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Control principles of micro-source inverters used in microgrid


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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.
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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
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
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
[35], 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 DGsdirect connection. Microgrids
can also operate in islanded mode, thus increase system
reliability and availability of the power supply.
Proper control is a precondition for microgridsstable
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 masterslave 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.
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 inverters DC bus.
* Correspondence:
Department of Electrical Engineering, Tongji University, Shanghai, China
Protection and Control o
Modern Power S
© 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (, which permits unrestricted use, distribution, and
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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 inverters power referencetrackingis realized by
adjusting the output currents. The control system calculates
the output current references based on the relationships
among the inverters 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
inverters output currents are represented by vector ias
Neglecting the power consumed on the filter inductor,
the output power of GFD inverter is calculated accord-
ing to instantaneous power theory [7]as:
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
, can be
obtained by solving the following equation:
Pref ¼viref
Qref ¼viref
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
, 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 inductors currents
can be controlled indirectly. If the potential at the mid-
dle point of the inverters DC bus equal zero and ignore
the delay of PWM process, the inverter is equivalent to
a proportional element with gain k
. Thus, the
closed-loop transfer function of the inverters current
control can be written as:
GcsðÞ¼ kpwmTcsðÞ
sL þkpwmTcs
ðÞ ð4Þ
(s) needs be designed in a way to ensure G
(s) have
sufficient bandwidth. Meanwhile, the gain and phase
shift of G
(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;dqUdq 0ωL
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 capacitorsvoltage,u. In natural reference
frame, there exists the following relation:
where iandi
are the inductor and grid currents,
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;dq0ωC
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-droopand ωU-droop. The PQ-
droop GS inverter adjusts its output power as a function
of the variation of the microgrids 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
and U
represent the no-load frequency
and no-load voltage, k
and k
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
and U
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:
where k
and P
(i = 1,2,,n) represent the active
power droop coefficient and output active power vari-
ation of the ith GS inverter, respectively. k
and Q
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 Uδ,and
the voltage at the microgrid bus is denoted by E0.
is the line impedance between the inverters filter
capacitor and the microgrid bus with an impedance
angle of θ.
Due to the small power angle δ, it is assumed that:
Thus, the output power of the GS inverter can be
expressed as:
P¼EU cosθE2cosθþEUδsinθ
Q¼EU sinθE2sinθEUδcosθ
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,Qand the transformer matrix T
introduced in [12, 13]:
hi ð12Þ
According to Eq. (11) and (12), it can be derived that:
The ωU-droop control based on the virtual power is
given as:
Similarly, transforming ω(δ) and Uwith the matrix
[14] gives the virtual frequency (phase angle) and
voltage as:
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
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:
In Eq. (17), ω
and U
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 inverters 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 ωand U,i.e.theR/X
of each line in the microgrid must be identical. In
addition, the variables directly controlled by Eq. (17) are
ωand 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 inverters PC is expressed as:
U¼GusðÞ Uref GusðÞ ZVIoð18Þ
where G
(s) is the voltage closed-loop transfer function
of the ωU-droop GS inverter, and Z
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
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
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 (cosθ0,sinθ1) as examples.
According to Eq. (11), the relation between the
output reactive power and the voltage of the GS
inverters PC is shown as:
In the Q-U plane, the intersection of the operational
curve described by Eq. (20) and the reactive power
droop curve is the GS inverters stable operating point
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 invertersequivalent voltage
sources and the microgrid bus are Z
and Z
, respectively.
If Z
is not equal to Z
,theinvertersoperating points will
be different. Increasing invertersdroop coefficient leads
to new operating points. The voltage of the microgrid bus
moves from Eto E, and the invertersoutput power
changes move from Q
and Q
to Q
and Q
, respectively
It can be seen that the reactive power sharing accuracy is
improved with the increase of the inverters droop coeffi-
cient. Decreasing the GS inverters no-load voltage can
also increase reactive power sharing accuracy, as shown in
Fig. 10. To adjust each inverters 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
is the inverter no-load voltage; Eis the
voltage of microgrid bus; k
is the reactive power droop
coefficient; K
is the integral gain. The transfer function
of the inverters output reactive power can be written as:
sZLþKukQEsðÞ ð21Þ
and its steady-state value can be calculated as:
limtQtðÞ¼lims0sQ sðÞ
In this method, the output reactive power of each GS
inverter is independent to the line impedance Z
. By de-
livering the voltage information of the microgrid bus to
each GS inverter, accurate reactive power sharing can be
realized. This method doesnt require a central controller
to participate, avoiding the usage of complicated algo-
rithms. Besides, the additional parameter, K
, can be
used to adjust the dynamic response of reactive power
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 microgrids
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.
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
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
Competing interests
The authors declare that they have no competing interests.
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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Guo and Mu Protection and Control of Modern Power Systems (2016) 1:5 Page 7 of 7
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... The integration of DG units at the distribution level poses multiple challenges that need to be solved [4]. The DG units are connected to the microgrid through voltage source inverter (VSIs) [5]. For the operation of the microgrid, proper control system mechanisms are required to stabilize the voltage and frequency of the inverters and to perform proper power-sharing among multiple DG units [6]. ...
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This paper presents the mathematical model and control of the voltage source inverter (VSI) connected to an alternating current (AC) microgrid. The VSI used in this work was a six-switch three-phase PWM inverter, whose output voltages were controlled in a synchronous (dq) reference frame via a sliding mode control strategy. The control strategy required only output voltages; other states of the system were estimated by using a high-gain observer. The power-sharing among multiple inverters was achieved by solving power flow equations of the electrical network. The stability analysis showed that the error was ultimately bound in the case of the real PWM inverter and/or with a nonlinear load in the electrical network. The microgrid was simulated using the SimPowerSystems Toolbox from MATLAB/Simulink. The simulation results show the effectiveness of the proposed control scheme. The output voltage regulation of the inverter and power-sharing was achieved with the ultimately bounded error for the linear load. Later, the nonlinear load was also included in the electrical network and the error was shown to remain ultimately bounded. The output voltage regulation and power-sharing were achieved with the nonlinear load in the system.
... GFM control is designed for autonomous operation or island mode, represented as ideal AC voltage sources with a fixed frequency. GS control can act both as a voltage and current source, providing basic support [9][10][11][12]. ...
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In the past decade, inverter-integrated energy sources have experienced rapid growth, which leads to operating challenges associated with reduced system inertia and intermittent power generation, which can cause instability and performance issues of the power system. Improved control schemes for inverters are necessary to ensure the stability and resilience of the power system. Grid-forming inverters dampen frequency fluctuations in the power system, while grid-following inverters can aggravate frequency problems with increased penetration. This paper aims at reviewing the role of grid-forming inverters in the power system, including their topology, control strategies, challenges, sizing, and location. In order to facilitate continued research in this field, a comprehensive literature review and classification of the studies are conducted, followed by research gaps and suggestions for future studies.
... either a conventional synchronous generator or power electronic converter. The grid-supporting voltage source inverters (VSIs) in AC microgrids are the most significant part which is able not only to harmonious operate in parallel with each other but also has high flexibility to connect or disconnect from the external power grid (Guo and Mu, 2016). ...
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In this paper, new optimal procedures are introduced to design the finest controllers and harmonic compensators (HCs) of three-level cascaded control for three-phase grid-supporting inverters based-AC microgrid. The three control levels, comprising primary, secondary and synchronization control levels, are developed in stationary αβ-frame and based on the proportional–integral (PI) controllers and the proportional-resonant controllers along with additional HCs. The new optimal design guidelines of microgrid’s controllers and HCs are aimed to fulfill the study requirements. The optimization objectives and constraints are employed to minimize both the total harmonic distortion (THD) and individual harmonics of microgrid’s voltage to enhance the quality of microgrid’s output power. The THD of microgrid’s voltage can be reduced to 0.19% under the nonlinear loads. Moreover, the microgrid’s voltage and frequency can be perfectly regulated with zero deviations. Furthermore, these new optimal procedures accelerate the speed of synchronization process between the external power grid and the microgrid to be accomplished in time less than 20 ms. Additionally, an accurate power-sharing among paralleled operated inverters can be achieved to avoid overstressing on any one. Also, seamless transitions can be guaranteed between grid-tied and isolated operation mode. The optimal controllers and HCs are designed by a new optimization algorithm called H-HHOPSO, which is created by hybridizing between Harris hawks optimization and particle swarm optimization algorithms. The effectiveness and robustness of the H-HHOPSO-based controllers and HCs are compared with other meta-heuristic optimization algorithms-based controllers and HCs. A microgrid, including two grid-supporting inverters based optimal controllers and optimal HCs, are modeled and carried out using MATLAB/SIMULINK to test the performance under linear and nonlinear loads, and also during the interruption of any one of two inverters. The performance is investigated according to IEC/IEEE harmonic standards, and compared with the conventional control strategy developed in synchronous dq-frame and based on only PI controllers.
... Supply-demand imbalance usually leads to voltage and frequency biases, and voltage and frequency control strategies are important to ensure the power quality of MMGs. The existing voltage/frequency control strategies focus on eliminating these biases within one single microgrid [130] - [132]. The droop control is the most widely-used method to regulate the voltage and frequency based on drop characteristics of DGs, including active power-frequency properties [132] - [136], reactive power-voltage properties [137], [138], DC power-voltage characteristics [139], [140], and the interlinking converter droop characteristics between two microgrids [141]. ...
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The increasing penetration of various distributed and renewable energy resources at the consumption premises, along with the advanced metering, control and communication technologies, promotes a transition on the structure of traditional distribution systems towards cyber-physical multi-microgrids (MMGs). The networked MMG system is an interconnected cluster of distributed generators, energy storage as well as controllable loads in a distribution system. And its operation complexity can be decomposed to decrease the burdens of communication and control with a decentralized framework. Consequently, the multi-microgrid energy management system (MMGEMS) plays a significant role in improving energy efficiency, power quality and reliability of distribution systems, especially in enhancing system resiliency during contingencies. A comprehensive overview on typical functionalities and architectures of MMGEMS is illustrated. Then, the emerging communication technologies for information monitoring and interaction among MMG clusters are surveyed. Furthermore, various energy scheduling and control strategies of MMGs for interactive energy trading, multi-energy management, and resilient operations are thoroughly analyzed and investigated. Lastly, some challenges with great importance in the future research are presented.
... Coordinating multiple master DGs to regulate frequency and voltage together adds more layers of complexity in islanded microgrid operation which, as previously highlighted, is already challenging. Voltage and frequency regulation for microgrid island operation is usually accomplished through droop control [12][13][14][15]. Systematic restoration formulation for microgrids operating in droop-controlled island mode is scarce as most existing literature have approached this restoration problem through a rule-based approach [16][17][18][19]. ...
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The need to restore and keep the grid running or fast restoration during emergencies such as extreme weather conditions is quite apparent given the reliance of other infrastructure on electricity. One promising approach to electricity restoration is the use of locally available energy resources to restore the system to form isolated microgrids. In this paper, we present a black start restoration method that forms islanded microgrids after a blackout. The master DGs in the formed microgrids are coordinated to work together through droop control. Several constraints, including incentive-based demand response (DR) with direct load control (DLC) and distributed generator (DG) operation constraints, were formulated and linearized to realize a mixed-integer linear programming (MILP) restoration model. To improve compactness and to ensure that the model is neither under-sized nor over-sized, a pre-processing graph analysis approach was introduced which helps to characterize the least number of restoration steps needed to optimally restore the microgrid. Studies were performed on a modified IEEE 123 node test feeder to evaluate the effects of demand response, non-dispatchable DGs, and choice of restoration steps on the quality of the restoration solution.
Chapter proposes a state‐based decision‐making model to enhance the distribution system resilience throughout the unfolding events. The varying system topologies are modeled as Markov states, and the probabilities between different Markov states depend on the component failure caused by the unfolding events. A Markov decision process‐based model is proposed to make state‐based actions, i.e. system reconfiguration, at each decision time. To overcome the curse of dimensionality, an approximate dynamic programming (ADP) approach with post‐decision states is employed to optimize the proposed model, and the proposed model is validated by two test systems.
Solid oxide fuel cell (SOFC) based integrated energy system (IES) is promising in the future low-carbon power generation market, due to the high efficiency and flexibility. However, it is challenging for the dynamic control design in dealing with the conflicting objectives in terms of fast power tracking and overall efficiency during the transient process of load response. To this end, this paper develops a multi-objective optimal droop control strategy for the real-time power dispatch of the IES. Firstly, a nonlinear implicit dynamic model consisting of SOFC, lithium-ion battery, photovoltaic array and DC-DC converter is developed. Then, a multi-objective optimization is formulated to balance the power tracking performance and transient efficiency. Non-dominated sorting genetic algorithm-II (NSGA-II) is adopted to search the optimal parameters for droop controller. Simulation results demonstrates that the electricity loss of the proposed method can be reduced by 96.26% with a slight compromise in power tracking performance.
Three-level diode clamped inverters have been widely used in high-voltage and high-power applications, which can effectively increase the voltage/current limit of power electronic devices. This paper proposes a dynamic space vector based discontinuous PWM (DSV-DPWM) to enhance the robustness to neutral point potential fluctuations (NPPF). The proposed method generates a dynamic space vector (DSV) by detecting the voltages across upper capacitor and lower capacitor of inverter in the real-time, which then accurately synthesizes the vector reference. In particular, gh-coordinate and parity sector conversion approach are adopted to reduce the complexity of DSV-DPWM. Moreover, switching vector sequence that has opposite effect on neutral point potential is employed to eliminate neutral point potential offset (NPPO). DSV-DPWM performance is thoroughly analyzed based on error current vector. Both PSCAD/EMTDC simulation and digital signal processor (DSP) based hardware experiment using TMS320F28335 chip have been undertaken, which verify the advantages of DSV-DPWM against traditional space vector based discontinuous PWM (TSV-DPWM).
A novel control strategy based on "virtual negative impedance" is proposed which is composed of virtual inductor and "virtual negative resistor". The virtual negative resistor is used to reduce the power coupling and output voltage drop caused by the resistive cabling grid. The virtual inductor is implemented to ensure the output impedance of inverter mainly inductive and also to make the system impedance matched to raise the accuracy of reactive power sharing. The value range of the virtual negative resistor is studied to meet the transient stability and an improved scheme is proposed to increase this value range. The new scheme enables the parallel inverters to work with the impedance angle located in the second quadrant and make the control strategy robust against line parameter drift and uncertain estimation. In addition, the bode-plot method is presented by which the stability analysis of power control loop of parallel inverter is studied. Simulation and experimental results verify the effectiveness of the proposed control strategy and method.
For parallel multi-inverters in island microgrid, the difference of equivalent output impedance and line impedance affects greatly on power sharing and circulating current restraining. From power transmission characteristics of parallel inverters, the influence of power sharing among resistive inverters was analyzed in this paper, where the conventional droop control was applied. Based on the analysis of circulating current characteristics of resistive inverters, a robust droop multiple loop control method was proposed, which included the outer power loop and the inner voltage and current loop. In the outer power loop, a robust droop controller is adopted to reduce the effects on accurate power sharing due to the impedance difference. Introducing virtual complex impedance including resistive component and inductive component, the equivalent output impedance of inverters is redesigned as pure resistance. Quasi proportional-resonant (QPR) control is applied to realize zero steady-state errors control of the output voltage with wide bandwidth for parallel inverters, which will reduce further the deviation of output voltage and restrain circulating current. The output voltage feedforward control and the proportional control of the capacitor current are adopted to improve the transient response and current disturbance. The effects on the equivalent output impedance in the different control mode and parameters were analyzed comparatively, and the proper parameters were selected. Simulation and experimental results show the correctness and validity of the proposed control method.
Due to the property of line impedance and other factors in microgrid, reactive power supplied by distributed generation units could not be shared in accordance with their voltage droop gains. This paper presented a reactive control strategy of microgird based on secondary control by analyzing the relationship between reactive power and output voltage at no-load. Potential functions which have the objective of reactive power sharing and deviation of output voltage at no-load were established in the proposed method. Microgrid central controller centrally regulated output voltage at no-load of distributed generation units via potential functions, and reactive power sharing among distributed generation units was effectively improved. Perfect dynamic response and steady performances of the proposed scheme were validated by simulation results. In addition, circulating normalised reactive power was introduced to study reactive power sharing among distributed generation units under different load levels, and the validity of the proposed method was further verified.
Power coupling is a common problem in a microgrid because of the differences in impedance ratios between low voltage microgrid lines and traditional high voltage transmission lines. A virtual power V/f droop control method based on coordinate rotational transformation is proposed to handle this problem. The method achieves the decoupled real and reactive power control through orthogonal transformation matrix. By improving the voltage-frequency droop control, a new drooping restriction control algorithm is also introduced. Furthermore, a low pass filter is designed specifically to reduce the harmonic effect and improve the control accuracy. Taking the load disturbance of microgrid in the islanded mode and its transformation from islanded mode to interconnected mode into consideration, the good adaptability of low-voltage microgrid controller is proved by simulation results. Experimental results indicate that the proposed method can achieve the load power sharing effectively in the low-voltage microgrid, with relatively stable frequency and amplitude, good dynamic effect, and enhanced accuracy and stability of system power sharing.
Distributed generation inverters have become a key element to improve grid efficiency and reliability, particularly during grid faults. Under these severe perturbations, inverter-based power sources should accomplish low-voltage ride-through requirements in order to keep feeding the grid and support the grid voltage. Also, rated current can be required to better utilize reactive power provisions. This paper presents a reference generator capable to accomplish these two objectives: to keep the injected currents within safety values and to compute the power references for a better utilization of the inverter power capacity. The reference generator is fully flexible since positive and negative active and reactive powers can be simultaneously injected to improve ride-through services. Selected experimental results are reported to evaluate the performance of the proposed reference generator under different control strategies.
For microgrid in islanded operation, due to the effects of mismatched line impedance, the reactive power could not be shared accurately with the conventional droop method. To improve the reactive power sharing accuracy, this paper proposes an improved droop control method. The proposed method mainly includes two important operations: error reduction operation and voltage recovery operation. The sharing accuracy is improved by the sharing error reduction operation, which is activated by the low-bandwidth synchronization signals. However, the error reduction operation will result in a decrease in output voltage amplitude. Therefore, the voltage recovery operation is proposed to compensate the decrease. The needed communication in this method is very simple, and the plug-and-play is reserved. Simulations and experimental results show that the improved droop controller can share load active and reactive power, enhance the power quality of the microgrid, and also have good dynamic performance.
The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature.
Voltage sags are one of the main problems in transmission and distribution grids with high penetration of distributed generation. This paper proposes a voltage support control scheme for grid-connected power sources under voltage sags. The control is based on the injection of reactive current with a variable ratio between positive and negative sequences. The controller determines, also, the amount of reactive power needed to restore the dropped voltage magnitudes to new reference values confined within the continuous operation limits required in grid codes. These reference values are chosen in order to guarantee low current injection when fulfilling the voltage support objective. Selected experimental results are reported in order to validate the effectiveness of the proposed control.
Advanced control strategies are vital components for realization of microgrids. This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends. This hierarchical control structure consists of primary, secondary, and tertiary levels, and is a versatile tool in managing stationary and dynamic performance of microgrids while incorporating economical aspects. Various control approaches are compared and their respective advantages are highlighted. In addition, the coordination among different control hierarchies is discussed.
The enabling of ac microgrids in distribution networks allows delivering distributed power and providing grid support services during regular operation of the grid, as well as powering isolated islands in case of faults and contingencies, thus increasing the performance and reliability of the electrical system. The high penetration of distributed generators, linked to the grid through highly controllable power processors based on power electronics, together with the incorporation of electrical energy storage systems, communication technologies, and controllable loads, opens new horizons to the effective expansion of microgrid applications integrated into electrical power systems. This paper carries out an overview about microgrid structures and control techniques at different hierarchical levels. At the power converter level, a detailed analysis of the main operation modes and control structures for power converters belonging to microgrids is carried out, focusing mainly on grid-forming, grid-feeding, and grid-supporting configurations. This analysis is extended as well toward the hierarchical control scheme of microgrids, which, based on the primary, secondary, and tertiary control layer division, is devoted to minimize the operation cost, coordinating support services, meanwhile maximizing the reliability and the controllability of microgrids. Finally, the main grid services that microgrids can offer to the main network, as well as the future trends in the development of their operation and control for the next future, are presented and discussed.