Flexible AC Transmission Systems (FACTS) Overview and Applications Claudio CañizaresDepartment of Electrical & Computer EngineeringPower & 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.
Compensation • Generator-system (generator-infinite bus) model:
Motivation • In steady state: • 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 • Steady state:
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 ois 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 • Steady state:
Compensation • As with series compensation, the system is more “stable”: • The maximum power transfer for the system is larger. • The generator operating angle ois 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 ois reduced.
TCR-FC • SVC and TCSC controllers are based on the following basic circuit topology: FC + v(t) - TCR
TCR-FC • Each thyristor is “fired” every half cycle. • The firing angle αis “synchronized” with respect to the zero-crossing of the voltage (or current). • As αincreases, the TCR-FC equivalent impedance changes from inductive to capacitive.
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 • 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 • 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:
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 • 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 50-60 USD/kvar (100 Mvar) 35-40 USD/kvar (200 Mvar) • STATCOM 1.2-1.3 SVC
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 • This controller is basically the STATCOM and SSSC combined, with independent controls, especially for PWM: • The STATCOM controls the sending-end voltage Vkand dc voltage Vdc. • The SSSC controls the power on the line Pland 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. 1139-1143]:
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.