Advantages of Electromechanical relays over Digital relays 16

[NickParker]

What are the advantages of EM relays over digital relays? Why would one use EM relays in this age?

 

[oldfieldguy]

davidbeach:

EM relays are why modern relays have torque control equations and time dials.

An SEL-411L is a device capable of may wondrous functions, but an HZ or a KD was the work of a genius. Once you have everything as numbers you can do anything that math and the available computational horsepower allows, but to make raw, undigitized, currents and voltages do all that, in a package that small, that's the mark of shear genius.​

I still marvel at the concoctions of iron and copper that make up an electromechanical relay to extract proper operation on phase angle, impedance, differential.

I learned (and understood) how and why a 50P was linked to a 21, why 87T might need a harmonic restraint, etc, all bits of knowledge that have served me well.

The fact that all those elements might be incorporated into a single microprocessor-based unit tells me how it's supposed to work and how to test its function.

The fact that employment of those functions in a microprocessor-based package often results in accuracy that taxes the capability of the test equipment is amazing to one who once calibrated using fiddly elements like torque springs and movable magnets. There's no adjustment to the microprocessor-based relay. It either works exactly, or not at all.

The multitude of functions and settings, though, can lead to some interesting "I didn't think it would do THAT!" episodes.

old field guy

 

[cranky108]

You wanted advantages of EM relays. One is with the plunger type of 50 element, when most fault studies calculate a fault value with symmetrical values, you need to add some time, or add current to the calculated value for the asymmetrical factor. With the plunger type of 50 element, you don't need to do this, because when the current is just over the pickup the EM relay will have a little hesitation, but just enough to over come the asymmetrical factor.
Microprocessor and solid-state relays will do exactly what they are told to do, even if that is not what you wanted.
This hesitation factor, is useful, but is not replicated in the microprocessor world.

Now maybe some people don't think this is an advantage, but it is a disadvantage of the microprocessor relays.

 

[cranky108]

Some disadvantages are that Electromechanical relays can be sensitive to off nominal frequencies as many did not have filtering, where microprocessor relays do have filtering, which maybe an advantage with inverter based systems. An advantage of Electromechanical relays is they have a long life. We removed some relays from a 1930's vintage unit for environmental reasons (mercury wetted contacts), after 75 years in service.
A disadvantage is the young technicians don't understand the older relays. For that matter, they don't understand the first generation microprocessor relays either.

 

[thermionic1]

EM relays have a limited setting range. Want a different range, get another relay(s). Settings on digital relays have a much wider range.
I'd also argue that digital relays today are far less expensive than a similar EM set up. SEL has a 751 which lists for less than $1000. I'm fairly certain that GE or ABB might sell you 3 or 4 EM relays for a feeder protection, but not for $1000.