Introduction to Power Sytem

Lecture 1

Instructor: Gary W. Chang

2007. 03. 05

Diagram of the Power Generation, Transmission and Distribution System

Power Generation Plant to Transmission Line A power generation plant is a facility designed to produce

electric energy from another form of energy, such as:

- Heat (thermal) energy generated from: fossil fuels;

coal petroleum natural gas

solar thermal energy geothermal energy nuclear energy

- Potential energy from falling water in a hydroelectric facility- Wind energy - Solar electric from solar (photovoltaic) cells - Chemical energy from:

fuel cells batteries

Major types generating electric power today

Fossil fuel power plant Nuclear power plant Hydroelectric power plant

Geothermal power plant Wind power towers Solar thermal power plant

A substation is a high-voltage electric system facility. It is used to switch generators, equipment, and circuits or lines in and out of a system. It also is used to change AC voltages from one level to another, and/or change alternating current to direct current or direct current to alternating current. Some substations are small with little more than a transformer and associated switches. Others are very large with several transformers and dozens of switches and other equipment. There are three aspects to substations:

Typical substation

Substation Types: Although, there are generally four types of substations there are substations that are a combination of two or more types. Step-up Transmission SubstationStep-down Transmission SubstationDistribution SubstationUnderground Distribution SubstationSubstation FunctionsSubstation Equipment

A step-up transmission substation receives electric power from a nearby generating facility and uses a large power transformer to increase the voltage for transmission to distant locations. A transmission bus is used to distribute electric power to one or more transmission lines. There can also be a tap on the incoming power feed from the generation plant to provide electric power to operate equipment in the generation plant.

A substation can have circuit breakers that are used to switch generation and transmission circuits in and out of service as needed or for emergencies requiring shut-down of power to a circuit or redirection of power.

The specific voltages leaving a step-up transmission substation are determined by the customer needs of the utility supplying power and to the requirements of any connections to regional grids. Typical voltages are:

High voltage (HV) ac:69 kV, 115 kV, 138 kV, 161 kV, 230 kV

Extra-high voltage (EHV) ac:345 kV, 500 kV, 765 kV

Ultra-high voltage (UHV) ac:1100 kV, 1500 kV

Direct-current high voltage (dc HV): 250 kV, 400 kV, 500 kV

Direct current voltage is either positive or negative polarity. A DC line has two conductors, so one would be positive and the other negative.

Step-up AC transmission substation Step-up transmission substation toAC transmission lines

Step-down transmission substations are located at switching points in an electrical grid. They connect different parts of a grid and are a source for subtransmission lines or distribution lines. The step-down substation can change the transmission voltage to a subtransmission voltage, usually 69 kV. The subtransmission voltage lines can then serve as a source to distribution substations. Sometimes, power is tapped from the subtransmission line for use in an industrial facility along the way. Otherwise, the power goes to a distribution substation.

Step-down transmission substation Step-down power transformer

Distribution Substation Distribution substations are located near to the end-users.

Distribution substation transformers change the transmission or subtransmission voltage to lower levels for use by end-users. Typical distribution voltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts.

34,500Y/19,920 volts is interpreted as a three-phase circuit with a grounded neutral source. This would have three high-voltage conductors or wires and one grounded neutral conductor, a total of four wires. The voltage between the three phase conductors or wires would be 34,500 volts and the voltage between one phase conductor and the neutral ground would be 19,920 volts.

From here the power is distributed to industrial, commercial, and residential customers.

Distribution substation

Underground Distribution Substation

Underground distribution substations are also located near to the end-users. Distribution substation transformers change the subtransmission voltage to lower levels for use by end-users. Typical distribution voltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts.

Underground Distribution Substation

Substations are designed to accomplish the following functions, althoughnot all substations have all these functions: Change voltage from one level to another

Regulate voltage to compensate for system voltage changes

Switch transmission and distribution circuits into and out of the grid system

Measure electric power qualities flowing in the circuits

Connect communication signals to the circuits

Eliminate lightning and other electrical surges from the system

Connect electric generation plants to the system

Make interconnections between the electric systems of more than one utility

Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt-amperes in the circuits

Transmission Lines Transmission lines carry electric energy from one point

to another in an electric power system. They can carry alternating current or direct current or a system can be a combination of both. Also, electric current can be carried by either overhead or underground lines. The main characteristics that distinguish transmission lines from distribution lines are that they are operated at relatively high voltages, they transmit large quantities of power and they transmit the power over large distances.

The types of transmission lines are: Overhead Transmission LinesSubtransmission LinesUnderground Transmission Lines

Overhead Transmission Lines

Some typical transmission line structures 345 kv transmission lines

DC voltage transmission lines Double set of transmission lines side by sideTransmission lines above subtransmission below

Subtransmission Lines

Subtransmission lines carry voltages reduced from the major transmission line system. Typically, 34.5 kv to 69 kv, this power is sent to regional distribution substations. Sometimes the subtransmission voltage is tapped along the way for use in industrial or large commercial operations. Some utilities categorize these as transmission lines.

Subtransmission lines

Subtransmission lines with distributionprimaries and secondaries

Subtransmission lines with distribution underbuild

Underground Transmission Lines

Underground transmission lines are more common in populated areas. They may be buried with no protection, or placed in conduit, trenches, or tunnels.

Transmission lines are installed in a tunnel,which enables many circuits in a limited area.

Heat is generated when electricity flows through cables, limiting the power transmission capacity in tunnels. Toincrease the capacity, a tunnel cooling system can be installed. The system above circulates cold water through the tunnels.

Transmission line laid in a trench

Cable installation of underground transmission line

Underground transmission line tunnel

Cross section of underground transmission line

Cable snaking through underground transmission tunnel

Distribution Systems

A distribution system originates at a distribution substation and includes the lines, poles, transformers and other equipment needed to deliver electric power to the customer at the required voltages. Customers are classed as:

Industrial CustomerCommercial CustomerResidential CustomerTransportation Customer

A distribution system consists of all the facilities and equipment connecting a transmission system to the customer's equipment.

A typical distribution system can consist of: SubstationsDistribution Feeder CircuitsSwitchesProtective EquipmentPrimary CircuitsDistribution TransformersSecondaries, and Services

Energy flow through a typical substation

Typical residential service drop Distribution primaries and secondarieson subtransmission pole

Industrial Customer

Most industries need 2,400 to 4,160 volts to run heavy machineryand usually their own substation or substations to reduce the voltage from the transmission line to the desired level for distributionthroughout the plant area. They usually require 3-phase lines to power 3-phase motors.

Industrial facility distribution transformer

Commercial Customer Commercial customers are usually served at

distribution voltages, ranging from 14.4 kV to 7.2 kV through a service drop line which leads from a transformer on or near the distribution pole to the customer's end use structure. They may require 3-phase lines to power 3-phase motors.

Distribution transformer to 3-phase service - commercial facility

Commercial service drop

Residential Customer The distribution electricity is reduced to the end use

voltage (120/240 volts single phase) via a pole mounted or pad-mounted transformer. Power is delivered to the residential customer through a service drop line which leads from the distribution pole transformer to the customer's structure, for overhead lines, or underground.

Residential distribution transformer and service drop Pad-mounted residential distribution transformer

Transportation Customer Currently the only electric transportation systems are

light rail and subway systems. A small distribution substation reduces the local distribution voltage to the transportation system requirements. The overhead lines supply electric power to the transportation system motors and the return current lines are connected to the train tracks.

Public transit train powered by overhead electric lines Electric cables carry electricity

to power the train's motors