External faults are those faults or hazards that occur outside the transformer. These hazards present stresses on the transformer that may be of concern and may shorten the transformer life. These faults include the following.
• OVER LOADS
Overloads cause the transformer to overheat and have the potential to cause permanent damage or loss of life to the unit. The time constant for overheating is long, however, and it may take many hours of exposure for the condition to become serious. In most cases, no protection is provided for overload, but an alarm will often be used to warn operating personnel of the condition. One cause of overload may be due to unequal load sharing of parallel transformers or unbalanced loading of three-phase banks.• OVER VOLTAGE
Over-voltage can be either due to short-term transient conditions or long term power-frequency conditions. Transient over-voltages cause end-tum stresses and possible breakdown. These transients are protected against by surge protective devices that are designed for this purpose. Power frequency over-voltages occur due to an emergency operating condition, such as a sudden loss of load on an isolated portion the system that causes the voltage to rise. This condition causes over-ftuxing of the transformer and an increase in stress on the winding insulation. Over-fluxing increases iron losses and may result in a large increase in exciting current. Such conditions result in rapid heating of the iron circuits of the transformer, with possible damage to core lamination insulation and even to winding insulation.• UNDER FREQUENCY
Under frequency also is caused by a major system disturbance that causes an imbalance between generation and load. The condition is similar to overvoltage in that exciting current is greatly increased at low frequencies, causing over-fluxing of the transformer iron circuits. The transformer may be able to continue operation at either high voltage or under-frequency, but the two conditions experienced at the same time could be very serious. Usually, the ratio of voltage to frequency should not be allowed to exceed 1.1 per unit, which is usually called a "volts per hertz" limit.• EXTERNAL SYSTEM SHORT CIRCUITS
System faults that are external to the transformer protection zone, but cause high transformer currents, can cause transformer winding damage. Large external fault currents cause high mechanical stress in the transformer windings, with the maximum stress occurring during the first cycle. This short time frame makes it almost impossible to protect the transformer from experiencing these stresses. The protection strategy for these events is, therefore, a matter of transformer design.Most of the foregoing conditions are often ignored in specifying transformer relay protection, depending on the criticality of the transformer and its importance in the system. The exception is protection against over-fluxing, which may be provided by devices called "volts per hertz" relays, which detect either high voltage or under frequency, or both, and will disconnect the transformer if this quantity exceed a given limit, which is usually 1.1 per unit.