Transient overvoltages, also known as voltage surges or spikes, can have various effects on transformer insulation. These overvoltages are typically of short duration but can have high amplitudes and can be caused by lightning strikes, switching operations, or other electrical disturbances. The effects of transient overvoltages on transformer insulation include:
Dielectric Stress: Transient overvoltages subject the transformer insulation to high levels of electric stress. This stress can cause localized breakdowns in the insulation material, leading to partial discharge (PD) activity.
Partial Discharge (PD): Transient overvoltages can result in partial discharge events within the insulation. PD is a localized electrical breakdown in the insulation that can degrade the insulation material over time, leading to reduced dielectric strength and eventually failure.
Insulation Breakdown: If the magnitude and duration of the transient overvoltage are severe enough, it can cause complete insulation breakdown, leading to flashover between transformer windings or to the transformer's core. This type of failure can be catastrophic and may require significant repair or replacement.
Increased Aging: The repeated occurrence of transient overvoltages can accelerate the aging of transformer insulation. The high-energy spikes cause thermal and mechanical stresses on the insulation, resulting in a reduction of its useful life.
Reduced Dielectric Strength: Transient overvoltages can lead to a reduction in the dielectric strength of the insulation. This means that the insulation may not be able to withstand normal operating voltages, leading to an increased risk of further insulation breakdown.
Electromagnetic Forces: Transient overvoltages can induce high electromagnetic forces within the transformer. These forces can cause mechanical stresses on the insulation and other transformer components, potentially leading to mechanical damage or misalignment.
Inter-turn Faults: In severe cases, transient overvoltages can cause inter-turn faults within the transformer windings. Inter-turn faults result from breakdowns in the insulation between individual turns of the winding and can lead to short circuits or reduced performance.
To mitigate the effects of transient overvoltages on transformer insulation, protective devices such as surge arresters are commonly used. Surge arresters divert the excess voltage to ground, protecting the transformer from high-energy spikes. Regular maintenance, insulation testing, and monitoring of the transformer can also help identify any degradation in insulation performance early on, allowing for timely repair or replacement before catastrophic failure occurs.