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HVDC System Solutions

N. M. Kirby, Senior Member, IEEE

than the conventional AC solutions. In particular, the need to

Abstract— This presentation will provide an introduction into connect renewable generation such as onshore and offshore
the main components which make up conventional Line wind from remote locations to population and load centers, is
Commutated Converter (LCC) HVDC, with ratings ranging making both long distance transmission and multi-terminal
from 75MW at the lower end, all the way up to 800 kV. This will HVDC transmission more attractive. The new VSC
include all of the HVDC configurations, including Back to Back, technology brings a more robust solution to the complexity of
Underground/Submarine Cable, Overhead Line, covering the
multi-terminal HVDC, and this is the focus of national,
main circuit components and other equipment which come
together to make up a complete working link. The presentation regional, almost continental scale grid developments using
will be supported by specific examples of each type of HVDC HVDC around the world.
project. In addition, the presentation will describe the new This paper and presentations will review the presently
Voltage Source Converters (VSC) technology which is available available LCC and VSC technology covering :
to power system planners. This technology offers benefits in • HVDC circuit configurations
function, footprint and flexibility when compared with LCC, and • Main circuit components and equipment
opens up whole new avenues of potential power system • Station Layouts
development in the future. VSC technology solutions will also be
described in similar detail to that of the LCC.
II. HVDC CONFIGURATION ALTERNATIVES
Index Terms-- HVDC substations, HVDC transmission, VSC, HVDC interconnections may be configured in a number of
Converter, Thyristor, IGBT different forms, namely:
• Back to Back : the converters are in the same
I. INTRODUCTION location, and the thyristor valves are normally in the

H VDC technology has been in mainstream use in power

systems for over 50 years and is now well matured, with
over 100 schemes in service worldwide, and this number
same building. The DC busbar is therefore short and
stays inside the valve hall building
• Cable Transmission: The converter stations are
continues to grow. The thyristor has been the exclusive geographically separated and connected by either
semiconductor in use for most of this period, with an LCC underground or submarine cable
HVDC link rating of ±500kV, 3000MW as the common • Line Transmission: The converters are generally
industry maximum. separated by longer distances and connected by an
However, in recent years there have been significant advances overhead line circuit.
in 2 directions: • Multi-terminal: 3 or more converters are
• Extending the LCC rating up through ±600kV, geographically separate and interconnected by either
±660kV and ±800kV, with planned development up to cables or lines.
±1100kVdc for China.
• Introducing VSC HVDC on a large scale, with ratings III. LINE COMMUTATED CONVERTERS
up to ±320kV, 1000MW, and increasing still further
as investment in development continues to take A. Main Circuit Components
advantage of new semiconductors. The main power circuit of an LCC HVDC converter station
The need for the development of power transmission systems consists of the following major areas and equipment:
to handle both a) the changing generation patterns from fossil- • Thyristor Valves – 12-pulse bridge configuration of
fuel based to renewables, b) ever increasing and moving load thyristors. Each Thyristor has individual auxiliary and
centers, and c) the lack of investment for many years, is gate interface circuits. Power transfer through the
bringing renewed focus on the benefits of HVDC and its converter is controlled by the HVDC Control System
ability to address many of these problems more effectively by adjusting the firing angle relative to the AC network
frequency. The valves are water-cooled.
• Converter Transformers – normally 3-winding star
N. Kirby is with Alstom Grid Inc., based in Philadelphia, USA (e-mail:
neil.kirby@alstom.com). primary with star and delta secondaries, to provide the
12-pulse phasing, providing a turns ratio designed in a
techno-economic optimization of valve costs. A

978-1-4673-1935-5/12/$31.00 ©2012 IEEE

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tapchanger adjusts the valve voltage to provide the

controls with a full firing angle range to maintain
power transfer under all network voltage conditions. In
addition it provides galvanic isolation between the AC
and DC circuits.
• AC Harmonic Filters – Provide shunt capacitance to
offset the reactive power absorption of the valves. Also
provide filtering of harmonics generated by the
thyristor valves, in the form of tuned, damped, or
bandpass filters.
• AC Switchyard – Breakers, disconnects and grounding
switches to connect each of the DC converters,
individual switching of shunt AC capacitor banks, as
well as AC lines and busbars, to provide as many
configuration options as possible to achieve the
necessary reliability of the overall HVDC link. Figure III-2 - HVDC Monopole Cable Converter Station
• DC Smoothing Reactor – reduces the 12-pulse ripple
which would ordinarily appear on the DC circuit, as
well as providing fault current limiting impedance.
• DC Harmonic Filters – tuned filters on the DC circuit to
reduce the 12th, 24th, etc. harmonics on overhead line
DC circuit in particular.
• DC Switchyard – provides reconfiguration alternatives
during normal operation as well as under fault
conditions in the DC circuit, including the HVDC line,
electrode line, converter, electrode, and in the DC area.

B. Station Layout Arrangements

Figure III-1, Figure III-2 and Figure III-3 show typical layouts Figure III-3 - HVDC Bipole Overhead Line Converter
for LCC converter stations, noting that the dominant area of Station
the footprint is the AC switchyard and the AC Harmonic
Filters. IV. VOLTAGE SOURCE CONVERTERS

A. Main Circuit Components

The main components in the power circuit of a VSC HVDC
system are as follows:
• IGBT Converters – commonly configured as Multi-
level Modular Converter (MMC) or Cascaded Multi-
level Converter, in a 6-pulse, symmetrical monopole
(i.e. converter bridge mid-point grounded, with +ve and
–ve DC conductor). Power transfer through the
converter is controlled by the HVDC Control System
by adjusting the voltage and phase relative to the AC
network frequency. The converters are water-cooled.
• Converter Transformers – Configured as 2-winding
units. IGBT converters do not generate harmonics to
the same extent as LCC converters, therefore the VSC
transformers do not have to handle this duty. Also,
Figure III-1 - Back to Back Monopole HVDC Converter since the symmetrical monopole configuration is used,
Station the transformer has no DC offset.
• Arm/Limb Reactors – These reactors are air-core, and
carry out several functions, (a) provide an impedance to
limit circulating currents within the converter, (b) fault
current limiting impedance, and (c) series impedance to
with voltage drop to facilitate power flow control.

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B. Station Layout Arrangements

V. CONCLUSION

Figures IV-1, IV-2 and IV-3 illustrate the layouts for VSC
converter stations. In VSC layouts, the main difference Several conclusions may be drawn from this discussion, based
between these and LCC is the absence of AC harmonic filters. on trends seen in the transmission and distribution world
today:
• The reliability and well-established performance of the
thyristor means that LCC HVDC will continue for the
foreseeable future to be the main choice for long distance,
bulk power transmission. This situation is likely to remain
until the alternative VSC semiconductors achieve similar or
better ratings and techno-economic performance than the
thyristor.
• The flexibility, additional features and reduced footprint of
VSC HVDC are already overcoming the obstacles of higher
cost and limited performance history. The use of VSC
technology will become predominant in the future in the
other end of the ratings range up to 1500 MW HVDC
systems, especially cable schemes (allowing the use of
lower cost XLPE cable).
Figure IV-1 - Back to Back VSC HVDC • The planned use of power electronics in all areas of
Converter Station transmission networks is increasing exponentially as remote
generation centers are connected to existing AC networks,
allowing more pro-active and automated control of the
flows of energy throughout the network.
• The use of power electronics in the form of AC/DC and
DC/DC converters in transmission and distribution
networks is likely to increase in the future, to manage the
increasingly diverse generation and load patterns in power
systems worldwide.

VI. BIOGRAPHIES

Neil Kirby joined Alstom in 1982 and has worked

Figure IV-2 - VSC Cable Monopole Converter Station
on the design of HVDC projects, in the areas of
control hardware and software, system design, and
overall project engineering. This has included
extensive periods of site work and commissioning
tests. In 2003 he moved to the USA where he is
presently Business Development Manager for the
Power Electronics business. He is a Senior Member
of IEEE and a member of CIGRE.

Figure IV-3 - Overhead Line VSC HVDC Converter

Station (Courtesy ABB)

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