Electrical Power Transmission of Bulk Power

Electrical Power Transmission of Bulk Power is a Seminar Report and PPT on electrical power transmission means high voltage power transmission using DC(Direct current) technique over AC(Alternating Current) Transmission. i.e. HVDC Transmission of bulk power.Most reference materials are used from IEEE (Institute of Electrical and Electronics Engineers) magazine of IEEE (Institute of Electrical and Electronics Engineers) website.Also we try to maintain Report format of documentation is suggested by IEEE (Institute of Electrical and Electronics Engineers)format.

Electrical power Transmission of bulk power

Electrical power Transmission of bulk power.HVDC ( High Voltage Direct Current) Power Transmission over AC(Alternating Current) Transmission( IEEE Format) ( Ms Word ) ( Cover ,Index, Chapters, List of Figures )

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With the development of the large-scale wind turbines and the rich wind energy resource, the progress of the offshore wind farms have become more attractive than the land wind power, especially the large offshore wind farms in the power range of several hundred megawatts are getting into focus. So the HVAC transmission has not met to the requirement of the offshore transmission, and the VSC-HVDC will be the new direction of offshore transmission by its advantage of power control.

The worlds of HVDC are currently taking place. For the first time ever, an off-shore wind farm will be connected to the grid by a dc cable. There are different types of HVDC system like voltage source converter (VSC) HVDC system, line commutated converter (LCC) HVDC, and self commutated HVDC. HVDC is increasingly being used in small-scale projects such as the feeding of off-shore oil and gas platforms. The VSC HVDC is believed to be a strong contender for the flexible transmission system and certainly has a promising future in the ever increasing number of application..

Introduction:

About Bulk Power Transmission

High-voltage direct current (HVDC) systems have been known for more than 50 years. Despite the maturity of the technology, interesting developments in the world of HVDC are currently taking place. In 2009, for the first time ever, an offshore wind farm called Borkum 2 in Germany will be connected to the grid by a dc cable. At the end of the same year, the Caprivi link in Namibia will set a new record for dc voltage in a voltage-source converter (VSC) HVDC system: 350 kV. This project will be the first to use overhead lines in conjunction with VSC. The 400-MW Trans Bay Cable project, scheduled to be in operation by 2010, will be the first application of a multilevel VSC HVDC topology. At the same time, HVDC is increasingly being used in small-scale projects such as the feeding of offshore oil and gas platforms. HVDC has evolved from a product with a very limited scope to a flexible system that can be economically used in an increasingly wide range of applications.

High Voltage Transmission

High voltages cannot be easily used in lighting and motors, and so transmission-level voltage must be reduced to values compatible with end-use equipment. The transformer, which only works with alternating current, is an efficient way to change voltages. The competition between the DC of Thomas Edison and the AC of Nikola Tesla and George Westinghouse was known as the War of Currents, with AC emerging victorious. Practical manipulation of DC voltages only became possible with the development of high power electronic devices such as mercury arc valves and later semiconductor devices, such as thyristors, insulated-gate bipolar transistors (IGBTs), high power capable MOSFETs (power metal–oxide–semiconductor field-effect transistors) and gate turn-off thyristors (GTOs).

About Offshore Wind Farms

Offshore Wind (Electrical power Transmission of bulk power)

Electric energy generated by offshore wind generating facilities requires one or more submarine cables to transmit the power generated to the onshore utility grid that services the end-users of this renewable energy source.

Because the power from the wind turbines is generated as an alternating current (AC) and the on-shore transmission grid is AC, the most straightforward technical approach is to use an AC cable system connection to facilitate this interconnection. Present state-of-the-art and the most cost effective AC technology for this type of interconnection is solid dielectric (also called extruded dielectric or polymeric insulated) cable, usually with cross- linked polyethylene (XLPE) insulation. This is the cable system technology presently used

Need of Earthing

Earthing, particularly for lightning protection, will need to be addressed as offshore structures may be more exposed to positive polarity lightning strokes. Positive downward lightning has higher peak currents and charge transfer, and is likely to be more destructive than the more common negative downward strike. Coupled with the difficulties of offshore access, this may lead to a much higher economic benefit of improved lightning protection. Also for directly connected wind farms with 33 kV collection circuits, some form of reactive power compensation/voltage control may be required.

History of High-Voltage Direct Current(HVDC):

History of HVDC (Electrical power Transmission of bulk power)

The history of dc technology dates back to the end of the 19th century. At that time, a debate was going on about the pros and cons of ac and dc for electricity transmission. Edison strongly advocated dc, whereas Westinghouse, Tesla, and Steinmetz were proponents of ac. This war of the currents was eventually decided in favor of ac. Three-phase ac can easily be transformed to high voltages, allowing far more economical transmission of power and has the advantage that it can produce rotating fields to drive rotating machines. Because of these advantages, dc technology was almost completely suppressed in the transmission of power, a few remnants, such as the small dc network near Pearl Street in New York City that was only shut off in 2007, notwithstanding. Research on dc was never completely abandoned, because, in some cases, dc transmission has a few distinct advantages over ac.

Principals of AC/DC Conversion:

HVDC transmission consists of two converter stations which are connected to each other by a DC cable or DC line. A typical arrangement of main components of an HVDC transmission is shown in figure.

Two series connected 6 pulse converters (12-pulse bridge) consisting of valves & converters transformer are used. The valves convert AC to DC, and the transformers provide a suitable voltage ratio to achieve the desired direct voltage and galvanic separation of the AC & DC systems. A smoothing reactor in the DC circuit reduces the harmonic currents in the DC line, & possible transient over currents. Filters are used to take care of harmonics generated at the conversion. Thus we see that in an HVDC transmission, power is taken from one point in an AC network, where it is converted to DC in a converter station (rectifier), transmitted to another converter station (inverter) via line or cable and injected in to an ac system.

By varying the firing angle & (point on the voltage wave when the gating pulse is applied & conduction starts) the DC output voltage can be controlled between two limits, positive and negative. When α is varied, we get,

Maximum DC voltage when α= 0°

Rectifier operation when 0°< α < 90°

Inverter operation When 90°< α < 180°.

Convertor:

Line-Commutated HVDC

The basic building block of the converter station is the well-known six pulse bridge.

The transformer of a 12-pulse bridge has a star–star–delta three-winding configuration or a combination of star–star and star–delta transformers. Both single- and three-phase units are used as shown in figure.

Six-pulse Converter Topology (Electrical power Transmission of bulk power)

Twelve-pulse Converter Topology (Electrical power Transmission of bulk power)

HVDC with Current-Source Converters (Electrical power Transmission of bulk power)

Self-Commutated HVDC

The principles of self-commutated converters have been known since motor control came into existence. Now that semiconductor technology has developed to the point that switching elements are available that have sufficiently high rating to economically build high-voltage valves, this same theory is applicable to HVDC.

the footprint is several times smaller, mostly because of the small filter requirements. The main drawback is that the losses are higher. This is because of the high-switching frequency of the power electronic components. Until the converter losses can be significantly reduced, long-distance bulk power transmission will remain the appendage of classic HVDC 

HVDC Convertor Transformer 

For six-pulse converter, a conventional 3-phase or three single phase transformers is used. For a 12 pulse converter bridge, the following converter transformer may be used,

2 X 3-phase 2-winding transformers

3 X 1-phase 3-winding transformers

6 X 1-phase 2- winding transformers

Converter transformers are specially designed power transformers differing in many respects from the usual power transformer.

DC Smoothing Reactors

Smoothing reactor is oil filled, oil cooled reactor having a large inductance (0.27H to 1.5H). A DC reactor is normally connected in series with the converter before the DC filter. The main objectives of the reactor are:

To reduce the harmonic currents on the DC side of the converter.

To smoother the ripples the direct current.

To reduce the risk of commutation failures by limiting the rate of rise of the DC line current at transient disturbances in the AC or DC system.

Filter

3 phase bridge converter employed in the HVDC transmission should convert pure AC to pure DC form but in practice the operation of converter generates harmonics in both AC and DC side. The harmonics does not interfere in the converter operation but they flow through the AC and DC line and produce several harmful effect such as overheating of capacitor and generator, over-voltage at points in the networks, interference with protective gear , interference with nearby communication systems.

HVDC converter may produce electrical noise in the carrier frequency band from 20 kHz to 490 kHz. They also generate radio interference noise in mega hertz range of frequency. High frequency filter is mainly aimed to reduce the interference to the power line carrier communication. Such filter is connected between the converter transformer and the station AC bus.

Reactive Source

No reactive power is transmitted over a DC line. However, the converters at the two ends draw reactive power from the AC system. The reactive power balance must be maintained at the two ends to ensure that AC voltage is held within specific limits. This requires installation of reactive compensation equipment in HVDC terminal stations.

Ground Return

A ground return means a return path through the ground. Most DC line use ground for their return path for reason of economy and reliability. The monopolar and homopolar link continuously use ground for the purpose of carrying return current.

Ground Electrode and Station Earth

The midpoint of the converters, called the neutral point, in each station is grounded with a suitable switching arrangement. The earthing is independent of station earthing. This electrode earthing is through electrode installed at a safe distance (about 5 to 25 km) from the terminal station, major pipe line, substations and populated areas. It is installed so far so as to avoid galvanic corrosion of substation earthing system, underground pipes, buried cables and structures.

Control Equipment

The control of firing angle is very important in HVDC system, electrical impulses, for firing, have to be sent simultaneously to all the thyristors connected in series. Since each thyristor is at different potential, the firing impulse must be transmitted to each one by means of insulating medium. A big difference between AC transmission and DC transmission is that power transmission in a DC link is always controlled. The valves of the two converter station are controlled in such a way that one of the station controls voltage and the other station controls current.

VSC BASED HVDC TRANSMISSION:

HVDC with Voltage Source Convertor

HVDC with Voltage-Source Converters (Electrical power Transmission of bulk power)

Voltage Source Converters (VSC) have for the first time been used for HVDC transmission in a real network. Experience from the design and commissioning of the transmission shows that the technology has now reached the stage where it is possible to build high voltage converters utilizing Insulated Gate Bipolar Transistors (IGBTs). Operation and system tests have proved that the properties that have been discussed for many years regarding VSCs for HVDC are a reality now. They include independent control of active and reactive power, operation against isolated ac. networks with no generation of their own, very limited need of filters and no need of transformers for the conversion process. This is only the first installation of VSC for HVDC.

The development of semiconductors and control equipment is presently very rapid and it is evident that this technology will play an important role in the future expansion of electric transmission and distribution systems.

VSC HVDC Equipment

A VSC HVDC system is a complex system that consists of several parts such as valves, transformers, phase reactors, filters, and a control system.

Equipment of a VSC HVDC (Electrical power Transmission of bulk power)

Valves

The valves are one of the most important parts of the VSC HVDC system. The semiconductor used is the IGBT. To increase the current carrying capability, six IGBTs are connected in parallel in a so-called six-pack. A typical IGBT submodule can handle large currents: they are operated at values well above 1 kA but can withstand much higher currents during faults. Water-cooled heat sink, IGBTs, diodes, and control electronics are combined in units that are connected in series to obtain high voltages.

Convertor Topology

HVDC Light

Basic Blocks of HVDC Light(Electrical power Transmission of bulk power)

PWM Technology HVDC Light (Electrical power Transmission of bulk power)

ABB uses a patented method that measures the voltage over each IGBT and applies a different boost signal at the gate depending on this voltage (Figure 10). Currently, all VSC HVDC systems in operation are HVDC Light

VSC HVDC was first commercialized by the ABB Group under the name HVDC Light. HVDC Light uses high frequency pulse-width modulation (PWM) as a switching technique to control the magnitude and phase of the voltage.

HVDC Plus

Siemens recently entered the market with HVDC Plus. The first application of HVDC Plus is the Trans Bay Cable, which will be commissioned in 2010.

HVDC Plus Switching Module (Electrical power Transmission of bulk power)

HVDC Plus (Electrical power Transmission of bulk power)

Advantages of HVDC over AC Transmission

The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1,000 km High-voltage direct current transmission allows efficient use of energy sources remote from load centers. Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I2R power losses when the cable is carrying AC. In addition, AC power is lost to dielectric losses.

DC cable transmissions have lower losses than a corresponding AC cable link. The converter station losses are normally as low as 0.6% per station, and DC cable losses are only around 0.3-0.4% per 100 km.

Long AC cables produce high amounts of reactive power requiring shunt reactors at both ends. In extreme cases the reactive current may seriously reduce the active power transmission capability. These drawbacks do not arise in a DC cable.

DC links can connect two asynchronous power grids in cases where it is impossible or impracticable to establish a synchronous interconnection.

For cable links longer than 40-50 km, DC provides lower Investment costs. The saving gained from installing only one DC cable instead of three AC cables more than compensates for the cost of the AC/DC converter stations.

Disadvantages of HVDC over AC Transmission

The disadvantages of HVDC are in conversion, switching and control. Further operating an HVDC scheme requires keeping many spare parts, which may be used exclusively in one system as HVDC systems are less standardized than AC systems and the used technology changes fast.

The required static inverters are expensive and have limited overload capacity. At smaller transmission distances the losses in the static inverters may be bigger than in an AC transmission line. The cost of the inverters may not be offset by reductions in line construction cost and lower line loss. With two exceptions, all former mercury rectifiers worldwide have been dismantled or replaced by thyristor units. Pole 1 of the HVDC scheme between the North and South Islands of New Zealand still uses mercury arc rectifiers, as does Pole 1 of the Vancouver Island link in Canada.

Applications of HVDC

Wind Farms

Connecting off-shore wind farms is technically challenging. The ac cable connections are problematic for large distances because of two reasons. First, losses of ac cables are high because of dielectric, sheath, and armour losses that are very low or nonexistent in dc cables.

Industrial Networks

VSC HVDC can be used to supply industrial networks. The converter can keep voltage and frequency constant. Disturbances in the grid, such as voltage dips, do not reach industrial installations. With suitable control strategies, voltage dips can be mitigated and power quality significantly improved. In case of disturbances, priority can be given to voltage control.

Weak Networks

It is not desirable to connect weak networks, characterised by a low short-circuit ratio (SCR), with classic HVDC systems. In practice, connections with network with SCR < 2.5 are avoided, as this can give rise to voltage fluctuations, an increasing risk of commutation failures and difficulties in recovering from failures.

CONCLUSION

The transmission of bulk power through HVDC is growing. Expansion of the HVDC network over large distances across the developing countries poses new challenges. The classic HVDC remains a cost effective alternative for such cases. The recent progress in power electronics heralded a new era for the HVDC technology. The VSC HVDC is believed to be a strong contender for the flexible transmission system and certainly has a promising future in the ever increasing number of applications.

Bibliography and References

[1]   Stijn Cole and Ronnie Belmans, ”Transmission of  Bulk Power”,2009 IEEE Industrial Electronics Magazine, September volume 3 number 3,Pg 19-21.

http://www.ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5268182&isnu ber=5268174

[2]   Xingjia Yao, Hongxia Sui, Zuoxia Xing,” The Study of VSC-HVDC Transmission System for Offshore Wind Power Farm”, Proceeding of IEEE International Conference on Electrical Machines and Systems 2007, Oct. 8~11, Seoul, Shenyang University of Technology, Korea.

http://www.ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4412247

[3]   Guanjun Ding, Ming Ding and Guangfu Tang,” An Innovative Hybrid PWM Technology for VSC in Application of VSC-HVDC Transmission System”, 2008 IEEE Electrical Power & Energy Conference.

http://www.ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4763385

[4]   http://www05.abb.com/global/scot/scot221.nsf/veritydisplay/34ec041beda66334 1256fda004c8cc0/$File/03MC0132%20Rev.%2000.PDF

[5]   http://www05.abb.com/global/scot/scot271.nsf/veritydisplay/bc2dc34ae02dae69c1257507004af875/$File/40-43%204M809_ENG72dpi.pdf

[6]   http://www05.abb.com/global/scot/scot221.nsf/veritydisplay/8cf1c44dbc522685c12574e9006051a5/$File/The%20early%20HVDC%20development.pdf

[7]   Jiuping Pan, Reynaldo Nuqui, Kailash Srivastava, Tomas Jonsson, Per Holmberg, Ying-Jiang Hafner” AC Grid with Embedded VSC-HVDC for Secure and Efficient Power Delivery”, IEEE Energy 2030, 17-18 November, Atlanta, USA,2008.

http://www05.abb.com/global/scot/scot221.nsf/veritydisplay/d732fdf148faffe8c15753e0061d8cc/$File/AC%20Grid%20with%20Embedded%20VSCHVDC%20Energy2030_1069-1.pd

[8]   http://www.ptd.siemens.de/PSCC2008_HVDC_VSC_Paper2.pdf

[9]   http://www.usa.siemens.com/answers/en/_assets/Prospects_for_HVDC_Cigre_Madrid_0611_V1.pdf

[10]http://www.energy.siemens.com/hq/pool/hq/power-transmission/HVDC/HVDC-Classic/TSQ-Valvehall-b.jpg

[11] Dr. P.S. Bimbhra, “Power Electronics”, Khanna Publishers, 4th Edition, Pg 249-317, 2004.

Electrical power Transmission of bulk power.HVDC ( High Voltage Direct Current) Power Transmission over AC(Alternating Current) Transmission( IEEE Format) ( Ms Word ) ( Cover ,Index, Chapters, List of Figures )

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