high voltage insulation diagnostics presented by: bostjan flis [email protected] welcome!
TRANSCRIPT
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
Riso Cpi
Rpi Ciso
Riss1
Riss2
Itot
+
Guard
U
ICiso IRCpi IRiso
IRiss 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
uA
0
10
20
30
40
t [s] 0 200 400 600 800
IRiso
Itot
IRiss
IRCpi
ICiso
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 systemU …test voltage
Discharge of Ciss
Itot
t [s] 10 20 30 40 50 60 0 0
Bad insulation
Good insulation
Discharging of Cpi
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/1kVb./ 5 MΩ for motors under 600Vc./ 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 > 2MΩb./ Additional insulation > 5MΩc./ Reinforced insulation > 7MΩ
Polarisation index (PI)
PI WertEvaluation
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)
DAR valueEvaluation
< 1 BAD
1 ≤ DAR ≤ 1.25 SUFFICIENT
> 1.4 VERY GOOD
kVMΩ ni UR
Thumb rule(motors and generators):
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
VNNN
G
VNNN
G
VNNN
G
Power transformer
HV vs LV
HV vs G
LV vs G
Device A Device B Device N
Power supply
PE - ground
Port A Port A Port A
Port B Port B
PE PE
Port C1
Port C2 Port C2
Port C1 C
Port C
Multiple devices
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)
rKRR
1// i10/75i10 0304,028,210rK
General requirements for HV insulation testers
Test functions:- Insulation resistance
- Step voltage test
- Capacitance
- PI, DD, DAR
- Current in µA and mA range
- Voltage AC/DC
Accessories:
- Shielded test leads
- High quality test terminals (HV, antistatic, high surface resistance)
- PC Software for downloading of test results and creating test reports
Other features:
- Guard terminal to eliminate surface insulation currents
- Automatic discharging of the test object after measurement
- Built-in timer
- On-Line graph R = f(t)
- EM immunity for usage in HV environments
- EM Filters for improved accuracy
- Good charging system (accus or mains or both)
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
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