Instantaneous Symmetrical Component Theory based Parallel ...load compensation, since this VSC...

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TSTE.2018.2845469, IEEETransactions on Sustainable Energy

1

Instantaneous Symmetrical Component Theorybased Parallel Grid Side Converter Control Strategy

for Microgrid Power ManagementRam Shankar Yallamilli, Student Member, IEEE and Mahesh K. Mishra, Senior Member, IEEE

Abstract—This paper proposes a centralized control strategyfor power management of hybrid microgrid connected to thegrid using a parallel combination of grid side converters (GSCs).An improved version of instantaneous symmetrical componenttheory (ISCT) is developed and is used for the control of paralleloperated GSCs, which results in reduced sensor requirement,control complexity, and communication bandwidth. In addition,a simple power management algorithm is developed to test theefficacy of the proposed parallel grid side converter controlstrategy for all the microgrid modes considering state of charge(SOC) limits of hybrid energy storage system (HESS), loadchanges, and renewable power variations. In the proposed system,a better dc link voltage regulation is achieved and usage ofsupercapacitor reduces the current stresses on the battery. Withthe proposed control strategy, the essential features of gridside converters like power quality, power injection, bidirectionalpower flow and proportional power sharing are achieved. Theeffectiveness of the developed control strategy for the proposedsystem is tested using MATLAB based simulink environment andvalidated experimentally using a laboratory prototype.

Index Terms—Battery, supercapacitor, power quality, grid sideconverters, state of charge, power management, power sharing,instantaneous symmetrical component theory (ISCT), microgrid.

I. INTRODUCTION

The rapid penetration of renewable energy based distributiongeneration (DG) into the present day power system is drivenby several factors like continuous increase in energy demands,growing global concerns about environmental issues, reducingthe dependency on fast depleting fossil fuels, and recenttechnological advancements [1]. These DG units along witha group of interconnected loads and energy storage systems(ESSs) with clearly defined electrical boundaries form a mi-crogrid and can operate in both grid connected or islandedmodes [2]. Microgrid which contains both ac/dc sources andac/dc loads is called as hybrid microgrid [3].

The interfaces between the DG units and the commonbus in a microgrid are often implemented by means of apower electronic converters [4] and the interaction betweenthe microgrid and main ac grid takes place through voltagesource converter (VSC) based dc/ac inverters called as gridside converters. To facilitate the proper interaction betweenDG units in a microgrid and main ac grid an effective control

Manuscript received June 15, 2017; revised December 14, 2017 and March30, 2018; accepted May 3, 2018. Ram Shankar Yallamilli is with ABBIndia Limited, Bangalore 560048, India (e-mail: [email protected])and Mahesh. K. Mishra is with the Department of Electrical Engineering,Indian Institute of Technology Madras, Chennai 600036, India. (e-mail:[email protected]).

strategy is necessary. The associated control strategies for gridside converter with respect to different reference frame theoriesare reported in the literature [5]–[7]. In general the controlstrategies applied to grid side converters consists of either onecurrent loop or two main cascaded loops, where one is theinternal current loop and the other is the external voltage loop[5]–[8].

Power quality is one of the important aspects in the mi-crogrid. According to the standards for interconnecting DGunits into the electrical power system, the injected current intothe grid should have a total harmonic distortion (THD) lessthan 5% [9]. Therefore different methods to compensate gridharmonics are presented in [10]–[15]. In addition, to the DGunits, presence of power electronic and unbalanced electricalloads further deteriorate the power quality. Different methodsto obtain an improved power quality are also addressed in theliterature [16], [17]. In a practical microgrid, both ac and dcsources coexist [3], [18] and these are interfaced by grid sideconverters, where bidirectional power flow [19] is necessaryalong with power quality improvement.

Control strategies discussed till now requires reactive powercommand, computationally intensive Clarke and its inversetransformations, phase angle of the grid voltage, higher ordercontrol and decoupling of active and reactive powers. Hence,it is necessary to consider a control theory which is compu-tationally less intensive and simple in formulation such as,instantaneous symmetrical component theory (ISCT). ISCTis primarily developed for unbalanced and non-linear loadcompensation by active power filters. Later, it has been usedfor the control of grid side converter in microgrids. In [20],an ISCT based multifunctional VSC controlled microgrid ispresented.

When providing multi-functionalities in a single inverter,either the real power injection or the load compensationcapabilities get compromised and the use of these invertersfor high power applications is limited by the current ratingof the semiconductor devices. Moreover, a large number ofDG units are installed in microgrids, where multiple invertersare connected in parallel for dc-ac conversion. Due to thesereasons parallel operation of grid side VSC is very commonin microgrids and in a grid connected microgrid parallel gridside converters (GSCs) are operated in current control mode(CCM) [21]–[23] where proper sharing of active, reactiveand harmonic powers is very essential [24]. In [24], a powersharing control strategy for parallel inverters using ISCT ispresented and in [25], a grid connected dual voltage source



2

PCC

DC Link Grid/SourceParallel Grid Side Converters High Gain Converter

Bidirectional Converters

Parallel DG Units

DC Loads

AC Loads

C0

Cb

Csc

S

D2D1 D3

Ci

L1

L2

Sb2Sb1

Ssc1Ssc2

Lb

Lsc

Cdc1a

Cdc1b

Cdc2a

Cdc2b

a b c

Converter-1

Converter-2

Rf1a, Lf1a

Rf1b, Lf1b

Rf1c, Lf1c

Rf2a, Lf2a

Rf2b, Lf2b

Rf2c, Lf2c

S11

S14 S16 S12

S15S13

S21

S24

S23

S26

S25

S22 Rl

Rla, Lla

Rlb, Llb

Rlc, LlcPb

Pdcl

Pgif1(abc)

if2(abc)

Pacl

ig(abc)

il(abc)

Ppv

vb

vsc

vpv

vdc

psc

HESS

Battery

PV

Source

Supercapacitor

Fig. 1. Proposed hybrid microgrid structure.

inverter scheme using ISCT is proposed to enhance the powerquality and reliability of the microgrid system. ISCT basedcontrol of parallel inverters mentioned above requires, the netactive power available in microgrid i.e., power measurementof all the sources and sinks. Moreover, in this study, thedc link voltage control is not explored. As well as, all theoperational modes of a typical microgrid consisting of hybridenergy storage system (HESS) are not addressed.

Apart from the effective grid side converter control, theother aspects required for continuous and reliable operationof microgrids are: control and operation of HESS [26], [27]within state of charge (SOC) limits and employing an appro-priate power management strategy [28], [29]. Considering theabove challenging aspects, in this work, authors have proposedan ISCT based centralized power management strategy forhybrid microgrid consisting of parallel operated DG units andgrid side converters with reduced sensor requirement.

II. SYSTEM DESCRIPTION AND MODELLING

A. System Description

The proposed hybrid microgrid considered for the studyis shown in Fig. 1. Renewable energy sources such as PVmodules have low terminal voltage. Hence, a high gain boostconverter (HGBC) is used as an interface between DG unit(PV) and the common dc bus (dc link). Due to the intermittentnature of renewable energy sources, storage systems havebecome an integral part of the microgrid. In this work, acombination of battery and supercapacitor called as hybridenergy storage system (HESS) is considered and it is in-terfaced to common dc bus using a bidirectional converters[30], which enables power flow into and from the energystorage elements. Various topologies of grid side converters areavailable for interfacing dc bus and ac bus (PCC). However, aneutral point clamped (NPC), voltage source converter (VSC)topology is used for both bidirectional power flow and forload compensation, since this VSC topology consists of lessnumber of switches, three legs have independent control, and

it can compensate unbalanced and non-linear loads in three-phase four-wire systems, where excessive neutral current is aproblem [31]. However, one of the drawbacks of NPC VSCis dc voltage imbalance, which occurs due to the presenceof dc components in neutral current [32]. In this work, thethree-phase ac load considered is assumed to be free from dccomponent. Hence, there would not be any voltage imbalancebetween the two dc link capacitors. The GSCs are interfacedto the PCC using filter impedances and the ac bus (PCC) isconnected to the utility grid and ac loads (linear and nonlin-ear). The control strategy for the proposed hybrid microgridis shown in Fig. 2, where the centralized controller consistsof power management algorithm (PMA) which receives inputsignals such as load currents, grid voltages, battery SOC,oscillating, and average components of effective current. Usingthese inputs PMA generates reference currents for variousconverters. Hysteresis controller is used to generate switchingpulses for both the GSCs and pulse width modulation (PWM)is used to generate switching pulses for all dc-dc converters. Inthis work, the filter parameters and hysteresis band consideredfor both GSCs is same i.e., both GSCs are identical. Hence,there wont be any zero sequence circulating currents flowingbetween GSCs [33].

B. System Modelling

The equivalent circuit of all parallel operated DG unitsincluding dc loads is shown in Fig. 3(a), where PV includ-ing HGBC, battery and supercapacitor including bidirectionalconverters, and dc loads are all modelled as current sources,where j = 1, 2 represents the number of GSCs connected to dclink. By applying KCL at node ’A’, the expression for currentiin is given below.

iin = ipv ± ib ± isc − idcl (1)

where ipv , ib, isc, and idcl are PV, battery, supercapacitor anddc load currents respectively. The per phase representation ofsingle grid side converter i.e., GSC-1 is shown in Fig. 3(b).



3

GSC-2 GSC-1

DC Link

DCDC DC

DC

DCDC

PV

BatteryDC Load

Supercapacitor

Unbalanced,

Nonlinear load

v* dc

Power

Management

Algorithm

PI

il (abc)

Lf1(abc)

Rf1(abc)

Lf2(abc)

Rf2(abc)

LPF

iosc

i*f2 (abc)

i f2 (abc)

S 2(123456)

Ssc1,2

Sb1,2

S

S 2

(1

23

45

6)

S 1

(1

23

45

6)

i f2

(a

bc)

i f1

(a

bc)

v dc

vg (abc)v g(abc)

Grid

iavg

vdc

Centralized Controller

i*sc

i*bat

i*pv

i bat

i pv

i*f1 (abc)

PW

M

PW

M

PW

M

i f1 (abc)

S 1

(12

34

56

)

S

Ssc1,2

isc

PI

PI

PI

Sb1,2

ief

impp

SOCbat

Fig. 2. Schematic diagram showing the control strategy for the proposed microgrid.

To derive the state space model of grid side converter, KVLis applied to Fig. 3(b), when switch S11 is closed, and theequation obtained is given below.

ipv ib isc idcl

Cdcja

Cdcjb

iin ioutja

ioutjb

A

S11

ilaS14ila

iin

Cdc1a

Cdc1b

Lf1

a, R

f1a

Rf1a, Lf1a

Vdc1

aS

a-

Sa

Vdc1

b

if1a

if1a

iin

(a)

(b)

vga

vga

iga igaiout1a

iout1b

Vdc1a

Vdc1b

(c)

Fig. 3. System modelling (a) DC side equivalent circuit (b) Single phaserepresentation of GSC-1 (c) Single phase equivalent of GSC-1.

dif1a

dt= −Rf1aif1a

Lf1a+Vdc1aLfla

− vgaLf1a

(2)

Similarly, applying KVL to Fig. 3(b), when switch S14 isclosed gives

dif1a

dt= −Rf1aif1a

Lf1a− Vdc1bLfla

− vgaLf1a

. (3)

Now combining above two equations with switching functions

[34] Sa and−Sa gives

dif1a

dt= −Rf1aif1a

Lf1a+SaVdc1a −

−SaVdc1b

Lfla− vgaLf1a

. (4)

Where, Sa = 0 and−Sa = 1 implies that top switch is open

and bottom switch is closed. The modified circuit of Fig. 3(b)

using switching signals is shown in Fig. 3(c). Similarly, forremaining phases

dif1b

dt= −Rf1bif1b

Lf1b+SbVdc1a −

−SbVdc1b

Lflb− vgbLf1b

(5)

dif1c

dt= −Rf1cif1c

Lf1c+ScVdc1a −

−ScVdc1b

Lflc− vgcLf1c

. (6)

The switching strategy to generate the switching signals Sa,−Sa, Sb,

−Sb, Sc, and

−Sc is implemented using a hysteresis band

current control (HBCC) [35] in the following;

If if1a≥i∗f1a + h then Sa = 0 and−Sa = 1

else if if1a≤i∗f1a − h, then Sa = 1 and−Sa = 0

else if if1a is within the band (i∗f1a + h and i∗f1a − h), thenthe present states of the switches are retained, where h is thehysteresis band.

The converter currents iout1a and iout1b can be expressedin terms of filter currents and switching signals as following.

iout1a = Saif1a + Sbif1b + Scif1c (7)

iout1b =−Saif1a +

−Sbif1b +

−Scif1c (8)

The relationship between iout1a, iout1b and Vdc1a, Vdc1b isgiven below.

Cdc1adVdc1adt

= − (iout1a − iin)

= − (Saif1a + Sbif1b + Scif1c − iin)(9)

Cdc1bdVdc1bdt

= iout1b + iin

=−Saif1a +

−Sbif1b +

−Scif1c + iin

(10)

Using (4), (5), (6), (9) and (10), the generalized state spacerepresentation of GSC is given below.

.xj = Ajxj +B1juj +B2jω (11)



4

where, xj is state vector, Aj is system matrix, B1j is inputmatrix, uj is input vector, B2j is exogenous input matrix, andω is exogenous input vector. Here j = 1, 2 corresponds to thetwo grid side converters.

III. GRID SIDE CONVERTER CONTROL USING ISCT

Instantaneous symmetrical component theory (ISCT) [36],[37] is one of the time domain control theories used to generatereference currents for active power filters, which are usedfor power quality improvement. According to this theory, thecompensator/filter reference currents are as given below.

i∗f(abc) = il(abc) −

(vg(abc) + β(vg(bca) − vg(cab))∑

i=a,b,c v2gi

)Plavg (12)

where β, φ+, i∗f(abc), il(abc), vg(abc), and Plavg are tan(φ+)√3

,power factor angle between voltage and current positive se-quence components, filter reference currents, load currents,grid voltages, and average load power on ac side. For unitypower factor operation β = 0. Therefore, the above equationis written as given below.


(vg(abc)∑i=a,b,c v

2gi

)Plavg (13)

To incorporate bidirectional power flow feature of grid sideconverter in a microgrid environment (13) is modified asfollows [25], [38].



2gi

)(Plavg − Pµg)(14)

where Pµg is the net power available in the microgrid, whichis nothing but the power available at the dc link. Accordingto the hybrid microgrid structure proposed in this paper, Pµgcan be formulated as given below.

Pµg = Psource − Psink (15)Psource = Ppv ± Pb (16)Psink = Pdcl. (17)

Substituting (15), (16), and (17) in (14) gives



2gi

)Plavg

+


2gi

)(Ppv ± Pb − Pdcl).

(18)

Hence, using this formulation of ISCT (18) for microgridapplications requires measurement as well as processing ofall source and sink power signals in a microgrid. This resultsin increased sensor requirement, increased control complexityand high communication bandwidth for centralized control.

To address the above-mentioned issues, authors have pro-posed an improved version of (14) for microgrid applications.Generally, from power quality perspective, grid currents shouldbe sinusoidal and in phase with respective phase voltage i.e.,

iga = k sin(ωt) (19)

Similarly, for remaining phases

igb = k sin(ωt− 120o) (20)igc = k sin(ωt+ 120o) (21)

where k is peak value of grid current. For further analysis,only single phase is considered. According to Fig. 1, fil-ter/compensator current can be written as

i∗fa = ila − iga. (22)

Substituting (19) in (22) gives

i∗fa = ila − k sin(ωt). (23)

From (14) equation (23) can be modified as given below

ila −

(vga∑

i=a,b,c v2gi

)(Plavg − Pµg) = ila − k sin(ωt). (24)

Cancelling the load current on both sides, we have(vga∑

i=a,b,c v2gi

)(Plavg − Pµg) = k sin(ωt). (25)

Substituting vga = Vm sin(ωt),∑i=a,b,c v

2gi = 3V 2

rms in (25)gives (

Vm sin(ωt)

3V 2rms

)(Plavg − Pµg) = k sin(ωt). (26)

Upon solving, equation (26) can be written as given below.

Plavg − Pµg =(3kV 2

rms

Vm

)= 1.5kVm (27)

Substituting (27) in (14) gives the improved version of refer-ence compensator/filter currents.



2gi

)(1.5kVm) (28)

From the above expression, it is clear that there is no needfor measuring PV, battery and load powers, this results inreduced sensor requirement and reduced cost of the signalconditioning and data acquisition stages. Due to the applicationof (28) for grid side converter control, the number of signalstransmitted and processed by the centralized controller isdecreased. Thereby, reducing the communication bandwidthand control complexity.

In (26) grid voltages are assumed as balanced because thecontrol algorithm for GSC reference current generation usesISCT, which requires balanced sinusoidal voltages. In case ofvoltage unbalance, the fundamental positive sequence compo-nents of grid voltages i.e., v+

ga1, v+gb1, and v+

gc1 are extractedusing PLL and dq0 transformation [25]. These voltages areused instead of vga, vgb, and vgc in GSC reference currentgeneration.

Based on the value of 1.5kVm in (28), grid side convertersoperate primarily in two different modes.

1) Grid sharing mode: Within the grid sharing mode thereare two sub modes namely rectifier mode and inverter modebased on the direction of power flow.

a) Inverter mode: When 0 < Ppv −Pdcl±Pb < Plavg, thena part of ac load demand is supplied by grid and remaining by



5

pdc

poutpin

pdcl

pacl

pb

pscvdc

ppv

Fig. 4. Power flow across DC link.

grid side converters. In this mode power flows through gridside converters from dc side (dc link) to ac side (PCC). Forthis, the range of 1.5kVm should be 0 < 1.5kVm < Plavg.

b) Rectifier mode: When Ppv − Pdcl ± Pb < 0, then a partof deficit power is supplied by the grid. In this mode powerflows through grid side converters from ac side to dc side. Forthis, the condition 1.5kVm > Plavg is to be satisfied.

2) Grid injecting mode: When Ppv − Pdcl ± Pb > Plavg,then the excess power is injected into the grid by grid sideconverters, where power flows from dc side to ac side. For this,the parameter 1.5kVm should be less than zero i.e., 1.5kVm <0.

Now this improved version of ISCT (28) has to be extendedfor control of parallel operated grid side converters ensuringproportional sharing of active, reactive, and harmonic powers.Let n be the number of parallel operated grid connected con-verters, then the generalized expression for reference currentsis as given below.

i∗fj(abc) = αj il(abc) − βj


2gi

)(1.5kVm) (29)

where j = 1, 2, ....n, if all the grid connected convertersare of same rating then α1 = α2... = αn = 1/n andβ1 = β2... = βn = 1/n. α represents the fraction of loadcurrent compensated by grid side converters and β representsthe fraction of excess/deficit power flowing through grid sideconverters.

IV. MICROGRID POWER MANAGEMENT

Power management strategy (PMS) is required for continu-ous and reliable operation of a microgrid with multiple energysources. The functionalities of microgrid PMS includes: 1)efficient sharing of real/reactive power requirements of loadsamong various energy sources, 2) responding quickly to sys-tem disturbances and transients, 3) power balancing amongvarious sources and sinks in microgrid, and 4) operating ESSwithin SOC limits. In this paper, power balance across dc linkis used along with power management algorithm to generatereference currents for various converters in microgrid.

A. DC Link Control

The dc link is implemented using a dc storage capacitor,which is used to buffer the input and output power differencesand consequently regulating the dc link voltage [39]. The

instantaneous power flow relationship across the dc link fromFig. 4 can be given as

pin(t) = pdc(t) + pout(t) (30)pin(t) = ppv(t)± pb(t)± psc(t)− pdcl(t) (31)pout(t) = ±pg(t)− pacl(t) (32)

where, ppv(t), pb(t), psc(t), pdcl(t), pacl(t), and pg(t) areinstantaneous PV, battery, supercapacitor, dc, ac load and gridpowers. The bidirectional power flow is denoted by the nota-tion ′±′. The dc link power can be further divided into threecomponents i.e., average power component pavg(t), oscillatorypower component pos(t) and transient power component ptr(t)as shown below.

pdc(t) = pin(t)− pout(t) (33)= pavg(t) + ptr(t) + pos(t) (34)

where, pavg(t) is the average power difference between PV andac, dc loads. This has to be shared between grid and battery,otherwise it results in linear increment or decrement of thedc link voltage. The transient power ptr(t) and the oscillatorypower pos(t) are provided by the supercapacitor.

B. Reference Current Generation

The reference currents for various power converters can bederived from the following effective current (ief (t)) equationusing PI controller as given below

ief (t) = Kpve +Ki

∫vedt (35)

= iavg(t) + itr(t) + ios(t) (36)

where, ve = vdcref − vdc is the difference between referenceand actual dc link voltage, which arises due to input andoutput power difference in the dc link. In (35) Kp and Ki

are proportional and integral gains of PI controller.Out of the three components of effective current, the average

component is obtained by passing through a low pass filter asgiven below. The cut-off frequency of the low pass filter ischosen to be 5 Hz as presented in [40].

iavg(t) =1

(1 + sτ)ief (t) (37)

This average component of current is a function of peak valueof grid current i.e., k in (19) and its derivation is as follows.The power available at the dc link voltage is given below.

Pdc = Vdciavg(t) (38)

This power has to be supplied/taken by the grid for a constantdc link voltage i.e.,

Pdc = Vdciavg(t) = Pg =3kV cos θ√

2. (39)

In (39), k and V are peak value of grid current and RMSvalue of grid voltage. Considering UPF operation i.e., θ = 0and solving (39), the expression for k is given below.

k =

√2Vdciavg(t)

3V= Ciavg(t) (40)



6

In (40), C is a constant. The average component of current hasto be delivered/absorbed by grid/battery. Similarly, the tran-sient and oscillating components supplied by supercapacitorare given below.

iosc(t) = ios(t) + itr(t) = ief (t)− iavg(t) (41)

C. DC Link Voltage Controller Gain Parameters Selection

The dc-link voltage is regulated by power balancing, whichis achieved by supplying/absorbing the deficit/excess powerby grid/battery. During microgrid deficit mode, the deficitpower is either given by grid alone or it is shared betweengrid and battery. Similarly, during excess mode, the excesspower available in microgrid is either taken by grid alone orit is shared between grid and battery. Therefore, there are twomodes effectively: 1) sharing mode (i.e., both grid and batteryshare the excess/deficit power) and 2) grid dominating mode(i.e., grid alone supply/take the deficit/excess power). In thiswork, voltage controller is designed in sharing mode and theobtained gain parameters are used for both the modes.

In this work, a two loop control strategy is employed with aninner average current control loop and an outer voltage controlloop. The inner current control loop changes depending on themode i.e., sharing mode or grid dominating mode. However,the outer voltage control loop remains same independent ofthe inner current loop. Therefore, designing a dc link voltagecontroller which is stable with any of the sources in the innercurrent control loop is a crucial aspect.

The supercapacitor has faster charge/discharge rates thanbattery, so the PI controllers are tuned based on the SCpower stage. To avoid oscillations, the inner current loopbandwidth of SC is limited to fsw/6 , where fsw is switchingfrequency. To divert the fast changing transient to SC, thebattery current loop bandwidth is maintained much lesserthan SC current loop bandwidth i.e., fsw/10. The voltagecontrol loop is slower than current control loop. Therefore, thebandwidth of voltage control loop is maintained lesser thancurrent control loop of SC i.e., fsw/20 [41]. The switchingfrequency considered in this work is 20 kHz.

The control block diagram in sharing mode is shown inFig. 5(a) where, Kp, Ki, Kppv , Kipv , Kpb, Kib, Kpsc,and Kisc are proportional and integral gains of dc linkvoltage controller, proportional and integral gains of PVcurrent controller, proportional and integral gains of batterycurrent controller, and proportional and integral gains ofsupercapacitor current controller. The small-signal model isderived as shown in Fig. 5(b), where Gcv(s), Glpf (s), Gh(s),Ggsc−1(s) = Ggsc−2(s) = Ggsc(s), Gcb(s), Gb(s), Gcsc(s),Gsc(s), Gcpv(s), and Gpv(s) are transfer functions of dc-linkvoltage controller, low-pass filter, hysteresis controller, gridside converter, battery current controller, battery converter,supercapacitor current controller, supercapacitor converter, PVcurrent controller, and PV converter, respectively. To derivethe loop gain from reference voltage to output voltage, otherdisturbances are made zero and the simplified model is shownin Fig. 5(c). The uncompensated open loop transfer function

v* dc

i*f2 (abc)

i f2 (abc)

S 2(123456)

v dc

i*sc

i*bat

i*pv

i avg

i pv

i*f1 (abc)

i f1 (abc)

S 1(123456)

S

Ssc1,2

isc

Sb1,2

PW

M

PW

M

PW

M

LPF

S

KK i

p

S

KK ib

pb

S

KK isc

psc

S1

1

S

KK

ipv

ppv

i l (abc)

0.5

cbai

gi

m

v

VK

,,

2

25.1)(abcgv

i bat

)(sGpv

)(sGb

)(sGsc

)(sGgsc

)(sGgsc

LPF

)(sGcpv)(sGcb

)(csc sG

)(sGlpf

)(sGcv

1K

1K

0.5

0.5

)(sGh

)(sGh

)(sGlpf)(sGcv

)(sGlpf

)(1 sGlpf

esr

dc

rSC

1

esr

dc

rSC

1

)(^

sV dcref

)(^

sV dc

)(^

sV dcref

)(^

sV dc

)(^

sib

)(

^

si g

)(

^

si sc

)(^

si l

)(^

sib

)(^

sibref

)(

^

si scref

)(^

si pvref

)(^

si pv

)(^

sib

)(

^

si sc

)(

^

si g

(a)

(b)

(c)

0.5

)(sGbcl

)(sGsccl

)(sGgsccl

Fig. 5. Control block diagram in sharing mode (a) Control block diagram(b) Small signal model (c) Simplified small signal model.

i.e., TV BG(s) has phase margin of 390 at 3 krad/s, where

TV BG(s) = Glpf (s)(Ggsccl(s) +Gbcl(s)) (resr +1

sCdc)

+ (1−Glpf (s))Gsccl(s) (resr +1

sCdc).

(42)

Now, the voltage controller is designed to achieve PM of 600

at 6.2 krad/s and the calculated gain parameters are Kp = 0.26and Ki = 2.47.

D. Power Management Algorithm

The proposed power management algorithm (PMA) shownin Fig. 6 decides the operating modes of the microgrid basedon the value of iavg(t). There are three microgrid operatingmodes namely excess, deficit and floating power modes.Floating power mode is merged with deficit power mode asthe HESS can be charged from the grid in both modes. Here,only SOC of the battery is taken into consideration whereupper and lower limits of SOC are considered as 0.9 and 0.2.Supercapacitor is provided to absorb transient and oscillatingcomponents in all the cases.



7

Start

iavg(t) 0

End

Deficit Power Mode Excess Power Mode

SOCb < 0.2

YSOCb < 0.9

Y

N

YN

Read iavg, iosc, SOCb, impp, il(abc), vg(abc)

ib = K1 iavg, isc = iosc, ipv = impp

ifj(abc) = il(abc) -(vg(abc)/Ʃi = abc v2

gi)(1.5K2Vmiavg)

ib =0, isc = iosc, ipv = immp


gi)(1.5Vmiavg)

ib = K1 iavg, isc = iosc, ipv = impp


gi)(1.5K2Vmiavg)

ib =0, isc = iosc, ipv = impp


gi)(1.5Vmiavg)

N

A

B

B

A

Fig. 6. Power management algorithm.

1) Deficit power mode: In this mode, PV power is lessthan the sum of ac, dc load powers. Hence, the deficit powerhas to be supplied either by grid/battery based on SOC limits.Considering SOC limits of battery there will be two cases.

Case-A: Here, SOCbat > Lower Limit, so the battery willsupply a fraction of deficit power and the remaining power issupplied from the grid.

Case-B: Here, SOCbat < Lower Limit, so the battery willbe kept idle to avoid deep discharging and grid will supplythe entire deficit power.

2) Excess power mode: In this mode, PV power is greaterthan the sum of ac, dc load powers. Hence, the excess powerhas to be taken either by grid/battery based on SOC limits.Considering SOC limits of battery there will be two cases.

Case-A: Here, SOCbat < Upper Limit, so battery will becharged with a fraction of excess power and the remainingpower is supplied to the grid.

Case-B: Here, SOCbat > Upper Limit, so battery will bekept idle to avoid deep charging and the entire excess poweris supplied to the grid.

The deficit/excess power available in a microgrid has to beshared between grid and battery for dc link voltage regulation.The amount of sharing between grid and battery is decided byK1 and K2, where K1 +K2 = 1. In this work, it is assumedthat K1 = 0.3 and K2 = 0.7, which means 30% of the availableexcess/deficit power in microgrid is taken/given by battery andremaining 70% is taken/given by the grid. However K1 andK2 can take any values between 0 to 1 and the conditionK1 +K2 = 1 must be satisfied. The ratio of K1/K2 dependson several factors like availability of grid, grid pricing andrating of battery units.

V. RESULTS AND DISCUSSION

The system parameters considered for both simulation andexperimental studies are mentioned in Table I.

TABLE ISYSTEM PARAMETERS

Supercapacitor specifications ValuesTerminal voltage (Vsc) 16 VNo. of packs in series 11Battery specifications ValuesAh Capacity 14 AhTerminal voltage (VB ) 12 VNo. of batteries in series 12DC load parameters Rdcl =320 ΩAC load parameters ValuesUnbalanced linear load Ra = 20 Ω,La = 65 mH

Rb = 10 Ω,Lb = 50 mHRc = 30 Ω,Lc = 80 mH

Nonlinear load 3-Φ, diode bridge rectifier withRl = 100 Ω,

Grid side converter parameters ValuesDC link voltage Vdc =220 VDC link capacitance Cdc1(ab) = Cdc2(ab) = 3300µFInterfacing inductors Lf1(abc) = Lf2(abc) = 20 mHInterfacing resistors Rf1(abc) = Rf2(abc) = 0.5 ΩHysteresis band h= ±0.1AHigh gain converter parameters Ll = L2 = 10 mH, C0 = 220 µFBattery converter parameters Lb = 10 mH, Cb = 220 µFSupercapacitor converter parameters Lsc = 10 mH, Csc = 220 µFGrid/source voltages 3-Φ, 50 V (P-N)

A. Simulation Results

The proposed system is simulated for a time duration of 4sec using MATLAB based simulink environment for differentoperating modes of grid and GSC, considering variations inPV power, load power, and SOC limits. The variations in dclink voltage are shown in Fig. 7(a), where the dc link voltagesettles at rated value of 220 V. The variations in PV, battery,supercapacitor and dc load currents are shown in Fig. 7(b),where ipv is increased to achieve both deficit and excess modesof operation. Based on the SOC values of the battery shownin Fig. 7(c), the fraction of deficit/excess power is given/takenby the battery under steady state and the transient change inPV and load powers is absorbed by the supercapacitor. Thevariations in the active power of grid, ac load and GSCs areshown in Fig. 7(d), where parallel operated GSCs are sharingpower proportionally. As ipv increases the power drawn fromgrid decreases and power flowing through GSCs increases. Thevariations in grid and GSCs currents are shown in Fig. 7(e)and 7(f). The unbalanced non-linear load currents are shownin Fig. 7(g). As the parallel operated grid side converters withcurrents shown in Fig. 7(h) compensate the load currents, thecurrents drawn from the grid are sinusoidal as shown in Fig.7(i). The microgrid mode transfer from deficit to excess modeand unity power factor (UPF) operation are shown in Fig.7(i) and 7(j). During the deficit power mode, power is takenfrom the grid hence source currents are in phase with theirrespective phase voltages. Similarly, in case of excess powermode, power is injected into the grid hence the source currentsare out of phase with their respective phase voltages. Reactivepower flows in grid, load and GSCs are shown in Fig. 7(k),where the load reactive power is supplied by GSCs and thereactive power taken from grid is zero.

B. Experimental Results

The scaled down laboratory prototype version of the pro-posed hybrid microgrid shown in Fig. 8 is considered forexperimental studies. The control strategy for the whole setup



8

0 1 2 3 4200

220

240

-4

4

8

0 1 2 3 4

0

0 1 2 3 4-4

-2

0

2

4

0.4

0.8

1.9 2 2.1 2.2

-2

-

1

0

1

2

0 1 2 3 4

-200

0

200

400

600

0 1 2 3 4

-2024

1.9 2 2.1 2.2

-2

0

1

2

Volt

age (

V)

Cu

rren

t (A

)

Cu

rren

t (A

)C

urren

t (A

)

Cu

rren

t (A

)

Cu

rren

t (A

)

Cu

rren

t (A

)V

olt

age (

V)

& C

urren

t (A

)P

ow

er (

W)

Time (S)Time (S)Time (S)

Time (S)

Time (S)

Time (S)

Time (S)

Time (S)

Time (S)

Vdcipvibat idcl

isc

ig(abc)

ig(abc)

1.9

-4

2 2.1 2.2

0

4

Time (S)

1.9 2 2.1 2.2-80

-40

0

40

80

il(abc)

vga

iga

Pgsc-1 Pgsc-2,

PaclPg

igsc-2(abc)igsc-1(abc) ,

igsc-2(abc)igsc-1(abc) ,

(a)

(g)

(i)

(f)

(h)

(j)

(d)

(b)

Excess ModeDeficit Mode

Grid Injecting ModeGrid Sharing Mode

Mode Transfer

(e)

0 1 2 3 40

Upper Limit

Lower Limit

(c)

Time (S)

-4

0 0.5 1 1.5 2 2.5 3 3.5 4

-50

50

150

Time (S)

Qacl

Qgsc-1 Qgsc-2,

Qg(k)

Reacti

ve P

ow

er (

VA

R)

Batt

ery S

OC

Fig. 7. Simulation results: (a) dc link voltage, (b) dc load, PV, battery, andsupercapacitor currents, (c) Battery SOC including upper and lower limits,(d) Various active powers as per labelling, (e) Grid current variations, (f)Variations in grid side converter currents, (g) Unbalanced and non-linear loadcurrents, (h) Zoomed view of GSC currents, (i) Zoomed view of grid currents,and (j) Phase-a grid voltage and scaled version of current representing unitypower factor (UPF) operation, (k) Various reactive powers as per labelling.

AC loads

GSC-1Filter resistors & inductors

ELGAR power source

Emulated PV source

DC loads

Supercapacitor packs

DC regulated supply

IGBT leg

Transducers

PQ analyzer

Battery packsGate driver

PCC

DC link

DSO

Host system

dSPACEGSC-2

Fig. 8. Experimental setup.

is implemented in real time using dSPACE Microlab Box.Tests are conducted to verify the steady state and transient

vga

igsc-1a

igb

igc

vga

iga

igb

igc

vga

igsc-1b

igsc-1c

iga

vga

igsc-2a

igsc-2b

igsc-2c

vga

igsc-1a

igsc-1b

igsc-1c

vga

igsc-2a

igsc-2b

igsc-2c

vga

iga

vga

iga

(b) (f)

(c) (g)

(d) (h)

(e) (i)

210 V/div

210 V/div 210 V/div

210 V/div210 V/div

210 V/div 210 V/div

4.2 A/div

210 V/div

4.2 A/div

4.2 A/div 4.2 A/div

4.2 A/div 4.2 A/div

4.2 A/div 4.2 A/div

vga

ila

ilb

ilc

(a)210 V/div

4.2 A/div

0.1 S/div

0.1 S/div0.1 S/div

0.1 S/div0.1 S/div

0.1 S/div0.1 S/div

0.1 S/div0.1 S/div

Fig. 9. Experimental results: (a) Source voltage (phase-a) and load/sourcecurrents before compensation, (b) Source currents after compensation duringdeficit mode, (c) Grid side converter - 1 currents during deficit mode, (d)Grid side converter - 2 currents during deficit mode, (e) UPF operation duringdeficit mode, (f) Source currents after compensation during excess mode, (g)Grid side converter - 1 currents during excess mode, (h) Grid side converter- 2 currents during excess mode, and (i) UPF operation during excess mode.

state performance of the proposed system and the detailedexperimental results are as follows.

1) power quality features: The unbalanced and nonlinearload currents along with phase-a source voltage are shownin Fig. 9(a). Power quality features during deficit mode areshown in Fig. 9(b)-(d). The currents drawn from the grid aresinusoidal as shown in Fig. 9(b), due to the compensation ofunbalance and non linear load currents by grid side converters

0 200 400 600 800 10000123456789

Frequency (Hz)0 200 400 600 800 1000

0123456789

Frequency (Hz)

Mag (

% o

f F

undam

enta

l)

Mag (

% o

f F

undam

enta

l)

(a) (b)

Fig. 10. Harmonic Spectra: (a) Load current (phase-a) harmonics, and (b)GSC-1 current (phase-a) harmonics.



9

idcl

ib ib

iscisc

idcl

Vdc

ig(abc)

vg(abc)

il(abc)

igsc(abc)

220

ipv

ib

isc

idcl

t1 t2 t3 t4 t5

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(V)

(V)(a)

(b)

(c) (d)

(e)

(f)

(g)

(h)

10 V/div

1 A/div

4 A/div

0.4 A/div

2 A/div

2 A/div

0.4 A/div

1 A/div 1 A/div

0.4 A/div

2 A/div

5 A/div

5 A/div

5 A/div

200 V/div

50 S/div

50 S/div

50 S/div

50 S/div

Fig. 11. Experimental results: (a) dc-link voltage, (b) PV, battery, superca-pacitor, and dc load currents, (c) Zoomed view of (b) at t1, (d) Zoomed viewof (b) at t2, (e) Source voltage, (f) Variations in grid currents, (g) Variationsin ac load currents, and (h) Variations in grid side converter currents.

with filter currents shown in Fig. 9(c) and 9(d). Moreover,UPF operation is achieved as shown in Fig. 9(e). Similarly,power quality features during excess mode are shown in Fig.9(f)-(h). The unbalance and nonlinear loads are compensatedby parallel operated grid side converters with filter currentsshown in Fig. 9(g) and 9(h). The grid currents shown in Fig.9(f) are balanced sinusoidal and have UPF operation with theirrespective phase voltages as shown in Fig. 9(i).

The harmonic spectra of load current and GSC-1 currentare shown in Fig. 10(a) and 10(b), where the 5th, 7th, 11th,and higher order harmonic components present in load currentare compensated by grid side converters.

2) power management features: In this section, powermanagement among various sources and sinks within themicrogrid to keep constant dc link voltage is discussed. Thedc link voltage for the entire range of operation is shown inFig. 11(a). The variations in PV, battery, supercapacitor, anddc load currents are shown in Fig. 11(b) and the zoomed viewof Fig. 11(b) at instants t1 and t2 are shown in Fig. 11(c)and 11(d) respectively. From Fig. 11(c) and 11(d), it is clearthat supercapacitor supplies the transient component of load

Pg (W)

Pg (W)

Pg (W)

Pgsc-2 (W)

Plavg (W)

Plavg (W)

Pgsc-1 (W)

Pgsc-1 (W)

Pgsc-1 (W)

Pgsc-2 (W)

Pgsc-2 (W)

Plavg (W)

Pg (W)

Pg (W)

Pg (W)

Plavg (W)

Plavg (W)

Plavg (W)

Pgsc-1 (W)

Pgsc-1 (W)

Pgsc-2 (W)

Pgsc-2 (W)

Pgsc-2 (W)

Pgsc-1 (W)

(a)

(c)

(e)

(b)

(d)

(f)

Fig. 12. Experimental results measured from control desk: (a)-(c) Variouspowers as per labelling during deficit mode for changes in dc load, ac loadand ipv , and (d)-(f) Various powers as per labelling during excess mode forchanges in dc load, ac load and ipv .

changes, whereas battery supplies the fraction of steady statecomponent. Till instant t3 the battery SOC is greater than thelower limit, therefore battery is supplying a fraction of deficitpower. From t3 to t5 battery SOC is out of the limits, thereforebattery remains idle. From instant t5 battery SOC is less thanits upper limit, therefore the battery is charging with a fractionof excess power. The variations in grid voltage, grid currentsand load currents are shown in Fig. 11(e)-(g). At instant t1there is an ac load change of 125 W as shown in Fig. 11(g) andthis increase in load demand is met by both grid and battery.The variations in GSC currents are shown in Fig. 11(h).

3) power sharing features: The active power sharingamong parallel operated GSCs in both excess and deficit modeare discussed in this section. Here, both the GSCs consideredfor experimental studies are of the same rating. Power sharingfeatures during deficit mode are shown in Fig. 12(a)-(c). InFig. 12(a), a dc load change of 100 W is introduced. Thisincrement in load demand is supplied by grid which flowsthrough the GSCs from ac to dc side i.e., GSCs are operatingin rectifier sub-mode of grid sharing mode. In Fig. 12(b),ac load changes by 125 W. This increment in load demand



10

is supplied by grid and there is no change in the powerflowing through the GSCs. In Fig. 12(c), PV power is changedby 100 W. This increased PV power flows through GSCsproportionally, thereby reducing the power drawn from thegrid. Power sharing features during excess mode are shownin Fig. 12(d)-(f). In Fig. 12(d), a dc load change of 100 Wis implemented. This increment in load demand is suppliedby the PV. As a result, the power flowing through GSCsdecreases. In Fig. 12(e), ac load changes by 125 W, andthis increment in load demand is supplied by the PV. As aresult, the power injected into the grid decreases and there isno change in the power flowing through the GSCs. In Fig.12(f), PV power is changed by 100 W and this increased PVpower flows through GSCs proportionally, thereby increasingthe power injected to the grid.

VI. CONCLUSION

In this work, a centralized power management controlstrategy that coordinates the parallel operation of GSCs withina hybrid microgrid is presented. The proposed parallel GSCcontrol strategy does not require power measurement ofvarious sources and sinks in a microgrid, thus making thecontrol strategy most suitable for microgrid power manage-ment. Power management among various sources and sinksis effectively achieved by considering the SOC limits ofbattery and renewable power availability. Apart from realpower flow control, additional power quality features likecurrent harmonic compensation, reactive power support, andunity power factor operation are achieved by compensatingunbalanced and nonlinear loads. Various features like reducedbattery current stress, fast and better dc link voltage regulation,and proportional power sharing among GSCs are illustrated.

ACKNOWLEDGMENT

This work is supported by the Department of Scienceand Technology, India (Project grant: IUSSTF/JCERDC-SmartGrids and Energy Storage/2017).

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Ram Shankar Yallamilli was born in Rajahmundry,India in 1991. He received his Bachelor degree fromGMR Institute of Technology, Rajam, India, in 2013and Master of Science from Indian Institute of Tech-nology Madras, Chennai, India in 2017. Presently heis working as Application Engineer in ABB IndiaLimited, Bangalore, India.

His research interests include microgrid powermanagement strategies, grid integration of renewableenergy sources and power converter applications inmicrogrids.

Mahesh K. Mishra (S00-M02-SM10) receivedthe B.Tech. degree from the College of Technology,Pantnagar, India, in 1991, the M.E. degree from theIndian Institute of Technology, Roorkee, India, in1993, and the Ph.D. degree from the Indian Instituteof Technology, Kanpur, India, in 2002, all in electri-cal engineering. He has about 28 years of teachingand research experience. For about ten years, hewas with the Department of Electrical Engineer-ing, Visvesvaraya National Institute of Technology,Nagpur, India. He is currently a Professor with the

Department of Electrical Engineering, Indian Institute of Technology MadrasChennai, India. His research interests include the areas of power distributionsystems, power electronics, microgrids, and renewable energy systems.

Prof. Mahesh is a Life Member of the Indian Society of TechnicalEducation. He received the IETE Prof. Bimal Bose Award in 2015 forhis outstanding contributions to Power Electronics Applications in PowerSystems. In Nov. 2017, he has been elected as Fellow of Indian NationalAcademy of Engineering. He is senior member of IEEE and serves as anEditor for the IEEE Transactions on Sustainable Energy.

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