Fast phasor estimation for digital bus differential protection under saturation conditions of current transformers

The article addresses the issue of non-selective operation of bus differential protection during external short circuits caused by current transformer saturation. To improve the reliability of digital bus differential protection, a novel algorithm is proposed for fast phasor estimation and correction of secondary current waveform of a current transformer under deep saturation conditions. This algorithm requires three instantaneous current measurements during the interval of accurate transformation at the initial moment of the short circuit before saturation occurs, allowing for the restoration of the amplitude and initial phase of the AC component without prior information about the current transformer parameters. The effectiveness of the algorithm was verified using a simulation model of a 500 kV bus protection system. The simulation demonstrated that the proposed algorithm eliminates non-selective operations during external faults, even under conditions of deep current transformer saturation and residual magnetic flux up to 90% of the maximum one, unlike conventional restrained differential protections, which may cause non-selective tripping under the same conditions. The results indicate that the algorithm can be implemented in real-time in protection IEDs through software upgrades, making its deployment an efficient solution for enhancing the reliability of bus differential protection.