Introduction
At its core, the power in the battery is in DC mode and the motor that drives the wheels usually uses AC power, therefore there should be a conversion from DC to AC by a power converter. Inverters can do this conversion. The simplest topology that can be used for this conversion is the two-level inverter that consists of four switches. Each switch needs an anti-parallel diode, so there should be also four anti parallel diodes.
Multilevel Inverter
In the dynamic realm of power electronics, the beginning of multilevel inverters has marked a revolutionary stride towards achieving higher power efficiency and quality. Power electronics 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 structure has been introduced as an alternative in high power and medium voltage situations. The term multilevel began with the three-level converter. Subsequently, several multilevel converter topologies have been developed.
However, the elementary concept of a multilevel converter to achieve higher power is to use a series of power semiconductor switches with several lower voltage dc sources to perform the power conversion by synthesizing a staircase voltage waveform. Capacitors, batteries, and renewable energy voltage sources can be used as the multiple dc voltage sources.
Background
The concept of multilevel converters has been introduced since 1975. The cascade multilevel inverter was first proposed in 1975. Separate DC-source full-bridge cells are placed in series to synthesize a staircase AC-out put voltage.
The term multi-level began with the three-level converter. Subsequently, several multi-level converter topologies have been developed. In 1981, diode-clamped multilevel inverter also called Neutral-Point-Clamped (NPC) inverter schemes was proposed. In 1972, capacitor –clamped multilevel inverters and in 1996 cascade multilevel inverter were proposed.
The simplest techniques involved the parallel or series connection of conventional converters to form the multilevel waveforms. More complex structures effectively insert converters with in converters. The voltage or current rating of the multilevel converter becomes a multiple of the individual switches, and so the power rating of the converter can exceed the limit imposed by the individual switching devices.
Key Benefits
Staircase waveform quality: Multilevel converters not only can generate the output voltages with very low distortion, but also can reduce the dv/dt stresses; therefore, electromagnetic compatibility (EMC) problems can be reduced.
- Common-mode (CM) voltage: Multilevel converters produce smaller CM voltage; therefore, the stress in the bearings of a motor connected to a multilevel motor drive can be reduced. Furthermore, CM voltage can be eliminated by using advanced modulation strategies.
- Input current: Multilevel converters can draw input current with low distortion.
- Switching frequency: Multilevel converters can operate at both fundamental switching frequency and high switching frequency PWM. It should be noted that lower switching frequency usually means lower switching loss and higher efficiency.
Key Applications
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.
- Energy storage devices like ultra capacitors and batteries can also be used with multilevel inverters.
- Another possible application of multilevel converters is their use in Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs). One reason is that multilevel converters, EVs, and HEVs are all ideally suited for utilization of a large number of relatively small-sized energy sources, such as batteries and fuel cells. Also, multilevel converters generally allow for smaller components, thus reducing weight.
Author :- Alka Yadav
Assistant Professor
Vikrant University, Gwalior
