9_borsi_pd_ intro and phenomena.pdf

Institute of Electric Power Systems – Schering Institute

Partial Discharge- Introduction and Phenomena

Prof. Dr.- Ing. habil. H. Borsi

Institute of Electric Power Systems, Division of High Voltage Engineering

- Schering-Institut -

Leibniz Universität Hannover

Germany

PD Classification

• Surface discharges appearing at the boundary of different insulation materials

• Corona discharge occurring in gaseous dielectrics in the presence of inhomogeneous fields

• Internal discharges occurring in voids or cavities within solid or liquid dielectrics

⇒ Continuous impact of discharges in solid dielectrics forming discharge channels (treeing) in organic materials


PD Classification

a) corona discharge b) surface discharges

c) discharge in laminated material d) cavity discharge

e) treeing

PD Patterns Source: J. Fuhr, Procedure for Identification and

Localization of PD, IEEE Transactions 2005


Why PD-Measurements ?

lPD measurements are an approved tool for insulation condition monitoring lKnown as a precise method lStatement about the actual condition of the insulation possible lAllows to detect very small defects in the insulation lAllows on time maintenance and repair

nPrevents failure and destruction nProlongation of life expectancy nMinimize new investigations nOptimization of the availability


Purpose

• Partial discharges are a sensitive measure of local electrical stress

• Partial discharge inception voltage gives an information on the limit of the electrical strength of an insulating material

• Partial discharge measurement is a typical non destructive test


PD-Monitoring and Diagnostic

• Major changes in the insulation systems are reflected in the partial discharges (PD)

• The partial discharge is a symptom of most (but not all)

deterioration mechanisms


Advantages of PD-Measuring

• Non-destructive diagnosis method

• Detection of locally limited defects

• Identification of PD sources

• On- or off-line measurement


Insulating Material Model

•Simplified failure within a solid insulating material

C3 C3

C2

C2

C1

C1 = cavity

C2 = part of insulating material (only marked cylinder)

C3 = part of insulating material


Cp Cp

Cs

Cs

C1

Cp'

Cs'

C1 RF

u

u1

i1

i

c)b)a)


Equivalent Circuit

•U = applied voltage at power frequency •C1 = capacitor representing the cavity •C2 = capacitor representing the insulating material around the cavity •C3 = capacitor representing the remaining insulating material •S = spark gap representing the discharge of the capacitor C1

UC 1

C 3

C 2

S


PD-Principle

Uz

U1(t)Ut(t)

U'1(t)-Uz

tUL

-UL

FCUq .1∆=

I1(t)

t

∫= dttiq ).(1

B

A

εr

B

A

CP

CS

CF

R1

S I1(t)

Ut U1

Is(t)

I2(t) I3(t)

ε0 CP/2 CP/2

2CS

2CS

CF


•Acoustic PD detection – No determination of apparent charge – Insufficient due to high damping of the solid

insulation •Electrical PD measurements

– Broadband • Large-scaled • Suppression of noises difficult

– Narrowband • Suppression of noises possible • Preferable despite complicated adjustment on-site


Measuring Circuit

•U∼ = high voltage supply Ca = test object (C1, C2 and C3) •Ck = coupling capacitor CD = coupling device •MI = measuring instrument Z = high voltage filter

U Ca

Ck

CD MI

Z


PD-Measurement Principle

Kt

KsK

KPS

S

Fm CC

CqCCCC

CC

qq+

≈++

= ...1.SFP CCC ,>>

Z

Ct U ~

A

B

Is

CK Im(t)

Cc Zm Um

Measuring Impedance

M Amp. Filter


Institute of Electric Power Systems Schering-Institut

Theoretical Background

Important PD-Parameters

Apparent charge Phase angle of the PD pulse related to the applied voltage PD sorce


Partial Discharge Parameter

•of a PD pulse is that charge, if injected within a very short time between the terminals of the test object in a specified test circuit, would give the same reading on the measuring instrument as the PD current pulse itself

• apparent charge q



•applied voltage at which repetitive PD are first observed in the test object when the voltage is gradually increased

• partial discharge inception voltage

applied voltage at which repetitive PD cease to occur when the voltage is gradually decreased

• partial discharge extinction voltage



• pulse repetition frequency N

• pulse repetition rate n ratio between the total number of PD pulses recorded in a selected time interval and the duration of this time interval

number of PD pulses per second in the case of equidistant pulses


Frequency Response of PD Measuring System

•A - bandpass of the measuring system •B - frequency spectrum of the partial discharge pulse •C - frequency spectrum of a calibration pulse •f1 - lower frequency limit f2 - upper frequency limit

frequency

-6 dB

A A

f 1 f 2

B C


PD Measuring Systems

•Wide-band measuring system 30 kHz ≤ f1 ≤ 100 kHz

f2 ≤ 500 kHz 100 kHz ≤ ∆f ≤ 400 kHz

•Narrow-band measuring system 9 kHz ≤ ∆f ≤ 30 kHz 50 kHz ≤ fm ≤ 1 MHz


On-site Detection Methods

Electrical measurements noise

Acoustic measurements sensitivity calibration

“Chemical measurements” gas-in-oil-analysis


Kind of Noise for Electrical Measurements

External noise • power line conducted noise like corona,

signals from electronic power devices, • irradiated noise like electromagnetic

interference due discharges, radio station, power line carrier, etc.

Internal noise • cross-talking between phases


PD-Measurement System

Ct

Zm C2

CK

R HV-Transformer

Amplifier

A/D Conversion Digital Signal Processor

Computer Interface Computer/Software/User

Panel

PD-Signal

Voltage Signal

PD Measurement System

Coupling Capacitor

Measuring Impedance

0-22

0 VA

C

Test Object


PD-Parameter

∑=i

iqT

I 1

∑=i

iqT

D 21

∑=i

ii uqE .

• Average Discharge Current (I)

• Quadratic Rate (D)

• PD Energy (E)

• PD Power Loss (P)

• PD Repetition Rate (R) ∑=i

ii uqT

P .1


PD-Parameter

• PD Inception Voltage (PDIV)

• PD Extinction Voltage (PDEV)

• PD Aparent Charge


PD Monitoring

Possible methods:

Apparent charge

PD- localization

Acoustic

UHF

Narrowband electrical

Wideband electrical


On line PD measurement • Difficulties by on line measurement

ØExpenditure digital signal processing because of the influence of ambient noises

Ü Continuously sinusoidal noise signals Ü Phase synchronous noise signals Ü Stochastically noise signals (Corona)

Alteration of the pulse shape ?

Before filtering

After filtering

PD-Measurement on

Transformers



Possible measurements

Measurement

on a

110kV/35kV

40MVA

transformer


Electrical fault detection Bushing

Capacitive sensor AKV FO-

Sender

FO- Receiver

DSO

Quadripole coupling

dB

amplification: 0 - 60 dB Bandwidth FO: > 10 MHz (3dB)

Patented sensor for decoupling of

capacitive signals


Online PD Measurement

Signal decoupling



Amplifier + FO-Sender

AKV

Sensor

FO-Cable



U (200mV/div)

V (200mV/div)

W (200mV/div)

MP (50mV/div)

Time Line = 10 Periods Time = 1µs/div

Good SNR

Perhaps PD-Signal



Impulse Charge > 5000 pC

Apparent Charge per Period > 500 nC

Impulse Charge < 200 pC

Apparent Charge per Period < 2000 pC 0 10 20 30 40 50

-1

-0.5

0

0.5

1

Time[ms]

0 10 20 30 40 50-4

-2

0

2

4

Time[ms]

0 10 20 30 40 50-1

-0.5

0

0.5

1

0 10 20 30 40 50-4

-2

0

2

4

0 10 20 30 40 50-4

-2

0

2

4

0 10 20 30 40 50-1

-0.5

0

0.5

1

Volta

ge [V

] Vo

ltage

[V]

• Online measurement A: Phase V (110kV/35kV/40MVA)

• Online measurement B: Phase W (220kV/118kV/150MVA)


Schematic diagram

winding PD ???

bushing

xD (t)

neutral

xS (t)

section PD ???

PD


Analysis of measured PD-signals

0

-5

5

0

-0.05

0.05at the bushing

at the neutral

calculated from bushing

calculated from neutral

0

-5

5

Origin 3

0

-5

5

Origin 5

0 50

Measured data Origin 1

10 20 30 40 0 5010 20 30 40

Time in µs

Sources of noise in a PD circuit


Dealing with noise - general options

• Avoiding noise: • Supply line filter, blocking impedance and PD-free setup • Optical isolation between measuring point and measuring system • Freely selectable measurement frequency for optimal SNR Suppressing noise: • Bridge circuit • Software gating (static and dynamic) • Antenna gating (external gating) • 3PARD for synchronous measurement at multiple phases • 3FREQ for synchronous measurement of multiple frequencies


PD Measurements on

Electrical Mashins


Typical PD sources in electrical machines

A. Slot discharge, semi-conducting paint abrasion due to loss wedges and bar vibration

B. Delamination in interface of main insulation and copper conductor due to load changes and

different thermal expansion of main insulation and copper

C. Internal discharge due to delamination between layers of main insulation due to overheating and

aging of the main insulation D. Local highest electrical field on sharp edges of

copper conductor E. Treeing in insulation layers

F. Internal discharges due to voids and cavities in layers of insulation

F

A B

C

E

D

Typical PD sources in electrical machines G. End-winding discharge due to defect on

grading coating, contamination in end-winding, aging or invalidate the proper

function of field grading H. Delamination of insulation in elbow (especially when manually manufactured)

I. Discharge on connection area between slot corona protection and end-winding

corona protection due to contact problem, contamination or potential shift

J. Insufficient spacing, tracking, especially between bars with big voltage difference,

different phases or bar and pressure fingers

K. Crack of the insulation directly at the slot exit due to mismatch of the thermal

expansion of the bar versus the stator core

G H

I

J

K

Testobject

Stator bar

Stator slot model

Cooling duct

Semi-conductive coating Corona protection coating Copper conductor

PD pattern recognition simulation of the slot discharge

without defect, 10kV, 25°C a=10 mm, b=10 mm, 10kV, 25°C

a=20 mm, b=20 mm, 10kV, 25°C a=30 mm, b=20 mm, 10kV, 25°C

b

Defect on semi-conductive screen

a

PD pattern recognition simulation of the insufficient spacing between bar and pressure fingers

HV

metal

PD-Measurements on

Cables


Differential Diagnostic Methods PD-sensor in a cable link

PD-Measurment on Cables

• a) Originalsignal b) filtert Signal

U

a)

b)

t

U

0 20 40 60 ms 100

0

200

mV

-200

-100

0

200

mV

-200

-100

PD-Measurment on Cables

• a) Originalsignal b) filtert Signal

U

t0 4 8 12 ms 20

U0

60

mV

-60

0

40mV

-40-20

-60

-30 5 µsC

Rog

b)

c)

PD Measurement

• PD measurement on a 3 phase cable system

9_borsi_pd_ intro and phenomena.pdf

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