Conventional and emerging converter technologies in hvdc power transmission system

By: Naveed Pirzado, Prov. Licensed Electrical Engineer, Canada
Conventional and Emerging Converter Technologies
in HVDC Transmission System around the World

Centre for Research in Applied Measurement and Evaluation
Contents
• Introduction- why HVDC?
• Current Source Converter (CSC)
– Operational Characteristic
– Limitation and Advancements
• Voltage Source Converter (VSC)
– Operational Characteristics
– Different type of topologies
– Advancement and Limitations
• Comparison between CSC and VSC topologies
• Modular Multilevel Converter (MMC) – New emerging technology
– Operational Characteristics
– Different type of topologies
– Advancement and Limitations
• Conclusion
© Naveed Pirzado, PMP, Electrical Engineer

Introduction
• The efficiency and rated power carrying capacity of DC transmission lines highly
depends on the converter for transforming the current (AC to DC and vice versa).
• Till to-date, different HVDC converter topologies have been built and utilised
throughout the world. Three dominant types of converter topologies
1. Line commutated converter current source converter (CSC)
2. Self Commutated voltage source converter (VSC)
3. Modular Multilevel Converter (MMC)
• In this presentation, the thorough literature review of several IEEE journals and recent
conference papers have been carried out and a summary has been developed providing:
“the overview of different topologies (CSC, VSC and MMC), their
operational characterises, advantages, limitations and the latest
developments”.

Introduction (Contd.,)
 HVDC stands for High Voltage Direct Current. It is an
efficient technology designed to deliver large amount of
electricity over long distances with negligible losses.
 The world’s first commercial HVDC link situated between
the Swedish mainland and the island Gotland was
delivered by ABB in the year of 1954 with the capacity of
20MW, 100kv.
 The Cost benefit analysis is shown in the graph.
 The longest HVDC link in the world is currently the
Xiangjiaba–Shanghai.
Total length - 2071km
Power ratting - 6400MW
DC voltage - 800KV

HVDC configurations and operating modes

Current Source Converter
• Until 1970, it was the dominant power electronics technology in HVDC system for converting
AC to DC at natural commutation.
• With current rating up-to 6250A combined with blocking voltage capacity of 10KV
makes CSC highest voltage and power rating level among all HVDC converter
technologies.
• Many of HVDC system have the configuration of 2, SIX pulse bridge, fed with AC source, in
which each bridge consist of thyristor valves, and each valve has series of thyristors. Key
features include:
– Switches are switched at line frequency 50/60 Hz.
– consumes reactive power at Ac side
– Need ac voltage for commutation
• Major Advantage:
– The most suitable way of transmitting bulk power
over large distances.

1: As, the ac outputs contain significant low order harmonic, which will cause various power
quality problems, leading to higher losses. Massive filters to be installed on both ac and dc side
for the smooth power outputs.
2: Due to absence of turn off capabilities of thyristor, the commutation of the switch is driven
by the interconnected ac grid and the current is always lagging i.e. converter keeps absorbing
the reactive power from the grid. As a result, massive reactive power compensation equipment
is essential on ac side.
3: CSC based HVDC is dependent on the strength of interconnected AC source. Particular, at
the inverter side, slight disturbance in the ac system may lead to commutation failure in the
converter, consequently leading to system breakdown.
Some major drawbacks related to CSC topology is mentioned below

• Major challenging problem still faced by line commutated current source converters is
commutation failure specially at inverter side.
• A new CCC topology (Capacitor Commutated Converters) has been introduced to
mitigate this problem as shown in Fig.
The highlights of this new topology include, but not limited to:
• Commutation Capacitor is installed between converter transformer and thyristor
valve
• CC provides part of the commutation voltage and reactive support.
• Reduced probability of commutation failure.
Major Challenge and Advancements in CSC topology

Voltage Source Converter
• Develop in the 1990’s with the first project
commissioned by ABB, 1997.
• It is self-commutated HVDC technology
• VSC converters operate at a high frequency with
PWM technique which allows simultaneous
adjustment of the amplitude and phase angle of
converter while keeping the voltage constant.
Line Commutated Converter (CSC) Self Commutated Converter (VSC)
• Use thyristors • Use IGBTs
• Higher Power Ratings: up to 10GW • Rating limited today: 1GW
• Required stronger AC system • Operated into weaker AC systems
• Required additional equipment for “Black” start
capability
• “Black” start capability
• Generated harmonic distortion AC & DC
harmonic filter requires
• Insignificant level of harmonic generation, hence
no filters required
• Power is reversed by changing polarity of the
converters
• Power is reversed by changing direction of
current flow
• Large site area, dominated by harmonic filters • Compact site area, 50-60% of LCC site area.

Comparison of equipment requirements for CSC (Conventional) HVDC and VSC (Light) HVDC

Voltage Source Converters (Cont.,)
• VSC can operate in all four quadrants of the P-
Q operating plan without external reactive
power compensation.
Some key benefits of VSC include:
– reduced risk of commutation failure
– improved harmonics performance.
– converters offer economic solution with
increased active and reactive power
controllability,
Following are main VSC converter topologies.
1. Two level converter
2. Three level converter
3. Modular Multi level Converter

Two Level Voltage Source Converter
• VSC has switching devices which operate to
produce two levels of voltage, where one switch
conducts at one time.
• Output voltage is (+Vdc/2, - Vdc/2)
• Major Draw Back:
– High switching losses
– Lower transmission efficiency
than CSC
• Despite several drawbacks, this topology is
popular for low voltage applications up to
1MVA

Three Level Voltage Source Converter
(i) Diode Clamped Neutral Point (NPC) topology.
– Can handle twice the direct voltage, twice the
power of a two level VSC converter.
• Drawbacks:
– Not feasible for higher level due to cost,
mechanical design complexities.
– An other draw back , outer valves face higher
losses than inner valves.
(ii) Active Neutral Point Clamped
– active switches connected in parallel to the NPC
diodes for clamping the neutral tap of the converter.
– It provided three extra switching states as
compared to the conventional three level NPC VSC
topology, which enables the possibility to
minimize the temperature imbalance and to
increase the output frequency or the converter
power

Five Level Voltage Source Converter
(iii) Five level Flying Capacitor topology.
– Produces same waveform as multilevel diode
clamped neutral point topology
– Has not additional diodes, but DC capacitors
– Phase redundancies which allows specific choice of
capacitor to be charged or discharged for voltage
balancing across different levels.
– Can control real and reactive power flow
• Drawbacks:
– As the level increases, so does the size of the
capacitors, and it becomes bulky
– the control to track the voltage for all the capacitor
becomes complicated as it requires high frequency
switches.

Modular Multilevel Converter (MMC)
• A new alternative of VSC HVDC topology
proposed in 2001, at the university of Bundeswehr
in Munich, Germany, by Dr. Rainer Marquardt.
• A ‘valve’ in the converter arm is no longer a single
IGBT/Diode module but a controllable VSC itself
which is usually termed as submodule (SM)
• Converter consists of six symmetrical arms, and
each arm has a series connection of N nominally
identical submodules and one inductor where each
submodule is comprised of two semiconductor
switches and a capacitor.
• This topology is scalable, and highly modular. It
provides high quality of the output voltage
waveform which requires almost no filtering.

MMC (Contd.,)
• MCC can adopt: (Half bridge) or (Full Bridge) configuration.
• (1) Half Bridge MMC:
– Most promising and wide known topology in HVDC system.
– The advantage of half-bridge structure is a smaller
switching element count and reduced power loss.
• Drawbacks
– this structure cannot limit the over-current resulting from a dc
side fault.
• (ii) Full Bridge MMC:
– Can produce a reverse voltage which opposes the driving ac
voltage and hence provides a negative back emf to the flow of
the fault currents resulting from the dc side faults.
• Drawbacks
– larger switching element count
– converter cost and operating losses higher as compared to the
half-bridge

The main problem with conventional half bridge MMC is absence of DC fault blocking capability and
below mentioned table provides an overview of the different configuration of MMC to mitigate this problem
and based on the IEEE paper “A Hybrid MMC Topology with dc Fault Ride-Through Capability, SDSM
configuration is more suitable to mitigate this problem.
New configuration: Series-double MMC

Conclusion & Future Works
Based on the discussions in previous sections, it is evident that VSC is the backbone
of todays modern system and remains the most preferred choice in transmitting
renewable energy such as wind, power and solar system either offshore or onshore
systems.
MMC is the most cost effective and promising technology being investigated in
HVDC system. As discussed in previous section, extensive research and efforts are
continued to improve the conventional MMC topology.
Some recent IEEE publication including such as :
1. Hybrid MMC Topology with dc Fault Ride-Through Capability for multiterminal DC
Transmission System
Have proposed the a cost effective and technical design improvements for
conventional MMC topology (as discussed previously) and is definitely a future.

References
1. Mohamed H. Okba, M. H. Saied, M. Z. Mostafa, and T. M. Abdel-Moneim, "High voltage direct current transmission - A review, part I," IEEE, 2012
2. M. P. Bahrman, "HVDC transmission overview," in Transmission and Distribution Conference and Exposition, 2008;IEEE/PES, 2008.
3. S. Mohamed Yousuf, M. Siva Subramaniyan, "HVDC and Facts in Power System," International Journal of Science and Research, 2013.
4. Vassilios G. Agelidis, Georgios D. Demetriades, and Nikolas Flourentzou, "Recent Advances in High-Voltage Direct-Current Power Transmission Systems," IEEE
inter. Coriference on Industrial Technology, ICIT 2006, pp. 206-213.
5. Nikolas Flourentzou, Vassilios G. Agelidis, and Georgios D. Demetriades, "VSC-Based HVDC Power Transmission Systems: An Overview," Power Electronics, IEEE
Transactions on, vol. 24, pp. 592-602, 2009.
6. Mohamed H. Okba, M. H. Saied, M. Z. Mostafa, and T. M. Abdel-Moneim, "High voltage direct current transmission - A review, part II – Converter Technologies”,
IEEE/ 2012.
7. Udana Niranga Gnanarathna, “Efficient Modeling of Modular Multilevel HVDC Converters (MMC) on Electromagnetic Transient Simulation Programs”, Ph.D.
Thesis, 2014.
8. Alf Persson , Lennart Carlsson, Mikael Aberg, “New technologies in HVDC converter design,” AC and DC Power Transmission, Sixth International Conference on
(Conf. Publ. No. 423), 1996
9. Lennart Carlsson, “‘Classical’ HVDC: Still continuing to evolve,” Modern Power Syst., vol. 22, no. 6, 2002.
10. James Varley, “HVDC: Fifty years on,” Modern Power Syst., vol. 24, no. 10, 2004.
11. Ying Xue, Xiao Ping Zhang and Conghuan Yang, “Elimination of Commutation Failures of LCC HVDC System with Controllable Capacitors”, IEEE
TRANSACTIONS ON POWER SYSTEMS, VOL. 31, NO. 4, JULY 2016.
12. Amirnaser Yazdani, “Modeling, Control and Applications of Voltage-Sourced Converters in Power Systems”, John Wiley & Sons, 2010.
13. Kamran Sharifabadi, Lennart Harnefors, Hans-Peter Nee, Staffan Norrga, Remus Teodorescu, “Design, Control, and Application of Modular Multilevel Converters”,
2016 John Wiley & Sons.
14. Oluwafemi E. Oni, Innocent E. Davidson, “A Review of LCC-HVDC and VSC-HVDC Technologies and Applications”, IEEE, 2016
15. Jih-S. Lai and Feng Z. Peng, “Multilevel converters—A new breed of power converters,” IEEE Trans. Ind. Appl., vol. 32, no. 3, Jun. 1996
16. B. R. Andersen, L. Xu, P. J. Horton, and P. Cartwright, “Topologies for VSC transmission,” Power Engineering Journal, June 2002.
17. Xinhan Meng, Ke-Jun Li, Zhuodi Wang, Wenning Yan, and Jianguo Zhao, “A Hybrid MMC Topology with dc Fault Ride-Through Capability for MTDC Transmission
System”, Mathematical Problems in Engineering Volume 2015, Article ID 512471.
18. Gregory J. Kish, Mike Ranjram, and Peter W. Lehn, “A Modular Multilevel DC/DC Converter With Fault Blocking Capability for HVDC Interconnects”, IEEE
TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 1, JANUARY 2015

Conventional and emerging converter technologies in hvdc power transmission system

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