PD are a partial breakdown of gas inclusions in insulationwhere the electric field intensity exceeds the breakdownfield strengthPD transforms part of the capacitive stored energy intoheat and radiation as well as mechanical and chemicalenergies which can degrade insulation materialsThis ageing process progressively reduces the insulationthickness and the breakdown voltage until the failureoccursInsulation erosion is very fast in organic materials (Type 1) slow in organic/inorganic insulation systems (Type 2)

Partial Discharge Measurements

PD are a partial breakdown of gas inclusions in insulationwhere the electric field intensity exceeds the breakdownfield strengthPD transforms part of the capacitive stored energy intoheat and radiation as well as mechanical and chemicalenergies which can degrade insulation materialsThis ageing process progressively reduces the insulationthickness and the breakdown voltage until the failureoccursInsulation erosion is very fast in organic materials (Type 1) slow in organic/inorganic insulation systems (Type 2)

Electric signals related to currents, electromagneticwaves, electroluminescence are recorded using propercouplers and processed in order to:

provide information on the discharge phenomenon establish PDIV as a function of the supply square waveparameters (mainly frequency and over-voltages) check the quality of the insulation and comparedifferent materials (PDIV and time-to-breakdown are currentlyadopted as parameters to be monitored during life tests)

Ref.- IEC 61934TS: Electrical insulating materials andsystems – Electrical measurement of partial discharges(PD) under short rise time and repetitive voltageimpulses

Electric signals related to currents, electromagneticwaves, electroluminescence are recorded using propercouplers and processed in order to:

provide information on the discharge phenomenon establish PDIV as a function of the supply square waveparameters (mainly frequency and over-voltages) check the quality of the insulation and comparedifferent materials (PDIV and time-to-breakdown are currentlyadopted as parameters to be monitored during life tests)

Ref.- IEC 61934TS: Electrical insulating materials andsystems – Electrical measurement of partial discharges(PD) under short rise time and repetitive voltageimpulses

PD Basics

The local breakdowngenerates a voltage dropand a consequent fastimpulsive currentabsorption from the supply

A suitable test circuit configuration allows to record the PDpulse signals. It is composed byA suitable test circuit configuration allows to record the PDpulse signals. It is composed by

Sinusoidal/square wave generator

Couplers and filters Synchro Units Digital recorders Processing unit

Waveform Generators

Sinusoidal : PWM-like : PWM+peaks :Square Unipolar

Square Bipolare.g.,Vpp= 07.5 kV

Unip./ Bip.dV/dt=075 kV/µsFreq.= 020 kHzDuty%=0.0190

Square Unipolar

Square Bipolar

with variable Vrms, V0p, Vpp, RR, RT

with selectable duty and wave-shape

able to supply high capacitive currents

Typologies of Samples Under Test

Twisted Pair (frame to testT2T insulation according toIEC 60851-5)

Motorettes (frame to testT2T, P2G and P2Pinsulation)

Motorettes (frame to testT2T, P2G and P2Pinsulation)

Complete coils & stators

PD Signal Characteristics

Partial Discharges generate impulsive signals having upto 1-2 GHz of frequency content

Fast voltage transition can reach 200 MHz depending tothe voltage RT

Suitable couplers and bandwidth must be selected toavoid the commutation interference during PDmeasurements

0 0.5 1 1.5 2 2.5 3 3.5 4-0.05

-0.025

0

0.025

0.05

PD

mag

nitu

de (V

)

Time (us)0 0.5 1 1.5 2 2.5 3 3.5 4

-1.5

-0.75

0

0.75

1.5

Gen

erat

or v

olta

ge (k

V)

PD pulsesGenerator voltage

Partial Discharges generate impulsive signals having upto 1-2 GHz of frequency content

Fast voltage transition can reach 200 MHz depending tothe voltage RT

Suitable couplers and bandwidth must be selected toavoid the commutation interference during PDmeasurements

Coupler and high-pass filters having a low cut-offfrequency higher than 250-300 MHz are required to avoidcommutation interferences

IEC 61934 Frequency Band Prescriptions

Volt. RT: 50ns

PD RT: 2 ns

8th order filterwith filter cut-offfrequency equalto 500 MHz.

Time [s]

-2 -1 0 1 2 3 4 5

Appl

ied

Volta

ge [V

]

-800

-600

-400

-200

0

200

400

600

800

PD Signal [V]

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Square Bipolar 10 kHz Applied VoltagePD signal

800

600

400

200

0

–200

–400

–600

–800–2 –1 0 1 2 3 4 5

–0,15

–0,10

–0,05

–0,00

0,05

0,10

0,15

Time s

PD signal V

Appl

ied

volta

ge

V

Square bipolar 10 kHz applied voltagePD signal

IEC 552/06

Time [s]

-2 -1 0 1 2 3 4 5

Appl

ied

Volta

ge [V

]

-800

-600

-400

-200

0

200

400

600

800

Filtered PD Signal [V]

-0.04

-0.02

0.00

0.02

0.04

Square Bipolar 10 kHz Applied VoltageFiltered PD signal–600

–2

–0,04

Time s

Filtered PD signal VAppl

ied

volta

ge

V

Square bipolar 10 kHz applied voltageFiltered PD signal

–400

–200

–800

0

200

400

600

800

–1 0 1 2 3 4 5

–0,02

–0,00

0,02

0,04

IEC 553/06

Volt. RT: 50ns

PD RT: 2 ns

8th order filterwith filter cut-offfrequency equalto 500 MHz.

Time [s]

-2e-6 -1e-6 0 1e-6 2e-6 3e-6 4e-6

PD s

igna

l [V]

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Time [s]

-2e-6 -1e-6 0 1e-6 2e-6 3e-6 4e-6

PD s

igna

l [V]

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Time [s]

-2e-6 -1e-6 0 1e-6 2e-6 3e-6 4e-6

PD s

igna

l [V]

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Time [s]

-2e-6 -1e-6 0 1e-6 2e-6 3e-6 4e-6

PD s

igna

l [V]

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

CommutationPD signal

Different Low Cut-Off Frequencies

Time [s]

-2e-6 -1e-6 0 1e-6 2e-6 3e-6 4e-6

PD s

igna

l [V]

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Time [s]

-2e-6 -1e-6 0 1e-6 2e-6 3e-6 4e-6

PD s

igna

l [V]

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Time [s]

-2e-6 -1e-6 0 1e-6 2e-6 3e-6 4e-6

PD s

igna

l [V]

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Time [s]

-2e-6 -1e-6 0 1e-6 2e-6 3e-6 4e-6

PD s

igna

l [V]

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

100 MHz200 MHz500 MHz10 MHz

Differential Configuration

A differential connection ofcoupler can be adopted tocancel the interferencesdue to voltage transitions

The capacitance of CC andthe SUT must be similar

Signal relevant to thevoltage transition reachesthe +/- input of the LF filterand it is cancelled

A differential connection ofcoupler can be adopted tocancel the interferencesdue to voltage transitions

The capacitance of CC andthe SUT must be similar

Signal relevant to thevoltage transition reachesthe +/- input of the LF filterand it is cancelled

F.Guastavino et al., “Measuring PD UnderPulsed Voltage Conditions”, IEEE Trans.on Diel.El.Ins. Vol.15, pp.1640-1648,December 2008

PD Detection Circuits and CouplersCapacitive Couplers

PD free devices

To be connected with a proper detection impedance toobtain high cut-off frequencies

Supply Testobject

Filter PD signalZ

IEC 540/06

Supply Testobject

Filter PD signalZ

IEC 540/06

Inductive Couplers

With very high cut-offfrequency

HFCTSupply Test

object

Filter PD signal

IEC 543/06

Supply Testobject

PD signalFilterHFCT

IEC 544/06

Antennas

High cut-off frequencies

Suitable for PDmeasurements in ASDapplications

SupplyTestObject

Antenna

AcquisitionSystem

Supply Testobject

Antenna

Acquisitionsystem

IEC 545/06

Light Detectors

Electrical Optical RF / EMI Acoustic

Description Electrical circuit that picks upcurrent pulse produced by

charge transfer during partialdischarge

Measures light emissionfrom partial discharges

Measures radiofrequency interference

generated by thedischarge

Measures the acousticemissions produced by

a partial discharge.

Advantage A good sensitivity and standardfor all HV equipment during

manufacture

Non-contact, applicablefor all voltage types.

Allows testing ofequipment in real

conditions

Non-contact, applicablefor all voltage types.

Allows testing ofequipment in real

conditions

Non-contact, applicablefor all voltage types.

Allows testing ofequipment of real

conditions

Disadvantage Sensitive to electrical noise.Cannot test circuit in operatingcondition in most cases. Most

commercial equipment canonly test at up to 400Hz

Insensitive to any form ofinternal partial discharge.

Sensitive to light andhighly directional.

Depending onequipment being tested,

EM emissions canprevent detection of PD

Sensitive to otheracoustic emissions.

Signals cannot alwayspropagate throughinsulation / 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

PD usingAntenna

VoltagePulse

PD usingVoltagedivider

PD usingAntenna

ResidualNoise

Commercial instruments areavailableMust of them requiresuitable input filters to tunetheir input requirements torecord PD and reject voltagecommutations (>200-500MHz)

PD usingAntenna

VoltagePulse

PD usingVoltagedivider

PD usingAntenna

ResidualNoise

Commercial instruments areavailableMust of them requiresuitable input filters to tunetheir input requirements torecord PD and reject voltagecommutations (>200-500MHz)

v

The amplitude and the phase of the PD signal areevaluated

The PD pulse sequence is transformed in a sequence ofDirach functions having the same phase

v

PD Patterns

A 3D histogram is obtainedconsidering the number ofdischarges having the sameamplitude and the same phaseof occurrence

φ [deg]

A [V]

Square wave Volt.Square wave Volt.DC=50 %DC=50 %SyncSync frequencyfrequency ==CommutationCommutationfrequencyfrequency

A 2D histogram can be derived from the PRPD patternprojecting n in A, plain and using different colors toevidence the different repetition rate

φ [deg]

Square wave Volt.Square wave Volt.DC=50 %DC=50 %SyncSync frequencyfrequency ==CommutationCommutationfrequencyfrequency

Twocommutations

per period

Two sub-patterns close tothe zero crossingAt least 1 PD per pulsePossible PD even duringthe “flat zone” of the voltagepulse

PDIV

Reference sin waveReference sin wave square wavesquare wave

PWM

VOLTAGE SUPPLY EUT

Voltagedivider

A low pass filter is required to obtain the phase referenceand to derive the so called “Phase Resolved” PD pattern

DETECTION UNIT

SYNC ChLP FILTER

Voltagedivider

Modulating wave

Modulating wave 50Hz

• Period of 20ms

Pulse Rep.Rate 1kHz

• 40 commutationsper period

Example of PRPD pattern when a PWM voltage is adopted

40 sub-patterns perperiod

Life-test Results

Weibull Plot:

)exp(1)( FttF

NF kVt

twisted pair specimens were fed till failure by differentwave forms at several amplitudes, in the presence of PDData were processed according to the standard procedure

PRPD patterns were recorded during the experiment

IPL model:

)exp(1)( FttF

Result 1: Unipolar-Bipolar Square Waves

Twisted pair of having insulation of different materialswere tested in air (with PD) and immersed in oil (no PD)

Sinusoidal, unipolar and bipolar square waves, the latterhaving the same RT, RR (50 Hz, 10 kHz), DT=50% anddifferent 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 tounipolar and bipolar pulses are completely overlapped

NO PD PD

The Jump Voltage is the real voltage stress that affects theinsulation ageingVEC can be adopted to compare the performances ofdifferent materials even in the presence of PDD.Fabiani et al. “Ageing acceleration of insulating materials forelectrical machine windings supplied by PWM in the presence and inthe absence of partial discharges”, IEEE ICSD pp.283-286, 2001

The picture is much more complicated when PWM andPWM+over-voltages are adopted to stress the materials in thepresence of PDDue to the “equivalent derivative effect”, there aredifferent Jump VoltagesV

VppVaa

Result 2: PWM wit and without Overvoltages

Vpp = 2(V0p +x)

x = overvoltage

V0p = voltageamplitude

Vaa = V0p +2x

t

VppVaax

F.Guastavino et al. “Life Tests on Twisted Pairs Subjected to PWM-likeVoltages”, IEEE ICSD pp.238-241, 2004

Sinusoidal

Result 3: Phase Resolved PD patterns

V = 1.83 kV V = 2.44 kV V = 3.05 kV V = 3.80 kV

Square

V = 1.9 kVV = 1.60 kV V = 4.32 kVV = 3.96 kV

Square

PWM

PWM+Over-voltage

V = 1.63 kV V = 2.73 kV V= 3.53 kV V= 4.32 kV

PWM+Over-voltage

V = 2.08 kV V = 2.50 kVV = 2.38 kVV = 2.90 kV

Discussion

PWM voltage waveforms can promote PD activity,reducing the life of magnet wires

The predominant factors explaining this behavior arepeak-to-peak voltage and switching frequency.

Even in the absence of PD, PWM voltage waveformscan accelerate the intrinsic aging of winding insulation(overvoltages). Very high slew rate (>5 kV/s), infact, plays also a non negligible role on accelerationdegradation due to increased voltage stress andheating.

This effect can be evaluate resorting to space chargemeasurements both in the presence and in the absenceof PD

Discussion

PWM voltage waveforms can promote PD activity,reducing the life of magnet wires

The predominant factors explaining this behavior arepeak-to-peak voltage and switching frequency.

Even in the absence of PD, PWM voltage waveformscan accelerate the intrinsic aging of winding insulation(overvoltages). Very high slew rate (>5 kV/s), infact, plays also a non negligible role on accelerationdegradation due to increased voltage stress andheating.

This effect can be evaluate resorting to space chargemeasurements both in the presence and in the absenceof PD

Space Charge and PD Activity

Space Charge = charge trapped inside the insulationor 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 insulationunless the waveform contains a DC component

IN HF pulse waveforms, SP is accumulated mainly by PD

Neglecting the SP accumulated in the bulk, the electricfield in air, at the insulation surface, E*

0 can be:

Where:E0 : electric field without accumulated charge

0 : the permittivity of air andinsulation

d l0 : the insulation thickness and the half air gap

0: the surface charge density

Space Charge = charge trapped inside the insulationor 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 insulationunless the waveform contains a DC component

IN HF pulse waveforms, SP is accumulated mainly by PD

Neglecting the SP accumulated in the bulk, the electricfield in air, at the insulation surface, E*

0 can be:

Where:E0 : electric field without accumulated charge

0 : the permittivity of air andinsulation

d l0 : the insulation thickness and the half air gap

0: the surface charge density

0000

*

ld

dEE s

Behaviour of the electric field in two enamelled wires in atwisted pair configuration

This configuration helps explain why the main ageingfactor is associated to the jump voltage for bipolar andunipolar 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>PDIVESC the drop of the electric field due to the PD chargeinjection

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>PDIVESC the drop of the electric field due to the PD chargeinjection

After PD, a residualvalue of the electricfield is:

00

0*

0* 0

ld

dE

EEE

s

scres

If the chargeinjected by PD isnot rapidlydepleted, E*

0(t) dueto SC deposited onthe insulationsurface remainsconstant

If the chargeinjected by PD isnot rapidlydepleted, E*

0(t) dueto SC deposited onthe insulationsurface remainsconstant

When E0 change itspolarity, E*

0(t)increases by 2Ep inboth cases

PDIVEEE resp 20*

The influence of the JV was experimentally supplying 4different kinds of enamelled insulated wires with sinusoidal,unipolar and bipolar square waves in the range of 50 to 10kHz and with a RT of 50 ns above the PDIV

SC measurement becomes a significant tool to evaluate theability of the different materials to deplete the SC injectedby PD and to compare different enamels

PEA SP Measurement System

A Pulse Electro-Acustic method ha been developed tomeasure the space charge accumulation on magnet wires

(D. Fabiani et al. “ Relation Between Space Charge Accumulation andPartial Discharge Activity in Enameled Wires Under PWM-like VoltageWaveforms”, IEEE Trans. on Diel.-Elect.Ins., Vol.11, pp.393-405, June2004)

The specimen is positioned analuminium ground plate and

a semicon-absorber and

fed by HV DC power supply

A voltage pulse of A=300 V, 10ns width, 110Hz of RR, is appliedthrough a 220 pF couplingcapacitor

The specimen is positioned analuminium ground plate and

a semicon-absorber and

fed by HV DC power supply

A voltage pulse of A=300 V, 10ns width, 110Hz of RR, is appliedthrough a 220 pF couplingcapacitor

220 pF

Pulse

2 M

HVDC

Adsorber (PMMA)

Piezoelectric (PVDF)

Semicon AdsorberEnameled Insulation

Copper

Oscilloscope

GPIB IEEE-488Acquisition Board

PC

Amplifier

PVC Insulation

Aluminum Ground Plate 220 pF

Pulse

2 M

HVDC

Adsorber (PMMA)

Piezoelectric (PVDF)

Semicon AdsorberEnameled Insulation

Copper

Oscilloscope

GPIB IEEE-488Acquisition Board

PC

Amplifier

PVC Insulation

Aluminum Ground Plate

Charges present in insulation are forced by the fastelectric pulse and a pressure wave is generated from theinteraction between charges and the material structure

The pressure wave propagates through the insulation andreaches the ground electrode under which the piezoelectrictrasducer (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 outputsignal to be correlated to the amount of trapped spacecharge

PEA signal is amplified and sent to a recording system

110 PEA outputs per second, synchronized with therectangular supply voltage, are processed to obtain thecharge profile

Charges present in insulation are forced by the fastelectric pulse and a pressure wave is generated from theinteraction between charges and the material structure

The pressure wave propagates through the insulation andreaches the ground electrode under which the piezoelectrictrasducer (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 outputsignal to be correlated to the amount of trapped spacecharge

PEA signal is amplified and sent to a recording system

110 PEA outputs per second, synchronized with therectangular supply voltage, are processed to obtain thecharge profile

Test Procedure

SC measurements are performed according to a specificpolarization/depolarization procedure

Polarization: the electric field is applied (volt-on, VP=1 kV) fora period tp=3600 s to achieve steady state conditions forthe accumulated charge

Depolarization: depolarization(volt-off) follows polarization. Itis obtained removing the supplyvoltage and grounding the highvoltage electrode, and lasted3600 s as well.

Depolarization: depolarization(volt-off) follows polarization. Itis obtained removing the supplyvoltage and grounding the highvoltage electrode, and lasted3600 s as well.

t =20 s:

PEA signal is mainly due to the electrode field-inducedcharge (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 ishidden by the electrode charge

t =3602 s: SC in the insulation bulk canbe observed looking at the PEAprofile under volt-off (shadedarea)

t =3602 s: SC in the insulation bulk canbe observed looking at the PEAprofile under volt-off (shadedarea)

when the poling field is removed the electrode charge isconsiderably smaller than under polarization, being onlydue to the image charge (of the internal charge)

Space charge profile at the beginning ofvolt on: no space charge present

Space charge profile 2s after voltageremoval: space charge = gray area

To quantify the charge accumulation:

Total absolute stored charge density(after grounding), QM

Depolarization characteristic

Space Charge Data Processing

1

0

),(1

)(01

x

x

VO dxtxQxx

tQ

Total absolute stored charge density(after grounding), QM

TIPO A - PROFILO CARICA: Volt-ON 3600 s (+) Volt-OFF 3600 s

Time [s]

1 10 100 1000 10000

Cha

rge

[p.u

.]

0.2

0.4

0.6

0.8

1.0

1.2

Depolarization characteristic

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 inType I insulation

Even in the absence of PD, jump voltage and switchingfrequency can accelerate the intrinsic aging of windinginsulation 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)

PD when active, are the most important ageing factor inType I insulation

Even in the absence of PD, jump voltage and switchingfrequency can accelerate the intrinsic aging of windinginsulation 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 VPItechnology even for small size random wound machines

The PD inception voltage is up to 60% higher comparedwith non-impregnated ones because air gaps are filled(not totally) by the impregnation

The time to breakdown of the inter-turn insulationdepends on the: PDIV and intensity of PD enamel thickness and its resistance against PD erosion

But due to small imperfections, longer lifetime is notguaranteed VPI process for small machines is expensive

The stator can be totally impregnated adopting the VPItechnology even for small size random wound machines

The PD inception voltage is up to 60% higher comparedwith non-impregnated ones because air gaps are filled(not totally) by the impregnation

The time to breakdown of the inter-turn insulationdepends on the: PDIV and intensity of PD enamel thickness and its resistance against PD erosion

But due to small imperfections, longer lifetime is notguaranteed VPI process for small machines is expensive

Mica-Films for Inter-Turn or Strand Insulation

They are also adopted in LVmachines

Improvements were found but

higher costs

slot efficiency reduction

increased dimensions

Currently adopted in MV and HVmachines fed with pulsating voltages

They are also adopted in LVmachines

Improvements were found but

higher costs

slot efficiency reduction

increased dimensions

Micro Fillers

Enamel or polymer film may also contain additives suchmetal oxides or ceramic materials with a naturalresistance to discharges (Al2O3, TiO2, SiO2, Fe2O3..), hasbeen adopted

The structure is a multi-coatingover the copper enameled wirewith the PD shield layer and thesurface protection coating

The structure is a multi-coatingover the copper enameled wirewith the PD shield layer and thesurface protection coating

The basic idea is to substitute the largemica flakes with a high densityinorganic 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 capabilityButIt become brittle and develop cracks when subjected totemperature variation in the presence of mechanicalstressesEach micro-composite material mustbe evaluated carefully

the endurance life is enhanced no variations in the ground-wall insulation thickness increased t2t PD and surge withstand capabilityButIt become brittle and develop cracks when subjected totemperature variation in the presence of mechanicalstressesEach micro-composite material mustbe evaluated carefully

AcceleratedAccelerated lifelife teststests onon twistedtwisted pairspairs withwithconventional andand coronacorona resistantresistant (CR)(CR) insulationinsulation

Tests:Tests: -- in the absence of PD (in oil), below PDIVin the absence of PD (in oil), below PDIV-- in the presence of PD (in air), above PDIVin the presence of PD (in air), above PDIV

Sinusoidal and distorted waveformsSinusoidal and distorted waveforms

Evaluation of Corona Resistant Materials

Objective: to draw life linesto draw life lines

life modelslife models

voltage endurance coefficient evaluationvoltage endurance coefficient evaluation

comparison among insulation systemscomparison among insulation systems

50 Hz sinusoidalAbove PDIV

Below PDIV

#A, #B,#C

Specimens: twisted pair insulationTraditional, #ACR #BCR #C

Above PDIV #A, #B,#C

#A, #B,#C

Procedure

Life tests10 kHz sinusoidal

10 kHz square unipolar

10 kHz square bipolar

Above PDIV

Below PDIV

#A, #B,#C

#A

Above PDIV

Above PDIV

#A, #B,#C

#A, #C

Duty cycle = 50%Slew rate = 1 kV/s

Sinusoidal Life Test on #1, AIR and OIL, 50 Hz 10 kHz

Failure time [h]

0.1 1 10 100 1000

Vol

tage

(rm

sva

lue)

[V]

1000

10000

50 Hz SIN AIR #A50 Hz SIN AIR #B50 Hz SIN AIR #C50 Hz SIN OIL #A50 Hz SIN OIL #B50 Hz SIN OIL #C10 kHz SIN AIR #A10 kHz SIN OIL #A

VEC = 11.7

VEC = 10.6

VEC = 7.1

VEC = 11.9

VEC = 8.7

VEC = 6.4

VEC = 4.5

•Dramatic PD effectespecially at HF on#A

•#B (CR) is worsethan #A at 50 Hz

•#C good VEC andlife times in thepresence of PD

•Increasing frequencyreduces lifeconsiderably, bothwith and without PD

#A, #C#B

#C

#B#A

Test Results

Life test results and life lines for sinusoidaltests (50 Hz-10 kHz ) in the absence (in oil)

and in the presence of PD (in air)

Sinusoidal Life Test on #1, AIR and OIL, 50 Hz 10 kHz

Failure time [h]

0.1 1 10 100 1000

Vol

tage

(rm

sva

lue)

[V]

1000

10000

50 Hz SIN AIR #A50 Hz SIN AIR #B50 Hz SIN AIR #C50 Hz SIN OIL #A50 Hz SIN OIL #B50 Hz SIN OIL #C10 kHz SIN AIR #A10 kHz SIN OIL #A

VEC = 11.7

VEC = 10.6

VEC = 7.1

VEC = 11.9

VEC = 8.7

VEC = 6.4

VEC = 4.5

•Dramatic PD effectespecially at HF on#A

•#B (CR) is worsethan #A at 50 Hz

•#C good VEC andlife times in thepresence of PD

•Increasing frequencyreduces lifeconsiderably, bothwith and without PD

Sinusoidal Life Test on #1, AIR, 50 Hz 10 kHz

Failure time [h]

0.1 1 10 100 1000

Vol

tage

(rm

sva

lue)

[V]

1000

1000050 Hz SIN AIR #A50 Hz SIN AIR #B50 Hz SIN AIR #C10 kHz SIN AIR #A10 kHz SIN AIR #B10 kHz SIN AIR #C

VEC = 10.6

VEC = 8.8

VEC = 6.4VEC = 7.1

VEC = 4.5

VEC = 12.6

•VEC decrease withfrequency for #A

•VEC increase withfrequency for #B &#C (life decreases)

•#B is the worst at50 Hz but the bestat 10 kHz (for highfields)

•#C good VEC andlife times both at50 Hz and 10 kHz

#B

#C#A

#B

Test ResultsSinusoidal Life Test on #1, AIR, 50 Hz 10 kHz

Failure time [h]

0.1 1 10 100 1000

Vol

tage

(rm

sva

lue)

[V]

1000

1000050 Hz SIN AIR #A50 Hz SIN AIR #B50 Hz SIN AIR #C10 kHz SIN AIR #A10 kHz SIN AIR #B10 kHz SIN AIR #C

VEC = 10.6

VEC = 8.8

VEC = 6.4VEC = 7.1

VEC = 4.5

VEC = 12.6

Life test results and life lines for sinusoidaltests (50 Hz-10 kHz ) in air for the three

tested materials.

•VEC decrease withfrequency for #A

•VEC increase withfrequency for #B &#C (life decreases)

•#B is the worst at50 Hz but the bestat 10 kHz (for highfields)

•#C good VEC andlife times both at50 Hz and 10 kHz

#B

#C#A

Life Test in AIR summary plot

Failure time [h]

0.001 0.01 0.1 1 10 100 1000 10000

Vol

tage

(pea

k-to

-pea

kva

lue)

[V]

1000

10000

#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

•Peak-to-peakvoltage is the mainstressing factor,besides pulserepetitionfrequency

•Effect of voltageshape negligible(experimentalpoints fit the sameline)

#C#A

#B#B#C#A

Test Results

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.

Life Test in AIR summary plot

Failure time [h]

0.001 0.01 0.1 1 10 100 1000 10000

Vol

tage

(pea

k-to

-pea

kva

lue)

[V]

1000

10000

#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

•Peak-to-peakvoltage is the mainstressing factor,besides pulserepetitionfrequency

•Effect of voltageshape negligible(experimentalpoints fit the sameline)

Tipo A - B valori di QM

Voltage frequency

DC 0.1Hz 50Hz 10kHz

QM

[C/m

3 ]

0

2

4

6

8

10

12

#A#B

Material s [s-1]#A 2.5#B 1.8

Speed of chargeexpulsion, s, for 50 Hzsquarewave voltage

(1000 V peak)

Space Charge Meas. On #A and #B

Tipo A - B valori di QM

Voltage frequency

DC 0.1Hz 50Hz 10kHz

QM

[C/m

3 ]

0

2

4

6

8

10

12

#A#B

Material s [s-1]#A 2.5#B 1.8

Total absolute stored charge density,QM, as a function of frequency. Bipolarsquarewave. Poling voltage: 1000 V

peak

Note that:QM decreases as the

frequency increasesQM (#B) > QM (#A) s (#B) < s (#A)

The CR solutions show different performance andbehaviorCR micro-composites must be evaluated carefully beforethey useD.Fabiani et al.”The Effect of Fast repetitive Pulses on theDegradation of Turn Insulation of Induction Motors”, Proc.of SDEMPED 2001, pp.289-293, Grado (I), 2001

Pulsating voltage accelerates degradation both inair and in oil (i.e. with and without PD), but differentVEC

CR materials show a longer life at higherfrequencies

The standard insulation, #A, seems to suffersignificantly 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

Discussion

Pulsating voltage accelerates degradation both inair and in oil (i.e. with and without PD), but differentVEC

CR materials show a longer life at higherfrequencies

The standard insulation, #A, seems to suffersignificantly 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

In the presence of PD, the external layer (organicmaterial) is eroded rapidly and the inorganic materialemerges and the enamel change its colour

The inorganic material is easily removed by themechanical stresses (vibrations).

The “Frost Effect”

Let

Type A: only organicmaterials

Type B: organicmaterials filled byinorganic particles

The surface erosion isevident (Type B1) while alocalized BD occurs inorganic enamel (Type A)

Fault Fault

Type A Type B1

Let

Type A: only organicmaterials

Type B: organicmaterials filled byinorganic particles

The surface erosion isevident (Type B1) while alocalized BD occurs inorganic enamel (Type A)

A new class of micro-fillers (Type C) has been developedwhere the inorganic filler chemically combines with theorganic enamel

Type B1

The chemical links delay the mechanical erosionand the insulation life is prolonged

Type C1

The “Frost Effect” combined with electrical stressesdetermine two types of breakdown:

Pinhole Type Massive Type

The two type of BD are related to the time exposition toPD, thus to the local electric field

In this example, BD occurred due to a defect on theconductor where the electric field was enhanced by thecopper protrusions

The “Frost Effect” due to PD is evident looking around thebreakdown site

The three layers are also evident in the picture

Another example of PD erosion

X-Ray spectrometry evidencedthe dominant presence of oxygenin the vicinity of the BD areawhile TiO2 was found around theBD crater

Nano Fillers

To improve the PD resistance of organic enamel, bymeans of the dispersion of nano-metric inorganic fillersare dispersed in the polymer matrix (under investigation)

Polymeric nano-composite: composite material withinorganic fillers having at least one dimension < 100 nm

Polymer NanofillersPolymer Nanofillers

Nanoparticles

Nanotubes, Nanofibres,Whiskers, Nanorods

Nanolayers

Nanocomposite

The filler rate is usually between 1%-10% of weight

The presence of inorganic nano-fillers can alter thedielectric properties of the materials. In particular,

• Permettivity

• Space charge accumulation

• Electrical Tree propagation

• Heat transmission

• etc.

This new technology must be handled carefully to avoidthat improving a property, worsening the others

The nano filler is selected taking into account theproperty to be improved (e.g., the use of nano micaflakes to delay the electric tree growth)

The filler rate is usually between 1%-10% of weight

The presence of inorganic nano-fillers can alter thedielectric properties of the materials. In particular,

• Permettivity

• Space charge accumulation

• Electrical Tree propagation

• Heat transmission

• etc.

This new technology must be handled carefully to avoidthat improving a property, worsening the others

The nano filler is selected taking into account theproperty to be improved (e.g., the use of nano micaflakes to delay the electric tree growth)

EL470+DEL72 (G21)After 22 hours

EL470 (G19)After 3 hours

Tree Growth: bush type

Nano-mica flakes form a wide and complex “labyrinth” wherethe length of the tree-channels are strongly increased andthe breakdown, delayed

EL470+DEL72 (G21)After 22 hours

EL470+MAE (G22)After 20 hours

Anomalous Tree Grouth due tothe barrier-effect of the nanofiller

Nano-fillers are added to improve the resin performances

mainly to withstand the PD erosion

Initialagglomerate

Conventionalcomposit

Intercalatednano-composite

Exfoliatednano-composite

The complete exfoliation ofthe nano-filler generates freecharges inside the insulationworsening e.g., thedissipation factor

RB standard resinN1 1% nano micaN3 3% nano mica

The intercalatedstructure is preferablethe ionic links betweenthe mica flakes arepreserved and no freecharges are introduced

Possible Barrier Effect

Conventional enamel

Inter-turn PDs

Initial stage of aging Further stage of aging

PD induce ablative degradation process leading to thescission of the polymeric chain, the formation of freeradicals and of volatile decomposition productsAdopt nano composite materials (Type C) that show astrong interaction between the nano particles

Nanocomposite enamel

Inter-turn PDs

Initial stage of aging Further stage of aging

PD

CeramicLike layer Increase of

nanofillerconcentrationon the surface

Aggregationforces between

inorganicnanoparticles

Interactions or bondsbetween filler and

carbonaceous residue

Conventional and nano-composite enamel have beenanalyzed and compared

Type A: double layer polyester-imide (PEI)

Type C1: double layer PEI and PEI+Barium SulphateBaSO4 (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 atdifferent voltage levels and temperatures

F.Guastavino et al. “Electrical Aging Tests on Different NanostructuredEnamels Subjected to Severe Voltage Waveforms”, proc.IEEESDEMPED, pp.283-287, Bologna (I), September 2011

The support of Elantas Deatech S.r.l. - Ascoli Piceno – Italy isgreatifully acknowledged

Conventional and nano-composite enamel have beenanalyzed and compared

Type A: double layer polyester-imide (PEI)

Type C1: double layer PEI and PEI+Barium SulphateBaSO4 (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 atdifferent voltage levels and temperatures

F.Guastavino et al. “Electrical Aging Tests on Different NanostructuredEnamels Subjected to Severe Voltage Waveforms”, proc.IEEESDEMPED, pp.283-287, Bologna (I), September 2011

The support of Elantas Deatech S.r.l. - Ascoli Piceno – Italy isgreatifully acknowledged

Test Set-Upoven

VAWADIT

ArbitraryWaveformGenerator

Twisted pair

oven

VAWADIT

ArbitraryWaveformGenerator

Twisted pair

PWM+peaksvoltage

waveform

Linear amplifier: 10 Hz– 3 MHz bandwidth atthe considered voltage

level, 60 dB gain

Temperaturetest: 150°C;120°C; 90°C;

60°C

The average time to breakdown (Tbd) is collected andrelated to the test voltage amplitude via the inversepower law:

Tbd = A (Vpp)-n

1000

10000

1000 10000 100000 1000000

Vte

st[V

]

Tbd [s]

PEI

PEI+ns

PEI+nb

1000

10000

1000 10000 100000 1000000

Vte

st[V

]

Tbd [s]

PEI

PEI+ns

PEI+nb

The fomation of nanostructured ceramic-like layer has beenobserved 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 evidencedfor Polymer layered silicate nanocomposites, but similarbehavior has been observed also in the case of polymer-SiO2 nanocomposites (Wu et al. 2005, Wang et al. 2006, also according to the work ofVaia).

Ceramic char formation during ablation

The described processes have been massively evidencedfor Polymer layered silicate nanocomposites, but similarbehavior has been observed also in the case of polymer-SiO2 nanocomposites (Wu et al. 2005, Wang et al. 2006, also according to the work ofVaia).

Qualitative Surface Analysis

Electricalaging tests

PDsactivity

Enamelerosion

Qualitativesurface analysis

Simple opticalmicroscope

Degradation area dimensionsfor conventional enamel after

electrical aging at 150°C

Degradation area dimensions fornanocomposite enamel after

electrical aging at 150°C

Degradation area dimensions are wider in the case ofconventional enamel than in the case of nanocomposite one

Degradation area dimensionsfor conventional enamel after

electrical aging at 60°C

Degradation area dimensions fornanocomposite enamel after

electrical aging at 60°C

Diminishing the temperature level, the eroded areadimensions is less wide

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

After aging test at 4.6 kVAfter aging test at 4.6 kV

PEI + nb PEI + ns

Thermal Ageing

Applying the Arrheniusmodel to the obtained

life timesLinearizing Life Curves

60 90 120 150

T [°C]

1000

10000

100000

1000000

0,00220,00240,00260,00280,00300,0032

1/T [1/K]

Tbd

[s]

Conventional

Nanocomposite

60 90 120 150

T [°C]

1000

10000

100000

1000000

0,00220,00240,00260,00280,00300,0032

1/T [1/K]

Tbd

[s]

Conventional

Nanocomposite

60 90 120 150

T [°C]

1000

10000

100000

1000000

0,00220,00240,00260,00280,00300,0032

1/T [1/K]

Tbd

[s]

Conventional

Nanocomposite

60 90 120 150

T [°C]

1000

10000

100000

1000000

0,00220,00240,00260,00280,00300,0032

1/T [1/K]

Tbd

[s]

Conventional

Nanocomposite

Conv.150°C

Nano150°C

Conv.120°C

Nano120°C

Conv.90°C

Nano90°C

Conv.60°C

Nano60°C

Conv.150°C

Nano150°C

Conv.120°C

Nano120°C

Conv.90°C

Nano90°C

Conv.60°C

Nano60°C

Tbd

[s]

0

2000

4000

6000

8000

1000030000

45000

60000

75000

90000

105000

120000

Conv.150°C

Nano150°C

Conv.120°C

Nano120°C

Conv.90°C

Nano90°C

Conv.60°C

Nano60°C

Conv.150°C

Nano150°C

Conv.120°C

Nano120°C

Conv.90°C

Nano90°C

Conv.60°C

Nano60°C

Tbd

[s]

0

2000

4000

6000

8000

1000030000

45000

60000

75000

90000

105000

120000

Conv.150°C

Nano150°C

Conv.120°C

Nano120°C

Conv.90°C

Nano90°C

Conv.60°C

Nano60°C

Conv.150°C

Nano150°C

Conv.120°C

Nano120°C

Conv.90°C

Nano90°C

Conv.60°C

Nano60°C

Tbd

[s]

0

2000

4000

6000

8000

1000030000

45000

60000

75000

90000

105000

120000

Conv.150°C

Nano150°C

Conv.120°C

Nano120°C

Conv.90°C

Nano90°C

Conv.60°C

Nano60°C

Conv.150°C

Nano150°C

Conv.120°C

Nano120°C

Conv.90°C

Nano90°C

Conv.60°C

Nano60°C

Tbd

[s]

0

2000

4000

6000

8000

1000030000

45000

60000

75000

90000

105000

120000

Data scatter is generally low; the minimum life time value obtained testing thenano-composite enamel at 150°C is considerablylonger than the maximum time obtained testing theconventional enamel at 60°C

Micro+Nano Composites

A combined use of micro and nano fillers has been alsoinvestigate to

improve different properties of the composite materials(thermal and mechanical in addition to PD resistance)

guarantee novel properties

Schematicrepresentation of PDerosion process due toPD for micro andmicro+nano compositematerials

Sample of micro-silica(60%wt) and nano-silica (5%wt) in epoxymatrixMicro-silica: black areaNano-silica: small withedots

Nano-silica (Dark gray spots)in epoxy resin matrix

CIGRE Working Group, “Characterization of Epoxy Microcomposite andnanocomposite Materials for Power Engineering Applications”, IEEEEl.Ins.Magazine, Vol.28, pp.38-51, March 2012

Nano-silica (Dark gray spots)in epoxy resin matrix

Discussion

It is possible to enhance the resistance to the action ofsurface PDs of organic insulating enamels used formagnet wire insulation by nano-structuration.

The application of nano-composite enamels is not aPanacea for inverter driven motor insulation:

1.Many matrix-filler combination may not lead to thedesired results; careful study of the chemical-physicalinteractions and degradation mechanisms

2.Nano-structuration does not prevent the inception ofPDs; rather it slows down the degradation of theenamel

It is possible to enhance the resistance to the action ofsurface PDs of organic insulating enamels used formagnet wire insulation by nano-structuration.

The application of nano-composite enamels is not aPanacea for inverter driven motor insulation:

1.Many matrix-filler combination may not lead to thedesired results; careful study of the chemical-physicalinteractions and degradation mechanisms

2.Nano-structuration does not prevent the inception ofPDs; rather it slows down the degradation of theenamel

Required further research investigation:1.Chemistry and physics of the degradation of nano-

composite enamels subjected to PDs2.Polymer – inorganic nano-particles interactions3.Interactions between nano-composite enamels and

secondary insulation (conventional or nano-structuredimpregnation resins)

4. Micro-nano composites

Required further research investigation:1.Chemistry and physics of the degradation of nano-

composite enamels subjected to PDs2.Polymer – inorganic nano-particles interactions3.Interactions between nano-composite enamels and

secondary insulation (conventional or nano-structuredimpregnation resins)

4. Micro-nano composites

The concepts of dielectric strength, Weibull distribution offailure times, lifetime and voltage endurance coefficientare the basis for the design of highly reliable insulationsystems in electrical apparatus

ASD introduced a new type of electrical stress arisingfrom high frequency harmonics due to repetitive voltageimpulses and motor-cable-converter impedancemismatch

Over-voltages and uneven voltage distribution along thewinding causes overstress mainly in inter-turn insulation

Over-voltages can cause PD that become the dominantageing factor mainly in Type I materials

Corona resistant materials (micro, nano composites) havebeen developed

Modeling for Insulation Design

The concepts of dielectric strength, Weibull distribution offailure times, lifetime and voltage endurance coefficientare the basis for the design of highly reliable insulationsystems in electrical apparatus

ASD introduced a new type of electrical stress arisingfrom high frequency harmonics due to repetitive voltageimpulses and motor-cable-converter impedancemismatch

Over-voltages and uneven voltage distribution along thewinding causes overstress mainly in inter-turn insulation

Over-voltages can cause PD that become the dominantageing factor mainly in Type I materials

Corona resistant materials (micro, nano composites) havebeen developed

It is necessary to study and model the lifetime behavior of new CRmaterials to design properly the insulation taking into account theASD specific stresses

The dominant aging factors

Some quantities extracted from the distorted voltagewaveforms and correlated with aging are introduced. Let:Some quantities extracted from the distorted voltagewaveforms and correlated with aging are introduced. Let:

N

hnfh thVtv

1

)sin()(

The Fourier series of the non-sinusoidal voltage supply.

N

hnfhf thVh

dt

tdv

1

)cos()(

The rms of the voltage variation is defined as:

If we consider the rms value of a 50 Hz sinusoidal voltagehaving the same amplitude of the fundamental (V0=V1),then:

N

hh

f

rms

Vhdt

tdv

1

22

2

)(

10

,50

0

2

)(V

dt

tdv

rmsHz

Considering their ratio where

10

,50

0

2

)(V

dt

tdv

rmsHz

N

hh

fs hK

1

22

0

1V

Vhh

Ks is the rms value of the derivative of thedistorted waveform and it is related to its RT

*1rms

rmsrms V

VK

*1P

PP V

VK where

VP and Vrms are the peak and rms valuesof the distorted waveform, V1

* is the reference voltage (V1P*=√2 ⋅

V1rms),

Additional parameters, related to over-voltages, can bedefined as:

*1rms

rmsrms V

VK

where VP and Vrms are the peak and rms valuesof the distorted waveform, V1

* is the reference voltage (V1P*=√2 ⋅

V1rms),

The Joule, Wj, and the dielectric, Wd, losses for a windinghaving a phase-to-ground capacitance C, can be writtenas:

N

h

haaJ V

VhVCrkW

1

2

1

221

221 )(

N

h

hd V

VhCVW

1

2

1

211 )(tan

Where ra is the resistance of the equivalent capacitor ka is a constant tan is the loss factorThe temperature rise is then given by:

Where ra is the resistance of the equivalent capacitor ka is a constant tan is the loss factorThe temperature rise is then given by:

thdJ RWW )( where Rth is the thermal resistance of the capacitorUsing the PWM technique, temperature increasesof about 10 to 20 K°

The Dominant Ageing Factors

With PD: It has been shown that PD is the dominantageing factor particularly at high pulse rate and frequency

Thus, the insulation system must be designed to workbelow the PDIV

PDIV depends on the adopted insulation

Without PD: Below the PDIV, the ageing mechanisms arevery different (related to RT, RR, temperature….)

Neglecting the interactions between factors as a firstapproximation, the simplest equation that can be used,based on an inverse power model, is:

The Dominant Ageing Factors

With PD: It has been shown that PD is the dominantageing factor particularly at high pulse rate and frequency

Thus, the insulation system must be designed to workbelow the PDIV

PDIV depends on the adopted insulation

Without PD: Below the PDIV, the ageing mechanisms arevery different (related to RT, RR, temperature….)

Neglecting the interactions between factors as a firstapproximation, the simplest equation that can be used,based on an inverse power model, is:

srmsP ns

nrms

nP KKKLL 0

where L is insulation lifetime, and L0, np, nrms, nsare adjustable parameters

KP, Krms, and Ks are further analyzed statistically, e.g., byusing the Standardized Pareto Chart (SPC) and the MainEffect Plot (MEP)

ssrmsrmsPp KnKnKnL

Lloglogloglog

0

Peak voltage is clearly the most influential factor of lifetime,followed by rms and voltage slope

nba NBALL 0

The experimental data suggested that an inverse powermodel, in the form of

can be applied to correlate life- time and aging factors, inparticular P2P voltage and temperature, that is, in logform

loglogloglog bVaLL PPD

where L=lifetime; VPP=P2P voltage; and a, b and L0 areparameters calculated through multivariable linearregression

Again, the jumpvoltage is still themost influential factorof lifetime

where L=lifetime; VPP=P2P voltage; and a, b and L0 areparameters calculated through multivariable linearregression

Peak-to-peak voltage has been recognized as dominantageing factorThe inverse power model correlating the lifetimes atdifferent stress values is slightly modified, that is

nppVLL 0

Life Modeling: a simplified version

The pulse RR of the applied voltage is also important, withlifetime decreasing with increasing RR. If Lf and L1 are thelifetimes at f (= 10 kHz) and f1 (= 50 Hz), respectively then

The pulse RR of the applied voltage is also important, withlifetime decreasing with increasing RR. If Lf and L1 are thelifetimes at f (= 10 kHz) and f1 (= 50 Hz), respectively then

)( 11 f

fLLf

where the exponent γ is estimated experimentally

These assumptions allow the lifetime of an insulationsystem under impulse conditions to be estimated usingdata obtained under sinusoidal voltage testing. Moreover:

The most important stressing factors are fundamentalfrequency, the RR and peak-to-peak voltage amplitude

The overvoltage is adiabatic

The system is operated in the stress range within which thepredominant degradation mechanism does not change duringageing

VEC is frequency independent, i.e., no significant frequencydependence of the number of impulses or voltage cycles beforefailure is observed. This corresponds to = 1 which isapproximately true for composite organic/inorganic insulation

These assumptions allow the lifetime of an insulationsystem under impulse conditions to be estimated usingdata obtained under sinusoidal voltage testing. Moreover:

The most important stressing factors are fundamentalfrequency, the RR and peak-to-peak voltage amplitude

The overvoltage is adiabatic

The system is operated in the stress range within which thepredominant degradation mechanism does not change duringageing

VEC is frequency independent, i.e., no significant frequencydependence of the number of impulses or voltage cycles beforefailure is observed. This corresponds to = 1 which isapproximately true for composite organic/inorganic insulation

If these conditions are satisfied and the measured lifetimeat test frequency f1 is L1, then the estimated lifetime L2 attest frequency f2 is given by:

2

112 f

fLL

It follows that

if lifetime line 1corresponds to testfrequency f1,

lifetime line 2 for testfrequency f2 (f2 = 10 f1) isobtained by translatinglifetime line 1 one decadehorizontally (arrow A)

It follows that

if lifetime line 1corresponds to testfrequency f1,

lifetime line 2 for testfrequency f2 (f2 = 10 f1) isobtained by translatinglifetime line 1 one decadehorizontally (arrow A)

to maintain the f1 lifetime, the applied stress at f2should be reduced as shown by arrow B

Combining the two simplified models

2

1

2

1,, )(

1122 f

f

U

ULL n

ufuf

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 VECdata can be generated for any desired frequency, e.g., thefundamental for motor drives, based on measured lifetimesfor appropriate insulation systems at f1 = 50 or 60 Hz

Experimental evidence validates this simplified approach forimpulse voltages up to 1 kHz because the variation of theVEC n with frequency is negligible

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 VECdata can be generated for any desired frequency, e.g., thefundamental for motor drives, based on measured lifetimesfor appropriate insulation systems at f1 = 50 or 60 Hz

Experimental evidence validates this simplified approach forimpulse voltages up to 1 kHz because the variation of theVEC n with frequency is negligible

At higher frequencies a decrease of n is observed, even forinorganic/organic insulation

The dependence of n with frequency can be modeled, butonly by introducing further parameters in the model.

Design Criteria

PD is a dominant deterioration phenomena that leads topremature BD of the insulation. Using conventional enamel,the electric stress must be below the PDIV

a moderate PD activity can be accepted when CRcomposite organic/inorganic insulating material is adopted

In the absence of PD, the peak of the distorted voltagewaveform and its repetition rate are the most importantageing factors

Modeling the long-term behavior is feasible in the firstapproximation

Detailed evaluation of the new materials through long-term voltage endurance tests is still strongly recommended,to maximize the reliability of the insulation system

PD is a dominant deterioration phenomena that leads topremature BD of the insulation. Using conventional enamel,the electric stress must be below the PDIV

a moderate PD activity can be accepted when CRcomposite organic/inorganic insulating material is adopted

In the absence of PD, the peak of the distorted voltagewaveform and its repetition rate are the most importantageing factors

Modeling the long-term behavior is feasible in the firstapproximation

Detailed evaluation of the new materials through long-term voltage endurance tests is still strongly recommended,to maximize the reliability of the insulation system

A. Cavallini, D. Fabiani, G.C. Montanari, “Power Electronicsand Electrical Insulation System – Part 1: PhenomenologyOverview”, IEEE Electrical Insulation Magazine, Vol. 26, pp. 7-15, May-June 2010

A. Cavallini, D. Fabiani, G.C. Montanari, “Power Electronics andElectrical Insulation System – Part 2: Life Modeling for InsulationDesign”, IEEE Electrical Insulation Magazine, Vol. 26, pp. 33-39, July-August 2010

A. Cavallini, D. Fabiani, G.C. Montanari, “Power Electronics andElectrical Insulation System – Part 3: Diagnostic Properties”, IEEEElectrical Insulation Magazine, Vol. 26, pp. 30-40,September-October2010

References:

A. Cavallini, D. Fabiani, G.C. Montanari, “Power Electronicsand Electrical Insulation System – Part 1: PhenomenologyOverview”, IEEE Electrical Insulation Magazine, Vol. 26, pp. 7-15, May-June 2010

A. Cavallini, D. Fabiani, G.C. Montanari, “Power Electronics andElectrical Insulation System – Part 2: Life Modeling for InsulationDesign”, IEEE Electrical Insulation Magazine, Vol. 26, pp. 33-39, July-August 2010

A. Cavallini, D. Fabiani, G.C. Montanari, “Power Electronics andElectrical Insulation System – Part 3: Diagnostic Properties”, IEEEElectrical Insulation Magazine, Vol. 26, pp. 30-40,September-October2010

In form wound MV and HV rotating machines, theground-wall and strand insulation, based on mica-tapesand mica-films, respectively, withstand to the PD activity

Turn insulation is stressed by the uneven voltagedistribution and can be designed according to the abovementioned composite materials and criteria

Design of the Stress Grading for HV Machines

The end-arm stress grading isthe weak point when a machinedesigned for 50/60 Hz is fed bypulsating of PWM supply. Thispromoted investigations to:

select proper materials

design properly the stressgrading

The end-arm stress grading isthe weak point when a machinedesigned for 50/60 Hz is fed bypulsating of PWM supply. Thispromoted investigations to:

select proper materials

design properly the stressgrading

Problemi nei sistemi digradatura

Solo nei motori form-wound (MT)• Il campo elettrico tangenziale

aumenta all’aumentare delcontenuto in frequenza dellatensione (f> 2 kHz si hanno scarichein testata)

• Il campo elettrico cala all’aumentaredella conducibilità della gradatura

Solo nei motori form-wound (MT)• Il campo elettrico tangenziale

aumenta all’aumentare delcontenuto in frequenza dellatensione (f> 2 kHz si hanno scarichein testata)

• Il campo elettrico cala all’aumentaredella conducibilità della gradatura

Media conducibilità Alta conducibilità

Campo elettricodi scarica Campo elettrico

di scarica

Typically:

• Insulation with no stress grading system(normally for Vn ≤ 4kV)

• Insulation systems with anticorona coating within theslot ( 4kV ≤Vn ≤ 6kV)

• Insulation with stress grading system (Vn ≥6kV)

Typically:

• Insulation with no stress grading system(normally for Vn ≤ 4kV)

• Insulation systems with anticorona coating within theslot ( 4kV ≤Vn ≤ 6kV)

• Insulation with stress grading system (Vn ≥6kV)

Due to material discontinuity, high values of electricgradient affect the surface of the coil at the edge of the slotgrading tape thus generating tangential surface discharges

The stress grading is designed solving the field equation

0)*(

U

tUcc

and its solution allows to draw the electric field outsidethe magnetic core

)cosh(

)(cosh1)(

kL

LxkVxU a

)cosh(

)(cosh1)(

kL

LxkVxU a

)sinh(

)(sinh)(

kL

LxkkVxE a

Hot spots due to PD, can be discovered considering theair breakdown strength (e.g., 2.3 kV/mm) and theelectric field gradient

Stress grading materials are characterized by theirresistance that can be constant or electric field dependent:

)exp( 3/20 nEs

High values of n and low values of 0 increase the gradingeffect

The stress grading is designed considering the number oflayers and their coating length outside a slot portion

After the material selection (0, n), the space distributionof the electric field is determined using FEM softwaretools considering the machine geometry and the ground-wall insulation characteristicsThe correct choice of the number of layers and theirlength is evaluated checking the electric gradient (belowof PDIV) and hot spots

High values of n and low values of 0 increase the gradingeffect

The stress grading is designed considering the number oflayers and their coating length outside a slot portion

After the material selection (0, n), the space distributionof the electric field is determined using FEM softwaretools considering the machine geometry and the ground-wall insulation characteristicsThe correct choice of the number of layers and theirlength is evaluated checking the electric gradient (belowof PDIV) and hot spots

Stress grading is currently designed for 50/60HzapplicationsAssuming the 0 reference the edge of the slot grading, thepotential and the electric field distribution can be derivedand analyzedThe different behavior of a single layer stress grading withconstant and exponential resistivity, is show

But the potential distribution is strongly related tofrequency of the applied stress.Stress grading designed for ac is not able to operate athigher frequencies

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

22000

24000

26000

28000

30000

32000

34000

36000

-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]

V [V]

GEN.3.1 GEN.3.2 GEN.3.3 GEN.3.4

250 kHz50 Hz

30 kVf variabile

20 kHz

1 kHz

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

22000

24000

26000

28000

30000

32000

34000

36000

-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]

V [V]

GEN.3.1 GEN.3.2 GEN.3.3 GEN.3.4

250 kHz50 Hz

30 kVf variabile

20 kHz

1 kHz

A multi-layer and longer stress grading having lower resitance,is required for higher frequencies

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000

16000

17000

18000

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50

Distanza x dal termine del ricoprimento conduttivo [mm]

SIM.1.2.1.A SIM.1.2.1.B

Mat.D

Mat.A

Vmax [V]

16 kV # 1,25 MHzMod.1

s = 2,7 mm

1

4

2

3

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

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15000

16000

17000

18000

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50

Distanza x dal termine del ricoprimento conduttivo [mm]

SIM.1.2.1.A SIM.1.2.1.B

Mat.D

Mat.A

Vmax [V]

16 kV # 1,25 MHzMod.1

s = 2,7 mm

1

4

2

3

Experimental Validation

Two different stress grading configurations designed forstandard (50 Hz) and PWM supply voltages have been testedby means of PD measurements

Both frames were supplied by HV rectangular wave-shape

PD measurements were performed using an antenna probeable to record PD pulses and the fundamental wave-shapeadopted as the phase reference of PD

Moving the antenna probe, PD were localized at the edge ofthe 50Hz stress grading while only signals due tocommutations were recorded on the other frame

Thus confirming the validity of the stress grading design

Besides the advantages in using ASD, new problemsrose due to the significant harmonic content of the powersupply and the over-voltages generated by mismatchimpedances in inverter/cable/drive connection

The insulation is subjected to increased electric,thermal and mechanical stresses and its life is shortened

Additional stresses, typical of ASD, have beenexamined

New composite materials (micro, nano fillers) havebeen proposed

Specific test methods are developed

Specific standards are under discussion

Conclusions

Besides the advantages in using ASD, new problemsrose due to the significant harmonic content of the powersupply and the over-voltages generated by mismatchimpedances in inverter/cable/drive connection

The insulation is subjected to increased electric,thermal and mechanical stresses and its life is shortened

Additional stresses, typical of ASD, have beenexamined

New composite materials (micro, nano fillers) havebeen proposed

Specific test methods are developed

Specific standards are under discussion

Thank you for yourattention!

Thank you for yourattention!