Transcript
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
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
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? Page 13
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 )
Page 14
Methods to avoid interferences
Optical isolation between measuring point and measuring system
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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
PAGE 18
Synchronous Multi-Channel Measurement
Methods to suppress interferences
Windowing methods
Static windowing
Dynamic noise gating (DyNG)
Antenna gating (external gating)
Methods to suppress interferences Page 22
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
-
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Measurement channel Gating channel
De-noised measurement
UHF Gating
Methods to suppress interferences Page 25
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
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A sensor inside the tank?
> October 13
Factory testlab
On-site
UHF-Sensor
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Meas. Tap
Combined UHF and IEC Meas.
IEC PD Measurement UHF PD Measurement
Corrected IEC PD Measurement
&
Methods to suppress interferences
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
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.
Methods to suppress interferences
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?
PAGE 38
Measurement at Three Different Frequencies with 3CFRD
Faulty joint
Needle
Surface discharge
22kV_Corona_Surface_Joint
Signals of 3 Different PD Sources
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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)
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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?
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