1. technology and computing

SSRG International Journal of Electrical and Electronics Engineering (SSRG-IJEEE) – volume 1 Issue 9 –November 2014
Simulation of Hybrid Solar-Wind Energy Storage System
and Its Power Control in the Modern Power System
Krishna Yadav*1, Neelima*2
M-Tech Student Department of EEE, VBIT, Aushapur, Ghatkesar, R.R (Dt), Telangana, India.
Assistant Professor, Department of EEE, VBIT, Aushapur, Ghatkesar, R.R (Dt), Telangana, India.
ABSTRACT
This paper presents a hybrid solar energy management strategy for a wind photovoltaic micro-grid with an energy
storage system (ESS) and it injected to PV array on the DC-link. The power or energy storage system consists of a
battery and a super capacitor is interface to the DC-link for the bi-directional DC-DC converters. To cascade PI or
PID controller is used to regulate current supplied by the ESS. The proposed Fuzzy logic controller is for efficient
power sharing between the battery and the super capacitor. This designed simulation model is used for the microgrid operation. In this paper the proposed system power supplied to grid is maintain reference value during changes
in wind speed and solar radiation. The simulation results are shows the ESS improves the DC-link voltage stability
during three phases to ground faults.
Keywords: DFIG, Energy Storage System, micro-grid.
I.
INTRODUCTION
Wind power plants require the presence of fast and
flexible control of other generators in the power
system so as to balance power generation with
demand. The impacts of wind power integration that
need to be considered include reactive power
generation/voltage support, fault ride through
capability, frequency control, and dispatch of
conventional generating units [1]. A hybrid wind-PV
microgrid with storage has a great potential to
provide higher quality and more reliable power than a
system based on a single resource [2].
In the recent past there has been an increased use of
the doubly fed induction generator (DFIG) in wind
energy conversion systems due to its special features.
The unique capabilities of DFIG are: (1) it can supply
power at constant voltage and frequency; (2) the rotor
can operate in both sub-synchronous or supersynchronous speeds; (3) the rating of the power
converter is approximately 30% of the rated wind
turbine power; (4) the generated active and reactive
power can be independently controlled; and (5) high
efficiency and reduced mechanical stress on the wind
turbine.
The operating performance of the DFIG depends on
the stability of the DC bus voltage, especially during
faults [3]. Large voltage fluctuations also lead to
increased power losses in the power converters and
injection of harmonics into the grid. Crowbar based
solutions are the most commonly used for protection
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of power converters [4, 5]. The crowbar protection is
also enhanced by a DC chopper circuit with a resistor
across the DC bus [6]. The resistor is used with the
crowbar to stabilize the DC-bus voltage by
dissipating excess power. The shortcomings of the
crowbar circuits are: loss of DFIG active and reactive
power controllability and magnetization is carried out
by the stator [4], and the need for extra hardware in
the DFIG which increase cost and reliability [7].
This study investigates the performance of the DFIG
and the stability of the DC bus voltage when a PV
array and an energy storage system are integrated on
the DC bus. An initial study in [8] proposes
integration of two sets of rechargeable batteries on
the DFIG back-to-back converter DC-bus in a wind
power generation scheme. One battery set is used to
supply rotor currents while the other is used to
smooth out wind turbine power output. Integration of
PV array and battery storage on the DFIG DC-bus is
presented in [9]. The battery in [9] is directly
connected to DC-bus and is used to control the DCbus voltage.
In this work, the energy storage system consists of a
battery and a supercapacitor. Integration of battery
and supercapacitor storage units offers a
complementary scheme due to their different power
and energy densities. A battery has a high energy
density but requires accurate regulation of
charge/discharge current within manufacturer
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SSRG International Journal of Electrical and Electronics Engineering (SSRG-IJEEE) – volume 1 Issue 9 –November 2014
specified range in order to increase its lifetime. A
supercapacitor has high power density and longer
lifetime but has the disadvantage of being more
costly and having low energy density.
II.
RELATED WORK
Out of the above natural sources wind energy
conversion systems include less conversion
equipment, land need, less maintenance, direct
coupling of wind turbine to generator shaft. There So
many WECS technologies available classified as:
fixed speed and variable speed WECS. Fixed speed
employed Squirrel Cage Induction Generator(SCIG)
as mechanical to electricity conversion element with
soft starter technology simple to construct but may
affect steady state stability of power system under
unbalanced conditions such as gust in wind, voltage
dip in the bus bar voltage , and Need a stiff power
grid and not tolerated by weak Grid [1-6].Variable
speed WECS employed mainly two technologies
such as SCIG in which Capacitor bank and softstarter are replaced by a full scale converter. It
requires 100% rating of power stability equipment
(FACTS for power factor correction) as that of
generator rating. It gives still less effective steady
state stability measures as constrained by high cost of
converter[1,4,5]. Second technology of variable
WECS is Doubly Fed Induction Generator (DFIG)
based wind power to electricity conversion element
as shown in fig. 1. This technology becomes so much
popular and opted by maximum number of countries
in the world. There are following advantages listed
for DFIG based WECS [4-6]:





Converter system provides reactive power
compensation and smooth grid integration.
The market shareof DFIG systems (75%)
many times the any other types of WECS.
Around 86% patents are on controlling of
DFIG. Woodward filed 10 patent
applications in the same field in year 2010,
as it has no previous record of Intellectual
Property activity.
Converter Rating is only 25%-30% inDFIG
as compared to 100 % of total nominal
power of the generator.
Stator feeds the remaining 70%-75% of total
power directly into the grid.
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Fig. 1DFIG with power converter.
In the present scenario due to penetration of large
number of DFIG based WECS and interconnection to
main grids gives rise to new steady state stability
challenges for the researchers and scientists due to
some power regulating issues under unbalance
conditions such as Voltage sags or faults which occur
in the network makes performance poor, fault ridethrough(FRT), Low voltage ride through (LVRT)
capabilities of DFIGWTs under transient periods,
Inter area oscillations in long distance transmission to
keep up constant output power to grid and to extract
maximum power from continually fluctuating power,
Sub-synchronous resonance (SSR) occurred in series
compensated electrical networks becomes new area
of research with DFIGWTs connected to series
compensated networks, Large oscillations of the DClink voltage cannot be avoided as the grid side
converter controller was not optimized, Suitable
choice of Insulated gate bipolar transistor (IGBT for
converter equipment) thermal impedances, SmallSignal Stability Problems and steady state problems.
Some other issues are also taken as research finding
by the researchers such as converter's battery energy
system optimization (BES), stator's harmonic current
control, Direct torque control, amplitude frequency
control, load frequency control, open loop rotor
control, Control based inertia contributed by DFIG,
Hysteresis-Based Current Regulators and Dynamic
Stability control using FACTS. This paper gives an
overview on the some emerging issues related with
DFIG based WECS taken by the researchers in past
and novel technologies proposed by them. There are
around thousands of research IEEE activities
(Research Publications) on DFIG control aspects
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SSRG International Journal of Electrical and Electronics Engineering (SSRG-IJEEE) – volume 1 Issue 9 –November 2014
during past few decades. Fig. 2 shows the major
IEEE's IP Activities on DFIG in approximation.
III.
PROPOSED WORK
The Emerging Issues and their Control of DFIG
based WECS are shown in the above fig.2 and
described one by one as follows:
(a) Coordinated control of frequency regulation
capability.
A (DFIG)-based WECS not provide frequency
response because of the decoupling between the
output power and the grid frequency. Power reserve
margin also problem for DFIG because of the
maximum power point tracking (MPPT) operation.
[8] presented a novel frequency regulation by DFIGbased wind turbines to coordinate inertial control,
rotor speed control and pitch angle control, under low
and high wind speed variations.
(b) Battery Control Operation (BESS)
[9] presented a new based on battery energy storage
system (BESS) and tried to reduce the power
fluctuations on the grid for uncertain wind conditions
and also, compared with an existing control strategies
like the maximum power point extraction at unity
power factor condition of the DFIG.[10] presented
the modified rotor side of DFIG with DC link
capacitor is replaced with the BES. The co-ordinated
tuning of the associated controllers using bacterial
foraging technique (based on eigen-value) to damp
out power oscillations.Furthermore, an evolutionary
iterative particle swarm optimization (PSO) approach
for the optimal wind-battery coordination in a power
system was proposed in [11], [12].
(c) Stator Current Harmonic Control
[13] proposed a sixth-order resonant circuit to
eliminate negative sequence 5th harmonic and
positive sequence 7 th harmonics currents from
fundamental component of stator current. A stator
current harmonic control loop is added to the
conventional rotor current control loop for harmonic
suppression. The affects of voltage harmonics from
the grid on the DFIG are also have been discussed in
[14]–[17]. Resonant controllers have been widely
used in harmonic control and unbalanced control for
both DFIG and power converter systems [17], [18]–
[25]. The use of resonant circuits aims to achieve
high bandwidth at certain frequencies and also
eliminate current harmonics in the three-phase power
converter systems [16]–[20] and the DFIG [17]
during grid voltage distortion. In [22]–[24], the
resonant controllers are used to keep the current
output balanced during a grid voltage imbalance.
(d) Fault Ride Through
A grid fault posed an overload condition to DFIG
when it trying to stabilize the wind farm. This would
check the fault ride through capability of the DFIG.
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[26] Proposed the dc-link chopper-controlled braking
resistor with the supplementary rotor current (SRC)
control of the rotor side converter of the DFIG and
series dynamic braking resistor (SDBR) connected to
the stator of the DFIG. [27,28] a study focused on
stabilizing FSWT without using any FACTS device.
A series dynamic braking resistor (SDBR) was used
to improve the FRT of large wind farms composed of
IGs in [29], while in [30] the SDBR was connected to
the rotor side converter of the DFIG to improve its
Fault Ride Through capability. A superconducting
fault current limiter (SFCL) [31], passive resistance
network [32], and series anti-parellelthyristors [33]
connected to the stator side of a grid connected
DFIG. [34] proposed a new control strategy using a
dc-chopper inserted into the dc-link circuit of the
DFIG and a small value of SDBR connected in series
in the
stator of the DFIG, the former of which acts as a
damping load to suppress the dc-link voltage during a
grid fault.
(e) Regulation of active/reactive power
[35] DFIG is a electromechanical device and is
modelled as non-linear system with rotor voltages
and blade pitch angle as its inputs, active and reactive
powers as its outputs, and aerodynamic and
mechanical parameters as its uncertainties. A
controller was developed that is capable of
maximising the active power in the maximum power
tracking (MPT) mode, regulating the active power in
the power regulation (PR) mode for simultaneously
adjusting the reactive power to achieve a desired
power factor. For MPPT adaptive controls [35–38]
and fuzzy methodologies [39–41] were proposed
despite not knowing the Cp-surface. In [42]
developed a non-linear controller that simultaneously
enables control of the active power in both the MPT
and PR modes with aerodynamic and mechanical
parameters were known. [43] presented a dynamic
model of BDFIG with two machines‘ rotor
electromechanically interconnected. The
method used to extract maximum power at any given
wind speed is to implement maximum power point
tracking (MPPT) algorithm based on the various
control strategies for the VSR have been discussed in
[44-46]. It has been demonstrated in [43] that the
proposed BDFIG system can be used for the large
off-shore wind energy application with reduced
system maintenance cost. [47] proposed a modelbased predictive controller for a power control of
DFIG and internal mode controller [48], [49] have
satisfactory performance when compared with the
response of PI, but it is difficult to implement one
due to the formulation of a predictive functional
controller and the internal mode controller. Fuzzy
based DFIG power control can be realized [50, 51].
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SSRG International Journal of Electrical and Electronics Engineering (SSRG-IJEEE) – volume 1 Issue 9 –November 2014
(f) Voltage Unbalance Control
[52] Wind energy is often installed in rural, remote
areas characterized by weak, unbalanced power
transmission grids. Voltage unbalance factor (VUF)
is defined as the negative sequence magnitude
divided by the positive sequence magnitude. The
control topology is fairly standard (based on statorvoltage-oriented dqvector control is used. This
orientation can be called ―grid flux oriented‖ control
[52]. [53] implemented new rotor current control
scheme which consists of a proportional–integral (PI)
regulator and a harmonic resonant (R) to suppress 5th
and 7th harmonics. The steady-state and transient
response of DFIG-based wind power generation
systemunder balanced [53]–[56] and unbalanced
[57]–[64] grid voltage conditions have been well
understood. [61] and [62] proposed proportional–
integral (PI). plus resonant tuned at twice the grid
frequency current controllers for both grid- and rotorside converters. For instance, standards IEEE-519–
1992 [65] and ER G5/4–1 [66] have, respectively,
recommended different practices and requirements
for harmonic control in electrical power systems. As
indicated in [67] and [68- 69], the presence of
harmonics in the supply system results in torque
pulsations and increased copper and iron losses in
electrical
machines.
[70]
presented
a
feedback/feedforward nonlinear controller for DFIG.
The mechanical and electrical parts of the wind
turbines are considered separately in most of the
current literature: [71]–[88] considered only the
mechanical part, while [96]–[103] considered only
the electrical part, focusing mostly on the DFIGs.
[104] considered both these parts, its controller was
designed to maximize wind energy conversion, as
opposed to achieving power regulation (i.e., only
operate in the MPT mode).
(g) Direct Torque Control
Direct power control (DPC) was based on the
principles of direct torque control [105], [106]. The
DPC applied to the DFIG power control has been
presented in [107]–[109]. This strategy calculates the
rotor voltage space vector based on stator flux
estimated and power errors. An alternative to DPC is
power error vector control [110]. This strategy is less
complex and obtains results similar to those of direct
control of power. A anti-jamming control has been
proposed by [111] to improve the controller
performance. The predictive control is an alternative
control technique that was applied in machine drives
and inverters [112], [113]. Some investigations like
long-range predictive control [114], general
predictive control [112], and model predictive control
[115]–[117] were applied to the induction motor
drives.
IV.
SIMULATION RESULTS
Simulations are performed to verify the performance
of the microgrid with the proposed power and energy
management strategies. The first study investigates
performance under normal operation with step
changes in wind, solar radiation, and grid power
demand. The next studies investigate performance
during three-phase short circuit faults. Simulations
are carried out in MATLAB/Simulink using a
discrete fixed step solver and a time step of . The
simulation model is implemented as shown in Fig. 9.
The system sizing is shown in Table 2. 10 s
4.1. Response under normal operation
The system is studied during normal operation with
an initial wind speed of 10 m/s, solar radiation of 0.5
kW/m2, ambient temperature of 25 oC, and a grid
power demand of 1.0 p.u. The initial battery and
supercapacitor state of charge are 70% and 50%
respectively. Simulations are carried out with the
following step changes: at t=1s solar radiation step
change from 0.5 to 1.0 kW/m2 at t=1.5s grid power
demand step change from 1.0 to 0.7 p.u., and at t=2s
simultaneous wind speed, grid power demand , and
solar radiation step changes from 10 m/s to 15 m/s,
0.7 to 1.0 p.u., and 1.0 to 0.8 kW/m2 respectively.
The system response is shown in Figs. 10-12.
It is observed in Fig. 10 that during start-up the
system reaches steady state at t=0.3s and is able to
supply the required power to the grid. The slow
variations in power generated by the DFIG stator (Ps)
due to the large wind turbine time constant is
compensated by power supplied to the grid via the
grid side converter Pgc. Between t=0.3s and t=1.5s,
power is supplied to the grid from the energy storage.
Fig. 11
Fig.2.Wind constant power 1
W1,w2,w3 scope
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SSRG International Journal of Electrical and Electronics Engineering (SSRG-IJEEE) – volume 1 Issue 9 –November 2014
Fig.3.Pw1 scope
Constant power 1 ess
Fig.6.Vabc p2,, Iabc p2
W1,w2,w3
Fig.7.Vabcp3,, iabcp3
Fig.4.Vabc-b575,,Iabc-b575
Fig.8.Vdcess1,, vdcess2,,vdcess3
Fig.5.Vabcp1,, Iabcp1
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SSRG International Journal of Electrical and Electronics Engineering (SSRG-IJEEE) – volume 1 Issue 9 –November 2014
Fig.9.Constant power1 ess powertracking
V.
CONCLUSION
[6] L. Fan, S. Yuvarajan, and R. Kavasseri, ―Harmonics analysis
of a DFIG for a wind energy conversion system,‖ IEEE Trans.
Energy Convers.,vol. 25, no. 1, pp. 181–190, Mar. 2010.
[7] J. Hu, H. Nian, H. Xu, and Y. He, ―Dynamic modeling and
improved control of DFIG under distorted grid voltage conditions,‖
IEEE Trans. Energy Convers., vol. 26, no. 1, pp. 163–175, Mar.
2011.
[8] M. Liserre, R. Teodorescu, and F. Blaabjerg, ―Multiple
harmonics control for three-phase grid converter systems with the
use of PI-RES current controller in a rotating frame,‖ IEEE Trans.
Power Electron., vol. 21, no. 3, pp. 836–841, May 2006.
[9] R. Teodorescu, F. Blaabjerg, M. Liserre, and P. C. Loh,
―Proportionalresonant controllers and filters for grid-connected
voltage-source converters,‖ IEE Proc. Electr. Power Appl., vol.
153, no. 5, pp. 750–762, Sep.2006.
[10] W. Qiao, W. Zhou, J. M. Aller, and R. G. Harley, “Wind
speed estimation based sensorless output maximization control for
a wind turbine driving a DFIG,” IEEE Trans. Power Electronics,
vol. 23, no. 3, pp. 1156-1169, May 2008.
[11] W. Qiao, G. K. Venayagamoorthy, and R. G. Harley, “Realtime implementation of a STATCOM on a wind farm equipped
with doubly fed induction generators,” IEEE Trans. Industry
Applications, vol. 45, no. 1, pp. 98-107, Jan./Feb. 2009.
[12] D. W. Novotny and T. A. Lipo, Vector Control and Dynamics
of AC Drives, Oxford University Press, 2000.
This paper proposed a solar energy
management strategy for hybrid solar-wind energy of
micro-grid with an energy storage system and PV
array for current injection in DC-link. Energy storage
system consists of a batter and a super capacitor is
connected to DC-link through a bi-directional DCDC converter. And then to cascade PID controller is
used to regulate current to DC-link. The proposed
fuzzy Logic Controller is an efficient power sharing
between battery and super capacitor. By using this
proposed scheme the active power is supplied to the
grid due to the variations in wind speed and solar
radiations is minimized. Then the power
controllability is stabile.
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