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8. Evaluation of Discharges

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Title: 8. Evaluation of Discharges


1
8. Evaluation of Discharges
  • 2003. 10. 18

2
8. Evaluation of Discharges
  • Ultimate goal is to evaluate
  • Must be ascertain
  • - Observed discharges Harmful or not ?
  • - Absence of discharges safe level ?
  • (minimum
    detectable discharge)
  • In connection with other test( tand, overvoltage
    test..)
  • make clear that observed dielectric will be
    sufficiently safe
  • Three section
  • - Recognition Type of discharge
  • - Mechanism deterioration of the insulation
  • - Evaluation discharge levels

3
8.2 Recognition
  • 8.2 Recognition
  • Oscillogram
  • - discharge pattern -gt valuable indication
    to the type
  • and origin
    of discharges
  • - distribution of phase angle
  • - ratio between positive and negative pulses
  • 2. X-Y diagram
  • - discharge magnitude (function of the test
    voltage)
  • - recorded in pC(logarithmic scale)
  • 3. Time effect
  • - in some cases discharge magnitude and the
    extinction
  • voltage change(when testing for sometime
    at high voltage)
  • - X-Y diagram(good practice) rising and
    declining voltage
  • - ignition effects and time effects can be
    shown

4
8.2 Recognition
  • 4. Ambience
  • - Character or location of the discharge
    determined
  • by making changes in or around the sample
  • a. Immersion in oil or applying grease
    surface discharge
  • b. changing pressure in GIS distinguish
    between
  • discharges in the pressurized gas
  • and isolated discharges
  • c. increasing the temperature fissures
  • - temperature cycles interstices
  • - good tool combined with antecedents of the
    sample and
  • with the
    results of other tests

5
8.2 Recognition
  • 8.2.1 Dielectric-bounded discharges
  • - Occur between dielectric bounds(internal or
    superficial)
  • fairly symmetric patterns(Fig8.1)
  • - Occur in advance of the test voltage peaks
  • - Impulse (same amplitude, number and location)
    appear at the positive and negative half cycles
  • - There is some random variation (amplitude and
    phase angle)

6
8.2 Recognition
  • 8.2.1 Dielectric-bounded discharges
  • 1. A squarely shaped diagram limited cavity(Fig
    8.2,3)
  • - Further discharges not possible (after
    ignition of the cavity, the available space is
    occupied)

7
8.2 Recognition
  • 2. A triangular diagram(Fig8.4) somewhat larger
    cavity
  • - discharges can expand first and remain
    constant (fill with discharge)

8
8.2 Recognition
  • 3. An ever increasing diagram(Fig8.5)
  • - surface discharge(Fig8.6 a)
  • - discharge in a large gap(Fig8.6 b)
  • 100,000pC observed
  • - discharge in an interface(Fig8.6 c)

9
8.2 Recognition
  • 4. Effect of time (1)
  • - at its maximum value keep for some
    time(e.g. 30min)
  • discharge magnitude may change
  • - magnitude decrease and higher extinction
    voltage(Fig8.7)
  • - longer period(e.g. 24hr) more

10
8.2 Recognition
  • - Elastomeric insulation fissures
  • (direction of
    the electric field, Fig 8.8)
  • - Round cavities in thermoplastics(Fig 8.9)
  • gradual increase in discharge magnitude (a
    number of cavities of various sizes)
  • higher extinction voltage (formation of
    electrically conducting layers at the surface of
    the cavity)
  • rested for sometime(e.g. 24h) restored

11
8.2 Recognition
  • 5. Effect of time (2)
  • - inception voltages are lowered instead of
    increased by testing the sample at high
    voltage(Fig 8.10,11)
  • Fig 8.10 Voltage is kept at maximum value,
    discharge level is gradually increased and
    becomes stable(e.g.15min)
  • - Extinction voltage is lower than the inception
    voltage
  • epoxy resin(small amount of moisture during
    manufacture)

12
8.2 Recognition
  • 5. Effect of time (2)
  • Fig 8.11 extreme case of deterioration by
    voltage
  • - oil-impregnated paper(not sufficiently dry)
  • - bubbles are generated and increase in size and
    number during testing.
  • - rested for some time discharges will
    disappear

13
8.2 Recognition
  • 8.2.2 Electrode-bounded discharges
  • pattern asymmetric(Fig 8.12)
  • Between electrode and dielectric
    boundary
  • - Accur in advance of the test voltage peaks
  • One half cycle small number of large
    discharges
  • The other cycle larger number of smaller
    discharges
  • Amplitude differece 3 10 times

14
8.2 Recognition
  • 8.2.2 Electrode-bounded discharges
  • - squarely shape X-Y diagram(Fig8.2) internal
    discharge in an

  • electrode-bounded cavity(Fig 8.13 a)
  • - Ever-growing X-Y diagram(Fig8.14) surface
    discharge
  • ( starting from
    one of the electrodes)

15
8.2 Recognition
  • 8.2.2 Electrode-bounded discharges
  • - Time effect small
  • Self-healing effect with surface discharge
    spurious
  • discharges are seen and
    extinguish again
  • (voltage is increased or kept constant
    for observation)
  • Fig8.15
  • internal discharge lower voltage
  • surface discharge higher voltage

16
8.2 Recognition
  • 8.2.4 Floating objects
  • Metallic parts peculiar discharge
    patterns(Fig8.16)
  • - Equal amplitude, number and phase angle at
    both half cycles
  • - More-or-less equally spaced, sometimes occur
    in pairs
  • - Move around the time base, disappear(certain
    point) and
  • restart(moment)
  • - X-Y diagram square (Fig8.2)
  • - Unaffected by time

17
8.2 Recognition
  • 8.2.4 Floating objects
  • Peculiar behavior is caused
  • - bad contact(floating parts or floating part
    and electrodes,
  • loose connection to a screen or
    unwanted metal particle)
  • - serious defect in insulation
  • Disturbances in a discharge test
  • - metallic objects laying on the floor
  • - Clean floor 30kV or more

18
8.2 Recognition
  • 8.2.5 Corona discharges in SF6 gas and air
  • Corona discharges Typical patterns(Fig 8.17)
  • - Negative half cycle sharp edge is at high
    voltage
  • - Positive half cycle
    at the earth side

19
8.2 Recognition
  • 8.2.5 Corona discharges in SF6 gas and air
  • X-Y diagram(Fig8.18) at higher voltage large
    corona discharge
  • - opposite half cycle and second step is added
  • Time effect discharge pattern is little affected

20
8.2 Recognition
  • 8.2.5 Corona discharges in SF6 gas and air
  • Corona discharge cause
  • - sharp edges in a construction
  • - disturbances(sharp edges in the high-voltage
    test circuit)
  • - Free of sharp edges or points high-voltage
    leads and

  • earth side(30 100kV)
  • 8.2.6 Corona in oil
  • Occur Both half cycles, Symmetric
  • about the voltage peaks (Fig 8.19)
  • - equally spaced in time
  • greater magnitude and spacing than the
    other half cycle


21
8.2 Recognition
  • 8.2.6 Corona in oil
  • - smaller discharges equal magnitude
  • - larger discharges equal magnitude or
    vary
  • - In contrast with corona in air larger
    discharges start first
  • - X-Y diagram similar Fig8.18
  • Time effect little affected, take a short time
    to stabilize
  • (first apply the voltage)
  • - Larger discharge on positive half cycle the
    point is at high voltage

22
8.2 Recognition
  • 8.2.7 Contact noise
  • - Coarse and irregular noise about zero points
  • Make clear distinguished from normal
    discharges
  • - Occur sample under test or itself from test
    circuit
  • - Frequently in capacitors connection to the
    foils
  • ? short-circuiting the capacitor after
    charging
  • 8.2.8 Interference
  • - external sources(Fig8.20)

23
8.2 Recognition
  • 8.2.9 Expert systems
  • - Can provide a possible diagnosis
  • After the discharge characteristics have
    been answered
  • Further automation Pulse height analyser is
    coupled to the
  • discharge
    detector
  • - Analyser builds pulse height histograms
  • - Algorithm programmed using the language of an
    expert system
  • Cannot give better answer than are allowed by
    the physical
  • correlation between pulse patterns and
    discharges
  • 8.2.10 Recording
  • - make of both the discharge pattern and the X-Y
    diagram

24
8.3 Mechanisms of deterioration
  • 8.3 Mechanisms of deterioration
  • Internal and surface discharges
  • cause deterioration to dielectrics
  • 1. Heating the dielectric boundary
  • 2. Charges trapped in the surface
  • 3. Attack by ultraviolet rays and soft X-rays
  • 4. Formation of chemicals such as nitric acid
    and ozone
  • Cause depolymerization, stress cracking, gassing
  • leading to erosion of dielectric surface

25
8.3 Mechanisms of deterioration
  • 8.3.1 Internal discharges in polymers
  • Three stages of deterioration
  • 1. A uniform surface erosion
  • thermal degradation, ultraviolet radiation
  • - ions laid down by consecutive discharges
    cannot neutralize electrons(be trapped below the
    surface)
  • - Close proximity of charges high field
    strengths in the dielectric -gt reach breakdown
    strength -gt surface erosion

26
8.3 Mechanisms of deterioration
  • 8.3.1 Internal discharges in polymers
  • 2. Discharges become concentrated
  • because stress cracking cause micro-crack
  • -gt deep pits are formed -gt discharges further
  • concentrate, carbonization occur
  • - First two stages major part of the time to
    breakdown
  • high stress(at 10 to 20kV/mm) a few hours
  • lower stress(at 3 to 5kV/mm) many years

27
8.3 Mechanisms of deterioration
  • 8.3.1 Internal discharges in polymers
  • 3. Field concentration around sharp tip
  • -gt approach dielectric strength over a
    distance of some microns -gt dielectric breakdown
  • -gt field concentration moves new tip
  • and narrow channels propagate
  • - found in polyethylene and rubber(Fig8.21)
  • - Third stage breakdown take place in a few
    voltage cycles

28
8.3 Mechanisms of deterioration
  • 8.3.2 Internal discharges in paper insulation
  • - Discharges in voids adjacent to the conductor
  • attack the insulation, penetrate the first
    paper layer
  • -gt surface discharges occur along the layer
  • -gt trees or carbonized tracks are formed
  • -gt tracks follow the weakest points in the
    insulation(butt gap) Fig8.22

29
8.3 Mechanisms of deterioration
  • 8.3.2 Internal discharges in paper insulation
  • - At the foot of the tree local overheating
  • -gt thermal breakdown -gt trees and corbonize
    track attain enormous lengths

30
8.3 Mechanisms of deterioration
  • 8.3.3 Surface discharges
  • - Deterioration by surface discharges
  • same patterns internal discharges
  • - Discharge resistance of different materials
  • - Time to breakdown taken as a measure.
    (Fig8.23)

31
8.3 Mechanisms of deterioration
  • 8.3.4 Corona discharges
  • - Occur around bare conductors
  • - Indirect action by ozone formed by corona
  • deteriorate neighbouring dielectrics
  • - Corona discharge In SF6 gas not acceptable at
    all
  • create aggressive by-products (very
    detrimental to dielectric surfaces)
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