Worst case Fault currents from Generator Decrement Curve

[Wfg42438]

Hello Everyone,

I was reading this article on generator decrement curves created by cummins and its was quite interesting.

On Figure 3 of the article, they show that in there example for a terminal fault at around 0.1 seconds both the line to line and L-G fault currents are larger than the 3Ph fault current.

https://www.stamford-avk.com/sites/stamfordavk/files/AGN005_D.pdf

I have seen articles on the L-G being largest but was surprised to see the LL current also being larger than the 3Ph current.

I wanted to ask in practice how is this accounted for and how common is this case?

I am thinking in practice your limiting factor is the LG fault current since the generator will have a maximum time it can sustain for under a LG fault which for the example in the article is much smaller than for the LL case.

Is the idea to simply use protection that operates prior to the time where this phenomenon occurs?

if anyone can shed some light on this it be much appreciated.

 

[dpc]

I didn't read the article, but since it's about generator decrement curves, I'm assuming these are transient values, calculated for a fault at the generator terminals. The decrement curves generally assume no change to generator excitation, but in the real world there are often fault current support systems (current boost) that will impact the resulting fault current. For longer duration faults, the problem becomes the lack of fault current since the synchronous reactance be greater than 100%, meaning the fault current is less than the rated current. The type of excitation also has an impact.

Larger medium-voltage generators are almost always grounded through some type of impedance to limit the SLG current for external faults. In addition, large generators are normally connected directly to the delta winding of a unit transformer to reduce the chances for SLG fault that is seen by the generator. For solidly-grounded machines, the generator can be braced to withstand the forces associated with full magnitude SLG faults (if specified this way).

LL faults are much less common than SLG. For very large machines, isolated phase bus duct is used to connect the generator to its unit transformer. This greatly reduces the risks of LL faults.

For generators breakers connected directly to the generator terminals, there are specific ratings and designs intended to allow the breaker to clear these faults at the very high X/R conditions that exist at the generator.