HV Switchyard design & Live tank CTs 1

[RRaghunath]

I am used to seeing Dead tank CTs in 400kV switchyards in India. I was told Alstom Grid has only Live tank design CTs in their range of offering and was just thinking about design changes that are required if Live tank CTs are to be used! One thing is that the live tank CTs have much bigger tank on top (to accomodate 5-cores etc.) and thus can impact the phase-to-phase and phase-to-ground clearances. This could mean slightly increased bay width.
Any thoughts / experiences from the experts!

 

[odlanor]

In my country typical 500kV substation are outdoor installation with Live tank CTs of 4,5 or 6 cores.
When there is problem of terrain they use GIS installation.

 

[RRaghunath]

Thanks odlanor.
That's what I too understand from the text books -
Live tank CTs are preferred for EHV switchyards with large current ratings and high fault levels and have a design advantage.
For 132kV switchyards with hardly 1600A current rating and 25kA fault level, I am not sure.
Which country you are in, any way.

 

[odlanor]

I am from Brazil. I guess we always use Live tank CTs at any level above 69kV. I think it's a matter of culture.

 

[scottf]

Outside of India, most places use live-tank/post-type CTs for HV/EHV applications. There are many technical benefits over tank-type/hair-pin designs. I've never seen a case where the head-size of the CT causes clearance issues.

 

[DTR2011]

Scottf,

I've always been curious about the advantages of live tank vs. dead tank. Seems to me that having the insulation system and possibility of failure vs. a bushing type ct is a lot to compare to.

Am I missing something? I come from the ANSI MR CT world of dead tank breakers as well.

 

[scottf]

small-geek-

A few quick ones off the top of my head:

- the primary conductor in head-type design relatively short and hence a very low inductance. This limits the voltage build up (L di/dt) is very low during fault currents.

- The primary conductor of dead-tank CTs passes down and then back up inside the insulator with primary insulation around the entire length. During a fault current the conductors tend to want to pull together, which places the entire HV insulation at risk of damage.

- with a dead-tank CT, the high current carrying primary conductor is inside the HV insulation, which means the insulation is exposed to much higher heat and the heat must be dissipated through the insulation. This gives a live-tank CT big advantage of increased thermal stability and longer life expectancy.

After re-reading your post, I believe you may be confusing dead-tank CTs with bushing CTs that are used on dead-tank breakers. Those are not the same thing.

Dead-tank CTs are insulated, free-standing CTs with the tank at ground potential...meaning on the bottom of the unit. Live-tank CTs/Post-type CTs have the tank at HV and at the top of the unit.

 

[davidbeach]

But why free standing CTs rather than bushing CTs on the breakers?

 

[prc]

The only advantage for live tank CT is less cost due to less insulation,smaller dia porcelain and less copper by way of primary winding( in live tank CT it is a copper bar passing through circular core centre).The disadvantage is less withstand strength from seismic,transport forces as the core is held in live upper tank and the centre of gravity of CT goes up.Heating /cooling issues are taken care in both designs and no special advantage can be claimed.In case of a blast the damage to surrounding equipment can be more with live tank CT.The price advantage for live tank Ct becomes less and less at lower voltages.

David, to my knowledge, dead tank breakers are preferred only in US and Japan and in other places it is live tank breakers where CTs cannot be accommodated.In Japan free standing CTs are avoided due to the frequent earthquakes and CTs are accommodtaed in dead tank breakers as BCTs.

 

[scottf]

Davidbeach....

I don't think bushing CTs would be more favorable that free-standing CTs, where they can be accommodated. Especially for protection applications.

There is one down-side to bushing CTs and that is they are limited to 1 primary turn (the bushing). That limits the ability to practically produce lower-ratio units that are often needed for proper revenue metering. In general, the practical cut-off for bushing CTs is 600:5A or so.

 

[prc]

scottf,one query.In free standing CTs(esp live tank ) aslo,only one turn primary is normally provided.So will not the limitation that you anticipate applicable to them also?

 

[scottf]

prc-

In free standing CTs, it is possible, and common, to have more than 1 primary turn. Most head-type/live-tank designs have a 1,2, and 4 turn primary configuration using solid bars and have wound primary designs using flexible conductor.

 

[RRaghunath]

Interesting / informative dicussion. Thanks.
I still wonder whether any layout issues are associated with Live tank CTs due to larger tank at top and is live!!

 

[prc]

Raghun, I dont think any clearance issue will come.The reduction will be marginal.Of course better to check with actual figures from product drawings.