How to prevent the failure of overhead line cables

There are many and complex branch lines of overhead lines, among which there are many agricultural grid lines, long power supply radius, and all of them are radial power supply lines
.
The safe operation of overhead lines is directly related to the daily life
of the majority of users.
After the transformation of urban and agricultural networks in recent years, the anti-accident ability of the line has been significantly improved, but the failure of overhead lines still occurs
from time to time.
The common faults of overhead lines are analyzed in order to improve the safe operation level of overhead lines and improve the reliability
of power supply.

1.
Short circuit fault: first, the line transient short circuit fault (generally the circuit breaker reclosing is successful); The second is a permanent short circuit fault of the line (generally the circuit breaker reclosing is unsuccessful).

Common faults are: line metal short circuit fault; Line lead jumper broken arc short circuit fault; Drop fuse, disconnector arc flash short circuit fault; small animal short circuit fault; Lightning flashover, short circuit faults, etc
.

Ground fault: transient ground fault of the line; Permanent ground fault
of the line.

2.
The cause of the failure

Line metal short-circuit faults are:

(1) Failure caused by external force damage, overhead line or rod equipment (transformer, switch) is short-circuited by external parabolic objects or short-circuited by external force; The car hit the rod and caused the rod to fall and break the wire; Typhoons and floods caused poles to fall and disconnections

(2) Line defects cause failures, arc sag is too large, typhoon caused by line collision or short circuit generated by electric force caused by line collision.

Line lead jumper broken arc short circuit fault: line aging strength is insufficient caused by line breakage; Poor contact of the line overload connector causes the jumper clamp to burn and break the wire
.

Drop fuse and disconnector arc short circuit faults are:

(1) The fuse of the drop fuse is blown and the melt pipe is exploded or the arc is pulled to cause the arc short circuit between phases;

(2) The line aging or overload causes the disconnector clamp to be damaged, burned and arc-pulled, resulting in a short circuit
between phases.

Short-circuit faults are:

(1) On the pier-type distribution transformer, the high-voltage down conductor from the drop fuse to the transformer is bare wire, and the transformer's high-voltage terminal post and high-voltage arrester are not equipped with insulation shields;

(2) On the bus bar of the high-voltage distribution cabinet, the busbar is not insulated, and the high-voltage distribution room is not strictly anti-rat;

(3) In the high-voltage cable branch box, the busbar is not insulated, and the cable branch box has loopholes
.

Lightning strike overvoltage
.

Line transient ground faults are:

(1) Single-phase grounding caused by artificial external parabolics or trees touching the wire;

(2) The line insulator is dirty, and in rainy days or foggy and high humidity weather, flashover to the ground occurs, and generally disappears
after the weather turns good or heavy rain.

The permanent ground faults of the lines are:

(1) External sabotage;

(2) Line disconnector and drop fuse are caused by insulation aging breakdown;

(3) Caused by the explosion of line arresters, mostly occurring in the thunderstorm season;

(4) Direct lightning strikes cause line insulators to explode, mostly occurring during the thunderstorm season;

(5) Caused by aging or defective breakdown of line insulators, it mostly occurs in coastal areas
with serious pollution.

Fault judgment

Regardless of whether the fault of the line is transient or permanent, the success of the circuit breaker reclosing, it is necessary to conduct an accident inspection of the fault line to find out the cause of the accident, especially the correct judgment of the possible fault point is particularly critical, it is the premise
of whether it can quickly isolate the fault and restore the power supply.

Short circuit fault:

(1) Substation lines generally adopt two-stage or three-stage current protection, that is, current speed off or time-limited speed off and overcurrent protection, we can make preliminary judgments
according to the protection action of the substation fuse.
If the line occurs in the current rapid cut-off protection action, it can be judged that the fault point is generally caused by a direct short circuit of the two-phase or three-phase line, and the fault point is more likely to be in the line near the main line or substation
.
Because the starting current of the quick-break or time-limited speed-off protection action is large, it is set according to the maximum operation mode (that is, avoiding the short-circuit current at the exit of the next line), so this fault has greater damage to the line and equipment, such as line metal short circuit or lightning strike short circuit.

If the line occurs an overcurrent protection action, it is generally caused
by a non-metallic short circuit or a short circuit of the branch line at the end of the line.

(2) Because the circuit breaker on the line column is generally only set with one overcurrent protection (the maximum time limit is), and it adopts the step-shaped time limit characteristic that increases step by step, we can judge which line is faulty
according to the line circuit breaker protection action.
Ground fault: For permanent ground fault of the line, the method of segmented test pulling of the line branch circuit breaker should be used to judge the faulty line segment
.
If it is a transient ground fault, every point of the line can occur
.

(1) During bad weather, during typhoons, heavy rain and thunderstorms, short circuits and ground faults often occur, such as broken rods, collapsed pole foundations, and tree pressure conductors
.

(2) During the first spring rain after winter, ground faults often occur, mostly occurring on lines erected along the sides of highways and streets with more serious dust, such as flashover breakdown of insulators due to excessive dirt deposits
.

Fault finding

The general principle of fault finding is: first the trunk line, then the branch line
.
For lines that are not found to be faulty after inspection, you can first test the power transmission after disconnecting the branch line circuit breaker, and then find and restore other lines
that are not faulty step by step.

Short circuit fault finding: a line trunk line and each branch line is generally installed with column circuit breaker protection, theoretically speaking, if the switch time limit at all levels is well coordinated, then the fault section is easy to judge and find
.
When the circuit breaker of the substation trips, first check whether the circuit breaker on the trunk column and the circuit breaker on each branch post trips, and then find the line after the tripping step by step according to the various faults that may occur as described above, until the fault point
is found.

In addition, for overhead lines equipped with a line short-circuit fault indicator, the fault section line
can also be determined by means of the indication of the fault indicator.
Another point is that when the fault point is detected, it is very wrong
to think that as long as the fault point is repaired, the line can be restored to power supply, and the line patrol is stopped.
Because when a short-circuit fault occurs on the line, the short-circuit current also flows through the line
above the fault point.

Ground fault finding: The search for permanent ground fault points of the line can be found according to the above after determining the ground fault segment, according to the causes of its possible formation and various environmental factors, while the transient ground fault can only be found
for the entire line.