high voltage direct current transmission system hvdc seminar
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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.