FLOATING GROUND 480 VAC 2

[AlejandroZuniga]

Line 1 to ground is 500vac, Line 2 to ground is 480vac and Line 3 to ground is 30vac. Why or how does this happen, my supervisor mentioned something about "floating ground in the facility"

 

[waross]

To make it more fun it is possible that the voltages will change depending on what machinery is operating.
Every conductor has some insulation resistance to ground.
Every conductor has some capacitance to ground.
These are often fairly well balanced, but the resultant current is very low.
It doesn't take much to upset the balance and drop one phase voltage close to ground potential.
If possible isolate the issue by dropping out each feeder in turn until you find the responsible feeder.
When the feeder is located drop out loads one by one until the load or circuit is located.
And, the issue may be on the utility side of your facility.
Readings like that are often a sign of developing failures.
You can leave. It may never fail.
If it does fail catastrophically, it may be easily located by the smoke and fire.

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

 

[oldfieldguy]

Everything WAROSS said, plus - finding a phase a very low voltage to ground, possibly zero, is not unusual in ungrounded systems. In fact some systems have a phase deliberately grounded as normal operating conditions, but since that isn't your case, disregard it.

One grounded phase is a fault condition and should be treated as such, even though the equipment may operate normally for a long period. However, if it's an intermittent fault, it can create overvoltage spikes that will wreak havoc with your system, blowing out weakened insulation in a seemingly random fashion. Because of this, locating the cause of a grounded phase is a priority.

However, if the cause of the first grounded phase is not located, a ground on a second phase will be, in effect, a phase-to-phase fault with a path through your grounding system, and that opens up a whole different set of cases, the easiest of which is one of the faulted circuits tripping off due to overcurrent.

old field guy

 

[ViCH]

What about load? Measurements was taken with or without load? How much current You see on each phase?

Viktor
Electrical Technician II

 

[AlejandroZuniga]

Thank you guys, and the load is a 20HP 3 phase motor connected to a VFD PLC system. The issue i am having is that the VFD is faulting out at the set threshold of 815VAC. However when the load is on (Motor) the VFD runs at a constant 640VAC which is wrong because when i measure the actual output of the VFD on my meter it is 480VAC which is the same reading i get phase to phase on my incoming power from the MCC panel. Right now we are running by keeping the load on the system but there is other things the circuit does that dosen't require the load odd the motor. However when i take the load off the BUS VAC begins rapidly climbing to the threshold of 815 VAC

 

[ViCH]

So the topic - is about floating ground voltage on VFD outputs? If so, then I think, U may have some problem with capacitors or controller itself in VFD module.

Viktor
Electrical Technician II

 

[waross]

You can easily build a grounding device.
Three 2 KVA, 480 Volt primary, lighting transformers connected in wye:delta.
The delta voltage is unimportant.
Connect 2 x 1000 Watt, 240 Volt strip heaters in series from the wye point to ground.
At least one of the heaters should be mounted on standoff insulators to increase the insulation to ground.
This will balance your voltages against unbalanced leakage and will limit ground fault current to about 4.16 Amps.
Either a voltage relay or a current relay and an alarm is strongly suggested and may be a code requirement.

By the way, are those voltages that you quoted at the MCC or at the output of the VFD?


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

 

[waross]

Are you measuring the DC bus voltage with an AC meter?
Are you measuring the output of the VFD with a multi-meter?

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

 

[oldfieldguy]

Ah-- VFD issue.

Had that issue with one of my stations. The old part of the facility was fed from an ungrounded delta transformer secondary. VFD's kept tripping on overvoltage. The VFD manufacturer offered to sell us isolation transformers for each drive, which would have basically put a grounded wye at each VFD.

I opted to change out the transformer that fed the old plant, installed a grounded wye secondary, the VFD issues went away.

old field guy

 

[AlejandroZuniga]

Thanks guys finally got it fixed you guys were a lot of help, fhe guys had just installed that VFD brand new powerflex 700 and the F.L.A was set too low as well as parameter 42 was wrong for the load it was running. But it is back up and running !