Power station ‘trips’ are normal, but blackouts are not

Tens of thousands of Victorians were left without power over the long weekend as the distribution network struggled with blistering temperatures, reigniting fears about the stability of our energy system.

It comes on the heels of a summer of “trips”, when power stations temporarily shut down for a variety of reasons. This variability has also been used to attack renewable energy such as wind and solar, which naturally fluctuate depending on weather conditions.

The reality is that blackouts, trips and intermittency are three very different issues, which should not be conflated. As most of Australia returns to school and work in February, and summer temperatures continue to rise, the risk of further blackouts make it essential to understand the cause of the blackouts, what a power station “trip” really is, and how intermittent renewable energy can be integrated into a national system.

Blackouts

Initial reports indicate recent blackouts in Victoria were caused by multiple small failures in the electricity distribution system across the state, affecting all but one of the five separately owned and managed systems that supply Victorians.

Across the whole of mainland Australia, very hot weather causes peak levels of electricity consumption. Unfortunately, for reasons of basic physics, electricity distribution systems do not work well when it is very hot, so the combination of extreme heat and high demand is very challenging. It appears that significant parts of the Victorian electricity distribution system were unable to meet the challenge, leading to uncontrolled blackouts.

Parenthetically, electricity distribution systems are vulnerable to other types of uncontrollable extreme environmental events, including high winds, lightning, and bushfires. Sometimes blackouts last only a few seconds, sometimes for days, depending on the nature and extent of the damage to the system.

These blackouts are very different from those caused by power station “trips”, although they have the same effect on consumers. When electricity is insufficient to meet demand, certain sections of the grid have to be strategically blacked out to restore the balance (this is known as “load shedding”).

It is the possibility of blackouts of this second type which has excited so much commentary in recent months, and has been linked to power station “trips”.

What is a ‘trip’ and how significant is it?

“Trip” simply means disconnect; it is used to describe the ultra-fast operation of the circuit breakers used as switching devices in high-voltage electricity transmission systems. When a generator trips, it means that it is suddenly, and usually unexpectedly, disconnected from the transmission network, and thus stops supplying electricity to consumers.

The key words here are suddenly and unexpectedly. Consider what happened in Victoria on January 18 this year. It was a very hot day and all three brown coal power stations in the state were generating at near full capacity, supplying in total about 4,200 megawatts towards the end of the afternoon, as total state demand climbed rapidly past 8,000MW (excluding rooftop solar generation).

Suddenly, at 4:35pm, one of the two 500MW units at Loy Yang B, Victoria’s newest (or, more precisely, least old) coal-fired power station tripped. At the time this unit was supplying 490MW, equal to about 6% of total state demand.

The system, under the operational control of the Australia Energy Market Operator (AEMO), responded just as it was meant to. There was considerable spare gas generation capacity, some of which was immediately made available, as was some of the more limited spare hydro capacity. There was also a large increase in imports from New South Wales, and a smaller reduction in net exports to South Australia.

By the time Loy Yang B Unit 1 was fully back on line, three hours later, Victoria had passed its highest daily peak demand for nearly two years. There was no load shedding: all electricity consumers were supplied with as much electricity as they required. However, spot wholesale prices for electricity reached very high levels during the three hours, and it appears that some large consumers, whose supply contracts exposed them to wholesale prices, made short-term reductions in discretionary demand.

This (relatively) happy outcome on January 18 was made possible by the application of the system reliability rules and procedures, specified in the National Electricity Rules.

These require AEMO to ensure that at all times, in each of the five state regions of the NEM, available spare generation capacity exceeds the combined capacity of the two largest units operating at any time.

In other words, spare capacity must be sufficient to allow demand to continue to be reliably supplied if both of the two largest units generating should suddenly disconnect.

Forecasting

AEMO forecasts energy demand, and issues market notices alerting generators about reliability, demand and potential supply issues. On a busy day, like January 18, market notices may be issued at a rate of several per hour.

These forecasts allowed generators to respond to the loss of Loy Yang B without causing regional blackouts.

What is not publicly known, and may never be known, is why Loy Yang Unit B1 tripped. AEMO examines and reports in detail on what are called “unusual power system events”, which in practice means major disruptions, such as blackouts. There are usually only a few of these each year, whereas generator trips that don’t cause blackouts are much more frequent (as are similar transmission line trips).

It has been widely speculated that, as Australia’s coal fired generators age, they are becoming less reliable, but that could only be confirmed by a systematic and detailed examination of all such events.

Managing variable generation

Finally, and most importantly, the events described above bear almost no relationship to the challenges to reliable system operation presented by the growth of wind and solar generation.

With traditional thermal generation, the problems are caused by unpredictability of sudden failures, and the large unit size, especially of coal generators, which means that a single failure can challenge total system reliability. Individual wind generators may fail unpredictably, but each machine is so small that the loss of one or two has a negligible effect on reliability.

The challenge with wind and solar is not reliability but the variability of their output, caused by variations in weather. This challenge is being addressed by continuous improvement of short term wind forecasting. As day-ahead and hour-ahead forecasts get better, the market advice AEMO provides will give a more accurate estimate of how much other generation will be needed to meet demand at all times.

Of course, AEMO, and the generation industry, do still get caught out by sudden and unexpected drops in wind speed, but even the fastest drop in wind speed takes much longer than the milliseconds needed for a circuit breaker in a power station switchyard to trip out.

At the same time, as the share of variable renewable generation grows, the complementary need for a greater share of fast response generators and energy storage technologies will also grow, while the value to the system of large, inflexible coal-fired generators will shrink.

This article was first published in The Conversation and has been reproduced with permission from the author.

About our Guest Author

HughSaddler Image

Dr Hugh Saddler is a consultant and an Honorary Associate Professor at the School of Public Policy at the Australian National University. For the past two decades Hugh has focused on a range of issues relating to energy and climate change policy while working in a number of roles in the private, government and academic sectors.

Hugh is the author of a book an Australian energy policy and has published a number of scientific papers and articles on energy, technology and environmental policy. He is also a regular commentator on energy and climate change policy issues in the media.

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6 responses to “Power station ‘trips’ are normal, but blackouts are not”

  1. John says:

    I seriously have to laugh, better weather forecasting and wind and solar only have small units go off line.

    No they have both, small units going off line and the whole plant due to weather and problems.

    Controlling wind and solar, compared to base load power is like adding another two numbers to power ball!!!!!!!! (Meaning its a lot more complex)

  2. Paul C says:

    Interesting article, but it seems to assume that all power generation is equal. As long as we can balance generation with demand, then all will be well.

    Hugh doesn’t take into account frequency stability. A key factor in the SA system black event was that SA wind generation couldn’t stay in sync with VIC, once SA lost a large amount of wind generation.
    The Heywood interconnector tripped because the frequency of SA generators fell below that of VIC, not because VIC couldn’t supply the load.
    What followed was a rapid sequence of under frequency load shedding events, as transmission systems tripped off the remaining renewable generation that was unable to remain synchronised.

    His conclusion that “the value to the system of large, inflexible coal-fired generators will shrink” seems to miss this point.

    There have been some interesting articles on WattClarity about how AEMO now manages the situation when the total SA generation becomes mostly wind. Following the system black event, AEMO can now direct gas generators to come on line, so there is a minimum of synchronous generation to ensure reliability (for example: 2 x Pelican Point generators, or 1 Torrens Island and 1 x Pelican Point generator), even though there is enough wind generation to supply the load.

    The value of large thermal generators is that they stay in sync, even when subject to a large changes in load, unlike wind generators.

    A great article all the same! I enjoyed reading it.

  3. James O’Brien says:

    Thanks Hugh for a nicely written and factual article. I wonder why you omitted the inconvenient truth behind the system black event in SA. In this case AEMO expected a strong contribution from wind turbines, but they shut down unexpectedly (tripped) because of multiple short circuits to ground (aka lightning).
    The operators of traditional generators and distribution systems have a social obligation to increase output, often under duress, to supply their customers loads. On the other hand operators of variable renewable energy generators are only obliged to maximise the revenue available to their owners and place protection of their assets ahead of supplying the load. Consequently SA is now a circus ring for spruikers like Elon Musk.

  4. Ken says:

    I have great difficulty getting my head around much of the commentary on the electricity system in Australia. I could follow much of the sentiment if it was 1925 and the grid and all the vagaries associated with the “new” electricity were not yet understood. But the system is now a very mature discipline in 2018. Mention was made about the high temperatures for example, with the implications that there were no materials capable of working at 50 degrees C or above. We (humans) might find 50 degrees hot and we do turn on our air conditioners more. But electrical systems which over heat under these conditions are themselves wrongly designed or maintained. Class H insulation work well above class A insulation of cotton-covered-wire. It is not 1925 !
    The public can reasonably expect to have electricity for a good deal of the year. They can also reasonable expect to be the sole determinant of the load – just as a customers chooses to go in to a shop to buy something – solely when the customer decides – not the shop.
    As the country transitions into a period when more and more electricity is provide from renewable sources they also a right to expect that the transitions will be done by professional engineers who typically get it right first time and hopefully on budget. Sadly this is not what is happening. We have policy written by people who are self styled experts who often have no idea of what they are doing. We have commentary offering derision of the public that are asking for nothing more than it works. Of course when it falls short of reasonable expectations the public offer their advice which often is wrong. But the public are not the cause of the problem because they offer the wrong advice – it is the silly people that have got into running the show who are behaving like what happens when the asylum inmates take over the asylum that are the problem not the public.

  5. Ken says:

    Paul C writes that when SA was blacked out that the SA frequency fell below that of Vic. This is not correct. The Haywood inter-connector is not a DC link – it is synchronous connection albeit a weak connection. The two grids must at all times share exactly the same frequency. It is true that in times of instability that there will develop an unacceptable phase difference between SA and the rest of Australia but the frequency remain the same. If frequency is defined the reciprocal of a single period then may be the problem could be seen as a frequency difference but that is not the normal meaning of frequency. The same thoughts apply when thinking about frequency modulation and phase modulation. Over a very short period of sampling then it is hard to tell the difference between the two but they are not the same. The wind turbines took themselves off line because someone had written software that asked for this to happen. But the owners of the turbines and the AEMO had no idea this was the case. After multiple lightening strikes like what happens on most days in Far Nporth Queensland in summer the wind turbines threw the towel in. The system was never deigned to loose so much capacity so quickly and that triggered a text-book cascade fault condition, which involved the Haywood inter connector also tripping. After the inter-connector tripped with the systems now separated the death-throws of the SA grid obviously involved frequency collapse but not until after the separation.

  6. […] The top panel is essentially a closeup of the same Reneweconomy chart that prompted all this, showing the rundown of the Loy Yang A unit (LH axis) and the response of the Tesla battery / Hornsdale Power Reserve (RH axis). The middle panel shows the increase in output from the whole subset of generators around the NEM that materially responded to the event (plenty didn’t). To be clear, except for the battery and the Jindabyne hydro unit in blue, these generators were already online and producing before the trip – they simply increased their output, using what’s known as “spinning reserve”. In terms of FCAS markets, this response would be primarily the fast raise (6 second) and slow raise (60 second) contingency services enabled by AEMO. For emphasis, I’ve highlighted in red the responses from the SA battery and also from Gladstone in Queensland. It’s very clear that they were only a small part of the overall story, nor was either the first to respond to the event. In fact most of the response came from old-fashioned coal-fired stations. That’s not at all surprising, because in this overnight timeframe with low demand it’s predominantly coal generation that’s online, and contrary to what you might expect from reading elsewhere, during low to moderate demand periods online coal fired generation can usually vary its output fast enough, in aggregate, to respond perfectly adequately to this kind of event. […]

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