another grounded B phase thread

[don_resqcapt19]

I’m sorry if I was misleading. You are 100% technically correct.
But for troubleshooting it creates more trouble than it’s worth.
Like I stated earlier, it seems like every one we have built over the last 30 years we have had to send service guys to because someone didn’t understand what they were dealing with.
Those with two fuses (legal) or two poles didn't cause as much confusion as those with three pole breakers (also legal as you stated)

I have seen a number corner grounded systems, and all but one had 3 pole breakers. The one that didn't had fuses and was originally an ungrounded system. They grounded B phase at the transformer and installed dummy fuses for B phase at all of the fused disconnects.

 

[augie47]

Heed the advice given in Post #10.

 

[kchap]

Wait. We talking about the same thing?
grounded b phase is a corner grounded delta.

Is it 480 delta or 240 delta?
There isn’t a center ground like an open delta or center grounded closed delta (high leg delta)

The op may have the naming wrong. He may have a center grounded delta.

It is a corner grounded delta.

 

[MTW]

Heed the advice given in Post #10.

He already said,

The three-phase loads are fine

Until they turn them on...

 

[tortuga]

Most VFD's will not like a corner grounded Delta supply, they are built to be applied on a Wye system.

Can you elaborate on this, I have never seen a corner grounded service that I can remember, but lots of things are going VFD these days.
It seems like all a VFD does on the line side is convert AC to DC, it would be just the ratings of the caps or built in surge protection or something?
Or are they sensing L-G voltage but not current looking for a dropped phase?

 

[winnie]

Can you elaborate on this, I have never seen a corner grounded service that I can remember, but lots of things are going VFD these days.
It seems like all a VFD does on the line side is convert AC to DC, it would be just the ratings of the caps or built in surge protection or something?
Or are they sensing L-G voltage but not current looking for a dropped phase?

1) Many VFDs have L-G TVS devices and filter components on the input. These won't be happy on a corner grounded system, but these can sometimes be disconnected.

2) The rectified DC voltage on a corner grounded system has the same + to - voltage as from a wye system, but higher common mode AC to ground and higher maximum voltage to ground. (Consider the point in the AC cycle where the grounded phase is most negative and connected to the - rail by the rectifiers, the - rail is at ground potential...then half a cycle later the + rail will be at ground potential.) This means higher insulation stress between any component and surrounding grounded metal, and higher switching frequency capacitive currents from motor winding to frame.

Jon

 

[tortuga]

1) Many VFDs have L-G TVS devices and filter components on the input. These won't be happy on a corner grounded system, but these can sometimes be disconnected.

Seems like a design flaw tm me, if its a 240Volt ot 480 Volt drive it should not assume G-N voltage is Y.

2) The rectified DC voltage on a corner grounded system has the same + to - voltage as from a wye system, but higher common mode AC to ground and higher maximum voltage to ground.

interesting, that seems like a big issue, Is 'common mode AC to ground' ripple that gets thru the rectifier?

 

[MTW]

Here is a couple of threads discussing VFD's and their power sources, you may find interesting.

Corner Ground delta and Danfoss drive

Power system types supplying VFD's & DC Drives, What's the Grounding Ramifications?

Open-Delta Systems Affect Variable Frequency Drives

There are only a few VFD manufacturers that are build drives for the N American market that have jumpers that are removable for delta and ungrounded systems, as the rest of the world uses wye systems exclusively.

And a clip from a Allen Bradley MCC selection manual.

Variable Frequency Drive Unit Selection

When variable frequency drives are used, the drive installation or user manual
should be consulted to determine the existence of protection circuitry that is
referenced to ground. If these circuits exist, they are typically rated for voltages
present on solidly grounded Wye power systems. When drives with these
protective circuits are used on ungrounded, impedance grounded, or corner
grounded Delta power systems, these circuits must be removed or isolated
from ground to prevent their continuous operation and shortening of life that
will occur in the presence of higher than expected line-to-ground voltages.

 

[winnie]

interesting, that seems like a big issue, Is 'common mode AC to ground' ripple that gets thru the rectifier?

There are two types of ripple that get through the rectifier, common mode and differential mode.

The differential mode ripple is in the + to - voltage. This is the ripple that the load sees directly. This ripple is the same for wye, delta, corner delta, etc.

The common mode ripple is the average DC voltage to ground. It is what the insulation system sees. It is lowest in wye fed systems, highest in corner grounded delta systems, and unpredictable in ungrounded systems.

Jon