Download Converter transformers: key HVDC systems component

Main feature
chapter IiI
Making energy available to all, today and tomorrow
800 kV hvdc transformer on test
Converter transformers: key HVDC systems
Converter transformers are crucial
to an HVDC transmission system. Their robustness
and reliability are of paramount importance for the
transmission system’s availability. The increase of power
demand creates new challenges for their design.
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Alstom Grid///Spring-Summer 2011
High voltage direct current (HVDC)
transmission is today widely recognised
as an effective and economic solution
for transporting bulk power over long
distances through overhead lines or cables.
Applications are steadily growing around
the world, especially in major emerging
markets like China, India or Brazil, where
there is a great need for transmitting large
alstom grid team at ptr manufacturing plant in wuhan, china
components
amounts of electric energy from remote
power generation sites (hydroelectric
plants, wind farms, etc.) to urban or industrial consumption centres situated hundreds or even thousands of kilometres
away. In other cases, HVDC systems may
be applied to interconnect alternating
current power grids that may differ in
frequency or stability level. With the
increased demand for electricity, DC transmission capacity can now reach figures of
up to 6,400 MW with ultra high voltage
levels (UHVDC) of up to ± 800 kV – or even
more in the future.
System availability depends
on transformer reliability
“Converter transformers are the key and the
most expensive components of HVDC transmission systems,” says Milan Saravolac, R&D
Director of Alstom Grid’s Power Transformer
Activity. HVDC converter transformers serve
to connect the AC system (of the power
generation side) to the DC transmission
system, and at the other end, the DC transmission lines to the AC system (of the power
consumption side).
In modern UHVDC bi-pole transmission
schemes utilising two 12-pulse converters
per pole, 24 large single-phase transformers are needed at each end of a line.
“Reliability of these UHVDC converter
transfor­mers is of paramount importance
as the outage of only one of them could
result in the de-energisation of the
Alstom Grid///Spring-Summer 2011
35
Main feature
chapter IiI
Making energy available to all, today and tomorrow
whole pole and associated transmission line,” explains Saravolac. In its latest
reliability surveys, the International Council
on Large Electric Systems (CIGRE) still points
to transformers as the dominant contributors
to failure statistics; however, there has been
significant improvement in recent years
thanks to the application of more stringent
IEC standards and test requirements, rigorous design reviews, use of modern condition
monitoring equipment, and improvements
in the design of the insulation system.
The size issue
HVDC converter transformer design faces
many challenges. As they are used as an
interface between AC and DC transmission
systems, they have to handle superimposed
requirements related to both sides. This
means that in addition to meeting the same
design and performance requirements as
any other power transformer in an HVAC
system, HVDC converter transformers need
to meet additional ones with regard to the
Converter
transformers are the
key components of
HVDC transmission
systems. connection to the converter station and
exposure to DC voltages and currents.
Another challenge is size. Since the HVDC
voltage and power levels are steadily rising,
there is an ever-increasing need to raise the
power and voltage rating of individual converter transformers. The size – and weight
– of UHVDC converter transformers is
gradually reaching physical limits imposed
by transport constraints (rail gauges, tunnel
width, load limitations, and so on). Special
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Alstom Grid///Spring-Summer 2011
Alstom Grid 256 MVA, 3-phase,
358 kV HVDC transformer on test.
M o r e
modular designs may therefore be required,
such as the use of separate tank compartments for housing the UHVDC connection
between the transformer active part and
terminals. “The requirement to contain the
active part of a UHVDC converter transformer within a limited
space, including all
UHVDC interconnections, is something of
a challenge in terms of
design and optimisation of the transformer
dielectric system,”
Saravolac adds. To fully
respond to this challenge without compromising quality and reliability, a full
understanding of the operational requirements and phenomena involved is critical,
together with the availability of sophisticated
design, modelling and simulation tools.
which enable the connection with the
UHVDC valve installation placed inside the
converter building. Like transformers,
bushings have to comply with combined
AC and DC requirements and stresses.
Additionally, for the UHVDC bushings
that are intended
to project directly into
the converter hall,
it is necessary that
oil is excluded from
the insulation system
in order to minimise
the risk of fire.
Drawing on its know­
ledge and experience
acquired with previous applications, Alstom Grid “has demonstrated a real team effort to rise to the
UHVDC challenge”. Converter transformers
designed by Alstom Grid have been selected
for the Rio Madeira project in Brazil, the
world’s longest HVDC system (2,375 km),
which is currently being constructed
between Upper Amazonia and the Sao
Paulo area.
Alstom Grid
has demonstrated
real team effort to
rise to the UHVDC
challenge. Specially designed bushings
UHVDC applications also create challenges
for critical components such as bushings,
David Wright
Testing an 800 kV
transformer in Wuhan
“There are few laboratories around
the world capable of testing bulky
800 kV DC equipment weighing over
160 tonnes,” says David Wright,
Development Project Manager.
Building on its European expertise,
Alstom Grid decided to transfer
the technology, processing and
manufacturing skills to its Wuhan
plant in China. There, a full scale
model of an 800 kV UHVDC
transformer was manufactured
by the local workforce and tested
at voltages up to 1,000 kV AC
and 1,300 kV DC. Ultra High
Frequency sensors and computer
analysis developed at Alstom Grid’s
Technology and Innovation
Centre in Massy, France, were used
in conjunction with ultrasonic guns
and an ultraviolet corona camera
to detect external discharge
and allow the elimination of sources
of noise in the test plant and its
connections. Testing was carried
out at each of the levels appropriate
for the transmission voltages of the
600, 660 and 800 kV DC schemes
being considered worldwide.
“The DC withstand and polarityreversal tests recorded no counts
of partial discharge at all, though
IEC specifications permit up to
30 counts of partial discharge
above 2,000 pC level. Testing was
a resounding success and a real
credit to the European and Chinese
teams,” concludes Wright.
Basic hvdc principle
Alstom Grid///Spring-Summer 2011
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