Transmission of Electricity

[raylewis]

Ohms law applies to all electrical circuit irrespective of distances
HV lines are a design feature(voltage drop and lower current for cable size)
If you increase the voltage up on 400kv the current will increase
Work your way back from the load(resistance)
eg
Lets say we have 400kv to 1000v transformer(400 to 1 ratio)
Lets say your end user works on 1000v and the load(constant) is 1 ohm
He will draw 1000A(ohms law)

At 1000A the 400kv has 2.5A (400 times less)
Remeber you transformer ration does not change from 400/1
If you increase primary the secondary also increases all the way down the line to the load.
If you increase the 400kv to lets say 500kv then the secondary side becomes
1250v and at 1 ohms thats 1250A and on the 500kv(400kv) now increases to
3.125A (1250 devided by 400)

Hope this clarifies the matter

Look forward to reply

 

[DPG]

I think I must have misunderstood your original post. You seemed to be saying that in transmision circuits when the voltage went higher the current went lower , but in standard circuits when the voltage went higher the current went higher. As you say, Ohms law applies equally to both.

 

[raylewis]

The transmission circuit forms part of the whole circuit, its just thet the HV was designed specifically with
voltage drop and current carrying capacity over distances in mind.

I have been lucky in my 35 yr career as an Elect Eng to have worked on nearly all aspects of electrical systems
and I am a great believer on passing on my knowledge to others.

Hope I have helped some Electricians out there
Please feel free to ask


Regards

Ray

 

[Des 56]

Reply to ray lewis

Quote
Please dont make the common electricians mistake - the higher the voltage the lower the current,

Your reply to my post seemed to be tainted with a little sarcasm,so to continue in the same mindset,my reply to your question Is this interesting enough for you ? will be as was


Thanks for the basic tuition in ohms law
I am glad that I will not be making the same mistake as the common electrican, having now been taught this most important basic law of electrical theory
I congratulate you on your knowlege in your chosen field but.

Do you not think it a bit presumptious to assume that electricians are unable to correctly use the formula for ohms law and that the term common electrician is a little derogatory, especially on a forum for electricians. Some probably many electricians on this forum may have 20, 30 or 40 years of experience and an array of technical knowlege to accompany that experience
I read and appreciated your input and technical prowess,but thought the whole post was tainted by the remark
If it was not meant as derogatory, then the wording of the last lines of your post were not in my opinion as clear as should have been about that remark

 

[ITerrell]

Hi guys, my first post on here.
Firstly, Ohms law always applies.
Secondly, I think that what we are referring to is the principle that for the transmission of any given amount of power, the greatest losses are those due to the resistance of the cable.
Power (W) = V x I = I^2 x R (Isquared x R).

In other words, the power lost due to cable resistance is proportional to the square of the current.
This is why it is important to keep the current as low as possible, and so the transmission voltage must increase for any given amount of power.

Of course, there are practical limits.
Too high a voltage leads to all sorts of problems with safety, losses due to discharge into the air around the conductors (Corona discharge), and design issues with transformers and insulating bushes etc etc, so a compromise of 400kV seems to be the most practical nowadays (used to be 275kV until improved SF6 switching technology, improved insulator technology and other factors made 400kV practical).
Some other countries do use higher voltages, but usually over Fjords and at DC.

hope this helps,

All the best

Ian

 

[raylewis]

Des 56 you forgot the Hope....this was interesting.....
This was not a comment but a question
Will re frase other to the way it was meant....... Electricians common mistake
Not here to be sarcastic just here to help
Anyway I appologise if it was taken the wrong way,as it wasnt meant
that way.
Only here to learn and help

Your quite right Terrell the higher the HV the worst the insulation problems with other things
Was privelaged to help design and work on 768KV line from J'berg to Capetown in South Africa and a DC
line from Pretoria to Mosambique.The main problem is with insulation not so much overhead but at ground
level in switching.


Regards

Ray

 

[sheppy]

Thanks for that

 

[mechelec]

Delta-Delta HV transformers are use because you dont need a neutral phase in transmission.


You do need a earthed star point for protection of the grid network though, that is why electricity is transformed delta - star eg. Grid transformer primary (delta) fed via three wire 132KV and feeds out star 11KV with neutral point earthed through neutral earth resistor (earth required by electricity distribution regs), 11KV distributed three wire to substation tx primary delta with secondary line voltage 400V wired in star. This means that each section of the network (any voltage) is fed from a star connected feeder with earth faults going back to that point (origin) where the protection of that feeder will be.