FACT - FLEXIBLE A.C. TRANSMISSION SYSTEM

FACT

(FLEXIBLE A.C. TRANSMISSION SYSTEM)

INTRODUCTION: 

  FACTS  A flexible alternating current transmission system (FACTS) is a system composed of static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability of the network. It is generally a power electronics -based system.

A FACT is defined by the IEEE as "a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance control-ability and increase power transfer capability.

For a time being let us forget the word FLEXIBLE, we are left with A.C Transmission System. In power system, there are various stages like Generation, Transmission & Distribution. Transmission can be done in both A.C & D.C. We are all aware of the advantages & disadvantages of A.C & D.C transmission system but all over it is seen that D.C transmission system has an upper hand over A.C transmission system due to numerous reasons.

In-spite of this in Northern Oregon, the 1^st FACTS installation was adopted at the C.J. Slatt sub-station. The answer is hidden in the word “FLEXIBLE”. FACTS technology can be used to optimize existing electrical transmission system & provides flexibility to it.

At present, electrical demands are not steady, but fluctuate widely with customer needs. Static transmission lines cannot quickly adapt to these changes which result in voltage losses to customers. Reduced voltages & blowouts can also damage equipment. FACTS technology includes software that responds to changing utility conditions & optimizing system performance.

ABOUT FACTS:

A flexible A.C. transmission system (FACTS) is a combination of hardware and software installed in power transmission substations designed to improve capacity and reliability. AC refers to alternating current the electrical power supplied by electrical utilities to homes and businesses. FACTS technology can be used to optimize existing electrical transmission systems and often prevent or delay construction of new transmission systems.

Electric utilities face continued demands for greater capacity as customers consume greater quantities of electricity. Conservation programs, higher efficiency appliances and lighting, and smarter consumer electronics can slightly reduce electrical use. Environmental concerns for overhead electrical lines, magnetic fields and damage from new construction can delay new equipment construction. A flexible AC transmission system can provide better performance of existing transmission lines, improve power quality and reduce electrical losses.

Installation of FACTS equipment in series, or in-line, with the transmission lines is used to control system voltage. Equipment designed in shunt, or parallel across transmission lines, controls current or electrical flow. FACTS include software, capacitors, voltage regulators and other equipment that manages the electrical system.

Increasing development and population density can make construction of new transmission equipment difficult. FACTS technology is compact and can be installed within the existing footprint of a substation or electric transmission facility. Existing transmission lines carry higher loads without overheating or exceeding their rated power limits. Customers typically see improved electric supply quality and reliability.

FACTS technology helps us to provide uninterrupted flow of power as is not possible in case of D.C. system & thus the installation of rectifiers, inverters; D.C. switchgear & circuit breakers are obsolete & does not cause harmonics injection in the system. It improves the capacity of existing electrical transmission system.

TYPES OF FACTS:

1. Series Compensation
2. Shunt Compensation

Examples of series compensation-

• Static synchronous series compensator (SSSC)
• Thyristor-controlled series capacitor (TCSC): a series capacitor bank is shunted by a thyristor-controlled reactor
• Thyristor-controlled series reactor (TCSR): a series reactor bank is shunted by a thyristor-controlled reactor
• Thyristor-switched series capacitor (TSSC): a series capacitor bank is shunted by a thyristor-switched reactor
• Thyristor-switched series reactor (TSSR): a series reactor bank is shunted by a thyristor-switched reactor

           Examples of shunt compensation-

• Static synchronous compensator (STATCOM); previously known as a static condenser (STATCON)
• Static VAR compensator (SVC). Most common SVCs are:
• Thyristor-controlled reactor (TCR): reactor is connected in series with a bidirectional thyristor valve. The thyristor valve is phase-controlled. Equivalent reactance is varied continuously.
• Thyristor-switched reactor (TSR): Same as TCR but thyristor is either in zero- or full- conduction. Equivalent reactance is varied in stepwise manner.
• Thyristor-switched capacitor (TSC): capacitor is connected in series with a bidirectional thyristor valve. Thyristor is either in zero- or full- conduction. Equivalent reactance is varied in stepwise manner.
• Mechanically-switched capacitor (MSC): capacitor is switched by circuit-breaker. It aims at compensating steady state reactive power. It is switched only a few times a day.

WORKING OF FACTS: 

• Series compensation

In series compensation, the FACTS are 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.

• Shunt compensation

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

• Shunt capacitive compensation

This method is used to 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.

• 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 (generally in case of very long transmission lines). To compensate, shunt inductors are connected across the transmission line. The power transfer capability is thereby increased depending upon the power equation.

ADVANTAGES OVER STANDARD TRANSMISSION SYSTEM:

A flexible AC transmission system has several advantages over standard transmission networks. Existing transmission lines have significant voltage losses over long distances. This system can boost line voltage and maintain proper supply voltage across a transmission network. Most transmission systems are interconnected networks of supply lines, where power imbalances can occasionally occur. FACTS shunt systems can balance power across parallel supply lines to optimize performance. Increasing development and population density can make construction of new transmission equipment difficult. FACTS technology is compact and can be installed within the existing footprint of a substation or electric transmission facility. FACTS technology includes software that responds to changing utility conditions, optimizing system performance.

The superiority of FACTS over present transmission system can be briefed as under. They are-

• Reliable
• Compact
• Simpler in construction & design
• Reduction in construction time
• Cheaper
• Reduced voltage loss
• Prevents harmonics injection in the system
• Maintain proper line voltage
• System efficiency is high

FACTS CONCLUSION:

Now a day, there are so many scopes for using FACT system. The population is increasing, for extra loads, FACTS is needed to increase system performance and efficiency. In this presentation, the FACTS & FACTS Controllers are discussed. In India, there is a bright chance to use FACTS Technology in National Grid system. This technology improves the power transfer capability as well as stability of the power system.

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