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Practical 1 - Characteristics of Doubly-fed Induction Generators (DFIG)
Aim: The aim of this practical is to evaluate basic operating characteristics of a Doubly-fed
induction generator directly connected to a grid using standard MATLAB/SIMULINK blocks given in Simscape.
Objectives: Familiarise with the block diagram of wind generating systems given in Simscape.
Develop a simple model of a wind power plant directly connected to a grid
Simulation of the developed model and familiarise with MATLAB (2019a) Workspace and graph plotting
Background: In contrast to aconventional power generation, wind power is intermittent type. Thus
the output power of a wind generator cannot be controlled on demand. In other words, wind power is not dispatchable. The fluctuating output power of a wind generator or a wind farm affects the voltage profile, losses, reliability and stability of the system. The output power of a wind turbine depends on wind speed which is stochastic in nature. For a given wind speed, the turbine efficiency or performance coefficient (Cp) depends on turbine speed and hence tip-speed ratio (λ), and blade pitch angle (β). The turbine power Pm can be expressed as:
Where, ρ is the air density, A is the blade sweep area and Vw is the wind speed.
In this practical, a Doubly-fed-induction-generator (DFIG) is used. To extract the maximum power at different windspeeds, the speed of the wind turbine and hence the generator is required to vary according to the wind speeds. This is done by employing partial size back-to-back power electronic converters (ac-dc-ac). The converters are connected to a common dc capacitor. The rotor side and grid side converters synthesize an ac voltage from a dc voltage source (represented by the dc capacitor). The speed control range of the DFIG depends on the size of the converters used. Usually size of the converter is about 30% of rated power of the generator. Blade pitch angle of the wind turbine is also adjusted to limit the output power (at the rated value) during higher windspeeds.
Description:
Fig.1 illustrates the single line diagram of a simple wind power plant connected to a power grid and supplying a local load. Fig. 2 illustrates the corresponding Simulink model or block diagram. The procedure of developing the block diagram is described in the following.
Instructions:
1. Open MATLAB from ‘Start’ menu.
2. Select ‘New’ → ’Simulink Model’ (or type ‘Simulink’ in the command window) to get a new Simulink window.
3. Click ‘Simulink Library’ icon in the toolbar to get the window of the Simulink library.
4. The DFIG block is located in Simscape → Electrical → Specialised Power System → Renewables
→ Wind → Wind turbine doubly - fed Induction Generator (Phasor type).
? Drag & drop the model on to the new file.
? Save the file.
5. Type ‘Powergui’ in the Simulink library search space. Drag & drop the ‘Powergui’ block into your model.
6. Search for the following blocks and drag & drop them into your Simulink file.
? Three-phase source, constant, step, scope, clock, Three-Phase series RLC load, Bus creator, Bus selector, simout block (To workspace).
Hint: Find simout block (To workspace) in ‘Sinks’ category.
7. Connect all the elements as shown in Fig.2. Save the output of simout block as ‘array’ instead of ‘timeseries’ . (Hint: Change the ‘save format’ option of simout block)
8. Right click the ‘Bus creator’ → Block parameter → Set no. of inputs to ‘6’ .
9. Right click on the ‘Bus selector’ → Block parameter → Select P (pu), Q (pu), Tm, wr→ Click ‘OK’ .
10. Choose the solver: Click on the ‘Variable step auto’ (bottom right corner) → Settings → Solver → ode 45 (Dormand-Prince).
11. Double click on the ‘Powergui’ block → Select the simulation type as ‘Phasor’ and Frequency as ‘60Hz’ → Click ‘OK’ .
12. Double click on the ‘Three-phase Source’ and enter the following parameters:
? Phase-to-phase rms voltage - 575 V, Frequency - 60Hz, Phase angle - 0, 3-phase short circuit level - 100 MVA, X/R ratio – 7, Base voltage – 575 V
13. Double click on the ‘Three-phase load’ block and enter the following parameters :
? In the ‘Parameters’ tab → Nominal phase-to-phase voltage - 575 V, configuration - Y (grounded), frequency - 60 Hz, active power - 100 kW, Inductive and capacitive reactive power - 0
? In the ‘Load flow’ tab → load type ‘Constant Z’