Interoperability of Line Differential Protection

Abstract

Line differential protection is used for a long time to protect overhead lines and cables in transmission and distribution systems. The basic principle of line differential protection is Kirchhoff's current law. Due to this principle line differential protection is strictly selective to clear faults on the protected line.To apply Kirchhoff's current law a line differential protection needs the currents from both ends of the protected line. The currents from the local end can be measured directly by means of current transformers, connected to the line differential protection device. The remote end currents however cannot be measured directly by the local line differential protection device. In general, the remote end currents are measured by a line differential protection device of same type and afterwards sent via communication link.Due to bandwidth restrictions of the communication link, differences in pre-processing of the measured values and other device specific implementations there is no interoperability of line differential protection. In general line differential protection requires both devices to be from the same manufacturer. Often the same device type or even the same firmware version is required. Today there is no interoperability for line differential protection. In case one substation gets an update of the line differential protection, the remote substation needs an update of the related line differential protection too.This paper describes an actual case how interoperability was achieved for line differential protection of different protection platforms of one manufacturer. The problems and limitations for this use case are explained in detail.In addition to this the paper suggests an implementation of line differential protection based on sampled measured values and GOOSE according to IEC61850 and IEC61869. With this approach, the communication interface between the line differential protection devices becomes interoperable. More flexible solutions are found to be possible. For instance, a line differential scheme might consist of only one line differential relay, receiving sampled measured values from a merging unit located at the remote end. The trip command for the remote end might be transferred back via GOOSE to the merging unit located at the remote end. For redundancy even two different line differential protection relays could be used feeding each other with the sampled measured values from the remote end.