Forskelsstrøm

Forskelsstrøm is a term used in electrical engineering to describe the net current that does not return along its expected path within a circuit or protected zone. In an ideal, balanced system, the sum of currents entering and leaving a node is zero, but in practice a nonzero forskelsstrøm indicates leakage, fault, or abnormal operation. It is most commonly encountered in differential protection systems, where currents measured by current transformers around a device are continuously compared.

In transformer differential protection, for example, the current entering the transformer windings is compared with the current leaving them. A nonzero forskelsstrøm signals a potential internal fault such as a winding fault, bad connection, or insulation problem, and can trigger protective action if the difference exceeds a preset threshold. Protection schemes account for normal inrush currents, magnetizing currents, and measurement or CT saturation with time delays, slope characteristics, or harmonic restraint to avoid nuisance tripping.

Forskelsstrøm can also describe current imbalances in multi-branch circuits or equipment where uneven loading causes unequal currents in different conductors. It is distinct from jordfejlstrøm (earth fault current), which is the current leaking to earth due to insulation faults and is typically monitored by residual current devices.

Measurement relies on current transformers that encircle conductors, with the differential current processed by protective relays or metering equipment. Understanding forskelsstrøm is essential for electrical safety, fault detection, and reliable protection of transformers, generators, busbars, and other high-power equipment. See also differential protection, current transformer, residual current device, and earth fault current.