Due to the nonlinear influence of electrical equipment at the fault point, there are harmonic signals in the fault current, of which the fifth harmonic component is the main one. Because the compensation effect of arc suppression coil on the fifth harmonic is only equivalent to 1/ 25 at power frequency, its influence can be ignored. Therefore, the fifth harmonic current of the fault line is greater than that of the non-fault line, and the direction is opposite. According to this phenomenon, the faulty line can be selected, which is called the fifth harmonic method. The disadvantage is that the fifth harmonic content is small (less than 10% of fault current), the detection sensitivity is low, and it is affected by intermittent arc phenomenon. Harmonic square sum method is to sum the squares of harmonic components such as 3, 5 and 7 of each line and compare their amplitudes, and select the line with the largest amplitude as the fault line. Although it can overcome the shortcoming of small single harmonic signal to some extent, it can not fundamentally solve the problem.

2. Active component method

The active component of zero-sequence current is based on the existence of conductance to ground and resistance loss of arc suppression coil. The fault current contains active components, and the active currents of non-fault lines and arc suppression coils are in the same direction, and both return through the fault point. Therefore, the active component of fault line is larger than that of non-fault line, and the direction is opposite. According to this feature, the faulty line can be selected. When designing a specific line selection device, zero-sequence voltage and zero-sequence current can be used to calculate and compare the magnitude and direction of zero-sequence active power of each line to determine the fault line.

The advantage of active component method is that it is not affected by arc suppression coil. However, due to the small active component in fault current and the unbalanced three-phase parameters of the line, the detection sensitivity is low and the reliability cannot be guaranteed. In order to improve the sensitivity, some devices increase the active component of fault current by instantaneously connecting grounding resistance in parallel on the arc suppression coil. The problem caused by this is to increase the grounding current, destroy the insulation of the fault point, and easily lead to the expansion of the accident, which is more prominent for cable lines.

3, steady-state zero sequence current comparison method

When a single-phase grounding fault occurs in an ungrounded neutral system, the zero-sequence current value left by the faulty component is equal to the sum of the grounding capacitor currents of non-faulty components in the whole system, that is, the zero-sequence current on the faulty line is the largest, and the direction of the zero-sequence current on the faulty line is opposite to that of all non-faulty lines. By comparing the amplitude and phase of zero sequence current, the fault line can be found.

Limitations:

(1), the measured value of zero-sequence current is affected by the unbalanced current generated by saturation of current transformer.

② In the neutral grounding system via arc suppression coil, there is zero sequence current in the fault phase. On the faulty line,

The current direction is the same as the zero-sequence current direction of the non-fault phase line, but it is inductive and can compensate the capacitive zero-sequence current of the line on the left side of the fault point. Considering the compensation effect of inductive zero-sequence current, the zero-sequence current measured at the head end of the fault line may be smaller than that measured at the head end of other lines.

(3), will be affected by the size of the transition resistance.

4. Injection signal tracking method

Injection signal tracing method is called injection method for short. After the grounding fault occurs, the current signal with a specific frequency (225Hz) will be injected into the grounding line through the neutral point of the three-phase voltage transformer (PT), and the injected signal will be injected into the earth along the fault line through the grounding point. Each line is detected by the signal detector, and the line through which the injected signal flows is selected as the fault line. The advantage of this method is that it is not affected by the arc suppression coil, and there is no need to install a zero-sequence current transformer (CT). It can also locate the fault point of overhead lines by detecting along the fault line with a detector.

Its disadvantage is the need to install signal injection equipment.