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Introduction to grid infrastructure:transmission, distribution, and micro
GRIDSCHOOL 2010MARCH 8-12, 2010 RICHMOND, VIRGINIA
INSTITUTE OF PUBLIC UTILITIESARGONNE NATIONAL LABORATORY
Prof. Joydeep MitraElectrical and Computer Engineering
Michigan State Universitymitraj@msu.edu 517.355.1876
Do not cite or distribute without permission
MICHIGAN STATE UNIVERSITY
Mitra - 02GridSchool 2010
Topics covered
• Grid overview• Major components• Grid operation – normal and abnormal• Microgrids• Emerging technologies• Concluding remarks
Mitra - 03GridSchool 2010
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 regionsFRCC - Florida Reliability Coordinating Council
MRO - Midwest Reliability Organization
NPCC - Northeast Power Coordinating Council
RFC - ReliabilityFirst Corporation
SERC - SERC Reliability Corporation
SPP - Southwest Power Pool, RE
TRE - Texas Regional Entity
WECC - Western Electricity Coordinating CouncilSource: Energy Information Administration
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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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Generator• 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.
Stator
Rotor
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Transformer
• A transformer is an electromagnetic 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-cooled; some may also have fans to cool the oil.
• Illustration: large three-phase transformer; windings on each phase.
Three-phase transformer(source: Mitsubishi Electric)
Windings
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Conductors
• 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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ACSR ConductorSource: American Wire Group
Source: NationalGrid, UK
Overhead lines• Overhead lines are bare
conductors, typically ACSR (Aluminum Conductor, Steel Reinforced).
• Line construction depends on voltage rating.
• There are about 160,000 miles of overhead lines in the US.
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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 problems.
• HVDC (High Voltage Direct Current) is used to transfer large amounts of power over long distances.
• 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
• 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 frequency.
• Under abnormal operation, protection systems attempt to prevent equipment damage. Some equipment may be disconnec-ted 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 time.
• 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 operationIn any complex system, abnormal operation is inevitable.Abnormal operation is caused by system disturbances, such as
– lightning– switching of components or devices– large changes in load– faults– 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 state.
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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 “demand-response” programs, which reduce customer demand for electricity.
• 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 man-made 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
• SAIDI: system average interruption duration index
• CAIFI: customer average interruption frequency index
• CAIDI: customer average interruption duration index
total duration of customer interruptionsSAIDI =
total number of customers served
total number of customer interruptionsSAIFI =
total number of customers served
total number of customer interruptionsCAIFI =
total number of customers interrupted
total duration of customer interruptionsCAIDI =
total number of customer interruptions
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Some more distribution reliability indices
• CTAIDI: customer total average interruption frequency index
• ASAI: average service availability index
• ASIFI: average service interruption frequency index
• ASIDI: average service interruption duration index
customer hours service availabilityASAI =
customer hours service demand
total duration of customer interruptionsCTAIDI =
total number of customers interrupted
connected kVA interruptedASIFI =
total connected kVA served
connected kVA duration interruptedASIDI =
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.)
Microgrids
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Distributed energy resources• Generating Devices
– Reciprocating engines– Microturbines– Fuel cells– Windmills– Biomass– Geothermal power– Photovoltaics– Solar power collectors
• Storage Devices– Batteries– Ultracapacitors– SMES– Flywheels
• 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 technologies
• 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 technologiesNew 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 century.
• 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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