Showing papers on "Three-phase published in 1991"

Three phase PWM converter/inverter by means of instantaneous active and reactive power control

Abstract: A novel PWM (pulse-width-modulated) technique for the three-phase PWM converter/inverter is proposed. The instantaneous active and reactive power are used as the PWM control variables instead of the three-phase line currents. They are directly controlled by the PWM switching from the appropriate voltage vector selection. The features of the proposed PWM technique are simple construction, high efficiency, high response, and wide application. The principle of the PWM technique, the PWM control system, the control circuit, the control range, and the design indexes are described. Some simulation results and experimental results are shown. >

Transient simulation and analysis of a three-phase five-limb step-up transformer following an out-of-phase synchronization

Abstract: Theoretical and experimental analyses are presented of the electromagnetic transient following the out-of-phase synchronization of a three-phase five-limb step-up transformer; this means an abnormal condition where the angle between phasors representing the generated voltages and those representing the power network voltages at the instant of closure of the connecting circuit breaker is not near zero, as normal, but may be as much as 180 degrees (phase opposition). When this happens, the peak values of the transient currents in the windings of the transformer might be sensibly higher than those of the failure currents estimated in a conventional way; in addition, they correspond to unbalanced magnetomotive forces (MMF) in the primary and secondary windings of each phase of the machine. The currents and fluxes during the transient are computed by a nonconventional circuital nonlinear model of the transformer simulated by the electromagnetic transient program (EMTP). The results of an experimental validation made on a specially built 100 kVA three-phase five-limb transformer are also reported. >

Space vector modulation with unity input power factor for forced commutated cycloconverters

Abstract: A new modulation technique for three phase to three phase (3 Phi -3 Phi ) forced commutated cycloconverters (FCCs) is presented. It is based on the space vector representation of the output voltages and input currents in the complex plane. The resultant output line voltages and input phase currents do not contain low frequency harmonics. The input current displacement factor and the converter voltage gain can be freely varied, regardless of the load power factor; the only restriction being the equality of input and output active powers. The modulation technique holds for any input and output frequencies, so that it can be also applied for 3 Phi AC to DC and DC to 3 Phi AC conversion, thus making FCC a universal power conversion module. >

Calculation of cage induction motor equivalent circuit parameters using finite elements

Abstract: A means by which the finite-element method can be used in conjunction with the conventional equivalent circuit model to determine the equivalent circuit components for a three-phase cage induction motor is described. The method uses separate finite-element models for the rotor and the stator, consisting of one slot pitch and one phase band, respectively. The use of minimal models leads to a fast execution time. The method is illustrated by comparing computer predictions of performance with test measurements made on four motors, ranging in size from 4 to 150 kW.

Minimizing the current harmonics RMS value of three-phase PWM converter systems by optimal and suboptimal transition between continuous and discontinuous modulation

Abstract: A given converter voltage space vector can be realized by a three-phase PWM converter system by switching only two bridge legs. There, the third phase is clamped to the positive or negative DC link voltage. This method is called discontinuous modulation, due to the discontinuous shape of the generating phase modulation functions. As a closer analysis shows, the discontinuous method allows an increase in the effective system pulse frequency, dependent on the phase angle between converter output voltage and output current. The basis for a comparison with the continuous method is chosen as equal average switching losses of any converter bridge leg. For harmonic-optimal operation one must change between continuous and discontinuous modulation (or between various variants of the discontinuous modulation), depending on the load status. The harmonic losses of the control methods are calculated directly in the time domain. For this purpose, the space vector calculus is applied and approximations are used which are sufficiently exact for PWM converter systems with high pulse frequency. >

A synchronous machine model for three-phase harmonic analysis and EMTP initialization

Abstract: A three-phase synchronous machine model is developed for unbalanced harmonic load flow analysis and for initializing EMTP (electromagnetic transient program) transient simulations. Two nonlinear effects, the frequency conversion and saturation, are represented in conjunction with the machine load flow constraints. The model is in the form of a frequency-dependent three-phase equivalent circuit. It can therefore be easily incorporated into existing harmonic programs for systemwide harmonic analysis. Thus, the generation of harmonies by synchronous machines under various load flow conditions and the harmonic interaction between machines and other harmonic sources can readily be analyzed. >

Dynamic response of power conditioning systems for superconductive magnetic energy storage

Abstract: The dynamic response of two power conditioning systems for superconductive magnetic energy storage (SMES) are presented. One power conditioning system is based on a hybrid current sourced inverter (CSI), the second is a combination of a DC chopper with a voltage sourced inverter (VSI). The response of both systems to a load change, a three phase fault, and start-up is presented. >

A three-phase resonant PWM DC-DC converter

Abstract: Two types of three-phase resonant pulsewidth-modulation (PWM) controlled DC-DC converters are presented. It is shown that the combination of resonant and PWM control methods can be used to tightly regulate the output voltage from no load to full load at constant switching frequency and with low switching losses. It is also shown that both topologies exhibit zero voltage switching in continuous conduction mode. In discontinuous mode the turn-on losses are very similar to PWM converters. Relevant input and output current waveforms and component design ratings are derived. Key predicted results are verified experimentally on a laboratory prototype unit. >

Three-phase flyback AC-DC convertor with sinusoidal supply currents

Abstract: A three-phase flyback AC-DC convertor which has the step-up and step-down characteristics of output voltage, is described. The introduction of pulsewidth modulation makes it possible to obtain the sinusoidal supply currents with a near unity power factor. Three gate-turn-off thyristors provide the necessary control over the currents for unidirectional power flow. To improve upon the oscillatory transient operations, the tested system includes the digital PID controller using a microcomputer for output voltage regulations. The experimental results and state-space simulations confirm the usefulness of the presented convertor. >

Control of three phase power supplies for ultra low THD

Abstract: Current regulators are proposed and investigated for high-power three-phase power supplies with ultra-low total harmonic distortion (THD) (less than 0.5%). Two valid approaches to realize three-phase power supplies can be proposed. In the first approach, current regulated pulse-width-modulated (PWM) inverters along with high bandwidth voltage control loops are proposed. In the second approach, an internal voltage synthesizer under closed loop current control is used. Both approaches have mechanisms to decouple the LC filter interactions and to lower the THD. The tradeoffs of these two basic approaches are investigated. The influence of different current regulators in case of the current-regulated PWM (CRPWM) converter is investigated as well as different approaches to control the voltage synthesizer under current control. The study compares the effectiveness of these current regulator in soft-switching PWM inverters and describes a method to measure accurately ultra-low THDs. >

Voltage regulator for variable speed permanent magnet alternators

Abstract: A voltage regulating system for a permanent magnet alternating current generator that has a rotor and a three phase Y-connected output winding. The system has first and second three phase full wave bridge rectifiers each comprised of a plurality of controlled rectifiers. The ends of the phase windings of the output winding are connected to the AC input terminals of the first bridge rectifier. The phase windings are connected to taps and these taps are located between the neutral and the ends of the phase windings. The taps are connected to the AC input terminals of the second bridge rectifier. The first bridge rectifier is enabled and the second bridge rectifier is disabled at speeds of rotation of the rotor below a predetermined speed. At rotor speeds above the predetermined speed, the first bridge rectifier is disabled and the second bridge rectifier is enabled.

Three phase AC-DC load and harmonic flows

Abstract: The validity of assuming that the fundamental frequency load flow conditions of an AC-DC conversion plant are unaffected by the presence of harmonics is questioned, and an algorithm is derived which is capable of determining the true load flow conditions in the presence of harmonics. The algorithm is unique in its three phase approach to the problem, which is essential if normally unfiltered uncharacteristic harmonics are to be represented. Results of a realistic and relevant test system are used to demonstrate the interaction which exists between the fundamental and harmonic frequencies in the power converter. >

Transient performance of linear induction launchers fed by generators and by capacitor banks

Abstract: Computer simulation is used to investigate the transient performance of induction-type coilguns as a function of the dimensions, material properties, type of supply, firing sequence of switching elements, and connections of drive coils. The performance of both generator-driven and capacitor-driven coilguns is addressed. It is shown that the generator-driven coilgun performs satisfactorily in the starting section. However, at high velocity, the transit time is close to the electrical transient time constant, and therefore the DC components produce a retarding force. To avoid this problem, the three phase voltages should not be switched on simultaneously, but rather phase-by-phase according to their zero current crossing points. This can also alleviate the problem in the transient between the sections. The capacitor-driven coilguns, instead, derive the alternating current needed to create a traveling wave from resonance with the inductance of the coils. Therefore, the initiation of the sinusoidal current oscillation coincides with the switch-on time. They are ideally suited for short-time, high-acceleration operation, but they are likely to require higher operating voltages than the generator-driven coilguns, because of the constraint imposed on the capacitance by the resonance condition with attenuation. >

Capacitance requirements for three phase self-excited reluctance generators

Abstract: A direct method to find the capacitance requirements for a three phase self-excited reluctance generator under any load or speed is introduced. Expressions for the load angle as well as for the minimum value of the excitation capacitance are derived. It has been found that there is a cut off speed, above which no excitation is possible no matter what the terminal capacitance value. Expressions for the value of this speed under open circuit, resistive and pure inductive loads are also derived and presented.

Three phase power monitor

Abstract: A three-phase power monitor operates a contactor between a supply of three phase electrical power and a three phase load. A phase integrity detector has three inputs connected respectively to the three phases of the electrical power. If a loss, reversal, or dissymmetry is detected, a photoemitting device is actuated. Actuation can also result from low or high line frequency. A power supply circuit is input with the three phases of power and has an output providing a dc level corresponding to the voltage level of the three-phase electrical power. A hold circuit has a first time constant associated with it and attains a hold level when current is applied to its input, but reverts to a release level after this current ceases. An interrogator circuit is coupled to the power supply and to the input of the hold circuit and provides a periodic recharge current to the hold circuit at a period that is shorter than the period of the hold circuit. A high/low voltage level detector interrupts the periodic recharge current if the dc level is outside a predetermined voltage limit. A photosensor-based discharge circuit is optically coupled to the photoemitting device of the three phase integrity detector, and is electrically coupled to the hold circuit for discharging the time constant circuit thereof if a phase loss or phase imbalance is detected.

Inverter control device capable of suppressing DC magnetization in three-phase transformer

Abstract: An inverter control device according to this invention comprises a current DC component detecting circuit (11-13) for detecting the current DC components contained in at least two of the three phase output AC currents of the inverter, a DC magnetization suppressor signal generating circuit (14) for producing two correcting signals from the current DC components, the two correcting signals being used to balance in amount the three voltage DC components contained in the three phase output AC voltage of the inverter respectively, an output reference correcting circuit (19) for correcting two of first output references in accordance with the two correcting signals and outputting three second references including the two corrected values and one of the first output reference, and a gate control circuit (20) for controlling the three phase output AC voltages of the inverter according to the three second output references.

Pulse-split concept in series resonant Dc link power conversion for induction motor drives

Abstract: Three phase simultaneous current pulse control for a high frequency series resonant DC link converter was achieved by utilizing a previously developed pulse trimming method and current peak limiting with a saturable core. Each resonant pulse is split to flow in three phases almost at the same time so that a very smooth sinusoidal output current waveform is attained. The duality of the series resonant DC link with the parallel resonant DC link is discussed. Simulation and the basic experimental results are presented. >

A passive input current waveshaping method for three-phase diode rectifiers

Abstract: A passive waveshaping method for a three-phase diode rectifier is presented. It is shown that application of the proposed passive method yields significantly higher input power factor, lower input current distortion factor, lower input RMS current, and a lower total volt-ampere (VA) rating of the reactive components than the conventional three-phase diode rectifier. Relevant input and output current waveforms, component ratings, and power factor values are derived. Different modes of operation are investigated as a means of improving performance. Predicted results are verified experimentally on a laboratory prototype unit. >

A novel high voltage three-phase monolithic inverter IC with two current levels sensing

Abstract: A 250-V 1-A three-phase monolithic inverter IC (ECN3010) for variable-speed motor drives has been fabricated. Six insulated-gate bipolar transistors (IGBTs) and fast recovery diodes are integrated on one chip, for the output stages, using dielectric isolation. The IC provides many functions for driving a DC brushless motor by itself. A two-current-level sensing circuit has been developed to limit the motor start-up current and to turn off all the IGBTs when a short-circuit load occurs. The accuracy of the sensing circuit and its temperature dependence are discussed. >

A new DC-side active filter for inverter power supplies compensates for unbalanced and nonlinear loads

Abstract: A DC-side active filtering system for inverter power supplied is presented. The proposed system effectively nullifies the pulsating currents in the inverter input terminals as a result of unbalanced or nonlinear type loads. In the case of a three phase four wire inverter power supply the circulating zero sequence currents in the neutral are also nullified. The proposed system is fast in response, high in efficiency, and does not necessitate excess capacitive kVA required to supply highly unbalanced or nonlinear type loads. Analytical procedure, simulation, and experimental results illustrate the effectiveness of the proposed DC-side active filtering system. >

A three-phase half-controlled rectifier with pulse width modulation

Abstract: A three-phase pulse-width-modulated (PWM) half-controlled rectifier using a novel PWM control strategy whereby the low-order harmonic content in both the input current and the output voltage is reduced is presented. The circuit operates with a unity displacement factor at its input and uses minimum power components. The PWM strategy developed can be implemented on a three-phase half-controlled rectifier bridge with only three controlled switches to obtain PWM controlled rectification. Although the circuit operation is explained with force-commutated SCR switches, the basic controlled PWM operation is valid for any type of switch control. The circuit has wide applications ranging from rectifiers to battery chargers to motor drives. Even if an input current filter is desired, its size will be small due to the PWM pattern used. >

Excitation scheme of brushless self-excited-type three-phase synchronous machine

Abstract: AC excitation schemes of a brushless self-excited-type three-phase synchronous machine are presented. The rotor field windings of the machine are short circuited with the diodes, and an exciter or other exciting equipment is not required in this machine. On the positive-phase-sequence rotating field of the stator, the plural rotating fields with different speeds from it are superimposed by using a machine-side voltage source PWM inverter/converter, and the field MMF is obtained by half rectifying AC voltages induced in the rotor field windings. Two excitation schemes are proposed that have the advantage of the reduction of the torque pulsation as compared with the scheme that superimposes the singular rotating field with a different speed from the positive-phase-sequence field. This is confirmed by simulation, and experimental results are given. >

Throughflow water heater with safety cut=out - has monitoring circuit to switch off power if one or more heating elements have voltage applied with no water flow

Abstract: The electric heating elements are in a triangular array over the three phase supply. Each end of each element is connected to the supply via a triac. A monitoring circuit measures the bias applied to each element. If any one element has bias applied, with no water flowing, then the fault detection circuit switches off the power to all the heating elements. The bias on the elements is compared with either the signal from the waterflow meter, or with the switch-off, switch-on signal from the control circuit. A simple failure mode immobilises the entire supply if a fault is detected. ADVANTAGE - No risk of burning out an element if triacs are faulty; increased safety.

Three phase voltage measurements with simple open air sensors

Abstract: A low cost, easy to install high-voltage measuring system is described for open air substations and overhead lines. Based on the Differentiating/Integrating (D/I) principle, three free-standing capacitive pickup electrodes are used to sense the three phase to ground voltages. Apart from the individual phase aimed at, each sensor also receives a parasitic contribution from the other two phases. After integration the sensor signals therefore have to be unscrambled to recompose the original phase voltages The system has a large bandwidth, is linear and has excellent Electro-Magnetic Compatibility (EMC) properties. With its low cost, simple installation and EMC quality, the system may be incorporated easily into various applications like e.g. monitoring and line protection.

Measurements of communication signal propagation on three phase power distribution lines

Abstract: Measurements of voltages and currents for distribution line carrier signals on a three-phase power distribution network are presented. Both symmetric and asymmetric lines are used under loaded and unloaded conditions. The measurements were made at 25 kHz on an unenergized 20847 ft overhead distribution line test site. The measurements show the existence of standing wave patterns on three-phase lines. Except in the simplest cases these standing waves patterns are difficult to predict using classical two-wire techniques along with the intuition, heuristics or Smith charts. There is significant interaction between phases. One phase can draw a disproportionate amount of current from the source; this is especially true when one of the phases is an odd number of quarter wavelengths. >

A novel three-phase quasi-resonant DC link inverter

Abstract: A three-phase quasi-resonant DC link inverter (QRI), which adopts the resonant circuit with one additional switch, is proposed. The circuit states for given switching and load current states in the inverter are analyzed, and then an equivalent circuit seen from the inverter input side is derived. By using this equivalent circuit the QRI is analyzed, and two control techniques, which are the initial current control technique of resonant inductor and the initial voltage control technique of resonant capacitor, are presented in order to obtain the desirable oscillation of resonant link irrespective of inverter operation. The operating characteristics of QRI are verified through both simulations and experiments. >

Active input current waveshaping method for three-phase diode rectifiers with zero switching losses

Abstract: A zero voltage switched active network which can be used in conjunction with single-phase or three-phase AC to DC diode rectifiers is presented. It is shown that application of the proposed switching network in a three-phase AC to DC boost converter yields zero switching losses while maintaining a unity input power factor. Active network capacitor, C/sub s/, diodes D/sub 7/ and D/sub 8/ maintain a zero voltage during turn-off of Q/sub 1/ and Q/sub 2/. Capacitor discharges through the boost inductors of the circuit, thus limiting the rate of rise of current during turn-on. Moreover, the advantage of the proposed active network is that it can maintain a zero voltage switching over the entire range of the duty cycle of the operation. Consequently, boost stage can be used directly to control the DC bus voltage by varying the duty cycle at constant switching frequency. >

Surge-free switching-on of inverter to 3 phase asynchronous machine - using current desired value-current actual value comparison across search current regulator

Abstract: The determination of the synchronism of teh inverter and the three-phase asynchronous machine is carried out using the comparison of a current desired value and a current actual value across a search current regulator. With an inverter, which normally presupposes a stator voltage, across a rotor flux oriented voltage model of the three phase asynchhronous machine, as well as the corresp. phase position of this voltage. A magnetising current regulator (6) reacts to correct the voltage model, for compensating the non-linearity, whilst a rotor current regulator (3) reacts on the stator frequency to be specified corresp. to the load requirement, with both regulators getting their actual values from a current actual value computer (7). The current regulator is used as a search current regulator, to which is specified the current desired value zero with faded out magnetising and rpm regulators, thereby specifying any selectable starting frequency, for the inverter. The inverter is operated by the voltage model with a reduced output voltage, until the current actual value has reached a min. value. With the reaching of this min. value, the frequency value established is used, for initiating the operational type control of the three phase asynchronous machine. USE/ADVANTAGE - Type method which uses far less costly synchronising circuit for surge free connection of three-phase machine to inverter.

Easy digital control of three-phase PWM convertor

Abstract: The authors describe the design of a digital controller using a digital signal processor (DSP) for a sinusoidal input current three-phase pulsewidth-modulated (PWM) converter. The controller adopts a proportional compensator for current control. The current control system lag-time caused by the DSP operation time is decreased to less than one sample period by a method which acquires the currents in the interval between consecutive samples. The PWM converter is modeled, and, using the modified z-transfer, the characteristic equation of the input current control system including the lag-time is established. Using the characteristic equation, the controller is designed by a conventional root locus method. The results of a simulation and an experiment indicate the validity of this analysis and the design method. >