FLEXIBLE AC TRANSMISSION SYSTEM FACTS

Flexible transmission system is akin to high voltage dc and related thyristors developed designed to overcome the limitations of the present mechanically controlled ac power transmission system.  Use of high speed power electronics controllers, gives 5 opportunities for increased efficiency.

1. Greater control of power so that it flows in the prescribed transmission routes. 
2. Secure loading (but not overloading) of transmission lines to levels nearer their required limits. 
3. Greater ability to transfer power between controlled areas, so that the generator reserve margin- typically 18 % may be reduced to 15 % or less. 
4. Prevention of cascading outages by limiting the effects of faults and equipment failure. 
5. Damping of power system oscillations, which could damage equipment and or limit usable transmission capacity. 

Flexible system requires tighter transmission control and efficient management of inter-related parameters that constrains today’s system including:

1. Series impedance- phase angle. 
2. Shunt impedance- occurrence of oscillations at various frequencies below rated frequency. 

This results in transmission line to operate near its thermal rating. E.g. a 1000kv line may have loading limit 3000-4000Mw .but the thermal limit may be 5000Mw.