Low Order Harmonic Reduction of Three Phase Multilevel Inverter

Transcription

1 Journal of Scientific & Industrial Research Vol. 73, March 014, pp Low Order Harmonic Reduction of Three Phase Multilevel Inverter A. Maheswari 1 and I. Gnanambal 1 Department of EEE, K.S.R College of Engineering, Tiruchengode, Tamilnadu Department of EEE, Government College of Engineering, Salem, Tamilnadu Received 11 August 01; revised 09 September 013; accepted 0 January 014 In this paper, a cascaded H-Bridge multilevel inverter has been proposed using unequal dc power sources and less number of switches. A standard cascaded multilevel inverter requires h number of dc sources for (h + 1) levels. This proposed scheme allows less number of unequal DC power sources without the requirement of transformers. Cascaded H- Bridge multilevel inverter fed induction motor shows the better performance due to fundamental frequency switching scheme. High quality output is derived due to the absence of lower order harmonics. High conversion efficiency is also achieved for induction motor drive when it is operated with the proposed inverter. The performance of three phase cascaded H-Bridge multilevel inverter with unequal dc sources is simulated by using MATLAB platform. Harmonic analysis is done on cascaded H-Bridge seven level inverter and experimental results are presented to demonstrate the superiority of the proposed system. Keywords: Fundamental frequency switching scheme, multilevel inverter, Total Harmonic Distortion (THD), Unequal dc sources. Introduction The recent development in solid-state electronics are widely used in industries to control motor drives, power systems, switched mode power supplies (SMPS),auto motives, etc. The inverter is one of the most extensive assemblies in power electronics. The main aspects for the development of multilevel inverters are multilevel voltage waveform; low total harmonic distortion (THD) and division of voltage to the switching devices 1. Multilevel inverters have received high attention because of their reliable operation, high efficiency and low electromagnetic interference (EMI). The desired output of a multilevel converter is synthesized by several sources of dc voltages. With an increasing number of dc voltage sources, the inverter output voltage approaches nearly sinusoidal waveform while adopting a fundamental frequency switching scheme. Transformers less multilevel inverters are uniquely suited for this application because of the high VA ratings is possible with this inverters -4. Structure of the multilevel voltage source inverters allows them to reach high voltages with low harmonics without the use of transformers or series-connected synchronized switching devices. Multilevel inverters also have * Author for correspondence dramaheswari@gmail.com several advantages with respect to hard switched two level pulse width-modulation (PWM) based inverter fed variable-speed drives. Motor damage and failure have been reported by industry as a result of some variable-speed drives operated by the inverters which has high voltage change rates (dv/dt), which produced a common-mode voltage across the motor windings. High frequency switching creates many problems because of common-mode voltage is impressed numerous times upon the motor at each cycle. The main problems of high frequency switching are failure of motor bearing and insulation breakdown in motor winding because of dielectric stresses, circulating currents, voltage surge and corona discharge 5, 6. Multilevel inverters can be able to overcome these problems because their individual devices have a much lower stress per switching action. They can also operate at high efficiencies, because they can switch at a much lower frequency than traditional PWM based inverters. There are variety of topologies are available in multilevel inverters. They are diode-clamped, flying capacitor and H-bridge cascaded multilevel inverters. Compared with diode-clamped multilevel inverter and flying capacitor multilevel inverter, the cascaded inverter needs less number of components and simple control methods. In the high voltage fields, the cascaded multilevel inverters are widely used. The

2 MAHESHWARI & GNANAMBAL: LOW ORDER HARMONIC REDN OF INVERTER 169 advantages of cascaded multilevel inverters are good output waveform and low switching stress. Its structure is suitable for modularization. Now a day, the existing PWM inverters are replaced by cascaded multilevel inverters. In the multilevel inverters, cascaded H-bridge multilevel inverter with unequal dc voltage sources is preferred, because it does not get affected from capacitor voltage balancing problems 7-9. This paper presents a new topology of seven level inverter which uses less number of unequal dc sources, switching devices and eliminates the need of capacitors. In this proposed work, a novel method is given to compute the switching angles for a multilevel inverter so as to produce the required output voltage while at the same time cancel out specified higher order harmonics. Particularly, a complete analysis is given for a seven level inverter and validated the performance with the hardware results. Cascaded H- Bridge Multilevel Inverter Topology A cascaded multilevel inverter consists of a series of H-bridge (single-phase full-bridge) inverter cells. The general function of a multilevel inverter is to synthesize a desired voltage from the separate dc sources. A single phase structure of cascaded asymmetric multilevel inverter is constructed with two H-bridges. The ac output of each level is connected in series such a way that the synthesized voltage waveform is the sum of the H- bridge outputs 10.For the asymmetric multilevel inverter, the value of the each dc source and voltage level is obtained as follows: V k = (k-1) V dc k=1,, h (1) N L = (h+1) 1 () Where, V k and N L denote the value of each dc source and number of output voltage levels. By having different values of dc sources, any number of voltage levels can be obtained at the output.the general function of a multilevel inverter is to synthesize a desired voltage from the separate dc sources, which may be obtained from fuel cells, batteries or ultra capacitors 11, 1. Symmetric multilevel inverter requires 1 switching devices to produce seven level output voltage whereas asymmetric multilevel inverter uses only 8switching devices for the same output voltage levels. This is the advantage of the asymmetric topology over the symmetric topology. However, this kind of multilevel inverter requires dc voltage sources with different values, providing of the dc voltage sources with different values which is a challenging issue. By opening and closing the switches of bridge-1 properly, the output voltage of bridge-1 can be made equal to V dc, 0, or V dc, while the output voltage of bridge- can be made equal to (V dc /), 0 or (V dc /). Therefore, the output voltage of the inverter can have the following values (1.5V dc,) (V dc,) (0.5V dc ), 0, 0.5V dc,v dc, 1.5V dc, which forms seven levels at the output voltage.with enough levels, the proposed fundamental frequency switching technique results in an output voltage of the inverter is almost sinusoidal. Quasi-square waveform is generated by phase shifting its positive and negative phase leg switching timings. By using this switching method, current stress of the active devices is made equal. Switching Angles Pulse width modulation control or space vector PWM methods are widely used techniques in the inverter control. These conventional methods will cause extra losses due to high frequency switching. To overcome this problem, low switching control methods is used. In this proposed method, fundamental frequency switching is used. Output voltage of the waveform can be evaluated by Fourier series expansion. It is clear that 5 th and 7 th order harmonic can be eliminated by solving the equations (3) to (5). The triplen harmonics will be cancelled automatically in the three phase system. In this case, the desire is to cancel the 5 th and 7 th order harmonics as they tend to dominate the total harmonic distortion. Mathematically, the statement of these conditions is then Cos (θ 1 ) + Cos (θ ) + Cos (θ 3 ) =V 1 (3) Cos (5θ 1 ) + Cos (5θ ) + Cos (5θ 3 ) =0 (4) Cos (7θ 1 ) + Cos (7θ ) + Cos (7θ 3 ) =0 (5) Three transcendental equation set can be solved to get the three unknowns θ 1, θ and θ 3. V 1 is the desired fundamental voltage. The widely used methods are resultant theory, iterative method such as the Newton- Raphson method. MATLAB nonlinear solver can also be used to solve the above set of equations. Among all the methods, resultant theory which produces possible solutions is selected to find the solutions of the above set of equations The transcendental equations characterizing the harmonic content can be converted

3 170 J SCI IND RES VOL 73 MARCH 014 into polynomial equations. The resultant method is used to find the solutions when they exist. Total harmonic distortion (THD) is calculated for the arrived set of solutions to select the set which generate the lowest harmonic distortion (mostly due to the 5 th and 7 th harmonics). The quality of the multilevel waveform depends on the selection of switching angles. Varying the switching angle also controls the magnitude of RMS value of output waveform and affects the total harmonic distortion. Simulation Study The simulation of three phase seven level cascaded H-Bridge multilevel inverter fed three phase induction motor was done using MATLAB/Simulink. In the simulation study all the switches are considered to be ideal. The frequency of the output voltage is 50 Hz. In this proposed multilevel inverter, only eight power switches with two dc sources are required to obtain the required output voltage for each phase. In the symmetrical type inverter, twelve power switches with three dc sources are required to achieve the same output voltage for each phase.the dc voltage sources used in the simulation studies are separate dc sources. In practice, these dc voltage sources are available via distributed energy resources like photovoltaic panels, fuel cells etc. If the available source is an ac source, then the required dc voltage sources can be obtained by a transformer with multiple secondary windings and rectifiers. There are different modulation strategies that can be applied for multilevel inverters. In this paper the fundamental frequency switching scheme is used. In this method, the switching angles are selected such that to eliminate lower order harmonics or minimization of total harmonics distortion. Since the proposed multilevel inverter uses unequal dc sources, the voltage stress among the switches will be asymmetrically distributed. Hence care should be taken while selecting power switches for this type of configuration. The simulation diagram of three phase seven level cascaded H-Bridge multilevel inverter is shown in Fig. 1. Simulation is carried out using the switching table shown in the Table 1. The spectrum of the output voltage and current is taken to determine the Total Harmonic Distortion (THD) of three phase induction motor drive. The simulation results of three phase voltage and current is presented in the Fig. (a) and Fig. (b). It clearly shows that the desired voltage levels are generated. From the figures, it is clear that the output current waveform is smoother than the output voltage. FFT spectrum for stator voltage and current were presented in the consecutive Fig. 3(a) and Fig. 3(b).From the normalized FFT analysis, it can be derived that the magnitude of lower order harmonics are very low and the magnitude of higher order harmonics are nearly equal to zero. Symmetric and asymmetric cascaded seven level inverter comparisons are given in Table. The proposed asymmetric cascaded H- Bridge Multilevel inverter topology has the advantage of its reduced number of switches and dc sources compared to conventional cascaded H-bridge multilevel inverter. Cost and weight is also reduced. Switching losses are considerably reduced intern heat produced in the converter is also reduces. It can be extended to any number of levels. The above features give the keen interest to use the proposed cascaded seven level inverter in the industry where the conventional inverters are used. Experimental Results and Discussion To experimentally validate the proposed topology, hardware of the three phase cascaded H bridge Fig.1 Simulation diagram of three phase cascaded H-bridge seven level Inverter Table 1 Switching table for cascaded H-bridge seven level inverter Voltage levels switches V dc V dc 3V dc 0 V dc Vdc 3V dc S S S S S S S S

4 MAHESHWARI & GNANAMBAL: LOW ORDER HARMONIC REDN OF INVERTER 171 Fig. (a) Three phase stator voltage waveforms (b) Three phase stator current waveforms Fig. 3 (a) FFT spectrum of stator voltage (b) FFT spectrum of stator current Parameters Table Comparison between conventional cascaded H-bridge seven level inverter and proposed cascaded H- bridge seven level inverter Topology Conventional cascaded H- Bridge seven level inverter Proposed cascaded H- Bridge Seven level inverter Percentage Reduction Number of switches per phase % Number of DC sources per phase % THD% 8.67% 6.73% 1.94% multilevel inverter has been built. Eight IGBT with freewheeling diodes are used as the switching device. Diode bridge rectifier is used to get the dc voltage. In this hardware, two diode bridges are used to produce 133V and 66V.ATmega16 Microcontroller is used to generate eight different gate pulses for IGBT power switch. Control algorithm for the switching is written in the high level language and then it is embedded in the ATmega16 microcontroller. Output of the inverter terminal is connected to the three phase squirrel cage induction motor. Fluke 435 Power Quality Analyzeris used to analyse the output voltage of the inverter. Output voltage and its FFT spectrum is exposed in Fig. 4 (a) and Fig. 4 (b).the result of the analyser shows a good accordance between the simulation and experimental results.

5 17 J SCI IND RES VOL 73 MARCH 014 variable speed ac drives (conveyors, rolling mills, printing machines, etc.,) are required and considerable amount of energy can be saved as the proposed system has lower harmonics. Fig. 4 (a) Phase voltage waveform (b) FFT spectrum of phase voltage Conclusion The proposed cascaded H-Bridge multilevel inverter fed three phase induction motor uses unequal dc sources is simulated and validated by hardware. A fundamental frequency switching control algorithm was developed and implemented. The total harmonic distortion is reduced considerably. The simulation result of stator current waveforms shows that the lower order harmonics have been reduced and also higher order harmonics are eliminated. Harmonic elimination reduces the heat generated in the stator winding of the induction motor. The torque of the induction motor is improved with the significant level due to the elimination of lower order harmonics which is the main cause for the production of negative torque.the proposed seven level cascaded H-bridge inverter can be used for the industries where the References 1 Malinowski M, Gopakumar K, Jose Rodriguez & Marcelo A Perez, A survey on Cascaded Multilevel inverters, IEEE Trans Ind Elect, 57 (010) Lai J S & Peng F Z, Multilevel converters-a new breed of power converters, IEEE Trans Ind Appl, 3 (1996) Tolbert L M, Peng F Z & Habetler T G, Multilevel converters for large electric drives, IEEE Trans Ind Appl, 35 (1999) Menzies W, Steimer P & Steinke J K, Five-level GTO inverters for large induction motor drives, IEEE Trans Ind Appl, 30 (1994) Bell S & Sung J, Will your motor insulation survive a new adjustable frequency drive? IEEE Trans Ind Appl, 33 (1997) Erdman J, Kerkman R, Schlegel D, & Skibinski G, Effect of PWM inverters on AC motor bearing currents and shaft voltages, IEEE Trans Ind Appl 3 (1996) Manjrekar M D, Steimer P K & Lipo T A, Hybrid multilevel power conversion system: a competitive solution for highpower applications, IEEE Trans IndAppl, 36 (000) Marchesoni M & Tenca P, Diode-clamped multilevel converters: a practicable way to balance DC-link voltages, IEEE Trans Ind Elect, 49 (00) Peng F Z, Lai J S, McKeever J W, & Van Covering J, A multilevel voltage-source inverter with separate dc sources for static var generation,ieee Trans Ind Appl, 3 (1996) Ebrahimi J, Babaei E & Gharehpetian G B, A new multilevel converter topology with reduced number of power electronic components, IEEE Trans Ind Elect, 59 (010) Dixon J, Pereda J, Castillo C & Bosch S, Asymmetrical multilevel inverter for traction drives using only one dc supply, IEEE Trans Vehicular Tech, 59 (010) Pereda J & Dixon J, High-frequency link: a solution for using only one dc source in asymmetric cascaded multilevel inverters, IEEE Trans Ind Elect 58 (011) Chiasson J N, Tolbert L M, Mckenzie K J & Du Z, Control of a Multilevel Converter Using Resultant Theory, IEEE Trans Control Sys Theory, 11 (1998) Chiasson J N, Tolbert L M, McKenzie K J & Du Z, Elimination of harmonics in a multilevel converter using the theory of symmetric polynomials, IEEE Trans on Control Sys Theory, 13 (005) Patel H S & Hoft R G, Generalized Techniques of Harmonic Elimination and Voltage Control in Thyristor Inverters: Part I Harmonic Elimination, IEEE Trans Ind Appl, 3 (1973)