a comprehensive treatment of power flow control using solid-state synchronous voltage sources for shunt compensation, series and phase angle control. It also describes the unique unified power flow controller that is able to control concurrently or selectively all three network parameters (voltage, impedance, transmission angle) determining power transmission. Comparison of the synchronous voltage source approach with the more conventional compensation method of employing thyristor-switched capacitors and reactors shows its superior performance (including the unmatched capability of using both reactive and real power compensation to counteract dynamic disturbances), uniform applicability, smaller physical size, and potentially lower overall cost. >

Comparison of multilevel inverters for static VAr compensation

Abstract: Inverter-based static VAr compensators (SVCs) can benefit from the use of a multilevel invertor structure which allows the elimination of the step-up transformer. The inherent high quality of the multistep waveform allows operation without PWM, thus high switching losses are avoided. This paper discusses the issues affecting the application of multilevel invertor structures as reactive power compensators and compares the device MVA and reactive component MVA requirements of two topologies that have been presented in prior literature. The modulation strategy strongly affects the voltage balancing in the DC bus capacitors as well as their ripple current rating and capacitance value. >

Torque and reactive power control of a doubly-fed induction machine by position sensorless scheme

Abstract: A novel control strategy to realize decoupled control of torque and reactive power for a doubly excited induction machine using a position sensorless scheme is proposed. The critical issues related to the torque angle estimation and control for a doubly excited induction machine are discussed. Compared to the other position sensorless schemes for a doubly fed machine, the proposed control method uses only the rotor voltages and currents as the feedback signals, which substantially reduces the costs and enhances the reliability of the position sensorless control of a doubly fed AC machine. Computer simulation and experimental results are given to verify the position sensorless control scheme. >

Arc-furnace model for the study of flicker compensation in electrical networks

Abstract: This paper presents an arc-furnace model consisting of nonlinear, time varying resistance where time-variation laws of arc length are considered. One consists of a periodic, sinusoidal law, the other of a band-limited white-noise law. The arc-furnace model is implemented by EMTP, referring to actual electric-plant configurations. Simulations are reported where the values of flicker sensation and short-term flicker severity P/sub ST/, are determined according to UIE specifications. The results show that the model based on the sinusoidal time-variation law can be useful for worst-case approximations, while the model using white-noise law is able to fit flicker measurements made in electric plants supplying arc furnaces. The models are used to investigate the effect on flicker compensation of the insertion of series inductors at the supply side of the furnace transformer. It is shown that considerable reduction of P/sub ST/ is obtained at the point of common coupling by series inductor installation at constant furnace active power. >

Compensation strategies for shunt active-filter control

Abstract: Compensation strategies for control of shunt active filters are compared in the paper. It is shown that the strategy based on unity-power factor control is appropriate when the supply voltage waveform of the plant where the active filter is connected shows significant distortion. As voltage distortion increases, this strategy provides compensated line current having lower harmonic distortion and RMS value with respect to the strategy generally used. This contributes to diminish the current and voltage distortion in networks. The unity-power factor compensation strategy conditions the current flowing in the plant where compensation is realized to fit the voltage waveform, thus reaching a unity power factor. Hence, the line current RMS value is minimum. The comparison of the strategies is performed by both Monte Carlo and ATP simulation. >

Distributed current and voltage sampling function for an electric power monitoring unit

Abstract: A low voltage AC electric power monitoring system includes a processor which samples the current and circuit breaker system includes a processor which samples each of the current components at seventeen samples per cycle to obtain current samples representing a single period of the AC power signal. The processor samples the voltage component at different points in the voltage cycle over several cycles to obtain voltage samples representing a single period of the AC power signal.

A new ZVS-PWM unity power factor rectifier with reduced conduction losses

Abstract: This paper introduces a new single-phase high power factor rectifier, which features regulation by conventional PWM, soft commutation and instantaneous average line current control. Furthermore, thanks to the use of a single converter, instead of the conventional configuration composed of a four-diode front-end rectifier followed by a boost converter, a significant reduction in the conduction losses is achieved. A prototype rated at 1.6 kW, operating at 70 kHz, with an input AC voltage of 220 Vrms and an output voltage of 400 V/sub DC/ has been implemented in the laboratory. An efficiency of 97.8 % at 1.6 kW has been measured. Analysis, design, and the control circuitry are also presented in the paper. >

Emergency load shedding to avoid risks of voltage instability using indicators

Abstract: This paper presents a new method to determine a viable load shedding scheme. This scheme presents to the system operator the location and amount of load to be shed to avoid a voltage collapse. In order to develop such a scheme in a timely fashion, indicators are developed to assess the risk of voltage collapse at a particular bus, and a sensitivity matrix coupling the amount of load to be shed and the indicator is computed. The method takes into account nonlinearities caused by generators reaching their limits of reactive power output. The advantages of the proposed method are viable network security, short computing times and low memory requirements due to the use of sparse vector techniques. The method developed has been tested with the 30 bus New-England test system and with a 206 bus 400 kV simplified French network with satisfactory results. >

An optimal reactive power planning strategy against voltage collapse

Abstract: This paper introduces a method of determining the minimum amount of shunt reactive power (VAr) support which indirectly maximizes the real power transfer before voltage collapse is encountered. Using a relaxation strategy that operates with a predictor-corrector/optimization scheme, a voltage stability index that serves as an indirect measure to the closeness of reaching the steady state voltage stability limits is obtained. Sensitivity information that identifies weak buses is also available for locating effective VAr injection sites. >

Single-stage single-phase parallel power factor correction scheme

Abstract: A new scheme of the single-phase parallel power factor correction (PPFC), which allows major input power to be processed only once to achieve both unity power factor and tight output regulation, is established in this paper. The new PPFC circuits based on this new scheme are much simpler than the previous one. Consequently, they are competitive to the conventional two-cascade-stage system concerning the efficiency and the cost. Large signal simulation and experimental results have proven the successful operation and good performances of the proposed new PPFC circuits. >

Single stage, high power factor, lamp ballast

Abstract: Unlike conventional ballasts requiring two cascaded stages, a new lamp ballast achieves near unity input power factor and high frequency sinusoidal lamp current in a single power conversion stage. A new discontinuous inductor current mode (DICM) of input inductor makes this possible by separating the input current shaping from high frequency output lamp ballasting function. The high efficiency is further enhanced by soft switching improvement, which is provided naturally through lagging current of the output resonant matching network. Design equations and experimental results verify all the advantages of the new lamp ballast. >

A new technique for loss reduction using compensating capacitors applied to distribution systems with varying load condition

Abstract: A new method based on a heuristic technique for reactive loss reduction in distribution network is presented. This method allocates capacitors to certain nodes (sensitive nodes) which are selected by first identifying the branch which has the largest losses due to reactive power. Then, the node therein, which has the largest reactive power is selected. The capacitor rating is determined by differentiating the system losses with respect to the load connected to that node. The compensating capacitors are placed at these optimal locations with appropriate VAr ratings to achieve maximum benefits in dollar savings. The variation of the load during the year is considered. The capital and installation costs of the capacitors are also taken into account. This method is applied to a 38 feeder distribution system of 27.6 kV, 560 MVA of the city of Windsor, Ontario, resulting in annual saving of about CAN $145000 after amortizing the capital and installation costs of applying the compensating capacitors, and using a figure for the cost of energy of 1.986 cents per kWh. >

Discussion of power definitions contained in the IEEE Dictionary

Abstract: This tutorial paper discusses the drawbacks of the definitions of various types of powers found in the IEEE Standard Dictionary of Electrical and Electronics Terms (IEEE Std. 100-88). With the exceptions of instantaneous power and active power, all remaining kinds of "powers" are nonphysical. The concept of power factor in polyphase circuits is ambiguous. Examples that illustrate the shortcomings of many power definitions are included. The impact of these definitions on current power/energy metering practices are discussed. It is recommended that some definitions be either changed or eliminated from the IEEE Dictionary. >

Generalized fast, power flow controller

Abstract: First and second dc linked, preferably voltage sourced, inverters are connected to an electric power transmission line in shunt and in series, respectively, by coupling transformers. Firing of the GTO-thyristors of the first inverter is controlled to regulate reactive power on the transmission line and to supply real power requirements of the second inverter. The second inverter is controlled to inject into the transmission line in series a voltage which can be adjusted in magnitude and from zero through 360 degrees in phase with respect to the transmission line voltage to selectively adjust any one or more of transmission line voltage magnitude, transmission line impedance and transmission line voltage phase angle. In a high power configuration of the controller each of the inverters includes a plurality of six-pulse inverter modules connected by interstage transformers to form balanced higher order pulse groups controlled through pulse width modulation or through adjustment of the phase between opposite groups to generate the appropriate ac voltages.

New single-phase active power filter

Abstract: A new algorithm for a single-phase active power filter, based on calculation of the real part of the fundamental load current, is proposed in the paper. The algorithm proposed can maintain the input power factor of the mains close to unity and force the mains current to be a sinewave under distorted or nondistorted mains voltage. A prototype is developed and tested to verify its performance. The experimental results show that the algorithm proposed can compensate for the reactive power and suppress the harmonics of the nonlinear load effectively.

Power system applications for phasor measurement units

Abstract: State-of-the-art technology now permits measurement and analysis of power system performance on a scale not previously possible. Synchronized sampling, derived from the GPS (Global Positioning System), and high accuracy sigma-delta analog-to-digital converters form the basis for a system that can measure the state of the power system at a given instant over any area. Samples are acquired 12 times per cycle and are processed by a recursive discrete Fourier transform (DFT) algorithm. This produces the magnitude and angle of the input signal for each sample. The resulting phasors can be combined to produce a positive sequence phasor for a set of three phase inputs. The positive sequence quantities can be used to calculate voltage and current phase and magnitude, real and reactive power, frequency, and impedance. Since the positive sequence parameters are acquired at the same instant (within 1 microsecond), the state of the system at the measured nodes is known at the sample time. Applications to fault recording, disturbance recording, transmission and generation modelling verification, and power system stabiliser testing, are discussed. >

Reactive capability limitation of synchronous machines

Abstract: Achievable generator reactive capability (GRC) is generally much less than indicated by manufacturers' reactive capability curves, due to constraints imposed by plant auxiliaries and the power system itself. The nature of these constraints is explained and a method for calculating them is provided and verified by field tests on a unit at high and low power system voltage levels. Several recommendations are made to enhance the GRC of the tested unit. >

Maximum frequency and optimum performance of class E power amplifiers

Abstract: An analytical method is proposed to discuss class E RF power amplifiers with finite choke inductors. All parameters in these amplifiers can be expressed in terms of circuit-component values and amplifier specifications. The component values of class E amplifiers required to achieve optimum amplification can be directly obtained after the amplifier specifications are fixed. An obvious relationship between the parameters of class E amplifiers with finite choke inductors in optimum performance can be obtained. Hence, the key function of each component in the circuit can be indicated clearly, especially in the case of a finite choke inductor. The finite choke inductor can also be used to compensate for the high intrinsic output capacitance of the active power device to extend operation frequency. To assure optimum performance and safe operation, a criterion is assigned. Based on this criterion and the optimum conditions, a maximum frequency for a transistor safely operating in class E amplifiers can also be obtained.< >

Q-V curve interpretations of energy measures for voltage security

Abstract: Energy methods have shown promise as measures for quantifying the vulnerability of power systems to problems of voltage instability and collapse. However, to make such measures more useful as a security assessment tool in an operational environment. It is important to provide physical interpretations of the quantitative measure. This paper demonstrates that for certain basic load models, the energy based security measure is equal to the area enclosed by a familiar Q-V curve, with a change of scale on the voltage axis. This interpretation has the added benefit of providing easily computed approximations to the maximum real and reactive power loadability both at individual buses and for the system as a whole. Results are demonstrated in detail on the IEEE 118 bus system, with additional tests on a 415 bus sample system. >

A utility interactive wind energy conversion scheme with an asynchronous DC link using a supplementary control loop

Abstract: The paper presents modelling, simulation and experimental verification of a utility interactive wind energy conversion scheme with an asynchronous link comprised of a diode bridge rectifier and a line commutated inverter. The control objective is to track and extract maximum power from the wind energy system and transfer this power to the electric utility. This is achieved by controlling the firing delay angle of the inverter. Since the diode bridge rectifier has no control on the DC link voltage, a supplementary control loop is used to limit the voltage within a preset voltage threshold. The proposed scheme for regulating the flow of power through the DC link ensures reduced reactive power burden on the self-excitation capacitor banks and better utilisation of available wind energy, while limiting the DC link voltage within a preset voltage threshold. The simulated results are experimentally verified and found to give good power tracking performance. >

Energy saving power control system

Abstract: Apparatus and methods for AC power regulation primarily intended for inductive loads (e.g., fluorescent lights, motors, etc.) which provide substantial reduction in power consumption while also providing a leading power factor, reduced harmonic distortion, reduced crest factor and reduced noise. The system is self-adjusting for a wide range of loads and can reduce power consumption by 25 percent in lighting loads while producing minimal reduction in light output. The system utilizes a Triac (32) and parallel capacitor bank (58) in series with the load (22). The Triac (32) is turned on in response to a near-zero differential voltage measured across the Triac (32) and is turned off near the peak of each AC half cycle by shunting current around the Triac (32). The capacitor (60) absorbs the inductive turn-off voltage spike caused by the collapsing magnetic field in the ballast at the instant of Triac turn-off. This energy, in turn, provides longer on-period for the lamp, thereby permitting more light and increased operating efficiency. The turn-off time is adjusted to regulate power at a reduced level responsive to line voltage, load current and load power-sense signals.

Harmonic measurements, analysis, and power factor correction in a modern steel manufacturing facility

Abstract: Maximizing electric power transfer directly affects the productivity of an electric arc furnace operation. Arc furnaces and rolling mill loads operate at power factors that result in penalty charges and lower bus voltages. In addition, the nonlinear characteristics of the furnace arcs and rolling mill drives generate significant harmonic currents that flow through the plant and utility power system. These harmonic currents cause system voltage distortion and power loss in the system, and can interact with power factor correction capacitor banks leading to equipment failures. This paper presents the analytical technique used to correct power factor in a modern steel manufacturing facility. The study included field measurements, harmonic analysis, and filter design work to reduce the amount of harmonic distortion in the plant. The modeling of arc furnaces and rolling mills for a harmonic analysis study is also discussed. The solutions recommended in this paper to increase power factor and reduce harmonics can be applied to other steel manufacturing facilities to improve power quality and therefore plant productivity.

ZETA converter applied in power factor correction

Abstract: This paper presents the analysis of the ZETA converter operating in discontinuous conduction mode (DCM) for power factor correction. The main attraction of the ZETA converter is that it is a naturally isolated structure, which allows a regulated output voltage with only one power processing stage. The principle of operation, mathematical analysis, design procedure and experimental results obtained from a laboratory prototype are presented. >

High speed power factor controller

Abstract: A microprocessor (42) is coupled to sense the current and voltage in an AC line (11) at a high sampling rate and to determine the power factor. In one embodiment the primary (83) of a transformer (84) is coupled across the power lines, and at least two capacitors (86, 88), forming separate switching paths, are coupled to the secondary (85) of the transformer. High speed switching circuitry under the control of the microprocessor alternately charges and discharges the capacitors (86, 88) at varying frequencies, to alternately store energy from the power lines (11) when the variable load (32) is returning energy to the power lines, and to supply the stored energy back to the power lines when the variable load (32) is absorbing energy from the power lines (11). This corrects the power factor since the frequency of the high speed switching circuitry is varied with the varying reactance of the load. In another embodiment power factor correction is accomplished using a capacitor ladder (324), with different values of capacitance being switched into the circuit (328, 330) under microprocessor (344) control to maintain the power factor close to unity under varying load (322) conditions.

Current control of a 3-level rectifier/inverter drive system

Abstract: The marriage of a three-level voltage source inverter with a force-commutated three-level rectifier is examined in this paper. Three-level inverters are capable of reducing the output current harmonics dramatically compared with typical two-level inverters whereas a three-level rectifier of this type allows nearly sinusoidal input currents at unity fundamental power factor on the utility side of the drive system. The dual capacitor split voltage bus can be regulated from either the inverter or rectifier side with neutral point balance maintained. This paper address the issues of neutral point voltage control and current regulation from both the rectifier and inverter perspectives. >

Series compensated PWM inverter with battery supply applied to an isolated induction generator

Abstract: This paper proposes a new series compensated induction generator/battery supply topology which provides a constant voltage and frequency at the terminals, allowing minimum current harmonic distortion while at the same time providing a source and sink of real and reactive power. With appropriate control of the reactive power, the speed of the generator is allowed to vary within a relatively wide range. This technique can be further expanded by applying ac capacitors in parallel with the load to lessen the burden of the PWM inverter. Both simulation and experimental results are in agreement with the theory which leads to the conclusion that the system could be feasible for isolated power generation systems (wind, hydro, diesel, or hybrid generation)

Synchronizing and damping torque coefficients and power system steady-state stability as affected by static VAr compensators

Abstract: The utilization of static VAr compensators (SVC) for supplying reactive power at certain points of an electric power system is an efficient way for fast control of transient and steady-state voltage changes following short-circuits, load rejection, opening of severely loaded transmission circuits, etc. Other SVC applications include the increase of power transmission capacity through interconnections between areas of a power system and the damping enhancement of local or inter-area electromechanical oscillation modes. This paper fundamentally deals with these last two issues by analyzing the results of synchronizing and damping torque coefficient calculations for a generation station connected radially to a much larger power system. The results presented pertain to a double-circuit, 800 km, 500 kV transmission system during a one-circuit out contingency. >