OVERVIEW OF THYRISTOR CONTROLLED SERIES CAPACITOR (TCSC) IN POWER TRANSMISSION SYSTEM E03421 MAZILAH BINTI A RAHMAN
1. Introduction In early 90s, a general concept in term of operation and planning of power systems was proposed and widely discussed since then. The Flexible Alternating-Current Transmission Systems (FACTS), incorporating a wide range of possibilities for better utilization. Improvement of voltage and current limits on the power electronics devices leads to a fast development of FACTS in the last decade. FACTS are defined by the IEEE as “AC transmission systems incorporating power electronics-based and other static controllers to enhance controllability and increase power transfer capability. And among the proposed FACTS devices, possibly the Thyristor Controlled Series Capacitor (TCSC) has given the best results in terms of performance and flexibility. It can have various roles in the operation and control of power systems, such as scheduling power flow; decreasing unsymmetrical components; reducing net loss; providing voltage support; limiting short-circuit currents; mitigating sub-synchronous resonance (SSR); damping the power oscillation; and enhancing transient stability. Research on control strategies for TCSC can be traced back to 1966 when Kimbark [3] analysed the improvement in transient stability of power systems by using switched series capacitors. This was a very simple control; the maximum amount of compensation was inserted at the same time that the faulted line was switched out. Ramarao et al .[4] .[4] proposed an optimal control of the capacitance. Pontryagin’s maximum principle was used to obtain bang-bang control. It was one of the earliest closed-loop control systems for series compensation.
2. TCSC in Power Transmission System
reactor
thyristor
Fig. 1 Simple Diagram of TCSC
I. Circuit and Function Figure 1 shows the simple diagram of TCSC. This structure is modeled as
X = X 0 where ∆ X =
+ ∆ X
f ( ∆ ω g ) . ∆ X is the capacitive reactance.
TCSC comprised of a series capacitor bank, shunted by a Thyristor Controlled Reactor (TCR), to provide a smoothly
ADVISOR
GORO FUJITA
variable series capacitive reactance. It is a one-port circuit in series with transmission line; it uses natural commutation; its switching frequency is low; it contains insignificant energy storage and has no DC-port. Insertion of a capacitive reactance in series with the line’s inherent inductive reactance lowers the total, effective impedance of the line and thus virtually reduces its length. As a result, both angular and voltage stability gets improved. Furthermore, in contrast to capacitors switched by circuit breakers, TCSC will be more effective because thyristors can offer flexible adjustment, and more advanced control theories can be easily applied.
II. Characteristics and Problems In this chapter, we will be discussing about some characteristics of TCSC that have been discussed all over the region for about 20 years ago. These chapter have been summarized and we could see clearly some part of the character and the problem occurs within it. Two major characteristic of TCSC that will be discussed below is sub-synchronous resonance that would appear in the generator oscillation and transient voltage stability controlling.
(i) Sub-synchronous Resonance (SSR) SSR has gained its name from the fact that the frequency of interest happen to lie in a region below the synchronous frequency of the network. The main concern in SSR studies is a significant oscillatory electromagnetic torque developed on the generator rotor and the possibility of shaft damage from torsional stresses [5]. As in general, any device that controls or responds rapidly to power or speed variations in the sub-synchronous frequency range is a potential source for excitation of oscillations [5]. Simulations of the IEEE First Benchmark Model [6] show that even without a fault, sub-synchronous oscillations appear in that system. They just take more time to develop. In other words, the occurrence of a fault may speed up the process ( Torque Amplification), Amplification), while the possibility of SSR is mainly due to the nature of the combined mechanical-electrical system. The eigenvalue results for the first benchmark model show that the thyristor switching can have a significant effect on system stability and that the effect varies with the firing angle [7]. The papers on the inclusion of the TCSC in SSR studies are centered on two field installations, Western Area Power Administration’s Administrat ion’s Kayenta site [8,9], and Bonneville Power Administration’s Slatt substation [10,11]. The Kayenta system was analyzed with time-domain simulation of a detailed model of the AC system, a machine and TCSC with controls [8]. While for the Slatt system, the effect of the TCSC was evaluate by measuring the electrical damping torque as a function of machine rotor speed. Analog and digital simulators modeled the AC system, machine and TCSC controls.
(ii) Transient voltage stabilizing control The inserted series capacitor will also affects the reactive power distribution in the interconnected power system. Thus, paper [12] suggests that TCSC be used to enhance voltage stability. In a transient voltage stability analysis, TCSC is operated in a big range for a long period, which is very different from small signal voltage stability. And this means that the
operation limits of a TCSC must be well considered. For other practical reasons, such as availability of the measurement and the burden of calculation, relatively simple controllers are used in the installed TCSC [13,14]. There are also other characteristics regarding TCSC and it’s such as power damping oscillation, fault current, non-linear control model, dynamic characteristic etc.
3. Discussions For two major characteristic that have been discussed above, MATLAB, the friendly user of technical computing have been used as study-system case. In this case, we used 3-Machine and 9-Bus System as transient stability study model. The analysis is done first with the model and the result are simulated by MATLAB.
Fig. 2
3-Machine and 9-Bus System Transient Stability Study Model (MATLAB).
Fig. 2 shows a MATLAB diagram of the study- system case. The research procedures are divided into two cases; that are study system with non-TCSC control and with TCSC control. The result has been simulated by using MATLAB programming. Then, we will see frequency deviation comparison for both studies. The result of the system model for non-TCSC control is that we can see after the fault, amplitude of frequency deviation is becoming large. Then, after the model were inserted with TCSC, the system becoming stepped-out. Further studies shows that TCSC will prevent the system becoming stepped out only if relatively simple weighting matrix been operated. The comparison of eigenvalue in the system also been studied. For a result, the stability level of the system is improved by TCSC. The difference of the improvement mode is according to TCSC installation location in the study-system. The result is the best installation for TCSC is near generator which is colce to the fault generator. The results of the small-signal investigations in this section have thus shown that in both modes of operation, the closed-loop power flow controller has an increasing influence on the damping of the system’s electromechanical oscillations as its time is made shorter.
4. Conclusion This paper has addresses a quick overview of thyristor controlled series capacitor as one the best proposed devices in FACTS family and its applications in power transmission system. There are also two major characteristic of TCSC and that are the existing of sub-synchronous resonance in generator oscillation controller and TCSC as transient voltage stabilizing controller. And for both characteristic, we proposed a 3-machine and 9-Bus System Transient Stability Study Model simulated by MATLAB friendly technical programming as study-case system. It has considered the application of TCSC for closed-loop control of power flow in both constant power and constant angle modes of operation. The results indicate that the power flow controller (TCSC) operation has an important influence on both the small signal and transient stability characteristic of the system． References [1] A.A Edris et al , “Proposed Terms and Definitions for Flexible AC Transmission System (FACTS)”, IEEE Trans. on Power Delivery, vol. 12, no 4, Oct. 1997 [2] L. A. S. Pilotto, W. W. Ping and et al, “Determination of needed FACTS Controllers That Increase Asset Utility of Power System,” IEEE Trans. Power Delivery, vol.12, no.1, Jan 1997, pp.364-371 [3] Kimbark, E.W.: “Improvement of System Stability by Switched Series Capacitors”, IEEE Trans. Power Apparatus Syst. 1966, PAS-85, (2), pp. 180-188 [4] Ramarao, N. and Reitan, D.K.: “Improvement of Power System Transient Stability using Optimal Control: Bang-bang Control of Reactance”, IEEE Trans. Power Apparatus Syst., 1970, PAS-89, (5/6), pp. 975-984 [5] IEEE Committee Report, “Readers Guide to Sub-synchronous Resonance”, IEEE Trans. On Power System., vol 7, no. 1, Feb 1992, pp. 150-157 [6] IEEE Commitee Report, “First Benchmark Model for Computer Simulation of Sub-synchronous Resonance.”, IEEE Trans. On Power Apparatus and Syst, vol. 96, no. 5, Sept/Oct. 1977, pp. 1565-1572 [7] S.G. Jalali, R.H. Lasseter, I. Dobson,: “Dynamis Response of a TCSC “, IEEE Trans. On Power Delivery, vol. 9, no. 3, July 1994, pp. 1609-1615 [8] N. Christl et al.: “Power System Studies and Modeling for the Kayenta 230KV Substation Advanced Series Compesation.”, IEEE Interl. Conference on Thyristor and variable static equipment for Ac and DC Transmission, London, 1991 [9] N. Christl, R. Hedin et al.: “Advanced Series Compensation (ASC) with Thyristor Controlled Impedance.”, Cigre 14/37/38-05, Paris, Aug-Sept. 1992 [10] S. Nyati et al.: “Effectiveness of TCSC in Enhancing Power System Dynamics: An Analog Simulator Study.”, IEEE Trans. On Power Delivery, vol. 9, no.2, April 1994, pp. 1018-1027 [11] E. Larsen, C. Bowler, B. Damsky, S. Nillson: “Benefits of TCSC “. Cigre Paper 14/37/38-04, Paris 1992 [12]A. E. Hammad, “Comparing the Voltage Control Capabilities of Present and Future VAR Compensating Techniques in Transmission System.”, IEEE Trans. On Power Delivery, vol. 11, no. 1, Jan. 1996, pp. 475-484 [13] X. Zhou, J. Liang, “ Overview of Control Schemes for TCSC to Enhance the Stability of Power Systems”, IEEE Proc. Generation, Transmission and Distribution, vol. 14, no. 2, March 1999, pp 125-134. [14]A. D. Del Rosso, C. A. Canizares, V. M. Dona, “ A Study of TCSC Controller Design for Power System Stability Improvement.”, IEEE Trans. on Power Systems, vol. 18, no. 4, Nov. 2003, pp. 1487-1496
