18_Aizam_HV IT in TNB Substations.pdf

Copyright 2013 TNB Research

Diagnostic And Condition Assessment Of High Voltage Instrument Transformer in

TNB Transmission Substation

Slide No. 1

Mohd Aizam Talib

Copyright 2013 TNB Research Slide No. 2

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


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


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


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


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


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


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)


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


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


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.


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


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


FDS response of current transformer

* higher response at B phase


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


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


PD laboratory experimental set-up

HV Source

Test Vessel HFCT

MPD600

Phase resolved PD pattern of void inside the pressboard sample


HFCT MPD600

External hardware gating

On-line PD measurement at site


PD pattern for instrument transformer with high PD activity, 800pC

PD pattern for instrument transformer with low PD activity, 100pC


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.

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

18_Aizam_HV IT in TNB Substations.pdf

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