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Abstract: This paper presents Artificial Neural Networks based protection for series compensated lines and the
problem faced by the distance protection scheme used for transmission lines protection when they are compensated in
series with the FACTS devices like TCSC. The use of TCSC creates certain problems to distance protection schemes as
there is a change in the apparent impedance measured by the relay. The change in the apparent impedance measured is
corrected by using Artificial Neural Networks (ANN) for the correct operation of the relay, which is extensively tested
using MATLAB Simulation for different conditions. The results obtained show that by using the Artificial Neural
Networks training methods the operation accuracy of the relay used for distance protection is improved.
Keywords: Artificial Neural Networks, Distance protection, FACTS devices, Thyristor Controlled Series Capacitor,
I. INTRODUCTION
Presently due to the deregulation of power industry and the restriction faced by power industry due to energy,
environmental and regulatory issues, the main challenge is to improve the power transfer capability and also to improve
the system integrity of the given transmission facility. The above mentioned problem can be addressed by using series
compensation. Series compensation when introduced in power systems influences the power flow in a particular
network segment which reduces active power losses and also prevents system - sub synchronous oscillations[1]-[3].
FACTS device are normally used in power systems which takes care of the power transfer capability, voltage
stability and power oscillation damping. Thyristor Controlled Series Capacitor (TCSC) is one of the series
compensation device used but, the use of TCSC creates certain problems for conventional distance protection schemes
due to the changes in the apparent impedance measured by the relay. The apparent impedance seen by the relay is
influenced by the uncertain variation of series compensation voltage [4]-[5].
Providers of transmission facilities normally prefer distance protection as the primary protection system, As series
compensation is introduced in power transmission lines it needs additional attention while choosing the protection
scheme as series compensation requires extra care in addition to the well know protection challenges on HV
transmission networks . In modern days power system protection systems has become intelligent has it uses micro
processor based techniques like Artificial Neural Networks to improve distance protection of transmission line with
TCSC compensation which uses back propagation algorithm and is extensively tested using MATLAB Simulation for
different conditions[6]-[11].
This paper is organized as follows: In the first section the introduction to Protection of Series Compensated
Transmission Lines using Artificial Neural Networks is discussed, in the second section brief introduction of Thyristor
Controlled Series Capacitor(TCSC) is presented, in the third section Artificial Neural Networks are discussed, in the
fourth section Distance Protection basics and effect of SC Compensation on Protection is presented, in fifth section
simulation methods used and results are presented and in sixth section conclusions based on the results obtained are
presented.
II. THYRISTOR CONTROLLED SERIES CAPACITOR
For many years series compensation technique is used to adjust the power transfer between two stations by
adjusting the net series impedance of the line. Installation of a series capacitor is a conventional and established method
of increasing transmission line capacity, by reducing the net series impedance, thus increasing power transmission. As
this method is well established method but, due to the limitation of its slow switching time it is replaced by Thyristor
controllers, which are fast acting devices due to which rapid and continuous control of line compensation is possible.
Thyristor Controlled Series Capacitor (TCSC) is one of the controllers used for series compensation. TCSC is a FACT
device which is a combination of thyristor-controlled reactor (TCR) parallel with capacitor. TCR is a variable inductive
reactor XL (α) controlled by firing angle α. The variation of XL (α) with respect to α can be calculated by [12]: