high voltage direct current transmission system hvdc seminar

Electrical and Electronics department SEMINAR REPORT 2013

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Seminar Report On

HIGH VOLTAGE DIRECT CURRENT

TRANSMISSION SYSTEM

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

2013


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ACKNOWLEDGEMENT

First of all I bow my head before God Almighty for the blessing he had

showered on me for the success completion of this work.

I would like to take this opportunity to extend my sincere gratitude to

Prof. X, Head of Department, Electrical & Electronics Engineering, for

extending every facility to complete my seminar work successfully.

I would like to express my sincere indebtedness to Prof. X,

Department of Electrical & Electronics Engineering, for her valuable guidance,

wholehearted co-operation and duly approving the topic as staff in charge.

I also extend my gratitude towards the staffs, students and parents for

their sincere support and motivation.

.


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ABSTRACT

High Voltage Direct Current (HVDC) technology has

characteristics which make it especially attractive in certain transmission

applications. The number of HVDC projects committed or under consideration

globally has increased in recent years reflecting a renewed interest in this field

proven technology. New HVDC converter designs and improvements in

conventional HVDC design have contributed to this trend. This paper provides

an overview of the rationale for selection of HVDC technology and describes

some of the latest technical developments


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CONTENTS

INTRODUCTION……………………………………………………….4

WHY WE USE DC TRANSMISSION………………………………….7

HVDC CONVERTER ARRANGEMENT……………………………...7

APPLICATION OF HVDC CONVERTER……………………………..9

ENVIORNMENTEL CONSIDERATIONS……………………………..10

HVDC CONTROL AND OPERATION………………………………...14

COMPARISON OF AC AND DC TRANSMISSION…………………..14

INHERENT PROBLEMS ASSOCIATED WITH HVDC………………16

CONCLUSION…………………………………………………………..17

REFERENCE ………………………………………………………..…..18


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INTRODUCTION

Electric power transmission was originally developed with direct

current. The availability of transformers and the development and improvement

of induction motors at the beginning of 20th

century, led to greater appeal and

use of AC transmission. Through research and development in Sweden at

Allmana Svenska Electriska Aktieboglet (ASEA), an improved multi-electrode

grid controlled mercury arc valve of high powers and voltages was developed

from 1929. Experimental plants were setup in 1930‟s.

DC transmission now become practical when long distances were to be

covered or where cables were required. The increase in need for electricity after

the 2nd world war stimulated research. In 1950, a 116km experimental

transmission line was commissioned from Moscow to Kasira at 200kv. The first

commercial HVDC line built in 1954 was 98km submarine cable with ground

return between the island of Gotland and the Swedish mainland.

Thyristors were applied to dc transmission in the late 1960s and solid

state values become a reality the highest functions dc voltage for dc

transmissions is +/- 600kv for the 785km transmission line in brazil. Dc

transmission is now an integral part of the delivery of electricity in many

countries throughout the world.


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WHY WE USE DC TRANSMISSION?

The question is often asked, “Why we use DC transmission?” One

response is that losses are lower. But this is not correct the level of losses is

designed in to a transmission system and is regulated by the size of the

conductor selected. DC and AC conductors either as over head transmission

lines or submarine cables can have lower losses but at higher expense since the

larger cross-sectional are will generally results in lower but cost more.

When converters are used for DC transmission, it is generally by

economic choice driven by one of the following reasons.

1. An overhead DC transmission line with its towers can be designed to be

less costly per unit of length than an equivalent AC line designed to

transmit the same level of electric power. However the DC converter

stations at each end are more costly than the terminating station of an AC

line and so there is a breakeven distance above which the total costs of

DC transmission is less than its AC transmission alternative. The DC

transmission has lower visual profile than an equivalent AC line and so

contributes to a lower environmental impact. There are other

environmental advantages to a DC transmission line through the electric

magnetic fields being DC instead of AC.

2. If transmission is by submarine or underground cable, the breakeven

distance is much less than overhead transmission. It is not practical to

consider AC cable systems exceeding 50km but DC cable system are in

service whose length is in the hundreds of kilometers and even distances

of 600km or greater have been considered feasible.

3. Some AC power systems are not synchronized to the neighboring

networks even though their physical distances between them os quite

small. Thais occur in Japan where half the country is a 60hz network and

other is 50hz system. It is physically impossible to connect the two


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together by direct AC methods in order to exchange electric power

between them. However if a DC converter station is located vthe required

power flow even though the AC systems so connected remain

asynchronous.


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HVDC CONVERTER ARRANGMENT

HVDC converter bridges and lines or cables can be arranged into a

number of configurations for effective utilization. Converter bridges may be

arranged either monopolar or bipolar as shown in 12 pulse arrangement.

Various ways HVDC transmission is used are shown in simplified form and

include the following.

BACK-to-BACK: there are some applications where two AC systems to be

inter connected are physically in the same location or substation. No

transmission line or cable is required between the converter bridges in this case

and the connection may be monopolar or bipolar. Back-to-Back links are in

Japan for interconnections between power system networks of different

frequencies they are also used as interconnections between adjacent

asynchronous networks.

Transmission between Two Substations: When it is economical to transfer

electric power through DC transmission or cables from one geographical

location to another, a two-terminal or point-to-point HVDC transmission is

used. In other words, DC power from a DC from a DC rectifier terminal is

dedicated to one other terminal operating as an inverter. This is typical of most

HVDC transmission system.

Multiterminal HVDC Transmission System: When three or more HVDC

substations are geographically separated with interconnecting transmission lines

or cables, the HVDC transmission system is a multiterminal. If the entire

substations are connected to the same voltage then the system is parallel

multiterminal DC. Parallel multiterminal DC transmission has been applied

when the substation capacity exceeds 10% of the total rectifier substation

capacity. A combination of parallel and series connections of converter bridges

is a hybrid multiterminal system.


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Unit Connection: When DC transmission is applied right at the point of

generation, it is possible to connect the converter transformer of the rectifier

directly to the generator terminals so the generated power feeds in to the DC

transmission lines. This might be applied with hydro and wind turbine driven

generators so that maximum efficiency of the turbine can be achieved with

speed control. Regardless of the turbine speed, the power is delivered through

the inverter terminal to the AC receiving system at its fundamental frequency of

50 or 60 Hz.

Diode Rectifier: It has been proposed that in some applications where DC

power transmission is in one direction only, the valves in the rectifier converter

bridges can be constructed from diodes instead of thyristors. Power flow control

would be achieved at the inverter and in the case where the unit connection is

used; AC voltage control by generator field exciter could be applied to regulate

DC power. This connection may require high speed AC circuit breakers

between the generators and the rectifier converter bridges to protect the diodes

from over currents resulting from a sustained DC transmission line short circuit.

MONOPOLAR CONFIGURATION

BIPOLAR CONFIGURATION


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APPLICATIONS OF HVDC CONVERTERS

The first application for HVDC converters was to provide point to

point electric power interconnections between asynchronous AC power

networks. There are other applications which can be met by HVDC converter

transmission which include:

Interconnection between asynchronous systems: Some continental

electric power system consists of asynchronous networks such as East, West,

Texas and Quebac networks in North America and island loads such as the

island of Gotland in the Baltic Sea make good use of HVDC interconnections.

Deliver energy from remote energy sources: Where generation has been

developed at remote sites of available energy, HVDC transmission has been an

economical means to bring the electricity to load centers.

Import electric energy into congested load areas. In areas where new generation

is impossible to bring into new service to meet load growth or replace

inefficient or decommissioned plant, underground DC cable transmission is

available means to import electricity.

Increasing the capacity of existing AC transmission by conversion to

DC transmission: New transmission rights of way may be possible to obtain.

Existing overhead transmission lines if upgraded to or overbuilt with DC

transmission can substantially increase the power transfer capability on the

existing right of way.

Power flow control: AC networks do not easily accommodate desired power

flow control. Power marketers and system may require the power flow control

capability provided by HVDC transmission.

Stabilization of electric power networks: Some wide spread AC power system

networks operate at stability limits well below the thermal capacity of their

transmission conductors.


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ENVIORMENTAL CONSIDERTAIONS

The electrical environmental effects from HVDC transmission lines can be

characterized by field and ion effects as well as corona effects (4), (5). The

electric field arises from both the electrical charge on the conductor and for a

HVDC overhead transmission line from charges on air ions and aerosols

surrounding the conductor. These gives rise to DC as well as due to ion current

density flowing through the air. A DC magnetic field is produced by DC current

flowing through the conductors. Air ions produced by HVDC lines from clouds

which drift away from the line when blown by the wind and may come in

contact with humans, animals and plants outside the transmission lines right-of

–way or corridor. The corona effects may produce low levels of radio

interference, audible noise and ozone generation.

Field and corona effects

The field and corona effects of transmission lines largely favor DC transmission

over AC transmission. The significant considerations are a follows

1. For a given power transfer requiring extra high voltage transmission, the

DC transmission line will have a smaller tower profile than the equivalent

AC transmission carrying the same level of power, this can also lead to

less width of right- of-way for DC transmission option.

2. The steady and direct magnetic field of DC transmission line near at the

edge of transmission right-of-way will be about the same value in

magnitude as the earth‟s naturally occurring magnetic field. For this

reason alone, it seems unlikely that this small contribution by HVDC

transmission lines to the background geometric field would be the basis

of concern.

3. The static and steady electric field from DC transmission at the levels

experienced beneath lines or edges of the right-of –way have no known

adverse biological effects. There is no theory or mechanism to explain


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how a static electric field at the levels produced by DC transmission lines

could effect human health. Electric fields from ac transmission lines have

been under more intense scrutiny than fields generated from dc

transmission lines.

4. The ion and corona effects of dc transmission line lead to a small

contribution of ozone production to higher naturally occurring

background concentrations. Exacting long term measurements are

required to detect such concentrations.

5. If ground return is used with monopolar operation, the resulting dc

magnetic field can cause error in magnetic compass readings taken in the

vicinity of the DC line or cable. This impact is minimized by providing a

conductor or cable return path in close proximity to the main conductor or

cable for magnetic cancellation. Another concern with continuous ground

current is that some of return current may flow in metallic structures and

intensify corrosion if cathodic protection is not provided.


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HVDC CONTROL & OPERATING PRINCIPLES

Conventional HVDC

For conventional HVDC transmission one line sets the while the other terminal

regulates the DC current by controlling its output voltage relative to that

maintained by the voltage setting terminal. Since the DC line resistance is low

large changes in current and hence power can be made with relatively small

changes in firing angle „α‟. Two independent methods exist for controlling the

converter DC output voltage. These are 1) By changing the ratio between direct

voltage and AC voltage by varying delay angle or 2) By changing the converter

AC voltage via load tap changers (LTC) on the converter transformer. Whereas

the former method is rapid the later method is slow due to the limited speed of

response of the LTC. Use of high delay angles to achieve the larger dynamic

range, however increases the converter reactive power consumption. To

minimize the reactive power demand while still providing adequate dynamic

control range and commutation margin, the LTC is used at the rectifier terminal

to give the delay angle within its desired steady state range .Example: 13-18

degrees and at the inverter to keep the extinction angle within its desired range,

E.g.: 17-20 degrees if the angle is used for DC voltage control or to maintain

rated DC voltage if operating in minimum commutation margin control mode.

VSC-Based HVDC

Power can be controlled by changing the phase angle of the

converter AC voltage with respect to the filter bus voltage. Whereas the reactive

power can be controlled by changing the magnitude of the fundamental

component of the converter AC voltage with respect to the filter bus voltage. By

controlling these two aspects of the converter voltage, operation in all four

quadrants is possible. This means that the converter can be operated in the


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middle of its reactive power range near unity power factor to maintain dynamic

reactive power reserve for contingency voltage support similar to a static var

compensator. It also means that the real power transfer can be changed rapidly

without altering the reactive power exchange with the AC network or waiting

for switching of shunt compensation.


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COMPARISON OF AC AND DC TRANSMISSION

Advantages of DC:

1. More power can be per conductor per circuit: The capabilities of

power transmission of an AC link and DC link are different. For the same

insulation, the direct voltage is equal to the peak value of the alternating

voltage. For the same conductor size, the current can transmitted with

both AC and DC, if skin effect is not considered. In practice, AC

transmission is carried out using either single circuit or double circuit 3

phase transmission using 3 or 6 conductors. For DC only one-half the

amount of copper is required for the same power transmission.

2. Use of Ground Return possible: In the case of HVDC transmission,

ground return may be used, as in the case of a monopolar DC link. Also

the single circuit bipolar DC link is more reliable, than the corresponding

AC link, as in the event of a fault on one conductor the other conductor

can continue to operate at the reduced power with ground return. For the

same length of transmission, the impedance of the ground path is much

less for DC than for the corresponding AC because DC spreads over a

much larger width and depth. In fact, in the case of DC the ground path

resistance is almost entirely depending on the earth electrode resistance at

the two ends of the line, rather than on the line length.

3. Smaller Tower Size: The DC insulation level for the same power

transmission is likely to be lower than the corresponding AC level. Also

the DC line will only need two conductors whereas three conductors are

required for AC. Thus both electrical and mechanical considerations

dictate a smaller tower.

4. Higher Capacity Available for Cables: In contrast to the overhead line,

in cable break down occurs by puncture and not by external flashover.

Mainly due to the absence of ionic motion, the working stress of the

DC insulation may be 3 to 4 times higher than under AC.


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5. No Skin Effect: Under AC conditions, the current is not uniformly

distributed over the cross section of the conductor. The current density is

higher in the outer region and result in under utiliasation of the conductor

cross section. Skin effect under conditions of smooth DC is completely

absent and hence there is a uniform current in the conductor, and the

conductor metal is better utilized.

6. Less Corona and Radio Interference: Since corona loss increases with

the frequency, for given conductor diameter and applied voltage, there is

much lower corona loss and hence more importantly less radio

interference with DC. Due to this bundle conductors become unnecessary

and hence give a substantial saving in line costs.

7. No Stability Problem: The DC link is an asynchronous link and hence

any AC supplied through converters or DC generations do not have to be

synchronized with the link. Hence the length of the DC link is not

governed by stability. In AC links the phase angle between sending end

and receiving end should not exceed 30 at full-load for transient stability.

8. Asynchronous Interconnection Possible: With AC links,

interconnections between power systems must asynchronous. Thus

different frequency systems cannot be interconnected. Such systems can

be easily interconnected through HVDC links.

9. Lower Short Circuit Fault Levels: When an AC transmission system is

extended, the fault level of whole system goes up, sometimes

necessitating the expensive replacement of circuit breakers with those of

higher fault levels. This problem can be overcome with HVDC as it does

not contribute current to the AC short circuit beyond its rated current.

10. Tie Line power is easily Controlled: In the case of an AC tie line, the

power cannot be easily controlled between the two systems. With DC tie

lines, the control is easily accomplished through grid control. the reversal

of the power flow is easy.


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INHERENT PROBLEMS ASSOCIATED WITH HVDC

1. Expensive Convertors: Expensive convertor stations are required at each

end of a DC transmission link, whereas only transformer stations are

required in an AC link.

2. Reactive Power Requirement: Convertors require much reactive power,

both in rectification as well as in inversion. At each convertor the reactive

power consumed may be as much at 50% of the active power rating of the

DC link. The Reactive power requirement is partly supplied by the filter

capacitance, and partly by synchronous or static capacitors that need to be

installed for the purpose.

3. Generation of Harmonics: Convertors generates a lot of harmonics both

on the DC side and the AC side. Filters are used on the AC side to reduce

the amount of harmonics transferred to the AC systems. on the DC

system smoothing reactors are used. These components add to the cost of

convertors.

4. Difficulty of Circuit Breaking: Due to the absence of a natural current

zero with DC, circuit breaking is difficult. This is not a major problem in

single HVDC link systems, as circuit Breaking can be accomplished by a

very rapid absorbing of the energy back into the AC system.

5. Difficulty of Voltage Transformation: Power is generally used at low

voltage, but for reasons of efficiency must be transmitted at high voltage.

Absence of the equivalent of DC transformers makes it necessary for

voltage transformation to carried out on the AC side of the system and

prevents a purely DC system being used.

6. Difficulty of High Power generation: Due to the problems of

commutation with DC machines, voltage, speed and size are limited.

Thus comparatively lower power can be generated with DC.


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CONCLUSION

HVDC transmission system is a very superior type of transmission system

topology, which serves for power transmission and thus contributes the

advantage like Use of ground return possible, Skin effect, Tower size etc.

Although HVDC posses some disadvantages. The extent of advantages

makes it a very suitable one for the transmission. For long distance transmission

of electricity HVDC transmission is the best one than Extra High Voltage AC

transmission.


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REFERENCES

1. “A Refined HVDC Control System”- Ekstrom. A and Liss. G (IEEE)

2. “Rapid City Tie-New Technology Tames The East, West

Interconnection”- M. Bahrman, D. Dickson, P. Fisher, M. Stolz.

3. “HVDC With Voltage Source Converter And Extruded Cables For Up to

+/-300kv and 1000MW”- B.Jacobson, V. Jiang-Hafner, Rey, G. Asplund

4. “Multiterminal Integration of the Nicolet Convertor Station into the

Quebec-New England phase II Transmission System”-D. McCallum, G.

Moreau, J. Primeau.

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