Developing Busbar protection with new differential characteristics to solve the breakpoint settings of digital commercial relays

The current commercial characteristics of the digital differential relays have many breakpoint settings of time delay due to Current Transformer Saturation “CT-SAT” and error. This can decrease the sensitivity of the Merz-Price circulating current protection relays. However, to enhance the stability of this type of relays, the operating threshold boundary is increased. This will cause serious limitations for the relays during faults occurrence inside the Bus-bar zone. The protection characteristic of such relays is based on the phasor quantities, in such case the secondary current during the CT-SAT equals to zero. To improve the Merz-Price circulating current protection characteristic, the relay operation should depend on the features of the faulted instantaneous current rather than the phasor quantities. The paper develops new Merz-Price circulating current characteristic based on the wavelet transform of the noises generated by the fault. The captured noises have valuable information during the CT-SAT. The study shows the performance improvement in system sensitivity and stability of the new characteristic. All types of the faults are successfully detected due to current saturation, ratio mismatch, early and late saturation.