Transformer Troubleshooting Question

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infinity

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

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?
 
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?
Click to expand...
did you find out the reason for the burned system bonding jumper?
 
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?
Click to expand...

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.
 
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?
Click to expand...
yes and yes
 
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.
Click to expand...

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...
 
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.
Click to expand...
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.
 
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?
Click to expand...
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.
 
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. :)
 
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. :)
Click to expand...
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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