Advanced Technologies in Power Transmission: FACTS and HVDC

This report provides a comprehensive overview of Flexible AC Transmission Systems (FACTS) and High Voltage DC (HVDC) transmission systems. It begins by defining FACTS and its role in enhancing power system controllability and increasing power transfer capability, including a discussion of series and shunt compensation. The report details various FACTS devices such as SVCs (Static VAR Compensators) and STATCOMs (Static Synchronous Compensators), explaining their functions in voltage regulation and power factor correction. It also covers HVDC transmission, highlighting its advantages for long-distance power transmission, including its use in smart grids and in conjunction with FACTS. The report explains the components of HVDC converter stations and describes the different configurations of HVDC systems (monopolar, bipolar, and back-to-back). The report emphasizes the importance of these technologies in modern power systems, improving grid stability, increasing transmission capacity, and minimizing line disturbances.

FACTS DEVICES AND HVDC
Flexible AC Transmission Systems (FACTS)
According to IEEE,” FACTS is a power electronic based systems and other static equipment that provide
control of one or more AC transmission system parameters to enhance controllability and increases power
transfer capability.” They are electronic based devices that provides fast voltage regulation and control of
power flow thus improving transfer capability of the system. There are three types of transmission lines
classified under compensation namely; uncompensated, series compensated and shunt compensated
transmission lines.
Series compensated transmission line
FACTS is used for series compensation by modifying the impedance whereby line reactance is reduced so
as to increase real power transmitted.
Real power; Ps = V 2
| X | sinδ and reactive power ; V 2
X ( 1−cosδ )
Series compensation application includes fixed series capacitor (FSC), Thyristor controlled series
capacitor (TCSC), thyristor protected series capacitor (TPSC) and Thyristor switched series reactor
(TSSR).
Transmission Line with Shunt Compensation.
FACTS is shunted in the power system to operate as current controlled source. This compensation
scheme is of two types namely; shunt capacitive compensation (SVCs & STATCOM) for power factor
correction and shunt inductive compensation(shunt reactor)
Static VAR compensator (SVCs)
SVCs has no moving parts and provides fast response reactive power on high voltage transmission
network. It regulates p.f, harmonics and transmission system stability. Components of SVC includes
thyristor controlled reactor (TCR), thyristor switched capacitors (TSC), mechancically switched capacitor
(MSC) and harmonic filters. Simplified SVC is as shown in the figure below.

Static Synchronous Compensator (STATCOM)
It is a shunt regulating device made of power electronic devices such IGBT, or GTO that provides or
consumes reactive power. It is based on voltage source converter (VSC) and a member of FACT family.
STATCOM has a faster response in operation compared to SVCs by providing a fast-acting, exact and
modifiable quantity of reactive power. It be used to compensate for the voltage at the receiver end of the
transmission line. It operates by improving power factor and suppressing voltage fluctuations.
High Voltage DC Transmission (HVDC)
Unlike in the past where mercury arc rectifiers were used, HVDC system uses thyristor bridges to convert
electrical power from ac to dc and vice versa. HVDC transmission is most suitable for long distances with
minimum losses as compared to HVAC systems. It also enables exchange of powers for the grid systems
operating at different frequencies. In smart grids, HVDC is very vital since it allows swift and precise
control of the power transmitted between critical grid nodes. Also, HVDC systems are used in
combination with FACTS to improve the controlled flow of electric power in large grid interconnected
systems.AC power grid is stabilized, carrying capacity of the former transmission line is also increased
and line disturbances are minimized when HVDC operates in conjunction with FACTS.
HVDC Transmission system.
HVDC transmission system is more advantageous over a long distance as compared to the AC systems as
it can transmit power over 1000km without voltage compensation. In addition, no need of
synchronization since power is transmitted in dc form. The figure below shows two substation linked with
HVDC transmission system.
HVDC converter substation can operate as a rectifier of converter depending on the direction of power
flow. Components of a converter station includes AC circuit breaker, AC filters, capacitor bank, on-load
tap changer(OLTC) transformers, thyristor/IGBT converter, smoothing inductor and dc filters.
There exist three configurations of HVDC namely; mono-polar, bipolar and back-to-back HVDC
transmission system. In monopolar, two converter substations are interlinked with a single conductor and
ground return. In bipolar, two overhead conductors positive and negative with respect to ground interlink
two converter substations. Finally, in the back-to-back, asynchronous ac power systems or independent
grids are linked by dc linkage.