Dynamic Performance Comparison of Synchronous Condenser and SVC Sercan Teleke, Tarik Abdulahovic, Torbjörn Thiringer, and Jan Svensson, Member, IEEE

1606 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 3, JULY 2008 Dynamic Performance Comparison of Synchronous Condenser and SVC Sercan Teleke, Tarik Abdulahovic, Torbjörn Thiringer, and Jan Svensson, Member, IEEE

Abstract—In this paper, a comparison of the dynamic performance between a conventional synchronous condenser, a superconducting synchronous condenser, and a static var compensator (SVC) is made in a grid setup by simulating different cases that affect the performance of reactive power compensation. The results show that the SVC injects more reactive power and has a better dynamic performance during faults that cause a moderate or minor voltage drop on its terminals, such as single-phase to ground faults in weak grids. The synchronous condensers, on Fig. 1. Single-line diagram with a synchronous condenser connected to grid. the other hand, bring the voltage to the nominal value quicker and show a better dynamic performance for severe faults such as three phase to ground faults in stiff grids. The superconducting synchronous condenser injects up to 45% more reactive power The purpose of this paper is to compare the dynamic perfor- compared to the conventional synchronous condenser during a mance of a conventional synchronous condenser, a supercon- nearby three phase to ground fault. ducting synchronous condenser (SuperVAR)and an SVC during Index Terms—Static var compensator (SVC), superconducting various grid set ups and fault types. synchronous condenser, synchronous condenser.

II. PRESENTATION OF REACTIVE I. INTRODUCTION POWER COMPENSATION DEVICES In this section, the synchronous condenser and the SVC are EACTIVE power compensation is defined as the reactive presented. R power management with the aim of improving the performance of ac power systems [1]. Reactive power compensation is A. Synchronous Condensers viewed from two aspects: load compensation and voltage support. In load compensation, the objectives are to increase the Synchronous condensers have played a major role in voltage power factor, to balance the load and to eliminate current har- and reactive power control for more than 50 years [7]–[12]. monics from nonlinear industrial loads [2], [3]. Voltage support In this section, conventional and superconducting synchronous reduces voltage fluctuation at a given terminal of a transmis- condensers are described. 1) Conventional Synchronous Condenser: sion line [1] and improves the stability of the ac system by in- A synchronous creasing the maximum active power that can be transmitted. It condenser is a synchronous motor without any mechanical load also helps to maintain a substantially flat voltage profile at all [13]. Its field is controlled by a voltage regulator to generate or levels of power transmission, increases transmission efficiency, absorb reactive power to support a system’s voltage or to keep controls steady-state and temporary over-voltages [4], and helps the system power factor at a specified level. Synchronous con- to avoid catastrophic blackouts [5], [6]. densers installation and operation are identical to large electric As reactive power compensation is an effective way to im- machines. A single-line diagram with a synchronous condenser prove the electric power network, there is a need for controlled is shown in Fig. 1. 2) Superconducting Synchronous Condenser (SuperVAR): reactive power compensation which can be done either by synchronous condensers or static var compensators (SVCs), which Only the field winding of the synchronous condenser utilizes utilize power electronic devices. a high-temperature superconductor winding cooled with a cryocooler subsystem to about 35–40 K [14]. The cryocooler modules are placed in a stationary frame and helium gas is Manuscript received April 16, 2007; revised September 9, 2007. This work used to cool the rotor of the machine. The stator winding is was supported by ABB FACTS, Västerås, Sweden. Paper no. TPWRD-00216- 2007. a conventional copper winding. However, the winding is not S. Teleke is with the Department of Electrical and Computer Engi- placed in conventional iron core teeth, since the iron core neering, North Carolina State University, Raleigh, NC 27606 USA (e-mail: saturates due to the high magnetic field, typically 1.5–2.0 T, [email protected]). T. Abdulahovic and T. Thiringer are with the Department of Energy and En- imposed by the field winding. Only the stator yoke (back iron) vironment, Chalmers University of Technology, Göteborg SE-412 96, Sweden uses magnetic iron to provide magnetic shielding and to carry (e-mail: [email protected]; [email protected]). flux between adjacent poles. The absence of iron in most of J. Svensson is with ABB FACTS, Västerås SE-721 64, Sweden (e-mail: jan.r. [email protected]). the magnetic circuits in these machines results in a very low Digital Object Identifier 10.1109/TPWRD.2007.916109 synchronous reactance (typically 0.3–0.5 p.u.). More details of

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Fig. 2. Single-line diagram of the SVC used in the simulations. Fig. 3. Single-line diagram of the used grid setup. the SuperVAR, including performance features, design config- urations, and maintenance challenges can be found in [15]. with 0.9 power factor; 10 kV with inertia constant of 2 s. These units are connected to a 36 kV substation via 25 MVA trans- B. Static VAR Compensator (SVC) formers (10% leakage reactance, 36/10 kV). Moreover, a resis- Maintenance requirements of conventional synchronous con- tive and an inductive load of 16 MW and 12 Mvar are also condensers rose interest in the development of static VAR systems nected to the substation via a 25 MVA transformer (10%, 36/10 [16]. Several papers have studied modeling [17]–[20] and the kV). A capacitor bank of 8 Mvar is connected to each 10 kV application of SVCs [16], [21], [22]. A typical SVC, composed load bus in order to keep the 10 kV bus within 95% to 100% of thyristor-switched capacitors (TSCs) and thyristor-controlled of nominal voltage during full load operation. The short-circuit reactors (TCRs), together with filters, is shown in Fig. 2. The ratio (SCR), which is the ratio between the short-circuit power filters (FCs) are used to remove low-frequency harmonics pro- measured at the 36-kV bus and the load total apparent power, is duced by the TCR and to produce reactive power. With proper 3.9 which represents a medium strong system. coordination of the capacitor switching and reactor control, the Four 8 MVA synchronous condensers or one 32 Mvar SVC reactive power output can be varied continuously between the are/is connected to the 36 kV bus via a 32 MVA transformer capacitive and inductive ratings of the equipment. (10%, 13.8/36 kV), to provide reactive power compensation, es- The compensator operates to regulate the voltage of the trans- pecially in case of faults occurring in the network that transfers mission system at a selected terminal. However, the maximum the power from the generator to the factory. The network con- obtainable capacitive current decreases linearly with the system sists of two 200 km lines with an X/R ratio of 15 where the fault voltage since the SVC becomes a fixed capacitor when the max- occurs in one of the lines, which will cause the breakers to open imum capacitive output is reached. Therefore, the conventional after a 250 ms delay, and accordingly to disconnect the faulted thyristor-controlled SVC rapidly deteriorates its voltage support line and connect it back 500 ms after the fault is cleared. capability with the decrease of system voltage. In the 36 kV substation, there is a 70 MVA step down transformer (10%, 130/36 kV) that adapts the voltage level to the factory. Finally, the 130 kV source is an infinite bus. III. SETUP OF PERFORMANCE STUDY

A. Setup of Compensators IV. SIMULATION RESULTS FROM THE STUDY For comparison of the SuperVAR [15], the conventional syn- To compare the synchronous condensers with the SVC, the chronous condenser and the SVC, a grid setup using PSCAD is parameters of the grid have been altered. The configuration made, where the parameters are given in the Appendix. The con- without any reactive power compensation is denoted by WOC. ventional synchronous condenser and the SuperVAR are using The cases with conventional synchronous condensers and the type DC2A exciter and the type ST1A exciter, respectively. SuperVARs connected to the 36 kV bus are denoted by CON The 32 Mvar SVC consists of two TSC banks where each bank and SCO, respectively. Finally, the configuration with the SVC has 35.8% of the rated power, two 3rd and 5th harmonic filters connected to the 36 kV bus is denoted by SVC. where each carries 10.2% of the rated power and two 7th and 11th harmonic filters where each carry 4% of the rated power. A. Different Fault Types For reactive power consumption, one TCR with the size of 38% In this section, single-phase-to-ground, two-phase-to-ground, of the rated power is employed. In the connection of the SVC and three-phase-to-ground faults are simulated and analyzed. to the network, a 10% coupling transformer with the size 1) Single Phase to Ground Fault: To determine the perfor- of rated reactive power is used, where the high voltage side was mance of the different compensators, a single phase to ground connected with Y grounded. fault in the middle of one of the two lines is applied. The resulting positive-sequence voltage, reactive power injection and B. Grid Setup the speed of the induction machines are displayed in Figs. 4–6. The grid setup used to compare the performance of the Su- From Fig. 4, it is clear that without any reactive power com- perVAR, the conventional synchronous condenser and the SVC, pensation, the system collapses and with compensation it sur- is shown in Fig. 3. The setup shows a factory, where the main vives. Moreover, after the fault, the SVC brings the voltage back load consists of two sets of 50 induction machines of 500 HP to 1 p.u. quickly and accurately.

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Fig. 6. Speed of the induction machines. Single-phase-to-ground fault.

Fig. 4. Positive-sequence voltage using the synchronous condensers and the SVC. Single-phase-to-ground fault.

Fig. 7. Positive-sequence voltage using the synchronous condensers and the SVC. Two-phase-to-ground fault.

Fig. 5. Reactive power injected (20 Hz LP-ﬁlter) by the synchronous condensers and the SVC. Single-phase-to-ground fault.

The reactive power injections are shown in Fig. 5. The SVC injects reactive power after a delay caused by the delay in the controller, whereas the synchronous condensers react instan- taneously due to the change in the terminal voltage. However, after the initial delay, the SVC injects more reactive power due to faster voltage control compared to the synchronous condensers that have significantly larger time constants due to Fig. 8. Reactive power injected (20 Hz LP-filter) by the synchronous con- their field windings. densers and the SVC. Two-phase-to-ground fault. It can be observed that at the instant of the fault, the Su- perVAR injects more reactive power than the conventional synchronous condenser, which is the consequence of the lower synchronous reactance. However, as we reach the instant of the fault clearance, the conventional synchronous condenser reaches the performance of the SuperVAR. This is the result of the quicker exciter utilized for the conventional synchronous condenser. It can be seen in Fig. 6 that the induction machines collapse without any reactive power compensation and the speed of the induction machines recover quicker with the SVC. 2) Two-Phase-to-Ground Fault: The resulting positive-sequence voltage, reactive power injection and the speed of the induction machines for a two phase to ground fault are displayed Fig. 9. Speed of the induction machines. Two-phase-to-ground fault. in Figs. 7–9. Now, the positive-sequence voltage drops more with the SVC compared to the case using the synchronous condensers. More- until the line is connected back. This is due to that the SVC pro- over, the positive-sequence voltage reaches the nominal value vides reactive power proportional to the square of its terminal quicker with synchronous condensers. The explanation is that voltage, so severe voltage drops on its terminals limit its reac- the SVC injects less reactive power during and after the fault tive power injection. And, this also means that the speed of the

TABLE I MINIMUM POSITIVE-SEQUENCE VOLTAGE DURING THE FAULT AT 36-kV BUS

TABLE II MAXIMUM INJECTED REACTIVE POWER BY SYNCHRONOUS CONDENSERS AND SVC DURING THE FAULT Fig. 10. Positive-sequence voltages in the presence of the synchronous condensers and the SVC. Three-phase-to-ground fault.

To observe the effect of reactive power compensation on minimum voltage levels more clearly during different fault types, minimum positive-sequence voltage levels at 36 kV bus during the fault are put on Table I. The maximum injected reactive power during the fault for the conventional synchronous condenser, the SuperVARand the Fig. 11. Reactive power injected (20 Hz LP-ﬁlter) by the synchronous con- SVC can be observed in Table II. densers and the SVC. Three-phase-to-ground fault. It should be mentioned that the values marked with * are obtained with a 40 MVA SVC (where the system did not collapse) due to collapse with a 32 MVA SVC during the three phase to ground fault. It can be seen from Table I and II that the SVC injects more reactive power when there is less voltage drop on 36 kV bus whereas, when the voltage drops more, such as the case observed in two phase and three phase to ground faults, synchronous condensers inject more reactive power than the SVC.

B. Different Fault Location To determine the sensitivity of the fault location, the fault location is changed to 50 km from the source which corre- Fig. 12. Speed of the induction machines. Three-phase-to-ground fault. sponds to 25% of the line length and 150 km from the source which corresponds to 75% of the line length and a single phase to ground fault is applied. The minimum positive-sequence induction machines drops more with the SVC due to the lower voltage during the fault for different fault locations is shown positive-sequence voltage. in Fig. 13, which displays that the minimum positive-sequence 3) Three Phase to Ground Fault: The resulting positive-se- voltage drops more when the fault location is far away from the quence voltage, reactive power injection and the speed of induc- source. Moreover, the synchronous condensers perform better, tion machines for a three-phase-to-ground fault are displayed while the performance of the SVC drops. This is in spite of the in Figs. 10–12. Here, the positive-sequence voltage drops even fact that the positive-sequence voltage only drops to 0.86 p.u. more with the SVC and it can be noticed that the system col- As the fault is closer to the load, the reactive power injec- lapses due to insufficient injection of reactive power. tion performance drops slightly with the SVC, while the per- However, the reactive power injected by the synchronous con- formance of the synchronous condensers improves during the densers is increasing. The larger the positive-sequence voltage fault (Fig. 14). However, after the fault has been cleared and the drop, the more reactive power is provided by the synchronous line is restored, the voltage rises up, which helps the SVC to in- condensers due to that the injected reactive power is propor- ject more reactive power due to its quicker response, while the tional to the difference of terminal voltage and the internal ma- synchronous condensers react slower due to their field winding chine voltage induced in the stator by the rotating magnetic field. dynamics. The speed of the induction machines drop more with the SVC The minimum speed of the induction machines drops more and hence the induction machines can not recover their speed when the fault is closer to the load, as shown in Fig. 15. The after the fault has been cleared. minimum speed is strongly related to the minimum positive-

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Fig. 13. Minimum positive-sequence voltage at the 36 kV bus. Different fault Fig. 16. Positive-sequence voltage at the 36 kV bus. g a PXR. locations on the transmission line.

Fig. 17. Positive-sequence voltage at the 36 kV bus. g a SXV.

It takes more time for the synchronous condensers to bring the voltage level back to the nominal value by decreasing the SCR to 2.4 (Fig. 16). Also it can be seen that the voltage during the fault drops only 10% which represents a small error for the ex- citers, resulting in a small increase in the ﬁeld current of the syn- Fig. 14. Maximum injected reactive power by the synchronous condensers and chronous condensers. When the fault is cleared, the induction the SVC. Different fault locations on the transmission line. machines begin to consume a lot of reactive power to recover their speed. Due to this fact and to the slow response of the synchronous condensers for low voltage changes, the voltage con- tinues to drop and it takes more time for the synchronous condensers to bring the voltage back to the nominal value. The SVC shows better performance in this case and keeps the voltage at 1 p.u. even during the fault, but due to the measurement delay it produces overvoltages during instants of the fault clearance and the line restoration. For the system when the SCR is increased to 5.8, which corresponds to a strong system, the system will not collapse even if it is not supported by reactive power compensation devices, Fig. 15. Minimum speed of the induction machines. Different fault locations which can be noticed in Fig. 17. However, reactive power com- on the transmission line. pensation units help in restoring the voltage in the 36 kV bus quicker. In Fig. 17, the advantages and disadvantages of each sequence voltage. When the fault is closer to the source, the device mentioned before can be conﬁrmed. speed is higher due to larger reactive power injection by the Injection of more reactive power by the SVC during and SVC. This advantage diminishes as the fault is closer to the load after the fault, which is seen in Fig. 18, explains the reason for and the voltage drop during the fault is increased. bringing the voltage back to the nominal value quicker with the SVC which can be noted in Figs. 16 and 17. Trends shown in C. Different SCR Fig. 18, clearly underline the better performance of the SVC in To observe the effect of different SCR, the short-circuit ca- the case of weak grid systems. pacity seen at the 36 kV bus is changed to have a SCR of 2.4 (double line length to 400 km) and 5.8 (half line length to 100 V. C ONCLUSION km). The results are obtained by applying a single phase to In this paper, it was found that the SVC injects more reactive ground fault while keeping the other parameters unchanged. power and shows better performance during faults that caused

TABLE VI PARAMETERS OF SUPERVAR

Fig. 18. Maximum injected reactive power by the synchronous condensers and TABLE VII the SVC. Various SCRs. EXCITER PARAMETERS FOR SUPERVAR

TABLE III PARAMETERS OF CONVENTIONAL SYNCHRONOUS CONDENSER

REFERENCES [1] T. J. E. Miller, Reactive Power Control in Electric Power Systems. New York: Wiley, 1982. [2] E. Wanner, R. Mathys, and M. Hausler, “Compensation systems for TABLE IV industry,” Brown Boveri Rev., vol. 70, pp. 330–340, Sep./Oct. 1983. [3] G. Bonnard, “The problems posed by electrical power supply to in- SATURATION DATA OF CONVENTIONAL SYNCHRONOUS CONDENSER dustrial installations,” Proc. Inst. Elect. Eng. B, vol. 132, pp. 335–340, Nov. 1985. [4] A. Hammad and B. Roesle, “New roles for static VAR compensators in transmission systems,” Brown Boveri Rev., vol. 73, pp. 314–320, Jun. 1986. [5] N. Grudinin and I. Roytelman, “Heading off emergencies in large electric grids,” IEEE Spectr., vol. 34, no. 4, pp. 43–47, Apr. 1997. TABLE V [6] C. W. Taylor, “Improving grid behaviour,” IEEE Spectr., vol. 36, no. EXCITER PARAMETERS OF CONVENTIONAL SYNCHRONOUS CONDENSER 6, pp. 40–45, Jun. 1999. [7] J. A. Oliver, B. J. Ware, and R. C. Carruth, “345 MVA fully water- cooled synchronous condenser for Dumont station part I: Application considerations,” IEEE Trans. Power App. Syst., vol. PAS-90, no. 6, pp. 2758–2764, Nov. 1971. [8] H. A. Landhult and B. Nordberg, “345 MVA fully water-cooled synchronous condenser for Dumont station part II. design, construction and testing,” IEEE Trans. Power App. Syst., vol. PAS-90, no. 6, pp. 2765–2777, Nov. 1971. [9] Y. Katsuya, Y. Mitani, and K. Tsuji, “Power system stabilization by synchronous condenser with fast excitation control,” in Proc. Int. Conf. Power Syst. Technol., Perth, Australia, Dec. 4–7, 2000, vol. 3, less voltage drop on its terminals, such as single phase to ground pp. 1563–1568. faults or faults in weak grids. However, during severe faults, [10] J. M. Van Coller, R. G. Koch, T. D. J. Hennessy, R. Coney, and G. such as three phase to ground faults and severe faults in stiff Topham, “The effect of a synchronous condenser on the voltage dip environment-as expressed in terms of the eskom ABCD dip chart,” in grids, the synchronous condensers perform better and bring the Proc. IEEE. AFRICON, Stellenbosch, South Africa, Sep. 24–27, 1996, terminal voltage to the nominal value quicker. This is especially vol. 2, pp. 620–625. true for the case with the SuperVAR, which injects up to 45% [11] S. Nakamura, T. Yamada, T. Nomura, M. Iwamoto, Y. Shindo, S. Nose, A. Ishihara, and H. Fujino, “30 MVA superconducting syn- more reactive power compared to the conventional synchronous chronous condenser: Design and it’s performance test results,” IEEE condenser. Trans. Magn., vol. M-21, no. 2, pp. 783–790, Mar. 1985. Moreover, for future studies, it will be very interesting to ex- [12] S. Kalsi, D. Madura, and M. Ross, “Performance of superconductor dynamic synchronous condenser on an electric grid,” in Proc. IEEE pand the analysis and comparison to include the STATCOM and Transm. Distribution Conf. Exhibit., 2005, pp. 1–5. the authors are planning to present a paper on this subject. [13] C. Corvin, “SLAC synchronous condenser,” in Proc. Particle Acceler- ator Conf., Dallas, TX, May 1–5, 1995, vol. 4, pp. 2114–2116. [14] S. Kalsi, K. Weeber, H. Takasue, C. Lewis, H.-W. Neumueller, and APPENDIX R. D. Blaugher, “Development status of rotating machines employing superconducting ﬁeld windings,” Proc. IEEE, vol. 92, no. 10, pp. Tables III and IV are used for the conventional synchronous 1688–1704, Oct. 2004. condenser. [15] S. Kalsi, D. Madura, R. Howard, G. Snitchler, T. MacDonald, D. Bradshaw, I. Grant, and M. Ingram, “Superconducting dynamic syn- Table V is used for the exciter DC2A of the conventional chronous condenser for improved grid voltage support,” presented at synchronous condenser. the IEEE Transm. Distrib. Conf., Dallas, TX, Sep. 10, 2003. Table VI is used for calculating parameters for the SuperVAR. [16] T. R. Boyko, K. A. Ewy, M. P. Hausler, A. Kara, C. S. Miller, D. R. Tor- genson, and E. P. Weber, “Integration of a static var system into Fargo substation,” in Proc. Int. Conf. AC DC Power Transmission, London, Table VII is used for the exciter ST1A of the SuperVAR. U.K., Sep. 17–20, 1991, pp. 241–247.

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[17] A. M. Gole and V. K. Sood, “A static compensator model for use with Tarik Abdulahovic was born in Srebrenik, Bosnia electromagnetic transients simulation programs,” IEEE Trans. Power and Herzegovina, in 1976. He received the B.S Del., vol. 5, no. 3, pp. 1398–1407, Jul. 1990. degree from the University of Tuzla in 2001 and [18] S. Lefebvre and L. Gerin-Lajoie, “A static compensator model for the the M.S. degree in electric power engineering EMTP,” IEEE Trans. Power Syst., vol. 7, no. 2, pp. 477–486, May 1992. from Chalmers University of Technology (CTH), [19] S. Y. Lee, S. Bhattacharya, T. Lejonberg, A. E. Hammad, and S. Gothenburg, Sweden, in 2006, where he is currently Lefebvre, “Detailed modeling of static var compensators using the pursuing the Ph.D. degree. electromagnetic transients program (EMTP),” IEEE Trans. Power The main focus of his research is the generation Del., vol. 7, no. 2, pp. 836–847, Apr. 1992. and propagation of high-frequency disturbances in a [20] IEEE Special Stability Controls Working Group, “Static VAR compen- sea-based wind park consisting of modern converter- sator models for power ﬂow and dynamic performance simulation,” controlled wind turbines. IEEE Trans. Power Syst., vol. 9, no. 1, pp. 229–240, Feb. 1994. [21] L. Gerin-Lajoie, G. Scott, S. Breault, E. V. Larsen, D. H. Baker, and A. F. Imece, “Hydro-Quebec multiple SVC application control stability study,” IEEE Trans. Power Del., vol. 5, no. 3, pp. 1543–1551, Jul. 1990. Torbjörn Thiringer received the Ph.D. degree in [22] A. E. Hammad, Applications of Static Var Compensators in Utility 1996 from Chalmers University of Technology, Power Systems 1987, pp. 28–35, Application of Static Var Systems for Gothenburg, Sweden. System Dynamic Performance, IEEE 87TH0187-5-PWR. Currently, he is a Professor in the Department of Energy and Environment at Chalmers University of Technology. His areas of interest are issues per- taining to grid integration of wind energy into power systems as well as power-electronic converters in general.

Jan Svensson (S’96–M’98) received the M.Sc., Lic. Eng., Ph.D., and D.Sc. degrees from Chalmers Uni- Sercan Teleke was born in Ankara, Turkey, in 1983. versity of Technology, Göteborg, Sweden, in 1991, He received the B.S. degree in electrical and elec- 1995, 1998, and 2002, respectively. tronics engineering from Middle East Technical Uni- From 1998 to 2002, he was an Assistant Professor versity, Ankara, in 2005 and the M.S. degree in elec- with the Department of Electric Power Engineering, tric power engineering from Chalmers University of Chalmers University of Technology. Currently, he Technology, Göteborg, Sweden, in 2006. He is cur- is with ABB Power Systems, Västerås, Sweden, rently pursuing the Ph.D. degree in electrical engi- involved in development of FACTS and HVDC neering at North Carolina State University, Raleigh. transmission, especially design and control of His research interests are in the areas of power- light-concept devices. His interests include control electronics applications to power systems and design of power electronics in power systems, power quality, energy storage, and wind and control of special purpose electrical machines. power.

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