‘Digital’ substation: Safer, reliable and economical

A substation is one of the elements of an electric power system, composed of high-voltage apparatus designed and sized for the substation operating voltage (called the primary system) and low-voltage equipment (referred to as a secondary system).

Given this definition, the term “digital substation” simply means using fiber-optic cables to communicate between the relays in the control house and the instrument transformers, breakers, and merging units in the station yard.

Substation primary equipment is not digital; there are no digital breakers, no digital transformers, and no digital surge arresters. Therefore, it is not possible to have a completely “digital” substation. The digital substation concept promises to digitise a portion of the substation secondary system by eliminating the majority of analog secondary circuits between the instrument transformers and protective relays.

SEL’s use of the term “Digital secondary systems”
SEL’s mission is to make electric power safer, more reliable, and more economical. We stay true to this mission by helping customers around the world protect, control, and monitor substation’s primary equipment using digital devices and communications protocols in substations.

With this in mind, we use the term “digital secondary systems” to more accurately define what portions of the substation can be digitised. As you browse, the SEL website, you will see this new term used in our literature related to SEL Time-Domain Link (TiDL) and Sampled Values technologies.

Robust solutions for digital secondary systems
Modernise your substation by choosing from two SEL digital secondary system solutions that advance how you protect and control the primary equipment in your substation.
• Apply SEL Time-Domain Link (TiDL) technology, which uses a secure point-to-point fiber connection to eliminate complex Ethernet network design
• Use an SEL Sampled Values (SV) solution with IEC 61850-9-2

These solutions improve safety by moving high-energy signals out of the control house and reduce the costs associated with running copper wiring for protective relaying. Placing SEL relays in the yard near the primary equipment is another way to achieve these benefits.

SEL TiDL technology: A simple, fast, and secure solution
SEL TiDL technology is an innovative digital secondary system solution engineered with simplicity in mind. This technology requires no external time reference, is more cyber secure, and is easy to implement, with no network engineering required.

In a TiDL system, SEL-2240 Axion TiDL nodes located close to primary equipment in the yard act as field modules that digitise analog signals into deterministic fiber communications. TiDL’s simple point-to-point architecture transports the data over a fiber-optic cable to a TiDL-enabled SEL-400 series relay in the control house.

The Axion’s synchronous sampling capability improves the accuracy of the protection signals, and its modular architecture allows you to select the number of analog and digital input modules to match the relay application.

Relay settings remain the same as those in the present SEL-400 series models, providing consistency and simplicity. Because TiDL uses a point-to-point architecture, implementation is simple and requires zero network engineering. TiDL maintains relative time; therefore, it does not rely on an external time reference for protection.

TiDL technology combines the advanced protection of the SEL-400 series relays with the modularity and flexibility of the Axion, providing a scalable and flexible solution. Implement SEL TiDL technology for a simple, fast, and secure solution.

SEL SV solution: Combining protection in the merging units with the flexibility of IEC 61850-9-2
SEL SV is the only solution in the world that combines protection in the merging units with the flexibility of IEC 61850-9-2 to increase the reliability of your power system. The new SEL-401 Protection, Automation, and Control Merging Unit and SEL-421 Protection, Automation, and Control Merging Unit digitize analog signals from primary equipment and transmit them to the relays in the control house via an Ethernet network. SEL merging units have integrated local protection that works in the event of a loss of communications between the merging unit and the relay. They can be paired with SV-supported relays (such as the SEL-421 Protection, Automation, and Control System) or with other devices that are IEC 61850-9-2–compliant, providing interoperability in user networks.

SEL SV technology allows you to create a robust and flexible Ethernet-based point-to-multipoint network using tools such as software-defined networks or VLANs to fit your application needs. You can use the SEL-2740S Software-Defined Network Switch to provide centralised traffic engineering and improve Ethernet performance. The switch acts as a transparent Precision Time Protocol (PTP) clock that supports the IEEE C37.238 power system profile, ensuring sub-microsecond time synchronisation of the end devices.

SEL relays in the substation yard: Providing the fastest protection speed
Customers from all over the world have placed SEL relays in outdoor cabinets in the substation yard to achieve copper reduction benefits similar to a digital secondary system. This is possible because SEL relays are built to reliably operate in harsh environments while providing unmatched protection. The relays transmit digital data, such as SCADA information, back to the control house via fiber-optic cables.

Article by:
Schweitzer Engineering Laboratories