1111Challenges for reliable offshore transformersTRANSFORMING YOUR NEEDS INTO SOLUTIONS17-8-2015 Challenges for Reliable Offshore Transformers Dr. Jan

1111Challenges for reliable offshore transformers TRANSFORMING YOUR NEEDS INTO SOLUTIONS 119-04-23

Challenges forReliable Offshore Transformers

Dr. Jan Declercq, Raymond Van Schevensteen

Pauwels International N.V.Belgium

2222219-04-23 Challenges for reliable offshore transformers TRANSFORMING YOUR NEEDS INTO SOLUTIONS

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


Operating conditions are different

Normal transformer in normal environment1000 kVA in apartment



Generator step-up transformer in WTG



Generator step-up transformer in wind turbineMulti Mega Watt offshore


So MMW WTG needs GSU Transformer

EHV transmission Generation 100 MW >220 kV

Transmission

Sub-transmission 110 – 70 kV Embedded generation

Distribution 0.5-4 MW 30 kV

Overloading, power quality, economics, environment, maintenance, reliability


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?


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


Operating conditions Thermal Stresses

Average 60% loading but … Full loading for several days Short term peak

overloads Sharp load fluctuations HarmonicsHigher air temperature in turbine

-500

0

500

1,000

1,500

2,000

0 200 400 600 800 1000 1200 1400 1600


Operating conditions Thermal Stresses

Load P

Current I

Losses RI2

Temperature

Lifetime

Tins 98 C

Thermal stresses give mechanical stresses and accelerated ageing

So verify insulation materials, overload tests, higher MVA, cooling inside


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

systemIncreased risk for initiating Partial DischargesSo verify electrical performance, specify extra testingExample PD tests, chopped wave


SWITCHING PRE-STRIKE OR RE-STRIKE

SOURCE VICTIM

RESONANCES IN TRANSFORMER WINDINGS

-6

-4

-2

0

2

4

6

8

0 2 4 6 8 10 12 14 16 18 20time [ms]

volta

ge [k

V]

0

1

2

3

4

5

6

7

8

0 0.1 0.2 0.3 0.4 0.5time [ms]

volta

ge [k

V]

VOLTAGE TRANSIENT

THROUGH CABLE

COUPLING

Switching surges

It is an EMC problem: breaker and transformer can be less compatible in the specific network arrangement


Operating conditions Mechanical Stresses

Mechanical environment Compact design Optimized dimensions for inside installation Optimized connections In case of nacelle design, vibration tests

including connection of cables

Verify mechanical design and test accordingly

Verify sensitivity of mechanical stresses on electrical insulation system


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


Specify good protection and materialsCast resin F1 or class K3 liquid

Cast resin transformer versus silicone liquid filled transformer tested according Cenelec

Operating conditions Safety


Operating conditions Safety

Tests performed at Ineris France, February and April 2004


New challenges

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

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


New markets

Wind offshore: >3 MVA transformers for offshore and nacelle

UK, 7 GW projects Belgium, NL 300 MW Spain, 2000 MW Denmark, 400 MW Germany, 68 GW


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)

Documents

1111Challenges for reliable offshore transformersTRANSFORMING YOUR NEEDS INTO SOLUTIONS17-8-2015 Challenges for Reliable Offshore Transformers Dr. Jan