REDUCED SENSOR BASED PV ARRAY FED DIRECT TORQUE CONTROL INDUCTION MOTOR DRIVE FOR WATER PUMPING

Abstract

This project aims to design, control and implementation of a solar photovoltaic (PV) array fed speed sensorless direct torque control (DTC) of an induction motor drive (IMD) for water pumping in standalone as well as battery connected hybrid mode. This stator flux estimated by proposed flux observer, is used for speed estimation. A DC link current sensor is used to reconstruct the motor phase currents by modified active voltage vector. One voltage sensor for DC link voltage sensing and only one current sensor for DC link current sensing, are used in this system for standalone operation of the system. All other required quantities are estimated through these two sensed signals. The IMD is energized by a photovoltaic (PV) array, which is operated at maximum power point (MPP). A perturb and observe control algorithm with additional flow rate controller, is proposed for MPP, which tracks MPP throughout the operating range and provides the facility to control flow rate. The suitability of the system is judged through simulated results in MATLAB/Simulink as well as test results obtained on a prototype developed in the laboratory.

Existing System

In the existing system, DTC of an induction motor drive (IMD) is even further improved by using H-bridge or multilevel inverter for high voltage and power applications. However, this switching table approach has some demerits. The sampling frequency of overall system, is kept high to limit the flux error and torque error in the desired error band.

Proposed System

An approach has been proposed here to estimate stator phase currents by a DC link current sensor mounted in the DC link and thus reducing the sensor cost, size and space requirement of the system. Moreover, the use of a single current sensor reduces the chances of discrepancy of current sensor sensitivities caused when two or more current sensors, are used and thus reduces the chance of phase currents unbalance. The space vector modulation (SVM) technique with slight modification, is used for current estimation. The modified SVM technique is necessary for the proper sensing of DC link current especially when the reference voltage vector (Vref) is in the vicinity of any one of the active vectors in any sector. The minimum time required for the reliable DC link current sensing must not be less than the sum of the inverter dead time (Tb), the reverse recovery time (Tr) of the diode and the sensor’s analog-digital acquisition time (TA/D). Therefore, active voltage vectors in each sector, are slightly modified to ensure the reliable detection of phase currents. An additional power source is required for continuous operation of the pump. This can be accomplished by a battery storage, in which power flow is controlled by a bidirectional converter through charging and discharging of the battery. When solar insolation is at its peak and the pump demands full operation at rated condition, the battery is not required. The battery is charged by the PV array, when water pumping is not required and is discharged when insolation is less and the water pumping is required at its rated volume. A 240 V, 300 Ah Lead acid battery is connected at the DC bus as a back-up support. Therefore, it enables a full utilization of both PV array installation and motor-pump system.

Architecture

Scheme of proposed PV array fed IMD