Grid Frequency [mains electricity]

Following the recent 'blackout' The Telegraph, reporting on the investigation, writes:

[quote]

The grid's frequency - a measure of energy intensity- normally sits around 50Hz. However, the paper said that in recent months the grid’s frequency has fallen below 49.6Hz on three different occasions, the deepest falls seen on the UK grid since 2015. On Friday the blackout was triggered when the frequency slumped to 48.88Hz.
In June, the frequency of the grid plummeted to within a whisker of National Grid’s legal limit of 49.5Hz after all three units of EDF Energy’s West Burton gas-fired power plant in Nottinghamshire tripped offline without warning.
In addition, the grid’s frequency fell to 49.55Hz on 9 May, and 49.58Hz of 11 July.

[/quote]

Is this just scaremongering, because they've found some numbers to quote; or is a 'slump' in frequency to 48.88Hz actually sufficient to trigger a large area blackout?

I believe we have some forum members with electicity industry experience ... so I would be grateful for an explanation [or a de-bunking] of the story.

Note: I am aware that the the contractual figure is 50Hz +/- 10% ... but I thought that was 'average over any 24 hours' or some such.

MichaelG.

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Ref: https://www.telegraph.co.uk/news/2019/08/12/national-grid-experienced-three-blackout-near-misses-last-months/

Edited By Michael Gilligan on 12/08/2019 23:38:58

Reporters often distort the facts and get it wrong. For a start "The grid's frequency - a measure of energy intensity" is incorrect. Frequency has nothing to do with a measure of energy intensity. It is the number of occurrences of a repeating event per unit of time.

Paul.

This is nicely implemented: **LINK**

http://mainsfrequency.uk/fm-meter

MichaelG.

[This posting has been removed]

This subject was broached some time ago I believe.

One of the problems with maintaining a stable frequency is the inputs from renewable energy. The wind and solar input to the grid will upset the status quo so that the base power generating equipment has to back down to prevent a rise in frequency.

When the wind drops and the clouds cover the sun the drop in frequency means the base power has to come on line quickly to maintain the power levels. With insufficient standby generating equipment there is likely to be brown outs or cuts happening more often.

Have a look at this web site for an interesting display of energy sources powering the national grid.

http://www.gridwatch.templar.co.uk/

Bill

The voltage can vary by +10% to -6% of the standardised european grid supply voltage of 230V. However, the frequency must be controlled within 1% of the nominal (see the introduction page on the link above @ 23: 53:12 where it states

”The GB mains frequency is nominally 50Hz. National Grid is obliged by its licence commitments to control the frequency within ±1% of 50Hz so it can fluctuate between 49.5Hz to 50.5Hz. However the normal operational limits are 49.8Hz to 50.2Hz.”

Our mains clocks with synchronous motors, which rely on the frequency for accuracy, would be rubbish if the frequency varied by as much as 10%!

Neither the supply side (turbines turning at, say,10% over design speed) nor the user side - machinery running at the wrong speed can tolerate wide variations. Large industrial motors are designed for that specific frequency and might well suffer damage if the frequency deviates greatly (and efficiency would not be optimum). Not sure of the effect on electronic items.

From the frequency website that micheal linked to I,d not heard of copper clad aluminium before I,d heard of alloy clad steel...

• A significantly higher frequency, e.g 400Hz as used in aircraft, would cause skin effects in transmission lines and this would reduce their carrying capacity or result in unwanted heating. At 50Hz, the electricity only travels through the outer 9.2mm of a copper conductor, so anything larger than 18.4mm in diameter is a waste of material unless it is for mechanical strength. To reduce weight and cost, many large conductors are made from copper-clad aluminium so that the electricity flows through the higher conductivity copper skin while the aluminium core provides the mechanical support.

I think the real embarrassment was to find some wag's program had dumped London off line first,

as a way of saving the rest of the UK from blackout.

Meanwhile, the demolition of our coal-fired back-up generation continues.

[This posting has been removed]

The news story is correct, mains frequency is important.

This is one of those counter intuitive things and is all to do with AC power theory.

Generators are basically synchronous motors that are driven rather than driving. If you don't know what a synchronous motor is see here

The important thing to know about synchronous motors/generators is in the name, they are synchronous with the mains frequency. That is with no load they run with the rotor spinning so that the rotor and stator magnetic fields coincide. N-S, S-N

As a motor is loaded, the angle between the rotor field and the rotating stator field increases and the rotor lags behind the stator field, so it is dragged along by the magnetic field. As the fields are not aligned, part of the stators field is cancelled by the the rotors other pole. ie A North stator sees mostly a South rotor pole plus a little bit of North rotor pole, and the magnetic circuit is slightly blocked. To keep it simple, less magnetic field, the more current flows in the stator, so more power is consumed and this power is converted into torque.

As the load increases eventually the rotor and stator break out of synchronisation and the motor stalls.

Now if we take the same motor but start to drive the shaft around., then the rotor will start to lead the stator field and the process is reversed. The torque is converted into power transferred to the grid, the amount generated is proportional to the difference between the grid frequency and the rotational frequency.

As you drive the shaft harder and harder, eventually the synchronisation will be lost and very nasty things possibly involving bright flashes and smoke happen. This is why they have automatic trips that drop out when the supply is out of tolerance and they say "it is to protect the rest of the system"

When more power is added to the grid with each generator leading the grid slightly, it has the tendency to increase the frequency. As more power is taken from the grid the reverse happens and the frequency drops.

To ensure the grid stays in balance we use 50Hz as the target frequency, each generator is synchronous and running with a leading angle so that the most efficient power transfer happens.

There is also a link between voltage generated and frequence, the higher the frequency (rotational speed) the higher the voltage. So at 50Hkz we will end up with the correct voltage on the grid.

As there is no way to store power in the grid and the load is varying, we have tolerances on the frequency and voltage. The national grid predict what power is needed at any time, and ensure generators come on and go off line to meet the load. However we have transitory loads, i.e. Advert breaks on the TV, every rushes out and puts the kettle on. Most of those are adsorbed in the tolerances, but some times it is necessary to increase the frequency just before a big spike, say an advert break in Coronation Street Xmas cliffhanger special.

So you can see Ofcom are correct in getting a little upset that the frequency has been bouncing around near the lower limit. It means the grid is running close to collapse. The loss of those two power sources resulted in the automatic protection tripping and the power cuts. If parts of the grid were not turned off we would have a cascade failure. Every power station in rapid sequence would drop off the grid as the remaining load was spread across fewer and fewer power stations.

The switch to wind an solar only make the situation worse, too much or too little wind and night time all mean the power going into the grid is reduced. So we need other sources, at the moment that is gas powered, eventually it will have to be battery storage. Nuclear is OK for base load, but it takes a while to warm up and cool down a reactor so it cant be used for transients.

We are already short of generating capacity, add in electric cars and houses switching to electric heating and we have a very real electricity crisis looming.

Adrian

Thanks for all the responses

I will digest them this evening.

MichaelG.

Read all about frequency control in AC power systems in this paper. It gives parameters for the Irish 50Hz system which isn't unusual:

Typically, the limits for frequency variation are ±0.4% (±0.2Hz in a 50Hz system or a frequency band of 49.8→50.2Hz). This is the normal range for the Irish system. The range during transmission disturbances is 48Hz to 52Hz and during exceptional transmission disturbances is 47Hz to 52Hz. The permissible variation in voltage is much greater, typically ±6%.

Reducing frequency can happen both as a symptom of overload and as a managed way of decreasing generator load during periods of high demand. Faced with excessive demand the operator can drop both voltage and frequency, but there are limits outside which the system shuts down.

Dave

I am under the impression that the supply also guarantees to supply the correct number of cycles per day to avoid clocks accumulating an error.

Mike

As a result largely of the campaigning by Mr. Frank Hope-Jones, of Synchronome fame.

Best regards,

Swarf, Mostly!

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Which begs the question that I asked

MichaelG

I wonder just how many clocks still get time from the mains?

Opps, I expect it is Ofgen that worry more about the mains supply than Ofcom. Maybe I should get all my posts checked by Ofof before posting.

Years ago, it was common to have electric clocks powered by the mains, they depended on the frequency being stable. My old boss back in the 70's had one of those clocks on the wall of his office with the live and neutral reversed, so the hands rotated anticlockwise. The numbers on the dial were reversed to match.