Current Differential Protection

Application of Current Differential Protection
to Tapped Transmission Lines

Microprocessor-based current differential relays offer superior protection for power transmission lines. The key advantages over distance relays include better sensitivity for high resistance faults, 100% line protection, and better performance in the single-pole-tripping mode, particularly during evolving and cross-country faults.Besides the well-known requirements of high-speed communication and sampling synchronization, a microprocessor-based differential relay requires ñ as per principle of differential protection ñ that the currents are monitored on all the circuits connected to a protected line including tapped transformers and lines. This may create a problem as the tapped connections are meant to provide a cost-effective alternative to actual substations. The taps are made outside the main substations and may not be equipped with proper protection means such a Circuit Breakers (CBs)
and Current Transformers (CTs). Also, high-speed communication from the tap site may be a problem. This either limits the application of line current differential relays or makes the tapped connections economically less attractive.
This paper discusses application problems and solutions permitting the application of current differential protection to tapped transmission lines without measurements at the tap point

The following sources of differential error signals need to be addressed to make the application possible:
• Total load current of the tapped transformers and lines.
• Faults on the low voltage side of the transformers.
• Magnetizing inrush of the tapped transformers on top of the line capacitive inrush current.
• External ground faults on the high voltage system causing a zero-sequence in feed from the line-side windings of the tapped transformers when connected in a grounded wye.