10_Prevost_PD with synch sys.pdf

Measurement of Partial Discharges with Synchronous Multi-Channel Systems

Conference on Electrical Power Equipment Diagnostics Bali, Indonesia Thomas Prevost

2013-10-31 Page: 2

Why PD Measurements ?

Non-destructive test method to:

• detect critical defects • localize defects • assess the risk

PD in HV Insulation Systems

Equivalent Circuit Diagram

Uz

U1(t)Ut(t)

U'1(t)-Uz

tUL

-UL

FCUq .1∆=

I1(t)

t

B

A

CP

CS

CF

R1

S I1(t)

Ut U1

Is(t)

I2(t) I3(t)

B

A

εr ε0 CP/2 CP/2

2CS

2CS

CF

∫= dttiq ).(1

2013-10-31

Measuring Partial Discharge

Applied Voltage

Voltage across void

Voltage breakdown across void

Page: 7 2013-10-31

Transformer under test

Coupling capacitor

Conventional Coupling of the Measuring Device

Testing transformer

IPD

PD

Fault (discharge)

Phase Resolved Partial Discharge (PRPD) Corona

PRPD Inner PD

PRPD Surface Discharge

PD Patterns

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

An example:

The source: 400kV = 4*105 V

PD signal: 100pC = 1*10-10 C (C=As) Under the influence of very high voltages and fields we are looking for very small signals.

OK so far – where's the problem?

Dealing with interferences

• Avoiding noise: • Supply line filter, blocking impedance and PD-free setup • Optical isolation between measuring point and measuring

system • Freely selectable measurement frequency for optimum SNR

• Suppressing noise: • Software windowing (static and dynamic) • Antenna gating (external gating) • UHF Gating • Synchronous measurement at multiple phases (3PARD) • Synchronous measurement of multiple frequencies (3FREQ )

Methods to avoid interferences

Optical isolation between measuring point and measuring system

Ground loop = Antenna for interferences

High Voltage area

PD Instrument (traditional) Measurement cable (electrical connection) Optical Fiber Cable

Ground loop minimization

Synchronous Multi-Channel PD Measurement System MPD600

PAGE 17

Synchronous Multi-Channel Measurement

Methods to suppress interferences

Windowing methods

Static windowing

Dynamic noise gating (DyNG)

Antenna gating (external gating)


Working principle of external gating

CD CD CD

PD pulse detected

PD pulse detected

PD pulse detected

External gating channel

PD pulse detected

CD CD CD

PD pulse detected

PD pulse detected

PD pulse detected

No PD pulse detected

External gating channel

The tank serves as a virtually perfect Faraday cage

Antenna/Unit-Gating: MINUS Operation

-

Measurement channel Gating channel

De-noised measurement

Presenter

Presentation Notes

The result of the IEC measurement by it own is show in here. The measurement is dominated by the external corona. The Discharge level is about 2.1nC! The measurement through the oil drain valve is focus on the PD inside of the transformer. The value is not possible according the IEC, but through the metallic shielded setup the external are highly damped. The correlation leads now to results with values which are calibrated. So the disturbances of the external noise is done by the combination of different methods.

UHF Gating


Principle of UHF-Gating

CD CD CD

No PD pulse detected

PD pulse detected

PD pulse detected

PD pulse detected

UHF sensor

The tank functions as a virtually perfect Faraday cage

CD CD CD

UHF sensor

PD pulse detected

PD pulse detected

PD pulse detected

PD pulse detected

A sensor inside the tank?

> October 13

Factory testlab

On-site

UHF-Sensor

Meas. Tap

Combined UHF and IEC Meas.

IEC PD Measurement UHF PD Measurement

Corrected IEC PD Measurement

&


Synchronous measurement at multiple locations to create 3PARD

Practical examples

Example: Induced voltage test

Battery Battery Battery

LV

HV

N

All stations measure SYNCHRONOUSLY (no multiplexer principle)

H1 H2 H3

Inner partial discharge

L1

L2

L3

L1

L2

L3

Inner PD source L1>L2>L3

3PARD

timeframe 1 µs

Inner PD source at L1

3PARD = Three Phase Amplitude Relation Diagram

Sources of external interference

L2

L3

3PARD

L1

L1

L2

L3

External interference L1 ≈ L2 ≈ L3

timeframe 1 µs

Êxternal interference

3PARD = Three Phase Amplitude Relation Diagram

Presenter

Presentation Notes

In the case of external interference, the amplitudes will be of approximately the same magnitude. [CLICK] [CLICK] [CLICK] This will lead to a single dot close to the origin of the diagram. Multiple PD sources and noise can be separated in this way. [CLICK]

Principle of 3PARD / 3CFRD

3PARD / 3CFRD

Noise Inner PD

Separated PRPD diagram: Noise signals

Separated PRPD diagram: Inner PD source

A single PD triple (signal ) is represented by one point in the diagram. Each aggregation of such points (cluster) represents a single PD source. The individual clusters can then be analyzed separately from one another in real time as a classic PRPD diagram without superimposition of other signals.


Synchronous multiple frequency measurement to create 3CFRD

PD source 1 PD source 2 PD source 3

FFT pulse 2

FFT pulse 3

FFT pulse 1

FFT of pulses above Filter 2

Filter 3 Filter 1

PD source 3

PD source 1

PD source 2

Filter 3 Filter 1 Filter 2

Every cluster can be analyzed separately

How does it work?

Measurement at Three Different Frequencies with 3CFRD

Faulty joint

Needle

Surface discharge

22kV_Corona_Surface_Joint

Signals of 3 Different PD Sources

PAGE 39

3CFRD and Back-Transformed Signals of the 3 PD Sources

PAGE 40 Stream

A practical example

Testing a 24 kV CT in the substation

The 3CFRD figure (3FREQ)

3CFRD (3 Center Frequency Relation Diagram)

...shows another source besides the background noise

The 3FREQ discovers the "hidden" PD

Before and after 3FREQ noise reduction

Background noise appr. 8pC

PD level appr. 6pC

Many thanks for your time! Questions?

10_Prevost_PD with synch sys.pdf

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