Abstract¬_Renewable energy is environmental friendly and popular alternative source of energy, which can be used to provide electricity and to meet the load demand during power outage. This paper deals with various Multilevel Inverter topologies used in Renewable Energy Sources. The four different commonly used Multilevel Inverter topologies are Neutral Point Clamped, Flying Capacitor, Cascaded Inverter and Hybrid Inverters1. Each topology have their own features with corresponding advantages and disadvantages when used in a Renewable Energy Power system.
The Review is made in the aspects of Construction Complexity, Total Harmonic Distortion (THD), Controlling methodologies and Components required. Multilevel inverter is used to completely remove the passive filtering requirement at the grid interfacing end result in grid improved efficiency and reduced cost1. I. INTRODUCTION Energy plays a vital role for development of a country. To fulfill the energy demand in condition of increasing population, urbanization and industrialization for development of country is major issue 3. The main source of energy is non-renewable resources like coal, petroleum and natural gas but, it will be exhausted in a few hundred years 3. It also causes the environmental issues like CO2 emission and greenhouse effect which deteriorates the environment3.Over the past several decades, depleting fossil fuels and environmental concerns have led electricity production from renewable energy sources as an essential alternative to the energy crisis5.
By interacting the renewable energy sources with the traditional grid we can overcome this energy crises. As some of the renewable energy sources produce DC or we convert some of AC into DC for storage purpose in case of fault in traditional grid it can standalone and fulfill the requirement. For conversion from DC to AC we need inverter.
Fig. 1 shows the block diagram of a renewable energy generation system using a multilevel inverter 4. It integrates a variety of renewable sources, such as solar energy, wind energy, tide energy and so on with the traditional grid and they are connected to a converter to generate DC power, which is stored in a capacitor or battery.
After connected to a multilevel inverter, DC power is converted into AC power 2. It is evident that the multilevel inverter capable of converting a single DC voltage source from a capacitor or battery into an AC voltage source is a key element of most stand-alone renewable energy generation systems. Various multilevel inverter topologies were proposed in the past decade which have been extensively studied for renewable energy integration systems 2. Efficiency of Renewable energy sources is comparatively less than the conventional fossil fuels, so improvements are made on either side for the purpose of power quality improvement and increase the usage of Renewable Energy sources. In the source side MPPT tracking is implied whereas in the converter side increasing the output levels of Multilevel Inverter and reduction of Total Harmonic distortion is done to increase the performance of Renewable Energy Sources. II .
LITERATURE SURVEY For integration and interfacing renewable energy sources (RES) to the existing grid the multilevel inverter is one of the important and necessary components to convert the DC power of the RES to the required AC level. Due to reduced disturbances and operational at lower switching frequency makes the Multilevel Inverter more suitable for Renewable Energy sources. Multilevel Inverter provide smoother output waveforms when the levels are increased simultaneously the Total Harmonic distortion is also reduced 9,14. The number of levels is inversely proportional to Total Harmonic Distortion such that at infinity levels the THD becomes zero15. Increasing the levels increase component requirements and control complexity, so selection of appropriate topology is necessary to overcome the above disadvantages.
Many multilevel inverter topologies are used in the literature for this purpose The basic topologies of the existing multilevel converters are divided into four types: The cascaded multilevel inverter topology8, the diode clamped multilevel inverter topology 10, the flying capacitor multilevel inverter topology13 and the hybrid multilevel inverter topology8. Every topology has its own advantages and disadvantages. Research and efforts is for the topology which have good voltage and frequency control, greater number of AC levels and small number of switches and their drive circuits etc. Several modern topologies with different features are proposed in the literature their detail study is as follow.
(1) Diode Clamped Multilevel Inverter Diode Clamped or Neutral point Clamped Multilevel Inverters has diode which clamps the source DC voltage to achieve steps in the output waveform 6. In a diode clamped Inverter to achieve N levels 2(N-1) switches, (N-1)(N-2) Diodes for Clamping and (N-1) capacitors for DC link is required6. The structure of a three phase six level Diode clamped Multilevel Inverter is shown in figure 2. The source Vdc is split into different voltage levels by using capacitor C1-C5 connecting them in series. The semiconductor switches S1-S5 and S¬1- S¬5 should allow the entire DC voltage from capacitors when switched on using PWM pulse respectively, but the diode D1- D4 should block different voltage levels such that D1 should block 4 levels decreasing down so that D4 should block 1 level so the step waveform is achieved in the output. Figure 2. Six level Three Phase diode clamped inverter Advantages: (i) The number of sources required is low as the capacitors can share a single source.
(ii) At higher output levels there are filters required because the THD is reduced when the levels are increased. (iii) During fundamental frequency switching the efficiency is high. (iv) Pre-charging of the capacitors in group is possible. Disadvantages: (i) Difficult to monitor and control overcharging of DC levels in the inverter thus affecting the Real power flow (ii) To achieve higher number of levels higher number i.e., (N-1)(N-2) clamping diodes is required