maulaca

March 5, 2015

Field diagnostics and diagnostic monitoring of HV insulation at generators, motors, transformers and cables (part I)

Topics

Page 2

> Introduction

> Maintenance

> Measurement, Diagnostics and Diagnostic Monitoring

> Rotating Machines (Generators / Motors)

Lifetime expectations

Faults and Aging

Measurement & monitoring parameters /quantities Insulation Resistance

Polarization Index

C / tan (power factor)

Partial discharge

Meta-Trends

1. HV asset fleet / installed base becomes older

2. Knowledge is a limited resource

3. HV assets are designed with lower margins than the generation before

4. Cost pressure

Page 3 OMICRON

Introduction

Page: 4March 10, 2015

but on the other hand ...

Blackouts

Page: 5March 10, 2015

millions ofpeople affected location date

July 2012 India blackout 620 India 30 July 2012-31 July 2012

January 2001 India blackout 230 India 2 January 2001

November 2014 Bangladesh blackout 150 Bangladesh 1 November 2014

2015 Pakistan blackout 140 Pakistan 26 January 2015

2005 JavaBali blackout 100 Indonesia 18 Aug 2005

1999 Southern Brazil blackout 97 Brazil 11 March 1999

2009 Brazil and Paraguay blackout 87 Brazil, Paraguay 1011 Nov 2009

Northeast blackout of 2003 55 United States, Canada 1415 Aug 2003

2003 Italy blackout 55 Italy, Switzerland, Austria, Slovenia, Croatia 28 Sep 2003

Thailand Nation-wide blackout of 1978 40 Thailand 18 Mar 1978

Northeast blackout of 1965 30 United States, Canada 9 Nov 1965

What to do ?

maintenance

Topics

Page 7

> Introduction

> Maintenance




Faults and Aging


Polarization Index


Partial discharge

Historical stages of maintenance practices

No maintenance (run to failure)

Preventive (time based)

Predictive (condition based)

Condition and Time based maintenance

Time

Insu

latio

n st

reng

th New condition

Operating stress

Time based maintenance

Diagnostic levelCondition based maintenance

Condition based maintenance requires reliable diagnostic tools

Equipment, that can be easily replaced within a

month

Critical equipment that should be repaired or replaced within 1 year

Critical Equipment with long term delivery time

(>1 year)

Run to Failure

Predictive Maintenance

Real Time Monitoring

Non critical equipment, that can be replaced

within few months

Preventive Maintenance

Eas

e of

repa

ir/re

plac

emen

tC

ost for unforeseen shut down

Reliability Centred Maintenance


Reliability cantered

Maintenance RCM

Preventive Maintenance

PM

Condition Based

MaintenanceCBM

Real Time Monitoring

RTM

Predictive Maintenance

PdM

Run to Failure

RTF

Topics

Page 13

> Introduction

> Maintenance




Faults and Aging


Polarization Index


Partial discharge

Measurement is a process of making a quantitative assessment of an unknown quantity.

Monitoring is the systematic observation of repeated processes or systems in order to draw conclusions through the comparison of results.

The comparison of repeated measurements is defacto monitoring.

Challenges:Boundary conditions, comparability, measurement uncertainty, repeatability

Page 14 OMICRON

Measurement vs. Monitoring

Why conduct monitoring when design, type, series and commissioning tests are performed?

> Design, type and serial tests are conducted prior to asset installation

> Possible damage via transport, assembly and installation that could lead to eventual failure

> These tests do not assess online condition when asset is in service

A commissioning test is:

> Limited in time

> Often limited to specific stress factors, but not all

> A snapshot in time, not continuous

Example: Cable after laying test: 1h @ 1.1 Un, no load, no temperature stress

Page 15 OMICRON


Monitoring enables measurement comparisons to be made in the presence of all operational stress factors (3-phase , asymmetries, couplings, loads, vibrations, temperatures, etc..) during real operation conditions over longer periods of time.

Monitoring helps to predict severe failure to extend reliable operation of equipment throughout its service life.

Page 16 OMICRON


OMICRON


> Periodic offline diagnostic measurements> Allows the variation of different measurement parameters > Does not take place under normal operating conditions> Execution is highly dependent on human interaction > Provides only a snapshot of condition state

> Periodic online monitoring (temporary monitoring)> Does not allow the variation of different measurement parameters> Takes place under operating conditions (load, temperature, vibration) > Execution required a minor degree of human interaction > Provides a snapshot of condition state over a specified period of time

> Permanent monitoring (continuous monitoring)> Does not allow the variation of different measurement parameters> Takes place under operating conditions (load, temperature, vibration) > Execution does not require human interaction after setup> Provides continuous (on-going) condition assessment

Page 18 OMICRON


> Continuous or periodic monitoring of HV equipment is an essential tool for effective maintenance management

> Monitoring answers questions about the present condition of the equipment and its future performance

> The large amount of real-time data gathered by monitoring systems can be used for precise insulation condition assessment

> Effective comparison of historical data is enabled via an easy-to-use and extendable database solution

> Exact knowledge of insulation state at any time saves money, as maintenance schedules can be specifically optimized and the service life of assets can be extended

Page 19 OMICRON

Diagnostic monitoring

> Involves no operational variables > Provides information to evaluate the aging condition of equipment> Allows assessment of future developments in the aging process > Supports decision making for ongoing maintenance or the replacement

of components > is a "planning tool (asset management)

Page 20 OMICRON

Diagnostic monitoring

OMICRON

What we would like to discuss today...

OMICRON

aging of HV assets means in general aging of HV insulation systems...

therefore we like to discuss today

> measurement methods / diagnostically methods to determine health indicating parameters of HV insulations

> monitoring of diagnostically parameters YES

> monitoring of operational parameter NO

What we would like to discuss today...

Topics

Page 23

> Introduction

> Maintenance




Faults and Aging


Polarization Index


Partial discharge

Seite 24 OMICRON

Field Diagnostics & diagnostic monitoring of rotating machines

Copyright "Siemens Pressebild

Seite 25 OMICRON

Field Diagnostics & diagnostic monitoring of rotating machines

Topics

Page 26

> Introduction

> Maintenance




Faults and Aging


Polarization Index


Partial discharge

Seite 27 OMICRON

Life Time expectation

Opinions & Statements:

> 20 ... 30 years (till rewinding) ?

> economically 15 years (depreciation) ?

> operated at determined temperature limit 20.000 hours (= 2.3 years)

all above may be correct what we learn: prediction is difficult ... impossible, therefore diagnostically methods or monitoring is needed

Seite 28 OMICRON


Insulation design evolution the last 100 years

Glew C.N.:The Next Generation A Review of the Factors influencing the Output of an Electrical Machine in the New Millenium., INSUCON/ISOTEC 98, p. 231-242

Seite 29 OMICRON

Generator

Topics

Page 30

> Introduction

> Maintenance




Faults and Aging


Polarization Index


Partial discharge

Seite 31 OMICRON

Faults & Aging

Faults

Source: Brtsch et al. "Insulation Failure Mechanisms of Power Generators", DEIS July/August 2008

CIGRE 2009 / Survey of Hydro Generator Failures

Seite 32 OMICRON

Faults & Aging

Insulation of Stator Winding generalCoil Type Winding Roebel Bar Type winding

Seite 33 OMICRON

Faults & Aging

Insulation of Stator Winding general

Seite 34 OMICRON

Faults & Aging

Stator slot

Seite 35 OMICRON

Faults & Aging


A. Covering TapeB. Spacer, Coil-End BracinC. Groundwall Insulation, Mica TapesD. Grading/ Silicon Carbide Coating

E. Slot Semi conductive CoatingF. Inner Semicon. coatingG. Turn InsulationH. Slot Wedge / SealI. Stator Core

Seite 36 OMICRON

Faults & Aging


Slot Semiconductive Coating prevent PDfrom occurring in any air gap that might bepresent between the coil/ bar surface andthe stator core. Carbon-loaded paint or tape

End winding Grading potential grading,silicon carbide, extends only fewcentimetre beyond the end of each slot.

Seite 37 OMICRON

Faults & Aging


Standards

IEC 60034 Rotating electrical machinesIEC 60034-18 Functional evaluation of insulating systemsIEC 60034-18-31 Test procedures for form-wound windings Thermal evaluation and classification of

insulating systems used on machines up to and including 50 MVA and 15 kVIEC 60034-18-33 Test procedures for form-wound windings Multifactor functional evaluation; Endurance

under combined thermal and electrical stresses of insulating systems used in machines up to and including 50 MVA and 15 kV

IEC 60034-18-34 Test procedures for form-wound windings - Thermomechanical cycle endurance evaluationIEC 60085 Thermal evaluation and classification of electrical insulationIEC 60216 Guide for the determination of thermal endurance properties electrical insulating

materialsIEC 60371 Specification for insulating materials based on micaIEC 60270 High-voltage test techniques Partial discharge measurement, Version 2000, 3rd Edition IEC 60034-27 TS, Ed.1, Rotating electrical machines Part 27: Off-line partial discharge measurements

on the stator winding insulation of rotating electrical machines, Version 2006 IEEE 1434-2000 IEEE Trial-Use Guide to the Measurement of Partial Discharges in Rotating Machinery,

Version 2000

Seite 38 OMICRON

Faults & AgingBroken Solder Connection

Seite 39 OMICRON

Faults & AgingInsulation with Cavity

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Faults & AgingDischarge Propagation Path

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Faults & AgingLoose coils in the slot, semi conductive coating abrasion

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Faults & AgingDelamination within Mica-Insulation

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Faults & AgingDelamination within Mica-Insulation

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Faults & Aging40 years old Insulation

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Seite 45 OMICRON

Faults & AgingSemi conductive/ grading coating overlap failure

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Faults & AgingPartial Discharge at the End-Winding

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Faults & AgingPartial Discharge at the End-Winding

Sou

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VA

TEC

H, A

US

TRIA

Seite 48 OMICRON

Faults & Aging

Insulation faults are mainly caused by aging and overloads

thermal> overload> extreme high and low ambient temperatures> Cooling air deficiency / wrong installation height above sea level; Management

errors for large generators (iron and copper different expansion coefficients)

ambient (chemical)> Moisture + (water with nitrogen oxides under PD influence generates nitric acid)> Oxidation by aggressive environment

electrical> Overvoltages, transients or permanent stress

mechanically> pollution> misalignment> Vibrations / relaxations> Exposure by foreign object damage> Forces caused by short-circuits & faulty synchronization actions

Seite 49 OMICRON

Faults & Aging

Highest and lowest temperature Increase of Current through Overload

Rated Voltage Test Voltage Over Voltage

water, ice aggressive and reactive chemicals abrasive particles: metal parts, ash,

carbon, lubricants

Oscillation in slot section mechanical forces in enwinding section Different thermal expansion

T

E

A

M

Topics

Page 50

> Introduction

> Maintenance




Faults and Aging


Polarization Index


Partial discharge

Seite 51 OMICRON

Life Time expectation & Aging

Diagnosis and Monitoring can not avoid faults, but it helps to plan and minimize the cost

A suddenly popped up insulation damage is not repaired on average less than 6 to 8 weeks (open end) - if all goes perfect.

A well-planned repair of a machine takes 2-3 weeks till re-commissioning (depending on the size of the machine)

Seite 52 OMICRON

Measurement & monitoring parameters /quantities

> Visual inspection

> Insulation Resistance

> Polarization Index

> Partial Discharge

> Capacitance

> TanDelta / Power Factor Tip Up

Seite 53 OMICRON


Insulation Resistance / Polarization Index

Insulation Resistance (IR) and Polarization Index (PI) are two universally accepted diagnostic tests for insulation tests. These have been in use for more than 75 years.

The IR test measures the resistance of the electrical insulation between the copper conductors and the core of the stator or rotor. Ideally the value of this resistance is infinite since the purpose of the insulation is to block current flow between the copper and the core. But in practice, it is not possible.However, the resistance should have a high value to avoid any appreciable leakage current. Lower value of IR indicates that the insulation has deteriorated.

Seite 54 OMICRON



In insulation resistance test, a high DC voltage is applied across, conductor and ground. The voltage is applied across the insulator. Due to this applied high DC voltage there will be a current through the electrical insulator. Insulators are dielectric in nature (capacitance). Due to that, initially there will be a charging current. After some time when the insulator is totally charged, the capacitive changing current becomes zero and then only resistive conductive current presents in the insulator. That is why it is always recommended to do insulation resistance test at least for 1 minute as it is proved that charging current totally becomes zero after 1 minute.

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Seite 55 OMICRON



Effect of Temperature on IR

Unfortunately, just measuring IR has proved to be unreliable, since it is not tenable over time and strongly dependent on temperature. A 10C increase in temperature can reduce IR by 2 to 10 times. When readings of temperature and insulation resistance are plotted on ordinary equally divided co-ordination, a curved characteristics is obtained. On the other hand if graph paper is used on which the insulation scale is laid out in logarithmic division, the graph becomes a straight line. Further, the effect of temperature is different for each insulation material and type of contamination. Although some temperature correction graphs and formulae are given in the IEEE-43 and some other books, they are acknowledged as being unreliable for extrapolation by more than 10C. The result is that every time IR is measured at different temperatures, one gets a completely different IR. This makes it impossible to define a scientifically acceptable IR value over a wide range of temperatures

Seite 56 OMICRON



Effect of Temperature on IR

Unfortunately, just measuring IR has proved to be unreliable, since it is not tenable over time and strongly dependent on temperature. A 10C increase in temperature can reduce IR by 2 to 10 times. When readings of temperature and insulation resistance are plotted on ordinary equally divided co-ordination, a curved characteristics is obtained. On the other hand if graph paper is used on which the insulation scale is laid out in logarithmic division, the graph becomes a straight line. Further, the effect of temperature is different for each insulation material and type of contamination. Although some temperature correction graphs and formulae are given in the IEEE-43 and some other books, they are acknowledged as being unreliable for extrapolation by more than 10C. The result is that every time IR is measured at different temperatures, one gets a completely different IR. This makes it impossible to define a scientifically acceptable IR value over a wide range of temperatures

Seite 57 OMICRON



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PI is a variation of the IR test. It is the ratio of IR measured after voltage has been applied for 10 minutes (R10) to the IR measured after one minute (R1), i.e.

PI = R10 / R1

PI was developed to make interpretation of results less sensitive to temperature. PI is the ratio of two IR at two different times. Temperature of the winding does not rise during the test period of 10 minutes. So it is fairly assumed that both R10 and R1 are measured at same winding temperature. Then the temperature correction factor will be same for both cases and will be cancelled during the calculation of Pl. Thus PI is relatively insensitive to temperature.

Interpretation of Polarisation Index results

PI Condition of item under test

4.0 Excellent

Seite 59 OMICRON



U-VWE 60 s 25.9809 nA

Measurement Time Current

U-VWE 600s 3.7637 nA

PI = 25.98 / 3.76 = 6.9

Polarization and frequency

> Due to different polarization methods the dielectric constant (r) and the dissipation factor (tan()) are frequency dependent

thermal losses light absorption (optics)

polarization losses tan()

dielectric constant

frequencyHz kHz MHz GHz

nr

nmmm mm

electrical engineering

optics

wavelength

IR light UV x-rays

losses caused by reloading of partial capacitances

dipoles follow the field with delay

grid gets into resonance

atoms get excited

orientation polarization

grid polarization

atom polarization

r = n

1

0

refraction index n

boundary layer polarization

Conductivity

> Caused by current flowing through the insolation

> Mostly caused by moving electrons> In liquids also caused by ion movement> In most insolation materials the water

concentration determines the conductivity

> Surface currents should be suppressed or relatively small

> Causes losses at all measuring frequencies> Determines the losses at very low frequencies,

below 0.1Hz

+

insulation

current through

insolation

surface current

Partial discharge (PD) losses

> PD in the Insulation can cause additional losses

> PD act as a current though the insulation

> PD losses only occur above the inception voltage

> With a higher voltage PD losses are higher due to higher PD intensity

> A precise PD detection is not possible> Only high PD activity leads to a visible increase

of the tangent()> This mostly happens far above the inception

voltage> PD sensitive insulations can get seriously

damaged at this PD level (e.g. PE)> Use a PD measurement system for PD

detection (e.g. the Omicron MPD600)

insulationcavity

with PD

Vacuum capacity

> Capacity of a test object which has as dielectric vacuum

> For the most objects it is only an ideal concept

> Has no losses

> Dielectric constant is 1

> Gas insolation is close to the vacuum insolation

vacuum

+

Polarization, conductivity and vacuum capacity

> The electric representation of an insolation consists of:

> An ideal capacity for the vacuum field

> Different polarizations, here shown as one

> A electric conduction through the dielectric

> This can be modeled as an equivalent circuit

Image Source: Andreas Kchler, Hochspannungstechnik, Heidelberg, 2009

vacuum E-field

polarization conductivity

equivalent circuit

Equivalent circuit

> The equivalent circuit can be summarized to two elements:

> A capacitor

> A resistor

> This gives the equivalent circuit for the tangent() measurement

> As parallel circuit> A series circuit is also possible

Image source Top Image: Andreas Kchler, Hochspannungstechnik, Heidelberg, 2009

What is the dissipation/loss factor tangent()?

> When using the equivalent circuit diagram of the test object, the tangent() is the relation between the resistive current (IR) and the capacitive current (IC)

>

> But IR and IC are not directly measureable

IC IR

test object

Itest

What is the dissipation factor tangent()?

> In the complex plane the angle delta can be seen between the test object current (Itest) and the capacitive current (IC)

> Using sinusoidal currents this gives a phase difference between the test object current (Itest) and the capacitive current (IC)

ICIR

Itest

U

t

i(t)

capacitive current

test object current

= phase difference

Difficulties measuring tangent()

> A good ground connection is essential> Low inductance and resistance> Good connection to the grounding point> Star shaped grounding of all used devices

> Use short connection cables

> Very good insolation against stray currents needed> Test object isolation to the ground (best use PTFE or PE)> High voltage to test object (use guard electrode if possible)

> All contacts in the measuring circuit have to be as good as possible

Typical tangent() values at 50Hz for different insolation materials

Insolation material Typical tangent()(50Hz)*

Polyethylene (PE and XLPE) 8*10-5 - 4*10-4

PTFE / Teflon ~1*10-4

Polyvinyl chloride (PVC) ~2*10-2

Cast resin (filling and humidity dependent) 5*10-3 - 2*10-1

Oil impregnated paper (0,1%-10% humidity) 2*10-3 - 1Mineral oil (humidity dependent) 1*10-3 - 8*10-2

Silicone oil ~2*10-4

Mica Lower than 1*10-3

Glass ~1*10-4

Pressboard ~3*10-3* Source: Andreas Kchler - Hochspannungstechnik

Dielectric losses, conductivity and temperature

> In most insulation materials the conductivity increases with the temperature> This gives a higher dissipation factor> Under certain conditions this can lead to a thermal break down

> The losses due to polarization may sink with higher temperatures due to better dipole movement

> This gives a lower dissipation factor

> In combination this mostly gives a rising dissipation factor with rising temperature, but other results may also occur

Seite 71 OMICRON


TanDelta Dissipation Factor Power Factor Gtan

PF)Factor(Powercos

DF)Factoron(Dissipatitan

SPQP

M

G

Insulation R C

IR

IC

S Q

P

Icos

Seite 72 OMICRON


Dielectric Losses

Conductive Losses Polarization Losses

.....

Seite 73 OMICRON


Dielectric Losses

> Conductive losses> Movement of conductive particles

> carbon in oil> Movement of ions and electrons

> leakage current through the insulation or on the surface of a bushing> Partial discharge

> Polarization losses> Interfacial polarization> Polarization of dipoles in insulation material (rotation/suspension)

> Increased moisture causes more dipoles and hence more losses

Seite 74 OMICRON


tanDelta = f(f)

tan

f [Hz]

Sum

Polarization Losses

Conductive Losses

Seite 75 OMICRON


tanDelta = f(f)

Freq/Hz0.0001 0.010 0.10 1.0 10.0

0.005

0.010

0.020

0.050

0.100

0.200

0.500

1.000

2.000

10000.001

5.000DF

New Moderate Aged

0.12

0.0024

50H

z0.0036

Measuring Principle

25 mm

2.5 km

Reference path: tan G = 10-5

G

Seite 77 OMICRON


Capacitance and TanD

tan G =conductor (tan GC)polarization (tan GP)losses caused by PD (tan GPD)

tan G = tan GC + tan GP + tan GPD

tan G = f (U)

Tip-Up Test IEEE 286 Tan-Delta Test IEC 60894

Seite 78 OMICRON


Power factor / dissipation factor (tan delta) tip-up test at 50 Hz / 60 HzDetermination of stator winding condition typical curves for aged and new rotating machines

Pow

er fa

ctor

/ di

ssip

atio

nfa

ctor

(tan

del

ta)

Del

ta ta

n de

ltaV V

Aged rotating machine

New rotating machine

New rotating machine

Aged rotating machine

The tip-up test and the delta tan delta diagram show the same thing.The delta tan delta diagram is frequency independent.

Seite 79 OMICRON


Power factor / dissipation factor (tan delta) tip-up test with variable frequency

Power factor / dissipation factor (V) at 62 HzPower factor / dissipation factor (f) at 2 kV

Pow

er fa

ctor

/ di

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atio

n fa

ctor

(tan

del

ta)

f VPow

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ctor

/ di

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atio

n fa

ctor

(tan

del

ta)

What is partial discharge ?

> Partial discharge (PD) is a localized dielectric breakdown of a small portion of a solid or liquid electrical insulation system under high voltage stress.

> Definition from IEC 60270 Specification: Localized electrical discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor.

Important units

Charge Q [1Coulomb] = [1As]:

Charge in movement = CURRENT!!!

The more electrons per TIME interval, the higher is the current

Current = Charge per Time[A] = [Coulomb] per [s][A] = [Coulomb] / [s]

Charge = Current times TimeCharge = Current x Time

+

nucleus1st orbit

2nd orbit

electron-

-

-

q =

Charge How to measure?

> Time Domain Integration q =

q

SI prefixes

kilo Volts [kV]

milli Ampere [mA]micro Farad [F]nano Coulomb [nC]pico Coulomb [pC]

Most used prefixes:

Types of PD

> Internal PD> Void discharges, electrical treeing

> External PD> Corona> Surface discharges

Solid insulation

Outer semicon

Solid insulation

Inner semicon

Image Source: Andreas Kchler Hochspannungstechnik

External PD

Surface discharge Corona discharge

Internal PD

Internal discharge in laminated material

Treeing

Cavity / void discharge

PD classification

Semicon layer protrusion(stress concentration at the tip)

Void (field strength doubling)

Lines ofelectrical field

En = 2

Breakdown theory in solid materials

1. Intrinsic breakdown (pure electrical breakdown)

> emerges in insulations stressed temporarily with surge voltage

2. Thermal breakdown

> thermal instability of the insulation caused by high temperature

3. Partial discharge breakdown (erosion breakdown)> as a result of aging processes in electrical high stressed insulation areas > mostly caused by manufacturing

Main differences in comparison to breakdowns in air

> Failure is destroyed by the breakdown channel

> Breakdown in solid insulations are dependent of:

> purity of different solid materials (e.g. compounding of cables)

> technological influences during manufacturing and contacting

> surface and boundary issues

> temperature, humidity and pressure inside the solid

Thermal breakdown

> dielectric losses are caused by conductivity and polarization phenomena

> material heats up (conductivity raises exponential with temperature)

> higher losses

Electrical insulationsare at the same timethermal insulations

tan()

conductivity losses

polarization losses

Thermal breakdown schematic process

Dielectric losses cause a warming in the insulation

> highest temperature occurs in the center plane

> not enough heat can be conducted

> heating effects increase

> thermal hot spot

hot spot

Thermal breakdown schematic process

> conductivity rises in heated area

> increasing e-field at the channel ends

> growth of channel to the electrodes

> breakdown

Partial discharge breakdown

> PD occurs as a consequence of :

> local heating (creation of voids)

> abrasion

> delamination

> mechanical stress (vibration)

> defects in the material

> water treeing

> high electric field strength

Partial discharge breakdown (treeing)

> The thinner the insulation, the faster follows on a channel growing a breakdown

A larger thickness of the insulation

increases the treeing time

> The channel building time is much higher than the channel starting time.

PD sources in solid insulations

cracks cleaving voids cavity delamination of fibers

PD treeing

Acrylic glass Epoxy resin

PD treeing stopped

AC frequency influences the electrical treeing

Prof. Dr. Ing. Daniel Pepper, Dissertation, Berlin, 2004

Partial discharge breakdown (treeing)

> PD tree reacts as pushed forward potential

> PD treeing is not a steady process:

1. treeing in a channel ends because of high pressure due to products of decomposition

2. high conductivity delays the growth of trees

3. free space charges lower the electric field locally

Eg

Eg: global electric field

+ +

How to analyze PD

> Well known approaches:

> PRPD

> Trend

> Q(U)

> PSA (Pulse Sequence Analysis)

> TDR and STDR (PD localization)

> OMICRONs new approaches:

> Frequency based pulse discrimination (3CFRD/3FREQ)

> Cross-talk evaluation (3PARD)

Phase resolved partial discharge PRPD

PRPD correlation between PD pulses and voltage phasePD nature might be identified

Phase resolved partial discharge PRPD

Am

plitu

de

1 2 3 4...

5 50

Time (ms)

Am

plitu

de

Time

Trigger TriggerTriggerTriggerTriggerTrigger

> How is a PRPD created?

1020

Further ways of PD analysis Trend

Charge vs. time

Applied voltage vs. time PD repetition rate vs. applied voltage

PD repetition rate vs. time

Further ways of PD analysis Q(U)

> Charge over Test Voltage

Further ways of PD analysis 3CFRD (3 Center Frequency Ratio Diagram)> Pulse Shape Analysis: 3CFRD or Time/Frequency map

T/W map

Influence of inverse gating on external disturbances

External disturbance PD Noise

Further ways of PD analysis 3PARD> Analysis of synchronous signal cross talk by using the

3 Phase Amplitude Ratio Diagram

500pC

850pC

900pC

Further ways of PD analysis 3PARD> Analysis of signal cross talk

Left Phase

Center Phase

Right Phase

500pC

850pC

900pC

Further ways of PD analysis 3PARD

Noise PD source 1 PD source 2

Further ways of PD analysis PD location> TDR: Time Domain

Reflectometry

> LOC: Statistical TDR

> Dual End:

Measure 2 PD pulse components directly without using reflections

Corona discharges on high voltage potential

Corona discharges on ground potential

Corona discharges on ground potential with multiple reflections (HV cable)

origin

reflection

Corona during online measurement

Ut(t)

tt

U1(t)

Partial discharge mechanism Void

Void discharges in solid dielectric material

some single cavities

Void discharges in solid dielectric material

Bigger cavities

Some single voids

Void discharges in XLPE cable Early stage (1/3)

Void discharges in XLPE cable Developing fault (2/3)

Void discharges in XLPE cable Developing fault (3/3)

Cable PD Defective outer semicon layer

Bubble in oil

Pattern is disappearing and re-appearing from time to time.

Spike on ground potential in oil

Surface discharges

Surface discharges Significant differences in amplitude

Contact PD

When the bad contact becomes better, the contact PD can disappear.

Floating potential

Delamination on outer semicon layer

Hopping particle

Hopping particles

Bushing (RBP) Early stages (88kV)

Bushing (RBP) Early stages (126kV)

Bushing (RBP) 156kV

Voltage transformer Inner PD

PD patterns and classification

Source: J. Fuhr, Procedure for Identification and Localization of PD, IEEE Transactions 2005

PD patterns and classification

CIGRE WG 21.03

Recognition of Discharges

Inception / extinction voltage

U/Umax

noise level

PDEV PDIV

Specified threshold for PDIV and PDEV

PD magnitude as a function of the normalized test voltage Qm = f (U/Umax)

Full bandwidth

oscilloscope signal time domain o

m bar indicator frequency domain

Low-pass

High-pass

Band-pass

> key element of the PD measuring system

Time domain vs frequency domain power quality

Time domain vs frequency domain


> easier to display

50 Hz: 1 V500 Hz: 0.1 V

Time Domain vs Frequency Domain

50 Hz: 1 V150 Hz: 0.3 V


50 Hz: 1 V150 Hz: 0.3 V250 Hz: 0.1 V350 Hz: 0.03 V450 Hz: 0.01 V


> the dirac pulse consists of all thinkable frequency components N x 50 Hz: 1V

The DIRAC pulse

The area below the curve is 1.

time (t) frequency (f)

A (f) = 1, for all f

amplitude (A)

Time domain vs frequency domain: i(t) and A(f) and charge

t

i

fA

area = q = 10pC

cut-off frequency

area = q = 10pC

area = q = 10pC

area = q = 10pC

all spectral curves show the same amplitude

charge = q = 10pC

Time domain vs frequency domain: i(t) and A(f) and charge

frequency f = 0Hz

The spectral amplitude @ 0Hz (f< fcut-off)

represents the impulse charge q!

f

A

charge = q = 10pC

PD pulse: time domain vs frequency domain

ideal PD-impulse:

a) time domain display of current i(t) T1 time to the maximum of the current (imax)T2 time to the declining half of imax

b) normalized amplitude density F()/F(0)1) T1/T2 = 1 s/5 s2) T1/T2 = 5 ns/50 ns3) T1/T2 = 5 ns/15 ns

a)

T1

i (t)

imax

imax/2

T2t

b)

0103 104 105 106 107 109Hz

0,5

1,5

F()

F(0)

1

2

3

Partial discharge breakdown

> PD occurs as a consequence of :

> local heating (creation of voids)

> abrasion

> delamination

> mechanical stress (vibration)

> defects in the material

> water treeing

> high electric field strength

This one...

Introduction

> ...For many years, the measurement of PD has been employed as a sensitive means of assessing the quality ... and localize sources of PD in used electrical winding insulation ...

> ... Compared with other dielectric tests (i.e. the measurement of dissipation factor or insulation resistance) the differentiating character of partial discharge measurements allows localized weak points of the insulation system to be identified...

Introduction

> ...The PD testing ... is also used when inspecting the quality of new assembled ... stator windings, new ... components (e.g. form-wound coils and bars, HV bushings, etc..) and fully impregnated stators...

> ...PD can provide information on> points of weakness in the insulation system> ageing processes> further measures and intervals between overhauls

Introduction

> Partial discharge testing of stator windings can be divided into two broad groups:

a. Off-line measurements, in which the stator winding is isolated from the power system and a separate power supply is employed to energize the winding;

b. On-line measurements, in which the rotating machine is operating normally and connected to the power system.

> Both of these approaches have advantages and disadvantages...

Annex Aand

IEC 60034-27-2

Introduction LIMITATIONS

> ... different types of PD measuring instruments will inevitably produce different results ...

> ... PD measurements will only be comparable under certain conditions ...

> ... absolute limits for the windings of rotating machines, for example as acceptance criteria for production or operation, are difficult to define ...

> Pulse propagation phenomena> Difficulties with calibration> Individual frequency response characteristics> Type of PD source> Location within the stator

Introduction LIMITATIONS

> ... Empirical limits verified in practice can be used as a basis for evaluating test results ...

> ... PD trend evaluation and comparisons with machines of similar design and similar insulation system measured under similar conditions, using the same measurement equipment

> ... not all insulation-related problems in stator windings can be detectedby measuring PD (e.g. insulation failures involving continuous leakage currents due to conductive paths between different elements of the insulation or pulseless discharge phenomena) ...

1 Scope

> Measuring techniques and instruments,

> The arrangement of test circuits,

> Normalization and testing procedures,

> Noise reduction,

> The documentation of test results,

> The interpretation of test results

> Test with AC ... bars or form wound coils with (without) conductive slot coating...

> Voltage rating >= 6kV

2 Normative references

> IEC 60060-1, High-voltage test techniques Part 1: General definitions and test requirements

> IEC 60060-2, High-voltage test techniques Part 2: Measuring systems

> IEC 60270:2000, Partial discharge measurements

3 Definitions

> ... general terms and definitions for partial discharge measurements given in IEC 60270 apply...

> Offline measurement > Measurement taken with the rotating machine at standstill, the machine being

disconnected from the power system > Test voltage is ... from a separate voltage source.

> Online measurement > Measurement taken with the rotating machine in normal operation

3 Definitions (Coating)

> Stress control coating > Paint or tape on the surface of the groundwall insulation that extends beyond

the conductive slot portion coating in high-voltage stator bars and coils

> The stress control coating reduces the electric field stress along the winding overhang to below a critical value that would initiate PD on the surface. The stress control coating overlaps the conductive slot portion coating to provide electrical contact between them.

> Conductive slot coating > Conductive paint or tape layer in intimate contact with the groundwall insulation

in the slot portion of the coil side, often called semiconductive coating

> This coating provides good electrical contact to the stator core.

3 Definitions (PD types)

> Slot discharges > Discharges that occur between the outer surface of the slot

portion of a coil or bar and the grounded core laminations

> Internal discharges > Discharges that occur within the insulation system

> Surface discharges > Discharges that occur on the surface of the insulation or on

the surface of winding components in the winding overhang or the active part of the machine winding

3 Definitions (Evaluation)

> Pulse height distribution > The number of pulses within a series of equally-spaced windows of pulse

magnitude during a predefined measuring time

> Pulse phase distribution > The number of pulses within a series of equally-spaced windows of phase

during a predefined measuring time

> Partial discharge pattern > PD distribution map of PD magnitude vs a.c. cycle phase position, for

visualization of the PD behaviour during a predefined measuring time, in which specific PD parameters are used for graphical representation

3 Definitions (Evaluation)

> Pulse height distribution

> Pulse phase distribution

> Partial discharge pattern

3 Definitions (Hardware)

> Coupling device > Usually an active or passive four-terminal network that

converts the input currents to output voltage signals

> These signals are transmitted to the measuring instrument by a transmission system. The frequency response of the coupling device is normally chosen at least so as to efficiently prevent the test voltage frequency and its harmonics from reaching the measuring instrument.

> PD coupling unit > A high voltage coupling capacitor

of low inductance design and a low voltage coupling device in series

3 Definitions (Numeric measures)

> Largest repeatedly occurring PD magnitude Qm > The largest magnitude recorded by a measuring system which has the pulse

train response in accordance with 4.3.3 of IEC 60270, or the magnitude associated with a PD pulse repetition rate of 10 pulses per second (pps), which can be directly inferred from a pulse height distribution.

3 Definitions (Numeric measures)

> Normalized quantity number NQN > Normalized area under a straight line fitted to the pulse counts in each

magnitude window of a pulse height analysis, in which the pulse counts are expressed as a logarithm of the pulses per second and the pulse magnitude window is a linear scale.

4 Nature of PD in rotating machines4.1 Basics of PD> ... where the dielectric properties of insulating materials are inhomogeneous ...

> ... local electrical over-stressing ...

> ... local, partial breakdown ...

> ... PD in general requires a gas volume to develop ...> Gas filled voids embedded in the insulation,> Voids adjacent to conductors> At insulation interfaces

> ... numerous PD pulses during one cycle of the applied voltage ...

> ... Stator winding insulation ... for HV machines will normally have some PD .., but are inherently resistant to partial discharges due to their inorganic mica components...

> ... PD in these machines is usually more a symptom of insulation deficiencies ...

> .. PD ... may also directly attack the insulation and thus influence the ageing process...

> ... The time to failure may not correlate with PD levels ...

4 Nature of PD in rotating machines

4.2 Types of PD> ... For a given machine, the various PD sources may be identified and distinguished

in many cases by their characteristic PD behaviour...

> Internal discharges> Internal voids> Internal delamination> Delamination between conductors and insulation> Slot discharges> End-winding surface discharges> Conductive particles

A - Small voids on edgeB - Delamination: conductor main insulationC - Delamination of tape layersD - Treeing in layersE - Broken strandsF - Micro voidsG - Slot discharge, semicon paint abrasionH - Discharges in Cooling DuctI - Delamination of Insulation in ElbowJ - End winding surface discharge - contaminationK - Insufficient Spacing, Tracking and SparkingL - Connection area between slot corona protection and end winding corona protection


4.3 Pulse propagation in windings > ... PD current ... is a transient pulse with a rise time of only a few

nanoseconds ...

> ... high frequency spectrum ...

> ... stator windings represent objects with distributed elements in which travelling wave, complex capacitive and inductive coupling, and resonance phenomena occur ...

> ... attenuation, distortion, reflection and cross-coupling ...

> ... transmission function from the PD source to ... sensor is unknown and depends on the specific design of the machine ... Therefore, the energy at the source of the PD, which can be taken as a measure of the erosion of the insulation, cannot be measured directly ...


PD pulse propagation

Two frequency components of the PD signals in a stator winding

Damping, reflection, attenuation

Amplitude of the pulse depends on:

> Calibration

> Ratio CK / CA

> Center frequency

> Propagation path


4.3 Pulse propagation in windings > ... the individual high frequency

transmission behaviour of a stator winding produces PD signals at the terminals that are a characteristicof the machine being tested and of the location of the PD source ...

> ... very high frequency components of PD signals are subject to ...attenuation ... and ... might not be detectable at the terminals of the test object ...

5 Measuring techniques and instruments

5.2 Influence of frequency response of measurement system> ... measurement in the lower frequency range ensures good sensitivity

not only for partial discharges in bars/coils close to the sensor but also for those that originate from further away in the winding. However, the lower frequency range is more subjected to noise and disturbances ...

> ... measurement in the very high frequency range may acquire only a very small proportion of the total PD energy, which results in sensitivity to signals originating only very close to the sensor. However, this frequency range may be less susceptible to noise and disturbance. ...

> ... For off-line PD testing to obtain appropriate sensitivity to PD from the whole winding it is advisable to use wide band PD measuring systems. The lower cut-off frequency should be in the range of several tens of kHz in accordance with IEC 60270. ...


5.3 Effects of PD coupling units> ... HV capacitor, coupling device, transmission system and input

impedance of the measurement system represent a high-pass filter. Therefore, increased input impedance or higher capacitance values lead to an increased sensitivity...


5.4 Wide-band and narrow band measuring systems

Superposition errors!

f >100kHz ... 1MHz

6 Visualization of measurements

6.2 Data presentation> PD magnitude [pC or mV]

> r.m.s. value of test voltage

> Inception voltage Ui (PDIV)

> Extinction voltage Ue (PDEV)

> Curve Qm = f(U)

U/Umax

Noise level

PDEV PDIV

Specified threshold for PDIV and PDEV


6.3 Additional means of PD data representation

> ... Additional quantities ... like integrated charge, discharge current, quadratic rate, PD power, and PD energy ...

> ... pulse height distribution, pulse phase distribution, phase resolved pulse height distribution, oscillograms of pulse trains, PD distribution maps, etc.... CIGRE technical brochure 226 ...

Part V: Generator Stator Insulation


> 6.3.2. PRPD pattern> ... a 2-dimensional PD distribution map (-q-n pattern) is employed for

visualization ...

7 Test circuits

> HV power supply conforming to IEC 60060-1 and IEC 60060-2; > A voltage measuring device; > A suitable PD coupling unit; > A connection cable from the measuring impedance to the PD device with

sufficiently low damping characteristics and good shielding; > A partial-discharge measuring system; > High-voltage connections.

> ... sufficiently PD free...

Example: IEC 60060-1

IEC 60060-2: Measuring systems

7 Test circuits

> ... sufficiently PD free...

> ... To ensure that the test circuit does not influence the measurement of partial discharges from the test object, the arrangement should first be tested up to the maximum test voltage in accordance with the test procedure given in 9.1.6. The noise level produced by the complete test circuit at maximum required test voltage shall not exceed 100 pC when using the normalization procedure in accordance with Clause 8...

> ... The whole test circuit should be of a low-inductance arrangement. It is essential that ground loops are avoided. Low inductance leads are recommended as ground connections...

7 Test circuits

MI

Z

CC

CD

Ck

Ca

Zmi

U~

7 Test circuits Complete winding

> ... HV source and the PD coupling unit should be connected to oppositewinding terminals whenever possible, to utilize the advantage of the damping effect of the winding phases to suppress conducted interference from the power supply...

> ... The PD coupling unit should be installed as close to the winding terminals as possible...

> ... The stator core should normally be grounded...


> Single winding energized, also measuring at the grounded windings


Single winding energized, also measuring at the grounded windings (ground connection at far end).

PD measurement on stator winding

Online measurement with permanent couplers


Phases not to be separated

Double length as filter

Phases not to be separatedPhases not to be separated

8 Normalization of measurements

> ... Due to pulse propagation, resonance and mutual cross coupling in machine windings, mentioned in 4.3, calibration is not possible...

> ... The aim of normalization is to ratio out various influences of the test circuit, for example power supply connections, stray capacitance, coupling capacitance and test object capacitance...

> ... Normalization is to ensure that the PD measuring system provides sufficient sensitivity to measure a specified value of PD magnitude correctly, as it appears at the machine terminals during the measurement, and to show that the PD detection system used, is responding in a repeatable fashion...

> ... comparisons between measurements on objects having the same design, taken with the same PD device...

> ... Normalization of the test circuit should be performed by injecting short-duration current pulses of known magnitude by means of a reference pulse generator (calibrator) ...

9 Test procedures

> Tests on complete windings

> Individual phases

> Individual winding components

> DUT disconnected from all external power supplies> Bus work> Surge arrestors> Surge capacitors> Excitation systems

> Test lead contact should always be at the machine terminals

> Same arrangement of all circuit components

> Same normalization procedure

9 Test procedures Equipment

> ... The waveform of the applied voltage should have Upp/Urms = 2 SQR(2) , 5% ...

> ... it is acceptable to perform PD tests at lower frequencies ... or at higher frequencies, 0.1Hz ... 400Hz

> ... PD results obtained from very-low frequency tests might significantly differ from that at power frequency and thus direct comparison might not be possible ...

9 Test procedures Preparation

> ... stator should be inspected for cleanliness ...

> ... sufficient air clearance between ...connections and ... cables ...

> ... check the insulation resistance before starting the PD test ... >100M ... [Mainly dirt, as dirt may influence PD behaviour]

> ... conductive slot coating should be in contact with ground potential throughout its whole length ... [for winding components]

> CONDITIONING!> ... PD will typically decrease during the first minutes ...> ... 5 min at the maximum test voltage is recommended ...> ... voltage may then be re-applied to start the partial discharge measurements

...

9 Test procedures Conditioning

Paschens Law

9 Test procedures Test voltages

> ... steps (e.g. U = 0,2 Umax) or ... continuous ramping (< 1 kV/s) up to ... Umax. ...

> ... dwell time on each step of at least 10s is recommended ...

9.1.6 PD test procedure

> ... level of background noise...should be obtained to ensure that the test arrangement has sufficiently low noise and PD up to the maximum testvoltage...

> ... replacing the test object by an appropriate discharge free capacitor ...

> ... Disturbances ... can be reduced or ... eliminated by appropriate measures ...

> ... localize such disturbance signals ...

> ... temperature detectors (RTDs) are anchored to the grounded stator frame ...

> ... the same connection point for the PD coupler, the test object and the measuring equipment should be used ...

> ... test arrangement as compact as possible ...

9.1.6.3 PD testing

> ... each voltage step, or during continuous ramping, the PD data ... should be recorded and processed ...

> ... Qm = f(U) curve ...

> PDIV

> PDEV

> ... distribution of pulse magnitudes, phase resolved distributions or specific PD patterns ... at various levels during test voltage increase and decrease ...

> ... Any comments or observations during the test should be recorded ...

> ... an appropriate sequence of standard measurements and ... extended measurements ... should be taken ...

9.2 PD location and ID

> ... Following the per-phase test... energize simultaneously all three phases of the winding ... [ No PD from end winding]

> ... subdivide the winding ...

> ... Electromagnetic probes ...> scanning of stator slots > machine end-winding> conduit boxes> cable routing> termination boards> stand-off insulators, etc.

> ... probes can disrupt the electric field ...

> ... additional safety requirements ...

10 Interpretation

> ... it has to be decided whether there are ... defects and ... what they imply regarding the performance of the insulation system, whether any supplementary tests are needed and the planning and/or implementation of any essential corrective maintenance. ...

> ... significant variations in the amounts of partial discharges depending on the individual properties of the machine being tested. As a result, a directcomparison of different types of machine in terms of absolute values is notpossible. ...

> ... Neither is it possible to establish any absolute limits for complete windings ...

10 Interpretation

> ... the PD site, which produces the highest PD magnitude is not necessarily the location in the winding being at most risk ...

> ... PD magnitude as a function of test voltage provides a simple and effective means of characterizing typical dominating PD sources ...

> ... Interpretation is always comparative ...

> ... meaningful interpretation for complete windings ...> Trending Qm on the same stator over time> Comparing Qm from several stators with the same design> Comparing Qm between different phases of one stator

> ... the higher the PDIV and PDEV ... the less insulation deficiencies ...

10 Interpretation

Trend in PD in a machine over time

> ... Initial fingerprint ... when the winding is new ...

> ... If the winding deteriorates ... Qm will usually increase over time ...

> ... Compare the PD quantities ... between windings ...

> Limitations> ... a new stator may have relatively high PD that decreases after the first 5000

to 10000 equivalent operating hours ...> ... same voltage, temperature and similar humidity conditions ...> ... variations of Qm of a certain percentage, for example 25 %, are normal

...

> Why?: ... That is, for example, Qm for the winding at the specified test voltage is lower than 95 % of the mean Qm magnitudes achieved by the manufacturer on the same windings they have made in the past....

10.3 Pattern recognition

> ... When using the -q-n patterns, it may be possible to separate various PD sources from each other, to assess the related risk and to trend them separately ...

> ... weight their risk separately ...

> Observe the trend behaviour of each PD source

> Localize the various PD phenomena

> Provide rough information concerning location for pinpointing

> Assess the insulation condition, depending on PD source and PD location.

3CFRD

Disturbance PD Noise

PRPD pattern shows multiple sources

L2

L1 L3

Separation of sources by 3PARD

3PARD = Three Phase Amplitude Relation Diagram

Inner PD source in L1 in 3PARD

Inner PD Source L1>L2>L3

L1

L2 L1

L2

L3

3PARD

timeframe 1 s

Inner PD Source in L1

L3

Outer noise in 3PARD

L2

L3

3PARD

Outer Noise

L1

L1

L2

L3

timeframe 1 s

Outer Noise L1 L2 L3

213

PD measurement on Stator Winding and calibrationLocalization: difficult

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March 5, 2015

Field diagnostics and diagnostic monitoring of HV insulation at generators, motors, transformers and cables (part II)

Topics

Page 2

> Transformer


Faults and Aging


> HV Cables


Faults and Aging


Seite 3 OMICRON

Field Diagnostics & diagnostic monitoring of transformers

Seite 4 OMICRON


When a Transformer is operated under ANSI / IEEE basic loading conditions (ANSI C57.96), its normal life expectancy is about 20 years. The ANSI / IEEE basic loading conditions for Transformer are:

i. The Transformer is continuously loaded at rated kVA (kilo Volt Ampere) and rated Voltages (Transformer must be operated at the rated Voltage and kVA)

ii. The average temperature of the ambient air during any 24-hour period is equal to 30C (86 F) and at no time exceeds 40C (104 F).

iii. The height where the transformer is installed, does not above 3300 feet or 1000 meters

Seite 5 OMICRON

Transformer Breakdowns

Seite 6 OMICRON

Faults in Transformers

Win

ding

Cor

e Bus

hing

Vess

el &

Oil

Acc

esso

ries

Tap

Cha

nger

Source: Cigre_WG 12-05 An international survey on failures in large power transformers in service, Electra No. 88, 1983

Condition and Time based maintenance

Time

Insu

latio

n st

reng

th New condition

Operating stress

Time based maintenance

Diagnostic levelCondition based maintenance

Condition based maintenance requires reliable diagnostic tools

Seite 8 OMICRON

Possible Impacts by lack of maintenance

Life of the Transformer

Baseline data not recorded Problems during warranty period not detected OLTC contacts wear Oil oxidation begins Fan and Pump bearing wear Visible effects of weathering and UV Oil decay products affect paper insulation Miss opportunity to intercept accelerated ageing Oxidation and hydrolysis enters accelerated ageing state OLTC and bushing failure rates increase Dielectric withstand diminishes Expensive failure (bushing, OLTC)

Seite 9 OMICRON

Overview: Transformer Measurements

Maintenance Intervals

Action Task Interval Remark

Light Regular IntensiveVisit 6 m 1 m 1 d In service

Detailed inspection (visual)

1 y 3 m 2 w In service

DGA 2 y 1 y 3 m Task interval may differ with monitoring

Oil Test 6 y 2 y 1 y

Cooling system cleaning

Conditional Conditional Any interval Outage may be required

Accessories verification

12 y or conditional

6-8 y 1-2 y Outage required

Electrical basic tests Conditional Conditional Any interval Outage required

Insulation tests (DF) Conditional 6-8 y 2-4 y Outage required

OLTC inspection 12 y 6-8 y 4 y

Seite 11 OMICRON

Overview: Transformer MeasurementsType of ProblemMagnetic Circuit Integrity

Magnetic Circuit Insulation

Winding Geometry

Winding/Bushing/OLTC Continuity

Winding/Bushing Insulation

Winding Turn to Turn Insulation

Diagnostic Technique

Bas

ic E

lect

rical

Winding Ratio x

Winding Resistance x

Magnetisation Current x x

Capacitance and DF/PF x x x x

Leakage Reactance x

Insulation Resistance x x

Core Ground Test x

Adv

ance

d El

ectr

ical

Frequency Response of Stray Losses x x

Frequency Response Analysis x x x x

Polarisation/Depolarisation x

Frequency Domain Spectroscopy x

Recovery Voltage Method x

Electrical Detection of PD x x

Acoustical detection of PD x x

UHF Detection of PD x x

Dissolved Gas Analysis x x x x x

Assessment and Interpretation

Indication RBP OIP RIP

increase of capacitance oil in cracksor partial breakdowns partial breakdowns partial breakdowns

high dissipation factor

partial breakdowns;insulator surface wet or dirty (clean the insulator);ageing of the inner insulation;water in the inner insulation;



dissipation factor is decreasing withincreasing voltage

bad potential connections;partial breakdowns



dissipation factor isstrongly increasing with increasing temperature

high moisture in the insulation;high degree of ageing



partial discharges normal, if constant

Discharges produce gasses;Errosion of the cellulose;production of x-wax

partial breakdowns;cracks or voids after electrical or mechanical stress;

Seite 13 OMICRON

Overview: Transformer Measurements

> Turns Ratio

> Exciting Current

> Winding Resistance (Dynamic and Static)

> Short Circuit Impedance (+FRSL)

> C & DF (Winding, Bushing and Oil sample)

> Moisture Determination in liquid and solid insulation

> Frequency Response Analysis Partial Discharge

0nF

5nF

10nF

15nF

20nF

25nF

30nF

0V

2000V

4000V

6000V

8000V

1000

0V

1200

0V

1400

0V

H(V) HL(V) L(V)

0.2%

0.25%

0.3%

0.35%

0.4%

0.45%

0.5%

0V

2000V

4000V

6000V

8000V

1000

0V

1200

0V

1400

0V

H(V) HL(V) L(V)

OMICRON

Tan delta(voltage sweep)

Capacitance (voltage sweep)

Cap

acita

nce

V

Pow

er fa

ctor

/ di

ssip

atio

n fa

ctor

(tan

del

ta)

V

Transformer power and dissipation factor (tan delta) + insulation capacitancePower transformer diagnosis

Automated testing with test templates and reporting with ExcelTM

0%

0.1%

0.2%

0.3%

0.4%

0.5%

0.6%

0.7%

0Hz

50H

z

100

Hz

150

Hz

200

Hz

250

Hz

300

Hz

350

Hz

400

Hz

450

Hz

H(f) HL(f) L(f)

OMICRON

0%

0.1%

0.2%

0.3%

0.4%

0.5%

0.6%

0Hz

50H

z

100

Hz

150

Hz

200

Hz

250

Hz

300

Hz

350

Hz

400

Hz

450

Hzff

New transformer Transformer with aged oil

Pow

er fa

ctor

/ di

ssip

atio

nfa

ctor

(tan

del

ta)

Pow

er fa

ctor

/ di

ssip

atio

nfa

ctor

(tan

del

ta)

Increased power factor / dissipation factor especially

at low frequencies

H(f) HL(f) L(f)

Transformer power and dissipation factor (tan delta)with variable frequency from 15 Hz to 400 Hz

Advanced diagnostics

Additional diagnostics revealed through frequency sweep from 15 Hz to 400 Hz.

OMICRON

Transformer power and dissipation factor (tan delta)with variable frequency from 15 Hz to 400 Hz


Advantages

> Faster and more reliable assessment of transformer and bushing conditions (aging)

> Shows additional details not obtainable from single power factor measurements:

> Determines whether moisture contamination is in the cellulose or if the oil is contaminated or otherwise compromised.

> Effective discrimination between a deteriorated power factor test result that warrants more frequent monitoring and one that requires immediate remediation.

> Faster fault localization

0.24%0.25%0.26%0.27%0.28%0.29%

0.3%0.31%0.32%

0 V

2000

V

4000

V

6000

V

8000

V

1000

0 V

1200

0 V

1400

0 V

A B C

0.5%

0.6%

0.7%

0.8%

0.9%

1%

1.1%

0V

2000V

4000V

6000V

8000V

1000

0V

1200

0V

1400

0V

A B C

New bushing Bad bushing

Pow

er fa

ctor

/ di

ssip

atio

nfa

ctor

(tan

del

ta)

V

OMICRON

Pow

er fa

ctor

/ di

ssip

atio

nfa

ctor

(tan

del

ta)

V

Inmost layer was not connected to the HV conductor properly.

Bushing power and dissipation factor (tan delta) + insulation capacitancePower transformer diagnosis

0.25%

0.35%

0.45%

0.55%

0.65%

0.75%

0.85%

0Hz 100Hz 200Hz 300Hz 400Hz 500Hz

f

Pow

er fa

ctor

/ di

ssip

atio

nfa

ctor

(tan

del

ta) only fNOM values

Aged bushing

Medium-aged bushing

New bushing

OMICRON

Condenser type bushing (220 kV RIP)

Bushing power and dissipation factor (tan delta)with variable frequency from 15 Hz to 400 Hz


Additional diagnostics revealed through frequency sweep from 15 Hz to 400 Hz.

OMICRON

0.4 %

0.45 %

0.5 %

0.55 %

0.6 %

0 H

z

50 H

z

100

Hz

150

Hz

200

Hz

250

Hz

300

Hz

350

Hz

400

Hz

450

Hz

1 2 3 4 5 6

f

Pow

er fa

ctor

/ di

ssip

atio

nfa

ctor

(tan

del

ta)

Oil-impregnated bushings (OIP)

Bushing power and dissipation factor (tan delta)with variable frequency from 15 Hz to 400 Hz


Looking at the whole frequency range shows that four bushings have aged considerably. At lower frequencies there is an

extreme increase in the dissipation factor (tan delta).

OMICRON

Insulating fluids (oil test cell) power and dissipation factor (tan delta)Power transformer diagnosis

Testing the insulation condition of oil

Exciting current per tap changer position

0.001 A

0.0015 A

0.002 A

0.0025 A

0.003 A

0.0035 A

0.004 A

0.0045 A

0 5 10 15 20 25 30

A B C

OMICRON

Tap changer position

Exci

ting

curr

ent

Exciting current and phase angle per tap

Exciting current per tapPower transformer diagnosis

OMICRON

OLTCPosition

DETCPosition

Phase AI out [mA]

WattLoss [W]

Reactance(+/-jX)[k]

Phase BI out[mA]

WattLoss[W]

Reactance(+/-jX)[k]

Phase CI out [mA]

WattLoss [W]

Reactance(+/-jX)[k]

PatternRating

1 0 61,56 573,22 67,62 42,81 396,23 88,42 59,00 540,12 67,43 NONE

2 0 62,62 569,87 66,09 44,31 410,38 86,00 61,12 559,15 66,15 NONE

3 0 64,79 588,69 64,45 45,79 422,97 83,65 63,23 577,36 64,53 NONE

4 0 67,05 608,41 62,72 47,44 438,60 81,21 65,46 596,85 62,81 NONE

5 0 70,51 625,79 60,10 49,08 452,48 78,84 67,80 617,27 61,04 NONE

6 0 71,89 651,07 59,04 50,92 469,99 76,44 70,24 638,84 59,23 NONE

7 0 74,44 673,86 57,11 52,74 486,24 73,79 72,79 661,55 57,33 NONE

8 0 77,18 698,20 55,26 54,69 503,53 71,31 75,56 685,89 55,56 NONE

9 0 80,07 724,07 53,37 56,78 522,56 68,86 78,47 711,65 53,72 NONE

10 0 82,97 750,83 51,35 58,97 542,90 66,22 81,55 739,10 51,88 NONE

11 0 86,33 780,10 49,64 61,28 564,43 63,40 84,81 768,08 50,05 NONE

0 mA

20 mA

40 mA

60 mA

80 mA

100 mA

1 2 3 4 5 6 7 8 9 10 11

A

B

C


Exci

ting

curr

ent

0 W

200 W

400 W

600 W

800 W

1000 W

1 2 3 4 5 6 7 8 9 10 11

A

B

C


Wat

t los

ses

Measurement and report table

Exciting current per tapPower transformer diagnosis

OMICRON

12

13

14

15

16

17

000 005 010 015 020

A

B

C

Rat

io

Tap

Settings: transformer ratio per tap

Transformer turns ratio (TTR) per tapPower transformer diagnosis

-0.08 %

-0.06 %

-0.04 %

-0.02 %

0.00 %

0.02 %

0.04 %

0 5 10 15 20

A

B

C

0.002 A

0.004 A

0.006 A

0.008 A

0.01 A

0 5 10 15 20

ABC

TRRatio: A

Date/Time: 02.05.2008 13:22

Overload: NO

Assessment: n/a

Range: AC 2kV

Nominal values:

Frequency: 50,0Hz

V test: 1000,0V

Tap V prim. Nom.Vsec.nom.

Ratio nom. V prim. Vsec. Ratio I prim.

001 449610,0/3V 15750,0V 16,4814:1 999,59V 0,0 60,636585V 0,0 16,4849:1 -0,02% 0,004566A -40,71

002 444890,0/3V 15750,0V 16,3084:1 999,74V 0,0 61,285938V 0,01 16,3127:1 -0,03% 0,004625A -41,18

003 440170,0/3V 15750,0V 16,1354:1 999,79V 0,0 61,947819V 0,0 16,1392:1 -0,02% 0,004721A -41,48

004 435440,0/3V 15750,0V 15,962:1 999,85V 0,0 62,622185V 0,0 15,9664:1 -0,03% 0,004791A -41,86

005 430720,0/3V 15750,0V 15,789:1 999,83V 0,0 63,309776V -0,01 15,7927:1 -0,02% 0,004902A -42,1

006 426000,0/3V 15750,0V 15,616:1 999,81V 0,0 64,010597V -0,01 15,6194:1 -0,02% 0,005008A -42,35

007 421280,0/3V 15750,0V 15,4429:1 999,78V 0,0 64,727524V -0,01 15,446:1 -0,02% 0,005097A -42,77

008 416560,0/3V 15750,0V 15,2699:1 999,89V 0,0 65,464005V -0,01 15,2739:1 -0,03% 0,005211A -42,96

009 411830,0/3V 15750,0V 15,0965:1 999,6V 0,0 66,197601V -0,01 15,1002:1 -0,02% 0,005334A -42,71

010 407110,0/3V 15750,0V 14,9235:1 999,78V 0,0 66,981621V 0,01 14,9262:1 -0,02% 0,005422A -43,33

011 402390,0/3V 15750,0V 14,7505:1 999,73V 0,0 67,760643V 0,0 14,7538:1 -0,02% 0,00568A -45,03

012 397670,0/3V 15750,0V 14,5775:1 999,83V 0,0 68,570297V 0,0 14,5811:1 -0,02% 0,005816A -45,37

013 392950,0/3V 15750,0V 14,4044:1 999,78V 0,0 69,396141V -0,02 14,4069:1 -0,02% 0,005939A -45,7

014 388230,0/3V 15750,0V 14,2314:1 999,74V 0,0 70,235367V 0,0 14,2341:1 -0,02% 0,006073A -45,83

015 383500,0/3V 15750,0V 14,058:1 999,79V 0,0 71,101517V 0,0 14,0614:1 -0,02% 0,006229A -46,1

016 378780,0/3V 15750,0V 13,885:1 999,72V 0,0 71,986328V 0,0 13,8876:1 -0,02% 0,006361A -46,33

017 374060,0/3V 15750,0V 13,712:1 999,78V 0,0 72,902046V -0,01 13,714:1 -0,01% 0,00651A -46,59

018 369340,0/3V 15750,0V 13,539:1 999,79V 0,0 73,832764V -0,01 13,5413:1 -0,02% 0,006659A -46,73

019 364620,0/3V 15750,0V 13,3659:1 999,75V 0,0 74,779701V -0,01 13,3693:1 -0,02% 0,006823A -46,88

Taps

OMICRON

Rat

io d

evia

tion

Exci

ting

curr

ent

Taps


Transformer in good condition

OMICRON

> Tremendous time-saver: removes the need to change test connections between tests

> Automatically operates the tap changer

> Quickly discharges the transformer

> Increases safety by reducing the risk of accidents

> Minimizes the likelihood of measurement errors

> Automated ratio and winding resistance tests

> Manual operation mode: allows measurements for special applications (e.g. auto transformer tests)


Automated measurement with switch box CP SB1

OMICRON

Leakage reactance / short circuit impedancePower transformer diagnosis

Determination of winding or core deformation due to high fault currents

OMICRON

Equivalent circuit diagram: effective resistance and reactance

Forces

10 kV winding 220 kV windingLeakage flux


Forces in a transformer, especially in case of a short circuit, can damage windings and the core.

Factory Test with W-Meter 3-Phase Measurement

SN [kVA] Pk [kW] f [Hz] Tap Position: UHV [kV] ULV [kV] uk [%] Uk [kV]

6500 34.5 50 1 51.7 27.2 6.250 1.9

Three Phase Equivalent Test CPC 100 Measurement

f [Hz] V1 AC [V] I AC [A] Z [] Phi [] R [] X [] L [mH]A-B 50 55.224 1.049 25.7019 85.01 2.2370 25.6044 81.50B-C 50 54.675 1.048 25.8922 85.24 2.1470 25.8030 82.13A-C 50 54.464 1.048 25.2547 84.96 2.2196 25.1570 80.08

Average Impedance

f [Hz] uk [%] Z [] Phi [] R [] X [] xk [%] L [mH]Factory Test Values: 50 6.250 25.7009 85.13 2.1826 25.6080 6.227 81.51Measured Values: 50 6.229 25.6162 85.07 2.2012 25.5215 6.206 81.24

Difference to Factory Test [%] -0.33 -0.33 -0.07 0.85 -0.34 -0.34 -0.34

Per-Phase Test CPC 100 Measurement

f [Hz] V1 AC [V] I AC [A] Z [] Phi [] R [] X [] L [mH]A-B 50 25.061 1.010 24.5892 84.37 2.4102 24.4708 77.89B-C 50 25.337 1.011 24.8397 84.54 2.3641 24.7270 78.71A-C 50 25.485 1.010 24.9888 84.44 2.4224 24.8711 79.17

Average Impedance24.8059 2.3989 24.6896 78.5896

Maximum delta of Average [%] 0.87 1.45 0.89 0.89

OMICRON

Factory test data

Measurement in accordance to IEEE Std C57.12.90

Factory test and measurement comparison

Per-phase test


OMICRON

Frequency

Mea

sure

d re

sist

ance

Frequency response of stray losses (FRSL)Advanced diagnostics

This test is automatically performed with the leakage reactance / short circuit impedance test.

R is frequency-dependent due to eddy losses (induced currents) in the windings which will increase with the frequency.

OMICRON

B

HV winding

LV winding

Losses through induced currents


Parallel strands without twisting

OMICRON

B

HV winding

LV winding

Induced currents are compensated

Parallel strands with twisting


Parallel strands with twisting compensate the induced currents.

OMICRON

B

HV winding

LV winding

Additional losses through induced currents


If the twisted strands are shorted additional losses will occur due to induced currents.

0

1

2

3

4

5

0Hz 100Hz 200Hz 300Hz 400Hz

ZkAB ZkBC ZkAC

18.8mH

19.2mH

19.6mH

20mH

20.4mH

0Hz 100Hz 200Hz 300Hz 400Hz

ZkAB ZkBC ZkAC

f

Three phase equivalent

f

R L

Three phase equivalent

OMICRON


The deviation of the resistance Rk,AC indicates short circuit parallel strands in the left diagram.

OMICRON

Tap changer

Connection clampsTap selector

DC winding resistancePower transformer diagnosis

Measurement of winding resistance and internal contacts

OMICRON

0.14

0.15

0.16

0.17

0.18

0.19

0 5 10 15 20 25 30

Rmeas

0.14

0.15

0.16

0.17

0.18

0.19

0 5 10 15 20 25 30

Rref Rmeas

Tap changer in good condition

Taps

Defective contacts in the tap selector

Win

ding

resi

stan

ce

Win

ding

resi

stan

ce

Taps

DC winding resistancePower transformer diagnosis

Measurement example

The contacts which were used in taps1, 3, 5, 23, 25, and 27 were defective.

OMICRON

Transformer demagnetizationPower transformer diagnosis

Influence of remanence

0.0A

0.002A

0.004A

0.006A

0.008A

0.01A

0.012A

0.014A

0.016A

0.018A

0.02A U with remanenceV with remanenceW with remanenceU without remanenceV without remanenceW without remanence

Initial FRA verificationAfter DC testAfter demagnetization

Exciting current Frequency response anaylsis

OMICRON

Dynamic LTC diagnostics with CP SB1 (OLTC test)Advanced diagnostics

4.7 A

4.8 A

4.9 A

5.0 A

5.1 A

-0.05 s 0 s 0.05 s 0.1 s 0.15 s 0.2 s 0.25 s 0.3 s 0.35 s

OMICRON

Ripple

Slope

Transient current during switching process

Time

Cur

rentDiverter switch switches to the

first commutating or transition resistor

Both commutating resistors are in parallel

Final contact of the diverter contact B is reached

Current control of the CPC 100 regulates the current to the nominal test current again

Dynamic LTC diagnostics with CP SB1 (OLTC test)Advanced diagnostics

The transient switch is recorded and graphically displayed to find faults in the OLTC,

maulaca

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