reasons behind this transformer current

[raadhatab]

i have an outdoor Transformer of 500KVA, 13.9 /.6 KV, The low voltage side Y is grounded by the transformer,and a feeder cable is connected to the main panel inside the building.
when i shut down the main panel, i found that there is a 0.9 amp flowing from the nuetral cable to the ground inside the building (which is the connected to the main water pipe).

can any body tell me the possible reasons behind this current and how to rectify it please

Regards

 

[ScottyUK]

There should one, and only one, connection between the earthing electrode and the system neutral. If this isn't the case then currents will flow.

The above case does not apply to networks employing PME.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

If the primary is grounded wye you may have part of the primary harmonics flowing in the secondary neutral.
Or
Although there is no load on the transformer there will still be capacitive charging currents. An unloaded transformer may have a non-sinusoidal voltage waveform which will develop harmonic components which will return from ground on the neutral. 0.9 is only about 0.19% of full load current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[raadhatab]

Thanks,

however, the transformer is away from the building, and the main building panel is grounded via the armored of the tech cable feeding the panel from the transformer,
do you mean that we have to connect the water pipe (or the grounding rods) of the building to the main transformer ground,

thanks in advance for your reply.

 

[waross]

Is your primary wye? How is the primary neutral connected?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

The neutral current is the vector sum of the phase currents. If there is current flowing from the neutral to ground, then there has to be current flowing in one or more phase windings. If the phases conductors are connected to an open breaker, then the only place the current can come from is distributed capacitance in the transformer and cable.

 

[alehman]

Sounds like this may be in Canada. I can only speak to the NEC, which requires the grounded circuit conductor to have a grounding electrode connection outside of the building, in addition to the system ground at the service disconnect. The CEC may be similar.

To avoid parallel paths and consequent current division, normally you would not want an equipment ground between the transformer and service equipment. The NEC specifically allows a continuous cable sheath to be bonded to the grounded conductor at only one point. If the sheath is connected at both ends and assuming the service equipment is properly grounded, and it is permitted by your codes, you might consider disconnecting the sheath ground at the service equipment.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[raadhatab]

- The primary of the transformer is delta and the star is solidly ground.
- Yes it is in Canada.
- Normally the outdoor transformer has its own ground, and the building has its own ground as well, by code how can we eliminate this current by connecting the building ground with the transformer ground?

any other ideas please

Thanks again

 

[davidbeach]

You're probably on a wild goose chase. At least in my experience, the utility will ground the wye point at the transformer and then take four wires to the service. Code then requires that the neutral be grounded at the service. This results in an unfortunate but near universal (at least in the universe of the NEC) double grounding of the neutral. If there is any potential difference between the two grounds there will be current flow. I doubt you'll find a code official who'd classify 0.9A as an objectionable current that might allow doing other things.

 

[rasevskii]

raadhatab:

Why are you bothered about this? Stray currents flow around all the time in ground loops if anything is energized around anywhere. I suppose that you are using a digital clamp-on meter to read this. Overkill, too much sophistication in your measurements. Harmonics, as said. If it was tens of amps then it would be worth looking at.

In energized substations, it is usual to find stray amps flowing around: in railings, water pipes, fences, ironwork, grounding wires to metal boxes...Take your clamp-on and do a few readings around the place in the ironwork, always an interesting but useless exercise.

regards, rasevskii

 

[raadhatab]

HI davidbeach, ScottyUK

Thank you for your response,

According to CEC yes two grounds are connected to neutral, one at the outdoor transformer side (four 10feet rods bonded together), and the other ground is inside the building connected to the main panel, (ground rod bonded to the water pipe0.

My concern is can I connect the Tr and building grounds together in order to eliminate this 0.9 amp, and is there a code that I can lean on?

Thanks

 

[PowerfulStuff]

While I have often seen the stray currents (up to 10's of amps) that Rasevskii mentions, with the panel off i.e. no phase current flowing, induced currents should disappear. Another potential issue is that of crossed neutrals where the neutral from another piece of equipment is inadvertently connected to the wrong supply. This can be fatal when the neutral to earth link is broken during maintenance work and thus goes up to the full applied potential. It's relatively straightforward to track down - just turn off each piece of neutral connected equipment and see when the current goes away. I've seen this quite a few times on lighting circuits, but could be anything.

Regards,
PowerfullStuff

 

[davidbeach]

Tying ground at the transformer to ground at the service could reduce, but not eliminate, the current. Some current will flow in every available path. Maybe they are already tied together. Why the concern over less than an amp of current?

 

[collies99]

Check if there are single phase PT or open corner delta PT's, or other single phse metering on the 600 bus ahead of the breaker, they could actual load current.

600/(root 3) * .9A is not a lot of power and could be real load.

GL

 

[PHovnanian]

Regardless of whether the transformer primary is delta or wye, its common practice for utilities to bond their multi grounded neutral to the transformer case and grounding system. So the primary neutral system just sees this path as one more parallel path through the ground back to the substation.

0.9A isn't much. There are a very few scenarios where this might not be tolerated. Services to diaries come to mind as one. Cows don't much like being part of the neutral return path through grounded milking machines. The solution here is to disconnect the secondary neutral bonding to the transformer case and primary neutral and depend only on the service entrance ground (your water pipe may need to be enhanced by a well engineered multi electrode premises ground system). Utilities don't like to do this (if they own the transformer). Ours insisted on warning labels at the transformer in the event the service neutral was to be disconnected. If something (like a cow) isn't being bothered by 0.9A, leave well enough alone.