Flexible AC Transmission Systems | FACTS

Flexible AC Transmission Systems (FACTS) – What and why?

FACTS is the acronym for “Flexible AC Transmission Systems” and refers to a group of resources used to overcome certain limitations in the static and dynamic transmission capacity of electrical networks. The IEEE defines FACTS as alternating current transmission systems incorporating power-electronics based and other static controllers to enhance control ability and power transfer ability. The main purpose of these systems is to supply the network as quickly as possible with inductive or capacitive reactive power that is adapted to its particular requirements, while also improving transmission quality and the efficiency of the power transmission system.

Features of Flexible AC Transmission Systems (FACTS)

• Fast voltage regulation,
• Increased power transfer over long AC lines,
• Damping of active power oscillations, and
• Load flow control in meshed systems,

Thereby significantly improving the stability and performance of existing and future transmission systems.
With Flexible AC Transmission Systems (FACTS), power companies can better utilize existing networks, increase the availability and reliability of their lines, and improve dynamic and transient network stability, ensuring a better quality of supply.

Influence of Reactive Power Flow on Power System Voltage

Reactive Power Compensation in Power Transmission System

Consumer loads need reactive power that varies continuously, increasing transmission losses and affecting voltage in the network. To prevent high voltage fluctuations or power failures, this reactive power must be balanced. Passive components like reactors or capacitors can supply inductive or capacitive reactive power. Fast and precise reactive power compensation, using thyristor-switched and thyristor-controlled components, can improve transmission efficiency and control, replacing slower mechanical switches.

Effects of Reactive Power Flow

Reactive power flow has the following effects:

1. Increase in transmission system losses
   • Adding to power plant installations
   • Adding to operating costs
2. Major influence on system voltage deviation
   • Degradation of load performance at under voltage
   • Risk of insulation breakdown at over-voltage
3. Limitation of power transfer
4. Steady-state and dynamic stability limits

Parallel and Series

Type	Shor-circuit level	Transmission phase angle	Steady-state voltage	Voltage after load rejection	Application
	nearly unchanged	slightly increased	increased	high	voltage stabilization at heavy load
	nearly unchanged	slightly increased	decreased	low	voltage stabilization at light load
	nearly unchanged	controlled	controlled	limited by control	fast voltage control reactive power control damping of power swings

Fig. Shows today’s most common shunt compensation devices, their influence on the most important transmission parameters, and typical applications.

Fig.: The active power/ transmission angle equation illustrates which FACTS components selectively influence which transmission parameters.

Protection and Control of FACTS

To improve redundancy management, special modules were developed to supplement the SIMATIC TDC automation system. These modules issue triggering signals to thyristor valves and take up less space than previous technology. SIMATIC TDC’s flexible interface design allows it to replace existing systems easily. This integration can be done with minimal delay, ensuring measured values from old systems are processed by the new control system. The space efficiency of SIMATIC TDC also allows parallel configuration with existing systems.

Human Machine Interface
The interface between the operator and the plant
(HMI = Human Machine Interface) is the standardized
SIMATIC Win CC visualization system, which further simplifies operation and facilitates the adaptation of graphical user interfaces to the operator’s requirements.

Hardware for Control and Protection

Siemens offers the latest in control and protection for FACTS – the tried and tested SIMATIC TDC (Technology and Drive Control) automation system. SIMATIC TDC is used worldwide in almost every industry and has been proven in both production and process engineering as well as in many HVDC and FACTS applications. Operating personnel and project planning engineers work exclusively with a standardized, universal hardware and software platform, enabling them to perform demanding tasks more rapidly. One of the main considerations in developing this automation system was to ensure the highest degree of availability of the FACTS – which is why all control and protection systems, as well as the communication links, are configured redundantly (if requested by the customer).
The new instrumentation and control technology also permits the use of a high-performance fault recorder operating at a 25 kHz sampling rate. New instrumentation and control technology reduces the period between fault recording and the printout of the fault report from several minutes (previously) to 10 seconds (now).

Converter for FACTS

LTT – Light Triggered Thyristors
Thyristors control passive components in reactive power compensation systems. Siemens’ direct light triggering system activates thyristors with a 10-microsecond light pulse at 40 milliwatts. This device includes overvoltage protection, making it self-protecting if forward voltage exceeds limits. The light pulse travels through fiber optics from the valve control to the thyristor gate. Conventional systems use electrically triggered thyristors, requiring pulses of several watts generated by nearby electronic equipment. Direct light triggering reduces electrical components in the thyristor valve by 80%, improving reliability and electromagnetic compatibility. Additionally, the new thyristor technology ensures long-term availability of electronic components for at least 30 years.
Thyristor valves from Siemens are assembled from 4-inch or 5-inch thyristors, depending on the current- carrying capacity/rated current required. Thyristor technology has been under constant development since the early 1960s. At present, thyristors can safely and economically handle blocking voltages of up to 8-kilovolts and rated currents of up to 4,200 amperes.