Spanish power grid

It was a matter of when, not if — and we'll see the same sort of thing on this side of the pond.

Saw that. Sounds like gas as it's hard to believe a 30 +/-degree delta T could cause such a difference in resistance. In any event, none of that would cause a frequency shift. Lack of inertia does. The article said Spain is about 80% non dispatchable/ asynchronous sources.

Tick, tick, tick.

BIG turbines. That's why we haven't seen synchronization failures here. With this , that makes 2 strikes against loading the grid with asynchronous sources

https://www.aer.gov.au/publications/reports/compliance/investigation-report-south-australias-2016-state-wide-blackout

But how would weather vary the frequency? That's my point. Nothing in the lines can vary frequency -only sources.

That's why you need a lot of rotating mass. If you have a sudden load change, a low mass source will change frequency quickly and the grid goes down. High inertia can absorb the fast changes without significant frequency shifting. GE, Siemens et.al. don't sell synchronous condensers for grins.

It will be interesting to see what the engineers come up with as a root cause. My money is going to be on a frequency droop or instability somewhere — why? Who knows at this point. At least that was what was found to be the root cause of the Texas grid blackout a few years back.

Thing is, if bits of the grid get out of phase — even slightly — with each other, huge currents can flow and things trip. Or if the frequency gets too far out of limits things trip. And then you have dominos…

The problem with a black start on a grid with a lot of asynchronous power is that you can't start any of that stuff without a power source which is synchronous running and feeding them, since all those gadgets are phase following.

Completely off grid systems, if they supply AC, will have a crystal controlled inverter. I wonder if the big battery energy storage systems could have that? Hmm… if they were big enough, they might be able to force stabilize things…

sarcasm/ Perhaps Edison was right and the grid should be DC. /sarcasm off

🙂

Had to find things the old way…

Many of the long distance transmission lines are DC because there is less loss(I would have to look up why and find the version that just describes it instead of using 4 pages of unnecessary calculus).

I'm confused Jamie…..

We have several DC transmission lines in the US, some of which are 40+ years old. So I'm confused by the "but nowhere near to practical engineering never mind installation yet." part.

They generate whatever frequency they need and lock it to the grid and since they're controlling it's frequency how is that any different than a typical generator turning at a set speed?

Well, @JDHW pretty well covered the problem. The short answer is that virtually all DC to AC inverters — whether on your rooftop panels or a wind turbine or a solar panel farm — or a gigawatt interconnector — track the frequency and the phase of the AC network to which they are connected. There are various ways to do this, but so far they all have the same basic principle: they vary their voltage and current output to exactly follow the voltage of the lines to which they are connected.

They do not generate a stable 50 or 60 hz output wave by themselves, never mind one which is exactly in phase with the lines to which they connect.

Further, most of them cannot absorb power from the network to which they are connected — only deliver it.

Thus if there are instabilities in the network to which they connect, they will only follow (and often worsen) those instabilities.

Contrast this with a rotating alternator. In such a device, the output frequency is determined by the rotation of the alternator — which is, in effect, a huge flywheel. Even your little home generator is pretty good at this with varying loads — but consider at the other extreme "Big Allis" — New York's Ravenswood No. 3 with a power output at full song of just under 1 gigawatt. (During the northeast blackout of 1965, she alone was able to keep the lights on in Manhattan for almost 10 minutes — she eventually quit when her frequency dropped too far and the lube oil pumps dropped out and cooked her bearings). Such units can maintain a stable frequency for a considerable time — several seconds at least, which is an eternity in grid terms — by either delivering a huge leading overload current if the grid freqency droops or absorbing a similar current if it starts to lead, solely by the rotating inertia of the machinery.

Various opinions exist as to how much power in rotating inertia is needed to keep a grid stable; I've seen estimates ranging from about 20% to 40% of the load.

Now we get to what might be… on small systems, such as an AC/DC/AC ultra pure power supply for instance, the output can be controlled with a crystal controlled oscillator (which, by the way, doesn't have to be at the same frequency output as input). But that doesn't scale all that well, and it can't absorb power. On a grid scale I've seen suggestions of controlling the output frequency using satellites, such as GPS signals — but there are some interesting problems with that, as well as some vulnerabilities to be considered.

It's an interesting problem…

You guys know more about power distribution than I do. I am just a dumb electrician.

I thought the reason George Westinghouse (AC Power) won out over Edison (DC Power) was because DC suffered more line loss and it couldn't transmit long distances as easily as AC.

Am I wrong or did I skip school that day?

the reason is that if they don't connect to the interstate grid the commerce clause of the constitution doesn't give the federal government jurisdiction over their power grid.

This is a really interesting site. Lots of posters that love and know lots about heating systems - including steam. Then there is an advanced discussion about how power systems operate - wow. Here is a slightly different take on what inertia means in a power system.

Before the days of modern power electronics everything that rotates was connected to the grid synchronously and contributed to frequency stability and this includes motors as well as the generators in the power stations. Motors are considerable fraction of the load on most grids, industrial and domestic (fridges/freezers/aircon). There is no doubt than thousands of tons of spinning metal does contain lots of energy.

However, AC power systems are very sensitive to frequency. The transmission system has distributed inductance and capacitance in the form of cables and to maintain stable voltages discrete capacitors and inductors are dynamically switched in and out of circuits and transformer voltage ratios are changed. Because of this frequency sensitivity, AC systems are controlled to very tight frequency limits - 1% or better. In terms of keeping a grid stable you can only use 0.5% of the considerable stored energy.

What really keeps a traditional power grid stable is some generators operating at less than maximum output that can rapidly change power output in response frequency changes (the technical term is spinning reserve). Inertia provides the energy while this ramping up of power output takes place. This balancing must happen within seconds. Things like generators failing or transmission lines failing need to be take into account when deciding how much generation capacity is running as spinning reserve and where it is located in the system. This is what power system operators do all the time and it normally works. When it goes wrong it hits the news.

Power electronics come of age and there are new ways of connecting power sources - PV, wind, DC power links that don't contribute to spinning inertia. On the load side inverter motor drives are very common in the industrial and domestic equipment that don't contribute to spinning inertia. Electronic power supplies for LED lighting and computers take the same amount power regardless of voltage.

With high levels of renewable energy on power grids operators can end up with all their traditional generation acting as spinning reserve. Or worse, having to run generators on no load to maintain inertia and curtailing renewable sources.

The way out of this is to make all sources of power and loads frequency sensitive. You don't need much actual storage from spinning metal or batteries when everything is frequency sensitive. If there was a requirement for PV or wind turbines to operate at 95% output when the grid frequency is 50 or 60Hz there would be an automatic reserve of 5% if the frequency started falling.

Converting all sources and loads to frequency sensitive versions is easy to do technically but virtually impossible to do retrospectively. You could start with the bigger renewable installations.

Renewable sources are intermittent so will always require backup from some source of dispatchable power.

I wonder why the industrial revolution really got going when steam power came along and we no longer had to rely on windmills and waterwheels? There is a working textile mill that is maintained as a museum near Manchester airport (uk) which contains a time clock for workers to clock in and out. It is linked to the waterwheel driving the mill - so lower water levels = longer working hours!

John

Old gen wind turbines were essentiall a big induction motor connected to the gear box running at fixed speed. Newer stuff is a doubly fed induction generator (think of them as a regular motor with an electric gearbox) which are pretty much the same thing but allows the motor speed to vary a bit from synchronous speed.

They are big motors spinning with the grid.

More info:

https://albertainnovates.ca/wp-content/uploads/2020/06/RAE-2429-N-ENERCON-Wind-Inertia-Value.pdf

@ChrisJ

Motors and generators are the same thing the difference is the mechanical input/output. A induction generator on a windmill will run at a slightly greater speed than the connected mains when there is enough wind. if the wind drops while the windmill is connected it will slow a little and take power from the grid acting as a motor. If you measure the power flowing on the connection between the windmill and the grid you can tell if it is acting as a generator or a motor. You can't make it run at different speed.

@Kaos

Modern wind turbines tend to be permanent magnet alternators connected to inverters. You can still run them as motors to store a bit of energy.

@ChrisJ You can get reverse power relays that sense the current and phase relationship with the voltage ,so the answer is sense reverse power and then disconnect the wind turbine. This is the sort of protection is used on conventional generators.

I guess you sense the speed of the wind turbine and reconnect when the speed is high enough to generate power.

@JDHW "I wonder why the industrial revolution really got going when steam power came along and we no longer had to rely on windmills and waterwheels? "

Easy. For the first time we had dispatchable power. Worried about "net zero"? Install a bunch of small fission plants and call it a day. 👍

Trying to use non dispatchable, variable output sources is creating problems for which we already have solutions.