3Ph Passive Harmonic Filters with 1 or 2 blown fuses.

[MiketheSparky]

Hello all,
First time poster here. I have a situation where my customer has asked that I find some independent information about what happens to the electrical system/harmonic filter mitigation when they have issues with their filters.

Backstory: Harmonic filters were installed per the contracted engineering firm after the equipment was installed. There are 45 filters throughout the building ranging from 15A 480V to 800A 480v. Since their installation, there have been multiple issues with contactors and fuse blocks creating arc faults. After finding manufacturing issues and having them corrected (properly torqued factory connections), I performed an IR scan of the building and found 10+ filters that had phase imbalances. Upon troubleshooting I discovered that all phase imbalances were on filters that had 1 blown fuse after the primary reactor.

My understanding of the filters is that the harmonics enter on 1 phase and leave on the other 2. (Correct me if I am wrong on that one.) So if there was a blown fuse then the harmonics would stack up on a single phase? If 2 fuses are blown then there is no filtering coming from the tuned reactor because it would just phase out correct?

What are the impacts to the electrical system in each scenario? The customer wants to remove all harmonic mitigation from their system due to the amount of money already spent in correcting the issues from the manufacturer. Do we have a good case to remove them? They are all on VFD's from air handlers and pump motors and they are all on a generator backed distribution. Thanks for your time.

 

[Jraef]

My understanding of the filters is that the harmonics enter on 1 phase and leave on the other 2. (Correct me if I am wrong on that one.)

No, that's not how anything works with 3 phase power. The harmonics would exist on all 3 phases, the filter just deals with them as they exist but without knowing the type of filter it is, we can only say that there is more to it going on inside. But there is no "in and out" involved. If one fuse blows, you are not getting the full harmonic mitigation is was supposed to be providing.

That said, passive harmonic filters are usually used on specific equipment that is creating harmonics, like VFDs, and tuned to the most prominent harmonic (or a couple of them) that the device is creating, i.e. 5th, 7th etc.. If these are just distributed randomly around a facility, it sounds like a scam job someone perpetrated... Fuses blowing on passive harmonic filters is not something that happens unless there is something seriously wrong with the filter assembly.

 

[MiketheSparky]

That is exactly what I thought. Thanks for your reply. I asked a few local engineers and most of them were stumped by the data I've collected. I was told that when a passive filter mitigates through the tuning reactor and capacitor bank, the balanced load has each phase's harmonics reduced by inducing those harmonics onto the other phases equally naturally via capacitive reactance. The Eaton Harmonic Solutions video by Dan Carnovale also stated something like that (though I may have misinterpreted it.) Regardless, what I am measuring load wise on the filters that have a single blown fuse, is really confusing. Here is an example of readings taken from the primary reactor in the harmonic filter: Line side-A 30A B 48.6A C 27.5A, Load side- A 30.4A B 35A C 32A. A phase fuse is blown. Units that have 2 fuses blown do not have any imbalance or energy loss through the reactor. All of the filters are installed on equipment with VFD's. And I have tested the capacitors and their resistors on all units. Any ideas on what direction I should take from here? I could give the manufactures info but I'm trying to avoid conflict between the customer and the engineering team while I get some more information on what solution to provide. Thanks again.

 

[Jraef]

So do these units have line contactors that switch them off when the VFD is not running? You mentioned contactors so I assume so. Do they have them properly interlocked with the drives then? The purpose of the contactors is to only engage the filters when the VFD is running, so that the filter caps are not on-line when the drive is off, causing a leading PF in the circuit. If they don't have the contactors being controlled by the VFD (which I have seen), then whatever they are doing to turn them on and off might be contributing to the fuses blowing.

As to your readings, harmonics exist on the phases differently, because the VFDs are not the only contributors. HID and LED lighting also cause harmonics and those are single phase loads, so they are not evenly distributed among the phases. I wouldn't be as concerned about that as about why fuses are blowing.

 

[MiketheSparky]

Yes, There are contactors that are controlled through a BMS system that activates when the VFD is running. The issue with the imbalance is that there is equipment from the same distribution that has all of the fuses intact that doesn't see the imbalance. It seems to follow each piece of equipment when there are fuses blown. I agree that the fuses blowing are just indicative of a larger problem. I also have power logger data showing the imbalance existing when the filter is installed and no imbalance when the filter has been bypassed via multitaps.

 

[synchro]

The only thing I can think of that could induce the fuse blowing is if there are resonances causing higher currents to flow.

I assume the harmonic filters are "notch" filters having a series connected L-C-R network that is shunted on each phase. The frequency of the notch is normally tuned a little bit below the harmonic frequency that is to be mitigated.
There are are also "ladder" filters like the L-C-L filters from Hammond:
https://americas.hammondpowersolutions.com/products/filters/passive-harmonic-filter

Are the fuses that are blowing in series with the capacitors?
Your comment below suggests that maybe the filters are of the L-C-L type, where the load current flows through two cascaded reactors that are in series with the load. If they were shunt L-C notch filters and a fuse in series with the capacitor was blown, then there would be no current through the inductor or capacitor. This is assuming it's a single tuned notch filter.

Here is an example of readings taken from the primary reactor in the harmonic filter: Line side-A 30A B 48.6A C 27.5A, Load side- A 30.4A B 35A C 32A.

Is the fuse blowing happening when powered from the POCO? Or on the generator?

 

[MiketheSparky]

We aren't sure when the fuses blew. We have only replaced fuses on units that had catastrophic failure due to the manufacturing issues that caused events on startup or shortly thereafter. These units are roughly 5 years old. There are no fuse indicators on these units and I believe if we continue to use them, I will be installing those moving forward. I've attached a picture of the inside of the larger 840A unit. A and B fuses are blown on this one. It looks like to me, that the trap reactor is tapped from the line reactor. The trap reactors on some of the units are really loud as well. Could they be delaminating and causing an issue? Thanks again for your knowledge so far.

 

[synchro]

It's not surprising that you would see current imbalances when there's one blown fuse on the LC trap filters. That would create an asymmetry in the AC voltage waveforms fed into the rectifiers of the VFDs, and therefore cause an imbalance on the currents drawn from the line conductors. I highly suspect that the harmonic filter is connected in a delta configuration, and so with two fuses blown it will not be conducting any current from the line conductors. Therefore it would not cause an imbalance in the line currents, as your testing has shown.

The trap reactors on some of the units are really loud as well. Could they be delaminating and causing an issue?

Do all the units where the trap reactors are loud have one fuse that is blown, or are there ones that are loud which have all fuses intact?
It's possible that some reactors could be loud because they are conducting a larger share of harmonic current, perhaps because the trap frequency is mistuned. Is it practical to measure the RMS current through the trap reactors and compare them?

We aren't sure when the fuses blew. We have only replaced fuses on units that had catastrophic failure due to the manufacturing issues that caused events on startup or shortly thereafter. These units are roughly 5 years old. There are no fuse indicators on these units and I believe if we continue to use them, I will be installing those moving forward.

... The customer wants to remove all harmonic mitigation from their system due to the amount of money already spent in correcting the issues from the manufacturer. Do we have a good case to remove them? They are all on VFD's from air handlers and pump motors and they are all on a generator backed distribution. Thanks for your time.

It certainly makes no sense to continue using the using the units as is when their fuses are blown. One thing that could be done is to disable the contactor that connects the trap filters on only the units that have had a problem. And then consider having the level of harmonic currents measured, by someone else if necessary if you don't have the equipment. The line reactors inside the harmonic filter units would still be in place, and are they more likely to be reducing harmonic levels than if the filter unit was completely removed. If the harmonic levels are adequate with the contactors disabled, then it seems like this could be an easier way out than trying to isolate and correct filter mistuning or resonances that might be present.

Are there power factor correction capacitors in the system that supplies these units? If so, perhaps they were needed with off the line motors but may now be causing problems such as resonances, etc. when used with VFDs which do not need power factor correction.

 

[MiketheSparky]

Not all of the trap reactors that are loud have blown fuses, but most that have a single fuse blown sound "cranky". I can take RMS measurements at the trap reactors and compare. The system is fully metered at every panel/distribution and main gear with Schneider PowerLogic so I have taken logs of the THD in the building previously. We could disable the contactors and see how much the THD changes. There are no power factor correction capacitors elsewhere. Also there is a huge discrepancy in Line/Load amperage on a few, mostly the loud units. Here is an example:

	Load A	Load B	Load C	Line A	Line B	Line C
Unit #1	31.3	29.9	30.7	57.8	57.1	55.5
Unit #2	41.9	41.3	41.5	60.3	59.6	59.2

These 2 units are identical. Seems like they are consuming a lot of power. Thank you.

 

[Jraef]

In my experience, loud reactors are usually a sign of the laminations getting loose, unless it is a manufacturing issue / quality control problem, but in that case it is generally there immediately. I haven’t seen a lot of it, but when I have, there was no fixing it in the field short of replacement.

Not having harmonic mitigation is likely not an option because the utility will insist on it. But I have been moving away from recommending passive filters on anything above 75HP now. It makes more sense to use Active Harmonic Filters at the PCC now. Not only have prices come down and reliability gone up, but I like the fact that the AHFs don’t care where the harmonics came from, they measure and react to whatever they see.

 

[synchro]

... Also there is a huge discrepancy in Line/Load amperage on a few, mostly the loud units. Here is an example:

	Load A	Load B	Load C	Line A	Line B	Line C
Unit #1	31.3	29.9	30.7	57.8	57.1	55.5
Unit #2	41.9	41.3	41.5	60.3	59.6	59.2

These 2 units are identical. Seems like they are consuming a lot of power.

Can you arrange for the load on the output of this harmonic filter to be temporarily shut off, and then measure the line current into the filter? Perhaps a portion of the high line current that you are seeing is from the traps sinking harmonic currents produced by other equipment on the power system.

 

[MiketheSparky]

@Jraef - When sizing AHFs, do you size them to the amount of mitigation you need or to the switchgear at the PCC? I've looked a little at some of these filters and they look like a solid solution, I'm just curious about cost for size. Customer had (2) 5000A switchboards.

@synchro - There is a lot of redundancy at this site. There is equipment in lead/lag right now that I could test on. I could also test the line side without the trap filter engaged and then after it's engaged to see if the trap is the cause. I'll let you know. Thank you.

 

[Jraef]

When sizing AHFs, do you size them to the amount of mitigation you need or to the switchgear at the PCC?

To the amount of mitigation needed, referred to as “corrective current”. So you do a detailed harmonic study, determine the amount of harmonic current you have or will have, then size the AHF to provide that. You can put it all at the PCC, or split it up into sub-systems, it all comes out the same. A lot of what I do involves MCCs, so I tend to put them in each MCC in accordance with the non-linear load in that unit. But I’ve also done it on switchgear that is feeding large groups of separate VFDs, it’s all worked out fine.

Also as to redundancies and back-ups, I tend to size to the actual running load. I did one project with 12 x 400HP pump drives at a mine, competition bid it with Active Front End drives, which would have worked fine. But I found out that there were 3 sets of 4 in different sites, and each site only ran 2 at a time, the other two were standby and alternated through the rotation. They never ran more than two, in fact their piping couldn’t handle it if they tried. So I used cheaper 6 pulse drives and an AHF at each site, sized for just two motors. We were 40% if the other bid and won the project. They then decided to add redundant AHFs (I thought it unnecessary, but oh well) and we were still at 60% of the other bid.

 

[MiketheSparky]

Do AHF's cause any issues with generator power? I'd like to suggest that as an alternative if we get to that point. It seems to me that with the amount of equipment at this facility, they need to have a better solution than what is currently installed. Also the I/THD on one of the switchboards is currently sitting at 10%. We know that some of the passive filters aren't doing any tuning mitigation so at this point if we start removing filters from the system it's just going to get worse.

Here are some readings of a filter that I took this morning. This filter doesn't have any blown fuses and everything looks good from my end. A little noisy but nothing like the ones that I said were loud.
Unit is rated at 156FLA. 480V. There are 6 VFD's downstream of the unit. I took measurements with a Fluke 375FC clamp meter. I toggled the trap reactor contacts via the BMS.

With trap engaged: A Load= 21.7A, B Load= 21.0A, C Load = 21.6A; A Line = 51.3A, B Line = 50.9A, C Line = 49.3A.

With Trap disabled: A Load = 20.3A, B Load = 20.1A, C Load = 20.6A; A Line = 20.7A, B Line 20.4A, C Line 20.7A.

I don't understand how the trap reactor is utilizing that much power. Just an observation that I thought I would throw out there. I have a Fluke 1738 that I am going to throw on there and see what the harmonics are with and without the reactor engaged as well.

 

[synchro]

Make sure that the harmonic filter is connected such that the traps are on the load side of the line reactor, and not on the power input side of the line reactor. Otherwise, the traps will be presenting a lower impedance at the power input side than to the load side at the harmonic frequency, which could allow the traps to draw a substantial amount of harmonic currents from the system and possibly overload them.

 

[MiketheSparky]

How can I confirm that the factory wired the taps for the line reactors correctly? In the photo you can see that the lugs are labelled so the electricians that installed it just followed that. What it doesn't do a great job of showing is that the trap circuit is directly tapped from the reactor, like a transformer is with taps from the windings. Most of the units are lug type connections, but some of the smaller units use a terminal block with line and load right next to each other (which has also caused some serious grief).

 

[synchro]

Where did you measure your line and load currents? On the conductors coming right off of the line reactor, or somewhere else?

 

[MiketheSparky]

Correct, I measured straight from the line reactor.

 

[synchro]

How can I confirm that the factory wired the taps for the line reactors correctly? In the photo you can see that the lugs are labelled so the electricians that installed it just followed that. What it doesn't do a great job of showing is that the trap circuit is directly tapped from the reactor, like a transformer is with taps from the windings.

Are the taps made on the line reactor to connect the traps located at or near the ends of the windings (i.e., near the L1, L2, L3 or T1, T2, T3 terminations)? Or are the taps made somewhere in between on the windings?

 

[MiketheSparky]

So I just went and looked at a bunch of them. Some have the load taps in the center and the trap taps at the bottom. Others have the trap tapped in the center and the load at the bottom. The manufacturers schematic shows the trap taps center. Ironically, the one using all of the extra power is the first image which matches the schematic. There are other sizes that I looked at that had the load taps in the center, but these two photos are from identical units. We received a replacement unit to one that failed on startup and I checked that one as well since it was from a different batch. It was load tapped in at the bottom as well. Some of the smaller 15A units have the load and trap both at the bottom and it's hard to tell which one is before the other in the windings.