Showing papers on "AC power published in 1989"

Optimal sizing of capacitors placed on a radial distribution system

Abstract: A capacitor sizing problem for capacitors placed on a radial distribution system is formulated as a nonlinear programming problem, and a solution algorithm is developed. The object is to find the optimal size of the capacitors so that the power losses will be minimized for a given load profile while considering the cost of the capacitors. The formulation also incorporates the AC power flow model for the system and the voltage constraints. The solution algorithm developed for the capacitor sizing problem is based on a Phase I-Phase II feasible directions approach. Novel power flow equations and a solution method, called DistFlow, for radial distribution systems are introduced. The method is computationally efficient and numerically robust, especially for distribution systems with large r/x ratio branches. DistFlow is used repeatedly as a subroutine in the optimization algorithm for the capacitor sizing problem. The test results for the algorithm indicate that the method is computationally efficient and has good convergence characteristics. >

An active power factor correction technique for three-phase diode rectifiers

Abstract: A novel active power factor correction method for power supplies with three-phase front-end diode rectifiers is proposed and analyzed. The implementation of this method requires the use of an additional single switch boost chopper. The combined front-end converter draws sinusoidal AC currents from the AC source with nearly unity input power factor while operating at a fixed switching frequency. It is shown that when the active input power factor correction stage is also used to regulate the converter DC bus voltage, the converter performance can improve substantially in comparison with the conventional three-phase AC-to-DC converters. These improvements include component count reduction, simplified input synchronization logic requirements, and smaller filter refractive components. Theoretical results are verified experimentally. The proposed method has the disadvantage of substantially increasing the current stresses of the switching devices and the high-frequency ripple content of the prefiltered AC input currents. >

Current waveform distortion in power factor correction circuits employing discontinuous-mode boost converters

Abstract: For active power factor correction circuits employing discontinuous-mode boost converters, the line current will automatically follow the sinusoidal line-voltage waveform. However, due to the modulation of the input inductor current discharging time, there is certain distortion in the AC line-current waveform. It is found that the modulation of inductor current discharging time is a function of the line voltage and the output voltage of the boost converter. For practical design considerations, the waveform distortion and the maximum power factor achievable are derived and presented in a graph form as functions of the ratio of line voltage to output voltage. >

Apparatus for reducing computer system power consumption

Abstract: A battery powered computer system monitors the address bus to determine when selected peripheral devices have not been accessed for a preset amount of time. When the preset amount of time has passed the system powers itself down and stops the system clock, placing it in a standby mode. The system is awakened by depressing a standby switch, unless there is insufficient energy in the batteries, under which circumstances an AC power source must be connected before the system can be awakened.

Application of Newton's optimal power flow in voltage/reactive power control

Abstract: The authors consider an application of Newton's optimal power flow to the solution of the secondary voltage/reactive power control in transmission networks. An efficient computer program based on the latest achievements in sparse matrix/vector techniques has been developed for this purpose. It is characterized by good robustness, accuracy and speed. A combined objective function appropriate for various system load levels with suitable constraints for the treatment of power system security and economy is also proposed. For the real-time voltage/receiver power control, a suboptimal power flow procedure has been derived by using the reduced set of control variables. This procedure is based on sensitivity theory applied to the determination of zones for the secondary voltage/reactive power control and a corresponding reduced set of regulating sources, whose reactive outputs represent control variables in the optimal power flow program. As a result, the optimal power flow program output becomes a schedule to be used by operators in the process of the real-time voltage/reactive power control in both normal and emergency operating states. >

Large-scale optimal power flow: effects of initialization, decoupling and discretization

Abstract: This paper presents the results of extensive numerical testing of a second-order OPF solution method. The testing was conducted using a 1500 bus network under various loading conditions. Three issues were studied: ? Sensitivity of the solution with respect to the initial conditions ? Accuracy of decoupling the original OPF problem into separate active power and the reactive power OPF problems ? Effects of the discretization of transformer taps on solutions. The first issue is particularly important in an EMS environment. An OPF must produce consistent solutions if it is to be used to guide the decision-making of power system operators. This requires that the OPF solution not be sensitive to the (arbitrarily or randomly) selected starting point used by the OPF program, and that changes in the OPF solution point be consistent with the changes in the power system operating constraints. Such changes include the evolution of bus loads over time, control variables which move to their limits over time, and changes in topology due to disturbances. Hence the first objective of the OPF testing was to study the sensitivity of OPF solutions to the choice of initial points. The second issue is of importance because most of the OPF approaches proposed by EMS vendors rely on decoupling of the active and reactive OPF problems in order to achieve acceptable computational performance.

Analysis and design of a three-phase synchronous solid-state VAr compensator

Abstract: A three-phase synchronous solid-state VAr compensator (SSVC) system that uses a three-phase pulsewidth-modulated (PWM) voltage-source inverter is presented and analyzed. The proposed SSVC system can compensate for leading and lagging displacement power factor. Pulsewidth modulation is used as a means of reducing the size of reactive components. The SSVC system is analyzed under self- and independently-controlled DC bus voltage operating conditions. Other areas of investigation include the design of SSVC filter components for both approaches and the closing of the loop around the reactive power command signal. Predicted results are verified experimentally for the case of the SSVC working with a self-controlled DC bus. >

Analysis of tap-changer dynamics and construction of voltage stability regions

Abstract: The destabilizing behavior of onload tap changers (OLTC) is an important mechanism responsible for the voltage collapse of interconnected power systems. A nonlinear dynamic model of the OLTC, impedance loads and decoupled reactive power-voltage relations is used to reconstruct the voltage-collapse phenomenon. Trajectories leading to a monotonic fall of bus voltages are obtained from initial conditions outside the stability region of a simple power network. The construction of voltage stability regions is desirable for the prevention of voltage collapse. Based on the proposed M-bus power network model, this research results in (1) a simple criterion for stability of an equilibrium, and (2) a method to obtain a stability region by forming the union of hyperbox subsets of the true region. The theoretical foundations of the proposed method, i.e., characteristics of the equilibria and monotonic behaviour of system trajectories, are thoroughly studied. >

Reactive and unbalanced currents compensation in three-phase asymmetrical circuits under nonsinusoidal conditions

Abstract: The present work is a continuation of the author's previous work (see IEEE Trans. Instrum. Meas., vol.37, p.30-34, Mar. 1988), where a new power theory of three-phase circuits under nonsinusoidal conditions was suggested. The main objective of the present work is to determine whether the theory suggested has practical applications that justify the need of measuring quantities introduced in it, particularly, whether it improves a theoretical basis for improving power properties of such circuits. It is shown that the three-phase linear asymmetrical load supplied from a symmetrical source of nonsinusoidal voltage composed of a finite number of harmonics can be converted with a reactance circuit into a symmetrical three-phase circuit of almost unity power factor. It is shown how such a reactance circuit can be designed and which quantities should be measured for this purpose. >

Direct frequency convertor with sinusoidal line currents for speed-variable AC motors

Abstract: A novel concept for a static three-phase to three-phase power converter for an AC drive with a unity power factor and reduced harmonics on the line side is presented. The power circuit comprises two back-to-back connected six-pulse bridges having no energy storage elements in the DC link. This permits pulse-width modulation (PWM) control in both bridges while requiring active turn-off semiconductor switches in only one bridge. The line-side harmonics are suppressed by a three-phase second-order filter. The method of predictive optimization is used for the control of the power converter. The complex control structure of the system is based on an online prediction of space vector trajectories. The steady-state operation of the system is exemplified by simulation results. >

Rectifier design for minimum line current harmonics and maximum power factor

Abstract: Rectifier line current harmonics interfere with proper power system operation, reduce rectifier power factor, and limit the power available from a given service. The rectifier's output filter inductance determines the rectifier line current waveform, the line current harmonics, and the power factor. Classical rectifier analysis usually assumes a near-infinite output filter inductance, which introduces significant error in the estimation of line current harmonics and power factor. A quantitative analysis of single and three-phase rectifier line current harmonics and power factor as a function of the output filter inductance is presented. For the single phase rectifier, one value of finite output filter inductance produces maximum power factor and a different value of finite output filter inductance produces minimum line current harmonics. For the three phase rectifier, a near-infinite output filter inductance produces minimum line current harmonics and maximum power factor, and the smallest inductance that approximates a near-infinite inductance is determined. >

Brushless generator having AC excitation in generating and starting modes

Abstract: An excitation system for a brushless generator having a main generator portion including a field winding disposed on a rotor and an armature winding disposed in a stator includes an exciter portion having a set of polyphase exciter field windings disposed in the stator and an armature winding disposed on the rotor and coupled to the main generator portion field winding. A first power converter is coupled to the main generator armature winding and a second power converter is coupled to the set of polyphase exciter field windings. Contactors are operable in a starting mode of operation to couple a source of electrical power to the first and second power converters and are operable in a generating mode to disconnect the source of electrical power from the first and second power converters. A control unit controls the power converters such that the power converters provide AC power to the main generator armature winding and to the set of polyphase exciter field windings during operation of the starting mode so that the rotor is accelerated. The control unit operates the power converters in the generating mode such that the second power converter provides AC power to the set of polyphase exciter field windings and the first power converter develops constant-frequency AC power.

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. >

A technique for reducing computational effort in Monte-Carlo based composite reliability evaluation

Abstract: There is a growing need within the electrical utility industry to refine the methods used in the allocation of capital resources based on reliability considerations, and to determine what are the tradeoffs between cost and reliability. The evaluation of the steady state adequacy of bulk power (composite) systems is widely recognized as an important part of this process. The computational tools currently developed for bulk system reliability evaluation differ basically with respect to the method of selecting the system scenario (step (1)) and the network model used in the adequacy assessment (step (2)). For example, models such as SYREL, GATOR, RECS and COMREL, are based on the successive enumeration of severe/likely scenarios and use an ac power flow for adequacy assessment. In turn, models such as SICRET, MEXICO and CONFTRA are based on Monte-Carlo sampling of scenarios, and use a linearized power flow model ("dc" power flow) for adequacy assessment. One possible limitation of Monte-Carlo methods is the strong dependence of computational effort (proportional to the number of samplings) with respect to the desired accuracy of the estimates. For example, a sample size of 104 would be enough to estimate a LOLP of 10-3 with a relative uncertainty of 30%. However, the same estimate would require 106 samplings if the desired accuracy was 3%. Some methods have been proposed to reduce this computational effort, such as stratification, Importance Sampling and bounding methods. This paper describes a new technique for reducing the computational effort in Monte-Carlo based composite reliability evaluation.

Capacitive coupled power supplies

Abstract: A rectifying power supply having circuitry for providing AC power, a plurality of capacitors responsive to the AC power for providing DC isolation and for providing capacitively coupled AC power, and a plurality of rectifying circuits responsive to the capacitively coupled AC power for providing respective DC outputs. The outputs of the rectifying circuits are coupled in series or in parallel.

VAr planning for power system security

Abstract: A method is presented for identifying a dispersed reactive power (VAr) supply that can enhance power systems' pre- and post-contingency voltage security, subject to technical and economic constraints. The problem is formulated in two stages. The first stage involves a nonlinear optimization problem which minimizes the amount of reactive supply. The second stage uses a mixed-integer linear program which optimizes the number of candidates buses for VAr support. Results of application of the method to an example system are presented. >

Power factor motor control system

Abstract: A device and associated method to control the electromechanical energy conversion process of a variable speed drive utilizing a doubly fed machine, which provides the mechanical energy as demanded by some mechanical load. Alternatively, a device and associated method is provided for a synchronous motor. The terminals of either the stator or rotor winding of the doubly fed machine are directly connected to a polyphase AC power grid of a certain grid frequency. The device establishes through electronic control a stable operation of the drive at a desired power factor at the terminals of these windings, irrespective of varying load conditions. This capability is provided without the need for computing or sensing this desired power factor and the subsequent feedback of its deviation from the desired value through a properly designed regulator. All signal processing as required can be conveniently and reliably implemented; only the easily accessible quantities of the stator voltage and real power supplied to the motor are sensed and subjected to simple algebraic manipulations. Torque speed characteristics produced by the drive are compatible with any demanded load condition within design limits. These flexibilities are retained along with the capability of electronically adjusting the drive speed without the need for detecting the mechanical load condition.

Analysis and design of a three-phase current source solid-state VAR compensator

Abstract: A reactive power compensation (RPC) system which uses a three-phase current-source force-commutated PWM (pulsewidth-modulated) rectifier is presented and analyzed Pulsewidth modulation is also investigated as a means of reducing the size of reactive components The proposed RPC system can compensate for leading and lagging displacement power factor Other areas of investigation include the selection of rectifier input and output filter components and the closing of the loop around the reactive power command signal finally, predicted results are verified experimentally >

An enhanced LQ adaptive VAr unit controller for power system damping

Abstract: Static VAR compensators have been installed in power systems primarily to function in the steady state regulation of voltage levels or reactive power flows. More recently however there has been much interest in utilizing these devices to improve the dynamic performance of power systems. This paper presents an adaptive linear quadratic Gaussian control strategy for static var systems to enhance power system damping and stability. The control strategy uses only local information to dampen oscillatory modes present in the network. The controller calculates an appropriate value of VAr unit susceptance to present to the network at each sampling instant. The calculation of the appropriate susceptance value is based on a reduced-order model of the power system which is obtained on-line by a least squares identification procedure. The controller consists of three main components: an identifier, an adaptive observer, adn an adaptive LQG regulator. The identifier users a recursive least squares type of algorithm to fit a linear, discrete transfer function model to a sequence of input and output signals obtained from the power system. This results in a reduced-order approximation to the actual power system. For this study, VAr unit susceptance is used as the input signal and bus frequency deviation is used as the output signal. The coefficients of the identified transfer function are then sent to both the adaptive observer and the adaptive regulator. The observer is an observable-cannonical representation of the system and it calculates a state vector representing system dynamics.

Power conversion system with stepped waveform inverter having prime mover start capability

Abstract: A power conversion system is operable in a generating mode to convert motive power developed by a prime mover into electrical power and in a starting mode to convert electrical power developed by a source of AC power into motive power for starting the prime mover. The system includes a rectifier having an output coupled to a DC link, first and second inverters, each coupled to the DC link and transformer including first and second sets of primary windings and a set of secondary windings wherein the second set of primary winding is coupled to an output of the second inverter. Contactors are provided for coupling the rectifier to the generator armature windings and the first set of primary windings to an output of the first inverter so that AC power produced by the generator is converted into DC power on the DC link and the DC power is converted into fixed frequency AC power which is developed in the set of secondary windings. The contactors are operable in the starting mode to couple the source of AC power to the set of secondary windings, the first set of primary windings to the rectifier and the output of the first inverter to the generator armature windings so that the AC power induced in the first set of primary windings due to application of AC power to the set of secondary windings is converted into DC power on the DC link and the DC power is converted into AC power at a controlled voltage and frequency which is applied to the generator armature windings.

Simulation studies of islanded behavior of grid-connected photovoltaic systems

Abstract: Simplified computer models are presented for predicting the run-on time of self-commutated inverters operating in a utility interactive mode. The inverter receives DC power from a photovoltaic (PV) array and delivers AC power to a local load or the utility lines. The models represent the dynamics of a phase-locked loop control circuit that is designed to destabilize the inverter operation and shut down the power-conditioning subsystem when a phase discrepancy between the line and some reference signal is detected. The sustained isolated operation (or islanding) of the PV system poses a possible safety concern to utility personnel and potential damage to utility-connected equipment. Two computer models, the TESLACO model and the APCC model, were implemented on an IBM PC using Pascal. They provide results compatible with experimental evidence and more elaborate computer modeling techniques. >

Excitation system for a brushless generator having separate AC and DC exciter field windings

Abstract: An excitation system for a brushless generator includes an exciter portion having a set of polyphase exciter field windings and an additional exciter field winding disposed in a stator of the generator, a source of polyphase, constant-frequency AC power and circuitry coupled to the set of polyphase field winding for connecting the source of polyphase, constant-frequency AC power thereto at a beginning of operation in a starting mode whereby AC power is induced in an exciter portion armature winding by transformer action. AC power is further provided to armature windings of a main generator portion to thereby cause a rotor of the generator to accelerate. An autotransformer is provided to reduce the voltage provided by the source of polyphase AC power prior to application of such power to the polyphase field windings so that the need for an auxiliary inverter is obviated. Thereafter, at least one of the windings of the set of polyphase windings is connected in series with the additional field winding when the speed of the rotor reaches a predetermined speed and a DC current of controlled magnitude is delivered to the series-connected exciter field windings to thereafter allow operation in a generating mode.

Method and apparatus for controlling a power converter.

Abstract: The invention controls a power converter (34) for interchanging power between first and second electric sources (30, 32). In the preferred embodiment, one of the power sources is a high power DC source (32), such as storage batteries, and the second source is a standard utility bus (30). A static power converter (34) controls power interchanges between the two power sources (30, 32) and a controller responsive to the utility voltage, a command specifying the required power interchange, a feedback responsive to the actual power interchange, a command specifying the reactive power interchange, and a feedback specifying the reactive power interchange to generate signals which control the static converter to assure the required power interchange.

Induction machine based hybrid aircraft engine starting/generating power system

Abstract: This invention relates to a hybrid AC/DC power source for aircraft power generation and aircraft engine starting. The power source employs an induction motor/generator and a converter for converting high frequency AC power into separate DC and low frequency AC power for use by aircraft electrical systems. The subject invention contemplates the provision of a driver for the motor/generator comprising a circuit for providing excitation to the motor/generator when it is operating as a generator, the circuit also converting AC power produced by the generator into the high frequency AC power for use by the converter. The converter comprises a first rectifier which produces the DC power and a second rectifier and an inverter which together produce the low frequency AC power. The generator produces uncontrolled frequency AC power. Bidirectional switches operate within the circuit to invert DC excitation power to produce power for the AC excitation and to rectify output of the generator into DC. The generator can function as an induction motor to provide motive power to start the aircraft engine. Either external DC power can provide power to the induction motor via the circuit or external AC power can provide power to the induction motor via the circuit.

Inverter input/output filter system

Abstract: A filter system for removing selected frequencies from AC power provided on at least two phase lines and a neutral line includes a phase-to-phase filter for removing a first selected AC component at a first frequency from the AC power wherein the frequency component has a line-to-phase displacement within first or second ranges. A phase-to-neutral filter is also provided for removing a second selected AC component at a second frequency different from the first frequency from the AC power wherein the second frequency component has a line-to-line phase displacement outside of the first and second ranges.

Analysis and design of a novel 3- phi solid-state power factor compensator and harmonic suppressor system

Abstract: A novel three-phase solid-state power factor compensation scheme is presented and analyzed. This scheme employs a PWM voltage-source inverter and has two important features. First, it can maintain a near-unity mains input power factor without sensing and computing the associated reactive power component, and second, it can substantially reduce any line current harmonics generated by nonlinear types of load. The proposed scheme is discussed in terms of principles of operation, power system design, and analysis under unbalanced operating conditions. Predicted results are verified experimentally. >

Robust damping controls for large power systems

Abstract: A perspective on the need for robust damping controls in the extensive power system that serves western North America is offered. Key issues include the dynamics of the power system, known threats to controller robustness, and the information-poor environment in which major damping controllers must be designed and operated. The discussion covers power system stability control, reactive-power modulation in the main area, small-signal response during system disturbances, system dynamics tests at the Celilo HVDC terminal, and HVDC modulation and interactions. >

Line voltage monitor and controller

Abstract: A line voltage monitor and controller disconnects AC-powered user equipment from an AC power line whenever the AC power line voltage is outside a predetermined operating voltage window. The equipment is automatically reconnected to the power line only after the power line voltage remains continuously within the operating voltage window for a predetermined delay period. The controller protects the user equipment from both over-voltage and under-voltage (brownout) conditions.

Power conversion system with stepped waveform DC to AC converter having prime mover start capability

Abstract: A power conversion system is operable in a generating mode to convert motive power developed by a prime mover into electrical power and in a starting mode to convert electrical power developed by a source of AC power into motive power for starting the prime mover. The system includes a stepped waveform inverter having a plurality of subinverters that are operable in the generating mode to produce rectangular waveforms wherein the waveforms are summed by a summing transformer to produce a stepped AC output waveform. During operation in the starting mode, the output of a rectifier bridge and an input of one of the subinverters are connected together by an interphase transformer. DC power is developed at a mid-tap of the interphase transformer and is delivered to the remaining subinverters which in turn develop AC power for operating a brushless generator as a motor.

EL operating power supply circuit

Abstract: In a power supply circuit for operating an electroluminescent (EL) element comprising a DC power supply, an inverter connected across the power supply, and the EL element connected to the inverter, an inductor is inserted between the inverter and the EL element wherein an AC voltage is applied from the inverter through the inductor to the EL element to emit light. Another EL operating circuit includes a DC power supply, an inverter, a transformer, and a bipolar capacitor connected in this order and an AC power output on a secondary winding of the transformer is applied to an EL element to generate light. Both the circuits enables a noticeable luminance increase and power saving.