Distortion with a non-linear load

[dave313]

Hello all

I'm having a problem with a non-linear load driven with an AC generator.

This is an off-grid system with a generator used for charging batteries through an inverter. When the gen is charging the batteries, there is interference from the 20khz switching frequency of the inverter that causes lights to flicker and has some electronic devices not working properly. The AC waveform is distorted when fully charging and cleans up as the rate of charge decreases as the batteries come up to full.

I have heard that a tank circuit will fix this problem but the inverter manufacture can not provide me with a schematic. My system is different as I use a 240vac generator and a step down transformer to get 120vac at the inverters. The gen is 250 feet away and I use the transformer to negate some of the voltage drop.

I was once told to put a small motor run cap (1-4uf) across the input of the inverters and that works for awhile but the cap eventually fails. My problem seems to be exacerbated by the transformer. I did put a 1500 watt resistive load across the output of the gen and that reduces the interference but I don't want to waste fuel running this load.

I was thinking to use the step down transformer as part of a tank circuit but I would need a +90uf cap to resonate at 60hz if this will even work at all. I measured the inductance of the transformer output at around 4.5mH. I don't know if I need to include the transformer input inductance in the calculations and 90uf seems rather large to put across an AC line. Also, this problem has been present for 15+ years and with several different generators.

Can anyone offer me any clues on how to fix this problem?

Thank you

 

[itsmoked]

Your description is good enough to explain the problem.

Your inverter is a simple unit that takes the input voltage at it's battery terminals transforms it to the peak secondary voltage and then generates the output from this 'stored' peak secondary voltage.

Next.

Your charger is a low-end rectify-the-input-and-send-it-to-the-battery-bank type.

The result is that this rectified charging voltage has a lot of 60Hz in it and the inverter front-end will automatically select for the peaks due to it's design. All these 60Hz peak humps get asynchronously stepped up and delivered out of the inverter to your confused loads.

Adding a capacitor improves things because it starts to filter the charger's output some, reducing the ultimate 120Hz charging peaks being consumed.

Let me guess; Lead Acid batteries?

You can fix this a couple of ways.
1) Replace the charger with one that outputs a filtered charging current instead of sucky 'pulsating DC'.
2) Build your own filter.

Do NOT touch trying some 'tank circuit'. Well, you could if you want to mess around a lot and replace your inverter a few times. That's a fools errand.

To greatly reduce the charging pulsation use an inductor. This trades the charging peaks for longer conduction angles which also reduces electrical stress on everything. It will remove the peaks the inverter is transforming.

We can maybe suggest an inductor if you can give us a lot more detail on what you have.
A) Confirm the voltage supplied to the charger.
B) What kind of batteries.
C) What are the bank amp-hours
D) Which bank voltage is your system?
E) What's the charger's maximum charging current?
F) Got a charger model number?
H) Any idea what the typical charging current runs at with your setup?
I) What is the maximum continuous DC draw your inverter requires?

No solar?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dave313]

Itsmoked
Yes solar, about 6kw worth. The generator is for cloudy days and it runs about 200 hours or so a year

OK here are some details you wanted:
A, 120vac to the inverter/charger (x2)
B,C,D, Lead acid battery @ 24vcd and 1700 amp hours
E, Max charging current for each inverter, ac input=20 aac, DC output=85 amps DC
F, Two Outback Power VFX-3524 inverters
H, I have each inverter input set to 15 aac and this gives me about 110 amps DC total to charge the batteries
I, Each inverter has a 250 amp breaker but I think I have never drawn more than 150 amps. Usual current is around 40 amps total.

Just to clarify things, this problem only happens when the generator is charging the batteries. When the gen is off, I have great quality power.

I was hoping to install the "filter" on the AC input side of the inverters, that is where the cap worked before. This has been a pain in the rear for a long time and I am now getting around to get this fixed. Thank you for your help.

 

[waross]

How about the Generator, gas, diesel, KVA?
For a diesel set at 6 KW the extra fuel to run 1500 Watts 200 Hours per Year will be about 2 gallons per month.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Ah.. well so much for the "cheap" comment.

When the gen is off, I have great quality power.

Which proves the mains based charging (generator) is the cause of the problem. Generally it's better to fix or prevent problems at their sources rather than trying to band-aid the problem farther down the line.

Let me get this straight.. Is the problem occurring while the generator input is being fed-thru (by-passed) to the inverter's output directly carrying the house loads while ALSO charging? Or is the generator output being strictly down converted and fed to the batteries and the inverter is synthesizing all the output to the house?





Keith Cress
kcress -

http://www.flaminsystems.com

 

[dave313]

The generator is a new 8kw diesel. It has not been the first gen I've used here.

When the generator is on, it runs the house loads (by-passed) and the inverter charges the batteries. It is when the inverter is charging the batteries the problem happens. If I disable the chargers, the problem is gone, also if I decrease the charge rate the distortion decreases proportionally. I was to understand that the 20khz switching of the charger causes harmonics that distort the 60hz sine wave.

Most users do not have this problem with generator charging and since I have had multiple generators and inverters in this system, I think it may be the step down transformer that makes this problem obvious. This is the last place to look...

 

[waross]

The AVR on the generator doesn't like producing the kind of current pulses that a charger demands.
A generator has quite high internal impedance. I suspect that the problem is the AVR response and the generator response to the choppy current.
Solution #1. Buy a battery charger and connect it directly to the batteries.
That will isolate your 120 Volt circuits from the flaky generator voltage caused by the battery charging.
Solution #2 Buy a 24 Volt, 100 Amp alternator and mount in place of the current alternator on the gen-set.
If there is no sun, use the generator to produce AC and use the oversized alternator to charge the batteries.
I've been there, and done that.
It worked well.
It was for a small hospital in the jungle of the Moskito Coast in Central America.
When the generator was running, it produced all of the AC power, and powered a large battery charger in addition to the set mounted alternator.
A 5 Amp battery charger was adequate to charge the starting battery.
The user power was either the inverter or the generator, never both.
I have worked with a few parallel generator setups.
I shudder at the thought of running a generator in parallel with an inverter.
That may be part of the problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dave313]

waross, I have looked into alternate charging systems. I would need a 24v 100a battery charger to replace my current charging requirements, lotsa bucks there. I can't run a DC gen because of the wire size and the distance plus I just got a new AC gen anyway.

I have thought about the AVR problem and when I put a 1500 watt resistive load across the gen's output, the problem gets much better but does not go away. Same thing happens if I put the load across the inverter inputs. A cap across the inverter inputs has worked in the past but the they fail too fast. I would imagine that there a certain value that would work but I'm not sure how to calculate it.

There are many users with the same inverter setup I have but do not have problems when charging batteries, this makes me think that my transformer amplifies the problem. Do you have any suggestions on a test I could run to determine the source of my problem?

Hey, thanks for the input

 

[waross]

You can keep guessing until you run out of money.
A typical generator when unloaded will produce about normal voltage when excited by a 12 Volt battery.
The same generator when loaded will need about 24 Volts excitation to produce a normal voltage.
Now replace the battery with an Automatic Voltage Regulator.
The AVR will produce about 12 Volts when the set is unloaded.
As the load is increased, the AVR will increase the voltage until it is about 24 Volts with the set loaded.
But, put a simple rectifier load on the set and as the cycle progresses, the current goes from zero or no load to a heavy load as the voltage nears the peak of the sign wave and then shortly past the peak drops to zero until the voltage nears the negative peak.
The AVR cannot cope with a subcyclic change from no load to full load and back to no load twice a cycle.
If a run cap is failing it may be rated for too low a voltage.
Try a cap with a much higher voltage rating.
Your transformer:
Itsmoked suggested using an inductor.
A transformer is an inductor.
It is helping you, not harming you.

Usual current is around 40 amps total.

That would be 4 Amps at the generator.
Not much of a buffer load on an 8 kW set.

I have each inverter input set to 15 aac

That is a 3.6 kW load.
But due to the choppy waveform your peak instantaneous kW will be much more than expected.
What are your distances?

If a run cap is failing it may be rated for too low a voltage.
Try a cap with a much higher voltage rating. (double)


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Bill; He's running the power 250ft at 240 then stepping it down at the charger/inverter to 120V.

If you've had success with caps then that's probably the most reasonable way to go. Just use better caps.

Perhaps one of these.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dave313]

waross, I hear you about the AVR and this may be the problem but Outback has sold thousands of these inverters and only a very small percentage has had problems with generator charging. When I contacted them years ago they recommended the cap but could not provide me with a value and said start with 1uf and go up from there. I used a 440vac motor run cap across the 120vac output of the transformer and it worked but seemed to draw some power and I don't remember if the cap got hot but after 50 hrs or so the cap would fail. At the time I only had a VOM to analyze the problem but today I have a scope, LCR meter, a clamp-on ac amp meter and some other test equipment.

Outback designed a "tank circuit" for a customer and it solved his problem but this happened many years ago and those engineers have left and there is no record of that tank circuit. I contacted the user that installed it on his system and he described it as a very heavy box and he thought there were just 2 wires that went across the input of the inverter input. He has no other information and does not have that system anymore. This is why I thought of using the transformer as part of a tank circuit. I do not have the experience to design this myself and thought I can get some help here.

I would like to use a cap but I think you need to take some measurements in order to select the correct value, I never liked the hit-and-miss approach especially on a mains circuit. I think simple is the way to go.

BTW, Over the years I have replaced generators and inverters as part of upgrades or replacements but not to correct this problem. I just purchased a good quality diesel generator thinking this would last me the rest of my life and since the flickering problem still exists, I came here for suggestions. You guys are great and are very helpful, I am getting some good information here. Thank you.

 

[waross]

Some generators in that size range do not use an AVR.
They use a capacitor based self excitation system.
The output voltage is trimmed by changing the capacitor value.
I have no idea how these sets accept a predominantly rectifier load, but probably not well.
Another suggestion:
100 Amps at 28 Volts charging voltage is 2800 Watts.
2800 Watts is 3.73 HP.
Look for a 5 HP, single phase motor on Ebay.
Look for a 100 Amp, 12 Volt alternator at the auto-wrecker's.
Use the motor to drive the alternator.
Hack the voltage regulator to boost the output from 13.8 Volts to 27.6 Volts more or less.
Given a choice between a heavy or a lighter pulley for the motor, use the heaviest pulley.
The flywheel effect will help.
In your next life, plan to put your batteries closer to the gen-set.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dave313]

Are there any suggestions on how to solve this problem with my existing setup? How would I go about selecting a capacitor value that would filter out the noise and have a reasonably long life?

 

[waross]

Capacitors worked before.
Buy a better quality capacitor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Check your inverter for a blown input rectifier.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

The last two caps of the 4 I recommended are rated for 480VAC service across-the-line. I'd add short leads to them and try one. Always shield yourself from capacitors or wear safety glasses. The 4X voltage safety factor should give you good service. If not they're inexpensive. If you find 480VAC caps blowing then you have other serious problems that need to be addressed because your inverter would be at risk.

You're looking for a qualitative answer but that is not going to happen in this case. There is a garbage sack of unknowns which shall-not-be-known in your circumstance. You don't have network analyzers to measure everything you'd need to go with a qualitative solution so work with the more useful empirical solution. Add a cap. If it blows add a better cap. (latest suggested solution) If they still blow then put in some spike protection to protect the cap better.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dave313]

I wish I had a network analyzer...

I guess I will get a few of those caps and try them out. If I use my clamp-on amp meter on one of the legs of the cap, will I be able to tell if one value is better than another?

 

[waross]

The current will be proportional to the value of the capacitor.
See if the lights flicker.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

As Bill sez, use what you need to kill the flicker.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dave313]

OK, I will start with a low value and work my way up. Thanks