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UK Grid problems 9th Aug
6

UK Grid problems 9th Aug

UK Grid problems 9th Aug

(OP)
The BBC is reporting widespread blackouts in London and the SE and I can see that a lot of emergency generation has started up (suggesting low frequency cut in). It'll be interesting to hear what happened and how low the frequency got. This is at least the 4th low frequency event so far this year.

Does anyone know any more details?

RE: UK Grid problems 9th Aug

Mbrooke: Another power outage for your record.

A preliminary estimate indicates that around 1 M people were impacted by the power outage in the UK.

Should be noted that some of the utilities involved in the power outage in the UK also own assets in the US such as National Grid (NG/former Niagara Mohawk) and Western Power Distribution (PPL in the US).

https://www.standard.co.uk/news/uk/uk-powercuts-la...

RE: UK Grid problems 9th Aug

@EnAppSys tweeting that it sagged to 48.814 Hz, alongside a fuel mix change plot that shows pumped storage kicking in to compensate, then being replaced by interconnect imports.

RE: UK Grid problems 9th Aug

From discussion on social media among ex-CEGB staff in the UK, something caused the CCGT plant at Little Barford to drop about 650MW off the system followed closely by Hornsea wind farm dropping a further ~800MW off the system. The big pumped hydro stations are fast to load up, but not that fast - the initial response has to come from machines already sync'd which have room to open their governors further or to over-fire their engines to maintain output as the frequency falls. The transmission system protection operated as intended to prevent either a system split or overall system collapse by dumping load from the system.

The UK system doesn't have many big high inertia machines left on the system as the last of the 1960's-vintage coalers retire - there's only the nukes and the largest single-shaft CCGT machines left of any real size - so the frequency fell really quickly, so fast that the generators charged with providing frequency correction couldn't increase firing fast enough to catch it. The inverters on the wind turbines just track the system frequency as it falls - they can't return stored energy into the falling system in the way that a large rotating machine does.

RE: UK Grid problems 9th Aug

National Grid do publish historic frequency data, so it will be interesting to look at this. unfortunately they publish it some months in arrears:

https://www.nationalgrideso.com/balancing-services...

Losing all of Little Barford is a surprise, since I assume it has duplicate step up transformers to 400 kV, so it might have been a 400 kV bus fault.

The subsequent loss of Hornsea is interesting, I assume that was because of ROCOF or Vector shift mains failure protection.

Vector Shift is being phased out in the UK (not a joke) because of perceived mal-operations (at least in the UK):

"On several occasions, it has been suspected that a transmission system fault that did not result in islanding resulted in the inadvertent tripping of embedded generation plants by their LoM (Loss of Mains) protection. A definite event was recorded on 22 May 2016 following a single transmission circuit fault. Further investigation of this event showed that a significant number of embedded generation plants had tripped as a result of the operation of VS protection. This event resulted in a loss of infeed and a frequency excursion that was bigger than that which was anticipated."

New protection settings are being used and there is a lot of work making this retrospective.
As Scotty says, wind generation. in particular, essentially has no inertia (because they are connected via inverters) so the system inertia is falling as the historic coal stations are being closed. This can result in potentially larger frquency excursions as may have been the case of yesterday’s event.

Well at least the transmission system load shedding worked, otherwise they could have lost the whole GB synchronous area!

RE: UK Grid problems 9th Aug

What is ROCOF and Vector shift and how does it work? I'm bad with acronyms and names LOL.

RE: UK Grid problems 9th Aug

Does this have anything to with that old joke about Lucas and the temp of English beer winky smile

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RE: UK Grid problems 9th Aug

ROCOF is Rate Of Change Of Frequency, which is what ScottyUK meant in regards to the machines can load up, but not that fast.
ROCOF is meant to be able to distinguish between slower changes in frequency due to load changes, versus abrupt changes due to loss of generating capacity.

EDMS Australia

RE: UK Grid problems 9th Aug

Hoxton,
Thanks for the details.
Little Bradford generation loss - even if it is 400kV bus fault, should it cause loss of both units?
I thought it is the practice to segregate the units, each unit connected to different busbar.
OR am I mistaken, it is only a single 650MW unit at Little Bradford!!

RE: UK Grid problems 9th Aug

The big ones are AC generators on a constant speed turbine. But also they convert to DC and back to AC to sync with the grid.

Similar to solar you can alter the power factor by +- 0.1 if you are having issues.

I am just a owner of a small 9.61kw solar plant but a mate in Scotland has a big turbine on his dairy farm.

The grid engineer that deals with him says the problem with these inverter systems is they only pump real power and the reactive has to be dealt with by the old school generators. So especially with loads of self producers and users it can knock the power factor hugely out for a site.

My power factor goes to poo when the heat pump kicks in when the solar is producing. 8.6kw ground. It's fine at night 0.9. Luckily its only out for under 10 seconds. If it was more that 20 seconds I would be automatically cut off from the grid.


He uses heatpumps as well on the farm which is why the subject came up when I installed my solar system. So I got an energy meter which also gives power factor from the start. Most solar plants they don't put them in until you get a battery. He has solar on his roofs as well and 44kw of high voltage battery's. He also has poo loads of led lights which apparently don't help.

I will state now I only have a mech engs worth of knowledge on AC so I can see the issue but don't know what the implications of this feature of inverters on the big picture.

But I would be extremely interested to find out from someone that does have a clue. Thinking about bringing my solar farm up to my 3p 32amp fuse limit.

RE: UK Grid problems 9th Aug

Another power outage in the UK.
The demolition by the implosion of cooling towers of a disused power station’s caused a power outage in Oxfordshire area on Sunday AM. Scottish and Southern Electricity Networks (SSEN) said material and dust related to the demolition had struck a 33 kV overhead power lines that affected 49,000 homes. Power restored a few hours later.

https://www.bbc.com/news/uk-england-oxfordshire-49...






RE: UK Grid problems 9th Aug

On 9th Aug incident, initial information was Gas fired power station tripped first but, today I see in the news that it is actually Wind power plant that tripped a few seconds prior to Gas fired plant.
I wonder why this confusion??

RE: UK Grid problems 9th Aug

Large wind farms are vulnerable to major load losses if the wind speed exceeds the wind turbine cutoff speed, usually about 55 mph. All wind turbines in the farm sequentially trip offline as their local windspeeds approach 55 mph, and this can lead to as much as a 1.5 GW loss in power within 45 minutes ( as occurred in texas Ercot 10 yrs ago and in the EU prior to that). The real fun starts when they then reload as the wind drops to below 54.999999 mph. This sort of major supply loss forces large industrial consumers offline, and consumer homes could also participate in the demand response if they had smart load centers . Priority house loads may be the lights and the communication/alarm systems, which are very low loads and a "blackout" can be averted in theory.

There should be scheduled sufficient spinning reserve to allow for one large central station to trip offline unexpectedly.The increase in spinning reserve can be provided by maintaining all CCGT units in a hot standby mode, and such a modification would imply each unit be provided with a small "pony" gas turbine & LP HRSG to maintain the steam turbine at 3000 rpm with a reheat steam flow of at least 10% MCR and the HRSG thick walled componetns withinm 120 C of desing temperature, to allow very fast restart of the main gas turbines. The improved VAR control provided by the sponning mass of the steam turbine generators is a bonus.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

RE: UK Grid problems 9th Aug

Thanks Hoxton. The findings are interesting and lots of learnings for utilities all over the world.
Good, comprehensive report produced in a short time of a month.

RE: UK Grid problems 9th Aug

Sounds like the substation level Low Frequency Demand Disconnection feature did not work very well. I wonder if it is time to move LFDD/(UFLS) function down to individual "smart" devices? It would be kind of interesting to have my TV shut down at 49.3 Hz, my refrigerator stop at 49.2 Hz, and my computer to shut down at 49.1 Hz.

Even better, my LED lights might ramp from 100% brightness at 49.5 Hz down to 10% brightness at 48.8 Hz. My VFD driven residential HVAC system could also have a linear ramp down of consumption. Setting the triggering threshold for linear ramping of lighting and residential HVAC systems above the first substation level LFDD/UFLS of 48.8 Hz might arrest frequency before resorting to tripping substations.

From Figure 2 in the report, it looks like amount of load tripped at exactly 48.8 Hz may have exceeded the amount of load needed to stabilize the system frequency. In the UK LFDD program, are there additional time delayed blocks? Within the western USA, there are additional small anti-stalling blocks disconnected at 15, 30 and 60 seconds.

As pumps and fans are moved behind VFDs, the grid looses both inertia and the load reduction frequency response characteristics of lower load at lower speeds during frequency depressions. Any idea what portion of VFD applications could be programmed with a frequency response for input frequencies lower than 49.5 Hz?

RE: UK Grid problems 9th Aug

Interesting point. Futuristic, when all the devices can be programmed to modulate the consumption based on frequency.
DOL connected agriculture pump sets show this characteristic.
Some utilities in India used to maintain grid frequency lower than 50Hz deliberately, to reduce the consumption of these heavily subsidised loads in India. I am talking of old days - 30 years back.

RE: UK Grid problems 9th Aug

Funnily enough... there's a proposal the other way in South Australia. When the grid gets too hot due to domestic solar they are proposing to limit the maximum feed in from each house. I think that is the way we are going to have to go, make both the supply and the consumption load based. I liked b4l's idea of using frequency to communicate this, for example life support systems and controllers for generators could have no limit, whereas toasters and so on could be switched out as soon as the grid is stressed.

Cheers

Greg Locock


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RE: UK Grid problems 9th Aug

In an inertia based system you can get to a very good place when load automatically reduces inversely proportional to frequency. Strong grids you may never see it, weaker grids it could work wonders. PV and type 4 wind have no inertia; type 3 has very little intertia. The sources follow the grid frequency down at full output to some point and then ... poof ... Oh, no, we didn't disconnect, that was just a momentary cessation of energization. That's how a single line fault, the clearing of which doesn't interrupt any of the PV, can result in the loss of 1.2GV of various PV sources (hard to call it generation in the traditional sense). Non-inertial resources may supply a tremendous amount of load in the 99.999...% of the time that everything is good, but they can be worse the useless during the 0.00...1% of the time that the power system is actually interesting. Massive amounts of iron at 3600 RPM (or 3000 RPM for some of you) can do wonders. I'm thinking it could even be massive flywheels to go along with the PV.

Or there has to be a Grid 2.0 in which nothing ever counts on the traditional performance of high inertia rotating mass. I'm pretty sure that could be specified and designed. I'm much less sure that there's a continuous path between here and there. The future may feature certain discontinuities...

RE: UK Grid problems 9th Aug

"I'm thinking it could even be massive flywheels to go along with the PV."

I had been thinking about that possibility as well. I presume they could be setup to deal with the reactive power issues as well with PV installations?

RE: UK Grid problems 9th Aug

Flywheel connected to a synchronous machine. It could function as a synchronous condenser as needed. Real power can be extracted as the system slows and absorbed as the system speeds up, but reactive can be what it needs to be within the machine capabilities.

RE: UK Grid problems 9th Aug

Reactive power control, spinning reserve capacity, and VAR control can be improved if several combined cycle power plants are retrofit with small "pony" gas trubines exhausting over smnall LP HRSGS that provide overnight spinning reserve steam to the IP steam turbine.

In many cases, the LP steam turbine L-1 stage blades can avoid "flutter" or aerodynamic instability if the P steam turbine is bypassed but maintained warm via a reverse flow valve and a casing vent valve to the condenser, as currently comfigured on large steam turbines in China and India. Roughly 6% MCR steam turbine power output plus perhaps 12% MCR CTG output ( via the pony turbine) would occur overnight or during long term spinning resereve operaton. The main HRSG would also be kept warm , within 120C of design temperature, using sparge steam . This allows a very fast restart of the main CTG , STG adn HRSG if a system wide loss of load occurs, and the HRSG thick walled components can then limit fatigue damage during the fast restart. This also can be configured to allow black start capability.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

RE: UK Grid problems 9th Aug

Flywheels? Someone needs to show the calcs to support that.

RE: UK Grid problems 9th Aug

Greg- The new IEEE 1547 requires DERs to have a high-frequency droop of 5% (i.e reduce to zero output at 63.036 Hz).

David,et al- Although inertia slows down the system dynamics, even very large flywheels only contribute energy for short duration. The frequency will continue to fall until load matches generation. This MPDI report lists flywheel applications having 4 to 60 seconds of supply capability. In the UK event, the frequency minimum was 75 seconds after the event began. Due to the additional trip of Little Barford GT1b tripping at 85 seconds into the event, it took 90 seconds into the event until having monotonic frequency increase. In order to achieve the load & generation match, either loads need to drop or generation needs to ramp up. I suspect the loss of frequency response from motors and the loss of voltage response from incandescent lighting is a much more dangerous to grid reliability than the loss of inertia.

When talking about inertia from traditional generators, we also need to be careful to distinguish between inertia and governor response. Many traditional generators often run at less than 100% output, so their governor droop characteristic automatically increases their output in response to frequency events. I am a little spoiled working with hydro units with fast governor response. Sometimes the base loaded machines have enough droop increase that the AGC system actually has to reduce output of the swing machine before the frequency minimum is reached.

I am also somewhat worried about "synthetic" inertia from wind turbines being used to slow an initial frequency decline, then having the turbine switch into reacceleration mode part way through the frequency event. In Synthetic Inertia from Wind Turbine Generation, the authors suggest a 10 second boost period followed recovery back to nominal frequency within 60 seconds. In the UK event, this would have had the wind turbines switching to recovery mode even before the initial local frequency minimum occurred 25 seconds into the event.

RE: UK Grid problems 9th Aug

In response to bacon4life's question. See attached from one of our DNO's. Table one gives the LFDD structure for one DNO.

There is no phased application of this, ie time delays within each tranche.

"The LFDD relays are fitted at 132 kV substations and are designed to trip the lower voltage side of the incoming 132 kV transformers or some or all of the outgoing feeders."

I assume that this means that the LFDD relays operate to trip the outgoing 66 kv or 33 kv feeders.

Apparently there is a review system so that hospitals and other significant infrastructure is not affected.

What is apparent that there has been no assessment of the positioning or power effect of distributed generation. It seems that LFDD could have tripped feeders that were actually sending power to the 132 kv system, which hardly helps!

One other effect that caused major transport disruption was the software upgrade to some electric trains which ensured that the trains tripped and locked out, such that the driver could not reset the train.

RE: UK Grid problems 9th Aug

a couple off-topic but related questions …

1) the grid in UK is connected to the European grid, yes? (I bet you can see where this is going) …

how will this be affected by Brexit ?

2) I can see that all the turbines in a farm acting at once (turning on, turning off) would upset the system.
can't the system have some AI in it to stagger these "simultaneous" events.
I can see that you could possibly have a "crash" due to over-wind, though I think you could reduce this by monitoring a distance away, preparing for the gust, …
but you should be able to better control re-starts (let's not re-start at 54.9999 mph only to have to crash again when the wing gets to 55.0001 mph) ?

another day in paradise, or is paradise one day closer ?

RE: UK Grid problems 9th Aug

rb1957,

yes it is, but the links are Dc and converter-fed at both ends. This essentially isolates one system from the other in terms of providing inertia (rotating mass) to slow the rate of frequency change. The principle reason is that the losses for long cables are very high.

How will it be affected by Brexit? Probably not much - there's a market and a supplier. The economics might shift depending on how the GBP / Euro exchange rate moves.

RE: UK Grid problems 9th Aug

1) UK Grid

Scotty is correct, but to be pedantic, he is referring to the 'British Synchronous Area'

'Synchronous area' means an area covered by synchronously interconnected Transmission System Operators (TSOs), such as the synchronous areas of Continental Europe ('CE'), Great Britain ('GB'), Ireland-Northern Ireland ('IRE') and Northern Europe ('NE') and the power systems of Lithuania, Latvia and Estonia, together referred to as 'Baltic' which are part of a wider synchronous area.

Ref: Article 2(1) of the Network Code on requirements for grid connection of generators (NC RfG)

So the UK is part of two synchronous areas, Northern Ireland being part of the Irish area, and the island of Great Britain being separate.

Any connections between synchronous zones are by HVDC links.

2) After BREXIT

Who knows? As Scotty says, the interconnectors are unlikely to be turned off, as they are commercial enterprises.

The government have checked this area out, allegedly, and did report that in the event of a Northern Ireland / Ireland network split, then there were not enough rental generating sets available to support the Northern Ireland network...

We do not know if the UK will remain associated with EU long term energy planning and if there will continue to be long term EU support for new interconnectors to GB.

3) Hornsea Windfarm trip

This seems to have been some form of software issue, now corrected...

4) Stop Press

A Mandatory retrospective change to UK 'G59' settings is being imposed for sub 5 MW sets, the larger sets having already had their settings changed.

http://www.energynetworks.org/electricity/engineer...

RE: UK Grid problems 9th Aug

I reckon Hoxton sorted this in the last sentence of his last post.

I've got a sub 5MW generator sync-ced to the mains at 11kV and having run for many years quite happily has tripped several times in about 2 months due to ROCOF.

The ROCOF discussions are well buried but they are there to be found and it seems the powers that be realised they were sitting on a potentially serious problem some while ago and revisited G59.

Sub 5MW has just been addressed and the ink is still wet on the signatures.

Seems to be the lack of system inertia, how much inertia does a wind turbine and inverter have, is failing to prevent the voltage and frequency drop, our machines try to catch up but the rate of change is such they come out on protection.



RE: UK Grid problems 9th Aug

I haven't come across tripping of generators on RoCoF in my service and don't think it is appropriate.
RoCoF concept is to save the grid during disturbances by accelerating the disconnection of loads and prevent tripping of conventional generators on under frequency.
Tripping of generators on RoCoF is worsening the grid distress and best avoided.

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