Transmission Line- Thermal Limit vs short circuit current
Transmission Line- Thermal Limit vs short circuit current
(OP)
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.
I=2624A
Is this my short circuit limit for breaker installed for this transmission line operation/protection?
Thanks in advance.






RE: Transmission Line- Thermal Limit vs short circuit current
RE: Transmission Line- Thermal Limit vs short circuit current
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.
RE: Transmission Line- Thermal Limit vs short circuit current
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.
RE: Transmission Line- Thermal Limit vs short circuit current
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?
RE: Transmission Line- Thermal Limit vs short circuit current
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.
RE: Transmission Line- Thermal Limit vs short circuit current