Lecture 25Power Distribution Overview

Dr. Lei WuDepartment of Electrical and Computer Engineering

EE 431POWER TRANSMISSION AND DISTRIBUTION

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Outline Primary distribution

Radial Loop Primary network systems

Secondary distribution Individual distribution transformer per customer Common secondary main Secondary network Spot network

Transformers in distribution system Distribution system construction

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Distribution System Construction Subtransmission lines

Different voltage levels and topologies, major concerns are costs and reliability

Distribution substation Including costs of all equipment (transformers, CBs, switches,

station buses and insulators, reactors, CT/PT, capacitors, grounding systems…), land and easements, and labors.

Distribution primary feeders Distribution transformers Secondary circuits Service drops

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Distribution Line Construction Distribution lines connect the distribution substation to the

ultimate customer. Overhead distribution line consists of

Conductors Structure to support conducts, including wood/steel poles, pole

foundation, framing (cross arms, brackets, insulator support) and miscellaneous hardware)

Insulators. Underground distribution line consists of

Buried ducts Vaults Cables

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Distribution System Topics Distribution protection – how to protect the distribution system

against faults. Power quality and reliability Interconnection issues

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Distribution Protection

Homework 5 will be posted online.Dr. Lei Wu

Department of Electrical and Computer Engineering

EE 431POWER TRANSMISSION AND DISTRIBUTION

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Outline Objective of distribution system protection Protection devices Time current characteristic (TCC) coordination rules

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Objective of Distribution System Protection The main objectives of distribution system protection are

Minimize the duration of a fault Minimize the number of consumers affected by the fault

Fault could be transient (temporary) or permanent. 75-90% of faults are temporary in nature.

Permanent faults are those require repairs by a repair crew in terms of Replacing damaged devices Removing tree limbs from the line Manually reclosing a circuit breaker or recloser to restore

service.

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Protective Device Protective devices are used to sense fault condition and

disconnect malfunctioning equipment Circuit breakers Reclosers Sectionalizers Fuses Lighting arresters Relays

Overcurrent relay Under-voltage relay Differential relays

Disconnect switches

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Circuit Breakers Include both medium (600 V-34.5kV) and high voltage (above

34.5kV), high current devices that must automatically disconnect faulted equipment

Usually classified by voltage, continuous current, interrupting capacity (maximum fault current the breaker can interrupt without becoming dangerous), and methods of extinguishing arc. When current carrying contacts open, the high electric field

between the contacts causes the gas to ionize and support current flow through it.

Length the arc or open the arc in a medium like air, oil and insulating gas.

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Circuit Breakers Include both medium (600 V-34.5kV) and high voltage (above

34.5kV), high current devices that must automatically disconnect faulted equipment

Usually classified by voltage, continuous current, interrupting capacity (maximum fault current the breaker can interrupt without becoming dangerous), and methods of extinguishing arc. Air circuit breakers Air blast circuit breakers Vacuum circuit breakers Oil circuit breakers Sulphur Hexaflouride circuit breakers

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Reclosers Most transmission and distribution line faults are temporary,

lasting from a few cycles to a few seconds. Recloser is a self-controlled device for automatically interrupting

and reclosing an ac circuit with a reset sequence of openings and enclosures.

Recloser can be programmed to sense an overcurrent, open and reclose several times, and after the preset number of operations remain open (lock out).

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Primary Radial Systems Recloser: Remote operation switch:

Source: http://www.langley-eng.co.uk/langley_products/pole_mounted_switchgear.html

Sectionalizers A device used to automatically isolate faulted line segments

from a distribution system. It counts the number of overcurrent followed by line de-

energization sequences and after a preset number of them it opens and locks out. It must be manually reset after lock out. If the fault is cleared while the reclosing device is open, the

sectionalizer counter will reset to its normal position after the circuit is reclosed.

If the fault persists when the circuit is reclosed, the fault-current counter in the sectionalizer will again prepare to count the next opening of the reclosing device.

If the reclosing devise is set to go to lockout on the fourth trip operation, the sectionalizer will be set to trip during the open-circuit time following the third tripping operation of the reclosing device.

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Sectionalizers Circuit breakers, reclosers, and sectionalizers are used together

to provide better protection of lines.

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Fuses Fuses are one-time devices that must be replaced each time

they open a fault. Low voltage fuses use zinc, copper, or silver, while high voltage

fuses use tin, cadmium, or silver. Current limiting fuses limit the peak fault current, current liming

is valuable because both the heating and mechanical damage caused by a fault is proportional to the square of the current.

The time-current characteristics (TCCs) of a fuse are presented by The minimum-melting curve The total-clearing curve

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Time Current Characteristic (TCC) Coordination Rules

Rule 1: Fuse-fuse coordination: For proper coordination, the total clearing time of the protecting fuse should be less than 75% of the minimum melting time of the protected fuse through the coordinating region.

Rule 2: Recloser Fast Trip-fuse coordination: The clearing time of the recloserfast trip should be less than 75% of the minimum melting time of the fuse throughout the coordination region for fast trip to fuse coordination.

Rule 3: Recloser Slow trip-fuse coordination: The total clearing time of the protecting fuse should be less than 75% of the recloser slow trip sensing time throughout the coordination region.

Rule 4: Recloser Slow trip to recloser slow trip coordination: For proper coordination between two reclosers on their respective slow trips, a suitable margin must be allowed between the curves throughout the coordination interval. This margin should be

Total margin= recloser operating time + error tolerance

An error tolerance of 0.15 seconds is recommended. A typical recloser operating time is 0.1 seconds.

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Time Current Characteristic (TCC) Coordination Rules

5. Circuit breaker-recloser slow trip coordination: A suitable margin must be maintained between the circuit breaker’s protective relay and the recloserslow trip sensing curve. This margin should be

Total margin= recloser operating time + error tolerance + allowance for overtravel (electromechanical relays only).

Overtravel allowance for electromechanical relays can be determined from manufacturer’s data. It is often sufficient to allow 0.1 seconds for overtravel on these relays. No overtravel allowance is needed for computer based relays.

6. Circuit breaker-circuit breaker coordination: Follow Rule #5, with the protecting circuit breaker taking the place of the recloser.

7. Circuit breaker-fuse coordination: Follow Rule #3.

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Lighting Arresters Clip the induced voltage transient caused a lightning strike.

Lighting hits line, setting up an ionized path to ground Lighting strikes power lines about 60-250 times per miles each year on

average in US. A single typical stroke might have 200,000 amps, with a rise time of 10 s,

dissipated in 200 s. Conduction path is maintained by ionized air after lightning stroke energy

has dissipated, resulting in high fault currents (often > 15,000 amps!) Within one to two cycles (16 ms) relays at both ends of line detect high

currents, signaling circuit breakers to open the line nearby locations see decreased voltages

Circuit breakers open to de-energize line in an additional one/two cycles breaking tens of thousands of amps of fault current is no small feat! with line removed voltages usually return to near normal

Circuit breakers may reclose after several seconds, trying to restore faulted line to service 20

Shunting and shielding are two basic methods to protect lines. In shunting method, lightning is permitted to strike the phase

conductors, and the lightning current is shunted to ground either by a flashover of by lightning arresters.

With shielding, a separate conductor ( called overhead ground wire) is installed above the phase conductors, and the lightning current is routed to ground without flowing through the phase conductors.

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