Our power is fed from a delta-delta tranformer,
and since we use a lot of VFDs, 5th and 7th harmonics are becoming an issue. I've investigated possibly installing a Zig Zag transformer...anyone else run into this problem? How did the zigzag work?

 

Responding to Captain Matlab's Sat, Jun 29, 10:41 am. query:

Without knowing your specifics, I can tell you that the Zig-Zag transformer alone, presumably mounted on the VFD-side of the Delta-Delta is not a viable solution. It will do nothing more than decrease the BCR ( Cost-to-Benefit Ratio) for the original VFD project!

Negativity aside, Zig-Zag's are used to provide a physical neutral point for ungrounded systems. They can be useful if your solution is to install a shunt-type tuned-filter (for removal of 5th and higher harmonics) between the VFD power source bus and ground.

Your situation is TYPICAL for those industries that experience wide-spread growth of VFD's with disregard to their impact on the power system. A good start would be:

IEEE Standard 519-1992
Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.

If you need additional information, please provide power system parameters.

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

As I see the few better ways to solve the harmonics problem is:

1)to have a passive harmonic filter before each VFD, the maximum improvement could be about 8% on current distortion or
2)an active harmonic filter that can limit upto 2 % of current distortion. This is more expensive and big in size. or
3)Replace your VFD with a good brand that has the lowest harmonic distortion like ABB or
4)supply all your VFD from a separate power transformer
and of course using screened cables for the supply and connection to the motors (watch for the maximum legth of cables)and totally avoiding neutral conductor in the control etc will reduce radiation from the cables to the system. All sensitive equipment you must use special power supply or filters.

Hope this helps,
Sekar

 

At Delphi we have 31 2000KVA delta-delta power transformers with most of our loads being nonlinear: VFDs, servo and other variable speed drives. We have determined that DELTA-DELTA power transformers do more harmonic filtering than small line reactors recommended by manufacturers and suppliers. As for the real problem, servo and other CNC applications, we have found that establishing a neutral reference, while balancing the phase voltage to ground and filtering harmonic noise(voltage and current) with respect to ground can be achieved using the same device. Balancing the phase voltage it is impossible for the phase voltage to float, arc or drift and no other surge suppression device is required. The device we use is Phaseback EMTVSS from APPLIED ENERGY LLC. The URL is www.applied-energy.us
One Phaseback unit is required per substation transformer. Monitoring for transient events over the past year, we experienced none.

 

Take out your thyristor drives and replace with good old star delta starters on the AC motors and Ward Leonard on any DC motors.