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Robust and Fragile Electricity Systems

October 21, 2016

By Paul Homewood 

 

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The recent South Australian blackout has triggered a debate about the manifest risks of wind farms to the security of electricity networks. National Grid’s 2016/17 Winter Outlook reinforces previous concerns that low-carbon policy mandates are resulting in electricity systems that are likely to be fragile in the face of external shock, and are therefore more difficult and consequently more expensive to manage.

The UK’s National Grid has just published its Winter Outlook for 2016/17, in which it describes the situation this winter as “tight but manageable” (Overview section, p. 14).

The margin of “derated capacity” over expected peak load is roughly 3.4 GW over about 52.7 GW expected peak load, or a margin that National Grid quotes as 6.6%. However, the constitution of this margin both undermines confidence in its resilience, and reminds us that security of supply is increasingly dearly bought in the United Kingdom.

The margin in fact is critically dependent on the 3.5 GW of contingency balancing reserves (defined on p. 14, as “additional capacity held outside the market”, meaning all sorts of odds and ends). It is interesting to note that, on page three, that National Grid observes that some units in the supplemental balancing reserve need more than one day’s notice of operation, which is not encouraging, either for reliability or for cost (plant brought in good time may become surplus to requirements, but will have to be paid in any case).

Excluding the (derated) contingency balancing reserve the margin is only 1% over a peak load of 52.7 GW, ie a margin of about 580 MW, with Loss of Load Expectation of 8.8 hours per year. This is not attractive, and shows how reliant the system has become on expensive contingency balancing reserves.

Furthermore, net interconnector imports are assumed as 2 GW. It must be questionable whether that is a safe assumption. We know from ample empirical evidence in Europe that interconnectors should not be relied upon in a tight corner, since they must reduce transit or even disconnect to protect themselves. In any case, if the market on the other side of the interconnector is also somewhat tight, they may be very little use at all. The news that the French system is likely to be itself experiencing tight capacity margins this winter, due to the safety inspections in the nuclear fleet is a reminder that this is no merely theoretical concern.

National Grid has de-rated grid connected windpower using the arguably generous Equivalent Firm Capacity of 21% (i.e. 0.21 x 10 GW = 2.1 GW). Given the overall narrowness of the margin, an error here could be critical.

Another point of concern is the fact that National Grid appears to have netted the derated capacity of embedded wind generation from the load estimate, i.e. 0.21 x 4.6 GW = approximately 1 GW. Again this could be badly wrong, and in any case, as one engineer has put it to me “This does not capture the combined probabilities of high demands and low availabilities of generation.”

In summary, the Outlook shows that the UK system is heavily dependent on costly contingency balancing reserves, the interconnectors, and on arguably optimistic assumptions about wind. As National Grid’s own summary, “tight but manageable”, suggests, it is now obvious that the UK has a fragile electricity system.

The potential consequences of such fragility, arising for similar reasons, have recently been made painfully evident in South Australia, which suffered a total system blackout on the 28th of September, with full service to all consumers only being restored completely by the 12th of October. The preliminary report and its update by the Australian Energy Market Operator (AEMO) present the current state of knowledge.

What we can infer at present is that the distal cause of the blackout was a state policy that discouraged conventional generation, and resulted in a system that was heavily dependent on wind turbines and on a single interconnector to the neighbouring state of Victoria. Experience has revealed this as a fragile system. The proximal causes of the blackout were a major storm with wind speeds in excess of those forecast. It appears that these winds caused grid damage resulting in voltage disturbances. The wind generators were not programmed to ride through such serious faults, and disconnected, resulting in the loss of 445 MW of generation, about 23% of the system load at that time (1,895 MW). This disconnection transferred the burden to the Victoria interconnector, which could not sustain that load, and itself disconnected to prevent damage, with a further loss of 900 MW of supply. The whole burden now transferred in a split second to the online conventional generation, which needless to say could not meet it and also disconnected, resulting in a black system. The entire event lasted about ninety seconds.

System operators world-wide will now be reviewing their policies on wind turbine Fault Ride Through, and AEMO itself is as a matter of urgency requiring the wind turbine operators to be programmed to provide more robust FRT, but this is more than changing the settings on a dial. Generators disconnect to prevent mechanical damage, which is a real hazard during a fault, so if higher levels of FRT are required, the wind turbines will probably have to be modified to make them less susceptible to gross physical harm in the event of major voltage disturbances. Conventional generation is, as a rule, already engineered to withstand fairly extreme faults, as witness the fact that none of the conventional generation that was online during the South Australian event disconnected until the interconnector came offline. Ensuring that the wind turbines are similarly robust may not be cheap, and there already signs that wind operators in South Australia are reluctant. The chief executive of one of the operators has been quoted Australian Financial Review that he was uncertain whether enhanced Fault Ride Through requirements would expose his equipment to damage: “It could have zero effect, it could have longer term operations and maintenance costs, it could have any number of issues”.

In fact the likelihood of it having zero effect on the wind turbines is small; these devices, already very expensive, will have almost certainly been engineered to be adequate, though not very much more than adequate to the levels of Fault Ride Through generally required. More demanding levels of FRT will thus mean that they are de facto under-engineered, with impacts on maintenance costs and reliability. Re-engineering will not be cheap, and improving the standards for new wind turbines will have a significant effect on their capital cost, pushing the hoped for independence of subsidy still further off into the future.

Read together, National Grid’s UK Winter Outlook and AEMO’s reports on the South Australian case, suggest that systems heavily exposed to wind generation tend to be fragile, and rendering such systems adequately robust is both difficult and, crucially, expensive.

http://www.thegwpf.com/robust-and-fragile-electricity-systems/

15 Comments leave one →
  1. October 21, 2016 4:55 pm

    The system fell over quite easily. It was not a major 50 year event as some would have us believe, no issues on the other side of the border, nothing at all – power all ok, no damage to the network. In fact if you read closely the little damage that did occur happened after the windmills stopped and occurred to two old transmission towers.
    It is a beat up to blame the weather and nowhere will you see the speed of wind that these windmills operate at. The facts are they trip out at a low wind speed to stop damage – usually 30 mph which is a speed commercial aircraft operate with all the time. They only start up at around 10 mph so they really are useless. But you won’t hear that from the climate and green lobby!

    • October 21, 2016 6:51 pm

      It was foolhardy to be maxing wind generation when you knew with certainty the storm would have winds above the turbine cutoff speeds. And because of that, not enough gas turbine on line in spinning reserve mode to provide grid inertia.
      UK coild well experience a similar scenario this winter. Plus, with the French interconnector unlikely to supply because of safety checks, there will likely be overall capacity shortfalls when the wind isn’t blowing. Quite the mess.

      • It doesn't add up... permalink
        October 21, 2016 10:49 pm

        Never mind the French interconnector not being able to supply – it could represent 2GW of additional demand, and in a severe case a further GW on the BritNed one too. I noted that National Grid regarded as a positive factor that the E-W connector to Ireland would be out of action for repairs – so not being a further source of demand.

        We saw interconnector flows like this during a cold snap last winter:

        If supply is tight on the Continent it could get somewhat worse.

  2. AlecM permalink
    October 21, 2016 6:17 pm

    By about 20.30 tonight, I too shall be ‘tight but manageable’.

    However, after the second bottle of strong wine, it takes the smallest push to make me collapse.

    A bit like the UK Electricity Grid perhaps………..:o)

  3. nzrobin permalink
    October 21, 2016 7:40 pm

    It is true that one of the key differences between wind generation and the good old salient pole synchronous machine is fault ride through, and coupled with that the ability to provide a contribution to fault current. However, there is a more important factor. And that is the presence of a governor control on the prime mover. The governor measures and responds to system frequency. (Similar to the cruise control in your car). Overall network energy balance is necessary requirement for stability. And energy balance is observed by change in system frequency. This is why we have RoCoF (rate of change of frequency) relays, and AUFLS (automatic under frequency load shedding). When the system loses a chunk of energy somewhere, it needs to be compensated for automatically within a few seconds. The loss of energy is noticed by a dip in frequency, and those salient pole machines which have governors in service, and which are operating a point allowing room to turn up the input to the machine can compensate. This is point that wind and solar will never be able to achieve, because you can’t ask for more wind and sun on demand. But you can open the wicket gates to a hydro, or turn up the gas to a thermal.

    • AlecM permalink
      October 22, 2016 2:07 pm

      The gate valve at the top of the hydro penstock can respond in 30s. The time constant for a CCGT is much longer.

      Therefore we need an equivalent to hydro but gas fired. I propose mass deployment of domestic fuel calls tied to frequency; responding to lower frequency by increasing output and to higher frequency by reducing output.

      Their time constant is similar to hydro plants.

  4. Graeme No.3 permalink
    October 21, 2016 8:44 pm

    There are 2 interconnectors from Victoria to SA, with the Murraylink supplying the NE with 220MW and the Heywood which is/has been up-graded from 460 MW to 650 MW. The query is because the project completion hasn’t been announced, unlike the delays in the scheduled date. Certainly the unavailability of the extra capacity was cited by the Premier as one of his excuses. The load was running close to 650MW when the demand was raised to 900MW, in excess of the combined capacity of 670MW or 870MW in any case.
    Ristvan (above) is quite correct that depending on the turbines with no reserve available was poor judgement, but there have been frequent puff pieces about how much SA’s demand was supplied by wind (the number of hours an minutes not emphasised).

    Only 5 of the 27 collapsed pylons failed before the blackout. Several seemed to have been blown over with their legs (and the small concrete plugs) were pulled out of the ground, due to the heavy rain this year saturating the ground. There were multiple transmission lines to the North and 3 of the 4 were (eventually) broken. They are about 50 years old, and the State Gov. refused to allow the operator, Electranet, a price rise to pay for their up-grading some years past.

    The actual strength of the storm is the subject of debate, and it certainly wasn’t “unprecedented”. Stronger storms hit in 1947 and 1953, and much of the State had nothing at all unusual wind speeds.
    The wind supply has driven the coal fired plant (550MW) and most of the CCGT out of business. There was a blackout in July which had to be fixed by getting the Pelican Point plant back in service, and it was the same plant that was restarted to provide the black start.

    On a personal note I lost supply for 4-5 hours (asleep when it came back on) but the town 5km north was out for 20 hours and the larger towns 10-15km. south had a similar wait, with another interruption of 14 hours later that day. A little puzzle as all were thought locally to be on the same transmission.

    The local paper yesterday (friday) had some fallout. Starting with a suburb being blacked out the previous day, and criticism of the State Treasurer (an ally of the Premier), the paper devoted 2 pages to criticism of the Premier, then 2 pages with an unfavourable Editorial and letters to the editor, with 12 unfavourable balanced by 2 saying what a tough job he has. Then 2 more pages with articles about the wheeling and dealing that got him the job, and listing those who might replace him. Not what he, nor his excessive number of media massagers, would have enjoyed.

  5. Bruce of Newcastle permalink
    October 21, 2016 9:01 pm

    Politics could add some fun to the UK situation. Hollande has been quite caustic about Brexit, as has Juncker, so it is conceivable that the French and Dutch interconnectors might be used as leverage.

    • It doesn't add up... permalink
      October 21, 2016 10:56 pm

      The best response would be to re-open coal fired stations (after all, BritNed is supplied by the coal fired Maasvlakte power stations anyway) – but since we would be leaving the EU, and not subject to the LCPD, no limit on their hours, and no requirement for FGD, let alone anything as fancy as CCS. Tall chimneys and prevailing winds would do the rest of the export job…

    • AndyG55 permalink
      October 22, 2016 12:18 am

      Totally OT…🙂

      Hey, Bruce, I notice you also have a semi-pet magpie.

      They love milk arrowroot biscuits.. so do the mynas, butcher birds etc.

      I crumble one up (biscuit that is) on the front veranda each morning.

    • Bruce of Newcastle permalink
      October 22, 2016 8:41 pm

      Andy – Mine have the luxury of hamburger mince, which is so liked that now I’ve many birds eating from my hand: the magpie pair, currawongs, butcherbirds, kookaburras and even a koel cuckoo. The noisy miners and the rainbow lorikeets will also eat from my hand, but they get bread instead. It’s a fun hobby.

      Enviros say you shouldn’t feed them, but my experience is they do very well with the extra food. It wouldn’t be the first thing they’ve gotten wrong.

  6. October 22, 2016 9:38 am

    It has been claimed that the SA wind shutdown was due to software errors and/or incorrect settings.
    http://cleantechnica.com/2016/10/19/south-australian-blackout-still-not-fault-wind-energy-variability/

  7. Gerry, England permalink
    October 22, 2016 11:54 am

    The Short Term Operating Reserve is mainly made up of banks of diesel generators that can start up immediately but they also sign up places that have back up generators that may also be diesel. If it takes a day to come online then it must be thermal as this would be required to warm it up slowly to avoid damage. If Europe is to have a very cold winter then there won’t be much to spare there via interconnectors and wholesale gas will become more expensive.

  8. October 22, 2016 3:24 pm

    I’m not too sure what the 21% capacity credit for wind actually means, but hope it does not represent “firm” capacity, because that firm capacity has all but vanished on several very cold days in recent winters:

    https://climanrecon.wordpress.com/2016/10/22/gb-wind-power-in-recent-cold-winters/

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