Linear vs. non-linear loads while using a generator

ELYNN4

I recently increased the charging amps on my 9kw Northernlights generator from 70 amps to 100 amps.  I have a 2200 amp hour 48v battery bank that I am charging with a Conext 6848+ inverter.  I noticed that the charge amps started cycling up and down by +/- 10 amps.  In trying to figure out what was going on I contacted Northernlights- they stated that non linear loads such as the charger would cause the voltage to vary and that the charger was reducing the load to keep the voltage in spec.  Their recommendation is to limit non linear loads to 20% of the capacity of the generator - anything higher than that is hard on all of the equipment.

In talking with Northernlights they admitted that they are exploring DC options as this would be much more efficient, but they have not got anything that they are ready to market.

Curious if anyone is aware of any aftermarket conversion kits to replace an ac generator with a direct coupled permanent magnet dc alternator that has the same capacity?  I would be interested in a system with active rectification with a rated voltage of +/- 65 volts and tailor the charge voltage down to what the battery bank needs rather than trying to change the rpm of the generator to meet the voltage requirements.

Thanks in advance for any info or thoughts folks might have.

BB.

I am not sure non-linear loading during charging is an issue here...

If I have the correct unit/manual, PDF page 135:

https://solar.schneider-electric.com/wp-content/uploads/2019/10/ML201908_Conext-XW-120V-Owners-Guide-975-0240-01-01_Rev-I-ENG.pdf



Usually, anything over 95% PF is usually pretty much "perfect"...

Maybe other issues? Can you monitor the battery bus voltage and see if you have varying voltage causing variation in charging current? (Conext output voltage varying? Battery voltage varying during gassing/bubbles under/around plates?--Guess guessing...).

-Bill

ELYNN4

Thanks Bill.  What is curious is that everything works fine at 70 amps.  The generator carries additional linear loads from the house just fine, so I don’t think that the generator is being overloaded.  The battery bank is large enough that it can absorb anything I can throw at it.  I’ll try to see if I can measure a voltage swing.

Here’s a link that describes the harmonics that develop with non linear loads.  https://youtu.be/K3ml5CH0p90  I can’t help but think that this what is happening with my system, when the voltage gets “dirty” the inverter reduces the load until it cleans up - then tries to increase the load again and the cycle continues…

mike95490

The Charger in the 6848 is a very efficient charger, nearly ideal.   That is not your problem.  The house loads that the generator is seeing passed through the inverter is the problem. Refrigerator compressors,  Blower motors for HVAC, even all the little 3w LED bulbs all have generally poor Power Factor, and I suspect that's the root cause.    Charging batteries is a pretty intense load for generators and I would never load a genset past 70% for battery charging ( and the house loads )
 The Conext Gateway should be able to show you what the house loads are, their PF and phase 1 - 2 imbalance. Compare the house loads Before you start the generator, and don't forget to add them to the total load the generator sees

70 A  @ 54V =  3780 watts,   100A = 5400 watts,  about 50% of your generator spec.

I've got 2,  3kw diesel gensets, one is 80 years old and can produce an honest 3kw all day long ( but I have to manually oil it )
the other modern one starts loosing it's waveform at about 500 watts,  the stove won't light, garage door opener resets, water pumps fault off-line  - terrible, but's it's key start and my wife can run it

This article
http://www.screenlightandgrip.com/html/emailnewsletter_generators.html
talks about movie production and the non-linear lighting that the stage generators can't handle.  Charts, O'scope shots
all sorts of stuff that reading can take an afternoon.

RCinFLA

The inverter has a maximum AC input current.  It is based on what you have setup as limit or max current of inverter pass through relay whichever is less.  If this AC input exceeds generator capability the generator may slump in voltage and/or drop in rpm if synchronous rpm generator which may cause inverter to release from generator.  Inverter can only track small rate of change in input AC frequency, about 0.3 Hz per second.  That translate to a synchronous rpm generator speed variance of about 17 rpm at 3600 rpm nominal. 

When running on generator you should set max Gen AC input current to 60-85% of max generator AC output capability to better allow for AC_out surge loading.  This depends on generator margin in its maximum output spec.  Toughest thing is a sudden increase in ACout surge current, like an inductive motor startup surge. Inverter backdown on charging may not react fast enough and surge current passes through to AC input overloading gen.

The inverter automatically backs down charger to keep from exceeding maximum AC input amperage to inverter.  AC output loads have priority on the total AC input current limit and inverter will back down charging to provide AC output load and stay within input AC limit.

Second factor is battery voltage.  As battery voltage rises during charging the same charging amps rate takes more input AC current.

Conext 6848+ inverter will on-the-fly change from charging batteries to drawing from battery to supplement AC output load current if AC output load exceeds AC input current limit.  You can theoretically have an AC output load of max generator AC input power plus max inverter power capability. 

ELYNN4

Thanks to both of you for the good information.  I have my AC2 breaker set for 35amps.  I have the charging limited to 50%.  When I set the charging rate to 75% I have the cycling or throttling problem.  When I am charging the battery bank my household loads are between 200 and 300 watts, each leg is within an amp of the other.  The cycling/throttling begins immediately with the bulk charge - battery voltage at +/- 54 volts.  The cycling goes away as the battery voltage gets closer to the absorb setting.

My generator spins at 1800.  When the charger starts it’s cycling I can hear a slight change in rpm when it loads it up.  I can cycle my well pump, which is a VFD, there is no perceptible change in the generator’s rpm until the pump turns off - then there is a slight flare because it’s doing less work.  I can run my microwave and will hear a slight change in the rpm as it compensates for the load.

I can limit my charge rate to 50%, but it seems like the 9kw generator could carry more if the system allowed it to.  I am interested in learning/understanding more on this issue so that I can continue to dial the system in.

Thanks again for the information - appreciate any and all suggestions.

Dave Angelini

AC2 should be 30 amps (period)  You will be in trouble when the AC2 relay get damaged !

  A 9KW gen is small for XW !

ELYNN4

Thanks Dave - I will make the change!

RCinFLA

AC1 and AC2 pass through relays are 60-amp rating.  

ACin limit can be set from 3A to 60A.

You would want to limit Gen input to safe load limit for generator.  That is likely in 30 amp range for 9 kW gen.  There is an inverter reaction time for surge current so greater than AC in limit can occur for a brief moment when surge load hits.

If you find inverter disconnecting from generator when a sudden surge current load is applied you may have to back down AC input max level on inverter AC input in attempt to allow inverter to pick up more of the initial surge current to prevent too much surge current getting through to generator.

If inverter does not disconnect from generator on sudden surge load then the rpm slump and AC load is acceptable.

You can check rpm with most newer DVM's as they include low frequency counter.  The DVM will unlikely detect the slight change in frequency however during a load surge.  You have a 4 pole alternator which is suppose to run at 1800 rpm to produce 60 Hz. This is common for diesel engines with their low rpm maximum torque profile.

Low freq PWM hybrid inverters have excellent AC input power factor for charging batteries.

ELYNN4

Thanks for the information RC.  Can you help me understand the mechanics of the balancing of the generator load during the charging process?  I’m curious what role the AC2 relay plays.  When I see the charger load varying up and down +/- 10 amps is the AL2 relay opening and closing?

I’ve had an issue with the inverter qualifying the power from the generator when it first comes on line after the programmed 5 minute warm up.  After lowering the qualifying voltage to 95 volts the inverter qualifies the AC and puts the generator on line.  I’ve never been able to get a voltage reading that low, but it must be.  The charger applies the load gradually, as it’s supposed to, but if it’s going to reject the AC quality it occurs immediately on applying the load.

Thanks again for the information

RCinFLA

Usually the voltage slump is not the problem with generator connect.  It is usually the rpm slump which corresponds to AC freq slump that the inverter is not quick enough to track so it just releases from generator.  Inverter will try again after a little time delay. 

One thing to be sure of is not having the inverter maximum AC input limit setting too high as that can hit the generator immediately after inverter sync's and connect relay engages causing the gen to bog down too much.

Not sure what you are meaning by balance, but maximum set inverter AC input limit current is distributed between AC output loads which has first priority on available AC input current. Any leftover input AC current from max allowed is available for charging.  If your charging settings are greater than what is available from AC input, then charging will be backed down to only consume what is available from AC input.

AC input current to charging can be calculated by AC watts input to DC watts output for charging.  Derate input AC watts by inverter efficiency number (91-93%).  The limit on maximum power is just a bit lower than inverter wattage capability.

For example, 50.0 vdc battery charged at 40 amps dc is 2000 watts to battery.  Inverter efficiency is 91% so AC power will be 2000w / 0.91 = 2200 watts.  If AC voltage is 240vac then charging AC current is 2200 watts / 240vac = 9.2 A rms.

Inverter/chargers usually spec their max battery charging current at lowest battery voltage.  As battery charges to higher voltage the charger's max current is usually less than specified in spec sheet.

ELYNN4

Thanks for the great feedback RC.

My AC2 breaker setting if currently 35 amps, which is probably too high so I will drop it down to 30 amps.

I had the charge rate set to 65%, which provided 85 charging amps to the battery.  The inverter would occasionally not qualify the power on the first try, but would always qualify on the second try.  I dropped the qualifying voltage to 95 volts and that seemed to solve the problem.

I then bumped the charge rate to 75% which provided 100 charging amps, but I noticed that the charging amps were fluctuating down to 90, then back to 100.  The inverter was continually reducing the charging rate and then bringing it back up only to drop it back down again.  I assumed because the generator couldn’t keep up, even though it sounds fine and no perceptible change in rpm’s.

I reduced the charging rate back to 65%, 85 amps, but noticed that it was still dropping the charge rate by 10 amps and then increasing back up only to drop it back down again.   This doesn’t make any sense to me since the inverter/charger could run for hours at 90 - 100 amp charge rate.  This makes me think that there is another parameter that the inverter/charger is not happy with or could it be that the voltage is getting distorted by the harmonics of the non linear load of the charger?

I dropped the charge rate to 50%, which provides 70 amps and now finally the charge rate remains steady thru the bulk charge cycle and then slowly tapers off as it goes thru the absorb phase.

I’m trying to understand why I can only get 13 amps at 240 volts out of the 9 kw generator.  The household loads are typically 200 - 300 watts, but they can go up to 600 when my boiler and circulating pumps are on.  The SCP never shows the generator putting out more than 6000 watts.

When I contacted the generator manufacturer they stated that the inverter/charger was a non linear load and that they only recommended running the generator at 20% of its capacity with non linear loads.  I believe that this is their conservative approach to keeping harmonic distortion to a minimum.  That’s when I started wondering if I should start thinking about going to a DC generator.

So far I’ve heard that I should drop the AC2 breaker setting to 30 amps to protect the AC2 relay, so I will be sure to do that.

I’ve also heard that the 6848+ is a very efficient charger and shouldn’t be a problem for the generator, and that a 9 kw generator is on the small side for the inverter/charger.

Bottom line - I’m trying understand my system and its limitations as best I can so that I can get it to perform in the most efficient and reliable manner possible.

Thanks in advance for any more insight you might have.