Module 4

1. What is microgrid ?
Microgrid is a group of interconnected loads and distributed energy resources with
clearly defined electrical boundaries that act as a single controllable entity with
respect to the grid .A micro grid can connect and disconnect from thr grid to enable it
to operate in both grid connected or islanding mode
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. 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 voltage and frequency
3. Discuss the benefits of micro grid
Enables grid modernization-Enables integrartion of multiple smartgrid technologies
Enhance the integration of renewable and distributed energy resources
 Facilitates integration of combined heat and power
 Promote energy efficiency and reduces losses by generation near demand
 Potential to reduce large capital investment by meeting increased consumption
with locally generated power
 Potential to reduce peal load
Encourages third party investment in the local grid and power supply
Meets End user needs
  Ensure energy supply for critical load
 Power quality and reliability controlled locally
 Promote demand side management and load levelling
 Promote community energy dependence and allows for community involvement in
the power supply.Designed to meet local needs and increase customer participation
 Technical Features makes it suitable to supply power for any remote areas where
feeding power from gird is difficult or disturbances due to climate or man-made
disturbances,
 From Grid point of view – it’s a controlled entity that can be operated as a single
aggregated load.
 From customers’ point of view - beneficial for locally meeting their electrical/heat
requirements.
o They can supply uninterruptible power, improve local reliability, reduce
feeder losses and provide local voltage support.
 From environmental point of view – reduces environment pollution & global
warming through utilization of low-carbon technology

4. List the major componenets of a microgrid

1.Distribution system
2. DG sources
3. Enerrgy storage
4. Controllers and loads
5.Briefly discuss the microgrid concept
Microgrids are small-scale, LV CHPsupply networks designed to supply electrical and heat
loadsfor asmall community, suchasavillage locality.
Microgrid is essentially an active distribution network because it is the interconnection of
DGsystems 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.
6.Classify the anti Islanding methods
7. List the major issues of microgrid
The key issues of microgrid including:
(1) The planning and design of microgrid (including DER)
(2) Operating characteristics of micro sources
(3) Microgrid operation and its energy management (including energy storage
technology) (4) Interconnection of microgrid to the bulk power system
(5) Island mode of microgrid
(6) Protection of microgrid

8. What is the use of static switches in microgrid

Microgrid is connected to the main utility system through static switches. The static
disconnect switch (SDS) is a key microgrid component for islanding and synchronization; it
can be programmed to trip very quickly on overvoltage, undervoltage, overfrequency,
underfrequency, or directional overcurrent
Usually Static switches of SCRs antiparallel configuration makes bidirectional power flow.
They allow for many open/close operations. They act much faster than conventional circuit
breakers (in the order of half a cycle to a cycle). Sometimes IGBTs are more preferred than
SCR because IGBTs much faster than SCRs and their current is inherently limited. Still
power flow cannot be controlled. There are some conduction losses in the devices. Fast
response DSP based switches and relays is used.
9 What is Passive Islanding method?
.Passive Islanding Methods The passive techniques look for some parameter deviations like
voltage magnitude , rate of change of frequency , harmonics , or Phase angle displacement .
In the islanding mode, these parameters vary largely at the PCC. The difference between the
grid-connected and islanding mode depends on the setting of the threshold values. A lower
setting for the threshold for the permissible disturbances in these quantities may cause
nuisance tripping. On the other hand, if the setting is too high, then the protective devices will
not respond to the islanding condition. The merits of these methods are the fast response and
the absence of system disturbances. However, these methods have the disadvantage of having
a large non-detectable zone (NDZ) and these protective methodologies might have variable or
unpredictable reaction times.

10. What is Passive Islanding method?

Detection Methods Active islanding methods (AIMs) send and receive a periodical
perturbation signal between the PV inverter and grid to make sure that the inverter is still
connected to the power grid. That is to say, with this method, even a small disturbance signal
will become clear and noticeable when inferring the islanding mode of operation so that the
inverter will deal with the power change . On the other hand, these methods, which depend
on using disturbance signals, degrade the power quality because they can cause a variation in
the magnitude of the inverter output current or output frequency. The PV inverter output
current is given by the following equation
Iinv = Im sin(2π f t + θ)
where f is the frequency, Im is the inverter current amplitude, and θ is the initial phase angle.
All the perturbation signals can be set or modified based on these three parameters
11. What are the disadvantages of Microgrid
 In microgrid, that must be considered and controlled voltage, frequency and power quality
parameters to acceptable standards whilst the power and energy balance is maintained.
 Electrical energy needs to be stored in battery banks thus requiring more space and
maintenance.
 The difficulty of resynchronization with the utility grid.

 Microgrid protection is one of the most important challenges facing the implementation of
Microgrids.
 Issues such as standby charges and net metering may pose obstacles for Microgrid.

 Interconnection standards needs to be developed to ensure consistency. IEEE P1547, a

standard proposed by Institute of Electrical and Electronics Engineersmay end up filling the
void.
12. Discuss the need of power electronics devices in microgrid
Microgrids often include technologies like solar PV (which outputs DC power) or
microturbines (high frequency AC power) that require power electronic interfaces like
DC/AC or DC/AC/DC converters to interface with the electrical system. Inverters can play an
important role in frequency and voltage control in islanded microgrids as well as facilitating
participation in black start strategies . The interface with the main grid can be a synchronous AC
connection or an asynchronous connection using a direct current coupled electronic power
converter.

13 write a note on microgrid controllers

  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 and protection
through MCs.
14. Compare Islanding and Anti Islanding
Islanding is the condition in which a distributed generator (DG) continues to power a location even though
external electrical grid power is no longer present. Islanding can be dangerous to utility workers, who may
not realize that a circuit is still powered, and it may prevent automatic re-connection of devices.
Additionally, without strict frequency control, the balance between load and generation in the islanded
circuit can be violated, thereby leading to abnormal frequencies and voltages. For those reasons,
distributed generators must detect islanding and immediately disconnect from the circuit; this is referred to
as anti-islanding.

PART B

1.Explain the key drivers of Microgrid Development

Energy Security
Severe Weather
There is a growing concern that weather-related disruptions will become more frequent and
more severe over time across the United States due to climate change, lending a sense of
urgency to addressing grid resilience. Microgrids can provide power to important facilities
and communities using their distributed generation assets when the main grid goes down.
Cascading Outages
Because electrical grids are run near critical capacity, a seemingly innocuous problem in a
small part of the system can lead to a domino effect that takes down an entire electrical
grid .Microgrids alleviate this risk by segmenting the grid into smaller functional units that
can be isolated and operated autonomously if needed.
Cyber- and Physical Attacks
The grid today increasingly relies on advanced information and communications
technologies, making it vulnerable to cyberattack . The centralized grid also contains large,
complex components that are expensive and slow to replace if damaged. Microgrids, through
their decentralized architecture, are less vulnerable to attacks on individual pieces of key
generation or transmission infrastructure. Natural or man-made electromagnetic pulse (EMP)
events could also have potentially catastrophic results.
Economic Benefits
Infrastructure Cost Savings
Investment in the U.S. electricity grid has not kept pace with generation. As a result, capacity
is constrained in many areas and components are quite old, with 70% of transmission lines
and transformers now over 25 years old. The average power plant age is over 30 years .
Microgrids could avoid or defer investments for replacement and/or expansion.
Fuel Savings
Microgrids offer several types of efficiency improvements including reduced line losses;
combined heat, cooling, and power; and transition to direct current distribution systems to
avoid wasteful DC-AC conversions. Use of absorption cooling technology in a combined
heat and power application could help address summer critical peak electrical demand.
Ancillary Services
Traditional ancillary services include congestion relief; frequency regulation and load
following; black start; reactive power and voltage control; and supply of spinning (due to
their ability to mimic the inertia of traditional generation), non-spinning, replacement
reserves . Power quality (reactive power and voltage harmonics compensation). When
discussing microgrids, intentionally islanded operation should be added to this list
Clean Energy Integration
Need to firm variable and uncontrollable resources
Important clean energy sources to address climate change like solar PV and wind are variable
and non-controllable, which can cause challenges like overgeneration , steep ramping and
voltage control problems for the existing grid if deployed in large quantities. Microgrids are
designed to handle variable generation, using storage technologies to locally balance
generation and loads
2.Categorize the Migogrid based on Power technology
Microgrids can also be classified based on the type of power distribution technology used,
AC, DC, or hybrid. While several considerations go into the eventual selection of a power
technology for the microgrid, the most suitable option is likely to be an AC microgrid
because it needs minimal modifications on the existing installations. However, DC or hybrid
microgrids often have better performance and should be considered for new installation
greenfield microgrids. As of existing installations, AC microgrid technologies dominate when
it comes to integration of local power generation and consumption for both on-grid and off-
grid applications, but as DC microgrid technologies develop further, there is likely to be
greater adoption as well combinations of the two approaches, or hybrid configurations.
AC microgrid.
In an AC microgrid, all the DER (renewable and nonrenewable), energy storage devices and
enduse loads (both AC and DC) are connected to a common AC bus (backbone network).
When the microgrid’s energy generation exceeds all the loads on it, the microgrid can send
out (export) energy to the utility power grid, or charge its energy storage devices, such as
batteries, via a bidirectional AC/DC converter. On the other hand, if the loads on the
microgrid exceed its internal generation, the microgrid can either take energy from the utility
grid or from its own charged energy storage sources. This is the grid-connected (normal)
mode of operation. However, when a fault occurs on the utility grid and the load demands are
not met, the microgrid disconnects itself from the utility grid at the PCC and begins to
function in islanded mode. The merits of AC microgrids include its adaptability that makes it
easier to be integrated with the original AC power infrastructure, and the simplicity of its
voltage transformation. Also, the AC microgrid equipment costs less compared to DC
microgrids. AC microgrids are a feasible option for both cities and rural areas. If disturbances
or faults occur, reliable power can be generated in isolated mode. Consequently, as of today,
the number of AC microgrids far exceeds that of DC microgrids.

DC microgrid.
The operating principle of a DC microgrid is similar to an AC microgrid, but it is connected to a DC
bus. Similar to AC microgrids, DC microgrids can also be operated in grid-connected or isolated
modes. Although it typically has a higher capital cost, the operating costs and system losses are
usually lower due to direct connection to DC loads via single stage power conversion. DC microgrids
are likely to see increased popularity in the coming years as it gives several operational advantages
over AC microgrids. In DC microgrids, the generation and distribution system comprises mostly PV
units, wind turbines, fuel cells, and other renewable energy sources used to meet energy demands.
From its storage devices, it utilizes the DC output voltage, and voltage regulation is better. As there is
no need for frequency control, additional system synchronization is not required.
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.
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
A hybrid AC/DC.
Hybrid microgrid is a combination of both AC and DC microgrid, bidirectional converters, and
control equipment. Hybrid AC/DC microgrids offer the best solution for grid integration of different
DER. A hybrid AC/DC microgrid has minimal conversion losses.
3. With a neat diagram explain the typical configuration , modes of operation
and controllers of microgrid
 Components of Microgrid
Radial Feeders
CHP & non-CHP micro-sources
Controllers – Central (CC), Micro source (MCs)
Circuit Breakers (CB)
Sectionalizing circuit breakers (SCB)
Storage Devices
• Microgrid consists of electrical / heat loads and micro-sources connected through a LV
network
• Heat Loads are placed near sources to avoid heat loss during transportation
• Micro-sources have plug & play features
• They are provided with Power Electronic Interfaces (PEIs) to implement the control, metering
and protection functions during stand-alone and grid-connected modes of operation.
• These features also help seamless transition of Microgrid from one mode to another
• Radial Feeders
• A, B & C
• A & C – have priority loads – needs uninterrupted power supply
• B – have non-priority electrical loads.
Operating Modes of Microgrid
• Grid Connected
• Stand-alone
 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 by either
(i) disconnecting the entire Microgrid by opening CB4
(ii) disconnecting feeders A and C by opening CB1 and CB3.
For option (i), the Microgrid will operate as an autonomous system with all the microsources
feeding all the loads in feeders A, B and C, whereas for option (ii), feeders A and C will
supply only the priority loads while feeder B will be left to ride through the disturbance.
Stand Alone Mode
• Microgrid will be disconnected from entire utility by opening CB4 or by opening CB1 & CB3
so that feeders A & D in which priority loads are connected.
• By opening CB4
- Microsources will feed A, B&C
• By opening CB1 & CB3
- Microsources will feed A & C
- B will be fed from utility grid
Grid Connected Mode
• Microgrid remains connected partially or totally
• Exports or imports power from or to utility grid
• In case of disturbances in utility grid, microgrid switches over to stand-alone feeding power to
priority loads.
Control of of Microgrid
1.MCs – Microsources controllers
• 2. CCs – Central Controllers
MCs – Microsources controllers
• Key function: To control
• voltage profile at local end
• Power flow of micro source independently (Without any communication from the
CC), in response to any changes / disturbances in load.
Assist in Demand Side Management (DSM), load tracing (Management) & economic
generation scheduling by controlling the storage devices.
Makes microgrids to pickup generation rapidly to supply its share of load in stand alone mode and
come back to gird-connected mode automatically with help of central controller.
 Significant Feature of MCs
Quick response to locally monitored currents & voltages irrespective of data from CC or
neighbouring CCs
• Enables plug & play operation of microsources.
• Facilitates addition of new microsources & removal of faulty microsources at any
point of the microgrid without affecting the control & protection of existing units.
• CC directives that are dangerous for its microsources will be overridden by individual
MC.
• Will interact with other MCs in the microgird independently.
Central Controller (CC)
• Main Feature
• Overall control of microgrid operation
• Protection of microgrid through MCs
• CC guarantees energy optimization & maintains specified frequency & voltage at the
consumer though voltage & power-frequency (P-F) control.
• Overall control of microgrid operation
• Protection of micro grid through MCs
• Provides power dispatch and voltage set points for all MCs
• Designed to operate in automatic mode with provision for manual intervention
• Two main functional modules
• Energy Management Module (EMM)
• Protection Co-ordination Module (PCM)

4.Explain the Protection Issues of Microgrid

The main goal of microgrid is to provide quality and reliable supply to the customers. Hence, it is
necessary to isolate the microgrid if fault occurs in main grid and to isolate the minimum faulty
part if fault occurs inside microgrid.

Faults during Grid Connected Mode – During the occurrence of fault in Microgrid the
protection devices in distributed energy resources (DER) must respond only after the activation of
protective devices provided at PCC. With the fault ride through capability (FRT), DER should
continue its work. For fault within the microgrid, the designed protection strategy should
disconnect the faulty portion from the rest of system. The conventional OC protection scheme is
set at fault current of 10-50 times of the full load current. Some non-fault cases result in voltage
unbalance at PCC. which are difficult to identify. Hence, there is a need for much effective
protection schemes to avoid such unwanted situations.
Faults during Island Mode– In this mode, the nature of issue is different from the previous one.
The fault current of an islanded microgrid is of 5 times of the load current.Here, the OC
protection scheme is set to get activated at 2-10 times of the full load current. This can be reduced
to 2-3 times of the full load current for converter based DERs in microgrid. This difference makes
the usage of fuses rather than OC protection devices in microgrid.

Changes in Fault current magnitude– The fault current level is high in grid connected mode
rather than island mode. According to types of DG the fault current contribution also varies. Fault
current of synchronous type DG is 5 times of the rated current and inverter fed DG is 1.5 times of
the rated current. Hence, prediction of fault current is difficult because the magnitude of fault
current depends upon many parameters including the mode of operation, type of DGs and number
of DGs.

Reduction in reach of impedance relay – The occurrence of fault in downstream of the bus DG
connected to utility grid, impedance measured by relay located in upstream is higher than real
fault impedance. This affects grading of relays and causes delay in operation or sometimes relay
does not operate at all.

Bi-directional Power flows – Unidirectional power flow (Substation to load) takes place in
conventional distribution system. The interconnection of DGs in distribution end makes the
power flow reverse and leads to power quality issues, voltage variation and protection issues.

False Tripping – It occurs when a DG is located near to the substation in a feeder and if fault
current in a healthy feeder is supplemented by the DG connected in neighboring feeder then the
protection device in the healthy feeder may isolate the circuit unnecessarily.

Blinding of Protection – When DG is connected to a network and if fault occurs in that feeder,
the impedance of the grid is much higher than the DG impedance. This makes the short circuit
current less than the pickup current of the feeder relay which leads to the failure in detection of
fault.

Re-Synchronisation – After islanding, the process of re-synchronization taking place to

reconnect the microgrid with main grid through the re-synchronization equipment at PCC. This
can be done either in manual mode or automatic mode. The microgrid synchronization schemes
are of three types:

 Active synchronisation
 Passive synchronisation
 Open-transition transfer synch-ronisation

4. Write a note on Islanding detection technics

(i) Islanding operation occurs when DG continues to supply power to the network even if
power is interrupted from the main grid . Islanding is a situation in which a distribution
system becomes electrically isolated from the remainder of the power system, due to a fault at
upstream side or any other disturbance, and yet continues to be energized by the DG
connected to it. Several researchers have proposed many methods for islanding detection.
There are four types of islanding detection techniques: 1) passive, 2) active and 3)
communication based and 4) hybrid techniques. Monitoring of the different system
parameters like voltage, frequency, impedance, THD at any desired location comes under the
passive techniques in which these parameters are compared with the pre-specified threshold
to decide about the islanding. Passive methods are preferred, since they use the information
that is available on the DG without influencing the normal operation of the DG. The major
demerit of passive techniques is dependency on threshold values. For higher threshold value,
islanding situations may not detected properly and for lower threshold value other
nonislanding conditions may be treated as islanding condition. Various passive methods that
have been presented so far are over/under voltage , the rate of change of frequency
(ROCOF) , the rate of change of power (ROCOP) -, total harmonic distortion of current , the
rate of change of voltage (ROCOV) and the phase shift method . In case of active method, a
small disturbance is intentionally introduced into the system and upon the feedback it can be
determined whether islanding occurs or not. But a large change in the system parameters will
occur in case of islanding as the main utility is absent. Active islanding methods have very
small NDZ. But the quality of the power is distorted due to the injection of external
disturbance. Some of the islanding methods are slip-mode frequency shift, active Frequency
Drift, current injection and voltage shift method in case of communication based technique,
communication based islanding detection methods depend on the communication links
between the DGs. This method has negligible NDZs along with the highest possible accuracy
but the drawback is that it is most expensive as it requires the high speed operation.