Flexible AC transmission system
FACTs devices are alternatives to traditional electric grid solutions and improvements, where building additional Transmission Lines or Substation is not economically or logistically viable.
While other traditional equipment can accomplish all of this, FACTs devices utilize Power Electronics that are fast enough to switch sub-cycle opposed to seconds or minutes.
[1] While HVDC focused on conversion to DC, FACTs devices used the developed technology to control power and voltage on the AC system.
The most common type of FACTs device is the Static VAR Compensator (SVC), which uses Thyristors to switch and control shunt capacitors and reactors, respectively.
When AC won the War of Currents in the late 19th century, and electric grids began expanding and connecting cities and states, the need for reactive compensation became apparent.
As operating a transmission line only at it surge impedance loading (SIL) was not feasible,[2] other means to manage the reactive power was needed.
Synchronous Machines were commonly used at the time for generators, and could provide some reactive power support, however were limited due to the increase in losses it caused.
In particular, shunt capacitors switched by circuit breakers provided an effective means to managing varying reactive power requirements due to changing loads.
This required either a careful study of the exact size needed,[4] or accepting less than ideal effects on the voltage of a transmission line.
Similar to a vacuum tube, the mercury-arc valve was a high-powered rectifier, capable of converting high AC voltages to DC.
As the technology improved, inverting became possible as well and mercury valves found use in power systems and HVDC ties.
Arc valves continued to dominate power electronics until the rise of solid-state semiconductors in the mid 20th century.
[6] As semiconductors replaced vacuum tubes, the thyristor created the first modern FACTs devices in the Static VAR Compensator (SVC).
[8] The thyristor dominated the FACTs and HVDC world until the late 20th century, when the IGBT began to match its power ratings.
Power flow must be calculated and controlled at each node (substation bus) to ensure the grid design and topology itself does not prevent generated electricity from reaching loads,[10] as when Transmission Lines reach dozens to hundreds of miles in length, they add significant impedance and voltage drop to the system.
Given two buses, each with their own voltage magnitude and phase angle, and connected by a Transmission Line with an impedance, the current flowing between them is given by[11]
Combining these two equations gives the real and reactive power flow as a function of voltages and impedance.
The goal shunt compensation is to connect a device in parallel with the system that will improve voltage and enable larger power flow.
TCRs produce large amounts of harmonics and require Filter Banks to prevent adverse effects to the system.
Series Compensation devices change the impedance of the Transmission Line to increase or decrease power flow.
Traditionally this is controlled by generators, however in large grids this becomes ineffective for managing power flow between distant buses.
The most straightforward phase angle compensation device would be to replace the tap changer on PAR with thyristors to switch portions of the winding in and out, forming a Thyristor-Controlled Phase-Shifting Transformer (TCPST).
Instead, this idea is expanded to replace a Quadrature Booster with a device referred to as a Thyristor-Controlled Phase-Angle Regulator (TCPAR), also known as a Static Phase-Shifter (SPS).
By linking the two sets of Power Electronics through a DC bus, typically by using GTO Thyristors or IGBTs, a TCPAR can be formed.
With the DC bus providing power from the shunt portion to the series portion, the device functions as a Phase-Angle regulator, however with the DC bus isolating the two side, the STATCOM can control the shunt voltage or the SSSC can control line impedance.
