Showing papers on "AC power published in 1968"

Optimal Power Flow Solutions

Abstract: A practical method is given for solving the power flow problem with control variables such as real and reactive power and transformer ratios automatically adjusted to minimize instantaneous costs or losses. The solution is feasible with respect to constraints on control variables and dependent variables such as load voltages, reactive sources, and tie line power angles. The method is based on power flow solution by Newton's method, a gradient adjustment algorithm for obtaining the minimum and penalty functions to account for dependent constraints. A test program solves problems of 500 nodes. Only a small extension of the power flow program is required to implement the method.

Optimum Control of Reactive Power Flow

Abstract: The general problem of minimizing the operating cost of a power system by proper selection of the active and reactive productions is formulated as a nonlinear programming problem in accordance with previous work by Carpentier of Electricitede France. This general problem is particularized to the minimization of transmission line losses by suitable selection of the reactive productions and transformer tap settings. An efficient computational procedure based on the Newton-Raphson method for solving the power-flow equations and on the dual (Lagrangian) variables of the Kuhn and Tucker theorem is discussed. This minimization procedure has been applied successfully to a 500-node system studied by the Bonneville Power Administration for which an effective power-flow program had been developed previously. The dual variables associated with the primary (electrical) variables are obtained in the course of the computation, and their engineering significance for power system design and tariffication is emphasized. The procedure has been extended to the general case of combined active and reactive optimization.

Voltage stability of radial power links

Abstract: The general phenomenon of voltage collapse or instability at the load end of transmission links is introduced with reference to previous work, and the induction motor is seen to be the critical constituent of system loads. The power/voltage and reactive-power/voltage characteristics of power-system loads are predicted and accurately represented by polynomial expressions which are used in the analysis of radial transmission links fed from infinite busbars. Graphs summarising the critical load voltages and the corresponding short-circuit levels for various transformer tap ratios are given and are general in application; these are based on the dV/dZ criterion for voltage stability. It is shown that, although tap changing to raise the load voltage increases the critical length of the link, it also reduces the margin between normal operating voltage and the voltage at the onset of instability. The effects of the injection of reactive power by static capacitors are discussed, together with the effect of the induction-motor content of the loads. The use of the dEs/dVr criterion as advocated in the Russian literature is discussed.

New Aspects of Power Factor Improvement Part I---Theoretical Basis

Abstract: Recent developments in energy conversion, including ever increasing use of controlled elements, have created new aspects with regard to power factor considerations. This paper describes and analyzes a novel approach to the reactive and distortive powers that characterize nonlinear control elements. It has been proved that these powers may be neutralized when using similar nonlinear elements which, however, are suitably interconnected.

A New Constant Extinction Angle Control for AC/DC/AC Static Convertors

Abstract: Maximum utilization of an invertor's capacity and minimum consumption of reactive power demands an accurate constant extinction angle control. A new scheme for such a control is presented that is based on the integration of each commutating voltage waveform from -s+ ?c, thereby obtaining a simulation of the steady- state commutation equation. The resulting control is capable of fresh computation of the firing angle every cycle. The control is extended to cover the convertor operation over its full range from a minimum permissible angle of delay to the minimum permissible extinction angle.

The Through-Pass Inverter and Its Application to the Speed Control of Wound Rotor Induction Machines

Abstract: A new inverter type suitable for variable voltage ratio dc-ac power conversion is introduced. It was developed particularly for the control of speed and torque of wound rotor induction machines. The inverter "short-circuit" and "inversion" modes are described and examples of some means for switching from one mode to the other and for their time ratio control are shown. The behavior of the induction machine-rectifier-through-pass inverter cascade is analyzed and a simplified block diagram is deduced. Experimental results confirming the theory are given. The problems associated with the application of thyristors to the scheme for the slip power recovery are pointed out.

New Aspects of Power Factor Improvement Part II---Practical Circuits

Abstract: This paper brings forward and analyzes the practical power-electronic circuits One of the more interesting configurations described is an inductive element in which, due to constrained reactive power flow, capacitive properties are achieved with regard to the supply network

Electric vehicle driving and controlling apparatus

Abstract: An electric vehicle driving and controlling apparatus, wherein during the low-speed operation, single-phase AC power is supplied to a synchronous motor type rotary machine through a thyristor group to thereby control the rotation of said rotary machine, and during the ultra-high-speed operation, three-phase AC power available at the armature terminals of said rotary machine is also applied to a linear motor to thereby accelerate the latter.

Brushless and self-excited 3-phase synchronous machine

Abstract: The paper describes a brushless and self-excited synchronous machine, which uses current compounding in auxiliary stator and rotor windings to produce an excitation current which increases as the load current is increased. With the machine used as an isolated alternator, the effect is sensitive to the power factor of the load, and the increase in excitation current is greater as the load becomes more inductive. The constancy with which the terminal voltage of an experimental machine is controlled as the load varies is comparable with that obtained with typical static current-compounded excitation schemes, and a feature of the new scheme is the very rapid recovery of the terminal voltage following a sudden increase in load. As with static schemes, the new system is readily adapted to control either the power factor or the reactive power of the input to the machine, operated as a synchronous motor on an infinite busbar. Results obtained from the experimental machine again indicate an accurate control, from no load to well beyond full load.

Frequency-dependent dynamic, representation of induction-motor loads

Abstract: For comprehensive transient and dynamic stability studies, induction-motor loads have usually been represented by fixed shunt impedances at system nominal frequency. A method is proposed here which allows single induction motors or groups of induction motors to be considered as frequency-dependent and dynamic. If the necessary parameters of the motors are known and if computation effort is not a problem, accurate assessment can always be made for single machines or groups of machines but may involve lengthy calculations. Alternatively, an approximate, rapid method is presented which gives the change in active-power input to an induction motor or group of induction motors due to inertia and frequency-variation effects to within ±2% of the actual change for known power factor, inertia factor and full-load slip; and to within ±5% of the actual change when the assumed parameters are within the following limits: power factor within ±5% inertia factor within ±10% and full-load slip within ±20% (where the actual change in power has been assessed by the use of an equivalent circuit). The change in reactive power, as assessed, is within ±20% of actual change in reactive power. ±5% change in operating frequency has been considered. For simplicity, windage and friction losses have been taken as an integral part of the load supplied by the motor, and core losses have been assumed constant. These methods can be applied to power-system stability studies, as illustrated by an example.

Performance of a predictive automatic load-dispatching system

Abstract: This paper describes the experimental use of a predictive automatic load-dispatching system, and discusses the experience gained. The objectives of control were the economic dispatch of active power only, while maintaining the security of the power system against loss of generation and transmission capacity. The control system used a digital computer, which allocated loads to individual generators for various times in the future, and transmitted instructions to the power stations, where they were automatically implemented by special machine controllers. The system was operational for about ten months. The aspects of control considered in the paper are prediction, the dynamic performance of the loading program, power-system security, frequency control, and operating economies. Other subjects treated in companion papers are the influence of machine-controller parameters on system stability, the performance of the machine controllers, and operator work load. Although it was only possible to examine the behaviour of a control system employing predictive techniques and regular instruction of generation, it was concluded that prediction of system conditions about 10min ahead was optimal for executive control of generation, but that regular instruction had serious drawbacks. Operating economies of a few tenths of one per cent seem feasible, but are markedly influenced by the way in which the constraints of the power system are quantified and handled.

The Controlled Slip Static Inverter Drive

Abstract: The ability to use the versatile characteristics of the semiconductor inverter to achieve optimized control of a squirrel cage induction motor is demonstrated. The technique has been made possible during the past few years due to the development of fast, powerful, and efficient inverters. These, in turn, are the direct result of advances in semiconductor technology, which have produced thyristors with improved qualities and the advent of integrated electronics. If the slip of an induction motor is constrained and controlled to values below breakdown, high efficiency, high power factors, and moderate currents result in performance comparable to that of a dc machine. General expressions defining torque and involving the quantities of slip and excitation are easily derived. Excitation can be expressed in terms of volts per cycle, current, or flux. Torque can be controlled by adjusting slip or excitation or both in combination. The ability to control slip and escitation precisely and accurately depends on the inverter which is used. The pulse width modulated (PWM) inverter is an extremely effective motor controller accomplishing voltage and frequency adjustment in a single circuit. It is a fast, linear device; its response is virtually instantaneous. As a power amplifier it is comparable to the duel converter of dc systems, and its speed of response makes it applicable to virtually any feedback loop. In considering the mating of the motor and inverter, several principal factors are involved in the optimization of the system.

Power System Voltage Control by Using a Process Control Computer

Abstract: Kyushu Electric Power Company (KEP) already has the automatic load dispatching control system by using a digital computer for process control In addition to the existing computer control system, KEP put into service the control of power system voltage and reactive power early in 1968 The necessity, principle, and method of the centralized control system of system voltage and reactive power in KEP is discussed

Circuit for braking and driving an ac motor in the forward and reverse direction utilizing semiconductor elements

Abstract: A controlling circuit for an AC motor having an AC motor, an AC electric power source for driving said AC motor, AC controlling elements connected between said AC power source and AC motor, a switching element connected between said AC motor and AC power source, a first means for detecting a condition of said AC controlling elements, and a second means for controlling said switching element in accordance with the result detected by said first means.

Alternative convertor for h.v. d.c. transmission. Three single-phase bridges connected in series

Abstract: The Graetz (3-phase) bridge circuit is the arrangement usually chosen for a 3-phase d.c. convertor, because it has the best valve and transformer utilisation, and hence lowest valve costs. This bridge has the disadvantage that the alternating current is fixed relative to the direct current, which means that the reactive-power requirement is dependent upon the active power. If three single-phase bridge circuits are used, however, it is possible to vary the relationship between the alternating and the direct currents, and hence it is possible to change reactive power without varying active power. This facility is particularly useful when a number of convertors are connected in series on the d.c. side, when the direct current is common to all convertors and voltage control is used to set the active power level. The operation of a convertor using three single-phase bridge circuits is analysed, and it is shown that, although the bridge is more expensive on valves, it has operational advantages over the conventional Graetz bridge. The theoretical and practical operating limitations are discussed, and it is shown that the convertor can be operated continuously over its complete power range with a reactive-power requirement of only 70% of the rated convertor apparent power, compared with 100% for a Graetz bridge. The cost of terminal plant other than valves is shown to be the same for both convertor circuits. In a brief economic assessment, it is shown that the overall cost of the new convertor may be only 14% higher than a similarly rated Graetz bridge

Instrumentation for Thyristor Control

Abstract: Since thyristor control produces nonsinusoidal periodic waveforms with high harmonic content, the electrical measurements associated with such controls require some adaptation of the existing techniques which have evolved mainly for the measurement of constant (dc) or sinusoidally varying (ac) electrical quantities. The basic principles and instruments discussed are relevant to voltage, current, real and reactive power, and power factor measurement in situations most likely to be encountered in applications of silicon-controlled rectifiers (thyristors).

Parallel operation of ac generators

Abstract: Means for connecting an AC generator driven by a prime mover to an AC power transmission line and operating the generator or generators on the power line. Each generator is connected to the power line by first bringing the generator up to approximately synchronous speed, with the generator field deenergized, and then connecting the generator winding to the power line. The power line voltage pulls the generator into synchronism, and the generator field is then energized. Means is also provided automatically to prevent connection of the generator to the power line until a certain generator speed is reached, and automatically to disconnect the generator from the power line in the event the prime mover is shut down.

Full wave phase control integrated circuit

Abstract: A semiconductor trigger-signal-generating circuit is provided for applications such as full-wave phase control of AC power to a load through a trigger-signal-responsive semiconductor switch. The circuit is particularly suitable for operating directly off an AC supply such as a 60 hertz AC line, and in a preferred embodiment is a semiconductor monolithic integrated circuit.

Control system and apparatus for a heating station

Abstract: A control system for regulating the power input to the heating station of a processing device. The system utilizes a gated, full-wave thyristor to regulate an AC power input to an electrical heating element. Using a thermistor as a temperature sensor, the control system functions to periodically sample heating station temperature at the commencement of each input power supply cycle. A signal threshold level is established within the sampling interval, below which full-power will be inserted through the thyristor.

Smooth control of reactive power using static devices

Abstract: A static system which provides smooth variation of reactive power, both inductive and capacitive, is developed. The chronological development of the circuit is outlined. The sequence of phenomena involved in the circuit is described. The system is analyzed and its basic properties are explored. The operation of the system as a variable inductor and capacitor is studied and results are presented. The behavior of the system is verified on an experimental model.

A Method of Power Factor Measurement for Circuit Interrupter Testing

Abstract: In testing load circuit interrupting devices for electric power systems, it is necessary to determine the power factor of the test circuit. There has previously been some difficulty in making this measurement. This problem is discussed, and a circuit is described which has been used to measure conveniently the phase angle between the source voltage and the load current.

Method and apparatus for reactive power regulation in electric networks

Abstract: 1,109,703. Power factor improvement. MOSKOVSKY ENERGETICHESKY INSTITUT. 17 Jan., 1966, No. 2031/66. Heading H2H. [Also in Division G3] In a capacitor bank used for power factor improvement and controlled by reverse parallel connected thyristors, current surges due to delayed angular firing of the thyristors are avoided by injecting into the capacitor a current pulse to produce a voltage greater than the supply voltage at that instant to extinguish the thyristor over the middle part of the cycle. Thyristor 1 is conductive from the beginning of a cycle until a voltage U2 is reached when the thyristor is extinguished by a pulse from a circuit 3. The capacitor voltage remains at this value until the thyristor 4 reconducts when the supply voltage U1 is again equal to U2 in the later part of the cycle. As the thyristors conduct when the supply voltage is approximately equal to the capacitor voltage large current surges are avoided. An automatic control circuit is described in the Specification, for producing timed pulses for two capacitor banks connected to a supply through star/star and star/zig-zag connected transformers respectively. Correction of individual phases is provided for.

Transformerless dc to ac power supply

Abstract: A transformerless DC to AC power supply which comprises a piezoelectric crystal dimensioned to be resonant at a given frequency, excited to resonance by the application of a sinusoidal signal derived from a multivibrator powered by a direct-current power source. A feedback loop adapts the output of the piezoelectric crystal to synchronize the operating frequency of the multivibrator at the resonant frequency of the piezoelectric crystal.

Error assessment in inadequately represented power-system networks

Abstract: In power-system load-flow and stability studies, transmission lines are usually represented in various forms which are not exact. Although errors in representation are apparently insignificant, it is shown that they can have a cumulative effect on the outputs of various synchronous machines in the power system. A method is presented, utilising the principle of moments, which enables the overall error in active-and reactive-power outputs of synchronous machines to be obtained with the help of a load-flow study under steady-state conditions. The method works equally well for active power and reactive power separately, as well as in complex form. Numerical examples are included to indicate the possible seriousness of such apparently small errors, and the desirability of including the effects is discussed, particularly in studies which employ a high degree of refinement.

Static Inverter Standby AC Power for Generating Station Controls

Abstract: The reverse transfer static inverter and static ac bus transfer switch system, as applied to power generating stations, are introduced. Equipment for which such protection is required and the power quality necessary are discussed. The proper protection methods for branch circuit coordination with static systems is described, using a particular power plant as an example.

Reactive Power and Its Control in a Large Metropolitan Electric Supply System

Abstract: In large metropolitan systems, the magnitude of reactive power can become so great that controlling it and system voltage demands more than the ordinary requirements in equipment and operating techniques. On the Consolidated Edison system, one of the major problems concerns an excess of charging Mvars from high-voltage cables during light load periods. At the outset this would seem to indicate that, with such excesses of capacitive Mvars, few capacitors would be required in the distribution area of the system. This unfortunately is not the case, since it is desirable and economical to have the reactive power for distribution located as near the load as possible, to maintain the megawatt capability of distribution stations and equipment at their maximum values. Additional capacitance is therefore required in the form of switchable capacitor banks at strategic locations, even though there may be an excess of charging Mvars on the high-voltage systems at the same time. Also, voltage control is usually held within relatively narrow limits in metropolitan areas and this aggravates the overall problem. On the system discussed here, a very large change has come about in the last two years with installation of approximately 65 miles of 345-kV cable. These situations and ways of handling them are discussed.