Certain phenomena can cause a substantial differential current to flow, when there is no fault, and these differential currents are generally sufficient to cause a percentage differential relay to trip.
However, in these situations, the differential protection should not disconnect the system because it is not a transformer internal fault.
Such phenomena can be due to the non-linearities in the transformer core. Some of these situations are considered below //
Magnetizing inrush current in transformers results from any abrupt change of the magnetizing voltage. Although usually considered as a result of energizing a transformer, the magnetizing inrush may be also caused by //
- Occurrence of an external fault
- Voltage recovery after clearing an external fault
- Change of the character of a fault (for example when a phase-to-ground fault evolves into a phase-to-phase-to-ground fault)
- Out-of-phase synchronizing of a connected generator
An example of inrush current following a reclose operation measured at the distribution substation breaker.
Since the magnetizing branch representing the core appears as a shunt element in the transformer equivalent circuit, the magnetizing current upsets the balance between the currents at the transformer terminals, and is therefore experienced by the differential relay as a “false” differential current.
The following summarizes the main characteristics of inrush currents //
- Generally contain DC offset, odd harmonics, and even harmonics.
- Typically composed of unipolar or bipolar pulses, separated by intervals of very low current values.
- Peak values of unipolar inrush current pulses decrease very slowly.
- Time constant is typically much greater than that of the exponentially decaying dc offset of fault currents.
- Second-harmonic content starts with a low value and increases as the inrush current decreases.
Read more about practical considerations of transformer inrush current //
Overexcitation of a transformer could cause unnecessary operation of transformer differential relays. This situation may occur in generating plants when a unit-connected generator is separated while exporting VARs. The resulting sudden voltage rise impressed on the unit transformer windings from the loss of VAR load can cause a higher than nominal volts per hertz condition and, therefore, an overexcitation event.
This could also occur in transmission systems where large reactive load is tripped from a transformer with the primary winding remaining energized.
When the primary winding of a transformer is overexcited and driven into saturation, more power appears to be flowing into the primary of the transformer than is flowing out of the secondary. A differential relay, with its inputs supplied by properly selected CTs to accommodate ratio and phase shift, will perceive this as a current differential between the primary and secondary windings and, therefore, will operate.
Since overexcitation manifests itself with the production of odd harmonics, and since the third harmonic (and other triples) may be effectively cancelled in Δ transformer windings, then, the fifth harmonic can be used as a restraining or a blocking quantity in the differential relay in order to discriminate between the over-excitation and the faulty state.
The effect of CT saturation on transformer differential protection is double-edged. Although, the percentage restraint reduce the effect of the unbalanced differential current, in the case of an external faults, the resulting differential current which may be of very high magnitude can lead to a relay male-operation!
For internal faults, the harmonics resulting from CT saturation could delay the operation of differential relays having harmonic restraint.
Reference // Transformer Differential Protection Scheme With Internal Faults Detection Algorithm Using Second Harmonics Restrain And Fifth Harmonics Blocking Logic – Ouahdi Dris, Farag. M. Elmareimi and Rekina Fouad