There are various sorts of transformers like two winding or three winding electrical power transformers, auto transformers, controlling transformers, earthing transformers, rectifier transformers and so forth. Various transformers request different transformer assurance plans relying upon their significance, winding association, earthing technique and mode. Activity and so on
It is normal practice to give Buchholz hand-off insurance to all transformers of 0.5 MVA or more. While for all little size appropriation transformers, just high voltage wires are utilized as the super defensive gadget. For all huge appraised and basic appropriation transformers, restricted earth shortcoming insurance with overcurrent security is applied.
Transformers evaluated over 5 MVA ought to be given differential insurance.
Depending on the normal service condition, nature of transformer faults, degree of continuous load, tap changeover scheme, and many other factors, suitable transformer protection schemes are selected.
Nature of Transformer Faults:
Although an electrical power transformer is a static device, internal stresses arising from abnormal system conditions must be taken into account.
A transformer is generally prone to the following types of transformer faults.
- Over current in light of over-loads and external shortcircuits,
- Terminal issues,
- Winding issues,
- Early faults.
All the above transformer faults cause mechanical and thermal stresses within the transformer winding and its connecting terminals. Thermal stress leads to overheating which eventually affects the insulation system of the transformer. Insulation faults lead to winding faults. An occasional malfunction in the transformer cooling system causes the transformer to overheat. Hence there is a great need for transformer protection schemes.
The short circuit current of an electrical transformer is usually limited by its reactance and for low reactance, the value of short circuit current can be very high. Duration of external short circuits which the transformer can sustain without damage as given in BSS 171:1936.
| Transformer % reactance: | Allowed shortcoming span right away |
| 4 % | 2 |
| 5 % | 3 |
| 6 % | 4 |
| 7 % and over | 5 |
Common winding faults in a transformer are either earth faults or inter-turn faults. Phase-to-phase winding faults are very rare in transformers. Phase faults in electrical transformers can be caused by bushing flash over and faults in tap changer equipment. Irrespective of the fault, the transformer should be isolated immediately at the time of fault otherwise a major fault may occur in the power system.
Primary defects are internal defects that pose no immediate threat. But if these defects are seen more and are not taken care of, they can lead to major problems. Faults in this group are mainly inter-lamination short circuits due to insulation failure between core lamination, low oil level due to oil leakage, obstruction of oil flow paths. All these defects lead to overheating. Hence a transformer protection scheme is required even for primary transformer faults. An earth fault very close to the neutral point of the transformer star winding can also be considered as a primary fault.
Effect of winding connection and earthing on magnitude of earth fault current.
There are basically two conditions for earth fault current to flow while winding earth faults,
- An ongoing exists for the current to stream into and out of the winding.
- Ampere-turns balance is kept up with between the wingdings.
The value of the winding earth fault current depends on the position of the fault on the winding, the method of winding connection and the method of earthing. The star point of the windings can be earthed either solidly or through a resistor. The system is earthed by an earthing transformer on the delta side of the transformer. A grounding or earthing transformer provides a low-impedance path for zero-sequence currents and a high-impedance path for positive and negative-sequence currents.
Star Winding with Neutral Resistance Earthed:
In this case the neutral point of the transformer is earthed through a resistor and its impedance value is much higher than the winding impedance of the transformer. This means that the value of the transformer winding impedance is negligible compared to the impedance of the earthing resistor. The value of the ground current is, therefore, proportional to the position of the fault in the winding. Since the fault current in the primary winding of a transformer is proportional to the ratio of the total turns on the primary winding to the short-circuited secondary turns, the primary fault current will be proportional to the square of the percentage of the wingdings short-circuited. The variation of fault current in both primary and secondary winding is shown below.
Star Winding with Neutral Solidly Earthed:
In this case the neutral point of the transformer is earthed through a resistor and its impedance value is much higher than the winding impedance of the transformer. This means that the value of the transformer winding impedance is negligible compared to the impedance of the earthing resistor. The value of the ground current is, therefore, proportional to the position of the fault in the winding. Since the fault current in the primary winding of a transformer is proportional to the ratio of the total turns on the primary winding to the short-circuited secondary turns, the primary fault current will be proportional to the square of the percentage of the windings short-circuited. The variation of fault current in both primary and secondary winding is shown below.
