Voltage Unbalance Vector Unknown 1

[dewone]

A customer has a utility transformer in a vault. All I can see is that the XO is left disconnected X1-X3 connects to bus duct feeding the facility.
The problem is that the voltages run A-B phase 515 V, B-C 515 V, and A-C 515. To ground the voltages get very unbalanced A 251 V, B 437, and C to ground is 260 V with no load. Loaded voltages came to A to ground 38 V, B 491 V , C 530 V phase to phase A-B 496 B-C 497 A-C 496.
I felt the system had a ground, now I think the utility didn't ground the transformer correctlly.
Any other thoughts would be appreciated?

 

[jghrist]

Is X0 grounded? Is there an equipment grounding conductor?

It sounds like the utility provided a 480 volt 3-wire service, but the customer connected some phase-ground loads which are causing severe neutral float. The unbalanced voltages to ground when no load is connected are because of different capacitance to ground for the phases.

If X0 is ungrounded, there may be an undetected ground fault when the load is connected.

Either the customer should disconnect any phase-to-ground loads or run a 4-wire service.

 

[dewone]

X0 has nothing connected to it. I can't see The H side to tell how it is connected or if there is an HO bushing.

 

[waross]

Is the system working well? Are the currents balanced?
You will normally have capacitive current to ground and insulation resistance to ground.
It sounds like your capacitance to ground is slightly unbalanced. This can be the result of something as simple as the position of the cables in the conduit. This is usually not a problem.
It may be that one load has even more unbalanced capacitance, or more likely, one or more loads have more leakage current in one phase.
You may want to ground X0 through a resistor, or a transformer winding to determine the magnitude of the ground leakage.
The leakage now is from each phase to ground and then back to the other phases. This ground current is probably distributed throughout the system and not at one place. Some of it may be concentrated at one or more motors.
You may use a small dry type transformer as a test ground. Connect a 600 volt winding from X0 to ground and limit the ground current by connecting light bulbs or other resistive load across a 120 volt winding. You may also short circuit the secondary of the test transformer through a fuse of the proper value to protect the secondary winding.
With a temperary test ground, the ground current should be negligible and the voltages to ground should be fairly well balanced. If the test transformer as showing current to ground and the voltages to ground are still unbalanced you may have a load that is developing a fault to ground.
respectfully

 

[dewone]

Thank you for your input.
The transformer is in a locked vault that we don't have access to. I took the readings at the main inside the building. The power comes through a wall via bus duct. Currents A 395 Amps B 348 Amps C 395 Amps. Taken from seperate panels and totaled.
Didn't have a large enough ammeter to fit over the Mains conductors.

 

[waross]

There are a lot of advantages to a grounded system. There are a few advantages to an ungrounded system.
I guess you have to live with what you have. It sounds as if the system is running normally. The large variations in voltages to ground may often be an indication of a system with very good insulation values.
A very small and begnign current to ground may cause a large swing in the voltage to ground on a system with excelent insulation.
The same small leakage may have little affect on a system with poor insulation to ground.
respectfully

 

[dewone]

Thank you everyone.
I'm going to say that there condition seems normal for the information on the transformer that we have. They have had a number of down stream failures. Blown fuses, motors, equipment...
So this must be a situation they have to live with, unless they change out the utility transformer to perhaps a 4 wire system.