Problems related to the increase in the transformer temperature above the assumed maximum atmospheric temperature require certain means of protection. Selectivity means that the device must clear any fault of current interruption being the nearest to the fault, even if the fault is detected by other protections associated with other interruption devices. Transformer issues need certain means of protection. Here’s how you can shield a high voltage transformer from a trip.
Protection of transformers
Stresses triggered by the supply
Lightning overvoltage attributable to lightning strokes landing on or above the overhead line providing the installation where the transformer is installed. Switching high voltage auto transformer is created by opening a circuit breaker or a load split switch. Depending on the application, protection against these two forms of voltage surges may be needed and is always assured by using ZnO surge arresters ideally attached to the transformer’s MV bushing.
Electrical devices and circuitry in a substation must be secured to mitigate damage due to intermittent currents and overvoltage.
Both instruments installed in an electrical power grid have uniform ratings for short-term resistance to current and short-term power frequency voltage. The protection’s task is to ensure that these thresholds of resistance can never be surpassed, thereby clearing the faults as soon as possible.
Stresses Due To the Load
The overload of the transformer is often attributed to a rise in the installation’s apparent power demand (kVA). This rise in demand will result either from a progressive addition of loads or an expansion of the installation itself. The consequence of any excess is an increase in the temperature of the oil and the transformer winding with a decrease in its lifespan.
Reliable security protects the transformer from overload, typically referred to as a thermal overload relay. This method of safety simulates the winding temperature of the transformer. The simulation is based on the calculation of the current and the thermal time constant of the transformer. Some relays may consider the effects of current harmonics due to non-linear loads such as rectifiers, machines, variable speed drives, etc. This relay method can also be used to determine the available time before the tripping order emission and the time delay before the re-energizing of the transformer. Besides, a high voltage autotransformer is fitted with high-temperature sensor thermostats.
Transformer Overcurrent Protection
Transformer overcurrent protection is required to protect primary windings from short circuits and overloads and secondary windings from overload.
Transformer overcurrent, such as those of other instruments, can be categorized as overload or short circuits. The transformer’s overload state is said to occur when it delivers its usual current rating from one to six times. In this case, the current is limited to its normal course, and the temperature increase in the transformer occurs.
If there is a high voltage autotransformer condition, the current is not limited to the load’s usual wiring channels. Short circuit currents can exceed hundreds of times higher than the usual full load operating current. Harm may be severe if the defensive devices do not respond to the current direction generated in milliseconds. As discussed above, the transformer’s impedance rating is used to measure the short-circuit current and assess the interrupting current power required by the protection system.