Optimal Control Design of Static Synchronous Series Compensator for Damping Power System Oscillation

Problem statement: In power systems, there exists a continuous challenge to improve dynamic performance of power system. Approach: The Static Synchronous Series Compensator (SSSC) is a power electronic based device that has capability of controlling the power flow through the line both in steady state and dynamic sate. This study applied the SSSC to damp power system oscillation. The optimal control design is applied to derive the control strategy of SSSC. The simulation results are tested on a Single Machine Infinite bus. The proposed method is equipped in sample system with disturbance. The generator rotor angle curve of the system without and with a SSSC is plotted and compared. Results: It was found that the system without a SSSC has high variation whereas that of the system with a SSSC has much smaller variation. Conclusion: From the simulation results, the SSSC can damp power system oscillation.


INTRODUCTION
The recent blackout in northeast United States, followed by similar incidents in England, Italy and Scandinavia have illustrated the vulnerability in modern power system. It challenges power engineering to find the new methods for improving dynamic performance of power system. Flexible AC Transmission System (FACTS) controllers, based on the rapid development of power electronics technology, have been proposed for power flow control in steady state and dynamic state.  (Barbuy et al., 2009;Prechanon Kumkratug, 2011;Rudez and Mihalic, 2009).
The control strategy of FACTS devices plays an important role for effective improvement of dynamic performance of a power system. Many research used in linear control schemes of SSSC for this purposes. However, modern power system is a large and complex network and disturbances usually cause in nonlinear response (Ahmad and Mohamed, 2009;Hafaifa et al., 2009;Hojat et al., 2010;Samimi et al., 2009;Majee and Roy, 2010;Bagher et al., 2009;Chamsai et al., 2010).
This study presents the control strategy of a SSSC for improving power system dynamic performance. The concept of optimal control is applied to derive control of SSSC. The control strategy is then applied to a SSSC placed in a power system to investigate the improvement of the power system dynamic performance.

MATERIALS AND METHODS
Mathematical model: Consider a single machine infinite bus system with a SSSC as shown in Fig. 1a. The equivalent circuit of the system is shown in Fig. 1b where the SSSC is represented by a variable synchronous voltage source of V s (Ahmad and Mohamed, 2009) and the generator is modeled by a constant voltage source behind transient reactance. Reactance X 1 represents the sum of generator transient reactance and transformer leakage reactance and X 2 represents the equivalent reactance of the parallel lines. The leakage reactance of the series transformer of SSSC can be included either in X 1 or X 2 . The phasor diagram of the system is shown in Fig. 2 for various operating conditions of the SSSC. It can be seen in Fig. 2 that, for given E' and V, the SSSC voltage V s changes only the magnitude of the current but not its angle.
When V s = 0, the current I 0 of the system can be written as Eq. 1: Here, X = X 1 +X 2 . The angle θ of the current can be written as Eq. 2: From Fig. 1b, the general equation of the current can be written as Eq. 3: Here, ∆I is an additional term appears because of the SSSC voltage V s . The electrical output power P e of the generator can be written as Eq. 4: Here, P e0 is the generator output power without SSSC (V s = 0) and is given by Eq. 5: e0 max P P sin = δ where, max P E V/X ′ = . The additional power term ∆P e appears in (4) is due to the SSSC voltage V s and can be written as Eq. 6: When V s lags the current by 90° (α = θ-90°), ∆P e becomes Eq. 7: After some mathematical manipulations of (2), the term cos(δ-θ) can be expressed as Eq. 8: The dynamic of the generator, in classical model, can be expressed by the following differential equations 12 and 13: Here δ, ω, P m , D and M are the rotor angle, speed deviation, input mechanical power, damping constant and moment inertia, respectively, of the generator. Equation 12 and 13 can be rewritten in the following general form Eq. 14: Where:

Control strategy:
The control strategy of the SSSC in a single machine infinite bus system is determined from the pole-placement design method. With liberalized swing equation of Eq. 12 and 13. The new equations in state variable from are given by Eq. 15: Here: X = Matrix consisting of δ and ω, respectively A and B = The constant matrix The u is the input control strategy of SSSC given by Eq. 16: Here, K is the constant gain control.
The quadratic performance index is given by Eq. 17: Here: R = Control weight coefficient Q = State weight coefficient The minimized value of Eq. 17 can be obtained by using Lagrange multipliers method. After some mathematical manipulations, we obtains Riccati equation (18): The gain controls the concepts of optimal control are the elements in P satisfied the Eq. 18.

RESULTS
The proposed control of a power system with a SSSC is tested on system of Fig. 1a It is considered that a three-phase self clearing fault appears at line 1 near bus m and it is cleared at 200 msec. Fig. 2 shows the swing curve of the system without and with a SSSC based optimal control.

DISCUSSION
It can be seen in Fig. 4 that, without the SSSC (k = 0), the maximum and the minimum rotor angle are around 124 and 3 degree, respectively. The damping of the system can be improved by using SSSC based optimal control. With the proposed method, the maximum and the minimum rotor angle are around 100 and 30 degree, respectively and the system can return to stable equilibrium point by 4 sec.

CONCLUSION
This study presents optimal control of a Static Synchronous Series Compensator (SSSC) in a power system to enhance power system dynamic performance. The control strategy of the SSSC is selected very carefully in the concept of optimal control. It is found that the SSSC control depends on both nonlinear function of machine angle and speed. The simulation results are tested on Single Machine Infinite Bus (SMIB) system. From the simulation results, it was fond that the SSSC with proposed control strategy can improve power system dynamic performance.