Analysis of Calculation Methods to Evaluate Losses and Efficiency of Wind Generators

Transcription

1 Analysis of Calculation Methods to Evaluate Losses and Efficiency of Wind Generators Author: Engr. Majid Ali Students of MS Electric Power and Energy Engineering School of Science and Technology University of Management and Technology Lahore 54000, Pakistan Co-Author: Mashood Nasir Assistant Professor School of Science and Technology University of Management and Technology Lahore 54000, Pakistan Abstract In this Paper Analysis of Calculation Methods to calculate the losses and output power of wind turbine generator systems with using two types first one is Squirrel-Cage Induction Generator and the second is Doubly-fed Induction Generator. By using the existing methods in this paper, we can evaluate the generated output power, total system losses, total energy generated, efficiency and capacity factor of wind turbine generator. The techniques presented in this paper wind speed as the input and then all state variables and conditions of the wind generator system, for instance wind turbine power, generator output power, grid input power from the system and various losses in grid interconnected. Most of the countries are coming to this adopt this energy sources because the Wind energy is a clean and renewable energy source in the world and most essential the viewpoints of global warming and reduction the fossil fuels. In adding the wind turbine generator system (WTGS) is very low cost in compare with other systems renewable energy sources. But the power achieved from wind generator (WG) is not continues constant due to wind speed variations in different area of the world. The generated electric power and the loss in wind turbine generator system (WTGS) change analogous w.r.t wind speed variations and so the efficiency and the capacity factor of the system are also changed. But we can analyze the optimal WTGS of different area of the world because the optimal WTGS annual energy production and capacity factor are very important factors. Index Terms: Renewable Energy Sources Wind Generator (WG) Wind Turbine (WT) Wind Turbine Generation Systems (WTGS) Squirrel-Cage Induction Generator (IG) Doubly-fed Induction Generator (DFIG) Maximum Power Point Tracking (MPPT) calculated WG input/output power, WG system losses, power converter loss, and efficiency. Firstly calculate efficiency of WG system by using Squirrel-Cage Induction Generator (IG) at a constant speed. By using this method we can calculate wind turbine features and induction generator electrical equivalent circuit, WG input/output power, WG system losses. Secondly we can calculate loss, power, and efficiency of WG with Doubly-fed Induction Generator (DFIG). Now a days most of the world used large size DFIG systems due to have quality to operate at variable speed by using power electronic converters in rotor circuit. II. Wind Turbine Power Calculation The turbine system characteristic is non-linear as seen in Fig. 2.1 and this characteristic similar to practical wind turbines. The wind power as given in Eq. 2.1 [2], [1] ( ) ( ) (2.1) = output power of turbine (w), = density of air (kg/ ) = Power coefficient, R = Radius of the blade (m), = wind speed (m/s) The tip speed ratio as given in Eq. 2.2 (2.2) = tip speed ratio, = angular speed of WT (rad per sec), = wind speed (m/s), R = Radius of the blade (m) The power coefficient as given in Eq. 2.3 ( ) ( ) (2.3) = blade pitch angular angle in deg and ( ) I. INTRODUCTION There are many countries which are using Wind Turbine Generation Systems (WTGS) from the vision of global warming and reduction the fossil fuels. It is most important to analyze the losses characteristics of WG which found from wind speed to capture more energy from the wind. Furthermore the most of the losses in wind turbine system is non-linear losses. If used average wind speed to get the more profit but they may cause many errors. In this paper represent many losses in WG depend on wind speed which is based on the Steady-state analysis. This method can be Fig. 2.1 Turbine System Characteristics

2 III. Squirrel-Cage Induction Generator In this part, we discuss a method of calculating the efficiency of WG. The system configuration for the analyze in this section as seen in Fig.2.2 and the losses of WG system with using Squirrel-Cage Induction Generator as shown Table 2.1 [7] The iron losses due to the flux change and there are two types, eddy current and hysteresis losses. = total iron losses as shown below Eq. 2.7 ( ( ) ( )) ( ) (2.7) B = flux density (T), σh = hysteresis coefficient, σe=eddy current coefficient, f = system frequency (Hz), and d = iron core thickness (mm) Bearing loss or mechanical loss due to the revolution of the rotor, this can be calculated as given below Eq. 2.8 Fig. 2.2 System configuration with Squirrel-Cage Induction Generator Table 2.1 Losses of WGs with Squirrel-Cage Induction Generator [4] Mechanical losses Gear box loss and Ball bearing loss Windage and Frication loss Copper losses Stator and Rotor winding copper loss Iron losses Eddy current loss and Hysteresis loss Stray load losses (w) (2.8) = parameters the rotor weight, revolution of the rotor and the axis diameter Windage loss occurs due to the air against rotor and is calculate as given Eq. 2.9 The electrical equivalent circuit of Squirrel-Cage Induction Generator shown in Fig By solving the Eq. 2.4 can calculate the input torque and copper losses. [7] speed. (w) (2.9) = parameters the rotor shape, rotor length and the rotor Stray load loss is calculated as follows Eq (w) (2.10) Fig. 2.3 Equivalent Circuit of Squirrel-Cage Induction Generator P is generated power (w) and generator. is the rated power (w) of Equivalent circuit of Squirrel-Cage Induction Generator parameters are given below r1 = Stator Resistance r2 = Rotor Resistance x1 = Stator leakage reactance x2 = Rotor leakage reactance rm = Iron loss resistance xm = magnetizing reactance s(slip) = (Ns-Nr)/Ns Nr = rotor speed, Ns = synchronous speed [2] Gear box losses are mainly due to tooth contact loss and viscous oil loss. Gear box efficiency is expressed as follows Eq ( ) (2.11) = output power of the gear box, = turbine power, and turbine rated power. ( ) ( ) (2.4a) ( ) ( ) (2.4b) A. Losses calculation in the Squirrel-Cage Induction Generator The input power of generator can be calculated given below in Eq. 2.5 from the equivalent circuit of Fig. 2.3 as seen above: Input power (generator) ( ) (w) (2.5) Copper losses can be calculated using the equivalent circuit as in Eq. 2.6 W (copper) (w) (2.6) Fig. 2.4 Power flow in WT system with Squirrel-Cage Induction Generator B. Algorithm for Squirrel-Cage Induction Generator Figure 2.5 shows the Algorithm of the method by using WT system with Squirrel-Cage Induction Generator, which is described below. [7]

3 1. Wind speed is taken as the input value and from this wind speed all states of WG are calculated. 2. Calculate the power produced by Wind turbine and multiply by the gear efficiency, 3. Ball bearing and windage losses which calculated in step 2 and stray load losses are subtract from the wind turbine output. Stray load losses are considered being zero in the initial stage. 4. In this step slip is changed using the Slip-torque characteristic curve until giving the same generated power as the power calculated in step Calculate the copper and iron losses by using equivalent circuit with iron losses adjusted. 6. Ball bearing loss and windage loss are calculated by using Eq.2.8 and Eq.2.9 and stray load loss is calculated from Eq In this step if the calculated losses required meet then stop the calculation but if not converge then it will go to step 2. IV. Doubly-Fed Induction Generator Fig. 2.6 System configuration and Equivalent circuit with DFIG Equivalent circuit of Doubly-Fed Induction Generator parameters are given below [2] r1 = stator winding resistance r2 = rotor winding resistance x1 = stator leakage reactance x2 = rotor leakage reactance rm1 = stator iron loss resistance rm2 = rotor iron loss resistance Table 2.2 Losses of WGs with Doubly-Fed Induction Generator [4] Mechanical loss Copper loss Iron loss Gear box, Windage and Ball bearing losses Stator and Rotor winding copper losses Stator and rotor iron loss resistances Stray load loss Power converter loss Transformer loss Fig. 2.5 Algorithm for Squirrel-Cage Induction Generator C. Calculated Results According to the calculated result the losses of this system are nonlinear w.r.t the wind speed variation. If the wind speed increases then the Iron loss and flux density will decrease. But the generator real power and reactive power increases and the generator internal voltage will decrease. A. Losses calculation in the Doubly-Fed Induction Generator Calculate the mechanical losses (Gear box, Windage and Ball bearing losses) approximate expressions for these losses are given below. [7] Gear box losses given in Eq.2.12 is the rated turbine power (2.12) Ball bearing losses given in Eq.2.13 (W) (2.13) = parameters of rotor weight, rotational speed and the axis diameter and is angular speed.

4 Windage losses given in Eq.2.14 (W) (2.14) = parameter concerning the rotor shape, rotor length and the rotational speed of the rotor. is angular speed. Iron losses given in Eq.2.15 [8] ( ) (2.15) the Iron losses changed due to the flux density changing and K1 and K2 are coefficients of hysteresis and eddy current losses respectively E0 reference voltage, E voltage (2.16) 3. Assuming generator stator active and reactive power equal to zero and calculate iron losses, copper losses and rotor power from electrical equivalent circuit 4. Iron loss is calculated by using Eq In this step we can compare iron losses which calculated in step 3 and step 4 if these both steps not equal then changed the iron loss resistance until the both steps equal 6. In this we calculate power electronic converter losses, transformer losses, total system losses, rotor power, wind generator output power and generator stator power. 7. In this step we can compare stator power which calculated in step 3 and step 6 if these both steps not equal then changed the stator power until the both steps equal 8. If the wind generator output power is greater than set power point in this like 5MW then we can reduced the wind turbine output by using changer and then go to step 2. (2.17) B0 = initial flux density and W = weight of iron core. Copper losses calculated using the equivalent circuit shown in Fig. 2.6 resistances and as in Eq and Eq.2.19 Stray load loss can be expressed approximately as Eq. (2.18) (2.19) (W) (2.20) power. = rated power of the generator and P is generator output The Power converter losses [6],[9] calculated by using giving in Eq.2.21 [ ] ( ) [ ] ( ) (2.21), = Phase current (A), = Carrier frequency (Hz), = IGBT duty ratio For easiness transformer is consider by leakage impedance and its loss equal to the resistance losses. But the iron loss of the transformer is not considered in this method. B. Algorithm for Doubly-Fed Induction Generator Figure 2.7 shows the Algorithm for Doubly-Fed Induction Generator which is described below. [7] 1. We used Wind speed as the input value and find out maximum power points devolved by wind turbine and then calculate the maximum power and rotor speed. 2. Calculate mechanical losses and stray load losses then deduct from turbine output power Fig. 2.7 Algorithm for Doubly-Fed Induction Generator C. Calculated Results According to the above calculation know that the stray load losses are greater than other losses but the gear losses are very big amongst all losses of this system.

5 V. Conclusion In this paper, firstly methods with using Squirrel-Cage Induction Generator according to the calculated result the all losses are nonlinear w.r.t the wind speed. If the wind speed increases then the Iron loss and flux density will decrease. But the generator real power and reactive power increases. Secondly method with using Doubly-Fed Induction Generator according to the above calculation knows that the stray load losses are greater than other losses but the gear losses are very big among all losses of this system. By using the presented methods in this paper it is way to calculate the generated output power, system losses, energy, efficiency and capacity factor of WG system. The presented method can be used successfully for betterment WG scheme, assembly planning, and economic situation of wind farms for definite areas all over the world. [13] Çetin, N. S., M. A Yurdusev, R. Ata and A. Özdemir, Assessment of Optimum Tip Speed Ratio of Wind Turbines, Mathematical and Computational Applications, Vol. 10, No.1, pp , [14] Walker, J. F., Jenkins, N., Wind Energy Technology, John Wiley and Sons, [15] Walker, J. F., Jenkins, N., Wind Energy Technology, John Wiley and Sons, VI. References [1] Anderson PM, Bose A (1983) Stability simulation of wind turbine systems. IEEE Trans Power Apparatus Syst PAS- 102(12): [2] N.W. Lane and W.T. Beale, Free-Piston Stirling Design Features. Presented at Eighth International Stirling Engine Conference, May 27 30, 1997, University of Ancona, Italy. [3] P. W. Carlin, A. X. Laxson, and E. B. Muljadi, The History and State of the Art of Variable-Speed Wind Turbine Technology, National Renewable Energy Lab., Tech. Rep. NREL/TP , Feb [4] Dynamic Modeling of Doubly-Fed Induction Machine Wind-Generators, DIgSILENT GmbH, Germany, Tech. Rep., Aug [5] Z. Chen and E Spooner, Grid power quality with variable speed wind turbines, IEEE Trans. Energy Convers., vol. 16, no. 2, pp , Apr [6] Hitachi, ltd.: High-voltage high-power IGBT, 0.html [7] Cotrell J (2002) A preliminary evaluation of a multiplegenerator drivetrain configuration for wind turbines. National renewable energy laboratory, NREL/CP [8] Polinder H, Van der Pijl FFA, De Vilder GJ, Tavner PJ (2006) Comparison of direct-drive and geared generator concepts for wind turbines. IEEE Trans Energy Convers 21(3): [9] Johnson GL Wind energy systems. Electronic edition, [online] [10] Hitachi, ltd. Power and industrial systems power semiconductor dept. power and industrial systems div., IGBT module application manual, [online] [11] Ragheb, M., Wind Power Systems. Harvesting the Wind [12] Calvert, N. G., Windpower Principles: Their Application on the Small Scale, John Wiley and Sons, 1979.