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University of the WITWATERSRAND

Insulation coordination of HVDC lines compared to HVAC lines

ELEN 7009

Alexander Jimmy George (691078)

3/19/2012

COMPARISON BETWEEN THE EMPHASIS OF INSULATION COORDINATION STUDIES FOR HVAC LINES AND FOR HVDC LINES AS PART OF THE MASTERS ENGINEERING DEGREE (POWER ENGINEERING).

Insulation coordination of HVDC lines compared to HVAC lines 2012

Abstract

High Voltage Direct Current, despite being invented first, was always the second choice to Alternating Current (AC) mainly due to voltage control through transformers and the improvement of induction motors during the 20th Century, this led to the appeal and common use of AC transmission. The use of HVDC transmission has been found to be more efficient, environmentally friendly, and economically attractive for power transmission over great distances. Inductive and capacitive parameters do not limitthe transmission capacity or the maximum length of a DC overhead line or cable. The conductor cross section is fully utilized because there is no skin effect. The use of DC requires more specialized equipment compared to AC. This report mainly focuses on the comparison of HVDC transmission to HVAC transmission regarding the above mentioned factors.

University of the Witwatersrand Page 2


Content

Abstract…………………………………………………….…………………………………………………………………………..2Content………………………………………………………………………………………………………………………………….3

1. Introduction to HVDC Transmission……………………………………………………………………………….………42. Principle of transmission for HVDC compared to HVAC……………………………………...…….….….……5

2.1 HVDC typical configuration…………………………………………………………………………………………..…52.1.1 Thyristor valves (in the rectifier and inverter)……………….…………………………………….52.1.2 Converter Transformers…………………………………………………………………………………..…62.1.3 AC filters and Capacitor banks…………………………………………………………….………………62.1.4 DC filters…………………………………………………………………………………………..…………………62.1.5 Transmission medium (DC line)……………………………………………………….………………….6

3. Surge performance on HVDC transmission lines………………………………….…………………………………73.1 Switching surges………………………………………………………………………………………………………………73.2 Lightning surges………………………………………………………………………………………………………………83.3 FOW (fast front of wave)…………………………………………………………………….…………………………..8

4. Equipment used in HVDC transmission lines compared to HVAC……………………...………………….94.1 Lattice Towers – ROW………………………………………………………………………………….………………….94.2 Insulators………………………………………………………………………………………………………………….……11

4.2.1 Dielectric resistivity of DC insulators compared to AC insulators……….………………114.2.2 Dielectric Material Selection…………………………………………………………….…..…….……..114.2.3 Ion Migration…………………………………………………………………………………….…………….…124.2.4 Attraction of Airborne particles due to unidirectional current………...….…………….13

4.3 Surge Arrestors………………………………………………………………………………………………………….…..145. Environmental Characteristics of HVDC lines compared to HVAC lines……………….….………..…..15

5.1 Effect of electric fields………………………………………………………………………………………….…………155.2 Effect of magnetic fields……………………………………………………………………………………….…………155.3 Radio Interference……………………………………………………………………………………………….…………165.4 Audible Noise……………………………………………………………………………………………………….…………165.5 Land use change for transmission line and substations……………………………………..……….…..165.6 Visual Impact………………………………………………………………………………………………………….….……16

6. Conclusion…………………………………………………………………………………………………………………………….177. References……………………………………………………………………………………………………………………..…….18

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1. Introduction to HVDC transmission

The most common perception is that HVDC transmission is used instead of HVAC because of lower losses. This is not entirely true as both DC and AC line losses is regulated by the size of conductor selected. Selection of a larger cross sectional area decreases the losses but at the same time increasing the cost. A HVDC power line can be designed to be less costly as compared to an HVAC line when transmitting the same amount of power. The graph below shows the cost vs. distance of HVAC and HVDC.

0 100 200 300 400 500 600 7000

50100150200250300350400

HVDCHVAC

Cost (million dollars) vs. distance (miles)

As seen on the graph, the initial cost of a HVDC system is higher, but as the distance increases it balances off and eventually becomes a more economic solution. DC converter stations at either end of the line are obviously more expensive than the terminating stations of an AC line. In this report a more detailed study and comparison is done explaining how cost is saved when opting for HVDC. Another important factor is the synchronizing of neighboring networks. Japan’s electrical grid is split into two, basically a 50Hz network and a 60Hz network. Interconnecting of these networks is physically impossible via HVAC. However, with the use of a DC converter station in each network, it is possible to connect the two networks by interconnecting a dc link between them. Research into HVDC by leading electrical R&D companies shows us the potential that HVDC has in the future. ABB is currently undergoing major HVDC projects in India and China. The graph below shows us the progress HVDC has made over the decades compared to HVAC.

1965 1970 1975 1980 1985 1990 1995 2000 20050

5000100001500020000250003000035000

HVACHVDC

MVA, MW vs. Year



2.0 Principle of transmission for HVDC compared to HVAC

2.1 HVDC typical configuration

A typical configuration drawing of a HVDC system is shown below:

The above configuration has three sections, namely, the transmitting converter station (left), the transmission medium (DC line), and the receiving converter station (right). The converter stations at each end are replicas of each other and therefore use similar equipment. The main equipment used are:

2.1.1 Thyristor valves (in the rectifier and inverter)

The thyristor is a controllable semiconductor that has the ability to carry very high currents (4000A) and is also able to block very high voltages (10kV). They are responsible for the conversion process. Thyristor valves contain a number of thyristors in series, operating at very high voltages (several hundreds of kV). Thyristor valves are constructed differently depending on the manufacturer. A common assembly used is a twelve-pulse group with three quadruple valves. The communication of the control equipment (control system) and the thyristor valve is generally done through fibre optics. A picture can be seen on the next page.



2.1.2 Converter Transformers

These transformers serve to connect the AC system (of the power generation side) to the DC system, and vice versa on the other end. They adapt the AC voltage level to the DC voltage level and contribute to the commutation reactance. Converter transformers are one of the most expensive components in a HVDC system. A picture can be seen below.

2.1.3 AC filters and Capacitor banks

Filters are installed in order to limit the amount of harmonics to the level required by the network, for a 12 pulse HVDC converter, current harmonics of the order 11, 13, 23, 25 and higher are generated. The capacitor banks and the filters compensate for the reactive power consumption by the converter. A typical filter and capacitor arrangement can be seen below.

2.1.4 DC filters

DC filters are used to reduce harmonics that can create disturbances in telecommunication systems. These harmonics are created by HVDC converters at all operational modes.

2.1.5 Transmission medium (DC line)

The remainder of this report will focus on the HVDC transmission line.

Three quadruple valves forming a 12 pulse converter

Reactive power AC filters

Converter Transformer



3.0 Surge performance of HVDC transmission lines

Insulation coordination is the examination and prediction of potential overvoltages and continuous voltage stresses occurring on a transmission line by determining the overvoltage, clearance in air, creepage against shed surfaces, rating and positioning of protective devices and the overvoltage insulation withstand limit (BIL, BSL). The main sources of overvoltages are switching impulses, lightning impulse, FOW (fast front of wave) and power frequency.

DC line fault clearing does not involve any mechanical action and therefore is faster than for an AC line. The DC fault current is lower (equal to the load current) than the AC fault current and therefore the time before the restart is shorter than for an AC line. Reduced voltage restart is also unique for HVDC. It should also be pointed out that DC line faults on a bipolar line affect only one pole (if fallen line towers is disregarded). The bipolar DC line is equivalent to a double circuit AC line.

3.1 Switching surges

Switching impulses are generally caused by circuit breaker operation on either protective switching or equipment energizing. They result from switching events on the AC system that get transferred through the converter transformer and also due to load rejection on the AC side. These surges are normally of the order 1kA to 3kA.

A switch impulse is generally defined as:

Voltage waveform 250/2500 sec (IEC) Current waveform 36/90 micro sec

The absence of sinusoidal current zero in a DC system makes it much more difficult for distinguishing arcs. The use of auxiliary circuits together with the breaker is the most common solution. A typical circuit diagram is shown above.

The auxiliary circuit basically consists of either a passive or active circuit depending on the DC range. For an active the capacitor C is pre-charged prior to the current switching, and the circuit breaker is inserted between the capacitor and inductor.

After the contacts in the circuit breaker are separated, an arc voltage is established inside the circuit breaker arc-quenching chamber. The arc voltage increases with the travel of moving contact and, starts a current oscillation if a parallel capacitor is fast used such as the HVDC circuit breaker with an auxiliary circuit. This oscillation current can lead to, depending on the arc chamber design and capacitance in parallel, an instable arc with oscillation current zeros or a stable arc without oscillation current zeros. For the circuit breaker design without oscillation current zeros an active auxiliary circuit has to be employed to create current zero crossing for breaker current extinguishing. In spite of different type of



auxiliary circuits in use, the capacitor and reactor combination of the auxiliary circuit, together with the arc voltage or pre-charged voltage, determines the current derivative at zero crossing and rate of rise of recovery voltage after current zero.

3.2 Lightning impulse

Lighting impulse is caused from either a direct lightning stroke to the line which is rare for shielded systems or a back flashover, that is when lightning strikes the pylon shield wires. A lightning impulse may be defined as:

Voltage waveform 1.2/50 micro sec (IEC 60071) Current waveform 8/20 micro sec (IEC 60090-1)

On the AC side, the switchyard shielding limits the direct strikes to less than 20kA. They are not very significant in the DC system (valve hall), as shielding by the converter transformer is done. The calculation of striking distance, attractive radius, shielding failure exposure width and shielding failure rate is similar to that of HVAC. Lightning may cause flashovers from either direct strokes or induced voltages from nearby strokes (indirect strokes). Direct strokes are a major concern for transmission lines whereas indirect strokes are more a threat on the distribution side. Protection from direct lightning is difficult due to voltages reaching the order of megavolts when the insulation is only capable of handling 100-500kV. Direct strike protection is possible by using shield wire with excellent insulation levels and the tight use of surge arrestors. A surge arrestor on the transmission side will have to be kept nearly on each pole.

On a HVDC transmission line, once a surge occurs, the DC fault protection detects the fault on the transmission line and protects the equipment. The protection circuits enable the rectifier to act as an inverter which discharges the line effectively. After some 80 - 100 ms, the line is charged again by the rectifier. If the fault was intermittent, due to e.g. a lightning strike, then normally the line can support the voltage and the power transmission continues. Full power is then resorted in about 200 ms after the fault. But if the fault was due to contaminated line insulators, there is a risk that re-charging of the line results in a second fault. Many HVDC transmissions are designed such that after a number of failed restart attempts, the following attempts are made with reduced voltage (80 %).

3.3 FOW (fast front of wave)

FOW are caused due to bushing flashovers within the valve. This results in discharge of stray capacitances through relatively short lengths of the conductor. They are typically of the order of 1kA. FOW impulse can be defined as:

Current waveform 1 / 2 micro sec



4.0 Equipment used in HVDC transmission lines compared to HVAC

4.1 Lattice Towers - ROW

One of the most significant advantages of DC transmission lines is the line ROW. For the same power capacity, AC transmission systems need a much larger space as compared to DC transmission. The AC transmission line has 3 conductors whereas DC only has 2. Consider the example of transmitting 10.000MW of power. Studies show us that the equivalent corridor width (ROW – Right of way) is less than half for HVDC transmission. The figure below illustrates this quite clearly. This plays a major cost and environmental impact on the project. Including less labor cost for erection, less visual impact on the environment, etc. Hence in the case of an 800kV system, the corridor width is 375m for HVAC and 120m for HVDC.

Two types of Tower structures exist namely:

Guyed Towers: These towers have a lower visual impact. Is a cheaper solution in terms of the weight and fabrication cost. It requires a larger space to mount, and hence is exposed to vandalism.

Self-supporting Towers: Which have a more consolidated design and technology. These towers are suitable for all environmental conditions.

Pictures of these types of towers are shown on the next page:



Self-supporting Tower

A typical lattice tower consists of: Guyed Tower

Insulators: insulating the conductors from the structure. Conductor wires: on either side of the tower. Shield wires: located at the top, just above the conductor wires. Neutral wire: located at the center, in between the two conductors.

The figure above shows the arrangement of the self-supporting tower. These towers are normally spanned 1200 feet (635 meters) apart. Depending on the required transmitting power, the number of conductors per set will vary.



4.2 Insulators

HVDC requires special care in insulation selection, the main areas of interest when selecting insulators are the materials being used and the specific stress conditions on the dielectric.

Common Insulator materials used: porcelain (ceramic) insulators, glass insulators and composite insulators.

4.2.1 Dielectric resistivity of DC insulators compared to AC insulators

Major suppliers for DC insulators significantly increase the electric resistance of the dielectric materials to provide more efficient insulators for DC transmission, due to the continuous transverse current crossing the body of the dielectric, this can generate high temperature and hence decreases the resistivity. If this continues it can lead to the puncturing or shattering through an avalanche phenomenon (which is worse for warmer countries). The unidirectional current going through the body of the dielectric can also generate some depletion of the atomic structure of the material, reducing the electrical and electromechanical properties of the dielectric.

Hence in the case of glass insulators, the resistivity of DC glass insulators are around 100 times greater than that of AC glass insulators at normal conditions.

Below shows a graph of the resistance vs. temperature of a typical AC and DC toughened glass insulator.

50 100 1500.1

1

10

100

ACDC

Resistance (GΩ) vs. Temperature (degrees Celsius)

4.2.2 Dielectric Material Selection

Selection of the materials used for insulators is based on the insulators life expectancy, failure rate, detection rate, electrical performance etc.

Porcelain/Ceramic Insulators

These types of insulators are of strong anti-aging ability and abundant operating experience, the only problem being the difficulty to detect after failure. The IEC 61325 standard describes the minimum requirements for this insulator, including its characteristics, the conditions under which the specified values of these characteristics shall be verified, and its acceptance criteria.



Glass Insulators

These insulators have outstanding mechanical and electrical properties. They are also arc – resistant and have a uniform voltage distribution. The problem is glass insulators have very little strength to withstand pollution flashover as compared to double and triple sheds porcelain insulators. The IEC 61325 standard also describes the minimum requirements for this insulator.

Composite Insulators

These insulators are light and have a good water repelling property. They have the strongest ability to withstand pollution flashover as compared to other insulators. They also resist mechanical shock. However, these insulators age quickly because they are composed of organic materials, and also have a low rate of detection after failure. Hence there are no international standards that cover composite insulators for DC applications due to the insufficient knowledge of their specific performance over time, and of the ageing mechanisms involved under those specific stress conditions.

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.120

102030405060708090

glassporcelaincomposite

Pollution flashover voltage (kV) per unit length (1m) vs. salt density (mg/cm^2) @ altitude 1970m

4.2.3 Ion Migration

Failure rate of the insulators due to ion migration on DC line is much higher than on AC line. This can be explained by the picture shown below:

Ions such as Na+ in the insulating body move to negative side under DC stress. The accumulation of ions at one side causes deterioration in electrical and mechanical performance. Hence dielectric material with excellent purity and homogeneity should be considered.



4.2.4 Attraction of Airborne particles due to unidirectional current

The risk of electrochemical corrosion is more common in DC transmission due to the attraction of airborne particles by the unidirectional current, which generates an alternate pollution level irrespective of the contamination factors of the environment. This mostly occurs on the metal end fittings (pins) of the insulator, thus degrading it. Attraction of airborne particles also occurs for AC transmission in very harshly polluted areas.

Today all DC insulators have sacrificial zinc sleeves at the pin and on the cap side, as shown in picture below. Various shapes of Zinc sleeves are available depending on the severity of pollution. The illustration below shows us the position of the sacrificial zinc sleeve on the insulator.



4.3 Surge Arrestors

Initially, DC surge arrestors were not available. The valves were protected by spark gaps connected across them. With the invention of MO surge arrestors for HVDC applications, spark gaps have been replaced. The invention of polymer housings for surge arrestors led to the wide selection of surge arrestors we have today in terms of mechanical properties, short circuit behavior and costs. There are basically four distinguished designs namely, porcelain design, polymer tube design, polymer cage design and polymer wrapped design. For HVDC applications we mainly focus on porcelain and polymer tube designs. The diagram below shows us the construction of the above mentioned surge arrestors.

Porcelain design surge arrestorPolymer tube surge arrestor

The porcelain and polymer surge arrestors both consists of hollow core housing, MO elements, and a flange including the sealing and pressure relief system. The polymer tube design is different as it consists of a hollow core compound insulator which has a fibre reinforced plastic (FRP), with polymer shreds due to its excellent chemical and physical properties. This results in higher cantilever strength and headload. The mechanical properties may be varied by adjusting the thickness of the FRP wall or fibre angle, hence enabling the customer to have a quite specific design to the required environment. Thus these types of surge arrestors are preferred for extreme mechanical requirements as opposed to porcelain. Even after a lightning surge the mechanical strengths is at least 75% of its initial value. Hence the Polymer tube surge arrestor may be used as both an arrestor and post insulator.

The main factors that govern the reliability and safety of surge arrestors are the sealing and pressure relief system. The sealing is designed to prevent the entering of moisture. The pressure relief mechanism releases pressure caused by arc heat that could destroy the surge arrestor if contained. Nowadays surge arrestor failure is quite rare as compared to the SiC surge arrestors, and is common to that of transformers and other instrument transformers. In most cases, the failure of MO surge arrestors is due to manufacturing or transportation defects.



5.0 Environmental Characteristics of HVDC lines compared to HVAC lines

The possible effects on the environment due to transmission lines include:

The effect of Electric fields The effect of Magnetic fields Radio Interference Audible noise The space required for transmission lines and substations Visual Impacts

HVDC transmission got some advantages over HVAC regarding the above mentioned factors.

5.1 Effect of electric fields

The electric fields created by HVDC are produced by the combination of the electrostatic field due to line voltage and the space charge field produced by corona. Corona of power transmission lines refers to the discharge phenomenon accompanied with light emission, produced by the ionization of air around the conductors, when the conductor surface potential gradient exceeds a critical value, namely, the corona onset gradient. Therefore any charge between the conductor and the ground has an effect on the total electric field produced by the DC line.

Results from a research lab in Canada, IREQ (Institut de Recherche d'Hydro-Québec, Quebec, Canada) shows us that the same effects of HVAC transmission lines on humans are not present for HVDC transmission lines. These effects include the spark discharge from humans to bushes, grass and other vegetation. For AC transmission lines, the contrast of discharges can be 100 discharges per second at 50Hz.

This phenomenon is of low scale for humans. When considering large machines with rubber tires (harvesters, automobiles etc.), for HVDC lines, the electrical resistance of the tires (about 10 ohm), is enough to prevent the accumulation of a dangerous charge. For HVAC overhead lines, inductive capacitance currents n large machines may be lethal.

5.2 Effect of magnetic fields

From various estimates, the limits on the magnetic field of an AC system vary from 10 to 50 micro Tesla. In the case of DC, there are no perceivable effects. The DC transmission lines magnetic field is in the same magnitude as that of the earth.



5.3 Radio Interference

This phenomenon takes place due to the corona discharge around the conductors, which is generated at positive voltages. Hence for DC transmission lines, only the positive pole conductors cause radio interfere, whereas for AC transmission lines, all the three phases generate radio interference.

Under rainy conditions, for AC transmission the radio interference increases by 10db, but for DC transmission it decreases. The radio interference level of a typical HVDC transmission line is 6 to 8 dB lower than that of the same HVAC transmission line, assuming equal capacity of the conductors.

5.4 Audible Noise

Audible noise is one of the important design parameters for both overhead lines and substations. The known measures to decrease audible noise from these sources are very expensive.

In HVDC substations, the main source of audible noise is the converter transformer. Audible noise from DC transmission lines is a broadband noise with contributions extending to high frequencies. The noise is most common in fair weather. Noise levels from a DC line will usually decrease during foul weather, unlike the noise levels on AC lines. The audible noise from transmission lines should not exceed, in residential areas, 50 dB during the day, or 40 dB at night.

5.5 Land use change for transmission line and substations

This is the most important environmental problem in the construction of overhead lines. The difference between HVAC and HVDC in terms of land use has been mentioned on page 5. The overall consumption is nearly the same due the additional land that DC converter stations need, but for a large scale HVAC requires more land.

5.6 Visual Impact

Visual impact is also mentioned on page 5 of this report, which is very important when it comes to transmission lines crossing national parks, resorts and other places. HVDC systems have a much less visual impact on the environment.

800kV AC

800kV DC



6.0 Conclusion

After investigations done on 800kV HVDC transmission, it is the most economical solution for the transmission of large amounts of power. Theoretically there should not be a problem for transmitting DC at even higher voltages. Research on 1000kV transmission lines are currently been done by leading R&D companies. Not only does HVDC transmission at 800kV compared to 800kV HVAC have a less capital costs for longer distances, the value of losses at 6400MW (1800km) is far less, which is shown on the graph below,

Investment and value of losses vs. line losses (6400MW, 1800km, 1400 USD/kW)

1 2 3 4 5 6 7 8 9 101000

1500

2000

2500

3000

3500

4000

800kV AC600kV DC800kV DC

MUSD vs. Percent line losses

The 800kV HVDC transmission line will be able to provide a lot more power, hence the society will have exceptional requirements on reliability of the complete system.



7.0 References

1. HVDC Transmission by Dennis A. Woodford, Manitoba HVDC Research Centre, Winnipeg, Manitoba, R3T 3Y6, Canada

2. Insulation co-ordination of UHVDC transmission lines by Su Fei, College of Electrical Engineering, Zhejiang University, Hangzhou, China

3. ENVIRONMENTAL CHARACTERISTICS OF HVDC OVERHEAD TRANSMISSION LINES by Prof. L. A. Koshcheev, St-Petersburg, High Voltage Direct Current Power Transmission Research Institute, Vladivostok, Russia

4. High Voltage Direct Current (HVDC) Transmission Systems Technology Review Paper by Roberto Rudervall, ABB power Systems, Sweden

5. Experience from First 800 kV HVDC Test Installation by Abhay Kumar, Dong Wu and Ralf Hartings, International Conference on Power Systems (ICPS – 2007), Bangalore, India

6. Three Gorges - Changzhou HVDC : Ready to Bring Bulk Power to East by Abhay Kumar, Mats Lagerkvist, Mårten Eklund, Yuan QingYun , ABB Sweden

7. Power Transmission with HVDC at Voltages Above 600 kV by U Åström, L. Weimers, V. Lescale and G. Asplund

8. The Xiangjiaba-Shanghai 800kV UHVDC project Status and special aspects by V. F. Lescale, U. Åström, ABB AB, Sweden

9. Technical Feasibility and Research & Development Needs for ± 1000 kV and above HVDC System by Gunnar Flisberg, ABB, Sweden

10. Power Transmission with HVDC at 800 kV by D. Wu, ABB, Sweden

11. Bulk Power Transmission at ± 800 kV DC by Lars Weimers, ABB Power Technologies

12. Comparative Evaluation of HVDC and HVAC Transmission Systems by Kala Meah, Student Member, IEEE

13. Advantage of HVDC transmission at 800 kV by Gunnar Asplund, Urban Åström, and Dong Wu, ABB Power Technologies

14. Outdoor Insulation Design for the Three Gorges-Changzhou ±500 kV HVDC Project by Urban Åström, Bengt Almgren and Dong Wu, ABB, Sweden

15. Measurement of Corona Characteristics and Electromagnetic Environment of +/- 800 kV HVDC Transmission Lines under High Altitude Condition by Zheng Zhang, Rong Zeng, and Zhanqing Yu, Department of Electrical Engineering, Tsinghua University, Haidian District, Beijing 100084, China



16. HARMONICS REDUCTION USING A CONTINOUSLY REACTIVE POWER COMPENSATION IN HVDC LINKS by Karim Shaarbafi, Ph D. student of power electronic Eng. Seyyed Hossein Hosseini, Ali Aghagholzadeh, Electrical Engineering Dept., Faculty of Engineering, University of Tabriz

17. Design and selection criteria for HVDC overhead transmission lines insulators by JM GEORGE, SEDIVER S.A. (France)

18. Impact of lightning on the reliability of future power systems by Prof. Mario Paolone, DESL ‐Distributed Electrical Systems laboratory

19. High Voltage Surge Arresters for Protection of Series Compensation and HVDC Converter Stations by Kai Steinfeld, Reinhard Göhler, Daniel Pepper Siemens AG, Berlin

20. Metal-Oxide Surge Arresters in High-Voltage Transmission and Distribution Systems by Volker Hinrichsen, Siemens PTD, Berlin/Germany

21. A Synthetic Test Circuit for Current Switching Tests of HVDC Circuit Breakers by Baoliang Sheng, Senior Member, IEEE

22. Bulk power transmission at extra high voltages, a comparison between transmission lines for HVDC at voltages above 600 kV DC and 800 kV AC by Lars Weimers, ABB Power Technologies

23. HVDC Transmission by M. P. Bahrman, P.E., Member, IEEE


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