Posted by daestrom on April 7, 2009, 9:25 pm
> Eeyore wrote:
>>
>> Alistair Gunn wrote:
>>
>>> In alt.energy.renewable Eeyore twisted the electrons to say:
>>>>> Actually it wouldn't be "them" switching your appliances on/off, your
>>>>> appliances would do it automatically based on their own state and the
>>>>> condition of the national grid. I haven't found conclusive proof as
>>>>> yet,
>>>>> but it seems that fridges with this sort of dynamic demand are now
>>>>> being
>>>>> sold in the UK ...
>>>> I'd like to know where they get their data from.
>>> It monitors the mains frequency ...
>>
>> The mains frequency is very rock solid. It HAS to be for the gris to
>> work. You're
>> talking crap.
>>
>> Graham
>>
> The instantaneous mains frequency isn't rock solid. The control system
> keeps the long term *average* mains frequency rock solid so that
> synchronous electric clocks stay in time, In the short term the frequency
> drifts by anything up to a Hz or so.
> The main reason is the time it takes mechanical generators to respond to
> changes in overall load.
Well, not exactly. The *governors* on regulating units can respond faster
than that (governors can respond on the order of a second or less). But
because the governors are simple proportional controls, they have to have a
fixed error to maintain the new output level.
Then the system operator and/or SCADA will slowly 'tell' the regulating
units to raise/lower their speed setpoints. Or if a regulating unit is near
a limit, the operator and/or SCADA will have to contact a base-load unit and
ask them to raise/lower their output. These operations are both
deliberately slower adjustments to avoid 'chasing their own tail' (hunting a
lot when load is ramping continuously up/down).
This is why the variations you usually see are on the order of 5 to 20
minutes.
daestrom
Posted by Don Kelly on April 8, 2009, 5:04 am
daestrom wrote:
>> Eeyore wrote:
>>>
>>> Alistair Gunn wrote:
>>>
>>>> In alt.energy.renewable Eeyore twisted the electrons to say:
>>>>>> Actually it wouldn't be "them" switching your appliances on/off,
>>>>>> your
>>>>>> appliances would do it automatically based on their own state and
>>>>>> the
>>>>>> condition of the national grid. I haven't found conclusive proof
>>>>>> as yet,
>>>>>> but it seems that fridges with this sort of dynamic demand are
>>>>>> now being
>>>>>> sold in the UK ...
>>>>> I'd like to know where they get their data from.
>>>> It monitors the mains frequency ...
>>>
>>> The mains frequency is very rock solid. It HAS to be for the gris to
>>> work. You're
>>> talking crap.
>>>
>>> Graham
>>>
>> The instantaneous mains frequency isn't rock solid. The control
>> system keeps the long term *average* mains frequency rock solid so
>> that synchronous electric clocks stay in time, In the short term the
>> frequency drifts by anything up to a Hz or so.
>>
>> The main reason is the time it takes mechanical generators to respond
>> to changes in overall load.
>>
> Well, not exactly. The *governors* on regulating units can respond
> faster than that (governors can respond on the order of a second or
> less). But because the governors are simple proportional controls,
> they have to have a fixed error to maintain the new output level.
> Then the system operator and/or SCADA will slowly 'tell' the
> regulating units to raise/lower their speed setpoints. Or if a
> regulating unit is near a limit, the operator and/or SCADA will have
> to contact a base-load unit and ask them to raise/lower their output.
> These operations are both deliberately slower adjustments to avoid
> 'chasing their own tail' (hunting a lot when load is ramping
> continuously up/down).
> This is why the variations you usually see are on the order of 5 to 20
> minutes.
> daestrom
Right on: I'd like to add the following:
Typically governors of units under regulation have a speed droop which
may be of the order of 5% from no load to full load if the machine was
running alone. This and the no-load speed setting determine how load
is shared between interconnected machines. The droop is necessary as if
there is no such droop, the sharing of load becomes indeterminate and
the fastest machine to respond will try to pick up all the load (and
even try to motor other machines). Hydro machines have an additional
temporary droop as because of hydraulic considerations- -if you open
the gates to pick up load, there can be a temporary decrease in output
as it takes time to change the flow in a penstock. In some cases this
constraint may result in full valve opening or closing taking minutes
where a steam valve can operate within a second in some cases.
In any case these governor settings may not be what is wanted so that an
external override (SCADA or other) is needed to re-adjust the load
sharing and frequency.
I recall one system which had a master clock which sent out control
signals every few seconds. Each machine had its own clock and a contact
arm. If the arm was between the signal contacts- nothing happened - but
if it was on a signal contact it got a signal to increase and decrease
the speed. Load changes could bias this - effectively it wasn't a load
frequency control but a load -time control. It was very ingenious and
worked well in the particular system with remotely controlled hydro
plants. This was discarded later on interconnection with the much larger
NW pool and conventional load frequency control was used.
As I recall, the frequency charts generally showed a band about 0.05 to
0.1 Hz wide.
In one system in Quebec, an excursion of 0.5Hz had to be reported as an
unusual incident
--
Don Kelly
dhky@shawcross.ca
remove X to reply
Posted by Eeyore on April 3, 2009, 3:15 am
Alistair Gunn wrote:
> Eeyore twisted the electrons to say:
> > Bill Carter wrote:
> > > All power generation methods HAVE to have backup sources. Even coal
> > > plants have limited uptime.
> > Which can be predicted and scheduled with regulat maintenance. Clouds
> > and the wind are NOT predictable.
> You can predict when a convential plant's going to need to be shut down
> for maintenance, you can't (necessarily) predict when it might end up
> shut down for other reasons :-
> <http://news.bbc.co.uk/2/hi/uk_news/7422817.stm>
> ... and even for your predicted maintenance, you're still going to need
> backup sources.
That's schedulable. Very easy to organise.
> Oh, and you can predict wind and sun levels. eg :-
> <http://www.metoffice.gov.uk/weather/uk/yh/york_forecast_weather.html>
Hopeless record.
Graham
Posted by Alistair Gunn on April 3, 2009, 11:15 am
In alt.energy.renewable Eeyore twisted the electrons to say:
> > You can predict when a convential plant's going to need to be shut down
> > for maintenance, you can't (necessarily) predict when it might end up
> > shut down for other reasons :-
> > <http://news.bbc.co.uk/2/hi/uk_news/7422817.stm>
> >
> > ... and even for your predicted maintenance, you're still going to need
> > backup sources.
> That's schedulable. Very easy to organise.
You still need the back-up plant available. Also Sizewell-B dropping
itself out of the UK's electricity supply (and thus taking 3% of supply
away) wasn't exactly predicted in the case I mention above.
> > Oh, and you can predict wind and sun levels. eg :-
> > <http://www.metoffice.gov.uk/weather/uk/yh/york_forecast_weather.html>
> Hopeless record.
Your prediction of supply from wind/solar doesn't need to be 100%
accurate, because your prediction of how much demand there's going to be
is never going to be 100% accurate either.
--
These opinions might not even be mine ...
Let alone connected with my employer ...
Posted by Eeyore on April 4, 2009, 3:28 am
Bill Carter wrote:
> Eeyore wrote:
> > Bill Carter wrote:
> >> Eeyore wrote:
> >>> Bill Carter wrote:
> >>>> Your wind power comes in at various times day and night.
> >>> Or not at all for days on end.
> >> Or you don't know what you're talking about. Wind power is very
> >> abundant in specific locations and lousy elsewhere. It is merely
> >> necessary site the turbines appropriately. See this resource map,
> >> much of the western US is rated good-superb and some locations
> >> in east as well.
> >> http://windeis.anl.gov/guide/maps/map2.html
> >
> > Still doesn't change the fact that some places are becalmed for days on end.
Both
> > solar and wind power HAVE to have conventional generation back-up further
> > increasing their cost.
> All power generation methods HAVE to have backup sources. Even coal plants
> have limited uptime.
Huge uptime compared to either solar or wind. I'm not averse to using some wind
but the
economics of *PV* solar are just daft.
Graham
>>
>> Alistair Gunn wrote:
>>
>>> In alt.energy.renewable Eeyore twisted the electrons to say:
>>>>> Actually it wouldn't be "them" switching your appliances on/off, your
>>>>> appliances would do it automatically based on their own state and the
>>>>> condition of the national grid. I haven't found conclusive proof as
>>>>> yet,
>>>>> but it seems that fridges with this sort of dynamic demand are now
>>>>> being
>>>>> sold in the UK ...
>>>> I'd like to know where they get their data from.
>>> It monitors the mains frequency ...
>>
>> The mains frequency is very rock solid. It HAS to be for the gris to
>> work. You're
>> talking crap.
>>
>> Graham
>>
> The instantaneous mains frequency isn't rock solid. The control system
> keeps the long term *average* mains frequency rock solid so that
> synchronous electric clocks stay in time, In the short term the frequency
> drifts by anything up to a Hz or so.
> The main reason is the time it takes mechanical generators to respond to
> changes in overall load.