When an alternator connected to the bus-bar is shut down the bus-bar voltage will

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remain unchanged

Explanation: In a large power system, an infinite bus-bar is maintained at a constant voltage and frequency regardless of the power input or the number of alternators connected to it. Therefore, when a single alternator is removed or shut down, the bus-bar voltage remains essentially unchanged due to the compensating effect of the remaining parallel-connected generators.

Working Principle: The bus-bar is an 'infinite bus' with very low internal impedance. When one alternator is disconnected, the load previously supplied by that machine is automatically redistributed among the remaining synchronized alternators through the action of their automatic voltage regulators (AVR) and speed governors, which maintain the system equilibrium.

Key Formulas: $V_{bus} \approx constant$ — The definition of an infinite bus system.; $P_{total} = \sum P_i$ — Total real power is the sum of remaining active units.

Key Points: An infinite bus-bar has zero internal impedance and infinite capacity.; The voltage and frequency are governed by the aggregate system capacity, not an individual unit.; Automatic Voltage Regulators (AVR) ensure the terminal voltage remains stable during unit disconnection.; Removing one unit only alters the load sharing (power factor/excitation) of the remaining units.

Advantages: High system reliability; Voltage stability for large-scale distribution

Disadvantages: Complex synchronization requirements; Requires high-speed protection systems

Applications: Grid-connected thermal and hydro power plants; Industrial distribution substations

Additional Info: The term 'infinite' refers to the ability of the bus to maintain constant voltage and frequency independent of the load variation.. If the alternator were connected to a small, isolated system rather than a bus-bar, the voltage would decrease.

Answer: D is correct — The bus-bar acts as an infinite source where voltage is maintained constant by the remaining parallel-connected generators.

Exam Tip: Always verify if the question specifies 'infinite bus' or 'isolated load'; in exams, bus-bar refers to the grid which is essentially an infinite system.

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