E.Rajendran et al. / International Journal of Engineering Science and Technology (IJEST)
LVRT SCHEME OF WIND ENERGY
SYSTEM USING PERMANENT
MAGNET SYNCHRONOUS
GENERATOR AND HYSTERESIS
CURRENT CONTROLLER
E.RAJENDRAN
Research Scholar, Anna University, Chennai, TN, India,
Assistant Professor, Department of Electrical & Electronics Engineering,
S K P Engineering College, Tiruvannamalai, Tamil Nadu, India,
E-mail: rajendran_electra@yahoo.com.
Dr.C.KUMAR
Academic Director, Senior Member IEEE,
Department of Electrical & Electronics Engineering,
S K P Engineering College, Tiruvannamalai, Tamil Nadu, India,
E-mail: drchkumararima@gmail.com.
G.PONKUMAR
ME Power system, Department of EEE
S K P Engineering College, Tiruvannamalai, Tamil Nadu, India,
E-mail:ponkumar21@gmail.com.
Abstract :
In this research paper provided the information about low-voltage ride-through (LVRT) scheme for
the permanent magnet synchronous generator (PMSG), and wind energy conversion system. The dc-link voltage
is uncomfortable by the generator side converter instead of the grid-side converter (GSC). Considering the
nonlinear correlation between the generator speed (ωm) and the dc-link voltage (Vdc), a dc-link Voltage
controller is anticipated using a hysteresis current controller. Among all, low-voltage ride-through has been
fundamental in the field, which is one of the most important challenges for wind energy conversion system. It is
essential to design an included controller to protect the converter from overvoltage/overcurrent and to support
the grid voltage during faults and recoveries. A unified dc-link voltage control scheme and hysteresis current
controller based wind energy conversion system is proposed. The controllers for grid-side converters are
coordinated to provide fault ride-through capability. The generator side is forced by space vector modulation
and grid side implemented hysteresis current controller. The Grid Side Controller controls the grid active power
and maximum power deliver to the grid. The ability of this control algorithm has been confirmed by simulation
results.
Keywords: Permanent Magnet Synchronous Generator (PMSG), Low -Voltage Ride-Through (LVRT), Grid
Side Converter (GSC). Wind Generator (WG), Wind Energy Conversion System (WECS), Hysteresis Current
Controller and DC-link voltage.
1. Introduction
Among various renewable energy sources, the wind power generation has been restless as one of the
largely rapidly rising energy sources. another way from the doubly fed induction generator (DFIG) wind
systems, a direct-drive wind energy conversion system depends on PMSG has a lot of advantages such as
reduced size in gearbox, soaring power density, largest precision, and easy control method, apart from initial
installation costs. As the scale of wind farms becomes larger and larger, the grid connection condition of the
wind turbine is more important. Recently, some countries have issued the dedicated grid codes for connecting
the wind turbine system to the electric grid. Also, the micro grid and the smart grid have been researched for the
efficient power management. However, in these systems, the grid voltage is much fluctuated in relationship with
the conventional one. In this proposed low- voltage ride-through control of the wind power generation system is
required for the grid irregular conditions.
The grid codes require the LVRT ability of the wind turbine system. For some national grid codes, the
wind power systems should keep on linked to the grid for the grid fault conditions. In the power system where
ISSN : 0975-5462
Vol. 5 No.04 April 2013
757
E.Rajendran et al. / International Journal of Engineering Science and Technology (IJEST)
the wind power generation is of a major portion, the grids will provided the information the power outage if the
wind farms trip off. Nowadays the number of solutions has been planned for the LVRT scheme of the variablespeed wind turbine systems. For this purpose, a crowbar System is linked in the rotor side of the DFIG to attract
the active power during the grid fault. The wind turbine continues its operation to manufacture the active power,
whereas the reactive power or the voltage at the point of common coupling (PCC) is controlled by the grid-side
converter (GSC). But, in the case of a weak grid and during a grid fault, the grid side converter cannot offer
sufficient reactive power or voltage support due to its small power capacity and the risk of voltage instability.
Also, a static synchronous compensator (STATCOM) has been used to guarantee the uninterrupted operation of
a DFIG wind turbine during the grid faults. The reactive power is injected to the grid by the STATCOM that is
installed at the PCC.
However, the STATCOM is not used alone for the DFIG ride through capability ever since it cannot
defend the rotor-side converter through a grid fault. On the other word, it should subsist used mutually with the
crowbar circuit which protects the rotor side converter from the rotor overcurrent when the grid fault happens.
For the PMSG wind turbine systems, a braking chopper (BC) with the low cost benefit and the easy control
concert has been applied for the LVRT. Conversely, it is complicated to enhance the power quality at the output
of the wind turbine systems seeing as the BC can just disperse the power. In other way, the STATCOM has been
applied to keep the wind turbine system connected to the grid during grid faults. With this method, the voltage
regulation is a lot enhanced in the transient state as well as in the steady state. However, the STATCOM has to
be used together with the BC.
In the PMSG wind turbine system, the generator is associated to the grid throughout a full-scale backto-back pulse width modulated converter and hysteresis current control, of which configuration is used. The dclink voltage is embarrassed by the grid source converter. However, the grid source converter may be out of
control in the case of the voltage sags in grid. At a grid fault, the dc-link voltage is improved excessively since
the wind turbine continues to generate the power but the grid cannot absorb the generated powerfully. In this
research, we have to inspect the maintaining the dc link capacitor at grid side converter is constant by using
hysteresis current controller.
2. Related Researches: A Review
Some of the recent researches related to power quality improvement for wind enegy conversion
systems using Hysteresis current control are discussed.
M. Chinchilla et al. [1] have proposed wind energy is a prominent area of application of variablespeed generators operating on the constant grid frequency. This paper describes the operation and control of one
of these variable-speed wind generators: the direct driven permanent magnet synchronous generator (PMSG).
This generator is connected to the power network by means of a fully controlled frequency converter, which
consists of a pulse width-modulation (PWM) rectifier, an intermediate dc circuit, and a PWM inverter. The
generator is controlled to obtain maximum power from the incident wind with maximum efficiency under
different load conditions. Vector control of the grid-side inverter allows power factor regulation of the windmill.
This paper shows the dynamic performance of the complete system. Different experimental tests in a 3-kW
prototype have been carried out to verify the benefits of the proposed system.
J.-H. Jeon et al.[2] have suggested the ability of the wind power plant to stay connected during grid
disturbances is important to avoid a cascading effect due to lack of power. Making it necessary to introduce new
code of practice, the grid operators require that wind turbines stay connected to the grid during voltage dips.
Low Voltage Ride through has emerged as a new requirement that system operators demand to wind turbines.
This paper analyzes the extent to which the LVRT capability of wind farms using squirrel cage generators can
be enhanced by the use of a Static Synchronous Compensator STATCOM. The ability of wind farms to stay
connected to grid during LVRT is investigated based on E-ON NETZ grid code. A simulation model of 9 MW
wind farm interconnected grid is carried out using the MATLAB Simulink Power Systems toolbox.
F. K. A. Lima et al. [3] have proposed a a new control strategy for the rotor-side converter of wind
turbines based on doubly fed induction generators that intends to improve its low-voltage ride through
capability. The main objective of this work is to design an algorithm that would enable the system to control the
initial over currents that appear in the generator during voltage sags, which can damage the RSC, without
tripping it. As a difference with classical solutions, based on the installation of crowbar circuits, this operation
mode permits to keep the inverter connected to the generator, something that would permit the injection of
power to the grid during the fault, as the new grid codes demand. A theoretical study of the dynamical behavior
of the rotor voltage is also developed, in order to show that the voltage at the rotor terminals required for the
control strategy implementation remains under controllable limits. In order to validate the proposed control
system simulation, results have been collected using PSCAD/EMTDC and experimental tests have been carried
out in a scaled prototype.
ISSN : 0975-5462
Vol. 5 No.04 April 2013
758
E.Rajendran et al. / International Journal of Engineering Science and Technology (IJEST)
3. Power Quality Improvement for WECS using PMSG and Hysteresis Current Controller
A few research results have been suggested that employ the dc-link voltage control strategies by the
generator-side converter instead of the GSC. Wind generator has been extensively used both in independent
systems for power supplying remote loads and in grid-connected application. The WG power production can be
mechanically proscribed by altering the blade pitch angle. This topology is based on the WG optimal power
versus the rotating-speed feature, which is frequently stored in a microcontroller memory. The WG turning
speed is calculated; the optimal output power is intended and compared to the actual wind generator output
power. The ensuing error is used to manage a power boundary. In a similar edition create in the WG output
power is calculated and the target rotor speed for optimal power generation is resulting from the WG optimal
power versus rotor-speed characteristic.
The target rotor speed is compared to the actual speed, and the error is used to control a dc/dc power
converter. The control algorithm has been executed in Lab VIEW running on a personal computer (PC). In
permanent magnet wind generator systems, the output current and output voltage are relative to the
electromagnetic torque and rotor speed, correspondingly. In the rotor speed is calculated according to the
measured WG output voltage, while the optimal output current is calculated by means of an estimate of the
current versus the rotational-speed most favorable feature. The error ensuing from the judgment of the deliberate
and the actual current is used to control a dc/dc converter. The disadvantage of all above methods is that they are
based on the understanding of the WG optimal power characteristic, which is typically not obtainable with a
high degree of correctness and also change with rotor aging. one more approach by means of a two layer neural
network updates online the pre programmed WG power feature by perturbation of the control signals just about
the values provided by the power feature.
3.1. Wind Energy Conversion System
Fig. 1. Block Diagram of Proposed Wind Energy Conversion system.
The figure 1. is being the control roles of the two converters, the dc-link voltage can be restricted to be
constant by swelling the generator speed during the grid voltage sag. However, the dc-link voltage reaction is
not high-quality even though a hybrid adaptive proportional–integral (PI) controller is used based on the power
and energy relationship. On the other hand, a hysteresis current controller using an input–output feedback has
been applied to various areas such as the dc-link voltage control of Space vector PWM converters and the output
voltage control of three-phase uninterruptible power system (UPS) inverters for the high dynamic responses.
This hysteresis current controller technique has been applied to the dc link voltage control for the PMSG wind
turbine system. In the first place, a nonlinear relationship between the generator speed and the dc-link voltage is
derived in below, where the dc-link voltage is chosen as output. Then, by applying the hysteresis current control,
coordination is obtained and then the dc-link voltage controller can be designed by the classical linear control
theory. In the meanwhile, the power of the PMSG is controlled by the GSC. The GSC dual current controllers in
the positive and negative-sequence reference frames are employed for grid unbalanced conditions.
ISSN : 0975-5462
Vol. 5 No.04 April 2013
759
E.Rajendran et al. / International Journal of Engineering Science and Technology (IJEST)
3.1.1. SIMULINK DIAGRAM
Fig. 2. Simulink Diagram of Wind Energy Conversion System.
The above figure 2. Matlab/simulink model of Grid connected wind energy conversion system using
permanent magnet generator and Hysteresis current controller is shown above figure. In the above simulation
diagram dc link voltage is controlled by using source voltage control is implemented. The dc link voltage
controller with low voltage ride through technique builds in subsystem. The GSC hysteresis current controllers
in the positive and negative-sequence reference frames are employed for grid unbalanced conditions. The
validity of the control algorithm has been verified by simulation results for the PMSG wind power system. The
grid voltage, and phase current are shown in below figure.
3.2. HYSTERIS CURRENT CONTROLLER
Fig. 3. Basic block diagram of CC-PWM converter.
The presentation of the converter system largely depends on the worth of the applied current control
strategy. Therefore, current control of PWM converters is one of the most essential subjects of present power
electronics. The block diagram of current controller procedure shown in the above figure 3.Whenever the
current vector touches the border of the surface, another voltage state is applied to force it back within the
square. Similarly, as in the case of the hexagon hysteresis control method, here the square acceptance band
ISSN : 0975-5462
Vol. 5 No.04 April 2013
760
E.Rajendran et al. / International Journal of Engineering Science and Technology (IJEST)
moves simultaneously with the reference current such that the current vector points always in the center of the
square. For this purpose two hysteresis comparators and components are employed. A simple consideration
makes it possible to control the current without any information about the load inner voltage.
Three main classes of regulators have been developed decades: Hysteresis regulators, linear PI
regulators and investigative dead-beat regulators. A short review of the accessible current control techniques for
the three- phase systems is offered in among the various pulse width modulation technique, the hysteresis
current control is a largely very often owing to its simplicity of execution. Also, moreover fast response current
loop, the method does not need any information of load parameters. However, the current control with a
hysteresis band has the drawback that the pulse width modulation frequency varies within a band owing to peakto- peak current ripple is essential to be prohibited at all points of the necessary frequency wave.
The method of adaptive hysteresis current control pulse width modulation technique where the band
can be planned as a function of load to optimize the pulse width modulation presentation is described in the
basic implementation of hysteresis current control is dependent on derive the switch the signals from the
similarity of the current error with a tolerance band. This control is dependent on the evaluation of the real phase
current with the tolerance band in the order of the reference current associated with that phase. On other words,
this type of band control is cynically reproduction by the phase current interactions which is typical in threephase systems. This is mainly due to the intervention between the commutations of the three phases, in view of
the fact that each phase current not only depends on the corresponding phase voltage but is also fake by the
voltage of the other two phases.
Depending on load conditions switching frequency may vary during the original period, resulting in
irregular inverter process. In the authors planned a novel technique that minimizes the effect of interference
between phases while maintaining the advantages of the hysteresis methods by using phase-locked loop (PLL)
technique to constrain the inverter switching at a pre determined frequency. In this paper, the current control of
PWM-VSI has been executed in the stationary orientation structure. The first method is based on prognostic
current control. Second method is based on space vector control using multilevel hysteresis comparators where
the hysteresis band appears as a hysteresis square.
3.2.1. Modeling and Controller Design of Hysteresis Current Controller
Fig. 4. Design of Hysteresis Current Controller.
The above figure 4. Three hysteresis bands of the width ± are starved of approximately each and every
reference value of the phase currents (ia; ib; ic). The goal is to stay the actual value of the currents restricted by
their hysteresis bands all the time. As the three currents are not self-governing from each other, the system is
unclear into coordinate system. By means of the transformation of the three hysteresis bands hooked on this
coordinate system, they result in a hysteresis hexagon area. I ref points that is the reference current points
vector toward the middle of the hysteresis what can be seen in steady state, the pour of the reference current
moves on circle in the area of the origin of the coordinate system . So, the hexagon gets moves on this circle too.
3.2.2. Structure of hysteresis current controller
The error of each and every phase current is prohibited by a two level hysteresis comparator, which is
shown in below figure (5). A switching logic is essential because of the combination of three phases.
ISSN : 0975-5462
Vol. 5 No.04 April 2013
761
E.Rajendran et al. / International Journal of Engineering Science and Technology (IJEST)
Fig. 5. Structure of Hysteresis Current Controller.
When the current error vectors ie touch the border of the hysteresis hexagon, the switch logic has to
decide next, the most best switching state according to the following:
1. The current discrepancy ie should be moved back towards the middle of the hysteresis hexagon as slowly as
possible to realize a low switching frequency.
2. If the angle of the current error ie is outside of the hexagon, it should be return in hexagon as fast as possible
(important for dynamic processes).
3.2.3. DC LINK CAPACITOR
Fig. 6. Dc Link Capacitor.
The above figure 6. The Dc link capacitor is major role of the proposed system. Because depends upon
the grid power level continuously tuned to adjust and maintains constant DC link capacitor voltage in all over
times.
DC link is modeled by relation:
CU dc/dt = idc* iL
(1)
Where
Idc- is DC link current
iL - Load current
i L = U dc
(2)
ISSN : 0975-5462
Vol. 5 No.04 April 2013
762
E.Rajendran et al. / International Journal of Engineering Science and Technology (IJEST)
3.2.4. Simulation of Hysteresis Current Controller
Fig. 7. Hysteresis Current Controller.
The above figure 7. Simulation diagram is done in matlab by using hysteresis current controller.
Using low voltage ride through technique, finding the grid current and voltage will be occurring in Sag.
Because, due to the any load demands or external disturbances may be occur. In this stage the dc link capacitor
voltage maintain at constant and GSC output power is adjust to maintain at constant. Otherwise varied the grid
power continuously and also power factor becomes lagging and entire system instability. Using the hysteresis
current control method, the grid side converter can adjust the inverter firing angle and maintain the power
quality in grid. Now the system becomes to working in stability mode of operation.
The grid side converter following advantages:
1. Control on the spot current waveform and high precision.
2. Hit the highest point current security.
3. Overload dismissal.
4. Extremely good dynamics responses.
5. Compensation of effects due to load parameter changes (resistance and reactance).
6. Compensation of the semiconductor voltage drop and dead times of the converter.
7. Compensation of the dc-link and ac-side voltage changes.
4. RESULT AND DISCUSSION
The above figure 2. PMSGs produced the AC power, in that AC input power is given to the rectifier.
Then the rectifier converts the AC input into DC output. Here rectifier controlled by using space vector pulse
width modulation technique. The DC output voltages are directly fed to the DC link capacitor circuit. The DC
link capacitor voltage buck-boost adjustment operation depends upon the required grid power level. This
process is done by generator side converter, using space vector pulse width modulation techniques. Then the DC
voltage is directly fed to the inverter circuit, for purpose of inverter converter DC to AC and then buck – boost
of the AC supply. Here inverter controlled by using Hysteresis current controller. The simulation outputs are
shown in below figures.
The below figure 8. Stand for in dc link capacitor voltage. Due to any load changes or external load
connected in the grid, the grid current will be suddenly changed. That time the capacitor voltage is unbalanced.
Using the hysteresis current control technique maintains the constant capacitor voltage and also maintains grid
required power to delivery in the load. The below figure 9. Current result is Space vector pulse width
modulation control of generator side converter. The inject power is obtained from the wind source. This is
attained from above simulation figure 2. The major propose work is to maintain and provided required power to
the dc link capacitor.
The below figure 10. Active power and reactive power injection result is during this time certain voltage
changes as well as Sag is occurred. This should be overcome by hysteresis current controller.
Because changing in the firing angles of inverter and tuned to controlled in the DC link capacitor voltage. So,
compensate the grid required active and reactive power. The below diagram 11. Correspond to the compensate
ISSN : 0975-5462
Vol. 5 No.04 April 2013
763
E.Rajendran et al. / International Journal of Engineering Science and Technology (IJEST)
waveform to the grid. If the reference currents and actual grid currents is to be compared, and then error should
be rectified by using PI controller. Finally, produce in the required grid power in the GSC inverter system. In
this inverter system triggered depends upon the error current basically.
Fig. 8. DC Link Capacitor Voltage.
Fig. 9. Generator Side Current Waveform.
Fig. 10. Active Power and Reactive Power Injection.
Fig. 11. Compensated Wave Form.
ISSN : 0975-5462
Vol. 5 No.04 April 2013
764
E.Rajendran et al. / International Journal of Engineering Science and Technology (IJEST)
5. Conclusion
In this paper, a PMSG-based WECS with dc link voltage is embarrassed by using hysteresis current
controller is proposed. Dc link capacitor is used for maximum power tracking control and delivering power to
the grid, concurrently. Compared to conventional WECS with feedback linearization technique, the number of
switching semiconductors is reduced by one and reliability of system is improved, because there is no
requirement for dead time in a low voltage ride though technique. For active power control methods, space
vector modulation method and Hysteresis current controller is proposed and compared. It is shown that with
feedback linearization method, constant of dc link voltage constant is increased only 7% compared to
conventional system, but there is more power fluctuations compared to source voltage control. With hysteresis
control, dc link voltage constant is increased based upon the substation requirement. Here grid power is
maintained at the rate of 100 amps, 440 volts and 20 amps, 440 volts. It was also shown that due to elimination
of dead time, reduction of harmonics is continued as well as conventional method.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
M. Chinchilla, S. Arnaltes, and J. C. Burgos, “Control of permanent magnet generators applied to variable-speed wind-energy systems
connected to the grid,” IEEE Trans. Energy Convers., vol. 21, no. 1, pp. 130– 135, Mar. 2006.
J.-H. Jeon, S.-K. Kim, C.-H.Cho, J.-B.Ahn, and E.-S. Kim, “Development of simulator system for micro-grids with renewable energy
sources,” J. Electr. Eng. Technol., vol. 1, no. 4, pp 409–413, Feb. 2006.
F. K. A. Lima, A. Luna, P. Rodriguez, E. H. Watanabe, and F. Blaabjerg, “Rotor voltage dynamics in the doubly fed induction
generator during grid faults,” IEEE Trans. Power Electron., vol. 25, no. 1, pp. 118–130, Jan.2010.
L. G.Meegahapola, T. Littler, and D. Flynn, “Decoupled-DFIG fault ridethrough strategy for enhanced stability performance during
grid faults,” IEEE Trans. Sustainable Energy, no. 3, pp. 152–162, Oct. 2010.
H. Polinder, F. F.Avan der Pijl, and P. Tavner, “Comparison of direct-drive and geared generator concepts for wind turbines,” IEEE
Trans. Energy Convers., vol. 21, no. 3, pp. 543–550, Sep. 2006.
F. Iov, A. D. Hansen, P. Sørensen, and N. A. Cutululis, “Mapping of grid faults and grid codes,” Risø Nat. Lab., Tech. Univ. Denmark,
Roskilde, Denmark, Tech. Rep. Risø-R-1617(EN), Jul. 2007.
C. Saniter and J. Janning, “Test bench for grid code simulations for multi- MW wind turbines, design and control,” IEEE Trans. Power
Electron., vol. 23, no. 4, pp. 1707–1715, Jul. 2008.
R. Pollin, H. G. Peltier, and H. Scharber, “Green recovery: A new program to create good jobs and start building a low-carbon
economy,” Center for American Progress, Washington, DC, Sep. 2008.
Q. Song and W. Liu, “Control of a cascade STATCOM with star configuration under unbalanced conditions,” IEEE Trans. Power
Electron., vol. 24, no. 1, pp. 45–58, Jan. 2009.
W. H. Zhang, S.-J. Lee, andM.-S. Choi, “Setting considerations of distance relay for transmission line with STATCOM,” J. Electr.
Eng. Technol., vol. 5, no. 4, pp. 522–529, Jul. 2010.
H. M. Pirouzy and M. T. Bina, “Modular multilevel converter based STATCOM topology suitable for medium-voltage unbalanced
systems,” J. Power Electron., vol. 10, no. 5, pp. 572–578, Sep. 2010.
G. Brando, A. Coccia, and R. Rizzo, “Control method of a braking chopper to reduce voltage unbalance in a 3-level chopper,” in Proc.
IEEE Int. Conf. Ind. Technol., 2004, vol. 2, pp. 975–978.
J. F. Conroy and R.Watson, “Low-voltage ride-through of a full converter wind turbine with permanent magnet generator,” IET
Renewable Power. Generation, vol. 1, no. 3, pp. 182–189, Sep. 2007
W. Li, C. Abbey, and G. Joos, “Control and performance of wind turbine generators based on permanent magnet synchronous
machines feeding a diode rectifier,” in Proc. IEEE Power Electron. Spec. Conf., Jun., 2006.
B. Singh, R. Saha, A. Chandra, and K. Al-Haddad, “Static synchronous compensators (STATCOM):Areview,” IET Power Electron.,
vol. 2, no. 4, pp. 297–324, 2009.
ISSN : 0975-5462
Vol. 5 No.04 April 2013
765