Maintenance Of Transformer

Any equipment in service, specially electrical equipment needs regular maintenance. Electrical equipments in service are subject to electrical, mechanical and thermal stresses. Periodical check – necessary to monitor condition. Measure to be taken to rectify defects to avoid total failure. 1

Transformer has two functional parts. a. Conducting material b. Insulating material Conducting material designed to carry rated current and insulating material designed to withstand the rated voltage. Currents and voltages in excess of rated values are taken care of by protective devices. In practice transformer is subjected to excessive current and voltages due to failure of protective equipments, leading to degradation of the material 2

Even during normal operating conditions the transformer is subjected to stresses resulting in degradation. Based on the design the transformers have normal life expectancy around 25 to 30 years. In practice transformers operate under abnormal conditions sometimes knowingly and many times unknowingly. This may lead to damage which, unless checked and corrective action taken may lead to total failures. 3

This process of checking and taking corrective action is known as maintenance. Types of maintenance: a. Break down maintenance – though not best – inevitable. b. Periodical/preventive maintenance – based on periodical tests. c. Proactive maintenance – The very cause which affects the transformer is anticipated and avoided. 4

• Predictive maintenance – modern technique – consisting of tests done using special equipments •

These tests can be termed as condition monitoring tests.

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No single test can confirm the fault in the transformer. Hence integrated approach is required taking into account results of various tests.

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Expertise and experience interpret the test results.

required

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Tests recommended for condition monitoring • Dissolved gas analysis of transformer. • Tan  and capacitance measurement of the insulation of the winding and of condenser type bushings. • Recovery voltage measurement • Insulation resistance and polarization index tests. • Frequency response analysis. • DC resistance of the winding. • Furan Analysis • Partial discharge tests.

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FAILURE OF 100MVA, 220/66/11KV POWER TRANSFORMER • 100 MVA, 220/66/11 KV Crompton Greaves make Power Transformer at Gowribidanur Station failed. • Physical inspection of the failed transformer • The entire oil of the transformer and the insulation of the winding are burnt. •

All the HV, LV & tertiary bushings are totally shattered.

• The main header connecting the radiator bank to the transformer tank and the radiator bank including its support attached to transformer tank have yielded. Marshalling box including the control cables have burnt. The conservator support structure has 7 yielded due to severe heat.

• 220 KV LAs, conductors of auxiliary bus have also burnt.   • The fire has commenced from the R-phase HV bushing and then it has spread to other areas. • B-phase HV bushing at the other extreme end is left only with the lead wire. •  LV bushing side is damaged to a much lesser extent the tap changer appears to be intact. •

Load on the transformer was 34 MW (current 80 Amps) on HV side 384A on the LV side.

• The voltage was 204 KV on HV side and 67 KV on LV side tap position at No.8. 8

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OLTC was operated about 3 hours before the failure.

  • The oil temperature and the winding temperature before failure was 420C. • Megger results were good. •

Since 2 years there is no interruption due to trouble in the transformer. • The day of failure was a clear day and no storm or lightning. • The details of relays provided for protection of the 9

Details of relays

 

Main protection: 1) 2) 3)

 

Differential REFR – HV – LV Over flux

Back up protection: 1) DOCR – HV – LV 2) DEFR – HV – LV 3) OCR – Tertiary

Relays: Temp HV Trip relay: LV Master Trip relay

Auxiliary 1)

Winding – –

2)

Bucholtz

Type & settings of relays

Operation of relays

    DTH32-bias – 30% CAG – 0.2 CAG – 0.2 GTT–K1.2/Time20 sec

    Operated Operated Not operated Not operated

  CDD CDD CDD CDD CDG

  Not Not Not Not Not

– – – – –

1 0.5 0.3 0.3 1.0

    30B – Trip 30C – High trip 30A – Trip 30D – High Trip   86

operated operated operated operated operated

    Operated Operated Operated Operated   Operated

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•   •   •   •   •   •

HV & LV side breakers tripped due to operation of the master trip relay. Internal fault in the transformer is ruled out. No Fault in OLTC. No Fault on 66KV bus and feeders. Failure of bushings. Over voltage due to lightning and switching surge. 11

• Due to weakening of the bushing.   • Reason for initiation of fire at R-phase bushing could be due to arcing.   • Arcing must have occurred due to shorting of phase to earth.   • Oil of the busing spilled over.   • Differential earth fault and transformer auxiliary relays operated and disconnected supply but fire spread to other parts. 12

 

 

• MEASURES TO BE TAKEN: a) Insulation Resistance and Polarization Index to be measured periodically. b) Tests on oil samples including DGA to be conducted periodically to monitor the condition of insulation inside the transformer. This is a very good test to detect incipient fault in their early stages. c) Tan delta and capacitance test of condenser bushings and windings to be conducted. d) Test for furan content in transformer oil. e) DC resistance test of winding.

 

f) Check tripping of 220 KV and 66 KV breakers for their proper functioning. 13

• Recovery voltage measurements.   • Frequency response analysis.   • Degree of Polymerization (D.P.).

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