8. Evaluation of Discharges

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( tanδ, 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

8.2 Recognition 1.Oscillogram - discharge pattern -> 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

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

8.2 Recognition 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)

8.2 Recognition 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)

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

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)

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

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

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)

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

8.2 Recognition 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

8.2 Recognition 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)

8.2 Recognition 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

8.2 Recognition 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

8.2 Recognition 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

8.2 Recognition 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

8.2 Recognition 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

8.2 Recognition 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) 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

8.2 Recognition 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

8.2 Recognition 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 Interference - external sources(Fig8.20)

8.2 Recognition 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 Recording - make of both the discharge pattern and the X-Y diagram

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

8.3 Mechanisms of deterioration 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 -> reach breakdown strength -> surface erosion

8.3 Mechanisms of deterioration Internal discharges in polymers 2. Discharges become concentrated : because stress cracking cause micro-crack -> deep pits are formed -> 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

8.3 Mechanisms of deterioration Internal discharges in polymers 3. Field concentration around sharp tip -> approach dielectric strength over a distance of some microns -> dielectric breakdown -> 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

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

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

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

8.3 Mechanisms of deterioration 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)