Study On Offshore Long Cable Frequency Conversion Drive System

In the current offshore oil extraction,submersible motor is an indispensable equipment,which has a wide range of applications.In order to better control the motor to make it in the ideal working condition,usually using PWM frequency drive to control the motor.However,due to underwater harsh environment restrictions,the inverter can only be placed on the platform.As the motor installed under water,so the need for long-term cable to connect the inverter and motor.When the voltage pulse signal is transmitted from the drive cable to the long end of the motor,since the motor side is often greater than the impedance of the cable characteristic impedance,the reflection wave at this time will generate an overvoltage in the motor terminal,generating a high frequency ringing.Meanwhile,when a PWM pulse signal carrying frequency harmonics close to the resonant frequency of the system,the motor terminal voltage will resonate over.High harmonics will also form a harmonic standing wave on long cable,resulting in long-range cable part of the harmonic is amplified.Over-voltage phenomenon will make the cable and the motor heat,resulting in insulation aging,and even a short time insulation breakdown caused by motor burned out,burst cable and other issues.In this paper,the transmission reflection process of the pulsed wave on the cable is analyzed from the theory of uniform transmission line.The influence of the wave reflection overvoltage is divided into three aspects:voltage amplitude,oscillation frequency and damping time.The influence of the parameters of the system on the three aspects of the wave reflection overvoltage is analyzed by the combination of theory and simulation.At the same time,the resonant and harmonic standing waves are analyzed by simulation,and the method of avoiding system resonance is proposed.Finally,for the three aspects of the wave reflection overvoltage,two criteria for judging the overvoltage condition of the motor are put forward,and several kinds of suppression measures are put forward.A kind of RLC filter structure and parameter selection method is designed and its simulation is carried out.