Multi-level inverters (MLI) are power electronic converters that convert DC power to AC power with high power-quality of output voltage waveforms. These MLI are the main streamline converters for integrating dc power of EV with microgrids. Thus, the recent interest of researchers is to investigate the MLI with a lower number of active and passive switch counts which could integrate the DC power of EV to the microgrid with the boosting ability. This chapter discusses various topologies of MLI for the integration of the DC power of EV to the grid for vehicle-to-grid (V2G) applications. MLI converts DC power to AC power with high quality of output voltage waveform. Thus, the recent interest of researchers is to investigate the MLI with a lower number of active and passive switch counts which could integrate the DC power of EV to the microgrid. Also, MLI must be capable of boosting the voltage level to meet the grid requirements. The aim of this chapter is to discuss the various topologies of MLI for the integration of DC power of EV to the grid for vehicle-to-grid (V2G) applications.
Top1 Introduction
The present environmental state of earth is degrading day-by-day due to emission of greenhouse gasses and particulate matters by the energy generation and transportation sector. These harmful emissions cause climate change and other health problems. To reduce these, the usage of renewable energy sources was introduced, like solar photovoltaic, wind, fuel cell, etc. Recently, integration of dc power of electric vehicles (EV) into microgrid is the emerging topic for the researchers. On integration dc power of EVs into microgrid, the reliability of grid is increased. The technology requires dc/ac converters which have the ability of converting dc power to ac power with high power quality, i.e. low contents of lower order harmonics. Figure 1 Block diagram of microgrid shows the block diagram for integration of renewable energy resources power along with EVs’ dc power into grid.
The development of multi-level inverter (MLI) was first reported in 1981 (Nabae & Akagi, 1981). The main purpose of the MLI is to synthesize output voltage waveform like a staircase. This is
Figure 1. Block diagram of microgrid
achieved by using several separate dc voltage sources at the input side. The low order harmonics in the staircase shaped voltage waveform are low; thus, the total harmonics distortion (THD) is minimum which further reduces electromagnetic interference (EMI) and the voltage stress on the switches (Shalchi Alishah, Sabahi, Nazarpour et al, 2014). In the literature there are three classical topologies namely (a) cascade H-bridge inverter (CHB), (b) flying capacitor clamped inverter (FC), and (c) neutral point clamped inverter (NPC) as shown in Figure 2. These topologies have simple control strategies and exclusive characteristics which makes their practical realization easy and simple. These MLI finds application in FACTS, induction heating, integrating renewable energy in microgrid, vehicle-to-grid (V2G) applications, UPS, variable frequency drives and small to large scale industries. To obtain large number of steps in the output voltage waveform, the required active switches increases this in turn makes the circuitry of MLI complex. At higher voltage levels, NPC and FC suffers the problem of capacitor voltage unbalancing. Cascaded H-bridge is most feasible and flexible among conventional topologies and utilizing a lesser number of components, however it requires large number of separate dc sources, which make MLI bulkier and costlier (Roy et al., 2019).
Figure 2. MLI (a) cascaded H-bridge (b) diode clamped and (c) flying capacitor
Initially, the requirement of large dc voltage sources was a big problem as this contributes to major portion of their size and overall cost. To overcome the issue the use of dc capacitor was introduced, which focused the research on balancing the capacitor voltage as the charging and discharging property of capacitor usually disturbs the voltage levels in the output voltage waveform. The requirement of more gate driver circuits and switches was also one major drawback, which further focused the research for the development of novel topologies with reduced active switches. The classical topologies and other topologies facing the problems discussed above were replaced by switched capacitor multilevel inverter (SCMLI) introduced by O.C. Mak and A. Ioinovici in 1998 (Mak & Ioinovici, 1998).