High Voltage Insulation Diagnostics
WELCOME!
Presented by: Bostjan Flis
bostjan.flis@metrel.si
Content overview
 Application areas of HV insulation testing
 What influences insulation resistance
 Symptoms of bad insulation
 Testing methods
 Insulation testing with DC test voltage - insulation characteristics
 Insulation testing with DC test voltage - test parameters
 Limit values
 Connection schemes
 Test data evaluation i.e. statistics keeping
 General requirements for HV insulation testers
 Metrel HV insulation testers
 Metrel's solution providing, services and development
Application areas of HV insulation testing
Insulation resistance testing is part of predictive maintenance of:
- Rotary machines (generators, motors)
- Transformers (industry, utilities, power distribution)
- Cables
- Measuring transducers
- Distribution boards
Benefits:
- Lower TCO (total cost of ownership)
- increased electrical safety
- prevention against fire hazard
What influences insulation resistance
- Thermal exposure
- Overload
- Moisture
- Electrical shocks
- Mechanical shocks (vibrations, bad transportation)
- Dirt (oil, dust, conductive materials)
- General ageing of insulation
Symptoms of bad insulation
- Insulation breakdowns in a form of electrical short-circuits
- Fire
- Long-term downgrading values of insulation resistance
- Insulation resistance values under by standards determined values
Insulation testing methods
- AC insulation testing (50/60Hz, 1 minute)
- Insulation surge breakdown testing
- Insulation testing with switching voltage
- Insulation testing with DC voltage
- Dielectric losses (tgδ)
- Partial discharges
- U – I characteristics
Insulation testing with DC test voltage - insulation characteristics
Itot
IRiss
Riss
1
IRiso
Riso
Guar
d
IRCpi
ICiso
Cpi
Cis
o
Rpi
Riss
2
+
U
uA
Itot
40
ICiso
30
IRiso
20
IRiss
10
IRCpi
0
0
200
400
600
800
t [s]
U
Test voltage
Riss1 and Riss2
Insulation surface resistance
Riso
Insulation resistance
Ciso
Capacitance of an insulation system
Rpi
Polarisation resistance
Cpi
Polarisations capacitance
Itot
Total insulation current
IRiss
Insulation surface leakage current
IRiso
Resisitve part of the insulation current
IRCpi
Polarisation current
ICiso
Capacitive part of the insulation current
Insulation parameters
Polarisation Index (PI)
PI = R tot (10 Min) / R tot (1Min)
Dielectric absoption ratio (DAR)
DAR = R tot (1 Min) / R tot (30 Sek)
Dielectric discharge (DD)
DD = I DIS (1 min) / (U * C ISO )
I DIS ….a discharge current measured 1 minute after disconnecting the insulation system from the test voltage
C ISO …. capacitance of the insulation system
Discharge of
U …test voltage
Discharging of Cpi
Ciss
Itot
Bad
insulation
Good
insulation
0
0
10
20
30
40
50
60
t [s]
Limit values for insulation testing
with DC voltage
Insulation resistance
Limit value
General applications
> 1MΩ per 1kV der Nennspannung des Prüflings
Rotary machines according to
IEEE 43-2000
a./ 1 MΩ + 1 MΩ/1kV
b./ 5 MΩ for motors under 600V
c./ 100 MΩ for motors above 600V
Switching- and control devices according to IEC 60439-1
> 1kΩ for 1V of the UN of the test object
Splitting transformers and safety low voltage transformers according
to IEC 61558
a./ Basis insulation > 2MΩ
b./ Additional insulation > 5MΩ
c./ Reinforced insulation > 7MΩ
Polarisation index (PI)
Evaluation
PI Wert
1 to 1.5
BAD (valid for older types of insulation materials)
2 to 4
SUFFICIENT (valid for older types of insulation materials)
>4
VERY GOOD (also for the modern type of insulation materials)
Dielektric absorption ratio (DAR)
Evaluation
DAR value
<1
BAD
1 ≤ DAR ≤ 1.25
SUFFICIENT
> 1.4
VERY GOOD
Thumb rule
(motors and generators):
Ri MΩ  U n kV
Connection schemes
Precondition!
- Measuring object has to be disconnected from power supply network (voltage – free)
- Surface of the insulation system has to be dry and clean
Power transformer
VN
HV vs LV
NN
G
VN
NN
HV vs G
G
Multiple devices
Power supply
PE - ground
Port A
PE
Port A
Port A
Port C1
Device N
Port C1 Device B
Device A
Port C2 Port C
C
Port C2
PE
Port B
Port B
VN
NN
G
LV vs G
Test data evaluation i.e. statistics keeping
Statistics
- Always assure the same measuring conditions
(connection, temperature, test voltage, diagnostics parameters)
- Regular test intervals (6 months or one year)
- Results keeping for the entire life period of the equipment
- Observation of downgrading trends
Temperature coefficient
(all values are to be calculated on 75ºC)
Ri10/ /75  Ri10/
1

Kr
K r  102, 280,0304 
General requirements for HV insulation testers
Test functions:
- Insulation resistance
Other features:
- Guard terminal to eliminate surface insulation currents
- Step voltage test
- Capacitance
- PI, DD, DAR
- Current in µA and mA range
- Automatic discharging of the test object after measurement
- Built-in timer
- On-Line graph R = f(t)
- Voltage AC/DC
- EM immunity for usage in HV environments
- EM Filters for improved accuracy
Accessories:
- Good charging system (accus or mains or both)
- Shielded test leads
- High quality test terminals (HV, antistatic, high surface resistance)
- PC Software for downloading of test results and creating test reports
Metrel HV insulation testers
Integrated features:
•Test voltage up to 10kV DC
•Resistance measuring range up to 10TΩ
•Automated calculation of PI, DAR and DD for profound insulation diagnostics
•On-Line R=f(t) display
•Test voltage setting in 25V steps
•Shielded 12kV test leads to reduce influence of external EM fields
•CAT IV/600V over-voltage category for utilities and industrial applications
•New TeraLink PRO Windows PC Software for TeraOhm 5kV Plus
•Handbook “Modern insulation testing” on CD
Metrel's solution providing, services and development
 Product training in Metrel training centre
 On-Line technical support
 Application training and publications
 Repair & Calibration training
 Calibration equipment
 Repair & Calibration services
 Advanced technical solutions for all product‘s segments
 (Hardware, Firmware, PC Software)
 Customised solutions (Instruments, Software, Accessories)
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