Protection communication

Protection communication and its problems with line differential relays

The line differential relays differentiate between an internal fault and external fault by comparing the magnitude and phase a=angle of the currents of the local end and the remote end(s). To do this one need to transmit the information about the current from the local end to remote end and Vice versa . To transmit the data to remote end and back . Relay uses different means of communication. In earlier designs, it was using pilot copper cables. But the latest design uses Fiber optic communication because of its higher bandwidth capacity and reduced interference to other electrical signals. There are also different type of converters available for changing the FO signals from a relay to electrical signals so that it can be transmitted thru electrical copper cables if there is non availability of FO cables

Methods of line differential communication

The important issue with the line differential communication is the time synchronization - meaning the data at the local end as well as the remote end required for differential communication shall be taken at the same instant of time. Otherwise it will lead to differential current during the normal operation as well as through faults

There are two methods used to achieve the time synchronization in line differential communication. One is use the internal clock of the relay and measuring the transmission / reception delay and compares the samples based on this time information and in other an external time ( GPS ) is used so that both the relays are synchronized in time

In the first method , the relay find out the total time taken for a signal to reach the remote end and back and divide this by two to find out the channel delay . Once the channel delay is found out , the relay after receiving the signal from the remote end compares the remote end sample with local end sample. It may be noted that , the local end sample is found out based on the time delay obtained for the channel delay . Hence the samples at the same time of local end as well as remote end is compared

Here the assumption is that , both the transmission channel delay and the reception channel delay is assumed to be same . This is called symmetrical channel delay . This is true in case of a FO cable connected directly between relay to relay . But if the Signal is transmitted thru the telecom net work of the utility with Multiplexers etc , different transmission and reception delay can occur due to possible different path of the transmission and reception and also due to switching delay in the communication network , buffering time in the Multiplexer etc . ( The communication network will use dynamic switching method where , the paths are switched depending on different conditions that includes traffic and channel failure ) . In this situation the communication between relays become unsymmetrical with different trasmission and reception time . If this time is known accurately , this can be compensated in the measurement . But it is not normally constant and hence , there could be measurement error in such a situation which leads to false differential current resulting to an unwanted trip.

Hence if the communication is done thru the utility communication network , the usual practice is to go for an external clock ( GPS) based communication where the relays at the remote as well as the local end is synchronized to same clock and the measurement become symmetrical . The issue with this communication is that , in case the GPS signal is lost , the relay will have to be blocked or switch to the internal clock based communication