94 Chapter-5 MODELING OF UNIFIED POWER FLOW CONTROLLER 5.1 Introduction There are a number of FACTS devices that control power system parameters to utilize the existing power system and also to enhance the dynamic performance and stability of the power system. Out of these, UPFC is the most flexible multi functional FACTS device. UPFC perform the functions of a shunt reactive current injection to control bus voltage and inject series reactive voltage to control power flow in transmission line [23][30][33]. If PI Controllers equipped by the UPFC shunt and series controllers are slow or if PI controllers are not properly tuned or if the UPFC operates manually, the UPFC is not in a position to effectively damp the power system oscillations [28]. To achieve this, power oscillation damping control stability loop or auxiliary controller is added along with power flow controller [23]. 5.2 Unified Power Flow Controller (UPFC) 5.2.1 Configuration The basic concept diagram of UPFC is shown in Fig.5.1. It contains two back to back AC to DC synchronous voltage sourced converters (VSC1 and VSC2) operated with common DC link capacitor [23] [28] [29]. VSC1 is connected in shunt through shunt-connected transformer and VSC2 is connected in series through series connected transformer. The shunt branch of UPFC comprised of a DC Capacitor,
95 VSC1 and a shunt-connected transformer corresponds to a STATCOM. It can absorb or generate only reactive power because the output current is in quadrature with the terminal voltage. The series branch of UPFC is comprised of a DC Capacitor, VSC2 and a series connected transformer corresponds to a SSSC. It can act as a voltage source injected in series to the transmission line through series connected transformer; the current flowing through the VSC2 is the transmission line current (I) and it is function of the transmitted electric power and the impedance of the line. The injected voltage (Vse) is in quadrature with the transmission line current (I) with the magnitude being controlled independently of the line current. Hence, the two branches of the UPFC can absorb or generate the reactive power independent of each other. If the two converters (VSC1 and VSC2) are operating at the same time, the shunt and series branches of the UPFC can basically function as an ideal ac to ac converter in which the real power can flow in either direction through the dc link and between the AC terminals of the two converters. The real power from VSC1 to VSC2 and vice versa, and hence it is possible to introduce positive or negative phase shifts between V1 and V2. The series injected voltage Vse can have any phase shift with respect to the terminal voltage V1. Therefore, the operating area of the UPFC becomes the circle limited with a radius defined by the maximum magnitude of Vse, i.e., Vse.max.
96 The VCS2 is used to generate the voltage Vse 0 Vse Vse.max and phase shift 0 θ 2π at the fundamental frequency. This voltage is added in series to the transmission line and directly to terminal voltage V1 by the series connected coupling transformer. The transmission line current passes through the series transformer, and in the process exchanges real and reactive power with the VSC2. This implies that the VSC2 has to be able to absorb and deliver both real and reactive power. The shunt-connected branch associated with VSC1 is used primarily to provide the real power demanded by VSC2 through the common DC link terminal. Also, it can generate or absorb reactive power independently of the real power, it can be used to regulate the terminal voltage V1; thus, VSC1 regulates the voltage at the input terminals of the UPFC. Another important role of the shunt branch of UPFC is a direct control of the DC capacitor voltage, and consequently an indirect regulation of the real power required by the series UPFC branch. The amount of real power required by the series converter plus the circuit losses have to be supplied by the shunt converter. Real power flow from the series converter to shunt converter is possible and in some cases desired, in this case, the series converter would supply the required real power plus the losses to the shunt converter.
97 Fig.5.1 The basic scheme of UPFC 5.2.2 UPFC transmission control capabilities The power transmission with UPFC based on the reactive shunt compensation, the series compensation and the phase angle regulation. The UPFC can meet multiple control objectives by adding the series injected voltage with appropriate magnitude and phase angle to the terminal voltage V1. Using phasor representation, the basic UPFC power flow control functions are illustrated in Fig.5.2. Voltage regulation with continuously variable in phase / anti phase voltage injection is shown in Fig.5.2 (a) for voltage increments Vse = ±ΔV (σ =0). Series reactive compensation is shown in Fig.5.2 (b), where Vse = Vq is in quadrature with line current I. functionally this is similar to series capacitive and inductive line compensation by the SSSC. Phase angle regulation is shown in Fig.5.2(c), where Vse = Vσ is injected with an angular relationship with respect to Vs that
98 archive the function as a perfect phase angle regulator, which can also supply the reactive power involved with transmission angle control by internal VAR generation. Multifunction power flow control executed by simultaneous terminal voltage regulation, series capacitive line compensation and phase shifting is shown in Fig.5.2 (d) where Vse = ΔV + Vq + Vσ. This functional capability is unique to the UPFC. No single conventional equipment has similar multi functional capability. Fig.5.2 Phasor representation of UPFC power flow control functions
99 5.2.3 UPFC control system The general control scheme of UPFC [31] [33] is as shown in Fig.5.3. The UPFC is a multi variable control device with four inputs (magnitude and phase angle of the shunt and series converter output voltages) and four outputs (real and reactive output powers of the shunt and series converters). The series converter controls the active and reactive powers flow through transmission line by adjusting the magnitude and phase angle of the series injected voltage. The shunt converter controls the dc voltage and the bus voltage (V1) at the shunt converter transformer. In this thesis, the shunt converter is used to control the sending-end bus voltage magnitude by locally generating and absorbing reactive power. The series converter directly controls real line power by the magnitude of the series injected voltage. Fig.5.3 Basic control structure of UPFC
100 5.2.3.1 Shunt converter controls The shunt converter has two duties, namely, to control the voltage magnitude at the sending-end bus (Bus V1 in Fig.5.1) by locally generating or absorbing reactive power, and to supply or absorb real power at the dc terminals as demanded by the series converter. It is possible to achieve real power balance between the series and shunt converter by directly controlling the dc voltage Vdc, as any excess or deficit of real power will tend to increase or decrease the dc voltage, respectively. By varying the magnitude and angle of the shunt converter output voltage the real and reactive power flow in and out of the shunt converter is controlled [23][31]. The PI bus voltage regulator as shown in Fig.5.4 (a) sets the reactive current reference and PI dc voltage regulator sets real current reference as shown in Fig.5.4 (b) This control scheme is basically the same as a STATCOM control. The d q decoupled current control strategy for shunt converter [45] is implemented as shown in Fig.5.3. The control system consists of: A phase-locked loop (PLL): it is used to synchronize the Shunt converter current with sending-end bus voltage (V1) at the point of UPFC connection. An AC voltage regulator (Bus-voltage regulator): it gives the reference reactive current Iqref required by the system to maintain bus voltage at constant value or in specified range.
101 A DC voltage regulator: it gives the reference active current Idref to maintain the capacitor voltage at a constant value or in specified range. The inner current regulator: it controls the magnitude and phase of the voltage generated by the PWM converter of Shunt converter to deliver or absorb required reactive current by the Shunt converter as per reference valve given by the AC and Dc voltage regulators. Fig.5.4 Shunt converter current controller The shunt converter controls the bus voltage by injecting reactive current in quadrature with sending-end voltage V1. The magnitude of the shunt voltage can be calculated by the following equation
102 Vsh = Vref + XS.I ---------------5.1 Where Vsh = Positive sequence voltage (pu) of shunt converter I = Reactive current (pu/pnom) XS = Slope (pu/pnom: usually between 1% and 5%) or the leakage reactance of shunt connected transformer and series reactance connected between converter and power system The voltage Vsh is controlled through the changes in the amplitude modulation ratio msh, as the output voltage magnitude is directly proportional to msh according to the following equation Vsh = (1/2 2)*msh*Vdc -------------5.2 5.2.3.2 Series converter controls Two different control schemes for the series converter were implemented. One scheme to control real power flow through transmission line and voltage magnitude at the receiving-end bus; another control scheme for controlling the real and the reactive power flows through the transmission line. From the basic principle of UPFC, series converter does main function of UPFC. The series converter active and reactive powers are controlled by using two separate PI controllers, taking advantage of the UPFC ability to independently control reactive and real power. The basic principle of real power flow being directly affected by changes in phase angles, while reactive power flow is directly associated with voltage magnitudes, is used here to design the UPFC control.
103 The outputs of the PI controllers are d and q components of the series injected voltage Vse, i.e., Vsed and Vseq respectively. The magnitude of the series voltage can be calculated by the following equation Vse-q = (Kp + Ki/S)*(Pref - P) ------------ (5.3) Vse-d = (Kp + Ki/S)*(Qref - Q) ------------ (5.4) Vse = (Vse-d 2 + Vse-q 2 ) ------------ (5.5) The amplitude modulation ratio mse = (8*Vse/Vdc) ---------------------(5.6) The phase angle of the series injected voltage with respect to the reference waveform, i.e., the sending-end voltage V1 is given as follows β = -tan -1 (Vse-q/Vse-d) Fig.5.5 Series converter injected voltage controller
104 The series converter controls active power flow in line by controlling the magnitude of the series injected voltage, injecting in qudrature with the line current I. 5.3 SIMULINK Modeling of UPFC The SIMULINK model of UPFC developed as a phasor model, to perform dynamic and transient stability studies in 3-Ph power systems. The series converter (VSC2) injected voltage (Vq) is controlled to meet the power demand in the line set by the reference power set point (Pref) and shunt converter (VSC1) delivers or absorbs the reactive current as per the output of ac voltage regulator. 5.3.1 PI Voltage Controller of shunt converter The SIMULINK model of PI voltage controller block diagram for UPFC shunt converter is shown in Fig.5.6. This controller gives appropriate shunt reactive current injected into the power system at which UPFC located for appropriate change in bus voltage with respect to the reference voltage. Fig.5.6 PI Voltage Controller block diagram of UPFC shunt Converter
105 5.3.2 FLPOD controller along with PI Voltage Controller of shunt converter The SIMULINK model of FLPOD controller along with PI Voltage Controller block diagram for Shunt Current Controller of UPFC is shown in Fig.5.8. FLPOD shunt controller is fed by one input namely change in power or difference in power (DP) of a constant resistive load connected parallel to the shunt converter to the UPFC. This gives the appropriate shunt current (Iq), which is required by the system during transient period and it gives zero output for steady state. The rules for the proposed FLPOD shunt controller are: i) If DP is DPN (DP Negative) Then Iq is IqN (Iq Negative) ii) iii) If DP is DPZ (DP Zero) Then Iq is IqZ (Iq Zero) If DP is DPP (DP Positive) Then Iq is IqP (Iq Positive) These rules are in matrix form as given below error (DP) DPN DPZ DPP Out put (IQ) IQN IQZ IQP The membership functions for input and output of FLPOD shunt controller, Change in power or difference in power (DP) and shunt injected current (Iq) are given in Fig.5.7.
106 Fig.5.7 (a) Input membership function (DP) and (b) Output Membership function (Iq) of FLPOD shunt controller Fig.5.8 FLPOD controller along with PI Voltage Controller block diagram of UPFC shunt converter 5.3.3 PI Power Flow Controller of series converter The SIMULINK model for PI Power Flow controller of series converter block diagram is shown in Fig.5.9. This controller gives appropriate series injected voltage for appropriate change in line power with respect to the reference power.
107 Fig.5.9 PI power flow controller block diagram of UPFC series converter 5.3.4 FLPOD controller along with PI Power flow controller of series converter The SIMULINK model for Series voltage controller of UPFC with FLPOD controller along with PI power flow controller is shown in Fig.5.11. FLPOD controller is fed by one input namely change in power or difference in power (DP). This gives the appropriate series injected voltage (Vq), which is required by the system during transients and it gives zero output under steady state. The rules for the proposed FLPOD series controller are: i) If DP is DPN (DP Negative) Then Vq is VqN (Vq Negative) ii) iii) If DP is DPZ (DP Zero) Then Vq is VqZ (Vq Zero) If DP is DPP (DP Positive) Then Vq is VqP (Vq Positive)
108 These rules are in matrix form as given below error (DP) DPN DPZ DPP Out put (Vq) VqN VqZ VqP The membership functions for input and output of FLPOD controller, Change in power or difference in power (DP) and series injected voltage (Vq) are given in Fig.5.10 (a and b) Fig.5.10 (a) Input membership function (DP) and (b) Output Membership function (Vq) of FLPOD series controller Fig.5.11 FLPOD controller along with PI power flow controller block diagram of UPFC series converter
109 5.4 Summary In this chapter details of UPFC have been discussed. SIMULINK implementation of the UPFC has been discussed. The UPFC with PI and FLPOD controllers allows the controls of the amplitude of both shunt reactive current and series injected reactive voltages.