R.muthukumar, Analysis of Dynamic Stability of Microgrid

Analysis of dynamic stability model of microgrid: A Survey
R.Muthukumar PG scholar *(Dept of Electrical and Electronic Engineering, Sree Sastha Institute of
Engineering and Technology,Chennai-123)

Abstract— MICROGRIDS, in particular, and smart grids, in
general, are new emerging power distribution infrastructures
with prominent potentials among them are realization of de-mand
response and efficient energy consumption, integration
of distributed energy resources (DERs), and high-reliability
electricity delivery. In a typical microgrid, the power
generation ca-pacity is similar to the maximum total load.
The low inertia of the system provides little margin for error
in the power balance, both active and reactive, and requires
rapid control response to load changes. Microgrid is defined
as the cluster of multiple distributed generators (DGs) such
as renewable energy sources that supply electrical energy.
The connection of microgrid is in parallel with the main grid.
When microgrid is isolated from remainder of the utility
system, it is said to be in intentional islanding mode. In this
mode, DG inverter system operates in voltage control mode
to provide constant voltage to the local load. During grid
connected mode, the Microgrid operates in constant current
control mode to supply preset power to the main grid
Index Terms—Distributed generators, dynamic stability,
energy storage system, microgrid, small-signal model.
.
I. INTRODUCTION
icrogrid is generally defined as a low or medium
voltage distribution networks that comprises various
distributed generations (DGs), storage devices, and
controllable loads.Microgrids are capable of operating
connected to the main utility grid (grid-connected mode) or
they can operate without the presence of a utility grid
(islanded or autonomous mode) without compromising
power quality. DGs, especially those based on renewable
energy sources such as solar and wind can be effectively
integrated into a microgrid to cater for rapid growth in
demand. This is one of the major advantages of forming
microgrids in a distribution network. Energy storage devices
can also be incorporated to enhance power management
strategies in the microgrid. , A microgrid should be able to
operate either in grid-connected or islanded modes.
Islanding occurs once the microgrid is disconnected from the
utility grid during a major disturbance in the main utility grid.
The microgrid should manage the islanded operation
maintaining the standard frequency and voltage Operation
Control and Energy Management of grid connected
Distributed Generation
MICROGRID RESEARCH CLUSTER
There are many research projects about the design,
control, and operation of the microgrid throughout the
world. In USA, the Consortium for Electric Reliability
Technology Solutions (CERTS) has proposed and tested the
control concepts of microgrid [3]. The field test hosted by
American Electric Power (AEP) demonstrates that the
control algorithm performs robustly during a variety of
transient events, such as separation from the utility grid and
the load changes during the microgrid is islanded. In Europe,
the ―MICROGRID‖ project funded by the European
Commission has been finished and the ―MORE
MICROGRID‖ project is in progress. The projects aim at the
increase of penetration of micro-generation in electrical
networks through the exploitation and extension of the
microgrid concept, involving the investigation of alternative
DG control strategies and alternative network designs,
development of new tools for multi-microgrid management.
In Japan, the New Energy and Industrial Technology
Development Organization (NEDO) started three research
projects, which deal with new energy integration to local
power system field test in 2004 [4]
MICROGRID ARCHITECTURE
Basic microgrid architecture is shown in Figure 1. This
consists of a group of radial feeders, which could be part of a
distribution system or a building‘s electrical system. There is
a single point of connection to the utility called point of
common coupling(PCC). Some feeders, (Feeders A-C) have
sensitive loads, which require local generation. The non-critical
load feeders do not have any local generation.
Feeders A-C can island from the grid using the static switch
that can separate in the less than a cycle. In this example
there are four microsources at nodes 8, 11, 16 and 22, which
control the operation using only local voltages and currents
measurements. When there is a problem with the utility
supply the static switch will open, isolating the sensitive loads
from the power grid. Non-sensitive loads (feeder D) rides
through switch will open, isolating the sensitive loads from
the power grid.generation on feeders A, B, and C to meet
the loads‘ on theses feeders. When the microgrid is grid-connected
power from the local generation can be directed to
the non- sensitive loads
M

.
Figure. 1. Microgrid Architecture Diagram.
OPERATIONAL MODES OF MICROGRID
Technical issues such as control, power balance
strategies, operation, protection and storage techniques
differ from one micro-grid to another. The main reasons are
the integration of high number of distributed power
generation units near to the electrical loads, the nature and
size of the micro-generation units, and availability of primary
energy sources for renewable power generation units.
MICROGRID CONFIGURATIONS
Fig. 2 shows a general configuration for power
electronics based microgrids. Normally a microgrid consists
of a static transfer switch (STS), distributed critical and
noncritical loads, multiple DER units with various power
electronics interfaces, protection devices, as well as
measurement, monitoring, and control units. In [12],
microgrid architectures are classified into three categories
depending on their applications which are: i) utility
microgrids, ii) industrial and commercial microgrids, and iii)
remote microgrids. However, recent advances in power
electronics and FACTS systems offer more options for
microgrid configurations with different customer
requirements
MODELLING OF MICROGRID
In this paper ,a comprehensive small-signal model of
a typical microgrid is presented including synchronous
generator, asynchronous generator, and voltage source
converter with corresponding control systems. The model
covers almost all the possible power interfaces of DG and
ESS existing in microgrid or distributed generation system.
All the subsystems are modeled individually and then
combined on a global reference frame. The model of different
kind of microgrids can be obtained easily based on this paper
with proper adjustment of the number and parameters of the
subsystems. The model is analyzed in terms of system
eigenvalues and their sensitivity to different operating state
and control strategies of DG or ESS .It is observed that the
dynamic stability of autonomous microgrid is influenced by
system configuration and the variability of intermittent
generation. It can be improved re- markably with ESS
equipped due to its ability to mitigate the instantaneous
active/reactive power unbalance. Therefore, properly
designed and controlled energy storage which is essential
solution to the reliable operation of microgrid as well the
effective utilization of renewable energy. Results obtained
from the model and eigenvalues analysis were verified
through simulation and experiment on a study microgrid
system. It can be observed that the model and its eigenvalues
successfully denote the relationship between the dynamic
stability and system configuration and the corresponding
operation state. This paper provides a theoretical foundation
for the analysis, configuration and operation of microgrid
especially in autonomous operation mode. It enables the
design and control of microgrids to achieve the objective of
both reliability of power system and effective utilization of
renewable resources.
A. Mathematic Model of DG1
DG1 is mainly composed of wind turbine,
transmission system, asynchronous generator and pitch
control system.
The electrical system of the three-phase symmetrical
asynchronous generator in its arbitrary reference frame of d1-
q1 is given by

Where
are the voltage and current vectors of the stator windings
(sq1,sd1)and the rotor windings (rq1,rd1), respectively, and
matrices and can be derived in [13].
The rotor mechanical model of DG1 is represented as
Where ωb and ωr are the base electrical angular
velocity and the rotor electrical angular velocity ,respectively.
Is the combined inertia constant of rotor and loads. is the
mechanical torque, and Te is the electromagnetic torque given
by
where Xm is the mutual inductance between stator and rotor
windings.
Wind turbine is connected to the asynchronous generator
through a coaxial shaft where the mechanical model can be
regarded approximately as a first-order time-delay system
due to its large physical inertia, as shown in (6):
(6)
Where TH is the inertia time constant.
The concept of slip is defined in synchronous rotating
reference frame by setting
The mathematical model of DG1 can be setup by (2)–
(7). The small-signal state space model of DG1 in the global
reference frame is
B. Mathematic Model of DG2
DG2 consists of a prime mover such as diesel engine
and gas turbine, and a three-phase synchronous generator
with excitation and governor control systems. The electrical
system of the synchronous generator in rotor reference
frame d2-q2 is
are the voltage and current vectors of the stator windings
(sq1,sd1) and the rotor windings(rq1,rd1) , respectively, and
matrices and can be derived in [13]. The rotor mechanical
model of DG2 is represented as
where ωe and ωr are the base electrical angular velocity and
the rotor electrical angular velocity, respectively.H is the
combined inertia constant of rotor and loads.T is the
mechanical torque, and Te is the electromagnetic torque
given by
where Xm is the mutual inductance between stator and rotor
windings
The mathematical model of DG2 can be established , and
transforming the global reference frame pro vided that the
synchronous generator operates at the grid-con-nected
mode.The state equations in the synchronous rotating
reference frame with the relation of ω and δs and given by

Combining two equations, the small-signal model of DG2 in
the global reference frame are
C. Mathematic Model of ESS
Fig.3 illustrates the schematic diagram of ESS, which
consist of battery bank and power electronic circuit with
corresponding control system, and filters. The filters can be
equivalent as the series connection of a resistor and an
inductance for each phase where denotes the lump resistor of
powerline and converter, and the effect of the dead-time of
IGBT bridges. Is the DC-bus voltage and can be considered
constant usually for dynamic analysis, which explains the
concept of VSC.
In the local reference frame of , the power circuit equations is
given by
where VN
DC and VN
QC are the voltage components of VSC
in dc-qc axis frame.Vdc and Vqc are the voltage components
of AC-bus in dc-qc axis frame. And idc and iqc are the current
components of VSC in dc-qc axis, respectively. Based on (14)
the small-signal model of the power circuit of ESS is
represented by
In the global reference frame, where Δδ can be derived
D. Mathematic Model of Network
As shown in Figure the model of network in , stationary
reference frame is represented by
where Rn and Ln are the line impedance of the th branch. is
the th branch current. , , and are the voltage vectors of PCC,
Bus-1, Bus-2 and Bus-3, respectively.Cp is the equivalent
capacitor of the loads,
E. System Model
Based on the above subsystem models of DG1,
DG2, ESS and network, the small-signal model of a multi-
DG microgrid system can be obtained. Fig. 6 describes the
block diagram
The complete small-signal model and system state matrix can
be obtained using the subsystem models de- noted by

Fig. 4. Block diagram of the global small-signal model of a
multi-DG microgrid system
DYNAMIC STABILITY ANALYSIS OF MICROGRID
To investigate the dynamic stability of microgrid
system,system stability margin the corresponding operation
strategies can be obtained by tracking the loci of the eigen
values with the variation of system parameters or steady-state
operating state.
CURRENT REALTIME EXPERIENCE OF
MICROGRID SYSTEM IN SPAIN
The Catalonia Institute for Energy Research (IREC)
microgrid is located in Barcelona, Spain.
The current microgrid power system is composed of:
1) Generation unit. Emulates different types of
generation such as wind and solar, reproducing real behavior,
and in the case of renewable energy sources, reproducing the
variable nature and dependence on external climatological
factors.
2) Energy storage unit. Emulates a storage system
which, according to the needs, can be either a battery or an
electric vehicle.
3) Consumption unit. Emulates the real behavior of
different types of consumption based of sensitive-loads
and/or non-sensitive-loads using various load profiles
Figure 4 Current configurable units of IREC’s microgrid
Figure 5 shows the internal view of the configurable units

BENEFITS OF MICROGRID
Microgrid has below mentioned enormous potential benefits:
i) It optimizes the value of existing production and
transmission capacity
ii) It incorporates more renewable energy
iii) It enables broader penetration of DERs and use of energy
storage options
iv) It reduces Carbon foot prints.
v) It improves power quality
vi) It improves power reliability, operational performance,
asset management and overall productivity of utilities.
vii) It enables two way communication with consumers by
enabling them to manage their energy usage
The traditional or classical system dispatch focused mainly
on:
• Unit commitment scheduling
• Economic dispatch
• Automatic generation control
• Grid security
• Local dispatch with some regional implications.
The market-based system dispatch in a Smart Grid has
additional sophisticated focus areas including:
• Formal day-ahead and real-time tasks
• Unit commitment and economic dispatch with more explicit
transmission security constraints
• Checks and balances to ensure transparency and
consistency
• Large-scale system dispatch that is regional and
multiregional in scope.
• Integration of distributed energy resources and demand
response resources.
• Shifting loads to more efficient generation using demand
response and distributed generation and storage with the aim
of saving energy and reducing carbon emissions.
• Integrating technological advances in transmission to
control power flows (FACTS,SVC, etc.).
CONCLUSION
A critical analysis of the literature review undertaken in
the present paper finally reveals that dynamic stability model
of System has a wide scope in operation and control of
microgrids. It enables the design and control of microgrids to
achieve the objective of both reliability of power system and
effective utilization of renewable resources.
The paper investigates technical & operational challenges
and probable solutions for Hybrid Microgrid systems
comprising various distributed energy resources. The load –
generation balance in such system is achieved through
Source control as well as Load control through microgrid.
FUTURE SCOPE
As seen from the trends in applications of microgrid the
use of microgrid for simulation, operation , control of Microgrid ,
Demand Response , Service restoration, Smart grid control, has been
observed. Micro Grid consists of numerous technologies. Apart from
previously discussed techniques some of the major technologies used
in microgrid can be enumerated as: Reliability, Smart Meters ,Smart
Sensors, Smart Appliances, Real Time pricing, Automatic Meter
Reading (AMR), Outage Management System (OMS), Plug in
Hybrid Electric Vehicles(PHEV), Vehicle to Grid
Technology(V2G), Home and Building Automation, Substation
Automation ,Feeder Automation / Reconfiguration ,Intelligent
Electronic Devices(IED) & their applications for monitoring &
protection , Smart storage (like Battery, SMES, Pumped Hydro,
Compressed Air Energy Storage),Wide Area Measurement
System(WAMS),Phase Measurement Units (PMU), Load
Restoration & Reconfiguration, Information & Communication
Technology (ICT) containing Advanced Metering
Infrastructure(AMI), Home Area Network(HAN), Neighborhood
Area Network (NAN), Wide Area Network (WAN), Bluetooth,
ZigBee, GPS, Wi-Fi, Wi-Max based communication, Cloud
Computing, Cyber Security, Broadband over power line (BPL) along
with Smart Micro Grid and Virtual Generation Technologies.
ACKNOWLEDGMENT
This literature survey paper work was supported in Sree
Sastha Institute of Engineering & Technology and various
research help from Department of Electrical and Electronics
Engineering are gratefully acknowledged
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