Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
DYNAMIC BEHAVIOUR OF DFIG-BASED WIND
TURBINES DURING SYMMETRICAL VOLTAGE DIPS
Almoataz Y. Abdelaziz, Amr M. Ibrahim, Ahmed M. Asim, Ahmed H. Abdel Razek
Electrical Power and Machines Department, Faculty of Engineering, Ain Shams
University, Cairo, Egypt
almoatazabdelaziz@hotmail.com,
amrmohamedhassan@yahoo.com,
asd.200n@gmail.com,
ahheizen@gmail.com
ABSTRACT
Recently, wind power generation has reached a significant penetration levels in some countries. This is the
case for instance in Denmark, Spain and Germany. This imposes new challenges to the transmission system
operators (TSOs). New grid requirements for wind generation are needed. These requirements stipulate
that wind farms should contribute to power system control, much as the conventional power stations and
focus on wind farm behaviour in case of grid disturbances. Isolation of wind turbines in case of voltage
dips is not allowed anymore as low voltage ride through (LVRT) capability is required. This paper will
discuss the dynamic behaviour of DFIG based wind turbine under severe three phase voltage dips without
and with the usage of crowbar protection system including simulation results to verify the discussion.
KEYWORDS
Wind Turbine, Low Voltage Ride Through, Crowbar &DFIG
1. INTRODUCTION
Doubly fed induction generators (DFIG) are the most commonly used wind turbine technology
[1-4]. It uses a wound rotor induction generator with slip rings to transmit electrical power
between the converter system and rotor windings. Variable speed operation is obtained by
injecting a controllable voltage into the rotor at desired slip frequency. The DFIG based wind
turbine can transmit electrical power to the grid through both the generator stator and the
converters. When the generator operates in super-synchronous (slip < 0) mode, power will be
delivered from the rotor through the converters to the grid, and when the generator operates in
sub-synchronous (slip > 0), the rotor will absorb power from the grid through the converters. The
generator slip ranges from -0.3 to 0.3, this means that maximum power transferred through the
converters is nearly 30% of the nominal power of the generator, therefore smaller and cheaper
converters can be used compared with full size converters used with other wind turbine
technologies [5].
In normal grid conditions, power electronic converters enable DFIG to operate at optimal rotor
speed. It maximizes electrical power generated by controlling the active and reactive power flow
into the grid [6].
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
In case of severe voltage dips [7-10], large transient currents are induced in the stator circuit. Due
to magnetic coupling between stator and rotor windings, large transient currents are induced in
the rotor circuit. Such high currents could damage the converters and increase the voltage of the
DC-link, therefore a protection is required. The protection of the converter is achieved by short
circuiting the rotor circuit through a crowbar, thus protecting the converters from high rotor
currents. Once the rotor currents are blocked from the converters, control on the DFIG is lost,
thus losing the active and reactive power control on the stator of the DFIG and the machine
operates as a typical induction generator [11-12].
The aim of this paper is to provide insight and understanding about the dynamic behaviour of
DFIG based wind turbines, its protection against overcurrent and overvoltage and its LVRT
capability.
This paper is organized as follows. Section 2 presents configuration of DFIG variable speed wind
turbine. Section 3 presents the dynamic model of DFIG. Section 4 describes effects of severe
voltage dips on DFIG. Section 5 presents simulation and results. Section 6 presents conclusions.
2. CONFIGURATION OF DFIG VARIABLE SPEED WIND TURBINE.
A schematic diagram of DFIG wind turbine is shown in Figure 1. The AC/DC/AC converter is
divided into two converters rotor side converter (RSC) and grid side converter (GSC) [13]. Both
converters are voltage source converters. They use forced commutated power electronic devices
(IGBTs) to convert from AC to DC and vice versa. A capacitor connected to the DC side of the
converter acts as a DC voltage source. A coupling inductor is used to connect the GSC to the grid.
The rotor winding is connected to the RSC by slip rings and brushes. Both the stator and the GSC
are connected to the grid through a transformer to convert low voltage into medium voltage. The
mechanical power captured by the wind turbine is converted into electrical power by the
generator and transmitted to the grid through the stator and the converters.
The control system of the DFIG based wind turbine consists of three parts [14].
•
Speed control which controls the electrical power reference of converter as well as of the
pitch angle.
•
RSC control which controls active and reactive power on the stator side.
•
GSC control that regulates the DC-link voltage to keep it constant and can be used to
inject additional reactive power to the grid.
3. DYNAMIC MODELLING OF DFIG
To demonstrate effects of severe voltage dips on the DFIG based wind turbine system, a detailed
time model is used [15-16]. This model preserves stator transients. It is called the fifth order
model.
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
Figure 1 A schematic diagram of DFIG based wind turbine
Figure 2 Equivalent circuit of an induction generator dynamic model
The equivalent circuit of the dynamic model is represented as in Figure 2. Stator and rotor voltage
equations can be written as follows
(1)
(2)
Where
and
represents the voltage, current and flux respectively,
is the synchronous
speed and
is the rotor speed. The subscripts and refer to stator and rotor respectively.
The relation between flux and currents is given by,
(3)
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
(4)
Where
represents the magnetizing reactance. Symbols
inductances respectively, where
and
represent the stator and rotor
(5)
(6)
Where
and
are the stator and rotor leakage inductances respectively. The electromagnetic
torque of the generator can be calculated as follows
(7)
The apparent power of the stator is given by
(8)
The mechanical equation is as follows
(9)
Where
represents the generator inertia and
represents the mechanical torque.
4. EFFECTS OF SEVERE VOLTAGE DIPS ON DFIG
At normal operating conditions, there is a stator flux rotating in the air gap of DFIG. This flux is
directly proportional to the stator voltage, if a voltage dip takes place, from equation (1), the flux
can't change instantaneously with voltage change. A transient flux component is induced in the
stator. This transient flux is a DC decaying component which depends on the time constant of the
stator circuit and the value of the rotor current. Due to mutual coupling between rotor and stator
of the DFIG, a transient emf is induced on the rotor circuit. This transient emf component is very
large as it depends on the rotor speed [17-18].
This large transient emf in the rotor winding saturates the RSC which leads to
• Loss of current control
RSC controls the rotor circuit current by changing output voltage of the converter. If the rotor emf
is very high so that maximum output voltage of the converter is reached, saturation takes place
and current control is lost. RSC can't compensate for large rotor emf. Current levels in the rotor
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
circuit increase to high values especially at the beginning of voltage dip. Unless a very oversized
converter is used large currents will damage the semiconductor of the converter.
• Increase of DC-link voltage
Due to large amount of power that the RSC absorbs, while being saturated, DC-link voltage
increases. During severe voltage dips this power can't be evacuated to the grid, because the power
transfer capability of the GSC has dropped due to low voltage of the grid, as a result the power
accumulates in the capacitor of the DC-link and its voltage increases.
Crowbar is to be used to protect the system from loss of current control and increase in DC-link
voltage, a schematic diagram of DFIG with crowbar system is shown as in Figure 3.
The crowbar shorts the rotor winding at severe voltage dip. It is connected at the first stage of the
dip when the rotor emf is at its highest level. Once the transient flux has decayed and rotor emf is
no longer dangerous, the crowbar can be disconnected, so that the machine can resume its normal
operation. When the crowbar is connected, it causes the rotor current to increase but this doesn't
affect the RSC, as it's disconnected and its current is zero. This means that DFIG is not controlled
anymore and it acts as typical squirrel cage induction machine. Once the crowbar is disconnected
the RSC can resume its operation and controls the stator power [19-23].
Figure 3 DFIG with crowbar system
5. SIMULATION AND RESULTS
To verify discussion in the previous section a simulation is conducted to study the electrical
dynamics of DFIG based wind turbine without and with the addition of crowbar to the system.
The simulation is carried out in MATLAB®/ SIMULINK/ SimPowerSystem software [24].
In this simulation the DFIG is subjected to a severe voltage dip of about 90% for 0.15 sec. An
EMT of DFIG is used to preserve the stator dynamics as discussed previously. Vector control is
used for controlling both RSC and GSC. It's assumed that the DFIG produces its nominal active
power and zero reactive power at normal conditions, also wind speed is assumed to be constant
during the simulation period. Turbine rated voltage and power are taken as base values. GSC is
rated at 30% of the turbine rated current. GSC controller limits the GSC current to 0.5 p.u.
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
5.1. Dynamic Behaviour of DFIG without Crowbar
A voltage dip of 90% takes place from 0.1 sec. to 0.25 sec. as shown in Figure 4.
This leads to transient overcurrents in stator and rotor circuits as shown in Figure 5 and Figure 6
respectively. Currents reach values of 6-7 p.u in both stator and rotor circuits, then decay
exponentially subjecting the RSC to very large stresses which are not acceptable because they
may destroy the RSC as shown in Figure 7.
Figure 4 Stator voltage under 90% voltage dip
Figure 5 Stator current without crowbar protection
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
Figure 6 Rotor current without crowbar protection
Figure 7 RSC current without crowbar protection
Figure 8 shows an increase in the DC-link voltage to reach a value 2-3 p.u that exceeds the
capacitor rating which is not acceptable.
Figure 8 DC-link voltage without crowbar protection
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
GSC tries to regulate the DC-link voltage at 1 p.u, its current increases to 50% of the wind turbine
rated current as shown in Figure 9, this causes a significant overload on the GSC. However, the
GSC is not able to regulate the DC-link voltage due to low grid voltage.
Figure 9 GSC current without crowbar protection
Figure 10 shows active and reactive power generated by wind turbine during severe voltage dip.
Active power drops nearly to zero due to low grid voltage. At the start of the dip DFIG inject a
burst of reactive power into the grid and at the end of the dip (voltage recovery) it absorbs a burst
of reactive power from the grid.
Figure 10 Active and reactive power without crowbar protection
Finally from the simulation results it is concluded that without crowbar severe voltage dips may
result in destruction of the converter system as fast separation of DFIG is not allowed as LVRT
capability is required.
5.2. Dynamic Behaviour of DFIG with Crowbar
To provide LVRT capability, a crowbar is used to prevent the DFIG from being disconnected
from the grid during severe voltage dips. The system is subjected to voltage dip of 90% as in the
previous case. Figure 11 shows the stator voltage.
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
Figure 11 Stator voltage with crowbar protection
The crowbar will not be connected to the rotor circuit till the voltage of DC-link reaches a value
of 1.1-1.2 p.u. When the DC voltage reaches 1.1 p.u the crowbar is connected and lasts connected
for 100ms. At the end of the voltage dip the crowbar is connected again when the DC-link voltage
reaches 1.1 p.u and it lasts connected for 40 ms.
Having the crowbar connected the converter is separated from the rotor circuit. Figure 12 and
Figure 13 show the rotor and stator currents with crowbar protection. They are quite similar to the
previous cases.
Figure 12 Rotor current with crowbar protection
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
Figure 13 Stator current with crowbar protection
Figure 14 shows the RSC current with crowbar protection. A sharp spike takes place till crowbar
is connected then the RSC currents drop to zero during the connection of the crowbar which
protects the RSC from over currents. After the disconnection of crowbar, rotor current returns to
its nominal values and it is controlled by RSC. Crowbar current is shown in Figure 15.
Figure 14 RSC current with crowbar protection
Figure 15 Crowbar current
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
Figure 16 DC-link voltage with crowbar protection
The effect of crowbar on DC-link voltage is shown in Figure 16, DC voltage is kept almost
constant at 1 p.u. Figure 17 shows the grid side converter current. The GSC current will not
saturate at 50% of rated current of the turbine which means that GSC is protected from large
stresses.
Figure 17 GSC current with crowbar protection
Active and reactive powers are nearly similar to what happen in the previous case as shown in
Figure 18.
Figure 18 Active and reactive power with crowbar protection
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Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013
6. CONCLUSION
DFIG based wind turbines are subjected to heavy stresses during severe grid voltage dips, which
may cause the deterioration of the converter system. Therefore, the whole system is to be
disconnected from the grid during severe voltage dips but this is not acceptable by the new grid
requirements, which require wind turbines to have LVRT capability. A crowbar protection system
is used to make wind turbines being able to stay connected to the grid during voltage dips while
limiting currents and voltages levels within acceptable values.
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Authors
Almoataz Y. Abdelaziz was born in Cairo, Egypt, on 1963. He received the B. Sc.
and M. Sc. degrees in electrical engineering from Ain Shams University, Cairo,
Egypt in 1985, 1990 respectively and the Ph. D. degree in electrical engineering from
Brunel University, England in 1996. He is currently a professor of electrical power
engineering in Ain Shams University.His research interests include the applications
of artificial intelligence and new heuristic and evolutionary optimization techniques
to power system protection, operation, planning and control. He has authored or coauthored more than 130 refereed journal and conference papers. Dr. Abdelaziz is a
member of the editorial board and a reviewer of technical papers in several
international journals. He is also a member in IEEE, IET and the Egyptian Sub-Committees of IEC and
CIGRE`. Dr. Abdelaziz has been awarded Ain Shams University Prize for distinct researches in 2002 and
for international publishing in 2011, 2012.
Amr M. Ibrahim, was born in Egypt in 1975. He received the B.Sc., M.Sc. and Ph. D.
degrees in electrical engineering from Ain Shams University, Cairo, Egypt in 1998,
2003 and 2008 respectively. He is currently an assistant professor in the department of
electric power and machines, Ain Shams University. His research interests include
power system protection and applications of AI in power systems.
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