"Stiff" Source verses a "Weak" Source

[umrpwr]

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.

 

[dpc]

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

 

[jghrist]

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 I_sc (not I_f;-)) to I_L. 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.