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Dc Microgrid Based Hybrid Power Generation Systems By Using Wind Power And
Wave
KALPANA, VITS, HYD
Abstract
In order to study the uncertainty and intermittent characteristics of wind power and
wave power, this paper proposes an integrated wind and wave power generation system fed
to an ac power grid or connected with an isolated load using a dc microgrid. The proposed dc
microgrid connects with a wind power generator through a voltage-source converter (VSC), a
wave power generator through a VSC, an energy storage battery through a bidirectional dc/dc
converter, a resistive dc load through a load dc/dc converter, and an ac power grid through a
bidirectional grid-tied inverter. The studied integrated wind and wave system joined with the
dc microgrid is modeled and simulated using the written program based on
MATLAB/Simulink.
INTRODUCTION
I N recent years, renewable energy and distributed generation systems (DGSs) have attracted
increasing attention and have been extensively researched and developed. They gradually
alter the concepts and operations of conventional power generation systems. The rise in
several countries makes it possible that this kind of DGS can be practically applied to a grid-
tied system or an isolated system with wind power, solar energy, hydropower, etc. The output
of DGS usually includes two kinds: dc and variable ac. Moreover, the generating capacity of
DGS comparing with conventional large synchronous generators is much smaller, and hence,
the dc microgrid can be practically applied to convert the generated time-varying quantities
of natural renewable energy and DGS into smooth dc electricity that can then be converted
back into ac quantities delivered to other power systems [1], [2]. Because of the intermittence
of renewable energy and DGS, bidirectional dc/dc converters are usually necessary to feed
the connected loads with smooth power [3].
In order to simulate a hybrid ac/dc micro grid system, photovoltaic and wind power generator
models, a doubly fed induction generator model, and an inverter model were established to
simulate the dynamic responses of the studied system in [4]. A practical low-voltage bipolar-
type dc micro grid was constructed using a gas engine as the power source, while a
bidirectional dc/dc converter shunting a super capacitor was utilized as an energy storage
device to balance the power demand of the studied system in [5]. Unexplored energy and
resources in ocean such as marine energy, tidal energy, ocean thermal energy, ocean wave
energy, salinity gradient energy, etc., are abundant. The simulated results of an Archimedes
wave swing (AWS) power convertor coupling with a linear permanent magnet generator
(LPMG) were compared with the experimental outcomes using the measured data obtained
from a 2-MW AWS test system along the coastline [6]. A configuration of a marine power
plant with two AWSs connecting to a power grid was proposed in [7], and the outputs of the
two AWSs were converted to dc quantity by individual diode bridge rectifiers and then
subsequently converted into ac quantity by an inverter to reduce the fluctuation of the
combined rectified output power. A hybrid electric vehicular power system in [8] utilized two
motors connected to a dc bus through a voltage-source converter (VSC), and a bidirectional
converter was connected between a battery and the dc bus. The dynamic average model was
used in [8] for all power electronics models by neglecting the switching phenomena to reduce
simulation computational intensity. A nonisolated bidirectional zero-voltage switching dc/dc
converter was proposed in [9], and the converter utilized a very simple auxiliary circuit
consisting of an additional winding of a main inductor and an auxiliary inductor to reach
zerovoltage switching and reduce the reverse-recovery problem of power diodes. Modelling
and testing the data centers of a dc microgrid using PSCAD/EMTDC were proposed in [10]
and [11] since most data centers were sensitive to the variations of electronic loads. The
proposed dc microgrid was also used to supply sensitive electronic loads during ac grid
outages in order to offer uninterruptible power system protection [11].
To achieve power sharing and improve economic benefit, a dcbus voltage control technique
for parallel integrated permanentmagnet wind power generation systems was proposed in
[12], and the technique was based on a master–slave control to solve controller discrepancy
problems. A 12-kW experimental system was constructed in [12] to confirm the effectiveness
of the proposed scheme. In order to achieve power sharing and to optimize the dc microgrid,
the control strategies for an islanded microgrid with a dc-link voltage control were developed
in [13] and [14], while the control strategies were combined with P/V droop control and
constant-power band to avoid frequent changes and voltage-limit violation on generation
devices. A battery/ultracapacitor hybrid energy storage system was proposed in [15] for
electric-drive vehicles. To satisfy the peakpower demands between the ultracapacitor and
battery, a larger dc/dc converter was necessary. The studied system utilized two storage
devices to compensate mutually in order to prolong the life of the battery. The simulated and
experimental results were carried out to verify the proposed control system [15]
Fig1: Configuration of the studied integrated wind and wave power generation system connected
to a power grid through the proposed dc microgrid.
RENEWABLE ENERGY SOURCES
Electric utilities and end clients of electric force are becoming progressively worried about
taking care of the growing vitality demand. Seventy five percent of aggregate global energy interest is
supplied by the smoldering of fossil powers. But increasing air contamination, an Earth-wide
temperature boost concerns, diminishing fossil energizes and their expanding expense have made it
necessary to look towards renewable sources as a future vitality solution. Since the previous decade,
there has been a gigantic interest in many nation on renewable vitality for force era. The market
liberalization and government's impetuses have further accelerated the renewable vitality part
development.
Development of force transformer innovation in the nation amid the previous five decades is
truly amazing. There are producers in the nation with full access to the most recent innovation at the
worldwide level. A percentage of the makers have great R&D set up to bolster the innovation.
2.2 RENEWABLE ENERGY SOURCES
2.2.1 Hydro Power
India is invested with a huge capability of hydro force, of which just 17% has been outfit in
this way. The hydroelectricity is a perfect and renewable wellspring of vitality. It has been felt that
there is a long development period in hydro extends because of deferrals in woods and environment
leeway, restoration of the venture affected individuals other than between state debate and
development burglaries because of a few reasons. Under RSE just little hydro ventures are considered
since they don't oblige huge poundage and have the ability to give energy to remote and bumpy
territory where expansion of the matrix framework is either un-financial or unrealistic. It has been
assessed that the potential accessible in the nation under little hydropower plans is of the request of
15000 MW in which the arrangements that are considered are up to 25 MW limit separately which are
named little hydro extends under the Ministry of Non-Conventional Sources of vitality. The little
hydro force stations are for the most part situated in sloping regions and are given need for nearby
advantages to the occupants which give them productive vocation through the vitality potential.
2.2.2 Solar Power
The climatic condition in India gives plentiful capability of sun based power because
of expansive scale radiation accessible amid a more extensive piece of the year because of
tropical condition in the nation. The sun based force can be produced for long haul use
through the utilization of sun oriented photograph voltaic (SPV) Technology which gives a
capability of 20MW for every sq. Km. The other system for Utilization of sun oriented
vitality is through the reception of sunlight based warm Technology. The projects are under
approach to use SPV by joining with matrix power frameworks.
The Solar Energy Technologies Office works to accelerate the market
competitiveness of solar energy by targeting cost reductions and supporting increased solar
deployment. Through its Sun Shot Initiative, DOE supports efforts by private companies,
universities, and national laboratories to drive down the cost solar electricity. The Solar
Powering America website makes it simple for communities, businesses, organizations and
state and local governments to both learn about and commit to choosing solar.
The other mainstream utilization is by stand-alone applications which incorporate
sunlight based controlled road lights, local lights, water pumps and so forth. The expense of
the photograph SPV modules is very lavish which is in the scope of $ 3-4 for each watt,
despite best endeavors, the cost couldn't descend in India, China and different nations. The
exertion is to convey the cost down to $ 1 for each watt when it might be more prevalent for
utilization. The endeavors to utilize indistinct silicon innovation were less expensive yet its
long terms utilization is not practicable. The SPV innovation if modest would be helpful for
individuals living in far - flung regions as broadening framework would include high cost.
2.5 WIND TURBINES
In the event that the mechanical vitality is utilized specifically by apparatus, for example, a pump or
crushing stones, the machine is typically called a Windmill. A wind turbine is a machine for changing
over the active vitality in wind into mechanical vitality. In the event that the mechanical vitality is
then changed over to power, the machine is known as a wind generator. As wind turbines increment in
size and ascend to more noteworthy statures to exploit higher vitality winds, their towers require more
materials and contain a bigger rate of the venture's expense. Proficient development strategies can
improve material amounts and decrease costs.
2.5.2 Types of wind turbines
Wind turbines are grouped into two general sorts: level hub and vertical hub. A flat pivot
machine has its edges turning on a hub parallel to the ground. A vertical hub machine has its sharp
edges turning on a hub opposite to the ground. There are various accessible outlines for both and
every sort has specific focal points and weaknesses. Be that as it may, contrast and the even pivot sort,
not very many vertical hub machines are accessible monetarily.
DC-DC Converter Basics
A DC-to-DC converter is a gadget that acknowledges a DC info voltage and produces a DC
yield voltage. Normally the yield delivered is at an alternate voltage level than the info. Also,
DC-to-DC converters are utilized to give clamor confinement, force transport regulation, and
so on. This is a synopsis of a portion of the prevalent DC-to-DC converter topologies.
3.1 BUCK CONVERTER
In this circuit the transistor turning ON will put voltage Vin toward one side of the
inductor. This voltage will tend to bring about the inductor current to rise. At the point when
the transistor is OFF, the present will keep coursing through the inductor however now
moving through the diode.
We at first accept that the current through the inductor does not achieve zero, in this
way the voltage at Vx will now be just the voltage over the leading diode amid the full OFF
time. The normal voltage at Vx will rely on upon the normal ON time of the transistor gave
the inductor current is persistent.
Fig 3.1 Buck Converter
T
Fig : Basic schematic diagram, dynamic average-value model, and control block diagram of
the employed bidirectional dc/dc converter. (a) Basic schematic diagram. (b) Dynamic
average-value model. (c) Control block diagram.
CONFIGURATION OF THE STUDIED SYSTEM:
A. System Configuration
Fig. 1 shows the configuration of the studied integrated wind and wave power generation system
connected to an ac grid through a dc microgrid. The wind power generation system simulated by a
permanent-magnet synchronous generator (PMSG) driven by a wind turbine (WT) is connected to
the dc microgrid through a VSC of VSC_PMSG. The wave power generation system simulated by an
LPMG driven by a linear permanentmagnet motor (LPMM) is also connected to the dc microgrid
through a VSC of VSC_LPMG. A resistive dc load RLoad is connected to the dc microgrid through a
load dc/dc converter. To achieve stable power flow (or power balance condition) and load demand
control of the dc microgrid under different operating conditions, a battery is connected to the dc
microgrid through a bidirectional dc/dc converter, while an ac grid is connected to the dc microgrid
through a bidirectional grid-tied inverter and a transmission line. When available wind power and/or
wave power can be injected into the dc microgrid with a fully charged battery, the surplus power of
the dc microgrid can be delivered to the ac grid through the bidirectional grid-tied inverter. When no
wind power or no wave power is delivered to the dc microgrid with a low-energy battery, the
insufficient power of the dc microgrid can be captured from the ac grid through the bidirectional
grid-tied inverter. The power of the resistive dc load RLoad can be obtained from the dc microgrid
through the load dc/dc converter only when the dc microgrid has enough power. The load dc/dc
converter with the resistive dc load RLoad can also slightly adjust the power balance condition of the
dc microgrid. The control functions of the bidirectional dc/dc converter, the bidirectional grid-tied
inverter, and the load dc/dc converter must be adequately coordinated with each other to obtain
stable operation of the dc microgrid. In this paper, the mathematical models of the studied
integrated system with the proposed dc microgrid are derived in detail, including the wind WT-
PMSG set with its VSC, the wave LPMM-LPMG set with its VSC, the bidirectional dc/dc converter
with the battery, the load dc/dc converter with the resistive load, and the bidirectional grid-tied
inverter. Both frequency-domain analysis and time-domain simulations are performed using
MATLAB/Simulink.
B. Models of WT and PMSG
The WT model employed in this paper includes the following operation conditions: the cut-in
wind speed of 4 m/s, the rated wind speed of 13 m/s, and the cut-out wind speed of 24 m/s. The
detailed characteristics and expressions for the captured mechanical power Pw, the dimensionless
power coefficient Cpw, the mechanical torque Tw, the tip speed ratio λw, and the blade pitch angle
βw of the studied WT can be seen in [16]. Fig. 2 plots the q–d-axis equivalent circuit of the studied
wind PMSG [17]. The per-unit (p.u.) q- and d-axis statorwinding voltages of the studied PMSG can be
expressed by, respectively,
Fig: Equivalent mass-spring-damper model of the studied AWS
Control Blocks of PMSG’s VSC:
Fig. 5 illustrates the control block diagrams of the indices mq1 and md1 of the studied
PMSG’s VSC. The d- and q-axis reference currents are generated by comparing the output
active power of the PMSG (PPMSG) with its reference value using maximum power point
tracking function. After subtracting the output currents of the PMSG (ig_PMSG) from their
respective reference values, the resultant differences pass through the respective first-order
lag controllers to obtain the deviations of the respective modulation indices that are added to
their respective initial values to acquire the VSC’s modulation indices. The limiters, namely,
md1_ max, md1_ min, mq1_ max, and mq1_ min, are included in the model to ensure normal
operation of the VSC.
Fig: Control block diagram of the modulation indices of the VSC of the studied PMSG.
Control Blocks of LMSG’s VSC & Control Blocks of Bidirectional Grid-Tied Inverter:
Fig. 6 plots the control block diagrams of the indices mq3 and md3 of the studied LPMG’s
VSC. After subtracting the output currents of the LPMG (ig_LMSG) from their respective
reference values, the resultant differences pass through the respective proportional-integral
controllers to obtain the deviations of the respective modulation indices which are added to
their respective initial values to obtain the VSC’s modulation indices. The limiters, namely,
md3_ max, md3_ min, mq3_ max, and mq3_ min, are included in the model to ensure normal
operation of the VSC.
SIMULATED AND EXPERIMENTAL RESULTS
To examine the operation characteristics of the studied integrated system joined with
the proposed dc microgrid, the results of the laboratory-grade experimental system and the
simulated outcomes using the developed system model are compared. Different experiments
are carried out, such as a load switching, speed variations of the wind PMSG, speed
variations of the forcer of the wave LPMG, etc. Only the results under a sudden load-
switching condition are shown due to page limit. Fig. 13 shows the responses of the studied
system when the connected load is switching from 500 to 1000 W at t = 10 s under the rotor
speed of the wind PMSG of 450 r/min and the forcer speed of the wave LPMG of 1.2 m/s. In
each subplot shown in Fig. 13, the left one is the measured result, while the right one is the
simulated dynamic response, where Park’s transformation is used to transform the d–q-axis
components into a–b–c three-phase components. For observing the output three-phase
voltage or current clearly, only a-phase quantity is shown. Fig. 13(a) and (b) [Fig. 13(c) and
(d)] shows the measured and simulated dynamic responses of the output voltage and current
of the wind PMSG (the wave LPMG), respectively. Since the rotor speed of the wave LPMG
and the forcer movement are constant before the load switching, the output voltage can
remain stable. After the switching of the connected load, the output current rises up
immediately to supply the load demand. Comparing with the measured waveforms, both
frequency and magnitude of the output current of the wave LPMG rise obviously. Fig. 13(e)
and (f) plots the measured and simulated dynamic responses of the dc-side current of the
wind PMSG and the wave LPMG, respectively. During load switching, the currents rise
instantaneously. Since the generator cannot supply sufficient load demands, the bidirectional
dc/dc converter is properly operated to compensate the power loss and maintain stable
operation of the studied integrated system. However, the practical high-frequency
components shown in the measured results cannot be clearly observed in the simulated
dynamic responses since the high-frequency switching in the simulated model is neglected.
Fig. 13(g) and (h) illustrates the measured and simulated dynamic responses of the dc voltage
of the dc microgrid and the output current of the bidirectional dc/dc converter, respectively.
During load switching, the dc voltage of the dc microgrid decreases instantaneously since the
generator cannot supply sufficient load demands. By using the bidirectional dc/dc converter
to compensate the power loss immediately, the system can still remain stable, and the high-
frequency components are due to the switching elements. Fig. 13(i) plots the measured and
simulated dynamic responses of the dc current flowing into the dc/ac VSI, while Fig. 13(j)
and (k) draws the measured and simulated dynamic responses of the output voltage and
current of the dc/ac VSI, respectively. During load switching, the dc current flowing into the
dc/ac VSI rises from 10 to 20 A, and the output peak ac current of the dc/ac VSI jumps from
5 to 10 A. Comparing with the practical measurements, the magnitude of the oscillations of
the simulated dynamic responses is smaller, and no obvious high-frequency component can
be found. Moreover, the converter can offer stable load voltage to make the load operate
normally.
Fig : Measured (left) and simulated dynamic responses of the studied integrated system
subject to a sudden load-switching condition. (a) Output voltage of PMSG. (b) Output current
of PMSG. (c) Output voltage of the wind LPMG. (d) Output current of the wave LPMG. (e)
Output dc current of the wind PMSG. (f) Output dc current of the wave LPMG. (g) DC
voltage of the dc microgrid. (h) Output current of the bidirectional dc/dc converter. (i) DC
current flowing into the dc/ac inverter. (j) Output voltage of the dc/ac VSI. (k) Output current
of the dc/ac VSI.
CONCLUSION
An integration of both wind power and wave power generation systems joined with a dc
microgrid has been proposed. A laboratory-grade test system has been presented in this paper
to examine the fundamental operating characteristics of the studied integrated system fed to
isolated loads using a dc microgrid. For simulation parts, the results of the root-loci plot and
the time-domain responses have revealed that the studied integrated system with the proposed
dc microgrid can maintain stable operation under a sudden load-switching condition.
Comparative simulated and measured results under a load switching have been performed,
and it shows that the studied integrated system with the proposed dc microgrid can be
operated stably under different disturbance conditions, while both measured and simulated
results can match with each other.
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