Question

Consider a system consisting of a synchronous generator working at a lagging power factor, a synchronous motor working at an overexcited condition and a directly grid-connected induction generator. Consider capacitive VAr to be a source and inductive VAr to be a sink of reactive power. Which one of the following statements is TRUE?
1. Synchronous motor and synchronous generator are sources and induction generator is a sink of reactive power.
2. Synchronous motor and induction generator are sources and synchronous generator is a sink of reactive power.
3. Synchronous motor is a source and induction generator and synchronous generator are sinks of reactive power.
4. All are sources of reactive power.

Answer 1

Correct Answer - Option 1 : Synchronous motor and synchronous generator are sources and induction generator is a sink of reactive power.

• Generally, synchronous machines are designed with over-excitation to improve stability.
• Synchronous motors are designed with leading power factors and synchronous generators are designed with lagging power factors to improve stability.
• An alternator or synchronous generator operating at a lagging power factor means it's overexcited and can supply lagging reactive power (Q) and real power (P). Hence synchronous generator operating at lagging pf is the source of reactive power factor.
• Over excited synchronous machine works at leading power factor and can supply reactive power, hence over-excited synchronous motor is source of reactive power
• The induction generator needs reactive power to function in the absence of rotor dc excitation. Hence it is sink of reactive power.