Issues Of Connecting Wind Farms Into Power Systems-PDF Free Download

Issues of Connecting Wind Farms into Power Systems
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P ow er p u S t a ll c o n t r o l P ow er p u A c t iv e s t a ll c o n t r o l P ow er p u P it c h c o n t r o l. 1 2 5 1 2 5 1 2 5, 1 0 1 0 1 0, 0 7 5 0 7 5 0 7 5, 0 5 0 5 0 5. 0 2 5 0 2 5 0 2 5, 5 10 15 20 25 30 5 10 15 20 25 30 5 10 15 20 25 30. W in d s p e e d m s W in d s p e e d m s W in d s p e e d m s. V in d h a s tig h e d m s, Fig 1 Power characteristics of fixed speed wind turbines a stall control b active stall control c pitch control. T o G r id, G ro u p 5 o u t o f 5, G ro u p 1 o u t o f 5. Fig 2 Basic configurations of Horns Rov wind farm Denmark. Direct connecting an induction generator may result in long cable at 150 kV would produce around 100 Mvar 9. transients the resultant inrush current could cause reactors will be needed to compensate the reactive power. disturbances to the grid and high torque spikes in the drive produced by the cable. train Such transient could limit the acceptable number of For long distance transmission the transmission capacity of. wind turbines a current limiter or soft starter based on cables may be mainly occupied by the produced reactive. thyristor technology is used to limit the inrush current to a power therefore ac transmission will meet difficulties In this. level below two times the rated current of the generator situation high voltage direct current HVDC transmission. which effectively dampens the torque peaks of the generator techniques may be used The new technology voltage source. and reduces the loads on the driving train In normal operation converter based HVDC system provides new possibilities for. states the soft starter is bypassed by a short circuited performing voltage regulation and improving dynamic. contactor to reduce the power loss associated with the stability of the wind farm as it will be possible to control the. semiconductors and reduce the required thermal capacity of reactive power of the wind farm and perhaps keep the voltage. the soft starter For full rated power electronic interfaced during the faults clearance and fast reclosures in the onshore. induction generators the current can be controlled transmission system. continuously from zero to rated current the disturbances to. the grid during switching operations are minimized III IMPACTS OF WIND FARMS ON POWER QUALITY. A modern wind turbine is often equipped with a, transformer stepping up from the generator terminal voltage A Voltage variations.
usually a voltage below 1 kV to a medium voltage at around On the local level voltage variations are the main problem. 20 kV or 30 kV for ac system connection associated with wind power This can be the limiting factor on. The grid connection may include two parts the local the amount of wind power which can be installed. electrical connection within a wind farm at a medium voltage In normal operational condition the voltage quality of a. level and the connection from the wind farm to the electrical wind turbine or a group of wind turbines may be assessed in. grid If the wind farm is large and the distance to the grid is terms of the following parameters 10. long a transformer is used to step up the medium voltage in Steady state voltage under continuous production of. the wind farm to the high voltage at transmission level power. Submarine cables with a lead sheath and steel amour are Voltage fluctuations. used for connecting an offshore wind farm to a on shore grid o Flicker during operation. either oil insulated cables or PEX insulated cables can be o Flicker due to switching. used The reactive power produced by the submarine cable of The influence of connecting a wind farm on the gird. connecting an offshore wind farm could be very high a 40 km voltage is directly related to the short circuit power level The. short circuit power level in a given point in the electrical represented with its output power at the PCC Ten minute. network represents the system strength If the voltage at a average data Pmc and Qmc and 60 s average data P60 and. remote point can be taken as constant Us and the short circuit Q60 can be calculated by simple summation of the output. power level SSC in MVA is defined as Us2 Zk where Zk is the from each wind turbine whereas 0 2 s average data P0 2 and. equivalent impedance between the points concerned Q0 2 may be calculated according to equations 3 and 4. Fig 3 illustrates an equivalent wind power generation unit below. connected to a network with equivalent short circuit N wt N wt. impedance Zk The network voltage at the assumed infinite. 0 2 i Pn i, busbar and the voltage at the Point of Common Coupling. PCC are Us and Ug respectively The output power and Q0 2 Qn i Q Q n i 4. reactive power of the generation unit are Pg and Qg which i 1 i 1. corresponds to a current Ig where Pn i Qn i are the rated real and reactive power of the. Pg jQ g 1 individual wind turbine Nwt is the number of wind turbines in. Us the group, The voltage difference U between the system and the C Voltage fluctuations. connection point is given by, Fluctuations in the system voltage more specifically in its. U g U s U Z k I g R k jX k 2 rms value may cause perceptible light flicker depending on. R k Pg X k Q g Pg X k Q g R k, the magnitude and frequency of the fluctuation This type of. j U p j U q disturbance is called voltage flicker or shortened as flicker. There are two types of flicker emissions associated with. The voltage difference U is related to the short circuit wind turbines the flicker emission during continuous. impedance the real and reactive power output of the wind operation and the flicker emission due to generator and. power generation unit It is clear that the variations of the capacitor switchings Often one or the other will be. generated power will result in the variations of the voltage at predominant The allowable flicker limits are generally. PCC If the impedance Zk is small then the voltage variations established by individual utilities Rapid variations in the. will be small the grid is strong On the other hand if Zk is power output from a wind turbine such as generator. large then the voltage variations will be large the grid is switching and capacitor switching can also result in variations. weak However strong or weak are relative concepts For a in the RMS value of the voltage At certain rate and. given wind power capacity P the ratio RSC SSC P is a magnitude the variations cause flickering of the electric light. measure of the strength The grid may be considered as strong In order to prevent flicker emission from impairing the voltage. with respect to the wind farm installation if RSC is above 20 quality the operation of the generation units should not cause. Pg Qg excessive voltage flicker, IEC 61000 4 15 specifies a flickermeter which can be used.
Ug to measure flicker directly 12 The flicker measurement is. W in d tu rb in e, based on the measurements of three instantaneous phase. Fig 3 A simple system with an equivalent wind power generator. connected to a network voltages and currents followed by using a flicker algorithm. to calculate the Pst and Plt where Pst is the short term flicker. B Steady state voltage severity factor and measured over 10 minutes and the long. Equation 2 indicates the relationship between the voltage term flicker severity factor Plt is defined for two hour periods. and power transferred into the system The voltage difference The flicker assessments can also be conducted with simulation. U can be calculated with load flow methods as well as other method 13. simulation techniques 11 The voltage at PCC should be Disturbances just visible are said to have a flicker severity. maintained within utility regulatory limits Operation of wind factor of Pst 1 The flicker emissions Pst and Plt may also be. turbines may affect the voltage in the connected network If estimated with the coefficient and factors cf k va and. necessary the appropriate methods should be taken to ensure kf k obtained from the measurements which are usually. that the wind turbine installation does not bring the magnitude provided by wind turbine manufacturers. of the voltage outside the required limits The flicker emissions from a wind turbine installation. It is recommended that load flow analyses be conducted to should be limited to comply with the flicker emission limits It. assess this effect to ensure that the wind turbine installation is recommended 3 that Plt 0 50 in 10 20 kV networks and. does not bring the magnitude of the voltage outside the Plt 0 35 in 50 60 kV networks are considered acceptable. required limits However different utilities may have different flicker. Depending on the scope of the load flow analysis a wind emission limits The assessments of the flicker emissions are. turbine installation may be assumed as a PQ node which may described below. use ten minutes average data Pmc and Qmc or 60 s average 1 Continuous operation. data P60 and Q60 or 0 2 s average data P0 2 and Q0 2 The flicker emission from a single wind turbine during. A wind farm with multiple wind turbines may be continuous operation may be estimated by. S 5 well as on the individual harmonics, P st c v n. S k Power electronic converters which operation in an on and. Where cf k va is the flicker coefficient of the wind off way are used in variable speed wind turbine systems 14. turbine for the given network impedance phase angle k at 15 The Pulse Width Modulation PWM switching. the PCC and for the given annual average wind speed va at frequency with a typical switching frequency of a few. hub height of the wind turbine thousand Hz shifts the harmonics to higher frequencies where. A table of data produced from the measurements at a the harmonics can be easily removed by smaller filters In. number of specified impedance angles and wind speeds can be general harmonic standards can be met by modern wind. provided by wind turbine manufactures From the table the turbines. flicker coefficient of the wind turbine for the actual k and va. at the site may be found by applying linear interpolation IV REQUIREMENTS OF CONNECTING WIND FARM INTO POWER. The flicker emission from a group of wind turbines SYSTEMS. connected to the PCC is estimated using equation 6 Integration of large scale wind power may have severe. N 6 impacts on the power system operation Traditionally wind. f i k a n i, turbines are not required to participate in frequency and. Where cf i k va is the flicker coefficient of the individual voltage control However in recent years attention has been. wind turbine Sn i is the rated apparent power of the individual increased on wind farm performance in power systems. wind turbine Nwt is the number of wind turbines connected to Consequently some grid codes have been defined to specify. the PCC the steady and dynamic requirements that wind turbines must. If the limits of the flicker emission are known the meet in order to be connected to the grid Examples on such. maximum allowable number of wind turbines for connection requirements are capabilities of contributing to frequency and. can be determined voltage control by continuous modulation of active power and. 2 Switching operations reactive power supplied to the transmission system as well as. The flicker emission due to switching operations of a single the power regulation rate that a wind farm must provide. wind turbine can be calculated as Some specifications have been worked out with regard to. Sn 7 the preparations for future large offshore wind farms as the. P st 18 N 0 31, S k following example 3, where kf k is the flicker step factor of the wind turbine for Active power and frequency control the active power is. the given k at the PCC regulated linearly with frequency variation between a certain. The flicker step factor of the wind turbine for the actual k range 47 Hz 52 Hz with a dead band 49 85 Hz 50 15. at the site may be found by applying linear interpolation to the Hz and the regulating speed is 10 of the rated power per. table of data produced from the measurements by wind turbine second. manufacturers The reactive power should be regulated within a control. The flicker emission from a group of wind turbines band at a maximum level of 10 of rated power absorption. at zero real power and production at the rated real power. connected to the PCC can be estimated from, Wind turbine will generally operate in normal conditions.
10 i f i k n i, 90 105 voltage and 49 51 Hz however it should also. be able to operate outside of the above conditions within. Where kf i k is the flicker step factor of the individual certain specified time limits. wind turbine N10 i and N120 i are the number of switching Under the condition of a power system fault a wind turbine. operations of the individual wind turbine within 10 minute would experience a voltage variation The severer degree of. and 2 hour period respectively Sn iis the rated apparent power the voltage variation and the time period of such voltage. of the individual wind turbine variation will determine whether the wind turbine must not. Again if the limits of the flicker emission are given the be disconnected ride through or may be disconnected or. maximum allowable number of switching operations in a must be disconnected. specified period or the maximum permissible flicker emission Also the wind turbine has to be able to withstand more than. factor or the required short circuit capacity at the PCC may be one independent faults occurred in a few minute intervals. determined There are also requirements related to rapid voltage. Abstract Wind power industry is developing rapidly more and more wind farms are being connected into power systems Integration of large scale wind farms into power systems presents some challenges that must be addressed such as system operation and control system stability and power quality This paper describes modern wind power systems presents requirements of wind turbine connection

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