direct integration of battery energy storage systems in distributed power generation

7/27/2019 Direct Integration of Battery Energy Storage Systems in Distributed Power Generation

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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 26, NO. 2, JUNE 2011 677

Direct Integration of Battery Energy Storage Systemsin Distributed Power Generation

S. D. Gamini Jayasinghe, Student Member, IEEE, D. Mahinda Vilathgamuwa, Senior Member, IEEE,and Udaya K. Madawala, Senior Member, IEEE

AbstractIn this paper, a wind energy conversion system inter-faced to the grid using a dual inverter is proposed. One of the twoinverters in the dual inverter is connected to the rectified outputof the wind generator while the other is directly connected to abattery energy storage system (BESS). This approach eliminatesthe need for an additional dcdc converter and thus reduces powerlosses, cost, and complexity. The main issue with this scheme isuncorrelated dynamic changes in dc-link voltages that results inunevenly distributed space vectors. A detailed analysis on the ef-fects of these variations is presented in this paper. Furthermore,a modified modulation technique is proposed to produce undis-torted currents even in the presence of unevenly distributed anddynamically changing space vectors. An analysis on the batterycharging/discharging process and maximum power point track-ing of the wind turbine generator is also presented. Simulationand experimental results are presented to verify the efficacy of theproposed modulation technique and battery charging/dischargingprocess.

Index TermsDual inverter, energy storage, non-integer voltageratio, wind energy.

I. INTRODUCTION

WITH the depletion of existing fossil fuel deposits, re-

newable resources such as solar, wind, and biomass

are emerging as alternative energy sources. However, the prob-

abilistic nature of these sources makes their grid integrationand reliability improvement difficult. Recently, microgrids have

been proposed to overcome this difficulty [1]. In order to make

such systems dispatchable, renewable sources should supply the

demand, at least for a limited period, irrespective of the fluc-

tuations present in the primary source. Certain fluctuations can

also be expected in the power demand. These fluctuations, irre-

spective of the origin, are the main reasons for instability and

instantaneous power imbalance of microgrids [2], [3]. The most

practical and effective manner in which these fluctuations can

be suppressed is by using an energy storage system [4][6].

Among all feasible energy storage technologies, battery sys-

tems can be regarded as the most developed and widely usedenergy storage device [7], [8]. The other technologies such as

Manuscript received August 19, 2010; revised November 30, 2010; acceptedJanuary 17, 2011. Date of publication April 7, 2011; date of current versionMay 18, 2011. Paper no. TEC-00314-2010.

S. D. G. Jayasinghe and D. M. Vilathgamuwa are with the School of Electri-cal and Electronic Engineering, Nanyang Technological University, Singapore639798 (e-mail: [email protected]; [email protected]).

U. K. Madawala is with the Department of Electrical and computer En-gineering, University of Auckland, Auckland 1142, New Zealand (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TEC.2011.2122262

flywheels, superconducting magnetic energy storage, and super-

capacitors are still in the development stage and require further

investigations [9], [10]. As a result, power quality enhancement

using battery energy storage systems (BESS) is actively pur-

sued in the field of distributed generation as evident from the

literature [11][21].

When it comes to system integration, the simplest way of

adding a BESS is the direct connection to the dc link of the grid-

side inverter [11][14]. Even though this connection is simple,

it suffers from several drawbacks such as uncontrolled voltage,

fixed current distribution governed by the internal resistance of

the battery bank, and lack of control over the power flow. Effects

of these issues can somewhat be reduced if an intermediate dc

dc converter is placed between the battery and the dc link. This

interfacing dcdc converter increases the system cost and power

losses. In addition to that, the low-pass filter, comprising of an

inductor and a capacitor, degrades the dynamic response. A 3-

level bidirectional dcdc converter has been proposed in [15] to

reduce voltage stresses on switching devices and to improve the

dynamic response with a reduced filter inductance. But it needs

four switches and a flying capacitor. Therefore, the interfacing

dcdc converter increases the system cost, power losses, and

complexity, even if an optimized design is used.A direct integration method, which is free from the aforemen-

tioned drawbacks and beneficial to both industry and academia,

is yet to be reported. This paper, therefore, presents a new direct

integration scheme for BESS with the use of grid-side inverter.

It utilizes the popular dual inverter topology, as shown in Fig. 1,

where two 2-level inverters are cascaded through a coupling

transformer. The two inverters are named as the main inverter

and the auxiliary inverter in line with their modes of opera-

tion. A battery bank is directly connected to the dc link of the

auxiliary inverter without an interfacing dcdc converter. Un-

like in the aforementioned simple direct connection topology,

the proposed system facilitates full controllability over charg-ing/discharging currents and voltage of the battery.

The main inverter operates at the fundamental frequency pro-

ducing square wave outputs. Harmonics of the square wave out-

put are compensated by the high-speed auxiliary inverter. This

particular frequency splitting arrangement can reduce switch-

ing losses as well as device ratings of the main inverter [34],

[35]. Another advantage of the proposed system is its ability to

produce up to 13 voltage levels in the phase voltage waveform

whereas the traditional 2-level inverter can produce only five

levels.

Redundancy and fault tolerance operation can be seen as

additional features of the proposed system where the faulty

0885-8969/$26.00 2011 IEEE


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678 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 26, NO. 2, JUNE 2011

Fig. 1. Proposed grid-side inverter with a BESS interface.

inverter can simply be disconnected by short circuiting corre-

sponding terminals of the transformer. In such situations, the

healthy inverter continues to be operational, but with less volt-

age levels [33]. Moreover, the 2-level inverter technology is wellmatured and standard modules are readily available in the mar-

ket. Therefore, the proposed system can easily be implemented

using off-the-shelf components.

In the proposed controller, charging/discharging currents of

the battery bank are controlled through the main inverter dc-

link voltage and as such it varies with the available wind power.

Furthermore, the auxiliary inverter dc-link voltage varies with

the state of charge (SoC) of the battery bank. The end result

of these uncorrelated variations is a noninteger dynamically

changing dc-link voltage ratio, which leads to unevenly dis-

tributed space vectors. Conventional modulation methods fail

to produce desired outputs under such situations [36], [37].Therefore, the challenge here is the generation of undistorted

current waveforms even in the presence of unevenly distributed

space vectors.

Extensive research has been done on modulation and control

of the aforementioned dual inverter topology, especially for mo-

tor drive applications [22][28]. They all have considered cases

where fixed-integer dc-link voltage ratios are present. A space

vector modulation (SVM) method with common mode rejection

is presented in [26]. A pulse width modulation (PWM) scheme

for this dual inverter is explained in [27] for 1:1 and 1:2 voltage

ratios. Although a power sharing controller is proposed in [28]

for dynamically varying dc-link voltages, it also assumes iden-

tical dc-link voltage variations thus making the ratio to be 1:1.A hierarchical modulation method is proposed in [29] to handle

dynamic changes in the auxiliary inverter dc-link voltage.

However, the authors have not found previous work address-

ing the issue of non-integer dynamically changing voltage ratio,

for simultaneous variations in both main and auxiliary inverter

dc-link voltages. Therefore, this paper presents an analysis on

the effects of such variations. Furthermore, modified modula-

tion and control techniques are presented to produce undistorted

current and deliver a constant amount of power to the grid even

in the presence of dynamic variations in both main and aux-

iliary inverter dc-link voltages. The viability of the proposed

direct integration method is tested with a wind energy conver-

sion system. Nevertheless, it can be used in the same way for

other renewable energy sources as well.

II. POWER SHARING AND MAXIMUM POWER POINT TRACKING

The output voltage vector of the dual inverter shown in Fig. 1

is equivalent to the addition of the main inverter voltage vector

and the auxiliary inverter voltage vector as in (1). Real powerdelivered to the load can be expressed as the dot product in

(2) [28]. Furthermore, the load power is equivalent to the sum

of the main inverter power and the auxiliary inverter power as

expressed in (3). Equations (4) and (5) show the relationships

between voltage vectors and corresponding switching states.

With the help of these five equations, an expression can be

derived for the power of the auxiliary inverter, in other words

battery power, as in (6). According to (6), it can be deduced that

the battery power PA has a linear relationship with the main

inverter dc-link voltage Vdc when the output power is constant

(i.e., i and vr are constant):

vr = vM + vA (1)

PL =3

2vr i (2)

PL =3

2(vM + vA ) i = PM + PA (3)

vM =2

3Vdc

SaM + SbMe

j (2/3) + ScMej (2/3)

(4)

vA =2

3Vdcx

SaA + SbA e

j (2/3) + ScA ej (2/3)

(5)

PA =3

2(vr vM) i (6)

where vr is the output voltage vector, vM is the main inverter

voltage vector, vA is the auxiliary inverter voltage vector, and i

is the current vector.

In order to verify this relationship, an experiment was carried

out using the setup shown in Fig. 2(a). A gradually increasing

dc voltage was applied to the main inverter while the battery

voltage was kept constant at 25 V. These input voltages are

plotted in Fig. 2(b). The controller was set to maintain constant

power dissipation through the load, and thus, the load current

was constant as shown in Fig. 2(d). Under this constant power

condition, the battery current showed a linear relationship to the

main inverter voltage with a negative slope as in Fig. 2(c). Since

the battery voltage is constant, the battery current is proportionalto the battery power. Therefore, these results confirm that the

battery power can be controlled by controlling the main inverter

dc-link voltage. The same argument can be extended to develop

a maximum power point tracking (MPPT) method for the wind

turbine generator (WTG) as follows.

According to (3), for a given output power PL , the main in-

verter power solely depends on the battery power. Therefore, the

maximum power point of the wind turbine can easily be tracked

by changing the battery power. Furthermore, the aforementioned

analysis reveals that the battery power can be controlled by con-

trolling the main inverter dc-link voltage. The usual practice is

to maintain this voltage at a constant level, with the help of a


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GAMINI JAYASINGHE et al.: DIRECT INTEGRATION OF BATTERY ENERGY STORAGE SYSTEMS 679

Fig. 2. (a) Experiment setup used to verify the linear relationship betweenmain inverter dc-link voltage and battery power. (b) DC-link voltage variations.(c) Battery current. (d) Load current of the a phase.

Fig. 3. MPPT controller block diagram.

controlled rectifier or a boost rectifier placed between the main

inverter and the WTG. The same controlled rectifier or the boost

rectifier can be used here to vary the main inverter dc-link volt-

age and thus indirectly track the maximum power point of the

WTG. The controller block diagram for this indirect MPPT is

shown in Fig 3. In this controller, the WTG model generates

a power reference based on the measured wind speed. Then, it

is compared with the instantaneous power of the main inverter.

The error is fed into a PI controller that generates a reference

for the main inverter dc-link voltage.

Fig. 4. Space vector diagrams and output voltage waveforms at differentvoltage ratios.

III. EFFECTS OF VARIABLE DC-LINK VOLTAGE RATIO

As explained in the previous section, the main inverter dc-link

voltage needs to be changed to track the maximum power point

of the WTG. This inevitably results in a non-integer dynami-

cally changing dc-link voltage ratio between the main inverter

and the auxiliary inverter. As the voltage ratio varies, space vec-

tors of the combined inverter get distributed unevenly as shown

in Fig. 4. The challenge here is to generate undistorted current

waveforms even in the presence unevenly distributed space vec-

tors. In the proposed system, it is carried out by using a modified

SVM technique. A detailed analysis on the proposed modulation


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method is given in Section IV and the purpose of this section

is to create the necessary background by analyzing effects of

variable voltage ratio.

All the space vector diagrams, shown in Fig. 4, contain seven

main inverter vectors, called main vectors. They are markedwith

large dots. The hexagon formed by these main vector points is

called the main hexagon. Auxiliary inverter vectors, named as

auxiliary vectors, are denoted by attached subhexagons at each

and every main vector point. From Fig. 4(a) to (c), only one

subhexagon is marked to avoid the diagram being too com-

plex. However, scattered dots in these three diagrams corre-

spond to the edges of missing subhexagons. The size of these

subhexagons depends on the battery voltage, which is assumed

to be constant in the following simulations. The circles found

in each space vector diagram represent the path of the refer-

ence voltage vector that remains unchanged for constant power

dispatch.

Fig. 4(a) shows the combined inverter space vector diagram

for the dc-link voltage ratio of 0 (Vdc = 0). At this point, due to

the absence of a power source in the main inverter dc-link, all theseven main vectors coincide at the origin turning the combined

inverter system into a conventional two-level inverter. In this

particular situation, the battery bank should supply the whole

demand or part of it depending on the capacity. Corresponding

inverter output voltage waveform is attached next to the space

vector diagram. When the main inverter dc-link voltage starts to

increase, corresponding main hexagon appears as in Fig. 4(b)

(f). At the ratio of 0.25, subhexagons overlap with each other as

shown in Fig. 4(b) producing four layers of hexagon rings. In

other words, the resultant space vector diagram is equivalent to

that of a 5-level inverter. But due to the uneven distribution of

the hexagons and the low resolution, only few voltage levels canbe seen in the attached waveform. At the ratio of 0.5, vectors get

distributed evenly and three layers of hexagon rings are formed

as shown in Fig. 4(c). Therefore, this particular state represents

a 4-level inverter. Corresponding voltage levels can clearly be

seen in the attached waveform in Fig. 4(c).

At the ratio of 1, the combined inverter resembles a 3-level

inverter (two layers of hexagon rings) with the space vector

pattern shown in Fig. 4(d). Further increase of the main inverter

voltage would expand the space vector diagram as shown in

Fig. 4(e) and (f). At the ratio of 1.5, four layers of hexagons

emerge again resulting in a 5-level inverter. But this time only

outermost vectors are distributed unevenly. Furthermore, the

resolution between voltage levels is high compared to that inFig. 4(b). The space vector diagram at the ratio of 2 is equivalent

to a 4-level inverter as evident from Fig. 4(f). The attached

waveform shows only five levels since the reference circle is

insidethe first layer of hexagon.If thereferencecircle is enlarged

to the next layer, more voltage levels would appear in the output

voltage.

The output voltage waveforms shown in Fig. 4 are in fact

the snapshots taken at six discrete values of the dc-link voltage

ratio. But as explained earlier, the voltage ratio can take any

value between 0 and 2. Therefore, it is important to see how

the output voltage waveform looks like at intermediate values.

Hence, a simulation was carried using the voltage ratio as the

Fig. 5. (a) Inverter output voltage, vas , at different voltage ratios. (b) Funda-mental component ofvas .

Fig. 6. Square wave output of the main inverter and smoothing effect of theauxiliary inverter.

independent variable and the result is shown in Fig. 5(a). At

the ratio of 0, the output voltage waveform is similar to that

in Fig. 4(a). The same scenario can be seen near the ratio of

1.125 as well. In between these two points, a varying waveform

can be seen and it gets optimized at the ratio of 0.5. Beyond

this point, the inverter output voltage waveform gets distortedgradually until the ratio reaches 1. This is due to the decrease

of available voltage levels. However, beyond the ratio of 1,

the waveform again starts to show a continuous improvement

as seen in the latter half of the waveform in Fig. 5(a). With

this result, it can be deduced that the proposed dual inverter

under a noninteger dynamically changing voltage ratio would

produce variable voltage levels similar to that of 2-, 3-, 4-, and

5-level inverters. But with the proposed modulation technique,

it still can synthesize the desired fundamental output voltage

waveform as shown in Fig. 5(b), until the ratio reaches 1.75.

Distortions can occur beyond this point.

IV. MODULATION STRATEGY

As mentioned in Section 1, the main inverter operates in the

six-step mode producing square wave outputs. The auxiliary

inverter is used to compensate harmonic power of the square

wave output and to regulate the real power exchange with the

grid. This combined operation is illustrated in Fig. 6 where the

auxiliary inverter is purposely turned off until 20 ms. During

this period, the only available output is the main inverter square

wave output voltage as shown in the first half of the waveform in

Fig. 6. Harmonic distortion of the output voltage is significant

under this operation. After 20 ms, the auxiliary inverter is turned

on, and consequently, the output voltage becomes smooth with


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Fig. 7. (a) Space vector diagram with the three-axis coordinate system. (b)Shaded triangle with dwell times.

Fig. 8. Block diagram showing the proposed modulation method.

low harmonic distortion as shown by the second half of the

waveform in Fig. 6.

A modified SVM method is proposed for the auxiliary in-verter, and thus, the overall modulation can be considered as a

combination of the six-step mode and SVM. The complexity

of this combined modulation process gets further amplified by

the dynamic variations present in dc-link voltages. To reduce

most of the computational complexities, the three-axis coordi-

nate system shown in Fig. 7(a) is used [30]. In this coordinate

system, each and every vector point is represented by three co-

ordinates that are projections on the axes ofVab , Vbc , and Vca . In

fact, these three axes represent line-to-line voltages of a three-

phase system. If dc-link voltages are fixed, these projections can

be normalized to get discrete values such as 1, 0, and 1. But inthe proposed system, dc-link voltages are not fixed, and hence,

projection can take any value within the operating range. As asolution to this issue, all the calculations are carried out using

actual voltages instead of normalized values, which should be

updated at every switching cycle. A simplified block diagram

of the proposed modulator is shown in Fig. 8.

Since the main inverter operates in the six-step mode, the

main vector selection and gate signal generation units in the

modulator would require only the angle information of the ref-

erence vector, i.e. r . Based on this angle, three normalizedcoordinates of the current main vector, Sab , Sbc , and Sca , are

defined as temporary variables that are then used to calculate

actual projections as expressed in (7). At the same time, three

axis coordinates of the reference voltage vector vr are calculated

TABLE ISWITCHING SIGNAL LOOKUP TABLE

as corresponding line-to-line voltages vab , vbc , and vca . Accord-ing to (1), the difference between the reference vector and the

main vector is the compensation vector that should be supplied

by the auxiliary inverter, and thus, it becomes the reference for

the auxiliary inverter. Projections of this new reference are cal-

culated using (8) and subsequently normalized by (9) to obtain

floor and ceiling values as floor(x) is the greatest integerxandceiling(x) is the smallest integer x. Those floor and ceilingvalues of the normalized reference are then used to locate the

triangle in the subhexagon, for example the shaded triangle in

Fig. 7(a), where the reference vector falls [30], [31]. The ends of

this triangle contain required auxiliary vectors V1 , V2 , and V3 .

But still they are defined using three axis coordinates. In order tomap three axis coordinates into gate signals, the transformation

in (10) and the lookup table in Table I are used:

[Vab Vbc Vca ]T =

Vdc3

[Sab Sbc Sca ]T

(7)

Sab , Sbc , Sca = {1, 0, 1}[vabx vbcx vca x ]

T = (Vab vab ) (Vbc vbc ) (Vca vca )]T

(8)

[vabx vbcx vca x ]T =

3

Vdcx[vabx vbc x vca x ]

T (9)

Ssn = 4x + 2y + z + 3 (10)

where the three axis coordinates are assumed to be in the form

ofVn (x,y,z), n = 1,2,3, and the resulting switching state numberis denoted by Ssn .

Once the three nearest vectors are identified, the next step

is to find corresponding dwell times for each and every vec-

tor. Usually, this part involves multiplications, divisions, and

trigonometric operations that consume significant amount of

processing time. But in the SVM technique adopted in this pa-

per, dwell time calculations are very simple and straightforward.

It is proven in [30] that dwell times are proportional to the dis-tance from the reference vector point to the opposite leg of the

triangle. For example, for the vector V1 in Fig. 7(b), the dwell

time is proportional to the distance d1 from the reference vector

to V2V3 leg of the triangle. In other words, it is proportionalto the difference between vab component (projection on the Vabaxis) of the reference and the corresponding floor value. The

other two dwell times, d2 and d3 , can also be calculated in the

same way. Therefore, to calculate dwell times only subtractions

are needed. This yields fast operation and simple implementa-

tion in hardware level. After finding auxiliary vectors and dwell

times, five-segment switching is used in the auxiliary inverter

as described in [32].


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Fig. 9. Voltage vector additions for different input power conditions at r =60: (a) case 1, (b) case 2, and (c) case 3.

V. ANALYSIS ON BATTERY CHARGING AND DISCHARGING

Charging and discharging process of the battery can be ex-

plained under three different cases where the available wind

power is less than the demand, equal to the demand, and higher

than the demand. In the first case, the battery bank should dis-

charge to supply the deficit and in the third case it should absorbthe surplus of power. However, in the second case, the battery

bank will not be affected. With the help of Figs. 2 and 4, a vague

relationship between these three cases and the dc-link voltage

ratio can be identified as follows.

Case 1: corresponds to 0 < dc-link voltage ratio < 1.Case 2: corresponds to dc-link voltage ratio 1.Case 3: corresponds to 1 < dc-link voltage ratio < 2.

To understand it further, voltage vector additions shown in

Fig. 9 can be used. The vector additions shown in Fig. 9 cor-

respond to r = 600 . At this angle, and its multiples, both the

main vector and the auxiliary vector get aligned making theanalysis easier. When the dc-link voltage ratio is less than 1,

the circular path of the reference voltage vector lies outside the

main hexagon as shown in Fig. 4(b) and (c). Hence, the magni-

tude of the main inverter vector vM is obviously less than that

of the reference vector vr , as shown in Fig. 9(a). As a result,

the auxiliary inverter voltage vector vA has to be used to fill the

gap. Since the same current flows through both inverters, cor-

responding share of power depends only on the magnitude of

the voltage vector. In certain cases, where both inverters should

equally share the output power, the main and auxiliary inverter

vectors are set to be equal [33]. But in the proposed system, the

maximum possible power should be extracted from the main

inverter, and thus, only the balance should be handled by theauxiliary inverter. This automatically happens when the main

inverter is operating at the six-step mode, and hence, no special

attention is required.

When both the circle and main hexagon are at the same mag-

nitude, the main inverter vector itself is sufficient to synthesize

the reference vector as in Fig. 9(b). This roughly corresponds

to the dc-link voltage ratio of 1, as in Fig. 4(d). However, it is

noteworthy to mention that the size of the reference circle, and

hence the dc-link voltage ratio, where the main hexagon meets

the circle, can change with the power demand. When the dc-link

voltage ratio is greater than 1, the main hexagon exceeds the ref-

erence circle as in Fig. 4(e) and (f). In this case, the direction

Fig. 10. (a) Wind speed. (b) Main inverter dc-link voltage Vd c and auxiliaryinverter dc-link voltage Vd cx . (c) Wind power Pw and dispatch power Pd . (d)Battery current Ib . (e) Inverter output current ias . (f) Inverter output voltagebefore filtering vas . (g) Inverter output voltage after filtering, vas ,f .

of the auxiliary inverter vector becomes negative as shown inFig. 9(c). This indicates that part of the available wind power

flows into the battery bank.

According to the abovementioned analysis, it can be deduced

that as long as the circle lies outside themain hexagon the battery

current is positive (discharge). When the reference circle and the

main hexagon are equal in size, the battery current tends to be

zero, which indicates that the available wind power is barely

sufficient to supply the demand. Further increase of the main

inverter dc-link voltage makes the main hexagon to exceed the

voltage reference circle and changes the direction of the battery

current. In other words, the battery bank absorbs part of the

available wind power.


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TABLE IISYSTEM PARAMETERS OF THE SIMULATION SETUP

VI. SIMULATION RESULTS

The proposed concept of ESS interfacing with the grid side

inverter has been tested in a MATLAB/SIMULINK-PLECS

digital simulation platform. The wind speed profile shown in

Fig. 10(a) is used in the simulation, which in turn produces

a dc-link voltage variation at the main inverter as shown in

Fig. 10(b). Corresponding wind power variation Pw and dis-patch power Pd are shown in Fig. 10(c). From this graph, it can

be concluded that the proposed system has the ability to supply

the demand amidst fluctuations present in the input power. The

surplus or deficit of power is supplied or absorbed by the battery

with a current profile as shown in Fig. 10(d). The output cur-

rent and voltage of the inverter are shown in Fig. 10(e) and (f),

respectively. Although the inverter output voltage shows some

fluctuations, once it is passed through a low-pass filter, a smooth

waveform can be observed as shown in Fig. 10(g). The system

parameters of the simulation setup are given in Table II.

VII. EXPERIMENTAL RESULTS

The schematic diagram and photographs of the laboratory

prototype are shown in Fig. 11(a) and (b), respectively. A pro-

grammable ac source is used to emulate the WTG and its output

was rectified using a bridge rectifier. The ac source was pro-

grammed in a way that the rectifier output voltage, in other

words the main inverter dc-link voltage, varies as shown in the

red graph (Vdc ) of Fig. 11(c). During the experiment, the bat-

tery voltage remained at 25 V as shown by the green line (Vdcx )

in Fig. 11(c). The two inverters were coupled through an RL

load and their parameters are given in Table III. The controller

was set to maintain constant power dissipation through the load

as shown in Fig. 11(d) by the graph marked with PL . The in-put power and battery power variations are plotted in the same

figure and are marked as PM and PA , respectively. The corre-

sponding battery current variation is shown in Fig. 11(e). The

shape of this graph is similar to that of the battery power dia-

gram since the battery voltage remains constant. Furthermore,

when the main inverter voltage is lower than the battery voltage,

the battery current is positive and it is negative when the main

inverter voltage exceeds the battery voltage. An enlarged view

of the inverter output current is shown in Fig. 11(f). An en-

larged view of the inverter output voltage is given in Fig. 11(g)

to show the multilevel operation of the proposed dual inverter

system.

Fig. 11. (a)Schematic of theexperimentalsetup. (b)Photographsof theexper-imental setup. (c) DC-link voltages, Vd c and Vd cx . (d)Maininverter power PM ,battery power PA , and output power PL . (e) Battery current Id cx. (f) Enlarged

view of the output current. (g) Enlarged view of the inverter output voltage.


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TABLE IIISYSTEM PARAMETERS OF THE EXPERIMENTAL SETUP

VIII. CONCLUSION

Additional switches and converters required to integrate en-

ergy storage devices into distributed power systems can be

avoided if the grid-side inverter itself can be used as the in-

terface. Accordingly, a modified topology of the popular dual

inverter system has been proposed to connect a battery bank

directly to the auxiliary inverter dc link. The challenge with

this topology is the uncorrelated and dynamic changes present

in dc-link voltages, which results in unevenly distributed spacevectors. A detailed analysis on the effects of such variations is

presented in this paper. Furthermore, a modified SVM method is

proposed to produce desired current waveforms even in the pres-

ence of unevenly distributed space vectors. The battery charg-

ing/discharging process with the proposed control method is

presented in this paper. Experimental and simulation results

are presented to verify the efficacy of the proposed modulation

method and battery charging/discharging process.

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S. D. Gamini Jayasinghe (S10) received the B.Sc.degree in electronics and telecommunication engi-neering fromthe University of Moratuwa, Moratuwa,

Sri Lanka, in 2003. In 2004, he joined the DialogUoM Mobile Communications Research Laboratory.He is currently working toward the Ph.D. degree inthe School of Electrical and Electronic Engineering,Nanyang Technological University, Singapore.

D. Mahinda Vilathgamuwa (S90M93SM99)received the B.Sc. and Ph.D. degrees in electri-cal engineering from the University of Moratuwa,Moratuwa, Sri Lanka, and Cambridge University,Cambridge, U. K., in 1985 and 1993, respectively.

In 1993, he joined the School of Electrical andElectronic Engineering,NanyangTechnological Uni-versity, Singapore, as a Lecturer where he is currentlyan Associate Professor. His current research interestsinclude power electronic converters, electrical drives,and power quality.

Dr. Vilathgamuwa is the Chairman of IEEE Section, Singapore, and a mem-ber of the Power Electronics Technical Committee for the Industrial ElectronicsSociety.

Udaya K. Madawala (M94SM06) received theB.Sc. (Hons.) degree in electrical engineering fromtheUniversityof Moratuwa, Moratuwa, SriLanka, in1987, and the Ph.D. degree in power electronics fromthe University of Auckland, Auckland, New Zealand,in 1993.

In 1997, he joined the Department of Electricaland Computer Engineering, University of Auckland,where he is currently a Senior Lecturer. His researchinterests include motor/generator design and control,power electronics, renewable energy, and super ca-

pacitor applications.Dr. Madawala is an active IEEE volunteer, and serves as an Associate Editor

and a member of the Power Electronics Technical Committee for the IEEEIndustrial Electronics Society.

direct integration of battery energy storage systems in distributed power generation

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