Why does SR469 Show 4000% Current Unbalance 1

[Jagger71]

I have three SR469's each feeding identical motors (250 HP, 575V). The current readings appear correct and visual inspection of the CT's appear correct (polarity & wiring - although wiring has not been confirmed by electrically tracing).

All three show current unbalance readings in the 2500%-4000% range. Any ideas what is wrong?

Voltages are approx. 612,608,610.

Thanks for any help.

 

[buzzp]

What are the current readings?
Seems strange to have such high unbalance readings since using the NEMA method of calculating you can only have a maximum of 100% unbalance (loss of a leg). Although, I never used these particular relays.

 

[jraef]

You probably have one of the CTs hooked up backwards.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Jagger71]

I thought so too but all three motors show the exact same problem.

Must be a CT wiring problem?

 

[dpc]

You might check the phase sequence configuration setting in the SR469. I believe it can be set to either positive or negative phase sequence. I have seen similar issues corrected when this setting is reversed.

You can also check the SR469 power and var readings.

 

[RalphChristie]

Is this a new or an old installation?

One of 4 possibilities:

1. Fault in the primary circuit
2. Fault at CTs
3. Fault in wiring (between CTs and relay)
4. Fault at Relay


Number 1 is not a possibility, because it happens on three different places. Thus your fault can be either at the CTs, the wiring, or the relay. Because this fault occurs on three different places, it has to be a common problem on all the places.

What kind of scheme do you use on this relay? (residual connection, core balance method, etc.) And are there not a metering feature on this relay that shows the different line currents and the unbalance current? Many times, by just looking to these currents, you can almost guest where your problem is or what kind of problem you have.

The most common problems I have encountered on this kind of fault is wrong connections at the back of the relay:
Connected on the wrong place/location.
Connected on the wrong element (1A instead of 5A and vice versa.)
Also, if you have a connection/test block, makes sure it is bridged out at the correct places.

 

[rsbuck]

With the current imbalances you posted the motor must have melted down. We can assume that this is erroneous data. I installed a CT with 115AC exciter current one time.

 

[Jagger71]

Unfortunately, I am not on site anymore so my information is limited.

I believe the currents were relatively balanced (less than 1%). I will check some more settings and try changing the system phase sequence to ACB from ABC to see the effect.

Thanks for all the responses thus far.

 

[Laplacian]

The phase rotation is specified incorrectly in the Multilin 469. Change that and you'll see the correct imbalance.

 

[Laplacian]

Forgot to mention; star for dpc

 

[Jagger71]

Thanks Laplacian, you are correct. Checking the phasor values on the Multilin is the best way to trouble shoot and confirmed the phase rotation was actually ACB not ABC as originally set. Switching corrected the problem. Now onto the next challenge.......