How Does a Synchronous Condenser Start Up?

"As the load on a synchronous motor increases, the stator current I_a increases regardless of excitation. For under and over excited motors, the power factor (p.f.) tends to approach 1 with increase in load. The change in power factor is greater than the change in I_a with increase in load.

The magnitude of armature current varies with excitation. The current has large value both for low and high values of excitation. In between, it has minimum value corresponding to a certain excitation. The variations of I with excitation are known as V curves because of their shape.

For the same output load, the armature current varies over a wide range and so causes the power factor also to vary accordingly. When over-excited, the motor runs with leading power factor and with lagging power factor when under-excited. In between, the power factor is unity. The minimum armature current corresponds to unity power factor."

"In electrical engineering, a synchronous condenser (sometimes synchronous capacitor or synchronous compensator) is a device identical to a synchronous motor, whose shaft is not connected to anything but spins freely.^[1] Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid's voltage, or to improve power factor. The condenser's installation and operation are identical to large electric motors."

http://en.wikipedia.org/wiki/Synchronous_condenser

Usually it is just a synchronous motor, which has what is called an "amortisseur" winding built into it that is used to dampen torque pulsations. But since it is essentially a squirrel cage induction machine built onto the synchronous machine, it is used to accelerate it to over 90% speed, then the synchronous exciter is engaged and the motor pulls in.

I have seen one synchronous motor that was instead accelerated with a "pony motor", a smaller motor mechanically linked to the shaft to accomplish the same task, then de-coupled once it is no longer needed. That one was almost 100 years old though, I don't know if anyone does that any more.

Thanks for your answers,...but How is the SC starting up using fRequency converter or solid started ? SC may be 200MVA, and high inertia

All about Synchronous Machines

You may find your answer here:

A Short Course on Synchronous Machines and Synchronous ...

[PDF] SLAC Synchronous Condenser

Think of a synchronous condenser as a generator without a prime mover or as a synchronous motor without a load, they are both same. Typically they are called into service because there is a low-voltage condition, that usually precludes any form of across the line starting especially for a machine of that size since the voltage drop during the initial ramp-up would be excessive.

The simplest, cheapest, and least disruptive to system stability method is to use a small (relatively speaking) motor sized to overcome the windage and friction losses of the accelerating machine along with an on-the-fly automatic synchronizer coupled to the excitation controller. A control system or VFD sized to start the small motor will cost much less than one sized for 200 MVA!