Showing papers on "Three-phase published in 2000"

A phase tracking system for three phase utility interface inverters

Abstract: The analysis and design of the phase-locked loop (PLL) system is presented for the phase tracking system of the three phase utility interface inverters. The dynamic behavior of the closed loop PLL system is investigated in both continuous and discrete-time domains, and the optimization method is considered for the second order PLL system. In particular, the performance of the three phase PLL system is analyzed in the distorted utility conditions such as the phase unbalancing, harmonics, and offset caused by the nonlinear load conditions and measurement errors. The tracking errors under these distorted utility conditions are also derived. The phase tracking system is implemented in a digital manner using a digital signal processor (DSP) to verify the analytic results. The design considerations for the phase tracking system are deduced from the analytic and experimental results.

Virtual flux based direct power control of three-phase PWM rectifiers

Abstract: In this paper, direct power control (DPC) of three-phase pulsewidth-modulated rectifiers without line voltage sensors is presented. The new system is based on virtual flux (VF) estimation. Theoretical principles of this method are discussed. The steady-state and dynamic behavior of VF-DPC are presented that illustrate the operation and performance of the proposed system compared to a conventional DPC method. Both strategies are also investigated under unbalance and predistorted grid. It is shown that the VF-DPC exhibits several advantages, particularly providing sinusoidal line current when the supply voltage is not ideal. Test results show the excellent performance of the proposed system.

Sharing of nonlinear load in parallel connected three-phase converters

Abstract: In this paper, a new control method is presented which enables equal sharing of linear and nonlinear loads in three-phase power converters connected in parallel, without communication between the converters. The paper focuses on solving the problem that arises when two converters with harmonic compensation are connected in parallel. Without the new solution, they are normally not able to distinguish the harmonic currents that flow to the load and harmonic currents that circulate between the converters. Analysis and experimental results on two 90-kVA 400-Hz converters in parallel are presented. The results show that both linear and nonlinear loads can be shared equally by the proposed concept.

Phase-locked loop for grid-connected three-phase power conversion systems

Abstract: Analysis and design of a phase-locked loop (PLL) is presented for the power factor control of grid-connected three-phase power conversion systems. The dynamic characteristics of the closed loop PLL system with a second order are investigated in both continuous and discrete-time domains, and the optimisation method is discussed. In particular, the performance of the PLL in the three-phase system is analysed under the distorted utility conditions such as the phase unbalancing harmonics, and offset caused by nonlinear loads and measurement errors. The PLL technique for the three-phase system is implemented in software of a digital signal processor to verify the analytic results, and the experiments are carried out for various utility conditions. This technique is finally applied to the grid-connected photovoltaic power generation system with the current-controlled PWM inverter as a subpart for generating the current reference of the inverter. The experimental results demonstrate its phase tracking capability in the three-phase grid-connected operation.

DC-bus voltage control of three-phase AC/DC PWM converters using feedback linearization

Abstract: In this paper, a fast voltage control strategy of three-phase AC/DC pulsewidth modulation (PWM) converters applying a feedback linearization technique is proposed. First, incorporating the power balance of the input and output sides in system modeling, a nonlinear model of the PWM converter is derived with state variables such as AC input currents and DC output voltage. Then, by input-output feedback linearization, the system is linearized and a state feedback control law is obtained by pole placement. With this control scheme, output voltage responses become faster than those in a conventional cascade control structure. For robust control to parameter variations, integrators are added to the exact feedback control law. Since the fast voltage control is feasible for load changes, it is shown that the DC electrolytic capacitor size can be reduced. In addition, the capacitor current is analyzed for size reduction of the capacitor. As is usual with PWM converters, the input current is regulated to be sinusoidal and the source power factor can be controlled at unity. The experimental results are provided to verify the validity of the proposed control algorithm for a 1.5 kVA insulated gate bipolar transistor PWM converter system.

A new approach to load balancing and power factor correction in power distribution system

Abstract: This paper presents a new approach for generating reference currents for an active filter and/or a static compensator. It is assumed that the compensator is connected to a load that may either be connected in star or in delta. The load can be unbalanced and may also draw harmonic currents. The purpose of the compensating scheme is to balance the load, as well as make the supply side power factor a desired value. The authors use the theory of instantaneous symmetrical components to obtain an algorithm to compute three phase reference currents which, when injected to the power system, produce desired results. They also propose a suitable compensator structure that will track the reference currents in a hysteresis band control scheme. Finally, the feasibility of such a scheme is demonstrated through simulation studies.

A novel approach to the design of a shunt active filter for an unbalanced three-phase four-wire system under nonsinusoidal conditions

Abstract: A new approach is presented for the design of a shunt active filter in a three-phase, four-wire distribution system with unbalanced, distorted sources and unbalanced loads. The purpose of the shunt active filter is to provide compensation currents such that the source needs to supply balanced (positive-sequence) fundamental source current at unity power factor even though the load consumes harmonic currents as well as positive, negative, and zero-sequence currents. A major feature of the proposed approach is that it does not require symmetrical component transformation to transform the three-phase voltages and currents to /spl alpha/-/spl beta/-o quantities. This makes practical implementation of the proposed method easier than the symmetrical component transformation approach. In addition, the power factor of the positive-sequence fundamental component is close to unity and only positive-sequence power is supplied by the source. The effectiveness of the proposed control algorithm is demonstrated by computer simulation and experimental results of a shunt active filter for a three-phase four-wire distribution system with distorted, unbalanced source voltages and unbalanced loads.

A novel control method for three-phase PWM rectifiers using a single current sensor

Abstract: This paper proposes a control method for three-phase voltage-source PWM rectifiers using only a single current sensor in the DC-link. A PWM modulation strategy for reconstructing three phase currents from the DC-link current is given. When 3/spl phi/ input currents cannot be reconstructed, a method for modifying the switching state of the PWM rectifier and a method for the predictive state observer is proposed. Compensation of the 2 sampling delays is also included, and this method is combined with all of the experiments. Performance differences between the two methods in a typical voltage source PWM rectifier are investigated experimentally.

Method and control circuitry for a three-phase three-level boost-type rectifier

Abstract: A synchronized control method for a three-phase three-level boost-type rectifier with reduced input current ripple and balanced output voltages is disclosed. The proposed control allows simplifying the control circuit as much as possible without compromising the rectifier performance. In fact, besides simplicity, the control method featured synchronized command signals to de switching devices, minimized input current ripple, full controllability of the output voltage, dynamic balance of the output center point, constant switching frequency, simplified design of EMC filters, good transient and steady state performance, and low cost. The invention described first the most important configurations that the three-phase three-level boost-type rectifier may assume and studied the converter's operation. The concept involved for output voltage, input current, neutral point balance and control system design was presented.

Nonintrusive identification of electrical loads in a three-phase environment based on harmonic content

Abstract: This paper describes a signature construction for identifying loads based upon harmonic content of the current measured at a metering socket. Data gathered from ten loads at a commercial site served to experimentally validate the signature construction. The repeatability of current harmonics was investigated during transients and steady-states of several single, split-phase, and three-phase loads monitored in isolation.

AC voltage and current sensorless control of three-phase PWM rectifiers

Abstract: In this paper, a novel control scheme of three-phase PWM rectifiers eliminating the AC voltage and current sensors is proposed. Both the phase angle and the magnitude of the source voltage are estimated by controlling the deviation between the rectifier current and its model current to be zero. The phase currents can be reconstructed from switching states of the PWM rectifier and the measured DC link currents. To eliminate the calculation time delay effect of the microprocessor, a predictive state observer is employed. The proposed control scheme reduces the system cost and improves its reliability. The feasibility of the proposed AC sensorless technique for PWM rectifiers has been verified through experiments using a high performance DSP chip.

New wide input voltage range three-phase unity power factor rectifier formed by integration of a three-switch buck-derived front-end and a DC/DC boost converter output stage

Abstract: A new three-phase unity power factor rectifier with a three-switch buck-derived input stage and a DC/DC boost converter output stage is presented. This system has a wide input voltage range and a continuous sinusoidal time behavior of the input currents lying in phase with the input voltages which is also guaranteed in case of a failure in one phase of the mains. The input currents are controlled using a switching state sequence showing minimum switching losses. A multi-loop system control is realized by an outer output voltage controller and an inner-loop buck+boost inductor current controller. Furthermore active damping of the input filter resonance is provided. For increasing the output power of the system a parallel connection of two interleaved units is proposed. There, a low input current ripple is achieved, and the cut-off frequency of the input filter can be shifted to higher frequencies (resulting in improved control dynamics and a more compact design downsizing of the inductors and of the input filter).

Hybrid active filter for harmonically unbalanced three phase three wire railway traction loads

Abstract: This paper investigates a hybrid active/passive topology and control strategy appropriate for removing harmonics caused by unbalanced nonlinear loads. Unbalanced loads such as railway traction loads may use static compensators to balance the fundamental frequency components. The harmonic currents produced by this combination are unlikely to be balanced. Purely passive structures are not sensitive to this harmonic imbalance, however some active structures are. Controllers aimed at balanced three phase loads using synchronous transformations will not remove all of the harmonic components. The controller presented in this paper uses harmonic sequence components to detect both balanced and unbalanced components and reduce both harmonic components to zero.

Tandem electrode welder and method of welding with two electrodes

Abstract: An electric arc welding apparatus comprising at least a first consumable electrode and a second consumable electrode movable in unison along a welding path between the edges of two adjacent, mutually grounded plates, a first power supply for passing a first welding current at a first low frequency between the first electrode and the two plates, a second power supply for passing a second welding current at a second low frequency between the second electrode and the two plates, where each of the power supplies includes a three phase voltage input operated at line frequency, a rectifier to convert the input voltage to a DC voltage link and a high frequency switching type inverter converting the DC voltage link to a high frequency AC current, an output rectifier circuit to provide a positive voltage terminal and a negative voltage terminal, and an output switching network operated at a given low frequency for directing a pulsating welding current at the given low frequency from the two terminals across one of the electrodes and the plates, and a circuit for independently adjusting the given low frequency so the value of the first low frequency of the first power supply is different from the second low frequency of the second power supply.

Extended speed range operation of permanent magnet brushless machines using optimal phase angle control in the voltage mode operation

Abstract: A system for controlling the torque of a sinusoidally excited permanent magnet motor (12). The system includes a rotor position encoder (14), speed measuring circuit (16), controller (18), power circuit (20) and power source (22). The position and speed signals (24, 26) and a torque command signal (28) are applied to the controller (18). The torque command signal (28) is indicative of the desired motor torque. The controller (18) determines a voltage amplitude (30) required to develop the desired torque by using the position, speed, and torque command signals (24, 26, 28) and other fixed motor parameter values. The controller (18) transforms the voltage amplitude signal (30) into three phase signals by determining phase voltage command signals (Va, V?b?, V?c?) from the voltage amplitude signal (30) and the position signal (24). Motor voltage command signals (32) of the controller (18) are applied to a power inverter (23) that is coupled with a power source (22) to apply phase voltages (34) to the stator windings of the motor in response to the motor voltage command signals (32). Whereby the power circuit can achieve required torque levels with lower currents for power switches.

Parallel operation of three-phase UPS inverters by wireless load sharing control

Abstract: A load sharing control based on the frequency and voltage droop concept for parallel operation of two three-phase uninterruptible power supply (UPS) systems with no control interconnection lines is presented in this paper. First of all, due to the use of active power and reactive power as control variables, the characteristics of output powers according to amplitude and phase differences between output voltages of two UPS systems are analyzed. Secondly, simulation results with different line impedance are demonstrated the feasibility of the wireless load sharing control. Finally, experiments are presented to verify the theoretical discussion with two three-phase 20 kVA UPS systems employed TMS320C32, a kind of real time digital signal processor (DSP).

Characterization of single-stage three-phase power-factor-correction circuit using modular single-phase PWM DC-to-DC converters

Abstract: The complete DC characteristics of three-phase modular power-factor-correction (PFC) converters using single-phase pulsewidth modulation (PWM) DC-to-DC converter modules for high-power applications are studied. Using circuit averaging, the converter input and output quantities are determined numerically. Both the continuous and discontinuous output current modes of operation (CCM and DCM) are studied in detail. Near-unity power factor can be achieved with the converter modules operating in the DCM. An averaged model was used to study and determine the boundaries between DCM and CCM over the full period of the three-phase input voltage. It is found that high power factor is inherent in the converter system provided that the converters are operated in the DCM and the voltage conversion ratio is selected properly. The criteria for obtaining high power factor are analyzed and the optimal circuit parameters are determined to obtain the best achievable power factor. Both simulations and experimental results from a 1.5-kW prototype using full-bridge converter modules have confirmed the analysis.

An improved starting strategy for voltage-source inverter fed three phase induction motor drives under inverter fault conditions

Abstract: Three phase voltage-fed inverter induction motor drives are prone to shoot through and other inverter faults that cause the drive system to shut down. The paper describes a novel strategy for restarting the drive in variable voltage variable frequency single phase mode in the presence of open base drive and shoot through fault in the inverter. The proposed method requires only the motor terminal voltages and currents to be measured. Therefore, it can be applied to even low performance open loop drives with the addition of two motor terminal voltage sensors. The starting algorithm has been verified by computer simulation and experiments on a 1 hp laboratory prototype. Experimental results are in good agreement with simulation predictions. The starting strategy described in this paper is expected to provide an economic alternative to more expensive redundancy techniques which find justification only in a few specialized applications.

Existence, uniqueness, and monotonic properties of the feasible power flow solution for radial three-phase distribution networks

Abstract: This paper analytically investigates the steady-state properties of a practical power distribution system with unbalanced loads, single-, two-, and three-phase lines and transformers of different connection types. It is shown that the three-phase power flow solution with feasible voltage magnitude for radial three-phase distribution networks with nonlinear load modeling always exists and is unique. Also, motivated by the practical use of three-phase power flow analysis as a basic tool for distribution system application functions, monotonic properties of the voltage magnitude at each node with respect to load changes are established. Implications and applications of these analytical results to distribution system analysis and design are discussed.

Reactive power compensation and load balancing for unbalanced three-phase four-wire system by a combined system of an SVC and a series active filter

Abstract: A combined reactive power compensation method of a static VAr compensator (SVC) and a series active filter is described for unbalanced three-phase four-wire distribution feeders with harmonic distortion. The SVC which consists of /spl Delta/-connected thyristor-controlled reactors, Y-connected thyristor-controlled reactors and passive filters, acts as a classic reactive power compensator for load balancing and power factor correction. The small rating active filter is used to improve the filtering characteristics of the passive filter in the SVC and suppress possible resonance between the system impedance and the passive filter. Simulation results performed by the Electromagnetic Transients Program show that the proposed reactive power compensation configurations can effectively balance currents, correct power factor and eliminate harmonic currents.

Two three-phase high power factor rectifiers that apply the third harmonic current injection and passive resistance emulation

Abstract: Two high power factor rectifiers that apply the third harmonic current injection principle and passive resistance emulation are presented in this paper. Two methods of current injection, simultaneous current injection in all three of the phases, and current injection to only one of the phases, are discussed. Conditions for the optimal third harmonic current injection are derived, structures of the rectifiers that apply passive resistance emulation, enabling recovery of the power taken by the current injection network, are presented. The proposed resistance emulator consists of a transformer, two diodes and a capacitor. Choice of the passive components is discussed, and their values are proposed, volt-ampere ratings of the magnetic components are derived. Influence of the higher order harmonics is discussed. Dependence of the input current total harmonic distortion on the load level is presented for both of the current injection methods. Analytically obtained results are verified on 1.5 kW experimental rectifiers.

Characterization of three-phase unbalanced dips (as easy as one-two-three?)

Abstract: A proposal is presented for characterization of voltage dips as experienced by a three-phase load. The primary result of the method is a so-called "characteristic magnitude" which corresponds to the magnitude (remaining voltage) as used for the existing methods to characterize dips experienced by a single-phase load. The proposed method may be extended by adding additional parameters where further accuracy is needed for characterization. The method is applied to the analysis of multi-stage voltage dips.

Four-legged three-phase PFC rectifier with fault tolerant capability

Abstract: A four-legged three-phase PFC rectifier is proposed to handle unbalanced source voltage and to provide fault tolerant operation. The four-legged PFC rectifier is controlled by a three-dimensional space vector modulation (3D SVM) scheme, and runs in the overmodulation region when the source is unbalanced and/or a leg failure occurs. Modeling and control of the four-legged PFC rectifier are presented. Under unbalanced source, operation of the four-legged rectifier with three control strategies-equal current, equal resistance and constant power are analyzed. Two rectifier failure modes are studied: (1) entire leg fails; (2) active switches fail with the anti-parallel diodes still functional. It turns out that in both failure modes, the PFC rectifier can have an equal or higher than 2/3 of the rated output power level. The operation of the four-legged rectifier with 3D SVM is demonstrated by experimental results. The simulation results show that the proposed four-legged rectifier is advantageous over directly coupled three single-phase PFC rectifier and three-legged three-phase PFC rectifier.

A novel three-phase zero-current-transition and quasi-zero-voltage transition (ZCT-QZVT) inverter/rectifier with reduced stresses on devices and components

Abstract: This paper proposes a new soft-switching strategy for three-phase inverter and rectifier applications. All the main switches and auxiliary switches are turned on and turned off under zero-current-transition (ZCT) conditions. Compared to existing ZCT techniques, the diode reverse recovery is reduced, and the main switch turn-on voltage is reduced to less than the DC link voltage from full DC link voltage-referred to as quasi-zero-voltage-transition (QZVT). In addition, the current stress in the auxiliary switches is reduced and evenly distributed, and the resonant capacitor voltage stress is reduced to 1.3/spl sim/1.4 times the DC link voltage from twice the DC link voltage. The operational principle, state plane analysis, and design aspects are described. Both simulation and experiment results are provided for verification. Based on the proposed technique, finally presented is an application example of three-phase soft-switching inverter for electric vehicle propulsion.

Modeling and Control of Parallel Three-Phase PWM Converters

Abstract: This dissertation studies modeling and control issues of parallel three-phase pulsewidth modulated (PWM) converters. The converters include three-phase boost rectifiers, voltage source inverters, buck rectifiers and current source inverters. The averaging of the parallel converters is performed based on a generic functional switching unit, which is called a phase leg in boost rectifiers and voltage source inverters, and a rail arm in buck rectifiers and current source inverters. Based on phase-leg and railarm averaging, the developed models are not only equivalent to the conventional threephase converter models that are based on phase-to-phase averaging, but they also preserve common-mode information, which is critical in the analysis of the parallel converters. The models reveal such parallel dynamics as reactive power circulation and small-signal interaction. A unique feature of the parallel three-phase converters is a zero-sequence circulating current. This work proposes a novel zero-sequence control concept that uses variable zero-vectors in the space-vector modulation (SVM) of the converters. The control can be implemented within an individual converter and is independent from the other control loops for the converter. Therefore, it greatly facilitates the design and expansion of a parallel system. Proper operation of the parallel converters requires an explicit load-sharing mechanism. In order to have a modular design, a droop method is recommended. Traditionally, however, a droop method has to compromise between voltage regulation and load sharing. After parametric analysis, a novel droop method using a gain-

Single-stage three-phase power-factor-correction circuit using three isolated single-phase SEPIC converters operating in CCM

Abstract: A single-stage three-phase power-factor-correction circuit is developed using three isolated single-phase SEPIC-based power-factor-correction circuits operating in continuous-current mode. This approach is found to be attractive for low to medium power applications and when stringent THD and EMI specifications have to be met, such as power supplies for avionics systems. Use of isolated single-phase circuits also avoids the problems associated with the interaction among three phases. Detailed design criteria for the power and control stages of the single-phase SEPIC power-factor-correction circuit are presented. Issues specific to the three-phase implementation, such as current sharing at the input, are discussed. Simulation and experimental results corresponding to a 500 W three-phase prototype switching at 100 kHz are reported.

Model of HTS three-phase saturated core fault current limiter

Abstract: A small (several hundred watts) model of a three phase saturated core HTS fault current limiter (FCL) was developed and tested. Iron yokes of all three phases were saturated by a single DC HTS coil. The coil comprised a 60 turns single pancake (ID 135 mm), wound after heat treatment from Bi-2223 multifilamentary tape in Ag matrix. The critical current of the pancake in liquid nitrogen was 8 A. The tests have shown that the limiting value of the AC current (at 50 Hz) can be easily adjusted in the range from 8 A to 20 A depending on the value of the DC current in the HTS coil. The optimum value of the latter is 4 A, corresponding to the 8 times increase of the differential resistance in the current limiting mode. The response time is very short (less that 1 ms). Under tests the short-circuiting event was made in one, two and all three phases. The case of short-circuiting of one phase in the three-phase FCL is especially favorable from the standpoint of the voltages induced in HTS coil compared to the one-phase FCL.

Three-phase zero-current-transition (ZCT) inverters and rectifiers with three auxiliary switches

Abstract: Zero current transition (ZCT) topologies are presented for three-phase inverters and rectifiers. Such devices are used for example in AC adjustable speed drives for so-called zero-emission vehicles (i.e., electric and hybrid combustion/electric automobiles). Compared to existing three-phase ZCT techniques, the number of auxiliary switches is reduced from six to three, while not altering the necessary device rating. Correspondingly, the number of gate-drivers for the auxiliary switches is also reduced to three. Meanwhile, the merits of the existing three-phase ZCT techniques are still retained, i.e., all the main switches and the auxiliary switches are turned on and turned off under zero-current conditions, and the independent commutation for each main switch is achieved. The desired soft-switching features are achieved. Therefore, this invention will contribute to more cost-effective, reliable, and efficient high-performance three-phase inverters and rectifiers.

Dynamic and steady state analysis of a three phase buck rectifier

Abstract: Current source rectifiers among other alternatives, offer several advantages over line commutated rectifiers. Advantages include displacement power factor control and reduced line current harmonic distortion. This paper analyzes the current source rectifier (CSR) in transient and steady state, the models are developed in a synchronous reference frame. The load behavior is characterized for two load conditions, resistive load or, in general, increasing current for increasing voltage, and constant output power, decreasing output current for increasing voltage. Constant power operation can occur for a converter system supplying a pulse width modulation (PWM) inverter with high dynamics. Several static converter characteristics such as power factor, real and reactive power are analyzed for both types of load. Transient characteristics are analyzed for both types of load by exact small-signal model with full set of equations.

An analytical study on steady-state performance of an induction motor connected to unbalanced three-phase voltage

Abstract: This paper presents an analytical study into the steady-state performance of a three-phase induction motor powered by unbalanced supply voltage. Positiveand negative-sequence equivalent circuits of the induction motor are used to ascertain phenomena caused by voltage unbalance such as line-current unbalance, derating, drop in machine efficiency, etc. This paper also proposes the ratio of motor positive- to negative-sequence impedance as an index that allows the sensitivity of line-current unbalance to voltage unbalance to be effectively evaluated. A numerical example is provided to show practical applications of the proposed analytical model.