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Transmission System Reactive Compensation and Stability Enhancement Using 48-pulse Static Synchronous Series Compensator

Transmission System Reactive Compensation and Stability Enhancement Using 48-pulse Static Synchronous Series Compensator (SSSC). A.M. Sharaf & M. S. El-Moursi Department of Electrical/Computer Engineering, University of New Brunswick PO Box 4400-UNB, Fredericton, N.B., Canada, E3B 5A3

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Transmission System Reactive Compensation and Stability Enhancement Using 48-pulse Static Synchronous Series Compensator

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  1. Transmission System Reactive Compensation and Stability Enhancement Using 48-pulse Static Synchronous Series Compensator (SSSC) A.M. Sharaf & M. S. El-Moursi Department of Electrical/Computer Engineering, University of New Brunswick PO Box 4400-UNB, Fredericton, N.B., Canada, E3B 5A3 Emails : sharaf@unb.ca , m.shawky@unb.ca

  2. Outline • Objectives • FACTS-Technology • SSSC-Device • Digital Simulation Model • 48 pulse-GTO VSC Converter • Decoupled d-q Current Control • Results • Conclusion • Future Work

  3. Paper Objectives • This paper presents the concept of Flexible Alternating Current Transmission Systems (Facts) and the Static Synchronous Series Compensator (SSSC) device comprising a 48 pulse-GTO DC-AC VSC converter model. • This SSSC-compensator device can provide a fully controllable series injected (Buck/Boost) compensating voltage over an identical specified capacitive and inductive range, independently of the magnitude of the transmission line current. In addition to series reactive/capacitive compensation using an external dc sustained power supply, it can also compensate for any feeder voltage drop across the inductive component of the transmission line impedance. • This paper presents a novel d-q decoupled controller using Phase Locked Loop (PLL). • SSSC-Device Performance and Control Validation.

  4. FACTS-Technology • Flexible AC Transmission System (Facts) is a new integrated concept based on power electronic switching converters and dynamic controllers to enhance the system utilization and power transfer capacity as well as the stability, security, reliability and power quality of AC system interconnections Use • Power Flow Control • Series Compensation • Voltage Regulation of Long Transmission System • Economic Operation • Voltage Stability Enhancement • Harmonic SSR Torsional Mode Damping by Detuning Resonance Conditions

  5. FACTS KEY DEVICE Static Synchronous Series Compensator (SSSC) • A static synchronous Series Compensator operated without an external energy source as Reactive Power with output voltage is in quadrature with and fully controllable independently of the transmission line current for the purpose of increasing or decreasing the overall reactive voltage drop across the transmission line and thereby controlling the electric power flow. • The SSSC FACTS device can provide either capacitive or inductive injected voltage compensation, if SSSC-AC injected voltage, (Vs), lags the line current IL by 90º, a capacitive series voltage compensation is obtained in the transmission line and if leads IL by 90º, an inductive series compensation is achieved.

  6. Theory of the SSSC • Figure 1 shows a single line diagram of a simple Transmission line with an inductive transmission reactance, XL, connecting a sending- end (S.E.) voltage source, , and a receiving end (R.E.) voltage source, respectively. Figure 1: A Sample Power Transmission System. S.E. R.E. (1) (2) (3) (4)

  7. (5) • The expression of power flow given in eq.1 and eq. 2 become (6) Where Xeff is the effective total transmission line reactance between its sending and Receiving power system ends, including the equivalent “variable reactance” inserted by the equivalentinjected voltage (Vs) (Buck or Boost) by the SSSC-FACTS Compensator.

  8. Digital Simulation Model • Figure 2 shows a simple power system 230-kV network grid equipped with SSSC rated at ±70 Mvar and its novel controllers which connected in series with the transmission system. The 48 pulse (VSC) SSSC connected in series with transmission line at bus B1 by coupling transformer T1. Linear Load Figure 2: The single line diagram representing the SSSC interface at S.E. of a Radial Distribution System.

  9. The feeding AC network is represented by an equivalent Thevenin is at (bus B1) where the voltage source is a 230 kV with 10000 MVA short circuit level (resistor 0.1 pu and an equivalent reactance of 0.3 pu) followed by the 230 kV radial transmission line connected to bus B2. The full system parameters are given in Table 1. Table 1 The power system parameter

  10. High pulse 48-Pulse Voltage Source Converter VSC-Building Block • Figure 3 shows the 48-pulse voltage converter comprises four identical 12-pulse GTO converters linked by the four 12-pulse transformers with proper phase-shifted windings to ensure the 48 pulse operation. Figure 3: 48 pulse GTO’s Voltage source Converter VSC-Cascade of (4) 12 pulse Converters

  11. Figure 4 depicts the net resultant 48- line-to-neutral voltage of the 48-pulse GTO-Converter scheme representing the SSSC compensator scheme. Figure 4: 48-pulse GTO’s converter output line to neutral voltage (Vph).

  12. Basic Control Scheme of the SSSC • The direct power flow control is not effective under some AC network contingencies. Therefore the equivalent (or injected voltage) control level that maintains the desired dynamic impedance of T.L is recommended. • Figure 5 shows the basic function of effective decoupled control system to keep the SSSC voltage, Vc, in quadrature with the transmission line current, and only control the magnitude of Vc injection to meet the desired reactive capacitive compensation level. Figure 5: Decoupled Control Structure of the SSSC.

  13. The SSSC equivalent impedance Xs is measured as the ratio of the q-axis voltage of the SSSC device, , to the magnitude if line current. This equivalent inserted impedance is then compared with reference level of the compensation impedance, (SXL). A PI controller generates the required small phase displacement angle, to charge or discharge the dc capacitor (C). The final output of the control system is the desired phase angle of the SSSC device output voltage, . • The novel decoupled control strategy for the SSSC device is validated for both capacitive and inductive operating modes under severe network disturbances and switching load contingencies.

  14. The main function of the SSSC device is to regulate the feeder power flow PL. This can be accomplished by either direct control of the AC line current or indirect control by compensating the impedance, Xs via a Buck/Boost compensating injected voltage, Vs. Xref = positive ; Vs lags IL by 90° plus (Capacitive Compensation) Xref = Negative ; Vs Leads IL by 90° plus (Inductive Compensation) Validation Testing • Capacitive Operation The SSSC device is connected at time t = 0.1 sec, while only load 1 ( P = 0.5 pu and Q = 0.15 pu) is in attached to the system. • At t = 0.5 sec, load 2 ( P = 0.235 pu and Q = 0.135 pu) is switched on for a duration 0.4 sec and then disconnected at t =0.9 sec. • The SSSC device operates in the capacitive mode with phase angle of at almost -90º. The SSSC device while operating in this capacitive mode also injects an equivalent capacitive reactance of -0.35 pu in series with the transmission line. • When load 2 is switched on, the capacitor Vdc and therefore the reactive power are increased in order to satisfy the specify . Since the SSSC device is in the capacitive mode, the injected voltage, Vq, lags the line current by 90º as shown in Figure 5(g).

  15. Digital Simulation Results (a) (b) (c) (d) (e) (f) (g) (h) Figure 6 : Digital simulation results of the sample study 230 kV radial transmission system attached to the SSSC device at bus B1 & operating in capacitive mode.

  16. The digital simulation is carried out for an inductive load 1 with (P = 0.167pu and Q = 0.017 pu) (at rated voltage) while this load is fully connected from the start point of the digital simulation. • In the case of an overvoltage state, an inductive series compensation is required to decrease the voltage at load bus. • When load 2 a capacitive load with (P = 0.6 pu and Q = -0.45 pu) is switched in at t = 0.5 sec for duration 0.4 sec to the distribution network, this is cause overvoltage so the inductive compensation is also required. • The SSSC FACTS device is switched to the power system at time t = 0.1 sec and the dc capacitor is charged by the real power flow from the transmission line to the dc-side capacitor. • When load 2 is switched on at t = 0.5 sec the SSSC device operates in the inductive mode and the series injected voltage, Vs, leads the transmission line current, , by 90º as shown in Figure 6(g). • The SSSC FACTS device provides a fast inductive series compensation for the power system.

  17. Digital Simulation Results (b) (c) (d) (a) (e) (f) (g) (h) Figure 7 : Digital simulation results of the sample study 230 kV radial system attached to SSSC device at bus B1 & operating in Inductive mode.

  18. Conclusions 1- The SSSC device is a controlled/injected voltage source that injects a near sinusoidal AC voltage in series with the transmission line. 2- This controlled voltage (Vs) is almost in quadrature with the transmission line current, thereby can effective as an inductive or a capacitive equivalent reactance in series with the transmission line. 3- The dynamic power flow in the Transmission line always decreases when the injected voltage by the SSSC in an inductive reactance mode and the power flow increases when the injected voltage by the SSSC in a capacitive reactance mode. 4- The Phase Locked Loop (PLL) has an block inherent time delay (about 8 millisecond). This has a great effect on the dynamic performance of the SSSC device. 5- The paper presents a novel high pulse 48 pulse GTO full model of the SSSC FACTS device as well as the new dynamic decoupled controller that minimizes the PLL-loop effect on controller fast response robustness.

  19. Future Work • Extensions of control strategies to reduce inherent nonlinearities and AC week network interactions with controller causing sluggish/unstable operation

  20. References 1- N.G. Hingorani, L. Gyugyi, Understanding FACTS, Concepts and Technology of Flexible AC Transmission Systems, IEEE press 2000. 2- L. Gyugyi, K.K. Sen, "Static Synchronous Series Compensator: A Solid-State Approach to the Series Compensation of Transmission Lines", IEEE Trans. on Power Delivery, Vol. 2, No. 1, pp. 406-417, 1997. 3- Amir H. Norouzi, A.M. Sharaf, " An Auxiliary regulator for the SSSC Transient Enhacement", IEEE 35th North American Power Symposium, Rolla, Missouri, Oct, 2003. 4- Kalyan K. Sen" SSSC-Static Synchrounous Series Compensator: Theory, Modeling, and Applications" IEEE Transactions on Power Delivery, Vol. 13, No.1, January 1998. 5- Amir H. Norouzi, thesis " Flexible AC Transmission Systems: Theory, Control and Simulation of the STATCOM and SSSC" Electrical & Computer Engineering Department, University of New Brunswick, 2003. 6- Ekanayake, J. B., Jenkins, N., "A three advanced static var compensator", IEEE Trans. on Power Delivery, Vol. 11, no.1, pp.540-545. 7- C.Schauder, H. Mehta, " Vector analysis and control of advanced static var compensator". Proc. IEE International Conference on AC and DC Transmission Paper No. 345, pp. 299-309, 1991.

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