12.15 Transformer Redesign

8/3/2019 12.15 Transformer Redesign

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Transformer ReTransformer Re--DesignDesignDavid Hayes

Rotek Engineering



Transformers form an essential part of any electrical supplysystem, and although inherently reliable and long lived,

they can represent a significant cost when circumstances

dictate their replacement.



The longevity of transformers lies in the fact that

they are essentially static devices, and thematerials of which they are constructed do not age

significantly.

The exception being the oil and paper

insulation systems.

10 MVA 22/6.6 kV

Construction steel

34%

Copper

13%

Electrical steel

26%

Insulation Materials

1%

Paint

0%

Porcelain0%

Transformer Oil

25%

Wood

1%

Aluminium

0%

Other

0%

250 MVA 400/132 kV

Construction steel

23%

Copper

12%

Electrical steel

34%

Insulation Materials

3%

Paint

0%

Porcelain1%

Transformer Oil

24%

Other

1%

Aluminium

1%Wood

1%



Transformer oil, tap changers and bushings can be replaced

onsite as part of a normal maintenance program however paper degradation is non recoverable and is generally

used as the primary indicator of an end of life condition.

Ultimate end of life is considered to be when the paper has

lost its tensile strength (DP of 200)

Typical reasons for transformer replacement:

1. Premature failure

2. End of life3. The requirements of the electrical system have changed.

4. Safety concerns

5. Consequential damage concerns



Repairing or remanufacturing the transformers can have asignificant cost benefit especially when multiple units are

involved.

Typically a transformer which is completely re-fitted with

new windings and insulation can cost as little as 60 – 70%

of the price of a new unit and may be considered to be the

equivalent of a new unit in terms of its expected lifespan.

This option can also be beneficial since it utilises a basicdesign which has already proven itself suitable for a

particular application.



Repair

•Lowest cost•Perpetuates existing design flaws

Re-Design

•Basic cost not significantly higher

•Corrects design shortcomings

•Can change or improve operational parameters

•Can utilise latest materials and technology

To achieve the best solution the Client must be fully awareof his actual requirements and not create an unrealistic

“wish list”



New transformer cost breakdown

Core steel

31%

Labour core

3%

Tank and coolers

15%

Labour assembly

4%

Labour Fabrication

9%

Accessories6%

Windings

23%

Labour winding

4%

Testing

5%

Redesigned transformer cost breakdown

Core

28%

Tank

13%

Assembly

7%Fabrication

8%

Windings

28%

Cleaning

2%

Testing4%

Accessories

10%

The above indicates a potential saving of 41 %

over the price of a new unit (Distribution class transformers)

With equivalent expected lifespan



Typical elements which can be modified with a re-design are:

•Voltage ratios and rated voltages

•Vector group

•Short circuit Impedance’s

•Short circuit withstand capability

•Cooling systems

•Losses – Stray and load losses

•Increased power ratings or lower operating temperatures.

( typically 20-30% )

•Improved built in protection and condition monitoringsystems

•Detection and elimination of potential problem areas in the

design



Insulation

Standard Kraft paper – Class A – 105 oC

Thermally upgraded paper – Class E - 120 oC

Nomex paper – Class H – 180 oC



Conductor

Standard rectangular conductor

Paper insulated CTC conductor

Netting tape CTC conductor

Epoxy bonded CTC conductor



Bushings

Resin bonded paper (Porcelain)

Oil impregnated paper (Porcelain)

Resin impregnated paper (Silicon rubber)

Resin impregnated fibreglass

•Improved safety

•Reduced fire risk

•High temperature class

•Still in development

•Obsolete



OLTC

High speed resistor type

Vacuum switch type •Low to zero maintenance

•Gaining acceptance

•Can be retro-fitted



OCTS

OCTS

Bolted link board

Fixed tap (No tapping)

•Problem with cokingdue to long term static

operation

•Robust bolted connections



Conclusion

Transformer design is basically a trade off of desired

performance criteria, and can be approached in anumber of ways depending on the relative importance of

the various requirements.

In re-designing a transformer utilising the existing coreand structural elements, it is possible to enhance critical

performance criteria to achieve an optimal design which

is equivalent to a new unit at a reduced cost.

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12.15 Transformer Redesign