Transformer Protection Types | Nature and Classes of Faults

Transformer Protection Types:

Nature of Transformer Faults: Power transformers, being static, totally enclosed and oil immersed develop faults only rarely but the consequences of even a rare fault may be serious, unless the transformer is quickly disconnected from the system. For the purpose of Transformer Protection Types, faults can be divided into three main classes:

Faults in the auxiliary equipment which is part of the transformer.
Faults in the transformer windings and connections.
Overloads and external short circuits.

Faults in Auxiliary Equipment:

The detection of faults in auxiliary equipment is necessary to prevent ultimate failure of the main transformer windings. The following can be considered as auxiliary equipment.

(i) Transformer Oil: Low oil is a dangerous condition in a transformer because live parts and the leads to bushings, etc. which have to be under oil, get exposed if the oil drops below the specified level. Oil level indicators with alarm contacts are available to give indication for immediate attention.

(ii) Gas Cushion: Deterioration of transformer oil and insulation is minimized if oxygen and moisture are excluded from the gas space. As the normal operating pressure within the tank varies rather widely, so sealing of the tank is not always recommended. A pressure vacuum gauge can be used on a sealed tank to provide a visual indication of pressure in the tank. Conservator tank is mounted on the main tank to take the expansion and contractions of the oil whereas silica gel is placed in the breathing vent to prevent moisture from entering. Sometimes a nitrogen cylinder is connected to the gas space, with regulating devices, to maintain the pressure between 0.5 and 0.8 atmosphere. This scheme maintains an inert atmosphere in spite of small leaks and provides some indication of presence of leaks by the rate of exhaustion of nitrogen in the cylinder. Alarm contacts can be provided in the system to indicate pressure inside the tank, above, or below predetermined limits.

(iii) Oil Pumps and Forced Air Fans: The top oil temperature normally gives indication of the load on the transformer. Increased oil temperature might be an indication of an overload or it might be due to a fault in the cooling system, such as failure of the oil pump or the blocking of a radiator valve, or non-operation of fans. A thermometer with alarm contacts will indicate rise in the oil temperature due to any of these An oil flow indicator is commonly used to indicate proper operation of oil pumps.

(iv) Core and Winding Insulations: Incipient faults may occur initially which may develop into major faults if not taken care of at the initial stages. Insulation failure may develop because of the following:

The insulation of the laminations and core bolts may be of poor quality or has been damaged accidentally during erection.
The insulation between the windings, between winding and the core, and the conductor insulation may be of poor quality; may have been damaged mechanically; may be brittle because of ageing or over-loading.
Badly made joints or connections.

These incipient faults need not be attended to immediately and, as such do not cause any interruption to the supply service, but must be taken care of as soon as possible. Gas actuated relays described later in this chapter provide indication alarms for incipient faults.

Winding Faults:

Electrical faults which cause immediate serious damage and are detected by unbalance current or voltage may be divided into the following classes:

Faults between adjacent turns or parts of coils such as phase-to-phase faults on the HV and LV external terminals or on the windings itself or short circuits between turns of HV and LV
Faults to ground or across complete windings such as phase-to-earth faults on the HV and LV external terminals or on the windings.

A short circuit between turns can start with a point contact resulting from mechanical forces or insulation deterioration due to excessive overload or a loose connection, breakdown of transformer insulation by an impulse voltage. The puncture of the turn insulation by an impulse is supposed to cause a path of destruction, through which the normal frequency voltage can maintain an arc. However, if the turn voltage is insufficient to maintain the arc it will be quenched by the oil at the first current zero.

Faults to ground, or across a large part of the winding will result in large values of fault currents as well as the emitting of large amounts of gas due to the decomposition of oil. This Transformer Protection Types is not difficult to detect, but rapid clearance of fault is essential to avoid excessive damage and to maintain system stability.

Overloads and External Short Circuits:

Overloads can be sustained for long periods, being limited only by the permitted temperature rise in the windings and cooling medium. Excessive overloading will result in deterioration of insulation and subsequent failure. It is usual to monitor the winding and oil temperature conditions and an alarm is initiated when the permitted temperature limits are exceeded. External short circuits may only be limited by the transformer reactance and where this is low fault currents may be excessive.