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Failures of poor design


phild

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Hello

 

Again I'm writing to you all again.

An existing substation with a new fugi frenic 5000 variable drive with a 120mm2 Varolex screened power cable, the screen is earthed with hose clamps with tails attached to the earths at both ends @ 400 meters in lengh to a standard 132 kw asychronous induction motor driving a conveyor to a rock reject bin.

This plant has been in existence for 6 months.

In that time, all the 500ma earth leakage protection namely the EL1 and EL2 in the substation at sometime has tripped randomly. The CT's for the MCC monitor the main neutral back to 11kv/415v transformer. As well, all the substation feeder supplies earth leakage protection in the vicinity also trip.

The VSD has a line filter, but has no filter on the output to the motor cable.

Recently the Toshiba 132kw motor failed with an earth fault

Words like parasitic currents, motor terminal over voltage, dv/dt, voltage reflection and inverter rated motors all come in to play. As well as 5% impedance load reactors , Sine Wave Filters and snubbers fitted at the motor to try minimise the problem of a long cable run.

My employer is playing hard ball with the contractors with the construction issues, not so much with the operational issues as outlined.

I'm not going into the politics or issues outlined, if anyone can suggest a direction to take it would be appeciated by waa, as i'm just a humble person who unfortunatly has been lumbered with the problem when at work, which is usually at night time.

 

Regards

 

PhilD

 

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Dear Phild,

please add the right type of Varolex cable between inverter and motor.

 

There are discussions about PE conductors inside screen, I usually suggest - for long cable as this - to use cables with PE conductors positioned externally (see picture).

If you want to test this solution, you can remove the connections at PE at both ends, and you can add a new PE conductor externally.

 

Other tips:

- EL1 and EL2 should be changed, for example to 1000 mA

- an output filter will help

 

What kind of problem don you find on Toshiba motor? A partial discharge problem on windings?

In this case you could add some voltage limiting device (as MOVs or gas tubes) near the motor, to absorbe short overvoltage at high voltage. Be carefull, this is a last-plage solution!

Regards

 

Mario Maggi

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Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Dear Phild,

please add the right type of Varolex cable between inverter and motor.

 

There are discussions about PE conductors inside screen, I usually suggest - for long cable as this - to use cables with PE conductors positioned externally (see picture).

If you want to test this solution, you can remove the connections at PE at both ends, and you can add a new PE conductor externally.

 

Other tips:

- EL1 and EL2 should be changed, for example to 1000 mA

- an output filter will help

 

What kind of problem don you find on Toshiba motor? A partial discharge problem on windings?

In this case you could add some voltage limiting device (as MOVs or gas tubes) near the motor, to absorbe short overvoltage at high voltage. Be carefull, this is a last-plage solution!

Regards

 

Mario Maggi

Hello Mario

 

Thankyou for your reply

 

In reply to your post, under the regulation employed the substation earth leakage EL1 and EL2 maximum is 500ma with delays up to 300msecs at the relays plus another 500msecs through a timer for the EL2 trip for the 11KV feeder. So we don't tweak these, as these have been setup to discriminate the earth faults in the substation zone.

The neutral fault current is limited to 1 amp through a reactor through the earth to the neutral star point.

Can you explain, how the PE inside the varolex cable on a long cable effects the asd drive.

As for the Toshiba motor the report on what destroyed it hasn't came back as yet.

The asd earths at the mcc and motor, will need looking at, as they aren't at the same point at the mcc. At the motor, they use a visual break isolator where the screen has hose clamps, then a earth tail connected to the frame.

 

Regards

 

PhilD

 

 

 

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