Damping of Power System Oscillations and Control of Voltage Dip by Using STATCOM and UPFC

Transcription

1 Volume 114 No , ISSN: (printed version); ISSN: (on-line version) url: ijpam.eu Damping of Power System Oscillations and Control of Voltage Dip by Using STATCOM and UPFC D.V.N.Ananth 1, G.V.Nagesh Kumar 2, D.Deepak Chowdary 3, K.Appala Naidu 2 1DADI Institute of Engg. & Technology, Anakapalli, Visakhapatnam, Andhra Pradesh, INDIA, nagaananth@gmail.com, ph: Vignan s Institute of Information Technology, Visakhapatnam, Andhra Pradesh, INDIA, gundavarapu_kumar@yahoo.com 3Dr. L. Bullayya Engg. College for Women, Visakhapatnam, Andhra Pradesh, INDIA gundavarapu_kumar@yahoo.com Abstract In this paper, Kundur s two synchronous machines in each of the two area systems are considered. A fault occurs in one of the double line and the synchronous generators behaviour in the two areas is studied without STATCOM, with STATCOM and UPFC. Reference voltage and current parameters are taken from area 2. If a symmetrical fault occurs in one of the line with fault resistance of 1 milli-ohm between phases and ground, how the system behaves is studied. Surge currents are formed and will flow in all lines, voltage decreases, oscillations in real power flow occurs and thereby overall stability decreases. The fault abnormal current so developed is preoccupied to the capacitor using proposed STATCOM or UPFC controller. The results prove that voltage is sustained by diverting surge inrush currents, the generator real power flow oscillations damped effectively and thereby system stability and reliability of supply are improved. The proposed shunt controller design for STATCOM or UPFC is so developed to perform as low impedance conduit for short circuit current, thus surge fault inrush currents are diverted towards the VSC. These disturbances are inherent and proposed STATCOM technique is having capability to maintain equilibrium during and post fault conditions. It is best suitable to maintain power quality with uninterrupted power supply and to damp inter-area oscillations. Keywords: Inter-area oscillations, Kundur two area test system, STATCOM, UPFC, Voltage Source Converter. 1. Introduction It is important to ensure proper steps in maintaining power system security while enhancing transfer capability and reliability. To achieve this advanced FACTS technology is promising; however it is very cost effective alternative. Among FACTS family SVC, STATCOM and UPFC are more suitable for 487

2 improving power system stability when a severe disturbance occurs in a system [1-6]. These authors considered UPFC is a better device to damp oscillations due to disturbances due to external faults. In fact UPFC is having feature of series and shunt controller, it can control both voltage and current and thereby reactive power of the system where it was connected. Static compensator (STATCOM) is capable of regulating voltage, control reactive power and also suitable for damping oscillations in the PSS incorporated synchronous generator during symmetrical fault conditions [7-9]. In series FACTS family, thyristor controlled series capacitor (TCSC) and static synchronous series compensator (SSSC) can damp inter-area oscillations [10] are suitable for hybrid applications. The disadvantage of these devices is removing from system because of any reason makes the work complicated and also continuity of supply disturbs. There are many types of disturbances occur for a synchronous generator (SG) in the power system network. It can be sudden change in the field excitation to SG, AVR or PSS failure, change in rate of fuel injection, load variations or because of internal or external disturbances with respect to generator. Internal disturbances are field excitation failure, inter-turn faults etc, external faults are like symmetrical or unsymmetrical faults, sudden addition or removal of big load. Because of these disturbances, stability of system may affect, leads to oscillation in generator rotor angle, output power and also load carrying capability of system decreases due to decrease in terminal voltage or increase in current. If proper protection is not taken, these disturbances may result in unsynchronous operation and finally lead to system collapse or damage to the machine, depending on type and magnitude of fault. But protective devices helps only to protect the system, can t ensure reliability of service. Therefore it is necessary to maintain stability and enhance reliability of service and must also increase life of machine and quality of supply. Aiming towards different objectives, maintaining stability, regulating voltage and mitigate harmonics, control of reactive power during and after transient operation, STATCOM or UPFC are chosen to be better alternatives. Optimal location of these devices also found important in the performance of the system during abnormal situations. The impedance of FACTS devices during fault time has to decrease so that fault current will be bypassed and when fault clears, this current has to be supplied to grid or respective load. So, these devices must be faster, bidirectional and has to operate dynamically during steady and transient situations [12, 13 and 14]. The paper first presents Kundur s test system in section II, modeling of synchronous generator and STATCOM are done in section III. Section IV compares the results for with and without STATCOM and section V gives conclusion followed by references. 2. Test System In example 12.6, page 813 from Kundur text book is considered for analysis [14]. There are four synchronous generators, two generators in area1 and other two in area 2, there is a double line between the buses 7 and 9. STATCOM is placed near bus 7 as shown in figure 1. If a three-phase to ground fault has occurred near the bus 8, how the system behaves with and without STATCOM were analyzed. The above test system consists of hydro-turbine based synchronous generators provided with advanced AVR and PSS. It generates 250MVA, 13.8kV and supplies to delta-star connected transformer which steps up to 230kV. The generator2 (G2) in area 1 is shifted in phase by 7.5 degrees and G3 and G4 488 are shifted by 15 and 22.5 degrees with respect to G1. The distance between

3 each generators and internal impedances are represented by equivalent RL parameters. The system considered for test system is modified Kundur s system to make system easy to design, but impedance of system near major buses remains unmodified. The block diagram representation of the Kundur two area system with STATCOM is shown in the Fig.1 Fig.1. Modified Kundur four machine two area system with STATCOM In the Fig.1, there are four subsystem blocks and each subsystem represents synchronous generator based power plant with hydraulic-turbine and governor for mechanical power input and excitation controller for field voltage control for synchronous generator with step-up transformer. The ratings and specifications are given in the Appendix. A 5th order power system stabiliser with speed controller is used in the system. A three phase to ground fault occurred in the line 7 to 8 with fault resistance of 1 milli ohm between phases and on to the ground. The intention of STATCOM is to provide lowest impedance path for fault current in either direction of the line flows so that the sag in the voltage can be compensated and also to mitigate surge currents and oscillations in the synchronous generator. As the system is with higher power and voltage rating, to completely mitigate the voltage sags, current swell and synchronous generator oscillations, STATCOM is unable to accomplish the task completely, UPFC is another option which can do work very effectively than STATCOM. The pulse output from pulse width modulator (PWM) is given to the STATCOM VSC for better control action during steady state and transient operation. In UPFC there are two controllers; one STATCOM connected in shunt and other is series connected called SSSC. These two controllers will be having different voltage and power ratings and control scheme will also be different. Hence cost incurred will be high and complexity comes when SSSC gets problem or has to be removed from supply. During such circumstances, system reliability may effect. Under normal conditions, this FACTS controller objective is to regulate the reactive power at (UPF) unity power factor in the system by absorbing current to the capacitor and during abnormal conditions; it will inject current to the system. In former action, the STATCOM acts as shunt regulated capacitor gives leading power factor by absorbing current and in latter case, acts as shunt inductor by absorbing current giving lagging power factor to system retaining the system to be in stable operating condition during normal and abnormal conditions. In this process, the real and reactive power from the STATCOM is sensed and thereby voltage regulation and power factor correction takes place by using PWM pulse generator. 3. Block diagram of synchronous generator and STATCOM 3.1. Synchronous Generator Block Diagram 489

4 Block diagram representation of single machine is represented by 4th order operational impedances as shown in Fig.2a. Damping in power system oscillations is done by using PSS by producing perturbations in electrical torque in phase with perturbations in rotor speed. The block diagram representation of 5th order IEEE DC1 exciter model is shown in Fig.2b with lead-lag compensator. Fig.2a. Block diagram representation of synchronous generator with voltage regulator and PSS, Fig.2b. block diagram of 5th order PSS with speed regulator 3.2. STATCOM Block Diagram STATCOM is a shunt device used to control voltage by controlling reactive power in the system. It injects current or changes the power factor to regulate voltage at the point it is coupled. The block diagram representation of general STATCOM controller is shown in the Fig. 3a. Fig.3a. block diagram and Fig.3b MATLAB implementation of STATCOM controller Dynamic equations of the above block are given by: = - = [ ( ) ] = [ ( ) ] = [ ( ) ] = [ ] = [ ] = [ ] ) + = 0 ) + = 0, for i =1 to m STATCOM equations 490

5 = = + + ( = + + ( = ( ( = = = = ( ) Modeling of capacitor [ ] = ( ) hence = [ ] From figure = Assuming sending end voltage is assumed to be constant The transfer functions are fine tuned to make the difference between, to be near to zero. This STATCOM controller output Vpd and Vpq are given to inverse parks transformation to get abc parameters which will be input to PWM pulse generator. With pulse with modulator (PWM), pulses will be given to IGBT converter, based on gate firing scheme, voltage will be controlled about the reference point. The design of STATCOM based on transfer function model as described in Fig.3 is implemented in MATLAB as shown in Fig.3b. 4. Results and Discussion The controller for STATCOM is shown in Fig.3a and its implementation in MATLAB in Fig.3b. The voltage and current are referred from area2 generator represented with Vabc and Iabc. With these parameters, real and reactive powers are calculated. These powers are controlled independently by using transfer functions to derive direct and quadrature axis voltages. These direct axis voltage can control real power and quadrature axis will control the reactive power flow from STATCOM. The two phase voltages are converted into three phase (dq -abc) voltages and this reference voltage is fed to PWM converter to generate pulses to STATCOM. This controls the direction of current flow from STATCOM to system or vice-versa based on difference in voltage magnitude at reference point at STATCOM DC voltage. If at reference point, voltage is higher, current will flow to STATCOM and when at reference point are low, current flows from STATCOM. The voltage at reference point can be high due to Ferranti effect or sudden load throw off, lightning and voltage may decrease due to heavy loading or due to faults. The aim of STATCOM is maintain constant voltage magnitude at reference point, minimise inter-area oscillations and to enhance stability and reliability. 4.1 Without STATCOM 491

6 Fig 4a (i) Voltage and current in area1 (ii) in area2 without STATCOM Fig. 4b (i) Generator parameters in area1 (ii) in area2 without STATCOM The results are compared without STATCOM and with STATCOM for the circuit shown figure 2. A three phase to ground fault occurs at 0.1 seconds and clears naturally at 0.2 seconds with fault resistance of 1mΩ respectively between phases and ground. Three winding transformers are used; primary winding is connected to area1, secondary winding is connected to area2 and tertiary winding is connected to STATCOM1. Similarly the connections were also done to STATCOM2. The transformer voltages on area1 and area2 are 230kV and on STATCOM side is 20kV. A capacitor of 1mF is connected common to both sides of IGBT based voltage source inverter. Fig 4c (i) Voltage and current in area1 (ii) in area2 with STATCOM Fig 4a (i) gives voltage in volts and current in amps for area 1, 4a (ii) area2 without STATCOM, 4a (iii) with STATCOM in area1 and 4a (iv) in area4 respectively. Initially during healthy period, voltage is nearly 200kV 492 and current is 200Amps, when fault occurred; voltage became nearly zero in

7 area1, where as in area2, it is 0.5kV and current was 500Amps. When STATCOM is placed as shown in Fig 2, it injects voltage and current as shown in Fig.4d. It can clearly be seen that fault current has mitigated and voltage on both sides are compensated by VSC. The voltage sag is less than 10% and current is almost uniform. There will be sub-transient and transient current waveforms with STATCOM controller. 4.2 With STATCOM Fig. 4d (i) Generator parameters in area1 (ii) in area2 with STATCOM (iii) DC voltage across capacitor with STATCOM The Generator stator voltage (quadrature and direct axis) are in per unit (pu) and pu rotor speed is shown in Fig. 4b (i) area 1, (ii) area 2 without STATCOM and (iii) for area 1 and (iv) in area 2 with STATCOM. The stator quadrature voltage for generator1 in area1 has decreased to 0.75p.u from 1p.u. during this transient time. With STATCOM, the quadrature and direct axis voltages are at equilibrium during and after fault. The stator output current in per unit quantities are shown in Fig 4c with out and with STATCOM for area1 and area2 generators. During equilibrium, stator output current is 0.5A; during transient is 3.6pu amps on area1 and 2.4pu on area2 without STATCOM. With placement of STATCOM, stator current is almost constant. The sub-transient and transient current can be reduced by taking a capacitor on VSC less than 3000uF. But it is not fully capable of mitigating oscillations caused due to such huge transients. In this analysis, the capacitor is 10000uF with 10kV rating. Fig. 4d, (i) give DC voltage across the capacitor bank at Voltage Source Converter (VSC-STATCOM). The voltage is nearly 8000V dc during normal conditions. When fault occurred at 0.1s, it has decreased to 7000V. This stored capacitor voltage is used to compensate the voltage on the source sides of area 1 and 2. Hence voltage is compensated as shown in Fig. 4a (iii). The STATCOM voltage in volts and current in amps for three phases are shown in Fig 4d (ii). The power system oscillations with and without STATCOM for output active power can be observed in Fig 4d (iii) and (iv). The damping s are very high without STATCOM, but is very less with it. 493 The ripples formed are due to the fact that STATCOM capacitor is taking

8 time to reach steady state. Figure 4(e) shows Voltage and current in area 1 and 2 with UPFC. Figure 4 (f) shows Generator parameters in area 1 and 2 with UPFC. 4.3 With UPFC Fig 4e (i) Voltage and current in area1 with UPFC (ii) in area2 with UPFC Fig. 4f (i) Generator parameters in area1 with UPFC (ii) area2 with UPFC 5. Conclusion If a severe three phase to ground fault occur in the midpoint of the system, voltage in area1 and area2 has dropped to zero and the current has drastically increased, results in large oscillations in generator real power (Fig 7d (iii) without any controller. It can be observed with STATCOM, these oscillations in real power during such transients were mitigated. The system regains its normal state after transients die out is due to the action of PSS and AVR. If STATCOM is not available, AVR or PSS fail to operate, this leads to instability and may also cause the generator to damage if proper action is not taken. In power stations, the relays will identify such situations and will trip the system from supplying power. This leads to load shedding and severe inconvenience to the customers. It can be observed that surge currents can be bypassed to STATCOM, which helps in maintaining nearly constant voltage and current in area1 and area2. Working of STATCOM depends on the impedance of the line, capacitor ratings, voltage that has to compensate, MVA rating of STATCOM transformer and reactive power of the system. Generator stator current has controlled with STATCOM and surge currents are mitigated. References 1. Amara, S.; Hsan, H.A., Power system stability improvement by FACTS devices: A comparison between STATCOM, 494 SSSC and UPFC, IEEE

9 2012 First International Conference on Renewable Energies and Vehicular Technology (REVET), Pp: Griffo, A.; Lauria, D., Some considerations on power system stability improvement by FACTS devices, IEEE SPEEDAM International Symposium on Power Electronics, Electrical Drives, Automation and Motion, 2006, PP: Kawabe, K.-i.; Yokoyama, A., Stability enhancement by multiple unified power flow controllers using wide-area information in the multi-machine power system, IEEE 2010 International Conference on Power System Technology (POWERCON), Pp: Yuma, G.P.; Kusakana, K., Damping of oscillations of the IEEE 14 bus power system by SVC with STATCOM, th International Conference on Environment and Electrical Engineering (EEEIC),PP: Grunbaum, R., FACTS for voltage control and power quality improvement in distribution grids, IET-CIRED. CIRED Seminar SmartGrids for Distribution, 2008, Pp: Kamarposhti, M.A.; Alinezhad, M.; Lesani, H.; Talebi, N., Comparison of SVC, STATCOM, TCSC, and UPFC controllers for Static Voltage Stability evaluated by continuation power flow method, IEEE Canada Electric Power Conference, EPEC 2008, Pp: Hu Y.; Che Z., STATCOM s effects on stability improvement of induction generator based wind turbine systems, Asia-Pacific Power and Energy Engineering Conference, APPEEC 2009, PP: Safari Tirtashi, Mohammad Reza; Rohani, Ahmad; Noroozian, Reza, PSS and STATCOM controller design for damping power system oscillations using fuzzy control strategies, th Iranian Conference on Electrical Engineering (ICEE), PP: Al-Ismail, F.S.; Abido, M.A., The impact of STATCOM based stabilizers on Power System Stability, using intelligent computational optimization approach,2011 IEEE PES Innovative Smart Grid Technologies Asia (ISGT), 10. Rai, D.; Faried, S.O.; Ramakrishna, G.; Edris, A., Damping Inter-Area Oscillations Using Phase Imbalanced Series Compensation Schemes,IEEE Transactions on Power Systems, Volume: 26, Issue: 3, 2011, Pp: Li Chun; Jiang Qirong; Xu Jianxin, Investigation of voltage regulation stability of static synchronous compensator in power system, IEEE Power Engineering Society Winter Meeting, Vol: 4, Pp: Suresh, Y.; Panda, A.K., Dynamic performance of statcom under line to ground faults in power system, 5th IET International Conference on Power Electronics, Machines and Drives (PEMD 2010),PP: P. Kundur, Power System Stability and Control, McGraw-Hill, 1994, Example 12.6, p CH.AppalaNarayana, D.V.N.Ananth, K.D.Syam Prasad, CH. Saibabu, S.SaiKiran, T. PapiNaidu Application of STATCOM for Transient StabilityImprovement and Performance Enhancement for awind Turbine Based Induction Generator, Volume-2, Issue-6, January 2013, pp

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