Basics of Conditional monitoring of power transformer

Types of monitoring

• Behavior monitoring
• Periodic monitoring
• Online monitoring
• Diagnostic monitoring

Behavior monitoring

• Operating temperatures
• Noise level
• Oil levels
• Flow indicators

Periodic monitoring

• Impedance measurement
• Capacitance& tan delta of windings /bushings
• No load losses
• Magnetic balance
• Mag current
• Oil tests
• Dissolved gas analysis

Oil tests

• Breakdown voltage
• Moisture content
• Acidity
• Tan delta
• resistivity

Dissolved gas analysis

• Type of fault gasses
• H2  partial discharges
• Co   cellulose involvement
• C2 h2     acetylene  arcing
• Ch4 , c2h4, c2h6,  thermal hot spots
• Frequency of DGA
• Decision on investigation

Online monitoring

• Online dga
• Online PD

Diagnostic monitoring

• Low voltage tests
• Loss measurement
• Voltage ratio
• Resistance
• Frequency response analysis
• Degree of polymerization index
• Furan analysis (furfural content)

Frequency response analysis

• To detect movement of windings under short circuit conditions
• Low voltage sinusoidal output to one end of winding from network analyser
• Voltage transfer function measured from 5hz to 10mhz
• NA programmed to measure amplitude  and phase shift at discreet intervals of frequencies
• Condition assessment based on

      1. Comparison  of responses from different phases for each winding and tap position

       2. Comparison of responses from different transformer of the same design response comparison

• Response for all three phases to be similar on a particular tap position for a single transformer and for different transformer of same design
• Appearance of new features or major phse shift is concern.
• Transformer windings LC  network responses governed by inductance/capacitance of network

Frequency bandwidths

• 5hz  to 2khz indicate mag circuit condition
• 2khz  to 200 khz   changes in leakage impedance
• 1mhz and above  indicate capacitance changes

DP INDEX

•  Kraft paper used as insulation is a mat of cellulose fibers which consist of cellulose molecules
• Cellulose a linear polymer of glucose molecules that are linked by glycosidic bonds.
• Length cellulose molecule is measured in terms of DP Index.

Mechanical strength  of paper

• Depends on dpindex of paper
• New paper   1200 to 2000
• Below 250  a matter of concern
• Below 150 little life left and close to failure

Furfural analysis

• Product of cellulose  degradation are furan compounds particularly 2-furfural dehyde
• Compounds stable up to 110 degree centigrade
• Presence of compounds in oil is certain indication of cellulose degradation