Abstract:

The introduction of power electronic converters has altered the fault characteristics of the traditional power grids. Traditional differential protection is inadequate to meet protection requirements when AC lines experience faults, leading to reduced sensitivity and even the risk of refusal to operate. To address this issue, a principle of protection for AC lines based on multi-stage mutation of total fault current has been proposed. This protection is founded on the characteristic changes in the waveform deformation and mutation features of current on both sides of the line after a fault occurs. The matrix gradient algorithm is employed to extract the mutation feature values of total current signals on both sides, describing the degree of mutation in the total current signals and thereby constructing longitudinal protection. In comparison to traditional fast protection based on transient quantities that do not require the extraction of fixed frequency band features, the proposed method overcomes the challenge of accurately extracting fixed transient quantities using the periodic component algorithm under controlled short-circuit current conditions. In contrast to protection using conventional transient signals directly, it exhibits good tolerance to transitional resistance and noise. Finally, a simulation model of the converter grid-connected system is built in PSCAD/EMTDC to validate the effectiveness of the protection. The proposed protection achieves fault identification in less than 7.5 ms and and maintains high sensitivity even in the presence of a transitional resistance fault of 100 Ω, meeting the requirements of fast response and selectivity for inverter-interconnected lines.

Key words: converters, matrix transformation, matrix gradient, pilot protection