# Clarification on Transformers - High voltage transmission

1. Aug 19, 2007

### scothoward

I seem to understand transformers very well, but one aspect of stepping up voltage for transmission is confusing me. I'm sure it is a very small oversight on my part.

In an ordinary electric circuit, when the voltage is increased, keeping the load resistance the same, the amount of current increases as well. Why is it that when we step up voltage for transmission, we are not also increasing the amount of current and thus the (i^2)(R) power loss?

Thanks

2. Aug 19, 2007

### NoTime

They put another transformer at the end. This changes the load impedance that the first transformer sees. Thus the current in the lines between transformers is lower.

You might note that the only positive thing this does is reduce the required size of the interconnect conductors.
Transformers are not 100% efficient.

3. Aug 20, 2007

### rbj

it's not the only positive thing. with the line current reduced, I2R losses in the lines are reduced. (i s'pose you could reduce the size of the conductors so much that the I2R losses are as bad as they are for the non-stepped up voltage and big fat conductors.)

not too much of whatever we engineer is 100% efficient.

4. Aug 20, 2007

### dlgoff

Just a note. Power transmission lines are just that. Since we are transmitting power, the configuration of the lines (and hence impedence) is important. If they are not "tuned", some power can be transmitted to the moon (power loss).

5. Aug 20, 2007

### Staff: Mentor

I have to admit that I'm fascinated by the complexity (and importance and technical achievement) of the power transmission line matrix system. I'd love to read a full technical description of how it all works, at an advanced EE level that discusses issues like this one about avoiding reflections at load distribution stations. Knowing what I know about communication transmission lines, it seems like there must have been so many problems to solve in distributing power over transmission lines into a redundant matrix network. Wow. It's one thing when you can put active repeaters at junctions, but it's totally another thing when you are trying to passively couple power through junctions into multiple distribution paths.

The simple descriptions of the power distribution networks at wikipedia and HowStuffWorks aren't what I'm looking for. Anybody got some pointers to EE level descriptions? Thanks!

6. Aug 21, 2007

### dlgoff

I'm no expert on power system networks and the like but I believe if you know about communication transmission lines, you know about power transmission lines also. Just bigger dimensions (since it's 60Hz), bigger transformers, etc. (oh, and much bigger hammers)

The trouble of matching the load with the transmitter is that the load keeps changing depending on how the customer uses his energy. Some customers with lots of motors, others with no load to high load in zero time.

When I worked for a power company, I was involved with real-time control of generators and maintaining area control error via a SCADA system. We modeled our system at various load levels and switched to the proper B-matrix (in software) when determining which and how much each generating unit should be moved. Not only the system loads needed to be considered, but the economics involved in selecting which unit to bring up (or down). Area Control Error is sort of like an error of a pid controller. The rule was, you had to cross zero at least once a minute. ACE is the total amount of power entering(-error) or leaving(+error) your system minus your total generation (with a bias for frequency variation). To measure the ACE you need to measure the watts and vars at each tie-line substation. The system I worked with had 17 tie-line substations and 5 power plants (if I remember correctly, 13 or 14 generators).

It was a fun job.

edit: definition of ACE didn't originally take generation into account.

Last edited: Aug 21, 2007
7. Aug 21, 2007

### Staff: Mentor

That does sound like a fun job! The economics of the power generation switch-overs is yet another layer of complexity that I wasn't thinking about. Very impressive.