Aalborg Universitet
An Improved Droop Control Method for DC Microgrids Based on Low Bandwidth
Communication with DC Bus Voltage Restoration and Enhanced Current Sharing
Accuracy
Lu, Xiaonan; Guerrero, Josep M.; Sun, Kai; Vasquez, Juan Carlos
Published in:
I E E E Transactions on Power Electronics
DOI (link to publication from Publisher):
10.1109/TPEL.2013.2266419
Publication date:
2014
Document Version
Early version, also known as pre-print
Link to publication from Aalborg University
Citation for published version (APA):
Lu, X., Guerrero, J. M., Sun, K., & Vasquez, J. C. (2014). An Improved Droop Control Method for DC Microgrids
Based on Low Bandwidth Communication with DC Bus Voltage Restoration and Enhanced Current Sharing
Accuracy. I E E E Transactions on Power Electronics, 29(4), 1800-1812.
https://doi.org/10.1109/TPEL.2013.2266419
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This paper is a preprint version of the final manuscript: X. Lu, J. M. Guerrero, K. Sun, J. C. Vasquez, “An improved droop control method
for dc microgrids based on low bandwidth communication with dc bus voltage restoration and enhanced current sharing accuracy,” IEEE
Trans. Power Electron., v. 99, no. PP, 2013.
Abstract—Droop control is the basic control method for load
current sharing in dc microgrid applications. The conventional dc
droop control method is realized by linearly reducing the dc
output voltage as the output current increases. This method has
two limitations. First, with the consideration of line resistance in a
droop-controlled dc microgrid, since the output voltage of each
converter cannot be exactly the same, the output current sharing
accuracy is degraded. Second, the DC bus voltage deviation
increases with the load due to the droop action. In this paper, in
order to improve the performance of the dc microgrid operation, a
low bandwidth communication (LBC) based improved droop
control method is proposed. In contrast with the conventional
approach, the control system does not require a centralized
secondary controller. Instead, it uses local controllers and the LBC
network to exchange information between converter units. The
droop controller is employed to achieve independent operation and
average voltage and current controllers are used in each converter
to simultaneously enhance the current sharing accuracy and
restore the dc bus voltage. All of the controllers are realized
locally, and the LBC system is only used for changing the values of
the dc voltage and current. Hence, a decentralized control scheme
is accomplished. The simulation test based on Matlab/Simulink
and the experimental validation based on a 2×2.2 kW prototype
were implemented to demonstrate the proposed approach.
Index Terms—Current sharing accuracy, droop control, dc
microgrid, low bandwidth communication, voltage deviation
I. INTRODUCTION
N order to integrate different types of renewable energy
sources and to electrify a remote area, the concept of the
microgrid was proposed several years ago [1]. Recent literature
Manuscript received January 16, 2013. This work was supported by the
National Natural Science Foundation of China under Grant 51177083.
Xiaonan Lu is with the Department of Electrical Engineering, Tsinghua
University, Beijing, 100084, China (e-mail: lxn04@mails.tsinghua.edu.cn).
Josep M. Guerrero is with the Institute of Energy Technology, Aalborg
University, Aalborg, 9220, Denmark (e-mail: joz@et.aau.dk).
Kai Sun is with the Department of Electrical Engineering, Tsinghua
University, Beijing, 100084, China (corresponding author, phone:
+86-10-62796934, e-mail: sun-kai@mail.tsinghua.edu.cn).
Juan C. Vasquez is with the Institute of Energy Technology, Aalborg
University, Aalborg, 9220, Denmark (e-mail: juq@et.aau.dk).
on this topic is mostly focusing on ac microgrids, since the
utility electrical grid relies on ac systems [2-6]. However,
various sustainable energy sources and loads, such as
photovoltaic (PV) modules, batteries, and LEDs, have natural
dc couplings, so it is a more efficient method for connecting
these sources and loads directly to form a dc microgrid by using
dc-dc converters without ac-dc or dc-ac transformations. In a dc
microgrid, there is no reactive power and there are no
harmonics, so higher power quality and system efficiency are
obtained compared to ac systems [7-15]. Therefore, there is an
increasing focus on dc microgrids nowadays. The typical
configuration of a dc microgrid is shown in Fig. 1 [7].
Since the renewable energy sources are decentralized
connected to the common bus in a microgrid, the interfacing
converters are connected in parallel. Power electronics
interfacing converter control is a key issue in the operation of a
microgrid, particularly for the load power sharing between
different modules [16-19]. Various control methods have been
proposed to achieve proper power sharing in a parallel
converter system, such as master-slave control,
circular-current-chain (3C) control, among others [20-21]. To
satisfy the requirements of a distributed configuration, droop
control without communication or with low bandwidth
communication (LBC) is commonly accepted as an efficient
power sharing method in a microgrid [22].
In a droop-controlled dc microgrid, the power sharing
method is realized by linearly reducing the voltage reference as
the output current increases [22]. Although droop control is
widely employed as a decentralized method for load power
sharing, its limitations should be noted. The output current
sharing accuracy is lowered down because of the effect of the
voltage drop across the line impedance. This effect is similar to
the reactive power sharing of ac microgrids with inductive line
impedances. To enhance the reactive power sharing accuracy in
the ac microgrid with inductive line impedances, several
methods have been proposed: the concept of virtual impedance
was proposed to match the unequal line impedance [23]; a
compensation method was proposed which used the remote
voltage signal and employed an integrator term in the
An Improved Droop Control Method for DC
Microgrids Based on Low Bandwidth
Communication with DC Bus Voltage Restoration
and Enhanced Current Sharing Accuracy
Xiaonan Lu, Student Member, IEEE, Josep M. Guerrero, Senior Member, IEEE, Kai Sun, Member,
IEEE, Juan C. Vasquez, Member, IEEE
I
2
conventional Q-V droop control [24]; the voltage amplitude in
Q-V droop control was replaced by
V
(V dot), which represents
the time rate of the change of the voltage magnitude [25]; the
voltage drop across the impedance was estimated in the
grid-connected operation to reach the modified slope in the Q-V
droop control [26].
In addition to the issue of current sharing accuracy in a dc
microgrid, a voltage deviation is produced because droop
control is realized by reducing the dc output voltage. To solve
this problem, a centralized secondary controller was proposed
to eliminate the voltage deviation; however, the influence of the
line resistance was not taken into account [22]. At the same time,
if there is a failure in the centralized controller, the function of
voltage restoration cannot be achieved. A control scheme based
on the average value of the dc output current in each of the
converters has been presented [27]. This method was useful for
restoring the dc bus voltage, while the effect of the enhancement
of current sharing accuracy was not obvious enough. The reason
for this is that only the average value of the dc output current
was considered, while the dc output current was not individually
controlled.
For the enhancement of power quality and the function of
protection, the communication system in a microgrid cannot be
completely removed. In order to meet the requirement of a
distributed configuration, high frequency communication is not
suitable enough for the practical microgrid. Power line
communication (PLC) or LBC is commonly utilized [22, 28]. In
this paper, an LBC-based, decentralized control method is
proposed for dc microgrid applications. Particularly, the load
power sharing is reached by using droop control. Meanwhile, a
hybrid control scheme with additional average current and
voltage controllers is employed in each converter module to
simultaneously enhance the current sharing accuracy and
restore the local bus voltage. The local controller of each
converter individually adjusts each output current, so the
current sharing accuracy is significantly improved. The LBC
system is used only for the interchange of the dc voltage and
current information, and all of the calculations and controllers
are realized locally. Therefore, the control system is suitable for
the distributed configuration in a microgrid and can provide
higher reliability. The proposed control scheme has been tested
for a range of communication delays. At the same time, the
accurate proportional load current sharing can be achieved with
different line resistances. The detailed model of the proposed
control scheme is derived, and the stability of the system is
analyzed. The simulation model based on Matlab/Simulink and
the 2×2.2 kW prototype based on dSPACE 1103 were
implemented to validate the proposed approach.
Solar
Array
Plug-in
Vehicle
DC
Loads
… …
Grid
DC Bus
Grid-Interfacing
Converter
DC
Loads
DC
Loads
Wind
Turbine
Energy
Storage
Fig. 1. Typical configuration of a dc microgrid.
The sections of this paper are organized as follows. Section II
reviews the limitations of the conventional droop control
method in dc microgrids. Section III introduces the principle of
the proposed control scheme. Section IV analyzes the stability
of the control system under different communication delays,
line resistances and expected output current sharing proportion.
Section V shows the simulation test of the control scheme by
using Matlab/Simulink. Section VI demonstrates the approach
by using a 2×2.2 kW prototype controlled by dSPACE 1103.
Finally, Section VII summarizes the paper and gives the
conclusions.
II. LIMITATIONS OF THE CONVENTIONAL DROOP CONTROL
METHOD IN DC MICROGRIDS
The conventional droop control method in a dc microgrid is
achieved by linearly reducing the voltage reference when the
output current increases. The first limitation of the conventional
droop control method is the degradation of the current sharing
accuracy. Since the output voltage cannot be exactly the same
due to the additional voltage drop across the line resistances, the
load current sharing accuracy is lowered down. Second, the
voltage deviation exists due to the droop action. The above two
limitations of the conventional droop control method are
analyzed in detail as follows.
A. Current Sharing Accuracy Degradation
The load current sharing in a dc microgrid is realized by an
I-V droop controller. This controller can be implemented by
means of a virtual resistance; this method is also named adaptive
voltage positioning [29]. If the line resistance is taken into
account, as in the reactive power sharing in ac microgrids with
inductive line impedances, the dc output voltages for the local
converters are not exactly the same. Hence, the current sharing
accuracy is degraded. A detailed analysis of this problem is
shown below.
A dc microgrid with two nodes is depicted in Fig. 2, where
each converter is simplified by using the Thévenin equivalent
model. The droop control method is expressed as
*
dci dc dci di
v v i R
(1)
where v
dci
is the output voltage of each converter, v
dc
*
is the
reference value of the dc output voltage, i
dci
is the output
current, R
di
is the virtual resistance, and i = 1, 2.
3
v
dc
*
R
load
v
load
R
line1
R
line2
R
d1
R
d2
+
-
+
-
+
-
i
dc1
i
dc2
v
dc1
+
-
v
dc2
+
-
v
dc
*
Fig. 2. Simplified model of a two-node dc microgrid.
Considering the relationship between dc voltage and current
in (1), the value of the output resistance in the Thévenin
equivalent model is equal to the virtual resistance, and the
output voltage of the voltage source in the model is equal to v
dc
*
,
as shown in Fig. 2.
From Fig. 2, the following can be derived:
*
load dc dc1 d1 dc1 line1
v v i R i R
(2a)
*
load dc dc2 d2 dc2 line2
v v i R i R
(2b)
These expressions then yield the following:
dc1 d2 line2 d2 d1 line1
dc2 d1 d1 line1
/i R R R R R
i R R R
(3)
In the conventional droop-controlled dc microgrid, the dc
output current of each converter is set to be inversely
proportional to its virtual resistance. Hence, it is concluded from
(3) that the current sharing error cannot be completely
eliminated unless the following expression is satisfied:
d1 line1
d2 line2
RR
RR
(4)
Usually in a dc microgrid, it is assumed that the system is not
so large that the line resistance only has a small value.
Therefore, a larger virtual resistance R
di
can be selected. Since
R
d
>> R
line1
and R
d
>> R
line2
, the following proceeds from (3):
dc1 d2 line2 d2
dc2 d1 line1 d1
i R R R
i R R R
(5)
However, the above assumption is only suitable for a small
system. If the dc microgrid is larger, (5) cannot be satisfied.
Meanwhile, with a large virtual resistance, the system stability is
challenged. This is the first limitation of the conventional droop
control method.
B. DC Voltage Deviation
As shown in (1), since droop control is employed, the dc
voltage deviation can be found from the following formula:
dci dci di
( =1, 2)v i R i
(6)
The analysis is also shown in Fig. 3. When the interfacing
converter operates in the open-circuit condition, the dc voltage
deviation is zero. When the dc output current is not equal to
zero, the dc voltage deviation exists and its value varies with the
load current. To guarantee that the voltage deviation does not
exceed its maximum acceptable value, the value of the droop
coefficient R
di
should be limited:
dcmax
di
dcfli
v
R
i
(7)
where i
dcfli
is the full-load output current of converter #i.
The deviation of the output voltage is the second limitation of
the conventional droop control method.
i
dc
R
d1
i
dc2
i
dc1
v
dc
dc2
v
R
d2
*
dc
v
dc1
v
Fig. 3. Droop curve in a dc microgrid with different virtual resistances.
III. PRINCIPLE OF THE PROPOSED CONTROL METHOD
In order to solve the problems imposed by the two limitations
of the conventional droop control method, an improved droop
control method based on LBC is proposed. In this method, the
enhancement of the current sharing accuracy and the restoration
of local dc bus voltage are realized simultaneously. The LBC
system is used for transferring the output voltages and currents
of different converters. The detailed configuration of the
proposed method is shown in Fig. 4. Here, the conventional
droop control method is used to achieve proportional load
current sharing approximately. Then, the output voltages and
currents in the dc sides of the converters are transferred to the
other converters using the LBC network. The average voltage
and average current proportional-integral (PI) controllers are
employed in each of the local control systems. For each average
voltage controller, the reference value is v
dc
*
and the average
value of the dc voltage is controlled; as a result, each output
voltage can be restored. At the same time, the reference value
for each average current controller is i
dc1
/k
1
or i
dc2
/k
2
, where k
1
and k
2
are the current sharing proportions, and the average value
of i
dc1
/k
1
and i
dc2
/k
2
is the feedback variable. Therefore, the
proportional output current sharing is guaranteed. All of the
calculations and controllers are achieved locally. Thus, the
proposed method is a type of decentralized method, which is
suitable for the distributed configuration of a dc microgrid.
The comparison of the proposed method to the conventional
current sharing method in parallel converter systems is shown in
Table I. For the HBC-based method, like master-slave control,
the control system is stable only when the HBC network is
employed, otherwise the current references cannot be
transferred among the converters. However, the proposed
LBC-based method is an improved version of the conventional
droop control. By using droop control, the current sharing is
achieved by regulating the local output voltage, so that the
communication is not necessary for guaranteeing the system
stability. Communication here is only employed to reach
auxiliary functionalities. The proposed LBC-based control
method is used to solve the two main problems produced by
droop control: current sharing accuracy degradation and voltage
deviation. Hence, considering the dependency of
communication, the viability of LBC-based methods in a
microgrid is higher than that of HBC-based methods.
The advantage of the LBC-based method lays also in the
reduced amount of data flowing in the communication network.
Assuming that the sampling frequency is f
s
, so the
communication frequency of the HBC network is f
s
. Meanwhile,
4
the communication frequency of the LBC network is selected to
f
s
/N. For an HBC-based control method, the data transferring is
accomplished every control period, while for an LBC-based
control method the data transferring is accomplished every N
control periods. Hence, during the same length of time period,
the amount of data on the communication network for the
LBC-based method is highly reduced to 1/N in contrast with the
HBC-based method. At the same time, when the scale of the
microgrid is enlarged with the increasing number of interfacing
converters, the data traffic for HBC network can be very busy.
For instance, in the master-slave control, the current reference
will be transferred to all of the slave converters. As a result,
when the number of the converters increases, more data is
required to flow through the communication network. If the
LBC-based control strategy is employed, since the data
transferring is performed every N control periods, the
communication stress is highly reduced. Therefore, the
LBC-based control method is more suitable than the
HBC-based control method in a microgrid.
Distributed Control
Diagram for Converter #2
-
+
dc1
v
δv
1
G
piv
1/2
*
dc
v
+
v
dc1
+
v
dc2
-
+
i
dc1
/k
1
dc1
i
δi
1
G
pic
1/2
+
+
i
dc1
/k
1
i
dc2
/k
2
-
+
+
δv
1
δi
1
+ +
v
dc1
-
G
lpf
i
dc1
-
*
dc
v
Voltage
Loop
+
-
*
o1
i
i
o1
Current
Loop
PWM
Generator
Power
Converter #2
Power
Converter #1
DG #1
DG Terminal
Load
Bus
DG #2
Line
Impedance
Line
Impedance
v
dc1
i
dc1
v
dc2
i
dc2
-
+
dc2
v
δv
2
G
piv
1/2
*
dc
v
+
v
dc2
+
v
dc1
-
+
i
dc2
/k
2
dc2
i
δi
2
G
pic
1/2
+
+
i
dc2
/k
2
i
dc1
/k
1
-
+
+
δv
2
δi
2
+ +
v
dc2
-
G
lpf
i
dc2
-
*
dc
v
Voltage
Loop
+
-
*
o2
i
i
o2
Current
Loop
PWM
Generator
v
dc2
v
dc1
i
dc2
i
dc1
Low-Bandwidth
Communication
Distributed Control
Diagram for Converter #1
R
d0
/k
1
R
d0
/k
2
k
2
k
1
Fig. 4. Detailed configuration of the proposed control system.
TABLE I
COMPARISON OF DIFFERENT POWER SHARING METHODS
Power Sharing
Method
Comm.
Dependency
Viability
Sharing
Accuracy
Voltage
Quality
HBC
1
-based
method [20-21]
High
Medium
High
High
Conventional
droop control [22]
Low
High
Low
Low
LBC-based
method
Low
High
High
High
It should be noticed that the dc input terminal is regarded as
the voltage source in the proposed method and the ideal I-V
droop curve is employed. In a real microgrid, the renewable
energy source and the energy storage unit, such as the
photovoltaic (PV) module and battery, have different droop
characteristics, as shown in Fig. 5 (a) and (b). However, they
can be combined to form an ideal dc voltage node which is the
input of the interfacing converter, as shown in Fig. 5 (c).
Especially in a dc residential microgrid, each house may have a
PV and a battery. It is an efficient way to make the battery
working in the voltage-controlled mode to form the local dc bus
voltage and the other renewable energy sources are controlled to
operate in the current feeding mode, which can inject or absorb
power from the local dc bus. In this way, the sub-grid consisting
of the PV and battery runs as a voltage node. Hence, the
proposed method can be used to enhance the control
performance of the interfacing converters between each dc
voltage node and the dc bus.
6 7 8 9 10 11 12 13
0
50
100
150
200
250
300
350
400
V
I
Operating
Range
Positive Current
Limitation
MPPT
(a)