Case studies and lesson learnt on Mis-operation of Power Line Carrier Communication (PLCC) & Digital Protection Coupler (DTPC) in EHV transmission lines.

PLCC has traditionally been used for Carrier-aided protection schemes. It utilizes the power lines of the electrical grid as communication medium to transmit analog or digital signals for various protection functions. Analog PLCC systems employ modulation techniques such as amplitude modulation (AM) or frequency modulation (FM) to encode and decode signals.

With advancements in optical fibre technology, DTPC has emerged as a digital alternative to analog PLCC. These devices utilize digital communication techniques through optical fibre network to transmit protection signals along with digital signals with higher accuracy and reliability. DTPCs offer advantages such as faster communication speeds, with improved noise immunity.

The integration of PLCC and DTPC in power system protection schemes has enabled accelerated & efficient fault detection and isolation. These communication technologies facilitate accelerated tripping of circuit breakers through coordination of both end protection relays. Additionally, PLCC and DTPC aid in the implementation of permissive-based protection schemes, where specific conditions must be met before allowing protective actions.

This paper presents several unique cases of maloperation of both PLCC & DTPC, during various scenarios of DC transient, earth fault, interference, and human error. Further, key findings & mitigation measures for eradication of these issues have been discussed in this paper

Even though the communication medium has various basic technique for security of data transfer. Some of the major key findings are.

1. Several tripping attributable to spurious transmission/receipt of Direct Trip (DT) signal through PLCC/DTPC system during DC earth fault & mixing has been observed across

POWERGRID substations. This spurious transmission/receipt of DT accounts of the major unwarranted tripping of transmission lines in the system. Root cause along with their corrective action has been discussed in this paper.

2. Migrating to digital protection couplers has seen challenges of potential false code transmission. In analog PLCCs, tele-protection signals are transmitted through dedicated carrier channel. However, in DTPCs, code transmission relies on the digital address configuration. If digital addressing is not properly set up or if multiple DTPCs share the same addresses, incorrect code transmission can occur. This transition brings the need for standardization of DTPC settings and addressing to avoid human errors and ensure proper implementation.

3. Few cases of single ended reception of phantom DT signals were observed due to interference. Same was analysed and remedial actions were taken to avoid the same.

These maloperation if not addressed may lead to erroneous tripping of transmission lines, causing disruptions and potential damage to the power system. The research conducted aimed to uncover the root cause of this issue and develop appropriate solutions.

In conclusion, operational experiences with both PLCC and DTPC have revealed several issues that require mitigative measures. False transmission/reception of PLCC/DTPC signals can result in erroneous tripping, causing disruptions and potential damage to the power system.

These issues were investigated to identify their root causes and remedial actions were taken as well as incorporated in the standard process of maintenance & project execution. This paper contributes to PS1 by documenting practical case studies and lessons learnt from mis-operations of PLCC and DTPC on EHV transmission lines within the POWERGRID network. By analyzing root causes such as spurious direct-trip signal transmission, digital addressing errors, interference, and human factors, and by identifying effective mitigation measures, the paper converts operational experience into structured knowledge. The findings have been incorporated into standard maintenance practices, project execution procedures, and DTPC configuration guidelines, supporting knowledge transfer, training of protection engineers, and prevention of recurrence. The work therefore reinforces experience-based learning and promotes best practices in communication-assisted protection systems.