| With the rapid development of economy in our country, the electric power as the leading industry has been into a period of rapid development. DC transmission with its unique technical and economic advantages plays a very important role in the nationwide power system development.AC-DC interconnected power grid has been gradually formed, especially in the in east China power grid and southern power grid as a typical case. Commutation failure is one of the most common fault of HVDC power transmission system, due to the tight electromagnetic coupling relationship between AC-DC system and the special operation and control mode of dc system, the dynamic behavior of AC-DC interconnected power grid fault is different from that in pure ac system, so it is of great significance to study the performance of relay protection under the AC-DC interconnection environment.It is of much importance for the safe and stable operation of AC-DC interconnected system to detect commutation failure both rapidly and precisely. Detailed analysis of the change rule of DC current during commutation failure is presented, and the cause of this change is also demonstrated. A customized mathematical morphology gradient is introduced to detect the DC current. When a morphological gradient that is greater than the threshold value is detected and the rate of DC current change is greater than zero simultaneously, it indicates commutation failure occurs at the moment; this method can also be employed to detect commutation failure caused by invertor faults. When severe faults occur (on the AC side), the detecting results of gradient can predict commutation failure which is about to occur soon after.The transient characteristics of the equivalent power-frequency variation current of HVDC system is obtained under different ac fault conditions through detailed theoretical analysis, which is the major issue to analyze influences of HVDC transmission on relay protection in AC-DC interconnected system. The coordinate plane is introduced to give a better description of the phase changes of the equivalent power frequency variation current. In addition, the range of phase variation can be determined by using the coordinate plane, and it laid a solid foundation of studying the effect of commutation failure on AC relay protection. In addition, the effect of commutation failure on directional pilot protection is taken as example to be analyzed in detail.The transient process caused by commutation failure in AC-DC interconnected system is not the same as that in pure AC system, which has effect on the performance of differential protection. The relation between the equivalent power frequency current of DC system and the converter bus voltage is obtained through simulations based on the CIGRE HVDC Benchmark model. The expressions of the current differential protection in AC-DC interconnected systems are deduced based on superposition principle. Then, the differential protection of the line that is connected to the converter station directly is analyzed in detail. The analysis results indicate that relay protections incline to reject acting when faults occur under the circumstance of AC-DC interconnected operation. Furthermore, a novel criterion to detect the internal fault only based on amplitude is proposed to solve this problem. PSCAD/EMTDC simulation results validate the correctness of the above analysis.Distance protection is one of the typical configurations of high voltage transmission lines. As the fault transient process in AC-DC interconnected systems is quite different from that in a pure AC system, the operation of distance protection will be affected to some extent. It is found that the effect of HVDC on distance protection mainly lies on the changes of additional impedance, which is due to DC current flow through the fault resistance. Moreover, compared to Fourier algorism, R-L algorism shows a great adaption to AC-DC interconnected systems with rapid and accurate calculation for measured impedance. Detailed analysis on the traditional AC-DC interconnected system model is presented, and a modified model is proposed based on the distribution of zero sequence current. With consideration of the connection mode of grounding impedance relay, the additional measured impedance caused by DC fault current flowing through fault resistance is deduced theoretically based on superposition theory. Thus its impact on distance protection is analyzed, and a countermeasure based on zero sequence directional element is put forward subsequently. Simulations based on PSCAD/EMTDC verify the validity of the proposed model and prove the theoretical analysis to be correct. |
