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"Stiff" Source verses a "Weak" Source

"Stiff" Source verses a "Weak" Source

"Stiff" Source verses a "Weak" Source

(OP)
On a similar note to a previous posting regarding faults and short-circuits another question was discussed regarding what constitutes a "stiff" source of fault current. Again opinions and thoughts were divided as were the responses.

Some defined a 'stiff" source in terms of bus voltage and ratio of short-circuit to load current with a ratio greater than 5 being considered a stiff source. While others preferred to define the "stiffness" of the source in terms of source impedance.

I would be interested in others opinion on this subject and how they would qualitatively define what they consider to be a "stiff" source of fault current. Thanks for your input.
 

RE: "Stiff" Source verses a "Weak" Source

How do you define a fast car versus a slow car?  They are relative descriptors, meaningless without context.   

RE: "Stiff" Source verses a "Weak" Source

One breakdown is the current distortion limits of IEEE Std 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems.  In this standard, the limits are different depending on the ration of Isc (not Ifwinky smile) to IL.  Ranges being <20, 20<50, 50<100, 100<1000, and >1000.

Stiff systems (high short-circuit current in relation to load current) can absorb higher harmonic currents without causing harmful harmonic voltages.

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