demag test card presentation

27 February 2014

Transformer demagnetization with CPC 100 + CP SB1

OMICRON

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

Structure





Page 3 OMICRON

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

OMICRON Page 4

Structure





Page 5 OMICRON

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

Influence of remanence on exciting current

Exciting current @ 2 kV

Page 7 OMICRON

0.0A

0.002A

0.004A

0.006A

0.008A

0.01A

0.012A

0.014A

0.016A

0.018A

0.02A U with remanenceV with remanenceW with remanenceU without remanenceV without remanenceW without remanence

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

Structure





Page 8 OMICRON

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.

Page 9 OMICRON


Interpretation ranges of the FRA measurement

Page 10 OMICRON

Remanence has a big influenceon the first resonance points.


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.

Page 11 OMICRON

Phase APhase C

Phase B

A B C

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


> 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

Structure





Page 13 OMICRON

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


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

CPC 100 Demag test card

Page 15 OMICRON


> 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

Power transformer demagnetization routine

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

Page 17 OMICRON


> 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

L =

u(t) = L*


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.

Page 19 OMICRON

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 and exciting current Iexc 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.

Structure





Page 20 OMICRON

Case study on a 350 MVA transformer

Page 21 OMICRON

Transformer dataManufacturer Trafo-UnionMan. year 1971Type TFSN 8556Vector group YNyn0Rating 350 MVAHV rating 400,000 kVLV rating 30,000 kV

Demagnetization routine

Demagnetization routine

FRA verification after demag. routine

FRA verification after demag. routine

Initial FRA verificationInitial FRA verification

DC winding resistance

DC winding resistance

FRA verification after DC test

FRA verification after DC test


Results

> Current: 6 A

> Max. flux: +/- 712.7 VS

> Iterations: 18

> Initial remanence: 98.2 %

> Remaining flux: -0.8 %

> Demagnetization time: 5 min

Page 22 OMICRON


Page 23 OMICRON

FRA after DC winding resistance test on phase B

FRA after demagnetizationwith the CPC 100


Phase A

Page 24 OMICRON

Initial FRA verification

After DC test

After demagnetization


Phase B

Page 25 OMICRON


After DC test



Phase C

Page 26 OMICRON


After DC test


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

Page 27 OMICRON

demag test card presentation

Documents