electric power transmisson

Keeks

How is electricity transmitted? What is the theory behind transmissons systems. I vaguly remember ohms law (V=IxR) but how exactly is electricity tranmitted in transmisson systems?

daveirl

This post has been deleted.

Keeks

I'm not exactly sure.....

There are two formulaes running around my head at the momnet. The one mentioned abouve, Ohms law (V=IxR) and the power formula (P=VI).

Now I known that electricity is tranmitted at high voltages so as to keep the current low. This fits the power formula but now Ohms law(?) or am I just not seeing it properly?

Can someone help my confusion?

Capt'n Midnight

V=IR as everyone should know
Power = IV = I2R (I.I.R)

Or in other words there is a voltage drop on power cables so you loose power over long distance links - losses up to 15% are not uncommon - in fact using electricity to electrolyse water and then pumping the Hydrogen in pipes and then converting it back to electricity in a Fuel cell starts to look like an alterntive over longer distances.

Overhead cables are made of aluminium - since it is the best conducting metal by weight - around a steel core for strength. Again to cut down on the weight overhead cables are not insulated !

Since the power lost in a cable through heating is I squared R, the best thing to do is to lower the R by using bigger cables - but in practice they reduce the current by using a higher voltage. A typical 220KV line will carry 1,000 times as much power as a 220V line carrying the same current - but the heating effect would be the same if the same type of wire was used. Big Power Transformers are very efficeient 99-99.5% of the energy is converted.

The current flowing depends on what people have switched on - eg the sudden surge of power when everyone plugs the kettle in after coronation street on wednesdays causes a voltage drop - remember V=IR. the R of the power cables stays the same so when more current goes down the line there is a higher voltatge across it so less voltage for the punters. In the UK they had a gas turnine station that was only used at peak demand - after corrie...

BTW: ESB meters measure POWER not Current - so there is the alligation that they up the voltage a bit so light bulbs take more current (V=IR, R=resistance of bulb / kettle / heaters) and so the ESB make more money and your bulbs die younger ( 5% over voltage halves the life of a hot bulb )

Underwater cables use DC because water acts like the other plate of a giant capicatior and you would loose a lot of power.

Then there is the whole power factor thing - hard to explain but if you have a factory with large motors eg: power factor of 0.8 then it means the ESB need to generate 125% of the power the motor uses ! - so obviously enough they charge such customers extra per unit.

Also if you generate more than an certain amount of power then the ESB have to accept any power you sell them (though you can't set the price) - which means if either of you want to do some maintainance then the other will be sending live electricity into you.

The biggest problem with power distribution is when a circuit breaker trips because of an overload - the load is then spread over many other circuits - but if they were fairly heavily loaded then they might also trip - and you get a domino effect. The biggest problem is how to start up again with getting tripped out...

Victor

Originally posted by Capt'n Midnight
BTW: ESB meters measure POWER not Current - so there is the alligation that they up the voltage a bit so light bulbs take more current (V=IR, R=resistance of bulb / kettle / heaters) and so the ESB make more money and your bulbs die younger ( 5% over voltage halves the life of a hot bulb )

Power is transmitted at so as to convert to 240V with the expectation that there will be some drop as you say, in effect targetting 220V. I doubt they ever push it up to 250-260V.

Originally posted by Capt'n Midnight
Then there is the whole power factor thing - hard to explain but if you have a factory with large motors eg: power factor of 0.8 then it means the ESB need to generate 125% of the power the motor uses ! - so obviously enough they charge such customers extra per unit.

Power conversion factor is none of the ESBs business - if you have inefficient machine, not their problem. Industrial customers actually pay a lower per unit cost (but higher supply cost) because (a) they take it at a higher voltage, typically 3kV (b) volume discount.

Capt'n Midnight

Originally posted by Victor
Power is transmitted at so as to convert to 240V with the expectation that there will be some drop as you say, in effect targetting 220V. I doubt they ever push it up to 250-260V.

Can't find a link - but was talking to someone in the consumers association a good few years back who reckoned they upped the voltage during off peak - since they were generating the stuff anyway..

http://www.oasis.gov.ie/public_utilities/electricity_services/electricity_services_in_ireland.html
The standard electricity voltage in the Republic of Ireland is 230V a.c., nominal, at 50Hz, - can't find the ltolerances they are allowed
NOTE:http://www.esb.ie/main/energy_home/customer_12.jsp?gaurantee=6

Power conversion factor is none of the ESBs business

ESB levies a surcharge if the power factor goes below 0.95 http://www.esb.ie/main/energy_business/power_factor_correction.jsp

Anyway the ESB still come out at or near the top in surveys on who the public trusts.. And they can maintain a pair copper wires in your house for a third of what eircom charge, and still have amongst the lowest consumer prices in Europe.... If they ever offer telephony over power cables

davros

Originally posted by Capt'n Midnight
The standard electricity voltage in the Republic of Ireland is 230V a.c., nominal, at 50Hz, - can't find the tolerances they are allowed

This was a European harmonisation thing a few years back. Some countries were operating at 220V, others (like us) at 240V. So the EU said everyone is now at 230V. But they changed the tolerances at the same time so that nobody really had to adjust their voltage.

Thus, in this country (and the UK), the voltage is:
230V -6% +10% (216.2V - 253.0V)

And in former 220V countries it is:
230V -10% +6% (i.e. 207.0V - 243.8V)