Showing papers on "AC power published in 1983"

An AC-to-DC Converter with High Quality Input Waveforms

Abstract: An ac-to-dc converter which draws sinusoidal and inphase current waveforms from the ac power source is described. Harmonic and power factor measurements obtained from a field- effect transistor (FET) converter operating at 45 kHz for a single- phase and a three-phase connection are presented. A stability analysis and design methodology are given.

Control Means for Minimization of Losses in AC and DC Motor Drives

Abstract: Motor drive losses can be reduced considerably by independently controlling the voltage and frequency for ac motors, and the field current and armature voltage for dc motors for any torque and speed operating point. The quantities to be controlled are already accessible in inverter ac power supplies and phase-controlled rectifier dc power supplies. The control signals can be supplied by an open- loop controller which solves the loss minimization equations using preset drive parameters, or by an optimizing controller that measures or calculates the losses and finds the combination of quantities that minimizes the losses.

Optimal Choice of Fixed and Switched Shunt Capacitors on Radial Distributors by the Method of Local Variations

Abstract: This paper presents a method of optimally choosing fixed and switched shunt capacitors on radial distribution feeders, considering load growth, growth in load factor and increase in cost of energy. Mathematical models to represent cost saving due to energy loss reduction taking the growth factors into account, cost saving due to release in systam capacities, capacitor cost and voltage rise during off-peek hours, as a function of capacitive current flows in the feeder sections have been formulated. Cost functions have been defined for optimizing the choice of both fixed and switched capacitors. A direct search technique known as the Method of Local Viariations has been employed for solving the resulting discrete variational problem. The problem has also been solved using Dynamic Programming Approach for comparison. The proposed method has been illustrated through some actual cases of radial feeders existing in an Indian distribution network. The results highlighting the influence of the growth factors have also been discussed in this paper.

A Quantitative Analysis of Induction Motor Performance Improvement by SCR Voltage Control

Abstract: Minimum input power and maximum efficiency operation occur at characteristic slip values which can be realized for any induction motor operating at part load by properly adjusting the amplitude of the applied stator terminal voltages. These two criteria are shown to yield perceptibly different results when the motor is driven from a silicon-controlled rectifier (SCR) voltage controller. In addition, it is demonstrated that a constant power factor controller results in an operating regime which is substantially poorer than operation at either minimum input power or maximum efficiency. It is further shown that minimum stator current and minimum power factor angle criteria yield results which are closer to the ideal than the constant power factor controller.

Power factor correction system

Abstract: A power-factor correction system for reactive power control and concomitant power factor correction adapted to cooperate with a line supplying power at a given line voltage from a power generator to an inductive load via a given path, includes a reactive power compensating device in shunt with the line supplying power, and is adapted to be connected in shunt with the inductive load. The reactive power compensating device, in turn, includes a fixed capacitance and a voltage applying circuit delivering a voltage of continuously variable magnitude in a stepless manner to the fixed capacitance in response to any change in lagging reactive power consumed by the load, so as to correct the magnitude of the power factor in the line supplying power to the inductive load to a sensed optimum correction value, as a result of the power compensating device generating thereacross a resulting compensating reactive power in proportion to the square of the voltage of variable magnitude delivered to the fixed capacitance so as to at least partly off-set the lagging reactive power, whereby power factor correction is optimized, yet does not produce any transient. The power-factor correction system is further capable of regulating the magnitude of the compensating reactive power to achieve optimization for line voltages having waveforms deformed from a sinusoid of the fundamental frequency and for non-linear loads. The system is also arranged to prevent destructive series resonance between a transmission line and the fixed capacitance.

Application of Power Transistors to Residential and Intermediate Rating Photovoltaic Array Power Conditioners

Abstract: An investigation into power conditioners that interface with photovoltaic arrays and utilities has been recently completed. The ratings for this investigation include residential systems (5-30 kW) that interface with a 240-V single-phase utility connection and intermediate systems (30-200 kW) that interface with a 480-V three- phase utility connection. Both systems mandated that an isolation transformer be provided between the array and the utility interface. A trade-off study was performed for many transistor and thyristor circuits and configurations. The weighting criteria included full- and part-load efficiency, size, weight, reliability, ease of control, injected harmonics, reactive power requirements, and parts cost. As the result of this study, a 10-kW high-frequency PWM transistor inverter feeding a high-frequency isolation transformer with a sinusoidally shaped current wave was selected. The output of the transformer is rectified with a diode bridge rectifier. Four thyristors, used as 60-Hz switches, reverse the polarity of the rectified current on every other half-cycle of the utility voltage. This reversal is accomplished slightly before the natural zero crossing of the voltage, thereby providing commutation for the thyristors. The results in the intermediate rating study targeted on a 100-kW design using power transistors in a full-bridge circuit connected to a 60-Hz three-phase transformer. Three bridge circuits are provided to interface with the polyphase utility. The switching strategy for the power transistors is based on a sine wave shape of current with a controlled deadband.

AC Elevator control system

Abstract: An AC elevator control system has a converter for converting a three-phase AC power to a direct current, an inverter for inverting the direct current to a three-phase AC power with a variable voltage at a variable frequency, and a three-phase induction motor for receiving the last-mentioned AC power to operate an elevator car connected to a counterweight through a traction rope trained over a sheave. A battery connected across the DC side of the inverter is enabled upon the occurrence of a power failure or a fault. A command emergency frequency generator responds to the occurrence of an emergency such as a power failure to deliver to the inverter a low frequency emergency frequency as determined by the relationship between a difference in weight between the elevator car and the counterweight and various losses of a motor driving system so as to cause the induction motor not to generate regenerative power.

3-Wire multiplexer

Abstract: The invention provides a 3-wire multiplexing system exhibiting high noise immunity, particularly suitable for industrial control applications. A master station provides AC power, and clocking and synchronization information, for a plurality of remote transmitter and receiver stations. A 3-wire cable links the master station and the remote stations. Two wires of the cable are power lines L1 and L2 and are isolated from each station by respective transformers. The third wire is a data line and is optically coupled to the remote stations. The remote stations are thus completely isolated from the cable, yet powered, clocked and synchronized therefrom without the need for local power supplies or local clocks. The signaling method switches the data line onto the power lines, and provides a high degree of data security by recognizing both the polarity and the position of a pulse on the data line within an AC cycle. The system further provides diagnostics in remote receivers including half cycle polarity and position decoding, error detection, fail state selection to fail OFF or fail ON or remain in the last valid state, and two scans alike detection.

Large Scale Reactive Power Planning

Abstract: A new methodology for planning future reactive compensation for large scale systems is presented. The planning technique is based on a restructuring of the network model and a special formulation of the revised simplex method. A preprocessor is incorporated for correcting systems which are not able to converge using conventional power flow or optimal power flow programs. A 600 bus example is provided which considers 285 transmission contingencies.

Economic dispatch of active power with constraint relaxation

Abstract: The economic dispatch of active power in large-scale power systems is considered and an algorithm for the solution of the problem by means of the sparse dual revised Simplex method is presented. Computational experience with large-scale problems, including dynamic dispatch over several time steps, is given, which indicates that the method has very low memory and processor time requirements and is suitable for implementation on minicomputer systems. The algorithm is extended to allow for hierarchical constraint relaxation and removal in cases where an infeasible problem has been specified inadvertently. The availability of such a constraint relaxation strategy permits the application of approximate methods for the inclusion of transmission losses and spare capacity constraints.

Characteristic Induction Motor Slip Values for Variable Voltage Part Load Performances Optimization

Abstract: Minimum input power and maximum efficiency occur at a characteristic optimum slip value which can be calculated for any induction motor using methods described. Efficiency is shown to be independent of output power when a variable voltage controller reduces the voltage approximately as the square root of the load torque to maintain the required slip during part load operation. Saturation effects do not significantly change these results.

A Current-Comparator-Based System for Calibrating Active/Reactive Power and Energy Meters

Abstract: A system for calibrating active/reactive power and energy meters under sinusoidal conditions using a current comparator bridge is described. Measurement can be made at any power factor from zero lag through unity to zero lead, positive or negative power, at 100 to 120 V, 1 to 5 A, and 50 or 60 Hz. The system features a digital oscillator, a thermal rms ac/dc voltage comparator, an automatically compensated capacitor for producing an accurate and stable reactive component, and a microcomputer for control and data reduction. The systematic uncertainty of the calibration system is estimated to be not more than 15 parts per million (ppm).

Novel topological alternatives to the design of a harmonic-free, utility/DC interface

Abstract: Circuit designs previously proposed for utility/DC power conditioning interfaces that draw a harmonic-free, unity power factor ac current have, for the most part, followed a certain topological approach. This paper discusses several alternative approaches that can greatly improve the interface's simplicity, performance, and cost. One suggestion involves the use of a dc current source instead of a voltage source. Another concerns the use of a tuned passive filter to remove the ripple from the dc system's waveforms. Results from an experimental 6 kW rectifier are included to demonstrate the feasibility and usefulness of the approaches suggested.

Tapped auxiliary winding for multi-speed operation of electric motor and method therefor

Abstract: A capacitor motor and method is disclosed which utilizes one or more tap points within the auxiliary winding of the motor which can be selectively connected to a source of AC power via a selector switch for multi-speed operation of the motor.

Optimal Design and Control Scheme for Continuous Capacitive Compensation of Distribution Feeders

Abstract: Distribution automation schemes being considered for implementation by electric utilities will rely heavily on substation-based computers for control of reactive power on primary feeders. To assist such implementation a new, continuously controllable, capacitive compensation scheme for primary distribution feeders is presented. This compensation scheme allows optimal placement and sizing of time- variable capacitors based on an optimal operating strategy which provides for power loss minimization as a function of time. A numerical example involving a realistic physically-existing feeder is provided to exemplify the design and control schemes.

Evaluation of Economy and/or Security Oriented Objective Functions for Reactive Power Scheduling in Large Scale Systems

Abstract: In this paper the security and the economy aspects of reactive power scheduling are emphasized and an automatic optimization procedure suitable for large scale systems is presented. Two objective functions have been implemented; the first one, based on security, distributes the reactive power generation among the units as proportional as possible to their ratings. The second one minimizes tle active losses expressed in terms of the reactive control variables. In both these optimization problems the control variables are partitioned into two subsets:-the terminal voltages at the generation buses-the LTC transformer tap settings.

AC-DC Switching regulator uninterruptible power supply

Abstract: A power supply for a load such as a computer, which is sensitive to power interruptions. A switching regulator is coupled to both the AC power line and a rechargeable battery, and makes automatic transitions between AC operation and battery operation, utilizing a transistor switch and an energy storage transformer coupled to both power sources and to the load. The transistor switch is driven by a variable duty cycle control signal provided by a programmed current feedback circuit responsive to both output voltage and instantaneous switch current. The feedback circuit cuts off the switch each time the peak current through the switch reaches a value corresponding to the desired output voltage--thus improving control loop stability and maintaining a constant peak current through the switch to substantially eliminate ripple. The circuit enables one terminal of the battery to be connected to one terminal of the switching transistor, to minimize noise. A charging circuit recharges the battery whenever the AC power line is operating within normal limits.

Standby power supply

Abstract: A relatively short-term standby power supply includes a relay which senses failure of the main AC power source, and in response to such a failure actuates contacts which turn on a first thyristor to connect a capacitor to the load. The capacitor has been charged to a higher voltage than the load requires and can consequently provide power to the load for a longer time period than would the same size capacitor charged to the normal operating voltage of the load.

A DC-AC Power Conversion Technique Using Twin Resonant High-Frequency Links

Abstract: The advantages of load commUtated high-frequency (HF) sinusoidal inverters as link stiiges in power conversion are well-known. An inverter configuration that employs parallel connection of the load to the resonating capacitor can be employed to realize the high- frequency link. A technique of dc to low-frequency ac power conversion using two, such high-fre4uency links is presented. The frequencies of the two links differ by twice the required output frequency. The difference between the two link voltages is then a high- frequency ac voltage whose envelope is a low-frequency ac wave at the required output frequency. The frequency of the envelope wave is decided by the difference between the two link frequencies while the amplitude depends on the amplitude of the individual voltages. Thus the output voltage can be regulated by varying the link frequencies, and so long as the frequency difference remains constant, the output (envelope) frequency is not affected. The output voltage can be obtained from the difference of the two link voltages by appropriate rectification/inversion. This process can be regarded as a form of cycloconversion in which the modulation process is incorporated in the input voltage instead of the switching.

Application of Static Var Compensators in Longitudinal Power Systems

Abstract: The application of a Static Var Compensator (SVC) in a longitudinal power system is presented and the technical studies performed to evaluate its effect on system operation are described. Steady state analysis is based on the combination of SVC and system characteristics. Using this approach changes in loading conditions and SVC adjustable parameters can be easily analysed. Results of simulations are presented that show the effect of SVC reactive power support on the system transient performance. Cases with supplementary controls to protect the system against SVC loss are also included.

Large Scale Security Dispatching: an Exact Model

Abstract: An exact optimal power flow model is presented for dispatching generation, transformer taps, and generator voltages to minimize operating costs while guaranteeing a steady state secure operating point after a contingency. A full ac power flow model is used, which permits including voltage and reactive power constraints in the optimization. The technique of this paper is unique in that all of the voltage and power constraints of all selected contingencies are enforced in a single optimization. No cycling between contingency evaluation and optimization is necessary in order to update the list of active constraints. Cases which have divergent power flow solutions are included in the problem with no difficulty. All non-islanding outages are guaranteed to converge to a feasible solution. The optimization algorithm solves a sequence of linearly constrained subproblems using a quasi-Newton search direction. The dispatching algorithm is tested on the IEEE 118 bus system power system.

Indentification and Ranking of Critical Contingencies in Dependent Variable Space

Abstract: Working in dependent-variable space, reliable yet simple computational procedures are presented for the purpose of identifying and ranking line outages which result in the violation of various limits related to power system security. Security limits defined in terms of real-power line flows, generator reactive powers and demand-bus voltage magnitudes are considered in normalised subspaces wherein critical contingencies are identified by a filtering algorithm using the infinite norm as a performance index. Critical contingencies are then ranked using performance indices which are defined in accordance with the subspace of interest. In the case of real-power line flows, ranking is based on a measure related to distance from the center of the corresponding security region whereas, in the case of generator reactive powers and demand-bus voltage magnitudes, ranking is based on measures related to distances from the boundaries of the corresponding regions. Furthermore, an operating point's violation of limits associated with generator reactive powers and demand-bus voltage magnitudes is measured in combined form form with due consideration being given to the relationships between these reactive powers and voltage magnitudes.

Reactive-power compensation

Abstract: For reactive-power compensation in hydroelectric generators (1), a loss reduction is achieved by connecting a fixed compensation circuit (6) to the secondary side of a mains transformer (5, 5'), which is connected via a converter (4, 7) to the generator (1) and is switchable in steps on the converter side, and the converter is adjustable over a wide voltage range.

Pulse width modulated inverter for unbalanced and variable power factor loads

Abstract: An n-phase inverter for developing polyphase AC power from DC power developed by a DC source (10) over first and second DC bus lines (16a, 16b) and a neutral line (30) includes circuitry for preventing the return of regenerative currents developed by a load (12) connected to the output of the inverter to the DC source The circuitry includes current sensors (32, 34) for sensing the presence of regenerative currents, and switches (Q7, Q8, Q9, Q10) which are operated when a regenerative current is sensed to disconnect the DC source from the inverter and to shunt the regenerative current back to the load The inverter may therefore be used with DC sources which cannot accept regenerative currents Furthermore, the size of a DC bus filter between the DC source and the inverter may be substantially reduced

DC-DC converter for remote power feeding

Abstract: A floating type DC-DC converter for remote power feeding is connected via a power separating filter to a signal line on which an AC signal is superimposed on DC power. The DC power separated by the power separating filter is stored in an input capacitor and is supplied by a switching operation of a switching element to the primary winding of a transformer and AC power induced in its secondary winding is supplied to an output capacitor after being rectified by a rectifying element. An electrostatic shield layer is interposed between the primary and secondary windings of the transformer and is connected to the capacitor connected to one of the primary and secondary windings. Rectifying element or switching element connecting means is provided by which an AC voltage induced in the winding portion on one side of the center winding point of the other of the primary and secondary windings and an AC voltage induced in the winding portion on the other side of the center winding point of said other winding are made substantially symmetrical with respect to the center winding point, opposite in polarity and substantially equal in amplitude to each other.

Processes and devices for providing a load with an electric AC supply without discontinuity of the AC signal

Abstract: An AC electric supply of a load capable of selectively connecting a main AC power source through a first primary transformer winding or an auxiliary emergency source such as from a battery power supply through a static DC-AC converter through a second primary transformer winding, the load being connected to the secondary transformer winding. The process includes sensing the voltage of the main source and automatically connecting the auxiliary emergency source when low voltage is detected and reconnecting the main source and disconnecting the auxiliary source when the disappearance of the failure of the main source is detected by first causing the auxiliary source frequency to drift, comparing the frequency between the auxiliary source and the resupplied main source, and when the two frequencies are in coincidence, connecting the main source and disconnecting the auxiliary source. An electric supply device comprises a transformer, a main electric AC source connected to the primary of the transformer and an auxiliary electric source connected to a second primary winding of the transformer, a voltage detector for detecting low voltage of the main electric source, and controlled switches for selectively connecting either the auxiliary power supply or the main power supply, and a synchronization detector which detects when the frequency of the main power supply and the auxiliary power supply are in coincidence and controls the switches to switch from the auxiliary power supply to the main power supply.