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?

 

[dpc]

There aren't really many advantages. EM relays are generally self-power via CT currents and VT potentials. The old instantaneous trip units can actually be faster than the digital function. With single-phase relays, there is some added backup. Trip contacts are pretty robust - output contacts are one of the most common failures in digital relays.

If this is a nuclear power plant in the US, EM relays may be required due to lack of seismic and NRC certification for digital relays. (unless something has changed in the last few years).

But many more disadvantages than advantages, overall.

 

[System Protection]

No cybersecurity concerns with an electromechanical relay.

Not many advantages otherwise.

 

[bacon4life]

Not sure why someone would install new EM relays besides the NRC issue. Perhaps EM might be less susceptible to EMP attack as well?

Some advantages of EM relays:
Retrofitting digital relays is a significant block of labor to implement every 5-20 years. EM relays can last decades (assuming the burdensome routine maintenance is performed). It can be an organization challenge to fund big blocks of capital spending, even when the capital spending will save on maintenance spending.

EM relays have the advantage of simplicity. Many of the misoperations associated with digital relays are due to folks incorrectly understanding the details of relay programming.

EM relays do not need firmware updates.

EM relays to not have dozens of optional features, so there is no temptation to enable lots of kinds of complicated protection schemes.

 

[cranky108]

Think of a small substation, where no batteries are needed. Maybe no control building.

 

[davidbeach]

With no batteries you're hoping that a capacitive trip device works...

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[edison123]

Good discussion.

Muthu

http://www.edison.co.in

 

[edison123]

david

Are batteries needed for EM relays to operate?

Muthu

http://www.edison.co.in

 

[davidbeach]

No, batteries are not needed for EM relay to operate, but I'm a firm believer in using batteries to trip breakers, at least in a utility environment. If you really don't want the expense and hassle of batteries, use fuses.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[edison123]

Got it, thanks.

Muthu

http://www.edison.co.in

 

[waross]

The batteries are not needed for the relays but they may be needed for the breaker trip coils.
A small plant may run without batteries.
The breakers may be controlled by AC trip and close with manual charging and closing for black starts.
As the plant or substation size grows larger, a point is reached where batteries are used to trip, charge and close breakers.
At one time all protection functions were performed by electro-mechanical relays, without batteries.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Thanks, Bill.

Muthu

http://www.edison.co.in

 

[che12345]

Dear All,
I understand that EM relays are no more in current production by GE, GEC, ASEA etc....These EM relays are out of the market some 30+ years !
So, a comparison is irrelevant, as an exact EM relay replacement is no further available in the market.
Che Kuan Yau (Singapore)















Dear All,

 

[thermionic1]

One disadvantage of certain Swedish EM high speed aux tripping relays is that they can be very susceptible to a small bump. This coupled with poor placement in the panel can cause a very bad day for someone. Very bad like a 345kV line from a Nuke being tripped at the remote end because an electrician bumped a ladder against a panel with one in it. No targets, just a call from Operations about an open breaker and zero indication why.

 

[cranky108]

All that said, the electromechinical lockout relay is still being made in the US, and is being used my most utilities.
I know most don't see that as electromechinical, but it is.

I do agree with David that fuses do have merit in many small substations, but a EM transformer diff, and a circuit switcher with a capacitor trip would be a good complement. Also add a 51 relay on the X0 bushing as a backup for the reclosers.
But maybe that concept is past it's time, sort of like me.

 

[dpc]

I understand that EM relays are no more in current production by GE, GEC, ASEA

...

In the USA at least, this is not quite accurate. ABB still offers a fairly complete line of EM relays (the old Westinghouse relays). And GE still sells new IFC overcurrent relays.

They will likely be expensive.

Cheers,

Dave

 

[System Protection]

One disadvantage of certain Swedish EM high speed aux tripping relays is that they can be very susceptible to a small bump. This coupled with poor placement in the panel can cause a very bad day for someone. Very bad like a 345kV line from a Nuke being tripped at the remote end because an electrician bumped a ladder against a panel with one in it. No targets, just a call from Operations about an open breaker and zero indication why.

Some of the old electromechanical distance relays would be supervised by overcurrent relays set just above load. This would prevent undesired trips from bumping the relay or from a loss of potential condition.

 

[RRaghunath]

For high impedance Busbar differential protection, Transformer winding Restricted Earth Fault protection, the attracted Armature type EM relays are the fastest and are easy on CT requirements.
For Lockout function of feeders, EM Trip/Lockout relays are still a standard with many utilities.

 

[thermionic1]

One very big advantage of the EM relay is training new, green relay techs. Due to the robust construction and single protection element, it's a good place to start. I'm fairly certain that in many places, new techs are started on a CO, IAC or equivalent. Many of the old manuals show a simple test circuit using variable resistors, meters, a lamp and maybe a timer. Knowing how to make a test set from basic components is useful later in life when you are far from home or the shop with your $50k test set not working and it's Friday afternoon.

 

[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’ll see your silver lining and raise you two black clouds. - Protection Operations