interview question related to multilevel inverter

viva questions on multilevel inverters

Inverter:

The Inverter is an electrical device which converts direct current (DC) to alternate current (AC). The inverter is used for emergency backup power in a home. The inverter is used in some aircraft systems to convert a portion of the aircraft DC power to AC. The AC power is used mainly for electrical devices like lights, radar, radio, motor, and other devices.

Multilevel Inverter:

Now a day’s many industrial applications have begun to require high power. Some appliances in the industries however require medium or low power for their operation. Using a high power source for all industrial loads may prove beneficial to some motors requiring high power, while it may damage the other loads. Some medium voltage motor drives and utility applications require medium voltage. The multi level inverter has been introduced since 1975 as alternative in high power and medium voltage situations. The Multi level inverter is like an inverter and it is used for industrial applications as alternative in high power and medium voltage situations.

General DC-AC Inverter Circuit

The need of multilevel converter is to give a high output power from medium voltage source. Sources like batteries, super capacitors, solar panel are medium voltage sources. The multi level inverter consists of several switches. In the multi level inverter the arrangement switches’ angles are very important.

Types of Multilevel Inverter:

Multilevel inverters are three types.

Diode clamped multilevel inverter
Flying capacitors multilevel inverter
Cascaded H- bridge multilevel inverter
Diode Clamped Multilevel Inverter:

The main concept of this inverter is to use diodes and provides the multiple voltage levels through the different phases to the capacitor banks which are in series. A diode transfers a limited amount of voltage, thereby reducing the stress on other electrical devices. The maximum output voltage is half of the input DC voltage. It is the main drawback of the diode clamped multilevel inverter. This problem can be solved by increasing the switches, diodes, capacitors. Due to the capacitor balancing issues, these are limited to the three levels. This type of inverters provides the high efficiency because the fundamental frequency used for all the switching devices and it is a simple method of the back to back power transfer systems.

Ex: 5- Level diode clamped multilevel inverter, 9- level diode clamped multilevel inverter.

The 5- level diode clamped multilevel inverter uses switches, diodes; a single capacitor is used, so output voltage is half of the input DC.
The 9- level diode clamped multilevel inverter uses switches, diodes; capacitors are two times more than the 5-level diode clamped inverters. So the output is more than the input.

Applications of Diode Clamped Multilevel Inverter:

Static var compensation
Variable speed motor drives
High voltage system interconnections
High voltage DC and AC transmission lines
Flying Capacitors Multilevel Inverter:

The main concept of this inverter is to use capacitors. It is of series connection of capacitor clamped switching cells. The capacitors transfer the limited amount of voltage to electrical devices. In this inverter switching states are like in the diode clamped inverter. Clamping diodes are not required in this type of multilevel inverters. The output is half of the input DC voltage. It is drawback of the flying capacitors multi level inverter. It also has the switching redundancy within phase to balance the flaying capacitors. It can control both the active and reactive power flow. But due to the high frequency switching, switching losses will takes place.

EX: 5-level flying capacitors multilevel inverter, 9-level flying capacitors multilevel inverter.

This inverter is same like that diode clamped multi inverter
In this inverter only switches and capacitors are used.

Applications of Flying Capacitors Multilevel Inverter

Induction motor control using DTC (Direct Torque Control) circuit
Static var generation
Both AC-DC and DC-AC conversion applications
Converters with Harmonic distortion capability
Sinusoidal current rectifiers
Cascaded H-Bridge Multilevel Inverter:

The cascaded H-bride multi level inverter is to use capacitors and switches and requires less number of components in each level. This topology consists of series of power conversion cells and power can be easily scaled. The combination of capacitors and switches pair is called an H-bridge and gives the separate input DC voltage for each H-bridge. It consists of H-bridge cells and each cell can provide the three different voltages like zero, positive DC and negative DC voltages. One of the advantages of this type of multi level inverter is that it needs less number of components compared with diode clamped and flying capacitor inverters. The price and weight of the inverter are less than those of the two inverters. Soft-switching is possible by the some of the new switching methods.

Multilevel cascade inverters are used to eliminate the bulky transformer required in case of conventional multi phase inverters, clamping diodes required in case of diode clamped inverters and flying capacitors required in case of flying capacitor inverters. But these require large number of isolated voltages to supply the each cell.

Ex: 5- H-bridge multi level inverter, 9- H-bridge clamped multi level inverter.

This inverter is also same like that diode clamped multi inverter.

Applications of Cascaded H-Bridge Multilevel Inverter

Motor drives
Active filters
Electric vehicle drives
DC power source utilization
Power factor compensators
Back to back frequency page link systems
Interfacing with renewable energy resources.
Advantages of Multilevel Inverter:

The multilevel converter has a several advantages, that is:

1. Common Mode Voltage:

The multilevel inverters produce common mode voltage, reducing the stress of the motor and don’t damage the motor.

2. Input Current:

Multilevel inverters can draw input current with low distortion

3. Switching Frequency:

The multilevel inverter can operate at both fundamental switching frequencies that are higher switching frequency and lower switching frequency. It should be noted that the lower switching frequency means lower switching loss and higher efficiency is achieved.

4. Reduced harmonic distortion:

Selective harmonic elimination technique along with the multi level topology results the total harmonic distortion becomes low in the output waveform without using any filter circuit.


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