Transformer Troubleshooting Question

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infinity

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A fellow electrician called me yesterday asking me about a transformer he was troubleshooting. Seems that the trans was sporadically blowing the primary CB. After opening the cover this is what he found:

-3 wire, 208 volt (Wye)primary/ 3 wire, 480 volt(Delta) secondary (Delta/Wye wired in reverse)
-XO bonded to the transformer case, burnt off completely.

Secondary voltages (approximate):
H1-H2= 490, H2-H3= 490, H1-H3= 490
H1,H2,H3 to transformer case=280 volts.

One question is why is there any voltage reading from H1, H2 or H3 to the transformer case if the Delta secondary is ungrounded? My only guess is that it had something to do with reading these voltages with a digital meter.

The other question is regarding the bonding jumper burning up on the XO. My guess is that it was carrying current back to the utility transformer XO secondary on the grounded metal raceway feeding the transformer. Any ideas?
 

infinity

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zog said:
Because it is not grounded, it is a "Floating ground" and depeneds on the capacitance of the system .

The secondary voltage readings were taken with the load completely removed from the secondary terminations. Is it safe to say that this is the reason the voltage reading were similar to that of a Wye secondary?


Do yo think an old solenoid type of voltage tester would have shown a different voltage to ground?
 

electricalperson

Senior Member
Location
massachusetts
infinity said:
The secondary voltage readings were taken with the load completely removed from the secondary terminations. Is it safe to say that this is the reason the voltage reading were similar to that of a Wye secondary?


Do yo think an old solenoid type of voltage tester would have shown a different voltage to ground?
did you find out the reason for the burned system bonding jumper?
 

jim dungar

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infinity said:
The other question is regarding the bonding jumper burning up on the XO. My guess is that it was carrying current back to the utility transformer XO secondary on the grounded metal raceway feeding the transformer. Any ideas?

The "0" bushing of a wye primary connected transformer should never be connected to anything, it should always be left floating (unless you are a utility). Load imbalances will negatively affect this bushing.
 

mivey

Senior Member
infinity said:
The secondary voltage readings were taken with the load completely removed from the secondary terminations. Is it safe to say that this is the reason the voltage reading were similar to that of a Wye secondary?


Do yo think an old solenoid type of voltage tester would have shown a different voltage to ground?
yes and yes
 

BAHTAH

Senior Member
Location
United States
Delta/Wye in reverse

Delta/Wye in reverse

infinity said:
A fellow electrician called me yesterday asking me about a transformer he was troubleshooting. Seems that the trans was sporadically blowing the primary CB. After opening the cover this is what he found:

-3 wire, 208 volt (Wye)primary/ 3 wire, 480 volt(Delta) secondary (Delta/Wye wired in reverse)
-XO bonded to the transformer case, burnt off completely.

IMO when you try and connect a transformer in reverse, if you have a connection (that bonding jumper) connected to ground the %Z of the windings to ground are not enough to allow you to connected the 208 as a Wye so typically the breaker would blow on the 208 supplying the transformer. The jumper most likely was blown early on and may be making contact causing the breaker to blow again. You would need to isolate the center tap of the Wye to back-feed. Just my thoughts...
 

mivey

Senior Member
jim dungar said:
The "0" bushing of a wye primary connected transformer should never be connected to anything, it should always be left floating (unless you are a utility). Load imbalances will negatively affect this bushing.
Agreed. The wye ground becomes a ground current source and will cause short circuit currents to flow through the transformer for ground faults. It also becomes a balance point for single phase loads.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
infinity said:
-3 wire, 208 volt (Wye)primary/ 3 wire, 480 volt(Delta) secondary (Delta/Wye wired in reverse)
-XO bonded to the transformer case, burnt off completely.

The other question is regarding the bonding jumper burning up on the XO. My guess is that it was carrying current back to the utility transformer XO secondary on the grounded metal raceway feeding the transformer. Any ideas?
Now I'm a believer. You cannot ground (or otherwise "neutral-ify") a Y-primary's neutral with a Delta secondary; it must be left floating.
 

infinity

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New Jersey
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This is a setup that has worked for years but since the recent addition of some new 3 phase, 480 volt equipment the nuisance tripping of the transformer main has been ongoing. From what I'm being told the secondary feeds a 3 wire three phase switchboard that only supplies 3 phase loads. Thanks for the responses. :)
 

mivey

Senior Member
infinity said:
This is a setup that has worked for years but since the recent addition of some new 3 phase, 480 volt equipment the nuisance tripping of the transformer main has been ongoing. From what I'm being told the secondary feeds a 3 wire three phase switchboard that only supplies 3 phase loads. Thanks for the responses. :)
While the secondary may only supply 3 phase loads, single phase loads on the primary can cause you problems with the grounded wye.
 
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