Unified Power Flow Controller BY Neha Kardam M.Tech (Power System) School Of Engineering Gautam Buddha University, Gr. Noida.
ContentsFACTS devicesBenefits of FACTS devicesTypes of FACTS DevicesIntroduction to UPFCCircuit DescriptionControl schemesConclusion
FACTSFlexible AC Transmission System (Facts) isa new integrated concept based on powerelectronic switching converters and dynamiccontrollers to enhance the system utilizationand power transfer capacity as well as thestability, security, reliability and powerquality of AC system interconnections .
BENEFITS OF FACTSRegulation of power flows in prescribedtransmission routes.Reduces the need for construction of newtransmission lines, capacitors and reactors.Provides greater ability to transfer power betweencontrolled areas.These devices help to damp the power oscillationsthat could damage the equipment.
Improves the transient stability of thesystem.Controls real and reactive power flow in theline independently.Damping of oscillations which can threatensecurity or limit the usable line capacity.
FACTS DevicesName Type Main function ControllerSVC shunt voltage control ThyristorTCSC series power flow control ThyristorTCPAR series & power flow control Thyristor shuntSTATCOM shunt Voltage control GTOSSSC series power flow control GTOUPFC shunt & voltage and power GTO series flow control
INTRODUCTION TO UPFCThe UPFC is a device which can control simultaneously allthree parameters of line power flow Such "new" FACTS device combines together thefeatures of two "old" FACTS devices: 1. STATCOM 2. SSSCThese two devices are two Voltage Source Inverters (VSI’s)connected respectively in shunt with the transmission linethrough a shunt transformer and in series with thetransmission line through a series transformer, connected toeach other by a common dc link including a storagecapacitor.
The shunt inverter is used for voltage regulationat the point of connection injecting an opportunereactive power flow into the line and to balancethe real power flow exchanged between theseries inverter and the transmission line.The series inverter can be used to control thereal and reactive line power flow inserting anopportune voltage with controllable magnitudeand phase in series with the transmission line.
CIRCUIT DESCRIPTION:The basic configuration of a UPFC, which is installed between thesending-end Vs and the receiving-end VR. The UPFC consists of acombination of a series device and a shunt device, the dc terminals ofwhich are connected to a common dc link capacitor . Fig1: Basic configuration of UPFC
FUNCTIONAL CONTROL OF SHUNT INVERTER The shunt inverter is operating in such a way to inject a controllable current Ic into the transmission line. This current consist of two components with respect to the line voltage: 1. the real or direct component id 2. reactive or quadrature component iq The direct component is automatically determined by the requirement to balance the real power of the series inverter. The quadrature component, instead, can be independently set to any desired reference level (inductive or capacitive) within the capability of the inverter, to absorb or generate respectively reactive power from the line. So, two control modes are possible: VAR control mode : the reference input is an inductive or capacitive var request; Automatic Voltage Control mode: the goal is to maintain the transmission line voltage at the connection point to a reference value.
FUNCTIONAL CONTROL OF SERIES INVERTERThe series inverter injects a voltage, Vse which is controllable inamplitude and phase angle in series with the transmission line. This series voltage can be determined in different ways:Direct Voltage Injection Mode: The reference inputs are directly themagnitude and phase angle of the series voltage.Phase Angle Shifter Emulation Mode: The reference input is phasedisplacement between the sending end voltage and the receiving endvoltage.Line Impedance Emulation Mode: The reference input is animpedance value to insert in series with the line impedance.Automatic Power flow Control Mode: The reference inputs are valuesof P and Q to maintain on the transmission line despite system changes.
OPERATING PRINCIPLE OF UPFC Fig2: Single phase equivalent circuit
(a) Active power control (b) Reactive power controlFig 3: Phasor diagrams in case of active and reactive power
Control schemesPHASE-ANGLE CONTROLAdjusting the amplitude of the 90" leading orlagging output voltage makes it possible tocontrol active power . The d-q frame coordinates based on spacevectors, the d-axis current id corresponds toactive power, and so it can be controlled by theq-axis voltage Vcq. Therefore, the referencevoltage vector for the series device is given by ………..
CROSS-COUPLING CONTROLThe "cross-coupling control" has not only anactive power feedback loop but also a reactivepower feedback loop.This control scheme is characterized bycontrolling both the magnitude and the phaseangle
GENERALIZED CONTROL SCHEME This "generalized control scheme." The referencevoltage vector for the series device, isgeneralized, as follows A voltage vector produced by the two terms is inphase with the current error vector i*-i. This meansthat the UPFC acts as a damping resistor againstpower swings.
CONCLUSIONConventional power feedback control schemes make theUPFC induce power fluctuations in transient states.The time constant of damping is independent of the activeand reactive power feedback gains Kp and Kq.The feedback gain Kr with a physical meaning of resistor iseffective in damping of power swings.The proposed control scheme achieves quick response ofactive and reactive power without causing power swingsand producing steady state errors.