Download 10EE751 HVDC TRANSMISSION

HVDC TRANSMISSION
Subject code:10EE751
Course by
A.Velu
Assistant Professor
Department of Electrical and Electronics Engineering
Text Books:


Direct current Transmission by EDWARD WILSON
KIMBARK(Wiley interscience, New york,1971).
High Voltage D.C.Power Transmission system by
K.R.PADIYAR IISc Bangalore, New Age International
Publishers Ltd.
Overview:





General Aspects of DC transmission and comparison of
it with AC transmission.
Converter Circuits
Analysis of the Bridge converter
Control of HVDC Converters and Systems.
Protection.
PART A (Unit 1 & 2)
General Aspects of DC Transmission and comparison of it
with AC Transmission
 Historical Sketch
 Constitution of EHV AC and DC links.
 Limitations and Advantages of AC and DC transmission.
Historical Sketch:
Evolution of Power Systems:
Late 1870s- Commercial use of electricity.
In 1882- First Electric power system which includes Generator,
Cable,fuse,Load designed by Thomas Edison at Pearl Street station in
New york.
It was DC System (Low Voltage 110V),underground cable is used to
distribute the power to consumers. Only 59 consumers are benefited by
this low voltage DC system. Incandescent lamps are used as a load.
In 1884-Motors were developed by Frank Sprague. After the invention
of motors electricity is used more effectively or it was appreciated.
In 1886 -Limitation of DC
High losses and Voltage Drop
Transformation of Voltage required.
Continues…
Transformer and AC distribution (150 lamps) developed by William
Stanley of Westing house.
 In 1889- First AC transmission system in USA between Willamette
falls and Portland, Oregon. It was 1-Phase,4KV,Over 21 Km.
 Before that in the year of 1888-N.Tesla developed Poly Phase
system and had patents of Generator,Motor,Transformer,
transmission lines. Later Westing House bought it.
 In 1890-Controversy on whether industry should standardize AC or
DC.
Edison-DC System
Westing House-AC System
Later because of features of AC System, its dominated
1.
Voltage increase is possible
2.
Simpler and cheaper generators and motors.

Continues…
In 1893-First 3-Phase line ,2.3KV,12 Km in California .
 Improvement in voltages year by year,
1922-165KV
1923-230KV
1935-287KV
1953-330KV
1965-500KV
1966-735KV
1969-765KV
1990-1100KV

Continues…
Standard voltages are 115,138,161,230KV preferred for High
Voltage (HV)lines.
 Remaining 345,500,765KV are Extra High Voltage(EHV)
lines.
 For interconnection of AC systems, We need fixed frequency.
 60Hz-US and Canadian countries
50Hz-Europe and Asian countries

Entry of HVDC system:
HVDC transmission was designed by a French Engineer,
RENE THURY. Simultaneously AC system was also
developed slowly.
 In between 1880-1911,atleast 11 Thury system were installed
in Europe. The prominent was Mouteirs to Lyons(France) in
1906. It comprises 180Km(4.5 km underground
cable),4.3MW,57.6KV,75A.
Features :
DC series generators were used.
Constant control current mode.

Continues….
In1920-Transverter(Mechanicalconverter-polyphase
transformer)were developed. Again AC system dominated.
 In 1938-All the Thury system were dismantled. Because in
DC system, we need frequent maintenance , cost also is not
effective.
 Again AC revolution back till 1950. In the year of 1950,
Mercury arc valves (Bulky converter) it was possible to
convert AC to DC.
 In 1954, first HVDC System between Sweden and Gotland
island was commissioned by cable. Conversion carried out by
Mercury arc rectifier. Again people think about DC
transmission because of the limitation in AC system.

Limitations of HVAC





Reactive power loss
Stability
Current carrying capacity
Skin and Ferranti effect
Power flow control is not possible.
Advantages of HVDC
No reactive power loss
 No Stability Problem
 No Charging Current
 No Skin & Ferranti Effect
 Power control is possible
 Requires less space compared to ac for same voltage
rating and size.
 Ground can be used as return conductor
 Less corona loss and Radio interference

Continues…
Cheaper for long distance transmission
 Asynchronous operation possible
 No switching transient
 No transmission of short circuit power
 No compensation problem
 Low short circuit current
 Fast fault clearing time

Disadvantages of HVDC
Cost of terminal equipment is high
 Introduction of harmonics
 Blocking of reactive power
 Point to point transmission
 Limited overload capacity
 Huge reactive power requirement at the converter
terminals.

Comparison of AC and DC Transmission
The relative merits of the two modes of transmission(AC
and DC) which need to considered by a system planner are
based on the following factors:
 Economics of Transmission
 Technical performance
 Reliability
A major feature of power systems is the continuous
expansion necessitated by increasing power demand .
This implies that the establishment of a particular line must
be consider as a part of an overall long term system
planning.
Economics of power transmission:






The cost of transmission line includes the investment and
operational costs.
Investment cost includes,
Right of way
Transmission towers
Conductors
Insulators
Terminal equipment
Operational costs includes
 It mainly due to cost of losses
Right of Way(RoW):


An electric transmission line right-of-way (ROW) is a
strip of land used by Electrical utilities to construct,
operate, maintain and repair the transmission line
facilities.
Rights of way may also include the purchase of rights to
remove danger trees. A danger tree is a tree outside the
right of way but with the potential to do damage to
equipment within the right of way. If the danger tree
falls or is cut down, it could strike poles, towers, wires,
lines, appliances or other equipment and disrupt the flow
of electricity to our customers.
Images for (RoW)
Continues…
Continues…
This Implies that for a given power level, DC lines
requires less RoW, Simpler , and cheaper towers and
reduced conductors and insulator costs.
 The power losses are also reduced with DC as there are
only two conductors are used.
 No skin effect with DC is also beneficial in reducing
power loss marginally.
 The dielectric losses in case of power cables is also very
less for DC transmission.
 The corona effects tends to less significant on DC
conductors than for AC and this leads to choice of
economic size of conductors with DC transmission.

Continues…


The other factors that influence the line cost are the cost
of compensation and terminal equipment.
In dc lines do not require compensation but the terminal
equipment costs are increased due to the presence of
converters and filters.
Variation of cost with line length:
Description:


AC tends to be more economical than DC for distances
less than Break even distance and costlier for longer
distances.
The breakeven distances can vary from 500Km to
800Km in overhead lines.
Technical performance

The DC transmission has some positive features which
are lacking in AC transmission. These are mainly due to
the fast controllability of power in DC lines through
converter control.
Advantages:
Full control over power transmitted.
The ability to enhance transient and dynamic
stability in associated AC networks.
Fast control to limit fault currents in DC lines.
This makes it feasible to avoid DC breakers in
two terminal DC links.
Continues…
 STABILITY LIMITS:
The power transfer in AC lines is dependent on the angle
difference between voltage phasors at the two ends. For a
given power level, this angle increases with distance.
The maximum power transfer is limited by the
considerations of steady state and transient stability. The
power carrying capability of an AC line as a function of
distance.
But in DC lines which is unaffected by the distance of
transmission.
Power transfer capability Vs. Distance
Continues…

VOLTAGE CONTROL
The voltage control in AC lines is complicated by line
charging and inductive voltage drops.
The voltage profile in a AC line relatively flat only for
fixed level of power transfer corresponding to surge
impedance loading (SIL) or normal loading.
The Voltage profile varies with the line loading. For
constant voltage at the line terminal, the mid point voltage
is reduced for line loading higher than SIL and increased
for loadings less than SIL.
Variation of Voltage along the line:
Continues…
Line compensation:
AC lines require shunt and series compensation in long
distance transmission, mainly to overcome of the line
charging and stability limitations.
Series capacitors and shunt inductors are used for this
purpose.
The increase in power transfer and voltage control is
possible through the Static Var Systems (SVS).
In AC cable transmission, it is necessary to provide shunt
compensation at regular intervals.
A whole picture of FACTS devices family:
Continues…
PROBLEMS OF AC INTERCONNECTION:
 When two power systems are connected through AC
ties(Synchronous interconnection),the automatic generation
control of both systems have to be coordinated using tie line
power and frequency signals.
 Even with coordinated control of interconnected systems, the
operation of AC ties can be problematic due to
a) The presence of large power oscillations which can lead to
frequent tripping.
b) Increase in fault level
c) Transmission of disturbances from one system to the other
Continues…


The controllability of power flow in DC lines eliminates
all the above problem. In addition, for asynchronous DC
ties, there is no need of coordinated control.
It is obvious that two systems which have different
nominal frequencies cannot be interconnected directly
with AC ties and require the use of DC links.
Continues…
GROUND IMPEDANCE:
In AC transmission, the existence of ground(Zero
sequence)current cannot be permitted in steady-state due to
high magnitudes of ground impedance which will not only
affect efficient power transfer, but also result in telephone
interference.
But ground impedance negligible for DC currents and a DC
link can operate one conductor with ground return(
Monopolar operation). The ground return is objectionable
only when buried metallic structures (Such as pipes) are
present and are subject to corrosion with DC current flow.
Reliability:
The reliability of DC transmission is quite good and
comparable to that AC systems.
 An exhaustive record of existing HVDC links in the
world is available from which the reliability statistics
cab be computed.
 It must be remembered that the performance of Thyristor
valves is much more reliable than mercury arc valves
and further developments in devices, control, protection
is likely to improve the reliability level.

Continues…
There are two measures of overall system reliability
a) Energy availability
b) Transient reliability

Energy availability:
Equivalent outage time is the product of the
actual outage time and the fraction of system
capacity lost due to outage.
Transient reliability:
This is the factor specifying the performance of HVDC systems
during recordable faults on the associated AC systems.
Recordable AC system faults are those faults which cause one or
more AC bus phase voltages to drop below 90% of the voltage prior
to the fault.
It is assumed that the short circuit level after the fault is not below the
minimum specified for satisfactory converter operation.
Both energy availability and transient reliability of existing
DC systems with thyristors valves is 95% or more.
HVDC outage statistics:
The average failure rate of thyristors in a valve is less than 0.6%
per operating year. The maintenance of thyristor valves is also
much simpler than the earlier mercury arc valves.
Types of HVDC links:
Monopolar link
 Bipolar link
 Homopolar link

Monopolar link:
Having one conductor (-Ve Polarity) and ground is used as return
path.
We can operated either in +Ve or –Ve polarity,but usually
preferred -Ve polarity in order to reduce the Corona effect.
The major drawback in this system is power flow is interrupted
due to either converter failure or DC link.
The ground return is objectionable only when buried metallic
structures (Such as pipes) are present and are subject to corrosion
with DC current flow.
Bipolar link:
There are two conductors , one is operates at positive and other is
negative. During fault in one pole it will operate as monopolar
link. This is very popular link in HVDC
Homopolar link:
In this link, two or more conductors have same polarity.
Normally negative polarity are used(to less corona loss and radio
interference).
Ground is always used as return path.
During fault in one pole it works as monopolar.
Application of HVDC:
The main areas of application based on the economics and
technical performances, are
 Long distance bulk power transmission.
 The underground of submarine cables.
 Asynchronous connection of
AC system with different
frequencies.
 Control and stabilize the power system with power flow
control.
Based on the interconnection, three types of HVDC is possible.
 Bulk Power transmission
 Back to back connection
 Modulation of AC system

Purpose of HVDC based on interconnection:
Bulk power transmission
(Transfer the power from one end to another end without
tapping power in between).For this DC system is the best
option. (Or) HVDC transmission where bulk power is
transmitted from one point to another point over long distance.
 Power flow control (Back to Back HVDC)
If two regions are very nearby, we can monitor the
power flow from one region to another to control, emergency
support as per our requirement.(Or)Back to Back link where
rectification and inversion is carried out in the same converter
station with very small or no DC lines

Continues…
To provide stability to AC system
This is basically used to control the power and stabilize
the system. It is also used to connect two different frequencies
system.
(Modulation of AC) AC system is connected parallel with DC
system.(or)Parallel connection of AC and DC links. Where both
AC and DC run parallel. It is mainly used to modulate the
power of AC lines.

HVDC is the better option for above cited purposes while
compare with its AC system.
Principle parts of HVDC Transmission:
Various Parts of HVDC transmission:







Converters
Converter transformers
Smoothing reactors
Harmonic filters
Overhead lines
Reactive power source
Earth electrodes
CONVERTERS





Converters are the main part of HVDC system.
Each HVDC lines has atleast two converters, one at each end.
Sending end converter works as Rectifier (It converts AC
power to DC power). However converter as receiving end
works as Inverter ( it converts DC power to AC power).
In case for reversal of operation, Rectifier can be used as
inverter or vice versa. So generally it is call it as
CONVERTERS.
Several thyristors are connector in series and/or in parallel to
form a valve to achieve higher voltage / current ratings.
Note*- Valves (Combinations of several thyristors) .
Various Thyristor Ratings:
Continues…
How to achieve required voltage and current ratings?
The current rating of converter stations can be increased by
putting
 Valves in parallel
 Thyristors in parallel
 Bridges in parallel
 Some combinations of above.
The voltage ratings of converter stations can be increased by
putting
 Valves in series
 Bridges in series
 Combination of above.
Bridge converters are normally used in HVDC systems.

Main requirement of the Valves are:
To allow current flow with low voltage drop across it
during the conduction phase and to offer high resistance
for non conducting phase.
 To withstand high peak inverse voltage during non
conducting phase.
 To allow reasonably short commutation angle during
inverter operation.
 Smooth control of conducting and non conducting
phases.

Continues…





Two versions of switching converters are feasible
depending on whether DC storage device utilized is.
An inductor-Current source converter
A Capacitor-Voltage source converter.
CSC is preferable for HVDC system
VSC is preferable for FACTS like STATCOM,SVC,etc
Comparison of CSC and VSC:
CSC
VSC
Inductor is used in DC side
Capacitor is used in DC side
Constant current
Constant voltage
Higher losses
More efficient
Fast accurate control
Slow control
Larger and more expensive
Smaller and less expensive
More fault tolerant and more reliable
Less fault tolerant and less reliable
Simpler control
Complex control
Not easily expandable for in series
Easily expanded in parallel for
increased rating
CONVERTER TRANSFORMERS:
For six pulse converter, a conventional three phase or
three single phase transformer is used.
 However for 12 pulse configuration, following
transformer are used.
 Six single -phase two windings
 Three single- phase three windings
 Two three- phase two windings
In converter transformer it is not possible to use winding
close to yoke since potential of its winding connection is
determined by conducting valves.
Here entire winding are completely insulated.

Continues…
As leakage flux of a converter transformer contains very high
harmonic contents, it produces greater eddy current loss and
hot spot in the transformer tank.
 In case of 12-Pulse configuration, if two three phase
transformers are used, one will have star-star connection, and
another will have star delta connection to give phase shift of
30°.
 Since fault current due to fault across valve is predominantly
controlled by transformer impedance, the leakage impedance
of converter transformer is higher than the conventional
transformer.
 On-line tap changing is used to control the voltage and
reactive power demand.

SMOOTHING REACTORS:





As its name, these reactors are used for smoothing the
direct current output in the DC line.
It also limits the rate of rise of the fault current in the
case of DC line short circuit.
Normally Partial or total air cored magnetically shielded
reactor are used.
Disc coil type windings are used and braced to withstand
the short circuit current.
The saturation inductance should not be too low.
Harmonic filters






Harmonics generated by converters are of the order of np±1in AC
side and np is the DC side. Where p is number of pulses and n is
integer.
Filter are used to provide low impedance path to the ground for the
harmonics current.
They are connected to the converter terminals so that harmonics
should not enter to AC system.
However, it is not possible to protect all harmonics from entering
into AC system.
Magnitudes of some harmonics are high and filters are used for
them only.
These filters are used to provide some reactive power compensation
at the terminals.
Overhead lines:






As monopolar transmission scheme is most economical and the first
consideration is to use ground as return path for DC current.
But use of ground as conductor is not permitted for longer use and a
bipolar arrangement is used with equal and opposite current in both
poles.
In case of failure in any poles, ground is used as a return path
temporarily.
The basic principle of design of DC overhead lines is almost same
as AC lines design such as configurations,towers,insulators etc.
The number of insulators and clearances are determined based on
DC voltage.
The choice of conductors depends mainly on corona and field effect
considerations.
Reactive power source
As such converter does not consume reactive power but due
to phase displacement of current drawn by converter and the
voltage in AC system, reactive power requirement at the
converter station is about 50-60% of real power transfer,
which is supplied by filters,capacitors,and synchronous
condensers.
 Synchronous condensers are not only supplying reactive
power but also provide AC voltages for natural commutation
of the inverter.
 Due to harmonics and transient, special designed machines is
used.

Earth electrodes:







The earth resistivity of at upper layer is higher (~4000 ohm-m) and
electrodes cannot be kept directly on the earth surface.
The electrode are buried into the earth where the resistivity is
around (3-10 ohm-m) to reduce transient over voltages during line
faults and gives low DC electric potential and potential gradient at
the surface of the earth.
The location of earth electrode is also important due to
Possible interference of DC current ripple to power lines,
communication systems of telephone and railway signals,etc.
Metallic corrosion of pipes, cable sheaths ,etc.
Public safety.
The electrode must have low resistance (Less than 0.1 ohm) and
buried upto 500 meters into the earth.
Constitution of EHV AC and DC links:
EHV transmission links, superposed on a lower voltage AC
networks, or interconnecting two such networks, or
connecting distant generating plants to an ac system, are
compared as to their principle components and arrangements
thereof, according to whether the line operates on AC or DC.
 Below single line diagram, is single circuit three phase AC
line. In such system requires transformer at both ends-step up
transformers at the sending end and step down transformer at
the receiving end.
 Most long overhead AC lines require series compensation of
part of the inductive reactance.(one bank of series capacitor)

Continues…


The three phase AC lines cannot be operated, except for a very short
time(less than 1 sec) with one or more conductors are open, because
such operation causes unbalanced voltages in the AC system and
interference in phone telephone lines.
Therefore three-pole switching is always used to clear the
permanent faults, although such fault may involve in any one
conductor. This being so, two parallel three phase circuits required
for reliable transmission.
Continues…






The line itself usually has two conductors, although some
lines have only one, the return path being in the earth or sea
water or both.
At both end of the lines are converters, the components of
which are transformers and group of mercury arc valves.
The converter at the sending end- Rectifier.
The converter at the receiving end-Inverter.
Either converter can function as rectifier or inverter,
permitting power to be transmitted in either direction. Of
course it is preferred for AC line, also has this reversibility.
The circuit breaker are installed only on the AC side of the
converters. These breakers are not used for clearing faults on
the dc line or misoperations of the valves, for these faults
Continues…
Can be cleared more rapidly by grid control of the valves.
However breaker is also required for clearing the faults in
transformers or taking the whole DC link out of service.
 Harmonic filters and shunt capacitors for supplying reactive
power to the converters are connected to AC sides of the
converter.
 Large inductance called dc smoothing reactors are connected
in series with each pole of the DC line.
Continues…
If higher reliability is required of a DC line than that provided
by two conductors, three or four conductors may be provided.
 Here one pole of four conductor line is shown with two
converters per terminal.
 The bus-tie switches 1 are normally open. If a permanent
fault occurred on the lower conductor, the converters
connected to it would be controlled so as to bring the voltage
and current on it to zero. Then switches 3 would be opened,
isolating the faulted line.

Continues…
Next the converter voltages would be raised to equality with
those of the respective adjacent converters, after which
switch 1 would be closed.
 The capability of all converter would be usable, and the
power normally carried by two conductors would then be
carried by one.
