Transmission Line- Thermal Limit vs short circuit current 3

[IndianTechEngg1001]

For 220kV line, the thermal limits are normally 300-1000MVA with economic power 350MVA. If I calculate current using formula P=1.732 x V x I (P= Thermal Limit=1000MVA)

I=2624A

Is this my short circuit limit for breaker installed for this transmission line operation/protection?

Thanks in advance.

 

[Sanchez01]

Thermal limits indicate max VA that can be passed through a conductor without damaging the insulator. Short Circuit limit is different and denotes maximum current that a breaker’s interrupter can withstand. Refer to IEEE Std 551-2006 for SC calculation.

 

[tlrols]

The current you calculated is 2624 Amperes...this level of steady state current may be useful for determining what size conductor to use or what size disconnect switch to use.

It IS NOT the short circuit current. Typically a 220 kV class circuit breaker will have interupting capabilities above 50,000 amperes. Breaker ratings of 63 kA and 80 kA are not at all unusual.

 

[bacon4life]

Short Circuit ratings for breakers are typically in the 20,000 to 63,000A range. The short circuit values at the breaker depend on the strength of the attached grid, not so much on the size of conductor used.

Assuming these are overhead lines, the thermal limit is based on either:
- a maximum allowable conductor sag,
-a maximum conductor temperature before the aluminum begins to anneal and loose strength,
-a maximum temperature for hardware and fittings.

 

[IndianTechEngg1001]

I go it,, thanks everybody.

Just one more clarification.

When we select high tension cables we see the short circuit current while selecting the cable size. Like if current is I=22kA then size of Al cable will be S=I/K where k=0.094. So cable to be selected is > 235sqmm (say 300sqmm)/

We select over head conductor size keeping in view the thermal limits, sag, mechanical strength. Do we need to keep short circuit levels in mind while selecting over head conductors?

 

[tlrols]

Generally short curcuit currents are not the primary consideration for sizing overhead conductor.

However, there ARE damage curves for conductors subject to fault currents. For example, a 556 kcmil conductor at 40C has a 2 second fault current limit of 33,920 amps (per my Alcan spreadsheet). Typically the need for the fatter wire based on thermal considerations more than covers the short circuit current carrying requirements--you should check however.

Note that using a more reasonable clearing time of say 6 cycles (i.e. 0.1 seconds at 60 Hz) the short circuit withstand for 556 kcmil is 151,595--good luck finding a breaker that can clear that kind of fault current. Also, good luck sourcing that kind of fault current as well!

In summary, from a practical basis, the fault current withstand capability of a conductor usually (but not in every case) is already dealt with by some other non-fault related need for conductor size.

 

[Sanchez01]

Agree with the posting above. Typically we don't worry if we are not talking about a long line which could have its own contribution.