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Increased risk of grid instability ?
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PS_RalphW



Joined: 24 Nov 2005
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PostPosted: Wed Jun 07, 2017 1:00 pm    Post subject: Reply with quote

From a warehouse that I am currently sitting in.

https://midsummerwholesale.co.uk/buy/moixa

Made in the UK offering local grid stabilisation
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BritDownUnder



Joined: 21 Sep 2011
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Location: Hunter Valley, NSW, Australia

PostPosted: Thu Jun 08, 2017 8:38 am    Post subject: Reply with quote

I think that only Adam and myself are getting this.

We are not talking about the ability to put more power into the grid on a seconds to hours scale but about a millisecond scale. It's like a flywheel, and unfortunately coal, gas and nuclear plants are like big flywheels that have a lot of momentum and don't slow down very quickly on a millisecond scale. Anything where the generator is synchronous, has a high mechanical moment of intertia and is 'locked' to the grid can do this.

Unfortunately solar and and to a lesser extent wind can't replicate this stabilizing influence as well. The reason being where there are generators which are absent in solar grid-tie inverters they are asynchronous and only follow the grid and can go faster or slower and are not locked to the grid frequency. It could be that pumped storage, tidal or even giant flywheels themselves will cover the problem in the future or something new gets invented that will.

At the moment it is not a big problem in the UK but in South Australia which is very wind, solar and import dependent it is becoming a big problem at certain times and when then synchronous generation suddenly trips.
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adam2
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PostPosted: Thu Jun 08, 2017 11:42 am    Post subject: Reply with quote

BritDownUnder wrote:
I think that only Adam and myself are getting this.

We are not talking about the ability to put more power into the grid on a seconds to hours scale but about a millisecond scale. It's like a flywheel, and unfortunately coal, gas and nuclear plants are like big flywheels that have a lot of momentum and don't slow down very quickly on a millisecond scale. Anything where the generator is synchronous, has a high mechanical moment of intertia and is 'locked' to the grid can do this.

Unfortunately solar and and to a lesser extent wind can't replicate this stabilizing influence as well. The reason being where there are generators which are absent in solar grid-tie inverters they are asynchronous and only follow the grid and can go faster or slower and are not locked to the grid frequency. It could be that pumped storage, tidal or even giant flywheels themselves will cover the problem in the future or something new gets invented that will.

At the moment it is not a big problem in the UK but in South Australia which is very wind, solar and import dependent it is becoming a big problem at certain times and when then synchronous generation suddenly trips.


Yes, you are correct.
Sorry if this seems disrespectful to other members, but it does seem to me that most contributors to this thread have not fully understood that we are talking about very short term instability.

The only likely cure is more rotating machinery, flywheels are a distinct possibility.

The behaviour of large grid systems is still not fully understood, perhaps surprisingly. They are too complex for truly accurate computer modelling, and much too big and expensive to build a spare one for experiments.
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fuzzy



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PostPosted: Sat Jun 10, 2017 10:00 am    Post subject: Reply with quote

Let's ask a different question then; what problems can you see as a consequence of a less stiff mains supply?
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vtsnowedin



Joined: 07 Jan 2011
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PostPosted: Sat Jun 10, 2017 11:51 am    Post subject: Reply with quote

[quote="adam2"]
BritDownUnder wrote:
I think that only Adam and myself are getting this.
.....
Yes, you are correct.
Sorry if this seems disrespectful to other members, but it does seem to me that most contributors to this thread have not fully understood that we are talking about very short term instability.

The only likely cure is more rotating machinery, flywheels are a distinct possibility.

The behaviour of large grid systems is still not fully understood, perhaps surprisingly. They are too complex for truly accurate computer modelling, and much too big and expensive to build a spare one for experiments.

Perhaps you are correct in that. I consider instability to be anything that can knock the lights off line long enough for me to notice or to make all the clocks reset. Of course millisecond flickers here and there if not corrected might well lead to a full outage so it is a matter of degree and duration. What could be more stabilizing then having fifty percent or more of your generation be from fossil fuel powered steam turbines?
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Pepperman



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PostPosted: Mon Jun 12, 2017 8:51 am    Post subject: Reply with quote

In the op there's a reference to conventional plant responding in seconds rather than milliseconds but I'm also keen to know what these problems are. Is there any literature out there that you can link to?

I don't know to what extent they're used in the UK but in Germany solar inverters are far from dumb devices and can provide quite sophisticated grid stabilisation services. I would expect that they can respond to frequency fluctuations extremely quickly.
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fuzzy



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PostPosted: Mon Jun 12, 2017 10:17 am    Post subject: Reply with quote

The good old mains we know is only generated to a close voltage and frequency range with a low impedance, because of the practical problems of joining ac generators together - not because it's good for the consumer. It has allowed filament bulbs to have a long life and motors to give a predictable power output. Electricians are obsessed with earth resistance, because they have this concept that everything will be OK if you can trip a fuse [except that very few faults involve a short to earth], but I can't recall much about live-neutral impedance. This site:

http://www.acoustica.org.uk/other/mains_Z.html

reckons mains is 0.25 ohms resistive and 0.23 ohms inductive . Combining these gives 0.34 ohms total at 50Hz and 2.31 ohms total at 500Hz if you want millisecond current changes. A high powered shower is 6-7 ohms resistance, and low powered devices will be 100s or 1000s of ohms load.

I can't see them allowing the mains to vary much because the wear and tear on suppliers eqpt would increase [variable tap transformers at power stations, rotors on generators etc]. I think there will be safety disconnects when supplies become less stable.
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adam2
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PostPosted: Mon Jun 12, 2017 12:16 pm    Post subject: Reply with quote

Pepperman wrote:
In the op there's a reference to conventional plant responding in seconds rather than milliseconds but I'm also keen to know what these problems are. Is there any literature out there that you can link to?

I don't know to what extent they're used in the UK but in Germany solar inverters are far from dumb devices and can provide quite sophisticated grid stabilisation services. I would expect that they can respond to frequency fluctuations extremely quickly.


PV inverters don't help, indeed they are part of the problem.
No matter how clever may be the electronics in an inverter, they simply CAN NOT supply more power in the very short term than is being provided by the PV array.
A short term dip in the frequency requires that more power be supplied, rotating machinery can supply extra power instantaneously by utilising inertia.
A million grid tie inverters supplying a similar amount of power cant. From where is this to come from ?
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adam2
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PostPosted: Mon Jun 12, 2017 12:23 pm    Post subject: Reply with quote

fuzzy wrote:
The good old mains we know is only generated to a close voltage and frequency range with a low impedance, because of the practical problems of joining ac generators together - not because it's good for the consumer. It has allowed filament bulbs to have a long life and motors to give a predictable power output. Electricians are obsessed with earth resistance, because they have this concept that everything will be OK if you can trip a fuse [except that very few faults involve a short to earth], but I can't recall much about live-neutral impedance. This site:

http://www.acoustica.org.uk/other/mains_Z.html

reckons mains is 0.25 ohms resistive and 0.23 ohms inductive . Combining these gives 0.34 ohms total at 50Hz and 2.31 ohms total at 500Hz if you want millisecond current changes. A high powered shower is 6-7 ohms resistance, and low powered devices will be 100s or 1000s of ohms load.

I can't see them allowing the mains to vary much because the wear and tear on suppliers eqpt would increase [variable tap transformers at power stations, rotors on generators etc]. I think there will be safety disconnects when supplies become less stable.


The supply impedance is determined by purely local factors, principly the size of the transformer, the length and size of the low voltage main, and the length and size of the service cable between the customer and the main

The actual achieved impedance varies a good bit, but it is of purely local importance to avoid excessive voltage drop and to ensure the prompt blowing of fuses in case of faults.
It is of no wider significance regarding grid instability.
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RenewableCandy



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PostPosted: Mon Jun 12, 2017 1:15 pm    Post subject: Reply with quote

<deletes bluster about a Strong and Stable electricity grid...>
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adam2
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PostPosted: Tue Apr 16, 2019 4:27 am    Post subject: Reply with quote

I think that instability has occurred during the last hour or so*

I suffered a short power cut at about 03-00, this happens fairly regularly and is seldom worthy of much comment.
However when the supply was restored, my UPS still made unhappy noises due to the "mains being out of tolerance"

I then observed the real time frequency meter shewn on the "dynamic demand" website.
This showed a very unstable frequency that was "bouncing around". There was nothing remarkable about the actual frequency, it remained within the normal range.
What WAS exceptional was the number of frequency changes and the rate of change, 0.3 cycles in less than a second, and this being repeated many times a minute.

Of course there might be a power cut at the offices of Dynamic Demand, in which case what I was observing was not the grid frequency, but a local standby generator ! It certainly looked more like a generator than a grid supply.

*From about 03-00 until about 04-00 BST on 16/04/2019.
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fuzzy



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PostPosted: Tue Apr 16, 2019 9:01 am    Post subject: Reply with quote

If it was a genuine reading of out of sync generators, it must cause tremendous strain on the grid equipment. Luckily they have a bottomless pit of money for maintenance - our's!
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adam2
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PostPosted: Tue Apr 16, 2019 10:50 am    Post subject: Reply with quote

fuzzy wrote:
If it was a genuine reading of out of sync generators, it must cause tremendous strain on the grid equipment. Luckily they have a bottomless pit of money for maintenance - our's!


I was suggesting that the "grid frequency" reported on the Dynamic Demand site might in fact have been a local standby generator, not linked to the grid.
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BritDownUnder



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PostPosted: Tue Apr 16, 2019 9:11 pm    Post subject: Reply with quote

I once saw the Bangladesh grid change from 51Hz to 49Hz in a few minutes. At some locations, one in Indonesia (which has a very highly fragmented and unsynchronised grid) the frequency changed too fast for the auto synchroniser that enables generators to sync to the grid could not keep up and we had to call 'head office' for advice.

GE gas turbines cannot generate power below 48Hz (maybe 48.5Hz) as they cannot keep cool enough. Technically this is wrong as all GE gas turbines are based on 60Hz and they have a 6:5 ratio gearbox to the generator where they are in 50Hz countries.
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