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1 MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp Power Quality Issues and its Improvement in Wind Energy Generation Interface to Grid System Sharad W. Mohod Dept. of Electronic Engineering, Prof. Ram Meghe Institute of Technology & Research Badnera-Amravati, India Mohan V. Aware Dept.of Electrical Engineering, Visvesvaraya National Institute of Technology, Nagpur, India Abstract Injection of the wind power into an electric grid affects the power quality. The performance of the wind turbine and their power quality are determined on the basis of measurements, and according to the guideline specified in International Electro-technical Commission, IEC The influence of the wind turbine in the grid system concerning the power quality measurements arethe active power, reactive power, variation of voltage, flicker, harmonics, and electrical behavior of switching operation and these are measured according to national / international guidelines. The paper study demonstrates the power quality problem due to installation of wind turbine with the grid. The pulse width modulation (PWM) inverter scheme for the grid connected wind energy generation for power quality improvement is simulated using MATLAB/ SIMULINK in power system block set. Due to improvement in technologies, the wind turbine is expected to support the grid and therefore the wind turbine has to control the reactive power over a wide range, and also to deliver the reactive power, in case of voltage drop and can remain connected during short-term voltage drop. Hence to ensure its fulfillments, the power quality measure is specified in IEC The scheme for improvement in power quality has been presented in the paper. KEYWORDS Component power quality, Wind Energy Generating System (WEGS) I. INTRODUCTION The power quality is used to describe how closely the electrical power delivered to customers corresponding to the appropriate standard and so operate their end-use equipment correctly. It is an essential customer-focused measure and is greatly affected by the operation of a distribution and transmission network. The growing importance of power quality is due to the widespread use of power electronic equipments such as information technology, power electronic converter, programmable logic controller and energy-efficient lighting. These loads are sensitive in nature and simultaneously it is a major cause and major victim of power quality problems. Due to this non-linear nature of such load, causes the disturbance in the waveform. This issue of power quality is of great importance to the wind turbine. The individual units can be of large capacity, up to 2 MW, feeding into distribution network with high source impedance, particularly with customers connected in close proximity [1]. In case of fixedspeed wind turbine operation, all the fluctuation in the wind speed are transmitted as fluctuations in the mechanical torque, electrical power on the grid and leads to large voltage fluctuations. Thus the network needs to manage, the excessive voltage transients which are to be avoided. Today in the variable-speed wind turbine designs, the uses of power electronic converters are mostly used. Thus the issue of harmonic distortion of the network voltage should be considered. During the normal operation, wind turbine produces a continuous variable output power. These power variations are mainly caused by the effect of turbulence, wind shear, and tower-shadow and of control system in the power system. These effect leads to periodic power variation at the frequency with which the blade passes through the tower, which are superimposed on slow variation caused by changes in the wind speed. There may be a high frequency power variation caused by the dynamics of the turbine. Today, the use of variable speed wind turbine operation has got the advantage, the fast power variations are not transmitted and can be made smooth. Thus the power quality issues can be viewed with respect to the wind generation and the transmission and distribution network can cause the variation in voltage to which the wind turbine is connected, such as voltage sag, swells, etc. However the wind generator introduces disturbances into the distribution network. The connection of power converter into the power system may inject the harmonic current into the grid and cause a reduction in power quality. Today the PWM inverter control technology has been developed and it can technically manage to control the power level associates with the commercial wind turbines. The wind turbine and their quality are assessed according to the national and international guidelines, and so it is important to evaluate the grid connection with wind generating system [2]-[4]. II. MOTIVATION FOR POWER QUALITY CONCERN (1) The lasted equipments in the power system are with microprocessor based control and power electronic

2 MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp devices, are more sensitive to power quality than used in past. (2) To increase overall efficiency in the system, use of adjustable speed motor, power factor correction are result in increase of harmonics level in power system. (3) Deregulation of utilities, distributed generations have increase the power quality problem (4) Awareness of end user for interruption, switching transients. (5) Globalization of industry around the world. III. POWER QUALITY ISSUES AND ITS CONSEQUENCES A. VOLTAGE VARIATION The voltage variation mainly results from the wind velocity and generator torque. The voltage variation is directly related to real and reactive power variations. The wind generating system equipped with an asynchronous generator consumes the reactive power and can cause additional negative problem for the grid. Switching the wind turbine generator ON and OFF also varies the voltages. The voltage variation is commonly classified as short duration and long duration voltage variation. (a) Voltage Sag /Voltage Dips It is the decreased of the nominal voltage level between 10% to 90% of the nominal rms voltage, at the power frequency, for the duration of 0.5 cycle to 1min. According to the European Standard EN 50160,the dips with depths of 10% to 15% are commonly due to the switching loads, where as large dips may caused by fault. Causes Start-up of wind turbines, Fault on the transmission/ distribution network, Fault in consumer installation, connection of heavy loads, start up of large motors. Consequences Malfunction of equipments namely microprocessor based control system, programmable logic controller, adjustable speed drives, that may lead to a process stoppage, tripping of contractors, relays trip for voltage sensitive load and loss of efficiency in electric machine. (b) Voltage Swells It is momentary increase of voltage at power frequency, with duration of more than one cycle and typically less than few seconds. Causes Start/stop of heavy loads, fault on the system, badly regulated transformer during off peak hours. Consequences-Flickering of light and screen, Damage of sensitive equipments. (C) SHORT INTERRUPTIONS It is total interruption of electrical supply for a duration from few milliseconds to one or two seconds. Causes Mainly due to the opening and automatic re-closure of protection devices. Consequences Tripping of protection devices, stoppage of sensitive equipments like personal computer, programmable logic control system. (d) Long duration voltage variation It is total interruption of electrical supply for a duration greater than 1-2 seconds. Causes- Equipment failure in power system, failure of protection equipments. B. FLICKER Voltage flicker describes dynamic variations in the network voltages caused by wind turbine or by varying loads. Thus the power fluctuation from wind turbine occurs during continuous operation. The amplitude of voltage fluctuation depends on grid strength, network impedance, phase-angle and power factor of the wind turbines. It is defined as a fluctuation of voltage in a frequency Hz. The IEC specifies a flicker meter that can be used to measure flicker directly. The flicker coefficient gives a normalized dimensionless measure of flicker, independent of network situation and independent of short circuit apparent power of the grid. It gives ratio of short circuit power and generated rated apparent power, which is necessary to achieve a long term flicker level. (P lt ), as the given equation (1). SK C( < K, Va) P (1) lt S n Where, C(Y k, V a )-flicker coefficient depends on grid impedance angle Y k and the average wind velocity V a. S K - Short-circuit power of grid at point of common coupling. S n - Apparent power of wind turbine at rated power. P lt - Long term flicker emission. The flicker standards are generally used to characterize the transient voltage variations.the short flicker is evaluated over a 10 min period and long term flicker is evaluated over 2 hours period. Causes: Fluctuation of active and reactive power of wind turbine, i.e. yaw error, wind shear, wind turbulence or fluctuation in control system, switching operations in wind turbine. In fixed speed wind turbine each time a rotor blade passes through the tower, the power output of the turbine reduces.this effect cause s periodical power fluctuation with a frequency of about 1 Hz, where as in variable speed turbine power fluctuation are smoothed.flickers are produced by arc furnace, arc lamps, capacitor switching. Consequences Degradation of power quality, damage to sensitive equipments.

3 MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp C. HARMONICS It results from the operation of power electronic converters. The harmonic voltage and current should be limited to the acceptable level at the point of wind turbine connection to the network. The emission of harmonic current during the continuous operation of wind turbine with power converter has to be stated. The relative harmonic current limit is stated in the Table 1. Table 1. Harmonic number Admissible harmonic (I h / I i ) where I h is the total harmonic current of h th order caused by the consumer and I i is the rms current corresponding to the consumer agreed power. The IEC does not require measurement of harmonic emission as till now no damage to equipment is reported. Thyristor based converter are expected to emit harmonic current that may influence the harmonic voltage. European Standard EN50160 includes the limit. To ensure the harmonic voltage within limit, each source of harmonic current can allow only a limited contribution, as per the IEC guideline. Modern variable-speed wind turbine uses a voltage source converter and normally switching is made by insulated bipolar transistor at several KHz to synthesize a sine wave and eliminates the lower order harmonics (<19th).The rapid switching gives a large reduction in lower order harmonic current compared to the line commutated converter, but the output current will have high frequency current and can be easily filter-out. system. The disadvantages of self excitation are the safety aspect and balance between real and reactive power. Causes If the sensitive equipment is connected to the generator during the self excitation, the equipment may be a subject to over load, under voltage and over frequency operation [5]-[7]. IV. GRID CODE FOR WIND POWER SYSTEM The arrangement of the technical requirements within grid code is required in the grid connected power system. The specific grid code are now being imposed for wind energy generating system, taking into account the development of wind power plant capabilities. The typical requirement for the wind generator in the system as follows: Control of reactive power-these requirements contributes to voltage control on the network. Control of active power. Protective devices. Power quality. It is important that these requirements are often specified at the point of common coupling (PCC) between the wind turbine and the electricity network (grid). V. GRID CONNECTED WIND GENERATION INTERFACE TO UNDERSTSND POWER QUALITY Wind generation interface system is connected to grid system with voltages on each side of the impedance shown in Fig. 1. D. WIND TURBINE LOCATION IN THE POWER SYSTEM The way of connecting the wind generating system into the power system highly influences the power quality. As a rule, the impact on power quality at the consumers terminal is located close to the load is higher, than connected far away from the load. When the wind generating system is connected to a medium voltage transmission line, the distance between the wind generating station and point of common coupling is small, such system are economical as compared to other location. Thus the operation and its influence on power system depends on the structure of the adjoining power network. E. SELF EXCITATION OF WIND TURBINE GENERATING SYSTEM The self excitation of wind turbine generating system (WTGS) with an asynchronous generator takes place after disconnection of WTGS with local load. The risk of self excitation arises especially when WTGS is equipped with compensating capacitor. The capacitor connected to induction generator provides reactive power compensation. However the voltage and frequency are determined by the balancing of the Fig. 1. Grid connected wind generator interface to power system. In the power system,the power is transmitted using three phase power that is as symmetrical as possible. The line-toline voltage is 3 times larger than phase voltage and total three phase power is constant. The voltage drop over the impedance can be written as in (2) V 1 V 2 3IZ (2) Where V 1 -rms voltage,z- impedance of transmission line, transformer in the feeding grid.

4 MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp At the point of common connection (PCC),wind farm and local load is also connected. The short circuit power S K,in wind connection is shown in (3) S K 2 1 V Z The change in wind power production will cause changes in the current through the impedance Z. These current changes cause the changes in the voltage V 2. In practice, connections with network with short circuit ratio < 2.5 are avoided, as this give rise to voltage fluctuations, called as weak grid. The impedance Z = R + JX at the fundamental frequency. Generally the impedance in presence of harmonics become as shown in (4) Zh ( ) R jhx L (4) Where h is the harmonic order,the inductive reactance changes linearly with frequency. The combination of wind power production and load are represented as P + JQ, where P is the active power and Q is the reactive power. The reactive power is depend on the phase shift between voltage and current, such as shown in (5) (3) tan 1 Q I (5) P The reactive power in the wind has an impact on voltage V 2, The impact is also depend on local load and on the feeding grid impedance. Today the wind generation equipped with induction generator, consume reactive power and reduces the voltage V 2 at PCC. It is necessary to manage reactive power for both customer and utility providers. The reactive power consumes, transmission and generation resources, incur real power loss on transmission system. Thus it is possible to control the reactive generation or consumption so as to maintain power quality at PCC. (3) Magnetic Synthesizers (4) On line UPS (5) Flywheel Energy storage system (6) Superconducting Magnetic Energy storage device. VII. A STUDY OF WIND ENERGY GENERATION INTERFACE WITH-STATCOM The proposed system for STATCOM interface with wind energy generation system as shown in Fig. 2. The system is simulated in MATLAB/SIMULINK in power system block set. The system parameter for given system is given Table I. Table 2: System Parameters S.N. Parameters Ratings 1 Grid Voltage 3-phase, 415V, 50 Hz 2 Induction Motor/Generator 3.35 kva, 415V, 50 Hz, P = 4, Speed = 1440 rpm, Rs = 0.01W, Rr = 0.015Ù, Ls = 0.06H, Lr = 0.06H 3 Line Series Inductance 0.05mH 4 Inverter Parameters DC Link Voltage = 800V, DC link Capacitance = 100 ìf. Switching frequency = 2 khz, 5 IGBT Rating Collector Voltage = 1200V, Forward Current = 50A, Gate voltage = 20V, Power dissipation = 310W 6 Load Parameter Non-linear Load 25kW. VI. GENERALISED MITIGATION TECHNIQUES The different technologies are available based on the specific requirement of the system as are: (1) Reactive power compensation technologies: (a) Synchronous condenser (b) Static VAR compensator (SVR) (c) Static synchronous compensator (STATCOM) (d) Static synchronous series compensator (SSSC) (e) Dynamic voltage restore (DVR) (f) Unified power flow controller (UPFC) (g) Interline power flow controller (IPFC) (h) Active Filters (2) Constant Voltage transformer Fig. 2. Wind energy generator interface with STATCOM The proposed system consists of the following main modules [7]-[11]. A. Wind turbine model The implemented model of wind turbine does not include mechanical dynamics and the detailed electrical model of induction machine. The uniform wind speed is considered so as to generate the same power.

5 MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp B. Induction generator Induction generator is connected to the distribution network, it needs an external reactive source connected to its stator winding to provide an output voltage control. It is sufficient that it works at a speed above synchronous speed. C. STATCOM The STATCOM is a three-phase voltage source inverter having the capacitance on its DC link. It is connected at the point of common coupling (PCC) through input inductance / transformer inductance. The STATCOM injects a compensating current of variable magnitude and frequency component at the bus, point of common coupling. shown as in Fig. 3. The power quality is observed at PCC, so that the source voltage and the source current are in-phase quantity. The fast response of STATCOM is observed in the grid system at t=0.7 sec. as shown in Fig. 5. From the technological point of view, the most effective location to install STATCOM is just directly at PCC bus. Fig. 5. Supply Voltage and Current at PCC. Fig. 3. VSC based STATCOM Three phase source current and three phase load current on the load side shows that the grid current is affected due to the effects of non-linear load, thus purity of waveform may be lost on both sides in the system. The simulated distorted voltage and distorted current with non linear load, is shown in Fig. 6. VIII. SIMULATION PERFORMANCE A. Dynamic performance of the system The performance of the induction generator is carried out by making a change in the torque from motoring mode to generator mode in the simulation. The wind variation will change the mechanical torque and the machine speed, therefore, the torque on machine changes. The induction machine starts as motor mode and is taken to a generating mode at t=0.4 sec. The rotor speed, electrical torque, threephase generated current and voltage are shown in Fig. 4. Fig. 6. (a) Load Voltage (b) Load Current (c) FFT of load current Fig. 4. (a) Rotor Speed (b) Electric Torque (c) Generated Current (d) Generated three phase s voltage of Induction Machine The performance of the system is measured by switching the STATCOM at time t=0.7 sec in the system and how the STATCOM responds to the step change command for increase in additional load, when applied at time t=1.0 sec. in the system. The source current, load current and injected current from STATCOM and generated current of induction generator is shown, when controller STATCOM is in an OFF condition and performance is shown in Fig. 7.

6 MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp The DC link voltage regulates the source current in the grid system, so the DC link voltage is maintained constant across the capacitor as shown in Fig. 10.The average DC capacitor voltage of about 760 Volt, is dynamically controlled and does not change due to the step change command in the load. Therefore the control topology is validated. Fig. 7 (a) Source Current (b) Load Current (c) Inverter Injected Current (d) Wind energy generator (Induction generator) current. The controller STATCOM is made ON at time t=0.7 sec., without change in any other load condition parameter and performance is shown in Fig. 8. Fig. 8. (a) Source Current (b) Load Current (c) Inverter Injected Current (d) Wind energy generator (Induction generator) current. As the STATCOM in operation, it injected the current into the power system and start mitigate for reactive demand, as well as harmonic current. The transient takes almost one fundamental period, until the source current resembles sinusoidal waveform. The controller response for additional increase in load is shown in Fig. 9. Fig. 9. (a) Source Current (b) Load Current (c) Inverter Injected Current (d) Wind energy generator (Induction generator) current. Fig. 10. DC Link Voltage across the capacitor IX. CONCLUSION The increase awareness of power quality issues of the consumer and electric utility for standardization and evaluation of performance is an important aspect in the wind generating system. The wind turbine generating system describes the electrical performance of the system, as it reflects on grid and influences the power quality. The International guidelines IEC sets the requirement and grid code for the power quality measurements. The paper simulates the scheme in MATLAB/SIMULINK for maintaining the power quality in such a way that it can cancel out the reactive and harmonic parts of the load current and maintain the source voltage and current in-phase at the point of common coupling in the grid system. Thus it fulfills the power quality norm on the grid as per the IEC standard. X. REFERENCES [1] Tande, J.O.Q., Applying power quality characteristic of wind turbine for assessing impact on voltage quality, Wind Energy, pp. 5, [2] Thiringer, T. Petrup P. Power Quality Impact of Sea Located Hybrid Wind Park IEEE Tran. on Energy Conversion, pp , [3] Z. Chen, E. Spooner, Grid Power Quality with Variable Speed Wind Turbines, IEEE Trans on Energy Conversion, Vol. 16, No.2, pp , June [4] Z.Chang, E. Spooner, Grid power quality with variable speed wind turbine, IEEE Trans. on Energy Conversion, Vol. 16, No. 2, June [5] F. Blaabjerg, Z. Chen, S.B. Kjaer, Power Electronics as efficient interface in dispersed power generation System. IEEE Trans. On Power Electron. Vol. 19, No. 5, pp , Sept [6] Sung-Hun, Seong R. Lee, Hooman Dehbonei, Chemmangot V. Nayar, Application of Voltage-and Current Controlled Voltage Source Inverters for Distributed Generation System., IEEE Trans. On Energy Conversion, Vol.21, No.3, pp , Sept. 2006

7 MIT International Journal of Electrical and Instrumentation Engineering Vol. 1, No. 2, Aug 2011, pp [7] Juan Manel Carrasco, Power Electronic System for Grid Integration of Renewable Energy Source: A Survey, IEEE Trans on Industrial Electronics, Vol. 53, No. 4, pp , August [8] S.W. Mohod, M.V. Aware, Grid Power Quality with Variable Speed Wind Energy Conversion. proceeding of IEEE Int. Conf. on Power Electronic Drives and Energy System (PEDES), Delhi, 5A-21, Dec [9] S.W. Mohod and M.V. Aware, A STATCOM control scheme for grid connected wind energy system for power quality improvement, IEEE, System Journal, Vol. 2, issue 3, pp , Sept [10] Chong Han, Alex Q. Huang, Mesut Baran, Subhashish Bhattacharya, Wayne Litzenberger, Loren Anderson, STATCOM Impact Study on the Integration of a Large Wind Farm into a Weak Loop Power System., IEEE Trans. On Energy Conv., Vol. 23, No. 1, March 2008, pp [11] P. Siemes, H.J. Haubrich, H. Vennegeerts, Concepts for the improved integration of wind power into German interconnection System, IET Review, Vol. 2, No.1, pp , Power generation 2008.