International journal of recent advances in engineering & technology (IJRAET)
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Induction Motor Drive System Using Push-Pull Converter and Three Phase Multilevel Inverter Fed from Solar Photovoltaic Panel
1Sridhar S, 2Darshini C
1,2Dept. of EEE, RNSIT, Bangalore, India
Abstract— The Project proposes a topology of Induction Motor drive system integrating a Push-Pull converter and 5-level Multilevel Inverter using a single solar photovoltaic panel. To match impedance between the solar panel and motor load and to step up the panel voltage, a dc-dc Push- Pull topology is employed. To obtain optimum motor performance and to reduce total harmonic distortion of the inverter output waveform, we employed sinusoidal pulse width modulation (SPWM) technique for switching of inverter power circuit. Maximum power point tracking algorithm (Perturb & Observe) is used to extract maximum power from photovoltaic panel. Finally the simulation results are discussed and analysed in order to verify the effectiveness of the proposed design.
Index Terms— DC-DC Push-Pull Converter, Induction Motor, Maximum Power Point Tracking, Power Electronics, Photovoltaic panel, SPWM, Three-Phase Multilevel Inverter
I. INTRODUCTION
Photovoltaic power provides an environment friendly green source of electricity, of which the fuel is sunrays, a renewable energy [1]. However, widespread use of fossil fuels has caused environmental pollution especially emission of Carbon-di-oxide from vehicles and power plants, that affect the global climate and temperature. The human concern for the environment and the way our modern technology deteriorates has aroused scientists and engineers all over the world to explore the cleaner renewable energy based electricity generation. Photovoltaic (PV) is one such technology first observed and Implemented by Alexander-Edmond Becquerel in 1839, where solar energy is converted to electricity. A power electronics interface is essential in order to run the three-phase induction motor using the solar panel. Thus using power electronics interface like dc-dc push-pull converter merge with a multilevel inverter, it is possible to transfer the power efficiently from the panel to the machine useable sinusoidal ac with the help of Maximum Power Point Tracking (MPPT) [2- 7], where MPPT works by changing the parameters of the power electronics components in order to obtain the maximum power available at that moment of the panel.
Hence, Perturb and Observe (P&O) technique is implemented to accomplish this task. According to
of induction motor. However, these techniques suffered from several drawbacks such as low fundamental output voltage, excessive amount of harmonic element, and higher value of Total harmonic Distortion (THD), which is harmful and resulting in poor performance of induction motor. To overcome this problem we employed SPWM technique in order to control the inverter and operate the motor drive system effectively.
II. PROPOSED TOPOLOGY
The proposed topology presents mainly an Induction Motor drive system integrating with a push-pull converter and 5-level multilevel Inverter using a single solar photovoltaic panel. To match impedance between the solar panel and motor load and to step up the panel voltage, a DC-DC Push-Pull topology is employed.
Sinusoidal Pulse Width Modulation (SPWM) technique is employed to control inverter switching in order to reduce the total harmonic distortion ofthe inverter output waveform and to obtain optimum motor performance.
Sinusoidal Pulse Width Modulation is commonly used multilevel inverter control method. This requires multiple triangular carriers which are modulated by sine wave. The different types of SPWM technique such as Phase Disposition (PD), Alternate Phase Opposite Disposition (APOD) and Gapped Alternate Phase Opposite Disposition (GAPOD) are employed. The simulation is carried out for different switching frequency and modulation indices. Speed of motor is controlled by proper switching of the inverter. THD under different switching techniques is determined.
Maximum Power Point Tracking technique is used to extract maximum power from the panel. The speed of the induction motor is controlled by switching the inverter as per the requirement.Fig.1 shows the block diagram of proposed system.
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2.1 Photovoltaic Panel
PV array is a p-n junction semiconductor, used to convert sunlight into electrical energy. When the incoming solar energy exceeds the band-gap energy of the module, photons are absorbed by materials to produce electricity. The cells in the PV array are tied in series or parallel and the electrical power of the PV array depends upon the solar irradiance, panel temperature and the operating current and voltage relationship. The current voltage characteristics, (VI)characteristic of PV array is a complex and non- linear function. The output voltage from the PV panel is 30V
2.2 DC-DC Push-Pull Converter
To achieve maximum power point tracking of the photo voltaic panel, the DC-DC push-pull converter topology is implemented and reported [3]. Switch mode DC-DC converters efficiently convert an un-regulated DC input voltage into regulated DC output voltages. Compared to linear power supply the switching power supply offers much more efficiency and power density. Switching power supply includes solid-state devices such as transistors and diodes to operate as a switch either completely turn on or completely turn-off. The basic push-pull converters consist of inductors, capacitors, diodes, transistors and transformer to step-up or step- down a voltage input. The Fig. 3 shows the push-pull converter circuit. When designing a push-pull converter, it is convenient to select the transformer turns ratio n such that duty cycle D does not vary in wide range [4].
The transformer turns ratio is 3:22. The push-pull input voltage is the MPPT panel array voltage. Thus given the motor output power, it is possible to numerically search the push-pull input voltage [5]. Thus, our design we implemented a DC-DC push-pull converter, that successfully steps-up PV arrays 30V DC output voltage into 220V DC in case of steady environmental condition, build in SIMULINK as shown in Fig.2.and the simulated output voltage of push-pull converter is as shown in the Fig 3.
Fig 2: Push-Pull Converter Circuit
Fig 3: Simulated Push-Pull Converter output voltage 2.3 Maximum Power Point Tracking (MPPT)
Maximum Power Point Tracking (MPPT) is very important in solar power system because it minimizes the solar array cost by decreasing the number of solar modules required to achieve the desired output power.
Fig.4: MPPT, P&O algorithm flow chart.
MPPT is a device that looks for the maximum power point of a source and maintains operating in that point.
The PV is not always operating in its maximum power point, but with the use of an MPPT it is possible to force the PV to extract the maximum power at the given irradiance level. We used P&O MPPT algorithm due to its simplicity and easy of implementation [2]. This technique by is easily implemented by an algorithm using the power-voltage characteristics of the PV module. Knowing that at the right and the left of the maximum power point the power decrease, the converters duty cycle is changed depending on the last change in power and if the duty cycle was increased or decreased. Fig.4: MPPT, P&O algorithm flow chart. To implement the P&O the power needs to be read at a time U, afterwards the voltage is changed. Next the power in time U+ I is read, if this power is incrementing we increment the duty ratio and by consequence the voltage in the PV. In the case that the power in the U+ 1 is lower than in the U time we decrement the duty ratio and by consequence the voltage. This technique operates in the boundaries of the MPP. The MPPT algorithm developed for this application is responsible for deploying the necessary adjustment in the Push-Pull Converter's duty cycle so that the optimum voltage is achieved, thus allowing maximum power delivery to the load [3]. Fig. 4 shows the P&O, MPPT algorithm varying the push-pull converter duty cycle to obtain the maximum power delivered by PV panel.
2.4 Multilevel inverters
MLI concept was first introduced in 1975. It is a power conversion device which produces an AC output voltage by using dc power sources. Multilevel inverters can produce a waveform of desire single or three phase voltage. Multilevel voltage source inverter reduces the THD of the output voltage and does not require transformers. With the increasing number of voltage levels, the power handling capability increases. Also the output waveform of staircase shape approaches to sinusoidal wave with minimum voltage stress. For increase in every level, the number of semiconductor power switches is increased.
Advantages: The advantages of multilevel inverters are as follows:
1) Output voltage generated is of less distortion and lower dv/dt.
2) Input current drawn is with less distortion.
3) Smaller common-mode voltage which is generated can be Eliminated by using Modulation Technique.
4) Size of the filter is reduced.
5) Generates lower electromagnetic interference.
Disadvantages:
One particular disadvantage is the greater number of power Semiconductor switches. Although lower voltage rated switches can be utilized in a multilevel converter, each switch requires a related gate drive circuit. Hence if the switches are more, the overall system becomes more expensive, reduces reliability and increases circuit complexity.
Applications:
In the area of power electronics, more development has taken in the field of multilevel power conversion technology. Multilevel inverters find applications in medium-to-high voltage range, motor drives, power distribution and power conditioning applications. In recent years, industry demands power in the megawatt level. Controlled ac drives in the megawatt range are usually connected to medium voltage network.
2.5 CASCADED H-BRIDGE (CHB) INVERTER Compared to diode clamped and flying capacitor inverter, CHB multilevel inverter does not require clamping diodes and balancing capacitors. Therefore it is widely used due to modular layout structure and lesser number of components. In this topology, the H bridge (single phase full bridge) inverters are connected in series. The desired output voltage is obtained by MLI from isolated dc source connected to each H-bridge.
Sources are batteries and renewable energy sources such as photovoltaic and fuel cells. Number of H-bridge inverter increases as the number of level increases. The cascaded H bridge inverter circuit is as shown in the Fig.
Advantages and disadvantages of CHB MLI:
Advantages
1) Requires least number of components among all types of Multilevel inverters.
2) Optimized circuit layout and packaging are possible.
3) Soft switching techniques can be used to reduce switching losses and device stresses
Disadvantages
1) Needs separate dc sources for real power conversions, thus its applications are limited
Fig.5: Three phase 5 level inverter
Fig.6: Phase voltage of 5 level Inverter
Fig.7: Line to Line voltage of 5 level Inverter 2.6 Pulse Width Modulation
Different control techniques are available for CHB MLI.
Among all techniques, PWM control strategy reduces the total harmonic distortion of the output voltage. In multi carrier PWM technique, modulating signal can be sinusoidal and carrier signal is triangular wave. When the sine waveform intersects the triangular waveform, triggering pulses to the inverter are generated. For m level MLI, (m-1) carrier signals are required. The Three
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each other. Multilevel sinusoidal pulse width modulation (SPWM) can be classified as shown in Fig.8.
Fig.8: Classification of Sinusoidal PWM In multicarrier SPWM, each triangular carrier wave is compared with modulating bipolar sine wave. If the sine wave amplitude is greater than the triangular wave signal amplitude, the switch related to that triangular signal is turned on, and if the sine wave amplitude is less than the triangular wave signal amplitude, the switch related to that triangular signal is turned off. The switching frequency of the inverter is defined by carrier frequency.
2.7 Types of SPWM
Sinusoidal Pulse Width Modulation is commonly used multilevel inverter control method. This requires multiple triangular carriers which are modulated by sine wave. Some types of SPWM technique [14] such as Phase Disposition (PD), Alternate Phase Opposite Disposition (APOD) and Gapped Alternate Phase Opposite Disposition (GAPOD) are explained below.
1) Phase Disposition (PD):
In PD-PWM, all the triangular carriers are in phase.
Modulating and carrier signal arrangement for the PD- PWM technique
With ma=1 is as shown in Fig.9. Total Harmonic Distortion is about 28.61%.
Fig.9 Sinusoidal reference with triangular carriers for a 3-phase 5-level PD-PWM scheme
2) Alternate Phase opposite Disposition (APOD):
In APOD-PWM technique, each triangular carrier signal is shifted by 180° from its adjacent carrier signal.
Modulating and carrier signal arrangement for the APOD-PWM technique with ma=1 is as shown in Fig.10. when ma=1.THD obtained in this method is about 21.87%.
Fig.10 Sinusoidal reference with triangular carriers for a 3-phase 5-level APOD-PWM scheme
3) Gapped Alternate Phase Opposite Disposition (GAPOD):
This is similar to APOD arrangement, but gap is maintained between successive alternate carriers. To obtain m level voltage output, where (m-1) carriers are required, the peak to peak carrier amplitude Vc =+/-(m- 1) and peak to peak amplitude of the modulating signal Vm=+/-(m-1) when ma=1. Modulating and carrier signal arrangement for the GAPOD-PWM technique with ma=1 is as shown in Fig.11. Since the value of Vm is higher than the APOD, it has smaller switching angle and thus further reduces THD than APOD PWM technique. Total Harmonic Distortion in this method is very less and is about 15.77%.
Fig.11 Sinusoidal reference with triangular carriers for a 3-phase 5-level GAPOD-PWM scheme 2.8 Induction Motor (IM)
An induction motor is an example of asynchronous AC machine, which consists of a stator and a rotor [10]. This motor is widely used because of its strong features and reasonable cost. A sinusoidal voltage is applied to the stator, in the induction motor, which results in an induced electromagnetic field. A current in therotor is induced due to this field, which creates another field that tries to align with the stator field, causing the rotor to spin. A slip is created between these fields, when a load is applied to the motor. The rotor speed decreases at the higher slip values. The frequency of the stator voltage controls the synchronous speed [11-12]. The frequency of the voltage is applied to the stator through power electronic devices, which allows the control of the speed of the motor. The research is using techniques, which implement a constant voltage to frequency ratio. Finally, the torque begins to fall when the motor reaches the synchronous speed. Thus, induction motor synchronous speed is defined by following equation, Ns = 120f / P Where, f is the frequency of AC supply, n, is the speed of rotor; p is the number of poles per phase of the motor.
By varying the frequency of control circuit through AC supply, the rotor speed will change.
III. SIMULATION CIRCUITAND RSULTS
The proposed design is verified by computer simulation using SIMULINK software package. Our proposed system design consists of a solar panel followed by a series of power electronics interface that controls the speed of an induction motor. At first, Sanyo HIP- 210HKHA6 [13] panel with 210 watts maximum power output under Standard Test Condition (STC) is used. At STC condition of 25-degree temperature and irradiance of 1000 W/m2 the panel is stimulated. Next, the push- pull converter is coupled with the solar panel which successfully steps-up PV panel 30V DC into 220 VDC and then fed into three-phase SPWM inverter. To keep push-pull converter duty cycle optimal for maximum
Fig.12: MPPT (P&O) Technique developed in SIMULINK
Maximum Power Point Tracking Technique is employed to extract maximum power from the PV panel. Usually Perturb and Observe method is used because of simple and easy for implementation. MPPT (P&O) technique developed in SIMULINK is shown in Fig.12 For proper switching of the Multilevel Inverter Sinusoidal Pulse Width Modulation Technique is employed. Compared to the conventional PWM Techniques SPWM is widely used which increases the performance and output of the system. The SPWM Switching for the multilevel inverter is developed in Simulink and shown in Fig.13.
And also harmonic behaviour of different types of Switching can analysed by observing the Waveforms as shown earlier.
Fig.13: SPWM Switching
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By controlling the Inverter Switching by using SPWM Technique the speed of the Induction Motor can controlled. The 5-level Multilevel Inverter with induction Motor build in SIMULINK is shown in Fig.14.
The waveforms of rotor current, stator current, speed of the induction motor and torque from the motor are shown in Fig.15, Fig.16, Fig.17 and Fig.18 respectively
Fig.15: Rotor current of Induction Motor
Fig.16: Stator current of Induction Motor
Fig.17: Speed of Induction Motor
Fig.18: Electromagnetic torque of induction motor
IV. CONCLUSION
The Induction Motor can be effectively driven by single photovoltaic panel. Sinusoidal Pulse Width Modulation Techniques usually increases the output voltage of the inverter and decreases the Total Harmonic Distortion Value. Finally the system performance is increased by the application of SPWM Technique. Perturb and Observe MPPT Technique is implemented successfully
which extract the maximum power from the PV panel by adjusting duty cycle of the Push-Pull Converter switches. The motor speed is controlled by controlling the switching sequence of the Multilevel Inverter and also by increasing the level of Multilevel Inverter the efficiency of the system is increased.
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