DESIGN AND DEVELOPMENT OF ENERGY CONSERVATION STRATEGIES FOR ASYNCHRONOUS MACHINE DRIVES

Abstract

Energy is the fundamental source for powering our lives, and growth of energy is linked to prosperity across the globe. At present, facing energy demand in environmentally affable way is a key challenge. Enhancing power generation of revolutionary renewable energy sources is the solution for reducing carbon emission. It is far better and easy if energy is used efficiently “Energy saved is energy generated”. Universally, electric motors are the greatest consumers of electricity and more than 90 percent of them are induction motors (asynchronous motors). Several factors which include robustness, low maintenance and low cost have made them popular for industrial applications. Electric motors in which spinning, coiling, grinding, crushing, and lifting applications, are frequently operated under part-loads results poor energy efficiency and power factor due to imbalance between copper and iron losses. Electric grid having many such partly loaded motors causes low power factor, resulting in voltage instability. Energy conservation (EC) can be obtained in greater extent with an efficiently utilised motor. Generally, electric machines are designed to attain maximum efficiency and elevated power factor in and around rated load. Hence, betterment of efficiency and power factor scales down the motor operating cost and reactive energy bill respectively. These part load efficiency and power factor can be improved by adjusting the motor excitation in accordance with load and speed. To adjust the excitation, induction motor should either be fed through an inverter or redesigned with optimisation algorithms. Therefore, enhancement of energy efficiency and conservation of constant speed IM drives is performed by regulating flux using various control techniques like simple state control, loss model control, search control, etc., In recent years, the use of variable speed drives in industrial plants has been steadily increasing. Unlike constant speed drives, variable speed drive provides precise speed control corresponding to load requirements. Advancement of variable speed drive (VSD) technology possesses energy efficient dynamic control. Doubly fed asynchronous motor (DFAM) based VSD fits for modern applications like variable speed pumped storage plants (with wide variation in water head), cooling, hoisting, traction, propulsion, etc., due to its effective controllability with partial rated power converters. In DFAM, energy optimisation techniques are implemented through controlling rotor power converters, and stator windings are directly connected to grid. As a result, full voltage is applied to stator windings irrespective to load changes which causes excessive iron loss at part-load. To reduce these losses, voltage in stator windings is altered according to load variations. In this aspect, two EC strategies are developed by switching the stator windings with respect to load variations: (i) Fuzzy Logic Control-Decision Making Algorithm EC strategy for DFAM, (ii) DC Injection EC strategy for DFAM. ii In developed EC strategies, rotor winding connections and control algorithm remain unaltered. Hence, these developed strategies are flexible to adopt in any existing doubly fed VSD system without any extensive modification. Energy optimisation of doubly fed system using fuzzy logic controller and decision making algorithm (FLC-DMA) is a good option for variable speed torque applications. In FLC-DMA based switching strategy, input voltage of stator is altered in accordance to load variations for reducing the losses. This algorithm predicts the switching point with speed and current input values for effective EC. In addition, an energy efficient starting technique is developed and implemented in DFAM. In the proposed strategy, DFAM is operated with synchronous speed, (i.e.) electromagnetic interaction between windings is created without slip losses. This zero slip ramping is executed by applying DC voltage to stator winding when the rotor starts to rotate and is continued till the machine is synchronized with grid. A low voltage variable DC supply is applied to stator winding through a transfer switch and the rotor converter pulses are adjusted to obtain speed variation with EC. This low voltage DC supply is tapped from DC link through a stepdown chopper. Such action makes the system to operate in variable speed mode with a substantial amount of energy saving without external DC supply. Developed DC Injection EC strategy can be adopted for any DFAM system. As the motor efficiency is directly related to energy loss and energy pricing structure, efficiency of existing DFAM drive can be improved with the proposed control strategy instead of adopting sophisticated techniques. Developed EC start-up strategy in a DFAM based PSP is analysed in economic perspective. In India, the first variable speed PSP is under construction in Tehri dam of Uttarakhand State with the capacity of 1000 MW (250 MW x 4 units). From the analysis, it is observed that implementation of start-up EC strategy in PSP leads to a saving of 6,62,976 kWhr per year. Advancement of power electronic topologies and control techniques possesses energy efficient operation on industrial drives, but these power electronic controls are not cost-effective in many cases. Moreover, power electronic switches could be the source of harmonics, causing vulnerability to machines and gird. Two cost effective energy conservation strategies are developed without power converters for induction motors serving in low or medium scale process industries: (i) Star- Delta EC strategy for singly fed asynchronous machine, (ii) Stator Short Circuit (SSC) and Rotor Short Circuit (RSC) EC strategy for singly fed asynchronous machine. In developed EC strategies, contactors are used to switch the winding connection for applying a reduced voltage and full voltage during light and high load torque demands, respectively. As the machine is operated with reduced rated voltage during stator short circuit operation, this strategy accomplishes EC in greater degree with higher efficiency and improvement in power factor. In addition, protective features against sensor fault, overloading and switchover instability is also taken care. Developed EC strategies are iii appropriate for motors with substantial load variation during their duty period, especially with longer duration of part load operation. Such process improves the efficiency and power factor without any extensive modification. On the other hand, these electrical drives in any industry are influenced by various electrical perturbations, and causes process interruptions and equipment failure. Power quality disturbances (PQD) are generally caused due to network instability that commit deleterious effect on electrical loads connected to it. Such instabilities are created by non-linear loads, system faults, motor starting, intermittent loads, power converter switching, uneven distribution of single phase loads, sudden variation in loads, etc., PQD mainly affects the industrial drives and the level of impact is based upon their age and winding connections. Hence, the quality of power received in any industry is the key factor for reliable and smooth operation. Therefore, EC schemes developed in the present research for asynchronous machines are analysed with various PQD and faults. Also, considering these PQD, the proactive solution is provided for uninterrupted operation through automatic adjustment of switch over point. In laboratory, based on theory and empirical evidence, experiments are conducted to identify the outcome of developed EC strategies in test machines (2.2 kW SFAM and 2.2 kW DFAM). Empirical evidence is obtained precisely by functioning the apparatus in a systematic procedure