Showing papers on "AC power published in 1976"

Reactive Compensation and Harmonic Suppression for Industrial Power Systems Using Thyristor Converters

Abstract: This paper discusses the problems and solutions of applying reactive compensation, either static or dynamic, to industrial power systems supplying large blocks of dc power from diode or thyristor converters. The resonance between power capacitors and system reactance can produce high harmonic voltages caused by the harmonic currents generated by converters. The interaction between these harmonic voltages and regulating systems can be minimized by the use of suitable filters. The design of these filters should eliminate the interaction between power system and load and reduce harmonic current flow. Different methods of reactive power control are also discussed.

Reactive power generation and control by thyristor circuits

Abstract: Generally, static VAr generators function as variable reactances (capacitive or inductive impedances) or controllable ac current and voltage sources. This paper reviews possible methods of VAr generation and control by static thyristor circuits and describes new approaches in which power frequency changers (cycloconverters) are employed. Oscillographic recordings illustrate the operation and performance of practical systems, including a 35 MVAR arc furnace compensator.

Probabilistic a.c. load flow

Abstract: The paper extends the presently available techniques for evaluating the load flow problem probabilistically. It presents two possible formulations of the problem that permit the probability-density curves of the angles, voltages, injected reactive powers and active and reactive power flows to be computed. Owing to the complexity of the problem, the load-flow equations were linearised. The errors introduced by this technique together with the results of a typical analysis are discussed.

RF Impedance of United States and European Power Lines

Abstract: Impedance measurements were made of 86 commercial 50-Hz AC power distribution systems throughout the LF-HF (0.02-30 MHz) spectrum in six European countries. The impedances are those presented to devices/appliances connected to the ac power source. The European results are presented in graphic and tabular form and are compared to impedances of U.S. power lines.

Three-Phase AC Power Control Using Power Transistors

Abstract: A three-phase ac power control circuit using power transistors which operate in a high-frequency chopping mode is described. The circuit is capable of handling several kilowatts of power at any lagging load power factor angle and the output voltage can be smoothly controlled from zero to full supply voltage. The circuit has inherently fast response and the high-frequency ripple at the output is easily filtered. The development, study, and experimental evaluation of the circuit with resistive and induction motor loads is described.

A Closed-Loop Control System for the Reduction of Reactive Power Required by Electronic Converters

Abstract: A closed-loop control system to reduce the reactive power required by electronic converters is proposed. The instantaneous reactive power which consists of both displacement of fundamental current and harmonic distortion current is measured and compensated by a reactive power source connected parallel between the power lines and the converter. A combination of a dc choke and a forced-commutated inverter is used for the reactive power source.

A Controllable Secondary Multikilowatt DC Current Source With Constant Maximum Power Factor in its Three-Phase Supply Line

Abstract: A type of industrial size dc converter for submegawatt applications is described and illustrated with the characteristics of a small scale 10 kW model. High conversion efficiency near 97 percent is maintained at full power and with an internal frequency of operation near 10 kHz. The system is characterized by the behavior of a true current source with inherent short circuit capability of its output terminals. All power is processed via a firmly controlled high Q series resonant circuit. Feasibility of power densities in excess of 1 kW/kg component weight is demonstrated. This high power density reflects itself in low quantities of material needed for construction of these systems. Operation of the dc converter as part of a rectifier-filter converter system, powered from a three-phase supply line is described. Test data demonstrate a high power factor PF?0.95 for all conditions of converter loading. Data obtained from a thyristor bridge-filter system are presented and subjected to comparative analysis.

Power brown-out detector

Abstract: Apparatus for computing an approximation of a running average of the peak voltage of an AC power wave. If this average drops below a predetermined level a brownout signal indicating potential failure of electronic units powered by the AC wave is generated.

Phase sequence correcting system for three-phase AC power

Abstract: The phase sequence of a three-phase AC power supply is detected and information representing that sequence is stored in a memory device, such as a flip-flop. Since the power supply, once it is energized, may require a few seconds to stabilize (such as may be the case when the power source is a diesel generator), the information fed to the memory device is likely to change during the stabilizing period. At the end of that period, however, the stored information represents the true phase sequence of the AC power supply, and at that time the information is read out and used to actuate switching apparatus to interconnect the three-phase AC power supply to the three terminals of a phase-sensitive load (such as a three-phase AC motor) in the manner necessary to provide the load with a desired phase rotation sequence. After the stabilizing period, the input circuitry of the memory device is inhibited so that the stored information representing the true phase sequence will be locked into the memory device and so that the device will be insensitive to any fluctuating voltages, frequency or noise that could otherwise cause an accidental phase reversal while the load is energized.

Equipment with controlled reactance

Abstract: Apparatus for power factor correction of alternating current networks in a continuous fashion. A first group of reactance devices are each serially connected to the network through a switching device, and a second reactance device is connected in the network through a control device for continuous control of the reactance provided by the second reactance device. A source of control signals responds to a signal determinative of the desired reactive power and controls the switching device for step-wise varying the reactance provided thereby and likewise controlling the control device for continuous control of the reactance contributed by the second reactance device to provide for overall continuous control of reactance as a function of the signal.

Chopper assisted uninterruptible power supply

Abstract: A bilateral chopper is used between an inverter and the standby battery of an uninterruptible power supply, the battery being charged through rectification of the AC power by the feedback diodes of the inverter under controlled voltage by the chopper, the load being supplied with AC power under controlled voltage by the chopper from the standby battery in case of failure of the AC power supply.

Reactive power control by least squares minimization

Abstract: An effective method is presented for performing reactive power control in a bulk power supply system by optimally rescheduling generator terminal voltage magnitudes and/or transformer tap settings. The optimization procedure is the familar least squares minimization technique and is suitable for on-line application in energy control center computers. Sensitivity factors provide the basis for the control function and are developed from a linearized reactive power model. Important features of this method include a model approximating the reactive power flow through a load-tap-changing (LTC) transformer and a Norton's equivalent circuit model approximating the reactive power output of a regulating generator. As a result of the linear models developed, optimally ordered triangular factorization is fully exploited to enhance computational speed and efficiency. Test results are presented to demonstrate the effectiveness of the method. A linearized active power model is briefly presented to illustrate its dual relationship to the linearized reactive power model. This is an important relationship, enabling both active and reactive power control by application of the same general model.

Method for simultaneous measurement of current and voltage on high-voltage lines using optical techniques

Abstract: A laboratory device for the simultaneous measurement of current and voltage on a high voltage busbar has been constructed. A laser beam may be directed upwards from the ground to interrogate a passive optical transducer attached to a high-voltage busbar. Real and reactive power indications can also be obtained from the output, thus providing complete simultaneous information on the line state. The device uses a combination of magneto-optic and electrogyration effects in crystalline α-quartz. The latter effect has only recently been discovered and should lead to a device with small temperature dependence. The measurement transducer operates in a differential mode and the light beam passes through the quartz at a chosen angle to the optic axis. Quartz is a rugged material, very suitable for use in a transducer. A more highly developed version of the device should provide a convenient, portable, large bandwidth, research tool for diagnostic use. Eventual development could lead to an acceptable instrument for long-term operational use on the power system. Such an instrument would offer the advantages of considerably improved performance over conventional devices, at much lower cost.

Line power distortion detector

Abstract: Apparatus for testing distortion of an AC power wave and providing an error signal if distortion of the type likely to cause DC output of a power supply receiving the AC power wave to fall below acceptable levels occurs.

Battery backup for AC powered DC supply

Abstract: A battery backup system for an AC energized DC supply has an AC line monitor for producing a pulse signal having a pulse width dependent on rise time of the AC signal and a frequency dependent on zero line crossings of the AC signal. The pulse signals are checked for frequency and pulse width by being used to control the charging time of a timing capacitor for a timer and to be compared with the pulse width of a predetermined reference pulse, respectively. The occurrence of a defect in the pulse signals stemming from either pulse testing criteria is effective to trigger an SCR circuit into conduction to connect a backup battery to the load devices to replace the DC power formerly being supplied by the AC line. After restoration of the AC power, the backup battery is removed following a fixed time delay to allow the AC powered DC supply to stabilize. A battery charging power supply is connected to the backup battery and is automatically switched between a battery "float-rate" and a "high-rate" charging operation following an AC outage and returned when the charging current indicates that the backup battery is fully charged.

Control circuit for hospital bed

Abstract: Beds, such as hospital beds, in which at least some portion of the bed is movably actuated by an electric motor (or motors) are provided with a control circuit to determine proper energization of the motor from an AC power source. Since the AC signal from the power source could be harmful to the person selecting a desired direction of travel of the movable portion of the bed, the selecting portion of the control circuit is isolated by appropriate transformers from the power portion that supplies the motor. Bidirectional switching devices, such as triacs, are utilized to convey the power to the motor. Other bidirectional switching devices, such as triacs, are used to gate the power handling triacs through appropriate gating transformers. A phase shifting arrangement is utilized in connection with the gating transformers to provide proper commutation of the power handling triacs. Supplemental features, such as additional locations of the control and limit switches to establish maximum distance of travel may be employed. When both head and knee movable portions are utilized, a contour circuit may be employed to automatically adjust the knee portion upon variation of the head portion, within certain limits of travel. A disconnect arrangement employing a self-gating triac is utilized to automatically open a hot line to the common of one of the motors unless the energizing circuit for that motor is completed.

DC to AC switching converter

Abstract: A regulated DC to AC power converter of the switching type for providing AC power at a selected output frequency, e.g., 50 Hz, is disclosed. The converter, which eliminates the need for an inverter transformer, is switched at a switching frequency which is considerably higher than the output frequency, e.g., 10 KHz. Due to the high switching rate the converter filter can be designed with small and lightweight components to provide a desired pass band, which includes the output frequency and harmonics thereof, and provides sufficient attenuation for the switching frequency and its higher harmonics, so as to produce an output with very low ripple or waveform distortion. Due to the high switching frequency the output waveform can be regulated with a response time of a fraction of a cycle. The converter includes a modulator section with a unique arrangement to control the switching of the switches between their On and Off states. In preferred embodiments of the converter the switches are implemented with transistors to form AC power at high power levels. Each switch includes a plurality of relatively inexpensive transistors which are connected in parallel with each transistor drawing only a small portion of the total current through the switch. A novel arrangement is employed to increase switch reliability by preventing its failure even though one or more of its transistors may fail in a short-circuit state. For switches implemented with transistors the extended turn-off time of the transistors, due to their saturated states, is accommodated by controlling the turning On of each switch to occur only after the transistors of the previously turned On switch returned to their Off state.

Thermal measurement of AC power in comparison with the electrodynamic method

Abstract: A thermal instrument for measurement of low-frequency ac power with a resolution of 1 ppm of maximum input is described. Due to its simple construction and, consequently, low number of error sources, it is well suited to precision measurements. The instrument has been applied to an accurate experimental comparison with the electrodynamic method carried out for identification of errors that cannot be traced by self-checks of each of the systems. The experimental configuration for the comparison is discussed and results of the measurements are given.

Electric arc furnace control system

Abstract: This disclosure relates to an electric arc furnace control system for positionally displacing the consumable furnace electrodes. Each electrode is coupled to an electric motor and arranged for vertical height displacement in relation to the material in the furnace. An arc current control signal is derived by comparing the actual arc current with a reference electrode arc current. In addition, a reactive power control signal is also derived by comparison of the (actual reactive power/actual real power) with a desired ratio of (reactive power/real power). The arc current control and the reactive power control signals are algebraically summed to provide an output error signal which is sent to the respective electric motors to provide rectilinear displacement of the electrodes.

Current regulation system for three-phase load

Abstract: The load currents, supplied by a three-phase AC power source to a three-phase load, are established and maintained at a preset desired level by sensing those currents and producing a feedback voltage which is compared with a reference potential. Anytime the load currents differ from the desired level the comparison produces a difference or error signal which causes an adjustment of the amplitudes of the three load currents such that they automatically return to the desired level, thereby regulating the current flow to the load. At start-up, the feedback arrangement is also effective in preventing instant current build-up to avoid sudden power surges. When the load is a three-phase motor, limiting the current flow facilitates a soft start with the motor gradually accelerating to full operating speed. Two overload protection circuits shut the system down in response to different overload or overcurrent conditions. One operates when the peak-to-average ratio of the load currents is higher than normal, and the other protection circuit actuates when the peak current is higher than desired.

Apparatus for compensating reactive power in a three-phase network

Abstract: An apparatus for compensating for the reactive power in a three-phase network. The apparatus includes an uncontrolled rectifier, an intermediate DC link connected to the rectifier, and a self-commutating inverter connected to the DC link and controlled so as to have a pulse-width modulated output.

Rapid power transfer system

Abstract: A system for rapidly substituting one source of AC power for another. A solid-state gated electronic valve is coupled in circuit with a principal power source or "line" and the current through the valve is monitored. Line voltage is also monitored and a timing signal produced to keep the system in synchronism with the principal power source. When the source voltage decreases or changes abruptly, or if its phase drifts unacceptably, the difference detected between the source and a reference effects the termination of valve gating signals, and allows the application of synchronous enabling signals to a standby power source. If the solid-state valve is non-conducting the standby source starts immediately in synchronism with the previously-supplied load voltage. If current is flowing in the valve, however, a commutating signal is produced which overrides the synchronous standby enabling signal to energize the standby power source with the proper polarity to extinguish current through the valve. When the current decreases below the holding current value the standby source is caused to enter the synchronous mode.

Symmetrical and Extinction Angle Control of Solid-State Series Motor Drive

Abstract: In this paper, two control schemes are proposed and investigated for solid-state dc series motor drive. Performance characteristics such as torque speed, input harmonic content, power factor variation, and peak motor current are obtained for both control schemes and compared with the commonly used phase-angle control scheme under identical operating conditions. The input power factor is always lagging in the phase and symmetrical angle control schemes, while it is leading in extinction angle control scheme over a wide range of speed and power. Each control scheme appears to have certain merits over the other schemes. Experimental results verify the basic principles of operation and demonstrate the feasibility of these control schemes.

Steady-State Analysis of the Series Resistance-Inductance Circuit with Controlled Switches

Abstract: The effect of forced-commutation on the ac phase controller with a series resistance-inductance load has been investigated. It has been shown that the total circuit power factor can exceed that of the load to sinusoidal current, with selected firing angles. Two modes of switching are introduced in which one maintains the fundamental power factor to near unity despite the load being largely inductive. In the second the fundamental power factor is kept to its most leading value and a relative improvement in distortion factor is achieved.

Synchronous switch control circuit

Abstract: A switching circuit for controlling the selective electrical energization of a resistive or inductive load by an AC power source such that a switching operation is performed only when the current from the AC power source is nearly zero. Pulses synchronous with the zero crossings of the AC power source are used to trigger one-shots to effectuate appropriately gated delay pulses when the switching circuit is ON and OFF, respectively. A latching circuit connected to an ON-OFF switch activates and deactivates the switching circuit.

A differential thermal wattmeter for the AC/DC transfer of power

Abstract: A differential thermal wattmeter for the ac/dc transfer of power is described. A differential multijunction thermal converter is used as a “sum and difference” multiplier, with both ac and dc voltage and current signals simultaneously applied to both heaters. Dc feedback to the current input speeds up the response and maintains thermal balance between the heaters, so that the output meter becomes a null indicator. This mode of operation reduces and can eliminate the need for quadratic response. The requirement for exact matching of the thermal converters is also eliminated by effective interchange of the heaters. The converter heaters are protected against burnout, and crest factors of 4 in the ac current or voltage signals are permissible without degradation of the accuracy. Maximum meter sensitivity is 100 ppm of nominal power for full deflection, with a resolution of better than 20 ppm, and a linear scale. The instrument is designed for use at power frequencies of 50–1000 Hz, with ac/dc transfer errors of less than 30 ppm over this range. The wattmeter can be used up to 20 kHz with reduced accuracy.

High harmonic current reducing apparatus in power converter

Abstract: This invention is applied to a power converter with a converting apparatus comprising one or more units including only controlled rectifiers or combination of controlled rectifiers and non-controlled rectifiers, said converting apparatus being connected with an AC power source, whereby high harmonic currents generated in the AC circuit during commutating periods are reduced. In the present invention, at the commutating period of the converting apparatus, the reactance in the AC circuit of the converter is changed to increase in accordance with change of the AC current of the converting apparatus, so that the change of the AC current during the commutating period is made more slowly thereby reducing high harmonic currents.

Circuit arrangement for the automatic de-excitation of a hysteresis motor

Abstract: A circuit arrangement for the automatic de-excitation of a hysteresis motor whose reactive power intake is compensated by capacitors and whose stator windings, which are fed from a three phase mains via a frequency converter, magnetize the rotor with their rotating field and thus produce a permanently magnetic excitation which continues in the rotor in the form of residual magnetic poles even after disconnection of the motor from the three phase mains. A voltage monitor monitors the mains side input to the frequency converter and responds upon interruption of the mains voltage due to malfunction or end of operation. Upon interruption of the mains voltage a capacitor of a buffer circuit in the d.c. intermediate circuit of the frequency converter transfers its stored energy, with decaying voltage, to the stator of the motor via an inverter which forms the output of the frequency converter, and upon response of the voltage monitor, the output frequency f 0 of the inverter is automatically increased by a predetermined amount Δf, whereby the residual magnetism in the rotor is quenched.

A current comparator bridge for power measurement

Abstract: A current comparator technique for the measurement of sinusoidal ac power at 60 Hz is described. The reference power is that generated by applying the voltage across a 10-kΩ resistor. The voltage itself is measured by comparing it directly to a dc reference with an ac/dc rms voltage comparator. The current is compared with the current comparator to the current in the resistor for the in-phase or active power component, and to the current in a capacitor for the quadrature or reactive power component. Six-digit resolution is provided for both components. The bridge can be operated over a voltage range from 50 to 150 V and a current range from 0.01 to 1 A at any power factor. An auxiliary current transformer provides extension of the current range to 10 A. Comparison measurements with a Drysdale wattmeter used as an ac/dc power transfer standard indicates agreement at unity power factor to within 25 ppm.