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Participant

Join Date: Feb 2021
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Power Quality Issue with Installed Passive Filter

02/02/2021 1:09 AM

Hello engineers, I am facing a PQ issue at one of Textile plant having non linear loads, when Switching ON certain stages by Automatic PF relay, the Installed 800 KVAR Passive filter at LV substation creates loud humming noise, overheating of reactors/capacitors and occasional blown fuses, etc, some times no noise is observed if some stages are turned ON manually; due to fear of fire customer has kept it turned OFF, P.F can’t be improved. Panel Analyzer shows THDI before switching Passive filter ON = 9% and after switching passive filter ON = 13% (all 12 stages On. Due to fear of fire customer has kept it turned OFF, P.F can’t be improved.

Does Resonance could still happen at installed anti-resonance passive filter with 7% detuning factor? Is the design of passive filter incorrect? or it has gone incorrect after installation of additional unplanned non-linear loads over a period, or there could be other reasons causing it, we want to avoid expensive AHF solution, opinion from forum members is requested.

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#1

Re: Power Quality issue with Installed Passive filter

02/02/2021 1:16 AM
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#3

Re: Power Quality issue with Installed Passive Filter

02/02/2021 5:50 AM

Start by calling the original equipment manufacturers of the <...Installed 800 KVAR Passive filter at LV substation...>. If the answer doesn't come during that phone call, then get them in to investigate and recommend.

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#4

Re: Power Quality Issue with Installed Passive Filter

02/03/2021 12:45 PM

I'm curious as to what type of loads you are referring to as "non-linear" because a THD (I've not seen the I at the end before but I think it might reference current as opposed to V for voltage) anyway a PF of 9% is something I've not seen before, 1%-1.5% but not 9% and to go to 13% with the power factor filter in would appear to me to be resonance in the filter at one of the harmonic frequencies caused by the load(s). The type of filter is important; the ones I've seen are usually composed of capacitors and no inductors. Depending on the filter configuration using LC combinations you can have resonance at one of the harmonics you are experiencing which will cause large currents in the filter which will blow the protection fuses.

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#5

Re: Power Quality Issue with Installed Passive Filter

02/03/2021 1:45 PM

I’m interested in what AHF is, not withstanding the list that AP#1 (Anonymous Poster No. 1) posted.

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#6
In reply to #5

Re: Power Quality Issue with Installed Passive Filter

02/04/2021 9:31 AM

There are far too many TLAs in this post.

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#7
In reply to #6

Re: Power Quality Issue with Installed Passive Filter

02/04/2021 10:41 AM

At least no ETLAs or STLAs, some restraint...

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#9
In reply to #5

Re: Power Quality Issue with Installed Passive Filter

02/06/2021 9:15 AM
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#8

Re: Power Quality Issue with Installed Passive Filter

02/05/2021 10:09 PM

Here is some information.

If you have non-linear loads, e.g. power supplies where current is absorbed at the peak of the AC cycle, third harmonics are generated. (Another culprit is fluorescent lighting.) A parallel circuit is tuned to absorb this third harmonic.

"...for industrial loads connected to stiff supply, it is difficult to design passive filters that can absorb a significant part of the load harmonic current and therefore its effectiveness deteriorates. Specially, for compensation of diode rectifier type of loads, where a small kVA passive filter is required, it is difficult to achieve the required tuning to absorb significant percentage of the load harmonic currents. For this type of application, the passive filter cannot be tuned exactly to the harmonic frequencies because they can be overloaded due to the system voltage distortion and/or system current harmonics.

The single-phase equivalent circuit of a passive LC filter connected in parallel to a non-linear current source and to the power distribution system is shown in Fig. 39.38. From this figure, the design procedure of this filter can be derived. The harmonic current component flowing through the passive filter ifh and the current component flowing through the source ish are given by the following expressions:"

https://www.sciencedirect.com/topics/engineering/non-linear-load

It sounds as if your passive filter is not properly sized to absorb all the harmonic currents that are being generated.

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#11
In reply to #8

Re: Power Quality Issue with Installed Passive Filter

02/10/2021 12:13 AM

Thanks.

The installed passive filter is a series L-C type, and it appears it has lost its tuning point to shunt high harmonic frequencies current and shall have to be re-designed after actual measurement of harmonics.

Just a question what is more beneficial in the given situation, Parallel L-C filters as you pointed out or commonly used series L-C filters, how the choice is made?

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#12
In reply to #11

Re: Power Quality Issue with Installed Passive Filter

02/10/2021 5:53 PM

Just a question what is more beneficial in the given situation, Parallel L-C filters as you pointed out or commonly used series L-C filters, how the choice is made?

Series harmonic filters are in series with the load and are tuned to have high impedance at the harmonic frequency, blocking the harmonic from the rest of the power system while allowing the primary power frequency to pass. Shunt harmonic filters are parallel to the load and are tuned to offer a low impedance to the harmonic frequency. It is also possible to employ both at the same time. I believe shunt filters are more economical because they do not have to handle the primary current.

"Harmonic filters come in many “shapes and sizes.” In general, harmonic filters are “shunt” filters because they are connected in parallel with the power system and provide low impedance paths to ground for currents at one or more harmonic frequencies. For power applications, shunt filters are almost always more economical than series filters (like those found in many communications applications) for the following reasons:

1. Series components must be rated for the full current, including the power frequency component. Such a requirement leads to larger component sizes and therefore costs.

2. Shunt filter components generally must be rated for only part of the system voltage (usually with respect to ground). Such requirements lead to smaller component sizes and therefore costs."

https://www.sciencedirect.com/topics/engineering/harmonic-filter

As PWSlack has suggested (#3), it's best to have a local expert make measurements and analyze your power system. We on this forum can only offer general information.

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#13
In reply to #11

Re: Power Quality Issue with Installed Passive Filter

02/11/2021 11:40 AM

Usman,

You write that the connection of the filters actually increases total harmonic distortion, 9% to 13%. Overloading will cause saturation; this will cause the filters to produce harmonics themselves.

Presumably, this is a three-phase system - do you have balanced voltages and currents?

The L-C networks can only be single phase - do the overloaded sections connect to a certain phase?

It would be useful for you to find which L-C units overload or increase THD when connected manually on/off.

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#10

Re: Power Quality Issue with Installed Passive Filter

02/06/2021 6:04 PM
  1. You need to establish the specification; frequency and amps for any filters shunting harmonic currents.
  2. Then you can measure actual currents to compare with ratings - some guide to what harmonics have increased & by how much. With Total Harmonic current indicating trouble you need more information.
  3. The mains frequency may have changed, or become less stable. However, if L-C networks resonant to harmonics are overheating/noisy, this shows they are on-frequency [possibly if a high harmonic, their resonance might be on a different harmonic].
  4. The supply source impedance may have reduced due to modifications, so your shunts are removing a smaller proportion of harmonics & more are going into the supply source.
  5. The supply source might have increased harmonics due to other consumers.
  6. You have not clarified if there are any rejection filters in the system, which reflect power back from the mains. This power has to go somewhere and a change on the load side could mean this adds to the harmonic L-C shunt losses.
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67model (2); Anonymous Poster (1); PWSlack (2); R_i_c_h (1); Rixter (3); rwilliams (2); SolarEagle (1); usman14865 (1)

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