Energy and Power Engineering, 2013, 5, 992-998
doi:10.4236/epe.2013.54B190 Published Online July 2013 (http://www.scirp.org/journal/epe)
A Summary of Optimal Methods for the Planning of
Stand-alone Microgrid System
Lei Qiao
School of Control and Mechanical Engineering, Tianjin Institute of Urban Construction, Tianjin, China
Received February, 2013
ABSTRACT
This paper describes the characteristics and optimal methods for the planning of stand-alone microgrid system, in order
to improve the power supply reliability, increase the coefficient of utilization of renewable energy and reduce the cost
of investment and operation. Next, the problems in the optimal planning for a stand-alone microgrid system are summa-
rized, including the unique operational control targets, the flexible combination approaches and the operation strategies
of distributed generation energy supply system, and the special requirements of the reliability of power supply quality
factor from the different users. And then, centering on the operational control and the advanced energy management
strategy, the optimal mathematical models and the solving methods, the reliability assessment approaches and the im-
provement measures of a stand-alone microgrid system, an overview of the general situation of the recent research at
home and abroad and the limitations of the study are summarized. Finally, several problems, existing in the optimal
planning of stand-alone microgrid system, to be urgently solved, are put forward.
Keywords: Stand-alone Microgrid System; Optimal Planning; Operation Strategies; Energy Management
1. Introduction
At present, considering geographical conditions, it is
rather difficult to build a conventional power distribution
system that connects with the power grid in some remote
areas and stand-alone islands. Diesel engine is usually
adopted as the main electrical source, nevertheless the
supply mode has many shortcomings, such as the low
reliability, the high operation and maintenance cost and
the environmental pollution, and so on. A stand-alone
microgrid system integrates the conventional power gen-
eration technology, the distributed generation technology
and the storage energy devices by a reasonable electric
network, and is the most effective way of improving the
supply reliability, increasing the utilization rate of re-
newable energy resources, saving the operation expense,
lowering the energy consumption, reducing the pollutants
discharge, and realizing the optimal use of multiple en-
ergy resources. Accordingly, the power energy demands
of remote areas and stand-alone islands can be met better
through a stand-alone microgrid.
Presently, some demonstration projects of stand-alone
microgid have already been built all over the world, such
as the microgid system with wind turbines, photovoltaic
arrays, diesel engines and energy storage devices on
Kythnos island in Greece [1] and the stand-alone mi-
crogid consisting of multiple energy forms and seawater
desalination installations on Dangan island in Zhuhai [2].
The above-mentioned projects only explored the question
about power supply to island from a feasibility viewpoint,
but the genuine optimal planning has been not still
achieved as a result of deficiencies in the theory and me-
thod of planning.
According to the character of optimal planning for a
stand-alone microgrid system, the article analyzes the
research status of optimal methods for the planning of
stand-alone microgrid system, summarizes the research
limitations, and at last proposes the pivotal problems
needed to be solved urgently.
2. A Brief Description of Optimal Planning for a
Stand-alone M ic r o gr i d Syst e m
2.1. Planning Objectives
The main planning objective for a stand-alone microgrid
is to decide the optimal system scheme that makes the
expense of construction and operation lowest on the basis
of the power energy demand, the renewable energy sup-
ply and the condition of existing power network in the
planning period.
Figure 1 shows a diagram of a simple radial 10 kV
AC stand-alone microgrid system, where DG (distributed
generator)contain diesel generators, photovoltaic system,
wind turbine generators and fuel cells, and ESS repre-
sents the energy storage system, they are connected to

L. QIAO 993
Figure 1. AC topology of a stand-alone microgrid system.
low voltage AC buses dispersedly and then input the
10kV or more high voltage grid through booster trans-
formers. Based on the structure, the main research con-
tents of the planning for a stand-alone microgrid include
the planning of capacity of distributed generation and
energy storage devices and the network frame planning.
The former carries out the optimal design to choose the
type, capacity and control strategy of distributed genera-
tors in a microgrid from the viewpoint of the balance
between the supply and demand, and the latter studies the
optimal planning about the structure of power network,
the optimal path and the connected location of distributed
generation. In the areas with grids, the key of the plan-
ning is how to choose reasonably the capacity and loca-
tion of the energy supply system with distributed gen-
erators to satisfy the established objectives, on the condi-
tion that the comprehensive energy demand is met, ac-
cording to the situation of local resources. Whereas, in
some areas that need the overall transformation or is
non-electric, the joint optimal planning should be devel-
oped.
2.2. Character of Optimal Planning
In a stand-alone microgrid system, many factors make
modeling and solving of the optimal planning very com-
plex, such as the input of intermittent renewable energy
resources, the flexible combination approaches, the vari-
ous control strategies and the different demand for the
power supply reliability. There are the main representa-
tions.
1) There are a mass of uncertain factors in a stand-
alone microgrid system, and hence the more flexible
model and solution algorithm of optimal planning need
to be adopted.
The planning period of stand-alone microgrid system
is usually 10 - 20 years. The planning scheme is estab-
lished based on the long period forecast of the compre-
hensive energy demand and some renewable energy re-
sources. Nevertheless, the result of forecast is uncertain
as a result of external conditions, such as the weather. In
addition, the fluctuation of the price of fossil energy and
the decreasing investment cost of distributed generators
also produce an effect on the optimal planning scheme.
2) A stand-alone microgrid system has the particular
operation and control objectives, the flexible combina-
tion schemes and the control strategies that influence the
result of the optimal planning, consequently these factors
should be considered adequately.
A microgrid system that is able to connect grid links
the power system in most of the time. The optimal objec-
tives of the maximal incomes and the minimal pollutant
emission can be achieved by managing scientifically the
different units in a microgrid system [3,4]. A stand-alone
microgrid system is not supported by the power system,
accordingly its main operational objective is to maintain
the long-term stability of system and meet the demand of
power energy. In a stand-alone microgrid system, the
combination schemes and the operational modes of dis-
tributed generators and energy storage devices are more
flexible, and as for the control strategies, the methods of
coordination control are multiform [5].
3) When a stand-alone microgrid system is pro-
grammed, the different users’ demands for the power
supply reliability should be considered, and then accord-
ing to the evaluation result of reliability, the planning
scheme needs to be modified.
A stand-alone microgrid system contains the multiple
energy input, the uncertainty of optimal planning, the
various combination schemes of distributed units, the
flexible system structure and the different demands for
the power supply reliability, which undoubtedly increase
the difficulty of the modeling and solution method of the
optimal planning.
3. Research Status of Optimal Planning for
Stand-Alone Microgrid System
The research status of the optimal planning for a stand-
alone microgrid system is introduced, including the op-
erational control and the energy management strategies,
the mathematical model and solution algorithm of opti-
mal planning, and the evaluation methods of reliability,
etc.

L. QIAO
994
3.1. Operational Control and Energy
Management Strategy of Stand-alone
Microgrid System
The existing researches demonstrate that a stand-alone
microgrid system with multiple energy systems can in-
crease the efficiency and the energy utilization factor.
However, due to the particular operational modes and the
various combination schemes of system, the feasible
control strategy need to be used in order to ensure the
system stability.
At present, the controls of microgrid are classified into
the control of distributed generators and the energy
management.
1) Operational strategies
The controls of distributed generators are divided into
constant power control, droop control and constant volt-
age /constant frequency control. The control strategies of
microgrid are classified into master-slave control and
equivalence control. Up till now, in the existing stand-
alone microgrid system, the layered management mode
based on master-slave control is adopted frequently. In
the control strategy, the adjustable generators such as
diesel generators, gas turbines and biomass generation
power, or the energy storage devices are disposed as the
main units of voltage regulation and frequent regulation,
besides other distributed generators are controlled with a
constant power.
The adjustable generators in a microgrid adopt the
plug and play equivalence control to realize the equipar-
tition of active current and reactive current without the
real-time communication, accordingly the reliability of
power supply is improved.
When a synchronous generator acts as distributed gen-
eration, the equivalence control is realized easily as a
result of the inherent droop character of synchronous
generator. Besides, some experts proposed many meas-
ures to achieve the equivalence control of inventor, and
the chief method was to simulate the droop control of the
regulation character of synchronous generator [6-8]. The
main disadvantage of droop control is that the frequency
and the voltage both have a steady state error. Conse-
quently, the energy storage devices were used in the
second frequency regulation in [9] and a dynamic voltage
recovery equipment was introduced in order to reduce
the voltage tolerance in [10]. The results of reference [11]
showed that the energy storage devices could ensure the
voltage stability and frequency stability of isolated sys-
tem by controlling the power appropriately in a stand-
alone microgrid where synchronous generators acted as
the main electrical source. For a stand-alone microgrid,
the deviation of frequency and voltage can be improved
by using the upper management system to dispatch the
power generation and supply.
2) Advanced energy management strategy
All kinds of distributed units in a system are inde-
pendent relatively and yet coupled, so the coordinated
relationship between energy resources and energy con-
sumption devices should be considered to realize the
comprehensive utilization of multiple energy resources.
Reference [12] suggested several operational strategies
around the energy management of diesel generator and
energy storage battery—the strategies decided the opera-
tional priority of diesel generator and energy storage bat-
tery based on the comparison the unit generation cost of
diesel generator with one cycle charging and discharging
cost of energy storage battery, the method was applied in
a optimal software—HOMER. The energy management
strategy of stand-alone microgrid with fuel cells, electro-
lytic water equipments and electrochemical cells was
researched, and a approach to coordinating and dispatch-
ing the hybrid energy storage was proposed based on the
battery’s state of charge in [13]. Considering the con-
straint condition of the equipment operation, the coordi-
nated control strategies of multiple energy resources
were suggested to ensure the long-term and reliable op-
eration of energy storage batteries in [14]. The achieve-
ments above indicated how to select a combination
scheme and control strategies of distributed generation
system depended on the local renewable energy re-
sources, the load demands and the cost of equipments
and fuel. In addition, at present some simulation soft-
wares can supply the coordination control strategies of
distributed generators. For example, simulation software
Hybrid2 developed by NERL suggests several schemes
that are divided into two classes [5], one is that diesel
generators play a role of net load following and storage
batteries are in the condition of floating charge as the
reserved power, and the other is that diesel generators
and storage batteries serve as the main power supply in
turn to meet the demand of net load.
3.2. Mathematics Model and Solution Methods
of Optimal Planning for Stand-alone
Microgrid System
Presently, in the aspect of optimal planning for a stand-
alone microgrid, many scholars at home and abroad
mainly focus on designing the capability of distributed
generators from the viewpoint of balance of supply and
demand. The most of researches calculate every index of
the combination scheme of different renewable energy
resources with the quasi steady state simulation program
according to the load data, the wind speed, the illumina-
tion intensity and the temperature in a life cycle—the
method is called the deterministic method. The advan-
tage of means is that the fluctuation of renewable energy
resources and loads in a life cycle and control strategies

L. QIAO 995
are simulated in detail during the optimal planning. But,
the disadvantage is that the forecast errors and the price
fluctuation influence the result of optimal planning, and
the uncertain factors can not be estimated. In addition,
the hourly data of weather and load need to be known in
the process of planning, however, it is difficult in prac-
tice.
Accordingly, the uncertain theory is applied widely in
the generators expansion planning and the system plan-
ning. The uncertain planning aims at the optimal prob-
lems in the uncertain conditions, and explains the inte-
grated theory of modeling and solving of stochastic plan-
ning, fuzzy planning and rough planning [15]. Reference
[16] researched the planning of distribution network
containing distributed generators with the fuzzy optimal
method. Reference [17] used Monte Carlo simulation to
simulate illumination radiant intensity and the initial ca-
pability of batteries and adopted the quasi steady state
simulation to analyze the reliability index during the op-
timal design, and then the result with meeting a certain
confidence level was regarded as the convergence crite-
rion of Monte Carlo simulation. Reference [18] advanced
the optimal configuration model of wind-hydro-solar
generation system based on the stochastic chance-con-
strained programming to obtain the optimal configuration
schemes and the evaluation indexes with meeting the
objectives and constraint conditions in all kinds of con-
fidence levels.
The optimal objectives involve the reliability, the sys-
tem cost and the pollutant emission. The reliability in-
dexes contain loss of load probability, loss of load prob-
ability and loss of load hours, and so on [19]. The costs
of system include the net cost and standardization cost in
a life cycle [20-22]. The problem is the hybrid optimal
planning with discrete variables and continuous variables,
so the artificial intelligent algorithm is fit to solve the
problem [23]. Reference [24-26] designed the type and
capability of distributed generators with the genetic algo-
rithm. Multiple constraint conditions exist at the same
time during the optimal program, but in the practical
multi-objectives optimal problem, the different objec-
tives always conflict one another. The multi-objectives
optimal design was achieved based on a fixed weigh in
[27] that converted the multi-objectives to the single ob-
jective. But, the multi-objectives solving does not mean
looking for the single optimal solution but searching a set
of equilibrium solution— Pareto optimal solution. The
multi-objectives genetic algorithm was adopted to design
the capability of stand-alone wind-solar-diesel-battery
microgrid, and aimed at the lowest cost of life cycle and
the minimum of carbon emission in [28]. Reference [29]
proposed a three-objective model containing the prob-
ability index of capacity deficiency. Reference [24,30]
described a layered optimal design, outer layer calculated
the capability of distributed generators, and the internal
layer optimized the key control variables of the selected
control strategy, and the method was applied in an opti-
mal software—HOGA.
Besides the researches mentioned, the distributed gen-
eration technology brings a new challenge to distribution
systems [31-32], accordingly, the comprehensive coor-
dination between distributed generators and distribution
systems should be considered adequately while the opti-
mal planning is laid out.
Reference [33] pointed out the planning of distribution
system with distributed generators was divided into the
location planning of distributed generators and the ex-
panding planning of distribution systems. Reference [31]
built a planning model of distribution grid with distrib-
uted generators considering the security restriction, the
randomness of distributed generator’s output, the power
penetration of distributed generators and the joint plan-
ning of distribution grids and distributed generators.
Reference [34] used the chance constrained program-
ming to set up a grid structure planning model for distri-
bution networks with distributed wind turbine generators,
considering the randomness of wind power generation
and the uncertainty of load forecast.
As already mentioned, the optimization of capability,
location and structure of distributed generation had al-
ready been carried out, however the optimal planning for
a stand-alone microgrid was hardly reported.
3.3. Reliability Evaluation and Improvement
Measures for Stand-Alone Microgrid System
The theory of reliability is applied in the planning in or-
der to establish the reasonable strategy and look for the
optimal balance between the economical efficiency and
the reliability, on the premise of meeting the reliability
evaluation index. For a stand-alone microgrid, the pre-
sent researches mostly focus on the reliability evaluation
of power output and load need, and the purpose is the
quantization and analysis of risk as a result of the random
invalidation of system, at the same time, not the index of
single load point but the general adequacy index should
be supplied. Reference [35] regarded the loss of load
expectation (LOLE) and the loss of energy expectation
(LOEE) as a reliability evaluation index, and discussed
the solving of reliability for a stand-alone microgrid with
wind turbine generators and batteries. Reference [36]
studied the effect of control strategy and configuration
scheme on the reliability index of stand-alone microgrid.
Another important aspect of reliability evaluation is
that the correct measure should be adopted to adjust the
output power of generators and loads during analyzing
the state of system. From the viewpoint of the balance

L. QIAO
996
between output power and load demand, besides the best
strategy of cutting load, the optimal strategy of switching
of renewable energy resources and rectification of power
need to be researched. According to the character of dis-
persed collocation and approaching load, when the ar-
ranged overhaul and the unexpected fault occurs, the
system can be divided into several stand-alone sub-mi-
crogrids, meanwhile, the divided principle and reliability
of sub-microgrid should be taken into account. Reference
[37] built an isolated model of distribution network in the
basis of the importance of loads, and aimed at the maxi-
mal equivalent effective load. Reference [38] researched
a computational method of probability of forming iso-
lated island. Reference [39] discussed the effect of iso-
lated island operation on reliability. Reference [40] could
obtain the feasible dividing scheme in shorter time with
the heuristic search, according to the request of load bal-
ance in the isolated mode.
3.4. Research Limitations
Through analyzing the above-mentioned research status,
we can see that there are some limitations around the
theory and method of optimal planning for a stand-alone
microgrid. The main representations are as follows:
1) In the aspect of mathematical model and solution
methods of optimal planning
Up till now, the researches at home and abroad have
not involved the joint planning of power source and
power system, which was the key of taking full advan-
tage of renewable energy resources. Multiple manage-
ment strategies of optimal planning don’t consider the
integrated optimization of single unit capability and
numbers of devices, the combined operation mode of
devices, and the calculation of reserve capacity, and so
on. The results gained only based on the energy balance
in a simulation step are infeasible in some ways.
2) In the aspect of methods of optimal planning in the
uncertain circumstance
Many works focus on the stochastic optimization
based on the uncertainty of probability and ignore the
effect of uncertain factors on the planning results. Be-
sides, because the problem is the joint planning of dis-
tributed generators and power system, and included the
selection of site and capability, the choice of control pa-
rameter and the system structure planning, if a single
layered optimal planning is used, the problem of curse of
dimensionality tends to occur.
3) In the aspect of reliability analysis and innovative
approach for a stand-alone microgrid
In the research of reliability evaluation, the reliability
is often analyzed from the viewpoint of the balance be-
tween supply and demand, and no attention is paid to the
factors of the reliability of non- electric components,
network structure and fault type. However, the reliability
of system can be improved effectively by designing and
installing reasonably the relay protection and automation
devices, so the reliability evaluation of the full system
with generators and electric components should be car-
ried out.
In the aspect of improving the reliability of stand-
alone microgrid, the division of stand-alone microgrid,
the correction measure of load in the condition of fault,
and the switch and adjustment strategy of power of re-
newable energy resources need to be researched in depth.
Compared with the developed countries, our re-
searches focus on the optimal design of complementary
power system structure, the control of rock-bottom de-
vice and the system simulation. Moreover, there is a lack
of the theory and guidance of stand-alone microgrid and
the corresponding optimal tools.
4. Research Prospect
The objective of optimal planning for a stand-alone mi-
crogrid is to look for the planning scheme of isolated
system with distributed generators, and meet the opera-
tional constraints and the load reliability constraints. The
optimal design needs to take account into the control
strategy, so the operation and planning are coupled each
other. Accordingly, the modeling and solving in the
planning become more complex. Future researches
should be developed as follows.
1) Research of multi-objective optimal planning for a
stand-alone microgrid in uncertain circumstances
All kinds of energy resources are not only independent
but also coupled. The emphases of research include the
energy management strategies of stand-alone
sub-microgrid, the modeling method considering uncer-
tainty factors of consumer demand, the condition of re-
newable energy resources and market price, the mathe-
matical model of the multi-objective optimal planning for
a stand-alone microgrid with distributed generators and
the solving method of uncertain planning theory.
2) Reliability evaluation and innovative approach for a
stand-alone microgrid
The reliability of stand-alone microgrid is influenced
by the type of fault, the system structure, the energy
management strategy and the operational mode, conse-
quently the optimal planning scheme needs to be evalu-
ated to advance the innovative measures. When the reli-
ability is analyzed, the intermittence of renewable energy
resources, the uncertainty of load, the variable operation
mode of energy storage system and the fault character of
devices must be investigated. The corrective actions
should be discussed, including the division method of
sub-microgrids, the optimal correction strategy of loads
in the condition of fault, the best switching way of re-

L. QIAO 997
newable energy resources generating equipment and the
power adjusting measure of adjustable units.
3) Two layered uncertain planning method with the re-
liability for a stand-alone microgrid
In the condition that the total capability of system is
known, and the load demand, the reliability index and the
operational restraint are met, Researchers are confronted
with a problem that is how to find a set of optimal deci-
sion variables results in the minimal total expense of in-
vestment, operation and loss and increasing the probabil-
ity of joining up renewable energy resources. Therefore,
the joint planning of power source and power system
should be studied.
The joint optimal planning for a stand-alone microgrid
contains not only the planning of system structure, but
also the optimization of the location and capability of
access of distributed generation. The solving of the
problem is complex, and the multi-layered uncertain
planning method is adopted to decrease the difficulty.
Consequently, the modeling and solving of two layered
uncertain planning should be developed.
5. Conclusions
This paper introduces the characteristics and problems of
the planning of stand-alone microgrid system, and sum-
marizes the general situation of the recent research at
home and abroad and the limitations of the study. Finally,
several problems, existing in the optimal planning of
stand-alone microgrid system, to be urgently solved, are
put forward.
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