MICROGRID

1
Its a low-voltage power distribution system integrated with
distributed energy resources (DERs) and controllable loads,
which can be operated with or without the main grid.
2
 Concept of Micro Grid
 Microgrids are small-scale, LV CHPsupply networks designed to supply
electrical and heat loads for a small community, such asa village
locality.
 Microgrid is essentially an active distribution network because it is the
interconnection of DG systems and different loads at distribution
voltage level.
 The generators or microsources employed in a Microgrid are usually
renewable/non-conventional networks with bidirectional electricity
transportation.
 From operational point of view, the microsources must be equipped
with power electronic interfaces (PEIs) and controls to provide the
required flexibility to maintain the specified power quality and energy
output.
3
Differences between - Microgrid and conventional power plant

• Microgrid consist of modular renewable DERs of small

capacity
while conventional power plants consists of large generators

• Power generated from micro sources is directly fed to distribution

network at distribution voltage.

• Micro sources are located close to customers premises ; power

supply can be carried out with less T & D losses and satisfactory
vtg and freq.

4
 Where micro grid suitable

Micro grids are suitable for supplying power

to remote areas where supply from the utility
grid s/m is either difficult to avail due to the
topology or frequently disrupted due to
severe climatic conditions or man made
disturbances.

5
ADVANTAGES
From Utility grid pt of view ,adv of MG is that it is treated
as a controlled entity within Power System. Hence MG can
be integrated into utility grid without hampering security and
reliability of the power utility.

From consumer pt of view ,MG helps the to meet their

power requirement locally with improved power quality
,reliability and reduced feeder losses.

From environmental point of view ,MG reduce the

emission of green house gases and carbon particulates their
by reduces environmental pollution and global warming.

6
ISSUES FACED:

A number of economic, regulatory and technical issues must

be resolved to achieve stable operation of MG.

Problem area that need attention- Climate dependent and

intermittent nature of DERs ,regulations for operating MGs
in synchronism with the power utility and low energy
content of fuels and lack of standards.

Extensive real time and off line researches must be carried

out to study such issues and to solve these problems.

7
How does a micro grid connect to the grid?

A micro grid connects to the grid at a point of

common coupling that maintains voltage at the same
level as the main grid unless there is some sort of
problem on the grid or other reason to disconnect. A
switch can separate the micro grid from the main grid
automatically or manually, and it then functions as an
island.

8
Typical Microgrid
configurations

9
 Typical Microgrid
configurations
 The Microgrid consists of three radial feeders (A, Band C)to supply
the electrical and heat loads.

 It also has two CHPand two non-CHPmicrosources

andstorage devices.

 Microsources and storage devices are connected to feeders A and C

through microsource controllers (MCs).

 The microsources have plug-and-play features. They are provided with PEIs
to implement the control, metering and protection functions during
stand-alone and grid-connected modes of operation.

 Some loads on feeders A and Care assumed to be priority loads (i.e.

requiring
uninterrupted power supply), while others are non- priority loads.

 Feeder Bcontains only non-priority electricalloads.

10
 Modes of operation
The Microgrid is coupled with the main medium voltage (MV) utility grid
(denoted as ‘main grid’) through the PCC (point of common coupling)
circuit breaker CB4asper standard interfaceregulations.
TheMicrogrid isoperated in two modes:
(1) grid-connected
(2) standalone.

In grid-connected mode, the Microgrid remains connected to the main grid

either totally or partially, and imports or exports power from or to the main
grid. In case of any disturbance in the main grid, the Microgrid switches over
to stand-alone mode while still feeding power to the priority loads.
This can be achieved byeither
(i) disconnecting the entire Microgrid by opening CB4or
(ii) disconnecting feeders Aand Copening CB1andCB3.
11
Operation and managementof Microgrid different modes is
controlled through
local micro source controllers (MCs) and the central controller
(CC):
Micro source controller (MC)
 The main function of MC is to independently control the power flow and
load-end voltage profile of the microsource in response to any disturbance
and load changes.
 MC also participates in economic generation scheduling, load
tracking/management and demand side management by controlling the
storage devices.
 It must also ensure that each microsource rapidly picks up its generation to
supply its share of load in stand-alone mode and automatically comes back to
the grid-connected mode with the help of CC.
 The most significant aspect of MC is its quickness in responding to the locally
monitored voltages and currents irrespective of the data from the neighbouring
MCs.
 Control feature facilitates the addition of new microsources at any point of
Microgrid without affecting the control and protection of the existing
units.
it will override the CC directives thatmay seem dangerous for its
23
 microsource.
MC will not interact independently with other MCs in the Microgrid and
(2) Central controller – main
function
• The overall control of
microgridoperation
• objectives
Its Protectionare
through the MCs.
 To maintain specified voltage and frequency at the load end
through power-frequency (p-f ) and voltage control

 To ensure energy optimization for the microgrid.

The CC also performs protection co-ordination and provides the power

dispatch and voltage set points for all the MCs. CCis designed to operate in
automatic or manualmode.

Two main functional modules of CCare

 Energy ManagementModule (EMM)

 Protection Co-ordination Module (PCM). 24

 Energy Management Module

– EMM provides the set points for active and reactive

power output, voltage and frequency to each MC.

It must ensure
(a)Microsources supply heat and electrical loads to customer
satisfaction.

(b)Microgrids operate satisfactorily as per the operational a

priori contracts with maingrid.

(c)Microgrids satisfy its obligatory bindings in

minimising system losses and emissions of greenhouse
gasesand particulates.

(d) Microsources operate at their highest

 Protection Co-ordination
Module
 PCM responds to Microgrid and main grid faults and loss of grid (LOG)
scenarios in away so asto ensure correct protection co-ordination ofthe
Microgrid.

 It also adapts to the change in fault current levels during changeover from
grid- connected to stand-alonemode.
 proper communication between the PCM and the MCs and upstream main grid
controllers. For main grid fault, PCM immediately switches over the Microgrid
to stand-alone mode for supplying power to the priority loads.

 Besides, if the grid fault endangers the stability of the Microgrid, then
PCM may disconnect the Microgrid fully from all main gridloads.

 Under-frequency and undervoltage protection schemes with bus voltage

support are normally used for protecting thesensitive loads.
 PCM also helps to re-synchronise the Microgrid to the main grid after the
initiation of switchover to the gridconnected mode of operation through
suitable reclosing schemes.
26
 The functions of the CCin the stand-alone
mode areas follows:
(1)Performing active and reactive power control of the
microsources in order to maintain stable voltage and
frequency at load ends.

(2)Adopting load interruption/load shedding strategies

using demand side management with storage device
support for maintaining power balance and bus voltage.

(3)Initiating a local black start to ensure improved reliability and

continuity of service.

(4)Switching over the Microgrid to grid-connected mode after main

grid supply is restored without hampering thestability of either grid.
16
The functions of the CCin the
grid-connected mode areas
follows : system diagnostics by collecting information from
(1)Monitoring
the microsources andloads.

(2)Performing state estimation and security assessment evaluation,

economic generation scheduling and active and reactive power
control of the microsources and demand
side functions by using collectedinformation. managemen
t
(3)Ensuring synchronised operation with the main
grid maintaining the power exchange at prioricontract
points.

17
 AC and DC Microgrids
DERs - wind, tidal and hydro produces variable AC
output voltage
DERs – photovoltaic (PV) system and fuel
cells produces DC output voltage
Interconnecting them gives AC or DC microgrids
30
 AC Microgrid systems
 A small AC microgrid is formed within power system by
interconnecting loads and DG units.

 DG units generating DC voltage are connected to the AC microgrid through

DC/AC converter.

 DG units producing AC voltages are connected through a transformer.

 During grid connected mode of operation, the two networks are

interconnected at the PCC, while the loads are supplied from microsources
and if necessary from the utility.

 If power produced by DG system is more than power demand by load,

surplus power will be exported to the utility grid.

 Comparing with conventional power grid, the major difference is the

 AC Mircogrid Systems

PV DG
arrays unit-2
Hydro- DG
Utility grid Micro grid turbine unit-1 MC

AC MC
AC
loads loads DC/AC
converter
PCC
CB2
From
6kV/415V
utility Distribution CB1 LVAC
grid line
transformer CC

CB3
LVAC Sensitive
line load

AC/DC PEI MC

converter
MC DG
WECS
unit-3
DC Storage
loads device

Typical AC Mircogrid configuration

 DC Microgrid
systems
 DC Power system have been Employed for over long distances via sea
cables, industrial power distribution systems, point-to-point transmissions,
telecommunication infrastructures and for interconnecting AC grids of
different frequencies.
 Devices like fluorescent lights, mobile chargers, computers adjustable speed
drives(ASDs), radio and many business and industrial appliances need DC
power for their operation.
 Available AC has to be converted to DC
 In conventional grid systems the DC generated from DGs has to be converted
to AC and connected to network. Then at consumer end, it has to be
converted to DC.
 Results in power loss from DC-AC-DC conversion. To avoid it DC micro
grids are formed, interconnecting loads and DC generating DGs.
 DC micogrid is made attractive due to the technical advancements in HVDC
operation.
DC Mircogrid Systems
Fuel PV
cells arrays
DG AC
Utility grid Micro grid DG
unit-1 loads unit-2
DC
loads MC
DC/AC
converter MC

PCC CB2
From
6kV/415V
utility Distribution CB1 LVDC line
grid
transformer CC

CB3
LVDC line Sensitive
load

DC/AC PEI AC/DC

converter converter

MC
MC

AC Storage
loads DG WECS
device
unit-3

Typical DC Mircogrid configuration

• Currently, LVDC network are coming into existence.
• Low voltage DC links are based on bipolar configuration where loads are
connected between tow polarities or across the positive polarity and the
ground.
• It facilitates
– More DG connections
– Guarantees higher power quality to the consumers.
• Measuring Instruments such as Demand Energy Managements (DEMs),
advanced Metering Infrastructures (AMIs) and protection systems can also be
incorporated into the power converters.
• Integration of these instruments
• Improve power quality
• Reduces system losses and down time
• Reduces protection malfunctions
Power from DC units or substations or storage devices can be transmitted
through

• Monopolar link configuration (single cable)

• Bipolar link configuration (two cables)
• Homopolar link configuration (three or more cables)
 Monopolar DC Link

HVDC CABLE

AC AC/DC DC/AC AC
SYSTE CONVERTER CONVERTER SYSTEM
M

Station 1 Station 2

MONOPOLAR DC LINK CONFIGURATION

 Monopolar DC Link

• Employs one HV conductor with a sea-return or ground-return

• Economic way of power transmission
• High current returning through ground causes corrosion of pipelines
and other buried metal objects.
• Metallic return can also be employed – concerns for
harmonic
interface and/or corrosion exist.
• Operated with negative polarity – as to reduce corona effects.
 Bipolar DC Link

AC
system
Industrial
1-Ф or 3-Ф
supply
AC supply
DC/DC DC/DC
converter converter

+LVDC
AC AC/DC N 220 DC/DC DG
system converter 2V20
converter unit
V
-LVDC
Station
DC/DC
converter
DC load
DC load

Bipolar DC Link configuration

 Bipolar DC Link
• Employs two conductors operating – one at +ve polarity and other at -ve
polarity
• At the ends the converters are grounded.
• Consists of two sets of power converters of equal ratings at each terminal
in series on the DC side.
• Under usual operation both poles works with equal current and so ground
current becomes zero.
• Also facilitates for a little time, the monopolar operation with half power
rating of the devices.
 Homopolar DC Link

AC
system
Industrial
1-Ф or 3-Ф
supply
AC supply
DC/DC DC/DC
converter
converter
+LVDC
AC AC/DC N 320 DC/DC DG
system converter 3V20
converter unit
V
-LVDC

Station
DC/DC
converter
DC load
DC load

Homopolar DC Link configuration

 Homopolar DC Link

• Employs two or more conductors with same polarity.

• Usually –ve polarities with metallic return or ground return is preferred.
• Advantages – reduced insulation cost.
• Disadvantages – earth return
• Employs three-wire system due to its highest efficiency factor for
DC distribution from substation to the consumers.
• Consists of 2 outer wires and 1 neutral wire.
• Voltage is divided between two sets formed by these three wires
 Comparison of AC and DC Micro
grid
Types of Micro grid AC DC
Cost of converters High Low
Controllability Difficult Simple
Difficult to guarantee Guaranteed smooth DC
Reliability power supply

Load availability High Low

Transmission efficiency Low High
Conversion efficiency Low High
Comparison of AC and DC
Microgrids
 Interconnection of micro grid

Utility Energy
grid router

Energy router based interconnecting

framework for the micro grids system
 ENERGY
ROUTER
• Energy router serves as an energy hub to setup an electrical connection
between microgrids and the utility grid.

• Its advantages includes :

1) Resolves the problem of instantaneous energy deficiency or surplus
by complementary energy exchanges between the neighboring
microgrids.
2) Isolation guarantees that any frequency or voltage variation at one end
of the energy router will have no direct impact on the systems on other
sides of energy router.
3) Extensive implementation of the energy routers will encourage the
shift of the power system architecture from the conventional
hierarchical framework to a more interactive and connective
framework.
DYNAMIC INTERACTIONS OF
MICROGRID WITH UTILITY
GRID
 Due to small capacity stability of utility will not affected much
when microgrid is connected.

 With higher penetrations MG influence the security and stability of

utility grid

 Dynamic interactions b/w grid and MG will major issue in management and
operation of both the grids.

 MGs have to be designed properly to take care of their impacts on utility

grid ,such that overall reliability and stability of the whole system is improved
 Technical and Economical Advantages
of Microgrid
• Related to environment – Integration of DERs
• Reduces no. of Thermal and nuclear power stations
• Reduces total particulate and gaseous emission and nuclear waste
• Reduces global warming and environmental pollution

• Related to Operation and Investment – Physical proximity of loads

and microsource helps in
• Enhancing the voltage profile by improving reactive power support
• Reducing T&D Feeder congestion and losses by 3%
• Reducing investments for expansion of generation and transmission systems by proper
asset management.
• Related to reliability and power quality
• Decentralization of power generating units
• Better match of Demand and Supply
• Reducing large-scale generation and transmission
• Enhancing restoration process and minimizing down times through black-start operation of
micro sources.
• Related to economy –
– Utilizes waste heat in CHP mode for heating purpose. Increases energy efficiency
above 80% as compared to conventional power system which has 40% efficiency.
– Integration of several microsources – reduces overall cost.

• Related to Energy Market

• Reduces cost of power
• Microgrids provide supplementary services
• Proper economic balance between DG utilization and network investment decreases
the
long-term electricity prices by about 10%.
 CHALLENGES AND DISADVANTAGES OF MICROGRID

• High cost associated with DERs:

High installation cost of DERs
• Technical difficulties:
 lack of technical in controlling a huge no of
experience microsources.
 Extensive real-time and off line research on management, protection
and control aspects of Microgrids and also on the choice, sizing and
placement of microsources
 lack of proper communication infrastructure in rural areas is a
potential drawback in the implementation of rural Microgrids.
 economic implementation of seamless switching between operating
modes is still a major challenge
 Solutions available for reclosing adaptive protection with
synchronism check are relatively expensive.
 Operational and Management
Issues of a Microgird
• To maintain power quality – balance between active and reactive power
must be maintained
• Operator must choose the mode of operation within the
proper regulatory framework
• Load demand, long term energy balance, generation, storage and supply
of energy must be properly planned.
• Control, protection and metering should be based on SCADA systems
• Economic operation must be guaranteed through generation scheduling,
economic load dispatch and optimal power flow operations
• System security must be maintained through contingency analysis and
emergency operations
• Suitable communication protocols and infrastructure must be employed
for overall energy management, control and protection
Control of Microgrids
 Control of Microgrids - Intoduction
 Wide-range of control is needed to ensure
 optimal operation
 system security,
 emission reduction
 seamless transfer from one operating mode to the other without going against
regulatory requirements and system constraints.

 Common Central Controllers (CC) and Microsource Controllers (MC)are

used to connect individual microsources and storage devices to
microgrids.

 MCs perform local control function of micro sources and storage devices

 CC perform the overall control function of micro grid operation

CC major function is
– to sustain reliability and power quality through voltage (Q-V)
control, power- frequency(p-f) control and protection coordination.
– perform scheduling economic generation from micro sources and
helps to sustain power intake from utility grid at jointly agreed
contract points.
– not only coordinates the protection scheme for the whole micro grid,
but also gives the voltages and power dispatch set points for all the
MCs to meet up the requirements of the consumers.

– guarantees energy optimization for the micro grid and sustains the
specified voltages and frequency profiles for the loads.
Central Controller
•
is designed to work automatic mode with a
provision of manual intervention when it is necessary.
• Monitors the functioning of MCs continuously
through two important modules i.e., the EMM and the
PCM.
 MICROSOURCE CONTROLLER
It guarantees,
• Addition of new microsources without modifying the present
microgrid configuration.
• Connection/disconnection of microgrid to/from the utility grid in
a quick and seamless fashion.
• Independent control of active and reactive power.
• Correction of voltage sag and system imbalances.
• Handling faults without the loss of stability.
• Meeting the necessities of load dynamics of the power utility.
Control functions for microsource controller

• Active and reactive power control

• Voltage control
• Storage requirement for fast load
tracking
• Load sharing through P-f control.
 Control Features of the MCs
Active and Reactive Power Control
• The microsources can be
(i) DC sources like fuel cells and SPV
DC power developed is converted directly into 50 Hz AC

(ii) AC sources like wind turbines and microturbines

The variable frequency AC output is first converted
to DC and then reconverted into 50 Hz AC.
• In both DC/Ac conversion takes place through an inverter (VSI) which is
the main component of the power electronic converter.
 Basic scheme for typical MC

Typical MC compressing of microsource and

power electronics converter.
Active and Reactive Power Control

• VSI in the converter system controls both magnitude (V) and

phase angle (δ1) of the output voltage (V< δ1) at Bus-1(converter
terminal)

• The microsource supplies controlled power to the Bus-

2(microgrid bus) at a voltage of E< δ2 via an inductor of reactance X

• V< δ1 leads E< δ2 by power angle δ,where δ= δ1 – δ2

Active and Reactive Power
Control
• By controlling δ the active power flow (P) is
controlled
• By controlling V the reactive power (Q) is controlled
P=3VE sinδ
2X
Q=3VE (V – E cosδ)
2X Vδ 1 L E
Power electronic
Microsources
converter
δ2
‖ ‖
Basic scheme for Typical MC
 Voltage Control
Why Voltage Control of Microgrid bus essential?
 Voltage Control

• Voltage-reactive power (V-Q) droop controllers can be used to control

circulating currents.
Voltage(V
)

Vset point

Capacitive Inductive
VAR VAR
Q max Q max

Droop characteristics for V-Q droop controllers

 Voltage Control
• The droop controller decreases the local voltage set point when the
microsource reactive currents become capacitive and increases the set
point when the current become mostly inductive.

• V-Q control performed by shunt capacitor banks installed at

substations,
is switching capacitor banks installed along the lines,
substation transformer load tap changers(LTCs), voltage regulators,
FACTS equipment, DERs and DG.
V-Q Control
• Centralized V-Q control: It is integrated with distribution
system which provides the information to the recording system
and determines how to proceed.
• Decentralized V-Q control: It is a stand alone system which
relies on local interaction with various devices associated with
V-Q controller.
 Centralized V-Q controller

Operation Substation

Historian Power flow

module

Volt/VAR
controller
SCADA RTU

Capacitor bank Voltage regulator Volt monitor Recloser

Field
 Decentralized V-Q
controller
Operation Substation

Volt/VAR
controller

Historian SCADA RTU

Capacitor bank Voltage regulator Volt monitor Recloser

Field
Storage Requirement for Fast Load Tracking
• Fast load tracking in stand alone is made possible with the help
of storage devices in Micro grids
• AC storage devices are directly connected to the micro grid bus

• DC storage devices are connected to the DC bus of the micro source

• The MC guarantees proper utilization of storage devices for

rapid load tracking
 Load Sharing Through P-f Control
• Microgrid controllers ensure smooth and automatic change over from grid-connected mode to stand-alone mode and
vice versa as per necessity.
• Local power balance at the new loading during changeover from grid to standalone mode is achieved by changing
the
operating point by exerting local P-f control through MC of each microsource.
• The controller does this autonomously after proper load tracking without waiting for any command from the CC or
neighboring MCs.

• Two modes of operation in P-f control:

(i) Isochronous mode

(ii) Droop mode

 Speed
measurement
unit

Steam Steam Prime

valve mover
Rotating
shaft
w

∆P
value +
[+ = open valve ∆w w ref
∫ KG -1 ∑
- = close valve] -

Basic architecture of isochronous governor

 Isochronous Mode
• Also referred as Frequency control mode .
• In Isochronous mode the machine is not affected by load and regardless of load it will
maintain the frequency.
• Systems are not connected to utility grid in this mode, hence it is necessary to run at least
one machine in this mode to take care of the load variation.
Fig. shows the basic architecture of isochronous governor.
 Isochronous governor varies the input valve to a bring frequency back to
nominal value.
 It cannot be used if more than one generator is electrically connected to the same system.
 A droop feedback is provided additionally to run more than one generating units in parallel
on a governors are provided with a feedback signal that causes the speed error to go to zero
at different values of generator output.
 By adding a feedback loop around the integrator this can be accomplished
 Speed
measurement
unit
Steam Steam Prime
valve
mover
Rotating
shaft
w

∆P +
value
∆w w ref
[+ = open valve ∫ KG ∑ ∑
+ -
- = close valve]
-
+
Load reference
∑ KR
point -

Basic architecture of isochronous governor in droop mode

 Droop Mode
• A new input, the reference is inserted to
load conventional
isochronous mode control.
• Slope of the droop characteristics is determined by the value of G R.

• G is equal to pu change in frequency divided by pu change in

R

unit output.

• Since microgrid frequency decreases with droop regulation, the MC

must integrate control functions to re-establish the operation at rated
frequency with correct load sharing.
• For example, it is assumed that two microsources operating with
their maximum capacities P and P
1max 2maxat common
minimum frequency w min

• In grid-connected mode they operate at a base frequency

w delivering powers P and P respectively
zero 01 02

• With variation in load demand, the microsources work at different

frequencies causing an alternation in relative power angles and the
operating frequency moves to a lower common value with different
proportions of load sharing
Frequency(Hz)

P02 P01
w0
P12 P11

w1

w min

Active power (P)

P2max P1max

Droop characteristics for P-f droop controllers

 Central Controller
The CC applies its control via two modules

 Energy Management Module - EMM

 Protection Co-ordination Module -

PCM
 Energy Management Module
• It incorporates different control functions to achieve finer control,
but increases design complexity.

• Basic Microsource Control Functions

-Voltage control
-Power factor control
-Prime mover speed control
-Frequency regulation
 Voltage control

• By varying the magnitude and phase angle of voltage, the microsources

usually control microgrid loads and their power factors via MCs.

• There is a chance of voltage rise on the feeders on microgrid when

distribution feeders are not fully loaded. MCs monitor the local voltage
rise and give feedback to EMM.

• Main aim- to make the microgrid appear to the utility grid as an

aggregate of loads and microsources working as a controlled unit at
utility pf.
 Power Factor
Control
• Microsources generally do not have any inherent pf control.

• Pf being load dependent, all the MCs are incorporated with

pf control characteristics as a function of load tracking.

• Pf control feature is completely built-in MCs, so that the

control does not need any command from EMM except for
voltage set point.
 Prime Mover Speed Control
• Incorporated for microsources such as wind turbines and
microturbines.

• To accommodate variation of load within the capacity of

microgrid, prime mover of microsource must change its speed
to achieve power balance for new loading. Fuel input should be
varied to achieve this which affects efficiency of prime mover.

• Prime mover speed control must guarantee power generation at

optimum efficiency for microsource.
 Frequency Regulation
• With the help of power electronic devices power generated from
microsources can be converted into any desired frequency.

• In grid connected mode- do not have to exert pf control.

• In stand alone mode- MCs exert this control.

• EMM monitors microgrid frequency. If MCs do not restore the

frequency drop within a set time, the EMM performs fast load
shedding to guarantee microgrid stability.
 EMM Operation in Typical Microgrid

• Number of EMM control functions is restricted

to keep the microgrid simple.

• Minimizes number of feedback signals needed

by EMM from MCs.
 Grid-Connected Operation
EMM control signals are limited to the local voltage and active
power set points.

EMM does not apply extra voltage control that may hinder the
functioning of voltage regulators and shunt capacitors of the
utility grid or with the MCs in microgrid.

Slight voltage rises in microgrid due to light load distribution

feeder will be arrested by utility controllers.
 Stand-alone operation
• Function of EMM
– provide voltage and active power set points for MCs.

• MCs autonomously controls frequency and reactive power

flow through V-Q and P-f drop characteristics.

• EMM- monitors the microgrid frequency and implements fast

load shedding via MCs if frequency is not re-established within
a pre-set time for ensuring system stability.
 Control of Heat Loads
• Higher priority than electrical loads in CHP microsources.

• EMM integrates a priority-setting factor for heat loads in

transmitting signals to the MCs.

• Electric power output is more valuable than heat for several

industrial cogeneration systems.

• Therefore, EMM should set the priority factor as per the

relative importance of heat and electrical loads.
 Energy Optimization with Maximum
Efficiency
• Microgrids must be interconnected to supply a big power pool.

• EMMs should make sure that optimum number of

microsources are running near to their rated capacities during
light load conditions.

• EMM performs this control smoothly because of their prior

knowledge of weather parameters, microsource generation
schedule and process condition and fuel information.
 Energy Storage Management
• EMMs control the non-priority loads by shedding them as and when
essential.

• By shedding the non-priority loads off, energy usage is reduced and

conserved for long term use.

• Only the short-term power needs are supplied by storage devices,

uninterrupted power supply is maintained to provide reliable supply
to prior loads.

• The power reserve for long term need is obtained by shedding

the non-priority loads without damaging the microgrid.
 Optional Control Functions for Intelligent EMM
Intelligent EMMs in a microgrid are used for
1. Focussing on energy consumption
2. Giving an overview of process control systems
3. Investigating energy saving opportunities depending
on weather conditions
4. Optimize the use of microsources and storage
automatically by using real-time market
5. To monitor power consumption
 Optional Control Functions for Intelligent
EMM
• Intelligent EMMs must have intelligent PEIs, wide information
handling capacity and enough communication networks.
• Control algorithm on AI based techniques must be also incorporated
with EMMs.
• They must have remote monitoring and control services.
• EMMs must also provide manual intervention facilities.
• EMMs must be able to manage information, provide operation
guidelines and set points to system operator.
• Their operation should aim at maximising availability, maintaining
high quality service and minimizing downtime.
• They can also supervise and control with SCADA systems.
 Protection Co-ordination Module
(PCM)
Overall protection of microgrid is supervised by PCM
1. Microgrids contain both generating units and loads resulting
in bidirectional flow of power
2. Due to presence of microsources passive distribution network
turns into an active one.
3. When it transforms from grid-connected mode to stand-alone
mode, microsources go through a significant change in its
short circuit capacity.
Typical Microgrid
configurations
 Protection scheme for Grid-Connected
Mode
• Normal Condition
• Utility Grid Fault
• Microgrid Feeder Fault
• Microgrid Bus Fault
• Re-Synchronization
 Protection scheme for Grid-Connected Mode
Normal Condition

• Microgrid remains connected to the utility grid via PCC CB, CBI.

• All the CBs remain closed.

• The loads are fed jointly by the utility grid and the microsources.
Protection scheme for Grid-Connected Mode Contd…

Utility Grid Fault

• By opening CB1, the microgrid disconnects itself from utility
grid.

• CB1 monitors the direction and magnitude of current on each

phase and sends a trip signal to CB1 if current limits go beyond
specified value within a pre-set time.

• Also guarantees that the microsources are not falsely tripped.

Protection scheme for Grid-connected mode Contd….
Microgrid Feeder Fault
• Fault power flow is unidirectional.
• By opening the feeder breaker, faults are cleared simply.
• The breakers have directional over current relays to detect the
faulty zone and clear fault.
• For all the relays the PCM grades relay settings such that the
faulty zone is isolated before all the microsources are
disconnected from the feeder.
• This guarantees microgrid stability and minimum loss of
generation.
 Protection scheme for Grid-connected mode Contd….
Microgrid Bus Fault
• If fault occurs on the microgrid bus, then by opening CB1 the
microgrid would be disconnected from the utility grid.

• PCM grades the CB1 relay to co-ordinate with the upstream

protection in the utility grid in case of any fault within the microgrid.

• CB1 is also graded with respect to the protective devices for the
microsources to minimize loss of generation, spurious tripping and
supply interruption.
Protection scheme for Grid-connected mode Contd….
Re-Synchronization
• It is responsibility of PCM to synchronize and reconnect the
microgrid to the utility grid through synchronism check
schemes.
• This may need a few seconds to minutes, depending upon the
nature of the loads and feeder.
• The PCM contains the control scheme to bring all
microsources into synchronization with utility grid.
• PCM provides both options for manual and automatic re-
synchronization as per requirement.