Harmonics & resonance

[DM7.ENG]

Looking for some clarity on the following topic, If anyone can help it would be much appreciated.


So pulled out the books to revise a topic studied a fewyears back at the beginning of my academic life which involved harmonics andresonance.


I find any online literature describing resonance talksabout LC filters for radio signal and signal transmission “Blab la bla”,


The issue I am having is regards to Power. I have a prettygood understanding of Harmonics and understand the concept of resonance in thesense that Series circuits at resonant frequencies will result in L&C cancellingout leaving Z=R. The possible result is a short circuit on the AC network asthe R value will be likely miniscule meaning high fault currents.


The opposite for parallel circuits as resonance will resultin Z = infinity and so no current flows causing the circuit to act as an open.Simple


The area of concern is I suspect I have harmonics on an LVnetwork consisting of an MCC with some soft starts and 2 x 250kW VSD’s. TheDrives have high quality passive filtering therefore, are not the cause forconcern. I suspect that the harmonics are coming from the network as the driveseem to clean up the contaminated line when they are running. As a result thesupply authority is the issue.


The phenomenon I am curious about is we have Power factorcorrection on the MCC therefore, I am curious as to whether the capacitors andmotors collaboratively are resonating causing these harmonics to amplify.


So in summary my question is. What effect does resonance haveon harmonics and how exactly do they intertwine?


A very basic view for me would be those harmonics that arenear to the resonant frequency determined by the Inductive loads andcapacitance of the PFC at a given instance would likely be amplified if inseries from looking at first principles? However, hoping someone out theresmarter than me can clarify.





Many thanks

 

[happysteve]

Eh up

I'm only looking at this from a theoretical perspective, I've no practical experience in this area. But I understand what you're talking about, and with a bit of help from Google, I reckon you're onto summat.

i found this article: PQ Corner: Power Factor Correction and Harmonic Resonance: A Volatile Mix

The long and short of it is that your pf correction capacitors will cancel out the inductive reactance at the fundamental (50Hz), and may well have a capacitive reactance at a harmonic equal to another inductive load. It transpires that this can lead to the types of issues you're having.

Another nice explanation: Risk of resonance due to power-system harmonics - Electrical Installation Guide

Sorry I've nowt practical to offer, hope that's helpful anyway.

 

[UKMeterman]

Hi,
Put simply if you excite a system close to it's resonant frequencies expect the possiblity of high voltages and currents. Think swaying people and the millennium bridge, or opera signers with wine glasses. If you think you have a problem hire in fluke 435 series 2 power quality analyser | hire | inlec and survey the supply, the connections to the PFC bank and also the connections to the drives.

When connecting up please remember suitable working methods for the expected fault levels and so on. You could also use the same kit to do a site energy survey.

 

[DM7.ENG]

Hi there

I Have previously hooked up a fluke analyser as we suspectedHarmonics. Found that ITHD went Sky high when the drives where off andsignificantly reduced when they ran. As there is no other non-linear loads onsite I assume the supply is contaminated.
There is PF monitors within each motor starter which tripout even when the parameter is set to max PF (0.99) with max debounce timer.This tells me at a particular instance PF is >0.99 i.e. unity or leading.
My suggestions for this is the switching of the capacitorsin the PFC bank is out or we have resonance acting upon these harmonics in thenetwork.
Problem being we are struggling to get a permit due to thetime of year. It is a problematic site with PLC issues so PM’s and those involveddo not want to touch it in case we can’t recover.
I assume I will have to investigate further after theholidays therefore, I would like to get an insight in to what’s happening.
My understanding is if the currents are being amplified atresonance then I can expects high voltages to appear on the fluke analyser?

 

[Marvo]

I'm not a PFC expert but I have some experience. All the problems I've encountered with resonance on LV networks with PFC are due to harmonics from external sources or from elsewhere on that network. I'd be worried about the effects on the drives if their filter stages seem to be taking the brunt of the external harmonics, the filters are designed to reduce harmonics caused by the drive itself, I'm not sure if external harmonics could damage them. Have you had any failures of drives, softstarters or PFC caps? Is the PFC bank switched or static? Have you measure the actual capacitor currents? I'm assuming your harmonics are in the triplen range but what specific order are they?

 

[marc8]

Hi All

I am no expert, but I remember at my last company we had problems with noise on the network being told it was us causing the issue, we had installed auto PF correction just after one of our LV transfomers location of our drive hook up's.
we were told we were causing harmonic noise on the 52nd harmonic, but we had a forced shut down due to the network operator wanting to shut down to service the HV switch set up & at same time we were advised to get our side serviced which we did.

Then found the problem on their side the harmonic distortion was being caused as part of their switch was not in a good way, they crimped through that side & would replace at a later date.
caused by network operators equipment at the time.
this is just a problem that we faced at the time.