Wednesday, August 4, 2010

The Xiangjiaba - Shanghai ±800kV UHVDC Transmission Line Project

Recent developments in High Voltage transmission and distribution systems have shown a gradual increase in DC transmission systems at very high voltage levels. High Voltage Direct Current (H.V D.C) has become common in developed countries due to reduced costs at high voltages as compared to Alternating Current (A.C) transmission. It is because of such reasons that the recently completed Xiangjiaba-Shanghai 800kV UHVDC transmission line was built.

 Project Details
Operating Voltage: ±800kV DC
No. of Poles: 2
Connection Point Xiangjiaba: Fulong Substation
Connection Point Shanghai: FengXiang Substation
Ownership: State Grid Corporation of China
Start of Project: December 2007
Commissioning Year: Pole 1 and Bi-pole 2010
Transmission Technology: UHVDC (Ultra High Voltage Direct Current)
Transmission Capacity: 6,400MW (6.4GW)
Length of Overhead DC Line: 2,071km
AC Voltage: 525kV (both ends)
Main Reason for Choosing HVDC: Long Distance and Low Losses
HVDC converter stations: Convert AC to DC and DC to AC
Design Supply and Installation: ABB

This new link is able to meet the electricity needs of about 24 Million people and sets a new benchmark in terms of voltage levels and transmission capacity, superseding the 600kV Itaipu transmission line in Brazil, also by ABB

DC Transmission and Distribution
In DC transmission technology the following terminologies are used:
    <300kV: High Voltage (H.V)
    300kV-765kV: Extra High Voltage (E.H.V)
    >765kV: Ultra High Voltage (U.H.V)
DC technology is used for large transmission and distribution of power at high voltages because at such large quantities, the systems are less expensive and suffer lower power losses. HVDC has found its place due to economic reasons of transmission.

Generation of DC (Direct Current)
Generally there are two methods of producing DC power:
1.    Standard DC Generators
2.    Converting AC to DC Using: Motor Generator Sets
                                                    6phase Mercury Arc Rectifiers
                                                    Rotary Converters
Standard DC Generators
A standard DC Generator is an electrical machine that consists of a magnetic  coil provided by permanent or electromagnets, and a coil of wire of a significant number of turns rotating in the magnetic flux. According to Faradays’ Law of Electromagnetism induction, a DC current shall be produced when the coil is rotated.

Converting AC to DC
The conversion of AC to DC is nowadays used in parallel to AC for specific loads that use strict DC. However this is done at a large scale in HV and EHV.
1.    A motor GenSet is simply a motor driven by AC to provide mechanical power to a generator producing DC.
2.    A rotary converter is a type of electrical machine where a motor-generator machine shares a single rotating armature and set of field coils.
3.    6phase mercury arc converters are solid state devices that are used for producing DC power to be sued by railways and for electrolytic processes.

Transmission & Distribution
DC Power maybe fed and distributed either by:
    2-Wire System
    3-Wire System

2-Wire System
In this system, one wire is the Outgoing or Positive wire and the Outers is the Return or Negative wire. The standard voltage between the conductors is 220V but this may depend in the project involved. This system has much lower efficiency and economy as compared to the 3-wire system.

3-Wire System
This system provides 2 voltages to be available at the consumers end. The Middle/Neutral is earthed at the generator end. It potential is midway between the outers. Motors requiring higher voltages are connected across the Outers whereas lighting and heating circuits requiring less voltage are connected between any of the Outers and Neutral.

Advantages of HVDC over HVAC Transmission
1.    No inductance, capacitance, phase displacement and surge problems in DC
2.    Due to the absence of inductance, the voltage drop in a HVDC line is less than A.C line for the same  load and sending voltage.
3.    No skin effect in DC therefore entire cross section of the line conductor is utilized.
4.    A DC line has less corona effect and reduced interference with communication circuits.
5.    HVDC can used to connect remote generating plants to the distribution grid.
6.    Facilitate power transmission between different countries that use AC at different operating voltages or frequencies.
7.    Synchronize AC produced by renewable energy sources.

Disadvantages of HVDC
1.    High cost of transmission and conversion equipment
2.    HVDC is less reliable and lower availability than AC systems
3.    HVDC circuit breakers are difficult to build because some mechanism must be included in the circuit breaker to force the current back to zero. Otherwise arcing and contact wear would be too great to allow reliable arching.
4.    Operating a HVDC requires many spare parts to be kept, often exclusively for one system since HVDC systems are less standardized.

Costs of HVDC
Normally, manufactures such as Alstom, Siemens and ABB do not state specific costs of information of a particular project since this is a commercial matter between the manufacturer and client. Costs are wide depending on the specifics of the project such as Power rating, Circuit Length, Overhead vs. Underwater route, Land Costs and AC Network improvements required at terminal.
For an 8GW 40km link laid under the English Channel, an approximate Primary equipment costs for a 2000MW 500kV bipolar convectional HVDC link (excluding way-leaving, onshore-reinforcement works, consenting, engineering, insurance etc) is as follows:
    Converter Stations= £110Million
    Subsea Cable + Installation =£1Million/km

THE XIANGJIABA - SHANGHAI ±800kV UHVDC TRANSMISSION PROJECTOwned by the State Grid Corporation of China, the Xiangjiaba-Shanghai Project has the capacity to transmit up to 7,200MW (7.2GW) of power from the Xiangjiaba Hydro plant in S.W China to Shanghai. This new link will meet the electricity need if 24Million people and set a new record in terms of voltage levels and transmission capacity, superseding the 600kV Itaipu transmission line in Brazil.
Transmission line losses on the new line are well under 7%. ABB, being the main technology supplier, is responsible for the overall system design. The scope of delivery of equipment to Xiangjiaba included 28 High and Ultra High Voltage converter transformers. 10 of which were delivered from Sweden and the rest manufactured with ABB components and technology in local partnership.  Other products delivered include: Thyristor Valves, DC and AC Switchyard equipment and the newly developed DCC800 HVDC Control System.

800kV UHVDC will play an important role in providing China with access to remote renewable energy, a key focus are for us”, Says Zheng Bosen, Executive Vice President, SGCC. One thing to note is that, the transmission line which went into commercial operation last month, was actually completed 30 Months 1year ahead of schedule. In response to this Zheng Bosen was very pleased to the good work done by ABB.

Highlights of the Project
1.    The new system voltage ±800kV is 33% higher than the voltage used for Itaipu ±600kV transmission line.
2.    The Power rating, 6,400MW (6.4GW) is more than double the power rating of the most powerful transmission line in operation today.
3.    Line losses shall be reduced to 7% compared with 10% if the line had been built with convectional 500kV DC transmissions.
4.    The overhead line length 2,071km will be one of the longest overhead transmission line in the world, surpassed only by the over 2,500km long Rio Madeira HVDC link currently under construction in brazil. For quite some time the 1,700km long Inga-Shaba HVDC line in Congo-Brazil was the longest transmission line.
An UHVDC link, 2,000 km long is 30% cheaper, partly because it reduced electricity loses by 30% as compared with 500kV DC or 800kV AC technology.

Future Trends in UHVDC
1.    India plans to build 5 UHVDC lines over the next 10years, each with a capacity of 6,000MW          (6 GW).
2.    China is planning one line every year for the next decade, each with a capacity of 5000MW (5GW) or 6,400MW (6.4GW).
3.    There are also plans to install 800kV UHVDC lines in Southern Africa in Brazil.

World Bank briefing document about HVDC systems


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    Wireless Energy Lights Bulb from Seven Feet Away
    Physicists vow to cut the cord between your laptop battery and the wall socket—with just a simple loop of wire

    EUREKA! WIRELESS LIGHT BULB demonstrates a new approach for transmitting power through the air between two coils of copper wire, even when separated by an obstruction [bottom].

    If you thought wireless Internet made life convenient, try wrapping your mind around wireless power. Researchers have successfully lit a 60-watt light bulb by transferring energy through the air from one specially designed copper coil to the bulb, which was attached to a second coil seven feet away [see image at right]. The ultimate goal: to shrink the coils and increase the distance between them so that a single base station emitting "WiTricity," as the inventors refer to the effect, could power a roomful of rechargeable gadgets, each containing its own small coil.

    Physicists have known for more than a century that a moving magnetic field produces an electric field and vice versa in an effect called electromagnetic induction, which makes motors turn and allows your, say, electric toothbrush to recharge when placed on its base station. But induction normally works only at very short distances, which is why the toothbrush and base station must touch.

    Last winter, physicists at the Massachusetts Institute of Technology proposed they could extend induction's reach by projecting a magnetic field from a length of wire coiled so its ends nearly touch. The gap between ends forces electrons to whoosh back and forth through the coild at a specific rate, creating a magnetic field that oscillates at a characteristic frequency. The electrons in a matching coil would have the same frequency, so they would pick up the magnetic field in the same way that a wine glass shatters when an opera singer belts out the right note.

    The MIT physicists expected that the moving electrons—a.k.a. an electric current—would carry enough energy to give a laptop the roughly 30 watts it needs, says team leader Marin Soljacic. As described online today in Science, they tested their theory by building a pair of 30-centimeter-wide copper coils and plugging a light bulb into the receiving coil.

    Although only 40 percent efficient, or about half as much as a laptop battery, the system worked as expected, says team member Aristeidis Karalis. Shrinking the coils will be challenging, he says, because less of the magnetic field will reach them.

    "The big showstopper for this," says theoretical physicist Douglas Stone of Yale University, who was not part of the study, "would be if people, entities or devices that are not supposed to absorb the radiation do absorb it."

    Stone says the most striking thing about WiTricity is its simplicity. "This is an idea that is based on principles that are more than 100 years old," he says. "We're not all thinking about 11 dimensions and the beginning and end of the universe."