circulating currents 1

[alanmitchell]

We have three generators feeding in one main switchboard with the neutral connected connection to earth at the switchboard.
Each generator has six 300mm cables per phase and three 300mm cables for the neutral The cables are glanded through and non ferrous plate at the generator and all the glands are earthed together.At the switchboard there are no glands and the cable braid is earthed to the switchboard earth bar.The problem is we are see currents flow in the earth braid as much as 70 amps at times which has burned the protective earth sleeving.Can anyone help with a solution to these currents.

 

[waross]

Don't ground the sheath/braid at both ends on single conductor cable. Run a separate grounding conductor.
Ground your braids at the feed end only and insulate/isolate the braids at the other ends of the cable. I hope there is an overall insulation of some type on the cables. Otherwise insulate the cable runs also. Years ago, when single conductor aluminum sheathed cable was popular for feeders, it was supported by wooden suports every 5 or 6 feet so as to insulate the sheath.
The currents in the main conductors are inducing voltages in the sheath/braid. Normaly this voltage is low enough to be ignored. If, however, you complete the circuit by grounding both ends and allow current to flow, the relatively low impedance of the braids can result in unexpectedly high currents in the braids.
respectfully

 

[Kiribanda]

alanmitchell,

Just a thought.

Have you had a chance to measure and verify whether this 70 Amps contain 100% fundemantal frequency (50/ 60 Hz) or THIRD harmonics?

If not, could you please verify and post it in the forum?

Kiribanda

 

[waross]

Hi Kiribanda;
That's a thought. I would expect any harmonics to travel in the neutral conductors. It will be interesting to see if alanmitchell is able to respond to your request.
respectfully

 

[alanmitchell]

Kiribanda,Thanks for the info.We have very large harmonics due to the running of a VFD to control a 800kw maotor.We have a passive harmonic filter in place which controls the high fifth harmonic, it reduces from 40% to about 9%.How would I measure the fundemantal frequency and what affects would the third harmonic cause.

 

[waross]

The third harmonic will flow in the neutral. If the braid is in parallel with the neutral, some of the harmonic may flow in the braid.
respectfully

 

[Kiribanda]

alanmitchell,

May be the filter has reduced the 5 th but not the 3 rd which will flow through the common neutral/ braid. Also by the winding design itself, the generators tend to produce 3 rd harmonics.

Therefore, in order to discuss further, you should use a clamp meter on the braid and the neutral conductors,and get two seperate complete harmonic spectrums. If you could post the same in the forum it would be much appreciated.

Kiribanda

 

[waross]

Kirbanda;
Don't forget that there are

three 300mm cables for the neutral

These are probably in parallel with the braids and probably carry most of any harmonics.
Induction from the phase conductors will be causing fundamental frequency currents in the braids. Once the induced currents are interupted, there may be harmonics flowing in the neutral. A before and after check for harmonics will be interesting, but harmonics aside, the present configuration will be causing fundamental frequency circulating currents.
respectfully