# Electricity transmission explained?

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1. Dec 17, 2014

### nz_caleb

I'm working an electricity broking desk at a finance firm this summer, and was hoping someone could clarify the science of it all for me.
-For example, a plant has 350MW of capacity. Say 350MWh are demanded for a one hour period. Assuming this plant bids successfully to supply the full amount, does this mean it simply needs to run at full capacity for an hour?
-Looking at transmission lines, they are listed in terms of kV. For example, one is listed as being 350kV HVDC. But after Googling kV to MW, 1MW is 1000kV amperes. I don't understand this at all, surely that woud mean you'd need thousands of transmission lines to transport a very small load of electricity (say 100MW)?
-Lastly, what is the difference between HVDC and AC in terms of transmission capabilities?
Thank you!

Last edited: Dec 17, 2014
2. Dec 17, 2014

### PhanthomJay

it is a simplification, but essentially, yes.
you are overlooking the current in the lines. Assuming a unity power factor, a megawatt (MW) is the same as a megavolt amp (MVA). Again oversimplifying, using power = voltage times current, a 100 kV line carrying 1000 amps can deliver 100 MW of power. You don't need a thousand lines to do this, just one (or say 2 for backup or to reduce line currents to 500 Amps each).
DC is economical for transmitting power over long distances with less line losses, but generally speaking, AC power can be easily transformed to lower voltages for distribution to customers(using step down transformers) and is the usual form of supply.
I am not an electrical engineer, so take this response lightly.

3. Dec 17, 2014

### jim hardy

You need to absorb the basic electrical terms for Energy, Power, Volts and Amps.

What's your background? Do you remember your high school physics?

That'll tell us where to start.

Power in an AC line = Volts X Amps X √3 X "power factor" which is usually between 0.8 and 1.
In a DC line it's just Volts X Amps.

So a 240 KV AC 3 phase line carrying 1000 megawatts would see
1000X10^6 /(√3 X 240X10^3) = 2405 amps
at power factor of 1.
That'd be a really stout line. 500 is more typical of that voltage.

A search on "power line capacity" turns up lots of hits, peruse them for one that suits your background.

"The Grid" is a pool into which everybody pours power. It is extracted by users and your office charges the appropriate accounts.
Inflow must equal outflow because electrical energy can't be stored in significant quantities (yet). Pumped hydro is about the only exception, and that's actually storing it as gravitational potential energy.
So the 'successful' bidder matches his generation to whatever he's sold.

Most plants cannot start and stop quickly. So be aware that apportioning generation solely by $per megawatt hour can put impossible demands on a plant and put a grid in an unstable condition. Surely some old hands will "show you the ropes". Meantime bone up on concepts of power and energy, and the electrical units. http://en.wikipedia.org/wiki/Electric_power 4. Dec 17, 2014 ### nz_caleb Cheers y'all, I definitely need to brush up on some basic physics! I'm an economics/finance major, can't remember anything from high school physics. I want to create a model showing how line outages can cause electricity to get bottlenecked at nodes, but when I started I realised the science of transmission was more complicated than I expected. 5. Dec 17, 2014 ### jim hardy \ It's a fascinating field. You might look into a book "Power System Analysis" by Charles Gross. This summer job can be a real learning opportunity for you. We humans do our best under pressure... 6. Dec 17, 2014 ### sophiecentaur It may be hard to understand how a particular power station can be controlled so that it supplies just the amount of Power it wanted to. All the generators on the grid are rotating at the same frequency but you can make one generator supply more than the rest if it is made to run just ahead of the others (which requires more coal to be fed to those boilers, basically). There will be some generators that are just running in sync and not supplying much power at all. If extra power is needed, the boiler is just fed with more fuel and the running spares can then input the extra necessary. It's a bit like a gang of men pushing a truck uphill. By pushing just a bit harder, one guy can deliver more power than the guy beside him, although they are both, apparently, walking at the same speed. With power stations, this can all be measured and the hard working one gets paid accordingly - which it's all about, as you well know. 7. Dec 18, 2014 ### anorlunda ### Staff: Mentor A better answer is that you don't need any physics or electrical knowledge at all to model your original question about price versus outages. Just consider the power grid as a black box that delivers energy from the seller to the buyer. It has a finite delivery capacity. An outage will reduce that capacity. How would that affect prices in different places? That is an economics problem and it could equally well apply to oil or soybean markets, or anything involving delivery of goods or services. 8. Dec 18, 2014 ### jim hardy He ought to learn a little about power systems. Utilities learned that "economic dispatch" sets you up for blackouts. When one big unit supplies 75% of a sub-region's power and that unit trips, you'd better have adequate replacement power instantly available and transmission capacity to get it in. A bottleneck then can put the lights out. 9. Dec 18, 2014 ### anorlunda ### Staff: Mentor What he ought to do is to attend the training offered by the market operator. The training might even be free. Those courses start at levels higher than ohm's law. I used to work for an ISO. Hardly a day went by without some naive trader loosing a few million (or a few dozen million) because of poor understanding, or sometimes just because of a typo. To be safe bidding on energy products, you must know the physics, the industry conventions and jargon, economic theory, and the pitfalls of the bidding system user interface. IMHO, it is easiest to start by learning the economics of markets. Bottlenecks are not just sometimes events. They influence prices 24x7. For example, as I write this, the prices in New York State range from$20,23 to \$31.65. Each zone has a different price, and the primary reason for the price differences is bottlenecks.

10. Dec 18, 2014

### Averagesupernova

I would suggest the same thing Jim Hardy has since the OPs questions are more centered around electricity than economics.

11. Dec 18, 2014

### jim hardy

At his level, summer intern, that was my thoughts exactly.

12. Dec 18, 2014

### zoki85

Lot of work waiting for you. Your time is important issue.

13. Dec 19, 2014

### dlgoff

It's a lot more complicated than that and to be fair to the OP he should understand this. North American Electric Reliability Corporation (NERC) is the regulatory authority in the US where you'll see just how complicated. One example is balancing and frequency control. Attached is a pdf file about this aspect.

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• ###### NERC Balancing and Frequency Control 040520111.pdf
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