Demag Test Card Presentation

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Transformer demagnetization

with CPC 100 + CP SB1

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Structure

> Why demagnetization

> Influence of remanence on the inrush current

> Influence of remanence on diagnostic measurements

> Exciting current measurement > FRA measurement

> Demagnetization with CPC 100 + CP SB1 > Case study on a 350 MVA transformer

> Demagnetization results > FRA verification

Page 2 © OMICRON

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Structure

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Influence of remanence on the inrush current

> When energyzing a transformer a transient current called “Inrush current” will flow for several cycles

> Remanence in the core can lead to too high Inrush Current and mechanical forces which can damage the transformer

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Structure

> Exciting current measurement

> FRA measurement

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Influence of remanence on exciting current

Exciting current measurement

> The exciting current measurement is a method to find failures or defects in the core.

> A magnetised core has a big influence on the exciting current and can lead to a wrong interpretation of the measurements.

> It is recommended to perform an exciting current measurement before the winding resistance measurements or to demagnetize the transformer.

Page 6 © OMICRON

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Influence of remanence on exciting current

Exciting current @ 2 kV 0.0A 0.002A 0.004A 0.006A 0.008A 0.01A 0.012A 0.014A 0.016A 0.018A

0.02A _{U with remanence}

V with remanence W with remanence U without remanence V without remanence W without remanence

A demagnetized core is essential for a relailable exciting current measurement.

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Structure

> Exciting current measurement

> FRA measurement

Page 8 © OMICRON

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Influence of remanence on FRA measurements

FRA measurement

> Remanence in a transformer has a big influence on the FRA measurment.

> Before performing a FRA measurement the transformer must be demagnetised.

> The FRA measurement is also a good method to verify whether a transformer is demagnetized.

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Influence of remanence on FRA measurements

Interpretation ranges of the FRA measurement

Page 10 © OMICRON

Remanence has a big influence on the first resonance points.

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Influence of remanence on FRA measurements

Typical FRA pattern without remanence

> The phases on the outer limbs (typically A and C) should be similar and should show two resonance points due to two different long magnetic paths.

Phase A

Phase C

Phase B

A B C

Remanence will move the resonance points to the right (higher frequencies).

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Influence of remanence on FRA measurements

> Why is the curve moving to the right if the core is magnetised?

> As explained in a previous slide, once the core saturates however, the winding inductance

appears greatly reduced. Or in other works Ldemagnetised is larger than Lmagnetised Ldemag > Lmag. A resonance point in the FRA cruve is always a combination of a Capacitance C and an inductance L which can be shown in an equivalent circuit diagram. A resonance condition is expressed by the formula fo=1/(2*phi*sqrt(L*C)). Therefore, if L is getting smaller, which is the case of a magentised core, the fo is getting larger and therefore the resonance point in the FRA curve is moving to the right

Page 12 © OMICRON

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Structure

> Demagnetization with CPC 100 + CP SB1

> Case study on a 350 MVA transformer > Demagnetization results

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Demagnetization with CPC 100 + CP SB1

Demagnetization

> The demagnetization is done on the primary side of the

transformer.

> For the measurement the V1 AC as well as the V DC

measurement inputs have to be connected.

Page 14 © OMICRON

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Demagnetization with CPC 100 + CP SB1

> The demagnetization routine is available as test card on the CPC 100s front panel and within the Primary Test Manager™ (PTM).

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Demagnetization with CPC 100 + CP SB1

> The demagnetization routine is available as test card on the CPC 100s front panel and within the Primary Test Manager™ (PTM).

Demagnetization test within PTM

Page 16 © OMICRON

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Power transformer demagnetization routine

> Demagnetization can be done with rated voltage at rated frequency or alternatively with reduced voltage at reduced frequency.

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Power transformer demagnetization routine

> What to consider if demagnetisation is done with reduced voltage @ reduced frequency.

> To calculate the flux in the core we need to know the winding resistance.

Page 18 © OMICRON

=

A

V

L i u

Ф

L

=

∗

u(t) = L*

∗

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Power transformer demagnetization routine

The right way of demagnetizating a power transformer with reduced voltage at reduced frequency

> The CPC 100 demagnetization algorithm is an intelligent iterative process depending on the hysteresis loop parameters. This enables the

demagnetization of small distribution transformers as well as for big power transformers.

1. Calculation of the hysteresis loop parameter by measuring the current, voltage and the core flux. 2. With the knowledge of the

hysteresis parameters and continuous monitoring the flux Ф and exciting current I_exc the flux is regulated to reach zero

magnetization.

3. Demagnetization process is finished when the remaining remanence flux is below 1 % of the maximum flux.

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Structure

> Demagnetization with CPC 100 + CP SB1

> Case study on a 350 MVA transformer

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Case study on a 350 MVA transformer

Transformer data

Manufacturer Trafo-Union Man. year 1971

Type TFSN 8556

Vector group YNyn0

Rating 350 MVA HV rating 400,000 kV LV rating 30,000 kV Demagnetization routine Demagnetization routine

FRA – verification after demag. routine FRA – verification after

demag. routine Initial FRA verification Initial FRA verification DC – winding resistance DC – winding resistance FRA – verification after DC test FRA – verification after DC test

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Case study on a 350 MVA transformer

Results > Current: 6 A > Max. flux: +/- 712.7 VS > Iterations: 18 > Initial remanence: 98.2 % > Remaining flux: -0.8 %

> Demagnetization time: 5 min

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Case study on a 350 MVA transformer

FRA after DC winding resistance test on phase B

FRA after demagnetization with the CPC 100

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Case study on a 350 MVA transformer

Phase A

Initial FRA verification After DC test

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Case study on a 350 MVA transformer

Phase B

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Case study on a 350 MVA transformer

Phase C

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Benefits of demagnetization with CPC 100 + CP SB1

> Efficient process because no additional wiring effort is needed when

already using the CP SB1 in combination with the CPC 100 > Short demagnetization time

> With the CPC 100 demagnetization algorithm the demagnetization of small distribution transformers as well as for big power transformers can be done

> Demagnetization will reduce the inrush current

> Demagnetization before routine or diagnostic tests will ensure correct results