FAULT RIDE-THROUGH CONTROL FOR A DOUBLY FED INDUCTION GENERATOR WIND TURBINE UNDER UNBALANCED VOLTAGE SAGS

Document Sample
FAULT RIDE-THROUGH CONTROL FOR A DOUBLY FED INDUCTION GENERATOR WIND TURBINE UNDER UNBALANCED VOLTAGE SAGS Powered By Docstoc
					INTERNATIONAL Electrical Engineering and Technology (IJEET), ISSN 0976 –
International Journal of JOURNAL OF ELECTRICAL ENGINEERING &
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME
                                TECHNOLOGY (IJEET)

ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 3, Issue 1, January- June (2012), pp. 261-281
                                                                             IJEET
© IAEME: www.iaeme.com/ijeet.html
Journal Impact Factor (2011): 0.9230 (Calculated by GISI)                ©IAEME
www.jifactor.com




       FAULT RIDE-THROUGH CONTROL FOR A DOUBLY FED INDUCTION
       GENERATOR WIND TURBINE UNDER UNBALANCED VOLTAGE SAGS

             NadiyaG. Mohammed 1, HaiderMuhamadHusen2, Prof. D.S. Chavan3

ABSTRACT

The aptness of Wind Turbines with Doubly Fed Induction Generator for original grid operator
that require low Voltage Ride Through operation is considered. This sophisticated grid technique
require the wind turbines to remain connected even under voltage dips in the grid due to fault or
any unpredicted perturbations. Ananalysis of such problems associated with voltage dips and
methods for Ride Through operation of such system are presented. Simulation results of Power
Error Vector Control method for the Voltage Source Inverter connected to the rotor during a
voltage dip are shown. The simulated results show that this type of control may eliminate the
need for a crowbar, or at least a great reduction in the rotor currents is obtained and activation of
crowbar may be limited to very extreme cases when very strong wind gusts and very deep voltage
dips take place at the same time. A DFIG system with series grid=side converter (SGSC) has an
excellent potential for voltage dips tolerance. This paper analyzes the reasons of a DFIG system
with SGSC for ride=through and presents a control scheme for operation under unbalanced grid
faults conditions. During grid faults, the each component of the generator’s stator flux is
effectively controlled through the SGSC. Also, the stator and rotor currents are further restricted
by controlling the rotor=side converter (RSC) to reduce the absorbing energy from wind turbine.
Then, successful ride-through of a DFIG system with SGSC under all types of severe unbalanced
grid faults at the point of common coupling (PCC) are achieved with reduced electromagnetic
torque oscillation. The proposed control scheme is validated by means of simulations.

Key Words: Doubly fed induction generator (DFIG), series grid=side Converter (SGSC),
unbalanced grid fault, wind turbine, low voltage ride=through (LVRT), wind power generation,
voltage dips, voltage sags.

1 INTRODUCTION

Wind power has become one of the most important and promising sources of renewable energy
that can partially solve the energy crisis and environmental dilemma we are confronting today
[1]. With the increased penetration of wind power into power grids, its impact on a power grid
cannot be neglected as before. Grid codes now usually require wind turbines to remain connected
to the grid even during extreme voltage dips such as a short-term low or even a zero voltage event
at the point of common coupling (PCC). This is the so-called “low voltage ride- through (LVRT)”
requirement adopted by most European countries [2,3].The majority of wind turbines above


                                                261
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

1MW are Doubly Fed Induction Generators (DFIGs) with a partially rated back=to=back
converter used between the rotor circuit and the grid [4=8]. While a saving is made on the size of
the power electronic converter, it is well known that a DFIG system with this configuration is
very sensitive to grid disturbances, especially to voltage dips. During a fault, the transient current
on the stator is reflected on the rotor windings. The resulting rotor current may exceed the
rotor=side converter (RSC) current rating and destroy the converter if no protection elements are
included. The mainstream scheme adopted by manufacturers to ride through grid faults is the so-
called “crowbar protection” [9,10]. When the crowbar is in operation, the high fault current will
flow through the crowbar instead of the converter. Thus, the RSC is well preserved during the
fault. However, under such a scenario, the DFIG behaves as a squirrel=cage induction machine
and consumes lots of reactive power from the grid for magnetization, which may further
exacerbate the stability of the connected grid.
Also, the operation of a crowbar is associated with large transient electromagnetic torques which
will increase the fatigues on the mechanical components of the wind turbine generator system
(WTGS), especially on the gearbox. Another LVRT technology is the rotor flux control technique
[11], which controls the RSC to counteract the undesired components in the stator flux. However,
the active and reactive power control over the machine is lost during the fault and the DFIG
system also consumes reactive power during the grid recovery. Furthermore this method is unable
to ride through the severe grid faults such as the voltage dips down to zero at PCC which means it
is not suitable for the new grid codes. In order to fulfill the new grid codes which require wind
turbines to ride through a short=term low or even a zero voltage event at PCC, modifications to
he traditional DFIG configuration for ride=through have become necessary. Petersson [12] has
proposed a new configuration with a grid=side converter in series with the stator windings of the
DFIG and has revealed that DFIG system with this configuration has an excellent potential for
voltage dips tolerance, but this scheme is not efficient in the power processing capability.
This paper describes a technique to control DFIG under un-balanced voltage sags. In comparison
with [15] and [16], it uses the approach based on separating the positive and negative components
of all the currents and voltages, as suggested in [12] and [13] for dc/ac converters and applied to
the DFIG as in in [14]. This paper introduces the following contributions: 1) The whole system is
analyzed, considering both the grid-side and rotor-side converters. The grid-side converter control
is not considered in in [14] or [16]. 2) A technique to keep the dc bus stable is proposed, based on
compensating the rotor power delivered by the rotor-side converter in the grid-side converter. 3)
The objective of the technique is to ride through voltage sags; hence, the main analyzed quantities
are the generator torque and the dc voltage bus. 4) Since this paper deals with ride through
voltage sags, the crowbar protection is considered (it is not considered in [14] or in [16]).
The compared to the balanced faults, due to the negative=sequence component injected by the
unbalanced grid faults, a more severe current and torque oscillation will appear when unbalanced
voltage dips occur [16]. Therefore, it is necessary to study the behavior of DFIG with this new
configuration under unbalanced grid faults. This paper proposes an unbalanced-grid-fault ride-
through control scheme for a DFIG wind turbine with the configuration explored in [14,15, 17].
The paper has been organized as follows. Section 2 reviews the configuration of the DFIG wind
turbine with SGSC. The behaviors of DFIG system with SGSC under unbalanced grid faults are
analyzed in Section 3. The proposed control scheme is designed in Section 4 and validated by
means of simulations in Section 5. Discussion is provided in Section 6 and finally Section 7
draws the conclusions.

II. CONTROL SCHEME UNDER BALANCED CONDITIONS

A DFIG wind turbine primarily consists of three parts: a wind turbine drive train, an induction
generator, and a power electronic converter (Fig. 1) [2], [4]. In the wind turbine drive train, the

                                                262
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

rotor blades of the turbine catch wind energy that is then transferred to the induction generator
through a gearbox. The induction generator is a standard, wound rotor induction machine with its
stator windings directly connected to the grid and its rotor windings connected to the grid through
a frequency converter. The frequency converter is built by two self-commutated voltage-source
converters, the RSC and the GSC, with an intermediate dc voltage link. The control in a DFIG
wind power plant has three levels: the generator level, the wind turbine level, and the wind power
plant level (Fig. 1) [4]. At the generator level, the RSC controller regulates the DFIG to achieve
one of the following two goals: 1) maximum energy extraction from the wind or 2) compliance
with a wind plant control demand; the GSC controllermaintains a constant dc-link voltage and
adjusts reactive powerabsorbed from the grid by the GSC. At the turbine level, thereare a speed
controller and a power limitation controller. At alow wind speed, the speed controller gives a
power referenceto the RSC based on the principle of maximum energy extraction. At a high wind
speed, the power limitation controller increases or decreases the pitch angle of the turbine blades
to prevent the wind turbine from going over the rated power. At thewind power plant level, the
power production of the entire plantis determined based on a grid requirement. The central
controlsystem sends out power references to each individual turbine,while the local turbine
control system ensures that the powerreference from the central control level is reached.

A. GSC Transient and Steady-State Models
Fig. 2 shows the schematic of the GSC, in which a dc-link capacitor is on the left and a three-
phase voltage source, representing the voltage at the point of common coupling (PCC) of the ac
system, is on the right. In the grid-side converter, the dc bus voltage and reactive power
references determine the current references, which determine the voltages to be applied in the
grid side.1) System Equations: In a synchronous reference frame, the grid-side voltage equations
can be written as




Fig. 1.General system scheme.

Active and reactive power provided by the grid-side converter can be written as Pz =3/2(vzqilq +
vzdild) and Qz =3/2(vzqild − vzdilq ).The dc bus voltage can be expressed as



2) Reference Quantities: The grid-side converter controls the reactive power and dc bus voltage.
The q-axis may be aligned to the grid voltage allowing active and reactive decoupled control. To
control the reactive power, aild reference is computed as



                                               263
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME




The active power, which is responsible for the evolution of the dc bus voltage, is controlled by
the ilqcomponent. A linear controller is usually designed to control the dc bus voltage. 3) Current
Loops Implementation: The current control is done by the following state linearization feedback
[18]:




where the ˆvlq and ˆvld are the output voltages of the current controller. The decoupling leads to




B. Rotor-Side Converter
In the rotor-side converter, the referenced torque and reactivepower determine the current
references, which determine thevoltages to be applied in the rotor side.
    1) Machine Equations: It is usually assumed that when thestator and rotor windings are
        placed sinusoidally and symmetrically, the magnetical saturation effects and the
        capacitance ofall the windings are negligible. The relation between voltagesand currents
        on a synchronous reference qd can be written as




    Linkage fluxes can be written as




    The torque can expressed as




                                                264
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

    The reactive power yields


    2) Reference Quantities: Orientating the synchronous reference qd with the stator flux
       vector so that λsd =0,therotor current references can be computed as




Current Loops Implementation: The control of the current is done by linearizing the current
dynamics using the following state feedback.




By neglecting stator current transients, the decoupling leads to




C. Current Controllers Tuning

Controllers have been designed using the so-called internal mode control (IMC) methodology
detailed in [19]. The parameters of a PI controller to obtain a desired time constant τ are obtained
as




The currents and voltages have been limited according to the converter operating limits. PI
controllers have been designed with anti-windup in order to prevent control instabilities when the
controllerexceed the limit values.

D. Crowbar Protection
The so-called crowbar is connected to avoid overvoltages in the dc bus due to excessive power
flowing fromthe rotor inverter to the grid-side converter, guaranteeing ride through operation of
the generator when voltage sags or other disturbances occur. The crowbar is triggeredwhen the dc
voltage reaches a thresh-old vcrow−c and disconnects when it goes below another thresh-old
vcrow−d .
During its operation, the rotor-side converter may be disconnected, as described in [20], or be
kept connected [9] to avoid losing control over the machine. In this paper, the rotor-side converter
is kept connected.

III. CONTROL SCHEME UNDER UNBALANCED CONDITIONS

In this section, nonsymmetrical voltage sags are considered. Such unbalanced sags imply negative
sequence components in all the relevant quantities. Therefore, important oscillations appear in

                                                265
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

torque, active and reactive power. Such oscillations have a pulsation of 2ωe . In order to mitigate
such oscillations, an approach taking into account the negative sequence quantities is required.
Such an approach has been discussed in [12] and [13], and has been applied to the rotor-side
converter of a DFIG in [14]. This section analyzes a whole back-to-back converter taking into
account both the positive and negative sequence components, and proposes a technique to control
optimally both the dc bus voltage and the torque when unbalanced voltage sags occur. As far as
unbalanced systems are concerned, it is useful to express three-phase quantities xabc = {xa ,xb
,xc }T in direct and inverse components as



Where x =2/3(xa + axb + a2xc), a = ej2π/3, xp= xpd +jxpq, and xn= xnd + jxnq . In this section,
voltages, currents, and fluxes are regarded as a composition of such positive and negative
sequences.

    A. Grid-Side Converter Analysis

    1) Voltage Equations: Considering two rotating reference frames at +ωe and −ωe , the
       voltage equations for the positive and negative sequences yield



    2) Active and Reactive Power: Active and reactive power can be written as [13]




Where




It can be noted that both active and reactive power have three different components each, and
hence with the four regulatable currents ipld , iplq , inld , and inlq , only four of such six powers can
be controlled.

    B. Machine-Side Converter Analysis

    1) Voltage Equations: Considering two rotating reference frames at +ωe and −ωe , the
       voltage equations for the positive and negative sequences can be obtained as




                                                  266
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME




   2) Stator Power Expression: The apparent stator power can be expressed as


       Using(14),wehave




       Taking into account xis= xisd+ jxisq, and rearranging it gives




       where




       Substituting stator currents in (27)




       it can be noted that both active and reactive power quantitities have three different
       components each, and therefore, with the four regulable currents iprd , iprq , inrd , and inrq ,
       only four of the six power quantities can be controlled.

       3) Rotor Power Expression: The apparent rotor power canbe expressed as




                                                267
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME




        Using(14),wehave




        Taking into account xis = xisd + jxisq , and rearranging and analyzing the active rotor power




        where




    3) Torque Expression: Analogously, electrical torque can be expressed as



        Where




    C. Reference Current Calculation

Since there are eight degrees of freedom (the rotor-side currents iprd , iprq , inrd , and inrq , and
the grid-side currents iprd , iprq , inrd , and inrq, eight control objectives may be chosen. This implies
that it is not possible to eliminate all the oscillations provoked by the unbalance. In this paper, the
main objective is to ride through voltage dips. Hence, it is important to keep the torque and dc bus
voltage as constant as possible and to keep reasonable values of reactive power. To this end, it
has been chosen to determine the currents to keep certain valuesof Γ∗0 ,Γ∗cos , Γ∗sin , and Q∗s0
for the rotor-side converter andP∗l0 , P∗lcos, P∗lsin and Q∗l0 for the grid-side converter. It can
benoted that P∗l0 ,P∗lcos, and P∗lsin are directly linked to the dc busvoltage.
The dc voltage E is regulated by means of a linear controllerwhose output is the power demanded
by the grid-side converter.Considering the power terms Pr0 ,Prcos , and Prsin in the rotorside
converter, Pr0 can be regarded as the average power delivered, while Prcos and Prsin are the rotor
power oscillating terms.Such terms will cause dc voltage oscillations, and hence theycan be
canceled by choosing

Pl0 can be computed as

                                                  268
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME


where P∗E is the output of the dc voltage linear controller.

3 Behaviors of DFIG System with SGSC under Unbalanced Grid Faults
3.1 Dynamic Response of Traditional DFIG System under Unbalanced Grid Faults

In the traditional DFIG configuration, the stator is directly connected to the grid. An abrupt drop
of the
grid voltage will directly transmit to the stator terminals of DFIG. According to the law of
constant flux, additional negative=sequence and transient DC flux components in the stator flux
will appear to guarantee the total stator flux constancy. Therefore, under unbalanced grid faults,
the generator stator flux will contain positive=sequence, negative-sequence and transient DC flux
components. The relationship of each component between stator flux and stator voltage can be
expressed as



                                                 (37)

where subscripts 1, 2, DC denote positive=, negative-and zero= (DC) sequence components in the
flux or voltage vectors, respectively. Ψs, ω0 and τs are the stator flux vector, synchronous
                                                                 '
angular speed and stator time constant, respectively. Us and U s are the stator voltage vectors
                                     '
just before and after the grid fault. u s represents the stator voltage vector after the grid fault.
Each component of the stator flux as shown in (1) will induce a voltage in the rotor according to
its amplitude and its relative speed with respect to the rotor. The induced rotor voltages can be
expressed in the stationary reference frame as [18]



                                         (38)

where Lm, Ls, ur and ωr are the mutual inductance, stator self inductance, rotor voltage vector
and rotor speed, respectively.
Substituting (1) into (2), the induced rotor voltages can be expressed in the rotor reference frame
as Table 1 Amplitudes of stator flux components and their induced voltages in the rotor side
under phase-to-phase grid fault.




                                                269
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME




                                              (40)

where s is the slip and superscript r denotes the rotor reference frame. Table 1 shows the
approximate amplitudes of each stator flux component and their induced voltages in the rotor side
under phase=to=phase grid fault. The symbol p denotes the depth of the grid fault. As can be seen
in Table 1 and (3), relative large voltages are induced in the rotor side. For the worst case (s=–0.3
and p=1), the maximum voltage amplitudes induced by the negative=sequence and the transient
DC flux components can reach 1.15pu and 1.3pu, respectively. Considering that the rotor
resistance and the transient inductance are usually very small, if the RSC cannot generate a
voltage equal to the sum of the three voltages induced by the stator flux components, the RSC
will lose current control transitorily and rotor over=current will appear. Fig. 2 shows the
simulation results of a 2MW traditional DFIG detailed in the Appendix under phase=to=phase
grid fault at PCC. The machine initially operates with full load and is at 30% super=synchronous
speed. The control scheme implemented in the simulation model is the vector control algorithm to
achieve the decoupling control of stator active and reactive powers and to keep the common dc
link voltage constant [4]. No additional action or control limit is included to constrain the fault
current. As shown in Fig. 2, due to the RSC cannot generate a voltage to counteract the induced
voltage by the stator flux components, high rotor over=current appears and further causes severe
oscillation of the electromagnetic torque.
Thus, to avoid the appearance of rotor over=current and guarantee the DFIG system to ride
through the grid faults, the stator flux each component should be effectively restricted, especially
for the negative= sequence and the transient DC flux components due to the relative speeds of
which with respect to the rotor windings are rather high. Besides, the severe torque oscillation,
which will increase the fatigues on the mechanical components of WTGS, should also be
considered.


                                                270
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

3.2 Principle for Suppressing the Rotor Over- Current
As shown in Fig. 1, a SGSC is introduced between the grid and the stator terminals of DFIG. The
generator’s stator terminal voltage us becomes the sum of the grid voltage vector ui and SGSC
series injected voltage vector ug which can be expressed in the stationary reference frame as


                      (41)
         Equation (41) indicates that the generator’s stator terminal voltage can be changed due to
the existence of the SGSC. Also, the transition of the generator stator flux imposed by the stator
terminal voltage during the grid faults can be changed through the output voltage of SGSC. As
analyzed in Section 3.1, under unbalanced grid faults, the negative-sequence and the transient DC
flux components will induce relative large voltages in the rotor side. If the SGSC can generate a
voltage to counteract the negative-sequence and the transient DC flux components in the stator
side, the rotor over=current caused by these two components can be eliminated. On the other
hand, if the DFIG equivalent mechanical output power Pmec imported from the rotor shaft is still
large, an abrupt drop of the grid voltage positive=sequence component will cause the increase of
the positive=sequence current which also maybe cause the rotor over=current. So the positive-
sequence current should also be restricted.
         On the other hand, if the DFIG equivalent mechanical output power Pmec imported from
the rotor shaft is still large, an abrupt drop of the grid voltage positive=sequence component will
cause the increase of the positive=sequence current which also maybe cause the rotor
over=current. So the positive= sequence current should also be restricted. Neglecting the
impedance of the series transformer, the electromagnetic power of DFIG can be written as



                                      (42)
Splitting the terms Rr/s and ur /s into Rr+(1-s)R r /s and ur /s+(1-s)ur /s, (42) can be rewritten as




                                        (43)
In (43), the first term is the power loss in the rotor windings. The third term is the power input
from the excitation power supply to the rotor side. The remaining terms are the DFIG equivalent
mechanical output power Pmec imported from the rotor shaft, which can be expressed as




                                          (44)
Fig. 3 shows the equivalent circuit of DFIG with Rs, Xls, Rr, Xlr, im and Xm the stator
resistance and leakage reactance, rotor resistance and leakage reactance, magnetizing current and
reactance based on (42)-(44). All the quantities in Fig. 3 are referred to the stator side.




                                                 271
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME




Neglecting the power loss in the DFIG, the stator active power output Ps is approximately equal
to the electromagnetic power Pem. If Ps is controlled to zero, Pem is also approximately equal to
zero. So do both the rotor active power output Pr and the DFIG equivalent mechanical output
power Pmec imported from the rotor shaft according to (5) and (7). Also, if Pem is zero, the
relative positive=sequence stator and rotor currents can also be restricted according to (5), which
can increase the possibility of the DFIG system to ride through the grid faults. It is also important
to note that the rest power will be stored in the mechanical inertia of the wind power generation
system, causing the speedup of the DFIG. If Pmec is reduced to zero in the super=synchronous
speed operation, the energy absorbed by the turbine rotor for a 2MW wind turbine system with a
grid fault is approximately equal to


                                            (45)
where t is the duration of the grid fault. The relationship between the energy and generator
speed can be expressed as [19].



                                (46)
where J is the moment of inertia, and ω1 and ω2 are the speed of generator before the grid faults
instant and after the recovery voltage instant, respectively. Also, the relationship between the
moment of inertia J and the inertia constant H is




                       (47)
wherePnom is the rated power of generator. Assuming the speed of generator before the grid fault
is 1.3pu. According to (8)=(10), and with the parameters provided in the appendix, the calculated
speedup of the generator with a 150ms grid fault is approximately 31r/min, which is about 1.5%
of the generator speed. Thus, the speedup of generator is quite small, and the scheme of
controlling the DFIG active power output Ps to zero is feasible during a short-time grid fault.
However, for a longer time grid fault, the emergency pitch angle regulation scheme should be
used to reduce the incoming torque. Otherwise, over=speed will occur and the over=speed
protection will trip the turbine.

3.3 Active Power Flow of DFIG System with SGSC under Grid Faults
        Fig. 4 shows the active power flow of the DFIG system with SGSC under grid faults
during super- synchronous speed operation. As shown in Fig. 4, if the output power is still large
enough during the grid faults, the surplus active power will flow into the common dc link.
Meanwhile, the active power from the RSC to the common dc link is also quite large.
Considering the limitation of the PGSC rating, these active powers cannot be delivered to the grid
through the PGSC. The common dc link voltage will rise sufficiently and exceed its safe

                                                272
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

limitation. Therefore, during the grid faults, the active power output of DFIG should be scaled
with the remaining voltage or reduced to an even lower value to protect the dc link capacitor.
Otherwise, an additional dc link crowbar is needed. Thus, the reduction of DFIG active power
output not only increases the possibility of the DFIG system to ride through the grid faults, but
also can effectively protect the dc link capacitance.
         Note that when the DFIG system operates in the sub=synchronous speed, the surplus
active power through the SGSC to the common dc link can be partially or completely consumed
by DFIG through RSC. Therefore the regulation stress of the PGSC in the sub=synchronous
speed is smaller than the one in the super=synchronous speed. Similarly, when DFIG system
operates in the synchronous speed, although the active power from the SGSC cannot be
consumedby DFIG through RSC anymore, the active power from the RSC to the common dc link
is zero. So the regulation stress of the PGSC in the synchronous speed is also smaller than the one
in the super= synchronous speed. So the super=synchronous speed operation condition is the
worst case scenario for thePGSC to keep the common dc link voltage constant. And for this
reason, we only show the simulations of DFIG in the super-synchronous speed operation for the
sake of brevity in Section 5.




4 Proposed Control Scheme for Grid Unbalanced Faults Ride-through

         During normal grid condition, the RSC and PGSC are controlled in conventional manners
to achieve the decoupling control of stator active and reactive powers and to keep the common dc
link voltage constant, respectively [4]. And for the SGSC, it is maintained in a zero voltage vector
switch state to eliminate harmonic losses as shown in [14, 17]. During the severe unbalanced grid
faults, Flannery and Venkataramanan [17] proposed that the negative-sequence component of the
stator flux is kept at zero to reject the undesirable effect to the rotor side while the
positive=sequence component of the stator flux is to scale in proportion to the positive-sequence
component of the remaining grid flux to eliminate the transient DC flux component and to
restrain the surplus active power flowing into the common dc link. As a result, in order to avoid
the appearance of the transient DC flux component and provide the required system response, the
positive=sequence stator flux controller must be carefully tuned. On the other hand, since the
transient DC flux component is not considered as the control target to be restricted, once the
transient DC flux component appears, it will only be damped with the stator time constant τs.
However, τs is relatively big (about a few seconds) for a MW=class DFIG, thus the attenuation
speed of the transient DC flux component is rather slow.
         Therefore, in order to overcome the problems highlighted above, we propose that the
transient DC component is controlled in the negative=sequence stator flux controller to suppress

                                                273
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

the adverse effect on the rotor side and accelerate its attenuation and the positive=sequence stator
flux controller is only used to control the generator stator positive=sequence flux component to
match the remaining grid voltage. A detailed description of the proposed control scheme of DFIG
system with SGSC for the grid unbalanced faults ride=through is given as follows: For RSC and
PGSC, the control schemes are stillthe same as the normal grid conditions. But for the RSC, the
active power output command of DFIG is set to zero to increase the possibility of the DFIG
system to ride through the grid faults and effectively protects the dc link capacitance as
mentioned in Section 3. For the PGSC, an area of concern with unbalanced grid faults is the
stabilization of the common dc link voltage. This was checked in simulation and it was found that
the grid-side converter could be controlled to avoid over=current during the unbalanced grid
faults [11, 20]. Also, with other improved control scheme of PGSC such as the one mentioned in
[21] the common dc link voltage can be more readily stabilized. We have focused our
investigations on the control scheme of SGSC, and the control scheme of PGSC isn’t discussed
too much for the sake of brevity. The SGSC is activated to control the each component of stator
flux as shown in Fig. 5 when a grid fault is detected.
         Fig. 5 shows the proposed controller of SGSC including three parts: 1) DFIG stator flux
estimation and decomposition; 2) Generation of stator positive= sequence flux component
command and the setting of stator negative=sequence and transient DC flux components
command; 3) Implementation of SGSC output series voltage control. The generator’s total stator
flux can be calculated as follows using the measured stator terminal voltage and current in the
stationary reference frame


                                       (48)
        Based on the analysis in Section 3.1, Ψs contains Ψs1, Ψs2 and ΨsDC. In the positive
reference frame (PRF) rotating at the speed of ω0, Ψs1 will be changed to a dc component while
Ψs2 and ΨsDC will be changed to ac components at the frequencies of 2ω0 and ω0, respectively.
Similarly, in the negative reference frame (NRF) rotating at the speed of –ω0, Ψs2 will be
changed to a dc component while Ψs1 and ΨsDC will be changed to ac components at the
frequencies of 2ω0 and ω0, respectively. Thus, Ψs in the PRF and NRF can be expressed as




                                               (49)
          Applying two notch=filters [16] at 2ω0 and ω0 to remove the negative=sequence and
transient DC flux components, the positive=sequence flux component in the PRF can be obtained.
Analogously, applying a notch=filter at 2ω0 to remove the positive=sequence flux component in
the NRF, the negative=sequence and the transient DC flux components can be obtained. Thus,
after filtering, the flux components in the PRF and NRF reduce to




                                        (50)
        In addition, during unbalanced grid faults, the grid flux Ψg (including stator ohmic
drop) can be expressed in the PRF as




                                                274
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME




                         (51)
Equation (16) indicates Ψg also contains positive-sequence, negative=sequence and transient DC
flux component, which can be expressed in the PRF as


                                      (52)
Similarly, applying two notch-filters at 2ω0 and ω0 to remove the negative=sequence and
transient DC flux components, the positive-sequence component of the grid flux can be obtained.
This is the command for the positive-sequence stator flux component controller, viz.:


                 (52)
For the negative=sequence and transient DC flux components, they are undesirable components
in the stator flux. Thus the command for these two flux components is zero, viz.:


                (53)
The proportional regulators are driven by the error between the command and their respective
estimated stator flux components expressed in (14) and (15). Note that the command for the
positive=sequence stator flux component controller has some important characteristics which are
summarized as follows: 1) Equation (18) also can indicate the grid flux during normal operation.
Thus when the grid fault is cleared, this command can bring the stator flux back to the normal
value. 2) A reduction of the stator positive=sequence flux component also causes a reduction of
corresponding induced rotor voltage. This means that there is more rotor voltage margin to
generate a voltage to control the stator active power output of DFIG to zero and to counteract the
remaining negative=sequence and transient DC flux components which cannot be rejected
completely by SGSC due to the proportional regulator.




1) Positive andNegativeComponentsCalculation: The positive and negative sequence components
calculation is done by using the Clarke transformation, rotating either ejωet or e−jωet, and finally,
applying a notch-filter at 2ωe to eliminate the opposite sequence. The technique is exemplified in
Fig. 4. For the rotor voltages and currents, the rotation applied is either ej (ωe −ωr )t or ej (−ωe −ωr )t.
2) ReferenceOrientation: The rotating references have been aligned with the stator voltage so that
vpsq =0. Nevertheless, vpsq has not been substituted in previous expressions for the sake of
describing general results. Orientation may be done computing the required θ0 assuming a
constant ωe or using a PLL [21] to determine both ωe and θ0 .




                                                   275
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME




3) Output Voltage Calculation
The output voltages calculation is done by summing the resulting positive and negative sequence
voltages in the stationary reference frame. For the line side


                                 (54)
Fortherotorside


                                             (55)
The resulting voltages are limited according to the converter rating. The final voltages can be
applied using standard space vector pulse width modulation (SVPWM) techniques. The technique
is exemplified for the rotor-side converter case in Fig. 5.
        5 SIMULATION RESULTS
In order to evaluate the ride=through capability of the proposed control scheme, the system has
been simulated on a 2MW DFIG wind turbine with SGSC.
In order to evaluate the ride-through capability of the proposed scheme, the system has been
simulated with severe voltage sags, making the control work at the maximum output voltage and
dealing with the triggering of the crowbar protection. The system under study is a 2- MW DFIG-
based wind turbine, where a two-phase 50% type E [22] voltage sag of 2s has been applied. The
data of the simulated system may be seen in Table I. In order to compare the presented control
scheme and some existing techniques, the following three cases have been studied.
In this study, there are two cases analyzed, 1st is without controller and 2nd is with controller
techniques.
Fig. 6 shows the simulation results of the DFIG system for double phase to groundfault near
PCC. It is of unbalanced grid faults double phase-to-ground fault. As analyzed in [22], the
propagation of voltage dips through different types of transformer connections results in a
different performance of voltage dips on the secondary side of the transformers. The
differentvoltage dips types through a delta/Yg transformer seen at the DFIG stator terminal
compared to all types of unbalanced grid faults at PCC are shown in Fig6. The proposed control
schemes have been evaluated by means of simulations with one balanced and one unbalanced
voltage sags.

Unbalanced voltage sag

A50% voltage sag has been applied to two phases leaving the third phase undisturbed. The
disturbance has been analyzed in a 2MW wind turbine was generating with a wind of 20m/s.The
DFIG stator and rotor side voltages are shown in Fig 6 without controller. There is a fault
between 0.1 to 0.2sec. The generator torque and DC bus voltage response are illustrated in Fig. 7
and 12. It can be seen that although the inverse sequence provokes an oscillating flux, an almost
constant torque can be achieved after a transient. As it has been stated, the constant torque implies
oscillating rotor power which can be compensated with oscillating grid-side converter power.

                                                276
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

Using the proposed technique, the resulting DC bus voltage has minimized the oscillations.
The stator voltages in positive and negative sequence and the abc stator voltages and currents are
illustrated in Fig. 8. Active and reactive power are illustrated in Fig. 9. It can be seen that while
the total power (depending on the torque) is almost constant, stator and rotor active power are of
oscillating nature.




Fig 6 DFIG stator and rotor side voltages




Fig 7 Torque response to aun-balanced voltage sag




Fig 8 Stator voltage in positive and negative sequence




                                                277
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME




Fig 9Rotor, stator and total active power




Fig 10 stator reactive power
When comparing figures 6 and 11, during a double phase to ground fault, there has been severe
change in the voltage waveforms. Without controller, the system after post-fault is very unstable
as seen in Fig 6, voltage has never reached its rated value amd is decreasing in nature. But in Fig
11, which is same system with same fault conditions, two unhealthy phases got disturbed, and the
system after post- fault is very stable. It has reached its pre fault state very immediately.
In Fig 8, positive and negative sequence components were shown. The positive sequence voltage
has changed very drastically and there been some surges for negative sequence components. It
can be observed from Fig 9 and 10, during the fault conditions, the real and reactive powers were
disturbed very drastically. The real power from rotor, stator and the composition of both were
changed as in Fig 9. I this, rotor real power which is nearly -10KW, has become oscillating and is
sinusoidal during the fault and many oscillations in stator real power.
It can also be identified that, voltage harmonics are very high, it means, there is some distortions
in voltage waveforms even under steady-state conditions.
In Fig 12, the DC voltage and grid voltage waveforms were shown. During the fault, excess
voltage is reached to dc supply and is stored in the capacitor bank. It can also be identified that,
with the proposed controller, the stator voltage and rotor voltage phase sequence is changed as in
Fig 11, there-by maintaining nearly constant voltage near the grid.




                                                278
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME




Fig 11 DFIG stator and rotor side voltages




Fig 12 C voltage and grid voltages

CONCLUSIONS

The present chapter has presented presents a control technique for doubly fed induction
generators under different voltage disturbances. The current referenceis chosen in the positive and
negative sequences so that the torque and the DC voltage are kept stable during balanced and
unbalanced conditions. Both rotor-side and grid-side converters have been considered, detailing
the control scheme of each converter while considering the effect of the crow-bar protection. The
control strategy has been validated by means of simulations for balanced and unbalanced voltage
sags.
  In this paper the modeling of a DFIG, considering the behavior of the generator when transients
in the stator voltage occur, was developed. Thanks to this model, that permits to predict the
performance of a DFIG under faulty scenarios, a novel control strategy for the rotor side
controllers, oriented to enhance its response during severe voltage sags, was proposed. This
strategy is based on using the measured stator current values as the set-point for the rotor current

                                                279
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

controller during the fault. As it has been demonstrated, in this way, it is possible to synthesize a
current in the stator in opposition to the currents generated during the fault, preventing thus the
stator/rotor windings from suffering over-currents, with no need of using crowbar circuits.
 The performance of this strategy was tested and compared with and without control algorithm,
already presented in previous works, which makes the active and reactive power references equal
to zero during the fault. The analytical equations and the simulation models based on MATLAB
have shown that the fluctuations in the stator and rotor currents, as well as in the electromagnetic
torque were reduced by the half, when using the new proposed strategy in case of severe balanced
and unbalanced voltage sags at the PCC.
Therefore the results presented in this paper, show that it is possible to control the stability of a
DFIG during severe contingencies in the power network, without the need of external auxiliary
circuits.

REFERENCES

[1] C. Luo, H. Banakar, S. Baike, and O. Boon-Teck, “Strategies to smooth wind power fluctuations of
wind turbine generator,” IEEE Trans. Energy Convers., vol. 22, no. 2, pp. 341–349, Jun. 2007.
[2] M.Kayikci and J.Milanovic, “Reactive power control strategies forDFIG-based plants,” IEEE Trans.
Energy Convers., vol. 22, no. 2, pp. 389–396, Jun. 2007.
[3] C. Eisenhut, F. Krug, C. Schram, and B. Klockl, “Wind-turbine model for system simulations near cut-
in wind speed,” IEEE Trans. Energy Convers., vol. 22, no. 2, pp. 414–420, Jun. 2007.
[4] K. Seul-Ki and K. Eung-Sang, “PSCAD/EMTDC-based modeling and analysis of a gearless variable
speed wind turbine,” IEEE Trans. Energy
Convers., vol. 22, no. 2, pp. 421–430, Jun. 2007.
[5] R. Pena, J. J. C. Clare, and G. Asher, “Doubly fed induction generator using back-to-back PWM
converters and its application to variable-speed wind-energy generation,” Proc. Inst. Elect. Eng. Electric
Power Appl., vol. 143, no. 3, pp. 231–241, 1996.
[6] P. Ledesma and J. Usaola, “Doubly fed induction generator model for
transient stability analysis,” IEEE Trans. Energy Convers., vol. 20, no. 2,
pp. 388–397, Jun. 2005.
[7] Y. Lei, A. Mullane, G. Lightbody, and R. Yacamini, “Modeling of the
wind turbine with a doubly fed induction generator for grid integration studies,” IEEE Trans. Energy
Convers., vol. 21, no. 1, pp. 257–264, Mar. 2006.
[8] D. Xiang, L. Ran, P. Tavner, and S. Yang, “Control of a doubly fed induction generator in a wind
turbine during grid fault ride through,” IEEE Trans. Energy Convers., vol. 21, no. 3, pp. 652–662, Sep.
2006.
[9] J. Morren and S. de Haan, “Ridethrough of wind turbines with doubly fed
induction generator during a voltage dip,” IEEE Trans. Energy Convers.,
vol. 20, no. 2, pp. 435–441, Jun. 2008.
[10] J. Morren and S. W. H. de Haan, “Short-circuit current of wind turbines
with doubly fed induction generator,” IEEE Trans. Energy Convers.,
vol. 22, no. 1, pp. 174–180, Mar. 2007.
[11] A. Junyent-Ferr´ e, A. Sumper, O. Gomis-Bellmunt,M. Sala, andM.Mata,“Digital simulation of
voltage dip characteristics ofwind turbine systems,”in proc. 9th Int. Conf. Elect. Power Quality Utilization,
Barcelona, Spain,2007, pp. 1–6.
[12] P. Rioual, H. Pouliquen, and J.-P. Louis, “Regulation of a PWM rectifierin the unbalanced network
state using a generalized model,” IEEE Trans.Power Electron., vol. 11, no. 3, pp. 495–502, May 1996.
[13] H.-S. Song and K. Nam, “Dual current control scheme for PWMconverterunder unbalanced input
voltage conditions,” IEEE Trans. Ind. Electron.,vol. 46, no. 5, pp. 953–959, Oct. 1999.
[14] L. Xu and Y.Wang, “Dynamic modeling and control of DFIG-based windturbines under unbalanced
network conditions,” IEEE Trans. Power Syst.,vol. 22, no. 1, pp. 314–323, Feb. 2007.




                                                    280
International Journal of Electrical Engineering and Technology (IJEET), IS  ISSN 0976 –
                                                         January-June
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January June (2012), © IAEME

                                                “Ride-through                                     wind
[15] S. Seman, J. Niiranen, and A. Arkkio, “Ride through analysis of doublyfed induction wind-power
              er                          disturbance,”
generator under unsymmetrical network disturbance,” IEEE Trans. Power Syst., vol. 21, no. 4, pp. 1782–  1782
1789, Nov.2006.
                                       “Control
[16] T. K. A. Brekken and N. Mohan, “Control of a doubly fed induction windgenerator under unbalanced
                                                                          129–135,
grid voltage conditions,” IEEE Trans. EnergyConvers., vol. 22, no. 1, pp. 129 135, Mar. 2007.
[17] R. Pena, R. Cardenas, E. Escobar, J. Clare, and P. Wheeler, “Controlsystem for unbalanced op   operation
         alone                       generators,”                                                    544
of stand-alone doubly fed induction generators,” IEEE Trans. Energy Convers., vol. 22, no. 2, pp. 544–545,
Jun.2007.
                 Ferr´
[18] A. Junyent-Ferr´ e, “Modelitzaci´ o i control d’un sistema de generaci´ oel` ectrica de turbina de vent,”
                          UPC,
Master’s thesis, ETSEIB-UPC, Barcelona,
Spain, 2007.
[19] L. Harnefors and H.-P. Nee, “Model based current control of ac machines using the
                                P.         “Model-based
                                                                          133–141,
internalmodelcontrolmethod,” IEEE Trans. Ind. Appl., vol. 34, no. 1, pp. 133 141, Jan./Feb. 1998.
                                     “Fault
[20] A. D. Hansen and G. Michalke, “Fa ride-through capability of DFIG
                                                    1594–1610, Jul. 2007.
wind turbines,” Renew. Energy, vol. 32, no. 9, pp. 1594
                              Bergas-Jan´ e, J. Candela, R. Burgos, andD. Boroye-vich, “Decoupled double
[21] P. Rodriguez, J. Pou, J. Bergas                                                vich,
synchronous reference frame pll for power converters control,” IEEE Trans. Power Electron., vol. 22, no. 2,
pp. 584–592,
Mar. 2007.
[22] M. H. Bollen, Understanding Power Quality Problems: Voltage Sags and Interruptions. Piscataway,
NJ: IEEE Press, 2000.
                                               data
[23] Texas Instruments Inc., “TMS320F2812 d manual,” Tech. Rep.SPRS174O, 2007.


                                       Mohammed,M.Tech. candidate, Electrical Engineering,
                     1. Mis. Nadiya G. Mohammed
                     DepBharatiVidyapeeth University, College of Engineering. Pune, India,
                     mmm.nadiya@yahoo.com




                           HaiderMuhamadHusen,M.Tech. candidate, Electrical Engineering,
                    2. Mr. HaiderMuhamadHusen
                    DepBharatiVidyapeeth University, College of Engineering. Pune, India,
                    haider_mhu@yahoo.com




                                                                   (Electrical),
                    3. Prof . D.S. Chavan : Ph.D. (Registered), ME (Electrical), BE (Electrical),
                    DEE Associate Professor, Co                               ordinator
                                                Co-ordinator (R&D cell),Co-ordinator (PH.D.
                    Programme management) BharatiVidyapeeth Deemed University College Of
                    Engineering Pune 411043. He is pursuing Ph D. He received ME
                                                Achieved          icate
                    (Electrical)(Power systems) Achiev rank certificate in Pune University for
                    ME       greenearth1234@yahoo.com




                                                    281

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:19
posted:11/23/2012
language:
pages:21