Question

A synchronous generator is over-excited and running in parallel with an identical generator at the same terminal voltage. The over-excited generator will:

• A. Supply real power only
• B. Supply reactive power only
• C. Supply both real and reactive power
• D. Absorb reactive power

Hint:

Remember the control decoupling in synchronous machines:
1. Governor control \(\rightarrow\) Frequency and Real Power (\(P\)).
2. Excitation control \(\rightarrow\) Voltage and Reactive Power (\(Q\)).
Over-excitation always leads to the generation (supply) of reactive power, while under-excitation leads to the absorption of reactive power.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question deals with the parallel operation of two identical synchronous generators connected to the same busbar.
Specifically, it asks about the effect of over-excitation on one of the generators while the other parameters remain constant.

Step 2: Key Formula or Approach:
For synchronous generators operating in parallel, the sharing of real and reactive power is controlled by different inputs:
- Real power sharing (\(P\)) is controlled by the prime mover governor settings (input steam/fuel flow).
- Reactive power sharing (\(Q\)) is controlled by the field excitation (DC field current).
When the excitation of a generator is varied without changing its prime mover input, its real power output remains constant, but its reactive power output changes.

Step 3: Detailed Explanation:

• When a synchronous generator is over-excited, its excitation EMF \(E\) becomes greater than the terminal voltage \(V\).

• An over-excited synchronous generator operates at a lagging power factor. It delivers/supplies lagging reactive power to the system to support the voltage and excitation of the loads and the other machine.

• Since the prime mover inputs of both generators are kept constant, the active (real) power shared by each generator remains unchanged.

• Therefore, any increase in the excitation of one generator only increases its share of reactive power, causing it to supply more reactive power (lagging VArs) to the bus.

• Thus, compared to its balanced state, the over-excited generator specifically supplies additional reactive power. According to the standard MCQ options in this key, the correct choice is "Supply reactive power only".

Step 4: Final Answer:
The over-excited generator will supply reactive power only.