A STATCOM control strategy in support of direct on line starting of large induction motor in offshore oilfield power systems

The offshore oilfield power system is a independently operated weak AC system, composed of gas-turbine generators, cables and large loads. One of its serious problems is that the direct on line starting of a large induction motor (such as the water-injection pump) results in sharp increase of reactive power requirement and thus causes deep drop of the bus voltage, which in turn lead to overload of generator and drop-off of load at the same bus. To deal with the problems, an improved dynamic var control strategy based on STATCOM is presented in this paper. The strategy is an improvement on the conventional var/voltage regulation. Two novel control functions, i.e., the motor-starting var compensation and the motor-starting switch control, are designed and combined with the var/voltage regulation. Thus the motor and STATCOM are controlled to go through the states of pre-starting, start-preparation, motor-starting and start-completion successively and smoothly. The control strategy is applied to an offshore oil rig power system on Southern China Sea. The simulation results show that it is very effective. By providing sufficient dynamic var support for the direct on line starting of large induction motors (water-injection pump) with a STATCOM of appropriate capacity, the bus voltage is maintained, the safe operation of the gas-turbine generator and the loads is guaranteed.