bus overvoltages on a high leg delta service

I work in a plant that has a High Leg Delta electric service, very old plant... we are having several VFD's tripping on bus overvoltage, across two lines. they are both fed from the same substation. when we measured the incoming voltage to the main panel we read 400 volts from each leg to ground with the high leg reading 763 volts. when you read phase to phase it reads 480 vac. last night electricians found condensation at the sub station and switched sides, problem went away, now its back...any ideas?

Are the VFD's driving a load that could be generating, like a downhill conveyor, this can lead to buss over voltage?

I'm having trouble picturing a "High Leg 480V delta" system. Even a corner grounded delta doesn't go 700V+ to ground.

What is it? Can you describe it?

Interesting...

https://www.google.com/search?q=Hig...e=UTF-8#q=High+Leg+Delta+electric+service+480

DickDV said:

I'm having trouble picturing a "High Leg 480V delta" system. Even a corner grounded delta doesn't go 700V+ to ground.

What is it? Can you describe it?

Click to expand...

The only way I could envision this voltage on High leg 480v delta is,
two transformers sharing two of the phases,
with the third phase on one transformer is high whilst the other is tied to earth,
either on purpose, or by a fault having blown that phase fuse and going to earth.

I am glad our supplies in NZ don't allow us to use of these arrangments

davidoeser said:

I work in a plant that has a High Leg Delta electric service, very old plant... we are having several VFD's tripping on bus overvoltage, across two lines. they are both fed from the same substation. when we measured the incoming voltage to the main panel we read 400 volts from each leg to ground with the high leg reading 763 volts. when you read phase to phase it reads 480 vac. last night electricians found condensation at the sub station and switched sides, problem went away, now its back...any ideas?

Click to expand...

Is it possible that the phase to phase voltage exceeds what you measured when some of the plant equipment is not running? I've seen this problem before in plants that have heavily loaded transformers.

Just so your not chasing the wrong problem: bus overvoltage may very well mean the internal DC bus of the VFD, not your incoming AC supply. In that case its almost always caused by motor regeneration: your controlled motor is generating power rather than sinking power. Very common problem when shutting down a system or anything that goes downhill: the energy from the momentum goes back into the VFD DC Bus and causes it to go overvoltage. Keep in mind that a VFD works by rectifying your incoming AC into its own internal DC bus, then converting that DC back to AC for the controlled motor: so 'bus overvoltage' refers to the internals of the VFD. This is frequently fixed by adding a braking resistor to the VFD. If someone has changed any of the VFD rampdown or shutdown settings, intermittent bus overvoltage faults can be a side effect of those changes.

Or it can even be as simple as the weight of the product you're moving has changed: stopping a heavier product can be regenerative. For example, I worked on one system that was fine and then we got a much heavier species of wood (or maybe is was wet too?) and we started getting these overvoltage faults when the conveyor would try to stop. I had to mess with the VFD rampdown settings to stop much slower to eliminate the overvoltage faults.

The cure for bus overvoltage is either braking resistors or lengthen the stop time.

It should be verified that this is not an incoming line voltage issue. If the line voltage is high, then the DC bus will be high. If a resistor is installed, then the chopper starts dumping bus to the resistor. If the resistor thermostat(should be one), isn't tied into a line side contactor, then high line voltage=resistor fire even if the drive isn't enabled.

Those are some weird voltages. Mathematically, they don't compute.

I have to agree that the voltages given don't make sense. Of course, if the 480V delta is truly floating, then any voltage to ground is possible. And, it can change on a moment's notice bepending on how it is induced into the power system.

I don't think there is much benefit to searching for regen voltage on the DC bus when the power to the VFD is all over the place.

Resolve the incoming power issues first before you blow something up.

I think nwboson has hit the nail. I've seen phase-to-ground voltages that high on an ungrounded delta. Inverters may (or may not-- it seems to depend on design) use ground as a reference point, and take the measurement of the DC Bus voltage over the limit.

I had the same problem with induction heaters in a previous life...

Our problem was with resistive heaters in a chemical bath eventually wearing through and shorting to ground. This would make the DC Bus voltage on the induction heaters climb right out of the limit. The VERY TEMPORARY WORKAROUND was to remove the ground from the induction unit until the faulty heater was replaced. The final fix was to install an isolation transformer at each induction heater.

EDIT: Also, keep in mind that these ground faults can be a real pain to locate, since it can happen anywhere on the bus.

Ah, the ungrounded delta. Great for submarines. Not so great for factories...

I think this is one of those cases where disconnecting any internal MOVs in the drive might be called for.

http://en.wikipedia.org/wiki/High-leg_delta

davidoeser said:

I work in a plant that has a High Leg Delta electric service, very old plant... we are having several VFD's tripping on bus overvoltage, across two lines. they are both fed from the same substation. when we measured the incoming voltage to the main panel we read 400 volts from each leg to ground with the high leg reading 763 volts. when you read phase to phase it reads 480 vac. last night electricians found condensation at the sub station and switched sides, problem went away, now its back...any ideas?

Click to expand...

I have never, in my 30+ years in the electrical industry, seen a "high leg delta" service that is anything but 240/120V 3 phase 4 wire.

I would guess that what you have is possibly a corner grounded 480V delta system where the utility has used only two transformers to get you 3 phases. Then you have lost your ground reference on the corner. So your system is going to an Open Delta condition, and the open leg is reading at some random high value that likely changes based on differences in capacitive coupling to that ground reference, but appears normal at other times. You need to shut down and call your utility IMMEDIATELY before someone gets killed!

If it is 480v Delta, I agree that 'normally', it's corner grounded, but with the voltages and description, It sounds like the stupid way most of the old 240v power is here in the US.

The Wikipedia link for high-leg delta shows it.

First, disconnect any internal EMI/RFI filters. They typically connect from +DC to ground, and can cause high voltage faults and eventually blow. I've seen this time and again on 240v systems.

The real solution is a drive isolation transformer, Delta to Wye.