Protection Operation Sequences and Risks in AC/DC Line Touch Fault

The advantages of DC in long-distance transmission and the scarcity of power transmission corridors will lead to an increase in the number of crossed AC and DC lines installations. Due to the small spacing between AC/DC lines, line touch fault are prone to occur. AC systems have no specific protection for it. Although DC systems have set up AC/DC line touch protection, but its applicability is still lack of research. Due to the entry of AC component, DC protection operations behave randomness. As a result, there is diversity of sequence of DC protection operation. Because different DC protection trip strategy can be different widely in different company, the fault treating procedure in DC system can be very complicated. By building up AC/DC crossover model with detailed AC and DC protection, this paper simulates AC/DC line touch fault considering influential factor. The simulation studied result shows potential risks in both AC and DC systems. The direct current component of the AC line will pose a challenge to the opening of the circuit breaker; at ABB's block strategy, there will be a large over-voltage due to Ferranti effect. Adding current zero detection in AC protection can help to solving the risks.