In modern differential relays the bias is applied to transformer differential protection to ensure stability for external faults while allowing sensitive settings to pick up internal faults. The situation is slightly complicated if a tap changer is present. With line CT ratios and correction factors set to achieve current balance at nominal tap, an off-nominal tap may be seen by the differential protection as an internal fault.
By selecting the minimum bias to be greater than sum of the maximum tap of the transformer and possible CT errors, maloperation due to this cause is avoided. Some relays use a bias characteristic with three sections, as shown in below Figure. The first section is set higher than the transformer magnetising current. The second section is set to allow for off-nominal tap settings, while the third has a larger bias slope beginning well above rated current to cater for heavy through-fault conditions.
Without on-load tap-changer regulator: 15% percentage characteristic – minimum operating threshold 30% of In
With on-load tap-changer regulator: 30% percentage characteristic – minimum operating threshold 30% of In