Showing papers on "Three-phase published in 2004"

Microgrid power quality enhancement using a three-phase four-wire grid-interfacing compensator

Abstract: This paper presents a three-phase four-wire grid-interfacing power quality compensator for microgrid applications. The compensator is proposed for use with each individual distributed generation (DG) system in the microgrid and consists of two four-phase-leg inverters (a shunt and a series), optimally controlled to achieve an enhancement of both the quality of power within the microgrid and the quality of currents flowing between the microgrid and the utility system. During utility grid voltage unbalance, the four-phase-leg compensator can compensate for all the unwanted positive-, negative-, and zero-sequence voltage-current components found within the unbalanced utility. Specifically, the shunt four-leg inverter is controlled to ensure balanced voltages within the microgrid and to regulate power sharing among the parallel-connected DG systems. The series inverter is controlled complementarily to inject negative- and zero-sequence voltages in series to balance the line currents, while generating zero real and reactive power. During utility voltage sags, the series inverter can also be controlled using a newly proposed flux-charge current-limiting algorithm to limit the flow of large fault currents between the micro- and utility grids. The performance of the proposed compensator has been verified in simulations and experimentally using a laboratory prototype.

Semiconductor losses in voltage source and current source IGBT converters based on analytical derivation

Abstract: A crucial criterion for the dimensioning of three phase PWM converters is the cooling of the power semiconductors and thus determination of power dissipation in the semiconductors at certain operating points and its maximum. Methods for the calculation and simulation of semiconductor losses in the most common voltage source and current source three phase PWM converters are well known. Here a complete analytical calculation of the power semiconductor losses for both converter types is presented, most parts are already known, some parts are developed here, as far as the authors know. Conduction losses as well as switching losses are included in the calculation using a simplified model, based on power semiconductor data sheet information. This approach should benefit the prediction and further investigations of the performance of power semiconductor losses for both kinds of converters. Results of the calculation are shown. Dependencies of the semiconductor power losses on the type of converter, the operating point and the pulse width modulation are pointed out, showing the general behaviour of power losses for both converter types.

Genetic algorithm-based design of the active damping for an LCL-filter three-phase active rectifier

Abstract: Active rectifiers/inverters are becoming used more and more often in regenerative systems and distributed power systems. Typically, the interface between the grid and rectifier is either an inductor or an LCL-filter. The use of an LCL-filter mitigates the switching ripple injected in the grid by a three-phase active rectifier. However, stability problems can arise in the current control loop. In order to overcome them, a damping resistor can be inserted, at the price of a reduction of efficiency. The use of active damping by means of control may seem attractive but it is often limited by the use of more sensors compared to the standard control and also by a complex tuning procedure of the controllers. This paper introduces a new active damping method that does not require the use of more sensors. It consists of adding a filter on the reference voltage for the modulator. The tuning process of this filter is easily done, for a wide range of sampling frequencies, with the use of genetic algorithms. This method is used only for the optimum choice of the parameters in the filter and an on-line implementation is not needed. Thus the resulting active damping solution does not need new sensors or complex calculations. Moreover, in the paper particular attention is devoted to the susceptibility of the system in a high polluting environment.

Prototype of multiphase modular-multilevel-converter with 2 MW power rating and 17-level-output-voltage

Abstract: A versatile prototype converter system of the new modular-multilevel converter (M/sup 2/LC) family is introduced. The new concept stands out due to its stringent modular construction as well as its extremely good control characteristics. This new M/sup 2/LC concept is well suitable for a wide range of high voltage applications. The basic working principle as well as the static and dynamic behaviour is explained in detail on a single-phase AC/AC-converter enabling four-quadrant operation. The M/sup 2/LC-family has already been introduced and examined for applications like traction converters, operating directly on the power line, and network interties [Marquardt, R et al., 2003 and Lesnicar, A et al., 2002]. With this prototype converter system, the characteristics of various different M/sup 2/LC-topologies can be investigated, including single or three phase DC/AC converters as well as multiphase AC/AC converters. The prototype converter has been designed in such a way that its configuration can be quickly changed between wide ranges of topologies. A flexible control system has been implemented, which is capable of controlling up to 32 IGBTs per converter arm, independently. Next to the presentation of the converter and its control system, measurement results are presented including converter operation.

Lagrangian modeling and passivity-based control of three-phase AC/DC voltage-source converters

Abstract: In this paper, we investigate the dc-bus voltage regulation problem for a three-phase boost-type pulsewidth-modulated (PWM) ac/dc converter using passivity-based control theory of Euler-Lagrange (EL) systems. The three-phase PWM ac/dc converters modeled in the a-b-c reference frame are first shown to be EL systems whose EL parameters are explicitly identified. The energy-dissipative properties of this model are fully retained under the d-q-axis transformation. Based on the transformed d-q EL model, passivity-based controllers are then synthesized using the techniques of energy shaping and damping injection. Two possible passivity-based feedback designs are discussed, leading to a feasible dynamic current-loop controller. Motivated from the usual power electronics control schemes and the study of Lee, the internal dc-bus voltage dynamics are regulated via an outer loop proportional plus integral (PI) controller cascaded to the d-axis current loop. Nonlinear PI control results of Desoer and Lin are applied to theoretically validate the proposed outer loop control scheme. The PWM ac/dc converter controlled by the proposed passivity-based current control scheme with outer loop PI compensation has the features of enhanced robustness under model uncertainties, decoupled current-loop dynamics, guaranteed zero steady-state error, and asymptotic rejection of constant load disturbance. Experimental results on a 1.5-kVA PC-based controlled prototype provide verification of these salient features. The experimental responses of a classical linear PI scheme are also included for comparative study.

A carrier-based PWM method for three-phase four-leg voltage source converters

Abstract: In this paper a voltage modulation method based on a triangular carrier wave for the three-phase four-leg voltage source converter is described. The four-leg converter can produce three output voltages independently with one additional leg. The proposed modulation method for the four-leg converter can be implemented with a single carrier by a simple but useful "offset voltage" concept. The method is equivalent to the so called three-dimensional space vector PWM method, but its implementation is much easier. The maximum magnitude of the balanced three-phase voltage and the maximum magnitude of zero sequence voltage, which can be synthesized simultaneously, are derived. The feasibility of the proposed modulation technique is verified by computer simulation and experimental results. These results show that a proposed carrier-based pulsewidth modulation (PWM) technique can be easily implemented without conventional computational burden.

Step-by-step design procedure for a grid-connected three-phase PWM voltage source converter

Abstract: The voltage source active rectifier is one of the most interesting solutions to interfacing dc power systems to the grid. Many elements are responsible for the overall system behaviour, such as value of the passive elements, sensors position, analog/digital filters and ac current/dc voltage controllers. In this paper a step-by-step design procedure, taking into account all these elements, is proposed and validated through the tests on an experimental prototype. The reported results are particularly relevant to evaluate the influence on the grid current harmonic content of the grid sensor position and of the use of analog filters in the feedback signals.

The generalized discontinuous PWM scheme for three-phase voltage source inverters

Abstract: This paper presents analytical techniques for the determination of the expressions for the modulation signals used in the carrier-based sinusoidal and generalized discontinuous pulse-width modulation schemes for two-level, three-phase voltage source inverters. The proposed modulation schemes are applicable to inverters generating balanced or unbalanced phase voltages. Some results presented in this paper analytically generalize the several expressions for the modulation signals already reported in the literature and new ones are set forth for generating unbalanced three-phase voltages. Confirmatory experimental and simulation results are provided to illustrate the analyses.

Influence of unbalanced voltage on the steady-state performance of a three-phase squirrel-cage induction motor

Abstract: The negative effects of a particular unbalanced voltage on the performance of an induction motor are studied in this paper. The paper suggests that the available definitions of unbalanced voltages are not comprehensive and complete. Therefore, the results of these analyses on motor performance are not very reliable. To prove this claim, a three-phase 25-hp squirrel-cage induction motor is analyzed under different unbalanced conditions. It is shown that it is necessary to define a more precise unbalanced factor for more accurate results. Experimental results verify the theoretical analysis.

A novel three-phase three-port UPS employing a single high-frequency isolation transformer

Abstract: A three-phase PWM rectifier and a three-phase PWM inverter are coupled via two four-quadrant full-bridge converter cells and a high-frequency isolation transformer. By employing a third transformer winding and further full-bridge cell battery energy storage is incorporated into the power transfer between rectifier and inverter resulting in a three-port UPS concept. The phase shift control of the power flow between the ports is analyzed for square-wave operation of the full-bridge cells. Furthermore, the utilization of the degrees of freedom of the system control, i.e. the extension to duty cycle control for optimizing the system behavior is discussed and control laws ensuring minimum overall system losses are derived. Finally, a control-oriented converter model is proposed and the decoupled control of the power flow of the ports is treated briefly. All theoretical considerations are verified by simulations using PSIM.

A novel three-phase single-stage distributed power inverter

Abstract: In this paper, a novel three-phase, DC/AC converter suitable for distributed power applications is proposed. The system consists of three DC/DC boost converters with a common point and operating as a three-phase inverter with intrinsic step-up capability. The converter obtained can invert, amplify and, where possible, regenerate bidirectional power sources such as fuel-cells, small gas turbines, and photovoltaic arrays. There are two main advantages to the system: the use of only six insulated gate bipolar transistors and small passive elements, and the fact that it does not need reverse voltage blocking capability. Simulation and experimental results show the effectiveness of the proposed system during both steady-state and dynamic operations.

Modelling, control design and analysis of VSC based HVDC transmission systems

Abstract: The development of power semiconductors, specially IGBTs has led to the small power HVDC transmission based on voltage source converters (VSCs). The VSC based HVDC transmission system mainly consists of two converter stations connected by a dc cable. This paper presents the modelling and control design of VSC based HVDC which uses twelve pulse three level converter topology. The reactive current injected by individual VSCs can be maintained constant or controlled to regulate converter bus voltage constant. While one VSC regulates the dc bus voltage the other controls the power flow in the dc link. Each VSC can have up to 4 controllers depending on the operating mode. The controller structure adopted for power controller is of PID type and all other controllers are of PI type. Each operating mode requires proper tuning of controller gains in order to achieve satisfactory system performance. This paper discusses a systematic approach for parameter optimization in selecting controller gains of VSC based HVDC. The analysis of VSC based HVDC is carried using both D-Q model (neglecting harmonics in the output voltages of VSC) and three phase detailed model of VSC using switching functions. While the eigenvalue analysis and controller design is based on the D-Q model, the transient simulation considers both models. The analysis considers different operating modes of the converters.

A novel three-phase DC/DC converter for high-power applications

Abstract: This paper proposes a new three-phase series resonant converter. The principle working of the converter is described analytically in detail for switching frequencies equal to the resonant frequency and for switching frequencies greater than the resonant frequency. Additionally, detailed simulations are performed to analyse the converter. Based on the analysis, design criteria for a 5 kW breadboard are derived. The 5 kW breadboard version of the converter has been built to validate the analytical investigations and simulations experimentally. Moreover, the breadboard is used to investigate the ZVS and ZCS possibilities of the topology and the influence of the deadtime and the switching frequency on the overall efficiency.

One-cycle control of three-phase active power filter with vector operation

Abstract: Active power filters (APFs) provides an effective measure to eliminate the power line harmonic/reactive currents generated by nonlinear loads or by distributed energy sources that are connected to the grid. Active power filters are typically connected in parallel to the harmonic/reactive current sources and cancel the harmonic/reactive components in the line current so that the current flow into and from the grid is sinusoidal and in phase with the grid voltage. Since the APFs process only the harmonic/reactive power, their power-handling capability can be much higher than that of the cascade power-factor-correction methods. In this paper, the one-cycle control method is extended to control three-phase APFs. The proposed control approach employs one integrator with reset along with several logic and linear components to control a voltage-source converter to achieve three-phase unity power factor for the current to and from the power grid. No multipliers or sensors for the load current and the APF inductor current are required. Furthermore, there is no need to calculate the reference for controlling APF inductor current so that complicated digital computation is eliminated. The operation switching frequency is constant that is desirable for industrial applications. The proposed control approach features great simplicity, excellent harmonic/reactive current cancellation, and solid stability. It is a cost-effective solution for power quality control for electronic equipment, buildings, industrial facilities, ships, airplanes, distributed power generation stations, etc. All findings are supported by experimental results.

A 12 kW three-phase low THD rectifier with high-frequency isolation and regulated DC output

Abstract: A robust 12 kW rectifier with low THD in the line currents, based on an 18-pulse transformer arrangement with reduced kVA capacities followed by a high-frequency isolation stage is presented in this work. Three full-bridge (buck-based) converters are used to allow galvanic isolation and to balance the dc-link currents, without current sensing or current controller. The topology provides a regulated dc output with a very simple and well-known control strategy and natural three-phase power factor correction. The phase-shift PWM technique, with zero-voltage switching is used for the high-frequency dc-dc stage. Analytical results from Fourier analysis of winding currents and the vector diagram of winding voltages are presented. Experimental results from a 12 kW prototype are shown in the paper to verify the efficiency, robustness and simplicity of the command circuitry to the proposed concept.

Mitigation of noneliminated harmonics of SHEPWM three-level multipulse three-phase active front end converters with low switching frequency for meeting standard IEEE-519-92

Abstract: Three-level neutral-point clamped (NPC) active-front-end three-phase (3LAFE) converters can be applied to high-power medium voltage applications, with four-quadrant unity-power-factor operation, allowing a broad range for power conversion systems, such as decentralized energy distribution and utilization. Switching losses and network interaction limit the operation of 3LAFE high-power converters, especially when GTO-converters are used. This is why low switching frequency and reduced harmonic distortion are the main goals when using optimum pattern pulsewidth modulation (PWM) where the selective harmonic elimination modulation method (SHEPWM) is used. This paper introduces a simple strategy to choose a good operating point for two 3LAFE converters in a 12-pulse configuration based on the behavior of noneliminated harmonics by SHE method. This strategy allows accomplishing the meeting of the IEEE Std. 519-92 and jointly, the minimization or elimination of harmonics filters. A complete characterization is given as a base of SHE method and for noneliminated harmonics behavior, as well as the full explanation of the optimization strategy. Experimental results show the strategy effectiveness for reducing harmonic distortion and meeting IEEE Std. 519-92.

Compensation in nonsinusoidal, unbalanced three-phase four-wire systems with active power-line conditioner

Abstract: For three-phase four-wire circuits, two compensation criteria have been established: one based on the instantaneous value concept and the other on the average value concept. Thus, according to the instantaneous value concept the non instantaneous power current is reduced, without altering the instantaneous active power. According to the average value concept, the nonactive average-current is reduced, without altering the average power. When the zero-sequence voltage component exists, both compensation types would not enable the zero-sequence (neutral) source current elimination. Then, two approaches are marked in this paper. The first one is for eliminating the non instantaneous power current or the nonactive average-current but the neutral current can still flow. The second one for eliminating the modified non instantaneous power current or the modified nonactive average-current, thus the neutral current component is compensated. According to recent recommendations in this work three-phase systems are considered as four-conductor systems. Experimental results are obtained to confirm the theoretical properties and to show the compensator performance.

Phase-decoupled method for three-phase power-flow analysis of unbalanced distribution systems

Abstract: This paper presents a three-phase power-flow solution method for unbalanced distribution networks, in which each phase is modelled in a decoupled way and therefore can be solved independently. All component models are described using only individual phase voltages, admittances and independent current sources. The component models are decoupled into each phase through compensation current injections whenever necessary. The solution method is based on the Zbus Gauss approach, with implicit factorisation of the Ybus matrix for each phase. The approach significantly reduces the overall amount of computation needed to solve sequential three-phase power-flows, becoming itself eligible for use on the real-time operation of modern distribution systems. Test results, showing both the accuracy and speed-up performances of the decoupled approach when applied to several networks, are presented.

Digital implementation of a line current shaping algorithm for three phase high power factor boost rectifier without input voltage sensing

Abstract: In this paper the implementation of a simple yet high performance digital current mode controller that achieves high power factor operation for three phase boost rectifier is described. The indicated objective is achieved without input voltage sensing and without transformation of the control variables into rotating reference frame. The controller uses the concept of resistance emulation for shaping of input current like input voltage in digital implementation. Two decoupled fixed frequency current mode controllers calculate the switching instants for equivalent single phase boost rectifiers. A combined switching strategy is developed in the form of space vectors to simultaneously satisfy the timing requirements of both the current mode controllers in a switching period. Conventional phase locked loop (PLL) is not required as converter switching is self-synchronized with the input voltage. Analytical formula is derived to obtain the steady state stability condition of the converter. A linear, low frequency, small signal model of the three phase boost rectifier is developed and verified by measurement of the voltage control transfer function. In implementation Texas Instruments's DSP TMS320F240F is used as the digital controller. The algorithm is tested on a 10-kW, 700-V dc, three phase boost rectifier.

A three-phase soft-switched isolated AC/DC converter without auxiliary circuit

Abstract: This paper describes an ac/dc converter system consisting of a voltage-source converter (VSC) with purely capacitive snubbers and a two-phase by three-phase cycloconverter, connected via a medium-frequency (MF) transformer. By alternately commutating the two converters, it is possible to achieve beneficial switching conditions for all semiconductor devices. A commutation and modulation algorithm is described, which allows for pulsewidth-modulation control of the output voltage while maintaining soft switching. Low-load operation of the converter is a potential difficulty because the load current may be insufficient for recharging the snubber capacitors of the VSC. However, if the cycloconverter is used to momentarily short circuit the transformer, a quasi-resonant commutation mode of the VSC can be achieved, making a fast and soft commutation of the VSC down to zero load possible, without an auxiliary circuit. Furthermore, the design and operation of a 40-kVA prototype converter system are described. The experimental results from the prototype clearly show the practical feasibility of the studied concept.

SVPWM-based current controller with grid harmonic compensation for three-phase grid-connected VSI

Abstract: Voltage space vector pulse-width modulation (SVPWM) is widely used in the current control of three-phase voltage-source inverters (VSI). However, as a voltage-type modulator, SVPWM brings some drawbacks to controllers, such as the compromised output current due to the back EMF disturbance and nonlinearity of the system, the lack of inherent overcurrent protection, etc. In this paper, SVPWM-based current control strategy with grid harmonics compensation and dual-timer sampling scheme is proposed and implemented in a 30 kW three-phase grid-connected VSI. Both simulation and experimental results verify that the proposed current controller offers a high performance of current control even under the influence of the grid harmonics. At the same time, the proposed dual-timer sampling scheme not only minimizes the system control delay but also offers an improved response for overcurrent protection to the system.

Real-time testing of a WPT-based protection algorithm for three-phase power transformers

Abstract: This paper presents real-time implementation and testing of a wavelet-packet-transform-based algorithm for power transformer protection. The proposed algorithm is implemented using a DS1102 digital signal processor board and tested on two different three-phase power transformers. Different magnetizing inrush, normal (through-fault) and internal fault currents are investigated for different loading conditions. The results show a complete independence from both the transformer parameters and current transformer saturation. In addition, the proposed algorithm has reduced computational burden, high speed, high accuracy, small required memory, and high reliability. In all cases of the investigated internal faults, the proposed algorithm is capable of identifying the fault and generating a trip signal in less than a quarter cycle based on a 60-Hz system.

Three-phase four-leg active power quality conditioner without references calculation

Abstract: Due to the use of diverse electric loads, the power quality problem in the power distribution systems can be very complex, especially in three-phase three-wire or four-wire distribution systems. This paper proposes a unified power quality conditioner (UPQC) with one cycle control (OCC) that has the functions of harmonic suppression, reactive power compensation, unbalanced load correction, and source neutral/zero-sequence current limitation. The advantages of this UPQC include no need for reference calculation, vector operation, and flexible structure for three-wire or four-wire system. Experimental and simulation results from a prototype confirmed the feasibility of the proposed approach.

An analysis of three-phase rectifiers with near-sinusoidal input currents

Abstract: An analysis of a three-phase low-harmonic diode rectifier equipped with inductors, capacitors, and diodes is presented. Inductors and capacitors are used in conjunction with the three-phase diode rectifier bridge to improve the waveform of the currents drawn from the utility grid. The operation of the proposed converter is analyzed and, on this basis, design considerations are commented upon. The converter characteristics are determined as a function of the load current. Comparisons between the studied converter and other rectifiers (classical rectifiers, with passive or active filters, and three-phase low-harmonic rectifiers applying the third-harmonic current injection) are also presented. Several possible applications of the three-phase rectifiers with near-sinusoidal input currents are mentioned. Analytically obtained results are experimentally verified.

Differential mode EMC input filter design for three-phase AC-DC-AC sparse matrix PWM converters

Abstract: The design of a differential-mode EMC input filter for a three-phase AC-DC-AC very sparse matrix converter intended for electrical machine drive applications is discussed in this paper. A review of the steps to be performed in the course of the filter design is presented and a detailed mathematical model of the EMI test receiver for quasipeak measurement of conducted emissions in the frequency range of 0.15...30 MHz is established. Furthermore, formulas for the estimation of the quasipeak detector output based on the LISN output voltage spectrum are provided. As experimentally verified by using a novel three-phase CM/DM separator this procedure allows an accurate prediction of the converter differential mode conducted emission levels and therefore could be employed in the design process for ensuring compliance to relevant EMC standards.

Reliability-oriented design considerations for high-power converter modules

Abstract: In this paper, proper and detailed reliability models for the power converter are addressed to assess the reliability of the system, taking into account the effects of stresses and environment on the reliability of the components. Based on that, a system design strategy is proposed in two different ways, either through a system or components point of view. The former implies the minimization of the number of components, thus simplifying the converter structure, and the minimization of stress on the different devices and components. The latter pursues the employment of the best possible technology per available component. Finally, a reliability-oriented design for a three phase AC to DC, bidirectional power converter module is presented.

Phase-decoupled method for three-phase power-flow analysis of unbalanced distribution systems

Abstract: This paper presents a three-phase power-flow solution method for unbalanced distribution networks, in which each phase is modelled in a decoupled way and therefore can be solved independently. All component models are described using only individual phase voltages, admittances and independent current sources. The component models are decoupled into each phase through compensation current injections whenever necessary. The solution method is based on the Zbus Gauss approach, with implicit factorisation of the Ybus matrix for each phase. The approach significantly reduces the overall amount of computation needed to solve sequential three-phase power-flows, becoming itself eligible for use on the real-time operation of modern distribution systems. Test results, showing both the accuracy and speed-up performances of the decoupled approach when applied to several networks, are presented.

System and method for unbalanced independent AC phase voltage control of a 3-phase, 4-wire output DC/AC inverter

Abstract: Independent phase output voltage control for a 3-phase 4-wire DC/AC inverter, for example, is provided. With the independent phase output voltage control, all three phase output voltages from the DC/AC inverter can be separately and effectively controlled to provide balanced 3-phase voltages from the DC/AC inverter to an unbalanced load, and/or to provide unbalanced 3-phase voltages from the DC/AC inverter to a balanced or unbalanced load. The independent phase output voltage control method can be applied to an inverter power system, with outputs of N-phase (N+1) wire configurations. Here, N can be any integer number greater than zero.

Multiple-frequency three-phase load flow for harmonic analysis

Abstract: A multiple-frequency three-phase load-flow model was developed in this paper. There are two new submodels including the fundamental power flow (FPF) and harmonic frequency power-flow (HPF) model. In FPF, models of electrical elements and PV buses were treated in the form of current injections in a transmission system. The standard Fourier analysis was used to deal with the harmonic loads to get injection currents. With harmonic currents as equivalent current sources, the HPF can be derived. Besides, the decoupled fast version of FPF and HPF, called DFPF and DHPF, were also proposed in this paper. Test results show that the proposed general-purpose methods are better performers than conventional power-flow solutions and are very robust.

Universal three phase controllers for power converters

Abstract: The systems and methods described herein provide for a universal controller capable of controlling multiple types of three phase, two and three level power converters. The universal controller is capable of controlling the power converter in any quadrant of the PQ domain. The universal controller can include a region selection unit, an input selection unit, a reference signal source unit and a control core. The control core can be implemented using one-cycle control, average current mode control, current mode control or sliding mode control and the like. The controller can be configured to control different types of power converters by adjusting the reference signal source. Also provided are multiple modulation methods for controlling the power converter.