Flexible AC Transmission Systems (FACTS) use power electronics to control power flow and increase transmission capacity. FACTS devices include SVCs, TCSCs, TCPARs, StatComs, SSSCs, and UPFCs. A UPFC can control both voltage and impedance to regulate active and reactive power flow bidirectionally. It does this by generating reactive power with shunt inverters and injecting real power with series inverters using PWM to control voltages. This allows increasing transmission line capacity and controlling power flows.

1. Flexible AC Transmission
System Overview

2. Flexible AC Transmission
System
Alternating current
transmission systems
incorporating power
electronics-based and
other static controllers
to enhance
controllability and
increase power transfer
capability

3. Constraints on Useable
Transmission Capacity
• Dynamic:
– Transient and dynamic stability
– Subsynchronous oscillations
– Dynamic overvoltages and undervoltages
– Voltage collapse
– Frequency collapse

4. • Steady-State:
– Uneven power flow
– Excess reactive power flows
– Voltage capability
– Thermal capability

5. 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

6. US FACTS Installations
San Diego G&E/
STATCOM/100 MVA
Mitsubishi
Eagle Pass (Texas)
Back-to-back HVDC
37 MVA/ ABB
CSWS (Texas)
STATCOM/ 150
MVA / W-Siemens
Austin Energy
STATCOM/ 100MVA
ABB
AEP/ Unified Power
Flow Controller /
100 MVA/ EPRI
TVA
STATCOM/ 100MVA
EPRI
Northeast Utilities/
STATCOM/ 150 MVA/
Areva (Alstom)
NYPA/ Convertible
Static Compensator/
200 MVA
Vermont Electric/
STATCOM/ 130
MVA/ Mitsubishi

7. • Power transfer between areas can be
affected by adjusting the net series
impedance.
• Transmission line capability can be
increased by installing a series capacitor
which reduces the net series impedance.
Power Flow Control

8. UPFC

9. UPFC
• may control voltage, impedance, and angle
• impacts active and reactive power flow in line

10. Basic Operation

11. UPFC Capabilities
• Increase transmission line capacity
• Direct power flow along selected lines
• Powerful system oscillation damping
• Voltage support and regulation
• Control of active and reactive power flow
at both sending- and receiving-end

12. Operation
• Reactive power is generated or
absorbed by the shunt inverter to
control bus voltage
• Reactive power is generated or
absorbed by the series inverter to
control the real and/or reactive power
flow on the transmission line

13. Cont’d
• A portion of the real power flow on the
transmission line is drawn from the bus by
the shunt inverter to charge the DC
capacitor.
• Real power is inserted into the line
through the series inverter.

14. jXSV RV
SRP
sinS R
SR
V V
P
X
δ=
Power flow in a transmission line
δTo increase PSR, increase
and R SV V jXI= +
jXI
SV
RV
δ

15. AV BV
injV- +
AV
injV
BV
α
jXSV RV
SRP
RV′injV- + ( )sinS R
SR
V V
P
X
α δ′
′
= +
jXI
injV
SV
RV′
δ
α

16. How is Vinj created?
V
+b1
a2
a1
b2
c1
c2

17. V
+b1
a2
a1
b2
c1
c2
a1 on, b1 on, c1 off
Vab=0, Vbc=V, Vca = -V
a1 on, b1 off, c1 off
Vab=V, Vbc=0, Vca = -V
V
+a1 b1 c1
c2b2a2
V
+a1 b1 c1
c2b2a2
a1 on, b1 off, c1 on
Vab=V, Vbc=-V, Vca = 0

18. Sine-triangle PWM
0 100 200 300 400 500 600 700
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 100 200 300 400 500 600 700
0
0.5
1
Va
0 100 200 300 400 500 600 700
0
0.5
1
Vb
0 100 200 300 400 500 600 700
-1
0
1
Vab