A stiff source or stiff bus has a high short-circuit level compared with the load connected to it, so motor starts, load steps and harmonic currents cause little voltage change. Engineers use the word as shorthand for a high short-circuit ratio: a 400 kV bus fed from several large stations is stiff, while a long rural feeder or a small island grid is soft or weak. Stiffness helps voltage quality but raises fault current and the duty on switchgear.
The 33 kV bus at the smelter is stiff enough that a 2 MW motor starting direct-on-line barely flickers the lights.
The infinite bus of textbooks is stiffness taken to the limit: a source whose voltage and frequency nothing on your side can move.
Also heard as
- strong
Do not mix it up with
- Brownout: A brownout is a sustained low voltage; a stiff supply is one that resists voltage change in the first place.
Related terms
Fault current
Abnormally high current that flows when insulation fails and a short circuit or earth fault forms.
Short circuit
Unintended low-impedance path between conductors at different potentials, bypassing the load.
Sag
Brief drop in RMS supply voltage, usually to between 10 and 90 percent of nominal, lasting up to about a minute.