Unified Power Flow Controller (Phasor Type)Implement phasor model of three-phase unified power flow controllerLibraryFACTS/Power-Electronics Based FACTSDescription

The Unified Power Flow Controller (UPFC) is the most versatile member of the Flexible AC Transmission Systems (FACTS) family using power electronics to control power flow on power grids [1]. The UPFC uses a combination of a shunt controller (STATCOM) and a series controller (SSSC) interconnected through a common DC bus as shown on the figure below.Single-line Diagram of a UPFC and Phasor Diagram of Voltages and Currents

P=V2V3sinX,Q=V2(V2V3cos)XThis FACTS topology provides much more flexibility than the SSSC for controlling the line active and reactive power because active power can now be transferred from the shunt converter to the series converter, through the DC bus. Contrary to the SSSC where the injected voltage Vs is constrained to stay in quadrature with line current I, the injected voltage Vs can now have any angle with respect to line current. If the magnitude of injected voltage Vs is kept constant and if its phase angle with respect to V1 is varied from 0 to 360 degrees, the locus described by the end of vector V2 (V2=V1+Vs) is a circle as shown on the phasor diagram. As is varying, the phase shift between voltages V2 and V3 at the two line ends also varies. It follows that both the active power P and the reactive power Q transmitted at one line end can be controlled. The UPFC controllable region in the P-Q plane is the area enclosed an by an ellipse as shown on the figure below.Controllable Region for a 100 MVA UPFC connected on 500 kV, 200 km line

This figure was obtained with a 100 MVA UPFC controlling active and reactive power at one end of a 500 kV, 200 km transmission line. The following parameters have been used: Line: length = 200km; reactance = 0.35 /km System voltage: 500 kV infinite sources V1 and V3; V1=1.0 pu, 0 degree; V3= 1.0 pu,7.22 degrees Series and shunt converter rating: 100 MVA Series converter: nominal injected voltage = 10% of nominal line-to-ground voltage (28.9 kV); impedance (transformer leakage reactance and filters) = 0.15 puWith V3 lagging V1 by 7.22 degrees, the natural power flow without compensation is 450 MW or 50% of the line surge impedance loading (SIL=900 MW). With an injected voltage Vs = 0.1 pu any operating point inside the larger ellipse can be obtained and active power can be varied by approximately +/- 300 MW.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 SystemThe shunt converter operates as a STATCOM. For a description of its control system, refer to theStatic 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

The series converter can operate either in power flow control (automatic mode) or in manual voltage injection mode. In power control mode, the measured active power and reactive power are compared with reference values to produce P and Q errors. The P error and the Q error are used by two PI regulators to compute respectively the Vq and Vd components of voltage to be synthesized by the VSC. (Vq in quadrature with V1controls active power and Vd in phase with V1 controls reactive power). In manual voltage injection mode, regulators are not used. The reference values of injected voltage Vdref and Vqref are used to synthesize the converter voltage.The UPFC block is a phasor model which does not include detailed representation of the power electronics. You must use it with the phasor simulation method, activated with the Powergui block. It can be used in three-phase power systems together with synchronous generators, motors, dynamic loads and other FACTS and Renewable Energy systems to perform transient stability studies and observe impact of the UPFC on electromechanical oscillations and transmission capacity at fundamental frequency.Dialog Box and ParametersThe UPFC parameters are grouped in three categories:Power data,Control parameters (shunt converter), andControl parameters (series converter). Use theDisplaylistbox to select which group of parameters you want to visualize.Power Tab

Nominal voltage and frequencyThe nominal line-to-line voltage in Vrms and the nominal system frequency in hertz.Shunt Converter ratingThe nominal rating of the shunt converter in VA.Shunt Converter impedanceThe positive-sequence resistance R and inductance L of the shunt converter, in pu based on the nominal converter rating and nominal voltage. R and L represent the resistance and leakage inductance of the shunt transformer plus the resistance and inductance of the series filtering inductors connected at the VSC output.Shunt Converter initial currentThe initial value of the positive-sequence current phasor (Magnitude in pu and Phase in degrees). If you know the initial value of the shunt current corresponding to the UPFC operating point you may specify it in order to start simulation in steady state. If you don't know this value, you can leave [0 0]. The system will reach steady-state after a short transient.Series Converter ratingThe ratings of the series converter in VA and the maximum value of the injected voltage V_conv on the VSC side of the transformer (see single line diagram), in pu of nominal phase-to-ground voltage.Series Converter impedanceThe positive-sequence resistance and inductance of the converter, in pu, based on the converter rated power and voltage. R and L represent the resistance and leakage inductance of the series transformer plus the resistance and inductance of the series filtering inductors connected at the VSC output.Series Converter initial currentThe initial value of the positive-sequence current phasor (Magnitude in pu and Phase in degrees). If you know the initial value of the series current corresponding to the UPFC operating point you may specify it in order to start simulation in steady state. If you don't know this value, you can leave [0 0]. The system will reach steady-state after a short transient.DC link nominal voltageThe nominal voltage of the DC link in volts.DC link total equivalent capacitanceThe total capacitance of the DC link in farads. This capacitance value is related to the UPFC converter ratings and to the DC link nominal voltage. The energy stored in the capacitance (in joules) divided by the converter rated power (in VA) is a time duration which is usually a fraction of a cycle at nominal frequency. For example, for the default parameters, (C=750 F, Vdc=40 000 V, Snom=100 MVA) this ratio1/2CV2dc/Snomis 6.0 ms, which represents 0.36 cycle for a 60 Hz frequency. If you change the default values of the nominal power rating and DC voltage, you should change the capacitance value accordingly.Shunt Converter TabModeSpecifies the shunt converter mode of operation. Select eitherVoltage regulationorVar Control.Reference voltage VrefThis parameter is not visible when theModeparameter is set toVar Control.Reference voltage, in pu, used by the voltage regulator. WhenExternalis selected, a Simulinkinput named Vref appears on the block, allowing you to control the reference voltage from an external signal (in pu). TheReference voltage Vrefparameter is therefore unavailable.Maximum rate of change of reference voltageThis parameter is not visible when theModeparameter is set toVar Control.Maximum rate of change of the reference voltage, in pu/s, when an external reference voltage is used.DroopThis parameter is not visible when theModeparameter is set toVar Control.Droop reactance, in pu/shunt converter rating Snom, defining the slope of the V-I characteristic.Vac Regulator Gains: [Kp Ki]This parameter is not visible when theModeparameter is set toVar Control.Gains of the AC voltage PI regulator. Specify proportional gain Kp in (pu of I)/(pu of V), and integral gain Ki, in (pu of I)/(pu of V)/s, where V is the AC voltage error and I is the output of the voltage regulator.Reactive power setpoint QrefThis parameter is not visible when theModeparameter is set toVoltage Control.Reference reactive power, in pu, when the shunt converter is inVar Control.Maximum rate of change of reactive power setpoint QrefThis parameter is not visible when theModeparameter is set toVoltage Control.Maximum rate of change of the reference reactive power, in pu/s.Vdc Regulator gains: [Kp Ki]Gains of the DC voltage PI regulator which controls the voltage across the DC bus capacitor. Specify proportional gain Kp in (pu of I)/Vdc, and integral gain Ki, in (pu of I)/Vdc/s, where Vdc is the DC voltage error and I is the output of the voltage regulator.Current Regulators gains: [Kp Ki]Gains of the inner current regulation loop.Specify proportional gain Kp in (pu of V)/(pu of I), integral gain Ki, in (pu of V)/(pu of I)/s, where V is the output of the d or q current regulator and I is the Id or Iq current error.The current regulator is assisted by a feed forward regulator. The feed forward gain in (pu of V)/(pu of I) is the shunt converter reactance (in pu) given by parameter L in theShunt converter impedance [R L]parameters.Series Converter TabBypass BreakerSpecifies the status of the bypass breaker connected inside the block across terminals A1, B1, C1 and A2, B2, C2. Select eitherExternal Control,OpenorClosed. If the bypass breaker is in external control, a Simulink input named Bypass appears on the block, allowing to control the status of the bypass breaker from an external signal (0 or 1).ModeSpecifies the series converter mode of operation. Select eitherPower flow controlorManual voltage injection.Reference powersThis parameter is not visible when theModeparameter is set toManual voltage injection.Specify references values, in pu. WhenExternalis selected, a Simulink input named PQref appears on the block, allowing you to control the active and reactive powers from an external signal (in pu). TheReference powersparameter is therefore unavailable.Maximum rate of change of reference powersThis parameter is not visible when theModeparameter is set toManual voltage injection.Specify maximum rate of change of Pref and Qref, in pu/s.Power Regulator gains: [Kp Ki]This parameter is not visible when theModeparameter is set toManual voltage injection.Gains of the PI regulators which control the line active power and reactive power. Specify proportional gain Kp in (pu of Vdq)/(pu of PQ), and integral gain Ki, in (pu of Vdq)/(pu of PQ)/s, where Vdq is the Vd or Vq injected voltage and PQ is the P or Q voltage error.Reference voltagesThis parameter is not visible when theModeparameter is set toPower flow control.Specify the direct-axis and quadrature-axis components of the voltage injected on the VSC side of the series transformer, in pu. WhenExternalis selected, a Simulink input named Vdqref appears on the block, allowing you to control the injected voltage from an external signal (in pu). TheReference voltagesparameter is therefore unavailable.Maximum rate of change for references voltagesThis parameter is not visible when theModeparameter is set toPower flow control.Specify maximum rate of change of the Vdref and Vqref voltages, in pu/s.Inputs and OutputsA1 B1 C1The three input terminals of the UPFC.A2 B2 C2The three output terminals of the UPFC.TripApply a simulink logical signal (0 or 1) to this input. When this input is high the shunt converter is disconnected and the series converter is bypassed. In addition, when the trip signal is high the shunt and series control systems are disabled. Use this input to implement a simplified version of the protection system.BypassThis input is visible only when theBypass Breakerparameter is set toExternal Control.Apply a simulink logical signal (0 or 1) to this input. When this input is high the bypass breaker is closed.VdqrefThis input is visible only when theExternal control of injected voltage Vdref _Vqrefparameter is checked.Apply a simulink vectorized signal specifying the reference voltages Vdref and Vqref, in pu.PQrefThis input is visible only when theExternal control of power references Pref _Qrefparameter is checked.Apply a simulink vectorized signal specifying the reference powers Pref and Qref, in pu.m