Power Quality Improvement on Wind Energy System by Using STATCOM Farah Atiqah Binti Ali Yusup1,a*, Shirley Anak Rufus2,b, Prashobh Kumar Karunakaran3,c, Nazreen Binti Junaidi4,d and Nurul ‘Izzati Binti Hashim5,e 1,2,3,4,5
Faculty of Electrical and Electronic Engineering, University Malaysia Sarawak (UNIMAS), 94300 Kota Samarahan, Sarawak. a
[email protected], brshirley@feng,unimas.my,
[email protected], d
[email protected],
[email protected]
Keywords: Power Quality (PQ), Point of Common Coupling (PCC), Static Compensator (STATCOM), Battery Energy Storage System (BESS), current Total Harmonic Distortion (THD).
Abstract. Wind energy known as a cheap, clean and uncontrolled resource and becomes an important green electricity source. However, installation of wind system into an electric grid introduces the Power Quality (PQ) phenomenon. These poses great challenges leads to various control schemes and new techniques to mitigate the PQ events. This paper described the improvement of PQ events in a grid connected wind energy system by using Static Compensator (STATCOM) with a Battery Energy Storage System (BESS) at the Point of Common Coupling (PCC). The performance of the proposed wind energy system is simulated via MATLAB/SIMULINK in power system block set. Introduction Renewable energy is the alternative sources like wind, biomass, hydro and solar that are necessary to sustain the growth and social progress in order to replace the non-renewable energy [1]. Compare to non-renewable energy, wind energy being a geographically and climatically uncontrollable resource that depends on the presence of wind in certain speed. [2-3]. Compare to Malaysia, other countries has a strategic location which most of the time has a windy climate. This made the installation of wind energy system suitable. Malaysia has equatorial climate which means receive sunlight and often raining nearly every day. With a poor wind sources, wind energy system will seldom be implemented. To capture the most wind energy, suitable height of tower for mounted the wind turbine is crucial [4]. Installation of wind turbine and structure of the adjoining into power grid also influences the PQ. Integration of wind energy system into existing electrical power system presents great technical challenges in the PQ phenomenon that essential for better improvement in order to improve power supply reliability and at the same time can mitigate the PQ disturbances. Continuous output power of the wind turbine depends on the fluctuation of wind speeds that caused the power variations and disturbances such as voltage sags, voltage swells, harmonics and flickers in the power system network. In this paper, the performance of STATCOM-BESS in facilitating the wind energy system is presented. PQ events occur due to the installation of wind turbine with the grid becomes major concern for most of the researchers [1-5]. Fast response time, low cost and superior voltage support capability, the STATCOM is considered in this proposed model [6-7]. Other than that, the STATCOM also can protect the sensitive devices at the non-linear loads sides from getting tripped and damaged. For the purpose of stabilized the grid system, a BESS is required in the proposed wind energy system in order to compensate the fluctuation generated by the wind turbine and maintain the reactive power produced by generator and non-linear load. Thus, STATCOM-BESS
plays an important role to make sure that reactive power can be injected and absorbed to wind induction generator as a power supply. Power Quality Issues Due to the fact that some modern electricity equipment that has more design and control features in it make some component more sensitive to PQ disturbances or events. [2] This is why PQ needs to be taken seriously to create and increase awareness among suppliers and consumers. The most common PQ problems that related are voltage sags, flicker, harmonics, dips and swells. As according to IEEE 1159-1995, voltage sags is define as decrease to between 0.1 and 0.9 per unit (p.u.) in r.m.s. voltage or current at the power frequency for durations of 0.5 cycles to 1 minute. Furthermore, voltage sags also known as a short duration reduction in r.m.s. voltage that can be caused by short circuit, overload or starting of electric motors [8-9]. In electrical power systems, THD is used to detect the PQ of electric power systems and it affects the reduction in peak currents. Harmonic distortion is not a new phenomenon on power system. It is caused by non-linear devices that connected in power system network that will produce a current that is not proportional to the applied voltage [10]. Equation of THD voltage is given as in Eq.1, where 𝑉𝑛 represents the nth harmonic voltage and 𝑉1 is the fundamental frequency. VTHD
hmax
Vn2 100 h 1 V1
(1) [10]
According to the International Electrotechnical Vocabulary (IEV) of the International Electrotechnical Committee (IEC), flicker is defined as impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time. From a more practical point of view one can say that voltage fluctuations on the supply network cause change of the luminance of lamps, which in turn can create the visual phenomenon called flicker. Power factor is a ratio between real power and apparent power in an electrical circuit. Basically, it is a measure of efficiency of power distribution system. It is in unity when voltage and current are in phase. Power factor usually are in its leading or lagging condition to show the sign of the phase angle of current with respect to the voltage. Power factor must be keep closer to unity that is between 0.95 closer to 1. This is because for a low power factor, more current flow in the system will increase energy lost. Power factor can be defined as stated in Eq. 2 where represents the phase angle for lagging and leading between voltage and current [11]. Power Factor, PF cos
(2) [11]
PQ Improvement in Wind Energy Generating System with STATCOM-BESS As illustrated in Fig. 1, grid connected wind energy system is implemented for PQ improvement at PCC. It consists of wind energy generation system and BESS with STATCOM. This system consists of turbine, induction generator, interfacing transformer, and rectifier to get DC bus voltage. Past researchers had analysed the wind energy system that being connected by STATCOM at the PCC together with BESS can help to reduce the harmonic distortion, and at the same time, maintain the power factor nearly to unity at the source side and reduce the voltage sags by provide a constant power support for wind generator and loads[12-14].
Fig. 1: Grid connected wind energy system with STATCOM-BESS [14] STATCOM-BESS STATCOM consists of Insulated gate bipolar transistor known as (IGBT) that able to reduce any distortion in the grid since it has a higher switching frequency for low on-state voltage drop Thus, this characteristic will protect generators from getting damage if it is being disconnected and connected in a short period. STATCOM shunt connected device that capable of generating and absorbing reactive power in which the output can be varied to control the specific parameters in the grid connected wind energy system. STATCOM is a regulating device that regulates the flow of reactive power in the system and used for power factor correction. Besides, it is acting as a voltage source converter and it also capable of converting dc power into ac power of variable amplitude and phase angle [14]. BESS is used as an energy storage element and this connection is a new trend of storage where the batteries are being used as a system to interchange energy with the grid. Battery is connected with STATCOM at PCC between user end and utility grid. Main purpose of using BESS is for voltage regulation and maintains DC capacitor voltage constant. In wind energy generation system, if voltage fluctuation had ever occurred in the system, BESS can be used to level the power fluctuation by charging and discharging operation [15].
Fig. 2: STATCOM controller in Simulink Fig. 2 shows the control scheme needed for STATCOM controller in order provide the appropriate current injection through Pulse Width Modulation (PWM) and PI controller. PWM technique in model enables very fast switching frequencies that can be used to improve on the efficiency of the converter. Apart from that, the PI controller will process any error signal and will generate the required angle to drive the error to zero. It is used to control the current injection at the PCC.
Simulation and Result
Fig. 3: Overall system: Grid connected wind energy system that being connected with STATCOMBESS Simulation model of the overall wind energy system is implemented in Matlab/Simulink is illustrated in Fig. 3. The simulation was carried out in a three phase, 415V with 50Hz grid. Asynchronous induction generator with nominal speed of 1440rpm, with a rotor resistance of 0.1 Ω, stator resistance 0.15 Ω and stator and rotor inductance carrying a value of 0.06H. STATCOM’s inverter has a DC link voltage of 800V, forward current of 50A, a gate voltage of 20V, non-linear load of 20kW and 310W of power dissipation.
(a) (b) Fig. 4 Voltage at Wind Generator (a) Without STATCOM-BESS and (b) With STATCOM-BESS As presented in Fig. 4(a), it is shows the voltage sag event that measured near the wind generator before being connected with STATCOM. In this paper, the voltage sag affected by a short duration reduction in r.m.s. voltage that caused by starting of induction motors. First voltage sags occur is within the first five cycles, which the voltage amplitude of 1.6p.u. reduce with a slow decay to 1.19p.u.. At 0.815s, then the voltage back to the pre-sag voltage of 1.6p.u. at 0.9s. It was found that the voltage amplitude drops to a value of 26% for three cycles. At 0.6s, the voltage drops from 1.6p.u. and slowly decaying to 0.5p.u. with value of 69% for one cycle. At Fig. 4(b) shows the
unbalance voltage sags can be mitigate as the system is connected with STATCOM-BESS and it has an equal magnitude for all phases.
(a) (b) Fig. 5: Power factor measured at PCC (a) Without STATCOM-BESS, (b) With STATCOM-BESS Performance of a power system is affected by power factor. Thus, it can be compare with peak value of current and voltage, as illustrated in Fig. 5(a). It found that the current leads the voltage by 45° that gives power factor of this system 0.7. However, by using the STATCOM with BESS, the current and voltage will be in phase or current leads voltage for exactly 25.84° that gives the power factor nearly unity at 0.89 as in Fig. 5(b).
(a) (b) Fig. 6: Current waveform and THD at non-linear load (a) Without STATCOM-BESS and (b) With STATCOM-BESS Harmonic current distortion in Fig. 6(a) shows the typical current waveform that distorted by non-linear loads before connected with STATCOM-BESS. It should be taken seriously as it will flow through the transmission and draw the harmonic currents in the power system. THD reading for the 10 cycles of harmonic current at non-linear load is analysed using Fast Fourier Transform (FFT) technique that gives 29.29% at the fundamental frequency of 50Hz. As connected with STATCOM-BESS, the current waveform distortion is slightly decreased and it can be evaluated by
FFT analysis, in Fig. 6(b). This is due to the current injected by the STATCOM which cancel the harmonic distortion occurred at the non-linear load. It is shows the decreased from 29.29% to 27.88%. Although the THD only decrease by 1.41%, it still proved that STATCOM-BESS can mitigate the harmonic effects even though it is not perfectly in sinusoidal form.
(a) (b) Fig. 7: Current waveform and THD at source (a) Without STATCOM-BESS and (b) With STATCOM-BESS Without the presence of STATCOM-BESS, current waveform at the source is slightly distorted and altered the original shape of the sinusoid that caused by the continuous variation of non-linear load and wind generator. It results of 6.97% THD current at source and was analysed for 5 cycles as shown in Fig. 7(a). By presence of STATCOM-BESS, distortion at the source can be mitigated as it results perfectly sinusoidal form with THD of 0.00%, illustrated in Fig. 7(b), compare to the waveform without the application of STATCOM-BESS.
(a)
(b)
Fig. 8: Current waveform and THD at wind generator (a) Without STATCOM-BESS and (b) With STATCOM-BESS In Fig. 8(a), it shows the current generated at the wind generator. It has been observe that the waveform generated is not a perfect sinusoidal form. Apart from that, noted that the amplitude of phase 1 (yellow line) is lower than both of the amplitude of phase 2 (purple) and phase 3 (blue). This is affected by the amplitude drop at phase 1 to 88%. THD generated for the current at wind generator without connected with STATCOM-BESS is 0.70% for a fundamental frequency of 50Hz. The FFT window is shown the distorted waveform for 10 cycles from of 0.8s until 1s of time sampling. After connected with STATCOM-BESS, the waveform is less distorted and the percentage of THD is differed slightly, 0.69% compared to percentage before connected with STATCOM-BESS, as in Fig. 8(b). Table 1 shows indicates the voltage sags, power factor and the THD value has improved after wind energy system being connected with STATCOM-BESS. Table 1: Summary of findings Findings Without STATCOM-BESS With STATCOM-BESS Power Factor 0.7 0.89 Voltage Sags 26% and 69% 0% THD at non-linear load 29.29% 27.88% THD of current at wind generator 0.70% 0.69% THD of current at source 6.97% 0.00% Conclusion A grid connected wind energy system with STATCOM and BESS at PCC with non-linear loads has been successfully implemented to mitigate the voltage sags, harmonics distortion and power factor. The presence of PQ events in the system is analysed by comparing the performance of the system with and without STATCOM-BESS in the shunt configuration. The system has been developed and simulated by using MATLAB Simulink power system block set. Based on the comparisons and simulations results, it can be conclude that the BESS technique is suitable and easy to implement other than provide a constant DC voltage to support the performance of STATCOM in mitigate the PQ events for the wind generation system. References [1]
Tenaga Nasional Berhad, Energy Savings at Home, (200866-W). 2014.
[2]
Gordon Walker, Renewable Energy and The Public, Division of Geography, Staffordshire University, Leek Road, Stoke on Trent, Staffs, ST4 2DF, UK, Vol. 12, Issue 1, pp. 49–59, 1995.
[3] Chong Han, Subhashish Bhattacharya, Wayne Litzenberger, Loren Anderson, Ander L. Johnson, Abdel-Aty Edris, Alex Q. Huang, Mesut E. Baran, STATCOM Impact Study On The Integration of A Large Wind Farm into A Weak Loop Power System, IEEE Transaction On Energy Conversion, Vol. 23, No. 1, March 2008. [4]
P. Ramanathan, A STATCOM-Control Scheme for Grid Connected Wind Energy System for Power Quality Improvement‟, Middle-East Journal of Scientific Research 20, Department of EEE, Bharath University, India, 2014.
[5]
Roger C. Dugan, Mark F. McGranaghan, Surya Santoso, H. Wayne Beaty, Electrical Power System Quality, Third Edition, McGraw- Hill Professional, 2012.
[6]
N. G. Hingorani and L. Gyugyi, Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems. Piscataway, NJ: IEEE Press, 2000.
[7]
C. Han, Z. Yang, B. Chen, W. Song, A. Q. Huang, A. Edris, M. Ingram, and S. Atcitty, System Integration and Demonstration of a 4.5 MVA STATCOM Based on Emitter Turn-off (ETO) Thyristor and Cascade Multilevel Converter, In Proc. IEEE IECON, Nov. 6–10, pp. 1329–1334, 2005.
[8]
Math H. Bollen, Understanding Power Quality Problems: Voltage Sags and Interruptions, P.M Anderson, IEEE New York, 10016-5997, 1999.
[9]
Kazibure, Wilson E., Sendaula, Musoke H., Electric Power Quality Control Techniques, NY: Van Notrtrand Ranhold, 1993.
[10] Lundquist, Johan, Harmonic Distortion in Power Systems, Chalmers University of Technology: Department of Electrical Power Engineering, 2001. [11] Venkat Krishnan, James McCulley, Role of Induction Motor in Voltage Instability and Conduct Reactive Power Planning, New York, 2000. [12] Aziz Ahmad, Anwar Shehzad, V.B. Shrivastava, Power Quality Improvement in IEEE 30 Bus System, Research India Publications, Advance in Electronic and Electric Engineering, Vol. 3, No. 6, 2013. [13] Juan Manual Carrasco, Leopoldo Garcia Franquelo, Jan T. Bialasiewicz, Eduardo Galvan, Ramon C. Portillo Guisado, Ma. Angeles Martin Prats, Jose Ignacio Leon, Narciso MorenoAlfonso, Power Electronics Systems for the Grid Integration of Renewable Energy Sources: A Survey, IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006. [14] Supriya Tripathi and C. S. Sharma, Power Quality Improvement in Wind Grid Energy System using STATCOM With BESS - Control Scheme, India, 2014. [15] B.T. Ramakrishnarao, B. Eswararao, L. Narendra, K. Pravallika, A STATCOM- Control Scheme for Power Quality Improvement of Grid Connected Wind Energy System”, Lendi Institute of Engineering and Technology, India, Vol. 2, Issue 3, 2013.
