Logo
Published on

Line current differential for industry?

Authors
  • avatar
    Name
    Ben Gibb
    Twitter

SEL-787L — What we know today

SEL is releasing the SEL-787L Line Current Differential protection relay. The unit is essentially a SEL-751 with added line current differential protection. Example applications are feeder protection of shaft cables, long runs of power cables, or other critical medium-voltage power cables.

The SEL-787L device's flagship ANSI protection is 87L. What is nice about line differential current on critical power cables is immediate fault clearing. Line differential does not require upstream and downstream trip coordination like instantaneous and timed overcurrent (ANSI 50/51). This is an immense advantage. The device quickly detects and isolates faults.

In mining especially, sympathetic ground faults can cause small cable faults to be big problems on long runs of power cable. A ground fault on one feeder cable can cause other faults elsewhere in the system. Line current differential protection helps reduce this risk by isolating and detecting the original fault quickly.

In mining applications, the length of feeders can be 15km or longer with many levels of trip coordination. Layers of coordination equate to non-ideal built-in trip delays. Line differential eliminates these delays by looking for differential current within the zone of the two SEL-787Ls.

The SEL-787L has essentially all the features of an SEL-751, therefore it can be used as a direct replacement to gain the 87L protection. I expect lots of end users will want to upgrade to the SEL-787L on their critical power cables. Protection elements such as overcurrent and arc flash detection can all copy over. Retrofits will be relatively straightforward given the unit is the same form factor as the SEL-751.

For potash mining end-users, if this product had been released 5-10 years ago, I don't believe there would have been much traction as fiber optic communication (or any reliable communication for protection purposes) was not installed.

The current differential could work well on multiple parallel cables that are fed individually (separate breakers). If one cable fails, the other cables can remain in service. If set up well, one could potentially avoid loss of power to downstream loads.

General commentary comparison between the SEL-787L, SEL-311L, GE L30 and GE L90

The SEL-387L, SEL-787L, SEL-311L, GE L30, and GE L90 all offer line current differential protection.

They all require one unit at each end of the cable/line. The relays talk through serial fiber or a multiplexer.

All five protection relays have line current differential, of course. Additionally, except the 387L, they all have 50/51 overcurrent elements, 67 directional elements, 25 sync check, and 81 frequency elements. The 387L is 87L protection only (the "zero settings" relay).

For SEL relays, one is in the 300 series rack-mount form factor and the other is in the 700 series form factor. Both GE relays are rack-mount.

The SEL-311L seems to be designed for utilities, whereas the SEL-787L seems targeted for industry. This is known through a few feature differences:

  • The 787L has incipient fault detection and thermal protection (IEC 49).
  • The 787L has industry communication protocols like Ethernet/IP.
  • The 787L, as it is part of the 700 series, offers fiber-optic arc flash detection (50PAF)

The SEL-311L and GE L90 provide backup line protection (ANSI 21), which the SEL-787L, SEL-387L, and GE L30 do not. However, the SEL-787L and GE L30 do have 50/51 overcurrent as a backup should the differential mis-operate.

The SEL-411 (not mentioned previously) is SEL's flagship transmission line protection relay. It offers 87L and 21, but also has advanced technologies for sub-cycle fault detection and locating.

Disclaimer: The author has no affiliation with SEL or GE. The content of this article is intended to be high-level. Not engineering advice. Consult an engineer for your specific application.