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
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
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
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
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.
A demagnetized core is essential for a relailable exciting current measurement. Page 7
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
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.
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
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
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
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.
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 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
Power transformer demagnetization routine > Demagnetization can be done with rated voltage at rated frequency or alternatively with reduced voltage at reduced frequency.
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. i
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
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
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