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Multilevel Inverter – Types & AdvantagesInverter:
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.
Multilevel Inverter
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.
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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. 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.
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.
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5- Level Diode Clamped Multilevel Inverter
Flying Capacitors Multilevel Inverter:
The main concept of this inverter is to use capacitors. The capacitors transfer the limited amount of voltage to electrical devices. In this inverter switching states are like in the diode clamped inverter. The output is half of the input DC voltage. It is drawback of the flying capacitors multi level inverter.
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.
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5-Level Flying Capacitors Multilevel Inverter
Cascaded H-Bridge Multilevel Inverter:
The cascaded H-bride multi level inverter is to use capacitors and switches. The combination of capacitors and switches pair is called an H-bridge and gives the separate input DC voltage for each H-bridge. 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, weight of the inverter are less than those of the two inverters.
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.
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5- H-Bridge Multilevel Inverter
Applications:
Power conducting Motor drives
Active filters
Electric vehicle drives
DC power source utilization
Advantages of Multilevel Inverter:
The multilevel converter has a several advantages, that is:
Common Mode Voltage:
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The multilevel inverters produce common mode voltage , reducing the stress of the motor and don’t damage the motor.
Input Current:
Multilevel inverters can draw input current with low distortion
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.
Concept of Pulse Width Modulation (PWM) TechniquesPulse Width Modulation (PWM) is a commonly used technique for generally controlling DC power to an electrical device, made practical by modern electronic power switches. However it also finds its place in AC choppers. The average value of current supplied to the load is controlled by the switch position and duration of its state. If the On period of the switch is longer compared to its off period, the load receives comparatively higher power. Thus the PWM switching frequency has to be faster.
Typically switching has to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive.
Switching frequency for audio amplifiers and computer power supplies is about ten to hundreds of kHz. The ratio of the On time to the time period of the pulse is known as duty cycle. If the duty cycle is low, it implies low power.
The power loss in the switching device is very low, due to almost negligible amount of current flowing in the Off state of the device and negligible amount of voltage drop in its off state. Digital controls also use PWM technique.
PWM has also been used in certain communication systems where its duty cycle has been used to convey information over a communications channel.
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Power delivery
PWM can be used to adjust the total amount of power delivered to a load without losses normally incurred when a power transfer is limited by resistive means. The drawbacks are the pulsations defined by the duty cycle, switching frequency and properties of the load. With a sufficiently high switching frequency and, when necessary, using additional passive electronic filters the pulse train can be smoothed and average analogue waveform recovered. High frequency PWM control systems can be easily implemented using semiconductor switches.
As has been already stated above almost no power is dissipated by the switch in either on or off state. However, during the transitions between on and off states both voltage and current are non-zero and thus considerable power is dissipated in the switches. Luckily, the change of state between fully on and fully off is quite rapid (typically less than 100 nanoseconds) relative to typical on or off times, and so the average power dissipation is quite low compared to the power being delivered even when high switching frequencies are used.
Modern semiconductor switches such as MOSFETs or Insulated-gate bipolar transistors (IGBTs) are quite ideal components. Thus high efficiency controllers can be built. Typically frequency converters used to control AC motors have efficiency that is better than 98 %. Switching power supplies have lower efficiency due to low output voltage levels (often even less than 2 V for microprocessors are needed) but still more than 70-80 % efficiency can be achieved.
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This kind of control for AC is power known delayed firing angle method. It is cheaper and generates lot of electrical noise and harmonics as compared to the real PWM control that develops negligible noise.
Look at the following projects which are based on PWM:
Sine Pulse Width Modulation Space Vector Pulse Width Modulation
Get good idea on various final year electronics projects for engineering students in various categories like embedded, electrical, robotics and communication.
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