ONLINE MONITORING APPLICATIONS OF POWER TRANSFORMER Deepak Subedi

Why monitoring of Power Transformer is required Ø Critical Assest of Power System (Transmission & Distribution System) Ø Transformer failure leads to power outage, financial loss, environmental hazards Ø Good tool to reduce power outage caused by Transformer failure Ø Can enhance the life time of the Transformer Ø Reduces the loss encountered due to Transformer failure Ø Can supplement to diagnose the reasons behind the Faiure

Methods for Condition Monitoring Ø Offline Monitoring o Accurate and useful information of the Transformer condition. o Transformer isolated from the network and may use intrinsive methods for measurement o Return Voltage Measurement, dielectric frequency response, gas analysis of oil sample Ø Online Monitoring o Temperature measurement, oil status, partial discharge o Winding movement, dielectric frequency response, load tap changers o Censors are used for the measurement o Censors may include optical fibers, vibration sensors, temperature sensors, UHF antennas, gas censors, capacitive censors and etc.

Temperature Measurement Ø Overload and malfunction in cooling system may lead to increase in temp. Ø Increase may temp. may cause ageing of insulation Thin Film Sensor • Advanced technique to measure temperature. • More accurate and reliable than traditional Pt 100 thermocouple devices • Thickness of film from 12 to 50 nm, significantly smaller than Pt 100 devices. • No air gap and changing of flux inside the core. • Can be used in crictical installation locations. Enhanced Fibre Optic Sensor • More accurate and reliable than traditional Pt 100 thermocouple devices • Sensing probe integrated with a plastic cladding large-core (200 m) optical fiber • Temperature detection is based on analog to digital processing hardware to monitor the power output of the optical fiber. • The resolution is 0, 2 C and accuracy is 0, 5 C for the measurement system

Gas Analysis • Electrical and thermal stresses such as arching, partial discharges and overheating cause degradation of dielectric oil and solid dielectric cellulose materials. • Different degradation mechanisms generate different gases thus making it possible to determine the degrading part of the transformer Online DGA • Traditionally DGA carried out using off-line measurements. • Recently, better techniques for gas analysis using a membrane or vacuum extraction. • Semiconductor sensors, infrared sensors, combustible gas detector and gas chromatography are commercially available. • Methods applied to DGA can be used to reveal apparent fault conditions. Online Gas Phase Monitoring • Carried out using infrared spectroscopy measurements • One application is to use a Clemet TM Fourier transform infrared spectroscope (FTIR) to measure the free gases in the inert gas blanket above the transformer oil. • FTIR can detect all hydrocarbons and even Furans too. • Measuring free gas quantities can provide helpful information on insulation status more readily than DGA.

Partial Discharge Measurement • Power transformer breakdowns are most frequently preceded by partial discharges (PD). • Every partial discharge occurring inside the transformer generates an electrical pulse, a mechanical pulse and electromagnetic waves which can be detected by using various sensors and techniques. UHF PD Detection • Partial discharges occurring inside the transformer excite electromagnetic waves with resonance at frequencies 500 – 1500 MHz. • Possible causes of PD: temporary overvoltage, weakness in the insulation introduced during manufacturing or due to various aging effects • Different types of data handling layers are used in this on-line application. • Four data layers: data monitoring layer, interpretation layer, corroboration layer and information layer. • PD sensor data can be analysed using these methods: time-energy mapping and clustering of PD data, time-frequency analysis combined with feature extraction and clustering and phase-resolved data representation. P. D. Contd……

New techniques for online PD Measurement: • On-line PD measurements of HV equipment suffer from heavy noise caused by various origin which effects the measurement accuracy. • Hardware and software approaches used to reduce the noises. • New sensors introduced with PD coupler board and multi channel PD detector. • Sensing board installed near of the HV conductor and the stray capacitance acts as a coupling capacitor. Vibration Measurement • Two major reasons: Core vibration and Winding Vibration • Core Vibration caused by excitation by magnetostriction and excitation generated in the air gaps. • Winding Vibration generated by Lorenz force due to correlation of leakage flux and winding current. • Piezoelectric accelerometer is positioned at different locations on the transformer tank wall. • On-line measurements are conducted under load and no-load conditions. • Spectrum of the signal is calculated for analysis. Spectrum of winding vibration can be found by subtracting the noload results from the loaded results • Different characteristics of the spectrum of the measured vibration of a power transformer in a good condition can be used as a reference during on-line monitoring.

On-Load Tap Changer Monitoring • Tap changers are the mechanical switching devices of the power transformer. • OLTC malfunction can destroy the complete transformer. • OLTCs cause more failures and outages than any other component of a power transformer. On-line monitoring of OLTCs • OLTC possible failures include: motor malfunction, loss of power and flaw in controlling circuits may cause the tap change to stop before its completion. • The tap changer needs 5 – 6 seconds to finish its operation. During this time, different sensors are used for on-line monitoring. • High sampling rate is needed. The system consists of a 30 k. Hz accelerometer, clamp-on current transformer and pair of thermocouples • Monitoring of OLTCs is carried out using measurements on vibration of the contact movements, temperature of the insulation oil, voltage and current of the drive motor. • The system is triggered by operation of the OLTC and above-mentioned measurements are conducted together with detection of the tap position.

Others Measurement and Monitoring • FURFURALDEHYDE MEASUREMENTS: – On-line FFA measurements using optical sensor • MECHANICAL FAULT DETECTION USING FREQUENCY RESPONSE ANALYSIS • On-line low voltage impulse test method – Low voltage impulse (LVI) also known as FRA is used to monitor possible winding movement on-line. • TAN- ON-LINE MONITORING – Measurement of the tan- of power transformer insulation used to determine the quality of the insulation as a standard test before initial installation.