Power System Transient Stability Enhancement by Tuning of SSSC and PSS Parameters Using PSO Technique

In this paper, the tuning design of SSSC and PSS was examined in increasing the damping of system oscillations and improve the stability of the power system during disturbances. The design problem of the SSSC controller and PSS is designed as problem of optimization and the technique uses (PSO) technique to find for optimal control parameters. By minimizing the objective function based on the speed deviation and time domain, which deliberately deviates at the oscillation angle of the alternator rotor to improve performance of transient stability of the system. The proposed controllers are tested on the system of weak bonding ability exposed to severe disturbance. Nonlinear simulation results are presented to demonstrate the proposed controller's effectiveness and its ability to give efficient damping. It is also noted that the proposed controllers of SSSC and PSS greatly improves the power system stability.

(FACTS) devices. FACTS controllers have the ability to quickly control the network mode in different operating conditions and have made it able to improve the stability of the power system (Sinha, 2011) "The Static Synchronous Series Compensator(SSSC) Devices are one of the family of facts devices" that can be installed with "power system in series with transmission lines". A SSSC is an electrical device for providing fast-acting reactive power compensation on high voltage transmission networks (Abdel-Magid,2004;Castro M, 2007).The SSSC is electrical controller have the ability to control the energy flow effectively and can be inject series capacitive or inductive voltage to provide fast reaction compensation on transmission lines of high voltage and it can help to enhance in transient state (Jowder,2005;Falehi,2011). The A SSSC can be controlled by the design of the PSS which can enhance the steady and dynamic performance of the state of the power system on a large scale (Ali, 2013).The tuning between PSS and SSSC can be increase the damping in some oscillation modes.
In some previous studies FACTS proposes stability controllers based on artificial intelligence (Shayeghi, 2010). In this paper, this design was made using particle swarm optimization (PSO) to obtain full efficiency and safety in "the single and coordinated design of PSS and SSSC for damping control. A strong design problem of PSS and SSSC-based damping controller is converted to an optimal problem to adjust the controller parameters, PSO is employed in the present work to optimally tune the parameters of the PSS and SSSC controller" . The designed objective is to increase the oscillations damping and improve the stability of power system, to test performance of the proposed controllers ,its applied with a single-machine power system, subjected to severe disturbance under different operating conditions ,the results simulation is carried out by using MATLAB Simulink.

Power System Modeling
"The single-machine power system shown in Fig. 1 is considered in this study" ."The system includes a generator, transformer, SSSC and two parallel transmission lines"

2-1 Generator model
Synchronous births are one of "the important components of any power system. There are three differential equations representing the dynamic behavior of a synchronous generator. These include mechanical, dynamic and electrical equations" (Jalilvand, 2011): Where, δ is angle of rotor , ω is speed of rotor, pm is input power ,peis output power;M is constant of inertia , D is damping coefficient, Efd is the field voltage,T´do is time constant of the open circuit field;iq isaq-axis armature current; id is d-axis armature current, ,x´d is the transient reactance of d-axis xd is reactance of d-axis .

2-2 The Excitation System And PSS Model
The function of the excitation signal is complementary to increase torque damping rotation in the rotor generator. Increasing the gain of AVR with Exciter leads to increase probability of maintaining the generator synchronization during the occurrence of large disturbances. Therefore the PSS used to improved vibrations damping in the generator rotor during disturbances , the dynamic model of excitation and PSS system in figure 2 can be described as follows (Chow, 2004;Kamwa , 2005): The five parameters of PSSs are gain KA and four time constants T1 to T4 must to be optimal values .

SSSC model
The SSSC "acts as a series compensator whose output voltage is fully controllable, independent of line current and kept in quadrature with it, with the aim of increasing or decreasing the voltage drop across the line, therefore controlling the power the basic voltage Vq is in quadrature with respect to line current, and can either provide capacitive compensation if V q leads I by π/2 rad or inductive compensation if V q lags I by π/2 rad". A relatively small active power exchange is required to compensate for coupling transformer and switching losses, and maintain the required DC voltage (Khadanga and Satapathy, 2015). Indeed, the SSSC can be controlled in two different operation modes: The SSSC relies on a dc capacitor fed voltage supply electrical converter that generates a three phase voltage at fundamental, that is then injected to a cable through a electrical device connected asynchronous with line ."The active and reactive power in transmission line are controlled by controlling the amplitude and the angle of Vq ,SSSC give the voltage Vq to the transmission line". The DC capacitor differential equation can be expressed as below (Shakarami , 2010): Where, C is the capacitor value, Kr is the ac to dc voltage ratio and Mr and the modulation ratio and, Vdc is the dc voltage, and Id and Iq are d and q axis the line current ,"The SSSC block diagram as shown in figure 3. "The lead-lag controller is preferred by power system utilities, due to the ease in its on-line training and the lack of guaranteed stability by some adaptive and variable structure approaches". "The input and output signals of the controller are Δω and Vq" ,"During dynamic conditions the series injected voltage Vq is modulated to damp system oscillations". The effective value of Vq in dynamic conditions is: Vq= Vqref+ΔVq"

Stabilizers Structure
The Transfer function of PSS and SSSC-based controllers will be: In structure of controller , TW is usually predetermined . Searching will done for optimal set of the stabilizer parameters are K, T1,T2,T3 and T4

Objective Function and Problem Optimization
The power system and improve transient performance of the power system after subjected to a large disturbance, during large disturbance the deviations in rotor speed Δω and power angle δ is happen, to minimize deviations we will use the objective function below: X is represents the controllers parameters and t1 is a range of time The design problem of SSSC and PSS can be framed as follows: Minimize J Subject to K min ≤ K≤ K max Ti min ≤ Ti≤ Ti max i=1,------,4

3-2 Application of PSO
Application the technique of Particle Swarm Optimization (PSO) for optimization the objective function of equation (8), can be used routines from PSO toolbox [Birge B.]. For calculation of objective function, we can calculation "the modified velocity and position of each particle by calculating the current velocity and the distance from the " pbest, to gbestby using equations (9) and (10) (Soliman , 2008;Valle, 2008): ( + 1) = + ( + 1) Where = , = ,W is weight of inertia ,c1is cognitive factor and , c2 "social acceleration factors r1 and r2 are random numbers the value of them between 0 and 1.
The flow chart of PSO algorithm" as shown in figure. 5.

4.The Tuning Of SSSC And PSS
The parameters of PSS and SSSC-based stabilizer can be coordinated tuned individually together to get the best performance of system . The tuning of PSS and SSSC-based stabilizer is addressed at the normal loading point. By using equations (9) and(10) PSO tune the parameters of the two controllers individually in order to minimize the objective function and also PSS and SSSC parameters are tuned at the same time using ( PSO) applying to the problem of optimization above to find the optimum settings for the proposed controllers .
In this study, the parameters of SSSC and PSS are tuned at nominal loading (P=1 pu ,Q=0.15pu) to study the effect of the proposed controllers, it is considered three different load conditions as shown in table (1),values of PSO algorithm parameters are set as shown in table (2) the ranges of the typical parameters that will optimize for PSS and SSSC-based controllers set as shown in table (3) for the tuning and the simulation.

4.1The Optimal Parameters of SSSC And PSS
The final optimum parameters for single and coordinated design of the two controllers are given in table (4)

Simulation of Non Linear Time Domain
The proposed designs for the SSSC and the PSS controllers have been simulated with power system in Figure 1.The 6-cycle 3-ph fault at midpoint one of parallel transmission lines (TL2)of the power system considered for studies of non-linear simulation with three operating conditions are shown in Table 1, for the study effect of the two optimal controllers SSSC and PSS when using individually and coordinated design with the power system.

5-Conclusion
When the disturbance is happening in power system, the parameters of this power system(V,P,Q) become unstable even with PSS operating ,if a SSSC is connected to a power system, then parameters of system can be stable but if the parameters of PSS and SSSC are tuned by using PSO Algorithm , then parameters of system which make it stable in a quick way. For the design problem of proposed controllers, the objective function was developed to increase damping the power system. The PSO technique used to find the optimal parameters of two controller. Then, the parameters of SSSC with PSS controllers are tuned in the same time make them more effective to increase the oscillations damping and improve the stability of power system . A non-linear simulation achieved by using a MATLAB version 13 SimPowerSystems toolbox for power system with optimal design of SSSC and PSS when the system subjected to 3-phase fault in one of transmission lines. The obtained results of simulation show that the tuning of two controller in same time more effectiveness to increase damping of system oscillations and their ability in improving power stability under severe disturbance with different conditions loading especially at large loading .It can be concluded that the stability of power system is improved when parameters of two controllers are tuned individually but with tuning design of PSS and PSS the oscillations damping of power system more increased and power transient stability is enhanced

APPENDIX
The data of power system in figure 1are: