Challenges of Integration of Wind Power on Power System Grid : A Review

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1 Challenges of Integration of Wind Power on Power System Grid : A Review Anil Gupta 1, Dr. Arun Shandilya 2 1 Assistant Professor, Department of Electrical Engineering, LNCT, Bhopal, MP, India 2 Professor, Department of Electrical Engineering, MANIT, Bhopal, MP, India Abstract The wind energy generation, utilization and its grid penetration in electrical grid are increasing worldwide. The wind generated power is always fluctuating due to its time varying nature and causing stability problems. This weak interconnection of wind generating source in the electrical network affects the power quality and reliability. The influence of the wind turbine in the grid system concerning the power quality measurements are the variation of voltage, flicker, harmonics.this paper provides a brief overview of the technical and operational issues related to integration.paper also considers grid codes that make wind energy more grid-compatible to ensure further growth of this promising renewable source of energy. Finally, potential technical challenges to the integration of large-scale wind energy into the power grid are reviewed with their available mitigation techniques. Keywords Electrical grid, Harmonics, Power quality, Flicker, Wind turbines, Grid codes. I. INTRODUCTION Electricity is produced in bulk by using power plants with natural gas, coal or oil as primary sources. Combustion of such fuels results in emission of pollutants like oxides of Carbon, Nitrogen and Sulfur. Due to shortage of energy resources, the energy crisis and greenhouse emission problems have been gradually increases day by day. Greenhouse gas emissions particularly carbon dioxide is the main cause of global warming. In recent years, a lot of energy policies and research projects focused on the energy-saving[1]. Large wind and solar farms have been installed in power systems around the world due to environmental problems caused by using fossil energy resources. The increasing environmental awareness requires that the system operator should supply electricity to consumers with minimum emissions. Environmental protection has been raised recently due to the concerns regarding global weather and air pollution. Renewable energy sources like wind energy, are widely applied to reach emission reduction with the increasing concern of environmental protection. Wind power generation does not produce harmful emissions. But with random nature of wind resources, the wind farm generates fluctuating electric power. 880 These fluctuations have a negative impact on stability and power quality in electric power systems[2].large scale Integration of DG units in the distribution grid not only affects the grid planning but also has an impact on the operation of the distribution grid[3]. Aspects which are influenced by the connection of DG units are 1) power quality 2) voltage control 3) grid losses 4) fault level 5) protection system. The increase in number of incoming induction generators to the grid, causes the power quality problems mainly in current Harmonics, reactive power and power factor. These problems will be more severe in weak grids. The simultaneous switching operation of Induction Generators results into excessive inrush of reactive power from grid, which is undesirable. The next sections describe the power quality issues and their mitigation techniques. II. POWER QUALITY ISSUES Power Quality has become very important issue over the last decade. A key reason for the increasing importance is the rapid spread of the use of equipments sensitive to power system disturbances and the widespread use of nonlinearly behaving power electronic converters. The addition of windturbines can have a significant effect and increases the complexity of this problem. Depending on the grid configuration and the type of wind turbine used, different power quality problems may arise.[4] The random nature of wind resources, the wind farm generates fluctuating electric power. These fluctuations have a negative impact on stability and power quality in electric power systems.large scale Integration of DG units in the distribution grid not only affects the grid planning but also has an impact on the operation of the distribution grid. Aspects which are influenced by the connection of DG units are 1) power quality 2) voltage control 3) grid losses4) fault level 5) protection system.the increase in number of incoming induction generators to the grid, causes the power quality problems mainly in current Harmonics, reactive power and power factor. These problems will be more severe in weak grids[5-8]. The simultaneous switching operation of Induction Generators results into excessive inrush of reactive power from grid, which is undesirable.

2 In this case an attempt is made to investigate the causes of poor power Quality in grid connected renewable Wind energy system. The voltage fluctuations, reactive power compensation, poor power factor and harmonics distortion are the main aspects of power quality problems in integrating wind energy with the smart power grid due to the inherent characteristics of these resources as shown in Fig. 1. This problem is considered in the power quality and wind turbine generating system operation and computed according to the rule given in IEC standard, Assessment of emission limit for fluctuating load. The relative % voltage change due to switching operation of wind turbine is calculated as d = 100K u ( k)s n /S * k (1) Where d - Relative voltage change, Ku( k) - Voltage change factor, Sn - Rated apparent power of wind turbine and S * k short circuit apparent power of grid. The voltage dips of 3% in most of the cases are acceptable. Fig. 1. Major potential technical impacts of integrating wind energy into the grid. III. VOLTAGE VARIATION A system experiences a state of voltage instability when there is a progressive or uncontrollable drop in voltage magnitude after a disturbance, increase in load demand or change in operating condition. If a large proportion of the grid load is supplied by wind turbines, due to output variations wind speed changes which can cause voltage variation and this affects the normal operation of system[9]. The voltage variation can occur in specific situation mainly as a result of load change and these canbe expected particularly in the case of generator connected to the grid at fixed speed. The large turbine can achieve significantly better output using variable speed operation, particularly in the short time range. IV. VOLTAGE DIP The Voltage dip is a very common and serious type of power quality disturbance due to its effects on sensitive equipment and industrial processes[10]. Voltage dip could occur when there is a large load such as motor start up, transformer energising, capacitor energising, switching of electronic load, momentary overload or a fault in the system network. It can cause the disconnection of wind generators, which could have a negative impact on the stability of the network due to loss of generation[11].it is a sudden reduction in the voltage to a value between 1% to 90 % of the nominal value after a short period of time, conventionally 1ms to 1 min. V. HARMONICS Harmonics can be injected both at the generation and the consumer end. At the consumer end, harmonics are caused by non linear loads such as television, personal computers, compact fluorescent lamps, and so forth.the harmonics distortion caused by non-linear load such as electric arc furnaces, variable speed drives, large concentrations of arc discharge lamps, saturation of magnetization of transformer and a distorted line current. The current generated by such load interact with power system impedance and gives rise to harmonics. The effect of harmonics in the power system can lead to degradation of power quality at the consumer s terminal, increase of power losses, and malfunction in communication system[12]. The degree of variation is assessed at the point of common connection, where consumer and supplier area of responsibility meet. The harmonics voltage and current should be limited to acceptable level at the point of wind turbine connection in the system. According to standard IEC guideline, harmonic measurements are not required for fixed speed wind turbines where the induction generator is directly connected to grid. Harmonic measurements are required only for variable speed turbines equipped with electronic power converters. In general the power converters of wind turbines are pulse-width modulated inverters, which have carrier frequencies in the range of 2-3 khz and produce mainly inter harmonic currents. The harmonic measurement at the wind turbine is problem due to the influence of the already existing harmonic voltage in the grid. The wave shape of the grid voltage is not sinusoidal. There are always harmonics voltages in the grid such as integer harmonic of 5 th and 7th order which affect the measurements. 881

3 Today s variable speed turbines are equipped with self commutated PWM inverter system. This type of inverter system has advantage that both the active and reactive power can be controlled, but it also produced a harmonic current. Therefore filters are necessary to reduce the harmonics. The harmonic distortion is assessed for variable speed turbine with a electronic power converter at the point of common connection. The total harmonic voltage distortion of voltage is given as in (2). 40 V THD = [ (V 2 h/v 1 )*100] (2) h=2 V h - hth harmonic voltage and V 1 fundamental frequency 50 Hz. The THD limit for various level of system voltages are given in the table 1.0 TABLE I VOLTAGE HARMONICS LIMIT System Voltage (kv) Total Harmonic Distortion (%) THD of current ITHD is give as in (3) 40 I THD = [ (I 2 h/i 1 )*100] (3) h=2 Where I h - hth harmonic current and I 1 fundamental frequency (50) Hz. The acceptable level of THD in the current is given in table 2. TABLE II CURRENT HARMONIC LIMIT Voltage level 66 kv 132kV I THD Various standards are also recommended for individual consumer and utility system for helping to design the system to improve the power quality. The characteristics of the load and level of power system significantly decides the effects of harmonics. IEEE standards are adapted in most of the countries. The recommended practice helps designer to limit current and voltage distortion to acceptable limits at point of common coupling (PCC) between supply and the consumer. 1. IEEE standard 519 issued in 1981, recommends voltage distortion less than 5% on power lines below 69 kv. 2. IEEE standard 519 was revised in 1992, and impose 5% voltage distortion limit. VI. FLICKERS The wind generators sometimes produce oscillatory output power, which could cause flickers in the power system network. Flicker is the one of the important power quality aspects in wind turbine generating system. Flicker has widely been considered as a serious drawback and may limit for the maximum amount of wind power generation that can be connected to the grid. Flicker is induced by voltage fluctuations, which are caused by load flow changes in the grid. The flicker is mainly produced by fluctuations in the output power due to wind speed variations. There are many factors that affect flicker emission of grid connected wind turbines during continuous operation, such as wind characteristics and grid conditions[12]. Variable-speed wind turbines have shown better performance related to flicker emission in comparison with fixed-speed wind turbines. The flicker study becomes necessary and important as the wind power penetration level increases quickly. Owing to smoothing effect, large wind turbine produced lower flicker than small wind turbines, in relation to their size. The flicker level depends on the amplitude, shape and repetition frequency of the fluctuated voltage waveform. Evaluating the flicker level is based on the flicker meter described in IEC Two indices are typically used as a scale for flicker emission, short-term flicker index, Pst and long-term flicker index, Plt. Plt is estimated by certain process of the Pst values. It is assumed that wind turbines under study is running at normal operation; hence, the long-term flicker index (Plt), which is based on a 120- min time interval, is equal to Pst and, therefore, Pst is only considered in this work. The normalized response of the flicker meter described in Figure

4 Fig. 2. Influence of frequency on the perceptibility of sinusoidal voltage change The flicker level (Pst 1) is a threshold level for connecting wind turbines to low voltage. The measurements are made for maximum number of specified switching operation of wind turbine with 10-minutes period and 2-hour period are specified, as given in (4) P lt = C( )S n /S K (4) Where P lt - Long term flicker. C( ) - Flicker coefficient calculated from Rayleigh distribution of the wind speed. The Limiting Value for flicker coefficient is about 0.4, for average time of 2 hours. VII. WIND TURBINE LOW VOLTAGE RIDE THROUGH CAPABILITY Fault ride through has come to play a role in strengthening power system security due to the increase in the integration of wind power in recent times. It requires the generators to remain connected in the likelihood of a disturbance on the network. A severe disturbance such as a fault could lead to a voltage dip and if the generators are unable to remain connected it could lead to an excessive loss of generation[5]. This could cause stability problems and may eventually lead to cascaded tripping of other generators. The impact of the wind generation on the power system will no longer be negligible if high penetration levels are going to be reached. The extent to which wind power can be integrated into the power system without affecting the overall stable operation depends on the technology available to mitigate the possible negative impacts such as loss of generation for frequency support, voltage flicker, voltage and power variation due to the variable speed of the wind and the risk of instability due to lower degree of controllability. 883 Fig. 3. Low voltage ride through (LVRT) capability Many countries in Europe and other parts of the world are developing or modifying interconnection rules and processes for wind power through a grid code. The grid codes have identified many potential adverse impacts of large scale integration of wind resources. The low voltage ride through (LVRT) capability, which is one of the most demanding requirement that have been included in the grid codes and shown in Fig. 3. It defines the operational boundary of a wind turbine connected to the network in terms of frequency, voltage tolerance, power factor, fault ride through is regarded as the main challenges to the wind turbine manufactures. The wind turbine should remain stable and connected during the fault while voltage at the PCC drop to 15% of the nominal value i.e. drops of 85% for the part of 150 msec. Only when the grid voltage fall below the curve, the turbine is allowed to disconnected from the grid. Wind farms using squirrel cage induction generators directly connected to the network will suffer from the new demands, since they have no direct electrical control of torque or speed, and would usually disconnect from the power system when the voltage drops more than 10 20% below the rated value. The LVRT basically demands that the wind farm remains connected to the grid for voltage dips as low as 5%. VIII. POWER QUALITY ISSUES MITIGATION TECHNIQUES Advanced custom power devices with adequate converter and control systems such as SVCs and STATCOMs can mitigate voltage instability, reactive power problems and harmonic distortion as well as improve the PQ of the network. In order to enhance the terminal voltage quality, SVCs were used for reactive power compensation of wind power induction generators but STATCOMs are superior compared to other flicker mitigation methods such as SVCs.

5 STATCOMs is faster, smaller and having better performance at low voltage conditions. A STATCOM based control mechanism is very useful to reduce the power quality problems on integrating wind energy into the grid. Simulation results show that an optimised STATCOM can cancel out the harmonic parts of the load current. Here the system canbe capable of meeting the reactive power demand from the wind generator and the load at the PCC to the grid. This scheme improved the quality of power significantly and fulfilled the PQ requirement. It is observed that the STATCOM can considerably improve the voltage profile at the PCC by regulating the reactive power of the grid during faults and maintaining an appropriate level of voltage sag on the grid and prevents the turbine from being disconnected from the grid during certain levels of voltage sag on the grid side. Efficient design of power electronic converters, adequate reactive power compensation and optimum design of wind turbines and the grid connection all play a key role in minimising the observed potential challenges and increases the efficiency of the system. IX. CONCLUSION In response to the energy needs and environmental concerns, electricity from wind generators is considered as one of the future solutions. However, the variability and the diffuse nature of the wind power can be challenging to the operation of a power system.the wind turbines connected to weak grids have an important influence on power system. The weak grid is characterized by large voltage and frequency variations, which affects wind turbines regarding their power performance, safety and allied electrical components. This paper gives a comprehensive literature review to explore these potential impacts and their available mitigation techniques. REFERENCES [1] Harry Davitian, Wind Power and Electric Utilities: A Review of the Problems and Prospects, Wind Engineering vol. 2,no. 4,pp. 1-28,1978. [2] John K. Kaldellis, D. Zafirakis The wind energy evolution: A short review of a long history. Renewable Energy vol.36,no.11,pp ,2011. [3] Z. Chen, Issues of Connecting Wind Farms into Power Systems, IEEE/PES Transmission and Distribution Conference & Exhibition: Asia and Pacific Dalian, China,vol.II, [4] Stefan Marko, Ivan Darul a, Large Scale Integration Of Renewable Electricity Production Into the Grids, Journal of Electrical Engg, vol. 58,no.1 pp.58 60,2007. [5] Inigo Martinez de Alegrıaa, Jon Andreua, Jose Luis Martına, Pedro Iban ezb, Jose Luis Villateb, Haritza Camblongc, Connection requirements for wind farms: A survey on technical requierements and regulation, Renewable and Sustainable Energy Reviews, vol.11, no.6,pp ,2007 [6] J. Charles Smith, Michael R. Milligan, Edgar A. DeMeo, and Brian Parsons, Utility Wind Integration and Operating Impact State of the Art IEEE Transaction On Power Systems vol.22,no.3,pp ,2007. [7] Doina Dragomir, The Aspects Related To The Grid Connection Of The Wind Farms Into National Power System,U.P.B. Sci. Bull. Vol.73,no.1,pp ,2011. [8] S.K. Khadem,M. Basu,M.F.Conlon, Power Quality In Grid Connected Renewable Energy Systems: Role Of Custom Power Devices International Conference On Renewable Energies and Power Quality, Spain,vol.III,pp March [9] Pavlos S. Georgilakis Technical challenges associated with the integration of wind power into power systems Renewable and Sustainable Energy Reviews vol.12,no.14, pp ,2008. [10] G.M. Shafiullah, Amanullah,Shawkat Ali, PeterWolfs, Potential challenges of integrating large-scale wind energy into the power grid A review Renewable and Sustainable Energy Reviews vol.20,no.8, pp ,2013. [11] I.WASIAK and Z. HANZELKA Integration of distributed energy sources with electrical power grid Bulletin Of The Polish Academy Of Sciences TECHNICAL SCIENCES vol.57,no.4,pp ,2009. [12] Sharad W., Mohod,Mohan, V. Aware, Power Quality Issues and Its Improvement in Wind Energy Generation Interface to Grid System, MIT International Journal of Electrical and Instrumentation Engg., vol.1,no.2,pp ,