Interconnection of an Isolated Power System to an External Power Supply: - Dimensioning, modelling and control
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In this master thesis, a possible solution on how to connect a jack-up rig to an external supply was proposed. Using a conceptual layout of a typical jack-up rig power system as the basis, a suitable solution was proposed. The solution consists of a three winding transformer, and a frequency changer consisting of a twelve pulse rectifier and a voltage source converter. The frequency changer was needed because of the different frequencies on the supply and jack-up rig systems. In addition, filters, and a simple overcurrent protection relay were designed. After presenting the theoretical basis for the design of the components, a model was made in Matlab/Simulink to test and verify the solution. It was assumed that the external supply was a stiff network, and the external supply was therefore modelled as a voltage source. The external supply supplies the jack-up rig through a sub-sea cable with voltage of 36 kV and frequency of 50 Hz. A simple RL equivalent was used to model this based on a suitable cable from Nexans. The twelve-pulse rectifier model was made using two premade six-phase rectifiers in Simulink connected in series. This to minimize DC link ripple. The DC link filter was dimensioned analytically with respect to allowed current and voltage ripple and the resulting values were inductance, L = 13.63 mH and, capacitance, C = 1.2 mF. A two level voltage source converter was used to convert DC to AC at the desired 60 Hz frequency. The control system was based on proven methods using a cascaded control structure with an outer voltage control loop, and an inner current control loop. The control was performed in the d-q reference frame allowing the use of PI controllers. These were tuned using the pole placement technique. The inverter output filter was tuned to attenuate the dominant inverter induced harmonics. A cut-off frequency of ω_o= 1257 [rad/s], with inductance, L = 6.3 mH, and capacitance, C = 100.36 µF proved to give good attenuation. All steady state simulations proved successful with respect to harmonic distortion, voltage level and frequency. The largest load side THD was the current THD on the HV switchboard. This was 0.59 %. The supply line current THD at nominal load was 9.28 % due to the twelve pulse rectifiers and the lack of a filter. During simulations of different load scenarios, step changes in load caused a maximum voltage dip of 20 V, and a current overshoot in one of the phases of about 1500 A. Using a linear ramp load instead, with a change in load power, dP/dt = ± 6 MW/s, and reactive power change, dQ/dt = ± 4 MVAr/s the supply followed perfectly, without any transients. Fault simulations were done with a three-pole short circuit on different places on the jack-up AC side, and a bolted pole-to-pole short circuit of the DC link. None of the faults at the load islands caused any overcurrent in the interconnection. The largest being 159.2 kA at the inverter terminals. The initial peak fault currents during jack-up AC side fault simulations was caused by the inverter filter capacitors. A short circuit of the inverter switches proved critical. This yielded a peak fault current of 747 kA, and would destroy the inverter switches. The short circuit on the DC link had a peak current of 23 kA, and would probably destroy the rectifier diodes. The overcurrent protection relays used proved successful in interrupting the fault currents, and selectivity was maintained. Studying the results, it is apparent that the proposed solution works during normal operation, but that a multilevel converter should be used instead to minimize filter capacitors and thereby peak fault currents.