1500 to 24V step-down converter  - Page 1
Forum section: electronics / projects

author: webgiorgio
author_rank: Regular Contributor
timestamp: June 27, 2018, 09:54:53 am
content: Hello,I would like to design a step down converter for an application in photovoltaic solar plants with solar tracking.In such solar plant, a 24Vdc 5A motor move 20-30 kW of PV panels to follow the Sun movement during the day.Normally, there is a small PV panel (60W) dedicated to charge a battery that supply 24V to the Sun tracking control system and motor.The problem with this system is the cost of panel+battery, and the life of the battery.So, in place of the 60W panel+battery, I want to make a DC-DC converter that will take the power directly from the string of the main panels.String voltage can be up to 1500 Vdc.Since one string is about 5-10 kW of nominal power, much more than the power demand of the DC motor (100-300W), I could probably avoid the use of the battery entirely (since in low radiation condition I have enough power anyway to operate the motor).The 24V side must be isolated from the 1500V side. 3000V isolation.This DC-DC is (obviously) in parallel with the grid-tied inverter.So, my questions:Does a similar DC-DC exist on the market? (I could not find one with such a broad and high voltage input)Which topology is best in this case? (something similar to the attached schematic?)Shall I consider to make the transformer, or design around a commercial transformer?Where can I find a commercial transformer for this DC-DC? One primary and one secondary, or double secondary?Which control chip would you use? (I would prefer the control of the switch on the low voltage side of the converter).Is the 1500V (3000 V isolation to the secondary) a big challenge?thanksGiorgio

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 27, 2018, 10:26:47 am
content: Would it be feasible to connect the primary side of the DC-DC converter both to taps in the 1500V string instead of between 1500V and GND?Then you could do with a much lower input voltage for the DC-DC converter, but youwill of course still need good insulation between primary and secundary.How many panels are there in your "string voltage of 1500V"?What is the physical setup? Do you even have the 1500V and GND connections available in the place where you need the motor?Why would your solution be preferable above running some separate wires for the motors?

author: ejeffrey
author_rank: Super Contributor
timestamp: June 27, 2018, 10:31:33 am
content: You have a grid tie inverter, so why not just power your motor off the grid? What is the output voltage of your grid tie inverter? Is it standard low voltage AC at ~230 V, or is it a much higher voltage? If it is 230 V or if you have 230 VAC available just use that with an off the shelf 24 V power supply. If not, you should at least consider if you can find a small step down transformer that would let you use a standard power supply.

author: d-smes
author_rank: Regular Contributor
timestamp: June 27, 2018, 11:08:58 am
content: Check out Mornsun PV200-29Bxx Series: 300 - 1500 VDC input, 4000 VAC isolation, 200Whttp://www.mornsun-power.com/uploads/pdf/PV200-29Bxx.pdfThey also have lower power units if you're just looking for a battery charger.

author: webgiorgio
author_rank: Regular Contributor
timestamp: June 27, 2018, 02:48:46 pm
content: QuoteThe set up is a large solar plant with about 1000 solar trackers of 20-30 kW each.See some photoshttps://www.google.com/search?q=solar+plant+one+axis&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjhqdKRh_TbAhXHaVAKHZ39CSAQ_AUICigB&biw=1274&bih=953.The drive pylon (the motor) is in the centre of each solar tracker (one row=one tracker).One tracker has about 80 panels. 40 to the north of the drive pylon, and 40 to the south.40 panels * 32V = 1280V.Quote. Not really. Generally the "ends" of the string are available near the drive pylon, where there is a trench with the power cables to the inverters.QuoteIn some cases (the present one) the inverter is far from the solar panels.Consider that the plant can have 500-1000 solar trackers, so, all those AC cables in the solar field would quite complicate the wiring.My goal is to identify a power supply 1500 Vdc - 24 Vdc at the same price of a 230 Vac - 24Vdc, and save the cost of all the AC cables.In some solar plants there is an inverter for each tracker, by the tracker, therefore there is the 400V AC near the motor, and using a 400 Vac-24 Vdc power supply is feasible.Mornsun: very relevant, I am having a look.Thanks!

author: max_torque
author_rank: Super Contributor
timestamp: June 28, 2018, 05:46:13 pm
content: If you keep a small amount of energy storage (small battery or supercaps) then the DC;DC only needs to trickle power down from the HV to the LV. The daily energy demand is going to be pretty fixed (that required to swing the panels by 180 deg) so you can trickle a small amount of power, stored as energy at 24v, and that provides the power to turn the motors to line up the panels. That small DCDC will probably pay for some caps or a battery or two (and the battery should be pretty robust as it won't get deep cycled so a cheap SLA or similar would work)

author: BrianHG
author_rank: Super Contributor
timestamp: June 28, 2018, 06:45:53 pm
content: Quote from: d-smes on June 27, 2018, 11:08:58 amDamn that's a nice supply, virtually all at 90% efficiency throughout the full 300v to 1500v range with a reasonable load on the output.