Transformer Protection System

There are different types of transformers, such as two or three winding electricity transformers, automatic transformers, regulating transformers, grounding transformers, grinding transformers, etc. Different transformers require different protection schemes depending on their importance, winding connections, methods and methods of earthing. operation etc.

It is common practice to provide Buchholz relay protection to all transformers of 0.5 MVA and above. While for all small distribution transformers, only high voltage fuses are used as the main protective device. For all the main and important distribution transformers, the overcurrent protection is applied together with the limited protection from earth faults.

A transformer generally suffers from the following types of transformer failure:

Overcurrent due to external overloads and short circuits,

Terminal failures,

windings,

Incipient failures.

All the transformers failures mentioned above cause mechanical and thermal stresses inside the winding of the transformer and its connection terminals. Thermal stresses lead to overheating, which ultimately affects the transformer insulation system. The deterioration of the insulation leads to the failure of the winding. Sometimes the failure of the transformer cooling system leads to overheating of the transformer. Therefore, transformer protection schemes are very necessary.

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

Transformer % reactancePermitted fault duration in seconds
4 %2
5 %3
6 %4
7 % and over5

The transformer general winding faults are ground faults or inter-turn faults. Phase-to-phase winding failures in a transformer are rare. Phase faults in an electrical transformer can occur due to a flash of the bushing and faults in the tap-changer equipment. Whatever the faults, the transformer must be immediately isolated during the fault; otherwise, a serious failure in the power supply system may occur.

Incipient faults are internal faults which do not constitute an immediate danger. But if these failures are overlooked and unresolved, they can lead to serious failures. The faults in this group are mainly an interlaminated short circuit due to an insulation error between the core lamination, which reduces the oil level due to oil leaks, blocking the oil flow paths. All these failures lead to overheating. Therefore, a transformer protection scheme is also required for transformer incipient failures. The grounding error, very close to the neutral point of the transformer star winding, can also be considered an incipient error.

Influence of the winding and grounding connections on the magnitude of the ground fault current.

There are mainly two conditions for the flow of earth fault current during winding to earth faults,

There is a current for the current that flows in and out of the winding.

Amp-turn balance is maintained between the windings.

The value of the winding earth fault current depends on the fault position in the winding, the winding connection method and the grounding method. The star point of the windings can be fixed solidly or through a resistance. On the delta side of the transformer, the system is connected to earth via a grounding transformer. The grounding or grounding transformer provides a low impedance path to the zero sequence current and a high impedance to positive and negative sequence currents.

Star cord with neutral resistance connected to earth

In this case, the neutral point of the transformer is connected to earth through a resistance and the value of its impedance is much greater than that of the winding impedance of the transformer. This means that the impedance value of the transformer winding is insignificant with respect to the impedance of the earth resistance. The earth current value is therefore proportional to the position of the fault in the winding. Since the fault current in the primary winding of the transformers is proportional to the ratio of the secondary turns in short circuit with respect to the total turns in the primary winding, the primary fault current will be proportional to the square of the percentage of the short circuit winding. The variation of the fault current in the primary and secondary windings is shown below.

Low vacuum protection

This protection, generally in the form of a regulator that compares the vacuum with the atmospheric pressure, normally adapts to the generator set above 30 MW. Modern practice requires that the regulator discharges the group through the secondary regulator until normal vacuum conditions are restored. If the vacuum conditions do not improve below 21 inches, the shut-off valves close and the main switch trips.

Protection against breakage of lubrication oil

This protection is not considered essential as the lubricating oil is normally obtained from the regulator oil pump itself and a regulator oil failure causes the shut-off valve to close automatically.

Protection against boiler leakage

Two methods are available to detect boiler combustion loss. In the first method, normally open (NO) contacts are provided with the fan motors which can trip the generator in the event of failure of more than two motors. The second methods use boiler pressure contacts which discharge the generator if the boiler pressure drops below approximately 90%.

Main motor failure protection

If the primary engine is unable to supply mechanical power to the generator, the generator will continue to rotate in engine mode, which means it absorbs electricity from the system rather than supplying it to the system.

In a steam turbine, the steam acts as a coolant by keeping the turbine blades at a constant temperature. Therefore, interrupting the power supply will cause overheating due to friction, with consequent distortion of the turbine blades.

Over-speed protection

Although it is common practice to provide overspeed mechanical devices on both the steam turbine and hydraulic turbine, which operate directly on the steam throttle valve or main shutoff valve, it is not common to back up these overspeed relay devices to steam powered appliances.

However, it is considered a good practice in hydroelectric units, since the regulator response is relatively slow and the whole is more subject to excessive speed. The installed relay is generally supplied by the permanent magnet generator used to control the regulator.

Protection against rotor distortion

The cooling rates after stopping at the top and bottom of the turbine housing are different and this uneven temperature distribution tends to cause destruction of the rotor. To minimize interruption, it is common practice to rotate the rotor at low speed during the cooling period. In view of the forces involved in a large modern rotor, it is now standard practice to install eccentric shaft detectors.

Star winding with neutral firmly connected to earth

In this case, the magnitude of the earth fault current is limited only by the impedance of the winding and the fault is no longer proportional to the fault position. The reason for this non-linearity is the unbalanced flow bond.

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