Power flow control, in an existing long transmission line, plays a vital role in Power System area. This paper employs the analysis and comparison of shunt FACTS Devices such as Static Synchronous compensator (STATCOM) and Static Var Compensator (SVC) in a long transmission line to control the voltage and the power flow. The proposed devices are used in different locations such as sending end of the transmission line, middle and receiving end of the transmission line. The PWM control strategy is used to generate the firing pulses of the controller circuit for both the Shunt FACTS Devices. Simulations were carried out using MATLAB Simulink environment. The suitable location and the performance of the proposed each model were examined. Based on a voltage-sourced converter, the STATCOM regulates system voltage by absorbing or generating reactive power. Contrary to a thyristor-based Static Var Compensator (SVC), STATCOM output current (inductive or capacitive) can be controlled independent of the AC system voltage. The simulation results reveals that the reactive power injected and absorbed is better at the middle of the transmission line for both STATCOM and SVC. Comparing the performance of both devices, the power flow is controlled when STATCOM is connected at the middle of the transmission line. The numerical results were obtained with and without compensation. Henceforth the analysis shows that STATCOM is superior to SVC for the power flow control in a long transmission line.
Key Words: FACTS device, STATCOM, SVC, PWM technique, MATLAB Simulink.
Introduction
With the rapid development of Power electronics in Power System, the transmission capability is increased and the stability of the power system is found to be improved. By the placement of FACTS devices in transmission systems, the technical solutions can be obtained thereby the voltage, active and reactive power flow was monitored and controlled . Traditionally, in order to control the power flow in the transmission line, the effective line reactance is controlled by using fixed or thyristor controlled series capacitors. The FACTS devices are introduced in the power system transmission for the reduction of the transmission line losses and also to increase the transfer capability. SVC is impedance based FACTS device to regulate the voltage in a transmission line. STATCOM is VSC based controller to regulate the voltage by varying the reactive power in a long transmission line. Mithulananthan.N et al [1] have explained the comparison of difference control techniques for damping undesirable inter area oscillation in power systems by means of PSS, SVC and STATCOM. Jong et al [2] have described the practical operation effect of KEPCO(Korea Electric Power Corporation) FACTS devices in the Korean power system. Le.C.D et al [3] have explained about the ride-through capability of a large Indiction Generator based wind park with different reactive power support solutions. Ding Lijie Liu et al [4] have compared STATCOM and SVC in voltage supporting, improving transient stability and transmission limit and damping low frequency oscillations. Musunuri.S et al [5] have presented a comparison of four FACTS Controllers, SVC, STATCOM, TCSC and SSSC on power system steady state voltage stability. Shayesteh.E, et al [6] have conducted an economic comparison between using FACTS devices and demand response programs implementation for the enhancement of ATC. Haue.M.H., [7] has fully exploited the features of STATCOM and SSSC to improve the stability limit of a simple power system. Arun Bhaskar.M., et al [8] have explained about the need for reactive shunt compensation to improve the voltage profile in the line by comparing SVC, TCSC and TCPST. Ugalde-Loo.C.E.,et al [9] have compared the series and shunt FACTS devices for using the frequency domain methods under the framework of individual channel analysis and design (ICAD). Albasri, F.A.et al[10] have investigated a comparative study of the performance of distance relays for transmission lines compensated by shunt connected flexible ac transmission system (FACTS) controllers. Tan Y.L.,et al[1] have demonstrated the effectiveness of SVC and STATCOM of same rating for the enhancement of power flow. Albasri, F.A.et al [12] have investigated the performance of distance protection of transmission lines when compensated with SVC and STATCOM and also studied the different fault types and fault locations and system conditions. Ozturk.A., et al [13] have examined the voltage stability of the bus load in various static and dynamic load systems that are fed by a wind farm. Kamarposhti.M.A., et al [14] have presented a study of series and shunt FACTS devices on steady state voltage and power stability. Phadke.A.R., et al [15] have compared several voltage stability indices in electric power system to identify the weakest bus of the system. Tull.H.K., [16] has explained the overview of an existing FACTS devices like SVC, STATCOM,UPFC and TCSC/TPSC regarding the reactive power compensation and system control.
In this paper performance strategy were conducted on STATCOM and SVC at different locations such as sending end, middle and the receiving end of the long distance transmission line. In every part of the location the power flow is tested with and without compensation strategy. A mathematical modeling approach and control design is presented in the proposed work. The simulink model of the standard system is developed and tested using MATLAB Simulink environment.
Operating Principle
STATCOM:
A STATCOM consists of a coupling transformer, an inverter and a DC capacitor is shown in Figure1.
Figure1. Structure of STATCOM. Figure2. Equivalent Circuit of STATCOM.
For such an arrangement, in ideal steady state analysis, it can be assumed that the active power exchange between the AC system and the STATCOM can be neglected, and only the reactive power can be exchanged between them. STATCOM is usually used to control transmission voltage by reactive power shunt compensation. Based on the operating principle of the STATCOM [16], the equivalent circuit can be derived, which is given in Figure2. In the derivation, it is assumed that (a) harmonics generated by the STATCOM are neglected; (b) the system as well as the STATCOM are three phase balanced. Then the STATCOM can be equivalently represented by a controllable fundamental frequency positive sequence voltage source Vsh. In principle, the STATCOM output voltage can be regulated such that the reactive power of the STATCOM can be changed.
SVC:
The schematic diagram of a TSC-TCR type SVC is shown in Figure 3. This shows that the TCR and TSC are connected on the secondary side of a step-down transformer. Tuned and high pass filters are also connected in parallel which provide capacitive reactive power at fundamental frequency. The voltage signal is taken from the high voltage SVC bus using a potential transformer. The TSC is switched in using two thyristor switches connected back to back. In steady state, TSC does not generate any harmonics. To switch off a TSC, the gate pulses are blocked and the thyristors turns off when the current through them fall below the holding currents. It is to be noted that several pairs of thyristors are connected in series as the voltage rating of a thyristor is not adequate for the voltage level required. However the voltage ratings of valves for a SVC are much less than the voltage ratings of a HVDC valve as a step down transformer is used in the case of SVC.
Figure3. Schematic Diagram of a TSC-TCR
Modeling Of STATCOM
From the equivalent circuit of the STATCOM shown in figure2. let , then the power flow constraints of the STATCOM are :
-
Where,
The operating constraint of the STATCOM is the active power exchange via the DC link. i.e.
Where,
Control Strategy
In the practical applications of the STATCOM and SVC, it may be used for controlling the voltage of the bus, reactive power injection, impedance, current magnitude and etc. Among these control options, control of voltage of the local bus, which the STATCOM and SVC are connected to, is the most recognized control function. The mathematical descriptions of the control functions are :
Figure4. Circuit Diagram without Compensation
Figure5 Circuit Diagram of STATCOM at Sending End
Figure6 Circuit Diagram of SVC at Sending End
4.1 Bus Voltage Control
The bus control constraint is
Where is the bus voltage control reference.
Reactive Power Control
In this control mode, reactive power generated by the STATCOM is controlled to a reactive power injection reference. The control constraint is
Impedance Control
STATCOM compensation can be equivalently represented by an imaginary impedance or reactance. In this control mode, Vsh is regulated to control the equivalent reactance of the STATCOM to a specified reactance reference:
Where, is the specified reactance control reference of the STATCOM and is the equivalent reactance of the STATCOM.
Simulation Of STATCOM And SVC
Circuit Description
The power grid consists of two 500-KV equivalents, respectively 3000 MVA and 2500 MVA, connected by a 600-km long transmission line. When the STATCOM and SVC are not in operation, the "natural" power flow on the transmission line is 925.8 MW from bus B1 to B3. STATCOM has a rating of +/- 100MVA. This STATCOM is a phasor model of a typical three-level PWM STATCOM. STATCOM is having a DC link nominal voltage of 40 KV with an equivalent capacitance of 375 µF. On the AC side, its total equivalent impedance is 0.22 pu on 100 MVA. This impedance represents the transformer leakage reactance and the phase reactor of the IGBT bridge of an actual PWM STATCOM. Figure4 explains about the circuit diagram without compensation. In this circuit the power is directly measured in the 600km long transmission line at the three stages like B1,B2 and B3 and also tabulated the result in table1. Figure5 explains about the circuit diagram when STATCOM is connected at the sending end of the long transmission line. Figure6 explains about the circuit diagram when SVC is also connected at the sending end of the transmission line. Similarly the connections are made when the STATCOM and SVC are connected at the middle and receiving end of the long transmission line.
Simulation Results
Initially Vref is set to 1 pu; at t=0.2 s, Vref is decreased to 0.97 pu; then at t=0.4 s, Vref is increased to 1.03; and finally at 0.6s, Vref is set back to 1 pu. And also the fault breaker at bus B1will not operate during the simulation. The results were obtained with and without compensation and also the numerical results were tabulated in table 1 and table 2. Figure7 explains about the real and reactive power without compensation and also the real power measured at different locations such as sending end, middle and receiving end of the long transmission line when STATCOM and SVC are connected separately.
Power Flow Without Compensation
Power Flow at B1,B2 &B3
Real Power at B1,B2 &B3
Power Flow With Compensation
STATCOM
Sending End
Real Power at B1,B2 &B3
Middle
Real Power at B1,B2 &B3
Receiving End
Real Power at B1,B2 &B3
Figure 7 Real Power When STATCOM & SVC connected in three different locations
Power Flow With Compensation
STATCOM
Sending End
Reactive Power at B1,B2 &B3
Middle
Reactive Power at B1,B2 &B3
Receiving End
Reactive Power at B1,B2 &B3
Figure 8 Reactive Power When STATCOM & SVC connected in three different locations
Figure 8 exhibits the flow of reactive power at different locations such as sending end, middle and receiving end of the long transmission line when STATCOM and SVC are connected separately. The graph results that the reactive power generated and absorbed is superior when STATCOM is connected at the middle of the long transmission line when comparing with the other ends. The results are also shown in table 1 and table 2.
Figure 9 shows that the comparison of voltage and reactive power control when both the FACTS devices are connected at the three different locations. This graph results that the voltage is regulated and also the reactive power flow is controlled at 0.4 sec when STATCOM is connected at the middle of the long transmission line when comparing with SVC and also other ends.
Voltage and Reactive Power With Compensation
Comparison of Voltage control
Sending End
Middle
Receiving End
Figure 9 Comparison of Voltage and Reactive Power Control when STATCOM & SVC connected in three different locations
Conclusion
The vital role of shunt FACTS devices, which are connected in long distance transmission lines, are to improve the power transfer capability and also to control the power flow in the power system network. In this proposed work STATCOM and SVC are employed as a FACTS device. STATCOM and SVC are connected at the various locations such as sending end, middle and receiving end of the long transmission line. The results were obtained with and without compensation. The simulation results reveals that the reactive power generated is superior when STATCOM is connected and also better at the middle of the transmission line when compared with the other ends of the transmission line and also the voltage is controlled at the middle of the line at 0.4 sec. The losses in the real power is also minimum when STATCOM is connected at the middle of the line.The numerical results of the system analysis were elaborated in the table 1 and table 2. The simulation results were carried in MATLAB Simulink environment.
