Learn it as soon as you learn it! 10kV line fault quick search formula!

The 10kV line in operation often trips due to natural disasters such as lightning, wind, snow, external forces, and environmental pollution. Once the 10kV line trips the accident, it will cause a large-scale power outage on the whole line or part of the line, which will inevitably bring economic losses to the power enterprises, so the sooner the location and cause of the accident are found, the hidden dangers of the accident are eliminated, the scope of the accident power outage is reduced, and the sooner the power is restored, the economic losses of both parties can be reduced. Therefore, when a 10kV line trips an accident, it should be organized as soon as possible to investigate and deal with it. In order to facilitate memory, the 10kV line fault investigation method is summarized as the following formula.

01

Quickly find 10kV line faults

(1) Mouth

The power is cut off when the line is faulty, and the protection action is skillfully judged;

The front end of the quick-break action check, about half of the total length;

The overcurrent action value is small, and the fault is far behind;

The speed is short and the overcurrent is tripped, and the fault is located in the middle of the line.

(2) Explanation of the formula

The accident occurred in different sections of the line, and its relay protection action is different.

Current quick break protection action tripping: The protection range of current quick break protection is generally the largest when a short circuit occurs in the maximum operation mode of the system, accounting for about 50% of the total length of the line. When the line is in the minimum operation mode, the protection range is the smallest, accounting for 15%~20% of the total length of the line. Therefore, if the current quick-break protection device trips, it means that most of the fault points are located in the front section of the line (near the side of the substation).

The over-current protection device trips: the protection range of the over-current protection is 100% of the protected line. However, usually the overcurrent protection device is equipped with a delay relay at the same time, and when it is used in conjunction with the quick-break protection device, it generally trips when the fault occurs in the rear section of the line.

Current quick-break protection and over-current protection trip at the same time: This situation generally indicates that the fault point is in the common range of quick-break protection and over-current protection, and most of the fault points are located in the middle of the line.

Therefore, if the circuit breaker of the substation trips, the relay protection action should be investigated in time. According to the action type and characteristics of the relay protection device, the nature and scope of the fault are roughly located.

02

10kV line grounding fault judgment

(1) Mouth

The grounding fault is skillfully judged, and one low, two high and three remain unchanged;

The load is broken and grounded, and one high and two low are also common.

It is difficult to distinguish between disconnection and grounding, and the user voltage is obvious.

There is only two phases of electricity when the disconnection is made, and the grounding user is not obvious.

(2) Explanation of the formula

After receiving the notice of the dispatcher on duty about the line grounding, the personnel of the power supply station or the electrician of the factory and mine should understand: which phase is grounded and what is the value of the grounding voltage of each phase? Numerical changes, where the values are constantly changing or stable, for further analysis of the grounding situation. Find the point of failure as soon as possible.

(1) The voltage of one relative to the ground is close to the zero value, and the voltage of the other two relative to the ground is increased by √ 3 times, and the voltage of the three-phase phase has not changed, which is metallic grounding.

One relative ground voltage decreases, but not zero, and the other two relative ground voltage increases, but does not increase to √3 times, which belongs to non-metallic grounding.

(2) One relative ground voltage increases, and the other two relative ground voltage decreases, which is the characteristic of non-metallic grounding and high-voltage phase failure.

i. High voltage disconnection, the load side wire falls on the wet ground, and the two phases are connected to the grounding wire through the load to form a non-metallic grounding. Therefore, the voltage to ground decreases, and the voltage relative to ground increases when the line is broken.

ii. The high-voltage disconnection does not fall to the ground or falls on the object with poor conductivity, or the fuse on the line blows one phase, and the disconnected ground line is longer, resulting in the unbalanced current of the capacitance of the three to the ground, and the voltage of the two to the ground is also unbalanced, the current of the capacitance relative to the ground becomes smaller, the voltage to the ground is relatively high, and the other two phases are relatively low.

iii. The winding of the burning phase of the distribution transformer touches the shell and is grounded, the high-voltage fuse is blown again, and the other two phases are grounded through the winding, so the voltage of the burning loss relative to the ground increases, and the other two phases decrease.

(3) It is necessary to distinguish the high-voltage phase loss from non-metallic grounding.

The high-voltage disconnection does not fall to the ground or falls on the object with poor conductivity, or the fuse on the line blows one phase, and the disconnected ground line is longer, resulting in the unbalanced current of the capacitance of the three opposite to the ground, and the voltage of the two to the ground is also unbalanced, and the capacitance current of the disconnected relative to the ground becomes smaller, the voltage to the ground is relatively high, and the other two phases are relatively low.

After receiving the notice from the dispatcher, the personnel of the power supply station should distinguish the high-voltage phase loss from the non-metallic grounding, and judge whether the voltage on the end user is balanced by checking whether it is a high-voltage phase loss or non-metallic grounding. The disconnected user only has two-phase electricity, and the load voltage of the grounding user does not change significantly.

03

10kV line grounding fault insulation judgment

(1) Mouth

Line fault insulation test, less than 40 unhealthy;

The distribution switch is not opened, and it is not safe under 30.

For a single overhang to measure insulation, 300 megaohms is the limit;

The needle porcelain vase is 200 trillion, and there is a hidden danger if the value is low.

(2) Explanation of the formula

The overall insulation shake test judgment method can quickly and effectively find the insulator with poor insulation and become the key to finding the line grounding fault.

The overall insulation shake test method is more suitable for 10kV lines with short length, small number of distribution transformers, and no cross-crossing of other 10kV and above lines. Before the implementation of the overall insulation shake test method of the line, safety measures should be taken first to ensure that there is no possibility of reverse power transmission to the test line, especially in the case that the short-circuit grounding wire can not be hung at both ends of the working line to ensure personal safety. On both sides of the maximum segmentation point of the line (the line can be divided into the breakpoint with the closest length before and after), of course, a branch line that meets the above conditions can also be regarded as the overall line insulation resistance shake test.

This method is not only suitable for monitoring the insulation level of the line and grasping the overall insulation of the line, but also suitable for the situation when the traditional treatment method cannot find the line grounding fault. When using the line overall insulation shake test method to find the line grounding fault, the insulation values on both sides of the swing measurement point are compared, and the lower side should be the fault section.

Before judging the fault phase of the fault section, it should be ensured that the line distribution transformer and capacitor are feasible to disconnect, otherwise, the three-phase insulation value measured by the insulation shake means that the three-phase insulation value measured separately is actually the insulation value of the three-phase communication, which is much smaller than the real single-phase insulation value.

Because the insulation values of A, B, and C phases on the same side are roughly the same under normal circumstances, the three-phase insulation values of all fault sections are compared after the shake test, and the one with the lowest insulation value corresponds to the fault phase. According to this method, the fault segments are searched in order until the fault point is found.

Single Suspension Insulator (300MΩ)

Strut Insulator (200MΩ)

For a certain section of a specific line, the insulation resistance value and ambient temperature at that time should be measured and recorded in detail when the line is put into operation, and a complete line insulation file should be established, which can lay a good foundation for the longitudinal and horizontal comparison of insulation data through the line preventive test in the future to determine whether the line insulation is good. The insulation value measured in the sunny line grounding fault finding, the statistical experience is that less than 40 MΩ is unqualified, if the distribution transformer switch is not pulled apart in the test, then less than 30 MΩ is unqualified.

For a specific section of a line, a longitudinal comparison should be made with the insulation value of the latest preventive test, and if the insulation value has decreased significantly, it can be determined as insulation damage. For the line with fewer line breaking points, the tension rod drainage line can be untied in the middle of the line, and the two sides of the suspension insulator are regarded as the breaking points, and the insulation is measured on both sides to determine the grounding fault point.