POWER FLOW CONTROL USINGFACTS FACTS Flexible AC Transmission System (Facts) is a new integrated concept based on power electronic switching converters and dynamic controllers to enhance the system utilization and power transfer capacity as well as the stability, security, reliability and power quality of AC system interconnections. Uses of facts devices • Facts devices in ac lines - power flow control. - increase of transmission capability. - voltage control. - reactive power compensation. - stability improvement. - power quality improvement Transmission on a no-loss line. In the case of a no-loss line :Vs = Vr=V. Transmission results in a phase lag δ that depends on line reactance X. Active power P is the same at any point of the line : LINE COMPENSATION USING FACTS : Series compensation : Series inductance causes large voltage drops. To compensate this FACTS are connected in series with the power system. FACTS for series compensation modify line impedance: X is decreased so as to increase the transmittable active power. However, more reactive power must be provided. Shunt compensation : FACTS are connected in shunt with the power system. Reactive current is injected into the line to maintain voltage magnitude. Transmittable active power is increased but more reactive power is to be provided. FIRST GENERATION OF FACTS Static var compensator(svc) : SVC comprises one or more banks of fixed or switched shunt capacitors or reactors, of which at least one bank is switched by thyristors. SVC typically include: 1. Thyristor controlled reactor (TCR) 2. Thyristor Switched Capacitor (TSC) 3. Harmonic filter(s) 4. Mechanically switched capacitors and/or reactors (switched by a circuit breaker). Thyristor-Controlled Series Capacitor (TCSC) :TCSC is an extension of conventional series capacitors through adding a thyristor controlled reactor. Placing a reactor in parallel with series capacitor enables a continuous and rapidly variable series compensation system. SECOND GENERATION OF FACTS Static Compensator (STATCOM): The variation of reactive power is performed by means of a Voltage-Sourced Converter (VSC) connected on the secondary side of a coupling transformer. The VSC uses forced-commutated power electronic devices (GTOs, IGBTs or IGCTs) to synthesize a voltage V2 from a DC voltage source. V1 represents the system voltage to be controlled and V2 is the voltage generated by the VSC. P = (V1V2)sinδ / X Q = V1(V1 – V2cosδ) / X Static Synchronous Series Compensator (SSSC ) : • It has a voltage source converter serially connected to a transmission line through a transformer. • The active power exchanged with the line has to be maintained at zero. • Hence in steady state SSSC is functionally equivalent to infinitely variable series connected capacitor. UPFC • Universal and most flexible of all other facts devices • Combination of 3 compensators -impedance -voltage magnitude -phase angle • Consists of two voltage source converters; -series converter -shunt converter • Converters are connected by a common dc link. Upfc-schematic diagram Operating principle of upfc • Consists of two voltage source inverters (VSIs) sharing a common dc storage capacitor. • VSIs are connected to the power system through coupling transformers. – One VSI is connected in shunt to the transmission system via a shunt transformer. – The other one is connected in series through a series transformer. Components Series inverter: • It injects an AC voltage VPQ with controllable magnitude and phase angle ρ(0 ≤ ρ ≤ 360) in series with the line. • The line current flowing exchanges the real and reactive power between it and ac system • The reactive power exchanged is internally generated by the inverter. • The real power must be supplied to it. DC Link : A capacitor works as a dc link between the two voltage source inverters. Shunt converter • It supplies the real power demanded by the series device at the dc link. • It can also absorb or generate the controllable reactive power. • It is used for voltage regulation at the point of connection injecting an opportune reactive power flow into the line. • Balances the real power flow exchanged between the series inverter and the transmission line. Control functions 1) Voltage regulation• Achieved by injecting a voltage V1, in phase (or out of phase) with VS • The magnitude of the sending bus voltage Vs is increased (or decreased) Control functions 2) Series reactive compensation• It is obtained by injecting a voltage V2, orthogonal to the line current • The effective voltage drop across the line impedance X is decreased (or increased) Control functions 3) Phase shift• A desired phase shift is achieved by injecting a voltage V3 • Shifts VS by + θ or - θ while keeping its magnitude constant. Basic upfc control scheme Conclusion : With the ever increasing demand for power and limitations on the power transfer capability of conventional transmission line,modelling of facts is the most viable and secure option to meet the power demand optimally.