3 PHASE INVERTER WITH 180 AND 120 CONDUCTION MODE

Transcription

1 3 PHASE INVERTER WITH 180 AND 120 CONDUCTION MODE Mahendra G. Mathukiya 1 1 Electrical Department, C.U. Shah College of Engineering & Technology Abstract Today most of the appliances and machine works on AC power. If the AC supply is not available for limited time period at that time we need to convert stored DC power in to AC power. This can be done by the power electronics equipment called as an Inverter. Basically inverter uses a power electronic switch as a form of an array. Different types of inverters are available in market for different purpose. By applying different patterns of switching of array gives an appropriate output. the simulation work for 3 phase Inverter with 180 conduction mode and 120 conduction mode for resistive load is presented. With the help of MATLAB simulation, control strategy for 180 conduction mode and 120 conduction mode is developed. Simulated phase voltage waveforms, line voltage waveforms and THD analysis of inverter for both the conduction modes are represented. Finally, both the conduction modes are compared by various parameters related to inverter. Keywords 3 Phase Inverter, 180, 120, Conduction mode, Mat lab I. INTRODUCTION From the late nineteenth century through the middle of the twentieth century, DC-to-AC power conversion was accomplished using rotary converters or motor-generator sets (M-G sets). In the early twentieth century, vacuum tubes and gas filled tubes began to be used as switches in inverter circuits. The most widely used type of tube was the thyratron. The origins of electromechanical inverters explain the source of the term inverter. Early AC-to-DC converters used an induction or synchronous AC motor direct-connected to a generator (dynamo) so that the generator's commutator reversed its connections at exactly the right moments to produce DC. A later development is the synchronous converter, in which the motor and generator windings are combined into one armature, with slip rings at one end and a commutator at the other and only one field frame. The result with either is AC-in, DC-out. With an M-G set, the DC can be considered to be separately generated from the AC; with a synchronous converter, in a certain sense it can be considered to be "mechanically rectified AC". Given the right auxiliary and control equipment, an M-G set or rotary converter can be "run backwards", converting DC to AC. Hence an inverter is an inverted converter. DC-to AC converter is known as an inverter. The function of inverter is to change a DC input voltage to symmetric AC output voltage of desired magnitude and frequency. The output voltage could be fixed or variable at a fixed or variable frequency. A variable output voltage can be obtained by varying the dc input voltage and maintaining the gain of inverter constant. On the other hand, if the DC input voltage is fixed and it is not controllable, a variable output voltage can be obtained by varying the gain of the inverter, which is normally accomplished by pulse with modulation control within the inverter. The inverter gain may be defined as the ratio of the AC output voltage to the DC input voltage. The output voltage waveforms of an ideal inverter should be sinusoidal. However, the waveforms of practical inverter is non-sinusoidal and contain certain Harmonics. For low and medium-power applications, square-wave or quasi-square-wave voltages may be acceptable; and for high-power applications, low distorted sinusoidal wave-forms are required. With the availability of high-speed power semiconductor devices, the harmonic contents of output voltage can be minimized or reduced significantly by switching techniques. DOI: /IJMTER RHSM1 113

2 II. DESIGN AND IMPLEMENTATION OF THREE PHASE INVERTER FED TO DRIVE THREE PHASE MOTOR Inverters are used in a wide range of applications; there are various type available by different aspects. By conduction mode there are also 150, 180 &120 conduction mode used. But the 150 is a recently advanced research. New modification for the most common, simple and wellknown three-phase six-switch voltage source inverter (VSI). In this modification, each one of the six transistors conducts for 150 instead of the known 180 or 120 conduction modes. For a star connected load, the output phase voltage becomes a seven level, 12 steps waveform. This result in a 50% reduction of the total harmonic distortion (THD), 75% reduction of voltage distortion factor (DF), and the lowest harmonic order (LOH) becomes 11 rather than 5. In IGBT based Inverter, we need to control the pulse of switching signal given to IGBTs Hex- Bridge Module. This job can be done by PIC16F72 Microcontroller. Easily developed PWM pulses calculation of the PWM period and inbuilt analog-to digital (A/D) converter module can have up to eight analog inputs for a device. So, overall the THD can be decrease by use of this type of technique and thee power loss can be minimized. THD will directly proportional to the step of output voltages.so the number of step increases in output causes low THD level. III. BLOCK DIAGRAM Figure 1: Block diagram of Inverter 3.1 AC Source An ac source is ordinary supply of rectifier, which has any frequency and voltage rating.which is practically constant supply of 3 phase or 1 phase. Before this block there is switch to operate supply and protection system by fuse. Rectifier A rectifier is converts alternating current (AC) to direct current (DC). A diode is like a oneway valve that allows an electrical current to flow in only one direction. This process is called rectification. Power circuit Power circuit consists of bridge of six switches. For 3 phases it has 3 legs. In each leg it has two switches. Basically the switch is IGBT or MOSFET. Driver Circuit In contrast to bipolar transistors, MOSFETs do not require constant power input, as long as they are not being switched on or off. The isolated gate-electrode of the MOSFET forms a capacitor (gate capacitor), which must be charged or discharged each time the MOSFET is switched on or off. As a transistor requires a particular gate voltage in order to switch on, the gate capacitor must be charged to at least the required gate voltage for the transistor to be switched on. Similarly, to switch the transistor off, this charge must be dissipated, i.e. the gate capacitor must be All rights Reserved 114

3 S 1 S 3 S 5 A DC SOURCE B C S 4 S 6 S 2 Figure 2 Circuit Diagram of Three Phase Inverter 3.2 Operation Of Three Phase Inverter A. 180 Degree Conduction Mode Mode S 1 S 2 S 3 S 4 S 5 S 6 1 st ON OFF OFF OFF ON ON 2 nd ON ON OFF OFF OFF ON 3 rd ON ON ON OFF OFF OFF 4 th OFF ON ON ON OFF OFF 5 th OFF OFF ON ON ON OFF 6 th OFF OFF OFF ON ON ON Table 1.Switching of 180 Degree Conduction mode In the three phase inverter of each switch conduct 180 of cycle, thyristor pair in each arm i.e. S 1, S 4 ; S3, S6 and S5, S2 are turned on with a time interval of 180. It means that S1 conduct for 180 and S4 for the next 180 of a cycle. Switch in the upper group i.e. S1, S3, S5 conduct at an interval of 120. It implies that if S1 is fired at ωt=0, then S3 must be fired at ωt=120 and S5 at ωt=240. Same is proved lower group of switches. On the basis of this firing scheme, a table in prepared as shown at the top. In this table, first row show that S1 from upper group conducts for 180, S4 for the next 180 and then again S1 for 180 and so on. In the second row, S3 from the upper group is shown to start conducting 120 after S1 starts conducting. After S3 conduction for 180, S6 conducts for the next 180 and again S3 for the next 180 and so on. Further, in the third row, S5 from the upper group start conducting 180 after S3 or 240 after S1. After S5 conduction for 180, S2 conducts for the 180, S5 for the next 180 and so on. In this manner, the pattern of firing the six switch is identified. Tis table show that S5, S6, S1 should be gated for step I; S6, S1, S2 for step II ; S1, S2, S3 for step III ;S2, S3, S4 for step IV and so on. Thus the sequence of firing the thyristor is S1, S2, S3, S4, S5, S6; S1, S2. It is seen from the table that is every step of 60 duration, only three switch are conducting one from upper group and two from the lower group or two from the upper group and one from the lower group. B. 120 Degree Conduction Mode Mode S1 S2 S3 S4 S5 S6 1 st ON OFF OFF OFF OFF ON 2 nd ON ON OFF OFF OFF OFF 3 rd OFF ON ON OFF OFF OFF 4 th OFF OFF ON ON OFF OFF 5 th OFF OFF OFF ON ON OFF 6 th OFF OFF OFF OFF ON ON Table 2.Switching of 120 Degree Conduction All rights Reserved 115

4 The power circuit diagrams of this inverter is the same as that shown. For the 120 degree mode VSI, each thyristor conducts for 120 of a cycle. Like 180 mode, 120 mode inverter also requires six steps, each of 60 duration for completing one cycle output AC voltage. For this inverter too, a table giving the sequence of firing the six thyristor is prepared as shown in the top. In this table, shown that even conducts for 120 and for the next 60 neither S1 nor S4 conducts. Now S4 is turned on at ωt=180 is further conducts for 120, i.e. from ωt=180 to at ωt=300. This means that for 60 interval from ωt=120 to ωt=180, series connected switch S1, S4 do not conduct. At ωt=300, S4 is turned off, then 60 interval elapses before S1 is turned on again at ωt=360. In the second row, S3 is turned on at ωt=120 as in 180 mode inverter. Now S3 conducts for 120, then 60 interval elapses during which neither S3 nor S6 conducts. At ωt=300, S6 is turned on, it conducts for 120 and then 60 interval elapses after which S3 is turned on again. The third row is also completed is similarly. This table show that S6, S1 should be gated for step I ; S1, S2 for step II ; S2,S3 for step III and so on. The sequence of firing the six thyristor is the same as for the 180 mode inverter. During each step, only two thyristors conducts for this inverter one from the upper group and one from the lower group; but in 180 mode inverter, three thyristors conduct in each step. 3.3 CIRCUIT SIMULATION & RESULTS Figure 3 Simulink Model of Three Phase Inverter A. Simulation Results for 180 Conduction Mode: Figure 4 Gate Voltage All rights Reserved 116

5 Figure 5 Voltage Waveform of Phase-A Figure 6 Voltage Waveform of Phase-B Figure 7 Voltage Waveform of Phase-C Figure 8 FFT Analysis of Line Voltage Waveform Fast Fourier transform analysis is shown in above fig. The contribution of 5 th harmonic is 19%, 7 th harmonic is 15% and so on. The total harmonic distortion is 30.90%. The effect of harmonics of multiple of 3 rd is minimized in line voltage All rights Reserved 117

6 IV. CONCLUSION From the above experiments it can be concluded that for 180 conduction mode magnitude of output voltages greater then 120 conduction mode. In 180 conduction mode it is require to provide a dead time mode between two switches in one leg while in 120 conduction mode there is no requirement of dead time mode.from the below table it can be say that THD in both the conduction mode is near to same. Table 1 comparision of result Parameter Theoretical Practical Phase Voltage Line Voltage Line/Phase Current Fundamental phase voltage Fundamental line voltage T.H.D.(Phase) T.H.D.(Line) REFERENCES [1] Bandana Bhutia,Dr S.M.Ali and Narayan tiadi, Design of Three Phase PWM Voltage Source Inverter For Photovoltaic Application, ijireeice,vol. 2, Issue 4,April 2014 [2] Navdeep Singh, Comparative Study between Traditional Inverers, ijareeie, Vol. 3, Issue 6, June 2014 [3] Ananya Dutta Some Aspects on 3 Phase Bridge Inverter (180 Degree Modes, ijeijournal., olume 3, Issue 4(November2013) PP:18-21 [4] Xiangli Li, Zhaoyang Yan, Yanni Gao, Hanhong Qi, The Research of Three-phase Boost/Buck-boost DC-AC Inverter, Energy and Power Engineering,SciRes 2013, 5, [5] Bhimbra P.S., Power Electronics, Khanna Publishers, 4 th edition [6] David Tam, New 1200 V Integrated Circuit Changes the Way 3-Phase Motor Drive Inverters are Designed, Internatinal Rectifier, E1 Segundo, All rights Reserved 118