When does resonance in electrical systems occur?

Resonance in electrical systems occurs when harmonic currents interact with system impedance. This phenomenon happens typically in AC circuits and is characterized by a condition where the inductive and capacitive reactances are equal in magnitude, resulting in a significant increase in current at specific frequencies known as resonant frequencies.

When harmonic currents, which are multiples of the fundamental frequency, are present in a system, they can interact with the system's impedance, leading to elevation in voltage and current levels. This interaction can produce unwanted effects such as voltage distortion, overheating, and even potential damage to components in the system.

The other options do not accurately describe the conditions for resonance. For instance, transformer overload typically refers to excess power being drawn, while high voltage may not necessarily induce resonance without the proper conditions of inductance and capacitance being met. The installation of capacitors alone does not create resonance; instead, it is the interplay of these components with inductive elements and the system's frequency response that leads to resonance.