Aiming at the fault traveling wave on the LCC-MMC HVDC transmission line, this paper analyzed its reflection and refraction characteristics at the DC buses and the fault point, plotted its propagation grid diagrams, obtained the relationship between the polarity of fault traveling wave and the boundary elements, and drew the conclusion that when the impedance of DC filter is far less than the wave impedance of transmission line, the existing fault location methods that need to distinguish these two waves according to their polarities are not applicable to the LCC-MMC HVDC system. On these bases, this paper proposed a modal traveling wave arrival time difference-based location principle for pole-to-ground fault, and a fault traveling wave propagation path ratio-based location principle for pole-to-pole fault, these principles neither need to identify the wave polarity, nor need the wave velocity value. In order to better extract the wave heads and their arrival time, this paper pre-treated the sampling data to make sure all the wave heads are extracted from the same frequency band, then the wave heads and their arrival time can be extracted with the empirical mode decomposition. The simulation results verified the effectiveness of the proposed fault location principle.

During the out-of-step oscillation process, the frequency of voltage at every point in power system changes continuously. In this paper, the voltage frequency variation regulation is analyzed in an equivalent two-terminal system whose two terminal potential amplitudes are not equal and all electrical component impedance angles are different. It is found that the bus voltages frequencies at different sides of out-of-step center own different characteristics. In order to use the frequency variation characteristics to construct the criterion of power system out-of-step protection, a fast frequency calculation algorithm is proposed, in which the frequency is calculated by toggling different formulas at different sampling points and the calculation accuracy are improved by the aid of CORDIC algorithm. The algorithm is easy to implement and the measurement errors can keep under a low level. Finally, a novel frequency based out-of-step protection with the wide area information is described. It can judge the occurrence of out-of-step oscillation in the first out-of-step oscillation cycle and determine the electrical component where the out-of-step center locates, which lays the foundation for the determination of out-of-step splitting section for complex power systems. The test results with IEEE39 system in PSCAD verify the correctness of the proposed protection.

Using the wide area information obtained from phasor measurement units, a novel out-of-step splitting protection is proposed. In order to analyze the different characteristic of bus voltage phase angles, a new coordinate system is introduced. In this coordinate system, all of the converted bus voltage phasor ends are stable, while only the common beginning is moving during the out-of-step oscillation process. Its conformal property guarantees the voltage phase angle differences in the old and new coordinate systems are equal. After the coordinate transformation, the formula about the converted power network neutral point trajectory is derived. The intersection of the trajectory and the broken line which connects the bus voltage phasor ends is the out-of-step center. All of the buses are divided into two parts by the out-of-step center. Through a series of proofs, it is found that the absolute values of the bus voltage phase angles in one part are always smaller than 110°, while the voltage phase angles of the rest buses vary from -180° to 180°. These different characteristics could be used to group these buses and locate the out-of-step center. It is very simple because only the voltage phase angle information is applied, and there are no complex mathematical operations. Finally, a novel out-of-step splitting protection scheme is given. The digital simulation results prove the proposed scheme is efficient.