Sine wave THD for Conext inverters

Ram

I'm trying to get some figures for the Total Harmonic Distortion for the Conext SW inverters as I am planning to buy a SW4024 shortly, but have been unable to get Schneider to respond to the question. The datasheet and manuals simply say "pure sine wave". I have played with several inverters including a Phocos 3kVA which stated a nice 5% THD with resistive loads, but demonstrated significant distortion and rapid flickering of lights as soon as I connected even a small SMPS load (a 200 watt Thinkpad laptop power supply!). I have had to throw away lots of money on some of these inverters. So I've been keen to get answers to the following:
(i) (Easy) What is the THD with resistive loads?
(ii) (Subtle) What is the THD when loaded at 50% of capacity with a load that has a crest factor of 3:1?

I suppose the basic electronics architecture being similar, the answers should apply to any of the SWs or XWs.

Anyone measured their Conexts - for (ii) with perhaps an inductive load such as a pump running?

Cheers - Ram

mike95490

When my 1Kw pump is running, I notice no other odd effects. XW6048

Dave Angelini

Most of the problems people have with inductive loads are battery issues not inverter issues. I am not saying that you will not have any flicker, but any of the XW's or the old SW's or the really old trace models are what deep well drillers use and used. They would run just about any pump reasonable. I do not use the new SW as it is not designed to last like the XW. The price difference does not mean much to me but I understand that cost is always an issue for some.

zoneblue

Led lighting is often a canary for this. Depending on the nature of the drivers, but the kind that use some kind of resistive/capacitive balast are the worst. I have seen this here with a desktop magnifier lamp, with a primitive driver. The Steca caused it to flcker quite a bit, The outback less so. Theres storys of poeple with 30K RE systems being plagued by light flicker. Dave what do you mean by "battery issues". Bad connections or just too small a bank?

http://forum.solar-electric.com/forum/solar-electric-power-wind-power-balance-of-system/off-grid-solar-battery-systems/334023-lights-flicker-on-xw6048-when-freezer-starts
http://forums.whirlpool.net.au/archive/2081139

Ram

Interesting input there, [USER="11848"]zoneblue[/USER]. Now what is interesting is that I have seen flicker on my Phocos 3kVA for example not just when appliances start, but continuously when there is an SMPS-powered item running somewhere - usually my Sony TV or Thinkpad laptop. SMPSes are known for drawing spikes of current in a nonlinear fashion, so the flickering is continuous - about 5 times a second, most noticeable with incandescent lamps and less with good LED lamps that have good regulators built in.

Curiously I have emailed Schneider people in 3 countries over the last 3 weeks trying to get a THD figure from them for the Conext SW inverters and there is no answer!

SkiDoo55

Manual states (<5%THD) at full rated power. In Spec section of the manual. Can Download from Schneider Web Site. at least on the XW can't find it on SW spec.
On LED lights I get flicker from the grid on some types but normally only when dimmed. Don't notice anything different on XW4024 when I am drawing form it to cycle the batteries.

Ram

SkiDoo55 wrote: »

Manual states (<5%THD) at full rated power. In Spec section of the manual. Can Download from Schneider Web Site. at least on the XW can't find it on SW spec.

Yes I saw it on the XW Spec, not able to see anything in the datasheet or the manual for the SW. Had a chat with an engineer here at Schneider India who seemed to think a THD spec was relevant only for grid-interactive inverters like the XW and not for off-grid inverters like the SW which I believe is wrong.

BB.

Ram, you are correct. THD applies to off grid inverters too. 5% or lessTHD is accepted as a TSW/PSW (true/pure sine wave) inverter. A MSW inverter (modified square/sine wave) is probably something like 20% or higher THD.

https://en.wikipedia.org/wiki/Power_inverter

2-level
square wave
~45%^[7]


3-level
"modified square wave"
> 23.8%^[7]


5-level
"modified square wave"
> 6.5%^[7]

For an induction motor, more or less the energy that is >0% THD is wasted as heat inside the motor. Only the "fundamental frequency" (50 or 60 Hz) is used to rotate the motor. Anything else is wasted as heat in the windings, circulating magnetic currents, etc.

For purely resistive loads (filament bulbs, electric heaters), the THD does not matter--All the energy is "used" (turned into light and heat).

-Bill

Dave Angelini

No wasting of energy unless we can control when we waste it ! Keep the induction motors happy!
Zoneblue " Dave what do you mean by "battery issues". Bad connections or just too small a bank?"
I would say both but it is pretty crazy when someone uses a 200 AH battery with a 4 to 6KW + xw inverter.
400AH has always been speced and for good reason. Less than that is just too small to do any reasonable work and any surges in load will cause electrical issues that may cause lights to flicker or microelectronics stress. We do not like stress!

Ram

By way of an update - I finally got an email from Eric Bentsen at Schneider acknowledging that while they have not published their THD spec for the Conext SW, their technical champion says it is less than 3%. Puzzling that an SW should have a better spec than its big brother the XW, and more puzzling why they do not publish it if it is really as good as 3%! He also said THD testing is done with resistive loads which leaves that all important question of nonlinear loads hanging :-(, but at least some answer is better than no answer.

BB.

I guess you could sort of verify the TSW/MSW of the AC inverter by using an induction motor (or non-Power Factor Corrected AC to DC power supply) and a Kill-a-Watt meter. You could use the VA/PF readings of the Kill-a-Watt meter and compare utility power readings to those on the "unknown" AC inverter output.

-Bill

Dave Angelini

For sure Bill it can be measured but again why? I can think of 3 or 4 different ways to make a THD measurement and get whatever spec I want if I know the desired outcome. Measurement standards
are for reference and then you write the spec. I would still take an XW over anything else Schneider puts their name on.

Ram

THD is very well defined and there are instruments to measure it. I use a Tektronix AA501 Audio distortion analyzer on the inverters I try. Then there are Power Quality Analyzers from Fluke etc. Your suggestion, @BB would give one a ballpark perhaps but if you already know you are dealing with a TSW like a Schneider SW, you need a more specialized instrument. What is important is that THD be measured and specified with a nonlinear load (a so-called 3:1 crest factor is good), which unfortunately Schneider has not done. Upgrading to a XW as a "safe" choice would make sense if only it did not cost 230% more :-(.

Dave Angelini

THD is well defined? OK You must be right. I use to make/test/sell/service/engineer the instruments that HP measured THD with.
That was long ago so I am glad that you have told me it us that it is well defined now.

BB.

The problem with companies going through multiple owners is the next marketing deparment wants to slap their name and model sysetm numbering on everything... Conext is now on new/old/etc. And now they add a "+" to the model for something "new". Add the reuse of old "SW" designations, and it is difficult to figure out what anyone is talking about over the years.

In general, from what I have read is 5% or less THD is "pure sine wave"... And the Conext SW says "pure sine wave" without qualification.

But, as you ask, the voltage wave forms of inverters (and gensets, and even utilities sometimes) can be dramatically affected by their loads. And the quality of the output wave form is quite dependent on the amount of copper and iron that the manufacturer invests in their product.

In the end, spec'ing products is always a game of marketing vs engineering. And since there are few folks out there that has the test gear and can spend the time and money it independently qualify the mfg. claims--We are sort of left with forums like this.

The person posting the question suggests the types of loads they need to run, and hopefully, people with experience running those loads can answer back and say how well their system runs them (or not).

Many designers do test their own and other mfg's equipment--But rarely let those test results out--They cost a lot of time and money to generate. And it is not in their best interest to give that information out for free.

-Bill

Ram

I agree with your observations, [USER="512"]BB.[/USER] Technically what I specified in my original post would be appropriate: measure THD with a nonlinear load of 3:1 crest factor. To make that sound more comprehensible and implementable, I would say "connect 1500W worth of loads that have SMPSes and then measure the THD". For those who really want to dig deep into the subject of nonlinear loads there is a paper called "Inverters and harmonics (case studies of non-linear loads)" written in 1993 by Jean Noël Fiorina in....Schneider(!) which I have attached.
I agree engineers do not want to divulge too much data, but THD with linear loads is something that I have seen specced for practically all inverters except Conext SW, and THD with nonlinear loads is something which is not uncommon to spec.
Last but not the least, with due attention paid to Dave's point - THD measured with any HP equipment would be more than useful enough for the purpose :-).

john p

Its almost impossible to give an accurate quote figure for THD.. as it is so dependant on the load and to some extent the battery supply.. the load on the inverter may be pure restive. inductive in varying amounts and may be a combination .of restive and inductive....Obviously the manufacturer cant give a figure for every possibility .. They settle for restive loads and if decent inverter will be 5% very seldom less as it makes it far more expensive to manufacture and the gain is simply not worth while...

Ram

Here is a detailed update I got on the Conext SW after chasing the matter further with Douglas Wicks, product manager for the battery inverter line.
best-Ram

...our product will probably NOT meet your requirements for audio equipment. For the IEC version, the THD is less than 5% with the normal input voltage range 44V-56V. When the input voltage range is 40V-44V, the THD is less than 8%. The tests are conducted under pure resistive load condition. (We are not sure where the % came from... )

I talked with our compliance team and they provided the following information.

For the CSW (IEC and UL models), the requirements for Vthd on OUTPUT of an inverter are summarized as follows.

IEC 62109-1 = no requirement
EN 50178 = no requirement
IEC 60240-1: (UPS safety) = no requirement
IEC 60240-2: (UPS EMC) = no requirement
IEC 60240-3 (UPS performance) =The effect on the output of the UPS by non-linear loads both in steady-state and dynamic is, in many cases, to cause deviation from the output characteristic specified by the manufacturer/supplier where these are quoted under linear load conditions. Due to the higher peak to r.m.s. steady-state current ratios, the output voltage total harmonic distortion may be increased beyond the manufacturer's stated limit. Compatibility with the load for higher levels of THD is a matter of agreement between the manufacturer and the purchaser.

IEC60335-1 (household equipment) = no requirement
CSA107.1 says 30% THD (mark a warning if higher)
UL1741 says same

ShinyShaman

I use this for my hifi audio gear. The SW didn't do it for me either in terms of THD.

http://www.psaudio.com/products/p5-power-plant/

Dave Angelini

I know plenty of audiophiles using the XW all the way back to 2007, even a recording studio. Offgrid really would not use the new SW because they just are not equal to the XW in quality and design life.