4 Flexible AC Transmission System (FACTS)

Flexible AC Transmission System (FACTS) is used to control current and voltage in the electric grid.  FACTS are built using quadrature tap transformers and voltage source converters.  Both of these devices tend to use thyristors (Muhammad 890).  FACTS systems “improve system stability, power quality and the overall reliability of the power systems.” (Ahmed 49)

FACTS systems control current by being placed in parallel (shunt) with transmission lines thus they can increase current via Kirchoff’s Current Law at the node on the transmission line where they are placed.

Example:  If I_{Transmission Line} =  95 A at 0 degrees  then if I_FACTS = 5 A at 0 degrees, then I_{Transmission Line} + I_FACTS = 100 A at 0 degrees

FACTS systems control voltage by being placed in series with transmission lines thus they can increase voltage since voltages in series add.

Example:  If V_{Transmission Line} =  116 v  at 7 degrees  then if V_FACTS = 14.95 v at -71 degrees, then V_{Transmission Line} + V_FACTS = 120 volts at 0 degrees

As can be seen in the above example, a FACTS system can help maintain a phase shift of 0 degrees and thus a power factor of 1.0.  In other words, FACTS can help a transmission line maintain purely real power or at least help a transmission line to have a power factor closer to 1.0.

Thyristors
A schematic of a thyristor is shown below.  Thyristors can use a very small voltage on the gate to control very large currents.  In the on state current conducts and in the off state it acts as an open (i.e. – no current flows).  An excellent video of thyristors is below.

PROBLEMS

1.   If I_{Transmission Line} =  50.14 A at 0 degrees  then if I_FACTS = 9.86 A at 0 degrees, then I_{Transmission Line} + I_FACTS = ?
2. If V_{Transmission Line} =  210 v  at 5 degrees  then if V_FACTS = 21.25 v at -59.5 degrees, then V_{Transmission Line} + V_FACTS =  ?  Would you have a unity power factor (with slight rounding)?
3.  If V_{Transmission Line} =  220 v  at 5 degrees  then if V_FACTS = 39.2 v at -71 degrees, then V_{Transmission Line} + V_FACTS =  ?  Would you have a unity power factor (with slight rounding)?