ANALYSIS OF DOUBLY-FED INDUCTION MACHINE OPERATING AT MOTORING MODE

Abstract

India, the land of agriculture is facing severe energy shortage even though the installed capacity of Indian power sector is increasing continuously, as demand outstrips incremental supply. India has an ambitious target to be “Energy Independent” by the year 2030 and the country plans to achieve it, by amalgamating various fuel sources and technologies. However, the main concern of policy makers across the world remains global warming vis-a-vis climate change. Use of conventional fuels for power generation is not gratified due to the emission of greenhouse gases. Emphasis is therefore laid on enhancing power generation through revolutionary renewable energy sources that augments power supply which is environment friendly. Taking cognizance of the opportunities and challenges ahead, the persistent growth in renewable energy generation, can moderate greenhouse gas emission to a larger extent. However, use of renewable resources offers its own set of problems. The increased penetration of renewable energy source (e.g. wind energy and solar energy) offers challenges to balance generation and consumption in the power system. The network interconnection regulation becomes more rigid when renewable power penetration becomes a non-negligible portion of the total power system. The power fluctuations in power system can lead to redundant change in system frequency and it may lead to failure of system if change in frequency is beyond prescribed limit. In interconnected power systems, the widespread deployment of energy storage systems becomes essential to effectively guarantee the power balance and improving operating capabilities of the grid, lowering cost, ensuring high reliability, feeder voltage and supply quality. From the catalog of energy storage systems, the solution for large storage (both for quantity and duration) reveals “pumped storage hydro-power plants” as one of the best in terms of storage potential, flexibility, reliability, response time, efficiency, risks and cost effectiveness. The pumped storage hydro-power plants can store power (charging) in the form of gravitational potential energy of water, by pumping from a lower elevation reservoir to a higher elevation reservoir. This can also generate power (dis-charging), by transferring water through turbine from higher elevation reservoir to lower elevation reservoir. Usually hydro-turbines are optimized for an operating point defined by speed, head and discharge. At fixed speed operation, limited deviations from head and discharge are allowed. But to improve the efficiency of operation of hydro turbines with part loads and with varying heads, variable speed operation of hydro turbine and generator offers an attractive solution. However, with variable speed operation, it is required to adapt the generator output voltage and frequency to match with the grid standards. In this regard the power electronic technology plays a vital role to match the characteristics of these generation units and the ii requirements of grid connections, including frequency, voltage, control of active and reactive power, harmonic minimization, etc. The synchronous machine (controlled converter connected to stator side) and more specifically Doubly-Fed Induction Machine (DFIM) with converters connected in rotor circuit are the main configurations used in pumped storage hydro-power plant for variable speed operation. The DFIM puts forward, the advantage of variable speed drive with four quadrants active-reactive power controls, lower rating (slip power) of converters in comparison with synchronous machine and hence lower power losses and converter cost. These DFIM based pump-turbine units are optimally suitable to deal with fluctuating power through sub and super-synchronous speed operations in motoring and generating modes. A comprehensive literature survey on converters to control DFIM shows single stage (AC-AC) cyclo-converters and matrix converters or double stage (AC-DC-AC) back-to-back converters. In the recent past, multi-level back-to-back converters have gained importance for the control of DFIM for industrial applications particularly at places where size is of much importance. The other applications of DFIM include wind power generation (generating mode only), land irrigation system (motoring mode only), naval propulsion and electric vehicle/hybrid electric vehicle (propulsion in motoring mode and braking in generating mode). Real power control (part-load operation at motoring mode) of DFIM is more significant in a variable speed pumped storage hydro power plant as the plant usually experiencing variation of surplus power available in the grid. In literature, various stating methods are discussed including rotor resistance starting, use of reactor and auto-transformer, rotor-chopper starting, rotor side converter based starting keeping stator short and two-way switch among others. The desired operation (sub-synchronous/super-synchronous) of DFIM in motoring mode can be obtained through controllers that control the rotor side converter and grid side converter. Various researchers and academicians have widely analyzed and designed several controllers based on scalar and vector control. An exhaustive literature survey has been carried out in this current work for control of DFIM in motoring mode i.e. decouple control of active, reactive power and speed control with stator side unity power factor. High performance sensor-less control is also reviewed. In practice, pumped storage hydro-power plants equipped with DFIM in motoring mode, experiences power quality problems that are dependent upon location of plant and length of the distribution feeder used. Therefore, the economic operation of the plant may affect due to power quality issues in pumping operation of pumped storage hydro-power plants. Analysis iii of DFIM under non-ideal supply voltages (voltage sag and unbalance) shall be received a good attention from both academia and industry as variable speed pumped storage plants are under development in many countries. Tehri hydropower development corporation (THDC) India ltd. is currently constructing (phase III) a variable speed pumped storage plants with a rated capacity of 1000MW (4x250MW) on Bhagirathi River, Uttarakhand state in addition with 1400 MW (phase I & II) commissioned capacity of fixed speed hydropower plants. This is the first project in India with variable speed technology. This present study helps to understand the performances of DFIM in motoring mode especially under voltage sag and grid unbalance so that the project authorities could be taken sufficient steps to improve the performance/efficiency of the plant during design stage. A three-phase slip ring induction machine is considered as DFIM to analyze the power quality issues in steady-state and dynamic performances. The machine is started at no load similar to squirrel-cage machine when rotor is short circuited. The two-way switch connected to rotor terminal is used to change over the connection from short circuit to converter (rotor side converter). Now the load is applied and desired control through rotor side converter is achieved for required speed (sub-synchronous/super-synchronous) with stator side unity power factor. The various causes and types of voltage sag are illustrated in this work. The doubly-fed induction motor’s performance during voltage sag depends on several voltage sag variables like type, duration and depth. Comprehensive computer simulations have been carried out in MATLAB/Simulink to investigate the performance of two DFIMs in motoring mode at different power ratings (2.2kW and 2MW) under voltage sag. The drive performance is analyzed in terms of shaft torque, transients in stator and rotor currents, total power taken from grid and speed loss in sub-synchronous and super-synchronous speed regions. The negative effects of voltage sag are system instability due to speed loss and transients in rotor current, resulting in system shut-down and converter failure, respectively. The drive speed loss and transients in rotor current depends on sag depth and duration. The over-dimension rotor side converter or its disconnection (rotor terminal short circuited hence the control is lost and machine runs as squirrel-cage motor) during voltage sag is the main idea to handle abnormal rotor currents. Although the over-dimensioned rotor converter needs not to be disconnected during sag but higher installation cost technique makes this technique undesirable. Moreover, various other methods are discussed for voltage-sag-ride through doubly-fed induction motor system. Finally, the small scale proto-type (2.2kW) is installed in laboratory with grid side rectifier and rotor side PWM inverter to validate the simulation results of DFIM. iv Furthermore, various causes and types of grid voltage unbalance are also illustrated in this work. Comprehensive computer simulations have been accomplished in MATLAB/Simulink to analyze the adverse effects of voltage unbalance in terms of grid and stator currents, variation in total power drawn from the grid, motor drive efficiency, overall grid side-power factor, rotor currents and mechanical ripples in sub-synchronous and super-synchronous speed regions. Furthermore, the performance of machine is analyzed for various magnitudes voltage unbalance factors, and for magnitudes of positive and negative sequence voltages. Unbalance causes increased losses due to time harmonics/negative sequence currents. These losses significantly reduce drive efficiency (short term effect) and drive’s life (long term effect) and hence drive de-rating must be applied to avoid damages. Due to reduced efficiency (financial loss) during unbalance grid voltage, plant authorities receive considerable attention to ensure consistent, efficient and reliable operation. During grid voltage unbalance, an extra burden is to be received by the rotor converter circuit including DC link. Finally, the experimental results on small-scale prototype (2.2kW) validate the above said analysis. A case study on irrigation pump system equipped with DFIM is carried out and the machine’s worthiness is analyzed for irrigation purpose where variable speed operation may be beneficial with limited speed ranges. Such irrigation units are suitable to deal with excess power generation from solar energy during day time by increasing pump output and may construct irrigation profitable