Study On Oscillation Identification Based On Frequency Characteristics And Zero-sequence Problems Of Parallel Transmission Lines

The high-speed development of electric power industry, when bringing tremendous economic benefits, has made power grid become increasingly complicated and raised the probability of power system oscillation greatly. In case of system oscillation, the correct action of out-of-step splitting device and relay protection device has great significance for avoiding the further expansion of accidents, and oscillation identification is just one of the core technologies. Meanwhile, along with the fast development of power grid construction and the increasingly severe shortage of land resources, parallel transmission lines have increased day by day, and line corridor has become dense gradually, so it’s necessary to research zero-sequence problems of parallel transmission lines. Based on analyzing the frequency characteristics of electrical quantities during oscillation process, the criterion of oscillation identification based on frequency characteristics is proposed; and meanwhile, based on analyzing the influences of parallel transmission lines on zero-sequence protection, the improved algorithm is proposed. The main contents and achievements are as shown below:The characteristics of current frequency and voltage frequency during power system oscillation are studied. The expressions of current frequency and voltage frequency at oscillation center and non-oscillation center are dedeuced, and the frequency characteristics during system oscillation are analyzed. Meanwhile, the influences of unequal amplitudes of electromotive forces at both sides and of unequal impedance angles of each component inside the system on frequency characteristics are studied. The study has shown that, in case of system oscillation, only the voltage frequency at oscillation center is equal to the current frequency; while the voltage frequency at non-oscillation center changes cyclically along with the change of power angle within the frequency range of electromotive force at both sides. It has no influence on the above-mentioned frequency characteristics no matter whether the amplitude of electromotive force at both sides is equal, or whether the impedance angle of each component inside the system is equal.An oscillation identification criterion based on frequency difference of electrical quantities is proposed. Oscillation factor is defined to token the comparison of voltage frequency and current frequency. This criterion can judge whether the system oscillates, and whether the oscillation center is inside the component. Meanwhile, the criterion can self-adapt to the change of system structure and operating mode, without being affected by change of middle loads.An oscillation identification criterion based on the characteristics of voltage frequency change rate is proposed. The derivate of voltage frequency at any point inside the system is deduced, then its characteristics are summarized and analyzed. According to the theory, the criterion can judge whether the oscillation center is inside the component in the first half cycle of oscillation; even not, it can still make judgment in the second half cycle of oscillation. Meanwhile, the criterion is not affected by the change of system structure, operating mode, and middle load.To prevent the influences of parallel transmission lines on pilot zero sequence directional protection, improvement measures based on negative-sequence directional protection are proposed. The action of zero-sequence directional components at each end in case of internal and external faults of mutual inductance lines is studied. Study shows that, the protection of the fault line is not affected by zero-sequence mutual inductance, and can act correctly; for the non-fault line with mutual inductance, the closer the electric connection is, the smaller the probability of zero-sequence directional components misoperation will be. Therefore, in condition of strong magnetic and weak electric connection, the misoperation of pilot zero sequence directional protection can be prevented with the policy of combining negative-sequence directional protection and zero-sequence directional protection.To prevent the influences of parallel transmission lines on inverse-time zero-sequence current protection, an improved algorithm based on modifying factor is proposed. The distribution of zero-sequence current and the action of protection components in case of internal grounding fault are studied. The study shows that, when the position of fault point and the zero-sequence parameter of system meet certain conditions, the zero-sequence current of non-fault line will be greater than that of fault line, and here, the protection will probably not meet selective requirements. Meanwhile, the improved algorithm can raise the selectivity and the speed of inverse-time zero-sequence current protection simultaneously.