Meaning of infinite bus

An infinite bus is an ideal bus whose voltage magnitude and frequency remain constant, whatever real or reactive power is drawn from or injected into it. It behaves like an ideal voltage source with zero internal impedance and infinite inertia.

Properties:

• Voltage and frequency are fixed; the phase angle is the reference.
• Thevenin impedance seen from the bus is zero ($Z_{th}\to0$, fault level $\to\infty$).
• Any single machine connected to it cannot change its V or f; the machine only changes how much P and Q it exchanges.

In practice, a large interconnected grid is treated as an infinite bus when the machine or load connected to it is small compared with the total system capacity.

Explanation

For a grid with Thevenin voltage $E_{th}$ and impedance $Z_{th}$, the bus voltage is

$$V = E_{th} - I Z_{th}$$

If the system is very large, $Z_{th}$ is very small (short-circuit MVA very high), so a change in current $I$ caused by one machine hardly changes $V$. Likewise, the total inertia $H$ of thousands of MW of rotating machines is so large that the power change from one machine does not change the frequency noticeably.

 E_th --[Z_th -> 0]--+---- Infinite bus
  (H -> infinity)    |     V, f constant
                     +--X_s--(G) small machine

Example: A 5 MW small hydro plant synchronised to the interconnected Nepal–India grid (hundreds of thousands of MW) sees an infinite bus. Raising its excitation only increases its reactive output; raising its turbine input only increases its active power; the grid V and f stay the same.

The concept simplifies analysis of a single machine connected to a large system (stability studies, V-curves).