This ppt is about the basic components of HVDC transmission system

1. HVDC
TRANSMISSION SYSTEM

2. CONTENTS
• Introduction
• History of DC power system
• Decline of DC system
• Limitations of HVAC Transmission
• Components of HVDC Transmission system
• Types of HVDC lines
• Disadvantages of HVDC system
2

3. Introduction
• High voltage direct current (HVDC) power systems use D.C. for
transmission of bulk power over long distances. For long-distance
power transmission, HVDC lines are less expensive, and losses are less
as compared to AC transmission. It interconnects the networks that
have different frequencies and characteristics.
• In a combined AC and DC system, generated AC voltage is converted
into DC at the sending end. Then, the DC voltage is inverted to AC at
the receiving end, for distribution purposes. Thus, the conversion and
inversion equipment are also needed at the two ends of the line. HVDC
transmission is economical only for long distance transmission lines
having a length more than 600kms and for underground cables of
length more than 50kms.

4. HISTORY OF DC POWER SYSTEM
The first complete electric power system comprising a generator, cable, fuse,
meter, and loads was built by Thomas Alva Edison – the historic Pearl Street
Station in New York City which began operation in September 1882.
This was a dc system consisting of a steam-engine-driven dc generator
supplying power to 59 customers within an area roughly 1.5 km in radius. The
load, which consisted entirely of incandescent lamps, was supplied at 110 V
through an underground cable system.
Within a few years similar systems were in operation in most large cities
throughout the world. With the development of motors by Frank Sprague in
1884, motor loads were added to such systems. This was the beginning of
what would develop into one of the largest industries in the world.

5. DECLINE OF DC
POWER SYSTEM
In the 1890s, there was considerable controversy over
whether the electric utility industry should be
standardized on dc or ac. By the turn of the century,
the ac system had won out over the dc system for the
following reasons:
• Power could not be generated at higher voltages.
• Transmission losses are
huge when transmitting power on dc.
• Voltage levels can be easily transformed in ac
systems, thus providing the flexibility for use of
different voltages for generation, transmission, and
consumption.
• AC generators are much simpler than dc
generators.
• AC motors are much simpler and cheaper than dc
motors.

6. LIMITATIONS OF HVAC TRANSMISSON
• As the distance increases, HVAC lines suffer from higher losses and increased
reactive power requirements, limiting the feasible transmission distance.
• HVAC transmission lines experience higher resistive losses compared to HVDC
lines, especially over long distances. This is primarily due to the skin effect and
proximity effect, which cause the current to concentrate near the surface of
the conductor in HVAC lines, leading to increased resistance and losses.
• HVDC transmission generally offers higher efficiency compared to HVAC
transmission, particularly for long-distance transmission and interconnection of
asynchronous grids.
• The Cables used for underground & underwater power transmission have
parasitic capacitance. It does not supply power unless it is fully charged.
• Integrating HVAC transmission lines into synchronous AC grids requires precise
synchronization of generators, which can be challenging, especially across large
interconnected networks.

7. COMPONENTS OF HVDC SYSTEM
The component used for this system is listed below.
• Converter Transformer
• Converters
• Filters
• Smoothing Reactor
• DC Transmission Lines or Cables
• Reactive Power Source

9. CONVERTER
TRANSFORMER
• The converter transformer is different than
the conventional transformer used in the HVAC
transmission line. Because this transformer is connected
with the power electronics equipment and designed to
withstand DC voltage stress and harmonic currents. In
the converter transformer, the harmonic content is
higher compared to the conventional transformer.
Hence, it causes more leakage flux and it forms local
hotspot in the winding. So, these transformers require
extra cooling arrangements to avoid the effect of a
hotspot.
• The step-up transformer is used to increase the voltage
level at sending end and the step-down transformer is
used to decrease the voltage level at receiving end of
the line. There are different configurations according to
the application i.e. two three-phase units or three
single-phase units used.
• Converter transformers are designed to withstand high
insulation levels to cope with the stresses associated
with HVDC transmission, including transient voltages
and switching surges.

10. CONVERTER STATION
• The converter provides the transformation
from AC to DC or DC to AC as required. The
basic building block of the converter is the
six-pulse bridge. However, most HVDC
converters are connected as twelve-pulse
bridges. The twelve-pulse bridge is
composed of 12 “valves” each of which
may contain many series-connected
thyristors in order to achieve the DC rating
of the HVDC scheme.
• The role of rectifier and inverter stations
can be reversed (resulting in power
reversals) by suitable converter control.

11. FILTERS
• The converters used here are power electronics switches.
The harmonics are produced due to switching in converters at
both ends of a transmission line. These harmonics are
transferred to the AC system. And that can be led to
overheating of equipment. Therefore, it is necessary to
reduce or eliminate harmonics.
• These filters are used on both the AC side and DC side. The
filter used in the AC system is known as the AC filter and the
filter used in the DC system is known as the DC filter. It
consists of series combinations of capacitors and inductors
and tuned to eliminate the expected harmonic frequencies.
• The AC filters provide low impedance and used passive
components. AC filters provide the reactive power required
for the operation of the converter. The DC filters small in size
and less expensive compared to the AC filters. The intensity
of harmonics is less in voltage source converters compared to
the line commutated converters.

12. SMOOTHING
REACTOR
• Smoothing Reactor is connected in series
with the converter at the DC side. It is
used to make current ripple-free and
decrease the harmonics in the DC system.
It is also used for protection purposes
by limiting the fault current. It can be
connected on the line side or neutral
side.
• The smoothing reactors are also used to
regulate the DC current. If a sudden
change occurs in the DC current, it will
oppose and allow the DC current to flow
at a fixed value. Hence, it reduces stress
on the converter valve by preventing
sudden changes. The smoothing reactor is
an oil-cooled reactor with high
inductance.

13. REACTIVE POWER SOURCE
• Reactive power is required for the operation of the converter. This power can be supplied through
the capacitor bank, synchronous condenser, or a suitable generating station locate near the converter.
• Capacitor Banks: Capacitor banks are one of the most common reactive power sources used in HVDC
systems. These banks consist of arrays of capacitors connected in parallel. By connecting or disconnecting
capacitor banks, reactive power can be injected or absorbed from the AC system as needed. Capacitor
banks are particularly effective for providing reactive power support during leading power factor
conditions.
• Synchronous Condensers: Synchronous condensers are rotating synchronous machines that operate
without any mechanical load. They are connected to the AC grid and generate or absorb reactive power
by adjusting their excitation level. Synchronous condensers are capable of providing both leading and
lagging reactive power support and are often employed in HVDC systems to enhance voltage stability and
control.
• Static Var Compensators (SVCs): SVCs are power electronics-based devices that can rapidly inject or
absorb reactive power to regulate voltage and improve power factor. They consist of capacitors and
reactors controlled by thyristor-based switching circuits. SVCs offer fast response times and precise
control over reactive power injection, making them suitable for supporting HVDC transmission systems.
• In the case of line commutated converter, the reactive power required is between 40-60% of its power
rating. This demand can be reduced by converter transformer having a sufficient range of on-load tap
changers. The harmonic filter supplies some amount of reactive power.

14. HVDC transmission lines and cables
• According to the type of HVDC system, the number of conductors
is selected. It is used to transmit the HVDC power from the
sending end to the receiving end. In the DC system, there is no
skin effect due to the absence of frequency. And compared to the
HVAC system, the size of the conductor is small for the same
power.
• Conventional conductors used in HVDC transmission lines are
often made of aluminum or aluminum alloy strands.
• Composite conductors consist of a combination of different
materials, such as aluminum strands reinforced with carbon or
glass fibers.
• HVDC transmission cables are another type of conductor used in
underground HVDC transmission projects. These cables consist of
multiple insulated conductors enclosed in a protective sheath.

15. TYPES OF HVDC LINES
• According to the arrangement of HVDC lines, the
transmission lines are classified into four types.
• Monopolar System
• Bipolar System
• Homopolar System

16. MONOPOLAR SYSTEM
• In this type of HVDC system, only one conductor is used to make a connection
between sending end and receiving end. And the ground or seawater is used for
the return path. Hence, the cost of this system is less compared to other systems.
But it is not useful for high power applications.
• The positive or negative polarity is used to transmit power. Generally, negative
polarity is used on overhead lines because the radio interference is lesser. The
block diagram of this system is as shown in the below figure.

17. BIPOLAR SYSTEM
• This system is the most commonly used compared
to other HVDC systems. In a bipolar HVDC system,
two conductors are required. One conductor is a
positive conductor and the other is a negative
conductor of the same magnitude with respect to
earth. This type of system arrangement is used to
transmit power for long-distance.
• Two converters are used at each end of the line.
The neutral point is grounded. Therefore, each
line can operate independently. Hence, it is a
combination of two monopolar systems. If a fault
occurs in one conductor, the second conductor
will continuously transmit power and increase the
reliability of the system. In this condition, the
ground is used as a return path and it works as a
monopolar system. When a fault is cleared, the
system again works as a bipolar system.
• In this type of system, normally the current is not
flowing through the earth. But sometimes the
balance current will flow through the earth. The
circuit diagram of this system is as shown in the
below figure.

18. HOMOPOLAR SYSTEM
• In this type of system, two or more conductors are used on the same tower. The
polarity for all conductors is the same and most probably it is negative polarity. For
return, path earth is used. In this type of system, corona loss and radio interference
are very less.
• If a fault occurs in one conductor, the second conductor will continue to transmit the
power. In this condition, more than 50% of rated capacity power can transmit by
overloading on a conductor. But it will increase the power loss. The main disadvantage
of this system is the large return current. The block diagram of this system is as shown
in the below figure.

19. ADVANTAGES OF HVDC TRANSMISSION
• In HVDC Line, the phenomenon of Skin Effect is absent. Therefore current flows
through the whole cross section of the conductor in HVDC
• there is no charging current involved in HVDC which in turn reduces many
accessories.
• As we know that, Corona Loss is directly proportional to (f+25) where f is frequency
of supply. Therefore for HVDC Corona Loss will be less as f=0. As Corona Loss is less in
HVDC therefore Radio Interference will also be less compared to HVAC.
• A DC link may interconnect two AC systems at different frequencies.
• The current carrying capacity of HVDC cable is considerably large.
• Insulation level of a DC line is lower than that of AC line and a DC line has two
connectors instead of three for AC lines.
• HVDC transmission generally offers higher efficiency compared to HVAC transmission,
particularly for long-distance transmission and interconnection of asynchronous
grids.

20. DISADVANTAGES OF HVDC SYSTEM
• The converters and filters are used at both ends of the transmission line
and the cost of this equipment is very high.
• The cost of a DC circuit breaker is very high compared to the AC circuit
breaker.
• The transformers are used to change the voltage level in AC systems.
But it is very difficult to change the voltage level especially in case of
high voltage for DC systems.
• Under abnormal conditions, it is very difficult to control the converter.
And it required advanced knowledge and technology of power
electronics. And also, there is a problem with the cooling of power
electronics switches used in the converter.
• Huge Reactive Power Requirements at Converter Terminals

21. CONCLUSION
We have explored the potential of High Voltage Direct
Current (HVDC) transmission systems in modernizing our
power grids and fostering a more sustainable energy
future. We have delved into the technical intricacies and
operational advantages of HVDC technology. We have also
seen how HVDC systems offer solutions to various
challenges faced by traditional AC transmission. As
techonlogy is growing and power demand is increasing,
HVDC could suitable for transmission of power.

22. THANK YOU