JNTUK IV-II EEE

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FLEXIBLE ALTERNATING CURRENT
TRANSMISSION SYSTEMS
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Presented By :
HARI MADHAVA REDDY. Y (Ph.D)., M.Tech
Assistant professor
UNIVERSAL COLLEGE OF
ENGINEERING AND TCHNOLOGY
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

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Contents
Unit–I: Introduction to FACTS
Unit–II: Voltage source and Current source converters
Unit–III: Shunt Compensators–1
Unit–IV: Shunt Compensators–2
Unit V: Series Compensators
Unit–VI: Combined Controllers

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Learning Objectives
 To learn the basics of power flow control in transmission
lines using FACTS controllers
 To explain operation and control of voltage source converter.
 To understand compensation methods to improve stability
and reduce power oscillations of a power system.
 To learn the method of shunt compensation using static VAR
compensators.
 To learn the methods of compensation using series compensators
 To explain operation of Unified Power Flow Controller (UPFC).

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Learning Outcomes
 Understand power flow control in transmission lines using FACTS
controllers.
 Explain operation and control of voltage source converter.
 Analyze compensation methods to improve stability and reduce
power oscillations in the transmission lines.
 Explain the method of shunt compensation using static VAR
compensators.
 Understand the methods of compensations using series
compensators.
 Explain operation of Unified Power Flow Controller (UPFC).

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Text Books:
1. “Understanding FACTS” N.G.Hingorani and L.Guygi, IEEE
Press.Indian Edition is available:––Standard Publications, 2001.
Reference Books:
1. “Flexible ac transmission system (FACTS)” Edited by Yong Hue
Song and Allan T Johns, Institution of Electrical Engineers,
London.
2. Thyristor-based FACTS Controllers for Electrical Transmission
Systems, by R.MohanMathur and Rajiv k.Varma, Wiley

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Unit–I:
Introduction to FACTS
Power flow in an AC System – Loading capability limits – Dynamic
stability considerations – Importance of controllable parameters – Basic
types of FACTS controllers – Benefits from FACTS controllers –
Requirements and characteristics of high power devices – Voltage and
current rating – Losses and speed of switching – Parameter trade–off
devices.
Unit–II:
Voltage source and Current source converters
Concept of voltage source converter(VSC) – Single phase bridge
converter – Square–wave
voltage harmonics for a single–phase bridge converter – Three–phase
full wave bridge converter– Three–phase current source converter –
Comparison of current source converter with voltage source converter.

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Unit–III:
Shunt Compensators–1
Objectives of shunt compensation – Mid–point voltage regulation for
line segmentation – End of line voltage support to prevent voltage
instability – Improvement of transient stability– Power oscillation
damping.
Unit–IV:
Shunt Compensators–2
Thyristor Switched Capacitor(TSC)–Thyristor Switched Capacitor
– Thyristor Switched Reactor (TSC–TCR). Static VAR
compensator(SVC) and Static Compensator(STATCOM): The
regulation and slope transfer function and dynamic performance –
Transient stability enhancement and power oscillation damping–
Operating point control and summary of compensation control.

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Unit–II:
Voltage source and Current source
converters
2.1. Concept of voltage source converter(VSC)
2.2. Single phase bridge converter
2.3. Square–wave voltage harmonics for a single–phase
bridge converter
2.4. Three–phase full wave bridge converter
2.5. Three–phase current source converter
2.6. Comparison of current source converter with voltage
source converter.

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Conventional thyristor device has only the turn-on control, its
turn-off depends on the current coming to zero as per circuit and system
conditions. Devices such as the Gate Turn-Off Thyristor (GTO),
Integrated Gate Bipolar Transistor (IGBT), MOS Turn-off Thyristor
(MTO), and Integrated Gate-Commutated Thyristors (IGCT), and similar
devices have turn-on and turn-off capability. These devices (referred to
as turn-off devices) are more expensive and have higher losses than the
thyristors without turn-off capability.
However, turn-off devices enable converter concepts that can
have significant overall system cost and performance advantages. These
advantages in principle result from the converters, which are self-
commutating as against the line-commutating converters. Compared to
the self-commutating converter, the line-commutating converter must
have an ac source connected to the converter, it consumes reactive
power, and suffers from occasional commutation failures in the inverter
mode of operation.

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Therefore, unless a converter is required to function in the two lagging-
current quadrants only (consuming reactive power while converting
active power), converters applicable to FACTS Controllers would be of
the self-commutating type.
There are two basic categories of self commutating converters:
1. Current-sourced converters in which direct current always has one
polarity, and the power reversal takes place through reversal of DC
voltage polarity.
2. Voltage-sourced converters in which the de voltage always has one
polarity, and the power reversal takes place through reversal of DC
current polarity.
Conventional thyristor-based converters, being without turn-off
capability, can only be current-sourced converters, whereas turn-off
device-based converters can be of either type.

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2.1. Concept of voltage source converter(VSC)
For reasons of economics and performance, voltage-sourced
converters are often preferred over current-sourced converters for
FACTS applications.
The direct current in a voltage-sourced converter flows in
either direction, the converter valves have to be bidirectional, and
also, since the dc voltage does not reverse, the turn-off devices need
not have reverse voltage capability; such tum-off devices are known
as asymmetric turn-off devices.
Thus, a voltage-sourced converter valve is made up of an
asymmetric tum-off device such as a GTO with a parallel diode
connected in reverse. Some turn-off devices, such as the IGBTs and
IGCTs, may have a parallel reverse diode built in as part of a
complete integrated device suitable for voltage-sourced converters.
However, for high power converters, provision of separate diodes is
advantageous.

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Converter topologies that are suitable for supplying and
consuming reactive power only and not for converting active power.
The internal topology of converter valves is represented as a
box with a valve symbol inside. On the de side, voltage is unipolar
and is supported by a capacitor. This capacitor is large enough to at
least handle a sustained charge/discharge current that accompanies
the switching sequence of the converter valves and shifts in phase
angle of the switching valves without significant change in the dc
voltage.
The dc capacitor voltage will be assumed constant. It is also shown
on the dc side that the dc current can flow in either direction and
that it can exchange dc power with the connected dc system in
either direction.

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Shown on the ac side is the generated ac voltage connected to the ac
system via an inductor. Being an ac voltage source with low internal
impedance, a series inductive interface with the ac system (usually
through a series inductor and/or a transformer) is essential to ensure that
the de capacitor is not short-circuited and discharged rapidly into a
capacitive load such as a transmission line. Also an ac filter may be
necessary (not shown) following the series inductive interface to limit
the consequent current harmonics entering the system side.
Voltage-sourced converter generates ac voltage from a de voltage. It is,
for historical reasons, often referred to as an inverter, even though it has
the capability to transfer power in either direction. With a voltage-
sourced converter, the magnitude, the phase angle and the frequency of
the output voltage can be controlled.

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SINGLE-PHASE FULL-WAVE BRIDGE
CONVERTER OPERATION

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SINGLE PHASE-LEG (POLE) OPERATION

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SQUARE-WAVE VOLTAGE HARMONICS FOR A
SINGLE-PHASE BRIDGE

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THREE-PHASE FULL-WAVE BRIDGE CONVERTER
A three-phase, full-wave converter with six valves, (1-
1') to (6-6'). The designated order 1 to 6 represents the
sequence of valve operation in time. It consists of three
phase-legs, which operate in concert, 120 degrees apart.
The three phase-legs operate in a square wave mode, in
accordance with the square wave mode described in the
section above and with reference to Figure 3.3. Each
valve alternately closes for 180 degrees as shown by the
waveforms Va, Vb, and Vc. These three square-wave
waveforms are the voltages of ac buses a, b, and c

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Fundamental and Harmonics for a Three-Phase Bridge Converter

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When the transfer is from one turn-off device to another turn-off device, i.e., 1to 4 or
4 to 1 during inverter operation with unity power factor, it is essential to delay the
turn-on for at least several tens of microseconds following the turn-off of the
complementary device is complete. This is to ensure that there is no chance of
simultaneous conduction of devices 1 and 4, which represents a direct short circuit
across the dc bus capacitor. It is important to note from the sequence of transfers
shown above with reference to Figure 3.5, that except for unity power-factor inverter
operation, all the current transfers are from a device to a diode, or from a diode to a
device. Thus there is very little risk of shoot-through in a topology that involves one
conduction pulse per half-cycle.

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BASIC CONCEPT OF CURRENT-SOURCED CONVERTERS
The current-sourced converter may be based on diodes, conventional thyristors
or the tum-off devices.
Thus there are three principal types of current-sourced converters

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THREE-PHASE FULL-WAVE DIODE RECTIFIER
The diode rectifier is very useful as a low-cost source of de
power obtained from an available ac source.
It is assumed that the de side inductance is very large and
therefore the de current is constant. The circuit consists of six valves,
numbered 1 to 6, the number sequence conveying the order of the
current transfer and the de output voltage. The current commutates
from valve to valve to turn it into an ac current.

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CURRENT-SOURCED VERSUS
VOLTAGE-SOURCED CONVERTERS

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