Distance protection algorithm for multiterminal HVDC systems using the Hilbert-Huang transform

Multiterminal high-voltage direct current (HVDC) systems still need advances in terms of protection in order to improve their reliability. In this context, the distance protection can play a major role by adding selectivity to the existing DC fault detection algorithms. Hence, the present work proposes a non-unit DC distance protection algorithm that uses the frequency of the DC voltage transient oscillation to estimate the distance of the fault. The DC voltage transient frequency is extracted using the Hilbert-Huang transform and compared with a pre-defined frequency/distance curve. The technique was evaluated by simulating faults in a four-terminal symmetric monopole multiterminal HVDC system. In the simulation environment the algorithm was fully selective for faults within the first protection zone and had a correct operation rate of 94% or more for faults located in the second protection zone. To further validate the presented technique, the proposed algorithm was embedded in a digital signal controller, running in real-time. In all performed tests in hardware, the faults were correctly detected and identified as being internal or external. The results indicate that the proposed algorithm could be used in real-world applications, in conjunction with fault detection techniques, adding selectivity to multiterminal DC protection schemes.