3 Phase Transformer Bank Impedance

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kwired

kwired

Electron manager
Location
NE Nebraska
How about an open delta with one transformer larger then the other?

My guess is higher impedance on the high leg only, otherwise for figuring maximum available fault current (which is what most of us need to know the impedance for) I have always just used the impedance of the larger unit, if anything it may yield a little higher max fault current then actual available current in certain situations.
 
mivey

mivey

Senior Member
GotShocked said:
I should know this but when you have a 3 phase bank of single phase transformers, are the individual transformer impedances summed to provide the 3 phase impedance?
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No. You can use the average. If calculating worst case 1phase fault, use the smallest if you want to be conservative. A matrix solution will allow you to use all three.
 
mivey

mivey

Senior Member
kwired said:
How about an open delta with one transformer larger then the other?

My guess is higher impedance on the high leg only, otherwise for figuring maximum available fault current (which is what most of us need to know the impedance for) I have always just used the impedance of the larger unit, if anything it may yield a little higher max fault current then actual available current in certain situations.
Click to expand...
I would run the calcs separately. For most 1 phase, use the impedance of the lighting pot, but also consider the 1-phase faults that involve the high leg pot. For 3-phase, you can account for the open delta with the correct formulas but for a scratch calc you could blend.

I don't remember the open delta formula off the top of my head but could look to see the dominant factor. A lot of times the high pot is smaller so the dominant one is the lighting pot.
 
W

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
ron said:
Yes, just like you would add the individual kVA for individual phases if they were singles to equate to a 3 phase.
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The impedances are not added together but the kVA is to provide a 3 phase equivalent. If you have different impedances on each transformer, you have a choice:

1. Use the average value
2. Use the lowest value to determine max fault current for equipment ratings
3. Use the highest value for arc flash calculations

The impedance values are usually closely matched so using the average value will usually provide acceptable results overall.
 
mivey

mivey

Senior Member
Delta Fault Data

Delta Fault Data

Just for fun, I ran some delta bank scenarios. Here are the results:

Code: 
[FONT=Courier New]

#1: Same % Z
#2: Lighting pot (AB) is Z/2
#3: Lighting pot (AB) is 2*Z
#4: Same % Z, lighting pot (AB) is 2*kVA
#5: Same % Z, BC open
#6: Same % Z, lighting pot (AB) is 2*kVA, BC open
																	

	#1				#2				#3				#4				#5				#6
	ABC fault			ABC fault			ABC fault			ABC fault			ABC fault			ABC fault	
	A	3.590 kA		A	5.465 kA		A	2.735 kA		A	5.465 kA		A	3.590 kA		A	5.426 kA
	B	3.590 kA		B	5.490 kA		B	2.747 kA		B	5.490 kA		B	2.036 kA		B	4.061 kA
	C	3.590 kA		C	3.590 kA		C	3.590 kA		C	3.590 kA		C	2.104 kA		C	2.140 kA
																	
	A-N Fault			A-N Fault			A-N Fault			A-N Fault			A-N Fault			A-N Fault	
	A	3.553 kA		A	6.373 kA		A	2.072 kA		A	6.372 kA		A	2.760 kA		A	5.511 kA
	N	3.553 kA		N	6.373 kA		N	2.072 kA		N	6.372 kA		N	2.760 kA		N	5.511 kA
																	
	B-N Fault			B-N Fault			B-N Fault			B-N Fault			B-N Fault			B-N Fault	
	B	3.553 kA		B	6.373 kA		B	2.072 kA		B	6.372 kA		B	2.760 kA		B	5.511 kA
	N	3.553 kA		N	6.373 kA		N	2.072 kA		N	6.372 kA		N	2.760 kA		N	5.511 kA
																	
	C-N Fault			C-N Fault			C-N Fault			C-N Fault			C-N Fault			C-N Fault	
	C	2.872 kA		C	3.191 kA		C	2.393 kA		C	3.191 kA		C	1.301 kA		C	1.505 kA
	N	2.872 kA		N	3.191 kA		N	2.393 kA		N	3.191 kA		N	1.301 kA		N	1.505 kA
																	
	A-B Fault			A-B Fault			A-B Fault			A-B Fault			A-B Fault			A-B Fault	
	A	3.109 kA		A	5.175 kA		A	2.071 kA		A	5.175 kA		A	2.068 kA		A	4.127 kA
	B	3.109 kA		B	5.175 kA		B	2.071 kA		B	5.175 kA		B	2.068 kA		B	4.127 kA
																	
	A-C Fault			A-C Fault			A-C Fault			A-C Fault			A-C Fault			A-C Fault	
	A	3.109 kA		A	3.454 kA		A	2.763 kA		A	3.454 kA		A	2.068 kA		A	2.068 kA
	C	3.109 kA		C	3.454 kA		C	2.763 kA		C	3.454 kA		C	2.068 kA		C	2.068 kA
																	
	B-C Fault			B-C Fault			B-C Fault			B-C Fault			B-C Fault			B-C Fault	
	B	3.109 kA		B	3.454 kA		B	2.763 kA		B	3.454 kA		B	1.032 kA		B	1.373 kA
	C	3.109 kA		C	3.454 kA		C	2.763 kA		C	3.454 kA		C	1.032 kA		C	1.373 kA[/FONT]
 
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