Pulse multiplication in AC-DC converters for harmonic mitigation in vector-controlled induction motor drives

In this paper, an autotransformer with reduced kilovoltampere rating for 24-pulse ac-dc converter fed vector controlled induction motor drives (VCIMDs) is presented for harmonic current reduction. The 24-pulse operation is achieved using dc ripple reinjection technique in 12-pulse ac-dc converters. The proposed novel harmonic mitigator is found capable of suppressing up to 21st harmonic in the supply current. The procedure for the design of autotransformer for proposed ac-dc converter is presented to show the flexibility in the design for making it a cost-effective replacement suitable for retrofit applications, where presently a 6-pulse diode bridge rectifier is used. The effect of load variation on VCIMD is also studied to demonstrate the effectiveness of the proposed ac-dc converter. A set of power quality indices on input ac mains and on dc bus for a VCIMD fed from other 24-pulse ac-dc converters are also given to compare their performance. The laboratory prototypes of proposed autotransformers based 12-pulse and 24-pulse ac-dc converters are developed and test results are presented to validate the developed design procedure and the simulation models of these ac-dc converters under varying loads.     

Suppression of converter introduced harmonic currents using aforced-commutated cycloconverter

A method of current harmonic suppression through application of a modified forced-commutated cycloconverter is described. The cycloconverter is operated using current-error control with hysteresis. Advantage is taken of the multiple source voltage switching choices available with the cycloconverter to produce corrective harmonic current injection that tracks the command with minimum error. A numerical simulation of the filter is made with a three-phase, six-pulse rectifier load. The results are presented in the forms of both harmonic residual components and Fourier spectra     

Apparatus for supplying an isolated DC load from a variable-speedself-excited induction generator

This paper describes the design and laboratory testing of novel generation apparatus for supplying an isolated DC load from a self-excited induction generator operable at variable speed. The variable-speed generating apparatus consists of a self-excited induction machine, a controlled Graetz bridge rectifier, a voltage-boost power converter, and a control system. The induction generator supplies the rectifier. The voltage-boost power converter interfaces the variable output voltage of the rectifier to the fixed DC voltage required for the load. The rectifier is operated at levels of average DC current and voltage which control machine voltage to the rated AC voltage and which also draw the necessary power to supply the DC load. Performance is enhanced with respect to earlier apparatus in that both the DC voltage supplied to the load and the AC voltage on the machine are simultaneously controlled to fixed reference levels over broad operating ranges of load and speed     

虚拟同步机接入弱电网的电能质量新问题及谐波抑制方法

对比了VSG和传统电流型并网逆变器(TGCI)接入弱电网下的谐波域模型,发现传统谐波电流抑制方法并不适用于VSG,同时弱电网条件下非线性负载谐波电流和电网谐波电压对VSG谐波输出阻抗的要求是相互矛盾的,进而导致VSG接入弱电网下的问题更加复杂.提出一种阻抗重塑型谐波电流抑制方法来提升VSG接入弱电网的友好性,主要包含无...     

T-connected autotransformer-based 24-pulse AC-DC converter for variable frequency induction motor drives

A new robust passive waveshaper based on 24-pulse ac-dc conversion is proposed to feed voltage source inverter (VSI) supplying squirrel-cage induction motor drive. The passive waveshaper consists of a newly designed T-connected autotransformer with less number of windings resulting in simplicity in design and manufacturing. The proposed 24-pulse ac-dc converter is found capable of suppressing less than 23rd harmonics in the supply current. The power factor is also improved to near unity in the wide operating range of the vector-controlled induction motor drive (VCIMD). Finally, the design of the autotransformer is modified to make it suitable for applications, where presently a 6-pulse diode bridge rectifier is used. A laboratory prototype of proposed T-connected autotransformer-based 24-pulse ac-dc converter is developed and test results are presented to validate the developed design procedure and the simulation models of this ac-dc converter under varying loads.     

Power quality improvement using fuzzy logic controller for five-level shunt active power filter under distorted voltage conditions

In this paper, a five-level inverter is used as a shunt active power filter (APF), taking advantages of the multilevel inverter such as low harmonic distortion and reduced switching losses. It is used to compensate reactive power and eliminate harmonics drawn from a thyristor rectifier feeding an inductive load (RL) under distorted voltage conditions. The APF control strategy is based on the use of self-tuning filters (STF) for reference current generation and a fuzzy logic current controller. The use of STF instead of classical extraction filters allows extracting directly the voltage and current fundamental components in the α-β axis without phase locked loop (PLL). The MATLAB fuzzy logic toolbox is used for implementing the fuzzy logic control algorithm. The obtained results show that the proposed shunt APF controller has produced a sinusoidal supply current with low harmonic distortion and in phase with the line voltage.     

Converter Fed Synchronous Generator System for Medium and Large Power Plants

Converter fed synchronous generators have the following advantages: The turbine can operate at any speed, independant of the frequency of the power system, the part load efficiency may be improved if the speed can be adjusted. The generator is protected and isolated. The stability is improved. This scheme is applicable to hydro, steam and wind generation. The paper gives details on a converter fed 650 MW unit with steam turbine having a synchronous generator with two 3-phase windings at 30° to each other which directly feeds a 12-pulse HVDC link. The usual converter transformer, var-compensators and current harmonics filters on the generator side are eliminated. The study shows the advantages and disadvantages of the system. The system stability is considerably improved compared to the conventional design. As the steam turbine can be operated only with constant speed, the improvements in the efficiency of the thermal system are limited. A cost reduction can be achieved only with a higher turbine speed. Details are given on the total system losses and the total system costs in comparison to the conventional design.     

A new synchronous generator based wind energy conversion system feeding an isolated load through variable frequency transformer

This paper aims to explore the possibility of synchronous generator (SG) based wind energy generation system feeding an isolated load using a latest power transmission technology i.e. variable frequency transformer (VFT). The proposed configuration does not employ any power electronics based interface as in conventional SG based stand-alone wind energy conversion systems (SWECS). For analysis, the simulation models of proposed configuration as well as conventional configuration have been developed under MATLAB-Simulink environment. A series of studies on power fed from the SG to the different loads at various SG input speeds has been carried out with the developed models. Further to analyze the effectiveness of the proposed method; the efficiency, total harmonic distortion (THD) of output voltage and THD of output current of the proposed method have been compared with those of the conventional method. From obtained results, it is observed that the proposed method is simple and does not produce harmonics. Moreover to validate the proposed scheme, an experimental analysis has been carried out. Further, the cost analysis of both systems has also been carried out. From the cost analysis, it is observed that the proposed system is cheaper than the conventional system.     

AC/DC power conversion interface for self-excited induction generator

A new AC/DC power conversion interface for the self-excited induction generator (SEIG) is proposed here. The proposed AC/DC conversion interface includes an excitation systemand a diode rectifier connected in parallel.The variable frequency AC power generated by the SEIG is converted into DC power by the diode rectifier.The DC power of the diode rectifier can charge a battery set and supply DC loads or be further converted into fixed-frequency AC power by an inverter for AC loads.The DC voltage is expected to be regulated in the above applications.The excitation system supplies an exciting reactive current to maintain the amplitude of the SEIG output voltage to be a constant value. Moreover, it can also serve as an active power filter to suppress the harmonic current generated by the diode rectifier. The excitation system is composed of an AC power capacitor and a power converter connected in series. The AC power capacitor is adapted to provide a basic reactive power, and it can also reduce the voltage rating and the capacity of the power converter. The salient point of the proposed AC/DC power conversion interface is that the capacity of the power converter in the AC/DC power conversion interface can be minimised, and the power loss of the AC/DC power conversion interface can also be reduced. A prototype is developed and tested to verify the performance of the proposed AC/DC power conversion interface.     

Mitigation of harmonic disturbance at pumped storage power stationwith static frequency converter

This paper investigates the harmonic distortion problem and mitigation method at the Mingtan pumped storage power station in Taiwan, where six 300 MVA synchronous generator/motors are started by a static frequency converter (SFC) before the pumping stage. Since the SFC uses a 6-pulse rectifier technique, a large amount of harmonic currents are produced during the starting period. The harmonic distortion level at each bus of the power plant was very high. Especially, the total harmonic distortion (THD) of current at the lighting feeder reached up to 184%, so that power fuses were burned out. At first a 5 mH reactor was inserted in the SFC feeder and a 5th order and high pass filter was installed. However, the harmonic distortion levels were still too high, but there is no space for additional higher-order filters. Finally, the SFC is feeded with an individual transformer and the harmonic disturbance problem is avoided. This paper also gives computer simulations to investigate the harmonic distortion problems and verify the mitigation methods     

Modeling and analysis of current harmonic distortion from grid connected PV inverters under different operating conditions

Due to the fast growth of photovoltaic (PV) installations, concerns are rising about the harmonic distortion generated from PV inverters. High current total harmonic distortion (THD) occurs when PV inverters operate under light load conditions due to low solar insolation. A general model modified from the conventional control structure diagram is introduced to analyze the harmonic formation process. Causes of the current harmonics are summarized and its relationship with output power levels is analyzed. The field measurement for current harmonics is carried out at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) microgrid to validate the analysis results. In particular for two-stage inverter, unlike previous papers that assume the DC-link voltage is constant, the DC-link voltage ripple is identified as the source of a series of odd harmonics. A mew periodic time-varying model is proposed by including the DC-link voltage ripple into the conventional current control loop model. This model is able to simulate the characteristics of the harmonic components and show their dependence on the DC-link voltage ripple.     

A novel stand-alone dual stator-winding induction generator with static excitation regulation

On the basis of the new idea of electric power integration, a novel stand-alone dual stator-winding induction generator (DWIG) system is built. In this generator, there are two sets of windings to be embedded in the stator slots. One, referred to as the 12-phase power winding, supplies power to the dc load via a 12-phase bridge rectifier, and the other, called the 3-phase excitation winding, is connected to a pulsewidth modulation (PWM) voltage source static excitation regulator (SER). A solid iron squirrel cage rotor is suitable for high speed generation. Experiments and simulations show the ac capacitors can reduce the inductance of rectifier loads and help to reduce the capacitance of the SER. A simple control methodology based on stator voltage orientation (SVO) is presented to regulate the output voltage of the 12-phase bridge rectifier in this paper. Moreover, the electric energy quality and the relative influence factors are studied by detailed experiments and analyses. The proposed system is especially suitable for self contained electrical systems, such as those found on electric vehicles, ships, and aircraft, where high performance and compact size are essential.     

Peak current mode control of three-phase boost rectifiers in discontinuous conduction mode for small wind power generators

This paper presents a peak current mode control scheme of a boost rectifier with low distortion of the input current for wind power systems based on permanent magnet synchronous generators with variable speed operation. The three-phase boost rectifier is operated in discontinuous conduction mode (DCM), and power factor correction techniques are applied. It is shown that the DCM operation significantly reduces the total harmonic distortion of the currents in the permanent magnet synchronous generator, increasing the power factor of the system, so that the vibrations and mechanical stress of the generator are minimized. The characteristics of the DCM boost rectifier are studied considering: (1) the series resistance of the inductors; (2) the modeling and adjustment of peak current mode control yielding a stable loop; (3) the design of an input filter that reduces the switching noise in the currents of the generator.     

Permanent-magnet machines

This paper explores the use of an interior permanent-magnet synchronous machine (IPM) as a source of controlled DC power. A three-phase diode rectifier converts the generated AC power into DC, which is further processed by a buck or boost DC-DC converter with a pulse-width modulation voltage controller for load voltage and output power regulation. The modeling and analysis of the generator system set forth are confirmed to accurately predict the generator characteristics by experimental results derived from a 2 hp interior permanent-magnet generator controlled separately by a buck and a boost DC-DC converter     

齿谐波励磁的混合励磁发电机带整流负载的定子铁耗计算

针对混合励磁发电机带整流负载时谐波含量增加对定子铁心损耗的影响,建立了齿谐波励磁的混合励磁发电机带整流负载的有限元场路耦合模型,分析了发电机带整流负载在齿谐波励磁系统断开和接通两种工况下的线电压谐波分量,借助传统定子铁心损耗计算模型计算出线电压谐波分量对定子铁心损耗的影响。同时利用有限元电磁仿真软件计算出电机空载及两种工况下的定子铁心损耗,仿真与计算结果比较表明,该计算方法在一定的误差内准确性较高。     

Harmonic reduction in adjustable-speed synchronous motors

Adjustable-speed synchronous motors (ASSM) are very popular in high-power high-speed applications. The Chilean mining industry uses them in milling operations in ranges of several thousands hp. Although a high pulse number for the inverter would be beneficial, it never goes beyond 12, due to complicated connections. Thus, a distorted harmonic-rich current feeds the synchronous motor, increasing the inverter commutation angle and causing additional rotor-heating and pulsating electric torque. Harmonic suppression by filters is not viable due to the variable frequency nature of the motor. In this paper, a new design for the inverter, without complicated circuitry, is proposed, so that it works with 36 pulses and then the motor is fed with a highly sinusoidal current. Also, the same technique is applied to the rectifier end, so that an effective harmonic reduction is carried out at both ends of the system, i.e., the motor as well as the AC supply system. A 20 kVA laboratory development drive system has been built and experimental waveforms for the conventional (12-pulse) and proposed (36-pulse) configurations are included     

Power quality enhancement of grid-connected fuel cell using evolutionary computing techniques

Fuel Cell (FC), as a type of new renewable energy sources grid-connected at Point of Common Coupling (PCC), is introduced in this study. This article presents the power quality improvement of the FC integrated to the power network through a chopper and an inverter using the conventional PI controller. Two PI controllers, tuned by three recent different evolutionary computing techniques namely Harmony Search (HS), Modified Flower Pollination Algorithm (MFPA) and Electromagnetic Field Optimization (EFO) methods are considered. The two PI controllers are used for driving the inverter connected the on-grid FC in order to govern the PCC voltage between the FC and the power network. These two controllers are exploited to drive the power and the current regulators at different voltage sag and swell conditions. The three optimization methods are compared to the Particle Swarm Optimization (PSO) with regards to voltage profile, power quality and execution time.Simulation results, using Matlab/Simulink^?, show the significance of the three optimization techniques in regulating the voltage at PCC with reduced harmonics during the system voltage sag and swell conditions when compared to the PSO. Through the numerical analysis, the superiority of MFPA method among the different optimization metaheuristic techniques is highlighted particularly for enhanced dynamic voltage response purposes.     

Performance investigation of artificial intelligence based controller for three phase four leg shunt active filter

In this paper, the choice of power quality compensator is a DSTATCOM which constitutes a three phase four leg voltage source converter (VSC) with a DC capacitor. The control strategy proposed for the DSTATCOM is a neural network based one cycle control (OCC). This control strategy involves neural network block, digital circuits and linear elements, which eliminates the sensors required for sensing the load current and coupling inductor current in addition to the multiplier employed in the conventional method. The calculation of harmonic and reactive currents for the reference current generation is also eliminated, thus minimizing the complexity in the control strategy. The control strategy mitigates harmonic/reactive currents, ensures balanced and sinusoidal source current from the supply mains that are nearly in phase with the supply voltage, compensates neutral current, and maintains voltage across the capacitor under unbalanced source and load conditions. The performance of the DSTATCOM with the proposed artificial neural network (ANN) controllers is validated and investigated through simulations using Matlab software. The simulation results prove the efficacy of the proposed neural network based control strategy under varying source and load conditions.     

小型风力发电机组优化控制策略与实验研究

提出一种小型风力发电机组功率的优化控制策略.根据选定的300 W/24 V永磁发电机,使用Wilson叶片设计计算模型,应用MATLAB语言设计了300 W风机叶片;并针对现有风机控制系统中将控制器的设计与叶片、电机的匹配特性彼此孤立、分离的现象,设计出与风力发电机的电机、叶片相互匹配的控制器.在风洞试验中测试了样机在8、10 、12 、15 m/s等风速一定条件下,功率随系统电压的变化规律,当降低系统电压时,风机输出功率会一直下降,在此过程中并没有出现功率增加的现象,也就充分证明了工作在峰前区域的风力发电机,当风速大于额定风速时,控制系统可以通过减小接入系统的负载电阻值来控制其功率.这对研究小型风力发电系统的可控性、可靠性和耐久性有一定的指导意义和实用价值.     

Voltage and Frequency Controller for a Three-Phase Four-Wire Autonomous Wind Energy Conversion System

This paper deals with control of voltage and frequency of an autonomous wind energy conversion system (AWECS) based on capacitor-excited asynchronous generator and feeding three-phase four-wire loads. The proposed controller consists of three single-phase insulated gate bipolar junction transistor (IGBT)-based voltage source converters (VSCs) and a battery at dc link. These three single-phase VSCs are connected to each phase of the generator through three single-phase transformers. The proposed controller is having bidirectional flow capability of active and reactive powers by which it controls the system voltage and frequency with variation of consumer loads and the speed of the wind. VSCs along with transformer function as a voltage regulator, a harmonic eliminator, a load balancer, and a neutral current compensator while the battery is used to control the active power flow which, in turn, maintains the constant system frequency. The complete electromechanical system is modeled and simulated in the MATLAB using the Simulink and the power system blockset (PSB) toolboxes. The simulated results are presented to demonstrate the capability of the proposed controller as a voltage and frequency regulator, harmonic eliminator, load balancer, and neutral current compensator for different electrical (varying consumer loads) and mechanical (varying wind speed) dynamic conditions in an autonomous wind energy conversion system.