PD_Presentation_1.pdf
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The document provides an overview of partial discharge detection as part of condition-based maintenance of electrical equipment. It discusses concepts like partial discharge mechanisms, the components used for partial discharge detection, and how online partial discharge detection can help monitor insulation health and provide early warnings of degradation. The document also covers topics like common types of partial discharges, how insulation defects can lead to partial discharges, and how partial discharges gradually deteriorate insulation over time if not addressed.
PD_Presentation_1.pdf
- 1. Making our world more productive Partial Discharge Part -1– Concepts and Basics Thomas Varghese 15th March, 2022
- 2. 2 MeetingAgenda 1. Meeting Protocol 2. Introduction 3. Partial discharge – A basic understanding 4. Partial discharge mechanism 5. Partial discharge Detection components 6. Online Partial discharge detection APAC Electrical Training
- 3. 3 MeetingProtocol – Thank you for taking time out of your schedule to attend this meeting – Administrative Items: – When not speaking, please place your microphone on mute – Feel free to express your doubts or clarifications – The slides and subject matter is expected to promote discussions on the topics being covered leading to an interactivesession APAC Electrical Training
- 4. 4 Electrical reliability Reliability of an equipment is determined by its initial design, manufacturing quality, installation and maintenance activities. Most of the participants being from the maintenance background, we will limit our scope to maintenance activities that ensure reliability of electrical equipment's. The graphs below show us the relation of overall cost against each type of maintenance strategies and the way each maintenance strategy restores the health of the equipment. Preventive Maintenance Condition based Maintenance Reactive Maintenance Total Cost RepairCost PreventionCost Number/Probability of Failures Cost PreventiveMaintenance(periodic) ReactiveMaintenance (At timeoffailure) Conditionbased Maintenance (Onlinemonitoring) Time Equipment health (%) Equipmentfailure (Majorrepair) APAC Electrical Training
- 5. 5 Condition based maintenance Condition based maintenance or Predictive maintenance along with a robust preventive maintenance schedule provides us with the optimum cost vs reliability scenario. Some of the common predictive maintenance tools available for electrical are as follows: APAC Electrical Training Tests Motor Transformer Switchgear Acoustical PD tests Yes Yes Yes Fluid analysis - DGA , BDV etc. - Vibration analysis Yes - - Insulation testing IR,PI, Tan Delta, PD . IR, Tan Delta, PD , FRAetc. IR, PD etc. Infrared imaging Yes Yes Yes Stator winding 60% Rotor winding 13% Stator core 1% Other 13% Bearing 13% FAILURE CAUSE OF LARGE ELECTRICAL MOTORS The graph alongside shows a breakup of the cause failures of large electrical motors. As can be seen, almost 60% of the failures are caused due to problems in the stator winding. So if we can predict and prevent some of these failures, it would increase the reliability of our electrical equipment by a good margin. Source: Field Current Signature AnalysisforFault Detectionin Synchronous Motors
- 6. 6 Condition based maintenance The distribution alongside shows the main cause or location of the electrical failures in motors. Some interesting facts emerge out of this distribution – Sensors (RTD’s , Vibration etc.) account for almost 1/3rd of the failures/trips – Cables and termination issues account for 28% – Insulation related failures account for another 27% – Overloading and power supply issues account for the remaining 13% APAC Electrical Training Protection relays which is usually considered the main and essential element for electrical equipment protection, only takes care of the last 13%, for the remaining 55% it only trips after the fault has already led to a magnitude where shutdown is essential to prevent major damage. Hence it does not improve reliability in these cases but only reduces the magnitude of the effects of the faultwhich leads to lesser time for repair. We do have 2oo3 logic etc. to prevent the top 32% that are sensor related , from causing unwanted/spurious trips thereby reducing this percentage by a significant margin. The industry does not really normally have systems in place for detecting the major 55% (insulation related)of the source of trips/failures. Sensor terminal boxes 32% Winding insluation failure 27% Cables 12% Power terminations 12% Over current 11% Contact resistance s 4% Phase unbalance 2% ELECTRICAL FAILURE CAUSES Source: CIGRE Motor failure surveyWG A1.19
- 7. 7 Insulation – A major component The insulation in electrical equipment is one of the main components, if not the main component, which determines its life and condition. We saw earlier that the winding faults form a large percentage of equipment failures in electrical. Any winding consists of mainly two components – Conductor and Insulation. Manufacturers have been able to ensure uniformity and quality of conductors and we rarely see any failure related directly to the conductor, like conductor breakage , soldering opening etc. Whereas it is a challenge to get that type of uniformity and quality in the various types of insulation that are applied/required during manufacture of an electrical equipment. Insulation may be in the form of tapes, sleeves, sprays, castings or liquid impregnated. Other than tapes and sleeves, it is very difficult to maintain and achieve a uniform quality of insulation in all batches and these lead to very different insulation properties and behaviors, even in identical machines. This has led to a condition where it is difficult or nearly impossible to come to a fixed standard value against which to compare to accept or reject an insulation. Most of the insulation related tests are best suited for comparison with the earlier test report values of the same equipment that had been taken when it was newly installed at site or at the OEM factory. Also, these tests are typically offline tests and require the equipment to be shutdown for these tests. If we are to use a predictive or truly condition based maintenance system, the tests should be possible to be done online so that we know the status of the equipment in real time and can start planningfor taking remedial measures as soon as we see the initial signs of any degradation to the insulation system. One of the main systems that gives us an early warning of degrading insulation properties is the online Partial Discharge equipment which can be used for most of the electrical equipment's like motors , transformers and switchgears. We shall be covering this topic in this training module to familiarize ourselves with the various components , their working and basics of analyzing those captured data. APAC Electrical Training
- 8. 8 Partial discharge – Introduction Partial discharge (PD) is a localized electric breakdown of a part of a insulation system under high voltage stress. As the failure is local and does not bridge the insulation, there is no complete failurein the insulation. Corona discharge is usually visible while the partial discharge happens within the insulation medium and is generally not visible PD is a an effect due to one of these defects in the insulation – Voids inside solid insulation or bubbles in fluids – Surfacedeposit of metal/contaminants – Spaces between Electrodes and insulation surfaces – Defective or loose joints PD Corona APAC Electrical Training
- 9. 9 Partial discharge – characteristics These defects can arise due to – Incorrect design or manufacture – Damage to equipment – Poor installation – Deterioration of insulation due to age – Overloading, heating, surges etc. PD tends to slowly increase with time and may not cause a complete insulation failure even after months or years. The deterioration is accelerated by – Elevated stress like high voltage, vibration – Temperature and humidity of the surroundings – High load leading to increased thermal and mechanical stress The change over time rather than the magnitude of the PD is a better evaluation of the condition of the insulation system. Hence a baseline value is very important to evaluate a PD reading. APAC Electrical Training
- 10. 10 Partial discharge – common types The common types of Partial discharge based on the various electrode configurations are as follows – HV HV HV HV HV HV HV Ideal Insulation Corona discharge SurfaceDischarge Lamination Discharge Treeing Looseconnection Cavity(void)discharge Normal PD APAC Electrical Training
- 11. 11 Partial discharge – Mechanism Consider the simplest case of one void in an insulator as shown alongside. The void contains a gas, typically air, which has a much lower breakdown voltage than the surroundinginsulation. Voltage “V” is applied across the insulation and is distributed linearly across spacing as per eq V =Vx + VPD + Vy As value of V increases, all the voltages proportionately increases till “VPD“ crosses the breakdown voltage “Vbd” of the medium inside the void. At this point, an arc is initiated between the ends of the surface leading to it becoming a resistive path instead of a capacitor. The charged insulators on top and bottom of the void also supply to the arc and get discharged themselves and the arcgets extinguished. There is a sudden inrush again, to charge the insulation capacitance, till the “VPD“ reaches the earlier breakdown point and the cycle repeats itself till the voltage “V” starts decreasing. Typical breakdown voltage for various materials – Air 24kV/cm – Transformer oil 150kV/cm – Epoxy Resin 300kV/cm – Polyethylene 500kV/cm Ratio between Air and cast resin > 20 V Vx VPD Vy V Vx VPD Vy Vmax Vbd Vresidual -Vresidual -Vbd -Vmax APAC Electrical Training
- 12. 12 Partial discharge – Mechanism These multiple arc’s in the medium lead to gradual degradation of the insulation property due to carbon deposits of the burning insulation. This region with the carbon deposit becomes a conductive part instead of the insulator. This leads to a voltage drop of nearly zero(0) across this defect, thus leading to increased voltage across the remaining insulators and hence more discharge frequency and intensity. As can be clearly seen, this is a process that leads to more and more discharges over time and finally a complete breakdown of the insulation. This is a simple illustration to understand the breakdown mechanism of the partial discharge. In actual life, we have multiple voids of different sizes distributed throughout the insulation material and having a PD due to these micro voids is inevitable. When the size of the void increases, the value of “VPD“ increases, which lead to more capacitive charging before breakdown but lower number of discharges. These high amplitude discharges damage the insulation much faster and are the main cause of concern. This voltage “Vbd“ is detected by the PD couplers as a value called “Qmax” in the graphs and the number of times it occurs is shown as PPS(pulse per second) or PPC(pulse per cycle). In the simple illustration used earlier, we can thus calculate the values as Qmax = Vbd ; PPC = 10; V Vx VPD Vy V=0 Vbd APAC Electrical Training
- 13. 13 Partial discharge – Mechanism The other common type of partial discharge is due to dust or foreign particle deposition on the insulation surface. This is a more of a environmental and maintenance issue and is one of the most easily rectifiable PD types. The foreign objects are in good probability more conductive than the main insulation or maybe a good conductor itself. The situation is again represented in the diagram for easy visualization. The normal voltage gradient of a clean insulation is represented by the green line while the gradient after deposition of dust/oil etc. is represented by the red line. As we can see, the red line has steeper gradient which if the accumulation is large enough may be more than the breakdown voltage of the insulation causing a local breakdown and arcing. A more common failure is breakdown of the air between the foreign particles causing local arcing which is seen as partial discharge. These arc causes ionization of air and local high voltage which serve as cause for attracting further dust particles. This type of surface PD is influenced highly by the moisture in air as water tends to make the dust particles more conductive while bringing down the breakdown voltage of air. This characteristic helps in identifyingthis type of PD. The photo shows end windings with dust deposition leading to PD activity. The other cause of PD is corona discharge where the mechanism is same as traditional corona. These typical types of PD in combination gives rise to all the other types of PD in general and can be used as a basis for understanding and troubleshooting the PD detected. V Vx VPD Vy V=0 V=0 APAC Electrical Training
- 14. 14 Partial discharge – Detection The partial discharge detection can be done online or offline. We shall concentrate on online first and then go to offline detection system. The online system consists of a coupling capacitor installed between the detection point and ground. The equivalent diagram is represented alongside for a clarity of understanding. Whenever there is a discharge across the void, all the capacitors discharge their stored energy through the gap resistance. This discharge is detected by the coupling capacitor and is transmitted as millivolt signal into the detection system electronics. The photos alongside provide the coupler constructional features. The high voltage is tapped from the bus and connected to the top of the coupler. As seen in the sectional view, the high voltage is dropped across numerous layersof mica sections and the bottom part is connected to ground and also to the signal processing card at the base of the coupler. A coaxial cable carries the signal to the main detection unit. The frequency response of the couplers is shown in the chart, where typically f1 is 500kHz and f2 is 500Mhz. This lower range of 500kHz corresponds to a signal width of 2microseconds. Thus for a 50Hz system with a cycle time of 20msec, the minimum resolution is 10,000 per cycle. Coupling capacitor Coupling capacitor Front view Bottomview Sectional view HV Frequency f2 f1 fm 500kHz 500MHz APAC Electrical Training Source: DR datasheet VPD Insulation inside machine V Vx Vy Voltagebus
- 15. 15 Partial discharge – Typical installation scheme This diagram shows the typical connection scheme for a online PD monitoring unit by Eaton. Other manufacturers also have a very similar scheme. 1 Partial discharge coupler is used in each of the 3 phases. They are the main source of detection for the terminal box connections, terminations and the initial coils of the motor high voltage windings. RTD’s are also a good source of partial discharge readings as they are present in the slot and any discharges in the slots or the nearby set of windings are transmitted through the RTD cables. A high pass filter is used to capture these high frequency signals from the RTD wires while the RTD sensing function is not disturbed. A RFCT arrangement captures radio frequency signals of the discharges to ground to give a PD reading but this arrangement is not opted for by Linde, traditionally. Typically not used in Linde APAC Electrical Training
- 16. 16 Partial discharge – Online monitoring readings The online monitoring system takes snapshot values of Qmax and PPS at the programmed intervals for creating a graphical trend over time which helps us visualize the gradual degradation of the insulation and generate alarms at predefined values. Typically Linde uses 4 hour intervalsbetween readings i.e. 6 readings per day. The PDI (PD intensity) in mW is calculated using the Qmax and PSS value along with the voltage of the system to arriveat an overall valueof PD activity. In general we see a gradual rise in PD over the years which is normal due to the expected degradation of the insulation system over time. APAC Electrical Training
- 17. 17 Partial discharge – Online monitoring readings A more detailed reading is taken at a higher interval, typically every 15days, which captures the full set of data over a period of one second. This reading is called a phase resolved partial discharge(PRPD) data which has details of every PD activity with the phase angle, amplitude and the number of times it has occurred over the cycle. This graphical representation is a very valuable tool in the interpretation of PD data to give us an indication of where the PD is occurring. The location of the dot indicates the phase angle on the x axis and the voltage on the y axis. The color of the dot indicates the count of that pulse in every cycle. The maximum defects are the micro voids in the insulation which are represented by a large count at lower magnitude which is a classical PD signature. As the defect size increases, in general, the voltage increases and count decreases. APAC Electrical Training
- 18. 18 Partial discharge – Online monitoring readings There are other forms of visualization also available for the experts but they are beyond the scope of this presentation. A sample of the available charts like polar graph, 3D PD graph as well as raw data is shown here as examples. The visualizations will be different for each manufacture but the basic information contained and the type of graphs remain more or less of a common nature. 3D graph Polar graph Raw data APAC Electrical Training
- 19. 19 Improvement of Insulation The diagram alongside gives us a visual depiction of how the insulation materials have advanced in the last century where the insulation thickness for the same voltage level(15kV) has shrunk from 15mm to 2.5mm (a factor of almost 5 or shrinking to 17% of original). This has led to smaller and smaller voids becoming critical as the voltage across the same size of void has now increased to 5 times and consequently, the possibility of breakdown has also increased by the same proportion. Evolution of StatorGroundwall Reduction in Thickness. Single wall thickness is shownat bottomof each cross section view APAC Electrical Training Year 2005 Source: Emery, F.T., Williams, M.: ‘Preliminary evaluation of flat glassbacked mica paper tape for high voltage coil groundwall insulation using vacuum-pressure-impregnation’
- 20. 20 Next meeting agenda – Data trending analysis – Difference between online and offline PD measurements – Tan Delta measurement basics APAC Electrical Training
- 21. Making our world more productive Linde CoE Thomas Varghese Tel +91 80-37991808 Mobile +91 9342502970 Thomas.varghese@linde.com www.linde.com