harmonic issues? arcing ice cube relays! 2

[vetlover]

Issue at hand, preparing for Hioki harmonics study, but thought I might ask. 2500 amp 480v 3phase service entrance straight to an MDP that feeds an 800 amp breaker to a sub-panel. There has been addition of plating apparatus at sub panel, requiring 480v feeds to full fledged rectifiers for process. While process is running,on the other side of MDP and upstream of new equipment, low voltage (24vdc) electronic equipment with ice cube and solid state relays are intermittently arcing/closing on points. Coil appears non-heated. Anyone done anything special besides line filtering?

 

[Skogsgurra]

Make sure that you have line reactors in front of the plating rectifiers. They cause heavy notching if the reactors are missing (or too small). I wouldnt do a harmonics study. The notches are quite high in frequency contents and elude a normal harmonics measurement. Do a normal wave-form study first. It is easier and you will see at once what is going on. And you can pull out any old scopemeter to do it.

 

[mc5w]

You might also need harmonic mitigating transformers. One form is a wye primary forked wye secondary autotransformer for use on 277Y480 volts. Under U.S. rules they can only be used on a grounded neutral system and the autotransformer neutral has to be connected to the supply neutral.

For you application you would connect say 1/3 of the load without a harmonic mitigating transformer, 1/3 of the load through a HMT that produces a 20 degree lag in voltage, and the other 1/3 through a HMT that produces 20 voltage lead. The net result is equivalent to an 18-pulse rectifier.

Another type of HMT is for stepping down from 480 volts to 120Y208 volts. Mirus International's Harmony-4 series has 4 secondary winding with 15 degree phase shifts. If nonlinear load is reasonably balanced between the 4 secondaries then the primary distortion is equivalent to a 24 pulse rectifier. Thier Harmony-2 series can be bought in pairs that function like a Harmony-4 split into 2 boxes.

Stay away from K rated delta primary wye secondary transformers. They no longer work because modern 277 volt fluorescent and HID ballasts do not generate enough 5th and 7th harmonic currents to neutralize the 5th and 7th harmonic currents from the K rated transformer.

There are all kinds of power quality products out there. Another kind that can be used with variable frequency drives is a Universal Harmonic Filter. These can be used with a diode type 6 pulse rectifier producing harmonic distortion equivalent to an 18 pulse rectifier. They CANNOT be used with SCR firing angle bridges such as 3 phase in DC out variable speed drives.