Transformer Protection and Transformer Fault

There are different kinds of transformers such as two winding or three winding electrical power transformers, auto transformer, regulating transformers, earthing transformers, rectifier transformers etc. Different transformers demand different schemes of transformer protection depending upon their importance, winding connections, earthing methods and mode of operation etc.
It is common practice to provide Buchholz relay protection to all 0.5 MVA and above transformers. While for all small size distribution transformers, only high voltage fuses are used as main protective device. For all larger rated and important distribution transformers, over current protection along with restricted earth fault protection is applied.
Differential protection should be provided in the transformers rated above 5 MVA.

Depending upon the normal service condition, nature of transformer faults, degree of sustained over load, scheme of tap changing, and many other factors, the suitable transformer protection schemes are chosen.

Nature of Transformer Faults

Although an electrical power transformer is a static device, but internal stresses arising from abnormal system conditions, must be taken into consideration.
A transformer generally suffers from following types of transformer fault-

1. Over current due to overloads and external short circuits,

2. Terminal faults,

3. Winding faults,

4. Incipient faults.

All the above mentioned transformer faults cause mechanical and thermal stresses inside the transformer winding and its connecting terminals. Thermal stresses lead to overheating which ultimately affect the insulation system of transformer. Deterioration of insulation leads to winding faults. Some time failure of transformer cooling system, leads to overheating of transformer. So the transformer protection schemes are very much required.

The short circuit current of an electrical transformer is normally limited by its reactance and for low reactance, the value of short circuit current may be excessively high. The duration of external short circuits which a transformer can sustain without damage as given in BSS 171:1936.

The general winding faults in transformer are either earth faults or inter-turns faults. Phase to phase winding faults in a transformer is rare. The phase faults in an electrical transformer may be occurred due to bushing flash over and faults in tap changer equipment. Whatever may be the faults, the transformer must be isolated instantly during fault otherwise major breakdown may occur in the electrical power system.
Incipient faults are internal faults which constitute no immediate hazard. But it these faults are over looked and not taken care of, these may lead to major faults. The faults in this group are mainly inter-lamination short circuit due to insulation failure between core lamination, lowering the oil level due to oil leakage, blockage of oil flow paths. All these faults lead to overheating. So transformer protection scheme is required for incipient transformer faults also. The earth fault, very nearer to neutral point of transformer star winding may also be considered as an incipient fault.
Influence of winding connections and earthing on earth fault current magnitude.
There are mainly two conditions for earth fault current to flow during winding to earth faults,

1. A current exists for the current to flow into and out of the winding.

2. Ampere-turns balance is maintained between the windings.

The value of winding earth fault current depends upon position of the fault on the winding, method of winding connection and method of earthing. The star point of the windings may be earthed either solidly or via a resistor. On delta side of the transformer the system is earthed through an earthing transformer. Grounding or earthing transformer provides low impedance path to the zero sequence current and high impedance to the positive and negative sequence currents.

Star Winding with Neutral Resistance Earthed

In this case the neutral point of the transformer is earthed via a resistor and the value of impedance of it, is much higher than that of winding impedance of the transformer. That means the value of transformer winding impedance is negligible compared to impedance of earthing resistor. The value of earth current is, therefore, proportional to the position of the fault in the winding. As the fault current in the primary winding of the transformers is proportional to the ratio of the short circuited secondary turns to the total turns on the primary winding, the primary fault current will be proportional to the square of the percentage of winding short circuited. The variation of fault current both in the primary and secondary winding is shown below.

Star Winding with Neutral Solidly Earthed

In this case the earth fault current magnitude is limited solely by the winding impedance and the fault is no longer proportional to the position of the fault. The reason for this non linearity is unbalanced flux linkage.

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