MULTI-LEVEL INVERTERS

INVERTERS

Inverter is an electronic device which converts direct voltage (DC) to alternating current (AC) voltage. Inverters are basically used in power supplies, for example, the inverter used in our home for emergency backup power also called Uninterruptible power supplies (UPS), when power is on , batteries are charged by DC Supply, produced from the alternating current (AC) and as soon as power fails, these batteries produce ac supply by providing DC supply. Inverters are also used in aircraft systems. The AC power produce from here is used for electrical devices.

Now, you might be wondering why Multi-level Inverters? Increasing industrial applications require high output power from less or medium power, this can be received through multi-level Inverters. Depending on the Industrial equipment, load of of the source is classified in high and medium power source, providing high power to the medium power source equipment can cause heavy damage too. Some examples of medium level-voltage source are solar panel, batteries,etc.

The multi-level inverter consists of several switches. It helps in producing better voltage waveform, to produce higher voltage, to reduce the switching frequency for the PWM generation.

So, How do they work? A two-level inverter is used here to create two different voltages for the load.

It is kind of modification of two-level inverter. Voltage level is directly proportional to the smoothness of the waveform and smoothness of the waveform is inversely proportional to the complexity i.e complexity increases with more smoothness.

TYPES OF MULTI-LEVEL INVERTERS:

There are three types of multi-level inverters:

1) Diode Clamped Multi-Level Inverter

2) Flying Capacitors Multi-Level Inverter

3) Cascaded H-bridge Multi-Level Inverter

Each type of multi-level inverters are divided into Introduction, advantages, disadvantages and application.

DIODE CLAMPED MULTI-LEVEL INVERTER

As the name suggests these types of inverters uses diodes and gives different voltage levels through multiple phases of the capacitors banks connected in series. Now, Diodes gives a limited amount of voltage which helps in reducing the stress on the other electrical devices. Now, the input dc voltage is twice the maximum voltage, this the drawback of this topology which can be solved by increasing the number of capacitors, diodes & switches. This type of inverters provide high efficiency. It uses 5-level and 9-level diode clamped multilevel inverter. The crucial difference between 5-level and 9-level is 9 level uses twice more capacitors than 5 level, which causes increase in output voltage.

Some of the advantages of Diode clamped multi level inverters is the low cost, efficiency of the inverter is high at fundamental frequency, capacitors of the inverter are precharged and low & inverters can be used simultaneously.

Some of the disadvantages of Diode clamped multi level inverters is when control and monitoring are not precise enough, dc level discharges and with the increase of multiple levels, no of clamping diode also increases.

Diode clamped multi level inverters can be used in high power motors, static VAR compensation, high power AC and DC transmission lines, etc.

FLYING CAPACITORS MULTILEVEL INVERTER

In this topology, It uses series connection of capacitors to transfer certain amount of voltage to the electrical devices. Just like in the diode clamped circuit, this inverter consist of switching gates, input DC voltage twice as maximum voltage but the clamping diodes are not required in this type of inverter. The active and reactive power flow both can be controlled by flying capacitors. Switching losses will take place due to higher frequency switching in the inverter.

C Flying capacitor multilevel inverter are reactive and real power flow, phase redundancies are available, no transformer is required.

Some of the disadvantages of Flying capacitor multilevel inverter are difficult voltage control of the capacitors, switching losses causing poor efficiency, expansive than diode and complex startup.

Flying capacitor multilevel inverter can be used in sinusoidal current rectifiers, AC-DC and DC-AC conversion applications, induction motor control using DTC, etc.

CASCADE H BRIDGE MULTILEVEL INVERTER

This type of inverter used both capacitor and switches, compared to previous two techniques, it requires less number of components in each level. It consists of series of power conversion cells and power which can easily be scaled. It is known as H-bridge as each cell of capacitor and switches can provide 3 different voltages.

Some of the advantages of Cascade H Bridge multilevel inverter is they are easy and quick manufacturing inverters, cheap cost, easily controllable with the transformer, less no of components and less weight as compared to other two inverters.

Some of the disadvantages of Cascade H Bridge multilevel inverter is, it needs a separate DC source, causing limited applications in the inverter as it increases large number of sources.

Cascaded H Bridge Multilevel Inverter can be used in motor drivers, electric vehicles, frequency link systems, renewable energy sources, Active filters.

ADVANTAGES OF MULTILEVEL INVERTER

Now, we have learnt about different types of multilevel inverter, let's dig into advantages of multilevel inverter such as:

Common Mode Voltage: It helps in reducing the stress and does not damage the motor.

Input Current: It helps in drawing input current with low distortion.

Switching Frequency: Multilevel inverter can operate at both higher and lower switching frequency. Lower switching here means Higher efficiency and lower switching loss.

Reduced Harmonic Distortion: It helps in making total harmonic distortion low in the output waveform w/o using any filter. This is achieved using Selective Harmonic technique along with multi level topology.