AN EFFICIENT ALGORITHM FOR POWER SYSTEM OSCILLATION DAMPING USING TCSC

CONTROLLER R.Sasikala

Assistant Professor/Electrical and Electronics EngineeringOxford Engineering College, Trichy-2

sasi_142k6@yahoo.co.in

ABSTRACT

This project presents a systematic procedure for modeling, simulation and optimal tuning the

parameters of a TCSC controller, for the power system stability enhancement. For the simulation

purpose, the model of single machine infinite bus power system with TCSC controller is

developed in MATLAB/SIMULINK. The design problem of TCSC controller is formulated as

an optimization problem and simulated annealing algorithm is employed to search for the

optimal TCSC controller parameters. By minimizing the objective function the stability

performance of the power system is improved. The results are obtained from simulations validate

the effectiveness of proposed modeling and tuning approach for power system stability

improvement. The simulation results also show that the proposed TCSC controller is effective in

damping the power system oscillations.

INTRODUCTION

Power system oscillatory stability refers to the damping of electro mechanical oscillations

occurring in power systems with oscillation frequency in the range of 0.2 Hz to 2Hz. Low

frequency oscillations are observed when large power systems are interconnected by relatively

weak tie lines. These oscillations may sustain and grow to cause system separation if no adequate

damping is available. Low frequency oscillations occur due to inadequate damping torque in

generators, causing both local –mode oscillations (1.0Hz-2.0Hz), Inter area mode oscillations

(0.2Hz-1.0Hz). These low frequency oscillations are the consequence of the development of

interconnection of large power systems. Low frequency oscillations in a power system

constraints the capability of power transmission, threatens system security, and damages the

efficient operation of the power system. Since system damping is small at best, it is reasonable to

use new devices for more damping.

This project presents a new approach to the implementation of the Facts devices based on

a single machine power system. The single machine infinite bus system is designed using the

transfer function approach. Thyristor controlled series compensator has been used for damping

low frequency oscillation in a single machine infinite bus power system. The TCSC controller

parameters are optimized using a simulated annealing algorithm. The simulation process is

carried out using MATLAB/SIMULINK. The system response with and without optimization is

compared to get an idea about their effectiveness in damping system oscillations.

THYRISTOR CONTROLLED SERIES COMPENSATOR (TCSC)

TCSC is one of the most important and best known FACTS devices, which has been in

use for many years to increase line power transfer as well as to enhance system stability. The

main circuit of a TCSC is shown in Fig. 1. The TCSC consists of three main components:

capacitor bank C, bypass inductor L and bidirectional thyristors SCR1 and SCR2. The firing

angles of the thyristors are controlled to adjust the TCSC reactance in accordance with a system

control algorithm, normally in response to some system parameter variations. According to the

variation of the thyristor firing angle or conduction angle, this process can be modelled as a fast

switch between corresponding reactance offered to the power system.

When the thyristors are fired, the TCSC can be mathematically described as follows:

Where iC and iL are the instantaneous values of the currents in the capacitor banks and inductor,

respectively; iS the instantaneous current of the controlled transmission line; v is the

instantaneous voltage across the TCSC. Assuming that the total current passing through the

TCSC is sinusoidal; the equivalent reactance at the fundamental frequency can be represented as

a variable reactance XTCSC. The TCSC can be controlled to work either in the capacitive or the

inductive zones avoiding steady state resonance. This mode is called venire control mode. There

exists a steady-state relationship between the firing angle α and the reactance XTCSC. This

relationship can be described by the following equation

Where,

Xc = Nominal reactance of the fixed capacitor C.

Xp = Inductive reactance of inductor L connected in parallel with C.

σ = 2(π-α) = Conduction angle of TCSC controller

MODELLING OF POWER SYSTEM WITH TCSC

The single-machine infinite-bus power system shown in Fig. 6 is considered. The

generator is equipped with a PSS and the system has a TCSC installed in transmission line. In the

figure XT and XL represent the reactance of the transformer and the transmission line

respectively, VT and VB are the generator terminal and infinite bus voltage respectively.

Fig: 2 Single machine infinite bus power systems with TCSC

STRUCTURE OF PROPOSED TCSC DAMPING CONTROLLER

The commonly used lead–lag structure is chosen in this study as a TCSC controller. The

structure of the TCSC controller is shown in Fig. 9. It consists of a gain block with gain K T, a

signal washout block and two-stage phase compensation block as shown in figure. The phase

compensation block provides the appropriate phase-lead characteristics to compensate for the

phase lag between input and the output signals. The signal washout block serves as a high-pass

filter, with the time constant TW, high enough to allow signals associated with oscillations in

input signal to pass unchanged. Without it steady changes in input would modify the output.

From the viewpoint of the washout function, the value of TW is not critical and may be in the

range of 1 to 20 seconds.

Fig. 5 Structure of the TCSC controller

The damping torque contributed by the TCSC can be considered to be in to two parts.

The first part KP, which is referred as the direct damping torque, is directly applied to the

electromechanical oscillation loop of the generator. The second part KQ and KV, named as the

indirect damping torque, applies through the field channel of the generator. The damping torque

contributed by TCSC controller to the electromechanical oscillation loop of the generator is:

ΔTD = TDω0Δω= KPKTKDΔω

Where, TD is the damping torque coefficient.

The transfer function of the TCSC controller is:

Where, u and y are the TCSC controller output and input signals, respectively. In this structure,

Tw is usually prespecified and is taken as 5 s. Also, two similar lag-lead compensators are

assumed so that T1=T3 and T2=T4. The controller gain KT and time constants T1 and T2 are to be

determined. In this study, the input signal of the proposed TCSC controller is the speed deviation

Δω and the output is change in conduction angle Δσ. During steady state conditions Δσ = 0 and

XEff = XT+XL-XTCSC(α0). During dynamic conditions the series compensation is modulated for

damping system oscillations. The effective reactance in dynamic conditions is: XEff = XT+XL-

XTCSC (α), where σ = σ0+Δσ and σ=2(π-α), α0 and σ0 being initial value of firing & conduction

angle respectively.

PROBLEM FORMULATION

In our project we consider three parameters to be minimized so that the damping is perfect.

Our aim is to maintain the roots in the left half of the s plane. More over the roots should be

sufficiently towards the – infinity. . A system with s=-3, will offer better stability than with s=-1.

Because even if the load fluctuations or speed deviation will not affect the system badly and the

damping will be faster. So here we concentrate on how to minimize the value of J for an

optimum value of sigma.

For objective function calculation, the time-domain simulation of the power system

model is carried out for the simulation period. It is aimed to minimize this objective function in

order to improve the system response in terms of the settling time and overshoots. The task in SA

algorithm is that we have to get the minimum damping from the controllers. This is analogous to

the minimum energy reached by the system with respect to cooling schedule.

APPLICATION OF SA TO THE PROPOSED CONTROL SCHEMES

Simulated Annealing Algorithm has been applied to search for optimal settings of the

optimized parameters of the proposed control schemes. In our implementation, the search will

terminate if the stopping criteria is achieved. The stopping criterion is occurring when set no of

iterations and consecutive rejections are reached. The final setting of the optimized parameters is

given in the Table1.

Optimal settings of the TCSC controller parameters

KT T1 T3

0.343 0.252 0.1281

TEST SYSTEM

Single machine infinite bus system

A 1000 MW hydraulic generation plant (M1) is connected to a load center through a

long500Kv, 700 km transmission line. The load center is modeled by a 5000 MW resistive load.

The load is fed by the remote 1000MVA plant. A load flow has been performed on this system

with plant M1 generating 950 MW. The line carries 944 MW which is close to its surge

impedance loading. To maintain system stability after faults, the transmission line is series

compensated at its center by Thyristor controlled switched compensator. The machine is

equipped with a hydraulic turbine and governor (HTG), Excitation system, and power system

stabilizer.

PROPOSED ALGORITHM:

Step 1:

The solutions are randomly generated for the objective function.

Step 2:

If the solutions results in the condition that new energy < min F, then stop the iterations

and display the solutions and the value of the objective function. New energy is the value of

object function for different solutions. Old energy is the same but in the previous iteration. Min F

refers to –inf.

Step 3:

If the new energy is less than the previous energy also the solution is accepted, the

previous solutions are actually replaced by the current solution which satisfies the best minimum

case.

Step 4:

If the randomly generated solutions does not give the minimum answer in the objective

function then the solutions are rejected.

Step 5:

Such rejections are counted by a counter.

Step 6:

The acceptance probability function P(e,e',T) was defined as 1 if e' < e, and exp((e − e') /

T) otherwise. Probability of acceptance is 1 only when the new energy is less than old energy.

Step 7:

The solution is accepted even when

rand < exp( (old energy –new energy)/(k*T) )

Step 8:

When the number of iterations reaches the set number or the consecutive rejections occur,

then we stop the process and display the best solutions and its value.

FLOW CHART

Flow chart for simulated annealing algorithm

SIMULATION RESULTS

Power deviation without controller

Power deviation with TCSC controller

TCSC reactance (reference and measured)

Deviation in power angle δ with controller

CONCLUSION

A method to analyze power system damping enhancement by the application of TCSC

has been developed and tested in a single machine infinite bus system. Power system based

controllers can damp the oscillations effectively only the particular operating conditions. In

addition with FACTS devices can damp the oscillations effectively all operating conditions.

From the result it is evident that the settling time and overshoot greatly reduced when using facts

devices. The TCSC controller design problem is formulated as an optimization problem. Then,

Simulated Annealing algorithm has been proposed to search for optimal settings of controller

parameters. The proposed approach has been applied to single machine power systems with

different loading conditions and system configurations. The simulation results shows that, the

TCSC controller provides good damping of low frequency oscillations and improves greatly the

voltage profile. The analysis reveals the effectiveness of the proposed SATCSC to damp out

local as well as inter area modes of oscillation. The nonlinear time domain simulation results

show that, the proposed SATCSC’s work effectively over a wide range of loading conditions and

system configurations.