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Transormer Technologies

Wind turbines require a grid connection where generation voltage is increased tomedium voltage by a step-up distribution transormer. This transormer can be in-stalled outside the turbine in a substation or as a pad-mounted solution. For insidetower or nacelle installation, the wind turbine industry has the choice between drytype transormers and high temperature liquid-flled t ransormers.Dry type transormers have been installed in wind turbines because o their frebehaviour and compact dimensions. Recently high temperature liquid-flled

transormers, such as SLIM®, with fre retardant liquid have been developed or themulti-megawatt turbines because o their compactness, perormance and high reli-ability. The insulation system consists o a silicone liquid and a high temperaturesolid insulation NOMEX®.

Functional specifcations

The electrical unctional specifcations or both technologies are the same, and bothtechnologies can meet the requirements such as low voltage, high voltage, power,impedance, load losses, no-load losses that a multi-megawatt turbine needs. Thehigh temperature design has more advantages :> On average the no-load losses or liquid-flled transormers are 50% lower lead-

ing to signifcant savings.> When high temperature insulation materials are used, the new transormers can

be designed much more compact and with improved thermal reliability.> Example : 4MVA SLIM® step-up transormer with 2 low voltages 3300V and690V, high voltage 33kV has dimensions o length 2800mm, width 1400mm andheight 2100mm. Load losses can be as low as 23000W and no-load losses6000W.

Operating conditions

The electrical environment o the step-up transormer is also dierent in a windturbine:> more switching surges with ast transient overvoltages in windings and system

voltage drops;> higher harmonic content;> overuxing due to voltage and requency uctuations.

> For transormers with open insulation systems such as dry type transormers,fltering and drying o the cooling air and maintenance is suggested especiallyor transormers in a humid and salty environment.

> For nacelle installations, vibrations with requency range 5Hz up to 250Hz.Taking all this into account, the transormer industry perceives liquid-flled trans-ormers in general to be more reliable.

Risk assessment and Standards

The likelihood o a transormer ailure is extremely low. This statement is justifedby our own experience (more than 10000 transormers in wind turbines), generaleedback and appreciation o users worldwide. The Mean Time Between Failures orliquid-flled transormers is > 500 years.International Standard IEC 60695-1-40 “Fire hazard testing: guidance or assessingthe fre hazard o electro technical products - insulating liquids” illustrates andconfrms the good fre saety record o liquid-flled transormers and that such in-cidents are very rare. Conventional transormers have a mineral oil class O1 liquid(liquid with fre point < 300°C and a net calorifc value > 42 MJ/kg). The SLIM® transormers use a silicone liquid and are classifed as class K3 (liquid with frepoint > 300°C and a net calorifc value < 32 MJ/kg). The IEC-guide states that fres

Fire behaviour o transormersChallenges or reliable step up transormers and substations or multi-megawatt turbines

temp

20’ 40’ timefre radiation+

radiation

temperature profle applied to thetransormers by fre and radiation

freproo transormer:did not contribute to fre

transormer partially contributed to fre,fre slowly dies out

non freproo transormer: largecontribution to fre, fre does not die out

> High temperature liquid-flled transormer aterfre test.

> Dry-type transormerater fre test.

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> Dry-type transormer with fre and heat radiation panel ater 2 min.

> Ater 40 min. No external heat generated. Transormer keeps burningor more than an hour.

> High temperature liquid-flled transormer ater 6 min. The highames are there because there is no air between the cooling fns andthe alcohol burns above the cooling fns.

> Ater 15 min. Only the alcohol burns. Later on the fre extinguishes.

Dry-type transormer

High temperature liquid-flled transormer

resulting rom class K liquid-flled transormers (such as silicone) are zero, whilesome 150000 o these transormers are in service since the late ‘70s.In case o a dry type transormer, the fre behaviour is classifed as F0 or F1 accord-ing to Cenelec Standard EN 60076-11 where a test is done on a set o a low and high

voltage winding.

Full-scale fre test on step-up transormers

Two transormers were tested at Ineris, France, namely a cast resin transormer1MVA 20kV 400 V with F1 fre properties and a 1.1MVA 10kV 400V SLIM® transormer.

The transormers were put in a thermal-aggressive environment characterized by afre rom alcohol below the transormers or 20 minutes and radiation or 2 panelsalong the side walls or 40 minutes with a heat input o 30kW/m2. It should benoted that this test uses the same conditions as the standard but the standard limitsitsel to one winding rom a dry type transormer. The temperature profle and sel-extinguishing eature result in its fre saety classifcation.

Discussion on the dry type transormer

Ater initiating the fre and radiation panels, the transormer leads to a combustionand smoke ater 6 minutes. The temperature o the external fre was 400°C whereasthe temperature on the central winding was between 800°C and 870°C ater 8minutes. This temperature dropped slowly to 700°C ater radiation and droppedurther to 400°C leaving the transormer burning on its own or more than an hour,producing dense black smoke.

Discussion on SLIM® and Bio-SLIM® transormers

The high temperature liquid-flled transormer has been put in identical conditionso fre and heat ux. The heat generated resulted in external temperatures o 500- 600°C and internal temperatures o 260°C. Due to the thermal expansion o the

liquid, the pressure rised to 1.6 bar. However, this pressure rise was not enough toopen the tank. The outside o the transormer suered no signifcant damage. Thetransormer did not participate in the external fre. In order to open the transormerand to note the eects, a more severe fre situation is needed. However, it isexpected that small leaks will occur and no rupture o the tank.

Conclusions

> Dierent transormer technologies are available or multi-megawatt turbines.> Dimensions, losses, reliability (MTBF), protection, electrical environment are

important because the operating conditions in a turbine are severe!> Fire behaviour o dry type and high temperature liquid-flled transormers are

comparable.> Good protection and prevention is always needed, independent o transormer

technology.> Other materials in turbines also have a fre load (nacelle, blades, cables, genera-

tor insulation, oil o gear box).

Contact

CG Power Systems Belgium NVAntwerpsesteenweg 167B-2800 Mechelen, BelgiumT +32 15 283 333 - F +32 15 283 300

belgium@cgglobal.com - www.cgglobal.com