What type of generator operates above synchronous speed when excitation is lost?

An induction generator operates above synchronous speed when excitation is lost due to its design and operational principles. In essence, induction generators convert mechanical energy into electrical energy by relying on the concepts of magnetic induction.

When connected to a power system, an induction generator typically runs below synchronous speed but, if the mechanical input exceeds the synchronous speed (for instance, due to increased wind speed or other mechanical means), it can operate in a generating mode. In this state, the rotor spins faster than the rotating magnetic field produced by the stator, creating a condition where it generates electricity.

This is distinctive from a synchronous generator, which requires precise control of excitation to maintain synchronous operation. If excitation is lost in a synchronous generator, it cannot sustain its operation and will instead slip below synchronous speed, leading to a loss of power generation.

Capacitive generators and thermal generators do not fit this context as they operate based on entirely different principles and applications. Capacitive generators are not commonly used in a typical context of electrical generation, and thermal generators focus on converting heat energy to mechanical energy, primarily in thermal power plants.

Thus, the correct answer is an induction generator, as it uniquely allows operation above synchronous speed even in the absence of excitation.