Ehv ac dc ppt u 5

1
UNIT – V
HVDC Transmission

Contents
2
1. HVDC Transmission: Overview of Unit- 5th
2. Objective of HVDC System
3. Principle of HVDC Transmission
4. Types of HVDC links
5. Advantages of HVDC transmission
6. Disadvantages of HVDC transmission
7. Basic scheme and equipment of converter station
8. Ground return
9. Basic principles of DC link control
10. Basic converter control characteristics
11. Application of HVDC transmission
12. Conclusion
13. Glossary

HVDC Transmission: Overview
of Unit- 4th
3
 Introduction: Initially DC used for generation, Transmission and
Distribution of electric power.
 The First Power System of 110 kV DC central Electric station was
installed by Edison in New York in 1882 for supplying power in the
range of 1.5 km.
 With the introduction of transformer and 3-phase system in 1886 and
afterward AC system became viable.
 Also the superiority of AC generators at high voltage, better performance
of AC motors etc. causes AC to supersede DC and AC has been used for
generation, transmission, and distribution since then.

HVDC Transmission: Overview of
Unit- 4th (Contd…)
4
 Now a days HVDC (High Voltage Direct Current) transmission has become
technically and commercially viable alternative to EHV/UHV AC transmission
particularly for long distance bulk power transfer.
 However it cannot be the substitute for AC, because of backbone of AC is its
superiority in case of generation, transmission, and distribution.
 The first HVDC link was set up in 1954 between Mainland of Sweden and the
Island of Gotland.
 It was a monopolar 100 kV, 90 km, 20 MW cable system with sea return path.
 In India first HVDC project was set up in 1989 in Vindhyachal as a Back-to-
Back link for Exchanging power of 500 MW between Northern and Western
region.
 First HVDC link in India was Rihand – Dadri commissioned in1990 of 814 km
length at ±500 kV, 1500 MW. The various Indian HVDC projects has
summarized in forthcoming slide .

HVDC in INDIA (Back-to-Back)
5
S.
No.
HVDC Project Connecting
Region
Capacity
(MW)
Year Voltage
(kV)
1. Vindyachal North – West 2 x 250 1989 69.7
2. Chandrapur West – South 2 x 500 1997-
1998
205
3. Vizag – I East – South 500 1999 205
4. Sasaram East – North 500 2002 205
5. Vizag – II East – South 500 2005 ±88

HVDC in INDIA (Transmission Line)
RAVI SHARMA/AIETM/DOEE
6
S. No. HVDC Project Year DC Voltage Line Length Capacity
1. National HVDC
Project-I
1989 100 kV 196 km 100 MW
2. National HVDC
Project-II
2000 200 kV 196 km 100 MW
3. Rihand - Dadri 1991 ±500 kV 816 km 1500 MW
4. Chandrapur -
Phagde
1998 ±500 kV 736 km 1500 MW
5. Talcher - Kolar 2003 ±500 kV 1400 km 2000 MW
6. Balia – Bhiwadi 2009 ±500 kV 780 km 2500 MW
7. Mundra –
Mohindergarh
2012 ±500 kV 986 km 1500 MW
8. Bishwanath – Agra 2015 ±800 kV 1728 km 6000 MW

Objective of HVDC System
8
 There are mainly three objectives of HVDC system:
 For transferring bulk amount of power over a long distance.
 For power exchange between two region or controlling power flow
between two region which is done with the Back-to-back system.
 For supporting weak AC line in which power controlling upto a large
extent is not possible and may cause stability failure.
 For this HVDC lines are used in parallel of AC line for improving the stability of system.

Principle of HVDC Transmission
9
 As we have both generation and distribution are in AC.
 So for HVDC transmission we need two converters, one at each end of
the transmission line.
 At sending end rectification will be done and the converter works as a
rectifier.
 At receiving end Inversion will be done and the converter acts as an
Inverter.
 The following are three basic steps:
1. Convert AC into DC (rectifier)
2. Transmit DC
3. Convert DC into AC ( inverter)

HVDC Transmission
Transmitting power at high voltage and in DC form instead
of AC is a new technology proven to be economic and simple
in operation which is HVDC transmission.
Since our
primary
source is A.C,
The three
basic
steps are:-
1. Convert AC into DC (rectifier)
2. Transmit DC
3. Convert DC into AC (inverter)
10

Wind
Power
AC
Transmision
Line
Converter
Station
Inverter
Station
AC Transmission
Line
Distribution
Line
HVDC
Transmission
Line
11

• Monopolar links
• Bipolar links
• Homopolar links
• Multiterminal links
HVDC links
can be broadly
classified into:
Types of HVDC Link
12

Monopolar Links
 It uses one conductor.
 The return path is provided by ground or water.
 Use of this system is mainly due to cost considerations.
 A metallic return may be used where earth resistivity is too high.
 This configuration type is the first step towards a bipolar link.
13

Bipolar Links
 It uses two conductor.
 Each terminal has two converters of equal rated voltage,
connected in series on the DC side.
 The junctions between the converters is grounded.
 If one pole is isolated due to fault, the other pole can operate
with ground and carry half the rated load (or more using
overload capabilities of its converter line).
14

Homopolar Links
 It has two or more conductors all having the same polarity,
usually negative.
 Since the corona effect in DC transmission lines is less for
negative polarity, homopolar link is usually operated with
negative polarity.
 The return path for such a system is through ground.
15

Advantages of HVDC Transmission
16
 Cheaper in cost:
 HVDC requires lesser conductors compared to AC for same power transfer.
 Insulation of conductor is 1/√2 times than that of AC system.
 It doesn’t need any intermediate sub-station as in case of AC system.
 Lower transmission loss because of less no. of conductors and less
resistance.
 No skin and proximity and Ferranti effect.
 Lesser Corona Loss and Radio interference.
 The voltage regulation problem is much less serious for DC, because of
absence of inductive voltage drop.
 It has greater reliability, because in case of fault on bipolar link it can work
as a monopolar link with reduced power transfer.
 Since only the IR drop is involved. For the same reason steady state stability is no longer a major problem.

Advantages of HVDC Transmission
17
 HVDC line can be built in stages i.e initially it is built with a single line
as a monopolar link later it can be converted in Bipolar link which
bifurcates investment.
 There is no limitation of loading of line because of absence of line
inductance.
 Surge Impedance loading in case of HVDC is higher than EHV AC line
because of limitation of line length in AC.
 Easy power flow control from converter station which doesn’t exist in
AC system.
 Asynchronous operation possible between regions having different
electrical parameters, mainly frequency.
 The towers of the DC lines are narrower, simpler and cheaper
compared to the towers of the AC lines.
 There is no requirement of compensating devices.

Disadvantages of HVDC
18
 The disadvantages of HVDC are in conversion, switching, control,
availability and maintenance.
 Power electronics based converters introduces harmonics on AC side and
ripples in DC side.
 Adequate cooling arrangement is required for thyristor valves.
 The required converter stations are expensive and have limited overload
capacity.
 Higher losses in static inverters at smaller transmission distances.
 The cost of the inverters may not be offset by reductions in line construction
cost and lower line loss.
 High voltage DC circuit breakers are difficult to build since some
mechanism must be included in the circuit breaker to force current to zero.
 More maintenance of line insulators.
 It requires some reactive power compensator at each end for providing
reactive power.

Comparison of HVAC & HVDC
19
 Conventionally power transmission is effected through HVAC
systems all over the world.
 HVAC transmission is having sever limitations like line length ,
uncontrolled power flow, over/low voltages during lightly / over
loaded conditions, stability problem, fault isolation etc.
 Considering the disadvantages of HVAC system and the advantages
of HVDC transmission , power grid has choosen HVDC transmission
for transferring 2000 mw from ER to SR.

Comparison of HVAC & HVDC
20
 More power can be transmitted in HVDC system than that of HVAC
with same tower structure as illustrated in fig.

Cost: HVAC Vs HVDC Transmission
21
Line Cost DC
Line Cost AC
Break Even Distance
Terminal Cost DC
Terminal Cost AC
Cost (Rs.)
Line Length( km )

Basic Scheme and Equipments of
Converter Station
ABHISHEK KUMAR/AIET/DOEE
22
1) Converters
2) Smoothing reactors
3) Harmonic filters
4) Reactive power source
5) Electrodes
6) Dc lines
7) Ac circuit breakers

Basic Scheme and Equipments of
Converter Station
23

Equipments of
Converter Station
Converters:
 They perform AC/DC and DC/AC conversion
 They consist of valve bridges and transformers
 Valve bridge consists of high voltage valves connected in a 6-pulse or 12-
pulse arrangement. See Video
 The transformers are ungrounded such that the DC system will be able to
establish its own reference to ground
Smoothing reactors:
 They are high reactors with inductance as high as 1 H in series with each pole
 They serve the following:
 They decrease harmonics in voltages and currents in DC lines
 They prevent commutation failures in inverters
 Prevent current from being discontinuous for light loads
24

Equipments of
Converter Station
Harmonic filters:
 Converters generate harmonics in voltages and currents. These harmonics
may cause overheating of capacitors and nearby generators and interference
with telecommunication systems
 Harmonic filters are used to mitigate these harmonics
Reactive power supplies:
 Under steady state condition, the reactive power consumed by the converter
is about 50% of the active power transferred
 Under transient conditions it could be much higher
 Reactive power is, therefore, provided near the converters
 For a strong AC power system, this reactive power is provided by a shunt
capacitor
26

Equipments of
Converter Station
27
Electrodes:
 Electrodes are conductors that provide connection to the earth for
neutral. They have large surface to minimize current densities and
surface voltage gradients
DC lines:
 They may be overhead lines or cables
 DC lines are very similar to AC lines
AC circuit breakers:
 They used to clear faults in the transformer and for taking the DC link
out of service
 They are not used for clearing DC faults
 DC faults are cleared by converter control more rapidly

Ground return
28
 A ground return means the ground or sea water or both as the return path either
continuously or for short times of emergency.
 Most DC transmission line use ground return path for reasons of economy and
reliability.
 The resistance offered by ground to DC is much lesser than that of AC.
 Since DC in the earth in a steady state, unlike transient or alternating current, does
not follow closely the route of the metallic conductor but spreads over a very large
cross-sectional area in both depth and width.
 Beside the advantage of economy ground return has reliability and less investment.
 In case of fault on one line of bipolar link it can be operated as monopolar link with
ground return.
 In starting stage of commissioning of new HVDC line it is built as a monopolar link
with ground return then later built as bipolar.

Ground return (Contd….)
29
 The resistance of this path is essentially independent of the length of the line.
 It may be regarded merely as the sum of the resistances associated with each
electrode unless the electrodes are near one another, which certainly would not
be true in long distance transmission.
 These resistances can be made low more by selecting the proper electrode with
suitable cross-section.
 The ground electrodes are put into earth from converting station within 5 km
radius.
 The electrode affects the aquatic animal of water in case of sea watwer return.
 It also causes rusting in underground pipes and induces currents in metallic
body under the earth.

Basic principles of DC link control
30
 In DC system the power transferred from one station to another is
controlled only by the magnitude of DC voltage at two ends.
 While in AC transmission system power transfer is governed by phasor
difference ( magnitude as well as phase ) of voltages at the two ends.
 Thereby power control in HVDC link is fast, easy and stable.
 If Vs and Vr are sending and receiving end voltages then line current
can be given as;
{ Let Vs > Vr}
where R= Resistance of line
R
VV
I rs
DC



31
 The sending voltage can be given as;
 The receiving end voltage can be given as;
where, α = firing angle of rectifier
β = extinction angle of inverter
Vac s = sending end voltage at AC side
Vac r = receiving end voltage at AC side
Xcs = Commutation reactance at sending end
Xcr = Commutation reactance at receiving end
)......(....................
3
cos
23
s iI
X
VV DC
cs
ACs


















 
)..(....................
3
cos
23
r iiI
X
VV DC
cr
ACr


















 

32
 Thus the power transferred is given as;
 Thus from above equation we see that power transferred through HVDC link mainly
depend on sending and receiving end voltage.
 This can be accomplished by:
 Controlling firing angles of the rectifier and inverter (for fast action)
 Changing taps on the transformers on the AC side (slow response)
 Therefore we can control the power flowing through HVDC link from converter
station only and also the power flow can be made reversible flow by altering
the firing angle.
 Power reversal is also obtained by reversal of polarity of direct voltages at both
ends
watt




 

R
VV
VIVP rs
sDCs

Basic converter control characteristics
33
 The ideal control system for an HVDC converter should have
following features:
 Control should be fast, reliable, and easy.
 Continuous operating range from full rectification to full inversion
 Control should be such that it requires less reactive power.
 Control should not sensitive to normal variation in voltage and
frequency of the supply.
 It can be used for protection of line and converter.
 To operate the converter in no commutation failure it is economical to
operate inverter at Constant Extinction Angle (CEA) control.
 Under normal condition rectifier operates at constant current control
and inverter at Constant Extinction Angle control.

Ideal Characteristic:
 Under normal Condition:
 Rectifier maintains CC (Constant Current)- α
 Inverter maintains CEA (Constant Extinction Angle) γ min
dciLdoid IRRVV )(cos  
34

Actual Characteristic:
 Abnormal Condition:
FA represents min. ignition angle (CIA mode)
AB represents Constant Current (CC mode)
Rectifier
*CIA shows maximum
rectifier voltage
35

Actual Characteristic:
 Abnormal Condition:
GD represents min. extinction angle (CEA mode)
GH represents Constant Current (CC mode)
Inverter
*CEA shows maximum
inverter voltage
Operating Point
Operating Point
at abnormal
36

37
 Each converter can work as a
rectifier as well as inverter.
 Operating Point 1:
 C1=rectifier(CC)
 C2=inverter(CEA)
 Operating Point 2:
 C2=rectifier(CC)
 C1=inverter(CEA)
Operating Point
2
Operating Point
1
Current is same

38
 The characteristics of converter control for both sending and receiving
stations are shown in fig. below;

Application of HVDC Transmission
39
 HVDC is used for transmitting bulk amount of power over
along distance via overhead line.

Back-to-Back HVDC Transmission
40
 HVDC is the unique solution to interconnect Asynchronous
systems or grids with different frequencies.
 It is used as a Back-to-Back system for exchanging power
between two region.

Submarine HVDC Transmission
41
 HVDC is also used for power transfer via medium and high
power submarine cable, where overhead lines cannot be
used.

Underground HVDC Transmission
42
 HVDC is used for medium and high power submarine or
underground cable for transmitting power.

Present Scenario of HVDC
43
 Very large investment for example in China and India shows that
HVDC very important in future especially in big new industrial
countries.
 To meet growing power transfer requirement over a very long distance,
±800kV HVDC transmission systems has been implemented in the
Indian Power System and under construction in China.
 The experience gained and the challenges faced in successful operation
of 800 kV HVDC project will not only improve the availability and
reliability of the HVDC systems but also possibility to build converter
stations above 800kV DC.

Present Scenario of HVDC
44
 To facilitate the transfer of bulk power various projects has been
planned which include the introduction of 800 kV, 3000 MW
upgradable to 6000 MW Multiterminal systems from Assam to Agra.
 Two more such corridors are likely to come up in India, which include
Champa-Kurukshetra (1,350 km) and Raigarh-Pugalur-New Trichur
(1,600 km).
 Now HVDC Light a trade name given by ABB and even though Siemens
kept that HVDC plus has been used which has basically HVDC a
different feature than the conventional HVDC, where they are using the
IGBTs.

Conclusion
45
 HVDC offers powerful alternative to interconnect two points in a
power grid, to increase stability of a power system, with its power flow
can be controlled rapidly and accurately.
 Substantial work has already been carried out by all the leading
manufacturers of HVDC and major equipment such as converter
transformer, Thyristor valves, wall bushings and other DC yard
equipment has been successfully type tested.
 This will built not only confidence in the mind of utilities but also
provide enough input for further research and development in this
field.

Glossary
46
 Back-to-back connection: In HVDC terms, links used to connect neighboring
grids are often referred to as “back-to-back” connections, indicating that the
distance between the two grids is minimal. Such connections are able to link
independent power grids, including those operating at different frequencies, and
enable power to flow from one grid to another.
 Blackout: A complete loss of power resulting from damage or equipment failure in
a power station, power lines or other parts of the power system. A blackout may also
be referred to as a power outage or power failure.
 Converter: An electrical device, comprising a rectifier and inverter, used to alter
the voltage and frequency of incoming alternating current in an electrical system.
The term may also refer to inverters, rectifiers or frequency converters.
 HVDC Light: An adaptation of classic HVDC, developed by ABB in the 1990s. It
can be used to transmit electricity in lower power ranges (tens of MW) to an upper
range of 1,100 MW. The superior controllability is achieved by using IGBTs (i.e.
transistors) as the power electronic device used for the conversion.

References
47
 HVDC Power Transmission System : (New Age Publication) – K R
Padiyar
 EPRI Power system Engg. Series Book: ( Mc Graw Hill Education
Publication ) – Prabha Kundur
 Siemens HVDC references
 http://www.energy.siemens.com/co/en/power-
transmission/hvdc/references.htm#
 http://www.energy.siemens.com/co/en/power-
transmission/hvdc/hvdc-classic.htm
 ABB HVDC references
 http://new.abb.com/systems/hvdc/references
 www.abb.com/glossary
 http://sarienergy.org/oldsite/PageFiles/What_We_Do/activities/HVD
C_Training/Presentations/Day_1/1_HVDC_SYSTEMS_IN_INDIA.pdf
 www.researchtrend.net/ijeece/pdf/17%20AHUTI.pdf

ANY QUERRY…..??
48
Thanks
for your attention !!!

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