Why Does Generator Fault Current Increase When Connected to the Grid?

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Discussion Overview

The discussion revolves around the phenomenon of increased fault current from a generator when it is connected to the grid, specifically during a single line to ground fault scenario. Participants explore the implications of generator and grid interactions in fault conditions, focusing on calculations and theoretical underpinnings.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that a standalone generator produces a fault current of 30kA, while in parallel with the grid, the fault current increases to 46kA from the generator side, raising questions about the source of the additional current.
  • Another participant questions whether the discussion is conflating different types of reactances, specifically transient and subtransient reactances, in the context of short circuit calculations.
  • A participant clarifies that the calculations are based on subtransient state conditions and that the increased fault current is due to the parallel configuration of generator and grid impedances, resulting in a lower total impedance.
  • One participant expresses uncertainty about the behavior of the additional current from the generator side and whether it flows through the generator windings.
  • Another participant, who admits to limited experience with short circuit calculations, speculates on the role of the transformer and the connection of unfaulted phases contributing to the fault current.
  • A participant confirms that the fault is indeed a single phase fault and that the circuit breaker is closed during both scenarios, indicating that the increased current is solely due to the grid's presence.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms behind the increased fault current, with some agreeing on the role of impedance while others remain uncertain about the implications for generator windings. The discussion does not reach a consensus on the exact nature of the additional current from the generator.

Contextual Notes

There are unresolved questions regarding the flow of the additional fault current through the generator windings and the specific contributions of the transformer configuration to the fault current dynamics.

ge.vasiliou
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Hi guys,
For one more time I need your opinion in a kind of strange issue, which escapes my understanding.

Performing a single line to ground short circuit calculations (Low Voltage Side) in a simple Generator power plant using a step up to transformer, we can observe the two following facts:

1. For a stand alone generator (not parallel to the grid) single line to ground fault current (low voltage side) equals to 30kA. This value actually represents that behavior of the generator itself.
Gen+Only.jpg


2. If generator works in parallel to the existed grid, results of the short circuit fault current calculation equals to 86,4kA. This total fault current splits to 46kA coming from Generator side, and rest 40,5 kA come from grid side.
Gen+Grid.jpg


The question is:
Since generator as a source is capable to provide 30kA on it's own, how is it possible in a parallel to grid configuration to have 46 kA coming from the generator side?
It seems 16 kA have been added to the generator side. Is this fault current actually going thru generator windings?

PS1 : Please notice that the above results in both cases can be reached either by a simulation software or by hand calculations using symmetrical components analysis.

PS 2: Quick Description of equipment:
Generator is 1,9MVA , cosF 0,8, 400Volts, 50Hz, Star connected, solid grounding, Xd'' = 0,13 p.u and Xo=0,02p.u. Step Up Transformer is 2,5 MVA, 6% impedance, Delta connection at LV Side, Star Connection (solid grounding) at 20kV Side. Grid is 20 kV , 250MVAsc, star – solid ground.
 
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No, we don't mix transient reactances. The currents appear in my sketches are short circuit current (symmetrical rms) during subtransient state of the fault, meaning actually the short circuit current for the first 0.2 - 0.5 seconds (before the whatever protective device operates). For the calculation of short circuit current - subtransient state we need Xd'' of the generator (subtransient reactance). The link you uploaded is nice, i already have this document. Thanks anyway.

I can understand why the fault level is increased when in parallel to grid:
Generator impedance are in parallel to the grid impedance resulting in a total smaller impedance meaning a bigger fault current.

What i don't know is if this extra current of 16kA which seems to come from my Generator side is actually going thru my generator windings.
 
i never have done short circuit calculations, so won't pretend to know.

But i look forward to learning from whoever does respond !This is a single phase fault?
What is state of CB1 for your two conditions? Which side of it is the fault on?
Perhaps because of the delta connected transformer the other two phases of generator can contribute current to the faullt through the transformer windings?
In other words, CB1 connects the unfaulted phases to the fault through transformer winding.

that's just a guess.

old jim
 
Jim,
Yes it is a single phase fault. CB1 is closed - normal operating conditions - in both sketches. Your guess doesn't apply to the second scenario that current is increased in Generator Side. This increased current is due to the existence of the grid (the only difference between my two schemes).
 

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