Transient analysis and modeling of wind generator during power and grid voltage drop

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 6, Issue 1, January (2015), pp. 41-48 © IAEME
41
TRANSIENT ANALYSIS AND MODELING OF WIND
GENERATOR DURING POWER & GRID VOLTAGE DROP
Mohamed Amer Abomahdi
Research Scholar in the Department of Electrical Engineering
Shepherd School of Engineering & Technology
SHIATS Deemed University, Allahabad (India)
Dr. A.K.Bhardwaj
Associate Professor in the Shepherd School, Department of Electrical Engineering
SHIATS Deemed University, Allahabad (India)
ABSTRACT
In recent years, wind energy has become one of the most important and promising sources of
renewable energy, which demands maintaining better means of system stability. The performance of
a grid-connected wind energy conversion system (WECS), based on a doubly fed induction generator
(DFIG) fed by an AC-DC-AC converter, is presented. The stator winding of the DFIG is connected
directly to the grid and the rotor of the DFIG is connected to the grid through a AC/DC/AC
converter. The main power is transferred through the stator windings of generator that are directly
connected to the grid. Around 65- 75% of total power is transmitted through the stator windings and
the remaining 25% of total power is transmitted using rotor windings (i.e.) through the converters.
The major advantage of speed control of variable speed wind turbine generators is usually optimized
which gives the maximum power in the system. This project focuses towards an efficient control
strategy to improve the overall efficiency and stability in Doubly Fed Induction generator using
converters.
Keywords: DFIG, Back to Back Converter, Wind Turbine.
1. INTRODUCTION
Due to increasing demand on electrical energy and environmental concerns, a considerable
amount of effort is being made to generate electricity from renewable sources of energy. The major
advantages of using renewable sources are abundance. Wind is the one of the most abundant
renewable sources of energy in nature. The wind energy can be harvested by using a wind energy
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 6, Issue 1, January (2015), pp. 41-48
© IAEME: www.iaeme.com/IJEET.asp
Journal Impact Factor (2014): 6.8310 (Calculated by GISI)
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IJEET
© I A E M E

42
conversion system, composed of a wind turbine, an electrical generator, a power electronic converter
and a control system. With increased penetration of wind power into electrical grids, DFIG wind
turbines are largely deployed due to their variable speed generation and they are the most important
generators for wind energy applications
Figure 1: DFIG and its power flow.
In conventional implementation of a grid connected DFIG, back-to-back converters are used
to connect the DFIG rotor to the utility. The rotor side converter controls the magnetizing and torque
rotor currents. The grid side converter regulates the voltage in the dc bus of the back- to-back
converters. It should be recognized here that thyristors are commonly used as switching devices in
the shaft generator system because of their higher reliability, efficiency and easy acquisition of large
ratings. Since the output frequency and voltage of the DFIG driven by the wind turbine are changed
due to unexpected fluctuation in the wind, ac power generated by the DFIG is converted once into dc
power with the thyristor rectifier, and then the dc power is converted again into ac power with
constant frequency and constant voltage with the inverter, which is supplied to the loads (or electric
power utility).
2. REASONS FOR USING DFIG
DFIG-based WECS are highly controllable, allowing maximum power extraction over a large
range of wind speeds. Furthermore, the active and reactive power control is fully decoupled by
independently controlling the rotor currents. Finally, the DFIG- based WECS can either inject or
absorb power from the grid, hence actively participating at voltage control. The synchronous nature
of PMSG may cause problems during startup, synchronization and voltage regulation and they need
a cooling system, since the magnetic materials are sensitive to temperature and they can lose their
magnetic properties if exposed to high temperature. Hence DFIG is dominantly used when compared
to PMSG.a three-phase wound-rotor induction machine can be set up as a doubly-fed induction
motor. In this case, the machine operates like asynchronous motor whose synchronous speed (i.e.,
the speed at which the motor shaft rotates) can be varied by adjusting the frequency frotor of the ac
currents fed into the rotor windings. The same wound-rotor induction machine setup can also serve
as a doubly-fed induction generator. In this case, mechanical power at the machine shaft is converted
into electrical power supplied to the ac power network via both the stator and rotor windings.

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Furthermore, the machine operates like a synchronous generator whose synchronous speed (i.e., the
speed at which the generator shaft must rotate to generate power at the ac power network frequency
(fnetwork) can be varied by adjusting the frequency of the ac currents fed into the rotor windings. In
a conventional three-phase synchronous generator, when an external source of mechanical power
(i.e., a prime mover) makes the rotor of the generator rotate, the static magnetic field created by the
dc current fed into the generator rotor winding rotates at the same speed (nrotor) as the rotor. As a
result, a continually changing magnetic flux passes through the stator windings as the rotor magnetic
field rotates, inducing an alternating voltage across the stator windings. Mechanical power applied to
the generator shaft by the prime mover is thus converted to electrical power that is available at the
stator windings.
3. MODELING OF WIND TURBINE
The modeling of wind turbine consists of three important parameters. They are
a) Transmission system b) Wind turbine system c) Distribution system.
TRANSMISSION SYSTEM
The electrical energy produced at various power stations are transferred to the consumers
through conductor lines. It consists of a voltage source, step-up transformer, transmission line, a bus
bar, step down transformer, V-I measurement block, scope and a resistive load.
1) AC Power Supply Scheme The large network of conductors between the power station and the
consumers can be broadly divided into two parts namely transmission and distribution systems.
Each part is subdivided into primary and secondary transmission.
2) Generating Station The generating station where electric power is produced by 3-phase
alternator is operated in parallel. The usual generating voltage is 11kv. For economy in the
transmission of electric power, the generated voltage is stepped up to 132kv at the generating
station with the help of 3-phase transformers.
3) Primary Transmission The electric power at 132kv is transmitted by 3- phase, 3-wire overhead
system. This forms the primary transmission.
4) Secondary Transmission The primary transmission line terminates at the receiving station. At
the receiving station the voltage is reduced to 33kv by step-down transformers. From this
station, electric power is transmitted at 33kv by 3-phase 3-wire over head System to various
sub-stations. This forms the secondary transmission.
5) Primary Distribution The secondary transmission line terminates at the sub-station where
voltage is reduced from 33kv to 11kv 3-phase, 3-wire. This forms the primary distribution.
From this the large consumers which are using 11kv are distributed from here.
6) Secondary Distribution The electric power from primary distribution line (11kv) is delivered to
distribution sub-stations. These sub-stations step down the voltage to 400V, 3-phase and 4-wire
for secondary distribution. The voltage between any two phases is 400V and between any
phase and neutral is 230V. From this the voltage is supplied to the feeders and to the
consumers.
Wind turbines convert the kinetic energy present in the wind into mechanical energy by
means of producing torque. Since the energy contained by the wind is in the form of kinetic energy,
its magnitude depends on the air density and the wind velocity. The wind power developed by the
turbine is given by the equation (1.1):

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P = ½ CpρAV3 … (1.1)
Where Cp is the power co-efficient, ρ is the air density in kg/m3, A is the area of the turbine blades
in m3 and V is the wind velocity in m/sec. The power coefficient Cp gives the fraction of the kinetic
energy that is converted into mechanical energy by the wind turbine. It is a function of the tip speed
ratio λ and depends on the blade pitch angle for pitch-controlled turbines. The tip speed ratio may be
defined as the ratio of turbine blade linear speed and the wind speed
λ = Rω/V … (1.2)
Substituting (1.2) in (1.1), we have:
P = ½ Cp (λ) ρA(R/λ) ω3 … (1.3)
The output torque of the wind turbine Turbines calculated by the following equation (1.4)
Turbine = ½ ρACpV/λ … (1.4)
where R is the radius of the wind turbine rotor(m). There is a value of the tip speed ratio at which
the power coefficient is maximum. Variable speed turbines can be made to capture this maximum
energy in the wind by operating them at a blade speed that gives the optimum tip speed ratio. This
may be done by changing the speed of the turbine in proportion to the change in wind speed.
4. CONVERTER METHODOLOGY
Wind turbines use a doubly-fed induction generator (DFIG) consisting of a wound rotor
induction generator and an AC/DC/AC IGBT based PWM converter. The stator winding is
connected directly to the 50 Hz grid while the rotor is fed at variable frequency through the
AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for
low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the
turbine during gusts of wind. The optimum turbine speed producing maximum mechanical energy
for a given wind speed is proportional to the wind speed. Another advantage of the DFIG technology
is the ability for power electronic converters to generate or absorb reactive power, thus eliminating
the need for installing capacitor banks as in the case of squirrel-cage induction generator. Where Vr
is the rotor voltage and Vgc is grid side voltage. The AC/DC/AC converter is basically a PWM
converter which uses sinusoidal PWM technique to reduce the harmonics present in the wind turbine
driven DFIG system. Here Crotor is rotor side converter and Cgrid is grid side converter. To control
the speed of wind turbine gear boxes or electronic control can be used.
5. CONTROL PARAMETERS
The block diagram for control parameters shows different modes of operation in which we
can select the voltage regulation mode and Var regulation mode. Also we can set the external
reactive current Iq ref for grid side to zero which gives flexibility to simulate various fault
conditions. Here we input the required values of voltage regulator gains (both proportional and
integral), power regulator gains, current regulator gains and their respective rate of change.

45
6. INDUCTION MACHINE MODEL
The model of the IG is an instantaneous values full order one, i.e. fifth order with the
derivative of the stator fluxes included. This is the best choice to accurately simulate transients in the
power systems when the electrical dynamics of the IG is of primary interest. The values of the IG
parameters have been chosen as per the requirements. The DFIG wind farm is composed of 6
aggregated units of 1.5MW each.
7. GRID SIDE CONVERTER
The GSC is modeled using a universal bridge model with IGBTs connected to the IG
terminals through a series RL filter. The control of the GSC aims at keeping the DC-link capacitor
voltage constant at nominal value. It does not contribute to grid voltage regulation or reactive power
injection (Iq = 0). The converter maximum power is half the IG rated power.
8. ROTOR SIDE CONVERTER
The RSC controls the active power delivered by the DFIG, by determining a torque reference
(following a power-speed curve). This torque reference is used along with a flux estimate to
determine the reference rotor current Idr. The RSC model has been modified so as to support the grid
voltage with reactive power injection. The reference for the reactive power is determined through a
PI controlled by measuring the grid voltage and comparing it with a reference.
9. SIMULINK DIAGRAM

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10. RESULTS AND DISCUSSIONS
In the "Wind Speed" step block specifying the wind speed. Initially, wind speed is set at 8
m/s, then at t = 5s, wind speed increases suddenly at 14 m/s.

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Start simulation and observe the signals on the "Wind Turbine" scope monitoring the wind
turbine voltage, current, generated active and Reactive powers, DC bus voltage and turbine speed. At
t = 5s, the generated active power starts increasing smoothly to reach its rated value of 9 MW in
approximately 20s. Over that time frame the turbine speed will have increased from 0.8 PU to 1.21
PU. Initially, the pitch angle of the turbine blades is zero degree and the turbine operating point
follows the red curve of the turbine power characteristics up to point D. Then the pitch angle is
increased from 0 deg to 0.76 deg in order to limit the mechanical power.
11. CONCLUSION
The wind farm is built with the rectifiers and the inverters. The features are low cost, low
commutation power losses and high reliability. The rotor side converter (RSC) usually provides
active and reactive power control of the machine while the grid-side converter (GSC) keeps the
voltage of the DC-link constant. So finally we simulated grid side and wind turbine side parameters
and the corresponding results have been displayed. The model is a discrete-time version of the Wind
Turbine Doubly-Fed Induction Generator (Phasor Type) of MATLAB/Sim Power Systems. Here we
also took the protection system in consideration which gives a trip signal to the system when there is
a fault (single phase to ground fault) on the system. The faults can occur when wind speed decreases
to a low value or it has persistent fluctuations. The DFIG is able to provide a considerable
contribution to grid voltage support during short circuit periods. Considering the results it can be said
that doubly fed induction generator proved to be more reliable and stable system when connected to
grid side with the proper converter control systems.

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REFERENCES
[1] Akhmatov V., Analysis of dynamic behavior of electric power systems with large amount of
wind power, PhD thesis, 2003, Ørsted DTU.
[2] Heier S. Grid Integration of Wind Energy Conversion Systems, John Wiley &Sons, Ltd.
Chichested UK, 1998.
[3] Hansen A.D., Michalke G., Fault ride-through capability of DFIG wind turbines, Renewable
Energy, vol 32, 2007, pp 1594-1610.
[4] Akhmatov V., Variable-speed wind turbines with doubly fed induction generators. Part II:
Power System Stability. Wind Engineering, Vol. 26, No. 3, 2002, pp 171-188.
[5] Akhmatov V., Variable-speed wind turbines with doubly fed induction generators. Part IV:
Uninterrupted operation features at grid faults with converter control coordination. Wind
Engineering, Vol. 27, No. 6, 2003, pp 519-529.
[6] Sørensen P., Bak-Jensen B., Kristiansen J., Hansen A.D., Janosi L., & Bech J. Power plant
characteristics of wind farms. Wind Power for the 21st Century. Proceedings of the
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[7] Eping C., Stenzel J., Poeller M., Mueller H., Impact of Large Scale Wind Power on Power
System Stability, Fifth International Workshop on Large-Scale Integration of Wind Power
and Transmission Networks, Glasgow, 2005, 9pp.
[8] Kayikci M., Anaya-Lara O., Milanovic J.V., Jenkins N., Strategies for DFIG voltage control
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[9] Akhmatov V., A small test model for the transmission grid with a large offshore wind farm
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[10] Ameer H. Abd and D.S.Chavan, “Impact of Wind Farm of Double-Fed Induction Generator
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[11] Ashish Dhamanda and A.K. Bhardwaj, “Automatic Generation Control of Thermal
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[12] Anas Abdulqader Khalaf, “Unified Power Control Under Different Grid Conditions for
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[13] Nadiya G. Mohammed, “Application of Crowbar Protection on DFIG-Based Wind Turbine
Connected to Grid”, International Journal of Electrical Engineering & Technology (IJEET),
Volume 4, Issue 2, 2013, pp. 81 - 92, ISSN Print : 0976-6545, ISSN Online: 0976-6553.

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