HVDC systems(introduction and benefits)

H.soltani.gshahid bahonar uni of kerman

(h.soltani.g.92@gmail.com)

Dec,2016

HVAC benefits:

AC technology was introduced as the standard for generation, transmission and distribution of electrical energy,because:-voltage conversion is simple

allows high power levels

- An AC transformer allows high insulation levels within one unit

has low losses-…

Introduction of HVDC system

• Monopolar links • Bipolar links• Homopolar links

HVDC links can be broadly

classified into:

HVDC System Configurations

Monopolar Links

It uses one conductor .The return path is provided by ground or water.Use of this system is mainly due to cost considerations.

Bipolar Links

Each terminal has two converters of equal rated voltage, connected in series on the DC side.

The junctions between the converters is grounded. If one pole is isolated due to fault, the other pole can operate with

ground and carry half the rated load (or more using overload capabilities of its converter line).

Homopolar Links

It has two or more conductors all having the same polarity, usually negative.

Since the corona effect in DC transmission lines is less for negative polarity, homopolar link is usually operated with negative polarity.

The return path for such a system is through ground.

HVDC Transmission Types

HVDC Transmission Types

HVDC Transmission Types

HVDC Benefits

HVDC Benefits

Economic Aspects

Suitable for long distance power delivery

HVDC Benefits Economic Aspects

But why it is economical in long distance transmission?

Transmitting power over DC lines requires fewer conductors and smaller tower

HVDC Benefits Economic Aspects

An example:

HVDC Benefits Economic Aspects

HVDC Benefits

Technological Aspects

HVDC Benefits Technological Aspects

-For a given cable conductor area, the line losses with HVDC cables is about 50% of that AC cables. This is due to carrying reactive component of current skin-effect

-suitable for asynchronous interconnections

- HVDC can carry more power per conductor

HVDC Benefits Technological Aspects

HVDC Benefits

Environmental Benefits

To recognize the benefits of HVDC let’s compare it with conventional AC technology(according to paper by L. A. Koshcheev (2003) on the potential environmental impacts of HVDC overhead transmission lines in comparison to HVAC lines)

1. Effects of Electric FieldsThe electric field produced by a HVDC transmission line is a combination of the electrostatic field created by the line voltage and the space charge field due to the charge produced by the line’s corona. -The discomfort to humans that is typically felt under HVAC transmission lines is not observed under HVDC lines.

-large machines with rubber tires (such as combine harvesters, automobiles,…) are not electrically charged to dangerous levels when the machines are standing under HVDC overhead lines.

Electric fields around HVDC line

2. Effects of Magnetic Fields-The maximum magnetic field strength of an AC power transmission system varies from 10 to 50 μT (micro Tesla) near the line, while exposure levels at residences, for example, are typically less than 1 μT.ButThe magnetic fields associated with DC lines produce no perceivable effects. The strength of the magnetic field around HVDC transmission lines is in the same range as that of the Earth’s natural magnetic field.

-Unlike AC magnetic fields, which continuously vary in strength and polarity with the associated electric current, DC magnetic fields are of relatively constant strength, orientation, and polarity.

Magnetic field around HVDC line

3. Radio InterferenceThe radio interference caused by electric power transmission lines is the result of the corona discharge around conductors at positive voltages.

-HVDC line radio interference is generated only by positively charged conductors, whereas HVAC interference is generated by all three AC phases.

-AC lines contribute up to a 10 dB (decibel) increase in radio interference under rainy conditions, while DC line radio interference decreases during rain.

-Radio interference levels from HVDC lines are typically 6-8 dB lower than those of HVAC lines of similar capacity.

4.land usage The towers of the DC lines are narrower, simpler and cheaper compared to the towers of the AC lines.

AC Transmission Line

DC Transmission Line

References:

1-ENVIRONMENTAL CHARACTERISTICS OF HVDC

OVERHEAD

TRANSMISSION LINES (Prof. L. A. Koshcheev

, May, 2003)

2-www.energy.siemens.com

3-www.abb.com

4-www. electrical-engineering-portal.com

5-http://www.slideshare.net/zunaibali/dc-transmission?qid=87715dd8-

1cc1-43d4-b51f-8cfbeaee25a9&v=&b=&from_search=1