This depends on your application and type of equipment. For large size equipment, usually there are primary and backup protection. For example, for a 20MVA transformer, you will have transformer differential and overcurrent protection. Transformer differential will be the primary protection and overcurrent will be the backup protection. Primary porotections operated faster then backup protection and if they fail to operate, backup protection takes care.
- Main-1 protection and Main-2 protection
or
- Main protection and Back up protection
or
- Primary protection and back up protection
or
- Primary protection and secondary protection
All should have same functionality. Used in different regions.
It's as good of a question as my coworker asking what is the difference between the A and B relay sets. For which he follows with which one is Primary.
There is no standard for the terms Primary, Secondary, or Backup. But I would assume the Primary has some function that the backup does not.
If you can read the settings then that will answer your question.
what I mean in the question is what is the job for each relay, primary and backup. How each of them protect the area? How do the primary and backup relay work ?
For anything we're putting in today both relays do exactly the same thing (set A & set B). We've moved away from the bad old days of a decent relay and a crappy relay (primary & backup).
We take primary relay to mean it does the reclosing functions, where the secondary pokes the primary relay to reclose.
In older relays the secondary does not communicate, but that is changing to both relays communicate with there counterpart.
Backup implies a slower relay, or in david's case a crappy relay by some company that does not bother talking to utility engineers.
We quit using the term backup, because it was expected to be slower than the primary, and many times it is as fast or faster than the primary. And this confused people.
The term set A and set B came out of two relays with the same settings.
David, do you put two 87L+21+67N+79 protective terminals?
Today, of course, two duplicated sets are best solution, for transformer, motors and generators.
For lines 87L is "problematic", without 87L of course duplicated too, 79 maybe not
How is dual 87L a problem? Our line terminal protection package, all voltages, is a pair of SEL-411L relays. We use as many of the features - step distance, DUTT/POTT, 87L - as that installation and the surrounding system (including comm availability) allow. What we don't use today, we might use tomorrow. The biggest single difference between the A and B relays is the event reporting; A only creates oscillography reports when it trips and B has additional event triggers.
There is also a custom to term the line protections as "Main-1" and "Main-2". Now a days people start using the term "duplicate protection" or "duplicated protection" (to be precise) as both relays have equal capability.
For utilities, there is 'A' and 'B' fully redundant protection. Each relay has the same protection functions and 'B' relay backs-up 'A'. There are also two tripping circuits for each breaker and separate CT's for each of 'A' and 'B'. They also use different manufacturers in case there is a hidden defect in one of the relays (Software or Hardware defect).
For industrial applications, there may be one relay or two with protection functions divided into primary protection (87T or 87B for example) and 50/51 for secondary protection.
The different manufacturer approach is used by some utilities, other utilities realize that the most likely common-mode failure is the protection engineer and the better they can understand the relays used the less likely the settings will have subtle errors. Twice as many relay types to keep up with means only half as much knowledge of any type. There are also issues where there is subtle difference between manufacturers where either relay would work perfectly fine on its own, but in combination they fight each other and produce unexpected results.
