Substation Insulator Strings 8

[luiscunha1978]

Hello!

My name is Luis. I've not any electrical background. I work in an engineering company (in the drawing area). Not long ago I saw some drawings of a Substation and I was curious to realize the following components:

I someone could explain my questions I'll be very happy.

Luis

 

[waross]

Sort of like calling two 120 Volt circuits with a common neutral 240 Volts.
They do carry a neutral with that line. The neutral is a lighter gauge and the insulators strings are much shorter than on the hot lines..

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[luiscunha1978]

Hello! Absurdly, if a person could climb up the metal structure and sit in Point B, no harm would happen to him?

 

[zeusfaber]

In theory, no harm at all - but you'd be sitting in an area with quite high fields, and making sure that the distance between all your extremities and all the live conductors is comfortably greater than the length of that spark gap is not so easy.

In practice, you'd wear a safety tether to stop you breaking your neck when you slipped and fell - then spend the whole time worrying about what happened if the dangling tether got too close to the 400kV line hanging underneath the insulator string (or if it was you dangling close to that line).

A.

 

[Mbrooke]

Not absurd at all, have a look at this:

https://www.youtube.com/watch?v=R9QJGXln1lE

https://www.youtube.com/watch?v=fchoI5vpBjE

https://www.youtube.com/watch?v=S_bjyFbEJaA

https://www.youtube.com/watch?v=6MXMJ_7mGxQ

Sorry for all the vids, but just to give various perspectives.

 

[luiscunha1978]

Hello!

@zeusfaber thank you very much for the explanation was quite enlightening!

Mbrooke, Thank you once again for your patience and willingness to help me understand this issue. This forum is certainly very useful especially as people like you who, however banal, who may seem the questions, seek to help and impart knowledge.

 

[cuky2000]

Below are some background associated with arcing horn applications for OH Transmission Lines

ARCING HORN OR PROTECTIVE SPARK GAP:
a) Is a protective element against overvoltage for insulator in OH transmission line & equipment bushings.
b) The arcing horn is set with the least path impedance than the insulators such as in the event of flashover overvoltage an arc strike between the air gap on the metallic elements instead of across the insulators.
c) Somehow, arcing horn and corona rings installed at the end of the insulator assembly help to reduce the number of the disk insulators while maintaining the desirable electrical strength (BIL & BSL).
d) In North America, many OH Lines use corona ring and less common to spec. out arcing horns. In Europe, it is a common practice for overhead T. Lines specify arcing horns in OH T. Line and equipment such as power transformer.
e) One of the benefits of using arcing horn is the cost and the capability to operate with high discharge energy of lightning strike or back-flashover overvoltage.
f) Some of the downsides of use of arching horns are the wide operation performance due to the weather-related dependance including air moisture, temperature, contamination among other factors that influence the system reliability.
g) In the last couple of decades, T. Lines metal oxide surge arresters are used as an option with less weather dependance performance than the arcing horn.

Below is an excerpt showing a sample of the field stress distribution between the air gaps of the arcing horn and across the insulator.
I hope this help to visualize the operation performance of the arcing horn.

 

[prc]

Major disadvantage of arcing horns is that after the surge discharge, there will be a power frequency power flow through ionized air that will cause power interruption. With arcing horns arc flash over voltages are quite high with steep fronted surges ie very little protection.In the days of old type surge arresters(silicon carbide blocks with series air gaps) arcing horns used to be provided with bushings due to erratic protection levels of arresters + to provide uniform protection margins over the entire surge rage. With modern metal oxide arresters, arcing horns are superfluous and rarely used with transformer bushings.

 

[luiscunha1978]

Very interesting! In this case, if I understood correctly, the distance between the arching horns should be such that in normal operating conditions, there is no arc between points A (230kV) and B (0V) and in the event of an overvoltage/surge discharge, that same distance, offers the least resistance path, that conduct the discharge by the arching horns and not the by insulator string.

 

[crshears]

I believe you are correct, luis; note however that the behaviour of arc gaps can be graphed similarly to a b-H curve for magnetic hysteresis, in other words once the air between the horns is sufficiently ionized and the air gap breaks down, the established arc will continue to conduct what is essentially fault current to ground, and it is this current that is detected by the circuit protective schemes, subsequently tripping the circuit breakers or other protective devices and taking the circuit off potential. Re-energization of the circuit in a short interval is normally performed by automatic reclosing features, nevertheless there will be a definite, even if very brief, interruption of the power supply.

In contrast to this, modern metal oxide varistors, as prc has noted, have a very linear [even if the line is curved] characteristic of conductance/applied voltage, meaning that as the overvoltage subsides and the voltage applied across the varistor returns to its normal range, the varistor's conductance will again fall to next to nothing / its resistance will again rise to a very high value, the upshot being that the line voltage is not collapsed below its normal values as with horn gaps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]