PWM STRATEGY FOR THREE PHASE VOLTAGE SOURCE ...

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International Journal of Electrical Engineering and Technology (IJEET) Volume 11, Issue 2, March-April 2020, pp. 286-302, Article ID: IJEET_11_02 33 _0Available online at http://iaeme.com/Home/issue/IJEET?Volume=11&Issue=2 ISSN Print: 0976-6545 and ISSN Online: 0976-6553 Journal Impact Factor (2020 10.1935 (Calculated by GISI) www.jifactor.com ): © IAEME Publication

PWM STRATEGY FOR THREE PHASE VOLTAGE SOURCE INVERTER WITH MINIMUM HARMONIC DISTORTION

Dr. B. Vaikundaselvan Professor, Kathir College of Engineering, Coimbatore, Tamilnadu, India

Dr. N. Prakash Assistant Professor, Kumaraguru College of Technology, Coimbatore, Tamilnadu, India

Dr. S.S. Sivaraju Professor, RVS College of Engineering and Technology, Coimbatore, Tamilnadu, India

Periyanayaki PG Student, Kathir College of Engineering, Coimbatore, Tamilnadu, India

ABSTRACT In this paper, comparison between two different Pulse Width Modulation (PWM)

techniques employed for three phase Voltage Source Inverter (VSI) is discussed. A suitable PWM technique is employed to obtain the required output voltage in the line side of the inverter. The PWM generation techniques used in this paper mainly based on Space Vector based Pulse Width Modulation (SVPWM) and Modified Space Vector

based Pulse Width Modulation (MSVPWM). MSVPWM method involves the comparison of the three phase reference modulating signals with a common triangular carrier wave to generate the pulses for the three phases. Whereas, in SVPWM method, a revolving reference voltage vector is provided as voltage reference instead of three

phase modulating waves. This paper analyses the two Pulse Width Modulation techniques for three phase VSI and the current THD is maintained at the level of

below 5%. The two Pulse Width Modulation techniques for three phase Voltage source inverter are compared by the current THD values to select the method with lower THD values. Keywords: Pulse Width Modulation, Voltage Source Inverter, Total Harmonic Distortion, SVPWM, MSVPWM.

Cite this Article: Dr. B. Vaikundaselvan, Dr. S.S. Sivaraju, Dr. N. Prakash and Periyanayaki, PWM Strategy for Three Phase Voltage Source Inverter with Minimum Harmonic Distortion, , International Journal of Electrical Engineering and Technology11(2), 2020, pp. 286-302. http://iaeme.com/Home/issue/IJEET?Volume=11&Issue=2

PWM Strategy for Three Phase Voltage Source Inverter with Minimum Harmonic Distortion


1. INTRODUCTION In the last years, new energy sources have been proposed and developed due to the

dependency and constant increase of costs of fossil fuels. On other hand, fossil fuels have a huge negative impact on the environment. In this context, the new energy sources are

essentially renewable energies. It is estimated that the electrical energy generation from renewable energy sources will increase from 19%, in 2010, to 32%, in 2030, leading to a

consequent reduction of CO2 emission. In rural areas particularly in the developing world, where most of the population up to 80% is located, more than 1 billion people lack the

essential energy services to satisfy the most basic needs and to improve their social and economic status.

The growing energy demand around the world led us to utilize these renewable energy resources. In recent years, the efforts to spread the use of renewable energy resources instead of pollutant fossil fuels and other forms have increased.

To utilize these renewable energy resources an inverter is essential which converts DC power to AC power as most of the renewable energy is found in DC form. In hybrid power system and micro-grid system the use of inverter is significant. In industrial applications, such

as single phase and Three Phase Induction Motor & other rotating machines, variable frequency & variable voltage supply is needed.

To vary the supply frequency and supply voltage, Voltage Source Inverter (VSI) is used. The Voltage Source Inverters (VSIs), where the independently controlled AC output is a

voltage waveform, behave as voltage sources required by many industrial applications. While the single-phase VSIs cover low-range power applications, three-phase VSIs cover medium to high-power applications.

Modern power electronics have contributed a great deal to the development of new powerful applications and industrial solutions; but at the same time, these advances have

increased the harmonic contamination present in line currents, which ends up distorting the voltage waveforms.

For high efficiency DC-AC conversion and peak power tracking it must have low harmonic distortion along with low electromagnetic interference and high power factor. An

inverter is evaluated after design by using the inverter performance and testing standards which are IEEE 929-2000 and UL 1741 in US EN 61727 in EU and IEC 60364-7-712.

The Total Harmonic Distortion (THD) generated by the inverter is regulated by international standard IEC-61000-3-2. The Total Harmonic Distortion (THD) of the voltage must be kept at minimum and according to recommended limit by IEEE Standard 519-1992 has to be kept at less than 5% for harmonic spectra up to 49th harmonic. For the partial loads THD is generally much higher. There are several switching techniques to control the VSI and

for harmonic reduction. Pulse Width Modulation (PWM) technique is the best one among them. Till now, many types of modulating modes have been brought forward in motion

control and power conversion, such as sinusoidal PWM, space vector PWM, current tracking PWM, harmonic elimination PWM and so on. These methods have some advantages and

disadvantages, but the most widely techniques used are the sinusoidal PWM and the space vector PWM. Pulse Width Modulation (PWM) has become the facto in industrial standard. This technique is the heart of the inverter system control signal

In this paper, firstly Voltage Source Inverter (VSI) and its components are discussed. Later the Sinusoidal Pulse Width Modulation (SPWM) techniques for harmonic reduction of three phase inverter are described. Then the techniques of generating SPWM switching signal using analog circuit (Operational-Amplifiers) are described for three phase SPWM Voltage

Dr. B. Vaikundaselvan, Dr. S.S. Sivaraju, Dr. N. Prakash and Periyanayaki


DC Supply Three Phase VSI

Load

Different PWM

Harmonic Elimination

Source Inverter (VSI). All the components of analog switching circuit are simulated and their simulation outputs are shown.

The switching circuit is checked by simulating it for different types of loads. The simulation outputs of the inverter are shown for both low voltage & high voltage system

before and after filtering step by step. Moreover, two structures of inverter based micro grid system are shown where one is common DC bus with a single central inverter and another one is common AC bus with individual inverters.

2. THREE-PHASE VOLTAGE SOURCE INVER TER In order to control three-phase Voltage-Source Inverters (VSI), there are two control

strategies: current control and voltage control. The voltage-controlled VSI use the phase angle between the inverter output voltage and the grid voltage to control the power flow. In the

current controlled VSI, the active and reactive components of the current injected into the grid are controlled using pulse width modulation (PWM) techniques.

A current controller is less sensitive to voltage phase shifts and to distortion in the grid voltage. Moreover, it is faster in response. On the other hand, the voltage control is sensitive to small phase errors and large harmonic currents may occur if the grid voltage is distorted. Consequently, the current control is recommended in the control of grid-connected inverter.

2.1. Three-Phase Grid- Connected InverterThe current controller of three-phase VSI plays an essential part in controlling grid-connected

inverters. Consequently, the quality of the applied current controller largely influences the performance of the inverter system. Many control mechanisms have been proposed to regulate the inverter output current that is injected into the utility grid.

Among these control mechanisms, three major types of current controller have evolved: hysteresis controller, predictive controller and linear Proportional-Integral (PI) controller.

Predictive controller has a very good steady-state performance and provides a good dynamic performance. However, its performance is sensitive to system parameters. The

hysteresis controller has a fast transient response, non-complex implementation and an inherent current protection. However, the hysteresis controller has some drawbacks such as variable switching frequency and high current ripples. These cause a poor current quality and introduce difficulties in the output filter design.

Figure 1 Block Diagram of Three Phase Inverter

The main objective of static power converters is to produce an AC output waveform from a dc power supply. These are the types of waveforms required in Adjustable Speed Drives

(ASDs), Uninterruptible Power Supplies (UPS), static compensators, active filters, VARFlexible AC Transmission Systems (FACTS), and voltage compensators, which are only a few applications. For sinusoidal ac outputs, the magnitude, frequency, and phase should be controllable.

According to the type of AC output waveform, these topologies can be considered as Voltage Source Inverters (VSIs), where the independently controlled ac output is a voltage



waveform. These structures are the most widely used because they naturally behave as voltage sources as required by many industrial applications, such as Adjustable Speed Drives (ASDs), which are the most popular application of inverters. Similarly, these topologies can be found as Current Source Inverters (CSIs), where the independently controlled ac output is a

current waveform. These structures are still widely used in medium-voltage industrial applications, where high-quality voltage waveforms are required.

For instance, the ac output voltage produced by the VSI of a standard ASD is a three-level waveform. Although this waveform is not sinusoidal as expected, its fundamental component behaves as such. This behaviour should be ensured by a modulating technique that controls

the amount of time and the sequence used to switch the power valves on and off. The modulating techniques most used are the carrier-based technique (e.g., Sinusoidal Pulse

Width Modulation (SPWM)), the space-vector (SV) technique, and the Selective-Harmonic-Elimination (SHE) technique.

2.2. Operation of Three Voltage Inverter Single-phase VSIs cover low-range power applications and three-phase VSIs cover the

medium- to high-power applications. The main purpose of these topologies is to provide a three-phase voltage source, where the amplitude, phase, and frequency of the voltages should always be controllable.

Although most of the applications require sinusoidal voltage waveforms (e.g., ASDs, UPSs, FACTS, VAR compensators), arbitrary voltages are also required in some emerging

applications (e.g., active filters, voltage compensators). The standard three-phase VSI topology is shown in Fig. 2 and the eight valid switch states are given in Table 1. As in

single-phase VSIs, the switches of any leg of the inverter (S1 and S4, S3 and S6, or S5 and S2) cannot be switched on simultaneously because this would result in a short circuit across the dc link voltage supply.

Similarly, in order to avoid undefined states in the VSI, and thus undefined ac output line voltages, the switches of any leg of the inverter cannot be switched off simultaneously as this will result in voltages that will depend upon the respective line current polarity. Of the eight valid states, two of them (7 and 8 in Table 1) produce zero ac line voltages.

Figure 2 Three Phase Voltage Source Inverter



Table 1 Switch States for a Full-Bridge Three-Phase VSI

State State Vab Vb Va Space Vector

2,6 ON4,5,3 OFF 1 v 0 -v V1=1+j0.5 3,1 ON4,5,6 OFF 2 0 v -v V2=j1.55 4,2 ON1,5,6 OFF 3 -v v 0 V3=-1+j0.5 5,3 ON1,2,6 OFF 4 -v 0 v V4=-1-j0.5 6,4 ON1,2,3 OFF 5 0 -v v V5=-j1.55 1,5 ON2,3,4 OFF 6 V -v 0 V6=1-j0.5 3,5 ON2,4,6 OFF 7 0 0 0 V7=0 6,2 ON1,3,5 OFF 8 0 0 0 V8=0

In this case, the ac line currents freewheel through either the upper or lower components. The remaining states (1 to 6 in Table 1) produce non-zero ac output voltages. In order to

generate a given voltage waveform, the inverter moves from one state to another. Thus the resulting ac output line voltages consist of discrete values of voltages that are V i, 0, and -Vi for the topology shown in Fig. 2. The selection of the states in order to generate the given waveform is done by the modulating technique that should ensure the use of only the valid states.

2.3. Filter LC Output voltage wave is synchronized with the grid voltage. So the PWM inverter will inject

ripple current in to the grid. The output LC filter is connected to remove high switching frequency components from output current of inverter.

The simulation design of LC filter is shown in Fig. 3. Where L= 10 e-3, C= 20 e-6. The value of L is design based on current ripple.

Smaller ripple results in lower switching and conduction losses. Typically the ripple current can be chosen as 10% - 15% of rated current. Considering 10% ripple at the rated current the designed value of inductor (L) in the system is given by Eqn 1.

The capacitor C is designed based on reactive power supplied by the capacitor at fundamental frequency. In this design reactive power can be chosen as 15% of the rated

power is given by Eqn 2.

Figure 3 LC Filter



3. PULSE WIDTH MODULATI ON Output voltage from an inverter can also be adjusted by exercising a control within the

inverter itself. The most efficient method of doing this is by pulse-width modulation control used within an inverter.

In this method, a fixed DC input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components. This is the most popular method of controlling the output voltage and this method is termed as Pulse-Width Modulation (PWM) Control.

The advantages possessed by PWM techniques are as under: The output voltage control with this method can be obtained without any additional

components. With the method, lower order harmonics can be eliminated or minimized along with

its output voltage control. As higher order harmonics can be filtered easily, the filtering requirements are minimized.

3.1. Space Vector Pulse Width Modulation The topology of a three-leg voltage source inverter is shown in Fig. 9. Because of the

constraint that the input lines must never be shorted and the output current must always be continuous a voltage source inverter can assume only eight distinct topologies. These

topologies are shown on Fig. 3. Six out of these eight topologies produce a nonzero output voltage and are known as non-zero switching states and the remaining two topologies produce zero output voltage and are known as zero switching states.

Space Vector Modulation (SVM) for three-leg VSI is based on the representation of the three phase quantities as vectors in a two-dimensional plane. This is illustrated here for the sake of completeness. Considering topology 1 of Fig. 4, which is repeated in Fig. 5, we see that the line voltages Vab, Vbc, and Vcaare given by,

��, plane as shown in Fig. 4, where voltagesab, Vbc, and Vca are three line voltage vectors displaced 1200 in space. The effective voltage vector generated by this topology is represented as V1(pnn) in Fig. 5��to the three legs/phases a, b, c being either connected to the positive DC rail (p) or to the negative DC rail (n). Thus ��rail and phases b and c being connected to the negative dc rail.

Proceeding on similar lines the six non-zero voltage vectors (V1- V6) can be shown to assume the positions shown in Fig. 6. The tips of these vectors form a regular hexagon (dotted line in Fig. 6).

We define the area enclosed by two adjacent vectors, within the hexagon, as a sector. Thus there are six sectors numbered 1 - 6 in Fig. 6. Considering the last two topologies of Fig. 6, which are repeated in Fig. 7 for the sake of convenience we see that the output line voltages generated by this topology are given by,



Figure 4 Eight Switching State Topologies of a Voltage Source Inverter

Figure 5 Topology 1-V1 (pnn) of a Voltage Source Inverter

Figure 6 ��

Figure 7 Non-��



These are represented as vectors which have zero magnitude and hence are referred to as zero-switching state vectors or zero voltage vectors. They assume the position at origin in the

��, plane as shown in Fig. 7. The vectors - are called the Switching State Vectors V1 V8(SSVs).

The desired three phase voltages at the output of the inverter could be represented by an equivalent vector Vrotating in the counter clock wise direction as shown in Fig. 7. The

magnitude of this vector is related to the magnitude of the output voltage (Fig. 8) and the time this vector takes to complete one revolution is the same as the fundamental time period of the output voltage.

Figure 8 ��

Figure 9 Output Line Voltages in Time Domain

Let us consider the situation when the desired line- -line output voltage vector Vis in tosector 1 as shown in Fig. 9. This vector could be synthesized by the Pulse-Width Modulation ��V1(pnn) and V2(ppn), the duty cycle of each being d1 and d2, respectively, and the zero vector (V7(nnn) / V8(ppp) ) of duty cycle d0:

Where, 0<m <0.866, is the modulation index. This would correspond to a maximum line-to-line voltage of 1.0Vg, which is 15% more than conventional sinusoidal PWM as shown.All SVM schemes and most of the other PWM algorithms use Eqn. 4 and 5 for the output voltage synthesis.



Figure 10 Synthesis of the Required Output Voltage Vector in Sector 1

The modulation algorithms that use non-��higher THD and/or switching losses and are not analysed here, although some of them, e.g. hysteresis, can be very simple to implement and can provide faster transient response. The duty cycles d1, d2, and d0, are uniquely determined from Eqn. 4 and 5, the only difference between PWM schemes that use adjacent vectors is the choice of the zero vector(s).

4. SIMULATION RESULTS The Three Phase Inverter with SVPWM is as shown in the Fig. 11 and the simulation

parameters values are tabulated in Table 2. The three phase VSI with Modified SVPWM is shown in the Fig. 12. The control circuit diagram is as shown in Fig. 13. The Input voltage of the inverter is set as 300V and the IGBT Switch is used for the Inverter.

Figure 11 Simulation Diagram of Three phase inverter with Space Vector Pulse Width Modulation

Table 2 Simulation Parameters

Input Voltage 300V Switch IGBT Modulation Index 0.9 Internal Resistance 0.001 Snubber Resistance 1e5 Frequency 50Hz



Figure 12 Simulation Diagram of Three phase inverter with Modified Space Vector Pulse Width Modulation

Figure 13 Control Circuit Diagram of the Inverter

4.1. Output o the Space Vector Pulse Width Modulation f The output of the SVPWM based three phase inverter, pulse generated waveform is as shown in Fig. 14. The corresponding output Phase voltage, line voltage and Current waveform is as shown in Fig. 15.

Figure 14 PWM Pulses of SVPWM



Figure 15 Output Phase Voltage, Line Voltage and Current of SVPWM

4.2. Output o the Modified Space Vector Pulse Width Modulation f The output of the Modified SPWM based three phase inverter Pulses generated waveform is as shown in Fig. 16. The corresponding output Phase voltage, line voltage and Current waveform is as shown in Fig. 17.

Figure 16 PWM Pulses of Modified SVPWM

Figure 17 Output Phase Voltage, Line Voltage and Current of Modified SVPWM



4.3. THD Waveforms of Space Vector Pulse Width Modulation The FFT analysis for finding Current THD of SVPWM based three phase inverter for a 300 V input voltage is as shown in Fig. 18.

Figure 18 ITHD of SVPWM for 300V Input Voltage


Similarly the Voltage THD and Current THD of SVPWM based three phase inverter for 400 V and 500 V are shown in Fig. 19 and Fig. 20.




4.4. Waveforms of Modified Space Vector Pulse Width Modulation THD The FFT analysis for finding Current THD of Modified SVPWM based three phase inverter for a 300 V input voltage is as shown in Fig. 21.

Figure 21 ITHD of Modified SVPWM for 300V Input Voltage





Similarly the Voltage THD and Current THD of SVPWM based three phase inverter for 400 V and 500 V are shown in Fig. 22 and Fig. 23. The THD Comparison values vare

tabulated in Table3.

Table 3 THD Comparison of SVPWM and Modified SVPWM

Input Voltage SVPWM Modified SVPWM 300V 5.81% 3.46% 400V 4.70% 5.9% 500V 5.01% 4.22%

5. HARDWARE RESULTS The hardware descriptions are,

A battery module working at the low-voltage side is employed as an energy-storage element, whose voltage rating is 12 Volts.

180W DC motor is used as a load whose rating is 12V, 1500 RPM. High frequency 2:1 transformer is used and the voltage range of a step down

transformer is 0-15V and the rated current is 1A. Regulated Power Supply (RPS) unit is used for the prototype in order to unregulated

AC into a constant DC whose range is 0-5V. The MOSFET is named as S1,S2,S3,S4 and the FCA76N60N is used The inductance value of the output filter is denoted as Lout and the value of the

inductance is 10 µH. The diodes are named as D1, D2, D3, D4 and the device name is RHRP3060.

Figure 24 Sine PWM Pulses of the Inverter Switches

Figure 25 Carrier wave for the Sine Pulse Width Modulation



Figure 26 Reference wave for the Space vector PWM Waveform

The PWM pulses of the three phase voltage source inverter waveforms shown in the Fig.24 and the output voltage and corresponding current waveform is shown in Fig. 25 and Fig. 26.

Figure 27 Space Vector Pulse Width Modulation PWM Pulses

Figure 28 THD value of the Space Vector PWM 1.46% �



Figure 29 THD value of the Sine PWM 3.4% �

6. CONCLUSION The three phase Inverter with Two different Pulse Width Modulation (PWM) techniques was

studied and the design procedure of the inverter also studied. The three phase inverter is designed and simulated using MATLAB Simulation. SVPWM and Modified SVPWM are

compared with the help of simulation. The analysis of both SVPWM and Modified SVPWM are presented in this paper. The Performance result of both PWM techniques has been studied

experimentally. The THD values of these two Pulse Width Modulation techniques is compared and analyzed. The Modified SVPWM has low THD (1.46%) compared to Sine PWM (3.4%) and hence it is chosen as the suitable method for the three phase inverter.

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