Impedance of 3 phase bank (3 single phase GrdY-GrdY) 4

[ffont]

I have a three phase pole mounted bank made out of 3 single phase transformers 100kVA 12470GrdY/7200-120/240V. The %Iz of each transformer is respectively 1.9%, 1.8% and 1.6% on the 100kVA base. The transformers are connected as a 12470GrdY-208GrdY bank (The 2 LV windings of each single phase unit are re-connected in parallel as 120V)

The consulting engineer for the building being serviced is requesting to have the actual % Iz of the above mentioned bank to calculate the S/C .... How do I calculate it in this case?

I read that if all 3 transformers had the same %Iz, lets say 1.9%, then the % Iz of the bank would be 1.9% (300kVA base).... but in this case each of the %Iz is quite different; can someone help and show the calculations to go through?

Thanks!

 

[jghrist]

Calculating the short-circuit current correctly with standard equations will not be possible with different %Z in each phase. Ø-gnd faults will depend on what phase is faulted. 3Ø faults will be unbalanced. You won't get this result with standard equations no matter what impedance you use.

For purposes of determining short-circuit duty vs equipment rating, use the lowest impedance to be conservative. For arc hazard analyses where a lower fault current might be conservative, check both with the lowest %Z and the highest.

 

[waross]

For line to neutral faults use the impedance of the faulted transformer. Base 100 KVA
For line to line faults it may be the average impedance of the shorted transformers. Base 200 KVA @ 208 Volts.
Open for comments and corrections.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

For 3-phase faults it would be on a 300 kVA base as that is the bank rating.

 

[waross]

I intended "For line to line faults it may be the average impedance of the shorted transformers. Base 200 KVA @ 208 Volts." to mean single phase line to line faults.
Is this correct David?
Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ffont]

Bill, David and Jghrist, thanks for the answers, we did indeed suggest to the consultant to use the lowest %Iz on a 300kVA base as the worst case scenario for a 3 ph short circuit... but he insisted! go figure!

With regards to the fact that the Nameplate %Iz is obtained using the LV winding as 240V and because these single phase units have a Lo-Hi-Lo design, should we recalculate the %Iz on the basis that both 120V windings are in parallel? if so, I forgot the rule of thumb to do so.

Thanks again!

 

[davidbeach]

It's ugly enough, what with mismatched impedances and all that, and you want to go and add in line-line faults? ;-)

I'd just go with jghrist's approach and assume a 3-phase, 300 kVA bank with the lowest impedance for equipment ratings and fault studies, and a 300 kVA bank with the highest impedance for load flow and arc flash studies.

If that doesn't get the necessary level of detail you'll need to use a single-phase analysis tool (EMTP comes to mind) or work through lots of math. But you'll never know the source impedance to a sufficient degree of accuracy to justify the extra work. If you calculate it all out, you'll wind up with a result something like phase fault currents that differ by 10's of amps, but the model source impedance accuracy requires the results to rounded to the nearest 500A or even 1000A. Why waste the time?

 

[odlanor]

ffont
your transformer is of 3 single phase transformers with 1.6, 1.8,1.9% at 100kVA base and 12470/V3 V base.
It is equivalent to a 1.6% , 300kVA base and 12470V base.

 

[magoo2]

Hey odlanor,

I think you're missing the point. The transformers have a series multiple connection on the secondary. As such they can be connected in series to get 120/240 V or in parallel to get 120 V.

With per unit, usually you select the phase to phase voltage and the three phase MVA. In this case, you get 12470 V for the primary and 208 V for the secondary. The MVA base is 0.3 or 300 kVA.

In this case, the secondary voltage is quite important.

 

[waross]

Use 120 Volts instead of 240 Volts to calculate the current at 120 Volts. It will probably be double the 240 Volt current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[odlanor]

magoo2,
I want to remember, is that per unit is a function of base power base and base voltage. The base voltage is different in grounded star connection.

 

[cuky2000]

Enclosed is an approximate graphical fault current adapted from Eaton Consulting Engineering Guide.

The value shown on the right-hand vertical scale is the fault current (kilo amperes) available at the fault point for the equivalent uniform impedance Green: 1.6% @100 kVA (4.8% @ 300 kVA), Blue: 1.8% @100 kVA (5.4% @ 300 kVA) & Red: 1.9% @100 kVA (5.9% @ 300 kVA).

As shown in the graph, the fault range between 18.8 kA to 15.8 kA (Delta Isc=3kA) at the transformer and 14kA to 12 kA (Delta Isc=2kA).

If the protective device is interrupting rating is above the upper range, this approach should be satisfactorily for practical purposes.

I hope this help.

 

[ffont]

cuky,

I see indications showing the %Iz of the 300kVA bank as being 3 times the % Iz of the individual single phase. This is really not the case..... eg if all 3 single phase units had a 1.6% IZ (on a 100kVA base), then the 3 ph bank has a 1.6% IZ as well (on a 300kVA base).

 

[waross]

100 KVA @ 400 Volts & 2.0% Z = 0.032 Ohms.
300 KVA @ 400 Volts & 2.0% Z = 0.092 Ohms.

Difference between Impedance in Ohms and Impedance in P.U. referred to different bases. (100 KVA Vs 300 KVA)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ffont]

Bill, thanks for clearing this up....