The strategy is based on an autonomous distributed control scheme in which the DC bus voltage level is used as an indicator of the power balance in the microgrid. The autonomous control strategy does not rely on communication links or a central controller, resulting in reduced costs and enhanced reliability. As part of the control strategy, an adaptive droop control technique is proposed for PV sources in order to maximize the utilization of power available from these sources while ensuring acceptable levels of system voltage regulation

1. 1096 IEEE TRANSACTIONS ON SMART GRID, VOL. 6, NO. 3, MAY 2015
A Novel Droop-Based Average Voltage Sharing
Control 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 droop
method. The proposed distributed secondary voltage control
utilizes an average voltage sharing scheme to compensate the volt-
age deviation caused by the droop control. Through nonexplicit
communication, the proposed control strategy can perform pre-
cise terminal voltage regulation and enhance the system reliability
against system failures. The distributed voltage compensators
that resemble a centralized secondary voltage controller are
implemented with the bi-proper anti-wind-up design method to
solve the integration issues that necessarily lead to the saturation
of the controller output efforts. The proposed concept of pilot bus
voltage regulation shows the possibility of managing the termi-
nal voltage without centralized structure. Moreover, the network
dynamics are illustrated with a focus on cable resonance mode
based on the eigenvalue analysis and small-signal modeling; ana-
lytical explanations with the development of equivalent circuits
give a clear picture regarding how the controller parameters
and droop gains affect the system damping performance. The
proposed derivative droop control has been demonstrated to
damp the oscillation and to improve the system stability dur-
ing transients. Finally, the effectiveness and feasibility of the
proposed control strategy are validated by both simulation and
experimental evaluation.
Index Terms—Decentralized control, droop method,
hierarchical control, microgrids (MGs), parallel load sharing.
I. INTRODUCTION
RECENTLY, dc microgrids (MGs) have been grasping lots
of attention with their flexibility and expandability. The
key of promoting dc MGs lies in the advanced technology
that enhances reliability during fault conditions, reduces over-
all costs and losses by removal of ac–dc conversion, as well as
achieves user-friendly operations [1]. In [2] and [3], the utiliza-
tion of dc networks has been addressed with their advantages
for industrial and commercial applications. Various manage-
ment and operation strategies for dc MGs have been proposed
in [4]–[10]. Among all technologies, the droop method is
commonly used to allow load sharing and voltage regulation
among parallel converters without communication. However,
the major drawback of the droop method is poor voltage
Manuscript received March 26, 2014; revised July 6, 2014; accepted
September 6, 2014. Date of publication September 23, 2014; date of current
version April 17, 2015. Paper no. TSG-00273-2014.
The authors are with iEnergy Center, Electrical Engineering and
Computer Science Department, Masdar Institute of Science and Technology,
Abu Dhabi 54224, UAE (e-mail: melmoursi@masdar.ac.ae).
Color versions of one or more of the figures in this paper are available
online 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 MG
operation; and 3) introduction of nonexplicit communication
infrastructure. Guerrero et al. [11], [12] proposed a hierar-
chical control strategy consisting of primary, secondary, and
tertiary controllers for both ac and dc MGs. A centralized
control scheme incorporated with low-bandwidth communi-
cation is applied for the primary controller to restore the
terminal voltage and to exchange power with external grids.
In such a case, when facing communication failures, the droop
control can still maintain the equal current sharing opera-
tion among converters (though inevitable voltage deviation
exists). The approach brings the potential of using limited
communication to enhance the voltage regulation capability
while securing the system reliability. In addition, many recent
papers [6], [12]–[14] have demonstrated the significant bene-
fits of communication for the control and management scheme
design to enhance the system reliability, power quality, and
stability.
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 line
communication (PLC). To incorporate different types of
devices, IEC61850 was suggested in [15] as a common proto-
col for exchanging data. Among different means, cost-effective
solutions using PLC can be seen in low voltage distribu-
tion networks. Moreover, Pinomaa et al. [16] proposed a
PLC based network architecture for low voltage dc (LVdc)
distribution system. However, it has been reported that data
transmission using PLC suffers signal attenuation in time-
varying or capacitive load conditions, resulting undesirable
communication [17]. An alternative way of employing con-
troller area network (CAN) protocol is proposed for the MG
control operation, showing the potential of reducing the cost of
communication due to its high availability in common power
electronics applications [18].
For the coordinated control approach in MGs, the cen-
tralized control structure may be of favorable solutions with
high-bandwidth communication. However, in remote area
applications sophisticated communication infrastructure may
add up the burden for securing a reliable and affordable
energy solution. As a result, using nonexplicit communica-
tion to assist MG management scheme is likely to be more
feasible. The droop control based on the decentralized struc-
ture offers a great highlight through parallel operation with
1949-3053 c
2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

2. HUANG et al.: NOVEL DROOP-BASED AVS CONTROL STRATEGY FOR DC MGs 1097
enhanced tolerance against failures in a single device. An
average current sharing (ACS) method is proposed in [19] to
achieve dc MG control based on the distributed configuration
by utilizing CAN bus communication. In this case, no external
voltage restorer is equipped to regulate the terminal voltage.
While the ACS scheme ensures good voltage compensation, it
cannot attain accurate voltage control compared to the hierar-
chical and centralized control scheme. To ensure better voltage
regulation performance, an improved control method is to be
investigated.
This paper proposes a novel decentralized control scheme
using the average voltage sharing (AVS) method to achieve
precise voltage regulation while securing system reliability
against communication or converter failures. The primary con-
trol loop is based on the droop method to manage load
sharing. The distributed secondary controllers employ the
bi-proper anti-wind-up design to allow parallel voltage reg-
ulator functionality, thereby restoring the average terminal
voltages back to the rated value. In addition to the secondary
compensation, pilot bus regulation is integrated into the dis-
tributed secondary control loop to adjust the reference set
point, allowing the proposed scheme to achieve voltage reg-
ulation of a single bus. Since the proposed scheme is fully
decentralized, there is no centralized controller in the MG. In
this case, the system has higher resilience compared to the
centralized system. The proposed scheme also fits within the
requirement of employing low-bandwidth communication by
periodically adjusting the voltage set-point to reduce the costs
of 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 results
are then carried out in Section V.
II. DC MG CONTROL STRATEGY
A. Droop Control
The major issue of the parallel converters supplying loads is
unequal load sharing among them. Ideally, when two convert-
ers operate in parallel, uniform current distribution between
them is expected since all converters are assumed to be
identical. In practice, however, factors such as nominal voltage
deviation, measurement errors, cable resistances, and unbal-
anced load distribution directly cause unequal current sharing
among sources. Fig. 1(a) shows the equivalent circuit of two
parallel 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 droop
gains are zero, the terminal voltages Vk is equal to reference
voltage V∗
k , and the relationship between output voltage and
current is shown in Fig. 1(b). Voltage deviations due to mea-
surement and reference errors are considered; cable resistance
is also presented by drawing the droop curves with different
slopes. Let I
j
k denotes the output current of the converter k.
The unequal current distribution is then shown by referring the
same output voltage; in this case, I1
1 is far higher than I1
2. 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 of
both converters become relatively large, reducing the current
deviation among them. The droop control aims to simulate
a virtual resistance by introducing the voltage drop into the
reference, which helps to mitigate the unequal current sharing.
B. Hierarchical Control Scheme
Although the droop control needs no communication to
achieve very fast response through primary voltage control,
the voltage reference is penalized by the droop term. Voltage
restoration through an additional PI compensator is not fea-
sible as the individual integrator actions in all players tend
to regulate their own terminal voltage, leading to controller
conflicts. One solution is to use a centralized voltage com-
pensator, namely the secondary voltage controller, to feed the
same compensation value to all converters and to shift the
reference set points simultaneously. Fig. 2 shows the configu-
ration of the hierarchical control scheme for dc MGs proposed
in [11]. A communication link is utilized to assure the same
compensation level to be received by all the converters. In
addition, the hierarchical structure can allows the tertiary con-
troller to replace the role of the secondary layer by acting as
a power manager that regulates the power flow in-between the
MG 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 to
achieve equal voltage compensation in order to avoid unbal-
ance of current distribution. However, failure of the centralized
units necessarily lead to malfunction of voltage compensa-
tion. Therefore, voltage compensation based on distributed
configuration shows the advantage of enhancing system relia-
bility and tolerance against failures. Fig. 3 shows the general

3. 1098 IEEE TRANSACTIONS ON SMART GRID, VOL. 6, NO. 3, MAY 2015
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 measured
current value is converted into voltage signal and multiplied
by the droop gain DnIn, which is linked to the positive input
of the op-amp. Since the current sharing bus connects to each
signal conditioner through a resistor Rn, the average voltage is
to appear on the bus. By choosing proper impedances of the
op-amp, the droop drop can be canceled out, indicating the
restoration of the output voltage to its nominal value. However,
the common bus carrying analog signals is distributed among
the converters and likely to suffer from noise for long-distance
applications. Therefore, an enhanced version of using digital
current sharing scheme (DACS) is proposed in [19] with the
low-bandwidth communication channel carrying digital sig-
nals to achieve the goal of voltage compensation. The DACS
structure is depicted in Fig. 4. The output current is scaled
down by the rated magnitude, which is then communicated
to other participants. By fetching the current signals sent out
from 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 compensation
and ensure the quality of compensation signals through digital
communication channels.
III. PROPOSED CONTROL STRATEGY
The DACS scheme provides voltage compensation against
the 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 control
loops, the DACS gains K1 . . . Kn are to be properly selected
so as to obtain exact voltage compensation. Although system
bus voltage approximates to the nominal voltage, the precise
voltage control capability is not given. In Fig. 5, the proposed
AVS 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 communication
channel with other converters. By receiving all the control
output signals from others, the average compensation value is
calculated and then fed into the primary control loop. This idea
is to emulate the same function of the centralized secondary
controller that regulates the dc bus voltage by aggregating
distributed 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, ki2
are the PI gains of the distributed secondary voltage con-
trollers (parameters are assumed to be identical among all the
inverters), and Vavg is the average voltage of the converter
outputs. In steady state, the integrator action of the secondary
controller approaches to minimize the average voltage error
Vref − Vavg, which indicates that the average output voltage
reaches the desired reference value.
A. Wind-Up Affect
Although the average error is to be minimized as described
in (1), the individual input errors necessarily differ from each
other. Hence, the integration wind-up effect appears in all

4. HUANG et al.: NOVEL DROOP-BASED AVS CONTROL STRATEGY FOR DC MGs 1099
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 that
opposite integration actions are induced when minimizing the
error between the average reference and bus voltage. When
the controller is implemented digitally, overflow occurs to the
controller output register, failing the compensation scheme.
To solve the problem, an anti-wind-up scheme should be
applied. The controller in (1) can be modified by considering
the average controller output effort Vavg as the actual output
to the primary loop, shown in Fig. 7. The secondary controller
is then reconstructed using the feedback form of the bi-proper
controller [22], which can be derived as
uk = C∞ek + C(s)ûk (2)
where C∞ is dc gain of C(s), C(s) is equal to C−1(s) − C−1
∞
(a strictly proper transfer function), ûk is the actual controller
output (Vavg in Fig. 7), and uk is the unconstrained controller
output. Since the average output of all the parallel controllers
is 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 like
a centralized voltage regulator without being affected by the
wind-up effect that occurs in individual controllers. The equiv-
alent effort of the controllers then regulates the average output
voltages by minimizing the average error signal eavg.
B. Discussion of Pilot Bus Regulation
The above mentioned control scheme displays only
the regulation capability of the average terminal voltage.
Adjusting the individual reference voltage set point reflects
directly on the average reference magnitude, which is then fol-
lowed by the average terminal voltage. Therefore, by adding an
additional 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 to
be sent through the communication channel with the com-
pensation signal to other converters. After receiving all the
additional bits p1 . . . pn, each controller compares the set with
its designated index (the minimum value indicates the priority)
in order to determine whether its terminal voltage is selected
to be regulated. The flowchart of the designation algorithm to
select the pilot bus is shown in Fig. 8. In this case, the sys-
tem can be initialized sequentially by the user to assign the
priority. Also, when a failure occurs to the pilot converter, the
role of performing pilot bus regulation can be transferred to
other converters based on the given index to enhance the sys-
tem tolerance against failures. The details of the performance
will be demonstrated in the evaluation section.
IV. ANALYSIS OF NETWORK DYNAMIC BEHAVIOR
In this section, the details of the system dynamic responses
will be investigated. A system based on two converters supply-
ing a load is utilized, shown in Fig. 9. The system parameters
are shown in Table I. The mathematical model of the system
can then be described in the state-space representation
Ẋ = AX + BU (5)
where the eigenvalues of A reveal the system modes. To form
the equation sets, the power electronic converters are mod-
eled as controlled voltage sources and the dynamics of high
switching frequency is ignored. By extracting the eigenval-
ues of the system transition matrix, a noticeable resonance
mode can be found at −849 ± 4574i, which is induced by
the resonance between the cable inductance and the converter
output capacitors (C-L-C). To investigate the impact of the
controller parameters and droop gains on the cable mode, the
eigenvalue loci are plotted in Fig. 10. It can be observed that
both the Kp and droop gains have noticeable influences on
the movement of the cable mode: increment of Kp moves
the eigenvalue toward LHP and improves the damping per-
formance; increase of the droop gain shifts the mode toward
RHP and lifts the oscillation frequency. The above observation
can be further explained by ignoring the load resistance in the
C-L-C circuit. Thus, the dynamic equation as seen from the
left side can be derived in the Laplace’s domain
V1 = (I1 − sC1V1) (sL12 + R12) + V2. (6)
Assuming both converters have the same proportional
gain kp, the inductor current of the converter 2 can be
represented as
I2 = Ti2(s)Iref2 = Ti2(s)Kp
V∗
2 − V2
(7)

5. 1100 IEEE TRANSACTIONS ON SMART GRID, VOL. 6, NO. 3, MAY 2015
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 the
current reference to the inductor current. With considera-
tion of fast current tracking performance, inductor current is
TABLE I
PARAMETERS OF SYSTEM IN FIG. 9
Fig. 10. Eigenvalue loci of the cable mode with respect to parameter
variations. Kp: voltageloop proportional gain; Ki: voltageloop integral gain.
assumed to be equal to the current reference. Thus, the current
perturbation I2, in small-signal views, can be derived as
I2 = −KpV2. (8)
Equation (8) shows that the proportional gain behaves like a
virtual resistor with resistance of 1/Kp. In addition, variables
in (6) are replaced by V1, V2, I1, and I2, shown in
V1 =
I1 − sC1V1

6. sL12 + R12 +
1
sC2 + kp
. (9)

7. HUANG et al.: NOVEL DROOP-BASED AVS CONTROL STRATEGY FOR DC MGs 1101
Fig. 11. Comparison of the eigenvalue loci and pole movements of the cable
mode by the variation of Kp (Ki = 0, droop = 0).
Hence, substituting I1 = −KpV1 + U1 into (9) gives
V1
U1
=
b2s2 + b1s + b0
a3s3 + a2s2 + a1s + a0
(10)
where
a3 = L12C1C2, a2 = R12C1C2 + KpL12 (C1 + C2)
a1 = KpR12 (C1 + C2) + L12K2
p + C1 + C2
a0 = 2Kp + K2
pR12, b2 = L12C2
b1 = C2R12 + L12Kp, and b0 = KpR12 + 1. (11)
By changing the proportional gain, in Fig. 11 the pole
movement 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 shifting
the cable resonance poles toward LHP. Similarly, the effect
of the droop gain can be explained by considering the outer
voltage loop
Io1 = I1 − IC1 = Kp(Vref − D1Io1 − Vo1) − C1dVo1
dt. (12)
Applying small perturbation terms, (13) can be obtained
− Io1 = sCeffVo1 + Vo1
Reff (13)
where
Ceff = C1
(1 + KpD1), Reff = (1 + KpD1)/Kp. (14)
Equations (13) and (14) show that the droop gain reduces
effective shunt capacitance; meanwhile, the virtual shunt resis-
tance caused by the proportional controller increases. This
exactly explains why increasing the droop gain obtains a
higher resonance frequency (smaller shunt capacitance) and
moves the real part of the cable mode toward RHP (larger
shunt 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 II
EIGENVALUES 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 larger
Leff1 and Reff2, indicating the increase of the damping and
reduction of the resonance frequency. The effect of the deriva-
tive droop control on the system damping performance is
given in Table II, showing that increase of the derivative gain
helps to improve the system damping performance. However,
it should be addressed that the selection of the gains should
consider the primary control loop stability, and the implemen-
tation of the derivative term is to include a low-pass filter
that rejects the high frequency noises while preserving the
resonance frequency.
V. EVALUATION
A. Simulation Results
To verify the proposed control strategy, the simulation
model of a dc MG system is established. The system con-
figuration is shown in Fig. 13 with the parameters shown in
Table 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 have
acceptable voltage deviations at the heavy-load condition. At
0.29 s, R3 switches on and the system reaches the heavy-load
condition; after 0.1 s the disconnection of R2 has the system
back to the original state. Shown in Fig. 14, the steady-state
voltages are 115.2 V:114.6 V:116.4 V (−4%:−4.5%:−3%) at
the 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, and
9.9 A:11.3 A:7.7 A in the heavy-load condition. This shows
that the selection of the droop gains has to sacrifice the sharing
accuracy 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 nominal
value. Fig. 15 illustrates the results of the proposed AVS
scheme. Since the voltage is well compensated, the droop
gains are selected as relatively larger (3:3:3) to mitigate the

8. 1102 IEEE TRANSACTIONS ON SMART GRID, VOL. 6, NO. 3, MAY 2015
Fig. 13. Configuration of a three-converter system.
TABLE III
SYSTEM 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 the
terminal voltages are restored to its rated value over the period
of the simulation. The steady-state terminal voltages can be
obtained 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%) in
full-load condition; steady-state converter currents are then
7.7 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 compensation
value VAVS based on the update rate of 5 ms. In addition, the
current distribution under different droop gains (based on the
rating of the converters) of 1.5:3:4.5 is shown in Fig. 16. It
can be seen that designated current sharing can be achieved
by 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 DAVS
scheme. (a) Converter terminal voltages. (b) Converter output currents.
(c) Compensated voltage.
Fig. 16. Simulation results of the dc MG based on the proposed DAVS
scheme 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 on
the ACS method. Also, significant improvement of the voltage
responses in Fig. 17(c) and (d) based on the higher update rate
demonstrates the main advantage of the proposed scheme that
provides the flexibility of controller design to accommodate
various system configurations and communication speeds in
order to achieve better dynamic performance.
Fig. 18 shows the pilot bus regulation capability, and in
this case the pilot bus is selected as the terminal voltage
of the converter 3. The precise voltage regulation of termi-
nal 3–120 V can be observed. In Fig. 19, current transient
responses are compared between the P droop and P-D droop
methods. The additional derivative droop gain significantly

9. HUANG et al.: NOVEL DROOP-BASED AVS CONTROL STRATEGY FOR DC MGs 1103
(a)
(c) (d)
(b)
Fig. 17. Comparison of the dynamic performance between the digital AVS
and ACS schemes based on the update periods of 5 and 1 ms (load switching
at 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 load
switching with the D droop sDdn/(τs + 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 affecting
the steady-state performance.
The system dynamic behavior in responding to the failure of
the 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, the
regulation of the converter 2 terminal voltage engages and the
system 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 output
currents. (c) Average compensated voltage.
Fig. 21. Diagram of the experimental setup.
the case considers that the system total load demand is less
than the remaining converters’ total rating, and if the system
is overloading, the voltage regulation cannot be achieved.
B. Experimental Results
A scaled-down experimental work is constructed with two
synchronous buck converters to verify the proposed scheme
with the system configuration and parameters shown in
Fig. 21 and Table IV, respectively. The control algorithm
is implemented based on the TI TMS320F2808 microcon-
troller for evaluating the droop control and the proposed AVS
scheme. Two control loops are independently executed for
the converters to regulate their output terminal voltages. The
delays caused by non-explicit communication is also consid-
ered and emulated through the slow update rate of 200 Hz for
the proposed AVS scheme.
Fig. 22 demonstrates the system response based on only the
droop control during the engagement of R2. The load changes
from 15 to 10 ohms. As can be seen that the voltage deviation

10. 1104 IEEE TRANSACTIONS ON SMART GRID, VOL. 6, NO. 3, MAY 2015
TABLE IV
PARAMETERS OF THE EXPERIMENT
Fig. 22. Dynamic responses of the output voltage and current during the
engagement of R2(droop control).
Fig. 23. Dynamic responses of the output voltage and current during the
disengagement 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, the
system performance responding to the disengagement of the
extra load is shown. Without the communication, the inherent
voltage 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 the
engagement of R2(proposed AVS scheme).
Fig. 25. Dynamic responses of the output voltage and current during the
disengagement of R2(proposed AVS scheme).
the extra resistor R2. The output voltage is well maintained in
the nominal level; the cable resonance is significantly reduced
by the proposed derivative droop method. Moreover, the case
for the sudden disconnection of R2 is shown in Fig. 25. The
voltage magnitude eventually reaches the nominal value after
the transient state. A similar cable resonance can be also seen
by 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 voltage
drops induced by the droop method; the derivative droop
method helps to damp the cable resonance. Finally, both
the simulation and experimental results have verified the
abovementioned functions of the proposed control strategy.
VI. CONCLUSION
The distributed AVS scheme is presented in this paper to
maintain the terminal voltage at the nominal value and secure
the uniform current sharing disregarding the variation of load-
ing conditions. The distributed secondary voltage controllers
are effectively constructed with the bi-proper anti-wind-up

11. HUANG et al.: NOVEL DROOP-BASED AVS CONTROL STRATEGY FOR DC MGs 1105
design method. Furthermore, the pilot bus regulation function
is achieved through low-bandwidth communication to mitigate
the voltage bias caused by the cable resistances at a chosen
terminal without any centralized compensator. In addition, it
has been observed that the cable resonance mode can be well
damped by the proposed P-D droop control. This has been ver-
ified by the theoretical analysis, which offers an insight into
the mathematical relationship between the controller param-
eters and the resonance phenomena in MG networks. The
improved voltage recovery performance during load switching
has been demonstrated by both simulation and experimental
results, which show the enhanced dynamic responses thanks to
the developed AVS scheme. Finally, due to the decentralized
structure of the proposed scheme, the system reliability can be
significantly enhanced based on cost-effective and nonexplicit
communication solutions.
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Po-Hsu Huang was born in Taiwan in 1985.
He received the B.Sc. degree from National
Cheng-Kung University, Tainan, Taiwan, and the
M.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 Power
Engineering, Masdar Institute of Science and
Technology, Abu Dhabi, UAE. He is currently pursu-
ing the Ph.D. degree from the Electrical Engineering
and 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 system
stability, and control.
Po-Chun Liu (S’13) received the B.Eng. degree
in electrical engineering from National Taiwan
University, Taipei, Taiwan, in 2011.
He is currently a Research Assistant with the
iEnergy Center, Masdar Institute of Science and
Technology, Abu Dhabi, UAE. His current research
interests include wind power systems and power
conversion in microgrids.

12. 1106 IEEE TRANSACTIONS ON SMART GRID, VOL. 6, NO. 3, MAY 2015
Weidong Xiao (S’04–M’07–SM’13) received the
Master’s and Ph.D. degrees in electrical engineering
from the University of British Columbia, Vancouver,
BC, Canada, in 2003 and 2007, respectively.
He is an Associate Professor with the Department
of Electrical Engineering and Computer Science,
Masdar Institute of Science and Technology,
Abu Dhabi, UAE. In 2010, he was a Visiting Scholar
with the Massachusetts Institute of Technology,
Cambridge, MA, USA, where he researched power
interfaces for photovoltaic (PV) power systems.
He was a Research and Development Engineering Manager with MSR
Innovations Inc., Burnaby, BC, Canada, focusing on integration, research,
optimization, and design of PV power systems. His current research interests
include 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 the
B.Sc. and M.Sc. degrees from Mansoura University,
Mansoura, Egypt, in 1997 and 2002, respectively,
and the Ph.D. degree from the University of
New Brunswick (UNB), Fredericton, NB, Canada,
in 2005, all in electrical engineering.
He was a Research and Teaching Assistant
with the Department of Electrical and Computer
Engineering, UNB, from 2002 to 2005. He then
joined McGill University, Montreal, QC, Canada, as
a Post-Doctoral Fellow with the Power Electronics
Group. He was with Technology Research and Development, Wind Power
Plant Group, Vestas Wind Systems, Arhus, Denmark, and was with
TRANSCO, Abu Dhabi, UAE, as a Senior Study and Planning Engineer, and
seconded as a faculty member with the Faculty of Engineering, Mansoura
University. He is currently an Associate Professor with the Electrical
Engineering and Computer Science Department, Masdar Institute of Science
and Technology, Abu Dhabi, UAE, and a Visiting Professor with the
Massachusetts Institute of Technology, Cambridge, MA, USA. His current
research interests include power system, power electronics, flexible alternat-
ing current transmission system technologies, system control, wind turbine
modeling, wind energy integration, and interconnections.