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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: The Spanish Grid Drop-out - recently released information.
Date: Thu, 15 May 2025 16:18:16 +1000
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On 15/05/2025 5:14 am, Carlos E.R. wrote:
> On 2025-05-14 16:16, Glen Walpert wrote:
>> On Tue, 13 May 2025 07:50:36 -0700, Don Y wrote:
>>
>>> On 5/13/2025 6:26 AM, Glen Walpert wrote:
>>>>> I don't think that using broadcast radio for real time mains grid
>>>>> control is a good idea. It would be far too unreliable.
>>>>
>>>> Point to point RF links have been in use in (mostly hard wired) grid
>>>> control SCADA (System Control And Data Acquisition) systems for at
>>>> least 30 years, which is when a former employer did some work on a
>>>> SCADA upgrade project for the Egyptian Electrical Authority.  A search
>>>> on 'SCADA radio link' will turn up several vendors.
>>>
>>> And they used these to control instantaneous frequency and phase?  Or,
>>> just coarse data collection and /supervisory/ control?
>>
>> SCADA is used to monitor and control the grid, where control is done in
>> real time by adjusting the set points for real and reactive power at all
>> controlled power sources.  A small percentage of sources being
>> inaccessible degrades control by an insignificant amount.
>>
>> Real power is regulated by the prime mover governors, which must be
>> operated in droop mode in order to share the load (the speed set point is
>> reduced with increasing load).  The grid Hz/Gw figure which has been
>> mentioned is not directly related to generator inertia, where
>> insignificant energy is stored, it is the aggregate droop setting of the
>> prime mover governors feeding the grid.  After speed/frequency shift (the
>> governors only measure shaft speed) due to a load change the grid control
>> center adjusts the governor set points to bring the frequency back to
>> normal.
>>
>> Voltage regulation is entirely separate and is done by setting the
>> reactive power output of sources, again by adjusting set points in real
>> time over SCADA (or one of the other approved standard comm protocols).
>> This is a bit complex for a quick explanation but is described in detail
>> in any decent text covering synchronous generator control.
>>
>> Voltage regulation is more likely to be a stability issue than frequency.
>> If you take two widely separated generators or sets of locally paralleled
>> generators at two power plants which are optimally tuned for stand alone
>> operation and connect them with a long transmission line, voltage will
>> begin to oscillate due to the time delay in locally detecting changes in
>> the other plants output.  Detuning the regulators to achieve stability
>> results in inadequate response to load changes - thus central control of
>> both plants over SCADA.
>>
>> Large generator voltage response time is limited by the L/R time 
>> constant,
>> which will be in the 10's of seconds, over a minute for some large multi-
>> pole hydroelectric generators.  Inverters can respond much faster, with
>> potentially worse stability issues, and dividing power sources into many
>> small units does not make stability any easier, you still can't set
>> optimal tuning parameters for widely separated groups of sources which
>> will be both stable and fast, central control of some percentage of 
>> source
>> set points is required.  This problem and it's solution is well 
>> understood
>> and the current generation of inverters are designed to allow remote
>> control of set points.
>>
>> In the US more than 2/3 of all solar power is utility owned 10 MW or
>> larger and all of this is under central grid operator control, so no
>> stability issues there.
>>
>> More intermittent power sources need more batteries, which are being
>> deployed by utilities as fast as they can be manufactured, being far
>> cheaper then peaking gas turbines, the most expensive of power sources.
>>
>> <https://www.tdworld.com/distributed-energy-resources/energy-storage/news/
>> 55287560/us-energy-storage-industry-commits-100-billion-investment-in-
>> american-made-grid-batteries>
> 
> Wouldn't it be easier to have the entire distribution network using DC?
> 
> just saying with a glass of wine in my hand :-)

That's probably correct, but it wouldn't be cheaper. It's probably true 
that taking DC down to the sub-station level could be cheaper, with 
today's technology, if you were starting from scratch, but since that 
kit is already there you would be throwing away a lot of big expensive 
transformers from the next level up, and replacing them with a lot of 
big, expensive - if less expensive - inverters. It would take a lot of 
capital investment to make the switch, and the people who operate the 
grid are adminstrators rather than entrepreneurs.

-- 
Bill Sloman, Sydney