PSI - Issue 36

Roman Dzhala et al. / Procedia Structural Integrity 36 (2022) 17–23 Roman Dzhala et al. / StructuralIntegrity Procedia 00 (2021) 000 – 000

20 4

4. Analysis of the possibilities of distinguishing local damage of insulation of UP To identify the possibility of distinguishing local damage of the insulation of UP by non-contact current measurements, it is necessary to analyse the effect of current leakage on CCM. In order to do this, it is necessary to investigate the nature of the distribution of magnetic field of the current flowing from UP in places of insulation damage. For rectilinear (along z -axis) insulated conductor with current J , part of which J s due to damage to the insulating coating at point z = z s flows into homogeneous infinite medium, magnetic field (MF) can be described by the following formula (Dzhala et al. (2012)):

   

   

J

+ − − ( s r z z z z 2

−

(2)

.

( ) , r H r z J J   = 2

1

+

−

s

s

4

r 

2

)

s

The earth's surface breaks the symmetry of the distribution of current leakage. For point current source J s located at depth h from the earth's surface, using known current density distribution, the expression of magnetic field is obtained by integration, which for our problem has the following form (Dzhala et al. (2018)):

   

   

y

(3)

,

2 2 x z = +  .

( ) H y J s s = , 

1

,

y h 

−

4



2

2

y

+



This field is equivalent to the field of linear current flowing from the point source (from the place of local damage to the insulation) to the depth of the earth (Dzhala et al. (2012, 2018)).The power lines of MF of the flowing current are concentric around the vertical passing through the point of current flow (Fig. 3).

Fig. 3. Components of magnetic fields H φ of transit current J and H s of current J s , which leakage due to local damage of the insulation of UP.

In the Cartesian coordinates x , y , z associated with the conductor, the components of this field have the following form ( )   cos , H H y s x = , ( )   , sin H H y s z = , H y =0, (4) where θ is angle between axis z and direction to the point of observation from origin (in y = constant plane), cos θ = – z/ρ . The formulas of magnetic field of UP with several insulation damages are described below. Let the transit current J flowing along UP at points z = z s of the insulation damage s = 1, 2,… decrease by the values of leakage J s . In Cartesian coordinates for magnetic field over UP, according to (Dzhala et al. (2012)) we get the formulas: