I’ve made measurements in a tissue factory fed from the secondary side of a distribution transformer 800KVA, 11/0.4 KV, X=5% after the erection of a fixed capacitor (100 kVAR)
The results were as follows:
With Capacitor Bank Disconnected
With Capacitor Bank Connected
Voltage Total Harmonic
Total Demand Distortion
The greatest harmonic was the fifth.
I want to know the rule that made the 6.8% TDD to become 31.75%? Is this due to parallel resonance? But the capacitor doesn’t burn nor its fuses blown. Does this factory need to erect a harmonic filter (detuned type)? Note: the same distribution transformer fed other factories that also have capacitor banks.
A. I believe your problem is due to the capacitor being a low impedance device therefore attracting any harmonics that are in the system. This will include any harmonics that are produced on the load-side of the 800kVA transformer. The fact that your greatest harmonic is the 5th tells me that you probably have some 6-pulse drives that represent a good portion of the load.
If you were experiencing resonance you would have burnt conductors, capacitor cells, or blown fuses so we don’t believe that this is a concern. Your fuses are not blowing because they are probably sized large enough to handle the 32% increase in amperage.
I would suggest that you obtain a complete one line diagram of every thing on the load side of this 800kVA transformer and examine your non-sinusoidal devices such as VFD’s, UPS’s, and all other devices that change AC to DC. This will give you a good idea of where the harmonics are being generated from and how to properly size a filtered bank. Since this is a tissue plant I would say that you have VFD’s that are spooling and un-spooling large rolls of paper.
You can learn more by reading our manual on solving harmonics. We would be happy to assist you in resolving this power quality issue and look forward to hearing from you in the future.