POWER FLOW CONTROL BY USING OPTIMAL LOCATION OF STATCOM S.B. ARUNA Assistant Professor, Dept. of EEE, Sree Vidyanikethan Engineering College, Tirupati aruna_ee@hotmail.com 305 ABSTRACT In present scenario, the optimal operation of load is one of the challenging aspects in power system. It may lead to large power flow with inadequate control, excessive reactive power in various parts of the system, large dynamic swing between different parts of the system and bottlenecks and thus the full potential of transmission interconnection cannot be utilized. So power flow control plays a major role in power systems. In this paper the shunt connected compensation (STATCOM) based FACTS device is incorporated for the control of voltage and power flow in long transmission lines. STATCOM can be controlled by employing thyristor controlled and switched reactors and capacitors. STATCOM regulates system voltage by absorbing or generating reactive power. Real power indices are obtained by simulating the STATCOM at the different locations of the transmission line by using MATLAB simulink environment. The results are analyzed in a test system with and without compensation of STATCOM. Keywords: Power flow control, FACTS devices, STATCOM, shunt compensation, Transmission line. 1. INTRODUCTION Three main aspects in electrical power systems are Generation, Transmission and Distribution. In the electrical utility sector with its ever growing and ongoing expansion, numerous changes are being continuously introduced. Figure 1: Illustration of electrical power system Transmission systems have many forms of limitations, involving power transfer between different areas, also referred as transmission bottlenecks or within a single area or region that is referred as a regional constraint. In earlier days, power flow was mechanically controlled. In effect from the point of view of both dynamic and steady state operation, the system is really uncontrolled So, FACTS technology is being used inorder to overcome the powerflow problem. FACTS devices are power electronic based equipments, which are used for the dynamic control of voltage, impedance and phase of high voltage AC transmission lines. There are basically two types of FACTS controllers: Thyristor-based controllers and converter-based controllers. Regulator or TCPAR employ conventional Thyristors to control one of the three parameters determining power transmission, voltage (SVC), transmission impedance (TCSC) and transmission angle (TCPAR). Consequently conventional Thyristor-controlled compensators the SVC and TCSC present variable reactive impedance and thus act indirectly on, the transmission network. The converter-based FACTS controllers, representing a new generation of transmission controllers, employ selfcommutated, voltage-sourced switching converters to realize rapidly controllable, static, synchronous AC voltage sources. FACTS devices can be classified as shunt, series and series and shunt connected. Figure 2: Types of FACTS devices
306 2. BASIC OPERATING PRINCIPLES OF STATIC SHUNT COMPENSATORS In a typical power system the existence of reactive power is prominent since most loads are naturally inductive. Reactive power represents energy alternately stored and released by inductors and/or capacitors. The power load flow of a system is considered with the following formula and its schematic representation in shown in Figure below... (1) Figure 3: Power flow diagram The basic principle of static shunt compensators is to generate reactive power. A voltage source converter basically converts the dc voltage from a capacitor into a three-phase voltage to be injected into the system, operating like a synchronous machine. For purely reactive power flow, the three-phase induced electromotive forces (EMFs) e a, e b and e c, of the synchronous rotating machine are in phase with the system voltages, v a, v b and v c. The reactive current I drawn by the synchronous compensator is determined by the magnitude of the system voltage V, that of the internal voltage E, and the total circuit reactance X.... (2) The corresponding reactive power Q exchanged can be expressed as follows:.. (3) In static shunt compensation, the reactive power flow can be controlled by the excitation of the machine (E) relative to the system voltage amplitude (V). Increasing E above V (i.e., operating over excited) results in a leading current that is the machine is seen as a capacitor by the AC system. Decreasing the E below V (i.e., operating under-excited) produces a lagging current, that is, the machine is seen as a reactor (inductor) by the AC system. A Flexible AC Transmission System (FACTS) is an ac transmission system in-corporating power electronicbased or other static controllers which provide better power flow control and enhanced dynamic stability by control of one or more ac transmission system parameters (voltage, phase angle. and impedance). The shunt controller basically consists of three groups. 1. Static variable compensator (SVC) 2. Static synchronous compensator (STATCOM) 3. Synchronous generator (SSG) or STATCOM with energy-storage system (ESS) Power quality improvement is possible to get if parallel connected STATCOM acts as a sinusoidal, with fundamental frequency, voltage source, therefore described conditioner makes possible to get: I) Sinusoidal source current ii) Reactive power compensation iii) Load voltage stabilization iv) Balanced source in conditions of the unbalanced load 3. STATCOM (STATIC SYNCHRONOUS COMPENSATOR) STATCOM is one of the most important shunt FACTS controllers, which have broad applications in electric utility industry. STATCOM has played an important role in power industry since 1980s and recognized to be one of the key technologies in future power system. STATCOM is based on the principle that a voltage source inverter generates a controllable AC voltage source behind a reactance so that the voltage difference across the reactance produces active and reactive power exchange between the STATCOM and the transmission network line in a similar manner of a synchronous condenser, but much more rapidly. Figure 4: schematic configuration of STATCOM.
307 In addition, in the event of a rapid change in system voltage, the capacitor voltage does not change instantaneously; therefore STATCOM effectively reacts for the desired responses. STATCOM is capable of high dynamic performance and its compensation does not depend on the common coupling voltage. 3.1. STRUCTURE OF STATCOM Basically, STATCOM is comprised of three main parts: a voltage source converter (VSC), a step-up coupling transformer, and a controller. In a very-high-voltage system, the leakage inductances of the step-up power transformers can function as coupling reactors. The main purpose of the coupling inductors is to filter out the current harmonic components that are generated mainly by the pulsating output voltage of the power converters. 3.2. BASIC OPERATING PRINCIPLES OF STATCOM The STATCOM is connected to the power system at a PCC (point of common coupling), through a step-up coupling transformer, where the voltage-quality problem is a concern. The PCC is also known as the terminal for which the terminal voltage is U T. The controller then performs feedback control and outputs a set of switching signals (firing angle) to drive the main semiconductor switches of the power converter accordingly to either increase the voltage or to decrease it accordingly. A STATCOM is a controlled reactive-power source. It provides voltage support by generating or absorbing reactive power at the point of common coupling without the need of large external reactors or capacitor banks. Using the controller, the VSC and the coupling transformer, the STATCOM operation is illustrated in Figure below. Figure 5: STATCOM operation in a power system The charged capacitor C dc provides a DC voltage, U dc to the converter, which produces a set of controllable three-phase output voltages, U in synchronism with the AC system. The synchronism of the three-phase output voltage with the transmission line voltage has to be performed by an external controller. The amount of desired voltage across STATCOM, which is the voltage reference, Uref, is set manually to the controller. The voltage control is thereby to match U T with Uref which has been elaborated. This matching of voltages is done by varying the amplitude of the output voltage U, which is done by the firing angle set by the controller. The controller thus sets U T equivalent to the Uref. The reactive power exchange between the converter and the AC system can also be controlled [6]. This reactive power exchange is the reactive current injected by the STATCOM, which is the current from the capacitor produced by absorbing real power from the AC system Where I q is the reactive current injected by the STATCOM U T is the STATCOM terminal voltage U eq is the equivalent voltage seen by the STATCOM X eq is the equivalent reactance of the power system seen by the STATCOM If the amplitude of the output voltage U is increased above that of the AC system voltage, U T, a leading current is produced, i.e. the STATCOM is seen as a conductor by the AC system and reactive with the AC system. The capability of controlling active as well as reactive power exchange is a significant feature which can be used effectively in applications requiring power oscillation damping, to level peak power demand and to provide uninterrupted power is generated. Decreasing the amplitude of the output voltage below that of the AC system, a lagging current results and the STATCOM is seen as an inductor. In this case reactive power is absorbed. If the amplitudes are equal no power exchange takes place. P sh = V i 2 g sh - V i V sh g sh cos (θ i - θ sh ) - V i V sh b sh sin (θ i - θ sh ) -------(5) Q sh = -V i 2 b sh - V i V sh g sh sin (θ i - θ sh )
308 + V i V sh b sh cos (θ i - θ sh ) ------(6) Where, g sh + jb sh = 1/Z sh 4. SIMULATION CIRCUITS AND RESULTS a. Implementation of STATCOM STATCOM is applied in considered transmission line with stages B1, B2, B3 and implemented in simulation. The power system was chosen from for simulation which has high source impedance and high transmission line impedance. This was essential to demonstrate the effect of voltage control and power increase in a highly impedance system by a STATCOM. The PWM control strategy is used to generate the firing pulses of the controller circuit b. 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. STATCOM has a rating of +/- 100 MVA. 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 10 MVA. This impedance represents the transformer leakage reactance and the phase reactor of the IGBT Bridge of an actual PWM STATCOM. 4.1 SIMULINK CIRCUIT WITHOUT STATCOM Figure 6: Simulink circuit without STATCOM Fig 4.1 shows the simulink circuit of the considered system without STATCOM. The simulink circuit consists of two equivalents of 500kv between 600km long 4.2. STATCOM AT SENDING END OF THE CIRCUIT: Fig 7: STATCOM at sending end of the circuit Fig 6 consists of the STATCOM at the sending end of the transmission line. STATCOM Consists of six GTO s shunted with the diode. Six pulse generator is used to trigger the GTO S. The reactive power absorbed is calculated at the three buses when STATCOM is Placed at the sending end of the long transmission line.
309 4.3. STATCOM AT MIDDLE OF THE CIRCUIT: Figure 8: STATCOM at middle of the circuit 4.4. STATCOM AT RECEIVING END OF THE CIRCUIT: Figure 9: STATCOM at receiving end of the circuit 4.5 SIMULATION RESULTS Results are obtained by simulating the STATCOM at the different locations i.e. sending end, middle and receiving end of the transmission line by using MATLAB simulink environment. Figure 10: Real power at B1, B2, B3 without compensation Figure 11: Reactive power waveforms at B1, B2, B3 without compensation Figure 12: Real power waveforms at B1, B2, B3 when STATCOM is connected at sending end Figure 13: Reactive power waveforms at B1, B2, B3 when STATCOM is connected at sending end
310 Figure 14: Real power waveforms STATCOM at middle Figure 15 : Reactive power waveforms STATCOM at middle 4.5 SUMMARY Table 1 : Comparison table position Fig 16: Real power waveforms STATCOM at receiving end Figure 17: Reactive power waveforms STATCOM at receiving end B1 B2 B3 P(MW) Q(MVAr) P(MW) Q(MVAr) P(MW) Q(MVAr) without STATCOM 0.353-0.4321 0.351-0.679 0.35 0.312 Sending end 0.362-0.5338 0.2038-1.513 0.1657-1.619 middle 0.885 0.364 0.05429-0.0554 0.0475-2.435 Receiving end 0.947 2.447 0.7897 1.69 0.3846-1.196 Table 1 shows the comparison of the reactive power absorbed when STATCOM is placed at the different locations in the transmission line. We can say that when STATCOM is placed at the receiving end of the transmission line with three stages B1, B2, B3 then real power is improved and reactive power is compensated when compared to other ends of the transmission line. 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 is employed as a shunt FACTS device. STATCOM is connected at the various locations such ascending end, middle and receiving ends of the transmission line. The results were obtained with and without compensation, from simulation results when STATCOM is placed at the receiving end of the transmission line real power is improved and reactive power is compensated when compared with the other ends of the transmission lines. Proper placing of STATCOM in the system helps to control power flow in the transmission lines, so that power losses can be minimized and performance of the power system will be improved. REFERENCES [1] M. Karthikeyan, Dr P, Ajay D Vimalraj, Optimal location of shunt facts devices for power flow control, The fifth International IEEE conference on power electronics and drive system, pp 154-159, sep 2011. [2] Tan, Y.L., Analysis of line compensation by shunt- connected FACT controllers: a comparison between SVC and STATCOM, IEEE Transactions on Power Engineering Review, Vol.19, pp 57-58, Aug 1999 [3] Xia Jiang Xinghao Fang Chow, J.H. Edris, A.-A. Uzunovic, E. Parisi, M. Hopkins, L. A Novel Approach for Modeling Voltage- Sourced Converter- Based FACTS Controllers, IEEE Transactions on Power Delivery, Vol.23(4), pp 2591-2598, Oct 2008. [4] K.R.Padiyar, N.Prabhu Design and performance evaluation of sub-synchronous damping controller with STATCOM, IEEE Transactions on Power Delivery, Vol.21 (3), pp 1398-1405, July 2006. [5] Albasri, F.A. Sidhu, T.S. Varma, R.K. Performance Comparison of Distance Protection Schemes for Shunt- FACTS Compensated Transmission Lines, IEEE Transactions on Power Delivery, Vol.22(4), pp 2116-2125, Oct 2007. [6] Yu Liu Bhattacharya, S. Wenchao Song Huang, A.Q. Control strategy for cascade multilevel inverter based STATCOM with optimal combination modulation, IEEE Conference on Power Electronics Specialists, PESC 2008, pp 4812-4818, June 2008.