Performance of T-Type Multi-level Inverter Based Open-End Winding Induction Motor Drive for Electric Vehicle Application

Performance of T-Type Multi-level Inverter Based Open-End Winding Induction Motor Drive for Electric Vehicle Application

B. Harshini

PG Scholar

EEE Department VNR VJIET Hyderabad, India

Dr. Ranjit. M

Assistant Professor

EEE Department VNR VJIET Hyderabad, India

Dr. V. Ramesh Babu

Associate Professor

EEE Department VNR VJIET Hyderabad, India

Dr. J. Srinivasa Rao Associate Professor EEE Department

VNR VJIET Hyderabad, India

Abstract:- Electric vehicles (EVs) plays a vital role to meet global demands on climatical variations. All EVs in todays market uses the classical two-level inverter as the propulsion inverter. To overcome constraints of conventional two-level inverter like electromagnetic interference,common mode voltage etc.,. Multi-Level Inverter topologies revolutionized in the industry because of its smart features over the conventional inverters. Now a days pulse width modulated voltage source inverters are used widely in applications like variable speed control drives in order to control magnitude and frequency of output voltage by using these voltage source inverter. But often in conventional two-level inverter fed induction motor drive operated at high frequency it yields to the production of common mode voltage (CMV) which leads to the production of shaft voltage and high bearing currents damages motor bearings. MIs have emerged to reduce CMV. In MIs with increase in levels of output it results in high dv/dt stress on individual switching devices and high switching losses. To getrid flow of bearing currents and to reduce dv/dt stress on individual switching devices open end winding induction motor drive can be implemented. This work proposes operational strategy of MI and T-Type inverter fed open end winding induction motor drive using MATLAB/SIMULINK environment.

KeywordsCommon Mode Voltage (CMV),Multi-level Inverter (MI),Open-End Winding Induction Motor (OEWIM)

1. INTRODUCTION

   Inverters are also called as power inverter which is an electrical power device,which is used to convert DC to AC whose output is of desired voltage and frequency. 2-level inverters are preferrable in low voltage and low power application which produces harmonic waveform which the affects the performance of the converter. The first solution is to add passive power filter at specific harmonic order which becomes complex. So, an alternative is to improve switching frequency of Voltage Source Inverter (VSI) which affects converter losses as well as efficiency. MIs became the better alternative for VSI to reduce the harmonic content without the need of passive filter or improving switching frequency. MI are more preferrable in high voltage and high

   power applications. The conventional topologies of MIs are Diode Clamped (DC) or Neutral Point Clamped Converter (NPC), Cascaded H-Bridge (CHB) Converter and Flying Capacitor (FC) converter. The disadvantages with these MIs are the circuit complexity which thereby increases the elements in the circuit. This paper deals with T-Type Inverter has advantages of less switching elements and better efficiency when compared with conventional MI topology. As conventional 2-level inverter generates voltage with high dv/dt stress and also produce CMV which results in leakage current and damages the motor bearings. In order to mitigate CMV in variable speed motor drives dual inverter fed open- end winding induction motor drive is advantageous.

2. DESCRIPTION AND OPERATION OF THE

   TOPOLOGY

   To obtain more number of levels in NPC MI, number of clamping diodes increases the power circuit complexity therefore reduces the converter efficiency. To overcome this problem presently researchers are focusing with reduced switching elements in MIs. T-Type Inverter is represented in Fig.1. It consists of 2-level inverter and are connected to auxiliary switches. The idea is to connect DC link midpoint with auxiliary switches which reduces switching conductors in the current path and thereby improves efficiency.

   Fig. 1. Circuit of T-Type Inverter

   Each phase of inverter has four switches.So,in each phase there exists three switching states and terminal voltages. Where P,O,N represents three terminal voltages i.e, Vdc/2,0,-Vdc/2 respectively. Operation of T-Type Inverter is to obtain three level voltages

   (P,O,N) which makes switches to turn on for longer time so that stress on the device increases as well conduction losses increases. To reduce stress on the switches modified switching scheme is helpful. The stress on the device can be reduced by means of any one of an auxiliary switches Q2 or Q4 can be turned ON so current in that phase is zero and results in zero conduction loss by conducting one of the auxiliary switch. Possible switching state with modified switching state wiring diagram and voltage levels is shown in Fig.2 and Table.I below respectively.

   Enhanced DC bus utilization

   SVPWM generates less harmonics distortion

   1. SVPWM for 2-level VSC:

      This technique uses the theory of revolving reference voltage instead of modulating wave as in case of Sinusoidal PWM technique. The circuit validates 2-level inverter as depicted in Fig.3. It comprises of six switches and a,b,c represents three phases. This converter has eight switching states out of which six states are active states and remaining corresponds to zero vectors.

      Fig. 3. 2-level inverter

      To implement SVPWM technique the voltage equations in abc reference frame is converted to dq reference frame. For each sector Vref is calculated with the help of two zero states and two non-zero states. The Space Vector (SV) diagram for 2-level inverter is represented in Fig.4. Sector-1 has been intended with vectors V0,V1,V2,V7 which is from zero degrees to 60 degrees.The timing duration calculations are shown below:

      Fig. 4. SV Diagram for 2-level inverter

      The general expression for sector-m is given as VrefT = VmTm+Vm+1Tm+1+V0T0+V7T7 (3.1)

      Where T = Tk+Tk+1+T0

      The period of active and zero vectors can be determined for each sector m is mentioned in below equations:

      Fig. 2. Operating modes of T-Type Inverter

      Tm =

      3||

      sin(

      3

      ) (3.2)

      Level	Q1x	Q2x	Q3x	Q4x	Output Voltage
      P	0	0	1	1	Vdc/2
      O	0	1	0	1	0
      N	1	1	0	0	-Vdc/2

      TABLE I. SWITCHING TABLE FOR T-TYPE INVERTER

      Tm =

      3|| (sin(

      1 )) (3.3)

      3

3. SPACE VECTOR PULSE WIDTH MODULATION(SVPWM)

   SVPWM strategy is one of the advanced , computational method which is better among PWM techniques.In recent days it has been widely spread because of its performance. 3.1.Advantages of SVPWM:

   SVPWM is one of the better strategy among PWM implementation with its merits such as :

   Better spectral performance

   1. SVPWM for 3-level T-Type Inverter:

   For 3-level inverter there are 3^3 i.e., 27 conduction states. Among 27 sectors 3 are null vectors and 24 are active vectors out of which 12 are small of magnitude Vdc/3, 6 are medium of

   amplitude Vdc/3 and 6 are large vectors of magnitude 2Vdc/3. The

   SV diagram of 3-lvel inverter is depicted in Fig.5 which has six

   sectors and each sector is broken to four sub-sectors. The reference voltage vector is generated with combinations of switching states existing in each sector. The expression is given as:

   = 1 + 2 + 3 (3.4)

   Where Total period is = + +

   Fig. 5. SV Diagram for 3-level inverter

4. OPEN-END WINDING INDUCTION MOTOR

   DRIVE

   The diagrammatic representation of 3-level converter, 4- level converter and T-Type converter is depicted in Fig.6, Fig.7, Fig.8 respectively. From Fig.8. it can been seen that Vaz,Vbz,Vcz represents the pole voltage of converter-1 and Vaz,Vbz,Vcz represents the pole voltage of converter-2. In this topology the converters are fed with distinct DC source to stop the flow of zero sequence currents into motor.

   Fig. 6. 3-level inverter fed OEWIM drive

   Fig. 8. T-Type inverter fed OEWIM drive

   Net phase voltage of this topology is given as: Vdzdz = Vdz -Vdz ,Where d = a,b,c

   The zero sequence voltage is estimated across the extremes of z and z is as follows as:

   Vzz = (Vaa+Vbb+Vcc) /3

5. SIMULATION RESULTS

   All parameter specifications as per Table.II which is used the verify the simulation results of various MI fed OEWIM drive using SVPWM are compared for different modulation indices.

   TABLE II. SPECIFICATIONS

   Parameters	Values
   DC Voltage Source	400V
   Resistance Load(R)	1 Ohm
   Switching Frequency(fs)	3KHz
   Modulation Index(m)	0.8

   Fig. 7. 4-level inverter fed OEWIM drive

   For Three-level inverter fed IM drive with SVPWM control:

   1. level inverter fed IM drive is simulated by SVPWM for m

      = 0.8 and modulating signal,Phase voltage,CMV are shown in Fig.9. The first waveform shows modulating signal of amplitude of 0.8. The second waveform denotes the phase

      voltage of various magnitudes such as 2Vdc/3, Vdc/3, 0. And

      last waveform represents the CMV (VCMV) which is attained

      as Vdc/6.

      Fig. 9. Modulating signal,Phase voltage and CMV of 3-level inverter

      Performance characteristics of IM drive with SVPWM controlled 3-level inverter i.e., stator currents, speed and torque is shown in Fig.10. The steady state is attained at 0.2 second and load is applied from 0.5 to 0.8 seconds stator current suddenly increased and speed got decreased to 1450 rpm and therefore reaches steady-state.

      Fig.10. Stator current,Speed and torque characteristics condition of 3-level inverter

      For 4-level inverter fed IM drive with SVPWM control:

   2. level inverter fed IM drive is simulated by SVPWM for m

   = 0.8 and modulating signal,Phase voltage,CMV are shown in Fig.11. The first waveform denotes modulating signal of magnitude of 0.8. The second waveform represents the

   phase voltage of amplitudes like 2Vdc/3, Vdc/3, Vdc/8, 0. And

   last waveform shows the CMV (VCMV) which is obtained as

   Vdc/3.

   Fig. 11. Modulating signal,Phase voltage and CMV of 4-level inverter

   The performance characteristics of IM drive with SVPWM controlled 4-level inverter i.e., stator currents, speed and torque is represented in Fig.12.The steady state is achieved at 0.15 second and load is applied from 0.5 to 0.8 seconds stator current abruptly improved and speed got reduced to 1380 rpm and therefore attains steady-state.

   Fig.12. Stator current,Speed and torque characteristics of 4-level inverter

   For T-Type inverter fed IM drive with SVPWM control:

   T-Type inverter fed IM drive is simulated by SVPWM for m

   = 0.8 and Phase voltage,CMV are shown in Fig.13. The first waveform denotes modulating signal of amplitude of 0.8. The second waveform represents the phase voltage of

   different magnitudes Vdc/2, Vdc/3, Vdc/4, Vdc/8, 0. And last

   waveform shows the CMV (VCMV) which is obtained as

   Vdc/6.

   Fig. 13. Modulating signal,Phase voltage and CMV of T-Type inverter

   Performance characteristics of IM drive with SVPWM controlled T-Type inverter i.e., stator currents, speed and torque is represented in Fig.14.The steady state is achieved at 0.1 second and load is applied from 0.5 to 0.8 seconds stator current rapidly amplified and speed got declined to 1475 rpm and therefore obtains steady-state.

   Fig.14. Stator current,Speed and torque characteristics of T-Type inverter

   The THD analysis for three-phase inverter fed OEWIM drive are listed in Table.III:

   TABLE III. COMPARISON OF HARMONIC ANALYSIS OF 3- LEVEL AND 4-LEVEL INVERTERS

   REFERENCES

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   S.No.	m	% THD of 3-level inverter	% THD of 4- level inverter
   1	0.8	1.82	1.41

   The THD analysis for NPC and T-Type inverter are listed in Table.IV:

   TABLE IV. COMPARISON OF HARMONIC ANALYSIS OF NPC AND T-TYPE INVERTERS

   S.No.	m	% THD of NPC	% THD of T- Type inverter
   1	0.8	10.92 [7]	2.6

6. CONCLUSION

The performance of MI and T-Type inverter fed OEWIM drive is presented in this paper. To get rid the flow of bearing currents and to minimize CMV dual inverter configuration is advantageous using SVPWM technique over other topologies. It can be concluded that T-Type inverter gives better peformance when compared to NPC MI. This work can be extended by using Discontinuous PWM techniques and also in real time.