solar smart grid

1 © Alexis Kwasinski, 2010
Microgrids and the grid interaction
• Microgrids could have a grid interconnection to
• Improve system economics
• Improve operation
• Improve availability
• With a suitable planning, grid planning can benefit from having
microgrids by
• Reducing conductor’s size
• Improving availability
• Improving stability
• Tools, strategies and techniques for an effective integration of a
microgrid into the main grid:
• Net metering – bi-directional power flow.
• Peak shaving
• Advanced communications and controls
• Demand response (?)

• Interconnection practice / recommendation: IEEE standard 1547
• Potential issues with microgrids integration into the main grid:
• Infrastructure long term planning / economics:
• There is no coordination in planning the grid and
microgrids.
• The grid is planned on a long term basis considering
traditional loads.
• Microgrids may “pop-up” afterwards “without notice.”
• Grid’s planning links economic (cost of grid’s electricity,
future demand…..) and technical aspects (line congestion….)
• Stability: microgrids are variable loads with positive and
negative impedance (they can act to the grid as generators)

• More potential issues with microgrids integration into the main grid:
• Safety: When there is a fault in the grid, power from the
microgrid into the grid should be interrupted (islanding)
• Availability: Microgrids can trigger protections (directional
relays) upstream in the grid and interrupt service to other loads
• Key issue: microgrids are supposed to be independently controlled
cells within the main grid.
• How much independence microgrids should have?
• Does independence apply also to planning?
• How much interaction / communications should be
between the grid and the microgrid?

• Example of microgrid development. Initial condition.
• Equipment and
financial planning is
done with all the load
in the figure in mind.

• Example of microgrid development. Planning issues. A microgrid is installed few years
later.
Transformers
and
conductors
can now be
oversized
(remember
this aspect
for PEV and
PHEV
integration)
Microgrid’s area

• Example of microgrid development. Initial normal power flow direction
Directional
Relay

• Example of microgrid operational issues. New power flow with a microgrid.
Directional
Relay
• The microgrid’s
power trips open the
directional relay
• Is it possible to
change the grid’s
state fast enough to
prevent voltage
collapse due to loss
of stability caused by
the sudden load
changes introduced
by the microgrid?
• What microgrid’s
control action
follows?
• Can the microgrid
stop injecting power
back into the grid
(i.e. prevents
islanding)?
Microgrid’s area

• Example of microgrid operation. Islanding.
• If islanding occurs
the microgrid will
continue to provide
power to a portion of
the grid even though
the grid connection
upstream has been
interrupted.
• Potential issues:
• Utility crews
safety.
• Power quality
at the energized
portion could be
poor. Loads
could be
damaged.
Microgrid’s area
“Island”

• Grid interconnection might be different for dc or ac microgrids
• For ac microgrids, grid interconnection can be done directly, with a
disconnect switch, and a transformer only.
• For dc microgrids an inverter is necessary
• Examples:
CERTS microgrid (ac) NTT Facilities Sendai project (ac and dc)

• dc microgrids integration with the grid
• The interface may or may not allow for bidirectional power flow.
Bidirectional power flow can be needed for:
•`Energy storage
• dc loads

Smart grids
• There are two similar but not equal approaches to the smart grid
concept.
• EU-led vision (customer and environmentally driven):
• Europe’s electricity networks in 2020 and beyond will be:
• Flexible: Fulfilling customers’ needs whilst responding to the
changes and challenges ahead;
• Accessible: Granting connection access to all network users,
particularly for renewable energy sources and high efficiency
local generation with zero or low carbon emissions;
• Reliable: Assuring and improving security and quality of supply,
consistent with the demands of the digital age;
• Economic: Providing best value through innovation, efficient
energy management and ‘level playing field’ competition and
regulation.
“European Technology Platform SmartGrids. Vision and Strategy for Europe’s Electricity Networks of the Future”
European Commission KI-NA-22040-EN-C EUR 22040

Smart grids
• US led vision (security
and consumer driven)
- Motivated by needs in
availability improvements
“The NETL Modern Grid Initiative A VISION FOR THE MODERN GRID”, US DOE

• There are many views of what is In reality, a smart grid is not a single
concept but rather a combination of technologies and methods intended
to modernize the existing grid in order to improve flexibility, availability,
energy efficiency, and costs.
• Smart Grid 1.0:
• Intelligent meters
• Smart Grid 2.0 (“Energy Internet”
enabler):
• advanced autonomous controls,
• distributed energy storage,
• distributed generation, and
• flexible power architectures.
• Distributed generation (DG), flexible power architectures, autonomous
controls and loads constitute local low-power grids (micro-grids).
The smart grid conceptThe smart grid concept

• Centralized
operation and control
• Passive
transmission and
distribution.
• Lack of flexibility
• Vulnerable
Smart grid evolution: dullSmart grid evolution: dull
past/presentpast/present

• Still primarily centralized
control.
• Limited active distribution
network (distributed local
generation and storage). Use
of virtual storage (demand-
response)
• Addition of communication
systems
• More efficient loads
• Flexibility issues
• Somewhat more robust
Smart grid evolution: present/immediateSmart grid evolution: present/immediate
futurefuture

• Distributed operation
and control
• Active distribution
network (distributed local
generation and storage).
• Integrated
communications
• Advanced more efficient
loads
• Flexible
• More robust
Smart grid evolution: FutureSmart grid evolution: Future

Smart grids
• Technologies and concepts:
• Distributed energy resources (generation and storage) are
fundamental parts. They provide the necessary active characteristics
to an otherwise passive grid.
• Advanced and distributed communications. All the grid
components are able to communicate. The grid operates like a
power-Internet (distributed, multiple-redundant, interactive and
autonomous). I.e. a Power-Net.
• Intelligent metering.
• Policies and regulatory actions. Necessary to achieve integration of
all the parts. Inadequate pricing models is a significant barrier to
introduce service-based business models (vs. energy-based).
• Grid modernization.

• DOE view for a smart grid:
- “An electrical grid is a network of
technologies that delivers
electricity from power plants to
consumers in their homes and
offices.”
• A Power-Net expands this
view based on paradigms
from the Internet.
• Some features compared with
conventional power grids:
more reliable, efficient, and
flexible.
The Power-Net

The Power-Net
• Like the Internet, the Power-Net involves diverse and redundant path
for the power to flow from distributed generators to users. Its control
resides in autonomous distributed agents.
• Power is generated in distributed generators, usually from alternative or
renewable energy sources. Power buffers are included to match
generators and loads dynamics. Energy buffers are added to make
variable sources dispatchable.
• Contrary to the Internet, the Power-Net involves a local approach for
power interactions.

• Desired Internet features:
• distributed and autonomous control,
• diverse information routing and redundant data or application
storage,
• performance degradation instead of full failure,
• link transmission rate control through temporary data storage
in buffers.
The InternetThe Internet
.M BT=
Buffer
size
Link
bandwidth
Maximum
(delay) time

• Key aspect: add distributed generation (fuel cells, microturbines, PV
modules, small wind, reciprocating engines) close to the load to make
power grids distribution portion an active electric circuit.
• Autonomous and distributed controls can be implemented with DG.
• Power vs. Energy buffers:
Extending the Internet into SmartExtending the Internet into Smart
gridsgrids
Ultracapacitors
or flywheels
(power buffer)
Batteries
(Energy buffer)
Predicted solar radiation
on PV module
.W PT=

Control and communication issuesControl and communication issues
• Coordination is needed in order to integrate variable generation sources (such
as PV modules) in the grid.
• Centralized control requires significant communication resources (i.e., large
bandwidth spectrum allocation) which in general is not available.
• The alternative is to provide all active nodes with an autonomous control that
allows controlling power interactions with the grid without dedicated
communication links. These more intelligent nodes become agents.
VS.

• Smart grid planning for disaster resiliency must consider disaster
impact on lifelines. During disasters special attention should be paid to
dissimilar ways in which disasters affect different DG technologies.
• Renewable sources do not have lifelines but they are not dispatchable,
they are expensive, and they require large footprints.
• Most DG technologies have availabilities lower than that of the grid.
• DG needs diverse power supply in order to achieve high availabilities.
• DG provides a technological solution to the vulnerable availability point
existing in air conditioners power supply.
•DG provides the active component to grid’s distribution portion,
essential for advanced self-healing power architectures.
Distributed generationDistributed generation
(DG)(DG)

Grid’s behaviorduringGrid’s behaviorduring
disastersdisasters» Power supply issues during disasters is
a grid’s problem transferred to the
load.
» Power grids are extremely fragile
systems.

Grid’s behaviorduringGrid’s behaviorduring
disastersdisasters» Common concept of damage to the electric grid during
disasters:
» Real sustained damage in more than 90 % of the area:

Conventional grid redundancy
• Redundancy is not common in distribution and sub-transmission
portions because redundancy may be very expensive.

Conventional grid diversity
• Diversity implies more than one different components performing
the same function.

Traditional Electricity Delivery Methods: Reliability
• With disasters affecting large areas, grid interconnection and/or
centralized control imply lack of diversity and a single point of failure.

• Lifeline dependencies can be reduced by extended local energy
storage. Lifeline’s effects on availability can be mitigated with diverse
local power generation.
• PVs and wind do not require lifelines but their variable profile leads to
added DG or extensive local energy buffers.
• Performance degradation: voltage regulation or selective load
shedding.
• Advanced (active) distribution through Power Routing Interfaces
Extending the Internet into SmartExtending the Internet into Smart
gridsgrids

• A hybrid ac (solid lines) and dc (doted lines) architecture with
both centralized and distributed generation resources.
Advanced Power Architectures
Monitoring points
Power routers

• Problem: Typical home peak power
consumption is below 5 kW. An electric
vehicle may require 1 kW to be charged in 8
hrs. or up to 8 kW for shorter charging profiles.
Also, PEV and PHEV penetration is not uniform (higher for
neighborhoods with higher economical household income).
Hence, grid’s distribution transformers can be easily overloaded
PEV and PHEV even if charging is done during nighttime.
• DG avoids overloading distribution transformers but economical
issues still need to be addressed
• Combination of DG and energy storage may be a suitable
solution.
Smart grids: PHEV and PEV integration

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