1. Electrical Checks and MeasurementsLEAP Standard on Stator Windings1.1       Polarization De-Polarization Current Analys...
Results will be analysed to obtain the following parameters:   • Discharging void volume ratio (if discharges are present)...
•    The contribution of the slot stress grading system, contamination and ageing to the non-             linear behaviour...
to the time of inspection and projected by the dashed green line, is made on the basis of “normal”expected parameters (as ...
2.1.2     The customer will have to ensure that the machine is adequately earthed while performing          the tests.2.1....
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Electrical Checks And Measurements And Customer Requirements LEAP

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Electrical Checks And Measurements And Customer Requirements LEAP

  1. 1. 1. Electrical Checks and MeasurementsLEAP Standard on Stator Windings1.1 Polarization De-Polarization Current Analysis (PDCA)A D.C. voltage is applied to the windings. The voltage is maintained for a time period of aminimum of 1000 seconds.The current flowing through the insulation is monitored during the charging period.After all relevant data is obtained, the windings are discharged and discharge currents aremonitored after the initial winding capacitance discharge (< 5 secs), over a total time period thatwill not be less than the charging time period.The charging and discharging currents are plotted on a log-log scale and analysed in the timedomain.The following parameters are computed from the measurements performed: • Ion Migration Time Constant • Slow Relaxation Time Constant • Interfacial Polarization Time Constant. • Ageing Factor / Mobility Index of the insulating resin. • Ion Concentration Index. • Dispersion Ratio. • Volume Resistivity of the Insulation. • Polarization Index.On the basis of the above an assessment of the winding insulation is made with regard to • General insulation quality • Sensitivity of the insulation system to moisture absorption • Presence of contamination in the windings • Condition of the binding resin • Extent of electrical contact of the coil with the slot1.2 ANALYSIS OF CAPACITANCE AND TAN DELTA MEASUREMENTSTan delta and Capacitance will be measured both below and above discharge inception voltage,at voltage levels that will be determined on the basis of discharge inception with the aim ofaccessing the maximum required data for analysis.Capacitance and tan delta measurements will be performed using a transformer ratio arm bridge.Measurements will be performed at increments that will not exceed 0.2 VL (line voltage) .Maximum test voltage will be (1/√3)*VL kV., r.m.s.ABBOffer Ref. Version Page 1 of 5 © Copyright ABB Limited 2009All data and information, both technical and commercial, contained in this offer is confidential and shall not be copied or disclosed toother parties without the written permission of ABB Limited. This proposal remains the property of ABB Limited and shall be returnedto ABB Limited or destroyed, at ABB Limiteds request, together with any copies.
  2. 2. Results will be analysed to obtain the following parameters: • Discharging void volume ratio (if discharges are present) • Effective phase of occurrence of discharges • Characterizing constants if variations are due to stress grading • Effective area involved in slot discharges (if slot discharges are present).The results will be analysed in order to assess the winding insulation with regard to: • Extent of De-lamination (if any) • Condition of the corona protection shield • Non-linear behaviour of insulation that would result in C and Tan delta variations in the absence of partial discharges.1.3 ANALYSIS OF RECORDED PARTIAL DISCHARGE PATTERNSPartial discharge pulse patterns will be monitored and recorded using a transformer ratio armbridge with appropriate coupling capacitors.The PD pulse patterns will be analysed with regard to pulse count, pulse magnitude, polaritydependence and phase to identify the nature of discharges which can then be classified as:• Internal Discharges• Surface Discharges• Slot Discharges1.4 NON-LINEAR INSULATION BEHAVIOUR ANALYSISThe tan delta and capacitance measurements vary with voltage even in absence of partialdischarges and one of the most obvious reasons for such behaviour is the presence of non-linearfield stress grading system at slot ends. Other reasons are space charge/interfacial polarizationdue to contamination, electrostatic forces on delaminated insulation, increased ionic mobility dueto ageing, surface partial discharges etc. It is evident that both the voltage supply acrossinsulation and the current passing through the insulation contain harmonics, which causeincrease or decrease in the measured tan delta and capacitance values. Thus, it becomesnecessary to understand this time varying effect of insulation admittance on the capacitance andtan delta measured.Non-Linear Analysis provides a understanding of such non-linear behaviour, and therebysupplements the tan delta analysis. The analysis provides additional insights into the aging ofinsulation.The machine insulation is tested by applying a known voltage across the insulation andmonitoring the voltage and the current flowing through the insulation, by capturing severalwaveform cycles of the voltage and the current. The insulation is tested at predetermined voltagelevels upto a maximum of (1/√3)*VL, r.m.s. The instantaneous admittance of the insulation iscalculated and the admittance patterns analysed for specific harmonic patterns.The extent of harmonics, predominance of odd or even harmonics, high or low frequencyharmonics is analysed to provide information on: • The integrity of the stress grading system used at the slot endsABBOffer Ref. Version Page 2 of 5 © Copyright ABB Limited 2009All data and information, both technical and commercial, contained in this offer is confidential and shall not be copied or disclosed toother parties without the written permission of ABB Limited. This proposal remains the property of ABB Limited and shall be returnedto ABB Limited or destroyed, at ABB Limiteds request, together with any copies.
  3. 3. • The contribution of the slot stress grading system, contamination and ageing to the non- linear behaviour • To confirm that observed anomalous tan delta variations can be physically related to the above non-linear phenomena1.5 WINDING RESISTANCE MEASUREMENTThe winding resistance will be measured to check for bad joints in the stator windings1.6 LIFE EXPECTANCY ANALYSISA combined stress phenomenological model is used to assess the extent of degradation of theinsulation and perform a lifetime expectancy analysis. The model accounts for thermal, electricaland mechanical stresses, whose relative effects on the life of the insulation are estimated on thebasis of the analysis from the measurements performed, and from the operating and historicaldetails made available.The theory stems from the fact that when stresses act on the insulation of the stator windings,there is a progressive deterioration of the strength of the insulation. In other words, ageing isexhibited by a progressive deterioration of the physical properties of the insulation. Not allphysical properties can give an indication of the progression of ageing, except perhaps thosedirectly related to failure, e.g. electrical breakdown strength and mechanical strength. If stressesunable to produce a failure are considered such as temperature or chemical exposure, otherfailure criteria are selected that are related to electrical or mechanical breakdown strengths. Inany case, failure essentially occurs when the selected property drops down to a limiting value, sothat a unique definition can be adopted for remaining life – the time for the selected property toreach that point. This point is generally related to the stresses that are developed in the statorinsulation during machine operation. In other words, when the strength of the insulationdeteriorates to a point where it equals the developed stresses in the insulation, the insulation willfail.The measurements performed during an inspection are converted into parameters that can berelated to the stresses that are developed in the insulation, for example the extent to air spacewithin insulation would affect thermal, mechanical and electrical properties of the insulation andthereby affect the thermal electrical and mechanical stresses that are developed in the insulation.This affects the rate of deterioration of the insulation. To give another example, when thetemperature of the insulation increases, the rate of deterioration of the mechanical strength of theinsulation also increases. Therefore, based on the knowledge of how parameters derived frommeasurements can affect stresses, it is possible to estimate the rate of deterioration of thestrength of the insulation.In the case of the LEAP Standard measurements performed, the parameters that have beenused as inputs from the analysis include the charge distribution parameters, the volumeresistivity, the discharging void content parameters, partial discharge parameters and data fromthe non-linear analysis. These parameters are used together with operating data such as windingtemperature and the number of starts, to draw the line describing the deterioration in insulationstrength, as indicated by the dashed red line.It is generally assumed that at the time of commissioning of the machine, there is no deteriorationof the relevant properties of the insulation. An initial life estimate, indicated by the solid red line upABBOffer Ref. Version Page 3 of 5 © Copyright ABB Limited 2009All data and information, both technical and commercial, contained in this offer is confidential and shall not be copied or disclosed toother parties without the written permission of ABB Limited. This proposal remains the property of ABB Limited and shall be returnedto ABB Limited or destroyed, at ABB Limiteds request, together with any copies.
  4. 4. to the time of inspection and projected by the dashed green line, is made on the basis of “normal”expected parameters (as derived from operating data and the normal range values ofmeasurements such as those specified for LEAP Standard inspections) at an early part of themachine life. LIFE EXPECTANCY ANALYSIS WITH 80 % CONFIDENCE LEVEL 100 90 80 70 Present Life % LIfe Used Up Estimate 60 50 40 30 Designed Life Estimate 20 10 0 0 10 20 30 40 50 60 70 80 90 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Equivalent Hours ( x 1000) Insulation degradation curve (designed) Remaining Life (present) Present LifeThe above estimation is based on the measurements performed and historical steady stateoperating data of the machine, made available to ABB. Models are used that describe knownwearing out and ageing processes of the ground-wall insulation of the stator windings. Withthe present levels of technology, it is not possible to model the degradation of the turn insulationand as such these defects are not within the scope of the study. It is assumed in the analysis,that future operating conditions of the machine, will be similar to the historical operating datamade available to ABB.Due to the part-theoretical nature of the analysis and the fact that such analysis is partially basedon unknown variables, any indicated life expectancy shall only be interpreted as an indicativeperiod of additional operability of the analyzed subject.2 Requirement on the site2.1 Preparation of machine for measurement:2.1.1 The machine will have to be prepared for test by the customer with the terminals made available, and with cables/busbars disconnected at the machine end. Other accessories in the terminal box such as surge arrestors, surge capacitors, etc, will also have to be disconnected by the customer. In general, all disconnection and reconnection will have to be carried out by the customer.ABBOffer Ref. Version Page 4 of 5 © Copyright ABB Limited 2009All data and information, both technical and commercial, contained in this offer is confidential and shall not be copied or disclosed toother parties without the written permission of ABB Limited. This proposal remains the property of ABB Limited and shall be returnedto ABB Limited or destroyed, at ABB Limiteds request, together with any copies.
  5. 5. 2.1.2 The customer will have to ensure that the machine is adequately earthed while performing the tests.2.1.3 During offline measurement, Stator winding temperature of machine offered for testing should not exceed 40 deg Celsius.2.2 Time for Measurements:2.2.1 Measurements for LEAP Standard on the stator winding of a single machine can be conducted on a day considering normal 8-10 hours working duration. (or) measurements can be conducted on maximum of two machines at single location considering 14 hours working time.2.3 Test Voltage: Stators will be tested upto a maximum of the earth voltage, (e.g. 3.8 KV AC in the case of 6.6 kV machines or 6.4 KV in 11 KV machines) - which is generally considered to be a safe test voltage level. However, if the insulation of the machine fails during test, it could only be attributed to a major defect in the insulation of the machine, and as such ABB will not be held responsible for such a failure.2.4 General Requirements from the Customer2.4.1 To comply with safety requirements, the end-customer will have to ensure that one other competent person in addition to our test engineer is present during testing, and that there is adequate lighting in the test area.2.4.2 The customer will depute one competent person who will assist our testing personnel during data collection and measurements.2.4.3 The customer will provide facilities to lift and shift the test equipment as and when required at site.2.4.4 Suitable/bench or any other adequate arrangements to set up the test equipment as close as is possible to the machine to be tested.2.4.5 Space for the safe storage of test equipments while not in use.2.4.6 Permission for use of photographic and video graphic recording during inspection within a framework of normal rules and regulation stipulated by the client.2.4.7 Power supply board with a minimum of three 5 amp sockets and switches (single phase, 3-pin, 220/240 V) and one domestic 32 amp socket and switch (single phase, 3 - pin, 220/240 V) should be arranged by the customer at the point or site of testing.ABBOffer Ref. Version Page 5 of 5 © Copyright ABB Limited 2009All data and information, both technical and commercial, contained in this offer is confidential and shall not be copied or disclosed toother parties without the written permission of ABB Limited. This proposal remains the property of ABB Limited and shall be returnedto ABB Limited or destroyed, at ABB Limiteds request, together with any copies.

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