UK Grid problems 9th Aug 6

[LSpark]

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?

 

[davefitz]

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

 

[Hoxton]

OFGEM issued the 'final' NG ESO report and appendixes today.

Some issues are unresolved

I will try and write a summary, sometime

 

[Hoxton]

Whoops, only the appendixes uploaded

Report here

 

[Hoxton]

In case you think that this was not forseen...

 

[RRaghunath]

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.

 

[Hoxton]

After our August 9th power outage the government discovered that it had an 'Energy Emergencies Executive Committee'

Government was so exited that it asked them to report! Sorry for cynicism!

Herewith report

 

[bacon4life]

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?

 

[RRaghunath]

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.

 

[GregLocock]

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


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[davidbeach]

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...

 

[Alistair_Heaton]

"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?

 

[davidbeach]

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.

 

[davefitz]

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

 

[3DDave]

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

 

[bacon4life]

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.

 

[Hoxton]

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.

 

[rb1957]

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 ?

 

[ScottyUK]

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.

 

[Hoxton]

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/elect...celerated-loss-of-mains-change-programme.html

 

[SilverfoxUK]

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.