HVDC stands for High Voltage Direct Current and
is today a well-proven technology employed for
power transmission all over the world.
HVDC technology is used to transmit electricity
over long distances by overhead transmission
lines or submarine cables.
It is also used to interconnect separate power
systems, where traditional alternating current
(AC) connections can not be used.
HVDC links can be broadly classified
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
This configuration type is the first step towards a bipolar link.
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).
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.
2. Smoothing reactors
3. Harmonic filters
4. Reactive power supplies
6. DC lines
7. AC circuit breakers
Components of HVDC 
Objectives of Control
◦ Efficient and stable operation.
◦ Maximum flexibility of power control without
compromising the safety of equipment.
◦ Principle of operation of various control
◦ Implementation and their performance during
normal and abnormal system conditions.
Direct current from the rectifier to the inverter
Power at the rectifier terminal
Power at the inverter terminal
Schematic diagram of control 
Internal voltages and can used be
controlled to control the voltages at any point on
the line and the current flow (power).
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).
Power reversal is obtained by reversal of polarity of
direct voltages at both ends.
To transmit a scheduled power, the corresponding
current order is determined by:
Bridge/converter unit control
Determines firing angles and sets their limits.
It coordinates the conversion of current order to a
firing angle order, tap changer control and other
BLOCK DIAGRAM OF CONTROL IMPLEMENTATION
HVDC links can stabilize AC system frequencies and
voltages and help with unplanned outages.
Renewable electricity superhighways in Smaller scale
A 500 V,1500 W,810Km bipolar HVDC transmission line
is set up between Rihand & Delhi.
In Vindhyachal back to back link is laid for exchange of
power between Northern & Western regions.
•For equivalent transmission capacity, a
DC line has lower construction costs than
an AC line.
•A double HVAC three-phase circuit with 6
conductors is needed to get the reliability
of a two-pole DC link.
•DC requires less insulation.
•For the same conductor, DC losses are
less, so other costs, and generally final
losses too, can be reduced.
•An optimized DC link has smaller towers
than an optimized AC link of equal
Price vs distance 
Reactive power requirement
Short Circuit Current
Independent Control of ac system
Fast change of energy flow
Lesser Corona Loss and Radio interference
• DC lines and cables are cheaper than ac lines or
• The towers of the dc lines are narrower, simpler
and cheaper compared to the towers of the ac
• Line losses in a dc line are lower than the losses in
an ac lines.
Cost vs distance  Distance vs losses 
The disadvantages of HVDC are in conversion, switching
Expensive inverters with limited overload capacity.
Higher losses in static inverters at smaller transmission
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
because some mechanism must be included in the circuit
breaker to force current to zero, otherwise arcing and
contact wear would be too great to allow reliable
HVDC is very important issue in transmission energy.
Problem of cascade blackout, can be reduced by
application of HVDC.
Very large investments in e.g in China and India shows
that high-voltage direct current will very important in
the future, especially in big, new-industries countries.
Recent studies indicate that HVDC systems are very
The data collected from 31 utilities says that forced
unavailability of energy due to the converter station is
The scheduled unavailability of energy is about 5.39%.
 Lips H P. “Aspects of multiple in feed of HVDC
inverter station into a common AC system”, IEEE
Trans. on Power Apparatus Sys. vol .2,pp 135-141,
 Xiao Wang, Boon-Tech Ooi. "High Voltage Direct
Current Transmission System Based on Voltage
Source Converters", IEEE Trans.on HVDC Sys.
 R. N. Nayak, R. P. Sasmal, Y. K. Sehgal, et al, "AC/DC
interactions in multi-infeed HVDC scheme: a case
study," IEEE Power India Conference, pp. 5-9, 2006