Challenges for Reliable Offshore Transformers

1
Challenges forReliable Offshore Transformers

• Dr. Jan Declercq, Raymond Van Schevensteen
• Pauwels International N.V.
• Belgium

2
Transformers as critical components

• Wind turbines are electricity generator units
  therefore all components are critical
• Mechanical blades, gearbox, tower,
• Electrical generator, converter, transformer,
  circuit breaker, control unit,
• Medium Voltage Transformer has to be designed as
  generator step-up transformer and fully
  integrated in wind turbine performance and
  specifications and now Multi Megawatt Turbines
  for offshore installation

3
Operating conditions are different

• Normal transformer in normal environment1000 kVA
  in apartment

4
Operating conditions are different

• Generator step-up transformer in WTG

5
Operating conditions are different

• Generator step-up transformer in wind
  turbineMulti Mega Watt offshore

6
So MMW WTG needs GSU Transformer

• EHV transmission Generation
• 100 MW gt220 kV
• Transmission
• Sub-transmission 110 70 kV
  Embedded generation
• Distribution 0.5-4 MW 30 kV

Overloading, power quality, economics,
environment, maintenance, reliability
7
Types of medium voltage transformers

• Mineral liquid-filled transformers
• Cast resin transformers
• SLIM and Bio-SLIM technologyFire
  bio-degradable liquid and Nomex insulation

Same operating range of voltages and power What
about other specifications?
8
Functional Specifications of transformers

• Power
• Liquid-filled up to 10 MVA and high voltages 36
  kV and above
• No load losses
• As low as possible, always energized
• Liquid filled 50 less losses compared to dry
  type
• Ex. 2 kW 8760 h/y 0.05 EUR/kWh per turbine
  per year
• Load losses at working temperature customized to
  needs
• Medium voltage
• 12 kV, 24 kV, 36 kV insulation level up to BIL
  200 kV
• Low Voltage
• 400 V, 660 V, 690 V, 1000 V, 3000 V, 6000 V
• Impedance
• 6 up to 14 for weak grids
• Dimensions
• Mass for nacelle, reduced dimensions, fits the
  door

These specifications are not sufficient to
guarantee performance and reliability
9
Operating conditions Thermal Stresses

• Average 60 loading but
• Full loading for several days Short term peak
  overloads Sharp load fluctuations HarmonicsHigh
  er air temperature in turbine

2,000
1,500
1,000
0
200
400
600
800
1000
1200
1400
1600
10
Operating conditions Thermal Stresses
Tins 98 C
Current I Losses RI2 Temperature Lifetime
Load P
Thermal stresses give mechanical stresses and
accelerated ageing So verify insulation
materials, overload tests, higher MVA, cooling
inside
11
Operating conditions Electrical Stresses

• Different electrical environment
• Switching surges from Circuit Breakers
• Overfluxing from overvoltages
• Lightning strikes
• Frequency variations
• Voltage dips
• Causing higher electrical stresses on electrical
  insulation system
• Increased risk for initiating Partial Discharges
• So verify electrical performance, specify extra
  testing
• Example PD tests, chopped wave

12
Switching surges
RESONANCES IN TRANSFORMER WINDINGS
SOURCE
VICTIM
COUPLING
It is an EMC problem breaker and transformer can
be less compatible in the specific network
arrangement
13
Operating conditions Mechanical Stresses

• Mechanical environment
• Compact design
• Optimized dimensions for inside installation
• Optimized connections
• In case of nacelle design, vibration
  testsincluding connection of cables

Verify mechanical design and test
accordingly Verify sensitivity of mechanical
stresses on electrical insulation system
14
Operating conditions Chemical Stresses

• Environment
• Cooling air
• Humidity
• Dust, salt
• Aggressive environment
• People

Verify protection of electrical system Specify
higher IP class, C5 offshore painting, air
filters, extra housing Maintenance interventions
for transformer or peripherial equipment
15
Operating conditions Safety

• Specify good protection and materialsCast resin
  F1 or class K3 liquid

Cast resin transformer versus silicone liquid
filled transformer tested according Cenelec
16
Operating conditions Safety

• Tests performed at Ineris France, February and
  April 2004

17
New challenges

• Wind offshore
• WTG go to higher MVA ratings from 2 MW to 6 MW
• Connection voltages mostly 33 kV
• Extreme reliability (MTBF gt 500 years)
• Reduced maintenance

So optimize transformer technology, design,
manufacturing and testing to meet the wind
turbine demands
18
New markets

• Wind offshore
• gt3 MVA transformers for offshore and nacelle
• UK, 7 GW projects
• Belgium, NL 300 MW
• Spain, 2000 MW
• Denmark, 400 MW
• Germany, 68 GW

19
Conclusions for offshore transformers

• Different transformer technologies are available
  for multi-megawatt turbines
• Dimensions, losses, reliability (MTBF),
  protection, electrical environment are important
  because the operating conditions in a turbine are
  severe!
• Fire behavior of dry type and high temperature
  liquid filled transformers are comparable
• Good protection and prevention is always needed,
  independent of transformer technology
• Verify compatibility for aggressive offshore
  environment
• Improve specifications for WTG transformers
• Improve testing specifications for WTG
  transformers
• IEC TC14 Transformer Committee will write
  standards for wind turbine use (approved 2005,
  start 2006)