PERFORMANCE INVESTIGATION OF SVM BASED CSI FED INDUCTION MOTOR DRIVE

Abstract

Current Source Inverters (CSI) are popular for the smooth speed control of large rating Induction Motors (IM) over complete operating range, i.e. from rated speed to very low speed. CSI based Adjustable Speed Drives (ASD) offer a number of advantages over Voltage Source Inverter (VSI) such as; short-circuit protection due to presence of the dc bus inductor, low inverter output dv/dt, resulting from the filtering effect of the output capacitor, and four-quadrant operation without any extra power circuit for regeneration. Due to the filtering effect of the output capacitor and the use of Pulse Width Modulation (PWM) techniques, the motor current waveforms can be made nearly sinusoidal. Therefore, harmonic losses and torque pulsation of the motor can be reduced. These features make the CSI-based ac drives attractive in medium to high power applications such as steel mills, elevator motors, etc., compared to VSI fed drives for medium and large horsepower applications, and also have the features of simple structure, more rugged and reliable. The present day CSI drives employ self commutating devices such as IGBTs with reverse voltage blocking capability. These drives, employing PWM techniques, can be used to get improved output current and voltage. CSI drives also use rectifier, based on self-commutating devices, as a source of variable dc power. The presence of large dc link reactor slows down the dynamic response of the drive. By virtue of the development of IGBT, which can operate at high switching frequency and up to medium power level, the size of dc link inductor and output filter capacitor can be reduced remarkably. The operation of CSI fed IM drive system in open loop leads to saturation in the machine, thus resulting in its poor performance. However, open loop operation at rated flux is not possible due to instability. Thus a closed-loop configuration for torque or speed control has to be employed. A number of investigations on speed control of CSI-induction motor drive systems were presented in the literature. In the new generation high power current source drives, Field Oriented Control (FOC), also known as vector control schemes are widely used. The FOC has the best dynamic behavior among induction motor drive control methods. Using proper field orientation, the stator current can be decomposed into a flux-producing component and a torque-producing component. These two components are then controlled separately. Various modulation techniques are used, which include Trapezoidal Pulse Width Modulation (TPWM), Selective Harmonic Elimination (SHE), and Space Vector Modulation (SVM). A combination of SHE over the upper frequency range and trapezoidal modulation for lower frequencies is required to provide low harmonic content in both stator voltage and current. By using the Space Vector PWM (SVPWM) instead of off-line PWM methods, harmonic elimination is possible in both the frequency ranges and the instantaneous current control capability is much improved. However the SVM algorithm requires complicated computations with complex numbers. To implement the SVM with simple algebraic equations the calculation process must be simplified. Also the phenomenon of resonance is reported in literature over a range of the operating frequency due to the interaction of filter capacitor and the machine inductance. An attempt has been made by the author to simplify the space vector modulation scheme and also to avoid resonance for a wider operating range. With the help of simple algebraic calculation a simplified duty ratio based space-vector modulation is developed. Added to this advantage, the calculation of modulation index is not required in the proposed SVM for CSI. The drive scheme under investigation consists of a front-end regenerative SVPWM converter and a SVPWM inverter on the induction motor side. In the converter part a smoothing reactor is placed on the dc side and an LC filter is inserted on the ac side to reduce the current harmonic injection resulting from the PWM operation. The drive power is drawn from the three-phase utility by the SVPWM converter and a regulated output current is produced at its dc link inductor. The converter draws sinusoidal input current at unity power factor, thus injecting negligible current harmonics into the utility. The SVPWM inverter draws power from the dc link terminal to produce variable frequency and regulated three phase ac currents for the induction motor. The firing command for inverter is generated using duty ratio based space vector technique for the different operating frequency ranges. The capacitor at the motor end absorb the rapid change in output current, which occurs on switching, and also provide a low impedance path for current harmonics. To investigate the performance of the drive a mathematical model of the induction motor, SVPWM inverter, SVPWM converter and controllers are developed. It is used to better understand the circuit in terms of mathematical expressions and to analyze the performance of the drive. The drive consists of two feedback loop namely inner loop for dc link current control of the front-end SVPWM converter and the other is outer speed feedback loop for the SVPWM inverter fed induction motor. The best-known controllers used in industrial control processes are proportional-integral (PI) controllers because of their simple structure and robust performance in a wide range of operating conditions. In current control, PI controller is used to regulate the magnitude of dc link current. The gating pulses for Current Source Converter (CSC) are generated in such a way to reduce the error of the PI current controller. In the outer loop speed control, another PI controller is used to bring the rotor speed equal to the set speed by processing the speed error of the drive. The reference electromagnetic torque calculated in this process decides the torque producing component of the stator current. The flux producing component of the stator current is also calculated with the help of measured rotor speed. The field angle is calculated from the measured stator current of the drive. Then the reference stator current is calculated by using flux and torque component of the stator current and the field angle. Generally current controller is faster than speed controller because of lower electrical time constant in the drive. The Ziegler-Nichols method, Hand Tuning, Tyreus and Luyben and Genetic Algorithm (GA) are the well known tuning methods available in the literature for calculating the gains of PI controller. The controller parameters are properly tuned using the above mentioned methods and their performances are compared. Using genetic algorithm search method, the optimum value of the controller gains is obtained by choosing a relatively higher stable region on the controller gains plane. Once, the current and speed controllers are designed, the closed loop drive is simulated. Transient as well as steady state performances of the SVPWM converter and the induction motor drive are analyzed with the optimized current and speed controller gains. Steady state performance of the proposed drive is obtained at different operating conditions; (i) variable capacitance at the motor terminals, fixed operating frequency, constant dc link current, (ii) variable frequency, fixed capacitance, , constant dc link current, (iii) variable dc link current, fixed capacitance, fixed frequency, (iv) full load torque, fixed capacitance, fixed dc link current, (v) step change in load torque, fixed capacitance, dc link current corresponding to load torque. By simulating the circuit for different ranges of capacitance value it is possible to find out the value of the capacitance at which resonance occur. The value of capacitance which offers sinusoidal current to the IM with less Total Harmonic Distortion (THD) is selected for further steady state analysis. The simulation of the CSI-fed IM drive is carried out with space vector modulation at both the converter and inverter end. Direct Vector control is implemented in the control part. The LC filter at the supply side, the dc link inductor and the output capacitor at the motor terminal are properly designed. In order to simplify the design of dc link inductor and the input filter capacitor, a maximum peak-to-peak ripple criterion is used. The stator current is measured for the calculation of the rotor field angle. In the normal SVM scheme which uses modulation index and trigonometric calculations the actual stator current is sensed and compared with the reference stator current. The error of these signals will be used for the generation of the pulses for the inverter. In the proposed space vector modulation scheme which uses algebraic calculation, the three reference currents and the actual dc link current is used as an input to the simplified SVM block. This simplified space vector modulation block is used for generating pulses for the inverter. To summarize the mathematical model of the induction motor, SVPWM inverter and SVPWM converter are developed. From the simulation of the closed loop drive system, steady state performance characteristics are obtained at different operating conditions. The equations for the current and speed PI controller are derived in discrete time domain. Ziegler-Nichols, Hand Tuning, Tyreus and Luyben are applied to obtain the proportional and integral controller parameters. Also PI controller gains for speed and current control loop are optimised using Genetic Algorithms for desired performance index. The stator frequency which is capable of producing resonance at harmonics 5, 7, 11, 13, 17 are predicted and during the steady state analysis it is found from the waveforms that the problem of resonance is not found for a wide range of operating frequencies. The LC filter at the supply side, dc link inductor and the terminal capacitor values are properly designed. A simplified control scheme utilizing space-vector modulation is developed to calculate the duty ratio required to synthesize the stator reference current. The proposed scheme has the advantage of space-vector modulation with fast dynamic response. An exhaustive simulation has been carried out on the CSI fed IM drive and results obtained are showing a better alternative to the conventional SVM scheme applied to CSI drives in terms of wide operating frequency range without resonance.