Voltage control for optimization of power transmission for a long subsea HVAC cable
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- Institutt for elkraftteknikk 
The objective of this thesis is to make a dynamic system representation of a long HVAC subsea cable, to investigate the use of controlling the operation voltage to optimize the active power transmission of the cable. The dynamic representation is based on the simplified model presented in Variable Transmission Voltage for Loss Minimization in Long Offshore Wind Farm AC Export Cables . The paper is a static analysis investigating the potential of minimizing the losses for a long HVAC cable, by using either fixed or variable voltage control.The system presented in  consists of an offshore wind farm (OWF) connected to a grid onshore through an AC cable. The analysis of the system only considers the cable side of the system by representing the OWF and the grid, with their respective transformers with on-line tap-changers (OLTC), as voltage sources, V1 and V2 respectively. This is made possible by assuming the transformers as ideal. The operation voltage is controlled by the voltage source V2. The variable operation voltage is optimized to the produced active power from V1. By continuously adjusting the voltage scaling to the instantaneous active power production, the operation current is kept within the operation limit of the cable. The result of the analysis is that the cable losses are minimized by controlling the operation voltage. The variable operation voltage is found to have the highest loss reduction. The dynamic representation of  made in the thesis, substitutes the transformers with OLTC, with back-to-back DC converters. The voltage regulation will then be faster. The dynamic representations for fixed and variable operation voltage were made in Simulink. The voltage source representing the OWF V1, was made as a voltage source with a voltage source converter (VSC) as a control loop to control the active power output of the voltage source. The voltage source representing the grid V2, was made as a voltage source with a fixed input for the fixed operation voltage model. The variable operation voltage model had a control loop, which adjusted the operation voltage based on the active power received at the grid side of the cable to create a more realistic model. It did not however continuously adjust the voltage scaling. The performance and viability of the system were investigated for several power production and fixed voltage levels. The performance was considered by calculating the cable efficiency. The viability of the system was considered by measuring the operation current and voltage to see if they were within the cable s operation limits. Most of the simulations were found to have exceeded the current limit of the cable, and none of the simulations had as high cable efficiency as in . The only simulations that did not exceed the current limit was the low production levels for variable operation voltage and low fixed operation voltage. The low fixed operation voltage simulations where found to have the highest operation area for power produced and the highest cable efficiency. The difference in results between the simulations in this Thesis and  can be explained by the high charging currents, which suggest that the cable selected for the simulations is not equal to the one in . The use of voltage control to optimize the power transmission was however found to work as the simulations showed that low operation voltages gave the highest cable efficiency for low production and high operation voltage gave the highest cable efficiency for high production.