SEL Time-Domain Link (TiDL) Process Bus Solutions

SEL-TMU TiDL Merging Units are installed close to primary equipment, such as within breaker control cabinets, to perform A/D signal conversion. They transmit digitized current and voltage measurements as well as breaker status and other binary signals using SEL T-Protocol over fiber-optic cable to TiDL-enabled relays. Each SEL-TMU supports direct fiber-optic connections to as many as four TiDL-enabled relays with no need for Ethernet networking.

Relays perform protective functions and other control actions based on the T-Protocol data streams from connected SEL-TMU devices as well as direct communications from other devices. They send communications-assisted protection signals, interlock signals, and control signals to SEL-TMU units via T-Protocol messages.

Each SEL TiDL-enabled relay has eight 100BASE-FX fiber-optic ports for acquiring digitized analog and binary data from SEL-TMU units to perform protective functions and other control actions. If a fault is detected, relays send a trip signal to the merging unit using the SEL T-Protocol over fiber-optic cable.

SEL TiDL merging units and TiDL-enabled relays communicate using the SEL T-Protocol, which maintains data synchronization among all connected merging units. The SEL T-Protocol combines several communication functions, which streamlines message encoding, transport, and decoding tasks and simplifies process bus configuration.

SEL TiDL-enabled relays communicate with other devices using common protocols, such as GOOSE, Mirrored Bits communications, synchrophasors, and SEL-2600 RTD Module communications.

TiDL relays measure the path delay between the relay and the merging unit on each connected port to align current and voltage measurements acquired from different merging units. This aligning procedure eliminates the need for external time sources, such as GPS clocks, for data synchronization.

TiDL relays can be time-synchronized with other devices in station bus networks via the Precision Time Protocol (PTP) or IRIG-B.

A TiDL system often needs to collect more than current and voltage measurements and binary statuses to perform protection actions. Many SEL protection, control, and monitoring devices collect and digitize ancillary data, such as temperature, alarms, control actions, and more, to supplement the protection data digitized by merging units. SEL offers devices that support traditional CT/PT, Rogowski coil, and digital message inputs and connect to a wide variety of protection, control, and monitoring sensors.

In addition to its protective functions, there are other important factors that must be considered when designing and implementing a process bus system.

Device protocols allow communication between digital devices and between process bus and station bus networks. Communications between SEL-TMU units and TiDL-enabled relays use the SEL T-Protocol exclusively.

SEL TiDL-enabled relays also communicate with non-TiDL relays and other devices using common protocols. This capability allows substations to integrate process bus systems with a variety of station bus protocols, such as DNP3, FTP, Telnet, HTTP, IEC 61850 MMS and GOOSE, IEC 61158 EtherCAT, synchrophasors, Mirrored Bits communications, and SEL-2600 communications. (EtherCAT® is a registered trademark and patented technology, licensed by Beckhoff Automation GmbH, Germany.)

Cybersecurity is a vital consideration in process bus systems. However, because operational technology (OT) and IT systems have different purposes, the cybersecurity solutions for one system may not be the most appropriate for another.

Preserve Availability with Privacy Methods

TiDL-based process bus solutions use privacy methods to preserve the availability of data and restrict access.

Direct connections provide privacy by physically isolating communications paths. Private links between devices enable immediate detection of intentional or unintentional disturbances to data flows, so that devices issue alarms and perform automatic corrective action to maintain security.

TiDL communications are inherently private through direct, point-to-point connections between SEL-TMU units and TiDL-enabled relays. Physically isolated communications paths prevent the capture or misdirection of SEL T-Protocol messages without detection. The T-Protocol cannot be encoded or decoded by conventional protocol analyzers to perform malicious or accidental insertion of simulated messages.

Secrecy Methods Reduce Availability

Network connections that are not engineered to maintain privacy often require coordinated secrecy methods like TLS within each device to obscure data, generally by encryption, making them unreadable to other devices and unauthorized users. This solution is undesirable in secondary systems because the additional processing load and time required to encrypt and decrypt data can slow protective action. Encryption also makes data unavailable to engineers, technicians, other devices, and systems that lack the appropriate decryption capabilities. 

In addition, the increased frequency of firmware updates required to stay current with secrecy technologies poses a serious obstacle for maintaining system availability, requires that devices be updated simultaneously, and introduces new supply chain and personnel vulnerabilities. Unintended consequences that affect protection reliability include device down time during firmware upgrades to address security vulnerabilities and a lack of communications among devices until they are all upgraded to the same firmware version. 

From our knowledge base:

• Do IT Cryptographic Security Controls Work for Energy Systems?
• Defense-in-Depth Security for Industrial Control Systems

As part of a digital secondary system implementation, TiDL-based process bus systems are as simple or sophisticated as an application demands, but many important factors must be considered in designing the system. Partner with SEL Engineering Services to create a turnkey solution that addresses the needs of your system and puts you in control. Our expert engineers provide thorough training in all products applied in a solution, ensuring that you can achieve your operational goals once a system is in place.

We’ve collaborated closely with our customers since the company was founded nearly 40 years ago—a major reason why we’re North America’s most trusted provider of protective relays and ranked #1 by international utilities in price, service, and support.