Monday, June 27, 2011

Analysis; Damage to a power network from multiple major earthquakes.

Christchurch has had 3 major, 10 moderate, and around 7000 minor earthquakes and aftershocks at this point in time. The following is a personal account of the damage based on my personal experiences and the different side-effects of earthquakes. 

I need to stress at this point in time, I am NO geologist, seismologist, or network analyst. So some of this information may not be accurate, it's simply based on my logic as an Electrical Tutor! I apologize in advance if I say anything incorrect, and feel free to post anonymous comments (this has been done, already, thank you!) and correct me.

In my opinion there have been four major side-effects of a quake (in Christchurch's case): 
  • Ground shaking 
  • Liquefaction
  • Lateral spread
  • Landslides/building collapse/rockfalls. 
Christchurch did not experience tsunami related to our own quakes at any stage. I will cover each category for some systems in the Christchurch distribution network that were affected, namely:
  • Substations 
  • Overhead lines 
  • Underground cables


Substations:

Ground shaking:
Large cast-iron circuit breakers and large transformers were affected by the shaking. Nothing is exempt from the ground acceleration and the following effects occurred:

Masonry bolts were removed from the concrete foundations by the ground shaking, which affected both circuit breakers and transformers. Also on multiple occasions, oil-filled transformers 'tripped out' due to the 'sloshing' of the internal insulating oil, making the float
switches think the transformer had developed a leak, which causes outages. I am not aware of any circuit breakers 'tripping' due to ground shaking, but it is a possibility.

Seismic bracing behind circuit breakers at Bromley substation 

Seismic bracing at Bromley substation 
During the last two decades, Christchurch's Network owner invested a significant amount of money in seismic strengthening their 270 brick substations which resulted in few sustaining serious damage. See HERE for more information.


Liquefaction:
Christchurch is effectively built on an existing swamp, particularly the eastern areas. The worst-hit example of a substation affected by liquefaction was New Brighton substation, which is approximately 200m from the Avon River. Lateral spread and a high water table were factors here also.


New Brighton substation sunk due to liquefaction


The February magnitude 6.3 quake caused the New Brighton substation to sink approximately 2m into the ground, damaging incoming and outgoing power cables. It also filled the switch room with approximately 700mm deep silt and water. This had to be cleaned out comprehensively and the two 66kV transformers were replaced with a single 66kV transformer fed from a new overhead line.

There were several smaller substation buildings that also sank and 'went out of level', but most appeared to continue operating without issue.

Lateral spread:

New Brighton substation may have been affected by lateral spread, but evidence was sparse to indicate that. As with liquefaction, lateral spread would have affected any substations that were near the Avon River. I'm not sure if that actually occurred at any stage.

Landslide/building collapse/rockfalls:
Sumner substation was hit by a massive boulder and partially disabled by this. It was built below a cliff composed of volcanic rock, some brittle, some boulders. I'm no geologist, so I couldn't describe the type of rock the cliff face was composed of, but the boulder that hit the substation was substantial and did serious damage.

The boulder that destroyed Sumner substation


Inside Sumner substation


Overhead lines:

Ground shaking:
Having seen up to 2.2G of ground acceleration from the magnitude 6.3 quake of February 22nd, Christchurch has seen unprecedented shaking due to an earthquake. Thousands of poles were heavily affected by this shaking and a large number were affected, predominantly by being left on leans, pulling barge boards off houses, straining lines, and damaging pole fittings such as cross arms and insulators. Conductors swung to within arcing distances and proceeded to do so. Also the ground acceleration caused the shafts of insulators to be bent and ties to be broken. 


Leaning poles due to a combination of shaking and liquefaction  in Kingsley St.


Video of using a helicopter to repair broken ties on an 11kV circuit that was inaccessible due to rockfall hazard in Sumner:





Liquefaction:
Many poles were affected by liquefaction, some sinking straight down, some leaning due to liquefaction coming up underneath the pole. 


Installed 3 weeks prior to the June 13 magnitude 6.3 quake, this pole was affected by liquefaction.

The same newly-installed pole.




Lateral spread:
Once again, many poles close to rivers were affected by this in a similar manner to liquefaction. Also, due to the lateral spread, the land sank, causing the water table to rise, which also caused poles to sink.


This building sank more than the pole did, ripping the termination bracket off the roof of the building.


Landslide/building collapse/rockfalls:
Poles above and below cliffs were affected by rockfalls and landsliding. Poles were hit by debris below, as well as their associated lines. This damaged poles by outright destroying the pole to breaking insulators and crossarms.


The Sumner boulder that fell on a pole.


The pole with the boulder on it.




Underground cables:


Ground shaking:
Two of the four types of dynamic force that travel underground during a large earthquake can be seen below.




A static description of waves is shown HERE.


As anybody can imagine, these massive forces can affect subterranean infrastructure systems, such as power cables, telecommunication cables, water pipes and waste pipes to name a few. In Christchurch all the aforementioned systems were affected by the quakes, power cables being in the firing line.


A minimum of four major 66kV underground cables were damaged, two beyond repair. The sheer ground acceleration ruptured the concrete the cables were embedded in in multiple locations, mostly where the liquefaction was at its worst. The other two cables had faults that were able to be repaired.



Partially completed 66kV XLPE repair joint


In the centre of the picture is a faulted 11kV PILCA cable, above is the concrete in which the one of the heavily damaged 66kV oil-filled cables can be partially seen.


3-in-a-row; three PILCA 11kV cables all bent and faulted.


Other cables were damaged also, approximately 800 underground cable faults occurred from the February quake. 11kV paper insulated lead covered armoured (PILCA) cables were bent into Z and S shapes, causing the insulation to fail on them. Other faults included previous faults pulling apart and terminations being removed from protective devices. Cables that transitioned up poles were bent by the pole swaying during the quake.


Liquefaction:
The makeup of the swampy soil in the eastern areas of Christchurch seemed to allow cables to be damaged as stated above, but the actual liquefaction process of water bubbling to the surface of soil did not affect cables as the shaking did. Liquefaction rather made travel impossible in some cases and created large potholes that vehicles fell into. The higher water table involved pumping out joint holes before work, which in some cases took hours.


An abandoned joint hole, this particular joint incomplete as several faults were found on the same cable.


Lateral spread:
Lateral spread caused cables to fault next to bridges that the cables passed under, and also caused joints to tear apart near rivers. Also cracks that appeared in land on hillside suburbs tore T-joints, other joints and terminations apart. 


Originally straight; this <still functioning> LV XLPE cable has been bent by lateral spreading.

Steve from Connetics showing the bent cable.


An 11kV XLPE joint torn apart by lateral spread.


A LV joint torn apart by lateral spread.



Landslide/building collapse/rockfalls:

I'm not aware of any faults due to these effects, however I understand many kiosks were damaged by falling buildings in the Christchurch CBD 'Red Zone' and the Lyttelton town centre.

Solutions:

Substations:
The circuit breakers inside Bromley substation were reinforced with steel braces to prevent further shaking and destabilizing. New Brighton substation was relocated to Rawhiti Domain, where there was little liquefaction and lateral spread. Sumner substation, I'm not entirely sure, but I think it was bypassed. 



Overhead lines:
Due to the ease of fault finding and maintenance, overhead lines were rapidly repaired. Severe unstable poles were immediately repaired, and ongoing pole straightening is continuing.



Underground cables:
Approximately 800 underground cable faults were repaired. Test Technicians and Cable jointers from all over New Zealand and Australia were employed to assist with the mammoth diagnosis and repair effort. Specialist 66kV jointers were bought in to repair oil-filled and XLPE insulated cables. The irreparable 66kV cables were replaced with temporary 66kV overhead lines, to keep the power supply running through winter until such time as a window of opportunity to replace the faulty cables exists.




See HERE for my blog post regarding the Bromley-Dallington 66kV overhead install.
See HERE for my blog post regarding the New Brighton-Rawhiti 66kV overhead install.




2 comments:

Selena said...

Hi Andrew,

Good post. I learnt heaps. This may blog post may be a good starting point for your students to work through solutions to the damage caused by the earthquakes.

The post provides good data but they could be asked to critique the post as to accuracy etc. and suggest alternatives :)

Keep up the good work, both on ths blog and with assisting to repair the electricity supply infrastructure. Selena

Andrew said...

Thanks Selena. GREAT idea about the problem-solving exercise! Will definitely use that one.

This post was really one I wanted to do that was separate to my usual 'work log' ones to record a bit of history from my experiences that I've witnessed on this unbelievably well-timed (for me) Academic Study Leave. It all comes to an end this Friday..

I'm not sure how I feel about coming back to CPIT, to be honest! But I'm sure aware that I have witnessed first-hand one of the world's most amazing phenomena, and as well as having my home damaged beyond repair by it, being left in sheer awe of the power of Mother Nature.

Thanks again,

Andrew ;D