Protection relay

[bob]

Hi,

 

We are in the process of installting one 4 MW 11 k V slip ring motor. What are the fundamental protections needed for a motor of this size ?

What are the most common type of relay for such an application ?

Actually, we have a 1 MW motor and the protection relay used is MPR1000.

 

Bob

[jraef]

Slip ring could mean synchronous or wound rotor induction.

 

Synchronous motors require very specialized protection relays. I recommend the GE / Multilin SPM, but that only does the specific tasks associated with the synchonous part. You still need some sort of motor protection relay for the amortisseur windings (stator).

 

If it is a wound rotor induction motor, it isn't that much different in terms of protection than any other induction motor.

 

I like the GE Multilin 369 relays, and if you are going to have RTDs in it, go with the 469 since it is about the same price as adding the RTD module to a 369. Schweitzer Engineering Labs also makes a fine Motor Protection Relay, the SEL-701. Siemens, ABB, Schneider all make them as well, none of them are bad. Most now include more functions than you probably want to bother with programming. If you can have RTDs put in the motor however, do that for the best protection. Make sure your relay has what is called "biasing" of the motor thermal model. This uses the RTD feedback to adjust the mathematical model of the motor thermal operating status to reflect what is really going on, some of which may not immediately show up as current anomalies.

[bob]

Hi jraef,

 

It is a wound rotor and we are ordering the motor with the six terminals out. Do you think it`s worth adding differential protection to a motor of such a size ?Is the multilin relay provided with differential protection ?

Besides will differential protection protect against negative sequence currents in the rotor ?

 

Bob

[jraef]

A common "rule of thumb" is to only consider differential protection on motors 2500HP and up, of which your's qualifies. The Multilin 469 relay can do differential rotection, but you will still need to add 3 more CTs into the circuit.

 

There is some debate about the value of dirrential protection for motors. Those that promote it do so based on the reaction speed increase in hopes, I suppose, that if taken off line fast enough, the damage will be reduced. I don't see it that way. While I agree that it can act faster on a current fault in the motor, I look at the likely causes, and the benefits to that faster reaction time.

 

The causes would be an insulation failure or physical damage. If it is physical damage, such as the rotor crashing into the stator because the bearings failed, there is nothing worse that could have happened. If only the insulation has failed, what additional damage is going to happen in waiting 1 or 2 more seconds? Either way, the motor is going to be off-line to be repaired. In the old days, it may have been related to the cost of a simple varnish repair etc. compared to a full rewind. Now, the real cost burden is in the cost to remove and re-install the motor, and the downtime. Once you have it out, you might as well rewind it anyway because you have already absorbed the major expense. Differential protection offers no quantifiable benefits in those costs in my opinion. If the application is that critical, have a spare motor nearby.

 

The other possibe reason to want differential protection is to help protect upstream systems such as transformers etc. My opinion on that is to make the investment in good transformer protection instead, and include differential protection there. The vast majority of differential protection relays are sold for transformers, not motors.