A Novel Droop-Based Average Voltage Sharing Control ...static.tongtianta.site/paper_pdf/be47c420-b835-11e9-9158...deviation among them. The droop control aims to simulate a virtual

1096 IEEE TRANSACTIONS ON SMART GRID, VOL. 6, NO. 3, MAY 2015

A Novel Droop-Based Average Voltage SharingControl Strategy for DC Microgrids

Po-Hsu Huang, Po-Chun Liu, Student Member, IEEE, Weidong Xiao, Senior Member, IEEE,and Mohamed Shawky El Moursi, Member, IEEE

Abstract—This paper introduced a decentralized voltage con-trol strategy for dc microgrids that is based on the droopmethod. The proposed distributed secondary voltage controlutilizes an average voltage sharing scheme to compensate the volt-age deviation caused by the droop control. Through nonexplicitcommunication, the proposed control strategy can perform pre-cise terminal voltage regulation and enhance the system reliabilityagainst system failures. The distributed voltage compensatorsthat resemble a centralized secondary voltage controller areimplemented with the bi-proper anti-wind-up design method tosolve the integration issues that necessarily lead to the saturationof the controller output efforts. The proposed concept of pilot busvoltage regulation shows the possibility of managing the termi-nal voltage without centralized structure. Moreover, the networkdynamics are illustrated with a focus on cable resonance modebased on the eigenvalue analysis and small-signal modeling; ana-lytical explanations with the development of equivalent circuitsgive a clear picture regarding how the controller parametersand droop gains affect the system damping performance. Theproposed derivative droop control has been demonstrated todamp the oscillation and to improve the system stability dur-ing transients. Finally, the effectiveness and feasibility of theproposed control strategy are validated by both simulation andexperimental evaluation.

Index Terms—Decentralized control, droop method,hierarchical control, microgrids (MGs), parallel load sharing.

I. INTRODUCTION

RECENTLY, dc microgrids (MGs) have been grasping lotsof attention with their flexibility and expandability. The

key of promoting dc MGs lies in the advanced technologythat enhances reliability during fault conditions, reduces over-all costs and losses by removal of ac–dc conversion, as well asachieves user-friendly operations [1]. In [2] and [3], the utiliza-tion of dc networks has been addressed with their advantagesfor industrial and commercial applications. Various manage-ment and operation strategies for dc MGs have been proposedin [4]–[10]. Among all technologies, the droop method iscommonly used to allow load sharing and voltage regulationamong parallel converters without communication. However,the major drawback of the droop method is poor voltage

Manuscript received March 26, 2014; revised July 6, 2014; acceptedSeptember 6, 2014. Date of publication September 23, 2014; date of currentversion April 17, 2015. Paper no. TSG-00273-2014.

The authors are with iEnergy Center, Electrical Engineering andComputer Science Department, Masdar Institute of Science and Technology,Abu Dhabi 54224, UAE (e-mail: [email protected]).

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

Digital Object Identifier 10.1109/TSG.2014.2357179

regulation due to significant voltage deviations. For the droop-based control strategy, three main trends are to be discussed:1) mitigation of voltage deviation; 2) reliability of the MGoperation; and 3) introduction of nonexplicit communicationinfrastructure. Guerrero et al. [11], [12] proposed a hierar-chical control strategy consisting of primary, secondary, andtertiary controllers for both ac and dc MGs. A centralizedcontrol scheme incorporated with low-bandwidth communi-cation is applied for the primary controller to restore theterminal voltage and to exchange power with external grids.In such a case, when facing communication failures, the droopcontrol can still maintain the equal current sharing opera-tion among converters (though inevitable voltage deviationexists). The approach brings the potential of using limitedcommunication to enhance the voltage regulation capabilitywhile securing the system reliability. In addition, many recentpapers [6], [12]–[14] have demonstrated the significant bene-fits of communication for the control and management schemedesign to enhance the system reliability, power quality, andstability.

When it comes to the implementation, the physical con-nection may vary from different devices based on Ethernet,optical fibers, wireless/radio techniques, or power linecommunication (PLC). To incorporate different types ofdevices, IEC61850 was suggested in [15] as a common proto-col for exchanging data. Among different means, cost-effectivesolutions using PLC can be seen in low voltage distribu-tion networks. Moreover, Pinomaa et al. [16] proposed aPLC based network architecture for low voltage dc (LVdc)distribution system. However, it has been reported that datatransmission using PLC suffers signal attenuation in time-varying or capacitive load conditions, resulting undesirablecommunication [17]. An alternative way of employing con-troller area network (CAN) protocol is proposed for the MGcontrol operation, showing the potential of reducing the cost ofcommunication due to its high availability in common powerelectronics applications [18].

For the coordinated control approach in MGs, the cen-tralized control structure may be of favorable solutions withhigh-bandwidth communication. However, in remote areaapplications sophisticated communication infrastructure mayadd up the burden for securing a reliable and affordableenergy solution. As a result, using nonexplicit communica-tion to assist MG management scheme is likely to be morefeasible. The droop control based on the decentralized struc-ture offers a great highlight through parallel operation with

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HUANG et al.: NOVEL DROOP-BASED AVS CONTROL STRATEGY FOR DC MGs 1097

enhanced tolerance against failures in a single device. Anaverage current sharing (ACS) method is proposed in [19] toachieve dc MG control based on the distributed configurationby utilizing CAN bus communication. In this case, no externalvoltage restorer is equipped to regulate the terminal voltage.While the ACS scheme ensures good voltage compensation, itcannot attain accurate voltage control compared to the hierar-chical and centralized control scheme. To ensure better voltageregulation performance, an improved control method is to beinvestigated.

This paper proposes a novel decentralized control schemeusing the average voltage sharing (AVS) method to achieveprecise voltage regulation while securing system reliabilityagainst communication or converter failures. The primary con-trol loop is based on the droop method to manage loadsharing. The distributed secondary controllers employ thebi-proper anti-wind-up design to allow parallel voltage reg-ulator functionality, thereby restoring the average terminalvoltages back to the rated value. In addition to the secondarycompensation, pilot bus regulation is integrated into the dis-tributed secondary control loop to adjust the reference setpoint, allowing the proposed scheme to achieve voltage reg-ulation of a single bus. Since the proposed scheme is fullydecentralized, there is no centralized controller in the MG. Inthis case, the system has higher resilience compared to thecentralized system. The proposed scheme also fits within therequirement of employing low-bandwidth communication byperiodically adjusting the voltage set-point to reduce the costsof communication infrastructure.

The paper is organized as follows. The introduction of dif-ferent MG control strategies is first illustrated in Section II.Section III elaborated on the proposed AVS control scheme,followed by the Section IV that provides the analysis of net-work dynamic behavior. Simulation and experimental resultsare then carried out in Section V.

II. DC MG CONTROL STRATEGY

A. Droop Control

The major issue of the parallel converters supplying loads isunequal load sharing among them. Ideally, when two convert-ers operate in parallel, uniform current distribution betweenthem is expected since all converters are assumed to beidentical. In practice, however, factors such as nominal voltagedeviation, measurement errors, cable resistances, and unbal-anced load distribution directly cause unequal current sharingamong sources. Fig. 1(a) shows the equivalent circuit of twoparallel converters connected to a load with RD1, RD2 repre-senting virtual resistance of the droop gains, R1, R2 symboliz-ing cable resistance, and RL being load resistance. When droopgains are zero, the terminal voltages Vk is equal to referencevoltage V∗

k , and the relationship between output voltage andcurrent is shown in Fig. 1(b). Voltage deviations due to mea-surement and reference errors are considered; cable resistanceis also presented by drawing the droop curves with differentslopes. Let Ij

k denotes the output current of the converter k.The unequal current distribution is then shown by referring thesame output voltage; in this case, I1

1 is far higher than I12 . When

(a)

(b)

Fig. 1. (a) Equivalent circuit of two parallel converters with droop control.(b) V-I characteristics of the system with and without droop control.

the droop gains are added, the equivalent series resistance ofboth converters become relatively large, reducing the currentdeviation among them. The droop control aims to simulatea virtual resistance by introducing the voltage drop into thereference, which helps to mitigate the unequal current sharing.

B. Hierarchical Control Scheme

Although the droop control needs no communication toachieve very fast response through primary voltage control,the voltage reference is penalized by the droop term. Voltagerestoration through an additional PI compensator is not fea-sible as the individual integrator actions in all players tendto regulate their own terminal voltage, leading to controllerconflicts. One solution is to use a centralized voltage com-pensator, namely the secondary voltage controller, to feed thesame compensation value to all converters and to shift thereference set points simultaneously. Fig. 2 shows the configu-ration of the hierarchical control scheme for dc MGs proposedin [11]. A communication link is utilized to assure the samecompensation level to be received by all the converters. Inaddition, the hierarchical structure can allows the tertiary con-troller to replace the role of the secondary layer by acting asa power manager that regulates the power flow in-between theMG and external grids.

C. Average Current Sharing Scheme

As mentioned in the previous section, a centralized con-troller is required for the hierarchical control scheme toachieve equal voltage compensation in order to avoid unbal-ance of current distribution. However, failure of the centralizedunits necessarily lead to malfunction of voltage compensa-tion. Therefore, voltage compensation based on distributedconfiguration shows the advantage of enhancing system relia-bility and tolerance against failures. Fig. 3 shows the general


Fig. 2. Hierarchical control scheme for dc MGs.

Fig. 3. Average current sharing scheme for dc MG.

configuration of the ACS scheme [20], [21]. The measuredcurrent value is converted into voltage signal and multipliedby the droop gain DnIn, which is linked to the positive inputof the op-amp. Since the current sharing bus connects to eachsignal conditioner through a resistor Rn, the average voltage isto appear on the bus. By choosing proper impedances of theop-amp, the droop drop can be canceled out, indicating therestoration of the output voltage to its nominal value. However,the common bus carrying analog signals is distributed amongthe converters and likely to suffer from noise for long-distanceapplications. Therefore, an enhanced version of using digitalcurrent sharing scheme (DACS) is proposed in [19] with thelow-bandwidth communication channel carrying digital sig-nals to achieve the goal of voltage compensation. The DACSstructure is depicted in Fig. 4. The output current is scaleddown by the rated magnitude, which is then communicatedto other participants. By fetching the current signals sent outfrom other converters, the average value is calculated and mul-tiplied by both the rated magnitude and Kn gain. In this case,the DACS scheme can achieve good voltage compensationand ensure the quality of compensation signals through digitalcommunication channels.

III. PROPOSED CONTROL STRATEGY

The DACS scheme provides voltage compensation againstthe droop drops and achieves the decentralized structure

Fig. 4. System configuration of digital average current sharing (DACS)scheme.

through low-bandwidth communication. Since the voltage reg-ulation is implemented based on individual primary controlloops, the DACS gains K1 . . . Kn are to be properly selectedso as to obtain exact voltage compensation. Although systembus voltage approximates to the nominal voltage, the precisevoltage control capability is not given. In Fig. 5, the proposedAVS concept is shown. It can be seen that the distributed sec-ondary control loop is implemented inside each converter unit.The controller output is then shared via the communicationchannel with other converters. By receiving all the controloutput signals from others, the average compensation value iscalculated and then fed into the primary control loop. This ideais to emulate the same function of the centralized secondarycontroller that regulates the dc bus voltage by aggregatingdistributed secondary controllers, shown as

�Vavg =n∑

k=1

uk

n= 1

n

(n∑

k=1

kp2(Vref − Vn)

+n∑

k=1

ki2

∫(Vref − Vn)

)

= kp2(Vref − Vavg) + ki2

∫(Vref − Vavg), k ∈ {1, 2...n}

(1)

where �Vavg is the average compensation voltage, kp2, ki2are the PI gains of the distributed secondary voltage con-trollers (parameters are assumed to be identical among all theinverters), and Vavg is the average voltage of the converteroutputs. In steady state, the integrator action of the secondarycontroller approaches to minimize the average voltage errorVref − Vavg, which indicates that the average output voltagereaches the desired reference value.

A. Wind-Up Affect

Although the average error is to be minimized as describedin (1), the individual input errors necessarily differ from eachother. Hence, the integration wind-up effect appears in all


Fig. 5. Conceptual diagram of proposed average voltage sharing (AVS)scheme.

Fig. 6. Illustration of wind-up effect in distributed PI controllers.

the distributed secondary PI controllers. Fig. 6 shows thatopposite integration actions are induced when minimizing theerror between the average reference and bus voltage. Whenthe controller is implemented digitally, overflow occurs to thecontroller output register, failing the compensation scheme.

To solve the problem, an anti-wind-up scheme should beapplied. The controller in (1) can be modified by consideringthe average controller output effort �Vavg as the actual outputto the primary loop, shown in Fig. 7. The secondary controlleris then reconstructed using the feedback form of the bi-propercontroller [22], which can be derived as

uk = C∞ek + C(s)uk (2)

where C∞ is dc gain of C(s), C(s) is equal to C−1(s) − C−1∞(a strictly proper transfer function), uk is the actual controlleroutput (�Vavg in Fig. 7), and uk is the unconstrained controlleroutput. Since the average output of all the parallel controllersis then fed into individual primary control loops. Therefore,the equivalent synthesized controller can be derived as

�Vavg =n∑

k=1

uk

n=

n∑

k=1

{C∞ek − C∞

(C−1(s) − C−1∞

)�Vavg

}

n

= C∞(

eavg − C−1(s)�Vavg

)+ �Vavg. (3)

Thus,

�Vavg = C(s)eavg. (4)

This indicates that the aggregated controller behaves likea centralized voltage regulator without being affected by thewind-up effect that occurs in individual controllers. The equiv-alent effort of the controllers then regulates the average outputvoltages by minimizing the average error signal eavg.

B. Discussion of Pilot Bus Regulation

The above mentioned control scheme displays onlythe regulation capability of the average terminal voltage.Adjusting the individual reference voltage set point reflectsdirectly on the average reference magnitude, which is then fol-lowed by the average terminal voltage. Therefore, by adding anadditional voltage regulator, the regulation capability for a sin-gle bus can be obtained with the configuration shown in Fig. 7.For selecting the pilot bus, additional bits pn are assigned tobe sent through the communication channel with the com-pensation signal to other converters. After receiving all theadditional bits p1 . . . pn, each controller compares the set withits designated index (the minimum value indicates the priority)in order to determine whether its terminal voltage is selectedto be regulated. The flowchart of the designation algorithm toselect the pilot bus is shown in Fig. 8. In this case, the sys-tem can be initialized sequentially by the user to assign thepriority. Also, when a failure occurs to the pilot converter, therole of performing pilot bus regulation can be transferred toother converters based on the given index to enhance the sys-tem tolerance against failures. The details of the performancewill be demonstrated in the evaluation section.

IV. ANALYSIS OF NETWORK DYNAMIC BEHAVIOR

In this section, the details of the system dynamic responseswill be investigated. A system based on two converters supply-ing a load is utilized, shown in Fig. 9. The system parametersare shown in Table I. The mathematical model of the systemcan then be described in the state-space representation

X = AX + BU (5)

where the eigenvalues of A reveal the system modes. To formthe equation sets, the power electronic converters are mod-eled as controlled voltage sources and the dynamics of highswitching frequency is ignored. By extracting the eigenval-ues of the system transition matrix, a noticeable resonancemode can be found at −849 ± 4574i, which is induced bythe resonance between the cable inductance and the converteroutput capacitors (C-L-C). To investigate the impact of thecontroller parameters and droop gains on the cable mode, theeigenvalue loci are plotted in Fig. 10. It can be observed thatboth the Kp and droop gains have noticeable influences onthe movement of the cable mode: increment of Kp movesthe eigenvalue toward LHP and improves the damping per-formance; increase of the droop gain shifts the mode towardRHP and lifts the oscillation frequency. The above observationcan be further explained by ignoring the load resistance in theC-L-C circuit. Thus, the dynamic equation as seen from theleft side can be derived in the Laplace’s domain

V1 = (I1 − sC1V1) (sL12 + R12) + V2. (6)

Assuming both converters have the same proportionalgain kp, the inductor current of the converter 2 can berepresented as

I2 = Ti2(s)Iref2 = Ti2(s)Kp(V∗

2 − V2)

(7)


Fig. 7. Diagram of the proposed AVS control scheme.

Fig. 8. Flowchart of the designation algorithm.

Fig. 9. Equivalent circuit of two parallel converters sharing a load.

where Ti2(s) is the closed-loop transfer function from thecurrent reference to the inductor current. With considera-tion of fast current tracking performance, inductor current is

TABLE IPARAMETERS OF SYSTEM IN FIG. 9

Fig. 10. Eigenvalue loci of the cable mode with respect to parametervariations. Kp: voltageloop proportional gain; Ki: voltageloop integral gain.

assumed to be equal to the current reference. Thus, the currentperturbation �I2, in small-signal views, can be derived as

�I2 = −Kp�V2. (8)

Equation (8) shows that the proportional gain behaves like avirtual resistor with resistance of 1/Kp. In addition, variablesin (6) are replaced by �V1, �V2, �I1, and �I2, shown in

�V1 = (�I1 − sC1�V1

) (sL12 + R12 + 1

sC2 + kp

). (9)


Fig. 11. Comparison of the eigenvalue loci and pole movements of the cablemode by the variation of Kp (Ki = 0, droop = 0).

Hence, substituting �I1 = −Kp�V1 + �U1 into (9) gives

�V1

�U1= b2s2 + b1s + b0

a3s3 + a2s2 + a1s + a0(10)

wherea3 = L12C1C2, a2 = R12C1C2 + KpL12 (C1 + C2)

a1 = KpR12 (C1 + C2) + L12K2p + C1 + C2

a0 = 2Kp + K2pR12, b2 = L12C2

b1 = C2R12 + L12Kp, and b0 = KpR12 + 1. (11)

By changing the proportional gain, in Fig. 11 the polemovement of the characteristic equation of the transfer func-tion in (10) identifies the result from the eigenvalue analysis.The larger proportional gain induces a lower shunt resistance(virtual), therefore increasing the system damping by shiftingthe cable resonance poles toward LHP. Similarly, the effectof the droop gain can be explained by considering the outervoltage loop

Io1 = I1 − IC1 = Kp(Vref − D1Io1 − Vo1) − C1dVo1/

dt. (12)

Applying small perturbation terms, (13) can be obtained

− �Io1 = sCeff�Vo1 + �Vo1/

Reff (13)

whereCeff = C1

/(1 + KpD1), Reff = (1 + KpD1)/Kp. (14)

Equations (13) and (14) show that the droop gain reduceseffective shunt capacitance; meanwhile, the virtual shunt resis-tance caused by the proportional controller increases. Thisexactly explains why increasing the droop gain obtains ahigher resonance frequency (smaller shunt capacitance) andmoves the real part of the cable mode toward RHP (largershunt resistance).

To mitigate the deteriorated damping, a proportional-derivative (P-D) droop control is introduced by adding a sub-traction term, −sDd1Io1, into the voltage reference. Thus, (13)can be further described as

− �Io1 = �Vo1/

Xeff1 + �Vo1/

Xeff2 (15)

where

Xeff1 = Reff1 + sLeff1, Xeff2 = Reff2 + 1/sCeff2

Reff1 = (1 + D1Kp)/Kp, Leff1 = Dd1

Reff2 = KpDd1/C1, and Ceff2 = C1/(1 + D1Kp). (16)

Fig. 12. Equivalent circuit of two parallel converters with P-D droop control.

TABLE IIEIGENVALUES AND DAMPING FACTORS OF CABLE MODE BY

DIFFERENT DERIVATIVE DROOP GAINS

The equivalent circuit representing (15) is shown in Fig. 12.As seen in (15), larger derivate droop gains result in largerLeff1 and Reff2, indicating the increase of the damping andreduction of the resonance frequency. The effect of the deriva-tive droop control on the system damping performance isgiven in Table II, showing that increase of the derivative gainhelps to improve the system damping performance. However,it should be addressed that the selection of the gains shouldconsider the primary control loop stability, and the implemen-tation of the derivative term is to include a low-pass filterthat rejects the high frequency noises while preserving theresonance frequency.

V. EVALUATION

A. Simulation Results

To verify the proposed control strategy, the simulationmodel of a dc MG system is established. The system con-figuration is shown in Fig. 13 with the parameters shown inTable III. Fig. 14 shows the simulation results of the sys-tem based on only the droop control during load switching.The droop gains are selected as 0.6:0.6:0.6 in order to haveacceptable voltage deviations at the heavy-load condition. At0.29 s, R3 switches on and the system reaches the heavy-loadcondition; after 0.1 s the disconnection of R2 has the systemback to the original state. Shown in Fig. 14, the steady-statevoltages are 115.2 V:114.6 V:116.4 V (−4%:−4.5%:−3%) atthe beginning of the simulation and 114.0 V:113.2 V:115.4 V(−5%:−5.6%:−3.8%) in the heavy-load condition; the con-verter output currents are 8.2 A:8.9 A:6.1 A initially, and9.9 A:11.3 A:7.7 A in the heavy-load condition. This showsthat the selection of the droop gains has to sacrifice the sharingaccuracy by reducing the gains, leading to a trade-off situa-tion between voltage regulation and uniform current sharing.Therefore, to achieve both objectives, the secondary compen-sation is necessary to restore the voltage back to the nominalvalue. Fig. 15 illustrates the results of the proposed AVSscheme. Since the voltage is well compensated, the droopgains are selected as relatively larger (3:3:3) to mitigate the


Fig. 13. Configuration of a three-converter system.

TABLE IIISYSTEM PARAMETERS

Fig. 14. Simulation results of the dc MG based on the droop con-trol. (a) Converter terminal voltages. (b) Converter output currents (unit:volt/ampere).

unbalance among converter currents. It can be seen that all theterminal voltages are restored to its rated value over the periodof the simulation. The steady-state terminal voltages can beobtained as 119.6 V:119.2 V:121.3 V (−0.3%:−0.7%:1.1%)and 119.6 V: 118.9 V:121.5 V (−0.3%:−0.9%:1.3%) infull-load condition; steady-state converter currents are then7.7 A:7.8 A:7.2 A and 9.7 A:9.9 A:9.1 A in heavy-load condition. Fig. 15(c) shows the voltage compensationvalue �VAVS based on the update rate of 5 ms. In addition, thecurrent distribution under different droop gains (based on therating of the converters) of 1.5:3:4.5 is shown in Fig. 16. Itcan be seen that designated current sharing can be achievedby choosing a different droop ratio.

The dynamic response of the average terminal voltage dur-ing load switching is presented in Fig. 17. Two cases are

Fig. 15. Simulation results of the dc MG based on the proposed DAVSscheme. (a) Converter terminal voltages. (b) Converter output currents.(c) Compensated voltage.

Fig. 16. Simulation results of the dc MG based on the proposed DAVSscheme under the droop gains of 1.5:3:4.5. (a) Converter terminal voltages.(b) Converter output currents. (c) Compensated voltage.

conducted based on different update periods of 5 and 1 ms.Improved transient performance can be observed with the pro-posed AVS scheme in comparison with the case based onthe ACS method. Also, significant improvement of the voltageresponses in Fig. 17(c) and (d) based on the higher update ratedemonstrates the main advantage of the proposed scheme thatprovides the flexibility of controller design to accommodatevarious system configurations and communication speeds inorder to achieve better dynamic performance.

Fig. 18 shows the pilot bus regulation capability, and inthis case the pilot bus is selected as the terminal voltageof the converter 3. The precise voltage regulation of termi-nal 3–120 V can be observed. In Fig. 19, current transientresponses are compared between the P droop and P-D droopmethods. The additional derivative droop gain significantly


(a)

(c) (d)

(b)

Fig. 17. Comparison of the dynamic performance between the digital AVSand ACS schemes based on the update periods of 5 and 1 ms (load switchingat 0.29 and 0.39 s). (a) and (b) 5 ms. (c) and (d) 1 ms.

Fig. 18. Simulation result of the system with prior bus voltage regulation(converter 3) during load switching at 0.39 and 0.69 s.

Fig. 19. Converter output currents (cable mode resonance) during loadswitching with the D droop sDdn/(τ s + 1). (a) Converter 1 output current.(b) Converter 2 output current. (c) Converter 3 output current. (Dd1 = Dd2 =Dd3 = 0.002).

improves the system damping performance without affectingthe steady-state performance.

The system dynamic behavior in responding to the failure ofthe converter 1 is shown in Fig. 20. When the failure occurs,the voltage regulation is automatically transferred to the con-verter 2 based on the given priority sequence. Hence, theregulation of the converter 2 terminal voltage engages and thesystem resumes normal operation. It should be addressed that

Fig. 20. Dynamic responses of the system during the failure of the converter 1(total load: 1.7 kW). (a) Converter terminal voltage. (b) Converter outputcurrents. (c) Average compensated voltage.

Fig. 21. Diagram of the experimental setup.

the case considers that the system total load demand is lessthan the remaining converters’ total rating, and if the systemis overloading, the voltage regulation cannot be achieved.

B. Experimental Results

A scaled-down experimental work is constructed with twosynchronous buck converters to verify the proposed schemewith the system configuration and parameters shown inFig. 21 and Table IV, respectively. The control algorithmis implemented based on the TI TMS320F2808 microcon-troller for evaluating the droop control and the proposed AVSscheme. Two control loops are independently executed forthe converters to regulate their output terminal voltages. Thedelays caused by non-explicit communication is also consid-ered and emulated through the slow update rate of 200 Hz forthe proposed AVS scheme.

Fig. 22 demonstrates the system response based on only thedroop control during the engagement of R2. The load changesfrom 15 to 10 ohms. As can be seen that the voltage deviation


TABLE IVPARAMETERS OF THE EXPERIMENT

Fig. 22. Dynamic responses of the output voltage and current during theengagement of R2 (droop control).

Fig. 23. Dynamic responses of the output voltage and current during thedisengagement of R2 (droop control).

increases due to higher output currents and the cable reso-nance can also be observed during the transient. In Fig. 23, thesystem performance responding to the disengagement of theextra load is shown. Without the communication, the inherentvoltage drops cannot be avoided. Fig. 24 shows the system per-formance with the proposed AVS scheme when switching on

Fig. 24. Dynamic responses of the output voltage and current during theengagement of R2 (proposed AVS scheme).

Fig. 25. Dynamic responses of the output voltage and current during thedisengagement of R2 (proposed AVS scheme).

the extra resistor R2. The output voltage is well maintained inthe nominal level; the cable resonance is significantly reducedby the proposed derivative droop method. Moreover, the casefor the sudden disconnection of R2 is shown in Fig. 25. Thevoltage magnitude eventually reaches the nominal value afterthe transient state. A similar cable resonance can be also seenby the waveforms of the converter output currents.

To sum up, with the proposed AVS scheme, the equal cur-rent sharing can be achieved without the necessary voltagedrops induced by the droop method; the derivative droopmethod helps to damp the cable resonance. Finally, boththe simulation and experimental results have verified theabovementioned functions of the proposed control strategy.

VI. CONCLUSION

The distributed AVS scheme is presented in this paper tomaintain the terminal voltage at the nominal value and securethe uniform current sharing disregarding the variation of load-ing conditions. The distributed secondary voltage controllersare effectively constructed with the bi-proper anti-wind-up


design method. Furthermore, the pilot bus regulation functionis achieved through low-bandwidth communication to mitigatethe voltage bias caused by the cable resistances at a chosenterminal without any centralized compensator. In addition, ithas been observed that the cable resonance mode can be welldamped by the proposed P-D droop control. This has been ver-ified by the theoretical analysis, which offers an insight intothe mathematical relationship between the controller param-eters and the resonance phenomena in MG networks. Theimproved voltage recovery performance during load switchinghas been demonstrated by both simulation and experimentalresults, which show the enhanced dynamic responses thanks tothe developed AVS scheme. Finally, due to the decentralizedstructure of the proposed scheme, the system reliability can besignificantly enhanced based on cost-effective and nonexplicitcommunication solutions.

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Po-Hsu Huang was born in Taiwan in 1985.He received the B.Sc. degree from NationalCheng-Kung University, Tainan, Taiwan, and theM.Sc. degree from National Taiwan University,Taipei, Taiwan, in 2007 and 2009, respectively,both in electrical engineering, and the M.Sc.degree from the Department of Electrical PowerEngineering, Masdar Institute of Science andTechnology, Abu Dhabi, UAE. He is currently pursu-ing the Ph.D. degree from the Electrical Engineeringand Computer Science Department, Massachusetts

Institute of Technology, Cambridge, MA, USA.His current research interests include dc/ac microgrids, power electron-

ics, wind power generation, linear/nonlinear system dynamics, power systemstability, and control.

Po-Chun Liu (S’13) received the B.Eng. degreein electrical engineering from National TaiwanUniversity, Taipei, Taiwan, in 2011.

He is currently a Research Assistant with theiEnergy Center, Masdar Institute of Science andTechnology, Abu Dhabi, UAE. His current researchinterests include wind power systems and powerconversion in microgrids.


Weidong Xiao (S’04–M’07–SM’13) received theMaster’s and Ph.D. degrees in electrical engineeringfrom the University of British Columbia, Vancouver,BC, Canada, in 2003 and 2007, respectively.

He is an Associate Professor with the Departmentof Electrical Engineering and Computer Science,Masdar Institute of Science and Technology,Abu Dhabi, UAE. In 2010, he was a Visiting Scholarwith the Massachusetts Institute of Technology,Cambridge, MA, USA, where he researched powerinterfaces for photovoltaic (PV) power systems.

He was a Research and Development Engineering Manager with MSRInnovations Inc., Burnaby, BC, Canada, focusing on integration, research,optimization, and design of PV power systems. His current research interestsinclude PV power systems, power electronics, dynamic systems and control,and industry applications.

Dr. Xiao is currently an Associate Editor of the IEEE TRANSACTIONS ON

INDUSTRIAL ELECTRONICS.

Mohamed Shawky El Moursi (M’12) received theB.Sc. and M.Sc. degrees from Mansoura University,Mansoura, Egypt, in 1997 and 2002, respectively,and the Ph.D. degree from the University ofNew Brunswick (UNB), Fredericton, NB, Canada,in 2005, all in electrical engineering.

He was a Research and Teaching Assistantwith the Department of Electrical and ComputerEngineering, UNB, from 2002 to 2005. He thenjoined McGill University, Montreal, QC, Canada, asa Post-Doctoral Fellow with the Power Electronics

Group. He was with Technology Research and Development, Wind PowerPlant Group, Vestas Wind Systems, Arhus, Denmark, and was withTRANSCO, Abu Dhabi, UAE, as a Senior Study and Planning Engineer, andseconded as a faculty member with the Faculty of Engineering, MansouraUniversity. He is currently an Associate Professor with the ElectricalEngineering and Computer Science Department, Masdar Institute of Scienceand Technology, Abu Dhabi, UAE, and a Visiting Professor with theMassachusetts Institute of Technology, Cambridge, MA, USA. His currentresearch interests include power system, power electronics, flexible alternat-ing current transmission system technologies, system control, wind turbinemodeling, wind energy integration, and interconnections.

Documents

A Novel Droop-Based Average Voltage Sharing Control ...static.tongtianta.site/paper_pdf/be47c420-b835-11e9-9158...deviation among them. The droop control aims to simulate a virtual