Suitable Cables for high harmonic (THD) generating machines

I had a similar situation to help sort out last week. A 130KW motor driving an irrigation pump, and fed from a VFD which had been supplied without input filtering. 35% harmonic distortion of the input current, with the lions share of these being 5th harmonic.

The 250A circuit breaker, and the cables connecting between MCCB and drive, between MCCB and main busbars, and the neutral conductor were all running much hotter than they should have been, the MCCB needing to be fan-cooled to prevent it from tripping.

The choices offered were :

1) Install bigger cables and a larger MCCB, wind up the trip on the MCCB, and the only sign of a problem remaining will be the extra charges levied by the power company. Of course, if the power company gets annoyed about the level of harmonics being injected into their network, they may well choose to make this option even less attractive by ramping up the charges.

2) Buy proper input filtering for the VFD.

They were not very happy about the cost of the input filters, but will probably come to the conclusion that this is the best option once they do their sums.

It is unfortunate, but there are plenty of short-sighted VFD salesmen out there who think it is a good thing to win tenders on price by omitting components such as input filters. When the customer finds that these are needed, they have to pay quite a lot extra for them. This tends to be an excellent tactic to drive the customers in the direction of more honest suppliers, and that is just what has happened in the case above.

If the harmonic presence is there in yr system, especially 3rd Harmonic, then the flow of current in neutral will be enormous (even in multiples of phase current) and it requires higher size of the conductor than the phase conductor. So, the consultant what he asked is correct only. If you want to know more about Reactive power,plz attend Reactive Power Management (RPM) classes conducted by L&T Technical training centre at Coonoor, Ooty. There you will be even practically experimenting the amount of harmonic current flowing in the neutral.

petjas,

The only harm you would experience in this recommendation is the added cost of the full-size core in the cable. Although Msamad has a lot of good and valid information regarding the problems caused by harmonics on a neutral core, it generally is invalid in your situation because the non-linear loads of the VFD's are line-line not line-neutral. I would be very surprised if any of your AC drives have a neutral connection, so it is impossible for them to impose harmonics on that core.

Regarding the decrease of the THD from 35% to 12% when the heaters are also running: If the heaters are switched on and off by contactors, then their load is purely resistive and has 0% THD. Adding a significant amount of total load at 0% THD to a portion at 35% THD will clearly reduce the aggregate % THD to the number you have observed. (If, however, the heaters were controlled through a variable voltage output solid-state control device, then they would also be imposing their own harmonics on the system.)

--John M.

instead of experimenting with cable why dont you try to eliminate the harmonics

could cut the time try using filter or cap bank

good information