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Protection Relay Replacement


PCE were engaged to replace GEC HO4 Cable Differential Relays on an 11kV Ring Main System due to the relays no longer being supported and the need to replace copper pilot wires. The switchboards in the ring system had high Arcflash levels as there was no fast, local protection.


Replacement Relay Evaluation

Several relay types were candidates for the replacement. Though plug in replacements were available, no additional protection would be provided by these.

Ideally, replacements would also have fibre optic pilots, OC & EF, Directional OC & EF, diagnostics & fault logging, modbus over ethernet communications, Arcflash detection and be physically small enough to fit into the existing panels.

The Siemens 7SD82 was found to be most suitable for this application though separate ABB RET Arcflash relays were also found to be necessary.


CT & VT Evaluation

The existing CTs were evaluated by calculation to ensure suitability with the 7SD82 relays at the site fault levels.

Switchboards in the ring system have no 11kV VTs so 415V reference voltages were used. If the 415V system was offline then non-directional OC & EF protection would still be in place. 415V VTs were needed to match relay input limitations.


Protection Settings

A protection study was needed to establish protection settings for the various elements. Directional OC protection is useful at the switchboards on the ring system but requires careful setting to retain grading.

Relay programming is now in progress using Siemens Digsi5 software.



Modifications to the existing panel wiring was designed and drawn to accommodate the new siemens relays and ABB AF relays.


Relay and VT Ordering

The relays and 415V VTs were sourced and purchased on behalf of the client.



A large number of candidate relays and an even larger number of options and part numbers made relay choice the main difficulty