So what am I even on about?

ABC News looked into the topic of grid stability back in 2017.

Power grids are complex machines, dependent on the laws of physics. The national grid is designed to operate at a consistent frequency of 50 Hertz, or 50 cycles a second.

Traditional coal, gas and hydro power stations are considered “synchronous” because they use turbines or spinning wheels to produce electricity. Those spinning parts need to stay close to 50Hz to help keep the grid in synch.

But most of Australia’s installed wind and solar systems are not considered to be. That’s because they use inverters to connect to the grid, rather than spinning wheels.

If too much power is fed in relative to demand, the frequency will increase. If demand outstrips supply, the frequency drops. Regulators rely on a suite of technologies to help keep the grid frequency close to 50Hz.

Synchronous condensers are one of the technologies used to maintain the frequency of the grid, with Energy Networks Australia explaining the technical details in their piece, The age of the syncons.

What is system strength?

System strength is important as it relates to the ability of the power system to withstand changes in supply or demand while maintaining stable voltage levels.

When system strength is low, generators may not be able to remain connected to the grid, control of the power system voltage level becomes more difficult and protection systems (which control and maintain the safe operation of the network) may not operate correctly. This can result in supply interruptions to customers.

System strength is typically provided by synchronous generation such as coal or gas-fired generation or pumped hydro.

What are synchronous condensers?

Synchronous condensers are an old technology, commonly used as far back as the 1950s to stabilise power systems.

They are large machines which spin freely and can absorb or produce reactive (Alternating Current – AC) power in order to stabilise and strengthen a power system.

Synchronous condensers help when there are changes in load as they increase network inertia. The kinetic energy stored in a synchronous condenser contributes to the total inertia of the power system and is beneficial from a frequency control perspective.

What is inertia?

Inertia in the energy system refers to the continuous momentum of energy typically provided by the large spinning turbines of synchronous generators like large coal-or gas-fired power stations. This type of generation helps withstand changes in generation output and load levels to keep the system stable.

The retirement of synchronous power plants and more renewable generation coming into the energy system means there is less inertia available, so flexibility or stability must be found elsewhere in the system to back it up.

And their usage in Victoria

Until the 1990s the electricity network in Victoria was managed by a single government entity – the State Electricity Commission of Victoria.

Brown coal from the Latrobe Valley was their fuel of choice.

But despite all of the old fashioned spinning metal in their power stations, in 1966 the SECV installed a 750 rpm +125 -75 MVar at 22 kV capacity synchronous condenser at the Templestowe Terminal Station in north-east Melbourne.

Photo by Mriya, via Wikimedia Commons

A few years later a second synchronous condenser was installed at the Fishermans Bend Terminal Station, south of the Melbourne CBD.

And in 1971 a third unit at Brooklyn Terminal Station, in Melbourne’s west, with a salient pole design rotating at 750 rpm, with a rating of +110 -64 MVar at 14.5 kV, and a short time overload rating of 140 MvAr (10 min).

Following privatisation the reliability of the synchronous condensers declined, with availability falling below the 91% target in 2003. As a result network operator SP AusNet launched a refurbishment program to address degradation of stator winding sidewall and rotor pole insulation, but by 2013 only the unit was Brooklyn had been upgraded.

As a result reliability declined, with the end coming in October 2016.

AusNet Services and AEMO agreed in October 2016 that it was prudent to retire, rather than replace, these three synchronous condensers on the transmission network. These assets were in extremely poor condition and studies confirmed that their replacement would not have provided a net market benefit.

Since the agreement to retire the synchronous condensers, all three units have failed due to their poor condition. Given that the synchronous condensers were due to be retired by 1 April 2017, AusNet Services and AEMO agreed that it was not efficient to repair and return the synchronous condensers into service.

With SP AusNet realising an additional $7.0 million depreciation charge in their 2017 annual report.

Everything old is new again

In 2017 synchronous condensers hit the news, when AGL flagged them as one part of their transition away from coal fired power.

Liddell Power Station is a 2000 MW black coal fired thermal power station, commissioned between 1971-73. The site also includes associated infrastructure required for power generation, including water, coal and transmission plant.

In April 2015 AGL released a revised Greenhouse Gas Policy. The Policy outlined AGL’s commitment to the decarbonisation of our electricity generation portfolio, confirming closure dates for our coal-fired power stations. The announced closure date for Liddell is the end of 2022.

AGL believes that the installed capacity and energy output from Liddell is best replaced with lower emissions and more reliable generation, with a longer lifespan.

As part of our NSW Generation Plan we are investigating the use of one Liddell generating unit as a synchronous condenser.

As part of new solar farm proposals.

Many other wind and solar projects in Victoria and elsewhere are having to go back to the drawing board because of connection requirements the developers either ignored, or didn’t know about.

The issue is most acute in western Victoria, but is also being felt in northern Queensland and south-west NSW.

Many new projects are being told that they face significant curtailment without either adding battery storage or old-style machinery known as synchronous condensers to deal with system strength issues.

Both options are causing headaches for developers, because either way they are trashing their financial models, and could cause extensive delays to projects that many expected would begin construction anytime soon.

As a high cost fix for system flaws.

RenewEconomy has been told that a synchronous condenser could add $8-$10 million in costs to projects already tight on margins. A group of solar farms in north-west Victoria have been told, RenewEconomy understands, that their additional costs could total $60 million.

And to reinforce the South Australian power grid.

As more energy sources such as wind and solar are connected to the grid, traditional power generation sources such as gas-fired units, operate less often. This has created a shortfall in system strength which was declared by the Australian Energy Market Operator (AEMO) on 13 October 2017 and a shortfall in inertia which was declared on 24 December 2018.

A secure power system needs adequate levels of system strength and inertia, which to date have been provided by traditional synchronous generators.

Following an analysis of these options, the installation of synchronous condensers on the network was determined to be the most efficient and least cost option to ensure there is adequate system strength and inertia.

On 20 August 2019, the AER approved $166 million to fund the capital cost of delivering the synchronous condenser solution.

The first two of four planned synchronous condensers will be installed at the Davenport substation in mid-2020 and the second two will be installed at the Robertstown substation by the end of 2020. They will be commissioned by early 2021.

How things change in the course of two years!

Update for 2022

In July 2022 a 60Mvr synchronous condenser supplied by GE has been switched on at the Murra Warra wind farm in the West Murray region, enabling the project to increase export capacity to 150MW, before moving towards full capacity of 209MW.

The synchronous condenser was required as part of the since abandoned “do no harm” rules that required new generation projects to address system strength issues in the transmission network. This is now the responsibility of network operators.

Footnote: alternate sources of voltage support

Static VAR compensators are another way of stabilising the electricity grid.

A static VAR compensator (SVC) is a set of electrical devices for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the Flexible AC transmission system device family, regulating voltage, power factor, harmonics and stabilising the system.

A static VAR compensator has no significant moving parts (other than internal switchgear). Prior to the invention of the SVC, power factor compensation was the preserve of large rotating machines such as synchronous condensers or switched capacitor banks.

Four SVC units are installed on the SP AusNet Network in Victoria – two +100 -60 MVar capacity units at Rowville Terminal Station, and one +50 -25 MVar unit at each of Kerang Terminal Station and Horsham Terminal Station.

Installed by the SECV during the mid-1980s and with a technical life of between 40 and 60 years, the control systems are now obsolete technology unsupported by the manufacturer, so an upgrade program is underway to replace them with modern equipment.

Footnote: and something really fruity

Down at Wonthaggi is a real power hog – the Victorian Desalination Plant.

It is supplied with electricity by a 88 kilometre long twin circuit 220 kV AC underground transmission line – the longest of its type in the world.

With the underground cable run requiring something odd at the halfway point – a ‘reactive compensation station’.

The yard full of high voltage switchgear contains three 52 MVAr oil-filled shunt reactors to compensate for the capacitance of the underground cables.

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They were located either side of Luddenham Road in the semi-rural suburb of Orchard Hills. One set to the east, the other to the west.

This is what they looked like from overhead.

In my travels around Victoria, I’ve never seen overhead line crossovers – the only examples I can think of place the lower voltage line underground, such as the Geelong-Portland 500kV line outside Bannockburn.

So what’s the story up in Sydney?

Wikipedia has something to say on everything, including overhead line crossings:

At crossings of overhead lines by other overhead lines, the two lines must be kept at the necessary safety distances between the lines and the ground. As a rule, the line with the lower voltage passes under the line with higher voltage.

Construction workers try to plan these crossings in such a way that their construction is as economical as possible. This is usually done by leaving unchanged the line that is crossed, if possible.

Undercrossings of existing lines are often constructed in proximity to the line’s pylons, since this can often be accomplished without raising the existing pylons and while keeping the necessary safety distances between the ground and the other line.

Luckily the Australian Energy Market Operator (AEMO) has plenty of information on their website, including a handy dandy map of overhead transmission lines, as well as a network diagram showing the individual circuits that make up the national grid.

That allowed me to identify the three transmission lines I had found outside Sydney:

• Line 38 and 32: double 330kV circuits between Sydney West and Regentville,
• Line 39: single 330kV circuit between Sydney West and Bannaby, and
• Line 5A1 and 5A2: double 500kV circuits between Kemps Creek and Eraring.

So the explanation from Wikipedia seems to hold here as well – the 500kV transmission line is on the top, with the lower voltage 330kV lines sneaking below: one via a non-standard pair of pylons, the other thanks to the extra clearance from a higher than normal pylon.

And a Victorian example

One of my Twitter followers pointed out this interesting setup at the Rowville Terminal Station in Melbourne’s eastern suburbs – what looks to be a two three phase bus bars running at ground level, while aerial transmission lines pass over the top.

But they aren’t conventional air insulated bus bars, but ducts filled with sulfur hexafluoride (SF6) gas.

500kV power lines in SF6 ducts at Rowville Terminal Station, Australia [2592×1944][OC]

Such technology is usually used for underground high voltage transmission lines, but at Rowville it was for a different reason – this whitepaper by CGIT Westboro has the details:

Rowville is a 550 kV above ground installation located in Melbourne, Australia.
The sole purpose of the gas insulated bus was to safely transmit two, three phase circuits of 550 kV power across an existing 230 kV overhead transmission corridor.

The customer’s main concern in this project was the possible mechanical failure of either the 230 kV or 550 kV (if overhead lines were used) transmission towers or lines. Failure of one or the other could result in damage to the lower feeder such that outages would be much more inclusive and costly.

The grounded enclosure sheath of the CGIT, however, would protect the ground level 550 kV line from breakdown and prevent further outages to result from 230 kV line failure.

A spare phase was installed at the request of the customer as a further precaution should a single phase of CGIT experience a breakdown. In the event of single-phase failure, this would reduce the outage time to a matter of hours by simply adjusting the line feeder leads accordingly.

The tech specs are as follows:

Commissioned 1979 (first circuit)
Additional circuit commissioned 1984

Voltage Rating: 550 kV
Current Rating: 3000 A
BIL rating: 1800A
2 circuits, including spare phase of GIL
Length: 235m

An expensive, though very ingenious solution!

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