A Study of Stress Grading System of Medium Voltage Motor Fed by Adjustable Speed Drives

The demand for medium voltage (MV) induction motors with an adjustable speed drive (ASD) has grown significantly over the past decade for many industrial applications. This is mainly because the applications of adjustable speed drives have clear advantages of enhanced efficiency of using electric power and precise control of the speed of industrial processes. However, the high frequency components of the output voltages of an ASD produce complex transients that stress the motor insulation. The fast rise time of repetitive impulse voltage creates additional electrical and thermal stresses on the motor’s insulation system. In addition, the overshoot voltage at the edge of the pulses, due to the impedance mismatch between the motor and the cable, increases the risk of insulation breakdown. The performance of the stress grading system under these fast-pulsed voltages is a critical issue for MV motors. The high electric field in the stress grading system and temperature rise due to Joule heat are the most important problems of a conventional form-wound coil stressed by an adjustable speed drive. Any local high electric field produces surface partial discharges (PD) on the stress grading system that may lead to immature insulation failure. Limiting the temperature rise and controlling the local electric field to avoid surface discharges and hot spots using an appropriate stress grading system is essential to prolong the life of MV motors. The conductive armor tape (CAT) and stress grading tape (SGT) are the two main components of the stress grading system of a form wound motor coil. The material properties and builds of the CAT and SGT applied to the conventional form-wound coils have been designed for power frequency voltages. However, it is less effective under pulsed width modulation (PWM) voltage that are typical from ASDs, because they have high frequency components that lead to elevated electrical and thermal stresses and thus faster ageing. The distribution of voltage and electric field along the coil in the overhang region are changed by the material properties; therefore, the enhanced electric field in the CAT or the SGT may lead to PD and hot spots in these regions. In this study, comprehensive electro-thermal coupled finite element method (FEM) using COMSOL® 5.3a has been developed in order to simulate the stress grading system with nonlinear field dependent materials. The actual dimensions of a 13.8 kV bar sample were applied in the simulation model along with the appropriate material parameters extracted from the experimental test results. The temperature rise associated with a one cycle of pulsed voltage is very small. However, a prolonged transient coupled electro-thermal FEM simulation, for example for one-hour, is impractical due to very long computation time. The simulation was run for three cycles and the heat source was calculated. Then the average heat source of domains during these cycles was calculated by another time transient ordinary derivative equation (ODE) interface. This average was used in a stationary study of heat transfer to obtain the temperature profile at steady state. To validate the simulation results, the temperature profile along the stress grading system were measured and simulated under pulsed voltage (2.5 kHz, 11.3 kV peak pulsed voltage) which shows good agreement between the simulation and the measurement studies. The electrical conductivity of CAT and SGT, which differs significantly by vacuum pressure impregnation (VPI), is the most important parameter affecting the voltage distribution, and it can change the temperature profile and the regions of hot spots along the stress grading system. This can also can be changed by temperature and tape builds. Therefore, the electrical conductivity of the tapes used was measured after VPI and under conditions of operation. The SGT works under high electric field, so, the conductivity of this tape must be measured under a high electrical field. However, it is impossible to measure the conductivity with DC voltage above 0.6 kV/mm, because of the excessive heat and temperature rise in the SGT material during the measurement. To reach higher electric fields, the measurement was carried out under pulse conditions. The conductivity of a one half-lap layer and a double half-lap layer of CAT and SGT at various temperatures was measured. Based on simulation and measurement results, this study presents the effect of conductivity of stress grading materials on the temperature profile and the electric field distribution along end winding region. One way of increasing the electrical conductivity of the tapes is to increase the number of layers of the tape. Therefore, simulation studies on various stress grading system builds on the electrical and thermal performances of the stress grading systems was done in this study. Reducing the maximum surface electric field is essential for prolonging insulation life. Simulations on the effect of floating metal foils, applied to the stress grading tape, on the electric field and temperature distribution, was studied in this work, under repetitive impulse voltages. Additionally, the evaluation of the thermal and electrical characteristics of the stress grading system under a reduced length of CAT from the slot exit was carried out. The temperature profile of the stress grading system under pulsed voltage at room and at elevated room and near typical operating temperatures are measured and simulated for several CAT lengths. The partial discharge inception voltage was also measured for different CAT lengths. A lower temperature rise is desirable, as this leads to a longer life, in the absence of partial discharges. The nonlinearity of SGT has an effect on the electric field distribution. Simulation studies on the effect of various SGT nonlinearities on the electrical and thermal performance of a stress grading system was evaluated in this study. The effect of using a stress grading system based on a micro-varistor characteristic on both temperature and electric field was also investigated. An optimization on the initial conductivity of the micro-varistor characteristic confirms that desired electric field and temperature rise are achievable by selecting an optimum conductivity. The effect of a proposed stress grading system, which is a combination of an optimized SGT conductivity and minimum CAT length, on the temperature and electrical performance under repetitive impulse voltage is evaluated. Finally, to practically evaluate the effect of micro-varistor type of stress grading system, the electrical and thermal performance of a cable termination based on a micro-varistor characteristic were evaluated by measurement and simulation