Grounding issues regarding medium voltage, delta distribution system.

[rck]

I am making recommendations for the upgrade of an existing power plant and distribution system on a military base. The system consists of generators with 2.4kV output, directly connected to an ungrounded delta, 2.4kV aerial distribution system. Feeder switchgear have 50/51 phase relays. Generator protection consists of 51V, 32 and 87 devices. I am tempted to recommend grounding one phase of the delta to provide ground fault protection although I am aware of other military installations that have ungrounded 2.4kV and 7.2kV delta systems.

I am wondering if there are any compelling reasons to justify the ungrounded delta system. One complication I can see with a grounded delta system is connections of single phase transformers. Some transformers would be connected to two ungrounded phases, while the other transformers would be connected to the grounded phase and one of the ungrounded phases, although I don't know if this is a problem. Are there any other caveats, potential problems, etc. that I need to be aware of if I were to ground one phase of this system?

 

[redtrumpet]

See IEEE Std. 142 (Green Book), Section 1.4, Methods of System Neutral Grounding and Section 1.5, Grounding at Points Other than System Neutral.

Green Book seems to indicate corner grounding of the delta is used only for 600 V or below.

 

[peterb]

Think in terms of a grounding transformer (zigzag or wye/delta). You can opt for a high resistance grounded system, which retains the ability to continue in service with one ground fault while removing the overvoltage possibilities of the ungrounded system.
Check out the Post Glover website

http://www.postglover.com

for some good reference material.
If you would rather have a ground fault trip system, consider medium resoistance grounding to give enough current to operate conventional relay schemes.

 

[busbar]

In ANSI territory, 3ø corner grounding would be very unusual for medium-voltage systems. Don’t think it's covered in IEEE guides/standards.

OTOH, high–resistance grounding seems well recognized for 5kV-class systems; less for 8kV class systems. Generally suited to retrofit at lower expense than low-resistance conversion.

If you need to document your recommendations, besides the green book, applicable IEEE standards may be─
C62.92 Grounding of Synchronous Generator Systems
C62.92.4 Neutral Grounding in…Distribution
C37.101 Generator Ground Protection
It is discussed in an IEEE/PCIC paper at 203.162.7.73/ieee/htmls/disk_31/593/4231/147_153_Selection%20of%20system%20neutr.htm

In the IEEE color book series, there is often a fair number of published references in each chapter.

See thread238-3370 thread238-21929 Thread238-4513