[time-nuts] Logging the grid frequency....

[Peter Gottlieb]

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
>