Delta Generation vs. Ungrounded Wye Generation

[Etope]

Just looking for some opinions on Delta generation vs Ungrounded Wye generation..

When would you use a Delta configured generator over an Ungrounded Wye generator? or vice versa?

 

[rcwilson]

I would always use a grounded generator. Solid groundied wye (low resistance) at lower voltages and sizes (> 1kV, > 1MW) and high resistance or high impedance grounded wye at higher voltages and sizes.

With an ungrounded delta or wye there is a much higher risk of voltage excursions and insulation damage.

Is there a reason you want to use a delta or ungrounded wye generator?

 

[Etope]

It is ungrounded so the plant operations can have greater continuity in the event of a ground fault.

The bus that the generators feed has a Zig-Zag grounding transformer with high resistance grounding so it complies with code.

Currently there are three 0.8 MW ungrounded wye generators feeding the main bus.

I was wondering why go with ungrouded wye generators over delta generators? or vice versa?

 

[rcwilson]

Your system is a reasonable design. The zig-zag high resistance grounding system limits fault current to a low but detectable value to minimize fault damage to the generator iron. If the generators were grounded, the ground fault current would change based on the number of units on line. A drawback is the generator windings are not grounded before synchronizing.

Wye wound generators are more common. Wye wound units are cheaper at higher voltage ratings. Less insulation is needed as the winding nears the neutral point but only if the wye point is grounded. Most low voltage windings use the same insulation throughout.