Based on the theoretical research of the characteristics of different transient process occurred on±800 kV UHVDC lines， such as internal faults， external faults and lightning， a new protection scheme for UHVDC lines based on transient energy is proposed. The solution can identify an internal fault and its faulted circuit reliably based on the characteristics of low-frequency transient energy differential of the two ends of DC lines. Comprehensive simulation studies using PSCAD/EMTDC show that the proposed scheme is simple， reliable and practical. It not only provides correct responses under various fault conditions including high-resistance faults， but also is of immunity to lightning， electromagnetic coupling between positive and negative circuits， and commutation faults of converters， which satisfies the protection performance requirements for UHVDC lines. Furthermore， the hardware of existing HVDC control and protection systems is enough to support the implementation of the proposed scheme.