Multi-level Inverter

What is an Inverter?

An inverter (or power inverter) is a power electronics device which used to convert DC voltage into AC voltage. Although DC power is used in small electrical gadgets, most household equipment runs on AC power. Hence we need an efficient way to convert DC power into AC power.

The inverter is a static device. It can convert one form of electrical power into other forms of electrical power. But it cannot generate electrical power. Hence the inverter is a converter, not a generator.

It can be used as a standalone device such as solar power or back power for home appliances. The inverter takes DC power from the batteries and converts into AC power at the time of the power failure.

A power inverter used in the power system network to convert bulk DC power to AC power. i.e. It used at the receiving end of HVDC transmission lines. This inverter is known as a grid-tie inverter.

 

 

How Does an Inverter Work?

Let’s understand the working of an inverter by an example. One bulb connected with a battery. It makes a close path. Hence the current will flow through the bulb.

The bulb has two terminals that are ‘A’ and ‘B’. The positive and negative terminal of the battery is connected with ‘A’ and ‘B’ terminal respectively and the bulb will glow.

Now, change the terminals of the battery. The bulb will glow in this condition also. So, what is the difference in both cases?

Here, one thing is different and that is the direction of the AC current.

Now imagine that you can rotate the battery at 50 or 60 rpm. What will happen? The direction will change 50 or 60 times. This is similar to AC power. And the frequency is 50 or 60 Hz.

This is just to understand the working principle of an inverter. Practically, inverter never works like this and it doesn’t have rotating parts.

The inverter uses the power electronics switches like IGBT, MOSFET. The number of switches depends on the type of inverter.

Let’s take a circuit diagram of a single-phase full-bridge inverter to understand the working.

There are four switches. A DC source connected with the switches and load.

When switch S1 and S2 are ON, S3 and S4 OFF, the direction of current through the load are positive in this condition. It gives a positive half cycle of the AC output.

Now, switch S3 and S4 is ON, S1 and S2 OFF. The current flowing in the opposite direction. It gives a negative half cycle of the AC output.

The ON and OFF time of switches decides the output frequency. The output of the inverter is a square wave. The filters used to generate a sine wave.

Types of Inverter based on the Load:

There are two types of AC power; single-phase and three-phase. Therefore, there are two types of load. And according to that, there are two types of inverters:

  • Single-phase inverter
  • Three-phase inverter

 

Single-phase Inverter

If the load is a single-phase, the inverter used to run the load that is the single-phase inverter. There are two types;

  • Half-bridge inverter
  • Full-bridge inverter

 

Single-phase Half-bridge Inverter

Two thyristors (S1 and S2) connected with two feedback diodes (D1 and D2) as shown in the below circuit diagram.

The supply voltage divides into two equal parts. The resistive load used to understand the working principle.

 

Mode-1

Thyristor S1 is ON and S2 is OFF during this mode. The current flowing path is V/2-S1-B-RL-A-V/2.

The current flowing through the load is B to A direction. And the voltage across the load is positive V/2. In this mode, the positive cycle of the output generates.

Mode-2

Thyristor S2 is ON and S1 is OFF during this mode. The current flowing path is V/2-A-RL-B-S2-V/2.

The current flowing through the load A to B direction. The voltage across the load is negative V/2. In this mode, the negative cycle of output generates.

Single-phase Full-bridge Inverter

In a full-bridge inverter, four thyristors and four feedback diodes used. One DC source applied to the circuit.

In a half-bridge inverter, one switch is in conduction at a time. And in a full-bridge inverter, two switches are in conduction at a time.

Mode-1

Thyristor S1 and S2 are ON and thyristors S3 and S4 are OFF during this mode. The current flowing path is V-S1-A-RL-B-S2-V.

The current flowing through the load is from A to B and make a positive half cycle.

Mode-2

Thyristor S3 and S4 are ON and thyristor S1 and S2 are OFF. The current flowing path is V-S3-B-RL-A-S4-V.

The current flowing through the load is from B to A and make a negative half cycle of output.

Three-phase Inverter

Generally, three-phase AC supply used in industries and the load is three-phase. In this case, a three-phase inverter used to run this load.

 

In a three-phase inverter, six diodes and six thyristors used. According to the conduction time of thyristor, this inverter divides into two types;

  • 120-degree mode of operation
  • 180-degree mode of operation

 

120-Degree Mode of Operation

At a time, two thyristors are in conduction. The conduction time for all thyristors is 120-degree. It means, a switch remains ON for 120-degree and OFF for the next 240-degree.

The shape of phase voltage is a quasi-square wave and the shape of the line voltage is three-stepped waveform.

180-Degree Mode of Operation

Three thyristors are in conduction at a time. The conduction time for all thyristors is 180-degree.

The shape of the line voltage and phase voltage is opposite to the 120-degree mode of operation. Here, for phase voltage, a waveform is a three-stepped wave and for line voltage, a waveform is a quasi-square wave.

In a 180-degree mode of operation, two thyristors of the common bridge are ON and OFF simultaneously. For example, in half cycle (180-degree) S1 is ON and the next half-cycle S4 is ON. So, at the same time, S1 is switching OFF and S4 is switching ON. Because of this simultaneous conduction, it is possible that the source may sort circuited.

This problem will not happen in a 120-degree mode of operation.

Introduction to Multilevel Inverter

  • A multilevel inverter is a power electronic device that is capable of providing desired alternating voltage level at the output using multiple lower-level DC voltages as an input.
  • Mostly a two-level inverter is used in order to generate the AC voltage from DC voltage.

Now the question arises what’s the need of using a multilevel inverter when we have a two-level inverter. In order to answer this question, first, we need to look at the concept of the multilevel inverter.

Concept of Multilevel Inverter

First, take the case of a two-level inverter. A two-level Inverter creates two different voltages for the load i.e. suppose we are providing Vdc as an input to a two-level inverter then it will provide + Vdc/2 and – Vdc/2 on output. In order to build an AC voltage, these two newly generated voltages are usually switched. For switching mostly PWM is used as shown in Figure 2.1, reference wave is shown in the dashed blue line. Although this method of creating AC is effective but it has few drawbacks as it creates harmonic distortions in the output voltage and also has a high dv/dt as compared to that of a multilevel inverter. Normally this method works but in few applications, it creates problems particularly those where low distortion in the output voltage is required.

The concept of multilevel Inverter (MLI) is a kind of modification of a two-level inverter. In multilevel inverters we don’t deal with the two-level voltage instead in order to create a smoother stepped output waveform, more than two voltage levels are combined together and the output waveform obtained in this case has lower dv/dt and also lower harmonic distortions. The smoothness of the waveform is proportional to the voltage levels, as we increase the voltage level the waveform becomes smoother but the complexity of the controller circuit and components also increases along with the increased levels. The waveform for the three, five and seven level inverters are shown in the below figure, where we clearly see that as the levels are increasing, the waveform becoming smoother.

A three-level waveform, a _ve-level waveform and a seven-level multilevel waveform, switched at fundamental frequency

 

Multilevel Inverter Topology

There are several topologies of multilevel inverters available. The difference lies in the mechanism of switching and the source of input voltage to the multilevel inverters. Three most commonly used multilevel inverter topologies are:

  • Cascaded H-bridge multilevel inverters.
  • Diode Clamped multilevel inverters.
  • Flying Capacitor multilevel inverters.

 

Cascaded H-bridge multilevel inverters

This inverter uses several H-bridge inverters connected in series to provide a sinusoidal output voltage. Each cell contains one H-bridge and the output voltage generated by this multilevel inverter is actually the sum of all the voltages generated by each cell i.e. if there are k cells in an H-bridge multilevel inverter then a number of output voltage levels will be 2k+1. This type of inverter has an advantage over the other two as it requires fewer components as compared to the other two types of inverters and so its overall weight and price are also less. Below Figure shows a k level cascaded H-bridge inverter.

 

Fig:One phase of a cascaded H-bridge multilevel inverter

In a single-phase inverter, each phase is connected to a single dc source. Each level generates three voltages which are positive, negative and zero. This can be obtained by connecting the AC source with the DC output and then using different combinations of the four switches. The inverter will remain ON when two switches with opposite positions will remain ON. It will turn OFF when all the inverters switch ON or OFF. To minimize the total harmonic distortion, switching angles are defined and implemented. The calculations for the measurement of switching angle will remain the same. This inventor can be categorized further into the following types:

  • 5 levels cascaded H Bridge Multilevel Inverter
  • 9 levels cascaded H Bridge Multilevel Inverter

In 5 level cascaded H Bridge Multilevel Inverters, Two H Bridge Inverters are cascaded. It has 5 levels of output and uses 8 switching devices to control whereas in 9 level cascaded H Bridge Multilevel Inverters, Four H Bridge Invertors are cascaded. It has 9 output levels and use and use 16 switching devices.plications of Cascaded H-bridge Multilevel Inverters

Cascaded H Bridge Multilevel Inverters are mostly used for static var applications i.e., in renewable resources’ of energy and battery based applications. Cascaded H Bridge Multilevel Inverters can be applied as a delta or wye form. This can be understood by looking at the work done by Peng where he used an electrical system parallel with a Cascade H Bridge. Here inverter is being controlled by regulating the power factor. Best application is when we used as photovoltaic cell or fuel cell. This is the example of Parallel connectivity of the H Bridge Multilevel Inverter.

Fig: Example of 3 phase Wye Connection

H Bridge can also be used in car batteries to run the electrical components of the car. Also, this can be used in the electrical braking systems of the vehicles.

Scientists and engineers have also proposed the multiplicative factor on Cascade H Bridge Multilevel. It means that rather than using a dc voltage with the difference in levels, it uses a multiplying factor between different levels of the multilevel i.e., every level is a multiplying factor of the previous one.

Advantages of Cascade H Bridge Multilevel Inverters

  1. Output voltages levels are doubled the number of sources
  2. Manufacturing can be done easily and quickly
  3. Packaging and Layout are modularized.
  4. Easily controllable with a transformer as shown in the Fig 2.5
  5. Cheap

Fig: Cascaded Inverter with transformer

Disadvantages of Cascade H Bridge Multilevel Inverters

  1. Every H Bridge needs a separate dc source.
  2. Limited applications due to a large number of sources.

Diode Clamped multilevel inverters

 

Diode clamped multilevel inverters use clamping diodes in order to limit the voltage stress of power devices. It was first proposed in 1981 by Nabae, Takashi and Akagi and it is also known as a neutral point converter. A k level diode clamped inverter needs (2– 2) switching devices, (k – 1) input voltage source and (k – 1) (k – 2) diodes in order to operate. Vdc is the voltage present across each diode and the switch. Single-phase diode clamped multilevel inverter is shown in the figure below:

 

Fig: One phase of a diode clamped inverter

 

The concept of diode clamped inverter can better be understood by looking into a three-phase six-level diode clamped inverter. Here the common dc bus is shared by all the phases, use five capacitors and six levels. Each capacitor has a voltage of Vdc and same is the voltage limit of switching devices. One important fact should be noted while considering the diode clamped inverter is that five switches will remain ON at any time. Six level, three-phase dc clamped multilevel inverter is shown in the figure below

 

Fig: Six level three phase inverter

Outputs of each phase can be understood by the following table. Here reference voltage is the negative Vo. Condition 0 means switch is OFF and vice versa. Output waveforms of six level dc clamped inverter is shown below:

Fig: Waveform of Six Level Inverter

Vab is the voltage due to the phase lag b and a voltage.

Applications of Diode Clamped Multilevel Inverters

The most common application of diode clamped multilevel inverter is when a high voltage Dc and Ac transmission lines are interfaced. This can also be used in variable speed control of high power drives. Static variable compensation is also an application of diode clamped multilevel inverters.

Advantages of Diode Clamped Multilevel Inverters

  • The capacitance of the capacitors used is low.
  • Back-to-back inverters can be used.
  • Capacitors are precharged.
  • At fundamental frequency, efficiency is high.

Disadvantages of Diode Clamped Multilevel Inverters

  • Clamping diodes are increased with the increase of each level.
  • The DC level will discharge when control and monitoring are not precise

 

Flying Capacitor multilevel inverters

 

  • The configuration of this inverter topology is quite similar to previous one except the difference that here flying capacitors is used in order to limit the voltage instead of diodes. The input DC voltages are divided by the capacitors here. The voltage over each capacitor and each switch is Vdc. A klevel flying capacitor inverter with (2k – 2) switches will use (k – 1) number of capacitors in order to operate. The figure below shows a five-level flying capacitor multilevel inverter.

    • If we compare above figures, it shows that the number of switches, main diodes and DC-bus capacitors are same in both the cases. The only difference between the two topologies is that the previous one uses clamping diodes in order to limit the voltage while this topology uses flying capacitors for this purpose, and as capacitors are incapable of blocking the reverse voltage, which diodes do, the number of switches also increases. Voltage on each capacitor is differing from the next as it has a ladder structure. Voltage difference between two back to back capacitors determines the voltage in the output frame.
    • Advantages of Flying Capacitor Multilevel Inverters
    • Static var generation is the best application of Capacitor Clamped Multilevel Inverters.
    • For balancing capacitors’ voltage levels, phase redundancies are available.
    • We can control reactive and real power flow.
    • Disadvantages of Flying Capacitor Multilevel Inverters
    • Voltage control is difficult for all capacitors.
    • Complex startup.
    • Switching efficiency is poor.
    • Capacitors are expensive than diodes.
    • So, that was all about Multilevel Inverters and their topologies

     

    AIM: Design a 5 level cascaded H-Bridge inverter using SIMULINK.

 

Results:

 

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