DC Inverter Power Supply - Page 1
Forum section: electronics / general

author: workshop
author_rank: Newbie
timestamp: August 26, 2013, 01:13:39 pm
content: In the absence of solar panels, what type of DC power supply would be suitable for testing household solar power inverters.

author: aargee
author_rank: Frequent Contributor
timestamp: August 26, 2013, 10:25:00 pm
content: Well, I guess something that replicated the output of the solar panels? and/or the maximum specs of said inverter?If it is a grid connect type, there's a whole host of other issues involved.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 27, 2013, 02:53:23 am
content: Actually, when testing grid tie inverters using a bench power supply, it is physically impossible for the setup to backfeed into the mains.

author: AG6QR
author_rank: Frequent Contributor
timestamp: August 27, 2013, 05:04:35 am
content: I'm no expert here, but from what I can find on the web, a solar cell is neither constant voltage nor constant current, nor constant voltage with a resistance. Its I-V curve is non-linear, so it has neither a Thevenin nor Norton equivalent. Search this page for the term "solar cell"http://www.facstaff.bucknell.edu/mastascu/elessonsHTML/Source/Source2.htmlTo me, it looks like it's not too far from being an ideal voltage source with a bit of resistance thrown in, combined with a hard current limit. So a typical linear bench power supply in series with a resistor, with both voltage and current limits set appropriately to mimic the solar panel in question, might be a close enough approximation for testing purposes. It won't be exact, though. Maybe you can omit the series resistance, depending on how faithful your model needs to be.But if it were me, I'd begin by measuring the I-V curve of the solar cells you're designing for. The current limit and voltage will vary depending on illumination and temperature, so it's worth doing a set of measurements under various conditions.

author: workshop
author_rank: Newbie
timestamp: August 27, 2013, 10:48:31 am
content: Therefore, a testing jig would involve, a variable DC power supply both in voltage and current to simulate Solar Panel Array voltages and currents. As to the Grid-Tie inverters because the output voltage amplitude and frequency must correspond with the grid AC parameters requires feedback, a constant load and steady AC conditions would satisfy the feedback requirements of the inverter.

author: bookaboo
author_rank: Frequent Contributor
timestamp: August 27, 2013, 05:21:45 pm
content: Solar panels should never be used to drive inverters directly, for starters it wont work and secondly will probably damage both inverter and panel.There's always a battery (usually some form of LA) and a solar charge controller. Essentially the controller is charging the battery and doesn't have anything to do with the inverter. All the inverter cares about from the DC side is that it can take current on demand from within it's specified voltage range.In theory you can do this with a bench PSU but I've often found that DC-AC inverters demand small surges even with no loads, this causes most PSU units to "bounce about" and hit their current limits, drop voltage, reset etc. etc.So the easy answer is just use a we charged 12v lead acid.

author: rollatorwieltje
author_rank: Unknown
timestamp: August 27, 2013, 05:41:36 pm
content: Quote from: bookaboo on August 27, 2013, 05:21:45 pmHousehold grid tied solar panels are directly feeding an inverter, over here Stecagrid and Sunnyboy inverters are quite common. No battery involved at all.

author: bookaboo
author_rank: Frequent Contributor
timestamp: August 27, 2013, 08:12:59 pm
content: Yes you are right, I assumed that the OP was referring to off grid inverters.I guess for on grid it might be hard to simulate a solar panel as any decent on grid inverter will have a MPPT charge controller at the input stage.

author: Harvs
author_rank: Super Contributor
timestamp: August 28, 2013, 03:30:21 am
content: You can do a first order simulation of a PV module by connecting a CC source across a string of diodes, to get a bit better approximation add the correct series and parallel resistance. It's easy to get a pretty good approximation of a PV module from the specs in the datasheet (short circuit current will be the current you put across it, open circuit voltage governs how many diodes you need.)Best bet may be to use something like the cheap 10W LEDs off ebay to make the diode string because they have ~10V forward voltage, but you may need multiple strings to get the current where you need it (depending on if you're just testing that they work, or actually want to put real power through the inverter.) You're also going to need a fairly high powered CC supply if you're going to simulate a proper array, but most inverters MPP track down to something like 90VDC, so two dual tracking lab supplies all in series could probably do the job.

author: workshop
author_rank: Newbie
timestamp: August 28, 2013, 10:11:02 am
content: While a SOLAR PANEL ARRAY, SPA for short, is not a linear device, in testing a household inverter, the DC power supply could be made to operate either in a linear or non-linear manner. In the linear situation, depending on the load placed on the output of the inverter, the current input would be adjusted accordingly. While in the latter, would require aprogrammableDC power supply to simulate SPA conditions. Perhaps, by recording the Voltage-Current relation of a real situation and inputing into the PPS.As everyone knows and asbookaboopointed out, whether the inverter is NON-GRID or GRID TIED, an inverter, in addition, supplying power for household use during day time, and/or pumping current into the grid, or charging a battery bank for night use, at night, it would require only DC voltage to make it operate (power been drawn from the battery bank).However, a battery bank would be un-economical to use, for obvious reasons, expensive and requires charging, therefore capital outlay. In the alternative, a single battery of 12V and a dc-dc converter, perhaps 2 batteries in parallel to supply enough currentIn my opinion, a programmable power supply (or PPS) is the better choice for NON-GRID or GRID TIED inverter. The only disadvantage in using a PPS or any other DC power supply, in a GRID-TIED inverter, the feedback system may not be open to testing. It all depends on the design.Here are a couple of lines describing briefly the input parameters of GRID-TIED inverter, an Aero-Sharp, taken from this URL:http://solar4living.com/aerosharp.htm,and I quote:"The PV input voltage range is from 150 to 400 volts. The inverter produces little to no output below 155 volts, and remains in stand-by mode at 150v or less. The unit will only start up (in stand-by mode) when the PV voltage rises above 85 volts.The maximum input current is 10A. With 8 x 190w 24v panels (1520w), the current would be 5A with an open circuit voltage of about 336v. However, a lower open circuit voltage could be used if the panels provide a higher current, such as 12 x 120w 12v panels (1440w), giving an amperage of 7A and an open circuit voltage of 210 volts. For best efficiency, a higher voltage and lower amperage string is preferable."Other GRID-TIED inverters would have similar parameters. I was wondering howHarvswould design the LED/RESISTOR combination for this particular inverter, in mind.Any particular PPS models either homemade or ready to buy??

author: SeanB
author_rank: Super Contributor
timestamp: August 28, 2013, 06:58:46 pm
content: All I did to make a small low power one was to use a transformer to give 19VDC with a bridge rectifier and about 15000uF of capacitance and then used a few 20W downlighter lamps to give a variable series resistance to both emulate the panel curve somewhat and to protect the output, as I was using a simple charge controller that regulated by shorting the panel to ground when it was fully charged. Worked well enough for a supply emulation of a 30W panel without having to actually place the panel outside. Lamps glowed bright with a discharged battery, fading to a dulll glowe as it neared full charge then flashed brightly every few seconds at full charge.

author: workshop
author_rank: Newbie
timestamp: August 29, 2013, 09:38:22 am
content: Fine SeanB. How can your jig can be used to test (identify faulty components) in a household inverter where the starting voltage is, as in the example, 150Vdc.Secondly, and more importantly, would you design a new power source for every inverter that comes your way, for repairs.

author: SeanB
author_rank: Super Contributor
timestamp: August 29, 2013, 06:46:52 pm
content: If I was having to deal with these inverters on a regular basis first I would get an isolating transformer of around 1kVA and a variac of about the same rating, about 8-10A. Then a 25A bridge rectifier on a heatsink and a fan and around 200uF at 450V or higher capacitors along with a bleed resistor to give a basic variable power supply capable of some serious power, along with a few 1kW heater elements to arrange in series or parallel as current limiting for the DUT, along with some basic metering of DC voltage and current. That would give me a setup capable of powering most inverters with reasonable chance of them working, you just adjust the input voltage to the minimum levels with some resistance in circuit so that the voltage will sag a little with load then wind up the wick with the unit connected to the mains to give power.Basically a similar set up to the old motor test bench where you can test a motor up to full load by using another or a synchronous motor as a variable load or drive source, with current circulating between them and only the losses being supplied by the mains.

author: workshop
author_rank: Newbie
timestamp: September 03, 2013, 10:33:56 am
content: ....well, an isolation transformer is, basically, any transformer because it is isolating the powered device from the power source. Be it 1:1 or some other ratio.A variable VOLTAGE / CURRENT power supply, effectively incorporates THE FUNCTIONS of both the ISOLATION TRANSFORMER and the VARIAC.Therefore, any particular models of commercial PROGRAMABLE POWER SUPPLIES suitable for testing inverters

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 03, 2013, 04:45:37 pm
content: Quote from: workshop on September 03, 2013, 10:33:56 amCareful there. Not all transformers are isolating. An autotransformer isn't isolating. A Variac is one example of a transformer that isn't isolating. The matching transformers I use in antenna tuners aren't isolating. The travel voltage doubler I have which uses an autotransformer to convert 120VAC to 240VAC for small electrical devices isn't isolating. All of these contain a DC path from input to output.

author: Harvs
author_rank: Super Contributor
timestamp: September 04, 2013, 02:19:10 am
content: Quote from: workshop on September 03, 2013, 10:33:56 amYes, but have you priced up a 1kW, couple of hundred volt, programmable power supply lately? Have a look around then compare the price to a used VARIAC and isolation transformer.