A Frame Work for Control of Gird Connected Wind Power Using Two Layer Control

45 International Journal for Modern Trends in Science and Technology
A Frame Work for Control of Gird Connected Wind
Power Using Two Layer Control
V.Sarath Kumar1
| D.Rupesh2
| K.Prudhvi Raj3
| Ch.Prasanth Kumar4
| K.Manasa5
| A.Jawahar6
|
Ch.Rajyalakshmi7
| Ch.Vishnu Chakravarthi8
1-8Department of EEE, Sanketika Institute of Technology and Management, Visakhapatnam, Andhra Pradesh, India.
To Cite this Article
V.Sarath Kumar, D.Rupesh, K.Prudhvi Raj, Ch.Prasanth Kumar, K.Manasa, A.Jawahar, Ch.Rajyalakshmi and Ch.Vishnu
Chakravarthi, “A Frame Work for Control of Gird Connected Wind Power Using Two Layer Control”, International Journal
for Modern Trends in Science and Technology, Vol. 03, Issue 04, 2017, pp. 45-52.
Recently, several large-scale wind generation projects have been implemented all over the world. It is
economically beneficial to integrate very large amounts of wind capacity in power systems. Unlike other
traditional generation facilities, using wind turbines present technical challenges in producing continuous
and controllable electric power. With increase in contribution of wind power into electric power grid, energy
storage devices will be required to dynamically match the intermitting of wind energy. When wind turbines
are connected to a grid, they should always maintain constant power. In order to maintain constant active
power, the use of Doubly-Fed Induction Generators (DFIG) with Energy Storage System (ESS) like super
capacitor (or) batteries can be used, with a two layer control scheme. In the two layers control there is a
high-layer controller known as Wind Farm Supervisory Control (WFSC), which generates the active power (P),
Stator Power (Ps), Energy storage power (Pe), DC voltage (Vdc) etc., references for the low-layer WTG
controllers. The low-layer controller has two different controls i.e., Grid side controller (GSC) and Rotor side
controller (RSC) which are used to control the AC/DC/AC converters of DFIG wind turbines and to generate
the desired active power demand specified by the grid operator. Simulation is carried out in Matlab to
evaluate the performance of wind farm equipped with 15 DFIG wind turbines with and without ESS to
provide a constant active power of 36MW.
KEYWORDS: Wind Energy, DFIG, Active Power, reactive power, Two layer control
Copyright © 2017 International Journal for Modern Trends in Science and Technology
All rights reserved.
I. INTRODUCTION
In recent years with growing concerns over
carbon emission and uncertainties in fossil fuel
supplies, there is an increasing interest in clean
and renewable electrical energy generation. Among
various renewable energy sources, wind power is
currently the fastest growing form of electric
generation. Although wind power currently only
provides about 3% of European electricity and 2%
of the U.S.'s electrical energy demands, it is
reasonable to expect a high penetration of wind
power into the existing power system in the near
future, e.g., by 2030.With the rapid increase in
penetration of wind power in the power system, it
becomes necessary to require wind farms to be
have as much as possible as conventional power
plants to support the network active power, voltage
and frequency.
Modern variable-speed wind power systems,
predominantly based on the Doubly-Fed Induction
Generator (DFIG) technology[1], are equipped with
back-to-back, AC/DC/AC power electronic
converters whose intermediate DC voltage and
excellent controllability renders them technically
attractive to incorporate energy storage devices
ABSTRACT
International Journal for Modern Trends in Science and Technology
Volume: 03, Issue No: 04, April 2017
ISSN: 2455-3778
http://www.ijmtst.com

Chakravarthi : A Frame Work for Control of Gird Connected Wind Power Using Two Layer Control
such as a flywheels, super capacitors, batteries,
etc., it is shown that a DFIG-based
wind-power/storage system can deliver
pre-specified amount of power to the grid, despite
windpower fluctuations. In this work a wind farm
equipped with doubly-fed induction generator
(DFIG) wind turbines, where each WTG’s is
equipped with a super capacitor energy storage
system (ESS) to maintain constant active power
control to the grid. Two different control schemes
are developed one for Rotor side control (RSC)
using stator flux reference frame and the other for
Grid side control (GSC) using current reference
frame are developed to provide the firing pulses to
the converters. A wind farm supervisory control
(WFSC) is developed to generate the active power
reference to the RSC and GSC. WTG controllers
then regulate each DFIG wind turbine to generate
the desired amount of active power, where the
deviations between the available wind energy input
and desired active power output are compensated
by the ESS.
Simulation is done in Matlab for the grid
connected wind farm, the wind farm consists of 15
DFIG wind turbines each with 4MW capacity at
different and constant wind speeds. A constant
active power Pref is taken at 38MW which can be
specified by the grid operator and the wind farm
has to supply this constant active power.
Simulation results are studied for the active power
supplied by wind farm with and without Energy
Storage System (ESS). Here we can observe that
with ESS the active power supplied by wind farm is
almost constant.
II. DOUBLY-FED INDUCTION GENERATOR SYSTEMS
For variable-speed systems with limited
variable-speed range, e.g. ±30% of synchronous
speed, the DFIG can be an interesting solution. As
mentioned earlier the reason for this is that power
electronic converter only has to handle a fraction
(20–30%) of the total power. This means that the
losses in the power electronic converter can be
reduced compared to a system where the converter
has to handle the total power. In addition, the cost
of the converter becomes lower. The stator circuit
of the DFIG is connected to the grid while the rotor
circuit is connected to a converter via slip rings.
DFIG system with a back-to-back converter can be
seen in Fig. 1.
Fig 1 DFIG system with a Back to Back Converter
The back-to-back converter consists of two
converters, i.e., machine-side converter and
grid-side converter that are connected
“back-to-back.” Between the two converters a
dc-link capacitor is placed, as energy storage, in
order to keep the voltage variations (or ripple) in the
dc-link voltage small. With the machine-side
converter it is possible to control the torque or the
speed of the DFIG and also the power factor at the
stator terminals, while the main objective for the
grid-side converter is to keep the dc-link voltage
constant. The speed–torque characteristics of the
DFIG system can be seen in Fig.3.4, as also seen in
the figure, the DFIG can operate both in motor and
generator operation with a rotor-speed range of
±Δωmaxr around the synchronous speed, ω1.
Fig 2 Speed-Torque Characteristics of DFIG
A typical application, as mentioned earlier, for
DFIG is wind turbines, since they operate in a
limited speed range of approximately ±30%. Other
applications, besides wind turbines, for the DFIG
systems are, for example, flywheel energy storage
system, stand-alone diesel systems, pumped
storage power plants, or rotating converters feeding
a railway grid from a constant frequency public
grid.

2.1 Modeling of the Wind-Turbine Doubly-Fed
Induction Generator
The wind turbine and the doubly-fed induction
generator are shown in Fig3. The AC/DC/AC
converter is divided into two components, the
rotor-side converter (Crotor) and the grid-side
converter (Cgrid). Crotor and Cgrid are Voltage-Source
Converters that use forced-commutated power
electronic devices (IGBTs) to synthesize an AC
voltage from a DC voltage source. A capacitor
connected on the DC side acts as the DC voltage
source. A coupling inductor L is used to connect
Cgrid to the grid. The three-phase rotor winding is
connected to Crotor by slip rings and brushes and
the three-phase stator winding is directly
connected to the grid.
Fig 3 DFIG connected to Wind Turbine
The power captured by the wind turbine is
converted into electrical power by the induction
generator and it is transmitted to the grid by the
stator and the rotor windings. The control system
generates the pitch angle command and the voltage
command signals Vr and Vgc for Crotor and Cgrid
respectively in order to control the power of the
wind turbine, the DC bus voltage and the voltage at
the grid terminals. An average model of the
AC/DC/AC converter is used for real-time
simulation The DC bus is simulated by a controlled
current source feeding the DC capacitor. The
current source is computed on the basis of
instantaneous power conservation principle: the
power that flows inside the two AC-sides of the
converter is equal to the power absorbed by the DC
capacitor.
III. DFIG WITH ENERGY STORAGE SYSTEM (ESS)
The ESS consists of a super capacitor bank and
a two-quadrant DC/DC converter connected to the
dc link of the DFIG as shown in figure 4. The ESS
serves as either a source or a sink of active power,
and therefore, contributes to control the generated
active power of the WTG.
Fig 4 DFIG of Wind Turbine connected with Energy Storage
System (ESS)
The dc/dc converter contains two IGBT
switches S1 and S2. Their duty ratios are
controlled to regulate the active power Pg that the
GSC exchanges with the grid. In this configuration,
the dc/dc converter can operate in two different
modes, i.e., buck or boost mode, depending on the
status of the two IGBT switches. If S1 is closed and
S2 is open, the dc/dc converter operates in the
buck mode; if S1 is open and S2 is closed, the
dc/dc converter operates in the boost mode.
The duty ratio D1 of S1 can be approximately
expressed as and the duty ratio D2 of S2 is D2 = 1 –
D1.
dc
SC
V
V
D 1
Also the nominal dc-voltage ratio Scan/Vdc,n is
0.5, where VSC,n and Vdc,n are the nominal
voltages of the super capacitor bank and the DFIG
dc link, respectively. Therefore, the nominal duty
ratio D1,nof S1 is 0.5.The duty ratio D1 of the
dc/dc converter is controlled depending on the
relationship between the active powers (Pr)of the
RSC and (Pg)of the GSC. If Pr is greater than Pg, D1
is controlled greater than 0.5. Consequently, the
super capacitor bank serves as a sink to absorb
active power, which results in the increase of its
voltage VSC. On the contrary, if Pg is greater than
Pr then D1 is controlled less than 0.5.
Consequently, the super capacitor bank serves as a
source to supply active power, which results in the
decrease of its voltage VSC. Therefore, by
controlling the duty ratio of the dc/dc converter,
the ESS serves as either a source or a sink of active
power to control the generated active power of the
WTG.
In Fig. 3.9, the reference signal Pg* is generated by
the high-layer WFSC.

Fig 5 Configuration and control of ESS for DFIG Wind
Turbine
3.1 Rotor side controller (GSC)
In the RSC, the independent control of the
stator active power Ps and reactive power Qs is
achieved by means of rotor current regulation in a
stator-flux oriented synchronously rotating
reference frame. The overall RSC control scheme is
shown in fig 6 consists of two cascaded control
loops. The outer control loop regulates the stator
active and reactive powers independently, which
generates the reference signals Idr* and Iqr* of the
d-axis and q-axis current components,
respectively, for the inner-loop current regulation.
The outputs of the two current controllers are
compensated by the corresponding cross-coupling
terms Vdr0 and Vqr0, respectively, to form the total
voltage signals, Vdr and Vqr. They are then used by
the PWM module to generate the gate control
signals to drive the RSC
Fig 6 Overall vector scheme of the RSC
3.2 Grid side controller (GSC)
The overall vector control scheme of the GSC, in
which the control of the dc-link voltage Vdc and the
reactive power Qg exchanged between the GSC and
the grid, is achieved by means of current regulation
in a synchronously rotating reference frame. The
equivalent circuit of grid connected inverter is
shown in fig 7.
Fig 7 Equivalent circuit of Grid side controller
Again, the overall GSC control scheme shown in
fig 8 consists of two cascaded control loops. The
outer control loop regulates the dc-link voltage Vdc
and the reactive power Qg, respectively, which
generates the reference signals idg* and iqg* of the
d-axis and q-axis current components,
respectively, for the inner-loop current regulation.
The outputs of the two current controllers are
compensated by the corresponding cross-coupling
terms vdg0 and vqg0, respectively, to form the total
voltage signals, Vdg and Vqg. They are then used
by the PWM module to generate the gate control
signals to drive the GSC.
Fig 8 Grid side controller (GSC) scheme
3.3 Wind Farm supervisory control
The objective of the WFSC is to generate the
reference signals for the outer-loop power
controllers of the RSC and GSC as well as the
controller of the dc/dc converter of each WTG,
according to the power demands from the grid
operator. The reactive power references of the RSC
and GSC controllers can be determined by
controlling the power factor at the PCC at the
desired value, which is not in the scope of this
paper. In this work, the reactive power references
of all RSC and GSC controllers are simply set as
zero.
IV. SIMULATION STUDY
Simulation studies are carried out to verify the
effectiveness of the proposed control schemes
under various operating conditions. Some typical
results are shown and discussed in this section. At
one end grid is taken as 120kv and it is stepped
down to 25kv and it is further stepped down to
575v and is given to different loads. At the other a
wind farm is connected which has 15 wind
turbines connected to it which each has a rating of
4 MW and 575v. The wind turbines WTG1, WTG6,
WTG9 are provided with different wind speeds
within the range of 10-14 m/s. And all the
remaining wind turbines are provided with
constant wind speed of 15 m/s.

Here the simulation is done in order to maintain
the real power supplied by the wind farm is to be
maintained constant. The constant real power is
given as Pref to the wind turbines under different
conditions like wind turbines operating without
any energy storage system, operating with energy
storage system with two layer conventional
controllers. The amount of real power that has to
be maintained constant i.e., Pref is specified by the
grid operator and in this case the power is to be
maintained at 36MW.Also we will observe the
variations of wind speed and variations in voltages
in the energy storage system for WTG1, WTG6,
WTG11 and the response of the system for step
changes in input power demand given by the grid
operator.
Fig 9 Diagram of Wind Farm connected to Grid
Fig 10 Diagram of grid connected to 15 WTG’s
Fig 11 Doubly-Fed Induction Generator (DFIG) of wind
turbine
Fig 12 Doubly-Fed Induction Geneerator (DFIG) wind
turbine with Energy storage
Fig 13 Control structure for Wind Turbine

Rotor-side convertercontrolsystem
1
Uctrl_rotor_conv
Vdq*
Vdc
Angle
Uctrl_rotor_conv
dq-->abc
Theta
Iabc_r
Iabc_s
Iabc_grid_conv
angle_rotor
Idq_r
Idq_s
Idq_gc
r_angle_transformation
Transformation
abc-->dq
wr
Idqr
Idqs
Idq_grid_conv
Freq
Vdqs
Idr*
Torquecontrol
Q_ref
Q_B1
Iqr*
QRegulator
Idqr* Idqr_ref
PriorityIqr
Idqr_ref
Idq_r
Idq_s
wr
Freq
Vdq*
Current
Regulator
11
Vdqs
10
Freq
9
angle_rotor
8
Q_B1
7
Q_ref6
Iabc_grid_conv
5
Iabc_rotor
4
Iabc_stator
3
Theta
2
Vdc
1
wr
Fig 14 Grid side Control
Fig 15 Grid side control Current regulator
Fig 16 Rotor side control
Fig 17 Rotor side current regulator
Figure 6.1 shows the grid is connected to a wind
farm, fig 17 shows that 15 WTG’s are connected in
the wind farm. Figure 6.3 and fig 6.4 shows a
Doubly Fed Induction Generator (DFIG) of a wind
turbine connected without and with Energy storage
System (ESS). Figure 6.5 shows the control
architecture of wind turbine to which the voltage,
current of the stator, current of rotor, speed and
Pref are given from the wind farm supervisory
control.
Figure 14 and fig 16 shows the grid side control
(GSC) and the rotor side control (RSC) which are
designed in chapter 4 to produce the firing pulses
to grid and rotor side converters. Figure 15 and fig
17 shows the current regulators used in grid side
control and rotor side control.
V. SIMULATION RESULTS
Simulation results are studied for some
operating conditions and are discussed below. A
constant active power of 36MW is considered and
this power can be specified by the grid operator.
Fig 18 Total power of wind farm without ESS and constant
grid power.
Fig 19 Total power of wind farm with ESS and constant
grid power
1
Vdq*
[idr]
[iqr]
[w_pu]
[idr]
[w_pu]
[iqr]
[iqr]
[idr]
[w_pu]
Fnom
Fnom
PI Demux
Demux
Demux
Demux
R_RL
R_RL
L_RL
L_RL
4
Idq_ref
3
Freq
2
Idq
1
Vdqs
v d'
Vd
Iq_ref
Id_ref
Iq
Id
v q'
Vq

Fig 20 Wind speed variations for WTG1, WTG6 and WTG11
Fig 21 Total active power of stator (Ps), GSC (Pg) and point
of common coupling (P)
Fig 22 Voltages of super capacitors of WTG1, WTG6 and
WTG11
Fig 23 Active powers of WTG1
Fig 26 Power tracking of wind farm with step change in
grid power
Figure 18 shows the total active power
supplied by the wind farm without Energy Storage
System (ESS) and the constant grid active power.
Here we can observe that the fluctuations are very
large the max total active power of wind farm is
about 60 MW to min of 1.3MW. Figure 19 shows
that the active power supplied by the total wind
farm with ESS has fewer oscillations and is near to
grid constant power, here the max power is 37MW
and min power is 35MW which is very near to the
grid power of 36MW.
Figure 20 shows the variations of wind speeds
provided to the Doubly-Fed Induction Generator
WTG1, WTG6 and WTG11. The variation of WTG1
is in between 8-12 m/s, WTG6 is between 10-14
Pe1 Ps1
Pe6
Ps6
Pg6
Pe11 Ps11
Pg11

m/s and that of WTG11 is in between 12-16 m/s.
All the remaining turbines receive a constant wind
speed of 15 m/s.
Figure 21 shows the total stator active power
(Ps) and GSC active power (Pg) and the total power
at the point of common coupling of all WTG’s (P).
Here we can observe that the variations of the
stator active power are exactly compensated by the
variations of the GSC active power consequently,
the total output active power of the wind farm is
constant. Figure 6.14 shows the voltages of the
super capacitor banks of WTG1, WTG6, and
WTG11. These voltages are always maintained
within the operating limits of [0.7, 1.1] p.u.
Figure 23 to fig 25 shows the stator power (Psi),
and the GSC active power Pgi of WTG 1, WTG6 and
WTG11 which are usually not constant. The
deviations between the RSC active power and the
GSC active power of each WTG are stored in or
supplied by the ESS (Pei).
Figure 6.18 shows the power tracking capability
of the wind farm, here step changes [38 20 30 25]
MW in active power at grid are given as Pref to the
wind farms and under these conditions also the
wind farm with ESS is able to supply the constant
power.
VI. CONCLUSION
With the penetration of wind energy in power
systems it is always necessary to maintain the
constant active power of the grid. With wind energy
connected to the grid it is difficult to maintain
constant power as the wind energy varies
continuously with wind speed and makes it
difficult to connect to the grid as it affects the total
grid power. The development of power electronic
devices like AC/DC/AC converters it is possible to
use a Doubly-Fed Induction Generator (DFIG) with
Energy storage system (ESS) to maintain constant
power to the total wind farm and makes it feasible
to interconnect the wind farm to the grid.
In the present work, the design of a wind farm is
done using 15 DFIG’s each producing 3.6MW
power. The proposed control strategies for
controlling the rotor and grid side converters are
also described. The simulation is done with a
120kv grid which supplies a constant power of
36MW which is connected to the wind farm.
Simulation results are observed for the power
supplied by the wind farm with and without ESS,
here observation has been made that without ESS
the total power generated by wind farm has high
variations compared to the wind farm with ESS
where we can observe a constant active power is
obtained. With step changes in power at the grid,
the power tracking performance of the wind farm
generates active power by the wind farm
dynamically by tracking the power demand with
good precision. This power tracking capability
cannot be achieved without using the ESSs or the
proposed control scheme. The proposed system
and control schemes provides a promising solution
to help achieve high levels of penetration of wind
power into electric power grids.
Acknowledgment
The authors would like to express their gratitude to
Dr.S.V.H Rajendra, Secretary, AlwarDas Group of
Educational Institutions, Dr.C.Ghanshyam,
Principal for their encouragement and support
throughout the course of work. The authors are
grateful to Dr.N.C.Anil, Vice-Principal, Sanketika
Institute of Technology and Management and staff
members for providing the facilities for publication
of the paper.
REFERENCES
[1] T. Ackermann, et al, Wind Power in Power Systems,
John Wiley & Sons, 2005.
[2] W. Qiao and R. G. Harley, “Grid connection
requirements and solutions for DFIG wind turbines,”
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[3] C. Abbey and G. Joos, “Supercapacitor energy
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[4] C. Abbey and G. Joos, "Integration of energy storage
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[5] R. Pena, J. C. Clare, and G. M. Asher, “Doubly fed
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