The document discusses planning for HVDC transmission and modern trends in HVDC technology. When planning HVDC transmission, the key factors to consider are cost, technical performance, and reliability. Modern trends aim to reduce converter station costs while improving reliability and performance. This includes advances in power semiconductors, converter control technology, development of DC breakers, conversion of existing AC lines, and operation with weak AC systems. Emerging technologies discussed are active DC filters, capacitor commutated converters, and ultra-high voltage DC transmission.

1. Planningfor hvdc & modern trends
content
 Cost
 Technical performance
 Reliability
 Power Semiconductors and valves
Prepared By
Mr.K.Jawahar, M.E.,
Assistant Professor
Department of EEE
Planning for HVDC& Modern Trends
Kongunadunadu College of Engineering and Technology Department of EEE

2. Planning for HVDC transmission
• The system planner must consider DC alternative in transmission
expansion.
• The factors to be considered are
(i) Cost
(ii) Technical performance
(iii) Reliability
• Generally last two factors are considered as constraints to be met and
the minimum cost option is selected among various alternatives that
meet the specifications on technical performance and reliability.
• The considerations in the planning for DC depends on the application.
Two applications can be considered.
1. Long distance bulk power transmission.
• The DC and AC alternatives for the same level of system security and
reliability are likely to have the same power carrying capability.
Kongunadunadu College of Engineering and Technology Department of EEE
Planning for HVDC& Modern Trends

3. 2. Interconnection between two adjacent systems.
• AC interconnection posses several problems in certain cases.
• For the same level of system security( reliability ), the capacity of AC
interconnection will be much more than that for DC.
The choice for DC interconnection will be based on the following
considerations.
(i) Small fluctuation in the voltage and frequency do not affect the power
flow which can be set at any desired value.
(ii) The system security can be enhanced by fast control of DC power.
Having settled on the DC link for interconnection, there are three
possibility configurations for interconnections are
1. A two terminal transmission where each terminal is located at a
suitable place somewhere within the network and connected by a DC
overhead line or cable.
2. A back to back HVDC station along with AC feeder
3. A back to back station located close to the border between the two
systems.
Kongunadunadu College of Engineering and Technology Department of EEE
Planning for HVDC& Modern Trends

4. • The specification and design of DC system require an understanding of
the various interactions between the DC and AC systems.
• The interruption of power in a DC system link can occur due to:
(a) DC line faults
(b) AC system faults
The following aspects require a detailed study of the system interactions:
1. Var requirements of converter stations and voltage stability
2. Dynamic over voltages
3. Harmonic generation and design of filters
4. Damping of low frequency and sub synchronous torsion oscillations.
The converter control plays a major role in theses interactions and the
control strategy should be as to improve the overall system
performance.
Kongunadunadu College of Engineering and Technology Department of EEE
Planning for HVDC& Modern Trends

5. Modern Trends in HVDC Technology
• Modern Trends are used to reduce the cost of converter stations while
improving the reliability and performance.
Power Semiconductors and valves
•The cost of the converters can come down if the number of devices to
be connected in series and parallel can be brought down.
•The size of the devices has gone upto 125mm and there is no need for
parallel connection.
•The increase in the current rating of the devices has made it possible to
provide higher overload capability at reasonable costs and reduce the
lower limits on transformer leakage impedance thereby improving power
factor. The voltage ratings are also increase.
•Light triggered thyristors (LTT) improve the reliability of converter
operation.
•The cost of the valves will reduced by the application of zinc oxide
gapless arrestors and protective firing methods.
Kongunadunadu College of Engineering and Technology Department of EEE
Planning for HVDC& Modern Trends

6. Converter Control
• The development of microcomputer based converter control equipment
has now made it possible to design systems with completely redundant
converter control with automatic transfer between systems in the case of
a malfunction.
• The micro computer based control also has the flexibility to try adaptive
control algorithms or even the use of expert systems for fault diagnosis
and protection.
DC Breakers
• The development and testing of prototype DC breakers will be possible
to go in for tapping an existing DC link or the development of new
MTDC systems.
•The DC breaker ratings are not likely to exceed the full load ratings as
the control intervention is expected to limit the fault current.
Kongunadunadu College of Engineering and Technology Department of EEE
Planning for HVDC& Modern Trends

7. Conversion of existing AC lines
• The constraints on RoW are forcing some utilities to look into the
option of converting existing AC circuits to DC in order to increase the
power transfer limit.
Operation with weak AC systems
• The strength of AC systems connected to the terminal of a DC link is
measured in terms of short circuit ratio (SCR).
• SCR is ratio between Short circuit level at the converter bus to Rated
DC power.
• If SCR is less than 3, the AC system is said to be weak.
• The conventional constant extinction angle control have been suggested
to overcome some of the problems with weak AC systems.
• The constant reactive current control or AC voltage control have been
suggested to overcome some of the problems with weak AC systems.
Active DC filter
• It can be eliminate both characteristics and low frequency non-
characteristics harmonics.
• In India chandrapur-padghe HVDC project uses an active filter in each
pole.
Kongunadunadu College of Engineering and Technology Department of EEE
Planning for HVDC& Modern Trends

8. Capacitor commutated converter(CCC)
• Improved voltage stability when operating with weak AC systems,
reduced risks of communication failures, less load rejection over
voltages and reactive power requirements.
• The CCC can combined with AC filters that have low reactive power
ratings. These can be automatically tuned passive filters or active filters.
• The CCC was first applied in Garabi BTB project for power exchange
between argentina and brazil.
UHV DC transmission
• Ultra High voltage Dc transmission at 800kV is being considered for
transmission of large blocks of power above 3000MW for distance
beyond 1500km, in China, India, South Africa/Congo and Brazil where
large hydro power resources available in remote areas are developed.
Kongunadunadu College of Engineering and Technology Department of EEE
Planning for HVDC& Modern Trends

9. Reference:
• Padiyar, K. R., “HVDC power transmission system”, New Age
International (P) Ltd., New Delhi, Second Edition, 2010.
Planning for HVDC& Modern Trends
Kongunadunadu College of Engineering and Technology Department of EEE