transmission lines

Unfortunately, there is insufficient information to answer your question. The purpose and potential of the 6 conductors depends upon the circuit, geometry, separation distance and how they are insulated from each other or tied together. It could, for example, be a six phase transmission line or some other compact circuit arrangement.

In a 3 phase, 3 wire delta system the phase and line voltages are equal and the phase and line currents differ by the square root of 3. The opposite relationship exists in a 3 phase, 4 wire Y-connected system.

The liquid coating applied to the glazed surface of ceramic insulators is silicone. It is applied to improve the contamination performance of the insulators by imparting hydrophobicity thereby preventing flashover and reducing surface leakage currents on contaminated insulators under conditions of light wetting.

I think the pairs of conductors you see are simply the same phase & polarity in parallel.

This video would back this up (and it's pretty darned amazing!): http://www.glumbert.com/media/highpower

Hi, I have a doubt that in high voltage transmission lines(220KV) why they are using 6 conductors. Is it two seperate conductors for each of the phases. for what purpose they are using this?.

REPLY

The 220kV line feeding our transformer station had quads on each phase. In other words, four conductors in a tight group and all of them at the same phase and potential. There are several reasons for doing this. Very high voltage tends to travel on the surface of a conductor. This is why all of our primary 220Kv switch gear was constructed from hollow pipe. No sense having metal in the center where it doesn't do much good. Eliminating the center gives lighter pieces for a given cross section of actual metal conductor. Likewise on the transmission line having two or four conductors instead of one single large diameter conductor provides more surface area. In addition, by spliting the total conductor into four smaller cables, you in effect create more surface area for a given cross section, This helps cool the wire and thus reduce sag from the heating effect of full load current.

And in areas which are prone to icing for at least pat of the year, the multiple smaller strands seem to stand up better than one large single conductor.

Hopefully a transmission line design engineer can fill in the rest of the details why multi strand is better than one large single cable.

************** Quote from ******************** REPLY ... There are several reasons for doing this. Very high voltage tends to travel on the surface of a conductor. This is why all of our primary 220Kv switch gear was constructed from hollow pipe. No sense having metal in the center where it doesn't do much good. Eliminating the center gives lighter pieces for a given cross section of actual metal conductor. ************** Quote Ends ******************** This effect is called "Skin-Effect" 7 is prominent in very-High Frequencies VHF & above. Not @ Power-Line Frequencies. More-over: It is the current which flows in conductors NOT the voltage

Go back and read your text book. Power line frequencies come in the VLF catagory.

The reason for the large conductors in to reduce the voltage gradient at the conductor surface and the resulting corona losses that also cause radio interference.

Reference: Transmission and Distribution Handbook Westinghouse Electric Co.

Snakers

Close, but no cigar.

It turns out that skin effect does indeed matter even at power line frequencies. Skin depth of copper at 60 Hz is about 8.5 mm, and for aluminum its 10.9 mm. Skin depth is an important design consideration for high current AC cables, transmission lines, and bus bars. Because of skin effect, very little current actually flows through the steel core within the steel reinforced aluminum conductors (ACSR) used in HV transmission lines...

ACSR is kind or becoming a thing of the past with the higher strength aluminum alloys. I am talking about 345 Kv and above. Can you give me an IEEE reference to this data?

My comment was in response to Haajee's post (#7) - I concur with your previous post. I was countering his statement that skin effect is unimportant at 50/60 Hz - this is a common misconception. Skin effect begins to have a significant impact at 50/60 Hz when conductor diameters exceed 1.25". This often occurs in substation buses and long-haul transmission lines. Skin effect (and other AC losses) are taken into account ("imbedded") within the AC resistance tables for various types of aluminum conductors. Although ACSR's steel core introduces additional ferromagnetic/hysteresis effects which further increase its effective AC resistance, skin effect impacts high strength 100% larger diameter aluminum conductors as well. For more information, see section 3-8 (Calculation of AC Resistance) in AA TH56, Aluminum Electrical Conductor Handbook. A discussion of conductor AC resistance is also contained in section 3.4.7 of IEEE 738-2006 (IEEE Standard for Calculating the Current-Temperature of Bare Overhead Conductors).

A very readable article about skin effect at 60 Hz also appeared in "Electrical Apparatus", August, 2006. A copy of the article can be found here (unfortunately without the figures). Select PRINT to see the article (less ads):

http://findarticles.com/p/articles/mi_qa3726/is_200608/ai_n17178657/pg_1

It appears that you have done the research and your home work well.

Providing accepted engineering reference's substantiates your comments at a professional level. This is the kind of engineering comment procedures that we need to inculcate, in order to improve the credibility of the forum.

I mention that the many Wanta-be engineers that make unfounded comments without doing the research or fail to provide references, do not favorably support our objectives.

Your statements have prompted me to reflect back and visualize the Quantum mechanics that might apply to this Phenomenon by the fact that any flow of current is associated with a magnetic field and in this case the fields would be in repulsion, so logic indicates that the current densities would be forced apart towards the conductor surfaces and become higher at or near the surface. Possibly as a form of "HALL" effect.

However, I would conclude that this would be a function of current and will occur in any conductor with current flow. I don't think that voltage comes into the equation, but has been inferred by some commenter's.

To conclude, where applicable, I concede my arguments.

Snakers.

*************** Quote **************

My comment was in response to Haajee's post (#7) - I concur with your previous post. I was countering his statement that skin effect is unimportant at 50/60 Hz - this is a common misconception. Skin effect begins to have a significant impact at 50/60 Hz when conductor diameters exceed 1.25".

*************** Un-Quote **************

Thanks for information about Skin-Effect @ 50/60 Hz.

I am a Communication & Power-Electronics man & learnt that it starts effecting after MW Band [after 1.5 MHz]. The practical experience was the Inductors used below 1.5 MHz were of normal single-conductor-enamalled while above that freq a "Litz-Wire" made of muli-strand individully-cotton-covered was used to aviod skin effect.

Later while working on Power-Electronics SMPS. the same "Litz-Wire" helped a lot.

Actually the depth of conductor was never a problem in Communication-System.

Regards

Small conductors at EHV voltages produce corona discharges because of the high voltage gradient (volts per meter) from the small surface area. This condition results in unwanted corona discharges from the conductor surface. Thats why lightning rods have sharp points.

To reduce corona discharge from the small surface of a single conductor the utilities install two or more conductors spaced apart so electrostatically, it becomes like a large oval conductor (three would make a circle) This increases the apparent size of the conductor and reduces the voltage gradient (volts per meter) surrounding the conductor below the point of ionization in the surrounding air.

Sharp objects like bolts and nuts on connectors also have sharp pointed surfaces that cannot be smoothed out so corona rings are installed on both sides of them to electrostatically shield them and reduce the stress.

in addition, the connections at the top ends of insulators require corona rings.

It is interesting to note that when a new line is constructed, great care is used by the workmen not to nick or make any burrs on the surface of the cable. Anything noted will be hand polished smooth with crocus cloth or sand paper before energization.

Even with great care, some extra corona will occur when a new line is initially energized. The corona can cause radio interference in some local radios, however, After a few months of operation, the natural corona discharge erodes off the little protrusions and the line becomes very quiet (no radio noise) The noise levels of a well designed line become very low and cannot be heard in a receiver a few feet away.

The higher the voltage, the more conductors required and the care needed.

Trivia .. Another reason for multiple conductors is to increase the current carrying capacity of the line.

The conductors are in parallel and each conductor shares the current.

Typically the normal current on these lines is around 1000 to 1200 amperes and up to 20 times that on a short circuit.

Just think.. 1000 ampers at 345,000 volts 3 phase is 1,000 * 345,000 * 1.732 = 597.54 million watts (megawatts).

Are you are talking about six individual wires, each suspended by its own insulator string? If so, then the transmission line likely has two sets of three-phase lines, where each set of three is wired in a delta configuration.

If you are talking about groups of lines strung together and insulated by a common insulator string, the multi-conductor set is called a "bundle". Conductor bundles are used to increase the "effective" electrostatic diameter of the group of wires to reduce the electric field seen at the surface of each individual wire. The bundle "looks" like a much larger diameter conductor, reducing the electric field stress and thereby reducing corona losses. Corona losses become increasingly important at higher (500 -765 kV) transmission voltage, so these lines always use two (or more) wires per bundle. Since all of the wires are connected in parallel, bundling also increases the overall current carrying capacity of a transmission line.

Power companies prefer using a delta configuration for long distance transmission since it only requires three lines instead of four (for a wye) to handle the same amount of power. For this reason, most HVAC transmission line towers are designed to handle groups of three, six, or nine high voltage lines. Using a delta configuration also provides greater transient stability for long distance lines.

Power companies may periodically "power wash" dirt, salt, or other contamination off insulator strings in order to prevent conductive channels (called tracking or dry banding) from growing across the surface of the insulator strings. This is a common problem in dusty or sandy areas, or on lines that may build up sea salt near coastlines. If this was not done, insulator damage can progress until the insulator string flashes over, potentially causing unplanned service outages and expensive insulator replacement. BTW, insulator washing is usually performed on LIVE systems...

In some cases, the power company may also apply a water repellent material or a protective (usually silicone-based) coating to provide supplementary protection. This is sometimes used in desert regions to partially repair strings that have had the insulator surfaces dulled by sandstorms. By washing the insulators and then applying a smooth protective coating, the life of the insulators can be extended.

Your comment is well stated and correct:

Another reason for Delta configurations in US systems is to allow for the application of protective relaying schemes that are more effective. Generally the lines are delta fed from grounded WYE transformers at each end. This enables the application of directional and distance reactive fault relaying applications. When a fault occurs, these relays can see and measure the distance from the station to the fault and determine it to be in one of 3 different trip zones. First zone (close in fault "instantaineous trip) Second zone far end fault (permissive trip or block) and Third zone (local block, fault is beyond next station), Stuck breaker, trip on time delay.

Permissive trip, permissive overreach, and Trip blocking (for third zone faults) are all schemes used to provide this service continuity and protection. Tripping only the necessary breakers to clear the fault optomizes the system continuity.

I mention that for those interested, a mathematical tool used by power engineers called "sequential components" is used for designing and setting these special relays. That's another subject.

LOOK UP "DIRECTIONAL TRIP PROTECTIVE DISTANCE RELAYING" and Mho relays.