1. 1. Electrical Checks and Measurements
LEAP Standard on Stator Windings
1.1 Polarization De-Polarization Current Analysis (PDCA)
A D.C. voltage is applied to the windings. The voltage is maintained for a time period of a
minimum 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 are
monitored after the initial winding capacitance discharge (< 5 secs), over a total time period that
will not be less than the charging time period.
The charging and discharging currents are plotted on a log-log scale and analysed in the time
domain.
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 slot
1.2 ANALYSIS OF CAPACITANCE AND TAN DELTA MEASUREMENTS
Tan 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 of
accessing 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.
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All data and information, both technical and commercial, contained in this offer is confidential and shall not be copied or disclosed to
other parties without the written permission of ABB Limited. This proposal remains the property of ABB Limited and shall be returned
to ABB Limited or destroyed, at ABB Limited's request, together with any copies.

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 PATTERNS
Partial discharge pulse patterns will be monitored and recorded using a transformer ratio arm
bridge with appropriate coupling capacitors.
The PD pulse patterns will be analysed with regard to pulse count, pulse magnitude, polarity
dependence and phase to identify the nature of discharges which can then be classified as:
• Internal Discharges
• Surface Discharges
• Slot Discharges
1.4 NON-LINEAR INSULATION BEHAVIOUR ANALYSIS
The tan delta and capacitance measurements vary with voltage even in absence of partial
discharges and one of the most obvious reasons for such behaviour is the presence of non-linear
field stress grading system at slot ends. Other reasons are space charge/interfacial polarization
due to contamination, electrostatic forces on delaminated insulation, increased ionic mobility due
to ageing, surface partial discharges etc. It is evident that both the voltage supply across
insulation and the current passing through the insulation contain harmonics, which cause
increase or decrease in the measured tan delta and capacitance values. Thus, it becomes
necessary to understand this time varying effect of insulation admittance on the capacitance and
tan delta measured.
Non-Linear Analysis provides a understanding of such non-linear behaviour, and thereby
supplements the tan delta analysis. The analysis provides additional insights into the aging of
insulation.
The machine insulation is tested by applying a known voltage across the insulation and
monitoring the voltage and the current flowing through the insulation, by capturing several
waveform cycles of the voltage and the current. The insulation is tested at predetermined voltage
levels upto a maximum of (1/√3)*VL, r.m.s. The instantaneous admittance of the insulation is
calculated and the admittance patterns analysed for specific harmonic patterns.
The extent of harmonics, predominance of odd or even harmonics, high or low frequency
harmonics is analysed to provide information on:
• The integrity of the stress grading system used at the slot ends
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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 phenomena
1.5 WINDING RESISTANCE MEASUREMENT
The winding resistance will be measured to check for bad joints in the stator windings
1.6 LIFE EXPECTANCY ANALYSIS
A combined stress phenomenological model is used to assess the extent of degradation of the
insulation and perform a lifetime expectancy analysis. The model accounts for thermal, electrical
and mechanical stresses, whose relative effects on the life of the insulation are estimated on the
basis of the analysis from the measurements performed, and from the operating and historical
details 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 is
exhibited by a progressive deterioration of the physical properties of the insulation. Not all
physical properties can give an indication of the progression of ageing, except perhaps those
directly related to failure, e.g. electrical breakdown strength and mechanical strength. If stresses
unable to produce a failure are considered such as temperature or chemical exposure, other
failure criteria are selected that are related to electrical or mechanical breakdown strengths. In
any case, failure essentially occurs when the selected property drops down to a limiting value, so
that a unique definition can be adopted for remaining life – the time for the selected property to
reach that point. This point is generally related to the stresses that are developed in the stator
insulation during machine operation. In other words, when the strength of the insulation
deteriorates to a point where it equals the developed stresses in the insulation, the insulation will
fail.
The measurements performed during an inspection are converted into parameters that can be
related to the stresses that are developed in the insulation, for example the extent to air space
within insulation would affect thermal, mechanical and electrical properties of the insulation and
thereby 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 the
temperature of the insulation increases, the rate of deterioration of the mechanical strength of the
insulation also increases. Therefore, based on the knowledge of how parameters derived from
measurements can affect stresses, it is possible to estimate the rate of deterioration of the
strength of the insulation.
In the case of the LEAP Standard measurements performed, the parameters that have been
used as inputs from the analysis include the charge distribution parameters, the volume
resistivity, the discharging void content parameters, partial discharge parameters and data from
the non-linear analysis. These parameters are used together with operating data such as winding
temperature and the number of starts, to draw the line describing the deterioration in insulation
strength, as indicated by the dashed red line.
It is generally assumed that at the time of commissioning of the machine, there is no deterioration
of the relevant properties of the insulation. An initial life estimate, indicated by the solid red line up
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other parties without the written permission of ABB Limited. This proposal remains the property of ABB Limited and shall be returned
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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 of
measurements such as those specified for LEAP Standard inspections) at an early part of the
machine 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 Life
The above estimation is based on the measurements performed and historical steady state
operating data of the machine, made available to ABB. Models are used that describe known
wearing out and ageing processes of the ground-wall insulation of the stator windings. With
the present levels of technology, it is not possible to model the degradation of the turn insulation
and 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 data
made available to ABB.
Due to the part-theoretical nature of the analysis and the fact that such analysis is partially based
on unknown variables, any indicated life expectancy shall only be interpreted as an indicative
period of additional operability of the analyzed subject.
2 Requirement on the site
2.1 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.
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All data and information, both technical and commercial, contained in this offer is confidential and shall not be copied or disclosed to
other parties without the written permission of ABB Limited. This proposal remains the property of ABB Limited and shall be returned
to ABB Limited or destroyed, at ABB Limited's request, together with any copies.

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 Customer
2.4.1 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.
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All data and information, both technical and commercial, contained in this offer is confidential and shall not be copied or disclosed to
other parties without the written permission of ABB Limited. This proposal remains the property of ABB Limited and shall be returned
to ABB Limited or destroyed, at ABB Limited's request, together with any copies.