Showing papers on "Three-phase published in 2002"

Control of circulating current in two parallel three-phase boost rectifiers

Abstract: One unique feature in parallel three-phase converters is a potential zero-sequence circulating current. To avoid the circulating current, most present technology uses an isolation approach, such as transformers or separate power supplies. This paper proposes a parallel system where individual converters connect both AC and DC sides directly without additional passive components to reduce size and cost of the overall parallel system. In this case, the control of the circulating current becomes an important objective in the converter design. This paper: (1) develops an averaged model of the parallel converters based on a phase-leg averaging technique; (2) a zero-sequence model is then developed to predict the dynamics of the zero-sequence current; (3) based on the zero-sequence model, this paper introduces a new control variable, which is associated with space-vector modulation; (4) a strong zero-sequence current control loop is designed to suppress the circulating current; and (5) simulation and experimental results validate the developed model and the proposed control scheme.

Novel three-phase AC-DC-AC sparse matrix converter

Abstract: A novel three-phase AC-DC-AC sparse matrix converter (SMC) having no energy storage elements in the DC link and employing only 15 IGBTs as opposed to 18 IGBTs of a functionally equivalent conventional AC-AC matrix converter (CMC) is proposed. It is shown that the realization effort could be further reduced to only 9 IGBTs (ultra sparse matrix converter, USMC) in case the phase displacement of the fundamentals of voltage and current at the input and at the output is limited to /spl plusmn//spl pi//6. The dependency of the voltage and current transfer ratios of the systems on the operating parameters is analyzed and a space vector modulation scheme is described in combination with a zero current commutation procedure. Furthermore, a safe multi-step current commutation concept is treated briefly. Conduction and switching losses of the SMC and USMC are calculated in analytically closed form. Finally, the theoretical results are verified in Part II of the paper by digital simulations and results of a first experimental investigation of a 10 kW/400 V SMC prototype are given.

Stability improvements of an LCL-filter based three-phase active rectifier

Abstract: Three-phase active rectifiers guarantee sinusoidal input currents and controllable dc voltage at the price of a high switching frequency ripple that can disturb and reduce efficiency of other EMI sensitive equipment connected to the grid. This problem could be solved choosing a high value for the ac inductors making them expensive and bulky. Moreover the dynamic of the system becomes poor with so high value of inductance. On the contrary to adopt an LCL-filter configuration allows to use reduced values of the inductances (preserving dynamic) and to reduce the switching frequency pollution emitted in the grid. However the stability of the system should be rigorously studied. A poor analysis made on qualitative considerations could lead to excessive damping (unnecessary increase of the losses) or insufficient damping (the system seems to be stable but it is not). In this paper the damping, both passive (based on the use of resistors) and active (based on the modification of the control algorithm), is studied using the z-plane root locus approach and looking to dynamic performances and losses. In fact it is necessary to verify the dynamic effects by the introduction of damping resistors or by the modification of the control algorithm to perform active damping. The analysis is validated both with simulation and experiments.

Space Vector PWM Control of Dual Inverter Fed Open-End Winding Induction Motor Drive

Abstract: The technique of space vector PWM control of dual voltage source inverter fed three phase open-end winding induction motor is presented in this paper. A space vector PWM technique is developed based on the combination of space vectors from dual inverters feeding the induction motor from both ends (open-end winding without neutral point). A total of 64 voltage space vector combinations are available for PWM voltage control of the inverter fed machine with open-end winding A space phasor based PWM scheme is proposed with minimum number of switching in a cycle per inverter coupled with equal number of switching for each inverter. All the voltage space vector combinations are utilized for generating the reference voltage vector to cover the entire speed range. The open-end winding structure can have significant third harmonic currents unless suitable third harmonic filters are used to suppress it. The third harmonic currents are suppressed by using isolated transformers for both inverters.

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 both the AC input voltage and current sensors is proposed. 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 input 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, the currents ahead one sampling period are estimated by a state observer and then are used for feedback control. The proposed control scheme reduces the system cost and improves its reliability. The feasibility of the proposed AC sensorless technique for three-phase PWM rectifiers has been verified through experiments using a high performance DSP chip.

A general three-phase PFC controller for rectifiers with a parallel-connected dual boost topology

Abstract: A general constant-frequency power-factor-correction (PFC) controller is proposed for three-phase rectifiers with parallel-connected dual-boost topologies. This paper shows that unity power factor and low current distortion in all three phases can be realized by one-cycle control using one integrator with reset along with a few near and logic components. This new extension of one-cycle control provides the core PFC function to the dual-boost topologies. It does not require multipliers, as used in most other control approaches to scale the current reference according to the output power level. In each 60/spl deg/ of AC line cycle, only two switches are switched at high frequency; therefore the switching losses are significantly reduced. All switches are switched at low current, which results in reduced current ratings. This control method is simple and general. It is applicable to three-phase rectifiers that can be decoupled into parallel-connected dual-boost topologies by slight modification of the logic circuit. This control method is verified by experimental results. The proposed controller is suitable to be integrated into a three-phase PFC control chip.

Apparatus and a method for voltage conversion

Abstract: An apparatus for converting direct voltage into three-phase alternating voltage and conversely comprises a VSC-converter (8) having a direct voltage intermediate link (9) and at least one phase leg (12, 13). Each current valve (14-17) of the phase legs has at least one semiconductor device of turn-off type and a rectifying member connected in anti-parallel therewith. A transformer (19) has two opposite ends of a first winding (20) thereof connected to an output (21, 22) each of the VSC-converter and a second winding (23) connected to a direct converter having at least three phase legs. Each of the current valves of the direct converter being able to conduct current and block voltage in both directions and to turn on by gate control. The midpoints (27, 27', 27') of the phase legs of the direct converters are provided with phase outputs for forming a terminal for the alternating phase voltage between these phase outputs.

Transient performance of three-phase self-excited induction generator during balanced and unbalanced faults

Abstract: A generalised dynamic model of a delta-connected three-phase self-excited induction generator (SEIG) has been developed using d-q variables in a stationary reference frame This model can handle symmetrical and unsymmetrical load and capacitor configurations, however, the SEIG performance during balanced and unbalanced faults has been studied, which include cases of extreme unbalance operation SEIG behaviour has been investigated considering the effects of main and cross flux saturation for load perturbation, three-phase and line-to-line short circuit, opening of one capacitor, two capacitors and a single line at the capacitor bank, opening of single-phase load, two-phase load, and a single line at load Simulated results are compared with experimental results and a close match between them validates the developed model

Novel Three-Phase AC-DC-AC Sparse Matrix Converter Part I: Derivation, Basic Principle of Operation, Space Vector Modulation, Dimensioning

Abstract: A novel three-phase AC-DC-AC Sparse Matrix Converter (SMC) having no energy storage elements in the DC link and employing only 15 IGBTs as opposed to 18 IGBTs of a functionally equivalent conventional AC-AC matrix converter (CMC) is pro- posed. It is shown that the realization effort could be further reduced to only 9 IGBTs (Ultra Sparse Matrix Converter, USMC) in case the phase displacement of the fundamentals of voltage and current at the input and at the output is limited to ±π/6. The dependency of the voltage and current transfer ratios of the systems on the operating parameters is analyzed and a space vector modulation scheme is described in combination with a zero current commutation proce- dure. Furthermore, a safe multi-step current commutation concept is treated briefly. Conduction and switching losses of the SMC and USMC are calculated in analytically closed form. Finally, the theoretical results are verified in Part II of the paper by digital simulations and results of a first experimental investigation of a 10kW/400V SMC prototype are given.

Fixed-frequency space-vector-modulation control for three-phase four-leg active power filters

Abstract: The paper presents a three-dimensional space-vector-modulation scheme for three-phase four-wire active power filters. The focus is on the implementation of a fixed frequency pulse width modulation (PWM) scheme with a minimum number of switch commutations per period and maximum DC bus voltage utilisation. For three-wire applications, space vector modulation is known to provide better utilisation of the DC voltage compared to a sinusoidal PWM approach. This concept is extended to four-wire applications by employing a four-leg active power filter. The largest symmetrical region in which the active filter's voltage space vector may reside is identified. Restricting the voltage space vector to this region avoids over-modulation and thereby prevents the production of low order harmonics. A digital controller is employed to provide deadbeat current control. The combination of the digital controller and the modulation scheme gives the four-leg active power filter the capability to independently track reference current waveforms in the three phases within one switching period. The four-leg active power filter may be used for harmonic compensation, reactive power compensation, load balancing, and neutral current compensation. Experimental results obtained from a 5 kVA laboratory active power filter validate the proposed modulation scheme as well as the control design.

System and method for dual mode control of a turbogenerator/motor

Abstract: A method of controlling a permanent magnet turbogenerator/motor includes providing a protected load connected in parallel with the turbogenerator/motor through a pulse width modulated inverter configured in a first operating mode to supply controlled current from the turbogenerator/motor to a utility electrical power source, and selectively connected to the utility electrical power source through an isolation device, monitoring the utility electrical power source, and automatically disconnecting the protected load from the utility electrical power source while reconfiguring the pulse width modulated inverter in a second operating mode to supply controlled voltage to the protected load when a fault is detected in the utility electrical power source. The present invention provides an external or integrated dual mode controller which can be used to provide automatic transitions of a turbogenerator/motor between grid-connected modes where a PWM inverter provides a controlled AC current source to stand-alone modes where the PWM inverter provides a controlled AC voltage source to a protected load. Three phase rotation sequence can be set to positive or negative rotation, or be set to the last measured rotation for the utility grid to enable automatic transitions from stand-alone to grid-connect modes. In addition, the present invention provides for detecting a reference phase angle of the three-phase power provided by a utility grid such that gradual synchronization/resynchronization of a protected parallel connected load can be performed by appropriately controlling the PWM inverter prior to re-connection to the utility grid.

Uninterruptible power system

Abstract: An uninterruptible power supply system senses quality of available three phase input power and determines if load requirements can be met therefrom. If not, it drives an inverter to generate acceptable quality three phase AC to supply to the load. In this mode, the energy is obtained from a battery array. A solar panel can be used as a primary or a secondary source of input energy.

Three-phase bipolar mode active power filters

Abstract: In this paper, a unified constant-frequency integration control method for a three-phase bipolar mode active power filter is proposed. The proposed control method eliminates the use of any multipliers, the need of sensing three-phase load current, and the nontrivial task of calculating the harmonies and reactive current components, as required by previously reported control methods. By applying one-cycle control and sensing the mains line current, unity power factor and low input current distortion can be realized by one integrator with reset along with a few linear components such as flip-flops, comparators, and clock. The proposed controller is simple, robust, and reliable. All findings are supported by simulation and experiments.

A new three phase time-domain model for electric arc furnaces using MATLAB

Abstract: This paper presents a new time domain model for electric arc furnaces using MATLAB The model can simulate both nonlinear and time varying nature of such loads, which create flicker, harmonics, and voltage/current unbalance In this model, Cassie and Mayr equations are used in three-phase mode and the effects of arc furnace transformer tap changer and flexible cables mutual inductances are considered The proposed model takes into account the system inter-relation of parameters and arcing conditions as well

Parallel operation of capacity-limited three-phase four-wire active power filters

Abstract: Three-phase four-wire active power filters (APFs) are presented that can be paralleled to enlarge the system capacity and reliability. The APF employs the PWM four-leg voltage-source inverter. A decoupling control approach for the leg connected to the neutral line is proposed such that the switching of all legs has no interaction. Functions of the proposed APF include compensation of reactive power, harmonic current, unbalanced power and zero-sequence current of the load. The objective is to achieve unity power factor, balanced line current and zero neutral-line current. Compensation of all components is capacity-limited, co-operating with the cascaded load current sensing scheme. Multiple APFs can be paralleled to share the load power without requiring any control interconnection. In addition to providing the theoretic bases and detailed design of the APFs, two 6 kVA APFs are implemented. The effectiveness of the proposed method is validated with experimental results.

Line current characteristics of three-phase uncontrolled rectifiers under line voltage unbalance condition

Abstract: Three-phase uncontrolled rectifiers with capacitive filters are highly sensitive to line voltage unbalance, drawing significantly unbalanced line current even under slightly unbalanced voltage condition. This paper presents an analysis of this "unbalance amplification effect" for an ideal rectifier circuit without AC- and DC-side inductors. A novel way of modeling the voltage unbalance is established by introducing a deviation voltage superimposed on balanced three-phase line voltages, which allows simplified analysis of the rectifier operation. With proper approximations, closed-form expressions for symmetrical components of the fundamental line current and correspondingly the current unbalance factor are derived in terms of the voltage unbalance factor, filter reactance, and load current. The equivalent RMS value of the third harmonic current is also derived. The analysis clearly shows high sensitivity of the current unbalance to the voltage unbalance, and provides a basic guideline for rectifier circuit design. The validity of the analysis is confirmed by simulation.

Design and control of a three-phase active rectifier under non-ideal operating conditions

Abstract: Industrial engineers face many nonideal operating conditions in the design of the active rectifiers: too long computation time, presence of acquisition filters, AC phases unbalance, location of the grid voltage sensors after a dominant reactance and passive damping if an LCL-filter is used. The phenomena generated by these conditions are generally nonlinear. Hence the first step is to find out how much each of them affects the system's overall performance. Then new rules for the tuning of the controllers can be developed on the basis of the analysis of the transient behavior and of the steady state harmonic content of the DC voltage and of the AC current. This procedure is developed in this paper through simulations and laboratory tests.

Thermal problems caused by harmonic frequency leakage fluxes in three-phase, three-winding converter transformers

Abstract: Harmonic frequency leakage flux can be a limiting factor in three-phase, three-winding HVDC converter transformers. Investigation of a three-phase, three-winding 240-MVA converter transformer failure indicated the failure was caused by harmonic fluxes. Calculations indicated that the magnitudes of these harmonic fluxes to be approximately 45% of the power frequency leakage flux for the transformer, and are little affected by the transformer impedance or the converter firing angle. A study of the failed transformer loading during its life was made and a calculation made of the hot spot temperature considering various insulation half-life factors. Based on published information on insulation half-life factors it was estimated that at full load the hot spot temperature of the transformer was about 159/spl deg/C. From examination of the insulation in the hot spot area, this estimate of hot spot temperature was considered reasonable.

Strategies of instantaneous compensation for three-phase four-wire circuits

Abstract: When the voltage source is not balanced for three-phase four-wire circuits, instantaneous compensation for the instantaneous reactive power does not eliminate the neutral current on the source side. In facti when the zero-phase voltage of the source exists, none of the present compensation strategies can guarantee the instantaneous elimination of the neutral current in three-phase four-wire systems. Two approaches are distinguished in this paper for instantaneous compensation. The first eliminates the instantaneous reactive current, thus neutral current can still flow. The second eliminates the instantaneous pseudo-reactive current, so that the neutral current component is compensated. In the latter case, a new control strategy is designed to avoid instantaneous power flowing through the compensator. It provides flexibility in compensating for the neutral current in a three-phase four-wire system including the zero-sequence voltage. Finally, simulated and experimental results are obtained to confirm the theoretical properties and show the compensator performance.

Current control implementation with deadbeat algorithm for three-phase current-source active power filter

Abstract: An effective system control method is presented for applying a three-phase current-source PWM convertor with a deadbeat controller to active power filters (APFs). In the shunt-type configuration, the APF is controlled such that the current drown by the APF from the utility is equal to the current harmonics and reactive current required for the load. To attain the time-optimal response of the APF supply current, a two-dimensional deadbeat control scheme is applied to APF current control. To cancel both the delay in the two-dimensional deadbeat control scheme and the delay in DSP control strategy, an adaptive line enhancer (ALE) is introduced in order to predict the desired value of three sampling periods ahead. The ALE also brings robustness to the deadbeat control system. Owing to the ALE being in a transient state, the settling time is short. By comparison in a steady state, the total harmonic distortion ratio of source currents can be reduced as much as compared to the case that ideal identification of controlled system could be made. The experimental results obtained from a DSP-based APF are also reported. The compensating ability of this APF is very high in accuracy and responsiveness, although the modulation frequency is rather low.

Reciprocity-transposition-based sinusoidal pulsewidth modulation for diode-clamped multilevel converters

Abstract: Modulation strategies for multilevel inverters have typically focused on synthesizing a desired set of three phase sinusoidal voltage waveforms using a fixed number of DC voltage levels. This results in the average current injection and hence the net power drawn from the multiple DC bus terminals to be unmatched and time varying. Subsequently, the DC-bus voltages are unregulated, requiring corrective control action to incorporated. In this paper, the principle of reciprocity transposition in introduced as a means for modeling the DC-bus current injection simultaneously as the modulation strategy is formulated. Furthermore, a new sinusoidal pulsewidth-modulation strategy that features constant and controllable current injection at the DC-bus terminals while maintaining output voltage waveform quality is introduced. The proposed strategy is general enough to be applied to converters with an even number of levels and an odd number of levels. Analytical results comparing the performance of the proposed modulator with a conventional multiple carrier modulator are presented for example multilevel converters with four and five levels. Computer simulation results verifying the analytical results are presented for a four-level converter.

Vector control system without sensor for inductive motor and method

Abstract: A sensorless vector control system of an induction motor includes: a magnetic flux and speed controlling unit for receiving a predetermined command value and generating two phase voltages of DC component; a first reference frame converting unit for converting the two phase voltages of DC component into three phase voltages of AC component; an inverter for receiving the three phase voltages of AC component and driving an induction motor; a current detecting unit for receiving the three phase power of AC component flowing between the inverter and the induction motor, and detecting and outputting three phase currents of AC component; a second reference frame converting unit for receiving the three phase currents of AC component, and converting and outputting two phase currents of DC component; a magnetic flux and speed estimating unit for receiving the two phase voltage of DC component and the two phase currents of DC component, estimating a magnetic flux and speed required for a vector control; and a primary resistance estimating unit for receiving the two phase voltages of DC component, the two phase currents of DC component and the magnetic flux and speed estimated values, estimating a primary resistance and outputting it. Since the speed and the torque of the induction motor can be controlled, the vector control system can be adopted even to the application sector which requires a tension control such as paper, metal film or fiber which allows only a vector control as well as the variable speed.

System for estimating the frequency of the power signal on a power transmission line

Abstract: The system (10) obtains the three phase voltages on the power line VA, VB and VC, either from a local primary (14) or local alternative source (16). The selected local source voltages (14, 16) are then applied through switches (20, 28), respectively, which are normally closed but are open in the event that the associated pole of each voltage phase is open. The voltages from the closed switches (20, 28) are applied to a calculating circuit (40) which produces a composite of the three voltages in accordance with a preselected formula. The composite voltage output is then normalized (44) and applied to a conventional frequency determination (estimating) circuit (46). This output (FREQ. OUTPUT) is an accurate system frequency for use by the protective relay.

A novel control strategy for unified power quality conditioner (UPQC)

Abstract: This paper presents a novel and easy to implement control strategy for unified power quality conditioner (UPQC). This control strategy is usable in three-phase three-wire utilities. The control strategy of parallel active filter (PAF) is based on combination of extended p-q theory and instantaneous symmetrical components theory while the control circuit of series active filter (SAF) is based on instantaneous symmetrical components theory. The problem of generating three phase imbalance reference currents of extended p-q theory has been solved. Operation of PAF using this method compensates for reactive power, current harmonics and imbalance currents while operation of SAF compensates for voltage imbalance and voltage harmonics. The operation of control circuit is explained using analytical analysis. The validity of operation of control strategy is studied through simulation results.

Alternative topology for three-phase four-wire PWM converters applied to a shunt active power filter

Abstract: This paper presents an alternative three-phase four-wire converter for active power filter applications. The topology of this converter consists of four switch-legs, and it uses a "split-capacitor" in the DC-bus. In this converter, three legs are destined to control the injected currents in utility lines, while the fourth leg assures a balanced voltage sharing in the DC-bus capacitors. This converter solves the problem of DC-bus voltage imbalance in the three-leg split-capacitor converter. Moreover, this topology allows each leg to be controlled independently, so injected current control is simpler than in the four-leg full-bridge conventional converter, which utilizes a single capacitor on the DC-bus side. In this sense, a simple constant-frequency current control technique is proposed. In this paper, an analytical study and a verification by simulation are conducted.

Design for torque ripple reduction of a three-phase switched-reluctance machine

Abstract: This paper describes the design of a three-phase switched-reluctance machine in which there are overlaps between the phase current pulses in order to minimize the torque ripple, especially for high-speed operation. The machine dimensions are optimized by the finite-element method coupled to a cyclic process. Theoretical and experimental results obtained with the angle control strategy are presented.

Comparative evaluation of three-phase PWM voltage- and current-source converter topologies in FACTS applications

Abstract: To date, all of the existing and planned converter-based FACTS controllers are realized based on the voltage-source converter topology. With the evolution of high-performance semiconductor switches and super-conducting materials for inductors, and also with the development of new modeling and control techniques, current-source converter topology is gaining ground in competition with the voltage-source converter topology in high-power applications, including FACTS controllers. In this paper, the voltage- and current-source converter topologies are compared from different viewpoints of device rating, DC-side energy storage requirement, AC-side waveform quality, start-up, and cost. The comparative evaluation of the two topologies is performed based on analysis, as well as experimental results.

Mitigation method of 3rd-harmonic voltage for a three-phase four-wire distribution system based on a series active filter for the neutral conductor

Abstract: This paper proposes a series active filter for mitigation of the 3rd-harmonic voltage in a three-phase four-wire power distribution system in a building. The active filter which consists of a single-phase inverter can suppress the harmonic voltage of the system. The active filter is characterized by acting not only as a capacitor but also as a resistor for the 3rd-harmonic components. A Hilbert transformer is applied to the controller of the active filter in order to realize the accurate 3rd-harmonic detection on a single-phase active filter. Measurement results of harmonic distortion of source voltage in a building are also shown in this paper. It is clarified in a simulation and experiment that the active filter can suppress the 3rd-harmonic voltage without increasing neutral conductor current.