Compared with the high voltage AC power transmission, HVDC power transmission has some advantages, such as larger transmission capacity, longer transmission distance, lower line loss, etc., which is becoming an important power transmission way on asynchronous grid interconnection and clean energy power integration [1, 2, 3]. Because of the long distance and passing through the valleys and mountains, HVDC lines easily suffer from the faults caused by lightning, storm, and so on. Therefore, to ensure the safety and stability of DC transmission system, a fast, reliable and sensitive HVDC line protection is indispensable.
At present, the primary protections used in HVDC line are traveling wave protection and voltage variation protection, which have poor ability to resist the high ground resistance fault . Additionally, their performances are also affected by other disturbance signals. HVDC line differential protection used for the back-up protection also has certaion shortcomings, such as slow operation speed and requiring the synchronous data [5, 6].
Aiming at these problems mentioned above, many protection methods have been put forward. In , authors analyze the performances of HVDC line protection for different fault transition resistance, fault position, etc., and finally present several improvement suggestions. Through analyzing the harmonic current characteristics of converter station, a protection scheme based on the transient harmonic current is present for HVDC line in . According to the boundary characteristics of HVDC line, some protection methods only using one-end information are proposed in [9, 10, 11, 12]. Although these methods have fast speed, their sensitivity and reliability are still influenced by the fault resistance, in addition, it is also difficult to set the threshold value. In [13, 14], authors propose the distance protection methods for HVDC line, but the high ground resistance fault still has a negative influence on their protection performances. To improve the reliability of protection, the hybrid protection schemes based on different principles are put forward in [15, 16, 17], but implementing these schemes is still a challenge in actual HVDC transmission engineering.
In  and , the transient-energy based protection methods are proposed for HVDC line, which can identify an internal or external fault by comparing the amplitudes of the transient energies on both ends of the line, these methods are similar to the current differential protection, and they need strict data synchronization. Different from the aforementioned methods, this paper presents a novel directional pilot protection method based on the polarity characteristic of transient energy. For a fault inside the DC line, the transient energies detected on both sides of HVDC line are negative; while the transient energy on one side is positive, and that on the other side is negative for an external fault. On the basis of this fact, an integral criterion identifying the fault direction is constructed. By analyzing the fault directions on both sides, the internal fault can be discriminated from the external fault. Due to only using the directional information, the proposed protection method does not require the synchronous data at all. To verify the performance of the proposed method, a bipolar HVDC transmission system model is built, and extensive simulations are carried out.