Comparing SCIG and DFIG for Wind Generating Conditions in Macedonia

Transcription

1 European Association for the Developent of Renewable Energies, Environent and Power Quality (EAEPQ) nternational Conference on Renewable Energies and Power Quality (CREPQ ) Las Palas de Gran Canaria (Spain), 3th to th April, Coparing SCG and DFG for Wind Generating Conditions in Macedonia Sanja Vitanova, e-ail:sanja_sr_@yahoo.co Vlatko Stoilkov, e-ail: stoilkov@feit.uki.edu.k Vladiir Dicev, e-ail: vladi@feit.uki.edu.k Contact address: Faculty of Electrical Engineering and nforation Technology, Karpos bb, Skopje, Macedonia Abstract The wind energy has becoe the ost potential renewable energy source recently. The technological and industrial developent in the wind power generation indicates that wind power should be seen as one of the ain doestic sources for electricity generation in all countries. This paper gives an overall observation of the ost coonly used electrical achines, i.e. the squirrel cage induction generators (SCG) and doubly fed induction generators (DFG) in the wind generation systes. sing the Matlab/Siulink, a siulation of wind far with the two types of generators has been ade in order to copare the results and to coent on the best option based on the output characteristics of the generator and wind turbine..ntroduction European dependency on iported fossil fuel as well as Macedonian as a part of Europe has becoe a threat to econoic stability increasing uncertainties over energy prices. Environental effects of fossil based power plants add another diension of the proble. These power plants load the atosphere with greenhouse gases resulting in global waring and cliate changes. For these reasons one of the key points on the European energy policy agenda is to increase the share of the energy deand that is covered fro the renewable energy sources. According to the European Coission by, 3% of E energy deands will be covered by renewable energy sources (RES) and wind energy will eet % of E electricity deands by. Most proising fro the RES are the wind and solar energy. The difference between the conventional sources and these sources is that the priary energy of the RES has stochastic nature and the fluctuations are uncontrollable and peranent. Wind power is the ost potential renewable source, very proising and ature renewable technology. Electricity is the coonly used energy source in the industry and in the private sector in Macedonia. Macedonia is highly dependent on energy iports (oil, natural gas, electricity) which have grown rapidly in recent years. The whole energy sources in Macedonia are based on conventional power plants. The approxiate structure of coverage is 7% doestic electricity production (where % fro coil TPP and % HPP) and % is provided by iport. nclusion of the renewable energy for electricity production will allow iproved and ore stable electricity supply and will reduce the energy iports..wind turbines Most of the wind turbines use induction generators because of their advantageously characteristics. nduction achines are siple and rugged in construction, offer ipressive efficiency under varying operating conditions, relatively inexpensive and require iniu aintenance and care. Characteristics of these generators like the over speed capability ake the suitable for the wind turbine application. The wind power captured by the turbine or and converted to echanical power is dependent on the average wind speed over the or surface and the ational speed of the or. Therefore axiu wind energy capture can be achieved only if the or speed is varied tracking the changes of the wind. Variable speed operation of the wind turbines is necessary to gain high efficiency in the generating systes. Additional advantages of the variable speed operation are the reduction of the drive train echanical stresses which perits the use of lighter transissions, the iproveent of the output power quality and the reduction of the noise eitted fro the wind turbines. The induction generators that are used in the wind turbine are usually squirrel cage induction generators (SCG) and in nowadays doubly fed induction generators (DFG). Doubly fed induction generators have windings on or and or where both of the windings transfer significant power between the shaft and the electrical syste. 3. Equivalent circuit of asynchronous achines t is often necessary to ake quantitative predictions about the behavior of the asynchronous achines under different operating conditions. For this purpose it is useful to present an equivalent circuit for the achines in operating ode in a ionary condition (Fig.). RE&PQJ, Vol., o., May

2 R R,, X where is the nuber of turns in the windings X, () Fig.. Equivalent circuit for G in ionary regie The asynchronous achines are built fro two separate electrical systes that are coupled by utual agnetic flux. n the priary winding W, voltage E is induced and in the secondary winding W voltage E is induced causing E s current in the or Z. s While the current is flowing through the or winding, it causes active (or) resistance and inductive (leakage) resistance voltage drop and the current that flows through the or circuit causes the sae resistances in the or circuit. n the or circuit there is an additional resistance beside the active resistance that defines the load of the achines. R and X refer to the resistance of the or and leakage resistance referred to the or side. The aplitude of the or current is s E () R + jx s Fro the equivalent circuit it can be seen that losses in R s represent losses in the or while losses in R are losses in iron. Energy consued by the or circuit represents all other fors of consuption of energy echanical output, losses in windings, frictional losses, losses in copper. A. Equivalent Circuit for DFG The figure below (Fig.) shows the equivalent circuit of a doubly fed achine in a fixed reference frae, for steady e operation. Fig.. Equivalent circuit for DFG in a fix reference frae The or quantities are referred to the or: The circuit allows to calculate the steady e values of or current and voltage. Special attention is paid to the calculation of the or apparent power as a function of or active and reactive current, as the or apparent power has a high ipact on the required rating of the or power electronic converter. The or and voltage equations in steady e regie are: R + j X + j X X ( R X ( ) + j s X + X, σ, X ) X + j s X + X, σ The or current can be calculated fro ( R + j X ) () j X Further the or voltage can be calculated as a function of speed, or voltage and or current R+ j s X ( R + j X )( R+ j s X ) j X j X + s X The apparent power processed by the or frequency converter is: S * abs ( ) (). Model description sing the progra Matlab/Siulink, siulation is ade for two wind generation systes whereas one of the is using SCG and the other uses DFG at siilar siulation conditions. The wind generation systes are connected to the grid. The odels that are exained are MW wind fars consisting of six. MW wind turbines connected to a kv distribution syste exporting power to kv grid through a k kv feeder (Fig.3). The behavior of the syste is estiated by several changes in the wind speed, naely for the noinal wind speed of /s as well as for iediate changes for to /s and fro to /s. nitially the wind speed is set to /s or /s respectively and then starting at t s wind speeds is raed to /s or /s in 3 seconds respectively. The sae gust of wind is applied to the second and the third turbine, respectively with seconds and seconds delays. The odels are phasor odels that allow transient stability type studies with long siulation tie and in this case the siulation tie is seconds. sta (3) () RE&PQJ, Vol., o., May

3 Fig.3. Siulation odel. Siulation results After the siulation the active and reactive power, the generator speed, the wind speed and the pitch angle for each turbine can be onitored fro the scope in the odel given. Also for the other scope blocks, the voltage, the current, the active and the reactive power of the kv bus can be onitored as well as the reactive power fro the co. The results for the wind generation syste with SCG can be seen fro the charts given in Fig. (where the green colour is for the first turbine, the pink is for the second and the blue is for the third turbine).... P_3 (MW) Q _3 (Mvar) W r _3 (pu) w ind _3 (/s) pitch_3 (deg) Vabc_B (pu)..... P_B (MW) Q_B (Mvar) V_B pos. seq. (pu) _B pos. seq. (pu/ MVA). Fig.. Siulation results for wind speed of /s for SCG The graphs show that when the wind speed is /s, which is the noinal speed of the wind in the case of SCG, the or speed Wr of the generator quickly reaches the value of about pu with sall variations in the beginning and then reains constant, that onitors wind speed which is also constant. The active power for a short tie with slight variations reaches its noinal value of 3MW and it is constant for all three turbines. Because in the beginning the value of the active power exceeds 3MW, the pitch angle is activated and returns the value of the active power to the noinal of 3MW. The reactive power also in a short tie reaches the value of about, MVAr and reains constant... P_3 (MW) Q _3 (Mvar) W r _3 (pu) w ind _3 (/s) p itch _3 (deg) - Vabc_B (pu) P_B (MW) Q_B (Mvar) V_B pos. seq. (pu) _B pos. seq. (pu/ MVA) Fig.. Siulation results for a change in the wind speed fro to /s for SCG The graphs show that with the change in wind speed of - /s in the case of SCG, the or speed Wr of the generator follows the change of wind speed and reaches the value of about pu with sall variations in beginning and then reains constant. The active power follows the change of wind speed with sall variations in the beginning but once the wind speed reaches the value of /s, the active power reaches its noinal value of 3MW and it is constant for all three turbines, so the pitch angle reains zero. Reactive power also follows the change of wind speed but once the wind speed reaches the value of /s, the reactive power reaches the value of, MVAr and is constant for all three turbines. 3 RE&PQJ, Vol., o., May

4 - -.. P_3 (MW) Q_3 (Mvar) wr_3 (pu) wind_3 (/s) pitch_3 (deg) Vabc_B (pu) 3.. P_3 (MW) Q_3 (Mvar) wr_3 (pu) wind_3 (/s) -. pitch_3 (deg) Vabc_B (pu)...7 P_B (MW) P_B (MW) Q_B (Mvar) V_B pos. seq. (pu)... Q_B (Mvar) V_B pos. seq. (pu)...7 _B pos. seq. (pu/ MVA) Fig.. Siulation results for a change in the wind speed fro to /s for SCG The graphs show that with the change in wind speed of - /s in the case of SCG, the or speed Wr of the generator is a constant value of pu except for the first and the second turbine where the or speed begins to raise after s or s respectively. The active power for a short tie reaches its noinal value of 3MW, and the pitch angles are activated when the value of the active power exceeds 3MW and returns that value to the noinal. Reactive power also follows the change of the wind speed, i.e. after the exclusion of the two turbines it stabilizes to the value of, MVAr. The first and the second turbine are excluded for the pection syste, ore accurate for the AC ndervoltage pection type. The results for the wind generation syste with DFG can be seen fro the following (where the green colour is for the first turbine, the pink is for the second and the blue is for the third turbine).. _B pos. seq. (pu/ MVA) Fig.7. Siulation results for wind speed of /s for DFG The graphs show that when the wind speed is /s which is the noinal speed of the wind in the case of DFG, the or speed Wr of the generator does not follow the change of the wind speed, in fact it grows over tie fro. to. pu. The active power follows that change of the or speed and later it reaches its value of 3 MW. Reactive power is alost constant, while the pitch angle reains zero since the active power does not pass the noinal value of 3MW. The graphs on Fig show that with the change in wind speed of -/s in the case of DFG, the or speed Wr of the generator does not follow the change of the wind speed, in fact it grows over tie fro. to. pu. The active power follows that change of the or speed and later it reaches its noinal value of 3 MW. Reactive power is alost constant, while the pitch angle reains zero since the active power does not pass the noinal value of 3MW. RE&PQJ, Vol., o., May

5 P_3 (MW) P_3 (MW) Q_3 (Mvar) Q_3 (Mvar)... wr_3 (pu) wr_3 (pu). wind_3 (/s) wind_3 (/s) pitch_3 (deg) pitch_3 (deg) -. Vabc_B (pu).... P_B (MW) Q_B (Mvar) V_B pos. seq. (pu) _B pos. seq. (pu/ MVA) Fig.. Siulation results for a change in the wind speed fro to /s for DFG The graphs on Fig. show that with the change in wind speed of -/s in the case of DFG, the or speed Wr of the generator does not follow the change of the wind speed, in fact it grows over tie fro pu to. pu. The active power follows the change of the wind speed but later it reaches its value of 3 MW. Reactive power is alost constant, while the pitch angle increasing with tie.. Conclusion Fro these siulation results the following conclusion can be estiated: in the wind generation syste with SCG the active power reaches its noinal value faster than in the wind generation systes with DFG where the active power reaches its noinal value later. The or speed of the generator in the DFG syste continues to grow in tie although the wind speed is constant.. Vabc_B (pu).... P_B (MW) Q_B (Mvar) V_B pos. seq. (pu) _B pos. seq. (pu/ MVA) Fig.. Siulation results for a change in the wind speed fro to /s for DFG References [] Wind energy fundaentals, resource analysis &econoics Sathyajith Mathew (Springer ) [] ntegrating Wind - Developing Europe s power arket for the large-scale integration of wind power EWEA February () [3] Analysis, Modeling and Control of DFG for WT Andeas Petersson (Chalers Bibliotek Sweeden ) [] Wind Energy - Renewable Energy and the Environent - Vaughn elson (Taylor&Francis Group ) [] Dynaic siulations of electric achinery using Matlab/Siulink Chee-Mun Ong (Prenticie Hall ) []Doubly Fed nduction Machine:Operating regions and Dynaic Siulation Joris Soens, Karel de Brabandere, Johan Driesen, Ronnie Belans (EPE 3- Touluse) [7] Variable speed wind power generation using DFG-a coparison with alternative schees Rajib Datta and V.T.Ranganathan (EEE Transactions on Energy Conversion) [] Wind energy syste Gary L.Johnson (Manhattan KS RE&PQJ, Vol., o., May