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G

gorilla

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Just thought I would post here and see if anyone could help me out regarding the transmission of electricity from power stations to the home/industry.

I have done this at college and I'm just doing a diagram of the system but unfortunately my lecturer missed out a stage and I would like to include it in my diagram.

This is what I have so far:

Electricity produced at power station using 3 phase generator that outputs 11kV to 25kV

This is then increased to the distribution voltage of 400kV using a transformer that outputs it as 3 Phase Delta

Once at 400kV the electricity is distributed to a Substation where it is reduced to 132kV remaining as 3 Phase Delta.

The electricity then moves onto another substation and reduced to 33kV and then to another where it is reduced to 11kV before it arrives at a local substation where it is reduced to 415V and distributed to our homes in 3 Phase Star where using one of the phases connected to the central neutral return gives a final output of 230V

Now that is the bit I know. Here is the bit where the lecturer stopped and told us we don't need to know it (it wouldn't hurt if we did though)

Electricity produced at power station using 3 phase generator that outputs 11 Kilo-Volts (kV) to 25kV

This is then increased to the distribution voltage of 400kV using a transformer that outputs it as 3 Phase Delta Once at 400kV the electricity is distributed to a Substation where it is reduced to 275kV remaining as 3 Phase Delta.

I know it needs to get brought down to 33kV, 11kV and then 415V but from my research it doesn't really do this directly. All though I have found a few places that say it goes from 275kV down to 33kV but I'm unsure of this.

can anyone help me out thanks, I would have asked another lecturer today but unfortunately didn't have time and I'm not back at college until Monday now.
 
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ian.settle1

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Mentor
Arms
just thought i would post here and see if anyone could help me out regarding the transmission of electricity from power stations to the home/industry.

I have done this at college and i'm just doing a diagram of the system but unfortunately my lecturer missed out a stage and i would like to include it in my diagram.

This is what i have so far:

Electricity produced at power station using 3 phase generator that outputs 11kv to 25kv

this is then increased to the distribution voltage of 400kv using a transformer that outputs it as 3 phase delta

once at 400kv the electricity is distributed to a substation where it is reduced to 132kv remaining as 3 phase delta.

The electricity then moves onto another substation and reduced to 33kv and then to another where it is reduced to 11kv before it arrives at a local substation where it is reduced to 415v and distributed to our homes in 3 phase star where using one of the phases connected to the central neutral return gives a final output of 230v

now that is the bit i know. Here is the bit where the lecturer stopped and told us we don't need to know it (it wouldn't hurt if we did though)

electricity produced at power station using 3 phase generator that outputs 11 kilo-volts (kv) to 25kv

this is then increased to the distribution voltage of 400kv using a transformer that outputs it as 3 phase delta once at 400kv the electricity is distributed to a substation where it is reduced to 275kv remaining as 3 phase delta.

I know it needs to get brought down to 33kv, 11kv and then 415v but from my research it doesn't really do this directly. All though i have found a few places that say it goes from 275kv down to 33kv but i'm unsure of this.

Can anyone help me out thanks, i would have asked another lecturer today but unfortunately didn't have time and i'm not back at college until monday now.

transformers
 
R

raylewis

  • Thread Starter Thread Starter
  • #4
The reason for generating between 11Kv - 25Kv is for altenator insulation properties,can you imagine the insulation req'd for 400kv in the altenator
Delta-Delta HV transformers are use because you dont need a neutral phase in transmission.
Standard is to transmit as high as you can(400KV) - main grid
Then as you get closer to the demand like Areas you step down to 275KV or 132KV for that area.Then depending on the area - city,industrial site,housing is the factor that determines the next step down according to the load demand.
Industrial main subs go down to 33KV and then 11kV as close as you can to the demand and then to 415/230v for light industry.buisness and domestic.
Pylons too big in built up areas and 275KV not good in built up area
Rule is get as close as you can with HV safely
As you can understand the closer you can get the HV to the load the better(lower losses).
Please dont make the common electricians mistake - the higher the voltage the lower the current,this rule only applies to transmission.
In standard circuits the higher the voltage the higher the current(Ohms Law)

Hope this helps you out
 
G

gorilla

  • Thread Starter Thread Starter
  • #5
The reason for generating between 11Kv - 25Kv is for altenator insulation properties,can you imagine the insulation req'd for 400kv in the altenator
Delta-Delta HV transformers are use because you dont need a neutral phase in transmission.
Standard is to transmit as high as you can(400KV) - main grid
Then as you get closer to the demand like Areas you step down to 275KV or 132KV for that area.Then depending on the area - city,industrial site,housing is the factor that determines the next step down according to the load demand.
Industrial main subs go down to 33KV and then 11kV as close as you can to the demand and then to 415/230v for light industry.buisness and domestic.
Pylons too big in built up areas and 275KV not good in built up area
Rule is get as close as you can with HV safely
As you can understand the closer you can get the HV to the load the better(lower losses).
Please dont make the common electricians mistake - the higher the voltage the lower the current,this rule only applies to transmission.
In standard circuits the higher the voltage the higher the current(Ohms Law)

Hope this helps you out
Thanks so the drop is from 275kV down to 33kV. That's all I needed to know. The rest of it my lecture told us but decided that 275kV wasn't important as it is being phased out but it would have been good to know for reference purposes at least.
 
S

shep

  • Thread Starter Thread Starter
  • #6
this is what we were given when i was at collegescan0015.jpg
 

Des 56

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Arms
Esteemed
Just having a quick nose through and couldnt let this go without comment

Please dont make the common electricians mistake - the higher the voltage the lower the current,this rule only applies to transmission.
In standard circuits the higher the voltage the higher the current(Ohms Law)


Please elaborate because the reply should be very interesting
 
R

raylewis

  • Thread Starter Thread Starter
  • #9
Reply to Des 56
Hope this is interesting enough for you
Ohms law V=IR
I=V/R
or R=V/I

or Z replaces R in AC circuits

Remember the load stays constant in normal circumstances for example a kettle
3kw measured as 18 ohms approx.This ohmic value does not change.
Use 230v
therefore I=V/R I=230/18 I=12.78A
Now use 250v
I=250/18 I=13.88A

Basic electrical formula(learnt this 1st year ONC)

In transmission over distances when transforming voltages using transformers on two sides of a circuits different rules (formulas) apply(winding turns of transformers,voltage drop over distance,conductor sizes types of transformer etc)

This is done to overcome losses over distances and lower current capacity for lower cable sizes
Learnt this the first month of my HND Electrical Engineering 25 yrs ago
Different set of rules and Elect formula come in to use
Guys ohms law isn't the only formula

Common mistake I have come accross in my Engineering career

Hope this reply was interesting enough for you
Have a think and give us a reply

Regards
 

DPG

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Arms
Esteemed
Advent Win
Mmm. Yep, fairly interesting. Are you saying ohms law applies only up to a certain distance of cable??

Also, transformer turns ratio is exactly that - a ratio of primary/secondary turns. This would apply for a large distribution transformer or a small mains transformer.

Just out of interest if you wound the grid supply (400kV) up a few thousand volts, what would happen to the current?

Look forward to replies :)

Darren
 
R

raylewis

  • Thread Starter Thread Starter
  • #11
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

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Arms
Esteemed
Advent Win
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.
 
R

raylewis

  • Thread Starter Thread Starter
  • #13
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

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Arms
Esteemed
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
 
I

ITerrell

  • Thread Starter Thread Starter
  • #15
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
 

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