6 replies to this topic

[chaterpilar]

We have a 250 hp motor 400 volts 60 hz which is started using a softsterter.

The supply to this motor comes from a panel which is at the output of a 13.8 kv/400 volts Dyn11 transformer.

The incoming panel to transformer has a Sepam relay which monitors overcurrent/earth fault etc.

Now, it is observed that during startup and before the starter is bypassed by the Bypass contactor the Io current is to the range of 50 amps (13.8kvside) but dissappears after the softstarter is bypassed.

What could be the cause of this abnormal Io current during startup?

Chaterpilar

[chris1373]

How much amps have you in the 400V side? Becouse if you have 50 amps in 13.8kv that means that you have nearly to 1725 amps in 400V side and that is 5 times the nominal current for your motor or am i wrong?

May be your soft starter isnt working and your motor is having a direct start but i am not so sure couse you will here it if such a motor is having a direct start.


Chris.

[jraef]

What does Io mean? Zero sequence current?

[chaterpilar]

Io is residual current. This current is monitored by Sepam relay.

I am suspecting the relay itself, as there are no effects on plant as well the generator is also not reading it.

Chaterpilar

[jraef]

Most likely the relay is not filtering the harmonics created by the phase angle SCR firing of the soft starter. That's not necessarily a design flaw, it's really difficult to do. I know on one of the soft starter designs I helped work on where we incorporated residual ground fault protection, we had to mask out the GF protection circuit during acceleration. What you most likely need to do is to see if the SEPAM can be programmed to ignore that Io current during acceleration.

[GGOSS]

A review of the discussion thread entitled Soft Starters & ELCB's & RTD's may also provide some clues. But I would have to agree with jraef, and that is most often the only solution is to inhibit ground fault protection ciruits during acceleration and deceleration.

Some of our regulations in Australia prevent us from doing that, and therefore it become more difficult and more expensive to find a resolution.

Regards,
GGOSS

[chaterpilar]

Sorry for a late feedback, but this was traced to a wrong CT connections to the SEPAM relay and hence the differential in phase currents was appearing as I0.

We corrected the CT connections and and I0 went back to zero.

Chaterpilar