International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal.com/
1305
Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for
PMSG Wind Turbine Systems
A Hybrid Control Scheme for Fault Ride-Through
Capability using Line-Side Converter and an
Energy Storage System for PMSG Wind Turbine
Systems
S.Rajkumar1, S.T.Suganthi2
1 Department of EEE,SNS College of Technology, Coimbatore-35 2 Department of EEE,SNS College of Technology,Coimbatore-35
Abstract— As the wind power installations are increasing in
number, Wind Turbine Generators (WTG) are required to have
Fault Ride-Through (FRT) capabilities. Lately developed grid
operating codes demand the WTGs to stay connected during
fault conditions, supporting the grid to recover faster back to its
normal state. In this paper, the generator side converter
incorporates the maximum power point tracking algorithm to
extract maximum energy from wind turbine system. A hybrid
control scheme for energy storage systems (ESS) and braking
choppers for fault ride-through capability and a suppression of
the output power fluctuation is proposed for permanent-magnet
synchronous generator (PMSG) wind turbine systems. During
grid faults, the dc-link voltage is controlled by the ESS instead of
the line-side converter (LSC), whereas the LSC is exploited as a
STATCOM to inject reactive current into the grid for assisting in
the grid voltage recovery. A simple model of the proposed system
is developed and simulated in MATLAB environment. The
effectiveness of the system is validated through extensive
simulation results
Index Terms- Boost converter, Braking Chopper(BC),dc-link control,
energy storage system(ESS),ride through, STATCOM, Permanent
Magnet Synchronous Generator.
I. INTRODUCTION
The development of various wind turbine (WT)
configurations in the last decade has been very dynamic and
has resulted in larger ratings and higher operating speed
ranges allowing them to be tied up to the grid more easily.
Variable speed operation of wind energy conversion systems
(WECS) make them more ‘grid-friendly’. Permanent magnet
synchronous generators (PSMG) based WECS are emerging
as strong competitors to the other variable speed technologies.
The power converter, whose rating is the same as that of the
generator, connected between PMSG and grid allows full
controllability of the system during normal operation and fault
conditions. Further, PMSG operates at higher efficiency and
better power factor than its counterparts especially when it
functions as a direct driven generator [1-3]. WECS based on
PMSG can be connected to the grid by using a voltage source
converter (VSC) on the grid side and by using either a diode
converter with a buck-boost converter or a VSC on the
machine side. Evidently, using VSC on both machine and grid
sides offer full control of active and reactive powers resulting
in the best performance [4-6] in terms of power output, quality
of power and performance during faults.
Different strategies have been presented by various
authors, to enhance fault ride through (FRT) capabilities of
PMSG based WECS. Many devices, such as static var
compensator (SVC), dynamic voltage restorers (DVR), Static
Synchronous Compensators (STATCOM) etc have been
shown to improve FRT of WECS, but they will also increase
the overall cost of the system [7-8]. In [9], a nonlinear
controller design for power converter based WT system is
presented which ensures that current levels remain within
design limits, even at greatly reduced voltage levels. A back to
back connected voltage source- converter (VSC) configuration
is discussed in [10] where the machine side converter (MSC)
controls the speed of the generator by using a flux vector
control technique and the grid side converter (GSC) controls
the power flow by PWM technique Transient analysis of a
grid connected wind driven PMSG is presented in [11] and a
comparison is presented with the other generators at fixed and
variable speeds. An electromagnetic braking resistor
controlled using power electronic switch is used to dissipate
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal.com/
1306
Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for
PMSG Wind Turbine Systems
the excess energy in the DC link Circuit, preventing DC link
over voltage in [12]. Flexible active power control of
distributed generation systems during Grid Faults is discussed
in [13]. Reactive power as well as real power manipulation
using current control is described in [14]. Control of grid
converter in synchronously rotating reference frame is
described in [15-16].
In this paper, the generator side converter
incorporates the maximum power point tracking algorithm to
extract maximum energy from wind turbine system. In
addition, FRT technique of the PMSG wind turbine system is
proposed during the grid fault. By switching the control mode,
the ESS is operated to control the dc-link voltage of the back-
to-back converters during the grid voltage sags. Meanwhile,
the LSC is utilized to supply the reactive current to the grid for
satisfying the reactive current requirements of the grid code.
By this, the grid voltage can be recovered rapidly without an
external STATCOM after fault clearance. Also, the generator
active power can be absorbed fully by the ESS and the BC
during the voltage sags. In addition, the output power
fluctuation of wind turbine systems operating in steady state is
smoothened by the ESS. With this control scheme, the system
can still operate well even though the grid voltage is fully
interrupted. The validity of the proposed control algorithm is
verified by simulation and experimental results.
II. SYSTEM DESCRIPTION
Fig. 1 presents a block diagram of the simulation
model used for the FRT and Maximum Power point tracking.
In this paper, the wind turbine converts the power of the wind
to mechanical power in the rotor shaft. This is then converted
to electricity using a permanent magnet synchronous generator
(PMSG). The output voltage is rectified using a three-phase
diode bridge rectifier. The dc-to-dc converter is used to
control the dc voltage Vdc . The MPPT controller delivers a
voltage reference that is compared to the actual value of Vdc.
The result is fed into a PI controller whose output is compared
to a triangular waveform to determine when to turn the dc -dc
boost converter switch ON or OFF.The ESS consists of a
Electric Double Layer capacitor bank and a bidirectional
DC/DC converter and is also connected to the dc-link. Super
capacitors are suitable for wind power applications, as they
present the features of high efficiency, high power density,
long cycle life and easy maintenance . A BC is connected in
parallel with the dc-link. The BC will be activated to dissipate
the excessive power beyond the capacity of the ESS in cases
of deep voltage sags or high wind speed variations. The
control objective of the DC/DC converter is to maintain the
dc-link voltage magnitude at a constant level, by absorbing
any mismatch between the generated power and the power
transferred to the grid. In normal conditions, the LSC controls
the dc-link voltage , and the ESS is able to smoothen the
power ripples. In grid fault conditions, on the other hand, the
LSC functions as a STATCOM, and the ESS controls the dc-
link voltage. The FRT control scheme is designed according to
the grid code requirements on FRT.
III. MODELLING OF PROPOSED CONTROL SCHEME
A. Maximum Power Extraction Algorithm
Due to its monotonic characteristics, wind turbines can be
controlled to yield maximum power using search control
methods. Before explaining the maximum power tracking
controller, it is important to understand the basic physics of
the system. The generated mechanical power is given by [17-
19]
Pmech=Tmech(t)ωR(t) (1)
Where, Tmech is the mechanical torque. For simplification, the
generated electric power of a one-phase generator is given by
Pe(t)=Va(t)Ia(t) (2)
Fig 1:Block diagram of the proposed MPPT and FRT Method
Va and Ia are the generator voltage and current respectively.
Assuming no losses in the system, then
Tmech(t).ωR(t)= Va(t)Ia(t) (3)
The basic electrical and motion equations are
Te = kIaIf (4)
Ia=(Va-Ea)/Ra (5)
Ea=kIaωe (6)
Where, ωe = (p/2) ωR and p is the number of poles of the
generator.
Maximum power is at
=0 (7)
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
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1307
Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for
PMSG Wind Turbine Systems
Fig. 2. Typical power versus speed characteristics of a wind turbine.
The power extracted from the wind can be controlled by
varying the dc bus voltage, which is a function of If and ωe.
Considering the wind turbine characteristics given in Figure 2,
we know that the maximum power point is obtained whe
ω = 0 (8)
This equation can be written as:
=
=0 (9)
According to equation (9),maximum power point is when:
=0 (10)
The function Pmech (Vdc) has a single point where
maximum power extraction is achieved. It also means that the
maximum power can be tracked by searching the rectified dc
power, rather than environmental conditions, such as wind
speed and direction. The MPPT algorithm is as follows. One
initiates the maximum power searching process by setting an
arbitrary dc side voltage reference Vref. The controller then
measures both the dc side current and voltage, and calculates
the initial electric power Po = VdcIdc. Next, the reference
voltage Vref is increased by ΔVdc so that.
Vref(k)= Vref(k-1)+ ΔVdc (11)
Then the dc power is calculated with P(k) =
Vdc(k)Idc(k).
If P(k) is bigger than P(k - 1), the maximum power
point has not been reached therefore, the voltage reference
needs to be increased by ΔVdc and the dc power needs to be
compared. This process will repeat until maximum power is
reached.
And if P(k) is less than P(k - 1), the dc voltage
reference is then decreased by ΔVdc. In order to search for
maximum power at any wind speed four conditions must be
met.
1. If P(k)≥ P(k-1) and Vdc(k)≥ Vdc(k-1), the dc side
voltage reference need to be increased by ΔVdc. This condition
is met when the turbine operates on the low speed side of the
power curve, shown on Fig 3.
2. If P(k)≥ P(k-1) and Vdc(k)< Vdc(k-1), the wind
turbine is being operated in the high speed side and the dc
reference voltage needs to be decreased by ΔVdc.
3. When P(k)< P(k-1) and Vdc(k)≥ Vdc(k-1), the
maximum power point is passed and a step back must be
taken, decreasing the reference voltage by ΔVdc. This
condition is met when the turbine is operated in the high
speed side of the dome and the power is decreasing.
4. When P(k)< P(k-1) and Vdc(k)< Vdc(k-1), the
power is decreasing on the low speed side, therefore the
voltage reference is to be increased by ΔVdc.
Fig 3 Maximum Power Tracking Process
In Figure 3, the power-speed plot is shown for three different
wind speeds, where υ1 < υ 2 < υ 3. The arrows show the
trajectory in which the turbine will be operated using the
maximum power tracking algorithm explained above. If the
wind speed is υ1, the controller will search for the maximum
power. If the wind changes to υ3 the turbine is no longer being
operated at the maximum power point so the controller will
search for the new maximum power point.
Fig 4. Flowchart for MPPT Algorithm
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal.com/
1308
Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for
PMSG Wind Turbine Systems
After reaching the maximum point it will operate the
wind turbine at the optimal point until wind changes, thus
searching for maximum power at any wind speed. In order to
optimize the maximum power search algorithm presented
above, a step that combines speed of convergence and
accuracy of results was developed. The variable step method
is based on the Newton Raphson method. The value of the
root can be calculated as,
Xn+1=Xn -
(12)
Where Xn is the current known value of X, ƒ(Xn)
represents the value of the function at Xn, and ƒ’(Xn) is the
derivative at Xn. The function ƒ (Xn) can be expressed as:
ƒ(Xn)= ƒ(Vdc(k)) =
=
= Slope(k) (13)
And ƒ’(Xn) as
ƒ’(Xn)=ƒ’(Vdc(k)=
=
(14)
Using (12,)(13) and (14), ΔVdc can be express as follows:
ΔVdc =
=
(15)
This variable step will allow the maximum power
tracker to converge faster to the maximum power point and
will decrease power oscillations due to large values of ΔVdc
when maximum power is achieved. For protection the value of
ΔVdc is limited. The ΔVdc limit can be changed based on the
generator size and design parameters.
B. DC/DC Converter Controller
Fig 5 Typical DC-to-DC Converter Controller
The maximum power tracker will generate a reference voltage
that will be used to control the dc voltage at the rectifier dc
side terminals. The dc-to-dc converter uses a simple feedback
controller. The dc voltage reference is compared to the actual
dc voltage, and the error signal is fed to a PI controller. The
output signal is compared with a fixed frequency repetitive
triangular waveform to deliver a signal that will turn ON or
OFF the switch. This is shown in Fig 5.
C. Control of ESS and BC
The ESS and the BC are used to suppress the
generator output power fluctuation in normal conditions by
absorbing or releasing the pulsated power component from or
to the grid, in which the power command, P*ESS, is obtained
through a high-pass filter to the generator power [20]. The
ESS power is regulated by an outer PI regulator, whereas the
EDLC current is controlled by an inner PI regulator.
Fig. 6. Control block diagram of the ESS and BC.
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Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for
PMSG Wind Turbine Systems
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D. DC-Link Voltage Control
During grid sags, the dc-link voltage of the back-to-
back converters is controlled by the ESS instead of the LSC.
Hence, an outer PI voltage controller is employed, which
produces a current reference for an inner PI current controller.
Fig. 6 shows the overview control block diagram of the ESS
and the BC in both normal and grid sag conditions.
Neglecting the power losses of the converters and considering
the active power negligible flowing into the grid, the dynamic
equation of the dc-link voltage is expressed as
Pgen-PBC-PESS=0.5C(dV2
dc/dt) (16)
where C is the dc-link capacitance, Pgen is the generator
power, PBC is the power dissipated by the BC, and PESS is the
power of the ESS computed from the ESS voltage, VESS, and
the EDLC current, IESS, as
PESS=VESS*IESS (17)
From (16 ) in order to keep the dc-link voltage constant, the
ESS and the BC should be able to absorb the generator
powerfully.
From the control block diagram shown in Fig. 6, the output of
the dc-link voltage controller, I∗ ESS, is given as
I*ESS=Kp2(Vdc*-Vdc)+
(Vdc*-Vdc) +
(18)
where Kp2 and KI2 are PI controller gains of the dc-link
voltage control. In Fig. 6, IESS_max represents the maximum
current of the ESS.
By expanding a Taylor series of the dc-link voltage at
operating point Vdc0, the following can be obtained:
V2
dc=V2
dc0+2Vdc0(Vdc-Vdc0) (19)
From (16)–(19), the dc-link voltage equation can be rewritten
in the “s” domain as
CVdc0sVdc=-VESSKp2(Vdc*-Vdc)-VESS
(Vdc*-Vdc) (20)
The transfer function of the voltage controller is derived as
[21]
∗ =
(21)
where ξ is the damping ratio, and ωn is the natural frequency.
It is indicated in (21) that the transfer function has a zero and
two poles, which are always located in the left-half plane.
Hence, control stability is achieved.
E. EDLC Current Control
To establish the current control law for the dc/dc converter, a
voltage across the inductance, VLf , is investigated.
The dynamic equation of the inductance voltage is expressed
as [22], [23]
VLf=Lf
=DESSVdc-VESS (22)
VLf=Lf
=DESSVdc-VESS (22)
where Lf is the boost inductance, and DESS is the duty cycle.
As shown in Fig. 6, the output of the current controller, V ∗Lf ,
is given as
V*Lf=Kpc(I*ESS-IESS)+
(I*ESS-IESS) (23)
where Kpc and KIc are PI controller gains of the current
control.
The duty cycle is calculated by
DESS=(VESS+V*Lf)/Vdc (24)
Then, the gating signals for switches S1 and S2 are generated
by comparing the duty cycle with the carrier wave of 2 kHz as
shown in Fig. 6.
F.BC Control
During the grid disturbance, the ESS may not absorb the full
generator power, and then the BC will be activated to dissipate
the rest of power, PBC as
PBC=Pgen-PESS (25)
The BC is controlled by the switch S3 shown in Fig. 6.The
duty ratio DS3 for the switch depends on PBC, which is
expressed as
DS3=
PBC (26)
where Rbc is the braking resistance.
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
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1310
Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for
PMSG Wind Turbine Systems
G.Control of LSC
The LSC controls the dc-link voltage, Vdc, to be
constant under normal conditions. Cascaded control structure
with an inner current control loop and the outer dc-link
voltage control loop is applied . During grid voltage sags,
however, the dc-link voltage is controlled by the ESS. Hence,
the LSC is exploited as a STATCOM to supply the reactive
current to the grid according to the requirements of the grid
code. For this, the control strategy of the LSC is a voltage-
controlled current source, in which the LSC is operated as a
current source .
In the case of unbalanced grid sags, the dual-current
controllers for positive- and negative-sequence components
are adopted for the LSC. The control block diagram of the
LSC is shown in Fig. 7. In normal operation, the current
references for positive- and negative-sequence current
controllers are calculated from the output of the dc-link
voltage controller as shown in Fig. 7 . During grid sags, dc-
link voltage control by the LSC is deactivated, and the LSC
injects the reactive current component. Hence, the reference of
the active current component, Ip*
qe is set to zero
Ip*
qe =0 (27)
Fig. 7. Control block diagram of LSC
The dq-axis current references In*
dqe, of negative-sequence
components are set to zero to eliminate the unbalanced current
components flowing into the grid, which are expressed as
In*
qe =0 (28)
In*
de =0 (29)
A proportional-integral (PI) controller is usually used
for dc-link voltage control. Hence, an error accumulation in
the integral regulator should be considered for fast transition
when the dc-link voltage controller is reactivated after fault
clearance. The PI controller is implemented in a discrete time
domain. The actual dc-link voltage is able to track its
reference by ESS control during the sag. Hence, an initial
accumulated error, ΔVdc_I_int, of the integral regulator is set
to zero as
ΔVdc_I_int = 0. (30)
IV SIMULATION RESULTS
To perform the feasibility of the proposed scheme, Matlab simulations have been performed for a PMSG wind turbine system. The system parameters for the simulation are listed in Appendix .
Fig 8 shows the variation of the wind speed and its
corresponding output voltage of the PMSG. With the increase
in wind speed the power fed to the grid also increases which is
indicated by an increase in magnitude of PMSG phase voltage.
At t =2.4 s, wind speed is changed from 8 to 12 m/s in step,
whereas tip-speed ratio is maintained at Cp maximum in
steady state conditions.
Fig 8 (a) wind Speed (b) Phase voltage
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
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1311
Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for
PMSG Wind Turbine Systems
Fig 9 (a) Grid voltage (b) Grid Current
Fig 9 shows the performance of the LSC at voltage sags. Fig.
9(a) shows the grid voltage, where three grid-phase voltages
drop to 20%, 20%, and 50%, respectively, during 0.5 s. Fig
9(b) shows the grid current which increased during the fault
condition
Fig 10(a) Grid Voltage (b) DC-Link Voltage
The two-phase grid voltage interruption is considered
as shown in Fig. 10(a) .With the proposed system, the dc-link
voltage is controlled well by the ESS, which is shown in
Fig.10(b). The increase in the dc-link voltage is less than
1.5%.
Fig. 11 shows the performance of the ESS and the
BC. The dc link voltage is controlled well as shown in Fig.
11(a), in which its transient value is less than 2.5%. Fig. 11(b)
shows the ESS powers, in which the control performance is
good for normal conditions. When the grid fault occurs, the
power controller is deactivated, and the power is absorbed by
the ESS as seen in Fig. 11(b) to maintain the dc-link voltage.
The current control performance is shown in Fig. 11(c). Since
the ESS is not able to absorb the full generator power, the rest
of the power is dissipated by the BC. The BC current is shown
in Fig. 11(d). When the EDLC absorbs the power from the
wind system, the EDLC voltage is increased as shown in Fig.
11(e).
Fig. 11. Performance of ESS and BC under unbalanced sag (a) DC-link voltage. (b) ESS power. (c) EDLC current. (d) Braking chopper current. (e)
EDLC voltage.
CONCLUSION
This paper has proposed the maximum power point
tracking algorithm to extract maximum energy from wind
turbine system and combines the ESS and the BC for the
LVRT in PMSG wind turbine systems which is an cost
effective solution. The maximum power was tracked by
searching the rectified dc power, rather than environmental
conditions, such as wind speed and direction. Controlling the
dc-link voltage by the ESS, the LSC is able to comply with the
reactive current requirements of the grid code. By this, the
grid voltage can be recovered rapidly without an external
STATCOM after fault clearance. Also, the output power
fluctuation of the wind turbine system operating in steady state
is smoothened by the ESS. This control scheme offers an FRT
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal.com/
1312
Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for
PMSG Wind Turbine Systems
capability for the wind turbines even though the grid voltage is
fully interrupted.
Appendix:
PMSG parameters: Stator resistance Rs=2.1Ω, Stator
Inductance: 0.00083mH, inertia J= 0.01197 kgm2, magnetic
flux Φ=0.118 Wb and number of poles=4.
Converter parameters: Low voltage side capacitor C1=500μF,
High voltage side capacitor C0=3600μF, Inductor L=200mH,
Switching frequency fd=20kHz, system frequency f=50Hz.
ESS and BC parameters: ESS power ratingPESS-rated -0.6 KW,
Capacitance of EDLCCEDLC-100 F, Operating VoltageVESS-
440 V,Power rating of BCPbc-rated-1.13 KW,Resistance Rbc-1.5
Ω
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