Abstract
For selective detection of earth faults in medium voltage networks, the general protection can be used, which unlike individual protections controls earth currents in all lines supplied from the substation. To detect high-resistance and arc faults, it is possible to use in such protections the details from wavelet decomposition of earth currents and the Bayesian conditional probability criterion. The paper presents a protection model using details of earth currents and the Bayesian criterion. Significant parameters of the model were determined: sampling frequency, wavelet types and decomposition levels, parameters of the probability density function and decision conditions. The effectiveness of the protection in the case of non-stationary high-resistance short-circuits in the network with natural asymmetry and harmonic distortion was also assessed.
Highlights
Earth faults in medium voltage networks can be detected by means of individual protections used on each outgoing line or a general protection, which controls earth currents of all outgoing lines in the switchgear [1]
Earth faults in medium voltage networks can be detected by means of general protection, controlling earth currents of all outgoing lines in the switchgear
This method allows to detect of earth faults with very high resistance up to 200 k
Summary
Earth faults in medium voltage networks can be detected by means of individual protections used on each outgoing line or a general protection, which controls earth currents of all outgoing lines in the switchgear [1]. The use of such a criterion is impossible, because as a result of compensation the fundamental harmonic of the earth current is small and often is smaller than own currents of healthy lines, especially long cable lines In such networks, it is possible to identify a faulted line basing on a comparison of mean, effective or amplitude values of higher harmonics that are not compensated. They are easy to use, as they do not require detuning from step changes in the capacitive currents of each outgoing line They are much more sensitive and more selective. Due to the limited accuracy of earth current measurements, they are able to distinguish currents with an accuracy of about 10% of the total network earth current Such a protection is activated after fulfilling a criterion for a zero sequence voltage, which limits its sensitivity. In order to determine the real possibilities of such a protection, its model was built in Matlab/Simulink and subjected to simulation tests
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