There is rapid growth in the demand of electrical power due to increase in the industrial and domestic load. Power demand is greater than the power generation. The solution is installation of new power stations and construction of more transmission lines. This may not be practicable or desirable due to many reasons like heavy cost, large time consumption and environment issues. Adjusting the reactance parameter of the transmission line improve the power flow profile of the power system. There is huge competition in the electricity market and this give scope to increase the capacity of power transmission of existing facilities.

In this situation the role of FACTS devices plays a vital role. The concept of FACTS devices has been given by N.G. Hingorani, in 1988. FACTS devices allow increased utilization of existing transmission network close to its thermal limits. It also avoids the need to construct new transmission lines and improves stability. FACTS devices use power electronics components and switches which are fast acting, more flexible to control the power flow in the transmission lines.

These devices can improve both dynamic and static performance of the system and have many advantages like enhancement of transmission capacity, mitigation of sub synchronous resonance, damping of power oscillation, power flow control, reducing the losses, increase in load ability, limiting short circuit currents, reducing unsymmetrical components, improvement in transient stability, voltage stability while enhancing system security.

TCSC is a series-controlled capacitance reactance that provide continuous power control on the AC line over a wide range. TCSC controllers use series compensating capacitor shunted by TCR. TCR is controlled by firing angle and thus it provides variable inductance and acts as variable inductive reactor.

The variation of X_L with respect to can be given as:

(1)

When α varies from 0 to 90, X_L (α) varies from actual reactance (X_L) to infinity. This variable inductive tuned reactor at firing angle is connected in parallel with the series capacitor, so that the variable capacitive reactance is obtained. This modifies the transmission line impedance. Effective TCSC reactance is given by:

(2)

(3)

(4)

(5)

(6)

TCSC has three different operating regions, viz., inductive, resonance and capacitive, which are based on the reactance provided by the TCSC. The TCSC characteristic curve of reactance is plotted between firing angle and reactance. It shows the operation in both capacitive and inductive regions. In between the two regions there is a resonance region.

A practical TCSC module consists of metal oxide varistor (MOV), for preventing the high capacitor voltages and also for transient stability series capacitor. Circuit breaker to control the insertion of capacitor is in the line. The role of current limiting inductor is restricting the magnitude and frequency of the capacitor current during the capacitor bypass operation. An ultra-high-speed contact (UHSC) across the TCSC valve is used to minimize the conduction losses. (Hingorani & Gyugyi, 2000; Kundur, 1994; Padiyar, 2007; Ramanujam, 2009; Sankar et al., 2010).

2.1 Based on Various FACTS Devices

This classification is based on the simulation and comparison of results using various FACTS devices. Simulink model for uncompensated power system has been developed and then simulated using various different FACTS controllers has been done. Simulation results showing real power and reactive power variation with change in the value of TCSC capacitance were plotted. The major devices from the FACTS family discussed were STATCOM, TCSC, FC-TCR, UPFC, SSSC. The performance evaluation of FACTS controllers for short transmission line has been done. TCSC Open loop control mode has been used for power flow control of transmission line. It has been done by firing angle variation and MATLAB based simulation. The aim of the simulation has been to understand the modeling and studying its influence on the electrical network. Active and reactive power flow, load current, load voltage variation has been observed and plotted. The variation of power flow with the change in value of capacitance were plotted. On increasing the value of capacitance there has been an improvement in real and reactive power flow. It has been concluded that FACTS devices controlled the power flow in a very fast and efficient manner, improved the voltage stability, power quality as compared to the uncompensated system (Akter et al., 2012; Archana & Basha, 2013; Chatterjee et al., 2014; Chavan & Lodhi, 2016; Joshi & Sahay, 2017; Khan et al., 2016; Kumar & Gupta, 2015; Mehta & Prakash, 2018; Mohmedhusein et al., 2017; Murali et al., 2010; Patel et al., 2017; Rakhonde et al., 2018; Saha et al., 2012).

Figure 1. TCSC Module

Different authors worked on various FACTS devices for controlling the power flow transmission in the transmission lines. The comparison of results on uncompensated case and compensation with different devices has been done. Here different firing control circuits were not developed. The response of TCSC to different operating modes and using TCSC power flow decreases in inductive mode and increases in capacitive mode.

2.2 Based on Single Phase Representation of TCSC using Pulse Generator Pulse generator

has been used for giving the firing pulses for analyzing the thyristor, capacitor current, capacitor voltage. For proper working of TCSC in capacitive and inductive mode appropriate pulses should be given. The modeling, simulation and analysis of the TCSC has been done. It has been found that the height of the load angle curve increases with more capacitive reactance of TCSC and decreases with more inductive reactance. Using TCSC, different operating regions were analyzed and plotted on the reactance characteristic diagram by firing angle variation of thyristor. Matlab, Labview and Model Sim were used to analyze the impedance characteristic as well as the transient stability of the system. There is a methodology to control the TCSC in inductive and capacitive region. Active and reactive power, the response of capacitor voltage, TCR currents and capacitor currents for different firing angles were studied. Using TCSC it has been observed that there is a smooth power flow control in transmission line over a wide range. TCSC can be used to increase the power transfer capacity of transmission line by changing the impedance of line. Device has been implemented in 3-phase 400 kV Kalpakam-Khammam line power flow profile analysis. The load has been suddenly increased and results were analyzed for both cases without and with TCSC controllers. Load condition has also been varied. Harmonic analysis, voltage sag and swell analysis has been done. It has been observed that using TCSC there is voltage stability improvement, power flow regulation and minimization of losses, power transfer capability has been enhanced closer to thermal limit without affecting the stability. The constant power control circuit developed has been tuned to for power flow improvement. The load voltage is maintained within limits under varying load conditions and sudden increase in load. TCSC is used for damping power oscillations with various input signals. (Arora et al., 2013a, 2013b; Chatterjee & Mitra, 2015; Choudhary, 2013; Dhawale & Gowda, 2017; Hitendrasinh & Makwana, 2015; Kanwar & Saxena, 2015; Khimsuriya & Solanki, 2016; Kumar & Shrivastava, 2016; Manjusha et al., 2016; Patel et al., 2016a; Prabhakar & Prabhu, 2016; Prudhviraj et al., 2013; Ram et al., 2015; Rathore & Singh, 2016; Sahu et al., 2014; Sangeetha & Padma, 2014; Sangeetha et al., 2015; Singh et al., 2013; Sivasankar & Sirish, 2017; Tiwari & Shukla, 2012).

Here the Simulink has been developed using pulse generator. Active and reactive power for different firing angles were measured. The different operating modes of TCSC, closed loop and open loop system were discussed. Here, the three-phase representation of TCSC, different ways of firing the circuit, fault analysis were not discussed.

2.3 Based on Three Phase Representation of TCSC using SIMULINK for Power Flow and Voltage Profile Improvement

For the proper working of TCSC, the values of TCSC capacitor and inductor should be chosen correctly. Capacitor value is affected by the degree of series compensation. The inductor value is affected by the range of capacitive and inductive region. There is a resonance region between the inductive and capacitive region. Resonance should be avoided as it produces harmonics in the system. It causes high voltage drop across TCR. The operating range of TCSC is lessened by multiple resonant points. To avoid this problem, the resonant factor which is defined as the ratio of resonant frequency of TCSC to power system frequency should be properly selected. When this factor is 1, it meets the condition of resonance and when less than 1 there is the occurrence of only capacitive region. So resonant factor 1 is not permitted and should be less than 3.

For the generation of appropriate conduction angle firing pulses SR flip flop has been used. Ramp signal has been generated by detecting the zero crossing of line current and it has been compared with the firing angle. To reduce the voltage sag using TCSC, PWM Generator Pulse Controller has been developed. The programmable voltage source has been used to vary the voltage with time. Testing with TCSC connected at different locations which comprised testing at sending end, testing at middle end and testing at receiving end of the transmission line, with different lengths of line, i.e.,100 km, 200 km, 300 km, 400 km long has been done. The power transfer capacity has been found to be better at the sending end as compared to other ends.

Power angle and P-V curves were used to assess the stability of the system which showed that there has been an improvement in both synchronous and voltage stability margins using TCSC. The receiving end voltage has been found constant under varying load conditions. Automatic control for TCSC has been done by a fuzzy controller. The controller also maintained the desired power flow in the transmission system.

Results showed that in the inductive mode the TCSC acts as a variable inductance providing device and thus acts as a controlled inductor. Overall inductance of the transmission line has been increased. Power flow decrease in inductive mode. The maximum reactance limit is selected properly to prevent the TCSC from operating in resonant region. It acts as variable capacitance providing device in the capacitive mode of operation. It decreased the reactance of transmission line depending on the firing angle. The power flow is increased in this mode. The efficiency and voltage regulation of the transmission line has been improved on using TCSC (Ankit et al., 2017; Chadar, 2013; Chirantan et al., 2017; German-Sobek et al., 2011; Karthikeyan & Eligo, 2019; Khattak & Khan, 2016; Mittal et al., 2011; Patel & Bhatt, 2012; Patel & Brahmbhatt, 2016; Patel et al., 2016b; Rao et al., 2013; Raut & Reddy, 2017; Raza et al., 2019; Shoiab et al., 2017; Singh et al., 2017; Sonam & Garg, 2016; Soreshjani et al., 2014; Suguna et al., 2016; Vashisth & Jamnani, 2016; Yadav & Dixit, 2017). Here detailed Simulink model has been developed for the three-phase representation of TCSC. It has been also shown that the parameters of TCSC should be properly selected. Here, the multi-bus systems, like IEEE-5 bus or IEEE-14 bus etc., were not analyzed.

2.4 Based on Simulation in Multi Bus System

FACTS controllers are widely used for congestion management in deregulated electricity market, to relieve transmission line overloading, and power flow enhancement. This has been analyzed by 5-bus system using MATLAB. Loss sensitivity index method has been used for the device placement. The simulation has also been carried on IEEE- 14 bus system with and without TCSC. After incorporating the TCSC device, the active and reactive power flow, transient stability has been improved. PSCAD software has been also used to investigate the performance of a double circuit line. Using TCSC the power transfer capability has been increased in the line. There has been an improvement of voltage stability and power flow control. TCSC based on PI, PID, FOPID and fuzzy controller has been designed and modeled. The controller has been tested on IEEE 9 Bus system with Matlab/Simulink software. The transient stability of the system has been improved by using local fuzzy based damping controller using Matlab/ Simulink program. Nine bus system with closed loop TCSC and FOPID controller and fuzzy controller has been discussed. Fuzzy controller has been proved to be more effective. The PID controller has been used to control the UPFC and TCSC performance in 30 bus system, IEEE 14 bus system using Matlab/Simulink platform. The TCSC device should be located at the optimum locations to have a better power flow transmission improvement. With the help of modeling and simulation of 14 bus system in Matlab voltage sag which has been created at the receiving end has been compensated using TCSC (Ashokkumar et al., 2013; Dafde & Bobade, 2017; Del Rosso et al., 2003; Dixit & Jhapte, 2013; Janhavi & Ratnakar, 2015; Kalaimani & Sundaram, 2017; Li et al., 2000; Marlin et al., 2015; Rani et al., 2015a, 2015b; Reddy & Ram, 2010; Selvarasu & Rajan, 2012; Shankar et al., 2015; Singh & Bhatt, 2018; Singh & Bharti, 2017; Srinivas & Krishna, 2016; Titus et al., 2013).

Here issues like the behavior of the system in a multimachine system for a very short period using FACTS device and without FACTS devices were discussed. The coordinated control by TCSC and PSS, the behavior of the system with different fault locations and fault clearing time were not discussed. Attention has not been given to real time simulation approach.

2.5 Based on Transient Stability Enhancement with Three Phase Faults after using TCSC

Analysis on three phase faults has been done without and with TCSC. With this three phase fault, problems occurs in generator voltage and current, infinite bus current and voltage and generator load angle. But after using TCSC transient stability enhancement, power flow improvement, enhancement of voltage stability and rotor angle improvement has been observed. Variation in load has been done from 5000 MW. It has been done till the system becomes unstable. Without the TCSC the system MW. It has been found unstable at 6000 MW, but after TCSC the system voltage stability gets enhanced and the rotor angle values also increased linearly.

The simulation of IEEE 3 machine 9 bus system has been done using SVC, SSSC and TCSC and the results were compared. Simulation results were developed for speed, rotor angle for synchronous machines of the system. It has been found that TCSC controlled the active and reactive power flow much better than SVC. The power curve showed oscillations when faults were applied which after applying TCSC were damped earlier without controller in the system. For real time simulation design, testing and analysis of STATCOM and TCSC has been done. Their relative performance has been studied using Matlab simulation. The results of the simulation were validated with OPAL-RT real time simulator which provide adequate and comprehensive modeling of electric power systems (EPS) containing FACTS. It has been observed that the settling time of relative load angles decreases in a much greater proportion while implementing TCSC as compared to STATCOM (Bainsla & Gupta, 2016; Desai et al., 2015; Garg et al., 2017a, 2017b; Kaur, 2016; Keerthivasan et al., 2005, 2014; Khan et al., 2016; Koundal & Shimi, 2019; Kumar & Surjan, 2013; Mahajan, 2008; Obi et al., 2016; Patel et al., 2002, 2006; Prakash & Tripathi, 2016; Rahman et al., 2021; Raju & Shubhanga, 2014, 2015, 2016; Saini et al., 2013; Salkuti, 2018; Sharma & Hooda, 2012; Shingare, 2015; Thampatty & Lakshmi, 2016).

Here the behavior of the system with the application of faults has been analyzed in detail while hardware issues and detailed mathematical analysis of the circuit has not been discussed.

2.6 Based on Hardware

A laboratory model has been created based on Arduino Uno for testing and implementation of single phase TCSC. The implementation of PLL has been done using Real-Time Application Interface (RTAI) on Linux platform. The variation of capacitance and inductance on the reactance characteristic graph were plotted. A system has been developed for a 2-bus system consisting of injection transformer, ZCD circuit, TCSC circuit having an inductor and capacitor and TRIAC for triggering. There has been improved power flow control, improvement in efficiency and voltage regulation of transmission line with TCSC.

Single machine infinite bus connection has been done through TCSC compensated lossless transmission line. The controller which gave the firing pulses for the TCSC thyristor to turn on has been Arduino ATmega controller. Oscillations were created and the controller has been tested for damping these oscillations. There has been an improvement in stability and oscillations were damped. Techniques like genetic algorithm optimization, ant colony optimization, honey bee algorithm firefly algorithm, particle swarm optimization and other AI based techniques can be used for developing the control algorithm for TCSC.

There are different models of transmission line. The Pi model of transmission line has been developed with lumped parameters. The length of line has been 136 km. It has been simulated in MATLAB with series capacitor and induction motor as load and firing circuit using Arduino controller. There has been a power flow enhancement, improvement in receiving end voltage, active power using TCSC.

Microcontroller 89S52 has been used to develop the closed loop control for TCSC. The different hardware components required were micro-controller 89S52, Microprocessor ADC 0808, Decade counter, LCD 16LC, Optocoupler, Voltage Regulator. The system needed two step-down transformers, one for providing power to microcontroller unit and another for measurement of output voltage. Input and output voltage has been compared using ADC. The triggering pulses to the thyristor were given using Arduino. The Simulink model consisted of ACS 712 (current transducer), OP AMP used for zero cross detection of line current (Arunachalam et al., 2005; Fuerte-Esquivel et al., 2000; Juncheng et al., 2002; Lal et al., 2008; Patel et al., 2016c; Pedekar, 2016; Sridevi, 2014; Tan et al., 1998; Xueqiang & Chen, 1999; Yarlagadda et al., 2012; Zaid, 2011).

Here different hardware set up were used for the power flowcontrol. The circuits were based on Linux, Arduino, PLL, etc. The detailed mathematical analysis, synchronisation techniques and the validity of the mathematical analysis has not been done by EMTP digital simulations.

2.7 Based on TCSC Circuit Analysis, Mathematical and Analytical Modeling and Synchronization Techniques

There are different reference on signals for thyristor firing angles namely line current or capacitor voltage. They have different impact on TCSC dynamic response. When the capacitor voltage has been used as the reference signal there were overshoot and oscillations but when line current has been used there have been no overshoot. Firing control method based on two stages has been developed for smooth switching between the capacitive vernier and inductive vernier modes and it has been done in less than a cycle. Detailed circuit analysis of TCSC using Laplace transformation equations for TCSC for any time period were precisely developed. The mathematical relationship between the fundamental impedance of TCSC and thyristor firing angle has been derived using Fourier analysis and it has been verified using EMTP digital simulations. A mathematical model has been developed which described the characteristics of TCSC.

The fundamental impedance has been calculated and characteristic of a TCSC using a normalized model has been established. The steady state equations of TCSC were given in normalized form which has been useful for design purposes. The finite quality factor Q has been incorporated in the equations and the use of Fourier analysis has been done. Earlier the device has been modeled using Poincare map. Poincare map linearization is used to calculate the passive damping of TCSC. In these approaches the inputs are assumed constant for one half cycle. This limited the effective bandwidth to a range that is adequate for SSR studies. An analytical, linear state space model of TCSC has been developed and the results were verified. The TCSC model is divided into linear and non-linear parts and then the non-linear model has been simulated in frequency domain. A good response and robustness to AC parameters were seen after using the controller (Bruno et al., 2015; Ghosh et al., 2001; Hu et al., 2004; Jindal et al., 2002; Jovcic & Pillai, 2005; Khederzadeh, 2007; Rigby et al., 1999; Rigby, 2002; Yang et al., 2017).

In the present paper extensive literature review has been done for the TCSC FACTS device. FACTS devices increases reliability, provides more flexibility, controllability and stability of power system. TCSC improves power system dynamic performance, boost power transfer capability of the transmission system, enhances the system transient stability, load sharing between parallel lines, improves dynamic stability, minimize system losses, mitigate subsynchronous resonance, damp power swings. This TCSC device is the most efficient and effective device in the modern FACTS power device used in transmission networks. The net var rating of series type of compensator is only 7.2% of that required for a shunt type for achieving the same power transfer. TCSC is based on state-of-the-art highpower electronics. It helps to mitigate transmission line capability constraints by controlling the line impedance and there by offering a better solution from technical, economic and environmental points of view.

Acknowledgment

The authors would like to express sincere thanks to Principal and Department of Electrical and Electronics Engineering, SJB Institute of Technology, Bengaluru, Karnataka, India. for providing extended support to carry out this work.