Slide 1

Introduction to grid infrastructure:
transmission, distribution, and micro
Prof. Joydeep Mitra
Electrical and Computer Engineering
Michigan State University
[email protected]  517.355.1876
Do not cite or distribute without permission
Topics covered
Grid overview
Major components
Grid operation – normal and abnormal
Emerging technologies
Concluding remarks
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Power system overview
Functional parts of a power system
Source: North American Electric Reliability Corporation (NERC)
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The major functional parts
• Generation system
– generates three-phase power at 4–25 kV
– interfaces with transmission system through the generating station
where voltage is stepped up to 115–765 kV range
• Transmission system
– meshed network transports 3-phase power at 115–765 kV
– interfaces with other transmission lines or sub-transmission lines at
switching stations
– switching stations connecting to sub-transmission system step down
voltage to the 20–69 kV range
• Distribution system
– mostly radial system transports power through primary (3-phase) or
secondary (1-phase) feeders to customers
– connects to the sub-transmission system through the distribution
substation where the voltage is stepped down to 33 kV and below
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The grid
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NERC regions
FRCC - Florida Reliability
Coordinating Council
MRO - Midwest Reliability
NPCC - Northeast Power
Coordinating Council
RFC - ReliabilityFirst
SERC - SERC Reliability
SPP - Southwest Power
Pool, RE
TRE - Texas Regional Entity
Source: Energy Information Administration
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WECC - Western Electricity
Coordinating Council
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Complexity of the power system
• Complex system of numerous mechanical,
electric and magnetic components operating
synchronously at same frequency (50 or 60 Hz).
• Approximately balanced voltages, currents and
power in three phases.
• At every instant of time there must be an
equality between power generated and power
consumed. This is true of both active and
reactive power.
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Major components
• Generators
• Transformers
• Transmission lines / feeders
– Overhead lines
– Underground cables
– HVDC lines
• Loads
• Monitoring, control and protection systems
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• Centralized generators are
rotational electromagnetic
machines, driven by prime
movers (turbines powered by
water, steam or gas).
• Stator has core, three-phase
winding, cooling system; rotor
has core and dc winding.
• Illustration: 800 MVA, 24 kV
(line-to-line) steam turbine
generator; rotor is 44” in
diameter, 277” long and
weighs 133,000 lb.
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• A transformer is an electromagnetic Three-phase transformer
(source: Mitsubishi Electric)
device, converting an ac voltage to a
higher voltage (step up) or a lower
voltage (step down).
• The main transformer itself has no
moving parts, but a core and two or
three windings in every phase.
• Transformers can be air-cooled or oil- Windings
cooled; some may also have fans to
cool the oil.
• Illustration: large three-phase
transformer; windings on each phase.
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• Transmission lines are three-phase, and almost
always overhead lines; distribution feeders can be
three-phase or single-phase, and overhead lines or
underground cables.
• Overhead lines have bare conductors, and shield
wires above for lightning protection.
• Cables are insulated and mostly buried.
• Conductor resistances cause voltage drops and
power losses.
• Long conductors have high inductances, and often
need compensating capacitors.
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Overhead lines
• Overhead lines are bare
conductors, typically ACSR
(Aluminum Conductor, Steel
• Line construction depends on
voltage rating.
• There are about 160,000 miles of
overhead lines in the US.
Source: NationalGrid, UK
ACSR Conductor
Source: American Wire Group
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Underground cables
• Used mostly in distribution systems.
• Sometimes used in transmission systems for
underwater transmission, or in highly urbanized areas.
Source: American Wire & Cable Company, Inc.
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HVDC transmission
• It is difficult to transmit ac over very long
distances because of dynamic stability
• HVDC (High Voltage Direct Current) is used to
transfer large amounts of power over long
• HVDC voltages are in the order of 500 kV.
• Must be connected to the ac grid through
converter stations at each end.
• The longest HVDC line in the world is the
Ingba-Shaba line (Congo); this 500 kV line is
1,100 miles long. In the US, the Pacific DC
intertie (PDCI) carries power from Celilo to
Sylmar at 500 kV and is 846 miles long.
The Pacific DC Intertie
Source: Wikimedia Commons
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• Loads are time-varying in nature, and have
temporal and spatial (geographical) diversity.
• Load composition (HVAC, lighting, digital, etc.)
is also diverse. Induction motors constitute a
very large portion of the total load.
• Most of the load is not controllable, except by
load shedding. However, there are demand
response programs (with limited penetration
at the present time) that allow some control
by the utility / load serving entity (LSE).
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Monitoring, control and protection systems
• The system is continuously monitored and
controlled during normal operation to ensure
real and reactive power balance and constant
• Under abnormal operation, protection
systems attempt to prevent equipment
damage. Some equipment may be disconnected for their safety. Control measures attempt
to ensure stable operation of those parts of
the system that remain in service.
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Normal operation and control
• Unit commitment ensures availability of sufficient
generation to meet anticipated load. Dispatch
balances generation with loads at every instant of
• All generators rotate at constant speeds to maintain
system frequency. Voltage magnitudes throughout
the system must also be within acceptable limits.
• Generated power must equal load plus losses at
every instant of time. A change in load causes a
frequency change, which triggers a governor
response to regulate frequency.
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Competitive market operation
In competitive markets, the functions of generation, transmission
and distribution are performed by independent entities.
Security constrained dispatch is performed by an independent
system operator (ISO).
• The generating company provides the ISO with generation
offers consisting of prices and quantities, for each hour.
• The distribution company aggregate loads for each hour and
provide this information to the ISO. If different rates are
offered, it also submits demand bids.
• The ISO clears the market every hour, based on bids received,
while avoiding transmission congestion. Market clearing and
congestion management may or may not be performed
simultaneously, depending upon the ISO.
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Abnormal operation
In any complex system, abnormal operation is inevitable.
Abnormal operation is caused by system disturbances, such as
switching of components or devices
large changes in load
failure of equipment due to weather, fatigue, or other causes.
Abnormal operating conditions can be
– faults,
– overvoltages, or
– instability.
There are control and protection mechanisms for reducing the impact of
abnormal operation and restoring the system to a normal operating
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Grid reliability
The North American Electric Reliability Corporation (NERC)
defines two components of system reliability:
• Adequacy – Having sufficient resources to provide customers
with a continuous supply of electricity at the proper voltage
and frequency, virtually all of the time. “Resources” refers to a
combination of electricity generating and transmission
facilities, which produce and deliver electricity; and “demandresponse” programs, which reduce customer demand for
• Security – The ability of the bulk power system to withstand
sudden, unexpected disturbances such as short circuits, or
unanticipated loss of system elements due to natural or manmade causes.
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Some grid reliability metrics
• Bulk power and distribution system reliability indices
– Bulk power system reliability indices are based on average
probabilities and frequencies of service interruption. In the
distribution system, load point indices are used.
• Some bulk power system reliability indices
– Loss of Load Probability (LOLP)
• dimensionless
– Loss of Load Expectation (LOLE)
• unit: hours/year
– Loss of Load Frequency (LOLF)
• unit: failures/year
– Expected Unserved Energy (EUE)
• unit: MWh/year
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Interpretation of indices
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Some distribution reliability indices
• SAIFI: system average interruption frequency index
total number of customer interruptions
total number of customers served
• SAIDI: system average interruption duration index
total duration of customer interruptions
total number of customers served
• CAIFI: customer average interruption frequency index
total number of customer interruptions
total number of customers interrupted
• CAIDI: customer average interruption duration index
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total duration of customer interruptions
total number of customer interruptions
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Some more distribution reliability indices
• CTAIDI: customer total average interruption frequency index
total duration of customer interruptions
total number of customers interrupted
• ASAI: average service availability index
customer hours service availability
customer hours service demand
• ASIFI: average service interruption frequency index
connected kVA interrupted
total connected kVA served
• ASIDI: average service interruption duration index
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connected kVA duration interrupted
total connected kVA served
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Microgrids (also known as active distribution
networks) are distribution systems with embedded
generation and storage.
‘Under this vision, integrated clusters of small
(<200kW) DERs provide firm power with a
guaranteed level of power quality through
operation in either grid-connected or island modes.’
(U. S. Department of Energy, “Transmission Reliability Multi-Year
Program Plan FY2001–2005,” July 2001.)
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Distributed energy resources
• Generating Devices
Reciprocating engines
Fuel cells
Geothermal power
Solar power collectors
• Storage Devices
• Combined heat and power
• Interruptible loads
(U. S. Department of Energy, “Transmission Reliability
Multi-Year Program Plan FY2001–2005,” July 2001.)
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Key benefits of microgrids
• Evolution of efficient and environmentally friendly generation
• Reduced stress on transmission network
• Avoidance of expensive and time consuming T&D expansion
(and reduced transmission losses!)
• Increased reliability (service continuity)
• Increased security (resistance to disruption)
• Availability of reliability differentiated products
• Permits use of CHP (combined heat and power) facilities
• Other forms of grid support (ancillary services)
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Emerging technologies
New technologies bring new benefits and new challenges.
• Renewables—wind, solar, tidal, etc.
• Flexible ac transmission systems (FACTS—technology is not so
new, but is finding new applications)
• “Smart Grid” technologies
– Advanced Metering Infrastructure (AMI)
– Phasor Measurment Units (PMU) / Synchrophasor
– “Smart” appliances / high penetration demand response
• Microgrids
• Distribution automation (has been happening for some time)
• Dynamic rating of transmission equipment (concept is not new,
but new devices and controls are being invented)
• New materials in transformers and transmission lines
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Concluding remarks
• Power systems is the oldest and most multi-disciplinary field of
electrical engineering.
• The North American grid had been described as the most
complex machine build by humans; electrification has been
designated as the greatest engineering achievement of the 20th
• Several factors have contributed to the recent renewed interest
in the field:
– Lack of investment in the system during the last three decades of
the 20th century;
– Deregulation;
– Depletion of fossil fuel reserves;
– Increasing consciousness of environmental issues.
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