By their nature, these "distributed generation" devices operate in current
injection mode. That is, they are synchronized to the line and inject enough
current at whatever voltage the line is at (subject to IEEE 1547 provisions,
e.g., -12% to +10% of nominal) to transfer the power they need to. So yes, they
generate their power at the exact frequency the line is at. They are all
four-quadrant devices and can also generate quadrature (imaginary) power as well
so can correct for power factor problems. All the decent sized inverters I have
seen are tightly phase locked to the line.
Now here's a fun thing to think about: due to safety concerns the power company
does not want any of these things to generate power if a line goes down. They
call this a power island. So every inverter must have detection for this
condition. It's trivial to detect if the load on the island is different than
the inverter output - the voltage will immediately go out of bounds - but not so
easy to detect if the island load matches the inverter output. so, what to do?
One thing manufacturers do is to wiggle the Q (imaginary power) output and see
if it shows up. If they're connected to the grid, well, they're way too small
to move the overall power factor but if they're the only source, it shows right
up! So many of these inverters are actually creating a small amount of phase
noise onto the mains. In terms of powering things it is insignificant and of no
consequence, but when you get to time-nut accuracy and measurement capabilities,
I'd bet you can actually see such modulation! If you are out on a long run from
the nearest substation it could even throw off your measurements as it will be
non-steady and depend on insolation or local wind conditions of the source inverter.
As for "huge profits," well, not in that industry.
Peter
On 2/24/2013 11:01 AM, Dr. David Kirkby wrote:
On 02/23/13 12:01 PM, John wrote:
All,
If you want a reason for logging the mains frequency, see the following
link to a news item which appeared on a BBC news program a few weeks ago
here in the UK.
http://www.bbc.co.uk/news/science-environment-20629671
There was also a full program about it which you can listen to at the
following link
http://www.bbc.co.uk/programmes/b01p7bxw
John G3UUT
Interesting. I never realized the UK national grid was just one grid, all
running at the one frequency.
I wonder if there are small variations in frequency at a local level due to
all these wind-turbines that seem to be cropping up everywhere? I doubt they
will generate a voltage at exactly the same frequency the voltage is coming
into them, especially as it is not a constant.
I personally object to funding these things so others can make a huge profit
from them. Perhaps I can argue they are a threat to national security!
Dave
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
