VSC-HVDC for Industrial Power Systems

Doctoral thesis, 2007

The recent developments in semiconductors and control equipment have made the voltage source converter based high voltage direct current (VSC-HVDC) feasible. Due to the use of VSC technology and pulse width modulation (PWM) the VSC-HVDC has a number of potential advantages as compared with classic HVDC, such as short circuit current reduction, rapid and independent control of active and reactive power, etc. With those advantages VSC-HVDC will likely be widely used in future transmission and distribution systems. One such application is to supply industrial systems characterized by high load density, high requirements on reliability and quality together with high costs after production interruption. In this thesis, the scenario of VSC-HVDC connecting two grids is studied as the starting point. Two different control strategies are implemented and their dynamic performances during disturbances are investigated in the PSCAD/EMTDC program. The simulation results show that the model can fulfill bi-directional power transfers, AC system voltage adjustment and fast response control. The VSC-HVDC model is further investigated for its ability to supply industrial systems with and without on-site generation. The frequency controller is utilized in the inverter station to ride through voltage disturbances, where the frequency of the VSC output voltage is slightly decreased and thereby the inertia energy of rotating masses in the system is exploited. The motivation for choosing this strategy is that the processing industries are much more sensitive to voltage drops than to frequency deviations. Simulation results show that a VSC-HVDC applied to an industrial system significantly improves the power quality of the industrial plant during voltage disturbances. Furthermore, the use of the proposed frequency controller can increase the ride-through capability of the system. However, the current limit of the converter, the power production level of the on-site generation and the generator size significantly influence its ability to mitigate voltage dips.