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M E T H O D O L O G Y Open Access
Control principles of micro-source inverters
used in microgrid
Wenming Guo
*
and Longhua Mu
Abstract
Since micro-sources are mostly interfaced to microgrid by power inverters, this paper gives an insight of the control
methods of the micro-source inverters by reviewing some recent documents. Firstly, the basic principles of different
inverter control methods are illustrated by analyzing the electrical circuits and control loops. Then, the main
problems and some typical improved schemes of the ωU-droop grid-supporting inverter are presented. In results
and discussion part, the comparison of different kinds of inverters is presented and some notable research points is
discussed. It is concluded that the most promising control method should be the ωU-droop control, and it is
meaningful to study the performance improvement methods under realistic operation conditions in the future work.
Keywords: Mirogrid, Micro-source inverter, Droop control, Control principle
Introduction
Recently, with the increased concern on environment and
intensified global energy crisis, the traditional centralized
power supply has shown many disadvantages.Meanwhile,
the high-efficiency, less-polluting distributed generation
(DG) has received increasing attentions [1, 2]. Microgrids
[3–5], which comprise micro-sources, energy storage
devices, loads, and control and protection system, are the
most effective carrier of DGs. When a microgrid is con-
nects to the utility grid, it behaves like a controlled load or
generator, which removes the power quality and safety
problems caused by DGs’direct connection. Microgrids
can also operate in islanded mode, thus increase system
reliability and availability of the power supply.
Proper control is a precondition for microgrids’stable
and efficient operation. The detailed control requirements
come from different aspects, such as voltage and fre-
quency regulation, power flow optimization etc. Since
these requirements are of different importance and time
scale, a three-level microgrid control structure is proposed
in [6]. As the foundation of microgrid control system, the
primary control is aimed at maintaining the basic oper-
ation of the microgrid without communication, which has
become a hot research topic recently. Since most micro-
sources utilize inverters to convert electrical energy, the
primary control is essentially the management of power
inverters. Micro-source inverters are required to work in a
coordinated manner based only on local measurements
and the control strategies decide the roles of each micro-
source. According to the principle of master–slave con-
trol, the micro-source inverters can be divided into grid-
feeding, grid-forming, and PQ-droop grid-supporting
inverters. From the perspective of peer control, the ωU-
droop grid-supporting invertershelp to realize microgrids’
plug and play function. Although being widely discussed
in the technical literatures, it still lacks a sufficient prac-
tical control method andexisting control technologies
need to be further studied and improved. This paper
describes the control principles of several typical micro-
source inverters and compares their characteristics so as
to provide a fundamental understanding of microgrids’
primary control.
Method
Grid-feeding inverter
The control objective of grid-feeding (GFD) [11] inverter
is to track the specified power references. Figure 1 illus-
trates the control block diagram of the most common
current controlled GFD inverter. For dispatchable micro-
sources, such as micro-turbine and fuel-cells, the inverter
power references can be set directly according to practical
requirements. For non-dispatchable micro-sources, such
as photovoltaic cells, the active power reference is usually
decided by the voltage controller of the inverter’s DC bus.
* Correspondence: gwmsch@163.com
Department of Electrical Engineering, Tongji University, Shanghai, China
Protection and Control o
f
Modern Power S
y
stem
s
© 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.
Guo and Mu Protection and Control of Modern Power Systems (2016) 1:5
DOI 10.1186/s41601-016-0019-8
In addition, this type of sources can also export reactive
power without affecting maximum power point tracking.
The GFD inverter’s power referencetrackingis realized by
adjusting the output currents. The control system calculates
the output current references based on the relationships
among the inverter’s output power, output current and the
voltage at the point of connection (PC). The three-phase
voltages at the PC are represented by vector v,andthe
inverter’s output currents are represented by vector ias
v¼vavbvc
½
T
i¼iaibic
½
T
Neglecting the power consumed on the filter inductor,
the output power of GFD inverter is calculated accord-
ing to instantaneous power theory [7]as:
P¼v:i
Q¼−vi
jj
nð1Þ
If the current controller in Fig. 1 is properly designed,
the output currents of the GFDinverter will follow their
references. Thus the current reference vector, i
ref
, can be
obtained by solving the following equation:
Pref ¼v⋅iref
Qref ¼−viref
jj
ð2Þ
The output currents of the GFD inverter are the same
as the currents flownthrough the filter inductor. In nat-
ural reference frame, there exists the following relation:
sLi¼uinv −uð3Þ
The voltages at the PC, u, are measured using voltage
transducers, the output voltages of the inverter, u
inv
, can
then be adjusted based on u(see the voltage feedforward
in Fig. 1) to control the voltage drop on the filter in-
ductor. This implies that the filter inductor’s currents
can be controlled indirectly. If the potential at the mid-
dle point of the inverter’s DC bus equal zero and ignore
the delay of PWM process, the inverter is equivalent to
a proportional element with gain k
PWM
. Thus, the
closed-loop transfer function of the inverter’s current
control can be written as:
GcsðÞ¼ kpwmTcsðÞ
sL þkpwmTcs
ðÞ ð4Þ
T
c
(s) needs be designed in a way to ensure G
c
(s) have
sufficient bandwidth. Meanwhile, the gain and phase
shift of G
c
(s) around fundamental frequency should be
close to 0 dB and 0 degree respectively. Therefore, the
output current of the GFD inverter can track their refer-
ences quickly and accurately.
For three-phase balanced operation cases, the control
system of the GFD inverter is usually designed in dq refer-
ence frame, where the voltages and currents are DC signals.
In this case, using PI controller can realize the output
current tracking without steady-state error. In dq reference
frame, the Park transformation will result in coupling
between the dand qaxis inductor currentcomponents, as
shown in Eq. (5). Therefore, the control system must com-
prise dq decoupling modules. The detailed control block
diagram in dq reference frame is illustrated in Fig. 2.
sLIdq ¼Uinv;dq−Udq −0−ωL
ωL0
Idq
ð5Þ
For unbalanced operation cases, the GFD inverters
need simultaneously controlthe positive and negative se-
quence currents [8, 9]. Under such condition, using PR
controller [10] in αβ reference frame might be a better
choice as a single PR controller can regulate both the
positive and negative sequence currents, and the control
effect is similar to that of using two PI controllers in
double positive/negative dq reference frames.
Fig. 1 Control block diagram of grid-feeding inverter
Fig. 2 Current control loop of GFD inverters in dq reference frame
Guo and Mu Protection and Control of Modern Power Systems (2016) 1:5 Page 2 of 7
Grid-forming inverter
The control objective of the grid-forming (GFM) [11] in-
verters is to maintain stable voltage and frequency in a
microgrid. GFM inverters are characterized by their low
output impedance, and therefore they need a highly accur-
ate synchronization system to operate in parallel with other
GFM inverters [11]. GFM inverters usually equips with
energy storage on their DC sides, therefore they can
respond to the change of load in a short time. The control
block diagram of a GFM inverter is shown in Fig. 3, includ-
ing an inner inductor current loop, which is identical to
that of the GFD inverter, and an outer capacitor voltage
loop. GFM inverters achieve their control objective by regu-
lating the filter capacitor’svoltage,u. In natural reference
frame, there exists the following relation:
sCu¼i−ioð6Þ
where iandi
o
are the inductor and grid currents,
respectively.
According to the above analysis, the GFM inverters can
also precisely control their inductor current by a properly
designed inner current loop. The impact of the grid current
on capacitor voltage is removed by current feedforward
and thus, uis fully controlled by adjusting i.
As shown in Eq. (7), in thedq reference frame, the dand
qaxis component of the filter capacitor voltage are also
coupled. Similarly, it is necessary to introduce dq decoup-
ling modules in the voltage control loop as illustrated in
Fig. 4.
sCUdq ¼Idq −Io;dq−0−ωC
ωC0
Udq
ð7Þ
Grid-supporting inverter
There exists an approximate linear droop relation between
the P-ωand Q-U of traditional synchronous generators. By
emulating this output characteristics, grid-supporting (GS)
[11] inverters, aimed at sharing load proportional to their
power capacities, can deploy two different droop control
structures, namely “PQ-droop”and “ωU-droop”. The PQ-
droop GS inverter adjusts its output power as a function
of the variation of the microgrid’s voltage and frequency.
In this case, the inverter behaves like a power source and
its control system is designed based on that of the GFD
inverter, as shown in Fig. 5(a). On the contrary, the voltage
and frequency at the PC of the ωU-droop GS inverter are
adjusted according to the variations of its output power.
The ωU-droop GS inverter behaves as a controlled voltage
source and its control system is based on that of the GFM
inverter, as shown in Fig. 5(b).
In Fig. 5, ω
0
and U
0
represent the no-load frequency
and no-load voltage, k
P
and k
Q
represent the active and
reactive power droop coefficients, respectively. In steady
state, the frequency of the microgrid is a global quantity,
and the voltages at different points of the microgrid are
almost identical. If “ω
0
”and “U
0
”of each inverter are identi-
cal, then both the PQ-droop GS inverter and the ωU-
droop GS inverter can share load variations as follows:
kp1ΔP1¼kp2ΔP2¼…¼kpnΔPnð8Þ
kQ1ΔQ1¼kQ2ΔQ2¼…¼kQnΔQnð9Þ
where k
Pi
and △P
i
(i = 1,2,…,n) represent the active
power droop coefficient and output active power vari-
ation of the ith GS inverter, respectively. k
Qi
and △Q
i
represent the reactive power droop coefficient and out-
put reactive power variation of the ith GS inverter,
respectively. Although both types of GS inverters shown
in Fig. 5have a good load-sharing performance, the PQ-
droop GS inverter cannot operate by itself. In contrast,
the ωU-droop GS inverter is controlled as a voltage
source, and thus can work independently regardless of
the microgrid operation mode. The ωU-droop GS in-
verter has acquired extensive attentions for its excellent
features though some problems still exist, including:
Fig. 4 Voltage control loop of GFM inverters in dq reference frame
Fig. 3 Control block diagram of three-phase grid-forming inverter
Guo and Mu Protection and Control of Modern Power Systems (2016) 1:5 Page 3 of 7
the line impedance of a low-voltage microgrid has a
large resistive component, thus P-ωand Q-U droop
control is no longer suitable.
the voltages at the PCs of each inverter are not
completely equal, thus the GS inverters cannot
share reactive power precisely.
Many researchers have proposed various improved
methods to deal with the above problems and some typ-
ical schemes will be presented in the following sections.
A. Decoupling transformation method
As depicted in Fig. 6, the voltage at the PC of
theωU-droop GS inverter is denoted by U∠δ,and
the voltage at the microgrid bus is denoted by E∠0.
Z
L
is the line impedance between the inverter’s filter
capacitor and the microgrid bus with an impedance
angle of θ.
Due to the small power angle δ, it is assumed that:
sinδ¼δ;cosδ¼1ð10Þ
Thus, the output power of the GS inverter can be
expressed as:
P¼EU cosθ−E2cosθþEUδsinθ
ZL
Q¼EU sinθ−E2sinθ−EUδcosθ
ZL
ð11Þ
If both the resistive and reactive components of the
line impedance cannot be ignored, the output active and
reactive power of the inverter will be dependent on both
δand U. In this case, the P-ω(δ) and Q-U decoupling
relation will no longer valid. To solve this problem, the
virtual power P’,Q’and the transformer matrix T
PQ
are
introduced in [12, 13]:
P0
Q0
hi
¼TPQ P
Q
hi¼sinθ−cosθ
cosθsinθ
P
Q
hi ð12Þ
According to Eq. (11) and (12), it can be derived that:
P0¼EU
Zδ
Q0EU−E2
Z
ð13Þ
The ωU-droop control based on the virtual power is
given as:
ω¼ω0−kPP0
U¼U0−kQQ0
nð14Þ
Similarly, transforming ω(δ) and Uwith the matrix
T
ωU
[14] gives the virtual frequency (phase angle) and
voltage as:
ω0δ0
ðÞ
U0
hi
¼TωE
ωδðÞ
U
hi
¼sinθcosθ
−cosθsinθ
ωδðÞ
U
hi ð15Þ
According to Eq. (11) and (15), it can be derived that:
Fig. 6 Simplified model of ωU-droop grid-supporting inverter
Fig. 5 Control block diagram of grid-supporting inverter. aControl
block diagram of PQ-droop grid-supporting inverter. bControl block
diagram of ωU-droop grid-supporting inverter
Guo and Mu Protection and Control of Modern Power Systems (2016) 1:5 Page 4 of 7
P¼EUδ0þU−U2−E2
ðÞ
Ecosθ
Z
Q¼EUU0þU−U2−E2
ðÞ
Esinθ
Z
ð16Þ
Since Eand Uare constant when the droop control
process reaches steady-state, the output active and reac-
tive power of the GS inverter will be regulated by the
virtual frequency and voltage respectively. Thus, a novel
ωU-droop can be established:
ω0¼ω0
0−kPP
U0¼U0
0−kQQ
nð17Þ
In Eq. (17), ω
0
’and U
0
’is the corresponding virtual no-
load frequency and voltage. The droop control block dia-
gram of the GS inverters applying two types of decoupling
transformation method is shown in Fig. 7.
It is worth noting that the first decoupling method is
designed to share the virtual power rather than the real
power. So there exists a complicated relation between the
variations of each inverter’s output power and their droop
coefficients when the load in the microgrid changed. The
second decoupling method avoids this problem consider-
ing that all inverters have the same ω′and U′,i.e.theR/X
of each line in the microgrid must be identical. In
addition, the variables directly controlled by Eq. (17) are
ω′and U′, and thus, it is necessary to carefully select the
droop coefficients [14] to ensure that the real frequency
and voltage are located in reasonable ranges.
B. Virtual impedance method
The coupling between the output active and
reactive power of the conventional ωU-droop
control can be mitigated by introducing virtual
impedance [15], as illustrated in Fig. 8. The voltage
at the inverter’s PC is expressed as:
U¼GusðÞ Uref −GusðÞ ZVIoð18Þ
where G
u
(s) is the voltage closed-loop transfer function
of the ωU-droop GS inverter, and Z
V
is virtual impedance.
The total impedance between the equivalent voltage
source of the inverter and the microgrid bus can be writ-
ten as:
Z¼GusðÞ ZVþZLð19Þ
where Z
L
is the line impedance.
If the magnitude of the virtual impedance is much larger
than the line impedance, the total impedance will be
largely decided by the virtual impedance. However, the
large total impedance may cause the microgrid voltage to
reduce substantially. In [16], a novel method was proposed
to solve this problem by introducing a negative resistive
component into the virtual impedance. As the virtual
negative resistor counteracts the line resistor, the total
impedance can be designed to be mainly inductive and of
small magnitude. According to Eq. (11), if the total imped-
ance is mainly inductive [17], the GS inverter should
adopt P-ωand Q-U droop control. However, if the total
impedance is mainly resistive [18], P-U and Q-ωdroop
control should be applied.
C. Reactive power sharing method based on
communication
To improve the reactive power sharing accuracy, a
common method is to revise the GS inverters’
droop control parameters, including no-load voltage
and droop coefficient. The following analysis takes
the inductive line (cosθ≈0,sinθ≈1) as examples.
According to Eq. (11), the relation between the
output reactive power and the voltage of the GS
inverter’s PC is shown as:
U¼EþZL
EQð20Þ
In the Q-U plane, the intersection of the operational
curve described by Eq. (20) and the reactive power
droop curve is the GS inverter’s stable operating point
[19].
Fig. 7 Control block diagram of ωU-droop Grid-supporting inverter
applying decoupling transformation method
Fig. 8 Control structure of virtual impedance method
Guo and Mu Protection and Control of Modern Power Systems (2016) 1:5 Page 5 of 7
As illustrated in Fig. 9, there are two inverters, namely
1# and 2#, with the same droop coefficient. The total
impedance between these two inverters’equivalent voltage
sources and the microgrid bus are Z
1
and Z
2
, respectively.
If Z
1
is not equal to Z
2
,theinverters’operating points will
be different. Increasing inverters’droop coefficient leads
to new operating points. The voltage of the microgrid bus
moves from Eto E’, and the inverters’output power
changes move from Q
1
and Q
2
to Q
1
’and Q
2
’, respectively
It can be seen that the reactive power sharing accuracy is
improved with the increase of the inverter’s droop coeffi-
cient. Decreasing the GS inverter’s no-load voltage can
also increase reactive power sharing accuracy, as shown in
Fig. 10. To adjust each inverter’s droop curve parameters
in a coordinated manner [19, 20], it is necessary to employ
a centralized control system.
Different with the method of adjusting droop parameter,
reference [21] proposed an improved control structure by
introducing integral module, as shown in Fig. 11.
In Fig. 11, U
0
is the inverter no-load voltage; Eis the
voltage of microgrid bus; k
Q
is the reactive power droop
coefficient; K
u
is the integral gain. The transfer function
of the inverter’s output reactive power can be written as:
QsðÞ¼KuUo−EsðÞ½EsðÞ−sE2sðÞ
sZLþKukQEsðÞ ð21Þ
and its steady-state value can be calculated as:
limt→∞QtðÞ¼lims→0sQ sðÞ
¼lims→0
U0−EðÞKuE−sE2
sZLþKukQE¼U0−E
kQ
ð22Þ
In this method, the output reactive power of each GS
inverter is independent to the line impedance Z
L
. By de-
livering the voltage information of the microgrid bus to
each GS inverter, accurate reactive power sharing can be
realized. This method doesn’t require a central controller
to participate, avoiding the usage of complicated algo-
rithms. Besides, the additional parameter, K
u
, can be
used to adjust the dynamic response of reactive power
control.
Results and Discussion
As can be seen from the above sections, the GFD inverter
behaves as constant power source and it participates
neither in voltage regulation nor in load variations sharing.
The GFM inverter behaves as constant voltage source and
it is responsible not only for maintaining the microgrid’s
voltage and frequency, but also for keeping power balance.
Load sharing among the GFM inverters is a function of
the impedances between the inverters and microgrid bus.
The PQ-droop and ωU-droop GS inverters can be
regarded as the upgraded version of the GFD and GFM
inverters, and they behave as controlled power source and
controlled voltage source, respectively. When the micro-
grid operation conditions change, they can adaptively
adjust the output power or voltage to achieve a more flex-
ible load sharing. Currently the most promising control
method is the ωU-droop control, because it can make the
system autonomy and achieve seamless mode switching.
When the microgrid is operated in islanded mode, any
Fig. 9 Reactive power sharing with different droop coefficients
Fig. 10 Reactive power sharing with different no-load voltages
Fig. 11 Large-signal representation of the proposed reactive
power control
Guo and Mu Protection and Control of Modern Power Systems (2016) 1:5 Page 6 of 7
addition or reduction of a single ωU-droop GS inverter do
not influence the configuration of the original system.
When the microgrid operated in grid-connected mode,
the ωU-droop GS inverter can output the specified power
by modifying its no-load voltage and frequency. However,
this autonomous control method is not widely applied
among numerous experimental microgrids, because there
still exist many practical problems, such as the dynamic
response speed, the impact of control parameters on
system stability, the capability to deal with unbalanced
and non-linear loads, and control strategies under fault
conditions. In addition, it can be seen from the above ana-
lysis that the performance of the ωU-droop GS inverter
operating with no communication is inferior. In order to
enhance the accuracy of reactive load sharing, it is worth-
while to study the design of the control algorithms with
reduced communication requirements.
Conclusions
This paper illustrates the control principles of micro-
source inverters, including grid-feeding, grid-forming, and
grid-supporting inverters. The PQ-droop and ωU-droop
grid-supporting inverters can be regarded as the upgraded
version of grid-feeding and grid-forming inverters with a
more flexible load sharing capability. Since the conven-
tional ωU-droop control exists some shortages, several
improved methods of ωU-droop based grid-supporting in-
verters are presented. The comparison of various inverters
is carried out and the valuable research points are also
discussed.
Acknowledgments
This work was supported in part by Nation Natural Science Foundation of
China (51407128) and the key technologies research project on distribution
network reconfiguration of State Grid Hunan Electric Power Company
(5216A1300JV).
Competing interests
The authors declare that they have no competing interests.
Authors’contributions
WG and LM conceived and designed the study. WG wrote the paper. All
authors read and approved the final manuscript.
About the authors
W. M. Guo was born in 1989 in Hunan, China. He received his B.S. degrees in
electrical engineering from Tongji University in 2011, where he is currently
working towards a Ph.D. degree. His current research interests are microgrid
protection and control.
L. H. Mu was born in 1963 in Jiangsu, China. He is currently a full professor in
the Department of Electrical Engineering, Tongji University, Shanghai, China.
His current research interests include protective relaying of power system,
microgrid and power quality.
Received: 12 May 2016 Accepted: 16 May 2016
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