21 Level Inverter with Reduced Number of Switches Using PV Energy Source
Mr. J. Arokiaraj, M.E., Assistant Professor
V.Vinitha,PG scholar,
Kings College of Engineering
IJTCSE Research /ISSN 2349–1582 conference publication
Abstract — An asymmetrical type of multilevel inverter for generating 21 voltage levels has been proposed in this paper. An asymmetrical multilevel inverter topology with PWM techniques using microcontroller with driver circuit and each panel of conventional PV voltage source added with multilevel inverter structure by additional switch and voltage source to obtain high voltage level. The objective of this project is to enhance the voltage level at the output with reduced number of switches. To improve the output voltage level. The advantage of proposed topology is to reduce the circuit complexity and also reduced numbers of switches, gate driver circuits, lower EMI and less Harmonic distortion in the inverter output voltage. The converter topology uses the midpoint voltage of the dc link to provide two more output voltage levels, decreasing switching power losses and EMI .The performance results of proposed 21-Level Asymmetrical multilevel Inverter are shown using MATLAB/SIMULINK software. The conventional control methods are mainly restricted to the direct and indirect control of the inverter.
Keywords- Asymmetrical model, Multilevel inverter, Pulse width modulation, Photovoltaic cell.
INTRODUCTION
Large electric drives and utility applications require advanced power electronics converter to meet the high power demands. As a result, multilevel power converter structure has been introduced as an alternative in high power and medium voltage situations. A multilevel converter not only achieves high power ratings, but also improves the performance of the whole system in terms of harmonics, dv/dt stresses, and stresses in the bearings of a motor. Referring to the literature reviews, the multilevel inverter (MI) with separated DC sources is clearly the most feasible topology for use as a power converter for medium & high power applications due to their modularization and extensibility. As a preliminary study the thesis examined and compared the most common multilevel topologies found in the published literature. Starting from the essential requirements, the
Different approaches to the construction of multilevel inverter are explained and compared. In particular, aspects of total harmonic distortion (THD) and modulation which are required or desirable for multilevel converters are discussed. Power electronic converters, especially dc/ac PWM inverters have been extending their range of use in industry because they provide reduced energy consumption, better system efficiency, improved quality of product, good maintenance, and so on. For a medium voltage grid, it is troublesome to connect only one power semiconductor switches directly. As a result, a multilevel power converter structure has been introduced as an alternative in high power and medium voltage situations such as laminators, mills, conveyors, pumps, fans, blowers, compressors, and so on. As a cost effective solution, multilevel converter not only achieves high power ratings, but also enables the use of low power application in renewable energy sources such as photovoltaic, wind, and fuel cells which can be easily interfaced to a multilevel converter system for a high power application.
The most common initial application of multilevel converters has been in traction, both in locomotives and track-side static converters [4]. More recent applications have been for power system converters for VAR compensation and stability enhancement [5], active filtering [6], high-voltage motor drive [3], high-voltage dc transmission [7], and most recently for medium voltage induction motor variable speed drives [8]. Many multilevel converter applications focus on industrial medium-voltage motor drives [3, 9], utility interface for renewable energy systems [10], flexible AC transmission system (FACTS) [11], and traction drive systems [12]. The inverters in such application areas as stated above should be able to handle high voltage and large power. For this reason, two-level high-voltage and large-power inverters have been designed with series connection of switching power devices such as integrated gate bipolar transistors(IGBTs), because the series connection allows reaching much higher voltages. However, the series connection of switching power devices has big problems [13], namely, non equal distribution of applied device voltage across series-connected devices that may make the applied voltage of individual devices much higher than blocking voltage of the devices during transient and steady-state switching operation of devices. As alternatives to effectively solve the above-mentioned problems, several circuit topologies of multilevel inverter and converter have been researched and utilized. The output voltage of the multilevel inverter has many levels synthesized from several DC voltage sources. The quality of the output voltage is improved as the number of voltage levels increases, so the quantity of output filters can be decreased. The concept of multilevel converters has been introduced since 1975. The multilevel inverter was first proposed in 1975 [14]. Subsequently, several multilevel converter topologies have been developed [16]. Although the multilevel inverter was invented earlier, its application did not prevail until the mid. The advantages of asymmetric multilevel inverters were prominent for motor drives and utility applications. The 21 inverter has drawn great interest due to the great demand of medium-voltage high-power inverters. The 21 level inverter is also used in regenerative-type motor drive applications [20, 21]. Recently, some new topologies of multilevel inverters have emerged.
MULTILEVEL INVERTER
In Proposed system, twenty one level inverter was obtained from less number of switches has been implemented in existing system. This unit produces a staircase waveform with positive polarity. It is connected to a single phase full-bridge converter, which particularly alternates the input voltage polarity and provides positive or negative staircase waveform at the output. Proposed System consists of 4 solar panel of renewable Energy sources and twelve switches of Multilevel Inverter and H-Bridge inverter. The gate signals are provided by proper switching (coding) sequences using PWM logic embedded MATLAB function. Voltage and current rating of the switches play an important role in inverters.
In this topology, current through all the switches are equal with the rated load current. This converter architecture, known as the H6 bridge, was originally developed in [22], in combination with a suitable PWM strategy, in order to keep constant the output common-mode voltage in case of a transformer less inverter for photovoltaic applications. With the same purpose, another PWM strategy for the H6 bridge was developed in [23] and [24]. In this paper, this converter structure is used to obtain a twenty one -level grid-connected converter for single-phase applications. In steady-state conditions, due to the low voltage drop across the inductance L of the output filter, the output voltage of the converter has a fundamental component very close to the grid voltage. The frequencies of these two voltages are identical, whereas the amplitude and their phase displacement are only slightly different. Depending on the modulation index value, the power converter will be driven by different PWM strategies and the output voltage levels of the power converter will be different. The comparison of multilevel inverter and conventional inverter as shown in Table 1.In each solar panel has two switches and which operates the positive semi period and negative semi period the switched transistors such as T1is ON and T2 is OFF. In T3 is ON and T4 is OFF commutates at the switching frequency. In finally the switching of the transistor of multilevel inverter generate either only positive or negative period of 21 level output voltage. Each inverter is able to generate three output voltage levels like +Vdc, -Vdc and 0.
Phase voltage (m) = 2n+1
the output signals is given to the H-Bridge inverter and which is operated also ON and OFF operation like inverter which produce proper positive and negative period of 21 level inverter of output voltage and also generate the load voltage and load current which is produced by using RL load. The gate a signal of switches takes from embedded MATLAB with PWM is used.
ASYMMETRICAL MULTILEVEL INVERTER
This proposed system presents a new topology of multilevel inverter which uses less number of switching devices(twelve switches).In level inverter used only eight switches(S1,S2…….S8) are operated at ON and OFF operation and to generate only 21 level of positive output voltage. Then another four switches (S9, S10….S12) are used in H-Bridge inverter and it finally produces proper 21 level output voltage. In proposed system of 21 level inverter with reduced number of switches using PV source is shown in fig 1. It exhibits several attractive features such as less components, simple circuit, and modular structure. In proposed work, a method is used to compute the switching angles for a multilevel converter. Harmonic analysis is done on twenty one levels by using capacitor based cross switched multilevel inverter. The proposed circuit generates a high-quality output voltage waveform and harmonic components of output voltage are low.
H-BRIDGE INVERTER
The H-bridge inverter consists of four switches and it should perform cross switched type o turn on and turn off operation in switches. The inverter is used to convert simultaneously ac voltage to pure multilevel output voltage (ac voltage level) and the all type of switches get gate signals from PWM logic embedded mat lab function. In H-Bridge inverter of simulation circuit is shown in fig 3. Finally which inverter produces 200 V output voltages (21 level output voltage).
H-BRIDGE INVERTER
The H-bridge inverter consists of four switches and it should perform cross switched type o turn on and turn off operation in switches. The inverter is used to convert simultaneously ac voltage to pure multilevel output voltage (ac voltage level) and the all type of switches get gate signals from PWM logic embedded mat lab function. In H-Bridge inverter of simulation circuit is shown in fig. Finally which inverter produces 200 V output voltages (21 level output voltage).
MATLAB FUNCTION
The MATLAB function consists of PWM logic embedded system and the gate signals of mat lab function are mainly based on coding process in mat lab. A switching sequence of coding operation for H-Bridge inverter and multilevel inverter is shown in table 2 and 3.It coding function which included the both H-bridge inverter and multilevel inverter operation
CONTROL CIRCUIT
The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4 Kbytes of Flash Programmable and Erasable Read Only Memory (PEROM). The device is manufactured using Atmel’s high density nonvolatile memory technology and is compatible with the industry standard MCS-51Ô instruction set and pin out. The on-chip Flash allows the program memory to be reprogrammed in-system or by a conventional nonvolatile memory programmer.
By combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C51is a powerful microcomputer which provides a highly flexible and cost effective solution to many embedded control applications. The AT89C51 provides the following standard features: 4 Kbytes of Flash, 128 bytes of RAM, 32 I/O lines, two 16-bit timer/counters, five vector two-level interrupt architecture, a full duplex serial port, on-chip oscillator and clock circuitry. In addition, the AT89C51 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes.
DRIVER UNIT
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 discharged.
When a transistor is switched on or off, it does not immediately switch from a non-conducting to a conducting state; and may transiently support both a high voltage and conduct a high current. Consequently, when gate current is applied to a transistor to cause it to switch, a certain amount of heat is generated which can, in some cases, be enough to destroy the transistor. Therefore, it is necessary to keep the switching time as short as possible, so as to minimize switching loss. Typical switching times are in the range of microseconds. The switching time of a transistor is inversely proportional to the amount of current used to charge the gate. Therefore, switching currents are often required in the range of several hundred mill amperes, or even in the range of amperes. For typical gate voltages of approximately 10–15V, several watts of power may be required to drive the switch. When large currents are switched at high frequencies, e.g. in DC-to-DC converters or large electric motors, multiple transistors are sometimes provided in parallel, so as to provide sufficiently high switching currents and switching power.
The switching signal for a transistor is usually generated by a logic circuit or a microcontroller, which provides an output signal that typically is limited to a few mill amperes of current. Consequently, a transistor which is directly driven by such a signal would switch very slowly, with correspondingly high power loss. During switching, the gate capacitor of the transistor may draw current so quickly that it causes a current overdraw in the logic circuit or microcontroller, causing overheating which leads to permanent damage or even complete destruction of the chip. To prevent this from happening, a gate driver is provided between the microcontroller output signal and the power transistor.
OUTPUT VALUE OF TOTAL HARMONIC DISTORTION (THD %)
The output value of total harmonic distortion (THD) in proposed system is around 2.96 % is shown in fig 7 by using FFT analysis. If the value of THD is based on output voltage of 21 level inverter (act as input value) and which total harmonic Value is reduced as compared to existing system. Multilevel inverters also have several advantages like reduced total harmonic distortion (THD) as compared to existing system operation. Compared to cascaded multilevel inverter, proposed structure needs less number of components and simple control methods.
The main advantages of capacitor switched multilevel inverters are good output waveform, low switching stress as compared to existing system. Higher voltages can be generated using the devices of lower rating and switching frequency can be reduced for PWM technique. The advantages of the proposed inverter are reduction of number of switches, reduction of gate driver circuits, reduction of isolated power supply unit for gate diver, lower EMI and less harmonic distortion in the inverter output voltage.
CONCLUSION
The proposed system which utilizes four asymmetric dc sources and twelve powers switches with load. By proper switching sequence, the proposed inverter produces a 21 level output voltage with low switching losses; by operating the switches at fundamental frequency. The advantages of the proposed inverter are reduction of number of switches, reduction of gate driver circuits, reduction of isolated power supply unit for gate diver, lower EMI and less harmonic distortion in the inverter output voltage. The computer aided simulation study has been carried out to validate the performance of proposed inverter. The proposed inverter can be recommended for distributed generators, stand alone and grid connected renewable energy power conversion applications.
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