240V AC utility feed has harmonic distortion, poor power factor?

[Opcom]

One of these might help peek into the dark corners of the mystery:

Do you connect the primary CT to neutral? What happens if you disconnect it and just go line to line across the 240V primary?

Are you sure all these transformers are 240V rated and not 208V rated?

Have you tried inserting a 50 Ohm resistor between the cap bank and the rectifier output to somewhat limit peak currents and does that change anything?

Can you use a big 240V variac to bring up the mains voltage from zero to full into the primary of the unloaded transformer and see when it starts buzzing? (secondary not connected and also the pri CT not connected)

[K1JJ]

One of these might help peek into the dark corners of the mystery:

Do you connect the primary CT to neutral? What happens if you disconnect it and just go line to line across the 240V primary?

Are you sure all these transformers are 240V rated and not 208V rated?

Have you tried inserting a 50 Ohm resistor between the cap bank and the rectifier output to somewhat limit peak currents and does that change anything?

Can you use a big 240V variac to bring up the mains voltage from zero to full into the primary of the unloaded transformer and see when it starts buzzing? (secondary not connected and also the pri CT not connected)

Hi Pat -

Those are good suggestions..

These are actually two identical transformers, 3500 @500 mA each. The primaries are in series and the secondaries in series.
Each has a tag showing a 115VAC primary.  I am running 119V under load on each primary and the balance between the two is nearly perfect.

The pole pigs were 240V pri each. I had only 120V on each one and it still buzzed under heavy current loads.

The pri CT is presently connected to neutral. I have tried the 240 across both without the CT and the buzz  noise was the same.  I also tried a large Variac rated at 50A, 120 VAC across one leg and the Variac started to buzz too, just like the xfmr and choke when I used them.  Any magnetic core buzzes in this supply.  I figured the pri CT would help the balance, but it buzzes whether the CT is connected to the neutral or not.

I will try the 50 ohm resistor between the diodes and the cap. I tried it with 10 ohms before but will increase it to 50 ohms to see what happens.


You said: "Can you use a big 240V variac to bring up the mains voltage from zero to full into the primary of the unloaded transformer and see when it starts buzzing? (secondary not connected and also the pri CT not connected)"

Reread my last post about this.  The xfmr stays quiet with a full pri and sec voltage and a full load across the diode stack. It only acoustically buzzes excessively when the last component, a filter cap, is added and the current draw is above 300 mA.  Otherwise the xfmr is near silent.  When everything is connected and working as shown in the schematic, it remains near quiet when pulling a small 100 mA draw.  The 240K bleeder pulls only 17 mA at 66 watts.

T

[KL7OF]

120V 60 Cycle  looks lumpy...

Edit:  My neighbor has a grid tie solar system........

[W7TFO]

Do any of your neighbors have a grid-tie solar power system?  Those use a switching topology to achieve the 60Hz, perhaps that is where you are getting the distortion, even across multiple distribution transformers.

73DG

[W1RKW]

interesting on the grid tie solar system.  I have 2 on either side of me.

[K8DI]

A cap input filtered power supply draws current only during the peak of the sine wave. It essentially clips it. Your 800mA current draw looks like the attached image to the diode. Look at the graph attached, from Duncan's PSU designer, a free tool on the Duncan amps page. The current is almost 3 amps, and it is for a small part of the time. This is with a generic HV diode type, there's no stacked up set of standard diodes available to simulate.
Point being, your peak current is a sharp peak, not a sine wave, and is much more than 800mA. So it makes the iron sing. If you put in a choke, what then? Or maybe much less capacitor, and a choke input? Check the second attachment...

Ed


PS, before you say anything about the shape of the waveform in #2, it is the current in ONE diode. The current in the inductor is a sine wave, the other half of which is provided by the other diode.

[K1JJ]

Guys,

Thanks again for the suggestions.

I spent some more time trying different chokes, cap input, choke input and found that a choke simply made the regulation worse due to IR drop. It did not help the buzzing much and brought my HV down below 4 KV to <3500, etc.    ( I need 4KV at minimum.)     A 50 ohm resistor from diodes to capacitor made little difference in noise - just poorer regulation.

So bottom line is I'm using a cap input supply like the schematic. The ripple and regulation is excellent. I built an insulated box around the two transformers inside the cabinet.  I sat the transformers on a 2" think piece of Styrofoam. I also used 3"  foam rubber around the sides and top of the transformers.  I ran some real load tests using the rigs and found the acoustical buzzing is now down to a quiet level. It is there but almost in the ambient room noise.

I ordered a bunch of 6' X 8' moving blankets and will cover the outside of the cabinet neatly with them. I'm expecting the noise to be close to non-existent once all sound paths are gone.  The insulation techniques were my last choice, but it will work FB.  

Also plan to look into the utility company harmonic distortion area. That will take time - but if it helps further, maybe I can get rid of the blankets.  Dennis and Steve - OK on the grid-tie solar system potential harmonic issues. I will check that out.

T

[KL7OF]

Tom...Was your power supply quiet when it was first in operation?  Your sinewave scope  pic looks like mine and the others.....I just built a powersupply capable of 9kv, I've run it as far as 7 KV with my lumpy sine wave input..it is a brute force cap input on a variac...no noise problems...I know you will figger it out...You may become an acoustical expert in the meantime...Good Luck  73

[W1RKW]

Tom,
I had a chance to load up the 813 rig  last night.  I turned off the fans and put the back cover back in place and although I hear a buzz, I have to be right on top of the rig to hear it and from a few feet away it's hardly noticeable at all. It's quiet for the most part and would not be picked up by a mic.

I'm wondering if your cabinet is resonating like that of a guitar string on an acoustic guitar at 60/120hz.  A vibration  table would be a nice thing to try here.  Probably a waste of time but it might be worth a try to put a loudspeaker in the cabinet and run some tones through it say from 20hz to 200hz and see if there is an audible peak especially around 60/120hz area.

[KA3EKH]

Am I confused or did you say you have two transformers and not one big transformer? If its two transformers and they are next to each other wont there be some weird things happening with the fields in close proximity? The fields will increase in relationship to the load.
Would speculate that although you have identical transformers maybe under heavy load weird things may happen that won’t happen with one transformer. If you are running just one transformer do you still get the noise under moderate load?

[K1JJ]

Steve and guys,

No, there was always buzz - when using the original used 10 KVA pole pig transformer, I had hum buzz back in 2010. I built an insulated cabinet around it. That kinda fixed it for 10 years. But last month I tore it down cuz I didn't need such high voltage or desire heavy weight anymore and replaced it with two smaller 3500V 500 mA (rated for each one) transformers. It buzzed again.  It buzzes no matter if I use one or two transformers. I tried tightening the laminations, NG.  I thought they were too close together causing magnetic anomalies, but moving them far apart shows no buzz improvement at all.

Bob - OK on the 813 rig being quiet. And, yes, the speaker cabinet is an issue. The xfmrs are sitting on a 1" piece of wood and 2" of styro, so did the best I can.

As for the original pole pig - it shud be quiet when sitting in a big tub of oil, especially when it was overkill at 10 KVA. But it buzzed terribly, and barely usable with the insulated enclosure.

So, bottom line is the newer, smaller transformers are fully packed in 2-3" styro or foam insulation.  I tried the 4X1 plate modulated rig and my big linear - and yes, the buzz level is acceptable now. Just a hint of buzz. This is a band-aid approach, but believe me, I tried everything. New parts, substituted parts, bridge rectifiers, fullwave rectifiers, different grounding, checked all connections, different caps, tried several big chokes, etc.

The bottom line is now I have virtually no ripple with 133 uF cap input and the regulation is within specs, from about 4900 to 4400V under normal load. Under excessive load it will drop to 4200V, but it's not needed.  The acoustical insulation solved the immediate problem until I improve it further by adding the insulated moving blankets when they arrive and/or call the utility to see what they can tell me about the harmonic sinewave.   The sinewave distortion isn't too much different than what others are seeing. But I still do not know the answer to the question:  Can SOME harmonic distortion like I am experiencing be enuff to cause acoustical hum/buzzing under heavy loads? I still don't know.  IE, if I started with a perfect sinewave, would the buzzing increase substantially as the waveform was changed to show some harmonic distortion on the peaks?  I still dunno.

But the bottom line is that the PS is now quiet enuff to move on and can only get better with the blankets or utility efforts.  However, for now, the REAL solution still remains as an unsolved mystery - and that kinda yellowfies me.

T

[Opcom]

I'm glad you have it working acceptably. Your power sine wave is really not very bad but there should be standards. I was surprised that it caused buzzing on three different transformers, but who knows. The only time I have had serious buzzing was on the older square wave inverters. 'modified sine wave' inverters are better but still buzz. On inverters that make trapezoid waveforms, which are only a little worse than your utility, the buzzing was minor. This was all 100-500W stuff so nothing like your levels. Real mystery!

P.S. can you share pictures of the transformers involved, the buzzers and non buzzers?

[K1JJ]

I tried yet another method to help reduce the transformer buzz as suggested by Frank / GFZ with great results:  

There are two HV transformer primaries in series and the secondaries in parallel to get the proper 4KV voltage and current required. When running the secondaries  in parallel, Frank thinks the circulating currents may be adding to the buzz problems. I put together two separate bridges -  one for each winding and tied the bridges together in parallel so the windings were isolated from each other.   I used (120)  6A , 1KV diodes; 15 in each leg of eight legs.

Fired up the 4-1000A plate modulated rig at full strap and gave the supply a close listen.  Lo and behold the transformer buzz was almost gone. I could hear a faint buzz - about the same level as a transformer running with no load.  Now the only buzz I hear is the breaker panel and the step start relays buzz, which is way down. IE, the HV transfomers are quiet enuff to get swamped by other things in the shack.  The RF carrier on the scope shows very little acoustical hum which I can probably cure with some foam around the breakers and step start.

So, if you ever come across this buzz problem when using two transformers, this simple circuit may help.


I think there are three reasons for less buzz:  

#1: There is no secondary circulating current. Frank says that no matter how closely matched the transformers are, there is always going to be imperfections enuff to make them different enuff to generate secondary circulating currents when wired in parallel.

#2:  With bridge rectifiers vs: fullwave, the transformer works on both sides at once, sharing the load.

#3:  With the secondaries  in parallel, each one runs at 1/2 the current for a given power output, thus less buzz.

Less peak power drawn - more average to keep transformers working together pulling the load, thus less noise.


It all adds up to quieter operation despite the slight AC sinewave harmonic distortion I have to deal with. I may still call the utility to see what they can do to improve it - now that I can live with this.

** One last thing:  The regulation is also better, which is very important to me.   I have gained about 300V less sag under full load. I have another 55 uF @ 5KV cap I might add making it a cap input filter at 188 uF total, which is a lot of storage capacity.  Can someone say "Jumbo Joules?"


T

[KL7OF]

That is excellent Tom....but why  do the breakers and relays still buzz?

[K1JJ]

That is excellent Tom....but why  do the breakers and relays still buzz?

Hi Steve,

The big relays (actually contactors) and AC breakers buzz partially cuz they are all running on AC in the coils and there is still some AC harmonic distortion and acoustic resonance generated by the contacts.   The breakers - I can't do much about except move them away (not)  or cover them with foam rubber.  But the step-start relays; I plan to add a diode, resistor  and capacitor to make the AC coil voltage pulsating DC and see how much that helps. I already have the step relays in a foam lined box. They are also generating more noise when the HV current increases, showing that the magnet sensors are creating bigger fields.

Essentially at this point I have no more hum and buzz that any other QRO shack - just that I am trying to achieve a quiet room with some audio processing.

T

[NC2W]

If I were at work, and this kind of problem was brought to my attention, I'd install a 'Dranetz Power Quality' recording analyzer at the utility entrance, and let it run.  Do you have any friends in the Instrument / Electrical shops of factories?  If so, they probably have a power quality recording meter.  They will show waveform (as you show), but also display stuff like THD of current & voltage.  They come with CT's & PT's (current transformers you wrap around your phase conductors, wearing your 8 cal PPE, and voltage sensors).  They usually record a value every few seconds over the period of a week or so, and you can know the worst case distortion, AND /OR the distortion timing.  If the noise is centered around 360Hz, it's (as was suggested above) some carrier noise from a VFD on a variable speed motor.  If the distortion rises and shrinks with the sun, it's either a solar array system, and/or it's someone's small business connected on the circuit common to your home.

A suitably size 'line reactor' applied is very helpful for this kind of problem.

Fluke 738 Power Quality Analyzer  / Dranetz DPBG106.   Be very careful using an O-Scope on utilities measurements.  Most of them aren't CAT III certified (which means if you inadvertently touch something, or have an insulation failure, the meter can blow up in your hand). 

I'll offer one more idea:  I don't know where you live, but most utilities have a power quality troubleshoot organization who will help with these issues for a modest fee.  DES (Duke Power Engineering Services) is the one I can remember off the top of my head.

 

[Opcom]

HV secondaries parallelled