1. Upgrading to 66kv: Switchgears and
Transformer Technology Challenges
By Dr. Magdalena Kurkowska
Transfer to 66kv inter-array voltage systems presents an attractive option, but also generates a number of
challenges to address. One of them is availability and compatibility of switchgears and transformers for 66kv
installations. Although there is an array of high voltage switchgears and transformers readily available on
the market, the offshore wind industry has its own unique requirements that cannot be compromised. These
specifications include: limited space available for installation, exposure to the high shocks and accelerations
at every stage, from transport, construction to installation, operation and commissioning on site, as well as
highly saline environment.
The question then arises: is the current switchgears and transformers technology adequate for 66kv offshore
installations? If not, what technological advancements would benefit the industry?
Wind turbine switchgears require downsizing
An increase of the inter-array voltage from 33kV to 66kV generates construction constrains and new
requirements for offshore wind turbine switchgears.
Although there are standard switchgears for rated voltages of up to 72.5kV available on the market, most of
them have too large dimensions to be installed inside the turbine tower. It is also possible to install a switch-
gear in a separate compartment placed underneath the nacelle or mounted in a lower location outside the
tower. These solutions, however, create incremental costs, as additional structural support structures are
required.There is a clear demand for small, compact-size switchgears suitable for offshore applications. Some
manufacturers (like Alstom and Siemens) already offer standard SF6 insulated high voltage switchgears that
can be installed within the transition compartment of wind turbine structure and able to accommodate
single, double or triple cable system connections.
Siemens compact gas-insulated switchgear launched last year only needs 200 cubic meters of space. When
used on an offshore platform, the platform size can thus be decreased by approximately 10 percent, the
company claims. Other makers are developing similar, space-saving products.
At present 66kv switchgears used in offshore wind projects are significantly more expensive than 33kv switch-
gears. Hopefully, new developments would lead to substantial price reductions.
Environmental and standards challenges
Insufficient level of standards for high voltage (HV) switchgears used in offshore wind turbines is another
issue. The gap covers such critical matters as: the shocks and accelerations, or the environmental
conditions the equipment is required to resist. Some developers, like Alstom used similarities in the
earthquake-withstand capability when designing and testing their products.
Another, difficult to ignore industry challenge is uncertain future of sulphur hexafluoride (SF6)-insulated
switchgear technology.
www.66kv-offshore-wind.com

2. Search of SF6 replacement
Perceived as almost perfect insulation material, sulphur hexafluoride (SF6) has been commonly used in
switchgears’technology from 1970s. Since then, the evidence of environmental impacts associated with SF6
has started to emerge: sulphur hexafluoride has been recognised as the most potent greenhouse gas, with
a global warming potential 23,900 times that of CO2. Moreover, if released to the atmosphere, the gas could
impact the climate for up to 3,200 years. As a result, SF6 has been placed on the Kyoto list of substances the
use and emission of which must be minimised.
In accordance with the European Union’s F Gas Regulation from 2014 aiming to bring
down the EU’s F-gas emissions by two-thirds from 2014 levels by 2030, the usage of
sulphur hexafluoride is now banned in most applications, with the exception of medium-
voltage and high-voltage (above 52 kV) switchgears. But even so, the future of SF6 remains uncertain.
Although the 2014 EU F Gas Regulation does not impose restrictions on the use of sulphur hexafluoride in
medium and high voltage systems, it does contain a provision for reviewing the situation in 2020. A complete
phase-out of SF6 cannot therefore be ruled out. Some industry experts go even further dubbing SF6 as a
‘yesterday’s technology’ and projecting the legislative changes are unavoidable.
This uncertainty makes a strong case to avoid SF6 switchgears in new installations, but are there any viable
alternatives? In fact, the situation is not so‘rosy’for the 66kv industry, with only few readily available SF6-free
models on the market.
Sulphur hexafluoride technology obsolescence creates demand for new solutions and some market players
have already responded to this requirement. Alstom, for example, has announced development of new SF6-
free switchgear suitable for use in 66kv offshore installations. Instead of SF6, the technology uses so-called g3
(green gas for grid), non-toxic, non-flammable gas mixture. Developed in cooperation with 3MTM, the new
solution is compliant with switchgears specifications, such as: voltage withstand, current interruption, heat
transfer, low temperature application, compatibility with switchgear materials, stability versus temperature
and time and easy handling for filling and topping-up.
Transformers installation challenges
When it comes to wind turbine transformers for 66kv, availability of products is not such a problem.
Transformers with 66kv as primary voltage represent a proven technology. Liquid-filled transformers are
available as required, while the choice of dry type transformer models suitable for 66kv installation is
somewhat limited at present, but growing. Instead, it is the installation where the challenge lies.
Transformers can be installed either in the nacelle, the transition piece, the turbine tower, or outside the
tower, within an external container, protecting the transformer against elements.The optimum solution is the
installation of transformer in the nacelle close to the generator. Due to limitedspace inside the nacelle,
installation of the transformer can be troublesome. Because of larger electrical design clearances and volume
of insulation materials required, 66kv transformers feature bigger dimensions than their 33kv counterparts of
the same transformation capacities.
www.66kv-offshore-wind.com

3. Therefore, slim and compact transformers are the best choice for 66kv applications. Suitable technologies
include forced cooled KFAF transformers. Other solutions, such as Bio-Slim transformers, using biodegradable
ester as the insulation material, can be also considered.
Maintenance regimes for switchgear components are similar to 33kv systems. Dry-type transformers
require visual inspections and checking of winding temperature controls. In exceptional cases cleaning of
windings and tightening of mechanical parts might be necessary. For oil filled transformers, the maintenance
requirements are higher and include ongoing oil analyses to ensure safe operation of the transformers.
Protection of steel tanks and cooling systems against corrosion caused by humid and saline marine air is also
necessary.
Dry type transformers are therefore easier to maintain. On the other hand, they are more sensitive to
electrical creep, condensation, partial discharges, temperature variations, cracks, and contaminations than
oil-filled models.
Substations equipment and organisation will be affected
As a result of switching to higher inter-array voltage, the substation equipment and organisation
will be also affected, as heavier and larger components will be required: the average weight of 33kv
substation switchgear is 1.4 t, while of 66kv: between 1.9 and 117t. They will also require more space. The
currently available models include: Siemens 8DN8-72.5kV, ABB ENK gas insulated switchgear, or Alstom grid
F35-72.5 kV gas insulated substation. There will be also a requirement for a larger auxiliary generator set.
Larger, heavier and more expansive reactive power compensation equipment will be also needed to
prevent outages and secure uninterrupted smooth operation of the wind farm. Industry experts estimate the
cost of enhanced reactive power compensation equipment for 66kv technology to be about 50% higher in
comparison with a 33kv equivalent. It will also demand increased maintenance and inspection efforts.
Summary
An increase of the inter-array voltage from 33kV to 66kV generates new requirements for offshore wind
turbine switchgears, transformers and other equipment. At present, the major problems the industry faces
are: lack of readily available compact switchgears models, high component prices due to limited competition
on the market and products availability, installation challenges due to space constrains, as well as uncertainty
about the future of SF6 switchgear technology. All this challenges creates demand for new models of switch-
gears and transformers, specifically adapted to offshore wind industry specifications.
www.66kv-offshore-wind.com