 Open Access
 Total Downloads : 337
 Authors : Midhun. G, P. Sandhya
 Paper ID : IJERTV4IS080531
 Volume & Issue : Volume 04, Issue 08 (August 2015)
 DOI : http://dx.doi.org/10.17577/IJERTV4IS080531
 Published (First Online): 26082015
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Comparison Study for SVPWM based CMV Elimination between Two Level Inverter, ThreeLevel NPC Inverter and Dual TwoLevel Inverter fed Induction Motor
Midhun. G
PG Scholar
Department of Electrical & Electronics Engineering MBCETKerala, India695015
P. Sandhya
Assistant Professor
Department of Electrical & Electronics Engineering MBCETKerala, India695015
AbstractPower Electronic AC Motor Drive Systems contain PWM inverters as one of the major parts. These PWM inverters when operated at high frequencies results in leakage current flowing through the parasitic capacitors to the ground which generates a voltage appearing between the neutral point of load and ground known as commonmode voltage (CMV). This voltage can affect the performance of the machine as it causes shaft voltage, bearing current and EMI issues. CMV elimination using conventional Threelevel NPC multilevel topology has certain drawbacks. An improved SVPWM strategy using open end winding configuration is introduced which eliminates alternating CMV and also makes CMV across machine windings zero such that no circulating commonmode current flows through it. A comparative simulation study is done between conventional and proposed configuration and verified using MATLAB.
KeywordsCirculating current, Commonmode voltage, Open end winding AC Drive.

INTRODUCTION
PWM Inverters in induction motor drives are known to cause commonmode voltage which results in leakage current flowing through the parasitic capacitors to the ground and creates drawbacks such as shaft voltage, bearing current, electromagnetic interference problems. Therefore it is extremely necessary to mitigate CMV in any AC motor drive systems [13]. Many studies were done to eliminate CMV which includes use of passive or active filters, Active common noise canceler and active commonmode compensator [45]. The most appropriate and effective way to eliminate CMV is to implement PWM techniques. Among the various PWM techniques space vector pulse width modulation (SVPWM) is proposed due to various advantages such as increased voltage utilization factor and less harmonic distortions at the output.
Conventionally used multilevel inverter in AC motor drive systems is Threelevel neutral point clamped (NPC) inverter where the CMV suppression is achieved by choosing those voltage vectors which produces low level of CMV[6]. But considering various limitations like requirement of neutral point clamping diodes with neutral point voltage fluctuations and increased use of dclink voltage, a new configuration is proposed by using two standard twolevel inverters to obtain
the same three level inversion as that of the conventional topology with only half the dclink voltage.
The proposed configuration is called openend winding induction motor fed with Dual Twolevel inverter [7]. Open end winding configuration can be easily obtained by opening the neutral point of the stator windings without any change in the structure of induction motor. Openend winding AC drives have several advantages compared to conventional star or delta connected machines which include suppression of switching CMV, increased voltage transfer ratio, no neutral point voltage fluctuations and reduced dclink input voltage.
An improved SVPWM is introduced such that the CMV of both inverters is equal and CMV across the machine phase windings is made zero. The circuit is implemented with a single dclink voltage. There are chances of circulating commonmode current to flow in openend winding induction machine with single dclink voltage source whereas this problem is absent in case of openend winding induction motor drives with separate dc sources. Therefore the circulating current and associated problems are eliminated in the proposed configuration since the CMV of both inverters at any instant are made same and CMV across the machine windings are always zero[89].
Even though the CMV across the machine phase is made zero, there are chances of alternating or switching CMV to be generated by the individual inverters which can lead to undesirable effects in AC motor drive systems. Therefore in the proposed work, the two inverters are modulated using SVPWM technique in such a manner that CMV across the machine phase is always zero and alternating or switching CMV of individual inverters is eliminated without any zero sequence current and the associated problems are also avoided.
Simulation of Conventional Threephase TwoLevel inverter, ThreeLevel Neutral Point Clamped Inverter fed induction motor and proposed Dual TwoLevel Inverter fed openend winding induction motor is done in MATLAB and the results are verified. Comparison between conventional and proposed drive is done based on THD point of view for CMV Elimination.

THE CONVENTIONAL TOPOLOGY
Fig.1 ThreeLevel NPC Inverter fed Induction motor Among multilevel inverter topologies, ThreeLevel NPC
Inverters have found broad applications in high power drives Fig.1 shows a Threelevel NPC Inverter fed induction motor where the commonmode voltage is calculated as shown below.
VAO = VAN+VNO VBO = VBN+VNO VCO = VCN+VNO
Adding the above equations and substituting under balanced condition (VAN+VBN+VCN=0), CMV can be obtained as:
CMV = VNO = (VAO+VBO+VCO)/3
The high frequency, high amplitude CMV generated by PWM inverters generate shaft voltage on the rotor side and when this induced shaft voltage exceeds the breakdown voltage of the lubricant in the bearings, it results in bearing currents. The problems associated includes erosion of bearing material, premature failure of motor bearing leading to its failure, increase in leakage current flowing through parasitic capacitors to the ground with associated EMI problems. PWM inverters which do not generate CMV are suggested as a solution to solve the above issues.
CMV suppression in threelevel NPC inverter fed induction motor is done by switching those states which produce low or zero CMV [6]. Out of 27 switching states, 7 of them produce zero CMV which when utilized alone for CMV suppression can reduce the maximum level of output voltage and introduce harmonic distortions at the output. Also the power circuit requires neutral point clamping diodes with neutral point voltage fluctuations. Among various CMV mitigation methods proposed earlier PWM techniques are more efficient. SVPWM technique is implemented here which effectively utilizes the supply voltage and produce less harmonic distortions at the output. Considering the drawbacks of the conventional topology, a new threelevel inverter configuration with CMV elimination is introduced which is termed as Dual inverter fed openend winding induction motor drive.

OPENEND WINDING AC DRIVE CMV MITIGATION
Fig.2 Dual TwoLevel Inverter fed openend winding induction motor with single DC source
Openend winding induction machine is obtained by opening the neutral point of conventional star connected induction machine which results in six terminals instead of three and requires two standard twolevel inverter on either sides of the machine as shown in Fig.2. The dclink voltage source required is only half the value as required for the conventional threelevel topology for producing the same three level inversions at the output. The CMV of individual inverters is calculated by taking the average of its pole voltages as:
Vcm1 = (VA1O+VB1O+VC1O)/3 Vcm2 = (VA2O+VB2O+VC2O)/3
The combined space vector diagram of Dualinverter fed openend configuration is similar to a threelevel NP inverter with 19 distinct voltage vectors out of the total 27 switching states. Among these 19 voltage vectors 7 of them does not contribute any CMV across the machine phase windings as shown in Fig.3.The two inverters are modulated using SVPWM strategy such that CMV of both inverters are equal and CMV across machine windings (Vcm) is zero.
Fig.3 Combined Space vector diagram of Dual inverter with zero CMV across machine windings
Space phasor locations S, H, J, L, N, Q and O as shown in Fig.3 do not produce any CMV across the machine phase. To eliminate switching or alternating CMV of individual inverters, SVPWM for Dual inverter is modified in such a manner that inverter 1 will switch in states 1, 3 and 5 or (2, 4
and 6) and inverter 2 will switch through 1, 3 and 5 or (2, 4 and 6. In the proposed work, space phasor locations with zero CMV across machine windings as shown in Tab.I are taken for CMV elimination.
Tab.I Voltage space vectors without alternating CMV and Zero CMV across machine windings
Space Vector Locations
Space vector combination
Vcm1
Vcm2
Vcm
S
13
Vdc/6
Vdc/6
0
H
15
Vdc/6
Vdc/6
0
J
35
Vdc/6
Vdc/6
0
L
31
Vdc/6
Vdc/6
0
N
51
Vdc/6
Vdc/6
0
Q
53
Vdc/6
Vdc/6
0
O
11,33,55
Vdc/6
Vdc/6
0
From Tab.I, it can be observed that there is no alternating or switching CMV for individual inverters as the CMV for individual inverters are always same and equal to Vdc/6 and CMV across machine windings is obtained by taking the difference of CMV of individual inverters.
Vcm = Vcm1 Vcm2 = 0
To achieve the proposed PWM scheme, the voltage space vectors OS, OH, OJ, OL, ON, OQ are chosen among the various voltage vectors of combined space vector diagram to obtain the space vector diagram without triplen contribution as shown in Fig.4.
Fig.4 Space vector diagram of proposed PWM scheme

SIMULATION
Simulation of TwoLevel Inverter fed Induction motor, ThreeLevel Neutral point clamped inverter fed Induction motor and Dual TwoLevel inverter fed openend winding induction motor are done in MATLAB. A Comparative study is done between the conventional and proposed work for the analysis of CMV reduction.

TwoLevel Inverter fed Induction Motor
Fig.5 TwoLevel SVPWM Inverter fed Induction Motor
The output of the inverter is fed to the induction motor through second order low pass filter bank as shown in Fig.5. CMV of the system is calculated by taking the average of pole voltages.

ThreeLevel NPC Inverter fed Induction motor
Implementation of space vector modulation for multilevel inverters is complex and computationally intensive due to difficulty in determining the location of reference vector, calculation of ontimes and determination of switching state vectors. The proposed multilevel space vector modulation method uses the basic twolevel modulation to calculate the ontimes; hence computation process for nlevel inverter becomes simpler and easier. Therefore a twolevel inverter based SVPWM algorithm for a multilevel inverter is used for developing the model for a Threelevel NPC inverter fed Induction motor [1011].

Dual Twolevel inverter fed openend configuration
The conventional dq model of a normal 3phase induction motor is modified to compute the motor phase current of the openend winding induction motor drive as shown in Fig.6. The inputs for this model are the PWM signals of the individual inverters and their DC link voltages. The pole voltages of the individual inverters are then computed. Subtracting the pole voltages of inverter2 from those of inverter1, the difference of pole voltages is obtained and supplied to the conventional dq model of induction motor to compute motor phase currents. SVPWM is implemented using
Matlab Function block where the pulses for upper switches of the inverter are obtained [14]. Inverter is modelled using the basic output voltage equations [13]. Also it is necessary to know in which sector the reference output lies in order to determine the switching time and sequence. The angle of the reference vector can be used to determine the sector as per Tab.II. Dynamic model of induction motor is implemented in MATLAB to study the transient as well as steady state behavior of the drive system. The dq equivalent circuit is used for obtaining the model equations in terms of flux linkage using one of the popular induction motor model known as Krauses model [12].
Fig.6 dq model of an openend winding induction motor Tab.II Sector Identification
Sector
Degrees
1
0 < < 60Â°
2
60Â° < < 120Â°
3
120Â° < < 180Â°
4
180Â° < < 240Â°
5
240Â°< < 300Â°
6
300Â° < < 360Â°


SIMULATION RESULTS
The output voltages of Threelevel NPC inverter and Dual Twolevel inverter are shown in Fig.7 and Fig.8. It can be observed that openend configuration with a dclink voltage of 200V produce the same threelevel inversion at the output similar to the conventional threelevel NPC topology with a dc link voltage of 400V. Therefore the proposed work utilizes only half the dclink voltage compared with conventional scheme.
Fig.7 Threelevel inverter output
Fig.8 Dual inverter output
For threelevel NPC inverter, space vector diagram obtained contains 27 switching states with 19 distinct voltage vectors as shown in Fig.9. Space vector diagram of the proposed configuration with CMV elimination is shown in Fig.10. The pole voltages and CMV of conventional and proposed topologies are shown in Fig.11, Fig.12 and Fig.13.
Fig.9 Space vector diagram of Threelevel inverter
Fig.10 Space vector diagram with CMV elimination
Fig.11 Pole voltages and CMV of Twolevel inverter
Fig.12 Pole voltages and CMV of Threelevel inverter
Fig.13 Pole voltages and CMV of Dual inverter
From Fig.11 and Fig.12, we can observe that peakpeak CMV for TwoLevel and Threelevel NPC Inverter fed induction motor is 400V and 266.6V respectively and from Fig.13 CMV of openend induction machine configuration is 0V. Therefore CMV can be effectively eliminated using openend induction machine configuration.
The machine parameters for a 2Hp induction machine is used for simulation as shown below:
Rr= 3.805; Rs=4.85; Lls=0.0062; Llr=0.0062; Lm=0.25; fb=50; p=4; J=0.03.
Fig.14 Machine outputs of TwoLevel inverter fed induction motor
Fig.15 Machine outputs of ThreeLevel NPC inverter fed induction motor
Fig.16 Machine outputs of openend configuration at no load (TL=0Nm)
Fig.17 Machine outputs of openend configuration under loaded condition (TL=2Nm)
Under no load condition, initially the machine accelerates and reaches synchronous speed (r=e=314.2rad/sec) when electromagnetic torque reaches load torque. Under load condition with TL=2Nm, rotor speed is reduced to r=289.9rad/sec as shown in Fig.16 and Fig.17 for openend configuration.

FFT ANALYSIS
The harmonic analysis of the conventional and proposed configuraton were done in FFT and compared based on THD point of view. THD of multilevel inverter is less compared to standard Twolevel inverter configuration as shown in Fig.18 and Fig.19. Also in threelevel NPC inverter, triplen contribution was predominant which is eliminated in the proposed Dual inverter fed openend scheme. In the proposed scheme, the third harmonic component is 0.13% with m=0.6 from Fig.20,whereas in conventional topology, third harmonic component is 1.3%(relative to fundamental) .
Fig.18 Harmonic spectrum of aphase pole voltage of TwoLevel Inverter
Fig.19 Harmonic spectrum of aphase pole voltage of Three level NPC inverter
Fig.20 Harmonic spectrum of aphase pole voltage of open end configuration: m=0.6

CONCLUSION
A detailed comparative study is done between TwoLevel, Threelevel NPC and Dual Twolevel inverter fed AC Drives in MATLAB and concluded that in Dual Twolevel inverter fed openend winding induction motor drive with single dc voltage source, CMV suppression is well achieved. Also harmonic analysis between conventional and proposed scheme is done in terms of THD point of view and observed that in the proposed topology, third harmonic or triplen contribution is negligible compared with conventional Threelevel NPC topology since the triplen contribution are eliminated in the present work using SVPWM and openend winding configuration.
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