Power System Analysis I

TRANSMISSION LINE PARAMETERS

Components of Overhead Lines

Transmission Line Parameters

An overhead transmission line consists of

• conductors,

• insulators,

• support structures,

• and, in most cases, shield wires.

Conductors

Virtually all overhead power transmission lines use bare aluminum conductors because of their economy, good electrical conduction properties and light weight.

For purposes of heat dissipation, overhead transmission line conductors are bare (no insulating cover).

AAC All aluminum conductor

AAAC All aluminum alloy conductor

ACAR Aluminum conductor alloy reinforced

Conductor types

ACSR Aluminum conductor steel reinforced

ACSR/Stalu

One of the most common conductor

types is aluminum conductor,

steelreinforced (ACSR).

Cross section of a composite

conductor 243-AL1/39-ST1A

ACSR consists of layers of aluminum strands surrounding a central core of steel strands. Each layer of strands is spiraled in the opposite direction of its adjacent layer.

Bundle conductors

EHV lines often have more than one conductor per phase; these conductors are called a bundle. Bundle conductors have a lower electric strength at the conductor surfaces, thereby controlling corona. They also have a smaller series reactance.

Configuration of double, triple, five, six and eight bundle conductros

Bundle Conductors

Six-bundle (765kV)

Double-bundle (150kV) Eight-bundle (1000kV)

Spacer

The distance between the conductors in the bundle is maintained by steel or aluminum bars (spacers).

In English units, conductor cross-sectional area is expressed in circular mils (cmil). A

circular mil is the area of a circle that has a diameter of 1 mil. A mil is equal to 1×10 3

in. The cross-sectional area of a wire in square inches equals its area in circular mils

multiplied by 0.7854×10 6.

Insulators

The live conductors are insulated from the towers by insulators which take two basic forms: the pin type and the suspension type.

This type is used for lines up to 33 kV.

Insulators for transmission lines above 69 kV are suspension-type insulators, which consist of a string of discs, typically porcelain. The number of insulator discs in a string increases with line voltage.

Because of continuous low velocity wind flow perpendicular to the phase or ground wires, those wires can start vibrating. This vibration can damage wires, insulators and other connected parts. Therefore often vibration dampers are installed. See photo: red circles.

Support Structures

These serve the purpose of keeping the conductors at a safe height from ground as well as at an adequate distance from each other. The construction of the support is dependent on the cost. The cost takes into account the design and the materials as well as

transportation and labor.

Supports (self-supporting)

concrete wood lattice steel

When specifying towers and lines, ice and wind loadings are taken into account, as well as extra forces due to a break in the lines on one side of a tower.

Highest located HV OHL within Europe (Vorabgletscher, GR), 380 kV

Shield Wires

Shield wires located above the phase conductors protect the phase conductors against lightning. They are usually high-or extra-high- strength steel, Alumoweld, or ACSR with much smaller cross- section than the phase conductors.

1.Insulator. 2.Bundle of two conductors 3.Spacer to hold the two conductors apart. 4.Earth wire at top of tower.

5.The three bundles on one side of the tower make up one electrical circuit. Most lines have two circuits, one each side. 6.Identity plate saying which line it is and who owns it. 7.Anti-climbing device -barbed wire to stop unauthorized climbing

Line Parameters

Line inductance ( L )

Line shunt capacitance ( C )

Line shunt conductance ( G )

Line resistance ( R )

Line Resistance and shunt Conductance

The resistance of the conductor is very important in transmission efficiency evaluation and economic studies.The dc resistance of a solid round conductor at a specified temperature is

𝑅𝑑𝑐 =𝜌𝑙

𝐴

The conductor resistance is affected by three-factors: Frequency, spiraling, and temperature.

When ac flows in a conductor, the current distribution

is not uniform over the conductor cross-sectional area

and the current density is greatest at the surface of the

conductor. This causes the ac resistance to be

somewhat higher than the dc resistance.

This behavior is known as skin effect. At 50 Hz, the ac

resistance is about 2% higher than the dc resistance.

Skin effect

Distribution of current flow in a cylindrical conductor, shown in cross section. For alternating current, most (63%) of the electrical current flows between the surface and the skin depth, δ, which depends on the frequency of the current and the electrical and magnetic properties of the conductor.

Spiraling

Since a stranded conductor is spiraled, each strand is longer than the finished conductor. This results in a slightly higher resistance than the value Rdc

The conductor resistance increases as temperature increases.

𝑹𝟐 = 𝑹𝟏

𝑻 + 𝒕𝟐𝑻 + 𝒕𝟏

-

Because of the above effects, the conductor resistance is best determined from manufacturers’ data.

Temperature

Conductance accounts for real power loss between conductors or between conductors and ground. For overhead lines, this power loss is due to leakage currents at insulators and to corona. Insulator leakage current depends on the amount of dirt, salt, and other contaminants that have accumulated on insulators, as well as on meteorological factors, particularly the presence of moisture. Corona occurs when a high value of electric field strength at a conductor surface causes the air to become electrically ionized and to conduct. Conductance is usually neglected in power system studies because it is a very small component of the shunt admittance.

Conductance