3 Phase Rectifier with input from a 480 VAC Ungrounded Delta System

[dithomas]

Hello all,
I have a short video attached of the ground indication lights on the secondary bus of a 1000kVA 13.8kV deltla / 480V delta transformer with ungrounded secondary. There is a 3 phase rectifier connected to one of the 600 amp feeder breakers. The DC bus of the rectifier is to be isolated from ground.
Since I have never seen the ground light react as they are in the video before I am wondering if an unintentional ground on either the + or - dc bus would cause the ground light to react this way. The blinking is intermittent.
I have only seen one light out when there is a solid ground on one phase which would be tracked down and repaired.
Thank you in advance for your thoughts and comments.
Dan

 

[LionelHutz]

I'd expect having the bus grounded to cause the issue continually.

One of the buses being grounded would cause each phase to also be grounded 60 times per second in a rotating pattern. I'm actually doubting you'd see this type of fault in the lights.

Does the rectifier feed a VFD? If so, maybe one phase on the output of the VFD is shorted to ground. This makes which bus is grounded follow the PWM waveform of the VFD while which phase is grounded follows which diodes are conducting. Together, this might create some kind of beat frequency that cyclically becomes visible in the lights.

 

[Mbrooke]

Regarding VFDs- they do not like being fed from ungrounded systems.

 

[waross]

Do the lights flicker when the rectifier is off?
There may be an intermittent fault on one of the DC buses.
What does the rectifier supply?


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

 

[dithomas]

Hello all,
know that drives and other power electronic devices do not belong connected to an ungrounded 3 Phase 480VAC delta supply but this is a plant built in the 1960's that does bottling and these systems were common at the time and obsolete for new tech but this is the hand that has been dealt!!

The rectifier feed a 300 VDC bus on a drum type collector system for a number of small servo drives on a multi station bottle labeling machine. As you can imaging a lot of moisture and hose down when cleaning.

My question has to do with getting some thoughts about the ground indicating lights seeing an intermittent ground on the DC bus on the drum.

My thinking is that as the rectifier commutation takes place each phase of the the 480 would go to ground but never creates a fault across any two phase. There is a filter on the output of the rectifier with appears to filter and regulate the DC bus voltage to 300 volt at 6.6 A.

The blinking of the ground lights varies in the length of time they blink and how often the blinking occurs.

 

[LionelHutz]

Is there a diode rectifier or is is an active front end controller with IGBT's and reactors and such? 3-phase, 480VAC in would create a DC bus voltage of about 680VDC, not 300VDC.

I already posted this but will try again. A simple ground fault occurring on the output of a 3-phase diode rectifier would affect all 3 lights equally so you likely wouldn't see them flicker in a pattern like that.

I wouldn't be surprised if you found one of the servomotors has a phase shorted to ground. As the motor is operated, the switching of the drive transistors will continually ground and un-ground the bus.