Modeling of a Static VAR Compensator (SVC) as a Voltage Regulator for a Heavy Loaded Transmission Line. Bekir Mumyakmaz1
Abstract In this study, a Static Var Compensator (SVC) is modelled by using Matlab/Simulink in order to regulate the receiving end voltage of a 230 kV transmission line. Only one phase of the transmission system is represented. The SVC consists of one 10 MVAr TCR bank and two 10 MVAr TSC banks connected to each phase of the load bus. The SVC controller monitors the receiving end voltage of the transmission line and successfully sends appropriate pulses to the compensator thyristors to obtain the susceptance required to keep the voltage level of the line end within ± % 5 tolerance limits. The desired voltage regulation at the transmission line end can be achieved in less than a half seconds using the modelled system under changing heavy and light loading conditions. Keywords: FACTS, SVC, Voltage Regulation, Simulation
1. INTRODUCTION It is known that the shunt reactive compensation can increase the steady-state transmittable power and control the voltage profile along the transmission line, if it is applied appropriately. Reactive compensation changes electrical characteristics of a line to provide compatibility with the load demand. The reactors are used to minimize line overvoltage under light load conditions, and the capacitors are used to maintain voltage levels under heavy load conditions. In practical applications, reactive shunt compensation is often used to provide voltage support for the load or to regulate the voltage at a given bus against load variations. It can also significantly increase the maximum transmittable power. Thus, shunt compensation is able to change the power flow in the system with suitable and fast controls to increase the transient stability limit and provide power oscillation damping [1]. In this study, a Static Var Compensator (SVC) is modelled in order to regulate the receiving end voltage of a 230 kV transmission line. A system consisted of two thyristor switched capacitors (TSC) and one thyristor controlled reactor (TCR) has been used. The line end load has been represented by step by step varying lagging power factor load. One phase equivalent of the system was simulated by Matlab Simulink [2] program.
2. VOLTAGE REGULATION USING SVC The voltage regulation system model is shown in Fig. 1. The circuit in Fig. 1 is a simplified one phase model of a 230 kV power system. The equivalent source is modeled by a voltage source (132.8 kV rms, 60 Hz.) in series with its internal impedance corresponding 2000 MVA short circuit level and X/R=10. Thus, the source inductance is Ls=70.2 mH and source resistance is Rs=2.645 ohms. The transmission line one phase 1
Corresponding author: Dumlupınar University, Department of Electrical and Electronics Engineering, 43100, Kütahya, Turkey.
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ICENS International Conference on Engineering and Natural Science, 24-28 May 2016, Sarajevo, Bosnia and Herzegovina equivalent model is one single pi section with a branch impedance of R=5.2 ohms and L=138 mH and two shunt capacitances of C=0.967 µF.
Fig. 1- The modelled voltage regulation system circuit layout.
The load at the line end has a total demand of 75 MW and 20 MVAr for each phase as seen in Fig 2. The load in the bottom of Fig. 2 is connected to the line at the beginning of the simulation and disconnected at 4 th second. The other two loads in the figure are not connected to the line at the beginning. The load in the middle is connected to the line at 1st second while the other is at 2nd. These two loads remain until the end of the simulation.
Fig. 2- The changing load model with a total demand of 75 MW and 20 MVAr.
The Static VAr Compensator (SVC) consists of two 10 MVAr tyristor switched capacitor (TSC) banks and one 10 MVAr thyristor controlled inductor (TCR) as seen in Fig. 1. The TCR capacity has been chosen to bring line voltage level to the specified limits under light loading conditions. The total TSC capacity is determined according to the full loading condition. The controllers of the TCR and TSC banks are explained in detail in the previous studies of [3 - 5].
The measurement and control block (not shown in Fig. 1.) measures the line voltage and the load current in order to calculate active and reactive power of the load and to decide the necessary number of the TSC units. The open-loop controller has been used to activate TSC banks. The TCR bank is used to regulate line voltage. The controller for the TCR is a “PI” type of controller that compensates the error between the reference line voltage and actual line voltage.
3. RESULTS AND DISCUSSION At the beginning of the study, only source and the line were included in the simulation in order to see the line voltage at no load condition. The voltage of the ideal AC voltage source in Fig. 1 has been chosen as 1.075 per unit (p.u). At no load, the receiving end voltage was 1.117 p.u (257 kV Line to Line) while the sending end voltage was 1.096 p.u (253 kV Line to Line). Then, full load has been added to the simulation to see the voltage profile of the system. The voltage of the ideal AC voltage source in Fig. 1 has kept constant at 1.075 p.u. This time, the voltage at the receiving end of the line is 0.95 p.u. (219 kV Line to Line) and the voltage at 2
ICENS International Conference on Engineering and Natural Science, 24-28 May 2016, Sarajevo, Bosnia and Herzegovina the sending end of the line is 1.03 p.u. The voltage level of the load is in the lower limit. Either at no load or at full load the receiving end voltage needs reactive compensation in order to remain inside the allowed limits.
When the 10 MVAr of TCR has been added to the circuit at no load, the receiving end voltage of the line is at upper allowable limit of 1.05 p.u. However, the receiving end voltage of the line is 1.024 p.u. and slightly bigger than rated value of 230 kV at full load when both 10 MVAr of TSC banks are included to the system. The TCR and TSC combination with the appropriate control can provide necessary voltage regulation on the receiving end of the line.
The proposed system in Fig. 1 has been tested in changing load conditions. The total simulation time is 5 seconds. The active and reactive powers of the load and the source are seen in Fig.3 and Fig. 4 respectively. Only 25 MW and 5 MVAr of load has been added in the simulation during the first second. Thus, there is light loading condition on the line and the receiving end voltage of the line is above the rated value (1.03 p.u.) because of the line capacitance as seen on Fig. 5. The only action option for the voltage regulator is using full capacity of the 10 MVAr of TCR. But, this is not enough to bring receiving end voltage to 1.0 p.u. value.
Fig. 3- The changing active powers of the load and the source during the simulation.
Fig. 4- The changing reactive powers of the load and the source during the simulation.
At the 1st. second, additional 15 MW and 5 MVAr of load comes in the simulation. This change causes the voltage to be below the rated value, so the voltage regulator lowers the current in the TCR by changing the ignition angle of the thyristors. Thus, the desired voltage (1.0 p.u) at the receiving end of the line can be achieved in a couple of periods of line frequency as shown in Fig. 5. Another load change occurs in 2nd. second and the demand of the load is now 75 MW and 20 MVAr. In order to bring the voltage within the tolerance limits, the voltage regulator takes both TCS units on then adjusts the ignition angle of the TCR thyristors. The rated voltage on the line-end is achieved in 0.35 seconds.
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ICENS International Conference on Engineering and Natural Science, 24-28 May 2016, Sarajevo, Bosnia and Herzegovina On the last part of the simulation, the load at the line-end is 50 MW and 15 MVAr. Only one TSC is enough to achieve the desired value on the line with the additional adjustment of TCR current. The current on the TCR is continuously controlled by changing ignition angle of the thyristors during the simulation and the resulting current of the TCR is shown in Fig. 6. The voltage regulation system in this study can affectively keep the voltage level at the receiving end of the transmission line within the specified tolerance limits of ± 5 %.
Fig. 5- The voltage of the receiving end of the line in p.u. during the simulation.
Fig. 6- The current of the Thyristor Controlled Reactor during the simulation.
4. CONCLUSION In this study, the voltage regulation of the receiving end of a 230 kV transmission has been made using a Static Var Compensator (SVC). The system that consisted of two thyristor switched capacitors (TSC) and one thyristor controlled reactor (TCR) has been modelled in Matlab Simulink environment. The controller of the static compensator successfully kept the voltage of the receiving end of the line at the specified tolerance limits of ± 5 % by adjusting susceptance of the SVC while the load of the line has changed fast.
REFERENCES [1]. Narain G. Hingorani and Laszlo Gyugyi, Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems, December 1999, Wiley-IEEE Press [2]. Mathworks - MATLAB and Simulink [Online], URL:http://www.mathworks.com, Access date: 15 April 2016. [3]. Mumyakmaz, B., Vardar, K., “Ark fırınları için fuzzy kontrola dayalı reaktif güç kompanzatörü tasarımı.” ELECO2002 Conference, 2002, Bursa, Turkey. [4]. Mumyakmaz, B., “Tristör anahtarlamalı kapasitörlerle reaktif güç kompanzasyonunun Matlab Simulink kullanılarak modellenmesi” ELECO2000 Conference, October 2000, Bursa, Turkey. [5]. Mumyakmaz, B., Jin, X., Wang, C., Cheng, T.C., “Static Var Compensator with Neural Network Control” IEEE / PES Transmission and Distribution Conference, April 1999.
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