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VK3XX > INFO 05.07.02 21:00l 160 Lines 7195 Bytes #999 (0) @ WW
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Subj: AC v DC power transmission
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Sent: 020704/0619Z @:VK3FRS.#MEL.VIC.AUS.OC #:25173 [Kilsyth] $:4B0939VK3XX
From: VK3XX@VK3FRS.#MEL.VIC.AUS.OC
To : INFO@WW
To all who might be interested.
I have been following some of the discussions on electrical power,
voltages and frequencies used in different regions with some interest.
Some of this has been of good value and some of no little amusement. But
some very good questions have been asked in the process.
I wish to reply to a question raised by Warren KB2VXA (directed to VK4LU)
who asks about the workings of DC transmission systems. This is indeed an
interesting and complex topic so I will endeavour to give a quick
overview first of the principles with the "how" at the end.
The factors which require consideration in the transfer of electrical
power in large power blocks and the virtue of DC being used can be
simplified to:-
1) Geographical.
2) Economic
3) Technical
so I will enlarge on each of these.
1) Geographical
In the generation of electric power geographical constraints dictate a
lot.
For example, The source of fuel for generation may be removed from the
system load by many miles or hundreds of them. AC transmission is
generally the most common. For example here in the state of Victoria the
main fuel for the major load which is in and around Melbourne is Brown
Coal the source of which is about 100 KM from the load. So compared to
shipping tons of the stuff by road or rail (and about 3 times as much is
required for a given energy output as with black coal) it makes sense to
generate around the source of the fuel. So AC transmission makes sense
particularly as relatively small amounts are tapped off on the way
between.
Another example is a hydro electric generating system which is a very long
way from the load and there is virtually no load in between in between.
Typical examples are hydro generation in Tasmania being transported across
about 250 KM of water to supplement power here in Victoria. This is a
strong case for DC transmission. Another case is surplus power generated
in France by their large nuclear industry being linked into England across
the English Channel. The former is still being argued here on
environmental grounds but the latter has been in operation for years.
Despite
2) Economic
Clearly the cost of major power lines worth millios per mile has to meet
economic criteria. Huge power flows require either high voltage or high
current (here we are dealing in megawatts, kilovolts and kiloamps) and
each has to be optimised for cost. For example if the voltage is not high
enough then the weight of copper to be drawn for the conductors and the
physical support of it will get less economical as the power flow
increases. As the voltage increases then the insulation becomes an issue
coupled with the loss of power by ionisation of the air around the wires
and would you believe, radiation from the wires. Power is not only lost in
the resistance of the wires. So what comes out at the remote end is a
complex evaluation.
3) Technical
Generally over short distances AC transmission has been the best bet and
is almost essential if there has to be T-off of power between the ends. So
where does DC transmission score.
Voltage: Typical maximum voltages for AC transmission are now reaching up
to about 650-700,000 volts. More power, more transmission lines in
parallel.This is the RMS voltage so the peak voltage is going to be in the
1.2 million volts area! So the insulation has to cope with this.
Ionisation loss (corona) becomes higher as the volts go up, but the cost
of insulation and towers to give adequate clearance becomes very
expensive. With the existing knowledge of insulation for AC lines the
voltage can be increased to around 1 million volts with proven technology
for overhead lines if transmission is by DC.
Current: Clearly the power lost in resistance is going to be least for the
least resistance of cable so this depends upon the voltage necessary to
transmit the power required. Lighter cable like aluminium instead of
copper is already in common use but at the expense of resistance losses.
AC Transmission lines: An AC transmission line is generally three phase
so three main conductors are required. These have to be held up and
insulated from tower structures for each phase. But this has already
existed back in the last century as common practice. These lines have
inductance as has been stated by Warren and the longer they are the
greater the inductance. Capacitance while not of insignificance gets less
as clearance distances increase with higher voltage and more as the line
length gets very long. It doesnt matter at all with DC except for initial
line charging.
AC/DC Transmission cables: For these environmental issues often determine
what can be done. Of course they are necessary for under water cables and
each cable has to be insulated for the system peak voltage over its entire
length in addition to protection against its environment. It is rarely
economic for large power flows except for short distances and
interconnected networks. Cables are used at the lower voltages like
132,000 volts is now considered a distribution voltage in most western
nations.
DC transmission:
So HOW?
This is much simpler than the previous bit which deals with the "WHY".
On some DC links power may be transmitted in either direction but I will
just describe a one way system which is probably the most common.
At the sending end the generated AC voltage is transformed to the DC
system voltage and then rectified. At the receiving end the DC is
"inverted" back to AC just like a UPS system without the batteries. This
is more familiar perhaps to computer oriented people.
Now we come to the real electronics interest.
When DC links first were planned the only suitable devices were large
mercury vapour diodes (the rectifiers) and equally large mercury vapour
"triodes" (called thyratrons) as the inverters. The "grid" turns them on
and off like a switch.
These had to be connected in voltage graded strings to suit the working
voltage. The current was much easier dealt with by parallelling.
Today the technology of high powered and high current solid state devices,
which do the same thing, is the practice. Again they need to be connected
in series and in parallel the former being the difficult bit.
These devises are huge physically yet are in essence nothing more than
silicon diodes and insulated gate FET's but on what a different scale from
the radio/electronics industry. But the principles are the same just built
for more amps and volts!
At the receiving end the AC from the inverter "valves" (NOTE Warren!) is
transformed to the system voltage and it can also be system frequency if
there should be a difference at both ends!
So it is all very simple until hundreds and thousands of volts and
thousands of amps become involved.
I hope that helps and Warren, I am going to write a bit to add to yours on
50 v 60 Hz for power systems! Be warned.
That is a very simplified story so I hope it was of interest. Power and
electronics boundaries are merging and in many other areas too.
Cheers and
73 - GORDON, VK3XX @ VK3FRS
Message timed: 15:46 AEST on 04 Jul 2002
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