ONLINE MONITORING APPLICATIONS OF POWER TRANSFORMER Deepak Subedi

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

Methods for Condition Monitoring  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  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  Overload and malfunction in cooling system may lead to increase in temp.  Increase may temp. may cause ageing of insulation Thin Film Sensor Enhanced Fibre Optic Sensor Advanced technique to measure temperature. More accurate and reliable than traditional Pt100 thermocouple devices Thickness of film from 12 to 50nm, significantly smaller than Pt100 devices. No air gap and changing of flux inside the core. Can be used in crictical installation locations. More accurate and reliable than traditional Pt100 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 no- load 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 kHz 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.