a) Dielectric Strength - The BDV of oil is its ability to withstand electric stresses without failure.
b) Resistivity - It is the measure of electrical insulating properties of oil. High resistivity reflects low content of free flowing particles.
c) Dielectric Dissipation Factor (Tan δ) - It is a measure to the ratio of the power dissipated in the oil to the product of effective voltage and current. It is tangent of loss angle & expressed in unit or percent. It determines the cleanliness of oil & is related to aging characteristic of the oil.
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Standard Values of New Oil as per IS : 335-1983
S/No. Characteristics Standard Values
1 Breakdown Value
a) > 145 KV 60 KV (min.)
b) 72.5 to 145 KV 50 KV (min.)
c) < 72.5 KV 40 KV (min.)
2 Dissipation Factor at 90°C (Tanδ)
0.05
3 Specific Resistance at 90°C 1 X 10 * 12 (Ω-cm) 4 Water content ppm
a) > 145 KV 15 ppm (max.)
b) 72.5 to 145 KV 20 ppm (max.)
c) < 72.5 KV 25 ppm (max.)
5 Inter Facial Tension 0.03 N/m (min.)
6 Density 0.89 g/cu.cm (min.)
7 Flash Point 140°C (max.)
8 Pour Point - 6°C (max.)
9 Total Acidity Test 0.03 mg KOH/g (max.)
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Dissolved Gas Analysis (DGA): - Transformer, in operation, is subjected to various thermal and electrical stresses, resulting in liberation of gases from the oil which is used as insulation media and coolant. The solid insulating materials like paper, wooden support, pressboard, etc. cause degradation and form different gases, which get dissolved in the oil. The most significant gases generated are Hydrogen (H2), Methane (CH4), Ethylene (C2H4), Acetylene (C2H2), Propane (C3H8), Propylene (C3H6), Carbon Monoxide (CO), Carbon Dioxide (CO2), and Ethane (C2H6). The gas connected in the relay will help to identify the nature of the fault. The greater the rate of gas collection, the more severe is the nature of the developing fault.
Colour of the gas helps in finding the affected material.
Colour --- Identification
White --- Gas of decomposed paper and cloth insulation Yellow --- Gas of decomposed wood insulation
Grey --- Gas of overheated oil due to burning of iron portion Black --- Gas of decomposed oil due to electric arc
Ratio Method used for Analysis of DGA results:-In this method three ratios of gases are used. They are methane / hydrogen, ethylene / ethane, acetylene / ethylene. If the ratio comes out more than one, it indicates abnormal deterioration and less than one indicates normal aging.
Particulars C2H2/C2H4 CH4/H2 C2H4/C2H6
a) Less than 0.1 0 1 0
b) 0.1 to 1.0 1 0 0
c) 1.0 to 3.0 1 2 1
d) More than 3.0 2 2 2
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Interpretation of the faults according to the observed ratios of Gases
Characteristic
Fault Ratio Code Diagnosis
C2H2/C2H4 CH4/H2 C2H4/C2H6
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Actions during failure / tripping of Transformer –
The action to be taken depends upon the size of the transformer, operation of protective relay, whether tripping is accompanied by loud noise, smoke or expulsion of oil from the transformer, etc. Observe the transformer external condition; look for any damage to the bushings, leads or cable box. Note the temperature of oil & check if the oil level in the conservator is right. Take megger readings between primary and secondary and also of each to earth. If everything is right, proceed as noted below:
- The failure may possibly be due to a sudden and heavy overload, or short-circuit.
If a DO fuse has dropped out, check if its ampere rating is right. If incorrect, replace by the correct size and energise the transformer, after switching off the load. If everything is all right, close the secondary circuit; if fuse blows again, the fault is obviously in the outgoing lines, which should be traced and rectified; if on the other hand, the primary circuit fuse blows out, even when the load is disconnected, an internal fault is indicated. This also apply, if an over current relay alone has operated and tripped the breaker.
- If a differential relay operates when a transformer is first switched on, it may be due to a switching surge. Check the harmonic restraint circuit and setting. If, on the other hand, relay operates when the transformer is in service, it is a sure indication of an internal fault.
Any tripping of buchholz relay requires to be carefully looked into. If the lower assembly has tripped due to sudden evolution of large quantities of gas, a major internal fault is to be inferred especially if either over current or differential or earth fault relay has operated. If, on the other hand, the upper assembly has operated due to slow release of gas it is necessary to find out its composition before any conclusions can be drawn. If it is air only, there is no cause for worry, as air can enter into the transformer in many ways.
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When transformer is commissioned, it sometimes happens that the buchholz relay upper assembly operates, after a few hours of run, due to the release of air bubbles entrapped within the windings, such as when hand filling is employed for filling of oil into the tank. If the accumulated gas is not air, an incipient fault is indicated.
DGA would help in identifying the nature of the fault, and this should be done as a routine measure. If the buchholz relay has tripped, without any gas being given out, it may be due to electrical fault in the wiring.
- Thorough checking is required if the earth fault relay has tripped, or there is an evolution of smoke or oil, and also PRV has operated in case of large transformer.
In such cases reclosing of breaker should not be permitted as it may cause further extensive damage. Detailed testing of transformer is to be carried out and compare the results with test certificate figures and consult the manufacturer.
In most of the cases, the cause of the fault can be found out if you carefully observe the condition of windings by lifting the core and coil assembly. The following notes may be of help in identifying the cause:
Lightning discharge or over voltage: This is characterised by breakdown of the end turns close to the line terminal. There may be a break in the turns or end lead, and also flash marks on the end coil and earthed parts close to it, but the rest of the coils will be found to be healthy.
Sustained overloads: The windings in one or all the phases would show signs of overheating and charring; the insulation would be very brittle and have lost all its elasticity.
Inter-turn short: The same signs as for sustained overloads would be noticed, but only on one coil, the rest of the coils being intact.
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Dead short-circuit: This can be identified by the unmistakable, lateral or axial displacement of the coils. The coils may be loose on the core; some turns on the outermost layer may have burst outwards and broken as if under tension. If, in addition to these signs, the windings are also completely charred, it is conclusive evidence that the short-circuit has continued for an appreciable period, not having been cleared quickly by the protective relays.
Visual checking of Transformer: -
- Check the colour of silica gel. If it is pink, reactivate or replace it. Also ensure proper quantity of oil in breather oil cup.
- Check oil level in Conservator of Main Tank & OLTC. It should be > ½ level marking.
- Check oil level in Bushings.
- Check for any oil leakage. Arrest leakages, if any.
- Check the working of OTI & WTI by taking hourly temperature readings. There should be changes in readings as per loading of transformer and atmospheric condition.
- Check the cooling system by making fans / pumps operation by manually.
- Check the tap position of RTCC panel and OLTC panel. It should have same position number.
Check the humming noise & vibration of transformer. If any abnormality found, it is to be referred to concerned manufacturer.
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