CIRED
17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
TRANSFORMERS FOR WIND TURBINES
NEED FOR NEW DESIGNS OR BUSINESS AS USUAL
Jan DECLERCQ
Pauwels International - Belgium
jan.declercq@pauwels.com
INTRODUCTION
The wind energy industry is booming at European and
international level with annual growth levels reaching up to
30%. The liberalisation of the electricity market, the need for
more generating power, the concept of decentralized
generation and distribution, the acceptance of wind turbines
in the landscape and the awareness of environmental
constraints were the major drivers. However, due to the
sharp increase in demand, the technology development
efforts increased and came out with new techniques leading
to estimated cost reductions of 5 % per year. The major
technology changes lead to an increased reliability, reduced
maintenance as well as an increased efficiency and power
output. This power output per tower has gone up from 400
kW over 1.2 MW to 2.5 MW nowadays and prototypes for 3
and 4 MW are being tested.
The eye-catchers of the technology are the wind blades and
towers but inside and also indispensable, the gearbox, the
generator and the link to the grid. This latter role is carried
out by switchgear and the transformer stepping up the
voltage from the generator (400-690 V) to a medium voltage
level for the distribution grid (10 000 – 33 000 V).
This paper will present an overview of technologies suitable
for step up transformers that can be used in wind turbines.
Dry type transformer, conventional transformers based on
the cellulose mineral oil insulation system and new
insulation systems will be discussed. Secondly, it will
highlight on the potential problems that can occur when
installing distribution transformers close to windturbine
generators. Numerous failures have already been reported in
Europe and this needs attention, especially when the
transformer goes off shore.
TRANSFORMER TECHNOLOGIES
For the transformer, the wind turbine industry has the choice
between dry-type transformers and liquid immersed
transformers. The fundamental difference is the electrical
insulation medium namely air and resin for dry type
transformers and an insulation paper and liquid for the liquid
immersed transformers. The conventional liquid immersed
transformer uses cellulose and mineral oil, an advanced
technology referred as SLIM® uses a high temperature
aramid insulation NOMEX® and silicone liquid. The
insulation structure is also needed for the heat management
and mechanical integrity.
The functional specifications for both technologies are the
same, and both technologies meet the requirements as shown
in table 1.
Power
MVA
High Voltage
kV
Low Voltage
V
Impedance
%
FIGURE 1. Typical windfarm with
step up- transformer inside
These step up transformers are subject over longer periods
to higher loads when the wind speed reaches more than 10
m/sec. The generator causes also more voltage and power
fluctuations stressing the integrity of the transformer.
Harmonics, fast transients and vibrations makes it even
harder.
PAU_Declercq_A1
Session 4 Paper No 25
Mineral
oil
cellulose
0.5 – 10
… 1000
10 – 36
…. 750
400–3000
4 – 10
Cast resin
and air
SLIM®
0.5 – 5
1 – 10
10 – 33
10 – 36
400 – 3000
400 – 3000
4 – 10
5 - 10
TABLE 1. Specifications for transformer technologies.
The specifications are generalised and every technology has
exceptions. The liquid immersed transformer with the
mineral oil and cellulose technology is the only technology
nowadays that allows voltages above 500 kV and power
ratings above 500 MVA.
So any transformer technology is allowed for its use in the
wind turbine. But what about the other specifications such as
-1-
CIRED
17th International Conference on Electricity Distribution
no load losses and load losses, dimensions, harmonics and
loading, fire behavior and last but not least the environmental
issues.
LOSSES AND DIMENSIONS
A comparative study is done in table 2 for SLIM® and
conventional transformers from 1 to 2 MVA.
No load losses
The levels of no load losses have a direct financial impact on
the transformer design and its operational use. Lower no load
losses require a larger magnetic circuit increasing the
dimensions and cost of the transformer.
The liquid immersed transformers have usually up to 50 %
lower no load losses for the same dimensions leading to an
attractive operational financial benefit. The annual savings of
for example 1900 W (the difference between 2 transformers
of 2 MVA ) is 1.9 kW x 8760 h/y x 0.05 €/kWh which equals
832 € for one turbine in service for one year.
Barcelona, 12-15 May 2003
Dimensions can be crucial for wind turbines. In towers
nowadays of 100 m height and a small door of 780 mm, the
electrical components have to fit in easily and perform their
job of delivering high power at severe operating conditions
for the expected life. Conventional mineral oil transformers
tend to become bulky because of their cooling equipment.
For outdoor installations and substations is this no problem.
Cast resin transformers can be made more compact but
enclosures are needed to respect the safety clearances for the
high voltage equipment. Consequently larger dimensions are
needed.
The new technology SLIM has been special designed for
wind turbines, with more power for less space and increased
reliability. It can be designed with the smallest dimensions
and lowest mass. This allows flexibility for tower and nacelle
installations and has to option to be replaced in case of an
emergency or fault as can be seen in figure 2.
Load losses
The three technologies can deliver the same load loss
performance e.g. high or low load losses. These losses are
proportional to the power squared and are only applicable
when the transformer is loaded. Note that the losses must be
specified at the expected operating temperature of the
transformer. In addition, the heat generated by the
transformer losses has to be evacuated from the turbine. This
can impose an extra requirement for lower load losses in
specific wind turbine designs. .
FIGURE 2. Compact transformer for in tower installation
Dimensions
POWER
Pnll
Pll @ 75°C
Pll @ 125°C
Uk
Temperature rise
Length
Width
Height
Height cover
Mass Liquid
Mass Total
W
W
W
%
K
mm
mm
mm
mm
kg
kg
1000kVA
Conv
SLIM
1450
1450
13000
11300
13000
6
6
60/65
80/120
1840
1640
1000
940
1805
1775
1305
1275
535
510
2415
2250
1250kVA
Conv
SLIM
1650
1550
16000
14500
16500
6
6
60/65
80/120
1860
1670
1000
970
1865
1880
1365
1380
620
620
2800
2700
1600kVA
Conv
SLIM
2000
1700
20000
17500
20000
6
6
60/65
80/120
2100
1900
1000
1000
2200
2000
1600
1500
825
715
3600
3250
2000kVA
Conv
SLIM
2650
1850
25500
22000
25000
6
6
60/65
80/120
2160
1900
1410
1230
2500
2200
1900
1600
1035
780
4440
4030
TABLE 2. Losses and dimensions of step up transformers, convetional versus SLIM design.
LOAD, AMBIENT AND HARMONICS
PAU_Declercq_A1
Session 4 Paper No 25
-2-
CIRED
17th International Conference on Electricity Distribution
As mentioned before, the high load will lead to an increase of
the losses and an increase of the transformer temperature.
This temperature rise is limited to 60 K for mineral oil and 65
K for cellulose-designed transformers. According IEC
standards, the maximum average monthly temperature for a
FIGURE 3 : Current I __ __ __ , losses ……., transformer temperature -.-.-.
and lifetime ______ in function of load.
Barcelona, 12-15 May 2003
100.0
1600
90.0
1400
80.0
1200
70.0
1000
60.0
50.0
800
40.0
600
30.0
400
20.0
Temp geno 1
Temp geno 2
T bearing A
T bearing B
Gearbox
Nacelle
Geno coll air
Gearb. Bearing
shaft bearing
Trafo 1
Trafo 2
Trafo 3
Outside door
Power
200
10.0
0.0
22:34:14
21:34:14
20:34:14
19:34:14
18:34:14
17:34:14
16:34:14
15:34:14
14:34:14
13:34:14
12:34:14
9:34:14
11:34:14
8:34:14
10:34:14
7:34:14
6:34:14
5:34:14
4:34:14
3:34:14
2:34:14
1:34:14
0:34:14
23:34:14
22:34:14
21:34:14
20:34:14
19:34:14
18:34:14
17:34:14
16:34:14
15:34:14
14:34:14
13:34:14
0
Time
Load P
transformer is 30ºC with a maximum daily temperature 40ºC
so the maximum temperature of the transformer is 40ºC + 60
K which totals to 100ºC. Above 100ºC, the insulation
materials start to age very fast according the Montsigner
relation.
L
In a wind turbine, transformers do operate in the vicinity of
this ageing point due to the higher air temperatures in the
tower or nacelle, the full load for several days and the content
of current harmonics. A power derating is usually advised e.g
a 1000 kVA transformer for a 900 kW turbine.
The SLIM® transformer has different insulation materials that
can withstand temperatures up to 200ºC without accelerated
loss of life. On site temperature measurements have been
performed on a 2 MVA 20 kV transformer to verify the
1600
40
1400
35
1200
30
1000
25
Po
we
r 800
20
600
15
400
10
200
5
0 10
:3
4:
14
14
:3
4:
14
18 22 2: 6:
:3 :3 34 34
4: 4: :1 :1
14 14 4 4
10
:3
4:
14
14
:3
4:
14
18 22
:3 :3
4: 4:
14 14
2: 6:
34 34
:1 :1
4 4
10
:3
4:
14
14
:3
4:
14
18
:3
4:
14
22 2: 6: 10 14 18 22 2:
:3 34 34 :3 :3 :3 :3 34
4: :1 :1 4: 4: 4: 4: :1
14 4 4 14 14 14 14 4
Time
6:
34
:1
4
10
:3
4:
14
14 18 22 2:
:3 :3 :3 34
4: 4: 4: :1
14 14 14 4
6: 10
34 :3
:1 4:
4 14
14 18 22 2:
:3 :3 :3 34
4: 4: 4: :1
14 14 14 4
6: 0
34
:1
4
FIGURE 4 : Loading of a 1600 kVA transformer at different windspeeds
compatibility of a transformer in a tower. Typical loading
profiles and corresponding temperatures are shown in figures
4 and 5 and are meant for illustration of the correlation
between temperatures and loading in turbines.
PAU_Declercq_A1
Session 4 Paper No 25
FIGURE 5. Transformer and air temperatures in ºC
in a windturbine
FIRE BEHAVIOUR
The Materials and the Transformer
The fire behaviour of the transformer depends on the
materials used inside the transformer. Conventional mineral
oil filled transformer contain mineral oil, which is a class O
product and has a fire point of 150ºC. A silicone filled
transformer has class K3 meaning a fire point above 300ºC
namely 360ºC. It is difficult to compare with the dry type
transformer technology but the fire properties are comparable
to a silicone oil filled transformer for a F0 cast resin
transformers and slightly better for a F1 cast resin
transformer.
In addition to the ignition values, smoke, heat release, toxic
gasses can also be important but in general, the silicone oil
filled transformers and cast resin transformer can be treated as
good fire retarding products.
Standards, Laws and Policies
All transformer are treated as passive to fire e.g. they will
very unlikely start the fire themselves. If an external fire
reaches the products, both will burn.
A lot of good information is available in the Standards such
as Cenelec standard HD637 S1:1999 on "Installation of
electric equipment" and IEC 60695-1-40,TS,Ed1 : Fire hazard
testing - Part1-40,"Guidance for assessing the fire hazard of
electro technical products - Insulating liquids".
In general, no formal guidelines do exist for installation of
electrical products in wind turbines, the three transformer
technologies are allowed. Class O filled transformers can
always be used but some countries could require fireretarding walls. Class K and cast resin transformers do not
need special equipment. It will therefore depend on the local
policies and laws.
-3-
CIRED
17th International Conference on Electricity Distribution
ENVIRONMENTAL IMPACT
Barcelona, 12-15 May 2003
Point 12 tank long side
Silicone liquid will degenerate slowly into natural existing
products and shows no harmful or contamination effects. If
silicone liquid is spoiled in water (salt or not salt) it will not
react with the water and sink to the bottom and degenerate
slowly into carbondioxide, water and other natural products.
The liquid will not take oxygen out of the water.
Mineral oil will react with the environment and oxidize. It has
to be mentioned that transformers can be considered as leak
free. They leave the factory leak free and can only be
damaged during transport or installation. Once in service,
they remain closed for their lifetime.
ENCLOSURES AND CABLE BOXES
Hermetic sealed transformers are considered as maintenance
free. Hermetic sealed transformers are IP55 and are treated as
EMC-passive elements. This is valuable for off shore or
aggressive environment applications where moisture and salt
contaminate the air.
Cast resin type transformers should be cleaned and checked
yearly since the air is the insulation medium. Cast resin types
are IP00 so ‘live’ surfaces and have higher EM values, thus
need additional protection such as an enclosure.
The connections of all transformers can be protected with
shielded connectors and cable boxes.
1
XFRfunc Mag
What about the end-of-life of the transformer.
The copper and steel can be recycled for 100 %. For the
moment, silicone oil is burned in incinerators at the same cost
as mineral oil. However, an European recycling business for
silicone liquid is already active since 2001. Mineral oil can
also be recycled but is usually burned and used for production
of electricity. The windings of the cast resin transformer have
to be treated as waste or need energy and high temperature
burning.
A recycling company with factories in 4 European countries,
recycles oil filled transformers at 75 – 100 EUR/ton. PCB
contaminated transformers at 1200 EUR/ton and dry type
transformers at 150-160 EUR/ton.
0.6
0.4
0.2
0
0
25
50
75
100 125 150 175 200
Hz
FIGURE 4 Frequency response of a finned tank for frequencies up to 200
Hz. The resonance frequencies of 5 Hz and 100 Hz are not harmful since
their amplitude is lower than 1.
ON SITE EXPERIENCE AND RELIABILITY
The windturbine industry started with distribution transformer
with conventional insulation system and dry type and these
transformers were designed according the practices for step
down grids. Thousands of transformers were successful
designed, manufactured and tested according IEC standards.
Many of them are successful in operation but several
transformers in Europe failed. This was applicable for
different manufactures, power ratings and voltages. An
increased failure rate of factor 3 is seen compared to a step
down grid.
The major reasons for failures are mentioned below :
•
•
•
•
VIBRATIONS
A few cases have been reported where heavy vibrations
occurred in the nacelle leading to damages in the transformer.
Vibrations have been reported from 5 Hz to 100 Hz and
accelerations up to 1 m/s2 or 0.1 g. These vibrations can lead
to fatigues of the material. Normally, transformers are
designed up to 3 g shock for transport reasons.
Additional tests can be carried out as shown in figure 4 to
verify if resonances are present in the frequency domain of
interest.
0.8
•
•
Voltage fluctuations due to generator and step up
function causes frequent saturation of magnetic
circuit
Very sharp load fluctuations (from 10 % to 100 %
load in less a minute) causing thermal stresses in
mainly the solid insulation
Average loading of transformer is mainly 50 % but
loading reaches 100 % for several days. This causes
accelerated ageing of insulation if designs are not
optimised.
Proximity of low voltage and high voltage circuit
breakers and frequent switching causing additional
switching electrical stresses. Failures have been
reported for more than 50 transformers.
Vibrations for nacelle installations causing
weakening of solid insulation, reduced connection
contacts and failures with even fires.
Moisture and reduced maintenance leading to start
up problems.
In some of the cases, the failures lead to fires causing a
burned out wind turbine. In other cases, it lead to a 50 %
failure rate for an off shore wind park where due to bad
weather conditions, some turbines could not be energized
again for 3 months.
Contractors and wind turbine manufactures must be aware
that although a transformer has its outstanding reputation in
reliability, it still can fail. When transformers are installed
PAU_Declercq_A1
Session 4 Paper No 25
-4-
CIRED
17th International Conference on Electricity Distribution
inside turbine towers and due to transformer dimensions and
door restrictions cannot be taken out in case of failure, serious
problems can arise. Worse case scenarios such as lifting of
tower in order to replace a transformer will lead to enormous
costs. Some wind turbine manufactures take therefore very
compact transformers for.
Barcelona, 12-15 May 2003
Transformers are treated passive to fire and otherwise;
precautions or selection of materials can be made in order to
be in line with the local policy and laws.
The transformer is very recyclable up to 98 % for liquid filled
transformers and up to 85 % for dry type transformers.
Enclosures and cable boxes can be installed in order to protect
the transformer or its live contacts from the environment.
In order to make a choice between different technologies, the
Total Cost of Ownership can be computed where all aspects
are taken into account and not just the purchase price. The
capitalization of losses and the extra costs in case of a
replacement need to be considered.
FIGURE 6 : Typical tower installation over a substation containing a
compact SLIM® transformer.
Consequently, various fine tuning in design, manufacturing
and testing is needed for a successful operation and
acceptable maintenance. The new generation of conventional
transformers as well as the SLIM® transformer has met
several of those new criteria.
CONCLUSIONS
Three transformer technologies have been presented which
can be used for step up transformers in the wind turbines.
Next to power, high voltage and low voltage specifications,
several additional features are highlighted.
Load losses and especially no-load losses have a direct impact
on the transformer design and its operational cost during its
lifetime.
Dimensions can be critical if in tower or nacelle applications
are required. The SLIM® transformer with advanced materials
can be made very compact.
PAU_Declercq_A1
Session 4 Paper No 25
-5-