Operate and Restrain in Relaying

[mbrooke]

What year did operate and restrain become the norm in differential relaying in power plants and substations? Or when was it realized that systems needed this? I ask out of curiosity looking at protective relaying books from the 1900s where bus, line, generator and transformer differential CTs were connected in parallel into a low impedance trip coil.

The text makes no mention of CT saturation, rather that as long as CT ratios are proportional and winding phase displacements are taken into account current will always be "zero" through the solenoid coil.

 

[xptpcrewx]

What year did operate and restrain become the norm in differential relaying in power plants and substations? Or when was it realized that systems needed this? I ask out of curiosity looking at protective relaying books from the 1900s where bus, line, generator and transformer differential CTs were connected in parallel into a low impedance trip coil.

The text makes no mention of CT saturation, rather that as long as CT ratios are proportional and winding phase displacements are taken into account current will always be "zero" through the solenoid coil.



View attachment 2557646

If you're referring to a balance beam 87 relay (percent differential), it's been in use since the early 1900's. The restraint coils are an inherent part of that design.

The benefit of this type of relay is to improve security for external faults outside the zone of protection, while still providing the sensitivity needed for internal faults (within the zone of protection).

Currents will never be zero. There will always be a difference with CT errors, manufacturing tolerances, AC/DC saturation, time-to-saturate, phase shifts, impedances, etc. Differences become even more pronounced when dealing with transformer inrush and CT mismatch.

Note: In your figure, A1 and A2 CT's would be mismatched in practice. A1 CT's would typically be located at (and integral with) the OCB, while A2 CT's would likely be furnished with the generator.

 

[mbrooke]

Note: In your figure, A1 and A2 CT's would be mismatched in practice. A1 CT's would typically be located at (and integral with) the OCB, while A2 CT's would likely be furnished with the generator.

Good catch. The gen's zone of protection would go through the breaker placing the CTs to the right while bus differential CTs would be to the left of the OCB.