Control of a VSC-HVDC system with large wind power Integration for power system transient stability Improvement

Abstract

Majority of wind farms are located far away from the load centers thus requiring long distance transmission of electrical power. One of the traditional methods of power transmission is based on alternating current (AC) and is not suitable for bulk power transmission over long distances. In addition, AC systems do not have full controllability of the active and reactive power compared to direct current (DC) systems. Besides, transient faults may cause high transient currents that can damage the power systems equipment. This necessitated the development of DC transmission based on line commutated converter-high voltage direct current (LCC-HVDC) and voltage source converter-high voltage direct current (VSC-HVDC) power transmission systems. Although, LCC-HVDC provides improved power controllability compared to AC systems, VSC-HVDC system is the most efficient and reliable for bulk and long distance transmission of electrical power. It also has the ability to operate in all the four converter quadrants allowing independent control of active and reactive power. Although VSC-HVDC systems are better than any other available transmission systems, they have challenges under short-circuit faults. The high current generated can cause the rated voltage across the diode to be exceeded thereby damaging the converter valves. In addition, the currently developed and commercialized DC breakers for protection of the VSC-HVDC system have a fault clearing time of 5ms. This is a major deterrent towards providing excellent protection for converters and the transmission network. This research provides solutions to converters and the transmission systems that often fail under the influence of short-circuit faults. Further, three scenarios were analyzed; conventional generation with and without wind power and that with wind power integrated through a VSC-HVDC system. In each of the three scenarios, a three-phase short-circuit fault was created on the transmission line and the critical clearing time was used to assess the performance of the proposed scheme. A novel fast DC breaker was developed in this work which has double protection against high voltage spikes provided by inclusion of the freewheeling diodes and high pass filters that divert the high frequency harmonics to the ground consequently protecting the converters and the AC network. Simulation results were done and from which a novel fast DC breaker developed in this research has a fault clearing time of 1ms. This is a great milestone in the development of a fast switching device. The findings of this research are major contributions towards the improvement of the power system transient stability of a VSC-HVDC system with large wind power integration.