This paper presents a control scheme and comprehensive analysis for a unified power flow controller (UPFC) on the basis of theory, computer simulation and experiment. This developed theoretical analysis reveals that a conventional power feedback control scheme makes the UPFC induce power fluctuation in transient states. The conventional control scheme cannot attenuate the power fluctuation, and so the time constant of damping is independent of active and reactive power feedback gains integrated in its control circuit. This paper proposes an advanced control scheme which has the function of successfully damping out the power fluctuation. A UPFC rated at 10 kVA is designed and constructed, which is a combination of a series device consisting of three single-phase pulsewidth modulation (PWM) converters and a shunt device consisting of a three-phase diode rectifier. Although the dynamics of the shunt device are not included, it is possible to confirm and demonstrate the performance of the series device. Experimental results agree well with both analytical and simulated results and show viability and effectiveness of the proposed control scheme </p

Akagi, Hirofumi

Fujita, Hideaki

Watanabe, Yasuhiro

English

Okayama University Scientific Achievement Repository

A new approach to harmonic compensation in power systems—A combined system of shunt passive and series active ﬁlters,”

An improved control scheme for a series capacitive reactance compensator based on a voltage source inverter,”

Dynamic control of a uniﬁed power ﬂow controller,”

Instantaneous reactive power compensators comprising switching devices without energy storage components,”

Optimal controller for the combination system of a UPFC and conventional series capacitors,”

Solid-state series capacitive reactance compensators,”

Static synchronous series compensator: A solid-state approach to the series compensation of transmission lines,”

Uniﬁed power ﬂow controller (UPFC) based on chopper stabilized multilevel converter,”

Uniﬁed power-ﬂow control concept for ﬂexible ac transmission systems,”

Control and analysis of a unified power flow controller

H. Fujita

Y. Watanabe

H. Akagi

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Okayama University Digital Information Repository

&lt;p&gt;This paper presents a control scheme and comprehensive analysis for a unified power flow controller (UPFC) on the basis of theory, computer simulation and experiment. This developed theoretical analysis reveals that a conventional power feedback control scheme makes the UPFC induce power fluctuation in transient states. The conventional control scheme cannot attenuate the power fluctuation, and so the time constant of damping is independent of active and reactive power feedback gains integrated in its control circuit. This paper proposes an advanced control scheme which has the function of successfully damping out the power fluctuation. A UPFC rated at 10 kVA is designed and constructed, which is a combination of a series device consisting of three single-phase pulsewidth modulation (PWM) converters and a shunt device consisting of a three-phase diode rectifier. Although the dynamics of the shunt device are not included, it is possible to confirm and demonstrate the performance of the series device. Experimental results agree well with both analytical and simulated results and show viability and effectiveness of the proposed control scheme &lt;/p&gt;</p

EngineeringElectrical Engineering fieldsOkayama University Year 1999Control and analysis of a unified powerflow controllerHideaki Fujita Yasuhiro WatanabeOkayama University Okayama UniversityHirofumi AkagiOkayama UniversityThis paper is posted at eScholarship@OUDIR : Okayama University Digital InformationRepository.http://escholarship.lib.okayama-u.ac.jp/electrical engineering/18Control and Analysis of a Unified Power Flaw Controller Hideaki Fujita, Member, IEEE, Yasuhiro Watanabe and Hir0fum.i Akagi, Fellow, IEEE Department of Electrical Engineering, Okayama University 3-1-1 Tsushima-naka, Okayama 700-8530, Japan Abstract - This paper presents a control scheme and com- prehensive analysis for a UPFC (unified power flow con- troller), on the basis of theory, computer simulation and experiment. This developed theoretical analysis reveals that a conventional power-feedback control scheme makes the UPFC induce power swings in transient states. The conventional control scheme has no capability of damping power swings, so the time constant of damping is inde- pendent of active and reactive power feedback gains inte- grated in its control circuit. This paper proposes a general- ized control scheme which is characterized by successfully damping power swings in transient states. Experimental results obtained from a 10-kVA laboratory setup agree well with both analytical and simulated results. Moreover, it is shown that the proposed control scheme is viable and effective in damping of power swings in transient states. I.  INTRODUCTION In recent years, environment, right-of-way, and cost con- cerns have delayed the construction of both power stations and new transmission lines, while the demand for elec- tronic energy has continued to grow in many countries. This situation has spurred interest in providing already ex- isting power systems with greater operating flexibility and better utilization. The FACTS (flexible ac transmission systems) concept [l], based on applying leading edge power electronics technology to existing ac transmission systems, improves stability to increase usable power transmission capacity to its thermal limit. A UPFC (unified power flow controller) [1]-[7], which is one of the most promising de- vices in the FACTS concept, has the potential of power flow control and/or voltage stability in power transmission systems . Little literature has been published on theoretical anal- ysis in transient states, although not only feasibility stud- ies but also practical implementation of the UPFC are presently under way. Undeland, et. al., have proposed a control scheme for the UPFC, with the focus on dynamic as well as static performance [4]. This control scheme, named “cross-coupling control,” adjusts the q-axis voltage v, to control the d-axis current id and the d-axis voltage vd to  control the q-axis current i,. Computer simulation and experimental results with emphasis on dynamic per- formance have been shown in [4], but theoretical analysis would not be enough to support the experimental results. This paper presents a control scheme and comprehensive analysis for a UPFC on the basis of theory, computer simu- lation and experiment. This developed theoretical analysis reveals that a conventional power-feedback control scheme makes the UPFC induce power swings in transient states. The conventional power-feedback control scheme has no capability of damping power swings, so the time constant of damping is independent of active and reactive power feedback gains integrateld in its control circuit. This pa- per proposes a generalized control scheme which is charac- terized by successfully damping power swings in transient states. The proposed control scheme makes the UPFC act as a “damping resistance” against power swings in tran- sient states. This stems from the concept that the series active filter proposed in [8] has the function of harmonic damping. Experimental results obtained from a 10-kVA laboratory setup agree with both analytical and simulated results. Moreover, it is shown that the proposed control scheme is viable and effective in damping power swings in transient states. 11. BASIC CONFIGURATION OF A UPFC Fig. 1 shows a basic configuration of a UPFC, which is installed between the sending-end Vs and the receiving- end VR. The UPFC consists of a combination of a series device and a shunt device, the dc terminals of which are connected to a common tlc link capacitor. The series de- vice acts as a controllable voltage source Vc, whereas the shunt device acts as a controllable current source IC. The main purpose of the shunt device is to regulate the dc link voltage by adjusting the amount of active power drawn from the transmission line. In addition, the shunt device has the capability of controlling reactive power. In the fol- lowing discussion, the shunt device is disregarded because the current flowing into the shunt device, IC is not as large 0-7803-4489-8/98/$10.00  1998 IEEE 805 Figure 1: Basic configuration of a UPFC. . -  -.lllllf VS I as the transmission line current I .  Fig. 2 shows a singlephase equivalent circuit of the UPFC, where the reactor L and the resistor R represent inductance and resistance in the transmission line, respec- tively. It is reasonable to remove the line resistance R from Fig. 2 because WOL >> R in the overhead transmission line. Thus, the line current phaser vector, I is given by (a) Active power control. (b) Reactive power control. Figure 3: Phaser diagrams in case of active and reactive power F~~ the sake of simplicity, the assumption of Vs = VR leads Thus, this control scheme is identical with the so-called to the phaser diagrams shown in ~ i ~ .  3. When the output , "phase-angle control" which is one of the most basic con- voltage V i  leads by 90" with respect to Vs,  i is in phase trol schemes for the UPFC. with Vs, as shown in Fig. 3(a). This results in active power flow from Vs to VR. Controlling Vc to be in phase with Vs B. Cross-coupling control makes I lag by 90" with respect to V s ,  thus resulting in vectors are applied, the above discussion is not applicable to analysis of the dynamic behavior. The "cross-coupling control" proposed in [4] has not only feedback loop. This control scheme is characterized by controlling both the magnitude of VM and the phase an- reactive power flow, as shown in Fig' 3(b)' Since the phaser an =tive power feedback loop but also a reactive power 111. CONVENTIONAL CONTROL SCHEMES A. Phase-angle control Adjusting the amplitude of the 90" leading or lagging out- put voltage makes it possible to control active power. On the d-q frame coordinates based on space vectors, the d- axis current id corresponds to active power, and so it can be controlled by the q-axis voltage w c q .  Therefore, the ref- erence voltage vector wg = [w&, w&Jt for the series device is given by (2) where Kp [VIA] is an active power feedback gain, and 2: and i: are active and reactive reference currents, respec- tively. Then the phase angle between VM and Vs,  6 can be controlled by VQ as follows: gle S. As a result, the cross-coupling control enables us to achieve both active and reactive power control. On the d-q frame coordinates, the q-axis current i, corresponds to re- active power and so it can be controlled by W C d .  Therefore, the reference voltage vector of the series device is given by [$:I = [ iP -?I [ iy] *- i  'I where Kp [VIA] is a reactive power feedback gain. (3) The phase-angle and cross-coupling control schemes ap- pear to be based on space vectors. However, both con- trol schemes may not render good dynamic performance, because their control concept comes not from space vec- tors but from phaser vectors. The series device injects a q-axis voltage to control the d-axis current or the active power in both control schemes. The q-axis voltage, how- ever, induces the q-axis current to flow in transient state. Therefore, the active power flow control is accompanied by 806 the reactive power flow control even though either control scheme provides instantaneous voltage references. IV. GENERALIZED CONTROL SCHEME AND TRANSIENT ANALYSIS A. Generalized Control Scheme This paper proposes a "generalized control scheme." The reference voltage vector for the series device, v; is gener- alized, as follows: (4) K, -Kq i ; - i d  [ z t ] = [ K p  K, ] [ i ; - i , ] ~  Note that the proposed control scheme comprehends the phaseangle and cross-coupling control schemes, so that the proposed control scheme can be considered a generalized control scheme for the UPFC. The scheme has two addi- tional terms with a gain K,. A voltage vector produced by the two terms is in phase with the current error vec- tor i*-i. This means that the UPFC acts as a damping resistor against power swings. B. Transient analysis The following assumptions are made in transient analysis: 1. The sending-end voltage vs is equal to the receiving- end voltage VR, and they are three-phase balanced sinusoidal voltage sources. 2. The series device is assumed to be an ideal controllable voltage source. Therefore, the output voltage wc is equal to its reference vT;. Invoking the first assumption yields vs as follows: cos wot cos(wot - 2 ~ / 3 )  ] , (5) [ vsw z]=gvs[  cos(wot + 2 ~ / 3 )  where Vs is an rms voltage at the sending-end, and WO is a supply angular frequency. The equivalent circuit shown in Fig. 2 provides the following equation: Applying the d-q transformation to (5) and (6)  leads to (7) R + L ~  - W ~ L  [ WOL R + L J  [ i t ]  = [:::I. The instantaneous active and reactive powers [9], p and q are given by VSd . i d  + V S q  ' i q  'USd ' iq - Usq . i d  Substituting (4) for (El), dong with invoking the second assumption, gives the following equation: The Laplace functions for the active and reactive powers produce &(s) and Iq(s:) as follows: Here, w, is an undamped natural frequency. Equations (11) and (12) conclude that the UPFC exhibits a second- order system, thus causing power swings in transient states. Damping factor S and power swing frequency w are given as follows: . (13) K,+R J(w0L + Kp)(woL + K p )  + (Kr + R)2 S =  w = w n 4 m  1 (14) - J m - = p )  (WOL + Kp) L Equation (13) tells us that the larger the gain K,, the closer < is to unity. Equation (14) leads to the fact that the larger the power feedback gains Kp and Kp,  the larger the power swing frequency N. The time constant of damping of power swings, r is given by (15) Note that K, acts as a damping resistor. Therefore, in- creasing K, reduces the time constant and improves the transient stability. Thle phase-angle control and cross- coupling control schemes have no capability of damping power swings because K ,  = 0. 807 V. EXPERIMENTAL SYSTEM CONFIGURATION Controllable power rating P Series device capacity PINV Peak voltage of U P  VPP A. Main circuit 10 kW 1.5 kVA (=15%) f 1 7  V f=15%nl Fig. 4 and Table 1 show a main circuit of an experimental system rated at 10 kVA and its circuit parameters. The main circuit of the series device consists of three single- phase H-bridge voltage-fed PWM inverters rated at 1.5 kVA. A PWM control circuit compares reference voltage v; with a triangle carrier signal of fsw = 1 kHz in order to generate twelve gate signals. An equivalent switching frequency is 2 kHz, which is twice as high as fsw because three H-bridge PWM inverters are used. A three-phase diode rectifier is employed as the shunt device. A reac- tor L and a resistor R, representing the impedance of the transmission line, are inserted between vs and U R .  In the experimental system, us and U R  are connected to a common power supply U .  Thus, no power flow occurs as long as the series device is not operated. The series de- vice always delivers a small amount of active power to the transmission line, which equals the power loss dissipated in the resistor R. A three-phase PWM converter should be employed in a practical system because the magnitude and phase of U S  may differ from those of uR. While the series device draws active power from the transmission line, the shunt device should regenerate the active power via the dc link capacitor to the transmission line. Utility angular frequency WO Line inductance L Line resistance R B. Control circuit Fig. 5 shows a block diagram of the control circuit. The three- to two-phase transformation obtains i, and io from the three-phase currents i,, i, and i,. The d-q transfor- mation gets i d  and i, from i, and ip with the help of sinusoidal signals of sin wot and cos wot taken from a read only memory (ROM). The phase information wot is gener- ated by a phase-lock-loop (PLL) circuit. Then U$ and U; are given by (4). The power feedback gains are set to Kp = Kq = 0.5 VIA in the following experiments. From (13), the feedback gain K, is obtained as 27r x 60 rad/s 1.0 mH (=lo%) 0.04s1 (=l%) In order to get a damping factor of < = 0.8, K, should be designed as O.Sd(27~ x 60 x 0.001 + 0.512 - o.04 K, = Jm = 1.13V/A. In the experimental system, therefore, gain K, is set to 1.2 fsw =1 lcHz Figure 4: Main circuit of an experimental system. I I -  U S  Figure 5: Control circuit. I Utility line-to-line voltage V I 200 V I VI. SIMULATED AND EXPERIMENTAL RESULTS Figs. 6 - 9 show simulated and experimental waveforms for a step change in the active power reference p".  Table 2 shows experimental and theoretical values of damping fac- tor <, power swing frequency w and damping time constant T. The experimental waveforms of the series device voltage VIA. uc are measured through a low pass filter with a cut-off 808 Table 2: Experimental and theoretical values of damping factor C, power swing frequency w and damping time con- stant r. Control Schemes phase-angle control cross-coupling control proposed control (Fig.8) (a) ExDerimental values. C w [rad/s] r [ms] 0.067 520 28 0.053 740 26 0.76 900 0.95 frequency of 400 Hz because the switching frequency of the series device is 1 kHz. Circuit simulation software based on PSpice is used to calculate the following simulated results. The series device is assumed to be an ideal controllable voltage source which is equal to the reference voltage in this simulation. The simulated and experimental wave- forms agree well with each other not only in the steady state but also in the transient state. Fig. 6 shows waveforms in the case of the phaseangle control scheme. Non-negligible power swing with a f r e  guency of 520 rad/s occurs in the waveforms of p and q. After the step change, p and q reach their final values in 70 ms. Equation (15) determines the theoretical time con- stant 1.0 mH = 25 ms, - L r=--- K, + R - 0 +0.04 f2 which agrees well with the experimental value in Table Fig. 7 shows waveforms in the case of the cross-coupling control scheme. It is clear from (14) that addition of the reactive power feedback gain K, increases the power swing frequency w from 520 rad/s to 740 rad/s. Since (15) shows that the time constant r is independent of Kp and K,, it takes the same time as that in Fig. 6 for p and q to reach their steady-state values. Fig. 8 shows waveforms in the case of the proposed con- trol scheme. No power swing OCCLUS in p and q. From (15), the theoretical time constant is 2 (4 .  1.0 mH 1.2 + 0.04 f2 - = 0.81 ms, L K, + R r=-- which is superior to that of the conventional control schemes. The proposed control scheme has the capability of damping power swings and improving transient charac- teristics. However, significant steady-state errors exist in p and q because Kp and Kq include no integral gain. Fig. 9 shows waveforms in the case of the proposed con- trol scheme with propolrtional and integral (PI) gains for K p  and K,. The integr<al gain is set to TI = 5 ms, so that the steady-state errors are removed from p and q. Note that addition of the integral gain has no effect on damping of power swings. VII. CONCLUSION This paper has proposed a generalized control scheme for the UPFC, along with a comprehensive analysis. The ex- perimental results obtained from the laboratory system rated at 10 kVA has slhown concurrence with analytical ones. This paper has revealed the following essentials: 0 Conventional power-feedback control schemes make the UPFC induce power swings in transient states. 0 The time constant of damping is independent of the active and reactive power feedback gains Kq and Kq. Therefore, the conventional control schemes based on only the power feedback loops are not capable of damping power swings . 0 The feedback gain K, with a physical meaning of a resistor is effective in damping of power swings. 0 The proposed control scheme achieves a quick re- sponse of active and reactive power without causing power swings and producing steady-state errors. RE:FERENCES [l] L. Gyugyi, “Unified Power-Flow Control Concept for Flexible AC Transmission Systems,” IEE Proceedings- C, V01.139, pp.323-331, July, 1992 [2] B. T. Ooi, S. Z. Dai, X. Wang, “Solid-State Series Ca- pacitive Reactance Compensators,” IEEE Trans. on Power Delivery, vol.7, No.2, pp.914-919, 1990 [3] B. S. Rigby, R. G.  Harly, “An Improved Control Scheme for a Series Capacitive Reactance Compensator Based on a Voltage Source Inverter,” IEEE/IAS An- nual Meeting, pp.870-877, 1996 [4] Q. Yu, S. D. Round, L. E. Norum, T. M. Undeland, “Dynamic Control of a Unified Power Flow Controller,” IEEE PESC ’96, pp.508-514, 1996 [5] L. Gyugyi, C. D. Schauder, K. K. Sen, “Static Syn- chronous Series Compensator:A Solid-state Approach to the Series Compensation of Transmission Lines,” IEEE Pans.  on Power Delivery, V01.12, No.1, pp.406- 413, 1997 809 10 ms (a) Simulated waveforms. 10 ms (a) Simulated waveforms. . .  10 ms (b) Experimental waveforms, 10 ms (b) Experimental waveforms. Figure 6: Simulated and experimental waveforms when the phaseangle control scheme is applied. ( K p  = 0.5 V/A, p* = 16.5 kW) Figure 7: Simulated and experimental waveforms when the cross-coupling control scheme is applied. (K,  = Kp = 0.5 V/A, p* = 16.5 kW, q* = 0)  [6] Y. Jiang, A. Ekstrom, "Optimal Controller for the Combination System of a UPFC and Conventional se- ries Capacitors," EPE'97 Vol.1, pp.372-337, 1997 "Unified Power Flow Controller (UPFC) Based on Chopper Stabilized Multilevel Converter," IEEE PESC '97, pp.331-337, 1997 [8] E'. 2. Peng, H. Akagi and A. Nabae, "A New Approach to Harmonic Compensation in Power Systems -A Com- bined System of Shunt Passive and Series Active Fil- ters," IEEE Trans. on IAS, Vol. 26, No. 6, pp. 983-990, 1990 [9] H. Akagi, Y. Kanazawa and A. Nabae, "Instanta- neous reactive power compensators comprising switch- ing devices without energy storage components," 1EEE %ns. on IAS, Vol. 20, No. 3, pp. 625-630, 1984 171 Y. Chen, B. Mwinyiwiwa, Z. Wolanski, B. T. Ooi, P [kWl 10: P [ k v  , r I . . . . ,  Figure 9: Simulated andl experimental waveforms when the proposed control scheme with PI gain is applied. ( K p  = Kq = 0.5 VIA, K, = l.2 VIA, TI = 5 ms, p* = 10 kW, Figure 8: Simulated and experimental waveforms when the proposed control scheme with P gain is applied. (Kp  = Kq = 0.5 VIA, K,  = 1.2 VIA, p* = 14.5 kW, q* = 0) q* = 0) - 10 811 9 b r I  0 .  I .  . . .  - 10 ' - \ '  " ' ' ' " ' ' " 

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Control and analysis of a unified power flow controller

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