Microgrids and energy storage system: An overview and control aspects

Microgrids and Energy Storage
Systems: An Overview and
Control Aspects
KRISHNAKUMAR R. VASUDEVAN, M.E
DOCTORAL RESEARCHER
POWER QUALITY RESEARCH GROUP
UNIVERSITI TENAGA NASIONAL, MALAYSIA
11/6/2020 POWER QUALITY RESEARCH GROUP 1

Agenda
1. History and Transition of Power System
2. Distributed Energy Sources
3. Energy Storage Systems
4. Microgrid Control Strategies
5. Challenges in microgrid
6. Success stories in India

History
of Power
System

Centralized Power System

Cause for Transition???
(Distributed to Centralized)
1. Location of power plants away from load centre.
i. They are located close to the fuel reserves.
ii. Near ports for easy logistics.
iii. Near water bodies.
2. Economical
◦ Bulk power generation is cheaper than generation through distributed small
power plants.
3. Reliability
i. Diverse mix of technology and interconnection of spatially distributed power
plants.
ii. Ability to ride through contingency events.
4. Use of old power plants for peak load
◦ Old plants with low efficiency can be used to meet the peak demand.

Future Power System

Cause for Transition???
(Centralized to Distributed)
1. High transmission losses in centralized grid.
2. Distributed generation offer high resiliency.
3. Improved end user participation.

AT&C Losses of Countries with
Highest Installed Capacity
Source: International Energy Agency

Microgrids

What is a Microgrid?
The US Department of Energy defines the microgrid as
‘‘a group of interconnected loads and distributed energy resources
within clearly defined electrical boundaries that acts as a single
controllable entity with respect to the grid. A microgrid can connect and
disconnect from the grid to enable it to operate in both grid-connected
or island mode”.

A Typical Microgrid
Source: [4]

Need for Power Electronics
1. Nature of power output – AC/DC
◦ It depends on the source of generation.
2. Non grid friendly AC
◦ Different generation frequency.
◦ Different voltage levels.
3. Maximum power extraction
◦ Maximum power point tracking is achieved through control of power
converters.
4. Control Flexibility
◦ Complete control on real power, reactive power, voltage and frequency.

Distributed
Generation
Sources
(DGS)
1. Solar PV
2. Solar Thermal
3. Wind Electric System
4. Small Hydro
5. Fuel Cell
6. Microturbine
7. Biomass
8. Diesel Generators
Note: DGS are not limited to renewable energy
sources (RES). However, RES is a boon to develop a
sustainable microgrid.

Solar PV
DC-DC DC-AC
ACDC

Solar Thermal Power
Generation; Central Tower

Solar Thermal Power Generation;
Linear Fresnel Collectors

Wind Energy Conversion System

Microturbine Generator

Energy Storage
Systems (ESS)

Energy Storage Systems
1. Stores energy when there is surplus power in the microgrid.
2. Delivers energy when there is a deficit power in the microgrid.
3. So, they act as controllable loads and sources.
4. They are very critical in the operation of microgrids.

Classification of Energy Storage
Systems Based on Technology
Source: [1]

Pumped Hydro Storage (High
Energy)
AC-DC DC-AC
ACAC
Source: [2]

Battery Energy Storage
(High Energy)
DC-DC DC-AC
ACDC
Source: [1]

Thermal Energy Storage
(High Energy)
Source: [1]

Compressed Air Energy
Storage (High Energy)
AC-DC DC-AC
ACAC
Source: [1]

Hydrogen Storage & Fuel Cell
(High Energy)
DC-DC DC-AC
ACDC
Source: [1]

Super Conducting Magnetic
Energy Storage (High Power)
DC-DC DC-AC
ACDC
Source: [1]

Flywheel Energy Storage
(High Power)
AC-DC DC-AC
ACAC
Source: [1]

Super Capacitor
(High Power)
DC-DC DC-AC
ACDC
Source: [1]

Role of Energy Storage in
Microgrid
1. Intermittency Mitigation
◦ Stochastic variation of meteorological variables requires additional reserves.
◦ RES have poor load following capability.
2. Power quality
◦ Fluctuations in power
◦ Voltage and current
3. Maintain Stability
◦ Provide voltage and frequency support.
4. Time Shifting
◦ Nocturnal demand.
◦ Energy arbitrage in a dynamic pricing environment.
5. Black Start & Backup
◦ Starting and synchronization of sources require firm power source.

An Important challenge to
overcome
1. Energy storage systems either have high power capacity or high
energy capacity.
2. Every application demands a storage which has high energy and high
power capacity.
3. None of the energy storage systems possess the ideal requirement.
4. It paved a way for the development of hybrid energy storage
systems.
5. A high power capacity ESS and a low power capacity ESS are
combined to form an ideal ESS.

Microgrid
Operation
Onsite Generation
Central Power Plant
Substation

Types of
microgrid
1. AC
2. DC
3. Hybrid
Note: Another classification
based on location of
microgrid.
1. Urban
2. Rural/Remote
3. Military
4. Industrial
Interlinking
converter
Source: [4]

Control
of
Microgrids

Control Hierarchy of Microgrid
Source: [5]

Role of Hierarchical Control in
Frequency Regulation (An example)

Inner Control or Level Zero
Control
1. Grid forming converters (Voltage Control)
1. Set frequency and voltage reference.
2. Stable sources like Diesel Generator, Fuel Cell.
2. Grid supporting converters (Voltage or Current Control)
1. Regulate the frequency and voltage.
2. Energy Storage Systems.
3. Grid following converters (PQ control)
1. Inject the real and reactive power set by higher control.
2. All renewable energy sources.
Source: [15]

Primary Control
Objectives of Primary Control
1. Frequency stability
2. Voltage stability
3. Plug and Play Capability of DGS
4. Avoid circulating currents

Primary Control
1. Active load sharing (With Communication)
i. Centralized control
• The load is shared equally by all the sources.
• The current references are equal for all the converters.
ii. Master slave
• One converter operates as grid forming converter (Master).
• Other converters (slave) follow the command from the Master.
Source:[6] [7],[9]

Primary Control
1. Active load sharing (With Communication)
iii. Average load sharing
• Weighted average current of all the converters is used as current reference to the individual
converters.
iv. Circular chain
• The converters are cascaded to form a chain.
• The reference of nth converter is determined by the (n-1)th converter.
Source:[6] [7],[9]

Primary Control
1. Advantages of Communication based controls
1. Accurate sharing of power.
2. Accurate frequency and voltage regulation.
2. Disadvantages of Communication based controls
1. High cost of reliable communication link.
2. Complex control structures.
3. Low reliability due to single point of failure.
Source:[6] [7],[9]

Primary Control
2. Droop control ( Without Communication)
i. P-f droop
ii. Q-V droop
Note: There are different variations of droop control refer to [8].
Source:[6] [7],[9]

Primary Control
1. Advantages of droop control
1. Simple control structure with less computation.
2. High reliability due to absence of communication link.
2. Limitations of droop control
1. Degree of freedom is limited to one (Only the droop coefficients).
2. Difficult to achieve trade-off between response and steady state deviation.
3. Droop relation fails for highly resistive networks.
Source:[6] [7],[9]

Secondary Control
Objectives of Secondary Control
Restore the frequency and voltage of the microgrid.
Types of Control
1. Centralized
2. Decentralized
Source:[6] [7],[9]

Centralized Secondary Control
Source: [12]

Decentralized Secondary Control
Source: [12]

Tertiary Control - EMS
(MGCC)
Objectives of Tertiary Control
1. Grid connected mode:
1. Follow the voltage and frequency set points of the grid.
2. Control the power exchanged with the grid.
3. Seamless transfer of modes.
4. Islanding detection.
2. Islanded mode:
1. Performs all the operation of a grid operator in the central grid.
2. Set the frequency and voltage references for the lower control.
3. Day ahead scheduling based on weather forecast. (Optimization)
4. Real time dispatching of DGs. (May not use optimization)

Challenges
in
Microgrids

Operation and Stability Issues
1. Stochastic Generation
1. Renewables are non dispatchable sources.
2. They predominantly operate at maximum power point (MPPT).
3. Off MPPT control results in reduced efficiency.
2. Small Signal Stability
1. Frequency stability
2. Voltage stability
3. No rotor angle stability due to the absence of rotating machines.
Source:[18],[19]

Operation and Stability Issues
3. Low Inertia
Overview:
1. Inertia is the critical entity which holds the stability of power system.
2. Large synchronous machines provide inertia to the grid.
3. Predominantly renewable energy sources and energy storage systems are static .
4. Variable speed wind turbines have hidden inertia.
Effects:
1. Operation of ROCOF relays for a small power unbalance.
2. Increased frequency nadir causes maloperation of under frequency load shedding
control.
Solution:
1. Virtual inertia emulation
2. Hidden inertia emulation
Source:[16],[17]

Economic Issues
1. High capital investment
2. Grid Parity
◦ The cost of energy of RES is higher than the cost of energy from grid.
3. Long payback period
◦ A renewable energy project has a project lifetime of 10 years or more.

Economic Justification
1. Social Cost of Carbon emission.
i. It is the marginal cost of impacts due to emission of an extra tonne of
CO2.
ii. In other words, it is the loss in revenue due to emission of CO2.
iii. India has the highest SCC of 86 $/tCO2.
iv. India faces a loss of about $210 billion/yr.

Economic Justification
2. Revenue generation by feeding in power to the grid.
3. Appropriate valuing of probable revenue generation in a dynamic
pricing environment will attract investments.
4. Exploiting the dynamic price signals
1. Energy Arbitrage
◦ The energy is consumed from the grid during low price period.
◦ The energy is injected to the grid during high price period.
2. Demand Side Management
◦ Control of loads in the microgrid to achieve various objectives.
◦ It can be controlled by microgrid controller.
◦ Users can also control their use.

Research Avenues in Microgrid
1. Microgrid Stability
2. Energy Storage Systems
3. Application of Energy Storage Systems
4. Energy Management Strategies (Tertiary Control)
5. Optimal sizing and placement of DGs
6. Integrated Energy System
7. Islanding detection
8. Protection of Microgrids
9. Heuristics and Metaheuristics play a major role in every field of
microgrid research.

Microgrids
and India
Can you guess the
relevance?
1. In OCT 2017-
24,847,762 (24.85
Million) houses were
yet to be electrified.
2. Bihar, Odisha,
Rajasthan and Uttar
Pradesh- 70% Un-
electrified
households.
3. Is Tamilnadu fully
electrified?
Source: SAUBHAGYA dashboard
Status as on 31 March ‘19

How real is
100%
electrification?
Hilly Hamlets are yet to be
electrified
Aliyar:
Sinnarpathi, Navamalai,
Keelpoonachi and Marapalam,
Veetaikaranpudhur:
Nagaroothu, Erumparai,
Poomathi and Sarkarpathy
Source: Times of India, May 1, 2018

Barriers to 100%
electrification?
1. Geographical constraints
2. Affordability to grid supply
3. High connection costs
4. Unreliable power supply

Success Stories
in India

Few Successful Microgrids Under
Operation in India
Project Location Capacity
(kW)
Source No. of
Houses
Cost of
Electricity ₹
/kWh
Dharnai Solar
City
Dharnai,
Bihar
100 Solar PV 350 12-14
Sagar Island
Microgrid
Sundarbans 26 Solar PV 1400 7
Amrita self-
reliant villages
Kerala 8 Mini hydro 40 -
Biomass energy
for rural India
Karnataka 500 Biomass 80 -
(source [20])

A Road not Taken!
9 kW Biomass-
Water Treatment plant
70% Savings in COE
2 kW Solar PV-
Street Lighting
350 kW Wind
Turbine- 2006
Electricity Trade
₹ 40 lakhs profit
₹ 5 Crore Corpus Power to 8000 residents
1996- Hunger
and Poverty
Stricken Village
2020-
Self Sufficient Village
Odanthurai Village,
Coimbatore, Tamilnadu

Concluding Remarks
1. Developed countries are fighting over the reliable and resilient of
power supply.
2. Whereas, people in developing countries like India are still fighting
for the lifeline power supply to cater for their basic needs.
3. Thus, microgrids are a boon to developing nations to power the
remote communities and decarbonize their grid.

References
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References
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References
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Queries?

Contact
Krishnakumar R. Vasudevan
vasudevkrishna.ceg@gmail.com / vasudevkrishna@ieee.org
+91 7299514208 / +60 163585420

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