Another Corner Grounded Delta

[mull982]

I have a corner grounded Delta question similar to the existing post but I thought I'd start a new one as not to hijack the other.

I have a brand new 4.16kV Delta to 480V Delta transformer being used as an step down / isolation transformer for a 480V VFD. For both the delta windings the namplate of the transformer shows a dotted line coming from the bottom left and corner of the delta and stopping in the middle of the transformer. I'm not sure what this line means? Does it mean the transformer is corner grounded or does it mean it has the option to be corner grounded?

I also noticed when meggering this new unit that on the secondary all three legs had a very high resistance to ground. I confirmed this with a meter, that there was not continuity to ground on any phases. This tells me that the unit is not corner grounded or else I would have seen a short to ground when performing these tests. I always thought that the winding internally would be tied to ground but it now looks like it is up to the customer to create this corner ground?

Looking at the other post is seems like I need to take a bonding jumper from the "B" phase busbar on the secondary of the transformer and bond this to the transformer frame which is tied into the ground grid as well as high side EGC? Is the correct method of corner grounding this unit?

 

[augie47]

That is the correct method, however, you do not necessarily have to ground the secondary. If if remains ungrounded, the Code requires you to install a ground fault monitor.

 

[mull982]

That is the correct method, however, you do not necessarily have to ground the secondary. If if remains ungrounded, the Code requires you to install a ground fault monitor.

That seems simple enough. I understand that we don not have to ground the secondary but if we dont then we need a ground fault monitor which is not currently called for.

For bonding jumper between "B" phase and ground I believe we look at 250.66. My feeder is (3) 500MCM conductors so using this total cross sectional area I would require a bonding jumper of 3/0. I will use 4/0. Is this correct.

The thing that is throwing me off is that there is no OCPD at the secondary of this transformer. The transformer secondary bus has the secondary feeder cables connected directly to it, and the feeder cables are routed directly to a breaker on the line side of a VFD. Does this change the scenario any? The breaker has a ground fault setting so I'm assuming this is designed to be a grounded system.

I have heard before that a VFD is a seperately derived system? If this is true then for a ground fault on the motor will it reference the VFD as the source or the corner grounded transformer in question. Even if VFD is seperatley derived source, I still need to have ground fault protection or warning for cables feeding the vfd correct?

I should probably also look if this VFD is designed for use on a corner grouned system.

 

[jim dungar]

For both the delta windings the namplate of the transformer shows a dotted line coming from the bottom left and corner of the delta and stopping in the middle of the transformer. I'm not sure what this line means? Does it mean the transformer is corner grounded or does it mean it has the option to be corner grounded?

Transformers almost never ever come with windings connected to ground.
The dashed lines are part of the ANSI standard for nameplates.

IEEE C57.12.70 IEEE Standard Terminal Markings and Connections for Distribution and Power Transformers.​

 

[templdl]

For what it's worth most aren't aware that those dashed lines represent the vector group, the relationship on the phase angle of the primary to secondary.

 

[netaguy]

That seems simple enough. I understand that we don not have to ground the secondary but if we dont then we need a ground fault monitor which is not currently called for.

For bonding jumper between "B" phase and ground I believe we look at 250.66. My feeder is (3) 500MCM conductors so using this total cross sectional area I would require a bonding jumper of 3/0. I will use 4/0. Is this correct.

The thing that is throwing me off is that there is no OCPD at the secondary of this transformer. The transformer secondary bus has the secondary feeder cables connected directly to it, and the feeder cables are routed directly to a breaker on the line side of a VFD. Does this change the scenario any? The breaker has a ground fault setting so I'm assuming this is designed to be a grounded system.

I have heard before that a VFD is a seperately derived system? If this is true then for a ground fault on the motor will it reference the VFD as the source or the corner grounded transformer in question. Even if VFD is seperatley derived source, I still need to have ground fault protection or warning for cables feeding the vfd correct?

I should probably also look if this VFD is designed for use on a corner grouned system.

I have come across the same situation as you and was curious about the VFD's as well.

You make a good point regarding the VFD's. Did you get answer to your question?

Thanks

 

[winnie]

Most common VFDs are _not_ separately derived systems.

Presuming a grounded source supplying the VFD, a ground fault on the VFD output (or motor) will result in current flowing back to the source ground point. The thing that makes VFDs interesting in terms of faults is the rectification stage.

The rectification stage connects each input phase to one of the 'DC' rails in turn. So when phase A is the most positive, it is connected to the DC rail; then when phase B is the most positive, it takes its turn, then phase C. Similarly when phase A is the most negative, it gets connected to the DC rail, and so on for B and C. (Depending upon the filtering, the capacitor bank, and the load, there will likely be periods when none of the phases are actually connected to the rails.)

This means that a fault on the output of the VFD will be connected to each input phase in turn, depending upon where things are in the input AC cycle and the output switching cycle.

Bringing this back to 'VFD on a corner grounded delta', when a given rail is connected to a given phase, the 'rail to groun' voltage is the same as the 'phase to ground' voltage. With a neutral grounded wye source, this means that the rail to ground voltage will vary, but will remain pretty close to 1/2 VDC. But with a corner grounded delta, for 1/3 of the AC cycle each rail gets connected to a _grounded_ supply conductor. This means that while the DC voltage (voltage between + and - rails) remains the same with either a grounded wye or corner grounded delta supply to the VFD, the AC 'rail to ground' voltage is quite different.

Since you are _not_ supposed to have a current carrying path between the rails and _ground_, then as long as the insulation system is sufficient, then this AC 'rail to ground' voltage should not matter. However I would not install a VFD on a corner grounded delta system without involving the VFD manufacturer.

-Jon