In addition to allow control of the line active and reactive power, the UPFC provides an additional
degree of freedom. Its shunt converter operating as a STATCOM controls voltage V1 by absorbing
or generating reactive power.
Both the series and shunt converters use a Voltage-Sourced Converter (VSC) connected on the
secondary side of a coupling transformer. The VSCs use forced-commutated power electronic
devices (GTOs, IGBTs or IGCTs) to synthesize a voltage from a DC voltage source. The common
capacitor connected on the DC side of the VSCs acts as a DC voltage source. Two VSC
technologies can be used for the VSCs:
VSC using GTO-based square-wave inverters and special interconnection transformers.
Typically four three-level inverters are used to build a 48-step voltage waveform. Special
interconnection transformers are used to neutralize harmonics contained in the square
waves generated by individual inverters. In this type of VSC, the fundamental component of
voltage is proportional to the voltage Vdc. Therefore Vdc has to varied for controlling the
injected voltage.
VSC using IGBT-based PWM inverters. This type of inverter uses Pulse-Width Modulation
(PWM) technique to synthesize a sinusoidal waveform from a DC voltage with a typical
chopping frequency of a few kilohertz. Harmonics are cancelled by connecting filters at the
AC side of the VSC. This type of VSC uses a fixed DC voltage Vdc. Voltage is varied by
changing the modulation index of the PWM modulator.
The UPFC (Phasor Type) block models an IGBT-based UPFC. However, as details of the inverter
and harmonics are not represented, it can be also used to model a GTO-based UPFC in transient
stability studies.
Control System
The shunt converter operates as a STATCOM. For a description of its control system, refer to the
Static Synchronous Compensator (Phasor Type) . In summary, the shunt converter controls the AC
voltage at its terminals and the voltage of the DC bus. It uses a dual voltage regulation loop: an
inner current control loop and an outer loop regulating AC and DC voltages.
Control of the series branch is different from the SSSC. In a SSSC the two degrees of freedom of
the series converter are used to control the DC voltage and the reactive power. In case of a UPFC
the two degrees of freedom are used to control the active power and the reactive power. A
simplified block diagram of the series converter is shown below.
Simplified Block Diagram of the Series Converter Control System
