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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