Protection of a voltage source converter (VSC) based HVDC system.

Abstract

To conserve energy and promote environmental sustainability, the power industry has invested a great deal into the generation of electricity using renewable energy (RE) sources. For such applications, high voltage direct current (HVDC) systems are considered a highly efficient alternative for bulk power transmission. Recent advances in technology favour the use of voltage source converter (VSC) based HVDC systems for the integration of RE sources. These schemes are favoured for their controllability and provide major reinforcements to the power systems. Despite its promising future, the technology is constrained by the unavailability of a reliable protection scheme, as the operating times for HVDC protection schemes are required to be ten to a hundred times faster than existing AC protection algorithms. VSC-HVDC networks are usually more vulnerable to DC-side faults. Selective protection against these faults is therefore essential for safe and reliable operation of the network. This study provides the necessary concepts to develop VSC-HVDC protection algorithms for multi-terminal (MT) meshed HVDC systems. DC fault characteristics were initially investigated. They provided a basic understanding of the VSCs natural responses to DC fault scenarios. The study also focused on analysing factors that may adversely influence the systems protection performance. These include the DC fault distance, DC-link conductor sizes and DC fault impedance. Results obtained from these variations show that the DC-link capacitor was one of the main sources that cause the high rise of DC fault currents and that these are the highest and the most dangerous when closest to the converter station. With a clear understanding of the DC fault characteristics, a protection scheme has been proposed. Initially, different methods are discussed with the intent of deciding on the scheme that is the most suitable. Detection techniques based on the discrete wavelet transform (DWT) for primary protection and the current derivative technique for back-up were chosen as the most promising. These techniques offer accuracy, speed and selectivity which are factors that are all important for the network. The single VSC terminal travelling wave technique was implemented to identify the exact position of a DC fault. This method reduces costs as it eliminates the need of communication links. Finally, to isolate the affected cables, the hybrid DC circuit breakers (CB) were implemented into the VSC-HVDC system. The CBs have been coupled with a reactance for fault current limiting and to isolate the system before it reaches an uninterruptable current magnitude. Back-up AC CBs were included on the AC side of the network and were stationed to separate the VSC network from the AC grid in cases where the implemented primary protection scheme fails.