Direct Torque Control for Three-Level Neutral Point Clamped Inverter-Fed Induction Motor Drive

Direct torque control (DTC) is a control technique in AC drive systems to obtain high performance torque control. The classical DTC drive contains a pair of hysteresis comparators and suffers from variable switching frequency and high torque ripple. These problems can be solved by using space vector depending on the reference torque and flux. In this paper the space vector modulation technique is applied to the three-level %eutral Point Clamped (%PC) inverter control in the proposed DTC-based induction motor drive system, resulting to a significant reduce of torque ripple. Three-level neutral point clamped inverters have been widely used in medium voltage applications. This type of inverters have several advantages over standard two-level VSI, such as greater number of levels in the output voltage waveforms, less harmonic distortion in voltage and current waveforms and lower switching frequencies. This paper emphasizes the derivation of switching states using the Space Vector Pulse Width Modulation (SVPWM) technique. The control scheme is implemented using Matlab/Simulink. Experimental results using dSPACE validate the steady-state and the dynamic performance of the proposed control strategy. KeywordsDirect torque control; nutral point clamped( PC); space vector pulse width modulation (SVPWM); three-level inverter


INTRODUCTION
The direct torque control (DTC) method has emerged as an alternative to Field Oriented Control (FOC) method for high performance ac drives since it was firstly proposed in the mid-1980 [1][2]. The merits of DTC are fast torque response, simple structure (no need of complicated coordinate transformation, current regulation or modulation block), and robustness against motor parameter variation [3][4][5][6][7].
On the other hand, multi-level inverters have become a very attractive solution for high power application areas [8][9][10][11]. The three-level Neutral Point Clamped (NPC) inverter is one of the most commonly used multi-level inverter topologies in high power ac drives. By comparing to the standard two-level inverter, the three-level inverter presents its superiority in terms of lower stress across the semiconductors, lower voltage distortion, less harmonic content and lower switching frequency [12]. Due to the above mentioned merits, the threelevel inverter fed DTC motor drive has become an important research topic in research and academic community over the past decade [13][14][15][16][17][18][19][20][21].
A variety of techniques have been proposed to overcome some of the drawbacks present in DTC [22]. Some solution proposed are: DTC with the Space Vector Pulse Width Modulation (SVPWM)) [23]; the use of a duty-ratio controller to introduce a modulation between active vectors chosen from the look-up table and the zero vectors [24][25]; use of artificial intelligence techniques, such as neuro-fuzzy controller with SVPWM [26][27][28]. However, the complexity of the control is considerably increased.
Among various modulation techniques for a multi-level inverter, SVPWM is an attractive candidate due to the following merits. It directly uses the control variable given by the control system and identifies each switching vector as a point in complex space. It is suitable for Digital Signal Processor (DSP) implementation and able to optimize switching sequences.
In this paper, Direct Torque Control-Space Vector Modulation (DTC-SVM) with three-level NPC voltage source inverter is investigated. The proposed scheme is described clearly. Simulation and experimental results are reported to demonstrate its effectiveness. The entire control scheme is implemented using Matlab/Simulink and hardware realization is done using dSPACE. The simulation and experimental results of the proposed scheme are found to be in close agreement, thereby indicating the feasibility of the proposed scheme in giving fast control of the induction motor torque.   s ρ ρ , are stator and rotor flux angles, respectively, and δ is the torque angle.

II. INDUCTION MOTOR MODEL
From the above equation, clearly, the electromagnetic torque is cross vector product between the stator and rotor flux vectors. Therefore, generally torque control can be performed by controlling torque angle δ with constant amplitude of the stator and rotor fluxes. An induction motor model is then used to predict the voltage required to drive the torque and flux to the reference values within a fixed time period. The required voltage is synthesized using space vector modulation. If the inverter is not capable of generating the required voltage, a voltage vector that will drive the torque and flux towards the reference values is used. These above fundamental equations with other equations and SVPWM switching states are used for implementing DTC using SVPWM in three-phase induction motor. A block diagram of this configuration is shown in Figure 1.  Table 1. In case of a two-level inverter there are a total of eight switching-states (six result into active vectors and two result into zero vector). The number of switching states for nlevel NPC inverter is given by Number of switching-states 3 n = (11) Hence, in case of a three-level NPC inverter, there are 27 switching-states and among them, three result in zero vector and 24 result in non-zero (active) vectors. Each switching state (combination of phase/leg switches) produces a defined set of three-phase voltages (eventually, voltage space vector), which

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can be represented in a hexagon form. The SVPWM method is an advanced, computation intensive PWM method and is possibly the best among all the known PWM techniques for variable-frequency drive applications. The principle of the SVPWM method is that the command voltage vector is approximately calculated by using three adjacent vectors. The duration of each voltage vectors obtained by vector calculations: V are vectors that define the triangle region in which * V is located. For simplicity here only the switching patterns for sector A will be defined so that calculation procedure for the other sector will be similar. Sector A is divided into 4 regions as shown in Figure 3a, where all the possible switching states for each region are given as well. SVPWM for three level inverter can be implemented by using the steps of sector determination, determination of the region in the sector, calculating the switching times T a , T b ,T c and finding the switching states. T a , T b , T c switching times for sector A is given in Table 2.

Region III Region IV Ta
Ts/2(1-2*1.1*m*sinߙ) Ts/2(2*1.1*m*sin(ߙ)-1)  The experimental setup of the proposed control system is represented by the block diagram shown in Figure 6. It consists of a dSPACE DS1104 controller board with TMS320LF2407 slave processor, ADC interface board CP1104, a four pole induction motor with its parameters listed in appendix. A three-phase VSI inverter is connected to the supply 300V dc bus voltage, with a switching frequency of 5kHz .The DS1104 board is installed in pentium IV 1.5 GHz PC for software development and results visualization. The control program is written in MATLAB/Simulink real time interface with sampling time of 100 µs. The experimental results are illustraed in Figures 7, 8 and 9.      The DTC schemes were simulated using the MATLAB/Simulink environment and the simulation results were found to be in agreement with the experimental results. Figure 10 represents the stator line voltage of two-level classical DTC and Figure 11 represents the line voltage of three-level SVM-DTC. The percentage of THD of stator line voltage of two-level DTC is 106.12% and that of three-level DTC is 34.48%. Figure 12 and Figure 13 represent the stator current of two-level and three-level DTC respectively. The percentage of THD of stator current in two-level scheme is 58.12% and that of three-level is 28.09%. During transient study of stator current (two-level), current was zero up to 0.01 second. At 0.01 second, current suddenly rises to 35 A and then decreases to -30 A and come to steady-state at 0.05 seconds. In three-level DTC , there is no current zero state but initially stator transient current rises to 60 A then come to steady-state at 0.1 second and flows smoothly. Figure 14 and Figure 15 shows the stator flux of two-level and three-level schemes respectively. Flux ripple of two-level is 0.03 Wb and that of three-level is 0.02 Wb under steady-state operation. Figure 16 and Figure 17 represent the torque response of two and threelevel schemes respectively. The reference torque is taken as 10 Nm. The step rise in torque command is from 0% to 100% of rated torque (10 Nm). The torque ripple for two-level schemes is found to be 13 Nm and that of three-level scheme is 7.5 Nm under steady-state operation. Figure 18 represents the steadystate stator d-q axis flux locus for two-level inverter and Figure  19 represents for three-level inverter. The stator d-q axis flux locus in the three-level scheme is more uniform compared to the two-level scheme. DTC-SVM with three-level inverter gives lower THD of voltage and current and reduced ripple in flux and torque compared to the classical two-level DTC. Therefore, it can be concluded that the DTC-SVM with threelevel inverter gives better results than classical two-level DTC in all aspects. • According to adopted definition, DTC operates with closed torque and flux loops but without current controllers

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• DTC needs stator flux and torque estimation and therefore, is not sensitive to rotor parameters • DTC has fast dynamic response, • DTC has simple and robust control structure; however, the performance of DTC strongly depends on estimation accuracy of the stator flux and motor torque.
DTC strategies using space vector modulation technique with detailed generation of space vector modulating signals have been discussed. Constant switching frequency SVPWM -DTC schemes considerably improve the drive performance in terms of reduced torque and flux pulsations, reliable start up, well-defined harmonic spectrum and radiated noise. Therefore, SVPWM-DTC is an excellent solution for general purpose IM drives in a very wide power range. The short sampling time required by the switching table DTC schemes makes them suited to very fast torque and flux controlled drives in spite of the simplicity of the control algorithm. In conclusion, it is believed that the DTC principle will continue to play a strategic role in the development of high performance drives.