Alternator Terminal Voltage on Infinite Bus Bar

When an alternator is connected to an infinite bus bar, the bus bar acts as a source of constant voltage and constant frequency, irrespective of the power or reactive power exchanged with it. Think of an infinite bus bar as an electrical grid so large that the actions of a single generator connected to it have a negligible effect on its overall voltage and frequency.

The terminal voltage of any machine connected to an infinite bus bar is dictated by the bus bar's voltage. The bus bar maintains its voltage at a fixed value.

Effect of Increasing Alternator Excitation

The excitation of an alternator controls the strength of its magnetic field and, consequently, the magnitude of the internally generated electromotive force ($E_f$). When the excitation is increased, the generated EMF ($E_f$) increases.

However, the alternator is connected to an infinite bus bar, which means its terminal voltage ($V_t$) is fixed by the bus bar voltage. The relationship between the generated EMF ($E_f$), terminal voltage ($V_t$), and the synchronous impedance ($Z_s$) of the alternator determines the power flow, specifically the reactive power flow, between the alternator and the bus bar.

Let's consider the phasor relationship in a simplified manner (ignoring resistance for clarity):

• $V_t$ is fixed by the infinite bus bar.
• Increasing excitation increases $E_f$.
• The difference between $E_f$ and $V_t$ across the synchronous reactance ($X_s$) drives the reactive power flow ($Q$).

If $E_f > V_t$, the alternator supplies reactive power to the bus bar (it acts as a generator of reactive power). If $E_f < V_t$, the alternator absorbs reactive power from the bus bar (it acts as a motor of reactive power). If $E_f = V_t$ (and assuming no load angle, $\delta=0$), the reactive power flow is zero.

When the excitation is gradually increased, $E_f$ increases. Since $V_t$ is held constant by the infinite bus bar, the increase in $E_f$ leads to the alternator supplying more reactive power to the bus bar. However, this change in reactive power flow does not cause the bus bar voltage (and thus the alternator's terminal voltage) to change because the bus bar is "infinite" – it can absorb or supply any amount of reactive power without its voltage changing.

Terminal Voltage Response

Therefore, even though increasing the excitation increases the internal generated voltage ($E_f$) and changes the reactive power output, the terminal voltage ($V_t$) of the alternator, being clamped by the infinite bus bar, remains unchanged.

In summary, when an alternator is connected to an infinite bus bar and its excitation is gradually increased, the terminal voltage will remain unaltered because it is fixed by the voltage of the infinite bus bar.