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Thread: large DC drives on large ungrounded delta-delta transformers

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    large DC drives on large ungrounded delta-delta transformers

    Not sure if this is the proper forum for this or not. Our plant was expanded in the early 90's when several large (150-450HP) DC drive systems were installed. All of these circuits were designed with ungrounded delta-delta step down and isolation transformers. I've always read that ungrounded secondary transformers feeding drive systems can lead to power quality problems(which we have). I'm just trying to figure out why this choice was made rather than using delta-wye with grounded secondary? There had to be some reason for this. Does anyone know why?

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    Quote Originally Posted by Tfret View Post
    Not sure if this is the proper forum for this or not. Our plant was expanded in the early 90's when several large (150-450HP) DC drive systems were installed. All of these circuits were designed with ungrounded delta-delta step down and isolation transformers. I've always read that ungrounded secondary transformers feeding drive systems can lead to power quality problems(which we have). I'm just trying to figure out why this choice was made rather than using delta-wye with grounded secondary? There had to be some reason for this. Does anyone know why?
    We have designed and manufactured DC systems up to about 4MVA. These were configured as 12 or 24 pulse arrangements. Not being grounded is not a power quality issue. Its a protection or safety matter. We fitted earth/ground balance detection circuits.
    Si hoc legere scis nimium eruditionis habes.

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    Besoeker, Can you please elaborate on this being a safety / protection matter?

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    Quote Originally Posted by Tfret View Post
    Besoeker, Can you please elaborate on this being a safety / protection matter?
    Without earth leakage detection, the DC busbar voltage to float to any level with respect to ground.
    That puts people and kit at risk.
    Si hoc legere scis nimium eruditionis habes.

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    Quote Originally Posted by Besoeker View Post
    Without earth leakage detection, the DC busbar voltage to float to any level with respect to ground.
    That puts people and kit at risk.
    Specifically, a restriking arc fault to ground can produce a DC offset several times the nominal voltage, threatening equipment and personnel. Unlike pure capacitive effects, the arc fault can deliver substantial current.

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    Quote Originally Posted by Tfret View Post
    Not sure if this is the proper forum for this or not. Our plant was expanded in the early 90's when several large (150-450HP) DC drive systems were installed. All of these circuits were designed with ungrounded delta-delta step down and isolation transformers. I've always read that ungrounded secondary transformers feeding drive systems can lead to power quality problems(which we have). I'm just trying to figure out why this choice was made rather than using delta-wye with grounded secondary? There had to be some reason for this. Does anyone know why?
    As a general rule, ungrounded Delta systems were popular in industrial settings because of fault tolerance. The first ground fault on the Delta system takes it from an ungrounded Delta to a corner grounded Delta and the factory keep chugging along spitting out parts or stuff. Their main DOWNSIDE was when power electronics, such as drives, came into play, because of the potentially destructive effects of that can have on the solid state power devices in the drives. So Drive Isolation Transformers were used to isolate those drives by giving them a "local" grounded Wye secondary without affecting the ungrounded Delta used in the rest of the plant.

    It seems very odd to me that someone would take the trouble to put in Drive Isolation Transformers, then NOT make them Delta-Wye. Seems like a possible oversight / mistake on someone's part. Generally when you look for a Drive Isolation Transformer from a manufacturer, they are ALL going to be Delta-Wye out of the catalog. Someone would have had to have the Delta-Delta made special, and I can't see why you would do that unless you had no idea what you were doing. The only advantage I can think of is that the secondary side of the transformer would be slightly smaller if it were Delta, so translate: cheaper. It might be something else I'm unaware of though...
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