Where "flywheels" in common parlance differ from synchronous condensers is that condensers run at zero torque - so they provide no active power to the system. T

Where "flywheels" in common parlance differ from synchronous condensers is that condensers run at zero torque - so they provide no active power to the system. They provide reactive power, which is needed to regulate and maintain the stability of the power system, but not active power which is used to match generation and demand or shift load. There's no actual source of energy being fed into a synchronous condenser. In a flywheel, you're drawing energy from the grid to spin up a really big mass and then storing it in rotational inertia so you can output it later very quickly. Flywheels can provide reactive power too, through their DC/AC power converters, but since you don't actually need any rotating mass to do that (recall, reactive power requires no torque), you can use a STATCOM - which is functionally like a synchronous condenser just without any moving parts.