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Braking resistor, single-phase earth fault line selection principle of the small current grounding system were reviewed
Mar 05, 2018

Braking resistor, single-phase earth fault line selection principle of the small current grounding system were reviewed

Braking resistor, the small current grounding system of single-phase grounding fault line selection principle of early single criterion principle

Because the capacitive current line itself may be greater than the sum of capacitive current of other lines in the system, so according to the size of the zero sequence current setting of the overcurrent relay is not perfect in theory, it is also influenced by system operation mode, the influence of many factors such as length, caused the mistake to choose, leakage, pops up; \"Power\" principle of zero sequence current i0 power with detailed measurement direction to complete the function of line selection, when used in short circuit, due to the small line of zero sequence current, plus power direction of interference, the line selection is not reliable to some extent, more wrongly, leakage and selection; With the comparison of the two zero sequence currents, choose the zero sequence current biggest line for fault line \"maximum\" principle, in the current circuit grounding or length of markedly different cases, is likely to cause false choice and choose more; \"In the first half wave\" principle based on ground fault occurred in the phase voltage is close to the maximum moment this hypothesis, using fault after fault transient zero sequence current of the circuit to each cycle of the first half wave and the characteristics of the fault line instead to realize the selective protection, but it can't reflect the grounding fault phase voltage is low, and greatly influenced by the grounding fault resistance, at the same time work dead zone; Using five or seven times the size of the harmonic current or direction selective earthing protection of the direction of \"harmonic\" principle, with five or seven times harmonic content is much smaller than the level of fundamental wave, and the harmonic content of the grid size is differ, so the zero sequence voltage values are high, low sensitivity, on the site there is a transition resistance will be rejected under the condition of dynamic phenomenon.

3.2 group than picture than principle

This method for multiple criteria and multiple criterion is to use two or more principle as criterion, increase the reliability and anti-interference ability, reduce the system operation mode, the influence of the length of lines, grounding resistance. [2] by using amplitude and phase method, with \"maximum\" principle from the first choose three or more lines of i0 zero sequence current biggest line, and then use \"power direction principle from the selected route lookup zero sequence current lagging i0 zero sequence voltage u0 lines, so as to choose the fault line. After the plan, known as the 3 c for queue to remove small current amplitude, avoided the same effect to a certain extent, queuing also avoids the set value, has the advantage that the value of follow-up \"a rising tide lifts all boats\". It can avoid the limitations of a single criterion, shorten the time of the line selection can also be relative, is the ideal way.

3 c solution, because the i3 may also smaller, thus the phase is i2 or i1 grounding may cause misjudgment, i3 is smaller, the higher the misjudgment rate, as proposed by the article [3] MLN series of line selection device extends the four line selection scheme, in addition to the 3 c solution, increased 2 c1v, 1 c1v scheme, 1 c, 2 c, by a computer according to different conditions to choose the appropriate solutions or artificial sentenced to line, line accuracy was further improved.

Small current system of single-phase grounding into protection tripping, require protection device has higher reliability. The fuzzy decision theory in paper [4] introduced the MLN - r series small current microcomputer protection screen, will be 5 kinds of line selection scheme according to the combination of fuzzy decision ruled that trip exports also print out the credibility is given.