Small signal stability analysis of a voltage source converter (VSC) based high voltage direct current system with onshore wind power generation.

Abstract

Advances in power electronics over several decades has led to high voltage direct current (HVDC) transmission emerging as a good option for electricity transmission in many circumstances, and it is able to integrate with, and augment, conventional high voltage alternating current (HVAC) transmission. This is because HVDC provides enhanced system operation and improved support in the development of onshore and offshore transmission networks. Because of its speed and flexibility, HVDC technology can provide transmission systems with several advantages such as transfer capacity enhancement, better power flow control, transient stability improvement, damping of power oscillations, enhanced voltage stability and control, rejection of cascading disturbances, and the absence of reactive power generation or absorption by the cable or line. In Africa, there are three HVDC systems. The Caprivi Link Interconnector between Zambia and Namibia is the first and only one on the continent employing voltage source converters (VSC), as opposed to conventional line commutated converters (LCC). HVDC VSCs use insulated gate bipolar transistors (IGBTs) technology as switches. IGBTs provide more controllability and flexibility than the thyristors used in LCC technology. These features make them suitable for implementation in multi-terminal HVDC networks; which are envisaged to form HVDC grids in future. The Caprivi Link interconnector is therefore a pioneering project in the development of electricity transmission in Africa. The southern African power pool (SAPP) currently has inadequate electricity generation capacity. Investing in more renewable energy options, such as wind generation, coupled with the use of HVDC transmission, could be possible remedies for this. The aim of this research was to investigate small signal stability issues that can be present in VSC-HVDC systems. The Caprivi Link Interconnector is used as the contextualization of the investigated system. Different conditions such as AC system strength, controller parameter and wind power generation levels are investigated for their influence on the small signal stability of the system. Small signal stability analysis is also investigated for different HVDC configurations. It is found that the systems experience both electromagnetic as well as sub-synchronous oscillations. The sub-synchronous oscillations are generally well damped and only affect the DC impedances. Power System Stabilizers (PSS) as well as Thyristor Controlled Series Compensators (TCSC) are implemented as mitigation for the electromagnetic modes.