Analysis of resonance frequency at six-step induction motor drive with long cable connection in a geothermal plant

In this paper, a six-step 120-degree induction motor drive with long cables connected to the induction motor at medium voltage level is analysed for predicting resonance frequency. These systems can be preferred to PWM-based drives in facilities such as geothermal power plants, mine plant and submarine oil wells, since the dead-time control is not issue in the inverter operation. The motor drive in the system contains semiconductor switching elements which generate the current and voltage harmonics in a wide range of frequency spectrum. The voltage and current harmonics at the inverter output lead to the resonance problems, torque and rotor speed fluctuations due to the capacitive effect of the long cables and the inductive effects of the cables, motors and transformers. The resonance in the network creates the overvoltage and overcurrent which can cause damage to the electrical components within the system. In this paper, analysis of resonance frequency at six-step 120-degree induction motor drive with long cable connection in a geothermal plant is performed first time in an analytical approach and C-type filter is suggested to suppress the resonance.