Showing papers on "Power factor published in 1993"

Power system voltage stability

Abstract: Voltage stability is a major concern in the planning and operation of electric power systems. This book provides a clear, in-depth explanation of voltage stability, covering both transient and longer-term phenomena and presenting proven solutions to instability problems. The book describes equipment characteristics for transmission, generation, and distribution/load subsystems of a power system, together with methods for the modelling of equipment. Readers will find static and dynamic computer simulation examples for small equivalent power systems and for a very large power system, plus an account of voltage stability associated with HVDC links. They will also get helpful planning and operating guidelines, computer methods for power flow and dynamic simulation, and descriptions of actual voltage instability incidents.

Point of collapse and continuation methods for large AC/DC systems

Abstract: The implementation of both point of collapse (PoC) methods and continuation methods for the computation of voltage collapse points (saddle-node bifurcations) in large AC/DC power systems is described. The performance of these methods is compared for real systems of up to 2158 buses. Computational details of the implementation of the PoC and continuation methods are detailed, and the unique problems encountered due to the presence of high-voltage direct-current (HVDC) transmission, area interchange power control, regulating transformers, and voltage and reactive power limits are discussed. The characteristics of a robust PoC power flow program are presented, and its application to detection and solution of voltage stability problems is demonstrated. >

The FBD-method, a generally applicable tool for analyzing power relations

Abstract: Starting from the most general case of m-wire unbalanced multiphase power systems with unsymmetrical loads under nonsinusoidal conditions, it is explained how the relations between currents, voltages and power quantities are analyzed in the time domain using the FBD method. Treatment in the frequency domain on the basis of Fourier series expansion of the time functions is also possible. It is shown that components of the total nonactive currents may be compensated without any time delay and without changing the collective instantaneous power. Rules are given for deriving simple equivalent circuits with m-equally structured branches from given voltages and currents. >

Small-signal modeling of average current-mode control

Abstract: A recently proposed average current-mode control method is analyzed. A complete small-signal model for the control scheme is developed. The model is accurate up to half the switching frequency. This control scheme is suitable for applications where the average inductor current needs to be controlled, as in power factor correction circuits and battery charger dischargers. The subharmonic oscillation, commonly found in peak current-mode control, also exists in this method. This subharmonic oscillation can be eliminated by properly choosing the proper gain of the compensation network in the current loop. Model predictions are confirmed experimentally. >

Power-factor correction with interleaved boost converters in continuous-inductor-current mode

Abstract: The operation and design trade-offs of the interleaved boost converter in continuous inductor-current mode in a high-power-factor preregulator circuit are investigated. By using interleaved converters, an overall reduction of boost inductor and electromagnetic interference (EMI) filter volume can be achieved, together with reduced switching losses. The problem of unequal load sharing between the interleaved power stages with average current control is pointed out, and a practical solution is recommended. EMI filter design considerations and experimental data taken from a 3 kW converter are presented. >

A flexible active and reactive power control strategy for a variable speed constant frequency generating system

Abstract: Variable-speed constant-frequency generating systems are used in wind power, hydroelectric power, aerospace, and naval power generation applications to enhance efficiency and reduce friction. In these applications, an attractive candidate is the slip power recovery system comprising a doubly excited induction machine or doubly excited brushless reluctance machine and PWM power converters with a DC link. In this paper, a flexible active and reactive power control strategy is developed, such that the optimal torque-speed profile of the turbine can be followed and overall reactive power can be controlled, while the machine copper losses have been minimized. At the same time, harmonics injected into the power network have also been minimized. In this manner, the system can function as both a highly-efficient power generator and a flexible reactive power compensator. >

Force commutated three level boost type rectifier

Abstract: A new force commutated three level boost type rectifier is proposed in this paper. The rectifier has the characteristics of drawing nearly sinusoidal current from utility with unity fundamental power factor. High reliability is possible due to its shoot-through free structure. A doubled dc bus voltage compared to a normal two-level rectifier is also possible because of the neutral point clamping. The operating principle, steady state analysis, input current and neutral point voltage control schemes, as well as detailed experimental results are presented in this paper.

Design consideration for power factor correction boost converter operating at the boundary of continuous conduction mode and discontinuous conduction mode

Abstract: A boost power converter operating at the boundary of continuous conduction mode (CCM) and discontinuous conduction mode (DCM) for power factor correction is described. Theoretically, the operation at the boundary of CCM and DCM is considered constant on-time for the boost switch. Due to finite switching frequency and capacitor filter effect, the switch turn-on time varies throughout the entire cycle. This variation of the switch on-time affects the average switching frequency and the circuit component selection criterion. The relationship between the circuit components and the switch on-time based on the fundamental principle of the power circuit operation is derived. A 44 W boost converter is designed and constructed accordingly. Experimental results show that the theoretical analysis along with practical design consideration can accurately predict the switch on-time and switching frequency. >

A novel single-phase power factor correction scheme

Abstract: A single-phase power factor correction scheme is proposed based on the power flow analysis. It is found that the conventional power factor correction (PFC) circuit must be designed to handle the rated power, although its purpose is only for power factor correction. With the proposed scheme, the PFC circuit is in parallel with the major power flow path, thus reducing its size and weight compared to a conventional two-cascade-stage scheme. A prototype circuit is built and tested to verify this concept. >

A new approach to improve power factor and reduce harmonics in a three phase diode rectifier type utility interface

Abstract: A novel approach to improving power factor and reducing harmonics generated by a three-phase diode-rectifier-type utility interface is proposed. This approach is passive and consists of a novel interconnection of a star/delta power transformer between the AC and DC sides of the diode rectifier topology. This interconnection, in combination with the 120 degrees conduction intervals of each diode, is shown to generate a circulating third-harmonic current between the AC and DC side of the rectifier bridge. This current is shown to drastically improve the performance of the diode-rectifier-type interface. The resulting input current is near-sinusoidal, with significant reduction in line current harmonics. The proposed system can be viewed as a cost-effective retrofit to the existing diode rectifier utility interface applications. A design example illustrates the sizing of the necessary passive components. Selected results were verified experimentally on a laboratory prototype system. >

Techniques for minimizing the input current distortion of current-controlled single-phase boost rectifiers

Abstract: Techniques for minimizing the input current distortion of current-controlled single-phase boost rectifiers are described. The switching patterns of several boost rectifiers are examined to identify the nature of their input current waveforms. This analysis is used to examine the low-frequency current distortion levels, and hence the power quality, associated with the rectifiers. A PWM (pulse width modulation) strategy that selectively switches between positive unipolar PWM and negative unipolar PWM, called phase-adjusted unipolar PWM, is shown to produce the lowest current distortion levels. A novel two-switch asymmetrical half-bridge rectifier is presented that draws an input current at a unity fundamental power factor and with the same low distortion as obtained with the four-switch H-bridge rectifier. The operation of the various rectifiers is examined with reference to theoretical predictions, circuit simulations, and experimental results. This analysis is used to compare the performances of the various rectifier switching patterns. >

New control scheme for AC-DC-AC converter without DC link electrolytic capacitor

Abstract: A novel concept for a static three-phase to three-phase power converter for an AC motor drive with unity power factor and reduced harmonics on the utility line is presented. The power circuit consists of two back-to-back connected six-pulse bridges having only a 5-/spl mu/F ceramic capacitor in the DC link. By controlling the active power balance between two bridges, the DC link voltage can be maintained within a 20-V deviation from the nominal value with the small ceramic capacitor regardless of the load variation. >

A super high speed PM motor drive system by a quasi-current source inverter

Abstract: The authors describe a surface-mounted PM (permanent magnet) motor with the ratings of 5 kW and 180000 RPM for super-high-speed drive applications such as machine tools. A much larger air gap, compared with that of the existing PM motors, is used to reduce the slot ripples and hence the resultant eddy current loss in the metallic sleeve that retains the permanent magnets; otherwise, this loss would be objectionable at such high speeds. The motor is supplied from a quasi-current source inverter with the DC current controlled by a chopper and operated at a near-unity power factor by the use of a simple position sensorless control, to improve the motor efficiency. Analytic results are presented showing the slot ripple reducing effect of increasing the air gap length. To investigate the losses of the sleeve, several motors with different sleeves were manufactured and tested. Typical experimental results on these prototypes are given, which show that a motor efficiency of higher than 90% can be realized at the rated torque over a wide speed range. >

A high performance single phase AC to DC rectifier with input power factor correction

Abstract: A high performance single phase AC to DC rectifier with input power factor correction is proposed. The proposed approach has many advantages which include fewer semiconductor components, simplified control, high performance features and satisfaction of IEC 555 harmonic current standards. Simulation and experimental results obtained on a laboratory prototype are discussed. >

A four-wire, current-controlled converter provides harmonic neutralization in three-phase, four-wire systems

Abstract: A four-wire active filter has been implemented. It neutralizes current harmonic effects, caused by any configuration of nonlinear loads in three-phase, four-wire systems. The results of a laboratory implementation are presented. These results are compared with those from computer simulations. Key design issues are discussed. >

High power factor boost rectifier apparatus

Abstract: A power supply for providing a single level dc output voltage, with input power factor correction, from a wide range of ac input voltages commonly available worldwide. The power supply rectifier circuit can be configured as a full wave bridge for high line voltage inputs and as a voltage doubler for low voltage inputs. Output voltage regulation and power factor correction are accomplished by a high frequency boost circuit having two inductors connected in parallel for the low input voltage configuration and in series for the high input voltage configuration. The high frequency switching semiconductor components are connected in a series circuit which limits their voltage requirement to one-half the output voltage.

Isolated boost circuit for power factor correction

Abstract: To provide a 48 volt DC bus for distributed power systems, AC-DC power converters with high power factor and isolation are required. Instead of using a conventional two stage scheme which has a boost front end followed by a DC-DC converter, a boost derived topology is chosen to achieve high power factor, isolation, and voltage step down. A newly proposed technology of soft switched insulated gate bipolar transistors (IGBTs) is used for this application to reduce the IGBT switching loss and achieve low conduction loss. The design considerations of the power stage and the control circuit are provided. A prototype converter is built to show the feasibility of the single stage scheme. >

Unity PF current-source rectifier based on dynamic trilogic PWM

Abstract: One remaining step in perfecting the stand-alone, unity power factor, regulated current-source pulse-width-modulated (PWM) rectifier is to reduce cost, by bringing the twelve-valve converter (consisting of three single-phase full bridges that operate with two-level or bilogic PWM) to the six-valve bridge. However, the six-valve topology requires a three-level or trilogic PWM strategy that can handle feedback signals. A general method of translating three-phase biologic PWM signals into three-phase trilogic PWM signals is presented. The method of translation retains the characteristics of the bilogic PWM, including the frequency bandwidth. Experiments show that the trilogic PWM signals produced by the method can handle not only stabilizing feedback signals but also signals for active filtering. >

Servo drive system and control characteristics of salient pole permanent magnet synchronous motor

Abstract: Permanent magnet (PM) synchronous motors fed by PWM inverters are considered. PM motors sometimes have a saliency, in which the q-axis inductance is larger than the d-axis inductance. A high-performance servomotor drive system for a salient-pole PM motor is described. An armature current vector is actively controlled according to load conditions in order to use a reluctance torque effectively, and, as a result, a large torque can be produced. The control algorithm for the armature current vector is described, taking the demagnetization of the PM and the magnetic saturation into account. Characteristics such as torque, power factor, efficiency, power capability, transient responses, etc. are examined in detail by computer simulations and experimentally. >

Single stage, high power factor, gas discharge lamp ballast

Abstract: A gas discharge lamp ballast provides near unity power factor simultaneously with high-frequency lamp ballasting in a single switching power conversion stage resulting in efficiency improvements, reduction in size and weight and reduced component count and cost. The single conversion stage comprises a fast-recovery diode connecting in series the input inductor, energy transfer capacitor and the resonant matching network, and switching means alternately connecting the first junction between the input inductor and energy transfer capacitor, or the second junction between the matching network and the other side of said capacitor to the return current path. The switching means comprises two current bidirectional switches driven out of phase, thus producing a square-wave high frequency voltage source, which is in turn converted by the resonant matching network into a sine-wave ac current source required by the gas discharge lamp. The fast-recovery input diode in conjunction with the input inductance chosen to be less than the critical inductance value forces the input inductor current into a new discontinuous inductor current mode (DICM). The average input inductor current is shown to closely follow the rectified line voltage when the ballast is operated at the constant duty ratio and at the constant switching frequency either open-loop or with the slow feedback loop of conventional PWM control. Zero voltage switching for the two current bidirectional switches is achieved by introducing two transition intervals during which both switches are OFF and utilizing the negative value of the lagging current of the matching network above resonance.

Apparatus and methods for generating uninterruptible AC power signals

Abstract: A broadband communications network having an uninterruptible power supply. The power supply has one input connection to AC line voltage and another input connected to a gas powered electrical generator. The power supply contains an uninterruptible power supply module having a controlled ferroresonant transformer and a battery. The power supply also contains a bypass module having a controlled ferroresonant transformer. Switches are provided to allow either of the inputs to be connected to either the UPS module or the bypass module for the generation of power signals for supplying power to the broadband communications network.

A new control algorithm for three-phase PWM buck rectifier with input displacement factor compensation

Abstract: A new control algorithm for the three-phase buck rectifier with an input filter is developed. The algorithm employs a separate control loop for compensation of the input current displacement factor in steady-state, in addition to the standard output voltage regulation loop. The algorithm allows separate design of the input filter and of closed loop output voltage control. The design procedure is explained and illustrated on an example. The algorithm is verified experimentally on an 1-kW, 100-kHz, three-phase isolated buck power converter. >

Power factor correction with flyback converter employing charge control

Abstract: The charge control concept is applied to a flyback power converter for the purpose of power factor correction (PFC). Using charge control, a flyback converter can operate in continuous conduction-mode (CCM) with unity power factor. The simplicity of the flyback circuit is maintained and the power handling capacity is increased. The properties of charge control related to flyback PFC application are studied. The design guidelines are presented. A 200 W prototype circuit is built. Experimental results show good application prospects. >

Harmonics and how they relate to power factor

Abstract: We are all familiar with power factor, but are we using it to its true potential? In this paper we investigate the effect of harmonics on power factor and show through examples why it is important to use true power factor, rather than the conventional 50/60 Hz displacement power factor, when describing nonlinear loads.

Apparatus and method for providing AC isolation while supplying DC power

Abstract: A source power coupler for coupling a power source to a first transmission line and a second transmission line. DC power from the power source is conducted by the source power coupler to the first and the second transmission lines while AC signals on the first and the second transmission lines are isolated from the power source by the source power coupler. The source power coupler is comprised of an operational amplifier having a non-inverting input and an inverting input. The gate of a transistor is coupled to an output of the operational amplifier. The transistor's drain is coupled to one of the transmission lines. And the transistor's source is coupled to the inverting input of the op-amp. Thereby, a control loop is formed, which provides an AC impedance between the power source and one of the transmission lines. A resistor is coupled between the source of the transistor and the power source, and a capacitor is coupled between the non-inverting input of the op-amp and the power source. Another resistor is coupled between the non-inverting input of the op-amp one of the transmission lines. The capacitor and the second resistor provide a second control loop. This control loop provides a constant direct current voltage across the transistor.

An approach to realize higher power PWM AC controller

Abstract: PWM controlled AC controllers exhibit difficulties in the commutation of inductive load current from one bidirectional switch to another due to finite switch on/off times. A four step switching strategy for repeated on/off operation of bidirectional switches in an AC controller is proposed. The approach permits safe transition of inductive load current at all power factors from one bidirectional switch to another, even in the presence of source side stray inductance. Results from an experimental prototype AC controller coupled to an inductive load are discussed. >

Harmonics: the effects on power quality and transformers

Abstract: The use of nonlinear devices such as rectifiers or power converters, power supplies, and other devices utilizing solid-state switching has increased in industry during recent years. Unfortunately, the effect of harmonic distortion has also increased on the industrial power system as a direct result. This work is based upon harmonic studies on 480 V, three-phase, variable-speed drives and the effects of harmonics on transformers in those systems. It also includes recommendations for correction of the problems resulting from harmonic distortion. >

A comprehensive design approach for a three-phase high-frequency single-switch discontinuous-mode boost power factor corrector based on analytically derived normalized converter component ratings

Abstract: In this paper the peak, mean, and RMS values of the component currents of a three-phase single-switch discontinuous inductor current mode boost rectifier are calculated analytically. The values are given in rated form in dependency on the output power and on the ratio of output voltage to the amplitude of the mains voltage. Furthermore, the influence of the voltage transfer ratio on the shape of the mains currents and on the power factor of the system is analyzed. The theoretical analysis is verified by digital simulation and a good consistency is achieved. Finally, the approach of the converter dimensioning based on the graphical representation of the calculation results is described and illustrated using a specific example. The correctness of the dimensioning is verified by measurements on a laboratory model. >

Battery operated computer having improved battery gauge and system for measuring battery charge

Abstract: A digitizer tablet computer has a host processor connected to a service processor which, in turn, is connected to a digitizer controller and to a power subsystem microcontroller. A battery supplies the primary power for operating the computer but the computer is also connectable to an external source of DC power to charge the battery and to operate the computer when it is not running on battery power. The service processor includes a battery gauge the contents of which indicate how much power is left in the battery and how much time remains at the current rate of power consumption before the power runs out. The amount of remaining power in the gauge is determined by keeping track of power consumed, power added, and power lost during off periods. The use of the second derivative of a voltage versus time discharge curve creates a point from which relative accurate estimates can be made of how much longer the computer can be operated.

Single-switch three-phase power factor preregulator under variable switching frequency and discontinuous input current

Abstract: A three-phase power factor preregulator with input inductor, diode bridge, and a single switch is analyzed. Operation with discontinuous input current and variable switching frequency is considered. Equations for average input current are presented, showing that quasi-sinusoidal waveforms are obtained. Also, the switching frequency equation shows an almost constant frequency operation. A design procedure is presented, where simulation and practical results show the good performance of the approach. Comparison with constant frequency operation also shows that a better harmonic distortion is obtained if the voltage in the switch (with switch off) is not far from the input voltage. Also, in this case, the high-order harmonic content is easily filtered. >