Infinite bus and national grid as 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.

National grid as an infinite bus

A national grid (e.g. Nepal's INPS connected with the Indian grid) links many generating stations and loads, with total capacity of thousands to hundreds of thousands of MW.

• Very low Thevenin impedance: Many generators and lines in parallel make $Z_{th}$ very small (fault level of several thousand MVA). So $V = E_{th} - IZ_{th}$ hardly changes when one load or generator changes its current.
• Very large inertia: The stored kinetic energy of all synchronised rotors is huge. Frequency changes as $df/dt \propto \Delta P/H_{total}$, so a change of a few MW causes negligible frequency change.
• Many controllers: Hundreds of governors and AVRs share any change in demand, so V and f are held near nominal.

Example: A 10 MW hydro plant connected to a 300,000 MW grid is only about 0.003 % of total capacity. Changing its excitation alters only its reactive output; changing its gate opening alters only its MW output; grid voltage and 50 Hz frequency remain practically constant. Hence, for that machine, the national grid acts as an infinite bus.