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Fast Frequency Service: Treating the symptom, not the cause?

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The National Electricity Market (NEM) is designed to operate at 50 Hz. Frequency deviation occurs when generation and load are mismatched.  It is important in a lightly meshed and long network such as the NEM to maintain tight frequency control and that frequency response is available throughout the network.

Changes in the approach to frequency control on the synchronous units has resulted in technical and regulatory changes which have compromised the resilience of the power system to frequency variations.  Consequently, an accumulation of small contingency events (as on 28 September 2016) can leave the power system exposed.

Prior to 1999 frequency deviation was addressed by mandated governor controls on the synchronous generating units.  The governors operated to control frequency deviations larger than 0.1 Hz around 50Hz.

The governors provided a continuous control of frequency as they commenced acting within the normal operating band when frequency deviated by 0.05Hz from 50 Hz (rather than commencing only when the operating band was exceeded).  This form of primary control, that provides fast response to frequency deviation, is mandated in other countries.

In 1999 the regulatory arrangements for the NEM were redesigned to create a real time market for the provision of all of its frequency control.  The changes introduced in 2001 included:

  • redefining the normal operating band from 49.9 Hz to 50.1 Hz to 49.85 Hz to 50.15 Hz
  • introducing eight frequency control ancillary services (FCAS) which operate in distinct bands.

Subsequent changes in the regulatory framework discourage tight governor control by:

  • attributing poor causer pays factors to synchronous generators with tight deadbands that are contributing to good frequency control
  • penalising generating units which ramp in a non-linear fashion because they are providing a primary governor response to frequency deviations within the normal operating band.

Over time, as a consequence of these changes and disincentives, governor deadband have been widened from 0.1Hz to 0.3Hz or more and units can disable their governors when not economically dispatched to provide frequency control.  This has resulted in a significant deterioration to the fast frequency response of the power system.

In contrast to the original efficient control action of governors, the FCAS services are inefficient because the action starts later requiring more service to control frequency and may not be located where it is required.  Due to the switching between services, the control is not optimal and does not provide good primary control of frequency.  For example, frequency needs to fall to below 49.85 Hz before contingency FCAS is provided to contribute to arresting the fall in frequency.

Fast primary control response to frequency deviation within a narrow deadband ensures that frequency deviations are effectively and continuously addressed.  By delaying the response to a fall in frequency synchronous generating unit momentum is lost because its rotating energy is extracted into electrical energy – this is the “inertial contribution” and deceleration sets in, therefore, frequency drops lower and will require more energy to recover.

An analogy to this is a truck has to accelerate in order to maintain its speed as it goes up a hill. It must accelerate (or increase its energy input) early enough to maintain its momentum. Failure to accelerate earlier enough will result in the truck slowing down.  It cannot easily regain its original speed and can only gear down and crawl up the hill or worse stall.

The resilience of the power system has been compromised by the shift to market based frequency control on the synchronous generating units.  The deterioration of fast acting governor control represents a significant deviation from Good Electricity Industry Practice and fails to meet the National Electricity Objective as more services are required to be dispatched than if services were re-designed with narrow deadbands.

Current proposals for improved frequency response remain focused on market mechanisms.  Implementing another market for fast frequency response will result in the same issues described above unless the market and the regulations reconsider how to incorporate good frequency control into the normal operating band.  The type of frequency control required cannot be centrally dispatched – it must be in continuous operation in the control systems – central dispatch responds too slowly to be effective.

Reassessment of the approach to frequency control should be undertaken with a view to:

  • reintroducing narrow mandated deadbands across the NEM
  • removing regulatory disincentives to tight governor action.

Further details and supporting data are set out in the accompanying paper published 3 Feb 2017.

On 10th February 2017 hot weather placed the NSW region at risk, the AEMO report on the event noted that (after a number of other heat related issues occurring in the region) at 16:22 a Tallawarra unit tripped from 408MW, this reduced the frequency to below 49.85Hz and for the following 7 minutes there was a sustained frequency oscillation on power system with a 24 second period.  Analysis of the 4 second data illustrates why there is an immediate primary control issue with the synchronous units on the power system.

The control (or lack of control) of the synchronous ‘baseload’ generation units are the cause of the instability of the frequency control on the power system.

synch

Kate Summers is the Manager of Electrical Engineering at Pacific Hydro, which she has been with for over 12 years.She specialises in technical regulation issues with renewable energy in the NEM and power system engineering.  

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  • Patrick Comerford

    Governor control is a complex issue. The author has raised a critically important aspect of its effect and consequence on the grid stability. The market like all capitalistic theory is only so right up to a point. When the market whether by ignorance or worse by design can be influenced by vested interests then sloppiness or lack of technical rigour will produce unintended consequences. With a regulator who is sometimes asleep at the wheel or closes its eyes to insidious commercial practices then beware. The issues will accumulate until the tipping point is reached and then it goes bang.

    • Alex Hromas

      What do you mean goes bang it has already done so and will do again. Buy candles!

    • Alastair Leith

      AEMO is 40% funded by member orgs (generators) and the rest comes from governments who are constantly lobbied by these same interests. Just sayin.

  • Chris Fraser

    Having understood the control system a little better after Kate’s piece, removing the governors makes no sense to me. But regardless, assuming governors can’t be brought back, the following probably should also be better understood;-
    Is it more efficient to match an overloaded main generator to a contingency FCAS than have a governor ?
    Do they need to be geographically close to present a consistent waveform at the point of dispatch ?
    Does bidding for both generation and FCAS at the same time provide a cynical opportunity to be greedy in the wholesale market ?

    • DJR96

      To the three questions:-
      1. No. If these large generators are relied upon to form the grid, then they should have the governor controls actively working to accurately maintain the grid form.
      2. No. Frequency is the same at Cairns as it is at Port Lincoln. There will be a phase angle difference if there is any significant load imbalances, but that’s more to do with ensuring generation capacity is reasonably matched to demand within each region.
      3. Absolutely yes. As it is now, frequency is allowed to drift further away from 50Hz and requires greater efforts and expense to correct. And as noted in the article, at greater risk to stability.

      Time for another service to be responsible for “forming” the grid and the generators can then only add energy to it and not have any responsibility for maintaining the grid form. That’s the direction it has gone, so let’s take the final step.

      • Chris Fraser

        I worry as I have garnered there is so much that can go wrong with spinning masses and matching to frequency, volt amplitude & phase angle that’s already in the transmission system running past.I feel all of them need grid-tie waveform ‘filters’ or ‘cleaners’ to avoid making a mess of a good existing situation online. According to the requirements of my grid-tie inverter it would be electronically controlled and automatic.The large generators can also plug in load-matching and fast-responding storage devices to cater for sudden drop-in loadings the same way, and be paid for providing actual energy and not some outdated frequency control service.

        • DJR96

          What I find truly staggering is that there really is nothing to “manage” frequency at the moment other than matching supply and demand. There is no governing what-so-ever.
          It is akin to two blindfolded people on an old-fashioned plank see-saw. If one moves the other must sense that and adjust accordingly. Truly cumbersome and imprecise.

          Only if it goes outside of the 49.85Hz to 50.15Hz range is anything done about it. And all that is done is a dispatch goes out to some generators to put in a bit more or less power. It doesn’t seem like anything more than a regular dispatch. And that is costing us all about $2 million per week. So yeah, it’s a pretty good scam by the incumbent generators. Which is why they don’t deserve to have anything to do with managing the grid. They should and could be relegated to simply following the grid like all inverter connected generators are now.

  • Peter G

    Thanks Kate for an excellent, informed article. The competing incentives here are diabolical and have allowed the public to be skinned, in addition to the weakening of system security. An example of a failure of governance where appropriate regulatory control has been sacrificed to the god of The Markets.

    • Alastair Leith

      The invisible hand was in your pocket lifting some change again!

  • Kate’s article was one of 3 about FCAS posted yesterday over at WattClarity:
    http://www.wattclarity.com.au/

    • Alastair Leith

      Thanks for providing these stories.

      “Late last week, the headlines like these in the two main newspapers of substance in the country (Ben Potter in the AFR on the 9th and Chris Griffith in the Australian on the 10th)…”
      I’m hoping that’s an ironic use of “the two main newspapers of substance”, Paul. On Energy issues today, and for the last 12 months in case of AFR and 12 years plus in the case of The Oz, these are the two main papers fossil and Liberal Party propaganda.

  • Ray Miller

    Thanks Kate for the article it is most disturbing. Confidence in AEMO and AEMC has fallen to an all time low, maybe we are long overdue for a major independent shakeup. Leaving things as they are seems increasingly not an option.
    Leaving the grid is an easy option for many maybe its about time to jump before the ship sinks.

    • DJR96

      Although to be fair AEMO do a pretty decent job given the regulations they have to work with.
      The AEMC on the other hand, has a board made up of coal industry stalwarts who are only interested in further entrenching their 20th century generation model and making the market work even better for that industry. They are are clearly working contrary to the NEO.

      • Ray Miller

        I acknowledge complexity of the task AEMO has, especially with questionable maintenance practices of many players, questionable technical standards, reliability and settings. But it seems to me we have all these engineers standing around with hands in pockets murmuring under their breath that they are unable to do anything about it. After reading the book QF52 about the new A380 plane which landed with no loss of life after a catastrophic engine failure, the pilot before he flew the plane made a nuance of himself learning as much as possible about the plane and its systems. Well maybe AEMO can lean something from aviation about flying the NEM. I would argue if AEMO has been given the important and critical job of managing they should do so. No hiding behind any lame excuses.

  • Ian Porter

    The article highlights the notion of baseload synchronous generation operating properly under poorly designed, outdate regulatory practice and which beggars belief. The NEM was properly designed for what it was intended years back but is suffering from technology change creep without a regulatory response. If they think they have problems at the moment, wait until critical mass in RE occurs in not too many years from now. All those distributed inverters fighting each other as sych-gen falls away. Maybe thats what big coal wants – glory in the chaos. Meanwhile the regulator fiddles while Rome burns. Lots of trouble ahead if something is not done and unfortunately in the absence of good policies, it’s not going to be limited to matters surrounding frequency control.