Transcript Folie 1 - zcu.cz

Developments in Diagnosis
of High Voltage Apparatus
Michael Muhr
Univ.-Prof. Dipl.-Ing. Dr.techn. Dr.h.c.
Institute of High Voltage Engineering and System Management
University of Technology Graz
Institute of High Voltage Engineering and System Management
Recording of technical
Monitoring
or characteristic values
Evaluation of
Analysis
Diagnosis
these values
Recognition of the condition and
proposal for problem-solving strategies
M. Muhr
Institute of High Voltage Engineering and System Management
Technical Diagnostics - TD
- Science of recognition of the condition of technical systems
- Aim of TD is to identify, to check and to classify characteristics of
the condition of technical systems 
to change or to influence the condition of these systems
- Quality assurance of technical systems
- Avoidance of an outage of technical systems
M. Muhr
Institute of High Voltage Engineering and System Management
Technical
Diagnostic
+ Prevent failure
- Costs
+ Increase reliability
- Effort
+ Economic use of equipment
- System
Optimized usage of diagnostic tools
• Technique / method
• Time interval
• Complexity
M. Muhr
Institute of High Voltage Engineering and System Management
Availability of technical
systems
Condition evaluation
Risk evaluation
Repair
Maintenance
Technical Diagnostics
M. Muhr
Modernization
Institute of High Voltage Engineering and System Management
Life Cycle
Condition
load
„lifetime extention"
by
maintenance
t
pollution
ageing
critical
threshold
„normal“
defect
failure
Lifetime
M. Muhr
Institute of High Voltage Engineering and System Management
Stress and Ageing Models
M. Muhr
Institute of High Voltage Engineering and System Management
Lifetime Estimation of Transformers
M. Muhr
Institute of High Voltage Engineering and System Management
Technical Diagnostic Evaluation
C
100
1
Section 1
2
O
3
CO
4
Section 2
5
CR
Section 3
100
C ... Condition of the equipment
I ... Importance of the equipment
O ... Equipment 1, 2, 3, 4, 5
M. Muhr
I
Institute of High Voltage Engineering and System Management
Aim of TD
Original condition
- Monitoring
- Analysis
- Diagnosis
- Ageing behaviour
Current condition
- Life time
- Preventive maintenance
- Repair
Modernization
Original condition with ageing
M. Muhr
Institute of High Voltage Engineering and System Management
Tasks of TD
Investigations at
equipment and models
Monitoring of equipment
Characteristic values,
current, voltage etc.
Life-time behaviour
Determining of limits
Setting of continuous
voltage strength
Optimization of life-time
Decision
- Continuation of operation
- Maintenance
- Renewing
M. Muhr
Institute of High Voltage Engineering and System Management
Determination of Early Failures
Bathtub Curve: from statistical analysis
Weibull Distribution:

  tD 
 1


t D 63  t D 63 
<1: early failure
=1: stochastic process
>1: ageing process
M. Muhr
Institute of High Voltage Engineering and System Management
Ageing of an electrical
insulation system
Ageing stress
• Thermal
• Electrical
• Ambient
• Mechanical
Ageing mechanism
• Instrinic
• Exstrinic
• Single stress
• Multi stress
Electrical insulation system
Failure
M. Muhr
Institute of High Voltage Engineering and System Management
TD at equipment of electrical power engineering
Thermal diagnostic
• Thermovision
• Temperature measurement
Analytic and spectrometry
• UV, VIS, IR - Spectrometry
• Gaschromatography
• Mass spectrometry
Mechanical diagnostic
• Sound emission analysis
• Dilatometry
• Mechanical spectrometry
• Analysis of mechanical quantities
Visual diagnostic
• UV - Videography
• Thermography
• Endoscopy
• X-ray graphy
Dielectric diagnostic
• Ue
• u(t)
• C(f)
• tan δ(f)
Breakdown diagnostic
• Ud
• φ (Ud)
• t (Ud)
Partial discharge diagnostic
• Ue
• Q (pd)
• φ (pd)
M. Muhr
Institute of High Voltage Engineering and System Management
Maintenance Strategies - Lifetime
M. Muhr
Institute of High Voltage Engineering and System Management
Blockdiagramm of a Diagnosis System
Excitation
• Measuring voltage
• Measuring temperature
Control
• Voltage
• Temperature
Reference
components
Diagnosis sample
• Original
• Mode
Data acquisition Sensors
Assessment and control unit
• Storage of quantities
• Run-off control
• Data processing
• Input and Presentation
• Further processing
• Interface
M. Muhr
Stress generation
• Voltage
• Temperature
• Vibrations
• Climate
Institute of High Voltage Engineering and System Management
Non – Destructive TD at Power Apparatus
Power apparatus
•Cables
•Solid
•Capacities
switchgears
•Transformers •Rotating
machines
•Instrumenttransformers
•Insulators
•Circuit
•Conventional breaker
switchgears
•Gasinsulated
switchgears
•Dielectric
•Dielectric
•Gas analysis
•Dielectric
•TD of
diagnostic
diagnostic
mechanical
quantities
•Visual TD
•Visual TD
diagnostic
•Dielectric
•Pd-diagnostic •Pd-diagnostic •Visual TD
•Sound
•Sound
•Pd-diagnostic diagnostic
•Visual TD
emission
emission
•Sound
diagnostic
diagnostic
•Pd-diagnostic
emission
diagnostic
•Sound
emission
diagnostic
M. Muhr
•Leakage
•Leakage
current
diagnostic
•Visual TD
•TD of
thermal and
mechanical
quantities
•Pd-diagnostic
current
diagnostic
•Visual TD
TD of thermal
quantities
•Sound
emission
diagnostic
•Pd-diagnostic
Institute of High Voltage Engineering and System Management
Basis Aspects of TD
- Aim
- Technique
- Method
- Quantity
- Connection between quantity and physical / chemical / technical
structure of diagnostic sample
- Diagnostic criterion - limit
M. Muhr
Institute of High Voltage Engineering and System Management
Technical Diagnostic
Mechanical
Dielectrical
Partial Discharge
Breakdown
Material analytic
and Spectrometry
Non-destructive
individual testing
Meaningfulness (stress)
Thermal
M. Muhr
Quasi non-destructive
Partly individual testing
Destructive
Type approval test
Random check
Institute of High Voltage Engineering and System Management
Electrical Diagnostic Methods
Dielectric
Breakdown
Partial Discharge
M. Muhr
Institute of High Voltage Engineering and System Management
Dielectric Diagnostics
Observation and evaluation of interaction of electric fields and material
•
•
•
•
•
Insulation Resistance
Capacity and Dissipation Factor
Isothermal Relaxation Current Analysis (IRC)
Polarisation / Depolarisation Current Analysis (PDC)
Frequency Response Analysis (FRA)
M. Muhr
Institute of High Voltage Engineering and System Management
Insulation Resistance
Guard Ring Arrangement
RD
RS
U=
RD
RS
U=
RO
RO
I
I
Insulations Resistance
Surface Resistance
M. Muhr
Institute of High Voltage Engineering and System Management
Capacity and Dissipation Factor
CX
CX
CN
CN
RX
RX
HV
A
A
D
R3
R4
CN
D
DSP
PC
M. Muhr
Institute of High Voltage Engineering and System Management
Analyse in Time Domain
voltage
40%
20%
A
V
A
IRC
PDC
RVM
current
0%
Recovery Voltage Analyse
60%
Discharge Current Analyse
80%
Discharge Voltage Analyse
100%
Charging current Analyse
120%
Schema
PDC
M. Muhr
Institute of High Voltage Engineering and System Management
Isothermal Relaxation Current Analysis (IRC)
A
V
A
Schema
M. Muhr
Institute of High Voltage Engineering and System Management
Polarisation / Depolarisation Current Analysis (PDC)
A
V
A
Polarisation
Schema
Depolarisation
Time
M. Muhr
Institute of High Voltage Engineering and System Management
Frequency Response Analysis (FRA)
1
normal
defect
In/Out
0.1
0.01
U, I
U, I
0.001
0.0001
10
HV
LV
M. Muhr
100
1000
frequency [kHz]
Institute of High Voltage Engineering and System Management
Probability [%]
Breakdown Diagnostic
HV-AC
99
98
95
90
80
70
50
30
20
10
5
2
1
96
100
102
104
Voltage [kV]
108
106
Voltage [kV]
98
104
102
100
98
96
1
6 11 16 21 26 31 36 41 46 51
Attempt
M. Muhr
106
108
Institute of High Voltage Engineering and System Management
Breakdown Test with Insulating Oil
80
80
60
60
40
40
20
20
0
0
1
2
0.5kV/s mean value
2kV/s mean value
5kV/s mean value
3
4
test series
5
coefficient of variation (%)
100
mean value (kV)
100
6
0.5kV/s coeff. of variation
2kV/s coeff. of variation
5kV/s coeff. of variation
M. Muhr
Institute of High Voltage Engineering and System Management
Partial Discharge (PD) Measurement
Sensitive nondestructive method
as important diagnostic tool
for evaluation of the insulation condition
PD-defect
Location and Detection
Electrical
Non electrical methods
M. Muhr
Institute of High Voltage Engineering and System Management
Partial Discharge Diagnostic
HV
Optical
IEC 60270
Electrical
HF/VHF/UHF
Mechanical
Acoustic
Chemical
M. Muhr
Opto-acoustic
Institute of High Voltage Engineering and System Management
PD-Detection Methods IEC 60270
• Conventional electrical measurement
• Integration at frequency domain
• Narrow-band
• Wide-band
• Integration at time domain
ZFilter
Test
object
Coupling
capacitor
HV
measure
PD
Pulse
G
PD-System
HV-Source
Quadripole
Measuring impedance
M. Muhr
Institute of High Voltage Engineering and System Management
PD-Measurement Results
Single impulses
Phase correlated
Time correlated
M. Muhr
Institute of High Voltage Engineering and System Management
PDC Analyse
M. Muhr
Institute of High Voltage Engineering and System Management
Unconventional PD-Detection
Rd
acoustic
CC
W
ST
Signalconversion
electrical
Surface
LWL
VP
optical
LWL fibre
RPA1
AMP
ICM
HST1
SYNC
Oscilloscope
ADU
Fibre in oil/board
M. Muhr
Institute of High Voltage Engineering and System Management
PD-Detection Methods
• Electrical measurements with high frequencies
- HF / VHF method – 3 MHz to 300 MHz
- UHF method – 300 MHz to 3 GHz
• Acoustic measurements – 10 kHz to 300 kHz
• Optical measurements – ultraviolet – visible – infrared range
• Chemical measurements
- Acoustic
HV
- Chemical
- Optical
PD
Directional
microphone
M. Muhr
Institute of High Voltage Engineering and System Management
HF / VHF – PD-Measurement
cable
inductive
current
cable
wire sheath
Rogowski coil
PD-signal
PD-signal
sheath interruption
measurement impedance
galvanic
cable
cable
HV
source
measurement
impedance
PD-signal
PD-signal
M. Muhr
Institute of High Voltage Engineering and System Management
HF / VHF – PD-Measurement
Rogowski coil
Rogowski coil
PD Analyser
Measuring for HF – PD-detection on machines, VHF – PD-coupler,
Split ring Rogowski coil
M. Muhr
Institute of High Voltage Engineering and System Management
HF / VHF – PD-Measurement
1
2
direct. coupling
A B
impulse
source / - No.
joint /1
left cable / 2
right cable / 3
direct. coupling
C D
signal at coupling output
A
B
C
D
–
X
X
–
X
–
X
–
–
X
–
X
Principle of the coupler sensor
M. Muhr
3
Institute of High Voltage Engineering and System Management
UHF – PD-Measurement
Narrow band
Wide band
PRPD
pattern
Spectrum
analyser
amplifiers
low-pass
filter
Peak detector
UHF – PD-measurement
M. Muhr
PD measuring
instrument
Institute of High Voltage Engineering and System Management
UHF – PD-Sensors
Detector
Dectector
Conductor
Conductor
Cage
Cage
- Mobile
UHF-window
sensor
- Conventional
UHF-sensors
Disc sensor
Cone sensor
Field grading electrodes
M. Muhr
Institute of High Voltage Engineering and System Management
UHF – PD-Fault Location
L1
L2
1
Amplitude
Defect
2
Sensor 2
Sensor 1
Measuring
instrument
Zeit
Pre amplifier
Time delay
Schematic arrangement
1 ... Sensor 1
2 ... Sensor 2
M. Muhr
Institute of High Voltage Engineering and System Management
Acoustic – PD-Measurement
• Acoustic Partial Discharge Detection
• Acoustic signal as a result of the pressure wave produced by PD
• Frequency spectrum 10 Hz up to 300 kHz
SF6
Box
AE Sensor
Air
Principle schematic to the acoustic PD-detection
M. Muhr
Institute of High Voltage Engineering and System Management
Acoustic – PD-Sensors
- Piezo-electric (sound emission)
- Structure-born sound-resonance
- Opto-acoustic-sensor
HV
HV
Sensor
Signal
PD
- Condenser microphones
- Accelerometer
PD
Time delay
Directional
microphone
Time
M. Muhr
Institute of High Voltage Engineering and System Management
Acoustic – PD-Measurement
PD-Detector
AE Sensor
amplifier
Filter
A/D Converter
Acoustic – PD-detection system
M. Muhr
Oscilloscope
Institute of High Voltage Engineering and System Management
Opto-Acoustic – PD-Measurement
Reference optical fibre coil
Laser
Beam
Splitter
Beam
Splitter
Detector
Oil tank
Sensing optical
fibre coil
High
Voltage
PD-source
Experimental setup of the optical interferometric detection of PD
M. Muhr
Institute of High Voltage Engineering and System Management
PD – Optical Detection
Peak
Rd
Lens
Optical fibre
Voltage supply
M
CC
LWL
W
ST
PR
optical
system
Signalconversion
VP
Medium oil
Observation area
Optical fibre with lens
Peak
IEC 270
Oscilloscope,
conventional
opt. fibre
PC
ADU
fluorescent
opt. fibre
Measuring circuit after IEC 60270
and optical measuring system
Plate
Fluorescent optical fibre
M. Muhr
Institute of High Voltage Engineering and System Management
Optical – PD-Detection in GIS
Photomultiplier
340
SF6
ADU
Oscilloscope
Test arrangement for optical PD-measurement
M. Muhr
560
670
780 nm
Emission spectrum of
corona discharges in SF6
I/U
Conv.
PD Detector
450
Institute of High Voltage Engineering and System Management
Optical – PD-Measurement
Operation diagram camera DayCor IITM
and application
M. Muhr
Institute of High Voltage Engineering and System Management
Chemical
- GIS
- Transformer
- Machines
Optical
- GIS
- Overhead lines
PD – Application
Acoustic
- High voltage
cables
- GIS
- Transformers
- Overhead lines
Electrical
-Transformers
- GIS
- Machines
- GIS
- Overhead lines
- ...
HF / VHF
UHF
- GIS
- GIL
- Transformer
monitoring
M. Muhr
- High voltage
cables
- Transformers
Institute of High Voltage Engineering and System Management
Monitoring, Data Processing, Announcement
Telephone lines
Alarm
Real time
announcement
Modem
Data archiving
Data input
and
monitoring unit
PC station
Display
M. Muhr
Data transfer
Report
Trend analysis
Institute of High Voltage Engineering and System Management
Pattern Recognition
Signal processing
Display
Data store
Data transfer
Report
Example for signal processing
M. Muhr
Trend analysis
Institute of High Voltage Engineering and System Management
Power Transformer Diagnostics
IEC 599
IEEE C57.1041991
Furane Values
Dörnenburg
Moisture
Rogers
Dissolved Gas
Analyses (DGA)
Neutralization Value
Duval
Other Methods
Oil Analysis
Fibre Optics
Loss Factor
Chemical
Endoscopy
Oil Analysis
Eye
UHF PD-Detection
Breakdown Voltage
Optical
Elektrisch
Electrical
Acoustics
PD-Evaluation
Partial Discharges
DAC-PD-Measurement
Mechanisch
Mechanical
Operating Noises
PD Location
Insulation
Thermisch
Thermal
Dynamics
Loss Factor
PDC-Analysis
Transient Oil Pressure
Operating Temperature
Insulation Resistance
Signal Analysis
Oil Stream
RVM, FDS Analysis
FRA-Analysis
Thermography
Temperature Monitoring
Transfer Function
Wavelet Technique
M. Muhr
Institute of High Voltage Engineering and System Management
Method
Information
Comment
Oil analysis
OA
Oil ageing, insulation property
Routine
Gas-in-oil analysis
DGA
Oil and paper ageing, flashovers, PD,
hot spots
Routine
tanδ
Dielectric losses in the insulation system
Routine
Furane values
FA
Ageing of the paper insulation
Condition determination
Degree of
polymerization
DP
Ageing of the paper insulation
Condition determination
Partial discharges
PD
Paper ageing, ageing of the
insulation system
Condition determination
Polarisation and
depolarisation
currents
PDC
Water content in the paper insulation
Condition determination
Infrared
measurements
IR
Overload, failure of the cooling system
Condition determination
and for fault location
Acoustic
measurements
AM
PD-detection and location
For fault location
Frequency response
FRA
Mechanical condition of the windings
After short circuits
Loss factor
(Dielectric dissipation factor)
M. Muhr
Institute of High Voltage Engineering and System Management
Oil Analysis
Moisture
Water content in paper
• Karl-Fischer titration
• Capacitive sensors
• Breakdown voltage
Breakdown voltage [kV]
• PDC
Low
tief
Temperature
High
hoch
Water content in oil
dilution
emulsion
Water content [ppm]
M. Muhr
Institute of High Voltage Engineering and System Management
Oil Analysis
Loss Factor (tanδ)
• Heat losses are
temperaturedependent
• High loss factor –
thermal breakdown
• Limit: tanδ ≤ 5 %
by T=100° C
Temperature dependace of tanδ for new oil
(1) and oils in operation (2,3,4)
M. Muhr
Institute of High Voltage Engineering and System Management
Oil Analysis
Dissolved Gas Analysis (DGA)
• Key gases: hydrogen, methane, ethane, ethylene, acetylene, propane,
propylene, carbon monoxide, carbon dioxide
• Detectable failures: thermal and electrical failure
M. Muhr
Institute of High Voltage Engineering and System Management
Oil Analysis
DGA - Evaluation
• CIGRE
• IEC
• IEEE
• Dörnenburg
• MSS / VDE-Diagnosis
• Rogers
• Duval
• Laborelec
• KEPCO
Duval triangel
M. Muhr
Institute of High Voltage Engineering and System Management
Paper Analysis
Polymerisation degree (DP)
- In direct correlation with the mechanical strength
new paper DP 1000 – 1200
paper at the end of life time DP approx.200
- A decrease in the DP value furane is going hand in hand with an increase of
components
- Furane dependence
• Oil temperature
• Type of oil
• Neutralization factor
• Type of paper
• Sludge content
• Moisture content of oil and paper
• Ratio oil / paper
M. Muhr
Institute of High Voltage Engineering and System Management
Oil Analysis
Furane Analysis
• Condition of paper insulation (mechanical strength) determined by degree of
polymerization
• High performance liquid chromatograph (HPLC)
M. Muhr
Institute of High Voltage Engineering and System Management
Partial Discharges
• Conventional
• Acoustic
Measurement
in laboratory
Measurement
on-site
• Optical
• Chemical
• Electrical
conventional
measurement
IEC 60270
Acoustic
PD - measurement
Optical
PD - measurement
electrical
High frequency
PD - measurement
Chemical
PD – measurement
(DGA)
HF/UHF
method
f ~ 10 - 500 kHz
Narrow band
Δf ~ 10 kHz
Broadband
Δf ~ 50 - 500 kHz
UHF
method
f ~ 0,2 - 2 GHz
Narrow band
Δf < 2 MHz
Broadband
Δf > 50 MHz
• New Systems: electromagnetic wave detection,
transfer functions, damped AC voltage (DAC)
M. Muhr
Institute of High Voltage Engineering and System Management
Dielectric Diagnosis
Charging and discharging measurements
M. Muhr
Institute of High Voltage Engineering and System Management
Diagnostics with Transient Test Voltages
• Frequency response analysis (FRA) and transfer function
• Low voltage impulse response (LVI)
• Wavelet analyse
• DAC-PD-measurement
FRA
M. Muhr
Institute of High Voltage Engineering and System Management
Thermal and Optical Diagnostic Methods
• Thermography
• Temperature measurement with fibre optics
• PT-100 elements
• Surface acoustic wave radio transponder
• Visual inspection
• Endoscopy
Ocular
OKULAR
LICHTQUELLE
Light source
FLEXIBLESend
ENDE
Flexible
BLICKBEREICH
Field of vision
M. Muhr
Institute of High Voltage Engineering and System Management
Mechanical Diagnostic Methods
• Transient oil pressure (short circuit)
• Stream analysis (oil and cooling air)
• Acoustic PD-locating
M. Muhr
No
stream
Stream
Institute of High Voltage Engineering and System Management
Diagnosis of Rotating Machines
Tasks
• Thermal monitoring
• Air cooling analysis
• Monitoring of vibrations
• Acoustic monitoring
• Chemical-analytic monitoring
• Magnetic monitoring
• Electrical monitoring
M. Muhr
Institute of High Voltage Engineering and System Management
Rotating Machines
- Electrical Diagnosis Methods
• Impulse voltage test at windings
ut   U .e .t . cost


ut  - Oscillogram of a decaying oscillation
a) Error –free
b) faulty
M. Muhr
R
2L
Attenuation factor
1
Resonance frequency of
L.CS
resonating circuit
Institute of High Voltage Engineering and System Management
Rotating Machines
- Dielectric Diagnosis
• Time domain
- Insulation resistance Riso
- Insulation current iiso = f (t)
• Frequency domain
- Capacitance
- Loss factor tan  = f (), C, U
M. Muhr
Institute of High Voltage Engineering and System Management
Rotating Machines
- Dielectric Diagnosis
Insulation time constant 10 as criterion for the dryness limit curve
10 = Riso (600s) x C (50Hz)
10 = f ()
(1) Dry condition
(2) Humid condition
(3) Dryness limit curve
Cooling
Heating
Insulation time constant in dependance of the slot temperature and the insulation condition
M. Muhr
Institute of High Voltage Engineering and System Management
Rotating Machines
- Partial discharge (Pd) diagnosis
• Pd’s
- In cavities of solid insulation
- In the are of the end winding
- At no-perfect working protect system
-
of mica
At vibrating parts of winding
M. Muhr
Institute of High Voltage Engineering and System Management
Physical Condition of Windings
M. Muhr
Institute of High Voltage Engineering and System Management
Discharge behaviour within Machine Windings
M. Muhr
Institute of High Voltage Engineering and System Management
Thermography as Diagnostic Tool
• Requirement
- Increasing efficiency of power grid
- Decreasing of inspection and maintenance
rate
• Load situation
- Increasing electrical power consumption
- Higher loads on equipment
• Demand of ON-line diagnostic tools
E.g. Contact less temperature measurement
M. Muhr
Institute of High Voltage Engineering and System Management
Thermography as Diagnostic Tool
Surface
Surface temperature
• Load current
Resistor
• Voltage
Conductivity
Dielectric losses
powe
r
outpu
t
temperature
Heat Generation
Temperature
Material/Structure
power output
Surface temperature
power
output
Surrounding
Temperature
measurement
M. Muhr
Institute of High Voltage Engineering and System Management
Temperature Measurement
Object
Temperature
Coupling
Sensor
High Voltage
Transmission
power supply
Signalling
Isolated Transmission
Isolated Coupling
Object
Temperature
Coupling
Sensor
High Voltage
M. Muhr
Transmission
power supply
Signalling
Institute of High Voltage Engineering and System Management
Spot Temperature Measurement
29.8°C 29.9°C 30.0°C 30.0°C 30.0°C
29.8°C 29.7°C 29.9°C 29.9°C 30.1°C
29.7°C 29.7°C 29.9°C 29.7°C 30.0°C
29.5°C 29.6°C 29.7°C 29.8°C 29.9°C
29.7°C 29.6°C 29.8°C 29.7°C 29.8°C
Transformer
M. Muhr
Institute of High Voltage Engineering and System Management
Temperature Distribution
33°C
23°C
Hot
Cold
M. Muhr
Institute of High Voltage Engineering and System Management
Thermography in HV - Systems
- Applications
• Connections
• Switches
• Power transformers
• Measuring transformers
• Overhead lines
• Cable terminations
• Bushings
• Fuses
• Rotating Machines
M. Muhr
Institute of High Voltage Engineering and System Management
Applications of Thermography
Poor contact resistance of a
joint
Poor contact resistance of a
disconnector
M. Muhr
Institute of High Voltage Engineering and System Management
Applications of Thermography
Compression connector of
an overhead line
Asymmetrical energy flow of
a cable
M. Muhr
Institute of High Voltage Engineering and System Management
Applications of Thermography
Bearing, machines
Slip-ring brushes
© FLIR
M. Muhr
Institute of High Voltage Engineering and System Management
Thank You for Your Attention
M. Muhr