The document provides information on high voltage direct current (HVDC) transmission systems. It discusses the history of HVDC transmission dating back to the 1930s. It also outlines some of the key reasons for the development of HVDC transmission including allowing power transfer between unsynchronized AC systems and different frequency grids. The document then describes different types of HVDC transmission technologies including monopolar, bipolar, and homopolar configurations. It also discusses multi-terminal and back-to-back HVDC systems. In conclusion, the rapid growth of HVDC transmission is driven by increasing renewable energy, grid interconnections, and more reliable electricity supply.

1. Concepts of High voltage DC
Transmission (HVDC)

2. History:
 The development of direct current (DC) transmission dates back to the
1930’s in Sweden and in Germany, and has been a proven technology since
the first major installations in 1954.
 Over the last 70+ years, DC projects have shown to offer significant
electrical, economic, and environmental advantages when transporting
power across long distances.
 Early commercial installations included one in the Soviet Union in 1951
between Moscow and Kashira, and a 100 kV, 20 MW system
between Gotland and mainland Sweden in 1954.
 The longest HVDC link in the world is currently the Xiangjiaba–
Shanghai 2,071 km ,±800 kV, 6400 MW link connecting the Xiangjiaba
Dam to Shanghai, in the People's Republic of China.
 Early in 2013, the longest HVDC link will be the Rio Madeira link in Brazil,
which consists of two bipoles of ±600 kV, 3150 MW each, connecting Porto
Velho in the state of Rondôniaia to the São Paulo area, where the length of
the DC line is 2,375 km.

3. What led to invention of HVDC
 HVDC allows power transmission between AC transmission
systems that are not synchronized.
 Since the power flow through an HVDC link can be
controlled independently of the phase angle between
source and load, it can stabilize a network against
disturbances due to rapid changes in power.
 HVDC also allows the transfer of power between grid
systems running at different frequencies, such as 50 and
60 Hz. This improves the stability and economy of each
grid, by allowing the exchange of power between
previously incompatible networks.

4. Concepts of High voltage DC Transmission
(HVDC)
 A high-voltage direct current (HVDC) electric power transmission system (also
called a power superhighway uses direct current (DC) for electric power
transmission, in contrast with the more common alternating current (AC)
transmission systems.
 Most high voltage links use voltages between 100KV to 800KV world wide.
 The modern form of HVDC transmission uses technology developed extensively in
the 1930s in Sweden and in Germany.
 UHVDC (ultrahigh-voltage direct-current) is shaping up to be the latest
technological front in high voltage DC transmission technology. UHVDC is defined
as DC voltage transmission of above 800 kV (HVDC is generally just 100 to 800 kV).
 In 2010, ABB Group built the world's first 800 kV UHVDC in China. The Zhundong–
Wannan UHVDC line with 1100 kV, 3,400 km length and 12 GW capacity was
completed in 2018. As of 2020, at least thirteen UHVDC transmission lines in
China have been completed.

5. Why HVDC is in rapid rise Globally
 The installation of HVDC systems is increasing at a rapid pace around the
world, including in Europe, North and South America, and China.
 Currently there are more than 300 different HVDC links world-wide.The
current rate of rise is around 10%.
 Factors leading to such high growth: Along with the increase in renewable
energy capacity, growth of cross-regional electricity trading, and rising
demand for a more reliable electricity supply, another factor behind this is
that the economical justifiability for using HVDC to strengthen grid
connections has been demonstrated by actual HVDC projects, as well as the
cost-benefit analysis. Rapid technical progress in voltage source converter-
type HVDC (VSC HVDC) has also contributed significantly to this outcome.

8. Methods and working

12. Monopolar HVDC Transmission System :
An HVDC link that uses only a single conductor is known as a mono-polar link.
Usually, in this type of link, only a single conductor with negative polarity is
used, in order to reduce corona and interference. Earth or water is used as the
return path.
However, a metallic conductor is used as a return path when earth resistivity is
very high. The power and current flows only in one direction. For monopolar
transmission systems, the rated current is from 200A to 1000A. The below
figure represents the mono-polar HVDC link.
Advantages of Mono-polar Link :
It uses only a single conductor. Hence, the design is very simple.
It requires less maintenance.
Because of high charging currents, these links are technically feasible than HVAC
systems.
It is economical.
Disadvantages of Mono-polar Link :
When a fault occurs on the conductor the entire transmission system is shut down.
These are used only for low-power rating links, like cable transmission.
It affects the magnetic compasses of ships when it passes over underwater cables.

18. An HVDC link that uses two conductors for transmitting the power and current
is known as bipolar links. Generally, these type of systems uses two conductors.
One with positive polarity and the other with negative polarity. The below
figure represents the bipolar HVDC link.
Under normal conditions, the current in the two poles is the same. Hence, the ground
current is absent. Whenever a fault occurs on these systems then they automatically
switch to the monopolar system by using earth as a return path conductor i.e., when
one pole undergoes fault condition, the other will continue to supply the load.
A single bipolar high voltage direct current line is equal to two ac transmission lines.
When compared to the monopolar link the voltage is twice between the poles in this
system. The mid-point of the converters are grounded. The voltage rating is given as ±
X KV where X represents voltage with its magnitude.

19. Advantages of Bipolar HVDC Link :
The transmission of power between two stations or on the mainline is
continuous.
The fault on one link does not affect the operation of another link.
During fault conditions, this link can also be used as the monopolar link.
The direction of power flow can be changed by changing the polarities of the
two poles.
The voltage in the bipolar link is twice between the poles when compared to
the voltage between the pole and the earth of a monopolar link.
Disadvantages of Bipolar HVDC Link :
Corona and radio interference is more when compared with a homo-polar
link.
The connection of a converter to a pole is complicated.
It is quite costly when compared to mono-polar links.

21. These links also use two conductors but of the same polarity. Usually of negative
polarity. When a fault occurs on the conductor the converters of the healthy pole are
quite enough to feed the remaining conductors. Which are able to supply more than
50% of the power. In this type of link, the earth is used as a return conductor. It also
acts as a monopolar link during faulty conditions.
Advantages of Homopolar HVDC Link :
It is comparatively cheaper than a three-phase ac line of the same ratings.
Corona and radio interference are greatly reduced with the use of negative polarity
conductors.
These links can be operated independently under faulty conditions.
The connection of the converter to the pole is not so complicated as the bipolar
link.
Disadvantages of Homopolar HVDC Link :
The presence of ground current may have an adverse effect on the pipelines
passing through the nearby areas.
It has limited applications due to the presence of ground currents.
The cost of the line increases for higher voltages.

26. A multi-terminal HVDC system consists of three or more converter substations in
which some of the converter stations act as the rectifiers and some of them as
the inverters. The substations are either connected in series or parallel according
to the requirements. The below shows the bipolar multi-terminal HVDC system.

29. Back to back systems:
It has no dc transmission line. Rectification and Inversion are done in the same
substation by a back-to-back converter. The figure below shows the back-to-back
HVDC coupling.
The back-to-back HVDC coupling is mainly used to interconnect two ac networks
operating at different frequencies. It also provides features like improving system
stability, rapid variations in the power exchange, and control over the magnitude
of voltage and frequency independently in two networks.
Advantages of Back to Back HVDC System :
1.The voltage and frequency can be controlled independently in two networks.
2.The power flow is fast, accurate, and fully controllable.
3.We can determine the power flow in the link.
4.Short circuit levels can be limited.
5.Coupling of two networks at different frequencies.
6.Daily and seasonal costs can be determined.
Disadvantages of Back to Back HVDC System :
1.Harmonics are generated.
2.These systems are very expensive because of complicated converters and dc switchgear.
3.When the system is nearer to the sea coast, insulators get contaminated with water.

31. Advantages of HVDC Transmission System :
1.HVDC transmission requires narrow tower whereas ac systems require lattice
shape towers, this makes the construction simple and reduces cost.
2.The ground can be used as the return conductor.
3.No charging current, since dc operates at unity power factor.
3.Due to less corona and radio interference, it results in an economic choice of
the conductor.
4.Since there is no skin effect in dc transmission the power losses are reduced
considerably.
5.Large or bulk power can be transmitted over long distances.
6.Synchronous operation is not required.
7.Low short-circuit current on dc line.
8.Tie-line power can be easily controlled.
9.Power transmission can be also possible between unsynchronised ac distribution
systems (interconnection of ac systems of different frequencies).
10.Cables can be worked at a high voltage gradient, which makes them more
suitable for undersea cables.
11.Power flow through the HVDC line can be quickly controlled.

36. Disadvantages of HVDC Transmission System :
1.It is very difficult to break the dc currents hence it requires a high cost of dc
circuit breakers.
3.Due to the generation of harmonics in converters, it requires ac & dc filters,
hence the cost of converting station is increased.
4.It requires continuous firing or triggering thyristor valves hence it is complex.
5.Converters have little overload capability.
6.HVDC system is not economical for primary transmission, sub-transmission,
and distribution hence it is not used.
7.HVDC substations have an additional loss at converter transformers and
valves. These losses are continuous.

38. conclusion
 Increasing demand of electrical power and
need for bulk efficient electrical power
transmission system lead to the
development of HVDC transmission system.
HVDC transmission system today become
one of the best alternative for transmitting
bulk power over long distance with very
less losses.

39. Thank You !

40. Comparison Between HVDC and HVAC System
Sl HVDC Transmission HVAC Transmission
1 It is economical for transmission of power above break-even point i.e.,
for long distance
It is economical for transmission of power below break-even point i.e., for small
distances.
2 The number of conductors required for transmitting power is less. The number of conductors required for transmitting power is more.
3 Does not require any intermediate substations for reactive power
compensation
Requires intermediate substations for compensation
4 Very fast and accurate power flow control is possible Power flow control is slow and is very difficult.
5 Skin effect is absent resulting in uniform distribution of current density
across the cross-section of the conductor
Skin effect is present due to which current density is non-uniformly distributed across
the cross-section f the conductor.
6 Corona loss and radio interferences are absent resulting in less
insulation level required for the transmission line
Corona loss and radio interferences are more due to which high insulation level is
required for the transmission line.
7 Voltage in the line does not fluctuate with the load Voltage in the line fluctuates with the load..
8 Transmission losses are less due to the absence of flow of reactive
power
Transmission losses are more due to the flow of reactive power
9 The fault levels of the two networks are unaffected and remain
unchanged when interconnected
Fault levels of two networks get added up and are increased after the interconnection.
10 The cost of right of way is less and the cost of supporting towers is less,
as this system requires narrow towers
The cost of right of way is more and the cost of supporting tower is more as this system
requires lattice-shaped towers.
11 DC breakers used in this system are of high cost, as it is difficult to
break dc currents
The circuit breakers used in this system are of low cost when compared to dc breakers.