Download II. Principle of HVDC Transmission System

International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Design Implementation of 250 kV HVDC
Overhead Transmission System
Phyu Win Win Ai ,Thet Tin

Abstract— - Most of the high voltage transmission in the world
is in the form of high voltage alternating current (HVAC). Since
the development of the transformer, AC power can be
generated, transmitted, distributed and used at different and
convenient voltages. However, with the proper equipment, AC
can be converted to DC electricity. The thyristor or silicon
controlled rectifier (SCR) valves make the conversion from AC
to DC and thus are the main component of any HVDC
converter. Therefore, in this paper, focus is made the thyristor
or silicon-controlled rectifier (SCR)s based HVDC system. In
this paper, HVDC system design is considered and then the
shweli-shwesaryan 110 miles, 250kV HVDC overhead
transmission system is implementation designed.
Index Terms— CSC, Thyristor, Design criteria, HVDC,
bipolar.
I. INTRODUCTION
High voltage DC (HVDC) Transmission system consists
of three basic parts:
 converter station to convert AC to DC
 transmission line
 second converter station to convert back to AC.
HVDC transmission systems can be configured in many
ways on the basis of cost, flexibility, and operational
requirements. Alternating current (AC) is the main driving
force in the industries and residential areas, but for the long
transmission line (more than 400 miles) AC transmission is
more expensive than that of direct current (DC). Technically,
AC transmission line control is more complicated because of
the frequency. By applying interconnections to the
neighboring systems, power systems have been extended to
achieve technical and economical advantages. During their
development, power systems become more and more
interconnected and heavily loaded. With the increasing size
and complexity of systems and as the result of the
liberalization of the electrical markets, needs for innovative
applications and technical improvements of the grids will
further increase. HVDC plays an important role for these
tasks.
II. PRINCIPLE OF HVDC TRANSMISSION SYSTEM
A direct-voltage system is a hybrid circuit incorporation
AC and DC components.
Manuscript received Oct 15, 2011.
Phyu Win Win Ai,Department of Electrical Power Engineering,
MandalayTechnologicalUniversity,(e-mail:[email protected]).Mand
al-ay , Myanmar, Phone/ Mobile No 09-400300801
Ld
Converter 6-pulse
transformer bridges
Converter
busbar
DC line
1
CB
2
F
11th,13th
HP filters
Electrode line
Figure 1. Principle of HVDC transmission system [1]
The incoming power is from an alternating source, which is
rectified and filtered before transmission through the DC
system, inversion taking place at the receiving end in order to
provide the usual AC supply conditions. The principle of
HVDC transmission system is shown in Figure 1.
A. Converters
A HVDC system requires an electronic converter for its
ability of converting electrical energy from AC-DC or vice
versa. There are basically two configuration types of
three-phase converters possible for this conversion process
(Figure 2):
 Current Source Converter (CSC), and
 Voltage Source Converter (VSC).
Modern HVDC transmission systems can utilize either
the traditional Current Source Converter (CSC) or the
Voltage Source Converter (VSC) as the basic conversion
workhorse. The two converters are actually duals of one
another. However, the choice of which option is selected for a
particular project is based upon economic and other factors.
At present VSC are still limited to below 250 MW capacities
due to commercial and practical limitations of the electronic
switches.
Figure 2. Converter of the CSC and VSC Types
B. Two-terminal HVDC Links
Two-terminal HVDC links are sub-divided into four
types:
 Monopolar link
 Bipolar link
 Homopolar link
 Tripole link
In the monopolar link arrangement, as shown in Figure
3, there is only one conductor, usually negative polarity and
the ground or sea is used for the return path. The current flows
between the earth electrodes at the two stations.
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All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Figure3. Monopolar Configuration [2]
Since one terminal of the converters is connected to
earth, the return conductor need not be insulated for the full
transmission voltage.
Figure4. Bipolar configuration [2]
The bipolar link as shown in Figure 4 has two conductors.
Each terminal has two sets of converters of identical ratings,
in series on the HVDC side. The junction between the two sets
of the converters is grounded at one or both ends. Normally,
both poles operate at equal currents and hence there is zero
ground current flowing under these conditions. From the
viewpoint of lighting performance, a bipolar HVDC line is
considered to be similar to a double circuit HVAC
transmission line.
Generators
Converter
Transformers
Surge Arrestors
Converters
DC Reactor
DC Filter
Surge Arrestors
Electrode Line
Ground
Electrode
DC Line
Surge Arrestors
DC Filter
DC Reactor
Converter
Surge Arrestors
Converter
Transformer
Shunt Capacitor
AC Harmonic Filter
Converter Breaker
Receiving AC System
Figure5. DC Transmission system operating in
bipolar mode
III. HVDC TRANSMISSION SYSTEM DESIGN CONSIDERATION
and ice loading on lines and towers is based on the design load
district. This affects insulator specifications as well as tower
dimensions, span lengths, tower design, and conductor
mechanical strength and wind dampening.
A. Tower Specifications
The towers support the conductors and provide physical
and electrical isolation for energized lines. The minimum set
of specifications for towers are the material of construction,
type or geometry, span between towers, weight, number of
circuits, and circuit configuration. The type of tower refers to
basic tower geometry. The options are lattice, pole (or
monopole), H-frame, guyed-V, or guyed-Y. The span is
commonly expressed in the average number of towers per
mile. This value ranges from four to six towers per mile.
The weight of the tower varies substantially with height,
duty (straight run or corner, river crossing, etc.), material,
number of circuits, and geometry. The vertical orientation
allows for a more compact right-of-way (ROW), but it
requires a taller tower.
B. Minimum Clearances
Clearances are specified for phase-to-tower, phase-to
ground, and phase-to-phase. Phase-to-tower clearance for 500
kV ranges from about 10 to 17 feet, with 13 feet being the
most common specification. These distances are maintained
by insulator strings and must take into account possible
swaying of the conductors. The typical phase-to-ground
clearance is 30 to 40 feet. This clearance is maintained by
setting the tower height, controlling the line temperature to
limit sag, and controlling vegetation and structures in the
ROW. Typical phase-to-phase separation is also 30 to 40 feet
and is controlled by tower geometry and line motion
suppression.
C. Insulators
Insulator design varies according to tower function. For
suspension towers (line of conductors is straight), the
insulator assembly is called a suspension string. For deviation
towers (the conductors change direction), the insulator
assembly is called a strain string. For 500-kV lines, the
insulator strings are built up from individual porcelain disks
typically 5.75 inches thick and 10 inches in diameter. The full
string is composed of 18 to 28 disks, providing a long path for
stray currents to negotiate to reach ground. At this voltage,
two to four insulator strings are commonly used at each
conductor connection point, often in a V pattern to limit
lateral sway.
D. Lightning Protection
Since the towers are tall, well-grounded metallic
structures, they are an easy target for lightning. To control the
effects of lightning, an extra set of wires is generally strung
along the extreme top points of the towers. These wires are
attached directly to the towers (no insulation), providing a
path for the lightning directly to and through the towers to the
ground straps at the base of the towers. The extra wires are
called shield wires and are either steel or aluminum-clad steel
with a diameter of approximately ½ inch.
E. Selection of Converter Transformer Rating
The RMS value of the transformer secondary current
(total and not just the fundamental frequency component) IRMS
is given by:
The altitude range of transmission tower is a rough
surrogate for weather and terrain. This is important, since
nearly all aspects of line design, construction, and
environmental impacts are linked to weather. The design wind
I T2 RMS 
1T 2
 i ( t )dt
T0
(1)
2
All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
The alternating line-current wave consists of rectangular
pulses of amplitude Id and width 2π/3 rad. Therefore,
I T2 RMS 
1 2 2
1 3 2
2
i
(
t
)
dt

I d dt  I d2


  2
  3
3
2
I d (2)
3
The RMS value of the line-to-neutral transformer
secondary voltage is given by:

(3)
E LN 
Vdo
3 6
Transformer volt-ampere rating is given by:
Three-phase rating = 3ELN ITRMS
  
2

Vdo
(4)
 3
I d  Vdo I d
3
3
3 6 
And hence, I T RMS 
F. Sizing of the Smoothing Reactor
While the current and voltage rating of the smoothing
reactor can be specified based on the data of the DC circuit,
the inductance is the determining factor in sizing the reactor.
Taking all design aspects above into account, the size of
smoothing reactors is often selected in the range of 100 to 300
mH for long distance DC links and 30 to 80 mH for
back-to-back stations.
In an HVDC long-distance transmission system, it seems
quite logical that the smoothing reactor will be connected in
series with the DC line of the station pole. This is the normal
arrangement.
G. Design Criteria for AC Filters
The reactive power consumption of an HVDC converter
depends on the active power, the transformer reactance and
the control angle. It increases with increasing active power. In
addition, a reactive band for the load and voltage range and
the permitted voltage step during bank switching must be
determined.
These factors will determine the size and number of filter and
shunt capacitor banks. Harmonic Performance Requirements
HVDC converter stations generate characteristic and
non-characteristic harmonic currents. For a twelve-pulse
converter, the characteristic harmonics are of the order n =
12k ± 1 (k = 1,2,3 ...). These are the harmonic components
that are generated even during ideal conditions, i.e. ideal
smoothing of the direct current, symmetrical AC voltages,
transformer impedance and firing angles.
H. DC Filter Design
Harmonic voltages which occur on the DC side of a
converter station cause AC currents which are superimposed
on the direct current in the transmission line. These
alternating currents of higher frequencies can create
interference in neighbouring telephone systems despite
limitation by smoothing reactors.
DC filter circuits, which are connected in parallel to the
station poles, are an effective tool for combating these
problems. The configuration of the DC filters very strongly
resembles the filters on the AC side of the HVDC station.
The interference voltage induced on the telephone line
can be characterized by the following equation:
Ieq 
2
 H  C  I( x ) 
m
1
(5)
Vin( x )  Z  I eq
(6)
where
Vin(x) = Interference voltage on the telephone line at point x (in
mV/km)
Hμ = Weighting factors which reflect the frequency
dependence of the coupling between telephone and HVDC
lines
Cμ = “C message“ – weighting factors
Iμ(x) = Resulting harmonic current of the ordinal number μ in
the HVDC line at point x as the vector sum of the currents
caused by the two HVDC stations
Ieq = Psophometric weighted equivalent disturbing current
Z = Mutual coupling impedance between the telephone and
HVDC lines
The intensity of interference currents is strongly
dependent on the operating condition of the HVDC.
IV. DESIGN RESULTS FOR250KVSHWELI-SHWESAR YAN
HVDC TRANSMISSION SYSTEM
The overall schematic connection diagram of proposed
Shweli-Shwe Sar Yan 250kV DC Transmission system is
shown in Fig 6
Shweli
Converter
Station
G
250kVDC T.L
110miles
(177.0278 km)
Shwe-SarYan
230/33/11kV
Converter
Station
11/230kV
500 MVA
Figure 6.Schematic Connection Diagram of Proposed 250kV
HVDC Overhead Transmission System
The line data for Shweli-Shwe Sar Yan 250kVDC
transmission line is as follows:
Voltage
- 250kVDC
Current
- 2kADC
Power
- 500MVA
Number of Circuit - Single, bipolar
Route length
- 110 miles
Selection of Voltage
We selected voltage as,
Line voltage=230 kV for HVAC and 250 kV for HVDC line
Then, we choose the equivalent spacing (Dm) = 8m for HVAC
and 7m for HVDC
Current rating, for HVAC overhead line is 1568.8866 A and
its power factor angle is 36.89 degree (lagging).
Current rating, for HVDC overhead line is 2 kA(p.f unity).
Choice of Conductor
(ACSR) conductors are used for high voltage work. The size
the conductors selected dependents on the length of the
transmission line, load on the line and voltage of the
line.ACSR conductor is selected. For ±250 kVDC, a 954
MCM, 1.196 inches diameter ACSR conductor would be
required for a single conductor configuration and therefore a
twin bundle conductor configuration would be required for
2000 A.
Cross-sectional area= 480 mm2 = 0.744 in2 = 0.0051667 ft2
Approximate Overall diameter, D=1.196 inches =3.03784 cm
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
=1.1519 cm
= 0.0117 Ω at 50°C
Earth
90.3224
Wire
2
0.478
mm
5 ft
inch
0.48
Design Results of HVDC Line Efficiency and Regulation
. Converter Design Results
Sending end HVDC voltage at Shwe Li Vs = 250 kVDC
Table 4 Design Results of Converter Transformer
Line
Receiving end HVDC at Shwe Sar Yan VR = 250 kV
-2000×R= 244.9 kV
Input power, Ps = 400 MW (500MVA×0.8)
Name
Output power, Pr = Ps – PL = 400MW- 20002×2.5492
= 389.8032 MW
Converter
Regulation
Transforme
Cros
(kV
(A)
g
DC)
250
2000
7m
s-
Current
secti
carrying
on
capacity
area
(A)
4.8
1010×2
cm2
ency
tion
following main data:
(%)
Inductance
97.45
Rated Voltage
512 kV DC
Rated Current
2500 A DC
170
2
Ball & Socket
16/20
20
20/24
damped filter types are shown in Figure 7
A
B
C
170
±150
±70
Voltage kV
250 mH
. The assembly of the selected AC tune filter and high pass
146
11th
50 Mvar
Q = 50
100
Mvar
Capacitor banks
13th
50 Mvar
Q = 50
Tuned Filters
±150
th
24 th
50
50 Mvar
Mvar
Q
Q=
= 33
High-pass
Damped filter
50 Hz filters 250 Mvars@ 230 kV
Figure 7. Selected AC Filter
±70
V. CONCLUSIONS
Accordingly the minimum number of discs is to be 7
(7×65=455kV).Thus, taking one numbers for suspension
string and two numbers for tension string in excess, the
numbers of insulator discs had been decided as follows:
Suspension Insulator –
7+1=8 units
Tension Insulator
–
18 units (Double string of 9 units)
Design Results of Sag for Line Conductor and Earth Wire
Table 3. Design Results of Sag for Line Conductor and Earth
Wire
Name
The smoothing reactors are of air core type and have
2.082
Unit Spacing mm
±65
650 MVA
regula
%
±145
1633 A
Effici
1
DC-Wet Flashover
kV
Design Results of Smoothing Reactors and AC Filter
CB-05
Voltage kV
kV
ntage
CB-16
DC-Dry Withstand
132.74
re rating
Line
CB-12
Coupling mm
230
volt-ampe
r
Identification
4
Current
Perce
Insulation of Line
Flashover Voltages = 250kV×1.8 = 450 kV. The flashover
voltages of HVDC insulator discs are shown in Table .
Table 2. Technical Particulars of HVDC Disc Insulators
3
voltage
C
cin
Volt
er
A
ent
er rms
B
age
Neutral
Transform
C
Spa
Line
A
Curr
Transform
A
B
C
Volt
Line to
age
Table 1. Design Results of HVDC Line Efficiency and
Line
to
B
Required radius, r
DC Resistance
This paper presents design consideration and calculation of
HVDC overhead transmission line for 250 kV Shweli-Shwe
Sar Yan in Myanmar. A high-voltage direct current (HVDC)
electric power transmission system uses direct current for the
bulk transmission of electrical power, in contrast with the
more common alternating current systems. HVDC allows
power transmission between unsynchronized AC distribution
Cross-secti
on area
Diameter
Line
480 mm2
1.196
Conductor
(954MCM)
inch
Max
Weight
: Sag
(lb/ft)
17 ft
1.227
systems, and can increase system stability by preventing
cascading failures from propagating from one part of a wider
power transmission grid to another.
The investment cost for HVDC converter stations are
higher than for high voltage AC substations. On the other
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
hand, the costs of transmission medium (overhead lines and
cables), land acquisition or right of way costs are lower in the
HVDC system. Moreover, the operation and maintenance
costs are lower in the HVDC system.From this paper, the
technical knowledge and design consideration and calculation
can be contributed to the students, researchers and other
engineers.
[1]
[2]
[3]
[4]
[5]
REFERENCES
Arrillaga J., Y.H. Liu, N.R. Watson, 2007, “Flexible Power
Transmission”.
Dennis A. Woodford, 1998, “HVDC Transmission”.
Kala Meah, SadrulUla, 2007, “Comparative Evaluation of HVDC and
HVAC Transmission Systems”.
Roberto Rudervall, J.P. Charpentier and Raghuveer Sharma, 1998,
“High Voltage Direct Current (HVDC) Transmission System”.
Hartmut Huang, Markus Uder, Reiner Barthelmess and Joerg Dorn,
2010, “Application of High Power Thyristors in HVDC and FACTS
Systems”.
Phyu Win Win Ai received her B.E (Electrical Power) degree from
Technological University, in 2009 and now pursuing M.E (Electrical Power)
at Mandalay Technological University. Her areas of interest are HVDC
overhead bipolar transmission system.
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