# Flexible AC Transmission Systems (FACTS) Overview and Applications

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Flexible AC Transmission Systems (FACTS) Overview and Applications
Claudio Cañizares Department of Electrical & Computer Engineering Power & Energy Systems (www.power.uwaterloo.ca) WISE (www.wise.uwaterloo.ca)

Outline Compensation. Thyristor Control:
Thyristor Controlled Reactor–Fixed capacitor (TCR-FC). Static Var Compensator (SVC). Thyristor Controlled Series Capacitor (TCSC). Thyristor Controlled Voltage Regulator (TCVR) and Thyristor Controlled Phase Angle Regulator (TCPAR). Voltage-Sourced Converters (VSC): VSC operation. Shunt Static Synchronous Compensator (STATCOM). Series Static Synchronous Compensator (SSSC). Unified Power Flow Controller (UPFC). Interline Power Flow Controller (IPFC). Convertible Static Compensator (CSC). HVDC light. D-FACTS: DSTATCOM. DSMES. Applications. Text can be adopted from the first column on p1.

Compensation Generator-system (generator-infinite bus) model:

Maximum power that can be transmitted is Pmax = E'V2/X The operating point is defined by Pm = PG = PL   = o

Compensation Series compensation:

Compensation The system is more “stable” because:
The maximum power that can be transmitted from the generator to the system is increased. The generator operating angle o is reduced. These can be associated with an increase in decelerating energy in the system (equal area criterion), i.e. a larger stability region.

Compensation Shunt Compensation:

Compensation As with series compensation, the system is more “stable”:
The maximum power transfer for the system is larger. The generator operating angle o is smaller. The stability region is larger.

Compensation For a phase shifter (phase-shifting compensation):

Compensation Hence for  = 10o:

Compensation As in the case of shunt and series compensation, the system is more stable because: The maximum power that can be transmitted from the generator to the system is increased. The generator operating angle o is reduced.

FACTS

TCR-FC SVC and TCSC controllers are based on the following basic circuit topology: FC TCR + v(t) -

TCR-FC Each thyristor is “fired” every half cycle.
The firing angle α is “synchronized” with respect to the zero-crossing of the voltage (or current). As α increases, the TCR-FC equivalent impedance changes from inductive to capacitive.

SVC

SVC The controller is connected in shunt through a step-down transformer to reduced the voltage level on the thyristors. The thyristor firing is synchronized with respect to the bus voltage. The main objective is to control the bus voltage magnitude. Filters may be used to reduced harmonics.

SVC The steady state control characteristics are:

TCSC

TCSC Somewhat similar to the SVC controller but connected in series with a transmission line. The thyristor firing is synchronized with respect to the line current. Filters are usually not used in this case, which lead to stringent limits on the firing angle α. In steady state, the device controller operates in the capacitive region; the inductive region is only used during transient operation.

TCSC The controller has a resonant point that must to be avoided, as the controller becomes an open circuit:

TCSC The typical controls for the TCSC are:

TCSC The power flow or “slow” control is designed to maintain a constant controller impedance. The stability or “fast” control is usually designed to reduced system oscillations after contingencies. The typical use of this type of controller in practice is for the control of inter-area oscillations (e.g. North-South ac interconnection in Brazil). For simple series compensation, MSC are a much cheaper option; however, these can lead to Sub-synchronous Resonance (SSR) problems.

TCVR & TCPAR The typical topology is:

TCVR & TCPAR TCVR and TCPAR are basically ULTC and phase-shifters, respectively, with thyristor switching as opposed to electromechanical switching. Thus, these controllers have better dynamic response, i.e. smaller time constants, than the corresponding electromechanical-based devices. Controls are typically discrete, but with certain designs these can be continuous.

VSC A typical six pulse VSC with GTO switches (IGBTs are used for “low” voltage applications):

VSC To reduce harmonics, multi-pulse converters and filters are used.
For example, for a 12-pulse VSC:

VSC

VSC

VSC Pulse-width modulation (PWM) control techniques may also be used (“popular” in low voltage level applications). Beside the control advantages, this technique eliminates certain lower harmonics, although it creates high level harmonics. For example, for a 6-pulse VSC:

Fire valves when carrier and modulation signals cross
VSC Fire valves when carrier and modulation signals cross CARRIER: MODULATION:

VSC This leads to: Changing the modulation ratio, i.e. the magnitude of the modulation signal, results in changes of the ac voltage magnitudes. Shifting the modulation signal leads to phase shifts on the ac voltages.

STATCOM

STATCOM It is basically a VSC controlling the bus voltage.
The phase-locked loop (PLL) is needed to reduce problems with spurious zero voltage crossings associated with the high harmonic content of the signals for this controller, especially with PWM controls.

STATCOM Two types of controls can be implemented:
Phase control in a multi-pulse VSC: By controlling the phase angle of the voltage, the capacitor can be charged ( <  ) controller absorbs P) or discharged ( >  ) controller delivers P), thus controlling the voltage output Vi.

STATCOM PWM control in a 6-pulse VSC: the voltage output Vi can be controlled through the modulation ratio m independently of its phase angle , which in turn controls Vdc.

STATCOM The typical steady state control strategy is:
The current limits are due to the valve current limitations.

STATCOM This device is typically model using a voltage source, neglecting dc voltage dynamics and losses; this is a rough approximation. There are several applications of this type of converter, but most of them at distribution voltage levels (using IGBT technology). Additional controls may be added to effectively damp system oscillations (the same applies to SVC).

STATCOM Compared to an SVC:
The STATCOM occupies significantly less space. There is more control flexibility (e.g. PWM, more reactive support at the limits). Costs are higher due to the cost of switching devices, i.e. installation costs: MSC  10 USD/kvar SVC  USD/kvar (100 Mvar) USD/kvar (200 Mvar) STATCOM  SVC

SSSC

SSSC Similar to the STATCOM but connected in series and synchronized with respect to the line current. A phase angle  control charges and discharges of the capacitor, thus controlling the output voltage Vi. PWM controls can be decoupled or coupled:

SSSC Decoupled PWM controls:

SSSC Coupled PWM controls (better overall performance):

UPFC

UPFC This controller is basically the STATCOM and SSSC combined, with independent controls, especially for PWM: The STATCOM controls the sending-end voltage Vk and dc voltage Vdc. The SSSC controls the power on the line Pl and Ql. There is a “demo” UPFC controller in Ohio (AEP-EPRI venture).

IPFC A combination of 2 SSSCs connected independently to 2 lines is referred to as an Interline Power Flow Controller (IPFC): In this case the power on both lines can be controlled independently.

CSC A combination of 2 SSSC and 2 STATCOMS connected to 2 independent lines is referred to as a Convertible Static Compensator (CSC). In this case the control possibilities are many, as it can work as a STATCOM, SSSC, UPFC and IPFC. The CSC has been implemented in NY to relief congestion (NYPA-EPRI venture) [E. Uzunovic et al, “NYPA convertible static compensator (CSC) application phase I: STATCOM,” Proc. Trans. & Dist. Conf. and Expo., vol. 2, 2001, pp ]:

CSC

HVDC Light Based on VSCs as opposed to the current sourced converters (CSCs) used in classical HVDC: IGBTs (Insulated-gate bipolar transistors have a FET gate and a BJT switch) instead of GTOs are used as switches; have lower losses, higher frequency switching capacity, are cheaper, but have less reverse voltage blocking capacity. These switches allow using PWM controls, which yield greater control flexibility.

HVDC Light The reduced reverse voltage blocking capacity of IGBTs versus GTOs reduces the overall power capacity of the link; this is the reason for the “light” label. The overall costs of the link are lower, allowing for wider applications of this technology. Classical HVDC is most cost effective at power ranges above ~250 MW, whereas HVDC Light ratings are typically in the order of a few tens of MW (the technology current upper limits are 1,200 MW and ±320 kV). Visit for more details and actual projects.

D-FACTS FACTS applied to distribution systems are referred to as D-FACTS. Since the voltage levels are lower, IGBTS are used as the switching valves. VSC-based controllers found in practice for voltage control: D-STATCOM: Basically the same as the STATCOM but VSC is based. D-SMES: IGBT-based Super-Magnetic Energy Storage (SMES).

D-FACTS Transmission system SMES:
Used for voltage and power (damping) regulation. Limited applications given the costs.

D-FACTS D-SMES: Better performance than D-STATCOM, since it has more active power capacity. More expensive than D-STATCOM, so its application is more limited.

Applications Oscillation damping with SVC and STATCOM: N. Mithulananthan, C. A. Cañizares, J. Reeve, and G. J. Rogers, “Comparison of PSSS, SVC and STATCOM Controllers for Damping Power System Oscillations,” IEEE Transactions on Power Systems, Vol. 18, No. 2, May 2003, pp

Examples IEEE 145-bus, 50-machine test system:

Examples A line outage yields:

Applications This has been typically solved by adding Power System Stabilizers (PSS) to the voltage controllers in “certain” generators, but FACTS (e.g. TCSC, SVC, STATCOM, UPFC) may also be used to address this problem. A power oscillation damping (POD) controller is added to the voltage regulation control, like in generator AVRs.

Examples The line outage with STATCOM POD located at the optimal voltage-stability placement Bus 77 (fairly similar results are obtained for the SVC POD):

Applications TCSC: SVC: STATCOM: UPFC CSC: HVDC light: SMES:
Brazil TCSC. SVC: Toshiba/Mitsubishi projects. South Australia POD. STATCOM: UPFC AEP. CSC: NYPA. HVDC light: ABB. SMES: BPA. Wisconsin Public Service.