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Available online at www.sciencedirect.com

Journal of Applied Researchand Technology

www.jart.ccadet.unam.mxJournal of Applied Research and Technology 15 (2017) 371–377

Original

Analysis of droop control method in an autonomous microgridAmir Khaledian ∗, Masoud Aliakbar Golkar

Department of Electrical Engineering, K. N. Toosi University of Technology, Tehran, Iran

Received 20 August 2015; accepted 9 March 2017

bstract

In this paper an analytical approach is conducted to evaluate the droop control method in an islanding microgrid. Droop control is the keyolution for sharing the demand power between generators in autonomous microgrids where there is no support from the electricity distributionrid. In the paper, three important load types are investigated to verify the droop control performance. First, coupling of active power and reactiveower is considered in the microgrid and a new method is proposed to facilitate separate control of powers. In the proposed method the effects ofroop gains on decoupling of active power and reactive power control, voltage regulation, power oscillation and system stability are studied. In theecond load type study, by applying the different types of faults, induction motor characteristics are observed. By simulation results it is shown thathe fault intensity and duration will determine how the microgrid attains to fast frequency convergence and how fast protection system operationan improve system stability. In the third case, imbalanced nonlinear load is studied in the microgrid and the influences of embedded controllers

n harmonic distortion, system balance and voltage regulation are observed.

2017 Universidad Nacional Autónoma de México, Centro de Ciencias Aplicadas y Desarrollo Tecnológico. This is an open access article underhe CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

eywords: Distributed generator; Droop control; Electrical load and microgrid

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pnBgmopi

. Introduction

The interconnection of small generation systems like solarhotovoltaics, micro turbines, fuel cells, wind turbines andnergy storage devices to low voltage distribution network willead to a dynamic power system. These power sources givehe capability of decentralized generation and are known asistributed generators (DGs) (Chowdhury & Crossley, 2009;umithira & Nirmal Kumar, 2013).

A microgrid which includes local DGs and loads, can operaten two different modes of operation. In interconnected mode its connected to the main upstream grid, being supplied fromr injecting power into it. Other mode is autonomous modef operation and the microgrid is disconnected from distri-ution network (Balaguer, Lei, Yang, Supatti, & Peng, 2011;ajumder, Ghosh, Ledwich, & Zare, 2009).

∗ Corresponding author.E-mail address: akhaledian@mail.kntu.ac.ir (A. Khaledian).Peer Review under the responsibility of Universidad Nacional Autónoma de

éxico.

ces(

http://dx.doi.org/10.1016/j.jart.2017.03.004665-6423/© 2017 Universidad Nacional Autónoma de México, Centro de CienciasC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

DGs improve the service reliability and decrease the needor future generation expansion planning. Moreover, in conceptf islanding microgrid it extends up the possibility of makingources responsible for local power quality factors in a way thats not conceivable with conventional centralized power gen-ration (Fu et al., 2012; Marwali & Keyhani, 2004; Zhengbo,inchuan, & Tuo, 2011).

Unlike conventional generators which almost exclusivelyroduce 50 or 60 Hz electricity, the majority of DGs are con-ected to the grid via voltage source inverters (Strzelecki &enysek, 2008). The basic control objective for DGs in a micro-rid is to achieve accurate power sharing while regulating of theicrogrid voltage magnitude and frequency. Centralized control

f a microgrid based on communication infrastructure is alsoroposed. However, in remote areas with long distance betweennverters, it is impractical and costly to use communication link.

Decentralized controllers are investigated to eliminateommunication links. Thereby power sharing for microgrid gen-

rators is achieved by means of droop controllers. In sometudies a static droop compensator is reported for power sharingChandorkar, Divan, & Adapa,1993; Katiraei & Iravani, 2006).

Aplicadas y Desarrollo Tecnológico. This is an open access article under the

3 pplied Research and Technology 15 (2017) 371–377

DfTe(&m&2pc

hapddicqci

2

dta(tf

oa

DG&

inverter

i ioLf

Vo

Lc

Cf

To the grid

PncII

2

eqpqraFivf

p

q

72 A. Khaledian, M. Aliakbar Golkar / Journal of A

roop control is enhanced featuring the transient response per-ormance (Guerrero, De Vicuna, Matas, Castilla, & Miret, 2004).o decrease the active power (P) and reactive power (Q) depend-ncy, droop controllers with virtual frequency–voltage frameLi & Li, 2011) and virtual output impedance (Chiang, Yen,

Chang, 2001) are also discussed. For nonlinear loads, har-onic based droop controllers are proposed (Borup, Blaabjerg,

Enjeti, 2001; Lee, & Cheng, 2007; Marwali, Jung, & Keyhani,004). Although these control methods can improve the overallower quality factors, but they have complexities in adjustingontrol parameters.

As it can be seen in literature review, there is no compre-ensive study of microgrid in presence of important load typesnd only one kind of load is considered in each article. In thisaper microgrid with droop control is analyzed in presence ofifferent kind of loads. Step change in active demand power,ynamic loads such as induction motors in fault condition, andmbalanced harmonic distorted loads are three types of loadsonsidered in this paper. Load characteristics effects on poweruality factors are observed in simulation results. These studiesan properly comprise the most important loads being utilizedn a sample microgrid.

. Droop control

Droop control for a sample microgrid is considered inirect-quadrature-zero reference frame which facilitates con-rol process by transforming time variant quantities of voltagend current in three phases reference frame to direct currentdc) quantities. Figure 1 illustrates the alternative current (ac)hree phases (abc) and direct-quadrature-zero (dq0) coordinaterames.

Reference voltage for pulse width modulation (PWM) signalsf inverters is generated by three back to back power, voltagend current controllers (Marwali & Keyhani, 2004; Pogaku,

b -axis

c -axis

d -axis

a -axis

q -axis

Figure 1. Three phases and direct-quadrature-zero coordinate frames.

tcp

P

Q

ω

v

ataRt

cdateo

Figure 2. DG connected to the microgrid via inverter.

rodanovic, & Green, 2007). Figure 2 shows a DG unit con-ected to the microgrid by inverter. Output filter (LfCf) andoupling inductance (Lc) are connected before the terminal bus.t is assumed that the input source of inverter is an ideal dc link.nner controllers are further discussed.

.1. Power controller

Droop control scheme mimics the operation of governor andxciter in synchronous generators and determines output fre-uency and voltage of DGs according to the active and reactiveowers derived from their terminals. In order to determine fre-uency and voltage by droop equations, instantaneous active andeactive powers (p and q) should be calculated from the gener-tor output voltage and current (v0 and i0) as they are shown inigure 2. i is the current of coupling inductance where its values

n dq0 reference frame are id and iq, vod, voq, iod and ioq are thealues of output voltage and current variables in dq0 referencerame along d-axis and q-axis respectively.

= vodiod + voqioq (1)

= vodioq − voqiod (2)

According to Eqs. (3) and (4) to derive P and Q, above quan-ities should be passed through low pass filter in which ωf is theut-out frequency. Reference voltage (v∗

od) and frequency (ω) ofower controller can be obtained by Eqs. (5) and (6) respectively.

= ωf

s + ωf

p (3)

= ωf

s + ωf

q (4)

= ωn − k1P (5)

∗od = Vn − k2Q (6)

In above equations ωn and Vn are nominal frequency and volt-ge of microgrid. K1 and K2 are droop gains; these gains relateo economic and technical features of each DG unit. Droop gainsre kept the same for all generators in this paper for simplicity.eference voltage along the q-axis (v∗

oq) is set to be zero in ordero have positive sequence components in three phase system.

According to droop characteristic, frequency of each DGhanges continuously by variation in its active power. When aisturbance occurs, frequency will reach the steady state amountfter transition time. DGs have different frequencies in compare

o each other during transient time because they face differ-nt impedances. As one frequency is possible for generatorsf a microgrid, active power is divided between DGs in a way

A. Khaledian, M. Aliakbar Golkar / Journal of Applied Research and Technology 15 (2017) 371–377 373

PIcontroller

PIcontroller

PIcontroller

F

F

PIcontroller

i ∗d

+

+

+

+

+

+

+ +++

+

+

+

+

++

i d

i q

i ∗q-

-

-

-

V∗id

i∗q

i∗d

V∗iq

Vod

iod

ioq

Voq

V∗oq

V∗od

∑

∑

∑∑

∑

∑

∑

∑

∑

∑

ωLc

ωCf

ωCf

ωLc

a

b

tp

2

t(eBbpelf

3

octup

t

Load1 Load2

X line2X line1

R line1 R line2

DG2

DG1 DG3

Figure 4. Single phase schematic of simulated microgrid.

Table 1Network and controller parameters of the test system.

Lf 1.35 mH Fs 8 kHzLc 0.35 mH Cf 50 �FF 0.75 ωf 31.41 rad/sK2 1.3 10−3 Hz/var k1 9.4 × 10−5 Hz/WX line1 0.1 � R line 1 0.23 �

X

pfifc

3

dbb1

bsBvp

pgoatimance. In Figures 8 and 9, active and reactive powers generatedby DGs are shown for new reactive power droop gain (K2 = 0.6)which is chosen by trial and error method.

DG1

Controlledcurrentsource DG2

DG3

Load1 Load2

X line1 X line2

R line2R line1

Bus1 Bus2 Bus3

Figure 3. (a) Voltage controller and (b) current controller.

hat produce same frequency for them. Therefore DGs shouldroduce same P in same active power droop gain (K1).

.2. Voltage and current controllers

Reference voltage and frequency generated by power con-roller are fed to back to back voltage and current controllersFig. 3) to create reference currents (i∗d and i∗q) and refer-nce inverter voltages (v∗

id and v∗id) in dq0 reference frame.

oth controllers are designed to reject high frequency distur-ances and provide adequate damping for the output filter. Innerroportional-integral (PI) controllers guarantee zero steady staterror and improve system transient response. Also feed-forwardoops isolate DGs from load disturbances. F is the current feed-orward gain and it has not unit.

. Simulation results

Observing DGs reactive power when a step change isccurred in active power, induction motor characteristics in faultonditions and power quality evaluation for nonlinear load arehree load scenarios applied to a sample microgrid. Matlab sim-lations evaluate the control scheme which was discussed in

revious section.

Figure 4 shows the microgrid single phase schematic. It con-ains three buses with a DG in each bus. Network and controller

line2 0.58 � R line 2 0.3 �

arameters like resistance and reactance of lines, inverter outputlter, switching frequency and droop gains are given in Table 1.

s is the inverter switching frequency. Coefficients of inner PIontrollers are tuned in each section by trial and error method.

.1. Step change in active power

In order to observe P and Q dependency, a step change in theemand active power is made by a controlled current source inus 1 as it can be seen in Figure 5. All the active power requestedy network and loads is 12 kW before the demand change. After

s 9 kW of total P is increased approximately.Figures 6 and 7 show active and reactive powers generated

y DGs in each three buses. Three DGs produce equal P inteady state time because their droop gains have the same value.ut in transient state the active power of DG1 has much greateralue with respect to two other DGs. It should be considered inrotection system design.

Although the change in active power is made in bus 1, reactiveower produced by each DG has noticeable change. In micro-rids with resistive lines P and Q are mainly coupled to eachther. In order to mitigate the coupling of DGs generated activend reactive power, droop gain should be tuned as well as main-aining system stability. Increasing the reactive power droop gains considered as a solution to improve the power sharing perfor-

Figure 5. Simulation of step change in active power.

374 A. Khaledian, M. Aliakbar Golkar / Journal of Applied Research and Technology 15 (2017) 371–377

8,000

6,000

4,000

2,000

00.2 0.4 0.6 0.8 1.2 1.4 1.6 1.81 2

Time (s)

Act

ive

pow

er (

w)

DG1 DG1DG2DG3DG2

DG3

Figure 6. Active powers generated by DGs.

Time (s)

Rea

ctiv

e po

wer

(va

r) 1,500

2,000

1,000

500

0

-500

-1,500

-1,000

DG1DG2DG3

DG1

DG3

DG2

0 0.2 0.6 0.80.4 1.8 21.61.41.21

owers

bcplc

3

control, induction motor as the most common industrial load

Figure 7. Reactive p

In this case active powers of DGs are the same as beforeut reactive powers are influenced less than previous case fromhange in the active power demand. Although P and Q areroperly decoupled by increasing K2, but it consequences oscil-

ation in Q and for higher values of reactive power droop gain itontributes to instability and low voltage regulation.

ir

10,000

8,000

6,000

4,000

2,000

0

Act

ive

pow

er (

w)

Tim0.2 0.3 0.4 00 0.1

Figure 8. Active powers genera

500

-500

0

0.1 0.2 0.3 0.50.4

DG1

DG3

DG2

DG1DG2DG1

Tim

Rea

ctiv

e po

wer

(va

r)

Figure 9. Reactive powers gener

generated by DGs.

.2. Induction motor in fault condition

In order to study dynamic loads in a microgrid with droop

s chosen to be examined. Motor characteristics like stator andotor current and rotor speed are observed in fault conditions.

e (s).5 0.6 0.7 0.9 10.8

DG1DG2DG1

DG1

DG2

DG3

ted by DGs for K2 = 0.6.

0.6 0.80.7 0.9 1

e (s)

ated by DGs for K2 = 0.6.

A. Khaledian, M. Aliakbar Golkar / Journal of Applied Research and Technology 15 (2017) 371–377 375

X line1R line1 R line2 X line2DG1

M1 M2

DG2

DG3

Bus2Bus1

Fault

Bus3

SMgam

fFahc

Table 2Induction motor parameters.

Equivalent circuit parameters Nominal values

rs 0.7384 � Line voltage 400 VLs 0.003045 H Frequency 50 Hzrr

′ 0.7402 � Speed 1440 rpmL′

r 0.003045 H Active power 7.5 kWL

itas

oD

Figure 10. Simulation of induction motor in fault condition.

ingle phase schematic of the circuit is shown in Figure 10 where1 and M2 are induction motors. Motor parameters are also

iven in Table 2 where rs and Ls are stator resistance and induct-nce. r′

r and L′r are rotor resistance and inductance. Lm is the

agnetizing inductance.After 0.6 s of the motor starting, a two phases to ground

ault is occurred in bus 1. After 3 cycles the fault is cleared.igure 11 shows motor 1 stator current as one of motor char-

cteristics. It can be seen that voltage and frequency controllersave the capability of maintaining the system stability after faultlearance.

lit

150

100

0

50

-50

-100

0.1 0.2 0.3 0.4 0.Tim

Sta

tor

curr

ent (

A)

Figure 11. Stator current in tw

150

100

50

0

-100

-50

0.2 0 0.4 0.6 Time

Sta

tor

curr

ent (

A)

Figure 12. Stator current in thre

DG

1 re

fere

nce

volta

ge (

Hz)

0.5 1.51 Time

51

50

49

48

47

46

Figure 13. DG1 reference frequency

m 0.1241 H Torque 50 N m

By increasing the fault extremity, 3 phases to ground faults applied to the microgrid. All the other conditions are kepthe same as the previous fault. As it can be seen in Figure 12,fter fault clearance, controllers are failed to maintain systemtability.

The main cause for instability is due to the extreme demandf current and power. According to the droop characteristic,Gs frequency will fail to converge because they generate

arge amount of active power and their generation differences remarkable. Reference frequency determined by power con-rollers for one of three DGs is shown in Figure 13.

5 0.6 0.7 0.9 10.8e (s)

o phases to ground fault.

1.2 1.410.8

(s)

e phases to ground fault.

2.52 3

(s)

in three phases to ground fault.

376 A. Khaledian, M. Aliakbar Golkar / Journal of Applied Research and Technology 15 (2017) 371–377

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Time

150

100

50

-150

-100

-50

0

Sta

tor

curr

ent (

A)

Figure 14. Stator current in three phases to

DG1

DG2

DG3

Bus1 Bus2 Bus3R line1 R line2 X line2X line1

LoadRectifier

Load1Load2

ftsFi

3

t

s1

iovild

tpoSDd

Figure 15. Simulation of nonlinear imbalanced load.

To enhance the microgrid stability in three phases to groundault, fast fault clearance is proposed as a solution. If the pro-ection system detects the fault and disconnects the part wherehort circuit is happened, system stability will be procured fast.igure 14 shows the stator current of the motor when the fault

s cleared fast.

.3. Nonlinear load

In this part imbalanced harmonic distorted load is studied inhe microgrid with droop control. Considering Figure 15, after

osi

0.1 0.2 0.3 0.4 00 Tim

310.0

311.5

312.5

311

312

DG

1 re

fere

nce

volta

ge (

v)

Figure 16. Reference voltage of d-axis

0.35 0.450.4 0.5 Time

Vol

tage

(v)

400

200

-200

0

-400

Figure 17. DG1

(s)

ground fault with fast fault clearance.

tarting the simulation a two-phase rectifier is connected to bus. This load is nonlinear and imbalanced.

Figure 16 shows reference voltage of d-axis created by DG1nner controllers in dq0 frame. After switching the rectifier,scillation is visible in this figure. Large deviations in referenceoltage absolutely for the DG of bus 1 where the nonlinear loads located are noticeable. This contributes to low voltage regu-ation because output voltage fails to track the reference voltageue to its fast deviations.

PI controllers of inner voltage and current controllers areuned to be operative in zero frequency in dq0 reference frame. Inresence of imbalanced harmonic distorted loads there would bether harmonic components such as 50 Hz caused by asymmetry.o as it can be seen in Figures 17 and 18, voltage and current ofG1 have high total harmonic distortion (THD), neutral pointeviance and low voltage regulation.

According to the simulation results, the impact of load type

n droop control method performance was analyzed. It washown that the reactive power sharing of inductive loads can bemproved by the proposed method in which the reactive power

.5 0.6 0.7 0.8 0.9 1

e (s)

created by DG1 inner controllers.

0.55 0.650.6

(s)

voltage.

A. Khaledian, M. Aliakbar Golkar / Journal of Applied Research and Technology 15 (2017) 371–377 377

0.4 0.60.5 0.55 0.650.45e (s)

40

30

20

10

0

-10C

urre

nt (

A)

. DG1

dag

qtdet

caadtvc

4

traato

ddctvSszc

C

R

B

B

C

C

C

F

G

K

L

L

M

M

M

P

S

S

Zhengbo, M., Linchuan, L., & Tuo, D. (2011). Application of a com-

Tim

Figure 18

roop can is increased with regard to the system stability. Thispproach facilitates the decoupling of active and reactive powereneration of power sources.

An in depth discussion was provided with regard to the fre-uency convergence in islanding microgrids. It was shown thathe main impact of fault occurrence on the system stability isue to large frequency deviation of generators with respect toach other. Fast fault clearance was introduced as a solution forhis problem.

Finally, for the nonlinear loads it was concluded that the mainause for the controller failure is due to the operation mech-nism of back-to-back voltage and current controllers. Theyre designed to be applied on voltage and current variables inq0 reference frame. In presence of harmonic distorted loads,ime variant (non dc) component would be appeared for controlariables. Therefore, the embedded PI controllers would fail inreating zero steady state error.

. Conclusion

Droop control based autonomous microgrid was analyzed inhis paper in presence of different types of loads. Simulationesults were shown for different case studies. Dependency ofctive and reactive powers generated by DGs was considered asn important challenge in isolated microgrids. Increasing reac-ive power droop gain was proposed as a solution and its effectsn power quality factors were shown.

In second study, induction motor characteristics in fault con-ition were shown. It was concluded that the fault intensity anduration determine how the microgrid attains to fast frequencyonvergence and fast response of protection system can increasehe system stability. Finally nonlinear load as an important con-entional industrial consumer was verified in the microgrid.imulation results showed that the main cause of high THD,ystem imbalance and low voltage regulation is due to non-ero frequency components of control parameters of inner PIontrollers in dq0 reference frame.

onflict of interest

The authors have no conflicts of interest to declare.

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