Energy and Power Engineering, 2013, 5, 1344-1346
doi:10.4236/epe.2013.54B254 Published Online July 2013 (http://www.scirp.org/journal/epe)
Discussion on the Factors Affecting the Stability of
Microgrid Based on Distributed Power Supply
Ye Xu, Zhao-hong Shi, Jin-quan Wang, Peng-fei Hou
Power and intelligence teaching and research center, PLA University of Science & Technology Nanjing, China
Received February, 2013
ABSTRACT
This paper summarizes the advantages of application of micro grid, analyzes the structure of microgrid, and discusses
the factors, which are important to the stable operation of micro grid. The factors include the technology of power
matching, harmonic suppressio n and the stability of electronic cascaded syste ms etc.
Keywords: Microgrid; Distributed Generation; Stability Analysis; Energy Storage
1. Introduction
With the progress of human society, electric power has
been the symbol of modern civilization. Power supply
reliability and power quality have become more and
more important. Facing pressures from traditional re-
source depletion and environmental pollution, power
generation methods based on fossil fuel and the central-
ized power supply mode have been difficult to meet the
requirements of economic and social development [1].
With the development of power electronic technology
and modern control theory, microgrid, which integrates
distributed generations, energy storage device, loads and
control modules, is considered progressive to meet the
growing power demand [2]. Micro grid provides a
method for distributed generation access to utility grid,
especially for photovoltaic cells, wind power generation,
fuel cell and super capacitor etc. Micro grid promotes the
development and utilization of renewable energy. There
is great significance to solve the problem of electricity in
remote areas of China.
The CERTS has set up a microgrid test platform as
shown in Figure 1. The definition of microgrid given by
CERTS (the Consortium for Electric Reliability Tech-
nology Solutions) is: Microgrid is composed of loads and
micro power generations, it can simultaneously provides
electricity and heat; power electronic devices are respon-
sible for the conversion of energy and the necessary con-
trol; microgrid is regarded as a controlled unit for utility
grid, and meet the requirements of power supply reliabil-
ity and power quality at the same time. The techniques
comprising the CERTS Microgrid concept are: 1) A me-
thod for effecting automatic and seamless transitions
between grid-connected and islanded modes of operation;
2) An approach to electrical protection within the micro-
grid that does not depend on high fault currents; and 3) a
method for microgrid control that achieves voltage and
frequency stability under islanded conditions without
requiring hig h - speed comm uni cat i ons.
Researches on microgrid are mainly concentrated on
the stable operation. The main factors that hinder the
development of the microgrid include nonlinear load
statistics theory, power matching technology, harmonic
hazard, stability of electronic cascaded systems etc [3].
Operation control and energy management technologies
based on correct theoretical basis are important to
achieve optimal and stable operation of microgr id.
2. Structure and Characteristics of
Microgrid
2.1. The Structure of Microgrid
Microgrid is mainly composed of micro power supply,
energy storage device and electrical / thermal load,
Figure 1. Micro power grid test platform in Dolan Tech-
nology Center.

Y. XU ET AL. 1345
through the static switch connecting to low-voltage dis-
tribution network. It can realize the smooth and seamless
transfer between islanding mode and grid-connected
mode. The control, measurement and protection function
of operation is based on the power electronic interface.
The main structure of microgrid can be DC microgrid,
AC microgrid and AC / DC hybrid microgrid and so on.
AC microgrid is the main form of microgrid. Its typi-
cal structure is shown in Figure 2. In AC microgrids,
distributed generations and energy storage devices are
connected to the AC bus through power electronic de-
vices. Through the control of the static switch which
connects microgrid and utility grid, can realize the mode
transfer between grid-connected mode and islanded op-
eration.
The structure of DC microgrid is shown in Figure 3. It
is obvious that distributed generations, energy storage
devices and loads are connected to the DC bus through
power electronic converters. DC microgrid connects to
the external AC network through the inverter device. DC
micro grid can provide power to AC, DC load of differ-
ent voltage-grades through power electronic conversion
devices. The energy storage systems connected to DC
bus, compensate the fluctuation of distributed genera-
tions and loads.
Compared with AC microgrid, DC microgrid adopts
only exist only one-level voltage conversion device be-
Figure 2. The structure of AC microgrid.
Figure 3. The structure of DC microgrid.
tween DGs and DC bus, reducing the cost of system con-
struction. As a result, the control methods become simple.
It is helpful to stable operation. At the same time, since it
is unnecessary to consider the synchronization problem
between each DGs, there is more advantages in circulat-
ing current restraining.
AC / DC hybrid microgrid contains both AC bus and
DC bus. It can supply power to AC loads and DC loads
directly. Actually, AC / DC hybrid microgrid can be re-
garded as AC microgrid, because DC microgrid is a cer-
tain power supply which connects to AC bus through a
power electronic inverter.
2.2. The Characteristics of Microgrid
Microgrid has kind of a dual role. For the power enter-
prises, microgrid can be regarded as a simple scheduling
load, which can make the response to meet the needs of
transmission system in seconds. For the users, microgrid
can be used as a customizable power, which can satisfy
the users' diversified requirements, increase the local
power supply reliability and improve the quality of elec-
tricity etc. Microgrid technology is the organic combina-
tion of the power electronics, distributed generation, re-
newable energy power generation technology and energy
storage technology[4]. Microgrid generally has the fol-
lowing advantages: Microgrid is different from the large
power system. It is a small system that is composed of
miniature distributed power supply and loads. It can re-
gulate the power supply capacity and the scale of the
load with a flexible strategy.
The type of distributed generation is different. It in-
cludes traditional power diesel engine, fuel cell, photo-
voltaic plant, wind power generation and other renewable
energy generation. Along with massive applications of
the power electronic equipments, the proportion of non-
linear loads in microgrid increases. Energy storage
equipment, such as super capacitor and battery, is neces-
sary condition for stable operation of the system.
According to the different operating conditions, the
microgrid can choose different modes of operation.
With the development of control strategy, the reliabil-
ity of power supply is improved[5]. There are some de-
fects in the system structure of microgrid. Microgrid
adopts a large number of power electronic devices as the
interface. As a result, the inertia of distributed genera-
tions is small, compared with the conventional generator.
Distributed generations do not good in the performance
of over load. Because of the small inertia, microgrid
needs energy storage equipments to achieve the power
balance between supply and load[6]. The two defaults of
microgird have a great influence on the stable operation of
the system.
3. Analysis of Stability Factor
After 10 years of development, although the microgrid
has made great progress in operation control, energy
management and conservation technologies, but small
inertia, nonlinear load ratio increasing and harmonic

Y. XU ET AL.
1346
harm are the problems that hinder the application of mi-
crogrid. Factors affecting the stable operation of micro-
grid include:
a) Microgrid itself is a small capacity power supply
system. Its power capacity is limited, and is comparable
with the load. While the different sorts of loads have
wide and frequent dynamic range, the bus voltage will
fluctuate in wide range. In order to k eep the stability, the
energy storage equipments have to provide a quick re-
sponse. Power supply and load matching is an essential
condition for the operation of power system, especially in
microgrid. However, traditional load statistical methods
are based on average power theory. They do not apply to
nonlinear loads, which introduce harmonics into the mi-
crogrid[7]. What’s more, the present theory does not con-
sider the effect of harmonics on load and power supply.
For an example, while a certain power supply system is
rich in harmonics, the actual output of active power of a
diesel generator of which the rated power is 120 KW, is
only 55 KW. The action principle between the harmonic
and Synchronous motor is not clear. In the engineering
application, the resolution is to configure larger power
motor leading to wasting of resources. Under the condi-
tion of large harmonics, power matching has been a
problem. At the moment, the study methods are based on
the average modules of converters and control system.
But the average module cannot describe the time-variant
characteristic of converters exactly[8]. According to the
analysis requirement of nonlinear electric power system,
the instantaneous power theory provides a feasible me-
thod to solve the time-variant and nonlinear problems. At
home and abroad, the scholars have applied the instanta-
neous power theory to make a lot of researches on the
nonlinear pow er syst em.
b) Distributed generation is based on power electronics
technology. Its impact on the power quality is mainly
caused by the voltage flicker and the introduction of a
large number of harmonics. Part of the distributed power
is greatly influenced by natural conditions, so that it is
difficult to generate stable power[9]. The great changes
in output and the interaction between distributed units
and medium voltage feedback devices both can cause
voltage flicker and impact to the power grid. To enhan ce
the reliability of distributed generation and the applica-
tion of energy storage equipment are effective ways to
solve voltage fluctu ation problems such as voltage pulse,
voltage flicker and power supply interruption etc. It pro-
motes the energy storage device development in energy
density, response time and cost performance.
c)The application of power electronic devices in mi-
crogrid, bring the problem of converter cascade stability.
as nonlinear switching circuits, its Impedance is
time-varying and nonlinear[10]. When the structure of
the power electronic devices is complicated, it is difficult
o predict the stab ility with conventional impeda nce ratio
methods. Besides adjusting the impedance ratio, adding
Intermediary filter and designing bus controller are ef-
fective methods to improve the cascade stability. At the
moment, the domestic study on cascade stability is very
limited. At abroad, only a few scholars are doing the re-
levant theoretical analysis and application research, such
as F. C. Lee of VT, M. Ehasani of Texas A&M Univer-
sity. In microgrid, converters cascade stability still needs
a lot of research work.
t
4. Conclusions
Microgrid based on distributed generation is powerful
supplement and effectively support of utility grid. Also,
it’s one of the important trends of power system in the
future. Power matching theory, energy storage technol-
ogy, converter cascade stability and harmonic suppres-
sion are the bases of operation control and energy man-
agement[11]. With the development of nonlinear power
system, microgrid will play an important role in applica-
tion of new energy.
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