18_Aizam_HV IT in TNB Substations.pdf
Transcript of 18_Aizam_HV IT in TNB Substations.pdf
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Copyright 2013 TNB Research
Diagnostic And Condition Assessment Of High Voltage Instrument Transformer in
TNB Transmission Substation
Slide No. 1
Mohd Aizam Talib
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1.0 Background High voltage instrument transformers form an integral part
of the power utility control circuits to monitor load currents and voltages for protection and measurement purpose
Inexpensive components and require minimal maintenance
Failure and explosion of instrument transformer - loss of assets, revenue and safety issue
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01234567
No.
of F
ailu
res
Year
Failure Cases of High Voltage Instrument Transformer
Over 36 failures and explosion of instrument transformers has been recorded at TNB Transmission substations from year 2001 until 2010.
There is a need to assess the condition of in service high voltage instrument transformers
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visual inspection cleaning of the porcelain Thermography hot spot
detection
Routine maintenance practices Limitations
Provide partial information on the condition of instrument transformers
Unable to detect abnormal degradation
Off-line and on-line diagnostic techniques
Verify the condition of in-service instrument transformer
Guarantee the reliability of instrument transformer
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2.0 Diagnostic of In-Service Instrument Transformer
Degradation of insulation system will increases the probability of instrument transformer failures
Important to assess the condition of in-service instrument transformer
Used of nonintrusive diagnostic tools to improve the maintenance practices for instrument transformers
Diagnostic techniques; dielectric dissipation factor, dielectric response analysis & on-line partial discharge measurement
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Dielectric Dissipation Factor / Power Factor
Ratio of the resistive current to the capacitive current under an applied AC voltage
Good insulation usually has a very low loss and increases in dielectric loss may indicate problems in the insulation structure.
IR
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Dielectric dissipation factor of new instrument transformer should not exceeded 0.5% and for in-service age units the measurement should less than 1.0%
Periodic measurement during the service life can indicate either the insulation is aging normally or deteriorating rapidly
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Dielectric Response Analysis
Used to estimate moisture and ageing state of oil-paper insulation from dielectric properties like polarization currents and dissipation factor.
Dielectric response techniques o Frequency domain ; Frequency Dielectric
Spectroscopy (FDS) o Time domain ; Polarization Depolarization Current
(PDC) & Recovery Voltage Measurement (RVM)
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Frequency Dielectric Spectroscopy
Measures the tangent delta over wide frequency range
An applied alternating voltage polarizes the molecules in the dielectric
As the frequency is increased, some of the molecule cannot follow the imposed fast change of polarity and stop their participation in polarization process
Display as dissipation factor or complex permittivity; which decrease with frequency
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Polarization Depolarization Current
A dc voltage is applied to the insulation for a time. The current arising from the activation of the polarization process measured.
Then, the voltage is removed and the object is short-circuited. Thus, the discharging current jumps to a negative value, which goes gradually towards zero.
Both kinds of currents called polarization and depolarization current
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On-line Partial Discharge Measurement All dielectric defects are accompanied by the presence
of partial discharges
PDs are in general are always generate electromagnetic signals and often accompanied by emission of sound, light, heat, and chemical reactions
Thus PD measurements (chemical, electrical and acoustic) are technically able to detect dielectric defects, even identify the presence of a specific defect.
When a PD occurs inside instrument transformer, an electromagnetic wave is radiated and can be detected using HF sensors.
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HFCT
fibre optical cable
BNC Cable Analysis software
Instrument Transformer under test
USB 2.0
Batt
ery
Test circuit for on-line partial discharge detection on current transformer
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3.0 Field Measurements
132kV current transformer was tested during routine maintenance testing
Tan delta test results
Power Factor and Dielectric Response Measurement
No Phase % tan delta
1 Red 0.5913 2 Yellow 0.5704 3 Blue 6.5288
Further analysis using dielectric response measurement
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FDS response of current transformer
* higher response at B phase
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The increasing of the responses at low frequency ranges indicated higher moisture content in blue phase of the instrument transformer
Moisture estimated in cellulose insulation
The instrument transformer was decided to replace
No Phase % moisture
1 Red 2.4 2 Yellow 2.6 3 Blue 4.3
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Newly diagnostic techniques adopted by TNB for condition assessment of in-service instrument transformers
Preliminary work in the laboratory using the technique showed encouraging results
Partial discharge measurement was carried out on in-service 132kV oil-filled high voltage current transformer.
On-line PD Measurement
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PD laboratory experimental set-up
HV Source
Test Vessel HFCT
MPD600
Phase resolved PD pattern of void inside the pressboard sample
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HFCT MPD600
External hardware gating
On-line PD measurement at site
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PD pattern for instrument transformer with high PD activity, 800pC
PD pattern for instrument transformer with low PD activity, 100pC
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4.0 Conclusions The condition of the instrument transformer can be
accurately assessed by various off-line and on-line diagnostic measurements
TNB has employed advanced diagnostic dielectric response measurement and on-line partial discharge testing as parts of their condition based maintenance strategies
Condition assessment of in-service instrument transformers - assess the reliability of aged units, and planning for the replacement to avoid any catastrophic failures occurs.
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Copyright 2013 TNB Research
Thank You
Mohd Aizam Talib Contact:- [email protected] TNB Research Sdn. Bhd. No. 1, Lorong Ayer Itam, Kawasan Institusi Penyelidikan, 43000 Kajang, Selangor, MALAYSIA Tel: +603-89225000 Fax: +603-89268828 / 29
Slide No. 22
Slide Number 11.0 BackgroundSlide Number 3Slide Number 42.0 Diagnostic of In-Service Instrument TransformerDielectric Dissipation Factor / Power FactorSlide Number 7Dielectric Response AnalysisFrequency Dielectric SpectroscopySlide Number 10Polarization Depolarization CurrentOn-line Partial Discharge MeasurementSlide Number 133.0 Field MeasurementsSlide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 204.0 ConclusionsThank You