Coordination with the transformer inrush curver?

[tmet]

What is or is there an importance to coordinate a high side fuse with the transformer inrush curve in a distribution substation (ex: 69-12.5 kv)? It is understood that the inrush current on a power transformer can be 8-12 times the magnitude of the load current for about .01-.1 seconds. Why then is the inrush current curve a "curve" and not an instanteous line for the .1 seconds on TCC graphs?

Thanks for any help.

tmet

 

[rbulsara]

I am not sure what 'inrush curve' you are referreing to, but on TCC generated by SKM program, shows xfmr inrush as a 'point' along 0.1 sec. Your other statement is true about 8-12 times FLA for 0.2 second. Do not confuse transformer damage curve as inrush curve.

Other rule of thumb is size the MV fuses (E rated) to 1.0 to 1.25 times the FLA. For example, for a MV transformer with 64A of primary side FLA, 65E min or 80E max fuse will suffice.

 

[rbulsara]

And yes it is important that the primary fuse curve lies to the 'right' and clear of the inrush current point on a TCC. This ensures that the fuse will not blow due to high inrush current when the transformer is energized .

 

[jbartos]

Suggestion: IEEE Std 242-1975 Buff Book shows transformer inrush point in a few coordination charts.

 

[jbartos]

Suggestion: Current IEEE Std 242-2001 Buff Book includes in Chapter 15 "Overcurrent Coordination" the Transformer Primary Fuse sizing/coordination with respect to the inrush point on page 615:
""1000 kVA transformer primary fuse coordination. The next device to be plotted is the 1000 kVA transformer primary fuse (see Figure 15-17). The 15.5 kV fuse (rated 65E) is selected for transformer protection. As much of the transformer through-faultcurrent curve should be to the right of the fuse curve as possible. Its rating has to be
less than 300% of the transformer full-load primary amperes of 42 A (65 A/42 FLA = 155%) in accordance with NEC Article 450-3. This device and all other upstream devices see the transformer inrush current. The fuse curve and the curves of all upstream devices should be above and to the right of the transformer inrush point; otherwise these devices could trip every time the transformer was energized. The downstream devices do not see the inrush current.
At times the short-time delay characteristic curve of the secondary circuit breaker extends into the primary fuse characteristic curve. In this example, loss of selectivity
is a better choice than installing a larger fuse because a larger fuse would compromise adequate transformer protection. Of course, when the main secondary circuit breaker trips, all transformer load is interrupted, and the fuse operation does not interrupt any load.""

and the transformer inrush point calculation on page 610.

 

[rbulsara]

its O.1 sec..and 8-12X...sorry for typo in my prev. response..

 

[peebee]

Red Book has similar information to Buff in this respect.

 

[cuky2000]

Inrush current on power transformer could be modeled by a single point, multiple points or less often available by a curve.

The typical inrush curve is an envelope that result to smooth the integrated rms equivalent of a magnetizing inrush wave. A simple approach to build the inrush curve is using the following set of points:
Time (sec) Inrush Factor
100 2
10 3
1 6
0.1 12
0.01 25

Beware that inrush is not a deterministic phenomenon that depend on voltage angle, transformer MVA rating, and other parameters.

See the encloe site showing a sample with a transformer inrush curve.

http://www.hubbellpowersystems.com/...rary/pdfs4lib/Chance-Construction/10-8901.pdf