wye-delta transformer question

[Bousman]

I just ran across a 480volt wye-delta service from my electric company. The delta is grounded at one corner. This results in Phase A reading 150v to ground, Phase C reading 150v to ground and phase B reading 0v to ground. Phase to phase on all readings is 480v. My question is: If phase B is equivalent to ground (no voltage difference) how is it that from phase A (or C) to phase B is 480 volts? This would seem to imply that phase B has voltage and if that is so why am I getting a reading of 0v to ground from phase B?

 

[augie47]

Your numbers don't seen to ring true for an intentionally corner grounded 480v system.
A true corner ground should read O on one phase to ground and 480 on the other two to ground.

 

[Bousman]

Ya, no kidding! This thing has got me stumped. I'm just going off the readings from my meter and information provided to me from the electric company. Side note: this service blew a hole through the service meter last night....good stuff!

 

[rcwilson]

Sounds like that delta is now ungrounded (floating) with B phase still grounded though a high impedance that is less than the A or C to ground impedance. (Maybe soot from the arc that blew the B-ground connection? Purely conjecture.)

The meter may draw enough microamps from A or C to pull the Vag voltage down to 150 V while you are measuring it. Vbg may jump to 330V when the meter's on Vag. If you had two meters you could monitor A-G & B-G and see what's happening.

 

[Jraef]

One common problem with corner gounded delta systems now is the increased use of electronic power systems, specifically the protection systems often built in to them. For example VFDs; 9 out of 10 VFDs are now made overseas. In places OTHER than the US and Canada, they NEVER see delta power systems, let alone grounded delta systems. So they use surge protection devices on the front-end of the VFDs to protect the diodes that are designed ONLY to be used on grounded Wye systems, i.e. 480/277V. The problem is, they use a gounded Wye connection of the capacitors on these surge protectors, and if you connect a grounded Wye system like that to a grounded Delta source, the reference voltage to ground on two of the caps jumps to line potential instead of Wye potential, i.e. the voltage that 2 of the caps see is now 480V instead of 277V and one sees zero volts. This has a tendency to pop the 2 caps in short order (because they would NEVER think to use caps rated for the higher voltage just in case, it would cost 23 cents more each). Then if one or more shorts instead of opening, you get all kinds of fun stuff happening.

So take a look around, this can be the case for VFDs, UPS systems, large 3 phase power supplies of any kind, some smaller computer or PLC power supplies, even some general machinery that has been imported from overseas where some smart ass engineer thought he might put in a surge arrestor just in case his machine ends up in Dumbunnystan somewhere where voltage is an on-again / off-again affair.

USUALLY they will have an instruction sheet that tells you something like
"WARNING! REMOVE THE SURGE PROTECTION IF USED ON A GROUNDED DELTA SYSTEM"... but nobody reads that stuff.

The quick way to start on this is to disconnect ALL of the loads on the transformer and measure again. Measure phase to phase, then phase to ground. If you still have the anomaly, you probably have a bad ground connection on the grounded phase, or a bad transformer. If the problem goes away when the load is disconnected, someone connected a formerly useful device to your system without reading the manual (or understanding what the impications were).

 

[Open Neutral]

Can you apply a test load to each phase while you measure? A KW or 2 of electric heater maybe?

 

[dana1028]

One common problem with corner gounded delta systems now is the increased use of electronic power systems, specifically the protection systems often built in to them. For example VFDs; 9 out of 10 VFDs are now made overseas. In places OTHER than the US and Canada, they NEVER see delta power systems, let alone grounded delta systems. So they use surge protection devices on the front-end of the VFDs to protect the diodes that are designed ONLY to be used on grounded Wye systems, i.e. 480/277V. The problem is, they use a gounded Wye connection of the capacitors on these surge protectors, and if you connect a grounded Wye system like that to a grounded Delta source, the reference voltage to ground on two of the caps jumps to line potential instead of Wye potential, i.e. the voltage that 2 of the caps see is now 480V instead of 277V and one sees zero volts. This has a tendency to pop the 2 caps in short order (because they would NEVER think to use caps rated for the higher voltage just in case, it would cost 23 cents more each). Then if one or more shorts instead of opening, you get all kinds of fun stuff happening.

So take a look around, this can be the case for VFDs, UPS systems, large 3 phase power supplies of any kind, some smaller computer or PLC power supplies, even some general machinery that has been imported from overseas where some smart ass engineer thought he might put in a surge arrestor just in case his machine ends up in Dumbunnystan somewhere where voltage is an on-again / off-again affair.

USUALLY they will have an instruction sheet that tells you something like
"WARNING! REMOVE THE SURGE PROTECTION IF USED ON A GROUNDED DELTA SYSTEM"... but nobody reads that stuff.

The quick way to start on this is to disconnect ALL of the loads on the transformer and measure again. Measure phase to phase, then phase to ground. If you still have the anomaly, you probably have a bad ground connection on the grounded phase, or a bad transformer. If the problem goes away when the load is disconnected, someone connected a formerly useful device to your system without reading the manual (or understanding what the impications were).

Jraef - you are a constant wealth of 'right on' information & you are well spoken - you communicate your information in a very understandable manner. Thank you