The Live Tank vs. Dead Tank Switchgear Debate 1

[thermionic1]

It is well established that the North American / ANSI world favors dead tank HV Swgr and the IEC world favors live tank. With that said, I'm curious about the reasons or opinions for these differences. Excluding HV Metering CT/VT.

Consider a 362kV DT Breaker (SF6 Type)

Single Large Foundation for Tanks & Control Cabinet
Six to 18 600V Class Multi Ratio Bushing CT's
Approx 350 lbs SF6
Some makes require extensive effort to install bushings & vac / fill gas
​

Compare to 400kV Live Tank (SF6 Type)

Three smaller foundations for breaker & Cabinet
Three smaller foundations for bus side CT's
Three smaller foundations for Line/Xfmr side CT's
Approx 100 lbs SF6
​

Is there a significant cost savings? Is one design inherently better / safer? Does seismic activity affect either? Does the SF6 volume matter that much? Is the installation cost a factor?

We recently had an outage in the area and root cause was a failed HV CT. I've read about these issues in Doble conference papers and elsewhere. I can't recall where a bushing CT has violently failed (not caused by human interaction).

 

[Mbrooke]

From what I've heard live tank CT failures are common with plenty of videos to go around.

https://www.youtube.com/watch?v=49HWJqpIeDA

https://www.youtube.com/watch?v=J8Z4UOQI-94

https://www.youtube.com/watch?v=9T5XEg3CyT8

https://www.youtube.com/watch?v=C9VkplUE8zw

Not a problem with dead tank breakers.

 

[davidbeach]

Bushing CTs vs. free standing (hairpin) CTs. Also, all those bushing CTs are each their own thing, with no oil, whereas the free standing CTs put all the cores into the same oil tank. Seems like an easy choice.

 

[prc]

Live tank CTs and breakers are impossible in high seismic zones eg. Japan, Chile. Any way this discussion seems no more relevant. GIS is replacing conventional switchgear rapidly with their prices coming down. Also manufacturers have come out with hybrid switchgear, incorporating isolators, CTs and breakers together in live tank models.

 

[cuky2000]

The name dead or live tank is associated with the potential level in the apparatus vessel.

Dead-tank breaker is a US invention started back during the early generation of bulky oil-filled units that later migrated to gas-filled and most recently start using vacuum in the interrupter chamber that is commercially available at least for 69 kV class breaker. The tank is a ground potential.

In the ANSI/IEEE marketplaces, ring bus, breaker 11/2 and occasionally Bkr 1 1/3 are the most common design for bulk HV & UHV facilities. Dead tank circ Bkr design since all the CT (3 to 4) are installed in the breaker bushing. This allows to simplify and make more compacted the substation design.

Live-tank breakers have a potential gradient from the connection of HV live part down to zero at the base interface with the metally structure. This design is better suited for a substation in main and transfers bus configuration popular in the IEC marketplaces requiring less number of free-standing CTs and somehow simpler P&C than a ring bus.

The reliability/cost ratio favors the ring bus design configuration and perhaps this was a major reason to select the death-tank design over the live-tank circuit breaker design. Occasionally, US designer selects live tank breakers (or circuit switchers)for transformer protection taking advantage of using the transformer bushing reducing substation space.

_{NOTEs:
1) For heavy seismic zones, Dead-tank breakers are more robust than the Live tank. However, there are accessories such as vibration dampers that could mitigate the mechanical stresses on the live tank breaker.
2) Most breaker manufacturers use the same interrupting mechanisms not only for dead and live design but also in GIS breakers.}

 

[Mbrooke]

@Cuky: If you ever wrote an encyclopedia it would be my Harry Potter. I love reading your posts being simple yet full of relevant spot on information.

 

[thermionic1]

So quite a few compelling reasons for dead tank. I could understand live tank minimum oil versus dead tank in terms of environmental and fire/explosion risk. I'm still left wondering why would someone choose live tank?

There is one GIS station being built locally in our downtown area, but at least here all other new stations are still AIS. Land is cheap and plentiful. We have a very large station we are doing in NY this summer that is AIS as well.

 

[bacon4life]

At lower voltage class than your example, we have used a number of live tank 115 kV breakers as space constrained retrofits in substations that used to have 115 kV fuses.

 

[Bill West]

What I saw in my time was bulk oil up to 138kv and air blast or minimum oil after that. In later years even 138kv moved over. I think the key was that the biggest utilities here are required to use open bidding, lowest evaluated price from a reputable supplier wins. With the GE and Westinghouse factories in Canada building lots of the lower voltages they could compete but after that importing low volume, higher voltage CBs from their US parents would be impeded by the US-Canadian exchange rates. Lower price is where European 138-230kv minimum oil breakers and 500kv air blast breakers moved in on our market.

The local standard switchyard layouts allowed for all breaker types because of the question of future expansions, land space was not enough of a problem. So live tank - dead tank was all economics. Oh, main & transfer bus at transmission levels was not used so CTs were not an issue either.

Bill

 

[thermionic1]

We had some special applications like this (series breakers), but this yard was rebuilt to something more modern (breaker and a half) a few years ago. The remote end still has 2 breakers in series.