Article on Impulse

7/28/2019 Article on Impulse

1/3

Article

The impulse design of transformer oil-cellulose structures

J.K. Nelson,C. Shaw

Dept. of Electr., Comput. & Syst. Eng., Rensselaer Polytech. Inst., Troy, NY

IEEE Transactions on Dielectrics and Electrical Insulation(impact factor: 1.09). 07/2006;

DOI:10.1109/TDEI.2006.1657958 pp.477 - 483Source:IEEE Xplore

ABSTRACT Transformer oil/cellulose structures are often designed based on a cumulative

stress criterion derived from experimental tests at power frequency. However, such structuresmust also meet stringent impulse requirements defined by a basic insulation level (BIL). The

industry has tried to establish an equivalence factor to permit power frequency cumulative stress

methods to be used to estimate impulse withstand strength. Since the mechanisms of failure

differ substantially under surge conditions, there would seem no good reason to suppose that auniversal equivalence factor is appropriate. Tests are reported using a 2.3 MV generator to

document impulse failure of a number of bulk, creep and hybrid structures to establish the natureof this relationship through statistical comparisons with the established 50/60 Hz methods.

Factors varied from 1.94 to 3.34, depending on the configuration. The methodology is described

and the results discussed in the context of the design of oil-cellulose structures, having regard to

complicating factors such as wave shape and electrode covering. The study permits somespeculation about impulse design under hybrid situations (i.e. failure paths involving both creep

and bulk liquid)

Transformer Insulation Design Based on the Analysis of Impulse Voltage Distribution

AbstractIn this chapter, the calculation of transient voltages over and between winding parts of a large power

transformer, and the influence on the design of the insulation is treated. The insulation is grouped into

two types; minor insulation, which means the insulation within the windings, and major insulation,

which means the insulation build-up between the windings and from the windings to grounded surfaces.

For illustration purposes, the core form transformer type with circular windings around a quasi-circular

core is assumed. The insulation system is assumed to be comprised of mineral insulating oil, oil-

impregnated paper and pressboard. Other insulation media have different transient voltage withstand

capabilities. The results of impulse voltage distribution calculations along and between the winding

parts have to be checked against the withstand capabilities of the physical structure of the windings in a

winding phase assembly. Attention is paid to major transformer components outside the winding set,like active part leads and cleats and various types of tap changers.

Chapter Preview

Top
http://www.researchgate.net/researcher/63698238_JK_Nelson/http://www.researchgate.net/researcher/63698238_JK_Nelson/http://www.researchgate.net/researcher/61106357_C_Shaw/http://www.researchgate.net/researcher/61106357_C_Shaw/http://www.researchgate.net/researcher/61106357_C_Shaw/http://www.researchgate.net/journal/1070-9878_IEEE_Transactions_on_Dielectrics_and_Electrical_Insulationhttp://www.researchgate.net/journal/1070-9878_IEEE_Transactions_on_Dielectrics_and_Electrical_Insulationhttp://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1657958http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1657958http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1657958http://www.igi-global.com/chapter/transformer-insulation-design-based-analysis/68882http://www.igi-global.com/chapter/transformer-insulation-design-based-analysis/68882http://www.igi-global.com/chapter/transformer-insulation-design-based-analysis/68882http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1657958http://www.researchgate.net/journal/1070-9878_IEEE_Transactions_on_Dielectrics_and_Electrical_Insulationhttp://www.researchgate.net/researcher/61106357_C_Shaw/http://www.researchgate.net/researcher/63698238_JK_Nelson/


2/3

Estimation Of Impulse Voltage Distribution Via Winding Ratio And Oscillating

Factor Method

The winding system of a power transformer consists generally of a minimum of two windings of

different nominal voltage levels. The simplest example is a two-winding transformer with a fixed

ratio, with (per phase) only one winding (in one part) for the LV winding and one winding (inone part) for the HV winding. Most of the time however, one of the two windings (usually theHV winding), has more than one part, because it needs to be adjustable in voltage. This means

that a winding will have a discontinuity in electrical properties in the connection point between

the two parts.

The impulse voltage distribution along a winding is usually not divided linearly according to the

turns ratio, which is in contrast to the voltages at nominal frequency. The initial distribution is

determined more by the series capacitances of the winding parts. The voltages tend to oscillatewith a level that is approximately proportional to the difference between the initial capacitive

voltage distribution and the final inductive voltage distribution, as shown in Figure 1.

Figure 1.

Initial-final transient voltage distribution along the height of a homogenous coil

For estimation purposes, and for a quick check of the correct behavior of a transient model, a

simple rule of thumb for the amplitude (peak-peak) of the oscillating voltage is assuming amultiplication factor of two, two times the nominal voltage.

The first winding type where this rule is applied is a layer winding. We take an example wherethe layer winding consists of six layers (of equal turns). See figure 2.

Figure 2.

Layer winding with impulse

voltage difference between layers

Nominal or induced voltages

between the layer ends are: (100%/ 6) x 2 layers = 33%.

But for impulse voltage

distribution, we multiply this figureby a factor of two: 2 x 33% = 66%.

Another example configuration has one high voltage main winding and one regulating winding.

It is assumed that the regulating winding has considerably higher series capacitance compared to

the main winding. The turns of the regulating winding can be connected to be additive (plus tapposition), or connected to be subtractive (minus tap position). In Figure 3, the four main

positions, usually relevant for acceptance testing are given.
http://www.igi-global.com/sourcecontent/9781466619210_63883/978-1-4666-1921-0.ch011.f02.png


3/3

Figure 3.

Plus/minus

regulation - four tappositions (usually

relevant foracceptance testing)

Transient voltages

are also referred to

as BIL, Basic

Impulse InsulationLevel, see IEEE Std,

C57.12.90(2006).

Transient voltageestimates or BILestimates across the

tap winding:

In tap Plus (a), the BIL level across the tap winding estimation: 2*60/(500+60)*100 = 21%.
http://www.igi-global.com/sourcecontent/9781466619210_63883/978-1-4666-1921-0.ch011.f03.png

Documents

Article on Impulse