Differential Protection Performance for Converter Transformer Intertap Short-Circuit Faults in On-Load Tap Changers
Yan Chenguang1, Zhang Peng1, Xu Ya1, Wu Juzhen2, Li Lingnan2
1. State Key Laboratory of Electrical Insulation and Power Equipment Xi’an Jiaotong University Xi’an 710049 China; 2. State Grid Economic and Technological Research Institute Co. Ltd Beijing 102209 China
Abstract：In recent years, explosions and fire accidents caused by converter transformer intertap short-circuit faults inside on-load tap changers (OLTCs) have occurred in succession, seriously threatening the safe operation of the power grid. As the main protection for converter transformers, the insufficient sensitivity and rapidity of differential protection under such faults have attracted widespread attention. However, due to the lack of effective theoretical models and calculation methods for intertap short-circuit faults in OLTCs, the fault process is difficult to reproduce, and the existing protection performance remains to be further studied. Given this background, this paper proposed a direct field-circuit coupling model and calculation method for converter transformer intertap faults in OLTCs. The bidirectional field-circuit coupling enabled the transient calculations of the winding leakage flux and the short-circuit current, and the improved black box model was adopted to simulate the time-varying arc-in-oil conductance characteristics. With this method, intertap fault simulations were carried out on a typical single-phase two-core-limb converter transformer, and the differential protection performance under short-circuit faults at different tap positions was quantitatively analyzed. First, the converter transformer intertap short-circuit fault process was analyzed based on a typical vacuum-type OLTC circuit topology and intertap short-circuit loop. Second, the governing equation of the internal electromagnetic field and the constraint equation of the external electric circuit were successively established and unified as coupled equations in matrix form. With an improved black-box arc model directly integrated into the short-circuit loop, the direct field-circuit coupling relations and a corresponding calculation method for converter transformer intertap short-circuit faults were developed to capture the transient interactions among the electromagnetic field, the electric circuit and the electric arc in the insulating oil. Third, with a simulation model of a ZZDFPZ-509400/500-400 single-phase two-core-limb converter transformer in a ±800 kV converter system, direct field-circuit calculations were carried out on the Ansys Maxwell and Simplorer platform, and the magnetic flux distributions, the short-circuit current and the terminal currents were accordingly obtained. Finally, on the basis of the calculation results in the study cases, three typical differential protection schemes configured in actual relay protection devices were evaluated and the operating behaviors under short-circuit faults between different taps were discussed. The calculation results show that under a serious intertap fault, the magnetic flux distributions inside the converter transformer are significantly distorted and the radial leakage flux density in the vicinity of the shorted turns reaches 3.77 T at maximum. Correspondingly, with a large electromotive force induced on the shorted turns, a high-amplitude circulating current is produced with a peak value of 92.87 kA. For the ratio differential protection and the fault incremental ratio differential protection adopted in certain protection devices, the severest intertap short-circuit fault in the middle of the winding can be detected within 17.6 ms and 5.6 ms after the fault occurs, respectively. When the shorted turns are at the winding ends, the root-mean-square value of the differential current is 0.23(pu), and the ratio differential protection exhibits inadequate sensitivity. The following conclusions can be drawn from the calculation and analysis: (1) The severities of short-circuit faults between different taps are remarkably different, and the shorted turns in the middle of the winding correspond to the most serious fault condition. (2) For intertap short-circuit faults at the winding ends, the differential current does not change significantly, and the ratio differential protection has the risk of rejection due to insufficient sensitivity. (3) Since the zero-sequence current under the intertap short-circuit fault manifests as a through current, the existing zero-sequence differential protection schemes also have difficulty reflecting such faults effectively.
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2023, Vol. 38 
