In isolated neutral system, when two-point grounding faults occurring on different phases take place and at least one of the transition resistances is large, the outgoing protection with the small fault current can not be directly removed, it still need to rely on the line selection device to achieve protection. At present, there are some problems in the research on the two-point grounding fault of the small current grounding system, such as lack of quantitative analysis, lack of line selection effect analysis, and difficult to be applied in practice et al. To address these issues, based on the fault equivalent circuit, this paper analyzes the power frequency fault characteristics for two-point grounding faults occurring on different phases in isolated neutral system. Based on the fault characteristics, the adaptability of the existing power frequency line selection methods to the two-point grounding fault of the different phase is analyzed and the development suggestion of line selection technology is put forward. Firstly, Setting up the compound sequence network of two-point grounding faults occurring on different phases, using the nodal impedance matrixes to respectively deduce the expressions of power frequency components, such as two grounding fault point currents, zero-sequence voltage, and so on. Secondly, analyze the characteristics of two-point grounding fault power frequency electrical quantity with the change of the transition resistances in ungrounded systems and compare them with the characteristics of single-point grounding faults. Finally, according to the fault characteristics, the limitation of the existing line selection methods are analyzed, and the development ideas of two-point grounding fault line selection technology are put forward. The results show that, when the transition resistance of at least one fault point is large, the amplitudes of zero-sequence voltage and zero-sequence current of power frequency in two-point grounding of the different phase show the characteristics of single-point grounding. At least one fault feeder has a power frequency zero-sequence current amplitude greater than that of the healthy feeder. The phase relationship of zero-sequence current of power frequency between the fault feeder and health feeder depends on two fault phase sequences, transition resistance of two fault points and total capacitance to the ground of the system. Generally, there is no definite phase relationship. In general, the zero-sequence voltage amplitude when two fault points are grounded is between the zero-sequence voltage when two fault points are grounded separately. When the transition resistance of two fault points is similar, the zero-sequence voltage when two fault points are grounded is minimum. For two-point grounding faults occurring on different phases in isolated neutral system, the zero-sequence voltage starting criterion still has high resistance to transition resistance and can reach the threshold value in most cases. The zero-sequence current at the outlet of two fault lines generally does not conform to the characteristics of both fault feeder and healthy feeder, and the existing line selection principle is not fully applicable. When the amplitude and phase information are used for line selection, missing selection or rejection will occur. Development suggestions of line selection technology: (1) As for the development of steady-state line selection technology for two-point grounding fault, based on the conclusions of power frequency characteristics, the change of power frequency characteristics of faulty line and non-faulty line before and after subsequent fault, or the change of power frequency characteristics of faulty line and non-faulty line before and after first fault line removal can be further studied. According to the difference of power frequency characteristics between fault line and non-fault line in two stages, two-point grounding fault detection is realized and the detection time is further shortened to improve detection efficiency. (2) As for the development of transient line selection technology for two-point grounding fault, based on the conclusions of power frequency characteristics, the intermittent arc grounding process after subsequent fault can be analyzed. © 2023 Chinese Machine Press. All rights reserved.
