Power Line Frequency Regulation

Discussion in 'Electronic Basics' started by Eric Immel, Jul 24, 2003.

1. Eric ImmelGuest

The physics professor who taught my freshman lab told us that, in the
US, the power line frequency is controlled to one part per million.
Naturally he insisted that we report our line-derived timing information
with that precision.

While maintaining close control of frequency seems like a good idea for
the health of the grid, is 1 PPM overkill? Was this PhD. confused?
Controlling the speed of a multi-ton turbine that accurately seems
challenging.

EI

2. Gerry AshtonGuest

Electric clocks, at least the older ones, use a synchronous motor
that is tied to the power line frequency. So to keep the clocks
accurate, the power line frequency is made to average 60 Hz
to a high degree of accuracy. I don't know how far the frequency
may depart from 60 Hz for a short time.

There may be other advantages to keeping the average frequency
accurate that I am not aware of.

Gerry Ashton

To reply, change invalid to net in address.

3. Eric ImmelGuest

For him, talking was the easy part. Listnening, now that's tricky.
AHA! I always suspected he was fullobull.

How do those 0.5Hz variations interact when different parts of the grid
or different generators connect to each other? That seems like it might
create as much as a 1 Hz beat. Where does that energy go?

EI

4. Rich GriseGuest

I've heard that they have fairly sophisticated control electronics
that sync-up the turbines before they put them on line. How in heck
you control the speed of a multi-megawatt turbine-driven generator
is kinda beyond me. A valve in the

5. Don KellyGuest

----------
The idea is that the frequency of the whole system floats up and down. If a
machine speeds up with respect to another, then there will be power transfer
between machines to bring them back to the same frequency. In cases where
this power transfer is insufficient, then the system will become
unstable -blackout time as the system breakers operate on power surges and
some areas shut down because of excess load and others overspeed and trip
out because of insufficient load. Not considered a good thing.

As for the control of a multi-megawatt machine- governors are used - new
ones are electronic and old ones are entirely mechanical. For a steam
machine, a steam valve is controlled (typically about 0.5sec from full
closed to full open) but fuel control follows for longer term control. For a
hydro unit, hydraulic pistons adjust the water control gates. In the case of
high head units, this is done slowly (30 sec to 2 minutes) to avoid water
hammer surges (or special surge towers are used). An exception is the case
in the Kemano plant in BC where Pelton wheels are used to drive 160MW units.
The head is about 2000 ft so gate/valve opening is very slow. In this case
as the load may change in 100MW jumps (aluminum pot lines are on or off, no
in-between), there is a deflector which can be put into or removed from the
water jet to quickly allow load pick up or dropping.
As for synchronisation- there are special control units to do this but it
can be done by a human operator simply watching a synchroscope and voltmeter
(a set of lights if no synchroscope), tweaking the speed and voltage then
closing a switch at the right time. A good operator can often do it faster
than the electro-mechanical control. The process is essentially the same as
would be done with 1 to 5KW units in a lab.

6. Fred AbseGuest

If it's hydro, guide vanes in the case of a Francis turbine (the ones with
the vertical axis and tangential entry). If it's a Pelton (bucket wheel),
a damn' great valve. Not sure on Kaplans, but it'd have to be some sort of
vane arrangement. If its a gas turbine, fuel valves.

Peltons make a lovely noise when you get gravel coming down

Everything from hydraulic servos (1930s originals still in use at Hoover),
to electronic control.

For years, they used to sync sets on line using just three lamps (wait
till they all go out then throw the switch and go change your underwear)

Then came the Synchroscope, now it's all automatic.

7. AC/DCdude17Guest

X-No-Archive: Yes

The number of cycles per day or whatever time frame they use is
regulated to a very close tolerance, but instantaneous frequency can hop
around a percent or two.

8. AC/DCdude17Guest

X-No-Archive: Yes

Some AC powered LED alarm clocks are tied to AC line also. It electronically
picks up the zero crossing under normal operation, but they usually have a
internal clock as well that runs on 9V battery when the power is out. The
internal clock is not very accurate and clock goes off a a min or so when
there's an hour long power outage.

Answering machines with an AC-AC adapter are most likely tied to the power
line as well.

Neither my answering machine nor my alarm clock loses a minute through out
the year. The only time I need to readjust them is daylight savings.

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