As Power Utilities continue to integrate legacy applications into their Ethernet/IP substation communication backbone networks, they are increasingly using TDM over IP/Ethernet instead of traditional TDM-based solutions for latency sensitive applications such as teleprotection. TDM over IP/Ethernet is a reliable, proven solution for teleprotection communications that produces deterministic results.
Tucson Electric Power Co. (TEP), for example, has been successfully using multiple TDM over IP/Ethernet links for protective relay communications between substations over fiber and microwave communications since 2011.
TEP’s teleprotection-over-IP links are based on a multi-service Ethernet platform, the JumboSwitch® from TC Communications, using a special "Turbo Serial" TDM over IP/Ethernet interface card to connect its protective relays with RS232 (mirrored bits®*) interfaces. (See diagram).
Deployment of the JumboSwitch® for TEP’s teleprotection links, SCADA, and metering over IP and conventional TDM links are part of TEP’s multi-service Fiber and Radio backbone for the Distributive Generation Projects. The advantages are both rack space and cost of deploying multiple boxes to provide the same function.
TEP’s initial search for a suitable product for teleprotection-over-IP communications was lengthy. Its goal was to find a low-latency industrial hardened Ethernet serial server to connect protective relays using mirrored bits between substations over fiber and microwave. It soon discovered that Ethernet teleprotection products vary greatly between communication vendors.
"We quickly learned to rely on our test results rather than vendor spec sheets," said Jon Otteman, Communications Engineer for TEP.
"Many hardened serial servers are unsuitable for teleprotection applications. For example, one popular hardened serial server from a major industrial Ethernet switch manufacturer tested out at 250 msec. of delay. When testing the delay, you must also ensure the measurements are consistently taken round trip or relay to relay; TEP’s standards are based on relay to relay."
Application & Implementation
TEP’s specs called for a delay (excluding relays) of less than 10 msec. and the TC3847-3 JumboSwitch Turbo Serial card passed all of TEP’s latency tests with typical readings of around 3 msec. delay.
In addition to meeting TEP’s stringent latency specifications for teleprotection, the JumboSwitch® solved connection issues for RS232, RS485, Ethernet, serial and dry contacts to monitor certain alarm points when an RTU is not available. It also provided a redundant power supply, required by TEP’s standard. Changing from 48VDC, 125VDC to 115VAC can be easily accomplished by switching the power module.
The JumboSwitch offers 25 different interface cards, including all popular interfaces used for teleprotection-over-IP applications. The G.703, C37.94, T1, and Turbo-Serial (RS232, RS422/485) teleprotection interface cards meet stringent real time requirements for protective relay communications in the Power Utility industry.
*Mirrored Bits® is a registered trademark of Schweitzer Engineering Laboratories Inc.