Flexible AC Transmission System (FACTS)

Flexible Alternating Current Transmission System (FACTS) is a static equipment used for the AC transmission of electrical energy. It is meant to enhance control ability and increase power transfer capability of the network. It is generally a power electronics based device.

FACTS is defines by IEEE as “a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability.

FACTS technology

FACTS could be connected:
· in series with the power system (series compensation)
· in shunt with the power system (shunt compensation)
· both in series and in shunt with the power system.

Series compensation

In series compensation, the FACTS is connected in series with the power system. It works as a controllable voltage source. Series inductance exists in all AC transmission lines. On long lines, when a large current flows, this causes a large voltage drop. To compensate, series capacitors are connected, decreasing the effect of the inductance.

 

Fig. 1:FACTS for series compensation- vector diagram 

Series compensation is a well established technology that is primarily used to reduce transfer reactances, most notably in bulk transmission corridors. The result is a significant increase in the transient and voltage stability in transmission systems. Series compensation is self-regulating in the sense that its reactive power output follows the variations in transmission line current, a fact that makes the series compensation concept extremely straightforward and cost effective. Series compensation is defined as insertion of reactive power elements into transmission lines and provides the following benefits:

• Reduces line voltage drops
• Limits load-dependent voltage drops
• Influences load flow in parallel transmission lines
• Increases transfer capability
• Reduces transmission angle
• Increases system stability

Thyristor controlled series capacitors add another controllability dimension, as thyristors are used to dynamically modulate the ohms provided by the inserted capacitor. This is primarily used to provide inter-area damping of low frequency electromechanical oscillations, but it also makes the whole series compensation scheme immune to subsynchronous resonance (SSR).

 

 

Fig 2. Series Compensation (Photo Coutesy: ABB)

Shunt compensation

In shunt compensation, power system is connected in shunt with the FACTS. It works as a controllable source. Shunt compensation is of two types:

1) Shunt capacitive compensation:

This method is used improve the power factor. Whenever an inductive load is connected to the transmission line, power factor lags because of lagging load current. To compensate, a shunt capacitor is connected which draws current leading the source voltage. The net result is improvement in power factor.

2) Shunt inductive compensation

This method is used either when charging the transmission line, or, when there is very low load at the receiving end. Due to very low, or no load — very low current flows through the transmission line. Shunt capacitance in the transmission line causes voltage amplification (Ferranti Effect). The receiving end voltage may become double the sending end voltage.To compensate, shunt inductors are connected across the transmission line.

 

Fig. 3: FACTS for shunt compensation- vector diagram 

Static VAR Compensator

. It helps in keeping steady state and dynamic voltages within their limits, but it does not provide any control of active power. The var output can be varied continuously between the capacitive and inductive ratings of the equipment. The necessary reactive power required for the compensation is generated or absorbed by capacitor or reactor banks. Thyristor switches are only used to control the combined reactive impedance the banks present to the system. An advanced static compensator has been recently presented . This device provide control for real and reactive power.

In general,

Benefits of Reactive Power Compensation

Reactive Power Compensation projects assist customers improve the technical reliability and economics of their transmission and distribution system

• Voltage support and improved regulation
• Increased transmission capacity
• Reduced system expansion costs
• Reduced kW and KVAr system losses
• Improved system power factor
• Improved economics of electricity supply
• Reduction of harmful harmonics
• Improved system stability.

What are FACTS devices?

FACTS stand for Flexible AC Transmission Systems. The term “FACTS” covers several power electronics based systems used for AC power transmission. FACTS solutions are particularly suitable in applications, which require one or more of the following qualities:

· Rapid dynamic response
· Ability for frequent variations in output
· Smoothly adjustable output
· Fast implementation to achieve considerable increase in transmission capacity.

FACTS involve devices such as:

· Static Var Compensators (SVC) which are a well know workhorses that have been around for the last 20 years and well utilized within Australia.
· STATCOM are new innovative devices based on utilization of high tech power electronic components, which result in a small installation footprint and very fast response times.
· Thyristor Controlled Series Compensation is a very cost-effective way to improve power transfer and counter power swings and SSR.