Partial Discharge Test - Switchgear
- 1. Predictive Maintenance of MV Switchgear with Partial Discharge R K Gupta Principal Consulting Electrical Engineer (Power System) Reliability Engineer BE Electrical Protection, Automation, PLC & SCADA, Level 3 MCSA, Colorado USA
- 2. IEC 60270 Section 3.1 Notes “Partial discharges are in general a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation…” ..“Corona is a form of partial discharge that occurs in gaseous media around conductors which are remote from solid or liquid insulation…” ..“Partial discharges are often accompanied by emission of sound, light, heat, and chemical reactions…” “Localized electrical discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor.” Partial Discharge Test
- 3. IEEE Standard 400-2001 Notes “Partial discharge measurement is an important method of assessing the quality of the insulation of power cable system” “A partial discharge is an electrical discharge (formation of a streamer or arc) that does not bridge the entire space between two electrodes.” “Partial discharges may occur in a “void…at a contaminant…or at the tip of a well-developed water tree...” Partial Discharge Test
- 4. Partial discharge Taxonomy Three basic types of PD sources: Internal PD Surface PD Corona PD Focus on Internal Discharges
- 5. PD Inception Condition A free electron is available in the gas inside the void. Mechanisms that allow a free electron inside the gas: a. Ionization by photon/gas molecule collision (background radiation) b. Schottky emission of electrons from metal and /or dielectric surfaces (electron injection). The external field, fE0, exceeds the inception field, Einc. The free electron, accelerated by electric field, can trigger an electron avalanche (PD).
- 6. Partial Discharge Process in Solid Insulator /Air Insulator Solid insulators are manufactured to give an even distribution of electrical stress between the conducting electrodes. Manufacturing processes invariably give rise to small cavities or voids in the insulation bulk. These cavities are usually filled with a gas of lower breakdown strength than the surrounding solid. The permittivity of the gas is invariably lower than that of the solid insulation, causing the field intensity in the cavity to be higher than that in the dielectric. Therefore under the normal working stress of the insulation, the voltage across the cavity may exceed the breakdown value and initiate electrical breakdown in the void.
- 7. Equivalent circuit for cavity in insulator A solid insulator of thickness d contains a disc shaped cavity of thickness t and area A, as shown fig. In the equivalent circuit the capacitance Cc corresponds to the cavity, Cb corresponds to the capacity of the dielectric that is in series with Cc and Ca is the capacitance of the rest of the dielectric. Given that capacitance C, in Farads/m2, is given by; Where; e0 = permittivity of free space = 8.854 x 10-12 Fm- 1 er = relative permittivity A = area between electrodes d = separation of electrodes
- 8. Breakdown Strength in Cavity If we assume that the gas in the cavity (of thickness t) in figure 1 has a relative permittivity of approximately 1, then: And, As Cb and Cc essentially form a capacitive divider, the voltage across the cavity, Vc, can be expressed as; Substituting into the above equation gives;
- 9. Breakdown Strength in Cavity Electrical field strength across the cavity (Ec) is given by the equation Given that in most circumstances t << d and er is greater than 1, it can be seen that electrical stress in the cavity is greater than that in the surrounding insulation. This, coupled with the fact that the breakdown strength of the gas is likely to be significantly lower than that of the insulation, makes the gas in the void liable to breakdown under normal working conditions.
- 10. Breakdown Strength in Cavity From the equations above it can be seen that the voltage across the dielectric at which discharge activity will initiate in the cavity, Vai, is given by; In practice voids in solid insulators are very often approximately spherical. In this case the field in the void is given by; Where erc = relative permittivity of gas in void. When er >> erc this approximates to:
- 11. Relative permittivity and breakdown strengths of some typical high voltage HV Insulating Material
- 12. Voltage and Current in discharging Cavity Each time a discharge occurs in the cavity, charge is transferred from one side of the cavity to the other until the potential difference across the cavity is too small to maintain the discharge. When the insulator is subject to a sinusoidal alternating voltage, charge builds up within the void as the applied voltage increases or decreases. This causes a series of discharges with charge first moving in one direction, then the other. Figure 2 shows how the voltage and current across a cavity changes with applied voltage.
- 13. The dotted curve shows the voltage that would occur across the cavity if the discharges did not equalize the potential difference across the cavity. As the voltage Vc reaches the value V+, a discharge takes place and the, the voltage Vc collapses and the discharge extinguishes. The voltage across the cavity then starts again increasing until it reaches V+ , when a new discharge occurs. In this way several discharges may take place during the rising part of the applied voltage. Similarly, on decreasing the applied voltage the cavity discharges as the voltage across it reaches V-, In this way groups of discharges are generated by a single cavity and give rise to positive and negative current pulses on raising and decreasing the applied voltage respectively. Voltage and Current in discharging Cavity
- 14. DEGRADING EFFECT IN SOLID INSULATION When the gas in a cavity breaks down, the opposite surfaces of the insulation momentarily become cathode and anode. Some of the electrons hitting the anode are sufficiently energetic to break the chemical bonds of the insulation surface. Similarly, bombardment of the cathode by positive ions may cause damage by increasing the surface temperature and produce local thermal instability. These degrading effects form small channels and pits in the surface that can elongate through the insulation. In addition to the ionic bombardment, chemical degradation may result from active discharge products, like O3 or NO2, formed in the air by the discharges. The net effect is slow erosion of the insulating material and a very gradual increase in the size of the cavity
- 15. Electrical Trees Electrical trees are comprised of a series of interconnecting channels or discharge paths with diameters ranging from less than a micron to tens of microns. Discharge activity in voids will eventually become centered at particular sites producing deep cavities in the surface. The cavities grow in length along the discharge axis and the energy of discharge impinging on their tips increases. This combined with electrical stress concentration by virtue of their point like form, produces increasingly intense electrical fields at the tips of the discharging cavities. Eventually the breakdown strength of the material in the immediate vicinity of the tip is exceeded. Breakdown follows with the evaporation, in the space of a few nanoseconds, of a small volume of material. This rapid conversion launches small shock waves into the insulation. These waves create, in time, a structure of fine cracks extending into the insulation. Their name comes from the dendritic patterns they from in the insulation.
- 16. Electrical trees emanate from points of stress enhancement in insulation. This can be a metal inclusion or a protrusion on a conductor but in practice they more usually originate from a void. The exact process by which electrical trees propagate is still not fully understood, however, it is generally accepted as being a combination of mechanical and thermal effects. There are two clear stages in the development of electrical trees under the application of an alternating voltage, the inception period, which may be considerable and a much shorter formative period. Eventually the tree will bridge the insulation. Discharges continue to occur without breakdown because space charge sets up a reverse field in the channels to counter the field between the electrodes. During this period the channels slowly widen. Eventually the field can no longer be maintained in the widened channels and catastrophic breakdown occurs, creating a very large channel though the insulation. Electrical Trees
- 17. Tracking Tracking is the formation of a permanent conducting path across an insulator surface. Usually the conduction path results from degradation of the insulation. For tracking to occur the insulation must be a carbon based compound. Most high voltage plant is situated outside. In industrial areas, insulators become contaminated with pollution and dirt from the atmosphere. Where substations are situated near the sea, salt very quickly covers the plant. In the presence of moisture, these contaminating layers gives rise to leakage current over the insulator surface. This heats the surface and through evaporation causes interruption in the moisture film. Large potential differences are generated over the gaps in the moisture film and small sparks can bridge the gaps. Heat from the sparks causes carbonization of the insulation and leads to the formation of permanent carbon tracks on the surface. Tracking as a phenomenon severely limits the use of organic insulators in outdoor environments. The rate of tracking depends on the structure of the polymers and can be significantly reduced by adding appropriate fillers to the polymer which inhibit carbonization.
- 18. CORONA Corona is a partial discharge in regards that gas breakdown begins at a position of high electric field but dies out as the electric field decreases very rapidly as a function of distance from the highly stressed position. The breakdown can die out for two reasons; The region of high field is too small to generate a fully formed breakdown channel. The field falls to such a low value, that even a fully formed breakdown channel cannot propagate.
- 19. Corona forms in partially ionised regions adjacent to conductors and causes a change in the electric field between the conductor and the ground. In effect, it can be seen as an extension of the conductor. As such, it will effectively reduce the capacitance between the conductor and ground, as their separation decreases. This causes a drop in the voltage on the conductor, a potential difference between the conductor and the voltage source and, therefore, a current flow from the voltage source to the conductor. The electric field in a corona is sufficiently high that when a free electron occurs, that electron will, on average, generate more than one additional electron (and positive ion). So a corona is full of positive and negative ions (electrons). Thus when the field reduces to the extent that the original current ceases to flow, the electric field does not immediately return to its previously high value. Before that can happen, the positive and negative ions must flow in the field toward the negative and positive electrodes respectively. As the negative charges are in the form of electrons, they can propagate sufficiently fast to contribute to the measured partial discharge signal. However massive positive ions flow so slowly that they typically generate a very small current over a long period of time. Thus a corona can be thought of as generating a PD signal though three mechanisms. First, the ionisation of a channel, which tends to look like an extension of the conductor and therefore increases the capacitance of the conductor to ground. Second rapid migration of electrons toward the positive electrode in a system where negative charge flows as electrons. And thirdly, flow of positive ions which tends to be too slow to be detected by most PD measuring systems. The time scale for the first two phenomena is nanoseconds to microseconds, while that for the third phenomena can be milliseconds or more. CORONA
- 20. Corona tends to be repetitive, as once the region is cleared of charge, it returns to the conditions which generated it in the first place. Corona in air is sensitive to air velocity and environmental conditions which affect space charge near the conductor. In many gases, including air, corona generated by positive and negative voltages differ substantially. This is due to the physical difference between negative charge carriers (electrons) and positive charge carriers (positive ions). Electrons being light and mobile gain kinetic energy very rapidly from an electric field, while positive ions are heavy and much less mobile. The outside surface of molecules is made up of electrons, so violent phenomena, which dislodge charge from a molecule, free an electron and simultaneously create a heavy positive ion. In corona from a negative conductor, electrons propagate away from the conductor in the direction of corona growth. Thus they can create further electrons through molecular collisions. In corona from a positive conductor, the electrons propagate towards the conductor and away from the direction of corona growth. In this case, electrons are generally detached ahead of the corona tip by photons generated within the corona. CORONA
- 21. Water Trees A water tree is a bush or fan like structure developing like an electrical tree, from points of stress enhancement. Water trees cause a reduction in the insulation's breakdown stress level which encourages breakdown. Electrical trees can, on occasion, be initiated from a water tree speeding the breakdown process. Water trees are more diffuse than electrical trees and generally grow at lower electrical stresses. Two types of water tree have been recognized according to where the tree initiates, ‘bow-tie’ trees and ‘vented’ trees. Bow-tie trees are initiated in the bulk of the insulating material, often from a void, and grow towards the conducting screens. They clearly derive their name from the pattern they form. Vented trees grow from one of the conducting screens into the insulation bulk
- 22. PD Testing Methods Online Partial Discharge Testing Ultrasonic Transient Earth Voltage High Frequency Current Transformer Electrical Method (Inductive & Capacitive) Offline Partial Discharge Testing Capacitance Voltage Divider( CVD) method
- 23. Ultrasonic Electrical arcs in the air and corona effects emit sounds and ultrasounds. The basic electrical problems that produce distinct ultrasound waves that can be detected by Ultrasonic Testing include partial discharge, corona and tracking. Ultrasonic measurement is most powerful on a comparative basis and will significantly increase the reliability of correct detection of partial discharge. Ranges of Ultrasonic detection Center frequency 40 KHz Bandwidth 38kHz-48kHz Acoustic Airborne Airborne acoustic (ultrasonic) radiation through air from corona and surface discharges in the plant
- 24. Ultrasonic / Acoustic Airborne Test Method Background Noise Measurement If any reading is made with the sensor flat on a surface (not over a vent or air gap) this can be discounted as background noise. AA/Ultrasonic Sensor Attachment Requirements There must be a clear air path (line of sight) from the sensor to the discharge source, i.e. a vent or hole in the plant housing. Fully enclosed air insulated switchgear with no grills, vents, air gaps etc, will not be suitable for Airborne Acoustic measurements.
- 25. Transient Earth Voltage Transient Earth Voltage (TEV) PD signals are generated by internal partial discharges in switchgear, cable terminations, motors and transformers. TEV signals are in a higher frequency range of between 4MHz – 100MHz and are oscillatory in general. The resultant PD signals are measured in dB (decibels), as is the convention for on-line s w i t c h g e a r t e s t i n g .
- 26. Background Noise Measurement TEV measurements can be affected by background electrical noise in the substation. Sources include: Power electronic switching, e.g. from DC power supplies. Corona from outdoor switchyards. High frequency communication systems, e.g. two-way radios TEV Test Method
- 27. The TEV sensor should be placed flat against the metal-clad switchgear close to vents or gaskets or seams on the metal- clad housing. The LEDs will light to show the measured PD level. The user MUST hold the unit whilst in use. It is recommended to place the TEV sensor at multiple points on the plant for example on the cable boxes and front and back of each switchgear panel. TEV Test Method
- 28. High Frequency Current Transformer (HFCT) sensor which is clipped around the earth strap of the cable. These pulses are generally in the frequency range of between 200kHz – 4MHz and are typically mono polar in shape. The unit measures the Cable PD pulses in pico Coulombs (pC’s) by measuring the charge content (area under the mono polar pulse). High Frequency Current Transformer
- 29. Procedure for HFCT PD Measurement Background Noise Measurement HFCT measurements can be affected by background electrical noise sources inside and outside the substation. Sources include: Radio frequency interference from local radio transmission Corona from outdoor switchyards. To measure the background noise level, attach the HFCT to the earthing conduit of a de-energized feeder, or a nearby LV earth.
- 30. Electrical Method Capacitive Sensor The Capacitive Sensor measures the electric field on the cable and cable joint. Sensors are designed to detects the quick variations of electrical fields caused by PD. The probe is a coated metal plate that create a capacitor between the cable and the plate. This capacitive coupler allows to read the PD activity on surface. The PD signal is detected by a capacitive probe, then amplified by the equipment in radio frequency bandwidth. The signal amplitude is proportional to the discharge energy.
- 31. ULD 40 ( Ultrasonic Detector) The ULD-40 is an ultrasonic detector designed for corona and arcing inspections for predictive maintenance in electric utilities. Main Applications Electrical Inspections: corona effect localization, arcs on shields. General Mechanical Inspections: motors, compressors, gears, bearing monitoring. Gas, air, pressure leaks, leak detection without pressure or vacuum. Aerospace Sector: airplane doors and windows, air tightness
- 32. Detection of Electrical Arcs and Corona Effects Electrical arcs in the air and corona effects emit sounds and ultrasounds. The role of the ULD-40 consists of capturing emitted ultrasounds and of translating them into the audible range. The ULD-40 accurately pinpoints and identifies corona effects and arcs that may be encountered on any type of high voltage installation simply by scanning around the suspected area.
- 33. Portable expert system for detection of partial discharges Increase your network reliability and workers security XDP
- 34. It is an instrument that enables to quantify the intensity of the high frequency electric field signal emitted by the partial discharge of a component, and to translate it into decibels (dB) A decibel (dB) is a unit that expresses the intensity of a signal in relations to a reference on the logarithmic scale XDP
- 35. With the XDP Follow up on ageing process of the critical components’ insulation Perform Quality Control of the insulators during installation or repair Safety control prior to work on energize equipment
- 36. XDP Advantages Portable Verification done under normal charge of the electric network – No service interruption Quick readings Very sensible Pinpoints the problem Our 3rd generation of PD Detector
- 37. XDP Technical Specifications Peak measurement value in dB relative to 15 picoCoulomb Peak detection with a fast numeric circuit Fast numeric processor (DSP) for instant analysis results Wave’s shape inspection in high frequency to determine PD proximity
- 38. XDP Technical Specifications… LCD (Liquid Crystal Display) displays wave form and dB reading Easy to read, big letter casings Upload/download capability PC (Windows) interface for PD readings analysis/trending
- 39. XDP Technical Specifications… Dynamic range of 40 dB Up to 64 readings recording capability Bandwidth of 300 kHz to 70 MHz Sampling frequency of 30 MHz Sampling period of 16 cycles (50 or 60 HZ) Built-in Real Time Clock
- 40. XDP Technical Specifications… Keeps in memory up to 10 PD signatures for quick PD recognition Quick auto-verification procedure for high reliability 8 hour battery autonomy The smaller PD Expert System on the market 203 X 114 X 51 mm (8 X 4.5 X 2 in.) 0,86 kg (1,9 lbs)
- 41. XDP Applications Cable & accessories Joint Elbow T Custom Switchgears Distribution Transformers Etc.
- 42. The Handling Rod The use of the Spatula, which is a surface sensor, with the 61 cm (24 in.) handling rod does not required direct access to the conductor
- 43. The Capacitive Sensor The Capacitive Sensor measures the electric field on the cable and cable joint Our Capacitive Sensor uses a flexible Spatula easily adaptable to the cable joint form
- 44. How it works The XDP detects the quick variations of electrical fields caused by PD The probe is a coated metal plate that create a capacitor between the cable and the plate This capacitive coupler allows to read the PD activity on surface It is moved along joints and on cable insulation jacket, in order to detect the presence of PD activity
- 45. The PD signal is detected by a capacitive probe, then amplified by the XDP in radio frequency bandwidth The signal amplitude is proportional to the discharge energy The energy is displayed on the XDP in dB (relative value)
- 46. How to use the Spatula Sensor Check the reference value on a regular basis to make sure that the XDP works well Turn off the XDP Connect the reference module Connect the Spatula Sensor Place the sensor on the module in a way to make it cling around it as much as possible
- 47. Check the reference value… Activate the XDP On the numeric display, you must read the displayed value on the sensor’s case
- 48. This technique will allow you to detect a PD activity even in an environment with normal to high ambient noise (12 – 20 dB) Always adjust the spatula the way it fit as much as possible around the cable and cable joint Always use the handling rod
- 49. Inductive Probe Applications Terminations Shielded cables Cable joints with grounding braid
- 50. A B C D
- 51. Quality control of a 12 dB ambient noise splice Without defect A = 10dB B = 12dB C = 12dB D = 10dB With defect A = 12dB B = 14dB C = 34dB D = 16dB
- 52. PD activities level classification Level 1: Free of Partial Discharge activity. No necessary action at this time. Retest within 12 to 18 months (< 12dB) Level 2: Moderate levels of Partial Discharge activity. Retest within 1 year (12 – 20dB) Level 3: High level of Partial Discharge activity. Repair or replacement required (20 – 35dB)
- 53. PD activities level classification… Level 4: Very high level of Partial Discharge activity (> 35dB) We strongly recommend to restrict the access to the area Repair or replacement required as soon as possible
- 54. Thank you