Wind Turbine Emulation Experiment Aim: Study of static and dynamic characteristics of wind turbine (WT) by emulating the wind turbine behavior by means of a separately-excited DC motor using LabVIEW and investigation of the performance of the wind turbine emulator (WTE) under the effects of: Constant wind speed profile Step variations in wind speed profile A random wind speed profile comprising of Sudden rise Sudden fall A gust Observation of the self-excitation phenomenon of induction generator connected to the WTE. Basic Structure of WTE: Fig. 1: Structure of wind turbine emulator using a separately-excited DC motor Many advanced technologies before implementation in actual sites are first tested in the laboratory. It is generally a difficult task to set up a wind turbine in the laboratory for testing procedures as there is a need for design and development of optimal control systems to enhance the quality of wind energy conversion systems. Thus, there is a need for wind turbine emulation in a laboratory. The structure of the wind turbine emulator using a DC motor is shown in Fig. 1. The wind turbine mathematical model shown in Fig. 1 is developed using appropriate mathematical equations describing the static characteristics of a wind turbine, explained in detail in [1-3]. 1
The inputs to the model are the wind speed, angular speed and pitch angle. The angular speed is applied after proper gear ratio conversion. The reference torque calculated from the model is multiplied with torque constant and gear ratio to calculate the reference current, which is compared with the actual DC motor armature current. The current error is tuned by a proportional-integral (PI) controller. It is then compared with a high frequency ramp signal to generate required PWM gate pulses to drive the power MOSFET which regulates the DC motor armature voltage. The variation in armature voltage in turn controls the armature current in accordance with the reference current and the system eventually reaches a steady state. Under steady state, the emulator power and current matches with the reference power and current, respectively. However, the shaft speed varies in accordance with the wind speed variation. The investigations of the emulator action under various wind profile ensures the effectiveness of the WTE. Apparatus/Equipment Required: Separately-Excited DC motor with DC supply 3-phase Induction Machine LabVIEW crio installed in a PC A SPST switch Excitation Capacitor Bank for Induction Generator 3-phase Gang Rheostat Digital Storage Oscilloscope Fig. 2: Experimental setup of complete laboratory based WECS using WTE In the laboratory setup, a complete WTE coupled with a self-excited induction generator (SEIG) along with all necessary hardware interface circuits has been developed. Fig. 2 shows 2
the developed setup. The hardware interface of WTE is achieved by means of LabVIEW crio. The mathematical model of WT and PI controller is developed in LabVIEW platform. The DC motor field and armature are given suitable supply as shown in Fig. 2. The actual speed and current of the motor are measured by means of tacho-generator and current sensor, respectively. The motor is coupled to the induction generator. Since it is a stand-alone setup, a capacitor bank is connected to the stator terminals for providing the required excitation. A 3-phase gang rheostat is used to apply a balanced load in order to get various operating points. The specifications of wind turbine, DC motor and the induction machine are given in the Appendix. Experimental Procedure: Connection diagram employing DC Machine for WTE is shown in Fig. 2. Connect the capacitor bank to the SEIG terminals. First Switch on the field power supply of DC motor and apply the rated voltage (220 V). Switch on the armature power supply of DC motor and gradually increase the voltage so that the motor starts to rotate. Ensure that the manual switch (S) shown in Fig. 2 is kept closed at the start. Open the switch (S) once the motor starts to rotate. Increase the armature voltage gradually to the rated value (220 V). Apply the load only when the SEIG has excited by means of a TPDT (T) switch shown in Fig. 2 (which is kept open till the SEIG excites) (1) Wind Turbine Characteristics: For study of static characteristics, the following assumptions are made: Wind speed is assumed to be constant. Effects of rotor inertia & tower shadow are neglected. To study the static behavior of WTE in real-time, a constant wind speed is maintained. The DC motor is coupled to self-excited induction generator. To obtain steady state operating point, a balanced variable 3-phase load is applied on the generator and is varied. This is repeated for different wind speeds to obtain power vs. speed characteristics. However, while studying the dynamic behavior, these nonlinearities (rotor inertia and tower shadow) are taken into consideration. Moreover, the effectiveness of the emulator should be examined with step variations in wind speed profile and arbitrary wind speed profile. Some typical wind speed profiles are shown in Fig. 3. 3
a) Constant Wind Speed: Apply a constant wind speed, as shown in Fig. 3(a), to the wind turbine mathematical model developed in LabVIEW. Apply a constant load to the SEIG (only after excitation). Ensure that the applied load should not cause de-excitation of SEIG. Compare the reference current (generated from the turbine torque by multiplying with gear ratio and torque constant as shown in Fig. 1) and power with the actual DC motor current and power by suitably tuning the PI controller parameters. Observe the shaft speed of DC motor. Repeat the above procedure for 5 different wind speeds and plot the power versus wind speed characteristics. b) Step Variations in Wind Speed: Apply a step variation in wind speed, as shown in Fig. 3(b), to the wind turbine mathematical model (taking nonlinearities into account) developed in LabVIEW. Apply a constant load to the SEIG (only after excitation). Compare the turbine current and power with the actual DC motor current and power by suitably tuning the PI controller parameters. Observe the shaft speed of DC motor and tabulate the values of steady state shaft speed with respect to wind speed at a constant load. Plot the variation in armature current, SEIG voltage, and shaft speed w.r.t. wind speed profile c) Arbitrary Wind Speed: Apply an arbitrary varying wind profile comprising of sudden rise, sudden fall and gust as shown in Fig. 3(c) to the wind turbine mathematical model (taking nonlinearities into account) developed in LabVIEW. Apply a constant load to the SEIG (only after excitation). Compare the turbine current and power with the actual DC motor current and power by suitably tuning the PI parameters. Observe the shaft speed of DC motor with respect to wind speed. 4
(a) (b) (c) Fig. 3: Sample wind profiles: (a) constant wind speed, (b) step variations in wind speed, and (c) arbitrary wind speed profile Self-Excitation of Induction Generator: Connect the 3-phase balanced load to the stator terminals of the SEIG. Maintain a constant wind speed for the emulation setup. Observe the magnitude of the self-excited voltage of induction generator for no load. Examine the change in the voltage level with gradual loading. 5
APPENDIX APPENDIX-I: SPECIFICATIONS Table A1: Wind Turbine Specification Rated Power Cut in wind speed Rated wind speed 500 W 3.5 m/s 7.5 m/s Turbine radius 1.25 m Turbine inertia coefficient 0.07 Optimum power coefficient (β = 0 0 ) 0.411 Optimum TSR (β = 0 0 ) 8 Table A2 : DC Motor Specifications Rated power 1 kw Rated voltage 220 V Rated speed 1450 rpm Armature current 4.1 A Field current 0.88 A Armature resistance 5 Ω Armature inductance 73 mh Field resistance 280 Ω Field inductance 15.29 H Mutual inductance 1.068 H Inertia coefficient 0.014 kg.m 2 Viscous friction coefficient 0.01 N.m.s Table A3: 3-Phase Induction Machine Specifications Rated power 1 hp Stator resistance 16.05 Ω Stator leakage inductance 753.43 mh Mutual inductance 703.43 mh Rotor resistance 7.46 Ω Rotor leakage inductance 753.43 mh Excitation capacitance (at full load) connected in 10 µf 6
APPENDIX-II: LabVIEW crio LabVIEW (Laboratory Virtual Instrumentation Engineering Workbench) programs are called virtual instruments (VIs), because their appearance and operation imitate physical instruments. LABVIEW contains a comprehensive set of tools for acquiring, analyzing, displaying, and storing data, as well as tools to help and troubleshoot code. In LABVIEW, firstly the user interface, or front panel, is designed with the controls and indicators. The controls are knobs, push buttons, dials, and other input mechanisms. The indicators are graphs, LEDs, and other output displays. After the user interface is designed, the code using VIs and structures is added to control the front panel objects. The block diagram contains this code and can be used to communicate with hardware such as data acquisition, control and modification. The main components in LabVIEW are as follows: crio: crio stands for compact reconfigurable I/O. It is an integrated system which combines a realtime processor and a reconfigurable field-programmable gate array (FPGA) within the same chassis. This also have analog input, analog output and digital I/O modules. It has a 266 MHz industrial realtime processor for control, data logging, and analysis, Analog Input/output Modules: Specifications of NI 9201 (analog input module) are: (i) 8 analog inputs, ±10 V input range (12-bit resolution single-ended) and (ii) 500 ks/s aggregate sampling rate. The NI 9263 (analog output module) has four simultaneously updated analog outputs, 100 ks/s (16-bit resolution). Interfacing Modes: LabVIEW has got two projects (modes of operation). They are: (i) real time project, and (ii) FPGA project. The real time mode comprises of two programming modes: scan interface mode and LabVIEW FPGA mode. These programming modes are basically the interfacing techniques. The scan interface mode is used for retrieving data of lower frequency with simpler processing techniques. In this mode the acquisition of data and processing of the blocks takes place in the crio device. When the frequency of the data acquired by the crio is higher and the processing demands accurate and faster processing, then LabVIEW FPGA mode is preferred. In this mode, the crio analog input and output modules are interfaced with the FPGA. Basically, in this technique, the real time data is acquired by means of analog input modules and the processing is done inside the LabVIEW FPGA. FPGA PROJECT: This project involves retrieving and processing of data by the LabVIEW FPGA. Data of the order of 70 KHz can be perfectly sampled in this project. But the FPGA can only be programmed by data having integral values. Floating point operation is not possible in this project. The operating frequency of several modes is given in Fig. A1. The interfacing between FPGA mode and real time mode takes place on interrupt based handshaking mode. The FPGA samples the high 7
frequency inputs from the analog input module and generates the interrupt signal. The real time (RT) mode reads the input from the FPGA input blocks after detecting the interrupt signal. The processing of inputs takes place inside the RT mode. The RT mode writes the output into FPGA output blocks. The RT mode then generates an acknowledge interrupt signal. The FPGA mode acknowledges the interrupt and waits for the next input. Fig. A2 shows the layout of interrupt based handshaking mode. Fig. A1: Comparison of operating frequencies in various modes Fig. A2: Interrupt-based handshaking mode Layout Design in LabVIEW: LabVIEW VIs comprise of a front panel and a control panel. The LabVIEW program is designed in the control panel and the program outputs are seen in the form of charts, graphs or digital displays in the front panel. Each panel has separate function palette which displays the list of available LabVIEW blocks. How to start LabVIEW: NI LabVIEW 8.6 ->> (Getting Started ->> Open) ->> E:\LabVIEW models\wte_modified\ Sum.lvproj ->> (Project Explorer) ->> NI-cRIO -Energy Lab (10.9.17.122) ->> target - single rate.vi - >> Run button displayed on the front panel. 8
APPENDIX-III: Symbols Wm Ww If Ia Speed of the DC Motor Speed of the Wind turbine Reference current DC Motor armature current REFERENCES 1. A.S.Satpathy, N.K.Kishore, and N.C.Sahoo, Emulation of Wind Turbine Characteristics Based on Separately Excited DC Motor Using LabVIEW, proceedings of CCEE, IISc Bangalore, 15-17 Dec, 2011. 2. L.Qihui, H.E.Yikang, and Z.Rende, Imitation of the Characteristics of the wind turbine based on DC motor, Frontiers of Electrical and Electronics Engineering in China, vol. 2, issue 3, pp.361 367,2007. 3. M.Monfareda,H.M.Kojabadi, and H.Rastegar. Static and dynamic wind turbine simulator using a converter controllerd dc motor, Renewable Energy, vol. 33, issue 5, pp. 906 913, 2008. 4. www.ni.com Getting Started with LabVIEW 5. www.ni.com LabVIEW Fundamentals 6. www.ni.com LabVIEW help Discussions:- 1. What are the different types of Wind Turbine Emulator? 2. Why DC motor is preferred for Wind Turbine Emulation? 3. Calculate the minimum value of capacitance required for excitation of induction generator. 9