[IEEE 2011 6th IEEE Conference on Industrial Electronics and Applications (ICIEA) - Beijing, China (2011.06.21-2011.06.23)] 2011 6th IEEE Conference on Industrial Electronics and Applications - Voltage compensation with Interline Power Flow Controller (IPFC) using all degrees of freedom

Voltage Compensation with Interline Power Flow Controller (IPFC) Using All Degrees of Freedom M. Fekri Moghadam, H. Askarian Abyaneh, S. H. Fathi Department of Electrical Engineering Amirkabir University of Technology Tehran, Iran M. Khederzadeh Department of Electrical Engineering Power & Water University of Technology Tehran, Iran Abstract—The Interline Power Flow Controller (IPFC) is a voltage-source-converter (VSC)-based flexible ac transmission system (FACTS) controller for series compensation in a multiline transmission system of a substation. The capability of injecting series voltages with controllable magnitude and phase angle makes it a powerful tool for better utilization of existing transmission lines in a multiline transmission system. IPFC is used to regulate active and reactive power flow in a multiline system, usually. In this paper, a control method for IPFC is proposed to control magnitude and phase angle of one sending bus of a substation. All degrees of freedom of IPFC and decoupled synchronous frame concept are used in the proposed control structure. Simulation results in Matlab/Simulink are presented to show the capability of IPFC in compensating the bus voltage. Keywords--Interline Power Flow Controller; Sereis Voltage compensation; Linear Controller; Decoupled PI Control; Matlab/Simulink I. INTRODUCTION Flexible Ac transmission systems (FACTS) technology has presented new opportunities for controlling power flow and enhancing the capacity of present lines and therefore, minimizing the gap between the stability and thermal levels [1, 2]. Among these, voltage-source-based FACTS devices like STATCOM, SSSC, UPFC and IPFC are able to inject voltages or currents into the lines through series or shunt connected transformers. These devices when compared to switched capacitor/reactor and thyristor-based FACTS controllers such as Static Var Compensator (SVC) and Thyristor-controlled Series Capacitor (TCSC) have the advantage of generating/absorbing reactive power using a single energy storage device such as capacitor or inductor. In addition, converter-based FACTS controllers are capable of independently controlling both active and reactive power flow in the power system. The UPFC and IPFC configurations consist of some (normally two) dc to ac inverters connected back-to-back with a common dc link, providing active power transfer between them [1]. Among these controllers, Interline Power Flow Controller (IPFC), presented in 1999 by Gyugyi is a back-to-back connection of some (normally two) inverters which are connected in series with different lines in a multiline transmission system of a substation. It addresses the problem of compensating a number of transmission lines at a given substation [3]. IPFC is able to transfer real power between compensated lines in addition to compensate reactive power for each individual line, independently. So it can equalize both real and reactive power flow between the lines, transfer power demand from overloaded to underloaded lines, compensate against resistive voltage drops, and increase the effectiveness of the system for dynamic disturbances [4]. In order to provide the proper regulation, there have been efforts to present control schemes meeting the desired objectives. Linear (PI) controllers are used in [5, 6, 7, 8, 9], with different coupled and decoupled concepts to provide IPFC system control. In [10], a fuzzy logic controller is implemented to enhance the dynamic performance of the system. The capability of transferring active power between different lines and independent control of reactive power has been considered in the reports [9] - [10]. Although the main objective of the IPFC is to optimize power flow among different lines, it can also be utilized to compensate voltage drops especially resistive voltage drops in transmission busses. This paper deals with voltage compensation feature of IPFC and uses its all degrees of freedom. The rest of paper is organized as follows. Section 2 describes the IPFC and corresponding dynamic equations. In this part the voltage injection concept and constraints imposed by the back-to-back connection of inverters are described. The linear control scheme used in this paper is presented in section 3. Section 4 describes voltage compensation with IPFC. Section 5 and 6 are devoted to simulation results and conclusion parts, respectively. II. INTERLINE POWER FLOW CONTROLLER A Static Synchronous Series Compensator (SSSC) is a series connected VSC based FACTS controller which can provide a series reactive power compensation for a transmission system [11]. Conventional SSSC can only inject a voltage which lags or leads the line current by 90° .The injected series voltage of SSSC can have desired phase angle if it is provided with Energy Storage System. Since the in phase component of injected series voltage corresponds to transfer (absorption or injection) of active power to the transmission 2179978-1-4244-8756-1/11/$26.00 c©2011 IEEE line, the ability of transferring active power in dc link will increase the effectiveness of SSSC in power flow control applications. The active power demand of series connected inverter could be provided by other inverter which is connected to the system. The other inverter connection can be parallel with the sending bus (UPFC) or series with another transmission line (IPFC). These two inverters should be connected together in a dc bidirectional link for active power transfer between them. An elementary IPFC consisting of two VSCs is illustrated in Fig. 1 [5] where R and L are the line resistance and line inductance of the system, V and δ are the bus voltage and phase angle respectively. Equivalent circuit of IPFC is also shown in Fig.2. Each inverter can compensate a transmission line by series voltage injection and the common dc link is represented by a bidirectional link for active power exchange between the two voltage sources [3]. The active power demand of inverter 1 is provided by another series inverter 2 which compensates the other line. The transmission voltages, impedances and power angles are assumed to be identical, ( 1 1 2 2 V V V V V s r s r = = = = , 1 2 δ δ δ= = , 1 2 L L L= = , ) 1 2 R R R= = . Also the ratings of compensating voltage sources are assumed to be identical. Although systems 1 and 2 could be (and in practice are likely to be) different, these simplifying assumptions will not cause loss of generality. System 1 is arbitrarily selected to be the prime system for which controllability of active and reactive power flow or voltage compensation are considered. The reason for this selection is to derive the constraints the free controllability of system 1 imposes on the power flow control of the system 2 [3]. A phasor diagram of system 1 defining the relationship between sending-end and receiving-end bus voltages and the injected series voltage by inverter 1 is shown in Fig. 3. The injected voltage has a controllable magnitude ( 0 1 1 ,maxV Vpq pq< < ) and phase angle ( 0 360ρ< < ) whose rotation modulates the active and reactive power flow in the transmission line. Figure 1. Interline Power Flow Controller with two Inverters Figure 2. Equivalent Circuit of Fig.1 The variation of active and reactive power at the receiving- end bus of transmission line 1 ( 1rP and 1rQ ) with the rotating 1 ,maxpqV is illustrated in the { 1rP -j 1rQ } plane shown in Fig. 4. It can be shown that the rotation of injected series voltage 1pqV produces a circular locus in the control plane, the center of which is located at the operating point of the uncompensated system. The operating point values (i.e. active and reactive power flow at receiving-end bus) are dictated by the system parameters (i.e. bus voltage magnitudes and phase angles and transmission impedance). The circular locus is the boundary of the defined control range, within which any values for active and reactive powers are achievable by proper control of the injected series voltage magnitude and phase angle. Line current flow through the series connected voltage source converter will cause a certain power exchange between the line and dc link source. If the line resistance is neglected, the active power exchange can be approximated as [6]: 2 sin( ) 2 . cos( ) 2ex pq V P V X δ δ ρ= + (1) A voltage compensation line which is parallel with the line of vector s rV V− can be defined as in [3]. It can be drawn that the amount of active power exchange is dependent to length of the line, originating from the end of sending-end bus vector sV , perpendicular with the voltage compensation line. If any operating point lies along this line, the active power demand will remain unchanged. In IPFC case this active power exchange should be provided by inverter 2 which is back-to- back connected to the inverter 1. Thus, the operating point of line 2 must lie in the complementary voltage compensation line in the opposite side whose position can be determined by the length of the perpendicular lines with the compensation line in Fig. 3. This complementary line is the constraint imposed by the prime system on system 2. This constraint leaves us only one control degree of freedom, which is usually devoted to the active or reactive power of the transmission line 2 at the receiving-end bus, conventionally. 2180 2011 6th IEEE Conference on Industrial Electronics and Applications Figure 3. Phasor diagram Figure 4. Control area III. SYSTEM MODELLING AND LINEAR CONTROL SCHEME Any control objective of IPFC or other FACTS controllers should be accompanied with a proper control scheme that guarantees its proper regulation. This part of the paper is devoted to mathematical modeling of the system and decoupled PI control used in the control system of IPFC as in [ 5, 6 ]. The analysis of the basic control system is based on a simplified mathematical model of the converter connected to the system. The converter, which is capable of exchanging active power with the system, is represented by a sinusoidal voltage source. The balanced three-phase system can be transformed into a synchronously-rotating orthogonal system. By applying the Averaged Model [12] for control, the system equations in decoupled control strategy d-q frame are [13]: 1 1 1 1 1 1 1 1 1 d d q ds d dc dr di L R i L i V u v V dt ω+ − = − − (2) 1 1 1 1 1 1 1 1 1 q q d qs q dc qr di L R i L i V u v V dt ω+ + = − − (3) 2 2 2 2 2 2 2 2 2 d d q ds d dc dr di L R i L i V u v V dt ω+ − = − − (4) 2 2 2 2 2 2 2 2 2 q q d qs q dc qr di L R i L i V u v V dt ω+ + = − − (5) where dcv is the dc voltage across the capacitor C , ω is the angular velocity of AC voltage and current vectors and du and qu are control variables. Neglecting the internal losses of the converter, the ac side power acP and the dc side power dcP are equal: 1 1 1 1 2 2 2 2 0 3 ( ) ( ) 0 2 ac dc d dc d q dc q d dc d q dc q dc dc P P u v i u v i u v i u v i v i − = + + + − = (6) From which the dc side current dci , can be obtained as follows: 1 1 1 1 2 2 2 2 3 ( ) 2 dc d d q q d d q qi u i u i u i u i= + + + (7) This allows the Kirchhoff's current law on the dc side to be written, which for IPFC circuit is: 1 1 1 1 2 2 2 2 3 ( ) 2 dc d d q q d d q q dv C u i u i u i u i dt = + + + (8) The control variables du and qu in the d-q frame are: )cos(σαmud = (9) )sin(σαmuq = (10) where m , σ and α are modulation index, modulation delay angle and series transformer turn ratio, respectively. The classical decoupled watt-var algorithm used in [14] is presented here and used in the control scheme. The two new variables introduced in (11) and (12) represent the output from the control system (13) and (14) consisting of two PI controllers. Additionally, a compensation for the cross- coupling of both current components was made in controller. The values * di and * qi are the reference values of the active and reactive currents, pK is the gain in the proportional part and iK is the gain in the integral part of an individual controller. The VSC voltages of each inverter of IPFC system are controlled as follows: .d dc q d ds dru v Li Lv V Vω= − + − (11) .q dc d q ds dru v Li Lv V Vω= − − + − (12) where *( )( )id p d d K v K i i s = + − (13) *( )( )iq p q q K v K i i s = + − (14) By putting above expressions for for du and qu in (2)-(5) the resulting equations will be decoupled. As described in [14], by introducing the simple condition (15), we obtain a transfer function in the form of (16) which is actually a first order system with time constant 1/ pKτ = 1i rVs V pqV pV qV 2011 6th IEEE Conference on Industrial Electronics and Applications 2181 .i p RK K L ≈ (15) * * 2( . ) q i p pd pd q i p i K sK Ki R K si i K s K s L + = = ≈ ++ + (16) IV. VOLTAGE COMPENSATION MODE OF IPFC Based on previous discussions, there can be three control degrees of freedom for IPFC system. Normally, the reactive power flow of both transmission lines and active power flow of the first or prime line are used as control objectives in the literatures [3] – [9]. However, it is possible to consider another feature for IPFC that can be implemented by series voltage injection. Since the injected voltage by the IPFC is fully controllable (i.e. in magnitude and phase angle), there can be another mode of operation which can be utilized for voltage regulation, especially resistive voltage drops, in sending-end buses. These voltage drops normally occur because of different reasons like contingencies and overload conditions in transmission network. Consider the IPFC system presented in Fig. 1. The prime system is taken to be equipped with voltage compensation mode of operation. IPFC control scheme can be set in a way that the series injected voltage by inverter 1, added to the sending-end bus voltage of line 1, results in a compensated voltage with desired magnitude and phase angle. The resultant voltage is named effective voltage in Fig.2. 1, 1,eff s inj V V V= + (16) The series injected voltage of inverter 1 can be adjusted so that to be in phase with sending-end bus voltage or in any phase angle that results in a desired magnitude and phase angle for effective voltage defined in Fig. 5. This means that the magnitude and phase angle of the injected series voltage of inverter 1 is determined by the operator or automatic control system of this mode of operation. So, only one degree of freedom will remain to be used by inverter 2. This can be utilized to control active or reactive power flow of the transmission line 2. As illustrated in Fig. 5, the end of injected series voltage phasor in the prime system is a particular point on the “voltage compensation line” of system 1, defined to derive the constraint system 1 imposes on system 2. The series injected voltage with required magnitude to compensate voltage drop in sending-end bus, interacts with the transmission line current. So at this particular operating point, inverter 1 has to exchange both active and reactive power with line 1. Voltage source converter based FACTS controllers can internally generate the reactive power but the active power must be supplied by the inverter 2 with an operating point lying along a complementary voltage compensation line. Any operating point on this line is achievable by proper control design. It is obvious that there is only one degree of freedom to be used in order that the operating point of inverter 2 lies along the complementary voltage compensation line. In practice, this condition is met by proper regulation of dc link voltage. Figure 5. Compensated System Phasor Diagram If the line resistance is neglected, the active power exchange can be approximated as follows: ' '2 sin( )2 . cos( ) 2ex pq V P V X δ δ ρ≈ + (17) It is important to note that (17) can be a good approximation if 0.95 . . 1.05 . . 1 p u V p u s ≤ ≤ . Again there can be a “voltage compensation line”, any point of which satisfies the constant value for the second term in (17), which means a constant active power exchange between two lines. The linear control scheme presented in previous section should be accompanied with proper decoupled d-q reference values in order to meet the desired voltage compensation mode of operation. The prime system can be controlled by decoupled PI control with d axis always coincidental with receiving-end voltage vector. The reference currents of the prime system will be obtained as follows: * * 1 1 , * * 1 1 , *' cos( ) *' sin( ) s ds eff s qs eff V V V V δ δ = = (18) then: * * 1 , 1 , 1 * * 1 , 11 , d inj ds eff ds q inj qsqs eff V V V V V V = = − − (19) Active power balance among the inverters is the restriction imposed on system 2 by system 1. The active power demand of the prime system must be compensated by the line 2. According to (17) and (18), the active power demand of inverter 1 is: * * * 1 , 1 1 , 1 3 ( ) 2 ex d inj d q inj qP V i V i= + (20) Then at steady state, the following equations can be derived with d axis always coincidental with the receiving-end voltage vector: * * * * * 2 , 2 2 , 2 3 ( ) 2 ex d inj d q inj qP V i V i− = + (21) 1i rV s V pqV 1,effV pV qV δ 'δ ρ 2182 2011 6th IEEE Conference on Industrial Electronics and Applications * * 2 2 2 3 2 r dr qQ V i= − (22) 2 2 2 2 2 ** 2 2 ,2 * * 2 2 , 1 2 2 0 0 qs Vi ds d injd i Vq q inj V V dr L V V qr R L R L ω ω = + − + + − − − − (22) So the reference currents for controlling * 2rQ can be derived as follows: * * 4 2 22 *2 2 ( ) 4 ( )2 2 , 2 2 2 ,2 , 2 2 3 329* 2 2 2 2 * 2* 2 2 3 2 Q Qr r V V V V R R V Pdr d eff q eff exd eff dr V rV dr i d R Q r i q V r − + − − + + = = − (23) The active power balance in dc link can be achieved by proper control of dc link voltage. By sampling the dc voltage and comparing it with the reference value, the dc voltage error can be obtained. A proportional controller is used as [14] to keep the dc voltage in the prescribed limits. According to the above discussion, the complete decoupled control architecture is shown in Fig. 6. Figure 6. Overal Linear Controler Design for IPFC V. SIMULATION RESULTS The proposed control mode of IPFC was tested in the Matlab/Simulink software program. A simplified two-line transmission system with IPFC controller was used as shown in Fig. 1. The sending ends are assumed as slack busses with power angles 1 2 30δ δ ° = = and receiving ends are selected to be angle references with power angles 0δ = . The line reactance is 0.5 . .p u .The inverter in line 1 is considered as the prime inverter which is able to inject a series voltage into the line with any phase angle with respect to the receiving end bus. The inverter 2 should satisfy the active power demand of inverter 1. Initially, the system is at steady state ( 1 1 30 ,sV ° = ∠ 2 1 30sV ° = ∠ and * 2 0.268 . . r p uQ = − ). Then the magnitude and phase angle of sending-end bus of line 1 is changed from 1 1 30sV ° = ∠ to 1 0.95 47sV ° = ∠ due to overload conditions or faults occurred in transmission system. At 0.2t s= , IPFC is made to follow the reference inputs as ( * 1 1 . . s p uV = , * 1 30 s δ °= , * 2 0 . . r p uQ = ) to compensate the voltage and phase angle. At 0.4t s= , IPFC is made to follow the reference inputs as ( * 1 1.05 . . s p uV = , * 1 10 s δ °= , * 2 0 . . r p uQ = ) to show its ability to increase the sending-end voltage. It should be noted that the complementary system can be set to control the active power flow as one degree of freedom is left for inverter 2. But this mode of operation for inverter 2 satisfies the “unity power factor” utilization of line 2.by inductive series compensation. Simulation results of the selected scenario are shown in Fig. 7, Fig.8 and Fig 9. Figure 7 shows the magnitude of the sending-end bus voltage and its reference value. It is evident that the IPFC is able to regulate magnitude of sending-end bus of the system of Fig.2. Figure 7. Simulation results 1 r V follows * 1 r V Fig. 8 displays the phase angle of the sending-end bus and its reference value. Again we can see the proper regulation of this parameter by the IPFC. Figure 8. Simulation results : 1 r δ follows * 1 r δ * 1 ,s eff δ 1 ,s eff δ * 1 ,ds effV 1 ,ds effV * 1di 1di * 1qi 1qi * dc V * 2di * 2qi 2qi dc V 2di ex P * 2rQ 0.1 0.2 0.3 0.4 0.5 0.6 0 0.2 0.4 0.6 0.8 1 1.2 P .U . 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 5 10 15 20 25 30 35 40 45 50 Time(s) A ng le (d eg re e) 2011 6th IEEE Conference on Industrial Electronics and Applications 2183 Figure 9 shows the actual and reference value of the reactive power of line 2. Figure 9. Simulation results : 2 r Q follows * 2 r Q VI. CONCLUSION In this paper, a new mode of operation of IPFC is proposed. In this mode, IPFC is able to control voltage drops in one of the sending-end busses of a multiline transmission system of a substation. There are three degrees of freedom for IPFC, to be used to control the transmission parameters. Voltage drop compensation leaves one control degree of freedom for the other line in a basic two line system. This degree of freedom is also used to meet the unity power factor utilization of the line 2. Averaged method is used to model the system equations and decoupled d-q frame control scheme is used to control the overall system. Simulation results show the capability of IPFC in voltage drop compensation in one transmission line of substation together with reactive power compensation in the other line. REFERENCES [1] N. G. Hingorani, L. Gyugyi, "Understanding FACTS: concepts and technology of flexible AC transmission system," IEEE PRESS, 2000. [2] Y.H. Song and A.T. Johns, Flexible AC Transmission System. IEE Book Series on Power Engineering, December 1999. [3] L. Gyugyi, K. K. Sen and C. D. Schauder, "The interline power flow controller concept: a new approach to power flow management in transmission systems," IEEE Trans. Power Del., vol. 14, no. 3, pp. 1115-1123, Jul. 1999. [4] J. Zhang, A. Yokoyoma and T. ide, "Application of interline power flow controller (IPFC) to power oscillation damping," IEEJ Transactions on Power and Energy, vol. 128, no. 10, pp. 1252-1258, 2008. [5] J, Chen, T. T. Lie and D.M. Vilathgamuwa, "Basic control of interline power flow controller," IEEE Power Engineering Society Winter Meeting, 2002. [6] J. Chen, T. T. Lie and D.M. Vilathgamuwa, "Design of an interline power flow controller," 14th PSCC, Sevilla, Jun. 2002. [7] S. Salem and V.K. Sood, "Simulation and controller design of an interline power flow controller in EMTP RV," Proceeding of International Conference on Power Systems Transients (IPST), Jun. 2007. [8] S.Sankarr and S.Ramareddy, "Simulation of Closed Loop Controlled IPFC System," International Journal of Computer Science and Network Security, vol. 7, no. 6, pp. 245-249, Jun. 2007. [9] R. Strzelecki, P. Smereczynski and G. Benysek, "Interline power flow controller- properties and control strategy in dynamic states". [10] D. Menniti, A. Pinnarelli and N. Sorrentino, "A fuzzy logic controller for interline power flow controller model implemented by ATP-EMTP," International Conference on Power System Technology, vol. 3, pp. 1898-1903, 2002. [11] L. Gyugyi, C. D. Schauder, K. K. Sen, "Static synchronous series compensator: a solid-state approach to the series compensation of transmission lines," IEEE Trans.on Power Delivery, vol. 12, no. 1, pp. 406-417, Jan. 1997. [12] S. R. Sanders, J.M. Noworolski, X.Z. Liu and G.C. Verghese, "Generalized averaging method for power conversion circuits," IEEE Transaction on Power Electronics, vol. 6, pp. 251-259, Apr. 1991. [13] B. Lu and B.T. 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Weinhold, "Basic control of Unified Power Flow Controller," IEEE Trans. on Power Systems, vol. 12, no. 4, pp. 1734-1739, Nov. 1997. 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 -3 -2 -1 0 1 Time(s) P .U . 2184 2011 6th IEEE Conference on Industrial Electronics and Applications /ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 150 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages false /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False /Description > >> setdistillerparams > setpagedevice