Shri Ramdeobaba College of Engineering and Management

(An Autonomous Institute affiliated to Rashtrasant Tukadoji Maharaj Nagpur University)

REPORT ON

“SOME STUDIES ON THREE LEVEL H-BRIDGE

INVERTER FOR STANDALONE SYSTEM”

Under the guidance of

Prof. P.A. Salodkar

Submitted by-

1. Abhijeet Joshi - 31

2. Abhinav Agrawal - 32

3. Anil Kumar - 43

4. Niket Goenka - 67

ABSTRACT

As compared to conventional two level inverters, multilevel inverters have been widely accepted for high-

power and high-voltage applications due to their added advantages of low switching stress and lower total

harmonic distortion (THD), hence reducing the size and bulk of the passive filters. In this report a three

level inverter topology is presented with various PWM techniques in single phase conventional cascaded

H-bridge multilevel inverter, and can be extended to any number of levels. The performance of the new

topology and its controller are validated with simulation results using the MATLAB/SIMULINK software.

Keywords: Multilevel inverter, SPWM, THD

AIMS & OBJECTIVE

1) Simulation and modeling of single-phase three-level inverter.

2) Simulation and modeling of single phase three-level inverter with various PWM techniques.

3) To design hardware for standalone Three level Invertor

INTRODUCTION

Inverter is a device that converts electrical power from dc form to ac form using electronic circuits.

The ‘inverter’ works reverse of what ac-to-dc ‘converter’ works.

Inverter can be broadly classified as:

Voltage source inverter(VSI)

Current Source inverter(CSI)

Further these can be classified into

Two level inverter

Multilevel inverter.

BLOCK DIAGRAM

Fig.1

MULTILEVEL INVERTER-

The inverters which produce which produce an output voltage or a current with levels positive and negative

voltage are known as two level inverters. The inverters which produce which produce an output voltage or

a current with levels positive, zero and negative voltage are known as three level inverters.

a

+Vd

-Vd

Vd

+Vd

-Vd

Vd

Vd

Fig.a Two level Circuit Fig.b Three level Circuit

Disadvantages of two level inverter:

In high-power and high-voltage applications the two-level inverters however have some limitations in

operating at high frequency mainly due to switching losses and constraints of device rating.

Harmonic content in the output is high.

The two-level inverters generate high-frequency common-mode voltage within the motor windings which

may result in motor and drive application problems.

Advantages of three level inverter:

Increasing the number of voltage levels in the inverter without requiring higher rating on individual

devices can increase power rating.

The unique structure of multilevel voltage source inverters’ allows them to reach high voltages with low

harmonics without the use of transformers or series-connected synchronized-switching devices.

The harmonic content of the output voltage waveform decreases significantly.

A variable output voltage can be obtained by varying the gain of the inverter, which is normally

accomplished by pulse-width-modulation (PWM) control within the inverter.

The general features of a multi-level inverter are as follows:

The output voltage and power increase with number of levels. Adding a voltage involves adding main

switching device to each phase.

1) The harmonic content decreases as the number of levels increase and filtering requirements are

reduced.

2) With additional voltage levels, the voltage waveform has more free switching angles which can be

pre-selected for harmonic elimination.

3) In the absence of any PWM techniques, the switching losses can be avoided.

4) Increasing output voltage and power does not require an increase in rating of individual device.

5) Power factor is close to unity.

6) No electromagnetic induction is produced.

7) These are meant for high power applications, such as in utility systems for controlled sources of

reactive power.

8) In steady-state operation, an inverter can produce a controlled reactive current and operates as static

volt-ampere reactive compensator (SVC / STATCON).

9) These inverters reduce the size of compensator and improve its performance during power system

contingencies.

10) Because of the use of a high voltage inverter, it can be directly connected to high voltage system

(e.g~13 kV) distribution system, eliminating the distribution transformer and reducing the system

cost.

11) The harmonic content of the inverter waveform can be reduced with appropriate control techniques,

and thereby the efficiency can be improved.

Basic operation of inverter:

Fig. 1 Cascaded H-bridge invertor

The basic single phase inverter circuit is shown. It is composed of two inverter legs with two IGBT devices

which act as a switch in each leg. The dc input is usually fixed. The output voltage, can be adjusted by

adjusting the gate signals of the switches. The upper and lower switches in the same inverter leg operate in

a complementary manner with one switch turned on and the other turned off. Thus we only need to

consider two independent gate signals, Vg1 and Vg2.

Switching techniques for inverter

Switching in inverter is required for switching of the semiconductor switches (e.g. thyristor, IGBT, etc).

The PWM is a very advanced and useful technique in which width of the gate pulses are controlled by

various mechanisms. We are using sinusoidal pulse width modulation (SPWM) technique in which the

sinusoidal waveform is compared with the triangular wave, the output of which is used to trigger the

switches.

The figure shows the basic configuration of bipolar switching with respect to fig.1.

Fig. 2

The upper and the lower switches in the same inverter leg work in a complementary manner with one

switch turned on and other turned off. Thus we need to consider only two independent gating signals vg1

and vg3.Signals vg1 and vg3 which are generated by comparing sinusoidal modulating wave Vmand

triangular carrier wave Vcr. The inverter terminal voltage is the output voltage across the load. Since the

output voltage waveform switches between the positive and negative dc voltages +Vd and –Vd, this

scheme is known as “bipolar pulse width modulation”.

Vm

Vcr

S1,S3

S2,S4NOT

C

Fig. 3 Circuit for bipolar switching

MATLAB simulation

Simulation of single phase bridge inverter using SPWM:

The single phase full bridge inverter has been simulated in MATLAB/Simulink as under:

Simulation parameters:-

The following figure shows the waveforms for the inverter using bipolar SPWM technique in

MATLAB

Fig.4 Waveform for bipolar switching scheme

CONNECTION DIAGRAM:

Fig. 5 MATLAB circuit diagram of Cascaded H-bridge invertor

DC bus voltage 100V

Carrier frequency 1KHz

Fundamental frequency 50Hz

Load R=10 ohm

Modulation index 1.0

Fig.6 Output Current & Voltage waveform of Cascaded H-bridge invertor

Total Harmonic Distortion:

Harmonics:

Harmonics distort current and/or voltage waves, disturbing the electrical distribution system and degrading

power quality.The nth order harmonicin a signal is the sinusoidal component with a frequency that is n

times the fundamental frequency.A distorted signal is the sum of a number of superimposed harmonics.

Figure shows an example of a sinusoidal wave affected by harmonic distortion; resulting into a square

wave. Harmonics are caused by non-linear loads

Fig. 7

FUTURE SCOPE:

Detailed simulation on three level H-Bridge standalone system.

Closed loop simulation of H-Bridge Invertor.

Hardware implementation of the inverter.

Methods of THD reduction using multilevel .