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Transcript of [IEEE IECON 2013 - 39th Annual Conference of the IEEE Industrial Electronics Society - Vienna,...
A Practical Load Sharing Control Strategy for DC
Microgrids and DC Supplied Houses
Po-Hsu Huang, Weidong Xiao, and Mohamed Shawky El Moursi
Electrical Power Engineering
Masdar Institute of Science and Technology Abu Dhabi, United Arab Emirates
Abstract— Microgrid (MG) research mainly focuses on AC-
based power flow control techniques. Nevertheless, the rise of
DC output sources such as photovoltaic (PV) systems, fuel cells,
and distributed batteries leads to the immediate need for DC
MGs. In this paper, a hierarchical control strategy for a droop-
controlled DC MG is proposed, which fits the smart house
infrastructure to adopt online renewable generation and load
sharing. The improved control strategies combined with the
hierarchical approach includes three loops of controllers: the
primary control, the secondary control, and the tertiary control.
The issues of internal current limiter and anti-windup are also
discussed in this study. The simulation results of the proposed
approach are presented to verify the feasibility, and the
experimental results are carried out to evaluate the droop
concept and secondary voltage compensation.
Index Terms-- Distributed generation, droop method, microgrid.
I. INTRODUCTION
In recent years, a new system concept, the microgrid (MG)
[1], has been proposed to integrate DG and storage devices.
The concept is developed to tackle the problems of new
strategies, coordination, and controls that could be very
different from conventional electrical grids. AC MGs [2-3]
have been proposed to facilitate the connection of renewable
power sources to conventional AC systems. However, the
inherent advantage of supplying DC power directly from DC
sources leads to the irreplaceable need of DC MGs. Also, the
development and deployment of distributed DC sources such
as PV, fuel cells, and batteries have brought attention to their
advantage for dc loads in commercial, industrial and
residential applications. Different applications in DC MGs
have been proposed [4-5] to integrate various distributed
resources. It is shown in [6] that the voltage control for DC
MGs can achieve super high quality power supply. Various
advantages of DC MGs such as higher efficiency, reliability,
and easiness of integration are also mentioned in [7-8].
When operating a DC MG connected to the utility grid,
distributed sources in the MG can supply a certain amount of
power. However, when the MG is operated in islanded mode,
it must perform voltage regulation inside the MG. Many DC
bus voltage control schemes have already been proposed in
[9-10]. Various control and operation strategies of DC MGs
are also proposed in [11]-[15] by using the droop method to
ensure load sharing and voltage regulation. However, the
inherent drawback of the droop method is its poor voltage
regulation. Thus, a proper control strategy will need to be
applied to handle such an issue.
A hierarchical control strategy for both AC and DC MGs
is proposed in [16]. The controller consists of the primary
control, the secondary control, and the tertiary control. The
controller is designed to operate in both islanded and grid-
connected modes. When the MG is islanded, the voltage
inside the MG is regulated to ensure high quality power
supply. Therefore, the voltage control and power sharing
method must be implemented to operate the MG. The most
widely used methods are the master-slave control and the
droop method. In [16], the author adopts the droop method to
perform voltage regulation and current sharing. In the case of
applying parallel DC-DC converters, the droop method
consists of negative proportional parts of the output current,
which can avoid output conflict between converters and to
achieve desired current and power sharing. However, the
subtracting part leads to the magnitude drop in voltage, so
called voltage deviation, indicating that the more output
power supplied the more voltage deviation induced.
To tackle this problem, the secondary control is utilized to
restore the voltage back to the nominal level. The secondary
controller is also responsible for synchronizing the MG
voltage with the external grid. It is stated in [16] that the
communication is necessary for the secondary control with
the droop method to avoid the output conflict. After
synchronization, the tertiary control then manages to dispatch
power flow between the MG and the external grid.
However, the operation between islanded mode and grid-
connected mode might cause the controller to fail or induce
lager transients. In addition, the desired power injection
reference from the tertiary controller might exceed the
maximum power capacity of the MG. Such issues should be
addressed and discussed. In this paper, proper solutions are
978-1-4799-0224-8/13/$31.00 ©2013 IEEE 7124
proposed to coordinate with the hierarchical control to obtain
an improved performance of the MG control. The concept of
implementing current sharing priority for both DC sources
and converters is also investigated. The method allows the
MG designer to choose the suitable current sharing strategy
according to the preference of different types of DC sources
and converters.
II. HIERARCHICAL CONTROL OF DC MG
The control diagram is shown in Fig. 1 including primary, secondary, and tertiary controls.
PI
Controller
PI
Controller
Current
loop
MGV
*
MGV
DCV
V
DC
Micro
grid
DC Stiff Grid
GI
2OV
Voltage
loopPWM
(Duty Cycle)
2DR
DC
source
Voltage
loop
PWM
(Duty Cycle)
1DR
DC
sourceCurrent
loop
Co
mm
un
icatio
ns R
equ
ired
Primary Control
Secondary Control
Tertiary Control
MGV
refV
refV
+
+ −
−
SVδ
+
+
V
V
V
A
A
+
−
Primary Control
GI
1OV
1OI
2OI
*
GI
TVδ
Bypass Switch
A
Fig. 1. Primary, secondary, and tertiary controls of a DC MG.
A. Primary Control
The goal of the inner loop controller is to maintain the
voltage level inside the MGs. When two converters work in
parallel, equal current distribution between them is expected
since all converters are assumed to be identical. However,
due to component tolerances and non-identical characteristics
of the inductors and capacitors, the exact same current
distribution and output voltage are impossible. As shown in
Fig. 2(a), even small amount of voltage difference can cause
comparably high circulating current among the converters.
Therefore, the droop method is implemented, being necessary
to reduce circulating current, and further to manage the
current sharing. Assuming each converter has the same value
of the droop resistance, 1 2D DR R= , the droop method
adjusting the voltage reference for the inner loop controller
can be expressed by the following equation: *
o ref D oV V R I= − (1)
where oI is output current, DR is the droop resistance, and
refV is the reference voltage. The resistance value can be
chosen by assuming the maximum allowed voltage deviation,
which can be shown as follows:
/ 2
/
ref n v
D v o
V V
R I
ε
ε
= −
= (2)
where nV is the nominal output voltage, vε is the maximum
allowed voltage deviation, eqR is the equivalent droop
resistance of all parallel converters, and maxI is the maximum
total output current. The droop characteristic is shown in Fig.
3(b). Each converter can choose different droop resistances
depending on its priority. By proper ratio of droop values
among the controllers, a diverse current distribution can be
obtained. The details will be discussed in the latter section.
LR
OV1OI 2OI
1OV
1R 2R
2OV
*oV
I
V
refV
oI limitI
(a) (b)
Fig. 2. Principle of load sharing and droop control: (a) Equivalent circuit of
two parallel converters supplying a load; (b) droop characteristic in function
of output current.
B. Secondary Control
From previous discussion, the droop method induces the
voltage deviation when the current sharing strategy shown in
Fig. 2 is implemented. To solve the problem, the secondary
control is proposed to compensate. The compensator senses
the voltage DCV in the DC stiff grid and compares the error
with the voltage MGV inside the MG to provide voltage
restoration by Vδ . As shown in Fig. 2, in order to achieve
an exact voltage restoration, which contributes to an identical
output voltage level, low bandwidth communications are
required to prevent circulating currents [16]. The Vδ is
shown as follows: * *
1 1( ) ( )S p MG MG i MG MGV k V V k V V dtδ = − + −∫ (3)
where 1pk and 1ik are proportional and integral terms of the
secondary controller. Note that the output SVδ should be
limited within the maximum voltage deviation vε . From (3),
(1) can now be modified as *
o ref S D oV V V R Iδ= + − (4)
The reason why the controller senses the voltage in the DC
stiff grid is that the MG is designed to be islanded in the
beginning and then be connected with the DC stiff grid.
When the voltage levels in both sides become synchronized,
the connection can be accomplished by the bypass switch.
C. Tertiary Control
In this stage, the tertiary controller provides the power
flow control by changing the voltage reference inside the
MG. The compensator senses the error between the reference
and the current flowing toward the dc stiff grid to control the
power flow across the bypass switch. The controller can be
expressed as: * *
2 2( ) ( )T p G G i G GV k I I k I I dtδ = − + −∫ (5)
where 1pk and 1ik are proportional and integral terms of the
tertiary PI controller. The TVδ is also limited within
DC VV ε± so that the value will not exceed the maximum
7125
voltage deviation.
III. DC MICROGRID CONTROL
In this paper, we adopt Buck converters to implement the
DC MG system. Fig. 3(a) illustrates the equivalent circuit of
buck converters. Applying Kirchhoff’s law, we obtain
LIN O
O OL
diL mV V
dt
dV VC i
dt R
= −
= −
(6)
Therefore, the small-signal average model of buck
converters can be expressed as
2
( ) 1
( ) / 1
O
IN
V s
mV s LCs L Rs=
+ + (7)
The mathematical model is useful in analyzing the system
and designing the controller. However, the details such as
switching transient states, conduction losses, and ripples on
the inductor are not well revealed. Thus, in order to design an
effective controller, such effects should be considered and
examined. L
+
−
+ −
i
−
+
V
i
i
C R
VL
C
L O
C
VINm
VO
⋯⋯1DR
2DR
DnR
V∆
oI
1oI
2oI
onI
(a) (b)
Fig. 3. Equivalent circuits (a) buck converters; (b) droop resistances with
converter output current.
B. Paralleling DC-DC converters
Unlike the single centralized power supply, paralleling
DC-DC converters require a reliable current sharing strategy
to ensure proper distribution and to prevent circulating
current among the converters. In order to achieve that, the
droop method is proposed to reduce the circulating current
and assist in distributing power supply among the converters.
As we mentioned in the previous section, the voltage-current
droop method includes a small virtual resistance which
allows the voltage set point to decrease when the load current
increases. If each converter has the same droop resistance,
identical current sharing is expected. When the values are not
the same, the equation (1) can be derived as *
1 1 2 2o ref D o ref D o ref Dn onV V R I V R I V R I= − = − = = −⋯ (8)
Therefore, *
1 1ref o eq o D o Dn onV V V R I R I R I− = ∆ = = = =⋯ (9)
where V∆ is voltage deviation, eqR is equivalent resistance
of all parallel droop resistors, OI is the total output current,
and DnR and OnI are the resistance and output current of the n-
th converter. Note that the equivalent circuit of equation (9) is
shown in Fig. 3(b) from which the output current of the n-th
converter is derived as
o eqOn
L Dn
V RI
R R
=
(10)
By properly selecting the droop values, the MG designer can
assign higher priority to the converters with higher efficiency
or the DC sources that is preferred to be utilized.
C. Current Limiter
Since the current sharing is based on the droop resistance
value, the controller itself cannot regulate the current output.
Therefore, the current limiter is required to prevent the
overcurrent induced in the converter output that could
damage the DC source or the converter itself. Especially, the
droop method permits some converters to inject more current
than others. When the heavy load changing happens, the
converters which are assigned to supply more power will
confront large output current so that large transients could be
induced. Also, when the tertiary control manages the current
flow in grid-connected mode, the desired injected current
reference could exceed the capacity of the DC MG. In such a
case, the current limiters will need to be applied to confine
the total output current injected to the external grid in order to
protect the MG.
D. Anti-Windup
When the MG resumes normal operation from current
limitation modes, special control action should be taken into
account. Otherwise, large transients might occur. The
transients happen because the tertiary controller proceeds to
normal operation without considering the occurrence of
current limitation. The output signal the tertiary controller
expectation is mismatched with the actual signal. Therefore,
the windup will be induced in the integrator. In order to
prevent such transients, the proper control strategy should be
applied to inform the controller the occurrence of saturation.
The anti-windup loop can be implemented by adding a
differential term between the real output and the expected
output of the controller into the integrator. By tuning the anti-
windup gain, the integral value in the integrator would reset
to the proper value before the system resumes normal
operation so as to avoid large transients. Besides, the anti-
windup loop can also be implemented in the secondary
controller and the inner loop controller when the output signal
of the controllers reaches the limit.
IV. EVALUATION
The feasibility of the proposed control strategy is verified
by simulation and experiment.
A. Simulation
The simulation consists of two dc/dc buck converters
connected in parallel supplying a load. The voltage level
inside the MG is selected at 400V, the droop values for
converter 1 and 2 are chosen as 1 Ω and 2 Ω respectively,
and the local load is designed to switch between 4kW and
12kW every 10ms during the entire simulation. The
simulation scenario is shown in Table I. Three cases are
simulated to verify the feasibility of MG operation.
Case 1—Injecting 10A to the external DC stiff grid: Fig 4
7126
(a) shows that, at 5ms, the secondary control is enabled to
restore the MG voltage back to 400V. Fig 4 (b) indicates that
the current sharing ratio is always maintained constant in both
standalone and grid-connected modes. In Fig 4(c), the tertiary
control is activated at 15ms to produce voltage deviation in
order to inject the desired current into the external grid.
TABLE I
SIMULATION SCENARIO
Time (ms) Description Mode
5 secondary control activated Islanded
10 load changing from 4kW to 12kW Islanded
15 connected to the grid Grid-connected
20 load changing from 12kW to 4kW Grid-connected
30 load changing from 4kW to 12kW Grid-connected
35 disconnected from the grid Islanded
40 load changing from 12kW to 4kW Islanded
50 simulation end Islanded
5 10 15 20 25 30 35 40 45 50
380
400
420
(a) MG Voltage
time (ms)
Am
plitu
de (
V)
5 10 15 20 25 30 35 40 45 50-10
0
10
20
30(b) Converter output current
time (ms)
Io1 Io2 (
A)
5 10 15 20 25 30 35 40 45 50-10
0
10
20(c) Current flow between the MG and the external grid
time (ms)
Ig (A
)
Converter 1
Converter 2
Fig. 4. Behavior of the voltage and currents of the dc MG in Case 1: (a) MG
voltage (b) Converter output current (c) Current flow between the MG and
the external Grid.
Case 2—Extracting 20A from the external grid: Fig. 5 (a)
and (c) show that the MG voltage drops at 15ms because the
tertiary control produces negative voltage deviation so that
the current can be extracted into the MG. At 20ms, the load
switches from 12kW to 4kW, causing an increase of the MG
voltage. Due to the decrease of the load, the MG no longer
needs 20A current to supply the loads. Therefore, the
converters stop supplying power and the extracted current is
limited at 10A.
Case 3—Current limiters: In this case, the current limiter
is equipped into the primary controller. The injected current
reference is chosen at 10A. The current limitations are
selected to be 10A and 25A for converter 1 and 2,
respectively. Fig 6(b) shows that converter 1 reaches the limit
at 10ms when the load is changing from 4kW to 12kW. The
remaining power, therefore, is supplied by the converter 2. At
15ms, the MG is connected to the external grid and the
injected current is limited to 5A due to the 35A total capacity
of the MG (30A supplied to the load). It can be seen at 20ms,
the load is reduced to 4kW so that the injected current returns
to the reference value.
5 10 15 20 25 30 35 40 45 50
380
400
420
(a) MG Voltage
time (ms)
Am
plitu
de
(V
)
5 10 15 20 25 30 35 40 45 50-10
0
10
20
30(b) Converter output current
time (ms)
Io1
Io
2 (
A)
5 10 15 20 25 30 35 40 45 50-30
-20
-10
0
10(c) Current flow between the MG and the external grid
time (ms)
Ig (
A)
Converter 1
Converter 2
Fig. 5. Behavior of the voltage and currents of the dc MG in Case 2: (a) MG
voltage (b) Converter output current (c) Current flow between the MG and
the external Grid
5 10 15 20 25 30 35 40 45 50
380
400
420
(a) MG Voltage
time (ms)
Am
plitu
de
(V
)
5 10 15 20 25 30 35 40 45 50-10
0
10
20
30(b) Converter output current
time (ms)
Io1
Io
2 (
A)
5 10 15 20 25 30 35 40 45 50-10
0
10
20(c) Current flow between the MG and the external grid
time (ms)
Ig (
A)
Converter 1
Converter 2
Fig. 6. Behavior of the voltage and currents of the dc MG in Case 3: (a) MG
voltage (b) Converter output current (c) Current flow between the MG and
B. Experimental Results
A small-scale DC MG is constructed with two DC/DC
buck converters. The system parameters are demonstrated in
Table II. The control algorithm is implemented based on TI
TMS320F2808 microcontroller for evaluating the droop
concept and the secondary voltage compensation. Fig. 7(a).
shows the system response with the droop ratio of 1:1 under
load disturbance. The load changes from 0.6 Ω to 1.1 Ω .
The voltage rises very less and gradually recovers to the
original value due to the secondary voltage regulation. The
current sharing ratio maintains the same and continues on
supplying to the load. The system response with the droop
7127
ratio of 1:2 under load disturbance is shown in Fig. 7(b). The
sudden change of load causes the unequal current drops
among the converter in the beginning of the transient period.
The higher voltage drop in one converter caused by the
feedback droop loop then induces lower voltage reference set
point. The controller output will decrease and reduce the
output current so that the voltage deviation is mitigated.
Therefore, the current sharing proceeds into the new steady
state and maintains the same ratio. Such self-adjusting
algorithm can ensure the current sharing as defined droop
ratio under load changing condition.
TABLE II
EXPERIMENTAL DATA
Parameters Value
Input voltage 9V
Rated output voltage 2V
PWM Switching frequency 200kHZ
Converter inductance 0.9 uH
Converter output capacitance 470 uF
Nominal load 0.6 Ω
Load resistance changes from 0.6 to 1.1 ohms
(a)
Load resistance changes from 0.6 to 1.1 ohms
(b)
Fig. 7. Voltage and current responses under load changing: (a) with droop
ratio of 1:1; (b) with droop ratio of 1:2 under load changing.
V. CONCLUSION
This paper has presented an improved implementation of the hierarchical controller for DC MGs. The DC MG can be operated in both islanded and grid-connected modes. The proposed control system coordinates the current sharing control of the two converters and performs power exchange management with the grid. In addition, the effects of load changing and mode switching on the system are also discussed and treated by the proposed methods. The simulation results
illustrate that the MG can properly switch between the standalone and grid-connected mode without large transients induced. The hierarchical controller is capable of providing good performance during the load changing and preventing the MG from injecting overcurrent. The performance after disconnected from the grid was also demonstrated by simulation to verify the feasibility. The advantage of using anti-wind up solution demonstrated that the oscillations can be effectively reduced when the system starts resuming normal operation. It also helps to increase the speed of the response after load changing or mode switching. A small scale MG prototype is constructed to verify the proposed control experimentally.
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