# Changing frequency

Discussion in 'General Electronics' started by prizark, Jan 28, 2004.

1. ### prizarkGuest

Hi guys,

I have a question. A power station is generating 100MW,3phase
50Hz at 440KV that must be connected to a grid of 220KV at 60HZ. How
can this be done.Like the step down thing can be done with a
transformer thing but how about the frequency change.Is this possible.

Thank You,
Prizark

2. ### svetoslav belchevGuest

I think if generator will bi spined fastly frequency will be more than 50Hz.
But and voltage and power will be more.Then you can use a transoformator to
slow the voltage.But it must be 3phase or you can use 3 trasfomators.

3. ### prizarkGuest

The way you suggested is not useful. Anyway what help do you need
regarding FM transmitter.You can find tons of ciruits at
www.discovercircuits.com.

4. ### GPGGuest

1 Run the generator at 60 Hz
2 Systems exist to rectify the output to DC and convert to AC but
for 100MW, not a trivial cost

5. ### EthanGuest

I know of a couple ways that this is done. The old school way is to
connect the shafts of two synchronous machines that have different
numbers of poles. Such as one with 10 and the other with 12. That
way one will run at 60Hz and the other at 50Hz, with the same rotor
speed. The current in the field windings is controlled to set one
machine as the generator and the other as the motor. Power can be
made to flow in either direction. I have heard of large stations like
this, I believe in Japan where part of the grid is on 60Hz and part on
50Hz. (I may be wrong about where these things are located)

The more modern method is to rectify the 50Hz station output, like for
a DC transmission line, then an inverter converts this to whatever
voltage and frequency you like at the other end. This allows better
efficiency but is much more complicated to control. The way this is
put together is much more complicated than this description, but if
you look up how DC transmission lines work you should find something
more usefull.

Either technique is primarily for changing the frequency, and is also
used to control power flow. Both methods will use transformers to
raise the voltages. Sychronous machines and the thyristors used for
the rectifier/inverter generally run at lower voltages (~10kV).

Ethan

6. ### GPGGuest

The more modern method is to rectify the 50Hz station output, like for
Here (NZ) there is a DC system that sends power north using DC
from the generators. This is converted to AC after crossing Cook
Strait. Capacity is 900 MW. DC is used to lower losses in transmission,
abt 700 KM

7. ### EthanGuest

Either technique is primarily for changing the frequency, and is also
Actually, I shouldn't say DC transmission lines are used primarily to
change frequency, there are a lot of other reasons to use them as
well. They are most common in situations where there is a lot of
capacitance in the transission line, so the current needed just to
charge and discharge the line at 60/50Hz becomes a significant loss.
You get this large capacitance in very long lines (several hundred
miles) or in lines that are much more closely coupled to ground, such
as an underwater cable. They are also used to transfer power between
two large power grids that are not running synchronously, such as the
eastern US and Texas, which for some reason needs to assert thier
independance this way.

Ethan

8. ### Jerry GreenbergGuest

You can step the voltage up or down with the proper type of
transformer. But, for the frequency difference, this is where it would
be very complex. You cannot mix 50 and 60 Hz from power sources. You
would have to do a frequency conversion, which is far more complex
than a simple transformer.

Jerry G.

9. ### prizarkGuest

Hi Ethan,

The information you provided is a bit closer to what I am
looking for. The first idea of motor-generator set is very old and
also expensive and requires extra maintainence. The latter approach of
converting it to DC and then invert it back to what we want is a good
idea.From what I've learned in the past few days is that the 3 phase
input is converted into 6 or even 12 phase using transformers so that
the output almost looks like DC and a bit of capacitor reservoir can
make it pure DC. From there a set of IGBT, thyristors or whatever
things that are necessary to re-make it into AC power will do the
work. But what I need is more detailed description of the actual
circuit workings and also some statistics on various ways of doing the
same thing.

Thank You,
Prizark

10. ### EthanGuest

Sounds like you are getting the idea. For more detail you are going
to have to find a real source. I havn't looked into this stuff in a
long time, and don't know what happened to the reference material I
had. You will have to find a good engineering library, or at least a
book on HVDC transmission lines. Amazon should have something if
nothing else.

Good luck.

Ethan