FACTS Technology and Controllers

1. FACTS Technology

Definition: Alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability.

2. Need for FACTS Technology

• To control the power flow by controlling the line impedance, voltage magnitude and phase angle. In an AC system, the power flow depends on the bus voltages (V1, V2), the reactance of the line (X), and the phase angle difference between the buses (δ). The power flow in AC system (Fig. 1) can be expressed by,

• To increase the power transfer (loading) capability of the line considering dielectric, stability limits and thermal limits.

  • • • Dielectric limit: From an insulation point of view, the normal operation voltage can go up to +10% of the nominal voltage or even higher. So, the FACTS technology can be used to ensure acceptable dynamic and transient overvoltages and power flow conditions.

      • Stability limit: Many stability issues that limit the transmission capacity such as transient, dynamic, and steady-state stability, frequency and voltage collapse and sub-synchronous resonance. The FACTS technology can be used to overcome the stability limits.

      • Thermal limit: The thermal capability of an overhead line is a function of ambient temperature, wind conditions, conductor conditions and ground clearance. FACTS controllers can enable a line to carry power closer to its thermal rating by providing added flexibility.

• Increase the system security by managing cascading blackouts and damping electromechanical oscillations of power systems.

3. Types of FACTS Controllers

The FACTS controllers can be divided into 4 categories:

• Series controller

• Shunt controller

• Combined series-series controller

• Combined series-shunt controller

3.1. Series Controller

• The series controller could be a variable impedance or power electronics-based variable source. The principle of a series controller is to inject the voltage in series with the line (i.e., variable impedance multiplied by current flow through the line). Fig. 2 shows the series controller.

• If the injected voltage has a 90-degree phase relationship with the line current, then the series controller only supplies or consumes reactive power. For any other phase relationship, the real power exchange is possible.

• The series controller can control the line voltage and hence it can control the current/power flow directly. So, the series controller is more effective for controlling the current/power flow and damping the oscillations.

• The examples for series controllers are static synchronous series compensator (SSSC) and thyristor-controlled series capacitor (TCSC).

3.2. Shunt Controller

• The shunt controller could be variable impedance or power-electronics based variable source. The principle of shunt controller is to inject the current into the system at the point of connection (i.e., variable shunt impedance connected to the line voltage causes variable current flows). Fig. 3 shows the shunt controller.

• If the injected current has a 90-degree phase relationship with the line voltage, then the shunt controller only supplies or consumes reactive power. For any other phase relationship, the real power exchange is possible.

• The shunt controller behaves like a current source and it is more effective for controlling the voltage by injecting the reactive current (both lagging and leading) and to damp the voltage oscillations. Also, the shunt controller provides a node bus independent of the line connected to the bus.

• The examples for shunt controllers are static synchronous compensator (STATCOM) and static VAR compensator (SVC).

3.3. Combined Series-Series Controller

• The combined series-series controller could be controlled in a coordinated manner or unified controller. Fig. 4 shows the unified series-series controller. 

• The combined series-series controllers provide series reactive compensation for each line independently. 

• The unified series-series controller transfers the real power among the lines via DC power link and it is called as Interline power flow controller (IPFC) which helps to balance both the real power and reactive power flow of the line.

• The series-series controller can be used for dynamic & contingency overloads and to bypass high fault currents.

3.4. Combined Series-Shunt Controllers

• The combined series-shunt controller could be controlled in a coordinated manner or a unified power flow controller (UPFC). Figs. 5 & 6 show the combined series-shunt controllers. 

• The shunt controller injects current into the system and the series controller injects voltage in series with the line. In the unified series-shunt controller, the real power exchange is possible between series and shunt controllers via a DC power link.

• The combined series-shunt controller is effective for both current/power flow control and line voltage control. 

• An example of a combined series-shunt controller is the unified power flow controller (UPFC). It is the combination of static synchronous series compensator (SSSC) & static synchronous compensator (STATCOM) that are coupled via common DC link.

References

• Narain G. Hingorani, Laszlo Gyugyi, “Understanding FACTS concepts and technology of flexible AC transmission systems”, 1999.