Analysis Of Resonance Over-voltage And Start Control Of Long Cables Frequency Drive System

In the offshore oil and gas power supply system, due to large pump load in use, out of production output and equipment reliability considerations, the motors have to be drived by frequency control devices. Offshore platforms and wells are usually long distance, so a long jumper cable is needed between the inverter and the motor.Because long cable has large self-inductance and ground capacitance, the cable itself has a resonant frequency. Ans due to inverter produces a large amount of high order harmonics, if the frequency of the harmonics is the same as or similar to the resonance frequency of the cable, it may cause the system to resonance and generate overvoltage on the motor, it will cause damage to the system. If the system is put in step-up transformer, the resonance may occur between the transformer and the cable. High order harmonics generated by inverter mainly concentrates nearby the switching frequency and multiple of switching frequency. Therefore, we can change the switching frequency and avoid the harmonic-rich region and the system resonant frequency overlap, then we can considerably weaken the system resonance. The transmission voltage of offshore oil and gas power supply system is relatively low, so the cable voltage loss is much serious, it may cause the motor can not be started. Therefore, we must compensate for system voltage. By analyzing the characteristics of the system load, we can select the appropriate frequency control mode and simulate the motor starting process. First, for the pump load, we select Constant V control, and then improve its control processes, adding voltage compensation measures, to meet the needs of long distance variable frequency drive system. As constant V control intrinsic defects, we propose speed sensorless vector control, and also improve its control processes. Then compare the two control methods,and clarify their advantages and disadvantages.