he challenge facing power system engineers today is to use the existing transmission facilities to greater effect [1] . Power system should retain its synchronism during and after all these kind of disturbances. Therefore the transient stability is an important security in power system design. So FACTS has come to help the power system engineer [2][3] . The SVC is one of the important FACTS devices whose effectiveness for voltage control is well known. The AC power transmission system has diverse limits, classified as static limits and dynamic limits [4][5] .Traditionally, fixed or mechanically switched shunt and series capacitors, reactors and synchronous generators were being used to enhance same types of stability augmentation [6] . For Authors ? ? ?: Department of EEE, Rajshahi University of Engineering & Technology Rajshahi-6204, Bangladesh. e-mails: [email protected], [email protected], [email protected] many reasons desired perfor-mance was being unable to achieve effectively. A static VAR compensator (SVC) power compensation on high voltage transmission networks and it can contribute to improve the voltage profiles in the transient state and therefore, it can improve the qualities and performances of the electric services [6] . An SVC can be controlled externally by using properly designed different types of controllers which can improve voltage stability of a large scale power system [7] . However, in this study, With a view to get better performance, A new PID has been designed & proposed for SVC to injects V qref externally for the improvement of power system stability. The dynamic nature of the SVC lies in the use of thyristor devices (e.g. GTO, IGCT) [6] . Therefore, thyristor based SVC with PID controllers has been used to improve the performance of 2-machine power system.

An SVC is a controlled shunt susceptance(B) which inject reactive power (Q net ) into thereby increasing the bus voltage back to its net desired voltage level. If bus voltage increases, the SVC will inject less (or TCR will absorb more) reactive power, and the result will be to achieve the desired bus voltage[Fig. 1]. Here, +Q cap is a fixed capacitance value, therefore the magnitude of reactive power injected into the system, Q net , is controlled by the magnitude of -Q ind reactive power absorbed by the TCR. The basis of the thyristor controlled reactor (TCR) which conduct on alternate half-cycles of the supply frequency. If the thyristors are gated into conduction precisely at the peaks of the supply voltage, full conduction results in the reactor, and the current is the same as though the thyristor controller were short circuited. SVC based control system is shown in Fig. 1 [6] . for secondary loop controller. Cascade control is mainly used to achieve fast rejection of disturbance before it propagates to the other parts of the plant.PID controller in cascade architecture is the best choice compared to conventional single loop control system for controlling nonlinear processs.

In voltage regulation mode (the voltage is regulated within limits as explained below). b). In VAR control mode (the SVC susceptance is kept constant). From V-I curve of SVC, From Fig. 2 [3], V=V ref +X s .I,: In regulation range(-Bc max <B<Bc max ) V=I/Bc max , , : SVC is fully Capacitive(B=Bc max ) V=1/Bl max , : SVC is fully inductive(B=Bl max )

This example described in this section illustrates modelling of a simple transmission system containing 2-hydraulic power plants[Fig. 3]. SVC has been used to improve transient stability and power system oscillations damping. The phasor simulation method can be used. A single line diagram represents a simple 500 kV transmission system is shown in Fig. 3[ 9]. Another machine is swing generator.PID is used in the model to add damping to the rotor oscillations of the synchronous machine by controlling its excitation current [5] . Any disturbances that occur in power systems due to fault, can result in inducing electromechanical oscillations of the electrical generators. Such oscillating swings must be effectively damped to maintain the system stability and reduce the risk of stepping out of synchronism.

V.

The load flow solution of the above system is calculated and the simulation results are shown below. Two types of faults: A. single line to ground fault & B. Three phase fault have been considered.

Consider a 1-phase fault occurred at 0.1s & circuit breaker is opened at 0.2s (4-cycle fault), Without SVC, the system voltage, power & machines oscillates goes on unstable[Fig. (5,7,9)]. But if SVC(without controller) is applied then voltage becomes stable within 3s [Fig. 6], power becomes within 3s[Fig. 8] & machines oscillation becomes stable within 4.5s [Fig. 10]. All results has been summarized in table-III

The Tyreus-Luyben procedure is quite similar to the Ziegler-Nichols method but the final controller settings are different. Tyreus-Luyben PID Controller, the values of delay time, rise time, and settling time are better in comparison with Modified Ziegler-Nichols method. Also this method only proposes settings for PI and PID controllers. These settings that are based on ultimate gain and period are given in table 1. For some control loops the measure of oscillation, provide by ¼ decay ratio and the corresponding large overshoots for set point changes are undesirable therefore more conservative methods are often preferable such as modified Z-N settings In this method, the parameter is selected as T i =?,T d =0. Using the proportional controller Action

Notice that the PID controller tuned by proposed Ziegler-Nichols tuning methods rules as follows,

The network remains same [

The performance of the proposed PID Controller with SVC has been summarized in the table -III. In table-III, ? (infinite time) means the system is unstable, SVC rating in MVA. The network is simulated in three steps; without SVC, With SVC, SVC with proposed CASCADE PID Controller.

The obtained results of the conducted investigations along with the associated simulation demonstrated clearly that the proposed (desinged) cascade PID controller enhanced significantly the effectiveness of the integrated SVC in the examined power controller because of shorter stability time, simple designed system. In cascade PID Controller may be highly suitable as a SVC, low cost & highly efficient controller. The proposed cascade PID for SVC is proved to be very effective of robust power system within very shortest possible time for both steady state & dynamic conditions. These proposed cascade PID Controller can be applied for any interconnected multi-machine power system network for stability improvement.

These controller can be applied to another FACTS devices namely SSSC, STATCOM, UPFC whose controllers may be controlled externally by designing different types of controllers which also may be tuned by using different algorithm i.e. Fazzy logic,ANN, Genetic algorithm, FSO etc. for both transient and steady state stability improvement of a power system.