When high voltage transmission line cross over

Out on the western edge of Sydney I came across a curious sight – not one, but two sets of high voltage transmission lines crossing over each other.

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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8 Responses to “When high voltage transmission line cross over”

  1. Moya White says:

    I’d be interested to know the level of emr at these intersecting lines. It’s known to be above safe levels in proximity to high tension transmission lines.

  2. […] unfortunately I was no closer to finding the reasons for the lines laying abandoned, until my recent post on transmission line crossovers. What started with an exploration of power lines in Sydney, expanded to Rowville Terminal Station […]

  3. Leigh says:

    There is another example on the Princes Highway at the Birregurra turnoff at Warncoort. The highway power lines go underground when the high voltage lines cross.

  4. Hi Marcus,

    Although I keep forgetting to go and have a look, there’s another example of a transmission line crossing another out at Nilma North. Unfortunately streetview images out there are 2008, so it’s not clear. Nearby is a transmission line that decends from towers to concrete poles. Right beside Nilma – Shady Creek Rd.

    Here’s the location – https://www.google.com.au/maps/@-38.1214604,146.0115764,700m/data=!3m1!1e3

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