Studies On The Complex Transient And The Key Technology Of Relaying Protection For AC/DC Hybrid Power Grid

With the rapid development of smart grid, the national power is becoming an unprecedented complex AC/DC hybrid power system, of which the scale is expanding rapidly, the structure is becoming more and more complicated, the technology continues to improve and the operation is flexible and changeable. If the single fault of the power grid cannot be prevented, there will be a complex and evolutionary accident process in the AC/DC hybrid power system. The protection relay, which is the first line of defense system for the safe operation of the power system, is of great significance for maintaining the stable operation of power grid and ensuring the power supply reliability. However, because of the complex structure of the power grid, there exists the sympathetic interaction between the non-linear ferromagnetic elements, such as the transformer and the CT, resulting in the complex transient process. In order to realize the long-distance and large-capacity power delivery, the new transmission technologies, such as the DC transmission and the series capacitor compensation, are widely applied in the system, causing the complex fault characteristic and its evolution process. Due to the above problems, the performance of the protection relay cannot satisfy the requirement of the safe defense requirement for the bulk power systems. The misoperation accidences of the protection are frequent, threatening the safe and sound operation of the AC/DC hybrid power system. Aimed at providing the safety protection for the smart grid, this paper makes an in-depth and systematic study on the complex transient process and the key technology of the protection relay of AC/DC grid.The energization of the transformer will cause the sympathetic inrush current in the nearby operating transformers, imposing a misoperation risk of the differential protection for the adjacent elements. The sympathetic inrush has become a research hotspot that had attracted extensive attention, because of its elusive, diversity and complexity. However, the previous mechanism studies on the sympathetic inrush current often assume that the operating transformer is un-loaded, which cannot match the real saturation of the operating transformers in the generator-transformer unit, the substation and the converter station. Hence, the researches on the sympathetic inrush current and its impact on the differential protection need to be deepened. This paper makes an in-depth analysis on the complex sympathetic inrush current by means of theoretical analysis, digital simulation and the dynamic tests. In order to study the basic model and the mechanism of the complex sympathetic, the analytical analysis is developed for the complex sympathetic inrush current, when the operating transformer is connected with the passive load, the active load and the generator. The dynamic tests for the sympathetic inrush current is carries out, and the influencing factors and the electrical characteristics of sympathetic inrush are summarized and distilled. Two methods for identifying the sympathetic inrush current, that are respectively based on the substation-domain information sharing and the fluxing calculating, are proposed to improve the reliability of the differential protection during sympathetic inrush current.Impacted by the magnetizing inrush current and the sympathetic inrush current of the transformers, the transient process of the primary current becomes more complex, which increases the difficulty for the transient characteristic assessment of CT. A simulation platform for analyzing the transient characteristic of the CT is established in this paper. The actual operating scene can be constructed flexibly in this platform. A variety of complex transient process in the power system can be simulated. The different CT simulation models can be selected for analyzing the transient characteristic of the CT. This simulation platform can achieve the analysis on the CT transient characteristic and its impact on the differential protection, and can provide an instruction for the design and the maintenance of the CT, and also for the accidence analysis in the field. The CT physical model is established based on the principle of magnetomotive force equivalence. The dynamic tests for the CT transient saturation is conducted, in order to deepen the study of CT transient saturation. Based on the dynamic test results, the accuract CT simulation model is established in the platform.Recently, it has been reported several cases of unusual misoperation of the differential protection for the transformer, the generator and the transmission line in service, caused by the adjacent transformer switching-on. This sort of misoperation is related to the sympathetic inrush current and the magnetizing inrush current of the transformers, and also interweaved with the CT saturation. The mechanism of the sympathetic interaction between the non-linear ferromagnetic elements has not been revealed so far, and the cause of this sort of misoperation is still unknown. This paper analyzes this complex transient process by theory analysis, digital simulation and the field recorded data. The misoperation cases of the generator, transmission line and the transformer during the adjacent transformer energization are analyzed in detail. The impacts of magnetizing inrush current, the sympathetic inrush current and the CT saturation are studied. It is disclosed that the CT transient or CT local transient saturation that are caused by the long-lasting dc component from the magnetizing inrush current, can lead to the misoperation. The commonly used CT saturation detection method in the field cannot effectively identify the CT saturation during the interaction of the non-linear ferromagnetic elements. To solve this problem, a new CT saturation detection algorithm by adopting the dc component in secondary current is proposed to prevent the differential protection from this sort of misoperation.The series capacitor compensation technology is widely used in the power grid, which can increase the transmission capacity of the transmission lines and improve the stability of the system. However, this technology will make the fault characteristic of the power grid more complex. The distance protection of the series capacitor compensated transmission line has to shorten the protected zone, in order to avoid the misoperation when the fault occurs behind of the series capacitor. To solve this problem, the boundary protection is proposed in this paper. This new method utilizes R-L differential equation and the least squares algorithm to solve the fault distance. The theory of equal transfer process of transmission lines (ETPTL) is introduced to eliminating the adverse impact of the high frequency component and CCVT. The method can judge the relative of the fault with respect to the series capacitor. Thus the protected range of distance protection can be extended to the whole series capacitor compensated transmission line, and the fast protection for the whole line will be achieved.With the wide application of DC transmission technology, the fault characteristic and its evolutionary process becomes more complex. The protection of the DC and AC system needs coordination and cooperation. According to the actual situation of a provincial AC/DC hybird grid, this paper studies on the impacts of short-circuit fault and the inrush current in AC system on the protection of the DC transmission system. The zero-sequence circulating current of the delta winding in YD-type converter transformer is utilized to improve the reliability of the bridge differential protection. The mechanism of the harmonic transmission in the DC system during the magnetizing inrush current is analyzed. Based on this, the linear dependence relation between the AC voltage of the rectifier station and the DC current is applied to identify the harmonic current in the DC system that caused by the inrush current, and the DC 50Hz harmonic protection will be prevented from misoperation during the magnetizing inrush current.At the end of this dissertation, the research work and results are summarized and the directions of further research are pointed out.