Shunt Compensation for Power Quality Improvement using a STATCOM Controller

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This paper addresses the issue of the modeling
and analysis of STATCOM controller. The transient
analysis and modeling is performed with the state-of-theart
digital simulator PSCAD 4.2.1. Simulation were
carried out for both cases where, STATCOM was
connected into the system and not, with simulation
interval 3-5 sec. The aim of the STATCOM is to provide
good power quality at the point of common coupling
(PCC). Some simulation results are presented, which
shows the compensation effectiveness of the STATCOM
controller at the connected bus.

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Shunt Compensation for Power Quality Improvement using a STATCOM Controller

  1. 1. ACEEE International Journal on Electrical and Power Engineering, Vol. 1, No. 2, July 2010 Shunt Compensation for Power Quality Improvement using a STATCOM Controller Parmar Hiren.S, Vamsi Krishna.K, Ranjit Roy M.Tech Student, Associate Professor Department of Electrical Engg S.V.N.I.T, Surat hrn_drj1010@yahoo.com, vamsi.svnit@gmail.com, rr@eed.svnit.ac.inAbstract— This paper addresses the issue of the modeling the SVC, which can supply only diminishing outputand analysis of STATCOM controller. The transient current with decreasing system voltage). This featureanalysis and modeling is performed with the state-of-the- of the compensator makes it highly effective inart digital simulator PSCAD 4.2.1. Simulation were improving the transient stability. Therefore,carried out for both cases where, STATCOM wasconnected into the system and not, with simulation STATCOM systems with GTO thyristors have beeninterval 3-5 sec. The aim of the STATCOM is to provide initially used for improving flexibility and reliabilitygood power quality at the point of common coupling of energy transmission in FACTS (flexible AC(PCC). Some simulation results are presented, which transmission system) applications [2]-[3].As theshows the compensation effectiveness of the STATCOM switching frequency of GTOs must be kept low, thecontroller at the connected bus. control with fundamental frequency switching hasKeywords- Point of common coupling (PCC), Power been used and multi-phase configurations have beenQuality, PSCAD, STATCOM, VSC . formed to reduce harmonics production. The newest applications of STATCOMs concern power quality I. INTRODUCTION improvement at distribution network level. Some examples given in the literature are the reduction of In recent years power quality issues have become flicker, voltage control and balancing single phasemore and more important both in practice and in load [4]-[5]. These are systems of a smaller powerresearch. Power quality can be considered to be theproper characteristics of supply voltage and also a where IGBT or IGCT technology can be appliedreliable and effective process for delivering electrical allowing fast switching with PWM control.energy to consumers. Binding standards and Although the possibility of using STATCOMs forregulations impose on suppliers and consumers the the reduction of influence of disturbing loads on theobligation to keep required power quality parameters at supply network has already been proved in practice,the point of common coupling (PCC). Interest in power there is still a lack of information about the complexquality issues results not only from the legal assessment of compensation effectiveness and aregulations but also from growing consumer demands. method of system selection and its control for aOwing to increased sensitivity of applied receivers and given network.process controls, many customers may experiencesevere technical and economical consequences of poor II. BASIC CONFIGURATIONpower quality. Disturbances such as voltage OF STATCOMfluctuations, flicker, harmonics or imbalance canprevent appliances from operating properly and make The STATCOM is a shunt device. It shouldsome industrial processes shut down. On the other therefore be able to regulate the voltage of a bus tohand, such phenomena now appear more frequently in which it is connected. The operating principle of athe power system because of systematic growth in the STATCOM in this mode has been termed as thenumber and power of nonlinear and frequently time- STATCOM in voltage control mode. This papervariable loads. shows that even though the structure of STATCOM When good power quality is necessary for technical used in both current control and voltage controland economical reasons, some kind of disturbance modes is the same, its operating principle is different.compensation is needed and that is why applications of In the current control mode it is required to follow apower quality equipment have been increasing. set of reference currents while in the voltage control The most recent approach for solid-state power mode it is required to follow a set of referencecompensators is based on self-commutated converters voltages. This paper discusses the reference voltageusing components with a current blocking capability. generation scheme and the control of STATCOM inSuch a compensation system is the static equivalent of the voltage control mode. In its most basic form, thethe synchronous compensator, hence the term STATCOM configuration consists of a VSC, a dcSTATCOM [1] (static synchronous compensator).A energy storage device; a coupling transformerSTATCOM can provide fast capacitive and inductive connected in shunt with the ac system, and associatedcompensation and is able to control its output current control circuits. Fig.1 shows the basic configurationindependently of the AC system voltage (in contrast to of STATCOM. The VSC converts the dc voltage 40© 2010 ACEEEDOI: 01.ijepe.01.02.01
  2. 2. ACEEE International Journal on Electrical and Power Engineering, Vol. 1, No. 2, July 2010across the storage device into a set of three-phase ac to capacitive mode occurs by charging angle δ fromoutput voltages. These voltages are in phase and zero to a negative value. The active power iscoupled with the ac system through the reactance of the transferred from the AC terminal to the DC capacitorcoupling transformer. Suitable adjustment of the phase and causes the DC link voltage to rise. The activeand magnitude of the STATCOM output voltages and reactive power may be expressed by the following equations:allows effective control of active and reactive power P = (VbusVvsc/XL)sin δexchanges have been made to remed the situation with Q = (V2bus/XL) - (VbusVvsc/XL)cos δsolutions based between the STATCOM and the ac Vvsc – out put voltage of STATCOMsystem. The VSC connected in shunt with the ac system Vbus – bus terminal voltageprovides a multifunctional topology which can be used XL - inductive reactance of coupling transformerfor up to three quite distinct purposes:• Voltage regulation and compensation of IV MODELING AND CONTROL DESCRIPTIONreactive power.• Correction of power factor. A typical ac system equivalent was used in this• Elimination of current harmonics. study to show the dynamic performance of the STATCOM. The simulated circuitry representing V bus this integration is shown in Fig 3. The detailed representation of the STATCOM is depicted in Fig 4. V vsc In the figures, the units of resistance, inductance, and V DC capacitance values are Ohms, Henry, and microfarad, respectively. A C s y s te m b u s C o u p lin g T ra n s fo rm e r D c E n e rg y S to ra g e V THE AC POWER SYSTEM VSC The ac system equivalent used in this study STATCOM corresponds to a two-machine system, where one Fig.1 Basic configuration of STATCOM. machine is dynamically modeled (including generator, exciter, and governor) to be able to III. PRINCIPLE OF STATCOM demonstrate dynamic oscillations. Dynamic oscillations are simulated by creating a three-phase Statcom is to suppress voltage variation and control fault in the middle of one of the parallel lines atreactive power in phase with system voltage. It cancompensate for inductive and capacitive currents BusD (Fig.1).A bus that connects the STATCOM tolinearly and continuosly. Fig.2 shows the vector the ac power system is named a STATCOM terminaldiagram at the fundamental frequency for capacitive bus. The location of this bus is selected to be eitherand inductive modes and for the transition states from acapacitive to inductive and vice versa.The terminal V vscvoltage (Vbus) is equal to the sum of the inverter (a )voltage (Vvsc) and the voltage across the coupling Itransformer reactive VL in both capacitive and V bus V bus V vscinductive modes. It means that if output voltage of δ VLSTATCOM (Vvsc) is in phase with bus terminal voltage(Vbus) and Vvsc is greater than Vbus, STATCOM VLprovides reactive power to system. If Vvsc is smallerthan Vbus, STATCOM absorbs reactive power frompower system. Vbus and Vvsc have the same phase, butactually they have a little phase difference to VL (b )component the loss of transformer winding andinverter switching, so absorbs some real power from I V bus VL V vsc V bussystem. δ Fig.2 is Statcom vector diagrams, which showinverter output voltage VI, system voltage VT, reactivevoltage VL and line current I in correlation with Imagnitude and phase δ. Fig.2. a and b explain how V I b V vscand VT produce capacitive or inductive power bycontrolling the magnitude for inverter output voltage bus A or B.VI in phase with each other. Fig 2.c and d show Fig.2 Vector diagram of STATCOM (a)STATCOM produces or absorbs real power with V I capacitive mode, (b) inductive mode, (c) activeand VT having phase±δ. The transition from inductive power release and (d) active power absorption. 41© 2010 ACEEEDOI: 01.ijepe.01.02.08
  3. 3. ACEEE International Journal on Electrical and Power Engineering, Vol. 1, No. 2, July 2010 Fig.3 AC system equivalent. Fig. 4 Detailed repersentation of the STATCOM. V . STATCOM SIMULATION As such a system produced harmonics and its practical application is limited. A multi-pulse scheme It is seen from Fig.4 two gate turn-offs (GTO) is one of the solutions used for harmonic reduction, based six-pulse voltage source inverters represent the in which several identical six-pulse bridges are STATCOM used in this particular study. The voltage connected to transformers having outputs that are source inverters are connected to the ac system phase displaced with respect to one another. Star- through two coupling transformers and linked to a dc and delta-connected winding give a relative 30° capacitor in the dc side. The value of the dc link phase shift and, with two six-pulse converter bridges capacitor has been selected as 1 µF in order to obtain connected, allows 12-pulse STATCOM operation to smooth voltage at the STATCOM terminal bus. be obtained. The most basic is a six-pulse system As stated in [6]-[7], a GTO-based inverter configuration, giving the rectangular output voltage. connected to a transmission line acts as an 42© 2010 ACEEEDOI: 01.ijepe.01.02.08
  4. 4. ACEEE International Journal on Electrical and Power Engineering, Vol. 1, No. 2, July 2010 alternating voltage source in phase with the line an exact moment of switching and is used by voltage, and, depending on the voltage produced by interpolation procedure which allows for switching the inverter, an operation of inductive or capacitive between time steps. mode can be achieved. It has also been emphasized To generate the gating signals for the inverters, that a dc link capacitor establishes equilibrium line to ground voltages are used for the inverter between the instantaneous output and input power of connected to the Y-Y transformer, whereas line-to- the inverter. The primary function of the STATCOM line voltages are utilized for the inverter connected to is to control reactive power/voltage at the point of the Υ-Δ transformer. connection to the ac system [6]-[8]. Fig.5 shows the control diagram of the STATCOM used in the VI CASE STUDIES simulation. In order to demonstrate the effectiveness of the STATCOM control system exerts reactive angle STATCOM case are simulated. A three- phase fault control as follows: an error signal is obtained by is created at Bus D of Fig.1. to generate dynamic comparing the reference reactive power Q ref with the oscillations in each case. The plot time step is 0.001 reactive power measured Qm at the load point. The PI s for all the figures given in these cases. Controller process the error signal and generates the required angle (alpha_ord) to drive the error zero. PLL (3-phase PI controlled phase locked loop), VII AC OSCILLATIONS AND STATCOM- MODE which generates a ramp signal theta that varies A two-machine ac system is simulated. The between 0 and 360°, synchronized or locked in inertia of machine I was adjusted to obtain phase, to the input voltage Va. approximately 3-Hz oscillations from a three-phase Interpolated Firing Pulses: The output is based on a fault created at 3.1 s and cleared at 3.25 s. When comparison of high (H) and low (L) input signals. there is no STATCOM connected in the ac power The low L input is normally a firing angle order system, the system response is depicted in the Fig.6. (order) and the high H input is from a phase-locked in the interval of 3-5 s where ac voltage is at Bus B. oscillator or equivalent. The response with STATCOM is connected shown Two signals are being sent to each switch, the first in Fig. 7. one tells to turn on or off, the second one, determines Fig. 5 STATCOM controls. Fig.6. AC voltage at BUS B when a STATCOM is not Fig.7 When a STATCOM is connected at BUS B connected 43© 2010 ACEEEDOI: 01.ijepe.01.02.08
  5. 5. ACEEE International Journal on Electrical and Power Engineering, Vol. 1, No. 2, July 2010 VIII. CONCLUSIONS [2] Mori, S.Matsuno, K.Hasegawa, T.Ohnishi, S.Takeda, M.Seto, M.Murakami and Ishiguro F. “Development This paper presents the modeling and control of of a large static var generator using self-commutated the STATCOM and its dynamics response to system inverters for improving power system stability”, IEEE Trans. Power Syst.,1993,vol. 8, no. 1, pp. 371– oscillations cause by a three-phase fault. It has been 377. shown that the STATCOM can be very effective in [3] Song,Y.H.,and Johns,A.T., “Flexible AC transmission damping power system oscillations. systems (FACTS)”, IEEE London 1999. The use of computer programs in the simulation [4] wich,G.: “Power quality enhancement using custom of STATCOM, including their controls, is extremely power devices”, Kluwer Academic Publishers, Boston important for the development and understanding of 2002. this power electronics based technology. The results [5] K.K. Sen, “STATCOM - STATIC synchronous achieved through the digital simulations clearly show compensator: The ory, modeling and applications”, the capability of the STATCOM to provide good IEEE Trans. Power Delivery, vol. 2, pp. 1177-1183, power quality at the point of common coupling Feb. 1999. (PCC). [6] L .Gyugyi ,“ Dynamic compensation of ac It was also observed that the location where the transmission lines by solid state synchronous voltage STATCOM is connected important for improvement source’,IEEE Trans.PowerDeliv ery, vol. 9, pp. 904- 911, Apr. 1994. of overall system dynamics performance. STATCOM compensator will become a good [7] K.V. Patil, J. Senthil, J. Jiang, and R.M. Mathur, “Application of STATCOM for damping torsional solution for power quality improvement. oscillations in series compensated AC systems”, IEEE Trans. Energy Conversion, vol. 13, pp. 237- REFERENCES 243, Sept. 1998. [8] IEEE Task Force on Benchmark Models for Digital [1] R. Mineski, R. Pawelek, I. Wasiak, Shunt Simulation of FACTS and Custom-Power Compensation for power quality improvement using a Controllers, T&DCommittee “Detailed Modeling of STATCOM controller: modeling and simulation”, Superconducting Magnetic Energy Storage (SMES) IEEE Proc. On Generation, Transmission and System”, IEEE Trans. Power Delivery, vol. 21, no. 2, Distribution, vol. 151, no. 2, March 2004. APRIL 2006. 44© 2010 ACEEEDOI: 01.ijepe.01.02.08

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