A flexible AC Transmission system includes the use of power electronics for controlling the voltage and the power flow of the AC transmission system. The power electronics include controlled shunt and series reactive power such as static Var compensator, static compensator (STATCOM), Thyristor controlled series compensation system (TCSC) as well as other devices that can put active power between the terminals while providing reactive power control and energy storage capability.
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According to IEEE, FACTS or Flexible AC Transmission system is, an “Alternating current transmission system incorporating power electronic based and another static controller to enhance the controllability and increase in power transfer capability”
Operating principles of flexible AC Transmission system controllers:
The main principle of the flexible AC Transmission system or FACTS, controllers is to achieve the following
Voltage Regulation:
The FACTS controllers are designed to provide automatic voltage regulation in the specified operational voltage range which is basically 0.95pu to 1.05 pu. When a small disturbance occurs in the system, the controller provides an excellent means of controlling the voltage. If the controller is operating close to zero Mvar, it is able to either go capacitive or inductive with full output to control the voltage at the set point. The operation of the transformer’s OLTC can be considerably reduced by the use of FACTS controllers like SVC and STATCOM.
Oscillation Damping:
By modulating the controller’s reactive power, oscillation damping is possible in the range of 0.2-2 Hz, which is basically the range of most electromechanical modes related to the power system. The controller’s power oscillation damping plays an important role in stabilizing the system after the disturbance, it can also postpone transmission reinforcement if the controller’s distance is well optimized. The power oscillation damping of the controller has a very simple structure with gain, washout filter, lowpass filters, and lead-lag blocks essential for dynamic compensation. Flexible AC Transmission system (FACTS) controllers like UPFC are capable of modulating the real and reactive power responding to oscillation, thereby attaining a stable system.
Power flow control:
Managing the transmission of power in a grid is to establish that the demand is met while maintaining the system’s stability. Flexible AC Transmission system (FACTS) controllers are capable of adjusting the impedance of the transmission line thereby controlling the power flow through the line. This is achieved by the use of capacitive reactance or by using the phase-shifting transformer to alter the phase angle. Flexible AC Transmission system (FACTS) controllers like SSSC are capable of injecting voltage in series with the line which influences the power flow direction and rate.
Reactive power compensation:
The reactive power compensation is important for maintaining the power factor thus improving the efficiency of the transmission. Controllers are capable of providing inductive and capacitive compensation. SVC achieves this by switching the capacitors and reactors in and out of service. The STATCOM on the other hand varies the output providing reactive power support. By supplying and absorbing the reactive power flexible AC Transmission system controllers improve the power factor of the system, thereby reducing loss and enhancing transmission capacity.
Flexible AC Transmission system (FACTS) Controllers
Static Var Compensator, SVC:
The most commonly applied flexible AC Transmission system (FACTS) Shunt controller is a static var compensator or SVC. It functions by absorbing and generating reactive power for controlling the magnitude of the system voltage to a preset value. The reactive power output can be changed very quickly and this acts as a great advantage for the AC system during transient and dynamic faults. It also assists in stabilizing the system voltage by decreasing and increasing the system voltage rapidly during any overvoltage or Undervoltage scenario.
There are different configurations of SVC but it mostly uses a thyristor-controlled reactor, thyristor-switched capacitor, harmonics filter with switched breaker, and fixed capacitors as basic branches.
Static Synchronous Compensator, STATCOMS
A static synchronous compensator STATCOM is a power regulating device, which is based on voltage source converter VSC technology. It is basically used to maintain the system voltage of AC and enhance the system stability. It performs the same functions as an SVC and because of its much smaller size, it is mostly used for applications in industry. The improvement of integrated gate commutated thyristor and insulated gate bipolar transistor IGBTs has made possible the use of pulse width modulation in VSCs.
The core of the STATCOM technology is an AC/DC voltage-sourced converter VSC providing the compensation current ‘I’ associated with the voltage ‘V0’ which is injected by the converter into the system. The current of the STATCOM either leads or lags the voltage by 90 degrees, thus acting as a synchronous condenser with no mechanical inertia.
SVCs v/s STATCOMs
| Attribute | SVC | STATCOM |
| Semiconductor Device | Thyristor | IGBTs or other high-power devices with turn-off capability |
| V/I Characteristic | Superior overvoltage performance | Superior undervoltage performance |
| Application | Bulk transmission systems, historically in industrial applications | High/medium/low voltage (T&D) |
| Reactive Power Range | Branches optimally designed for any range | Naturally symmetrical; asymmetrical ranges achieved with hybrid STATCOM/TSC/TSR |
| Short Circuit Level Requirement | hrated/SCC > 3–4a (lower values require advanced control strategies) | Virtually any Qrated/SCC |
| Valve Reaction Time (The inherent switching frequency of the valves) | 0.5 half-cycle | (1–2 ms) |
| Low-Frequency Harmonics | Higher content due to TCR harmonic generation | Negligible content if properly controlled |
| High-Frequency Harmonics (>30th) | Low content | Very low content but still needs to be analyzed |
| Power Quality (flicker, voltage sags, load balancing, active filters) | Good capability for voltage sags | Superior performance at fast load variations; active filter capability (when properly dimensioned) |
| Availability | High (>99%) | High (>99%) |
| Footprint | Larger depending on the rating and number of branches | Small; larger for hybrid STATCOMs |
| Losses | Lower total losses than STATCOM at full capacitive/reactive operation | Lower no-load losses than SVC at 0 Mvar |
| Renewables and Distributed Generation | More difficult to comply with some Grid Codes | Easier integration for achieving Grid Code compliance |
| Technology Status | Mature with limited scope for valve improvements; well-known among utilities | Technology is mature but still improving; increasing number of applications, particularly at lower voltages |
TCSC thyristor-controlled series capacitor:
The TCSC is a conventional fixed series capacitor that is in parallel with a thyristor-controlled reactor, similar to the ones in SVCs but with one reactor used for each of the three phases, which means the controlled reactor is not connected to the hv bus by means of coupling transformer. Metal oxide varistors are normally used for the protection of the series capacitors when the current passing through the capacitor bank increases as a result of a short circuit.
TCSC is meant for an application that requires sophisticated, continuous, and fast control of the series impedance of a transmission line. Each thyristor valve is triggered twice per cycle.
Static synchronous series compensator SSSC
It consists of a voltage source converter connected in series with the transmission line. It injects a voltage in series with the line which emulates either a capacitor or a reactor, defining the degree of series compensation. Since it injects a voltage with the line current in the quadrature instead of modulating the line current, it can proficiently operate with a wide range of reactive power. It also provides superior performance compared to thyristor-based series controllers.
Unified power flow controller:
Combined shunt series compensation has the ability to virtually decrease the transmission line length by using series compensation and control the line charging by shunt compensation. In a steady state, these compensations increase the surge impedance loading SIL of the transmission line, which permits an increase in the capacity, also controlling the power flow by controlling the phase angle. This type of compensation improves the reactive power, voltage control, oscillation damping, and dynamic load flow controls.
The unified power flow controller provides better control and flexibility for the AC power system. Active and reactive power is dependent on voltage, impedance, and phase angle of local and remote ends. UPFC develops control over these parameters. The UPFCs are capable of acting as a STATCOM and or SSSCs for a joint control strategy. It can inject active power in the line from shunt connected VSC in the STATCOM into a series connected VSC. Thus it can be used in both steady and dynamic states to improve power system performance.
Advantages of Flexible AC Transmission System Controllers
- The flexible AC Transmission system controllers can provide continuous control of reactive power.
- Some flexible AC Transmission system controllers can provide continuous control power flow in AC lines.
- Simultaneous control over both active and reactive power is possible via the flexible AC Transmission system (FACTS) controller
- Quick response by controllers to change in AC power system in less than a cycle.
- The flexible AC Transmission system controllers can be inserted disconnected, and reinserted with zero limitations.
Application of Flexible AC Transmission System (FACTS)
- FACTS devices can be used to enhance the transmission line’s capacity by increasing the surge impedance loading with series compensating devices.
- FACTS devices are extensively used for maintaining the voltage level within operational limits during peak load hours or other disturbances.
- FACTS devices can be used for both active and reactive power flow control to achieve better load sharing between parallel feeders or lines.
- Used in industrial power systems to improve efficiency and reliability.
- For incorporating the intermittent renewable energy sources in the grid it is the use of FACTS devices that maintain the variability and improve the grid stability.
