1077-2618/11/$26.00©2011 IEEE

BY RENATA CARNIELETTO,DANILO IGLESIAS BRANDAO,SIDDHARTH SURYANARAYANAN,FELIX A. FARRET,& MARCELO G. SIMOES

THE PRIMARY GOAL OF

this article is to discuss the

development of intelligent con-

trols for a power electronic

inverter capable of interfacing a photovoltaic

(PV)-based unit to the utility grid. The in-

verter is designed as a single-phase, full-bridge

converter operating at 120 V, 60 Hz ac. The

control functionalities of the inverter are defined

under the perspective of the Smart Grid Initia-

tive (SGI) of the U.S. Department of Energy and

substantiated via case studies in this article as the

ability to supply real and reactive power to local

loads, supply real and reactive power to other

utility loads up to the rated capacity of the

inverter, provide voltage support at the point of

common coupling (PCC), store energy in a lead-

acid battery bank, and enable the provision of control options

to the consumer based on near real-time electricity information

obtained from the utility through advancedmetering devices.

Integration of distributed generation (DG) sources to

the electric distribution system has potential advantages

including improved supply reliability, custom power

A multifunctional single-phasevoltage-source inverter

Digital Object Identifier 10.1109/MIAS.2010.939651

Date of publication: 28 June 2011

© ARTVILLE

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quality, local use of thermal energy, the ability to off-loadelectric energy from the transmission grid, and the provisionof an avenue to meet mandatory renewable portfoliostandards (RPSs) [1]. The U.S. federal government has rati-fied the SGI as its official policy to modernizing the electric-ity grid, which calls for increased levels of renewable energysources in the grid; provision of timely information and con-trol options to consumers; deployment of smart technologies,appliances, and advanced metering devices; and real-timepricing of electricity [2]. The perspective of SGI is to developthe functional controls for a power electronic inverter capableof interfacing PVinstallations to the utility grid.

For the purpose of this article, smart controls of a voltage-source inverter are defined as the combined functional abilityto supply power to local loads, supply power to other utilityloads up to rated capacity of the inverter, provide voltage sup-port at the PCC of the utility, store energy in a local lead-acidbattery bank, and provide control options to the consumerbased on near real-time electricity information obtained fromthe utility through advanced metering devices. A generalmodular design methodology for flexible inverters that maycater to increasing demands in the smart grid is presented in[3]; however, the combined smart functionalities describedhere are deemed unique. The smart inverter functionalitiesdescribed in this study look beyond the recommendations ofthe current national technical standard for interconnectingDG sources to the grid—IEEE Standard 1547 [4]—in pro-viding voltage support at the PCC—thus, offering an ancil-lary service in case of low-voltage scenarios. Traditionally,voltage sags in distribution systems are corrected usingutility-owned (or) -operated capacitor banks; however,with the advent of inverters with smart functionalities, theability to regulate voltage at the PCC is brought to thecustomer. The authors have not probed the safety issuesstemming from performing voltage control on the grid sideusing the proposed inverter setup. Based on real-time spotpricing of electricity obtained from the utility using anadvanced metering device, the inverter control algorithmdetermines the optimal operating mode. This algorithmenables the inverter to: 1) schedule local loads and 2) deter-mine either to locally store energy or sell energy to the grid.

Description of the Inverter ControlThe voltage-source inverter is designed as a 5 kVA one-phase, full-bridge converter operating at 120 V, 60 Hz acwith current control and voltage control to function in twomodes: grid tied and islanded, respectively [5]–[7]. Theentire control is developed in the DQ frame with a virtualQ axis (as the application is one phase). In the technicalliterature, this second virtual quantity is obtained either bythe derivative of the fundamental signal [8] or by delayingthe real-axis quantity by one quarter of the line period [9].In this implementation, the latter technique is employed.Figure 1 depicts the block diagram of the inverter simula-tion on a popular modeling and simulation platform.

Phase-locked loops (PLLs) are used to supply the controlloops with phase angle information [10]–[12], as shown inFigure 2. The current and voltage control shown use four pro-portional-integral (PI) controllers: two equal PIs for theinverter current (id and iq) and two equal PIs for the invertervoltage (vd and vq). The PI compensator is chosen for itssimplicity and ease in implementation, and the respective gains

PV

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smartfunc

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can be tuned through extensive simulations. In the case of thedevice shown in Figure 2, the PI compensators are used togenerate the references for a pulsewidth modulator (PWM).According to Figure 2, the main control consists of two loops:voltage loop, which is enabled when the inverter operates inislandedmode, and current loop for a grid-connected condition.

The Smart Functionalities of the InverterThe primary intent of the inverter development with smartfunctionalities is to enable an efficient interconnection andeconomical operation for dispersed PV-based DG installa-tions to the utility grid. The motivation of this study comesfrom a pilot program by a local utility in Colorado to imple-ment some of the smart grid recommendations in a candidatemedium-sized metropolitan area [13]. Such an implementa-tion is based on the ubiquitous deployment of PV installa-tions at a residential level of the candidate city. Somedistinctive aspects of this pilot program are smart metering,the incorporation of smart appliances, the provision of pric-ing information to consumers, the provision of some control

options to consumers, and information exchange on a fullynetworked system enabled by massively deployed sensors. Itis in this regard that the inverter with the aforementionedsmart functionalities is being proposed in this article.

The local load served by the inverter is modeled as twocomponents: primary and secondary VSI loads, which distin-guishes the critical loads from others that can be scheduled atthe location. So if the inverter is operating at the islandedmode and it does not have enough power to supply all localloads, only the VSI primary load will be supplied. Anotherconvenience of this load set is the ability to operate in the eco-nomic mode. This will be explained in the following sections.

The input to the smart inverter is a steady-state voltageof 350 Vdc, provided by PV panels [14], with a nominaloutput voltage of 192 V. A dc–dc boost converter has beenused in the model to raise the PV voltage level to 350 Vdc.The inverter setup also includes a lead-acid battery storagebank with nominal voltage of 192 V and 24 Ah cells [15]connected to the dc link through a bidirectional dc–dcbuck-boost converter, modeled as shown in Figure 3. A

2

Current Loop Control

p_ref

q_ref

(p.u.)

(p.u.)1

2

vVSI

iVSI iVSI iVSI_dq

vVSI_dq v2dq

vdvq_ref

vVSI

[1 0]

vd_ref vq_ref

PLL

thetaVSI

thetaVSI

pq_ref

pq_ref/Idq_ref

Id_refIq_ref

i2dqid_refIq_ref φ (degree)

m

φ (degree)m

m

φ P1

m

φ P1

Current Control

Voltage Control

PWM Modulator

PWM Modulator

Brkr

Voltage Loop Control

Conversionto DQ Frame

vVSI Switch

1pwm_out

Conversionto DQ Frame

vVSI thetaVSI

Current control and voltage control loops for the inverter with smart functionalities.

3

VoltageReference

350

in1

dc LinkSide (Vdc)

in 2

v +–

Voltage Control Loop

12,235×[1 2×683 683×683]

s3 + 2×32,509s2 + 32,509×32,509s

PWMGenerator 1

PWMGenerator 2

> 0 0 >

Out

InverterControl Algorithm

S1

C2 = 1.3 mF

C1 = 560 µF

out1

out2

BatterySide (VB)

CurrentReference

CurrentControl Loop

-C-–+–+

D1

D2

S2

+ –i

2

2

1

1

g

g

L = 430 µH

2778×[1 1,700]

s2 + 94,248s

3

1

4

2

Block diagram of bidirectional dc–dc buck-boost converter subsystem in the inverter setup.29

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battery model from a popular library simulation platform[16] was used for this purpose. The storage subsystem bringsflexibility to the system, e.g., the ability to supply localloads when the inverter is islanded without enough power,to store cheap energy, and to sell when the price is higher.

STATCOM FunctionAs affirmed by [17], if individual distributed energy (DE) sys-tems are allowed to regulate reactive power, they can also beused to provide voltage support at the low-voltage single-phasedistribution level. Simulations with inverters for DG systemsindicate that the invertermust be connected as a static var com-pensator (STATCOM), where the reactive power is injectedinto the ac grid to regulate voltage at the PCC.

The STATCOM function block used in this inverter setupis shown in Figure 4; this block measures the voltage sag (gridvoltage magnitudeminus nominal voltage reference), if any onthe grid side, to calculate the necessary reactive power for volt-age compensation. The PI controller keeps the sag voltagenull. There may be times in the inverter operation when itmay not be profitable to perform a voltage compensation, suchas during times when the real power spot pricing is higher;during such times, a controlled switch (see Figure 4) is usedfor bypassing the voltage compensation functionality of theinverter, and only real power is supplied to the grid.

Islanding and Reclosure FunctionThe islanding and reclosure function performs the task of con-necting (and islanding) the inverter-based DG to (and from)

the grid based on IEEE Standard 1547[4]. The functionality is modeled as asubsystem with the following inputparameters: frequency, phase, and DQframe voltage of the grid (denoted asFreq_Grid, Theta_Grid, andVdq_Grid,respectively in Figure 5) and inverterside (denoted as Freq_VSI, Theta_VSI,and Vdq_VSI, respectively in Figure 5).The algorithm compares the inputs withIEEE Standard 1547 recommendationsand generates an output signal (Brkr) inthe required time frame according toIEEE Standard 1547 to island or toreclose the inverter to the grid. It ispertinent to note that: 1) the 1547recommendations are not reproducedin this article and the attention of thecurious reader is pointed to [4] and 2)while the inverter looks beyond IEEE

Standard 1547 in its ability to regulate the voltage at thePCC, it conforms to IEEE Standard 1547 for grid connec-tion and disconnection.

Bidirectional DC–DC Buck-Boost ConverterThe bidirectional dc–dc buck-boost converter shown inFigure 3 is responsible for controlling the charge and dischargeprocesses of the battery setup. This converter can behave eitheras buck or as boost converter depending on which switch (S1or S2 in Figure 3) is ON.When the battery is charging, switchS1 is ON, and the converter runs in the buck mode. Whenthe battery is providing power to the inverter, the S2 is ON,and the converter works in the boost mode. The specificationsof the dc–dc converter are shown in Table 1.

The inverter control algorithm has been used to decidethe buck-boost converter action. For the buck functionality,a current control loop was designed based on the averagecurrent method [18]. For the boost functionality, a voltagecontrol loop was designed based on the K factor [19].

The buck-boost inductor nominal current IL is calcu-lated as shown in

IL ¼P

VB: (1)

Freq_GridFreq_VSI

Theta_GridTheta_VSIVdq_GridVdq_VSI

IEEEStandard 1547

Inputs Output Brkr

Islanding andReclosure Conditions

5Islandingand reclosure subsystemof the inverter-basedDGsetup.

freqg

sin cosg Sin_Cos

v3pu

Freq m

Inφ

DiscretePLL-Driven

Fundamental Value

1 p.u.

1

Voltage SagMeasurement

+– PI

DiscretePI Controller

Fix Inverter ApparentPower at 5 kVA

Switch

Qref

Pref

Pref_kW

1

2

Brkr_Q

[Qref]

[Qref] ×

sqrt25 +–

Saturation4

0

Block diagram of the STATCOM function used in the inverter-based DG setup.

TABLE 1. SPECIFICATIONS OF THE BIDIRECTIONALDC–DC BUCK-BOOST CONVERTER USED IN THEINVERTER-BASED DG SETUP.

Quantity (Symbol) Value

Buck-boost nominal power (P) 5 kW

DC link nominal voltage (Vdc) 350 V

Battery nominal voltage (VB) 192 V

Switching frequency (fs) 15 kHz

Resistance of battery bank (rB) 0.2 X

Inductor resistance (rL) 0.02 X

Resistance of the C1 and C2

capacitors (rC)0.05 X

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The inductance (L) of the buck-boost converter is designed to limit theripple current at the dc link to 50% ofthe nominal current (IL), as shown in

L ¼ VBðVdc � VBÞfsVdc0:5IL

: (2)

The buck capacitor, C1, is designedto limit the ripple voltage at the batteryto 0.1% of the nominal battery voltage(VB), as shown in

C1 ¼0:5IL

8fs0:001VB, (3)

and the boost capacitor, C2, is intendedto limit the ripple voltage at the dc linkto 0.1% of the nominal dc link voltage (Vdc), as shown in

C2 ¼PðVdc � VBÞfs0:001V

3dc

: (4)

Current Control LoopThe type 2 controllerG2(s), shown in (5), has been used for thismethod of current control of the buck-boost converter [20].

G2(s) ¼K2(sþ xz)

s(sþ xp): (5)

The high-frequency pole xp must be chosen close to theswitching frequency fs, the zero xz must lie in between onehalf and one third of the resonant frequency, and the K2 is atype 2 dc gain, as shown in Figure 3.

Voltage-Control LoopThe transfer function of the small signal dynamic boostmodel, Tboost(s), can be expressed as [20]

Tboost(s) ¼�1:33 10�4s2 þ 0:22sþ 3:53 104

9:43 10�5s2 þ 1:33 10�2sþ 47:2: (6)

Figure 6 shows the Bode plot of theopen-loop transfer function for the un-compensated system, Tboost(s), as perthe unit proportional control method.

The zero-crossing gain frequency,xGC, has been chosen as 1/20th of theangular switching frequency (xs ¼2pfs). The phase margin (PM) hasbeen adopted at 60�. Thus, theboost phase advance was calculatedto be 147� [20]. As the boost phaseadvance is higher than 90�, a type 3controller G3(s) shown in (7) waschosen [20] as

G3(s) ¼K3(sþ xz)(sþ xz)

s(sþ xp)(sþ xp), (7)

where the xz ¼ xGC

� ffiffiffiffiKp

, the xp ¼xGC

ffiffiffiffiKp

; and K3 is type 3 dc gain,as shown in Figure 3. K is a constantcalculated using the boost-phase ad-vance [20].

Figure 7 shows the Bode plot of thetransfer function for the open-loop com-pensated system, Tboost(s) as per unitsystem for the type 3 control method.

From Figure 6, it is observed thatPM and the gain margin (GM) are infi-nite and �2.82 dB, respectively, whichindicates an unstable system. FromFigure 7, it is observed that PM is60.3� and GM is 9.72 dB, which issatisfactory for a stable close systembecause PM is almost in between 30�and 60� and GM is higher than 6 dB

[21]. This represents good robustness, which is importantfor future implementation of the device in hardware proto-type. The prototyping of the device is beyond the scope ofthis article.

Premise of Operation of the Smart InverterThe smart functionalities of the inverter are aimed at theprovision of real and reactive power support to local loads,the provision of real and reactive power to grid loads up tothe rated capacity, the option to control voltage at the PCCduring voltage sags, and decision-making ability aided byinformation of real-time pricing obtained through advancedmetering devices from the utility grid. Based on these func-tionalities, the inverter operation is governed by certainrules, which determine the mode of operation, identified inthis article as supermodes and submodes. Depending on thestatus connection to the grid as determined by compliancewith IEEE Standard 1547, there exist two supermodes:stand-alone (S1) and grid-tied (S2) modes.

In supermode S1, i.e., the stand-alone mode, theinverter is islanded (isolated) from the electric distributionsystem, and it is subject to operation under one of the fol-lowing three submodes, namely, s1, s2, and s3, dependingupon the available inverter active power (PINV) and the

80

60

40

20

0

360

315

270

225

180102 103 104 105

Frequency (rad/s)

Pha

se (

degr

ee)

Mag

nitu

de (

dB)

Open-Loop Bode

6Open-loop Bode plot for the uncompensated system.

THE PICOMPENSATORS

ARE USED TOGENERATE THE

REFERENCES FORA SINUSOIDALPULSEWIDTHMODULATOR.

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local active power demand (ZINV), wherePINV represents the total power outputof the PV panels and the battery bankoutput, and ZINV represents the sum ofthe primary VSI load plus secondaryVSI load.

In submode 1 under supermode 1(identified as S1s1), when PINV is lesserthan ZINV, the power output of the PVpanels and stored energy are not enoughto supply the full demand of the localload. In such circumstances, prioritiza-tion of local demand is effected, andselected loads (primary VSI load) arepowered by the inverter. If, after theselection of loads, there is any remain-ing power from the PV panel, it will bedirected to storage in the battery bank.Typically, such a redirection to theenergy storage depends on the level ofthe state of charge (SOC). It is important to note that theSOC control of battery banks has not been developed bythe authors.

In submode 2 under supermode 1 (identified as S1s2),when PINV is greater than ZINV, the available inverterpower is greater than the local demand. In this case, theexcess power is routed to the battery banks for storage.

In submode 3 under supermode 1 (identified as S1s3),PINV is equal to ZINV, wherein the available inverter poweris equal to the local load demand. In this case, the inverterpowers the local loads without storage. In this case, priori-tization of loads may be effected if there is a need to storesome of the energy for later use.

In supermode S2, i.e., the grid-tied mode, the inverteris interconnected to the electric distribution system andis subject to operation under one of the following foursubmodes, s1, s2, s3, and s4, depending on the availableinverter active power (PINV), local active power demand(ZINV), and economic considerations for trading activeand reactive power with the grid on a spot-pricing basis.The variables for economic consideration include the spot

price to sell active power to grid ($PS),spot price to sell reactive power togrid ($QS), and a threshold value ofthe grid pricing of an electricity unitthat will enable the consumer to decidewhich loads will be powered using theinverter. In this case, it is assumed thatthe grid has infinite demand, i.e., thegrid will purchase whatever the inverterintends to sell.

In submode 1 under supermode 2(identified as S2s1), when $QS isgreater or equal than $PS, the inverteris controlled to provide voltage sup-port compensation to the grid. If thereshould exist additional inverter capa-bility to provide real power, then theinverter is controlled so that ZINV issupplied by the PINV, and any remain-ing power is sold to the grid or stored

in the battery bank [22].In submode 2 under supermode 2 (identified as S2s2),

when $QS is lesser than $PS, the inverter is controlled to fixthe reactive power reference to zero and to supply real powerto local loads ZINV, and any remaining power is sold to thegrid or stored in battery banks [22].

There is another submode under this supermode, i.e.,S2s3. This is related to the option of powering local loadsusing the inverter versus the option of buying activepower from the grid when there is power available fromDG. This can be chosen based on the comparison of thereal-time electricity pricing obtained from the grid ($PB)with a threshold value, such as the marginal cost ofelectricity production or a set customer preference, de-noted as MCP. If $PB is lesser than MCP, then the inverterload is supplied by the grid, and PINV is stored in a bat-tery storage for consumption or selling to the grid later,possibly during isolation from the grid or when real-timepricing of electricity is conducive to profitability, respec-tively. Or if there is no PINV available, then the electricenergy can be purchased from the grid and stored in a bat-

tery for later use. If $PB is greaterthan the MCP, then the inverter is socontrolled that ZINV is supplied byPINV, and the stored energy in thebattery bank and any remainingpower is sold to the grid. The use ofthe production marginal cost maynot be applicable in the case of PVsystems; however, if the installationconsiders customer preferences asinput as in [22], then the aboverationale can be upheld as shown incase study 1.

Submode 4 under supermode 2(identified as S2s4) refers to the oper-ation at an economic mode; this modeis being proposed as an alternativeunder the mode S2s3. This typicallyoccurs when the cost of purchasingelectricity from the grid is relativelyexpensive, i.e., such as described in

7

60

40

20

0

–20–40360

270

90

180

101 102 103 104 105 106

Frequency (rad/s)

Pha

se (

degr

ee)

Mag

nitu

de (

dB)

Open-Loop Bode

Open-loop Bode plot for the compensated system.

THE LOCAL LOADSERVED BY THEINVERTER ISMODELED AS

TWOCOMPONENTS,PRIMARY AND

SECONDARY VSILOADS.

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S2s3; thus, the inverter is set up forpowering all its local loads while con-nected to the grid. However, in such acase, if the inverter power is notenough to supply its loads, the cus-tomer has an option to operate at aneconomic mode, i.e., effecting priori-tization of primary VSI loads andshedding the secondary VSI loads(as in S1s1). This submode is achievedin the simulation by the use of aflag variable; if the flag is set to zero,it operates in the economic modeusing variable loads; and if the flag isset to one, it operates in an alwayssupplying loads mode, as the inverterpower is not enough, power from thegrid is purchased to supply theremaining loads.

It is pertinent to note that thisstudy does not consider the time linefor powering loads or charging stor-age, i.e., in terms of energy demandedand supplied. Consideration of theenergy supplied and demanded isinherently tied to hours of availablesunlight and to the PV installationcapacity and battery storage [22].Since this article is aimed at describ-ing the smart functionalities of aninverter, the authors feel that the useof power ratings is adequate for aproof of concept.

Simulation ResultsBased on the foregoing discussion,three case studies describing thesmart functionalities of the inverterare presented in this section. Thepower and voltage bases used were5 kWand 120V, respectively. The grid-side quantities are assumedly measuredat the PCC.

Case 1, Viz., S2s3In this simulation, the local demandfor an hour is 3 kW split into theprimary VSI load of 2 kW and thesecondary VSI load of 1 kW. As definedin mode S2s3, the real-time electricitypricing obtained from the grid ($PB¼US$1/kW) is lower than a set customerpreference (MCP¼ US$2/kW). In thiscase, the reference points of theinverter are set such that the grid-active power is purchased to powerthe local load and to charge the lead-acid battery bank. Therefore, theinverter is connected to the grid.The bidirectional dc–dc buck-boostconverter is operated in the buckmode.The voltage and current waveforms

associated with this operation modeare shown in Figure 8.

It is noticeable in Figure 8 that thevoltage and current waveforms on theinverter side are shifted by 180�; thisimplies that the inverter is buying electricenergy from the grid. As the voltage andcurrent are both at 1 per unit (p.u.), thenthe inverter-active power is 5 kW, i.e., atthe inverter nominal power. At the gridside, the voltage is 1 p.u. and the currentis 1.6 p.u. indicating that the grid-activepower is 8 kW. This corresponds to sum-mation of the VSI loads (3 kW) and thepower supplied to the battery bank(5 kW). The reactive power demandsat the inverter loads were set to zero.

1

0

–1

2

1

0

–1

–20.7 0.71 0.72 0.73 0.74 0.75 0.76 0.77 0.78 0.79 0.8

Time (s)

0.7 0.71 0.72 0.73 0.74 0.75 0.76 0.77 0.78 0.79 0.8Time (s)

Inverter Side

Grid Side

Vol

tage

(p.

u.),

Cur

rent

(p.

u.)

Vol

tage

(p.

u.),

Cur

rent

(p.

u.) Voltage (p.u.)

Current (p.u.)

Voltage (p.u.)Current (p.u.)

8Current and voltage at inverter and grid side for Case 1.

THE SMARTFUNCTIONALITIESOF THE INVERTERARE AIMED AT THEPROVISIONOF

REAL ANDREACTIVE POWER

SUPPORT.

10.5

0

–1–0.5

1

00.5

–0.5

–1

Time (s)

0.58 0.6 0.62 0.64 0.66 0.68 0.70 0.72 0.74 0.76

0.58 0.6 0.62 0.64 0.66 0.68 0.70 0.72 0.74 0.76

Time (s)

Inverter Side

Grid Side

Vol

tage

(p.

u.),

Cur

rent

(p.

u.)

Vol

tage

(p.

u.),

Cur

rent

(p.

u.) Voltage (p.u.)

Current (p.u.)

Voltage (p.u.)Current (p.u.)

9Current and voltage at the inverter and grid sides for Case 2.

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Case 2, Viz., S1s1In Case 2 (S1s1), the primary VSI load is 4 kW, and thesecondary VSI load is 2 kW. The PV panels are set up toprovide 3 kW. The inverter is initially in the grid-tiedmode until 0.65 s when the islanding mode is reinforced,possibly because of a far away fault in the grid. In theislanded mode, the PV panels are not able to supplyenough power to the loads, thus the bidirectional dc–dcbuck-boost converter will operate in the boost mode, andthe lead-acid battery bank will provide 2 kW. As the loadvalues are higher than 5 kW, the inverter (or thecustomer) must buy 1 kWof active power. Initially, in thegrid-tied mode, the inverter (or customer) purchases this1 kW from the grid. However, when the inverter func-tions in the islanded mode, the secondary VSI load dropsas described in mode S1s1. The voltage and current wave-forms associated with this mode of operation are shownin Figure 9.

When the inverter is connected to the grid, the invertervoltage and current values are both 1 p.u., indicating thatthe inverter-active power is 5 kW (3 kW from PV panelsand 2 kW from batteries). The grid-active power is 1 kW

as indicated by the grid voltage (1 p.u.) and grid current(0.2 p.u.).

At 0.65 s, the inverter begins operating in the islandedmode with the inverter voltage and current at 1 p.u. and0.8 p.u., respectively; therefore, the inverter-active poweris 4 kW (3 kW from PV panels and 1 kW from batteries).The grid-active power is 0.0 W (grid current is 0.0 p.u.).As in Case 1, the reactive power demand is taken as zeroin this case as well. The transients seen around 0.65 s inFigure 9 are caused by transition from the grid-tied modeto the islanded mode.

Case 3, Viz., S2s1In this simulation, the primary and secondary VSI loads areboth set at 1 kW. A voltage sag was induced on the gridside between 0.5 and 1.2 s. Operating in the S2s1 mode,the inverter provides voltage support to the PCC. This phe-nomenon, including the transient performance, can beobserved in Figure 10. The grid-voltage magnitude (Vmag)is kept at 1 p.u. by the action of the STATCOM functionblock of the inverter. The voltage and current waveforms asso-ciated with this mode of operation are shown in Figure 11.

Notice that the inverter is not islanded by the islandingand reclosure subsystem because the magnitude and dura-tion of the voltage sag do not fall within the limits recom-mended by IEEE Standard 1547 [4].

From the simulation start-up to 0.5 s, the inverter-active power is 5 kW, and the grid-active power is �3kW since 2 kW is consumed by the VSI loads. The sur-plus is injected to the grid side. When the grid has a volt-age sag from 0.5 to 1.2 s, the inverter voltage and currentare both at 1 p.u. and 32.4� out of phase (see Figure 11);this indicates that the inverter-active power is 4.2 kWand inverter-reactive power is 2.7 kvar. The grid-activepower is �2.2 kW, and grid-reactive power is �2.7 kvar.So the inverter is selling active and reactive powers togrid, and it is compensating voltage sag by supplyingreactive power to PCC. After 1.2 s, the voltage sag eventon the grid is assumed to be cleared; however, the inverterkeeps providing reactive power; hence, the voltage

magnitude on the grid side increasedquickly. However, the STATCOMaction decreased the inverter-reactivepower to 0.0 var and consequentlyincreased the inverter-active power to5 kW, helping the stabilization ofVmag at 1 p.u. Thus, the posteventgrid-active power is �3 kW and thepostevent grid-reactive power is 0 var.

It is observed in Figure 11 that thevoltage waveform on the inverter sideleads the current waveform; this iscaused by the injection of reactivepower from inverter to grid side. It isalso noticeable that the voltage andcurrent waveforms on the grid sideare out of phase by approximately231�. This phase difference is notexactly 180� as in Case 1 because inthis case the inverter injects (selling)both active and reactive powers tothe grid.

1

0

–1

1

0

–1

Time (s)

1.1 1.11 1.12 1.13 1.14 1.15 1.16 0.17 1.18 1.19 1.20

1.1 1.11 1.12 1.13 1.14 1.15 1.16 0.17 1.18 1.19 1.20

Time (s)

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Grid Side

Vol

tage

(p.

u.),

Cur

rent

(p.

u.)

Vol

tage

(p.

u.),

Cur

rent

(p.

u.) Voltage (p.u.)

Current (p.u.)

Voltage (p.u.)Current (p.u.)

11Current and voltage at the inverter and grid sides for Case 3.

1.1

1

0.9

0.8

0.5 1 1.5 2 2.5 3 3.5Time (s)

Vol

tage

Mag

nitu

de (

p.u.

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10Voltage magnitude on the grid side for Case 3.

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ConclusionsThis article describes the topology, control philosophy,operational algorithm, and simulation results of a volt-age-source inverter, interfacing a PV-based DG system tothe grid, with certain smart functionalities such as theability to supply real and reactive power to local loads,supply real and reactive power to other utility loads up torated capacity of the inverter, store energy in a batterybank, provide voltage support at the PCC, schedule loads,and provide control options to the consumer based onnear real-time electricity information obtained from theutility through advanced metering devices. This VSI iscapable of automatically choosing the operation modebased on a set of super and submodes corresponding tosystem conditions and real-time pricing of electricity.The smart functionalities are deemed to look beyond therecommendations of the current national technical stan-dard for interconnecting DG sources to grid, IEEE Stan-dard 1547, i.e., providing voltage support to PCC, thus,offering an ancillary service in case of low-voltage scenar-ios. This represents one of the tenets of the SGI, namely,enabling active participation of consumers in the demandresponse using timely information and control options.Several case studies are presented to illustrate the func-tionalities of the proposed device.

AcknowledgmentsThis work was supported in part by the Colorado CleanEnergy Project through the Colorado Renewable EnergyCollaboratory and the Xcel Energy Foundation and in partby the U.S. National Science Foundation under grant0757956. D. Brandao gratefully acknowledges F. Marafaoand D. Colon of the Universidade Estadual Paulista fortheir insightful comments about this article and for theirhelp in designing the controller parameters described inthe “The Smart Functionalities of the Inverter: Bidirec-tional DC–DC Buck-Boost Converter” section.

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[15] E. Lefter, L. M. Constantinescu, and C. Stoica. (2009, Nov.) Theexperimental study of lead acid batteries for an electric hybrid drivestand. [Online]. Available: http://electroinf.uoradea.ro/reviste%20CSCS/documente/JCSCS_2008/JCSCS_2008_31_Lefter_1.Pdf

[16] Mathworks Inc. SimPowerSystems—Model and simulate electricalpower systems. [Online]. Available: http:www.mathworks.com/products/simpower/

[17] B. Kroposki, C. Pink, R. DeBlasio, H. Thomas, M. Simoes, and P. K.Sen, “Benefits of power electronic interfaces for distributed energysystems,” in Proc. 2006 IEEE Power Engineering Society General Meeting,pp. 1–8.

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Renata Carnieletto and Felix A. Farret are with the Universi-dade Federal de Santa Maria in Santa Maria, Brazil. DaniloIglesias Brandao is with the Universidade Estuadal Paulistain Sao Paulo, Brazil. Siddharth Suryanarayanan is with Col-orado State University, Colorado. Marcelo G. Simoes (Msi-moes@mines.edu) is with Colorado School of Mines in Golden,Colorado. Suryanarayanan and Simoes are Senior Members ofthe IEEE. This article first appeared as “A MultifunctionalSingle-Phase Voltage-Source Inverter in Perspective of the SmartGrid Initiative” at the 2009 IEEE Industry ApplicationsSociety Annual Meeting.

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