Flexible AC Transmission System Overview

Flexible AC Transmission System

Alternating current transmission systems incorporating power electronics-based and other static controllers to enhance controllability and increase power transfer capability

Constraints on Useable Transmission Capacity

• Dynamic:– Transient and dynamic stability– Subsynchronous oscillations– Dynamic overvoltages and undervoltages– Voltage collapse– Frequency collapse

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

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

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 EnergySTATCOM/ 100MVA

ABB

AEP/ Unified Power Flow Controller /100 MVA/ EPRI

TVASTATCOM/ 100MVA

EPRI

Northeast Utilities/ STATCOM/ 150 MVA/

Areva (Alstom)

NYPA/ Convertible Static Compensator/

200 MVA

Vermont Electric/ STATCOM/ 130

MVA/ Mitsubishi

• 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

UPFC

UPFC

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

Basic Operation

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

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

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.

jXSV RV

SRP

sinS RSR

V VPX

Power flow in a transmission line

To increase PSR, increase

and R SV V jXI

jXI

SV

RV

AV BVinjV- +

AVinjV

BV

jXSV RV

SRP

RV injV- + sinS RSR

V VPX

jXIinjV

SV

RV

How is Vinj created?

V+b1

a2

a1

b2

c1

c2

V+b1

a2

a1

b2

c1

c2

a1 on, b1 on, c1 offVab=0, Vbc=V, Vca = -V

a1 on, b1 off, c1 offVab=V, Vbc=0, Vca = -V

V+a1 b1 c1

c2b2a2

V+a1 b1 c1

c2b2a2

a1 on, b1 off, c1 onVab=V, Vbc=-V, Vca = 0

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 7000

0.5

1

Va

0 100 200 300 400 500 600 7000

0.5

1

Vb

0 100 200 300 400 500 600 700-1

0

1

Vab