Multilevel Inverter

Introduction

The voltage source inverter produces voltage or current with levels either 0 or ±V_DC. They are known as the three level inverters. To obtain a quality output voltage or current waveform with minimum amount of ripple content, they require high switching frequency along with different PWM strategies. In high power and high voltage applications, these three level inverters, however, have some limitations in operating at high frequency due to the switching losses and constrains of device ratings.

A multilevel inverter is one in which a complete cycle of output waveform contains more than three DC levels. It may be easier to produce a high power, high voltage inverter with the multilevel structure because of the way in which device voltage stresses are controlled in the structure. Increasing the number of voltage levels in the inverter without requiring higher ratings on individual devices can increase the power rating. A multilevel inverter not only achieves high power ratings, but also enables the use of renewable energy sources. Renewable energy sources such as photovoltaic, wind, and fuel cells can be easily interfaced to a multilevel inverter system for a high power application.

Types of multilevel inverter

The multilevel inverters can be classified into three types

(i) Diode clamped multilevel inverter

(ii) Flying capacitor multilevel inverter

(iii) Cascaded multilevel inverter

Cascaded Multilevel inverter

A cascaded multilevel inverter consists of a series of H-bridge (single phase full bridge) inverter units. The general function of multilevel inverter is to synthesize a desired voltage from several separate DC sources (SDCSs), which may be obtained from batteries, fuel cells, or solar cells. Fig. 4.1(a) shows the basic structure of single phase cascaded inverter. The AC terminal voltages of different level inverters are connected in series. In general in cascaded multilevel inverter n DC source or n H-bridges are connected for 2n+1 level. So, in fig. 4.1(a) it is shown that three H-bridges are connected in series to produce seven levels in output voltage.

Fig. 4.1 (b) shows the synthesized voltage waveform of seven-level cascaded inverter with three SDCSs. The phase output voltage is synthesized by the sum of three different voltage outputs, V_AN = V_H1 +V_H2 +V_H3. Each inverter level can generate three different voltage outputs, +E, 0, -E, by connecting the DC source to AC output side by different combinations of the four switches, S₁, S₂, S₃, and S₄. Using top level as the example, turning ON S₁₁ and S₂₁ yields V_H1= +E. Turning ON S₃₁ and S₄₁ yields V_H1= -E. Turning OFF all switches yields V_H1= 0. Similarly AC output voltage at each level can be obtained in same manner. Controlling the conducting angles at different inverter levels can minimize the harmonic distortion of the output voltage.

Advantages of Cascaded inverter

The major advantages of the cascaded inverter can be summarized as follows:-

1. Compared with the diode-clamped and flying capacitors inverters, it requires the least number of components to achieve the same number of voltage levels.
2. Optimized circuit layout and packaging are possible because each level has the same structure and there are no extra clamping diodes or voltage-balancing capacitors.
3. Soft-switching techniques can be used to reduce switching losses and device stresses.

Disadvantages of Cascaded inverter

The major disadvantage of the cascaded inverter are as follows:

• It needs separate dc sources for real power conversions, thereby limiting its applications.

Applications of multilevel inverters

The most common applications of multilevel inverters are:

1. Reactive power compensation
2. Back-to-back intertie
3. Variable speed drives