12 Pulse T-Resistance Grounding

[Ranch]

NGR circuit on the secondary of a 4.2KV/480V 12 pulse 500kVA transformer. Let through per specification is to be 15A. Load is a 12 pulse AC VFD (VSI)

My experience up to now has been strictly with 12 pulse systems & DC Drives with let through at 1A. All work, but there there are circulating currents between the secondary windings due to a T Resistor circuit for the NGR. An example set up is with 2 x 120 ohm resistors, one connected to the zig zag derived neutral from the DELTA, and the other to the WYE secondary neutral. The ends of these resistors are then tied together and connected to a 277 ohm resistor that is then bonded to the frame ground. Hence the ?T? and the 1A let through in the event of a bolted fault. It is reported to me that if individual NGR circuits from each secondary are directly bonded to frame ground, circulating currents run through the frame and cause problems, problems I do not know the nature of.

I am cautioned that if the T resistor network isn't of a high enough resistance, circulating current can burn up the zig zag on the delta secondary winding.

So - discussion items that have come up in my office:

Deliver a system with 1A let through by interpreting the 15A specified as a maximum only and simply chose a value under this (only because we know the 1A works) but this is a rather large deviation from spec.

Let the delta float and use an NGR on the WYE secondary only. If we bolted one leg of the DELTA to ground, would we expect to see a fault current component flow through the NGR we could use for relaying?

Thanks in advance for anyones thoughts,

 

[jim dungar]

If this is a delta connected primary, it does not need a zig-zag transformer. In fact if a zig-zag is provided on the primary it will act as a ground reference point for the entire 4.4kV primary.

A neutral grounding resister on the 480Y secondary does not need to be connected to any "neutral" connection on the primary.

 

[winnie]

I do not know enough about these systems to answer the OP's questions, and want to learn more.

For others who are curious, this transformer has _two_ secondaries, one wye, the other delta. The reason for the two secondaries is that there will be a phase angle difference between them, so in essence you get _six_ phases, two three phase sets with the normal 120 degrees between each phase, but the two sets are 30 degrees apart. When you feed these _6_ phases to the rectifier bridge, you get 12 rectification pulses per AC cycle, rather than the normal 6 seen with a conventional 3 phase rectifier.

Essentially this is a very large DC source, used to supply power to a variable frequency drive.

At issue is how to properly ground the secondaries of this transformer. They are using 'high impedance' grounding rather than 'solid bonding', and they have _two_ neutrals to consider. They have the 'real' neutral of the wye secondary, and they have the 'derived' neutral obtained using a zig-zag transformer on the delta secondary.

The 'let through' is the amount of fault current expected from a ground fault, limited by the resistance. In the event of a ground fault, a reasonably well defined current will flow through the resistance, much lower than the fault current seen in the event of a fault on a solidly bonded system.

Sometimes high impedance grounding is used to permit the system to continue to operate in the event of a ground fault; another reason to use this system is to reduce the energy dissipated in a fault; you detect the ground fault and shut down anyway, but without tremendous fault currents.

Remember one of the reasons for 'grounding': to stabilize the voltage of the system relative to Earth and surrounding bonded metal? Well the higher the resistance, the lower the 'let through' current, and the less the stabilization effect. There will always be capacitive coupling between the system and earth, and the grounding resistance must be low enough to permit this capacitive current to flow and not throw system voltages all over the place.

Small 'zig-zag' transformers are used to derive a neutral on delta secondaries. These zig-zag transformers are much smaller than the secondaries that they protect, sized only to carry fault current.

Thus endith the limit of my firm knowledge.

My _guess_ is that the specification is calling for a high let through current because the VFD will generate _lots_ of capacitively coupled current. The VFD probably operates at a pretty high switching frequency, and there is probably a ton of current flowing at the switching frequency through the capacitance between motor winding and frame. If the grounding resistance is too great, then the entire secondary winding will be 'bouncing around' at the switching frequency.

I further _guess_ that if the wye secondary only were to be bonded, that the voltage on the delta side of things would be properly stabilized via the rectifier; I've not thought through the potential fault current path on the delta secondary. Consider the DC rail voltage versus ground when you feed a three phase rectifier with a corner grounded delta; the entire DC rail cycles up and down at line frequency, and my _guess_ is that if the grounding resistance were connected to the wye only, that you would still see fault current in the event of a fault on the delta.

You might consider using a transformer set that has a wye secondary and a full size zig-zag secondary, rather than using a delta. This should give the appropriate phase shifting, and solid neutrals an both secondaries. Again, this suggestion is a WAG, and outside of my comfortable range of knowledge.

Now I'll sit back and learn from others....

-Jon

 

[jim dungar]

That's what I get for answering questions at night. I forgot completely about a three-winding transformer. My apologies.

 

[Ranch]

Thanks for the replies -

More notes: This type of application is to keep our miners safe. I often come across two NGR criteria:

1. Long wall controls, permissible area systems (MSHA term for hazardous locations), and underground mining submerged pumps are examples where I expect to see 1.0 to 1.5A max ground fault current with relaying set to trip at 50-60% of this value. Thus on a 480V system, a simplified relaying circuit may consist of a 277 ohm NGR to feed a narrow band potential relay connected fail safe to either a contactor, shunt or under voltage release device upstream in a mine power centre.
2. For controls in non-hazardous locations we see 15A let through typically. So for the example above, change the NGR resistance to about 18.4 ohms, etc. (it is this resistance I have not tried ?.. yet)

At this time we have employed and continue to test derivations of 12 pulse resistance grounding with DC (dual DC converters with inter-bridge reactance to parallel the outputs to a DC motor). However, as many of you know DC motors are rather cost prohibitive whereas AC controls for 12 pulse systems (XFMRS/VFDS) have become much more available and reasonable than ever before.

Trying to calculate and predict all the current components in these circuits I believe is difficult. Example ? think downstream all the way to premature motor failure due to parasitic currents coupling motor bearing to frame. Perhaps we need to consider grounding brushes in the AC motors in these systems also.

But my expertise is more in the Newton Metres arena (sorry folks, that?s foot pounds) and applications these circuits drive. So my intentions are to record all I can to gain useful, empirical knowledge of multiple resistance ground circuits to insure the reliability of the overall systems, especially the GF relay circuit components.

I need a calculus junkie to do some predictions, I can measure the results. Someday we can build a spread sheet.

And one last opinion ? and it is just my opinion I invite comments on: It seems it is so competitive out there any more that some see it more cost effective to install controls and adapt after the fact rather than be burdened with the cost of this type of engineering up front, especially when it is done before a project is awarded and someone else gets your work.