Flexible AC transmission systems (FACTS) incorporate power electronics and static controllers to enhance controllability and increase power transfer capability in AC transmission systems. FACTS include static VAR compensators (SVC), thyristor controlled series compensators (TCSC), and static synchronous compensators (StatCom). SVC and StatCom are shunt devices that control reactive power flow, while TCSC and unified power flow controllers (UPFC) are series devices that control active power flow. Properly placing and coordinating multiple FACTS devices can improve power flow, increase usable transmission capacity, dampen oscillations, and provide other grid support functions. However, their control interactions must be analyzed using electromagnetic transient models to predict high frequency dynamics in large power systems

1. Flexible AC Transmission Systems:
Placement, Control, and Interaction
EPRI/NSF Workshop on Global Dynamic Optimization

2. EPRI/NSF Workshop on Global Dynamic Optimization
Flexible AC Transmission Systems
Alternating current
transmission systems
incorporating power
electronics-based and other
static controllers to enhance
controllability and increase
power transfer capability

3. EPRI/NSF Workshop on Global Dynamic Optimizationer
Flexible ac transmission system composed of a
static equipment. The Flexible Alternative
current transmission is used for AC
Transmission of electrical energy and increase
the power transfer capability. FACT is define by
IEEE as a power electronics based system and it
is provide one or more ac transmission system
are control and increase power transfer
capability.

4. EPRI/NSF Workshop on Global Dynamic Optimization
FACTS Controllers
• Static VAR Compensator - SVC
• Thyristor Controlled Series Compensator - TCSC
• Thyristor Controlled Phase Angle Regulator - TCPAR
• Static Synchronous Compensator - StatCom
• Solid State Series Compensator - SSSC
• Unified Power Flow Controller - UPFC

5. EPRI/NSF Workshop on Global Dynamic Optimization
SVC

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StatCom
• shunt device
• lower rated components since only carry a fraction of
the line current
• impacts bus voltage and reactive power support

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SSSC
• series device
• must have higher rated transformer and devices
• impacts active power flow

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UPFC
• combination of StatCom and SSSC
• may control voltage, impedance, and angle
• impacts active and reactive power flow in line

9. UPFC Topology
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10. EPRI/NSF Workshop on Global Dynamic Optimization
Series compensation
In series compensation, the FACTS is
connected in series with the power system. It
works as a controllable voltage source.
Shunt compensation
In shunt compensation, power system is
connected in Shunt (parallel) with the FACTS.
It works as a controllable Current source

11. EPRI/NSF Workshop on Global Dynamic Optimization
Steady-State
Power Flow Control
• UPFC
• SSSC
• TCSC
These devices can affect active power flow

12. EPRI/NSF Workshop on Global Dynamic Optimization
Useable Transmission Capacity
• Dynamic:
– Transient stability
– dynamic stability
– Dynamic overvoltage and under voltages

13. Power flow in ac
transmission line
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t
t
g
t
t
b ij
g
ij
b ∆
∂
∂
Λ+∆
∂
∂
Λ=∆α

14. EPRI/NSF Workshop on Global Dynamic Optimization
• Conventional eigenvalue analysis cannot
predict the high frequency self-modes of the
several FACTS devices in a large power
system network.
• High frequency control interactions among
the several FACTS devices must be checked
using an EMTP-type program
• A promising technique is based on the use of
high frequency eigenvalue calculation using
the different FACTS

15. EPRI/NSF Workshop on Global Dynamic Optimization
Series controllers
• low loop impedance - the series controllers
may experience a very strong interaction, and
therefore these controllers must be designed
in a coordinated way - the main linking
variable among the series controllers is the
ac current
• high loop impedance - no control interactions
may be expected among series controllers

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Dynamic Control
• transient stability improvement
• inter-area oscillation damping
• subsynchronous resonance

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HVDC
• HVDC converters in a large network will
not experience control interactions if the
transference impedances between their
commutation busbars are high. This
means that, in this case, the dc control
design of each station can be based
exclusively on the Short-Circuit Ratio
(SCR) at its ac connection point.

18. Any Queries ? ? ?
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