Partial Discharge Measurements
PD are a partial breakdown of gas inclusions in insulation
where the electric field intensity exceeds the breakdown
field strength
PD transforms part of the capacitive stored energy into
heat and radiation as well as mechanical and chemical
energies which can degrade insulation materials
This ageing process progressively reduces the insulation
thickness and the breakdown voltage until the failure
occurs
Insulation erosion is
 very fast in organic materials (Type 1)
 slow in organic/inorganic insulation systems (Type 2)
Electric signals related to currents, electromagnetic
waves, electroluminescence are recorded using proper
couplers and processed in order to:
provide information on the discharge phenomenon
 establish PDIV as a function of the supply square wave
parameters (mainly frequency and over-voltages)
 check the quality of the insulation and compare
different materials (PDIV and time-to-breakdown are currently
adopted as parameters to be monitored during life tests)

Ref.- IEC 61934TS: Electrical insulating materials and
systems – Electrical measurement of partial discharges
(PD) under short rise time and repetitive voltage
impulses
PD Basics
The
local
breakdown
generates a voltage drop
and a consequent fast
impulsive
current
absorption from the supply
A suitable test circuit configuration allows to record the PD
pulse signals. It is composed by

Sinusoidal/square wave generator
Couplers and filters
 Synchro Units
 Digital recorders
 Processing unit

Waveform Generators
Sinusoidal :
Square Unipolar
PWM-like :
PWM+peaks :
Square Bipolar

with variable Vrms, V0p, Vpp, RR, RT

with selectable duty and wave-shape

able to supply high capacitive currents
e.g.,
Vpp= 07.5 kV
Unip./ Bip.
dV/dt=075 kV/µs
Freq.= 020 kHz
Duty%=0.0190
Typologies of Samples Under Test
Twisted Pair (frame to test
T2T insulation according to
IEC 60851-5)
Motorettes (frame to test
T2T,
P2G
and
P2P
insulation)
Complete coils & stators
PD Signal Characteristics
Partial Discharges generate impulsive signals having up
to 1-2 GHz of frequency content
Fast voltage transition can reach 200 MHz depending to
the voltage RT
0.05
1.5
0.025
0.75
0
0
-0.025
-0.75
PD pulses
Generator voltage
-0.05
0
0.5
1
1.5
2
2.5
Time (us)
3
3.5
-1.5
4
Generator voltage (kV)
PD magnitude (V)
Suitable couplers and bandwidth must be selected to
avoid the commutation interference during PD
measurements
Coupler and high-pass filters having a low cut-off
frequency higher than 250-300 MHz are required to avoid
commutation interferences
IEC 61934 Frequency Band Prescriptions
Volt. RT: 50ns
PD RT: 2 ns
800
800
8th order filter
with filter cut-off
frequency equal
to 500 MHz.
800
800
0,15
0.15
0,04
0.04
600
600
600
600
0.10
0,10
400
400
-200
–200
-0.05
–0,05
AppliedVoltage
voltage V[V]
Applied
0.00
–0,00
–0,00
0.00
00
-200
–200
–0,02
-0.02
Filtered PD signal V
00
0,02
0.02
200
200
Filtered PD Signal [V]
0,05
0.05
200
200
PDSignal
signal [V]
V
PD
AppliedVoltage
voltage V[V]
Applied
400
400
-400
–400
-400
–400
Square
kHz
applied
voltage
Squarebipolar
Bipolar1010
kHz
Applied
Voltage
PDsignal
signal
PD
-600
–600
-0.10
–0,10
Square
kHz
applied
voltage
Square bipolar
Bipolar1010
kHz
Applied
Voltage
Filtered
FilteredPD
PDsignal
signal
-600
–600
–0,04
-0.04
–800
-800
-800
–800
–0,15
-0.15
-2
–2
-1
–1
0
1
2
3
4
5
–2
-2
–1
-1
0
1
2
3
4
5
Timee s
Tim
[s]
Time
Time [s]
s
IEC 553/06
IEC 552/06
Different Low Cut-Off Frequencies
0.15
Commutation
0.10
PD signal
PD signal [V]
0.05
0.00
-0.05
-0.10
10 MHz
100
200
500
MHz
-0.15
-2e-6
-1e-6
0
1e-6
2e-6
3e-6
4e-6
Time [s]
Differential Configuration
A differential connection of
coupler can be adopted to
cancel the interferences
due to voltage transitions
The capacitance of CC and
the SUT must be similar
Signal relevant to the
voltage transition reaches
the +/- input of the LF filter
and it is cancelled
F.Guastavino et al., “Measuring PD Under
Pulsed Voltage Conditions”, IEEE Trans.
on Diel.El.Ins. Vol.15, pp.1640-1648,
December 2008
PD Detection Circuits and Couplers
Capacitive Couplers
PD free devices
To be connected with a proper detection impedance to
obtain high cut-off frequencies
Test
object
Supply
Z
Filter
PD signal
IEC 540/06
Inductive Couplers
HFCT
Supply
Test
object
Filter
With very high cut-off
frequency
PD signal
IEC 543/06
Test
object
Supply
Filter
PD signal
HFCT
IEC 544/06
Antennas
Antenna
Supply
Test
Object
Suitable
for
measurements in
applications
Acquisition
System
Supply
High cut-off frequencies
Test
object
Antenna
Acquisition
system
IEC 545/06
PD
ASD
Light Detectors
Description
Advantage
Disadvantage
Electrical
Optical
RF / EMI
Acoustic
Electrical circuit that picks up
current pulse produced by
charge transfer during partial
discharge
Measures light emission
from partial discharges
Measures radio
frequency interference
generated by the
discharge
Measures the acoustic
emissions produced by
a partial discharge.
A good sensitivity and standard Non-contact, applicable
for all HV equipment during
for all voltage types.
manufacture
Allows testing of
equipment in real
conditions
Non-contact, applicable Non-contact, applicable
for all voltage types.
for all voltage types.
Allows testing of
Allows testing of
equipment in real
equipment of real
conditions
conditions
Sensitive to electrical noise. Insensitive to any form of
Depending on
Sensitive to other
Cannot test circuit in operating internal partial discharge. equipment being tested,
acoustic emissions.
condition in most cases. Most
Sensitive to light and
EM emissions can
Signals cannot always
commercial equipment can
highly directional.
prevent detection of PD
propagate through
only test at up to 400Hz
insulation / casings
Measurement Systems
Very large bandwidth oscilloscopes (up to 2GHz)
 Quite difficult to interpret the PD measurement results
 Data must be organized to synthesize the information
Commercial instruments are
available
Must
of
them
require
suitable input filters to tune
their input requirements to
record PD and reject voltage
commutations (>200-500MHz)
Voltage
Pulse
PD using
Antenna
Residual
Noise
PD using
Voltage
divider
PD Patterns
The amplitude and the phase of the PD signal are
evaluated
The PD pulse sequence is transformed in a sequence of
Dirach functions having the same phase
v
v
A 3D histogram is obtained
considering the number of
discharges having the same
amplitude and the same phase
of occurrence
A 2D histogram can be derived from the PRPD pattern
projecting n in A, plain and using different colors to
evidence the different repetition rate
A [V]
φ [deg]
Reference sin wave
Two
commutations
per period
Square wave Volt.
DC=50 %
Sync frequency =
Commutation
frequency
square wave
PDIV
Two sub-patterns close to
the zero crossing
At least 1 PD per pulse
Possible PD even during
the “flat zone” of the voltage
pulse
A low pass filter is required to obtain the phase reference
and to derive the so called “Phase Resolved” PD pattern
PWM
EUT
VOLTAGE SUPPLY
Voltage
divider
LP FILTER
SYNC Ch
DETECTION UNIT
Modulating wave
Example of PRPD pattern when a PWM voltage is adopted
Modulating wave 50
Hz
• Period of 20ms
Pulse Rep.Rate 1kHz
• 40 commutations
per period
40 subpatterns per
period
Life-test Results
twisted pair specimens were fed till failure by different
wave forms at several amplitudes, in the presence of PD
Data were processed according to the standard procedure
PRPD patterns were recorded during the experiment
Weibull Plot:
tF 
F (t )  1  exp( )

IPL model:
tF  kV  N
Result 1: Unipolar-Bipolar Square Waves
Twisted pair of having insulation of different materials
were tested in air (with PD) and immersed in oil (no PD)
Sinusoidal, unipolar and bipolar square waves, the latter
having the same RT, RR (50 Hz, 10 kHz), DT=50% and
different amplitudes were used in the test
Let V0p and Vpp the 0-to-peak and peak-to-peak values
When V0p = Vpp with or without PD, the life curves relevant to
unipolar and bipolar pulses are completely overlapped
NO PD
PD
The Jump Voltage is the real voltage stress that affects the
insulation ageing
VEC can be adopted to compare the performances of
different materials even in the presence of PD
D.Fabiani et al. “Ageing acceleration of insulating materials for
electrical machine windings supplied by PWM in the presence and in
the absence of partial discharges”, IEEE ICSD pp.283-286, 2001
Result 2: PWM wit and without Overvoltages
The picture is much more complicated when PWM and
PWM+over-voltages are adopted to stress the materials in the
presence of PD
Due to the “equivalent derivative effect”, there are
different Jump Voltages
V
Vaa
Vpp
V0p = voltage
amplitude
x
x
t
=
overvoltage
Vpp = 2(V0p +x)
Vaa = V0p +2x
F.Guastavino et al. “Life Tests on Twisted Pairs Subjected to PWM-like
Voltages”, IEEE ICSD pp.238-241, 2004
Result 3: Phase Resolved PD patterns
Sinusoidal
V = 1.83 kV
V = 2.44 kV
V = 3.05 kV
V = 3.80 kV
Square
V = 1.60 kV
V = 1.9 kV
V = 3.96 kV
V = 4.32 kV
PWM
V = 1.63 kV
V = 2.73 kV
V= 3.53 kV
V= 4.32 kV
PWM+Over-voltage
V = 2.90 kV
V = 2.08 kV
V = 2.38 kV
V = 2.50 kV
Discussion
 PWM voltage waveforms can promote PD activity,
reducing the life of magnet wires
 The predominant factors explaining this behavior are
peak-to-peak voltage and switching frequency.
 Even in the absence of PD, PWM voltage waveforms
can accelerate the intrinsic aging of winding insulation
(overvoltages). Very high slew rate (>5 kV/s), in
fact, plays also a non negligible role on acceleration
degradation due to increased voltage stress and
heating.
This effect can be evaluate resorting to space charge
measurements both in the presence and in the absence
of PD
Space Charge and PD Activity
Space Charge = charge trapped inside the insulation
or on interfaces (depends on the material, the poling field,
the temperature and the supply voltage waveform)
Above 10 Hz, SC is not trapped appreciably in insulation
unless the waveform contains a DC component
IN HF pulse waveforms, SP is accumulated mainly by PD
Neglecting the SP accumulated in the bulk, the electric
field in air, at the insulation surface, E*0 can be:
Where:
E0 : electric field without accumulated charge
E
0 : the permittivity of air andinsulation
d l0 : the insulation thickness and the half air gap
0: the surface charge density
*
0
 sd
 E0 
 0 d  l 0
Behaviour of the electric field in two enamelled wires in a
twisted pair configuration
This configuration helps explain why the main ageing
factor is associated to the jump voltage for bipolar and
unipolar voltage waveforms
Let E*0(t) the behaviour of the electric field in the air gap
before and after a PD event (gray line)
E0(t) the applied field (black line)
-E0p +E0p the bipolar voltage amplitude
2E0p the unipolar voltage amplitude
and let E0>PDIV
ESC the drop of the electric field due to the PD charge
injection
After PD, a residual
value of the electric
field is:
Eres  E *0  Esc  0
E
*
0
 sd

 0 d  l 0
If
the
charge
injected by PD is
not
rapidly
depleted, E*0(t) due
to SC deposited on
the
insulation
surface
remains
constant
When E0 change its
polarity,
E*0(t)
increases by 2Ep in
both cases
E*0 2Ep  Eres  PDIV
The influence of the JV was experimentally supplying 4
different kinds of enamelled insulated wires with sinusoidal,
unipolar and bipolar square waves in the range of 50 to 10
kHz and with a RT of 50 ns above the PDIV
SC measurement becomes a significant tool to evaluate the
ability of the different materials to deplete the SC injected
by PD and to compare different enamels
PEA SP Measurement System
A Pulse Electro-Acustic method ha been developed to
measure the space charge accumulation on magnet wires
(D.
Fabiani et al. “ Relation Between Space Charge Accumulation and
Partial Discharge Activity in Enameled Wires Under PWM-like Voltage
Waveforms”, IEEE Trans. on Diel.-Elect.Ins., Vol.11, pp.393-405, June
2004)
The specimen is positioned an
aluminium ground plate and
a semicon-absorber and
fed by HV DC power supply
A voltage pulse of A=300 V, 10
ns width, 110Hz of RR, is applied
through a 220 pF coupling
capacitor
PVC Insulation
Semicon Adsorber
2 M
Enameled Insulation
Copper
Aluminum Ground Plate
220 pF
Amplifier
HVDC
Pulse
Oscilloscope
Piezoelectric (PVDF)
Adsorber (PMMA)
PC
GPIB IEEE-488
Acquisition Board
 Charges present in insulation are forced by the fast
electric pulse and a pressure wave is generated from the
interaction between charges and the material structure
 The pressure wave propagates through the insulation and
reaches the ground electrode under which the piezoelectric
trasducer (PVDF) is located
 PVDF generates a voltage signal (PEA output signal)
proportional to the pressure wave propagating through it
 A proper calibration procedure allows the PEA output
signal to be correlated to the amount of trapped space
charge
 PEA signal is amplified and sent to a recording system
 110 PEA outputs per second, synchronized with the
rectangular supply voltage, are processed to obtain the
charge profile
Test Procedure
SC measurements are performed according to a specific
polarization/depolarization procedure
Polarization: the electric field is applied (volt-on, VP=1 kV) for
a period tp=3600 s to achieve steady state conditions for
the accumulated charge
Depolarization:
depolarization
(volt-off) follows polarization. It
is obtained removing the supply
voltage and grounding the high
voltage electrode, and lasted
3600 s as well.
t =20 s:
 PEA signal is mainly due to the electrode field-induced
charge (peaks indicate the electrode location, i.e., anode and cathode,
corresponding to the positive and negative signal peaks, respectively)
 the injected charge at the electrode-insulation interface is
hidden by the electrode charge
t =3602 s:
 SC in the insulation bulk can
be observed looking at the PEA
profile under volt-off (shaded
area)
 when the poling field is removed the electrode charge is
considerably smaller than under polarization, being only
due to the image charge (of the internal charge)
Space Charge Data Processing
Space charge profile at the beginning of
volt on: no space charge present
Space charge profile 2s after voltage
removal: space charge = gray area
To quantify the charge accumulation:
TIPO A - PROFILO CARICA:
Volt-ON 3600 s (+)
Volt-OFF 3600 s
1.2
Depolarization characteristic
Charge [p.u.]
1.0
 Total absolute stored charge density
(after grounding), QM
x1
1
QVO (t ) 
Q( x, t ) dx

x1  x0  x0
0.8
0.6
0.4
0.2
1
10
100
Time [s]
1000
10000
 the slope of depolarization characteristic,
s, is a measure of space charge dynamic,
i.e. the speed of charge recombination /
expulsion
Material Improvements
PD when active, are the most important ageing factor in
Type I insulation
Even in the absence of PD, jump voltage and switching
frequency can accelerate the intrinsic aging of winding
insulation due to increased voltage stress and heating
Solutions could come from the use of:
 VPI technologies
 mica-films for turn and strand insulation
 with metal oxide or ceramic fillers (micro-fillers)
 nano-scale technologies (nano-fillers)
VPI Technologies
The stator can be totally impregnated adopting the VPI
technology even for small size random wound machines
The PD inception voltage is up to 60% higher compared
with non-impregnated ones because air gaps are filled
(not totally) by the impregnation
The time to breakdown of the inter-turn insulation
depends on the:
 PDIV and intensity of PD
 enamel thickness and its resistance against PD erosion
But
 due to small imperfections, longer lifetime is not
guaranteed
 VPI process for small machines is expensive
Mica-Films for Inter-Turn or Strand Insulation
Currently adopted in MV and HV
machines fed with pulsating voltages
They are also adopted in LV
machines
Improvements were found but
 higher costs
 slot efficiency reduction
 increased dimensions
Micro Fillers
Enamel or polymer film may also contain additives such
metal oxides or ceramic materials with a natural
resistance to discharges (Al2O3, TiO2, SiO2, Fe2O3..), has
been adopted
The structure is a multi-coating
over the copper enameled wire
with the PD shield layer and the
surface protection coating
The basic idea is to substitute the large
mica flakes with a high density
inorganic materials that
 form barriers for the tree growth
 facilitate the space charge diffusion
 facilitate the heat transmission
 the endurance life is enhanced
 no variations in the ground-wall insulation thickness
 increased t2t PD and surge withstand capability
But
It become brittle and develop cracks when subjected to
temperature variation in the presence of mechanical
stresses
Each micro-composite material must
be evaluated carefully
Evaluation of Corona Resistant Materials
 Accelerated life tests on twisted pairs with
conventional and corona resistant (CR) insulation
 Tests: - in the absence of PD (in oil), below PDIV
- in the presence of PD (in air), above PDIV
 Sinusoidal and distorted waveforms
Objective: to draw life lines
life models
voltage endurance coefficient evaluation
comparison among insulation systems
Procedure
Specimens: twisted pair insulation
50 Hz sinusoidal
Traditional, #A
CR #B
CR #C
Above PDIV
#A, #B,#C
Below PDIV
#A, #B,#C
Above PDIV
#A, #B,#C
Below PDIV
#A
10 kHz sinusoidal
Life tests
10 kHz square unipolar Above PDIV
#A, #B,#C
Duty cycle = 50%
Slew rate = 1 kV/s
10 kHz square bipolar Above PDIV
#A, #C
Test
Results
Sinusoidal
Life Test on #1, AIR and OIL, 50 Hz 10 kHz
• Dramatic PD effect
especially at HF on
#A
10000
#A, #C
Voltage (rms value) [V]
#B
VEC = 11.7
#C
#A
VEC = 6.4
VEC = 10.6
#B
VEC = 8.7
VEC = 7.1
50 Hz SIN AIR #A
50 Hz SIN AIR #B
50 Hz SIN AIR #C
50 Hz SIN OIL #A
50 Hz SIN OIL #B
50 Hz SIN OIL #C
10 kHz SIN AIR #A
10 kHz SIN OIL #A
VEC = 4.5
1000
0.1
1
• #B (CR) is worse
than #A at 50 Hz
VEC = 11.9
10
Failure time [h]
100
• #C good VEC and
life times in the
presence of PD
• Increasing frequency
reduces life
1000
considerably, both
with and without PD
Life test results and life lines for sinusoidal
tests (50 Hz-10 kHz ) in the absence (in oil)
and in the presence of PD (in air)
Test Sinusoidal
Results
Life Test on #1, AIR,
• VEC decrease with
frequency for #A
50 Hz 10 kHz
10000
Voltage (rms value) [V]
#C
50 Hz SIN AIR #A
50 Hz SIN AIR #B
50 Hz SIN AIR #C
10 kHz SIN AIR #A
10 kHz SIN AIR #B
10 kHz SIN AIR #C
#A
#B
VEC = 7.1
VEC = 10.6
• #B is the worst at
50 Hz but the best
at 10 kHz (for high
fields)
VEC = 6.4
#B
VEC = 8.8
#A
VEC = 4.5
VEC = 12.6
#C
1000
0.1
1
10
• VEC increase with
frequency for #B &
#C (life decreases)
100
1000
Failure time [h]
Life test results and life lines for sinusoidal
tests (50 Hz-10 kHz ) in air for the three
tested materials.
• #C good VEC and
life times both at
50 Hz and 10 kHz
Voltage (peak-to-peak value) [V]
Test Results
Life Test in AIR summary plot
#C
#A
#B
10000
#A
#B
#C
• Peak-to-peak
voltage is the main
stressing factor,
besides pulse
repetition
frequency
#A 50 Hz SIN
#A 10 kHz SIN
#A 10 kHz UNIP
#A 10 kHz BIP
#B 50 Hz SIN
#B 10 kHz SIN
#B 10 kHz UNIP
#C 50 Hz SIN
#C 10 kHz SIN
#C 10 kHz UNIP
#C 10 kHz BIP
1000
0.001
0.01
• Effect of voltage
shape negligible
(experimental
points fit the same
line)
0.1
1
10
100
1000
10000
Failure time [h]
Life test results and life lines for sinusoidal (50 Hz-10 kHz )
and squarewave (unipolar and bipolar) tests in air for the
three tested materials as function of peak-to-peak voltage.
Space Charge Meas. On #A and #B
Tipo A - B valori di QM
12
#A
#B
10
Material s [s-1]
#A
2.5
#B
1.8
8
QM [C/m3]
Speed of charge
expulsion, s, for 50 Hz
squarewave voltage
(1000 V peak)
6
4
2
0
DC
0.1Hz
50Hz
10kHz
Voltage frequency
Total absolute stored charge density,
QM, as a function of frequency. Bipolar
squarewave. Poling voltage: 1000 V
peak
Note that:
 QM decreases as the
frequency increases
 QM (#B) > QM (#A)
 s (#B) < s (#A)
The CR solutions show different performance and
behavior
CR micro-composites must be evaluated carefully before
they use
D.Fabiani et al.”The Effect of Fast repetitive Pulses on the
Degradation of Turn Insulation of Induction Motors”, Proc.
of SDEMPED 2001, pp.289-293, Grado (I), 2001
Discussion
 Pulsating voltage accelerates degradation both in
air and in oil (i.e. with and without PD), but different
VEC
 CR materials
frequencies
show
a
longer
life
at
higher
 The standard insulation, #A, seems to suffer
significantly from PD activity and frequency increase
 CR material, #B, withstands PD better than #A
 PD increase due to PWM voltage waveforms
 #B tends to accumulate much more charge than
#A (>QM and <s) at frequency up to 50 Hz
 #B is worse than #A at 50 Hz
The “Frost Effect”
In the presence of PD, the external layer (organic
material) is eroded rapidly and the inorganic material
emerges and the enamel change its colour
The inorganic material is easily removed
mechanical stresses (vibrations). Let
Type A: only
materials
Fault
Type A
Fault
Type B1
by
the
organic
Type
B:
organic
materials
filled
by
inorganic particles
The surface erosion is
evident (Type B1) while a
localized BD occurs in
organic enamel (Type A)
A new class of micro-fillers (Type C) has been developed
where the inorganic filler chemically combines with the
organic enamel
Type B1
Type C1
The chemical links delay the mechanical erosion
and the insulation life is prolonged
The “Frost Effect” combined with electrical stresses
determine two types of breakdown:
Pinhole Type
Massive Type
The two type of BD are related to the time exposition to
PD, thus to the local electric field
In this example, BD occurred due to a defect on the
conductor where the electric field was enhanced by the
copper protrusions
The “Frost Effect” due to PD is evident looking around the
breakdown site
The three layers are also evident in the picture
Another example of PD erosion
X-Ray spectrometry evidenced
the dominant presence of oxygen
in the vicinity of the BD area
while TiO2 was found around the
BD crater
Nano Fillers
To improve the PD resistance of organic enamel, by
means of the dispersion of nano-metric inorganic fillers
are dispersed in the polymer matrix (under investigation)
Polymeric nano-composite: composite material with
inorganic fillers having at least one dimension < 100 nm
Polymer
Nanofillers
Nanoparticles
Nanocomposite
Nanotubes, Nanofibres,
Whiskers, Nanorods
Nanolayers
The filler rate is usually between 1%-10% of weight
The presence of inorganic nano-fillers can alter the
dielectric properties of the materials. In particular,
• Permettivity
• Space charge accumulation
• Electrical Tree propagation
• Heat transmission
• etc.
This new technology must be handled carefully to avoid
that improving a property, worsening the others
The nano filler is selected taking into account the
property to be improved (e.g., the use of nano mica
flakes to delay the electric tree growth)
Nano-mica flakes form a wide and complex “labyrinth” where
the length of the tree-channels are strongly increased and
the breakdown, delayed
Tree Growth: bush type
EL470 (G19)
After 3 hours
EL470+DEL72 (G21)
After 22 hours
Anomalous Tree Grouth due to
the barrier-effect of the nanofiller
EL470+MAE (G22)
After 20 hours
Nano-fillers are added to improve the resin performances
mainly to withstand the PD erosion
Initial
agglomerate
Conventional
composit
Intercalated
Exfoliated
nano-composite nano-composite
The complete exfoliation of
the nano-filler generates free
charges inside the insulation
worsening
e.g.,
the
dissipation factor
RB standard resin
N1 1% nano mica
N3 3% nano mica
The
intercalated
structure is preferable
the ionic links between
the mica flakes are
preserved and no free
charges are introduced
Possible Barrier Effect
Conventional enamel
Further stage of aging
Initial stage of aging
Inter-turn PDs
PD induce ablative degradation process leading to the
scission of the polymeric chain, the formation of free
radicals and of volatile decomposition products
Adopt nano composite materials (Type C) that show a
strong interaction between the nano particles
Nanocomposite enamel
Further stage of aging
Initial stage of aging
Inter-turn PDs
PD
Aggregation
forces between
inorganic
nanoparticles
Ceramic
Like layer
Interactions or bonds
between filler and
carbonaceous residue
Increase of
nanofiller
concentration
on the surface
Conventional and nano-composite enamel have been
analyzed and compared
 Type A: double layer polyester-imide (PEI)
 Type C1: double layer PEI and PEI+Barium Sulphate
BaSO4 (PEI+nb)
 Type C1: double layer PEI and PEI+Silica SiO2 (PEI+ns)
The TBD has been adopted as end-of-life criterium
Tests were performed applying a PWM like wave-form at
different voltage levels and temperatures
F.Guastavino et al. “Electrical Aging Tests on Different Nanostructured
Enamels Subjected to Severe Voltage Waveforms”, proc.IEEE
SDEMPED, pp.283-287, Bologna (I), September 2011
The support of Elantas Deatech S.r.l. - Ascoli Piceno – Italy is
greatifully acknowledged
Test Set-Up
oven
Arbitrary
Waveform
Generator
V
AWADIT
Twisted pair
PWM+peaks
voltage
waveform
Linear amplifier: 10 Hz
– 3 MHz bandwidth at
the considered voltage
level, 60 dB gain
Temperature
test: 150°C;
120°C; 90°C;
60°C
The average time to breakdown (Tbd) is collected and
related to the test voltage amplitude via the inverse
power law:
-n
Tbd = A (Vpp)
Vtest [V]
10000
PEI
PEI+ns
PEI+nb
1000
1000
10000
Tbd [s]
100000
1000000
Ceramic char formation during ablation
The fomation of nanostructured ceramic-like layer has been
observed for many ablative processes:
• Burning (Giannelis et al., Gilman et al) .
• Thermo-oxidative degradation (Mulhaupt. et al., Zanetti et al., Camino et al.)
• Exposure to combustion gases (Vaia et al.)
The described processes have been massively evidenced
for Polymer layered silicate nanocomposites, but similar
behavior has been observed also in the case of polymerSiO2 nanocomposites (Wu et al. 2005, Wang et al. 2006, also according to the work of
Vaia).
Qualitative Surface Analysis
Electrical
aging tests
PDs
activity
Simple optical
Enamel microscope
erosion
Degradation area dimensions
for conventional enamel after
electrical aging at 150°C
Qualitative
surface analysis
Degradation area dimensions for
nanocomposite enamel after
electrical aging at 150°C
Degradation area dimensions are wider in the case of
conventional enamel than in the case of nanocomposite one
Degradation area dimensions
for conventional enamel after
electrical aging at 60°C
Degradation area dimensions for
nanocomposite enamel after
electrical aging at 60°C
Diminishing the temperature
dimensions is less wide
level,
the
eroded
area
PEI+nb twisted pair
Before aging test
After aging test at 4.6 kV
PEI+ns twisted pairs
Before aging test
After aging test at 4.6 kV
Comparison Between PEI+nb and PEI+ns
PEI + nb
After aging test at 4.6 kV
PEI + ns
After aging test at 4.6 kV
Thermal Ageing
Applying the Arrhenius
model to the obtained
life times
Linearizing
Life Curves
Conventional
1000000
Nanocomposite
Tbd [s]
100000
10000
1000
0,0032
0,0030
0,0028
0,0026
0,0024
1/T [1/K]
60
90
T [°C]
120
150
0,0022
120000
105000
90000
75000
60000
Tbd [s]
45000
30000
10000
8000
6000
4000
2000
0
Conv.
150°C
Conv.
150°C
Nano
150°C
Nano
150°C
Conv.
120°C
Conv.
120°C
Nano
120°C
Nano
120°C
Conv.
90°C
Conv.
90°C
Nano
90°C
Nano
90°C
Conv.
60°C
Conv.
60°C
Nano
60°C
Nano
60°C
 Data scatter is generally low;
 the minimum life time value obtained testing the
nano-composite enamel at 150°C is considerably
longer than the maximum time obtained testing the
conventional enamel at 60°C
Micro+Nano Composites
A combined use of micro and nano fillers has been also
investigate to
 improve different properties of the composite materials
(thermal and mechanical in addition to PD resistance)
 guarantee novel properties
Schematic
representation of PD
erosion process due to
PD for micro and
micro+nano composite
materials
Sample of micro-silica
(60%wt) and nanosilica (5%wt) in epoxy
matrix
Micro-silica: black area
Nano-silica: small withe
dots
Nano-silica (Dark gray spots)
in epoxy resin matrix
CIGRE Working Group, “Characterization of Epoxy Microcomposite and
nanocomposite Materials for Power Engineering Applications”, IEEE
El.Ins.Magazine, Vol.28, pp.38-51, March 2012
Discussion
 It is possible to enhance the resistance to the action of
surface PDs of organic insulating enamels used for
magnet wire insulation by nano-structuration.
 The application of nano-composite enamels is not a
Panacea for inverter driven motor insulation:
1.Many matrix-filler combination may not lead to the
desired results; careful study of the chemical-physical
interactions and degradation mechanisms
2.Nano-structuration does not prevent the inception of
PDs; rather it slows down the degradation of the
enamel
Required further research investigation:
1.Chemistry and physics of the degradation of nanocomposite enamels subjected to PDs
2.Polymer – inorganic nano-particles interactions
3.Interactions between nano-composite enamels and
secondary insulation (conventional or nano-structured
impregnation resins)
4. Micro-nano composites
Modeling for Insulation Design





The concepts of dielectric strength, Weibull distribution of
failure times, lifetime and voltage endurance coefficient
are the basis for the design of highly reliable insulation
systems in electrical apparatus
ASD introduced a new type of electrical stress arising
from high frequency harmonics due to repetitive voltage
impulses
and
motor-cable-converter
impedance
mismatch
Over-voltages and uneven voltage distribution along the
winding causes overstress mainly in inter-turn insulation
Over-voltages can cause PD that become the dominant
ageing factor mainly in Type I materials
Corona resistant materials (micro, nano composites) have
been developed
It is necessary
 to study and model the lifetime behavior of new CR
materials
 to design properly the insulation taking into account the
ASD specific stresses
The dominant aging factors
Some quantities extracted from the distorted voltage
waveforms and correlated with aging are introduced. Let:
N
v (t )   Vh sin( h f t   n )
h 1
The Fourier series of the non-sinusoidal voltage supply.
N
dv (t )
  h f Vh cos( h f t   n )
dt
h 1
The rms of the voltage variation is defined as:
dv (t )
dt

rms
f
2
N
h V
2
2
h 1
h
If we consider the rms value of a 50 Hz sinusoidal voltage
having the same amplitude of the fundamental (V0=V1),
then:
dv 0 (t )
0

V1
dt 50 Hz , rms
2
Considering their ratio
Ks 
f
0
N
h 
2
2
h 1
h
where
Vh
h 
V1
Ks is the rms value of the derivative of the
distorted waveform and it is related to its RT
Additional parameters, related to over-voltages, can be
defined as:
VP
KP  *
V1P
Vrms
Krms  *
V1rms
where
 VP and Vrms are the peak and rms values
of the distorted waveform,
 V1* is the reference voltage (V1P*=√2 ⋅
V1rms),
The Joule, Wj, and the dielectric, Wd, losses for a winding
having a phase-to-ground capacitance C, can be written
as:
N
Vh 2
W J  k a ra  C V1  h ( )
V1
h 1
2
1
W d   1CV 1
2
2
2
N
2
Vh 2
tan   h ( )
V1
h 1
Where
 ra is the resistance of the equivalent capacitor
 ka is a constant
 tan is the loss factor
The temperature rise  is then given by:
  (WJ  Wd )Rth
where Rth is the thermal resistance of the capacitor
Using the PWM technique, temperature increases
of about 10 to 20 K°
The Dominant Ageing Factors
With PD: It has been shown that PD is the dominant
ageing factor particularly at high pulse rate and frequency
Thus, the insulation system must be designed to work
below the PDIV
PDIV depends on the adopted insulation
Without PD: Below the PDIV, the ageing mechanisms are
very different (related to RT, RR, temperature….)
Neglecting the interactions between factors as a first
approximation, the simplest equation that can be used,
based on an inverse power model, is:
 n rms
L  L 0 K P n P K rms
K s n s
where L is insulation lifetime, and L0, np, nrms, ns
are adjustable parameters
L
log
  n p log K P  n rms log K rms  n s log K s
L0
KP, Krms, and Ks are further analyzed statistically, e.g., by
using the Standardized Pareto Chart (SPC) and the Main
Effect Plot (MEP)
Peak voltage is clearly the most influential factor of lifetime,
followed by rms and voltage slope
The experimental data suggested that an inverse power
model, in the form of
L  L0 A B N
a
b
n
can be applied to correlate life- time and aging factors, in
particular P2P voltage and temperature, that is, in log
form
log L  log LD  a logVPP  b log 
where L=lifetime; VPP=P2P voltage; and a, b and L0 are
parameters calculated through multivariable linear
regression
Again,
the
jump
voltage is still the
most influential factor
of lifetime
Life Modeling: a simplified version
Peak-to-peak voltage has been recognized as dominant
ageing factor
The inverse power model correlating the lifetimes at
different stress values is slightly modified, that is
L  L 0V pp
n
The pulse RR of the applied voltage is also important, with
lifetime decreasing with increasing RR. If Lf and L1 are the
lifetimes at f (= 10 kHz) and f1 (= 50 Hz), respectively then
f1 
L f  L1 ( )
f
where the exponent γ is estimated experimentally
These assumptions allow the lifetime of an insulation
system under impulse conditions to be estimated using
data obtained under sinusoidal voltage testing. Moreover:
 The most important stressing factors are fundamental
frequency, the RR and peak-to-peak voltage amplitude
 The overvoltage is adiabatic
 The system is operated in the stress range within which the
predominant degradation mechanism does not change during
ageing
 VEC is frequency independent, i.e., no significant frequency
dependence of the number of impulses or voltage cycles before
failure is observed. This corresponds to  = 1 which is
approximately true for composite organic/inorganic insulation
If these conditions are satisfied and the measured lifetime
at test frequency f1 is L1, then the estimated lifetime L2 at
test frequency f2 is given by:
f1
L2  L1
f2
It follows that
 if
lifetime
corresponds
frequency f1,
line
1
to
test
 lifetime line 2 for test
frequency f2 (f2 = 10 f1) is
obtained
by
translating
lifetime line 1 one decade
horizontally (arrow A)
to maintain the f1 lifetime, the applied stress at f2
should be reduced as shown by arrow B
Combining the two simplified models
L f 2 ,u2
U1 n f1
 L f1 ,u1 ( )
U2 f2
where
 Lf1,u1 is the lifetime at frequency f1 and voltage U1
 Lf2,u2 is the lifetime at frequency f2 and voltage U2
 n is the VEC
data can be generated for any desired frequency, e.g., the
fundamental for motor drives, based on measured lifetimes
for appropriate insulation systems at f1 = 50 or 60 Hz
Experimental evidence validates this simplified approach for
impulse voltages up to 1 kHz because the variation of the
VEC n with frequency is negligible
At higher frequencies a decrease of n is observed, even for
inorganic/organic insulation
The dependence of n with frequency can be modeled, but
only by introducing further parameters in the model.
Design Criteria
 PD is a dominant deterioration phenomena that leads to
premature BD of the insulation. Using conventional enamel,
the electric stress must be below the PDIV
 a moderate PD activity can be accepted when CR
composite organic/inorganic insulating material is adopted
 In the absence of PD, the peak of the distorted voltage
waveform and its repetition rate are the most important
ageing factors
 Modeling the long-term behavior is feasible in the first
approximation
 Detailed evaluation of the new materials through longterm voltage endurance tests is still strongly recommended,
to maximize the reliability of the insulation system
References:
 A. Cavallini, D. Fabiani, G.C. Montanari, “Power Electronics
and Electrical Insulation System – Part 1: Phenomenology
Overview”, IEEE Electrical Insulation Magazine, Vol. 26, pp. 715, May-June 2010
 A. Cavallini, D. Fabiani, G.C. Montanari, “Power Electronics and
Electrical Insulation System – Part 2: Life Modeling for Insulation
Design”, IEEE Electrical Insulation Magazine, Vol. 26, pp. 33-39, JulyAugust 2010
 A. Cavallini, D. Fabiani, G.C. Montanari, “Power Electronics and
Electrical Insulation System – Part 3: Diagnostic Properties”, IEEE
Electrical Insulation Magazine, Vol. 26, pp. 30-40,September-October
2010
Design of the Stress Grading for HV Machines
In form wound MV and HV rotating machines, the
ground-wall and strand insulation, based on mica-tapes
and mica-films, respectively, withstand to the PD activity
Turn insulation is stressed by the uneven voltage
distribution and can be designed according to the above
mentioned composite materials and criteria
The end-arm stress grading is
the weak point when a machine
designed for 50/60 Hz is fed by
pulsating of PWM supply. This
promoted investigations to:
 select proper materials
 design properly the stress
grading
Problemi nei sistemi di
gradatura
Solo nei motori form-wound (MT)
• Il campo elettrico tangenziale
aumenta all’aumentare del
contenuto in frequenza della
tensione (f> 2 kHz si hanno scariche
in testata)
• Il campo elettrico cala all’aumentare
Campo elettrico
della conducibilità
della gradatura Campo elettrico
di scarica
di scarica
Media conducibilità
Alta conducibilità
Typically:
•
Insulation with no stress grading system
(normally for Vn ≤ 4kV)
•
Insulation systems with anticorona coating within the
slot
( 4kV ≤Vn ≤ 6kV)
•
Insulation with stress grading system (Vn ≥6kV)
Due to material discontinuity, high values of electric
gradient affect the surface of the coil at the edge of the slot
grading tape thus generating tangential surface discharges
The stress grading is designed solving the field equation



    ccU  ( * U )  0
t


and its solution allows to draw the electric field outside
the magnetic core
 cosh k ( x  L)
U ( x)  Va 1 

cosh( kL) 

sinh k ( x  L ) 
E ( x )  Va k
sinh( kL )
Hot spots due to PD, can be discovered considering the
air breakdown strength (e.g., 2.3 kV/mm) and the
electric field gradient
Stress grading materials are characterized by their
resistance that can be constant or electric field dependent:
s  0 exp(nE )
2/ 3
High values of n and low values of 0 increase the grading
effect
The stress grading is designed considering the number of
layers and their coating length outside a slot portion
After the material selection (0, n), the space distribution
of the electric field is determined using FEM software
tools considering the machine geometry and the groundwall insulation characteristics
The correct choice of the number of layers and their
length is evaluated checking the electric gradient (below
of PDIV) and hot spots
Stress grading is currently designed for 50/60Hz
applications
Assuming the 0 reference the edge of the slot grading, the
potential and the electric field distribution can be derived
and analyzed
The different behavior of a single layer stress grading with
constant and exponential resistivity, is show
But the potential distribution is strongly related to
frequency of the applied stress.
Stress grading designed for ac is not able to operate at
higher frequencies
V [V]
36000
34000
30 kV
f variabile
32000
30000
28000
26000
24000
22000
20000
50 Hz
18000
250 kHz
16000
1 kHz
14000
12000
20 kHz
10000
8000
6000
4000
2000
0
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100 110 120 130 140 150
Distanza x dal termine del ricoprimento conduttivo [mm]
GEN.3.1
GEN.3.2
GEN.3.3
GEN.3.4
A multi-layer and longer stress grading having lower resitance,
is required for higher frequencies
Vmax [V]
18000
4
17000
Mat.D
16 kV # 1,25 MHz
Mod.1
16000
15000
3
14000
s = 2,7 mm
13000
Mat.A
12000
2
11000
10000
9000
8000
7000
6000
1
5000
4000
3000
2000
1000
0
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
Distanza x dal termine del ricoprimento conduttivo [mm]
SIM.1.2.1.A
SIM.1.2.1.B
30
35
40
45
50
Experimental Validation
Two different stress grading configurations designed for
standard (50 Hz) and PWM supply voltages have been tested
by means of PD measurements
Both frames were supplied by HV rectangular wave-shape
PD measurements were performed using an antenna probe
able to record PD pulses and the fundamental wave-shape
adopted as the phase reference of PD
Moving the antenna probe, PD were localized at the edge of
the 50Hz stress grading while only signals due to
commutations were recorded on the other frame
Thus confirming the validity of the stress grading design
Conclusions
 Besides the advantages in using ASD, new problems
rose due to the significant harmonic content of the power
supply and the over-voltages generated by mismatch
impedances in inverter/cable/drive connection
 The insulation is subjected to increased electric,
thermal and mechanical stresses and its life is shortened
 Additional
examined
stresses,
typical
of
ASD,
have
been
 New composite materials (micro, nano fillers) have
been proposed
 Specific test methods are developed
 Specific standards are under discussion
Thank you for your
attention!