This paper proposes a double-fed induction machine with a flywheel energy storage system called multi-functional flexible power conditioner（FPC）. It consists of a doubly-fed induction machine（DFIM） with very large rotating inertia and a voltage-source pulse width modulation（PWM） rectifier-inverter used as an AC exciter. A 10 kW prototype of FPC is developed， and the corresponding parameters of the prototype are given. The simulation model for the prototype is built up and simulated in some operating states， such as phase modulation， subsynchronous， supersynchronous and so on. The prototype is also experimented in these proposed operating states. The simulation and experimental results show that the FPC is an effective way to compensate the unbalanced power and enhance the system stability. Satisfactory performance of the decoupled control for active and reactive power and dynamic operation is obtained.