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The Power Distribution Story

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The basic components of an electric power system consists of:

(1) The electric generating station,

(2) The voltage increasing transformer or substation,

(3) The high voltage transmission lines,

(4) The voltage decreasing transformers or substations,

(5) The primary distribution network, and

(6) The distribution service to the customer.

Components 2 through 4 are termed the transmission system, while 5 and 6 are the distribution system.

The transmission and distribution systems each contribute to carrying electrical energy from generating facilities to the customer. However, the functions are different. The transmission system, or bulk system, is designed to carry large quantities of energy over relatively long distances between the generating stations and to the main substations. Transmission system voltages are generally 115 kV to 230 kV and are called High Voltages (HV). When generating stations are very far from the service areas, Extra High Voltage (EHV) transmission lines are used. These typically range from 345 kV to 765 kV.

High Voltage Direct Current (HVDC) transmission lines are also used in special circumstances. HVDC systems are used to connect power regions together. Since each of the ac systems being connected might have a different voltage and phase, they cannot be directly connected without being synchronized. But, converting each ac system to dc and then connecting the dc systems together avoids the problem of synchronization and can provide current flow in either direction as needed. Several HVDC connections exist between the East and West power systems in the U.S. Other HVDC systems are used where very long transmission distances occur and the cost of the dc lines and towers are cheaper than the ac equivalent. Two dc power lines can carry the power equivalent to a set of three, three-phase lines at the same rms voltage. However, the distance must be great enough to offset the added cost of the ac-dc-ac conversion equipment at each end.

The distribution system is also called the local system. It typically consists of several individual systems, each connected to one or more distribution substations. The distribution system divides and carries energy over a network of low-voltage circuits to each customer. Distribution system voltages range from 120 V to 69 kV. Medium voltage distribution systems (2.4 kV to 69 kV) are used between substations and load centers. Low voltage distribution systems (120 V to 600 V) are used to provide power to residences, small industries, and most individual commercial buildings.

The role of generators and transformers in the power generation system.

The most basic power system components are generators, transformers, transmission lines, busses, and loads. They allow for power to be generated (generators), transformed from one voltage level to another (transformers), transmitted from one location to another (transmission lines), distributed among a number of transmission lines and power transformers (busses), and used by consumers (loads).

Transmission lines, distribution lines and service drop wires.       

Transmission Lines

 Transmission line carries energy more efficiently as voltage increases

o   If the generating station is within the load region, then the required transmission lines are relatively short

o   However, this system lacks reliability

o    Failure of a transmission line means energy cannot be supplied to one part of the region

o   To improve the reliability, additional lines can be constructed between distribution substations

o   The loss of any one of the transmission lines from the generating station will result in increased energy flow over the remaining lines in such a manner that the required energy reaches each of the substations

 

o   Generating plant is located at a considerable distance from the area it serves

o   It reduces the reliability due to the greatly increased exposure of the system to natural hazards

Distribution System

- consists of several individual systems, each connected to one or more distribution substations

- it divides and carries energy over a network of low-voltage circuits to each customer

- distribution system voltages range from 120 V to 69 kV

• Medium voltage distribution systems (2.4 kV to 69 kV)

- are used between substations and load centers

• Low voltage distribution systems (120 V to 600 V)

- are used to provide power to residences, small industries, and most individual commercial buildings

Service Drop Wires

Electric service

- the conductors and equipment that deliver electrical power from the electric power utility’s distribution system to the customers being served

Service entrance


- the interface between the overhead or underground cable from the local transformer where the power utility’s responsibility ends and the building owner’s responsibility begin

Difference between a two-wire and a three-wire service drop wire

Two-wire single phase or simply single phase power systems are defined by having an AC source with only one voltage waveform.

Three-wire single phase or split-phase power system is one with multiple (in-phase) AC voltage sources connected in series, delivering power to loads at more than one voltage.

 

 

 

 

 

 

 

 

 

 

 

 

References

Gustafson, Robert J., and Mark T. Morgan. 2004. Power Generation and Distribution. Chapter 4 in Fundamentals of Electricity for Agriculture, 4rd edition, 73-113.

 

Lumen (n.d.). Electric Generators. Retrieved from https://courses.lumenlearning.com/physics/chapter/

23-5-electric-generators/

 

 

Lumen (n.d.). Generators. Retrieved from https://courses.lumenlearning.com/physics/chapter/23-7-transformers/

 

 

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