A power cable approximately 200-300 meters long will be used to operate my underwater vehicle from land. Would it be more appropriate for the land-supplied power system to be VAC or VDC? In both systems I will convert to 48V 80 amps inside the robot.
I tried a different route like this and ended up frying the rov electronics. I switched to batteries for now.
Hi @Fly, welcome to the forum 
There’s a significant amount of risk in developing a high power system, to both the electronics and life involved, and especially so when it’s meant to operate in and around water. This isn’t something I’m comfortable providing recommendations on, particularly in a public forum where they may be misinterpreted and acted on by people without electrical training.
Some factors that should be considered in a design:
- total electrical efficiency
- vehicle operates on DC, so it must at least end as DC
- depending on use-case there may be a preference for AC or DC as the topside input
- voltage drop through the wires
- we have a Voltage Drop Calculator, although it is DC only at this stage
- the water conductivity may be relevant for AC - I’m not sure
- tether size and weight requirements
- insulation and wire density and thickness
- a nimble vehicle may have more stringent tether drag requirements
- operating risk
- during both assembly and usage
- including cases like the tether getting cut/severed in the water
We sell a power over tether setup from @Outland which converts to 400V DC over a tether up to 250m long, that may be relevant to refer to. It has a continuous output capacity of 1kW, compared to the ~4kW you’ve specified for your use-case.
I use a 300m tether to operate the ROV. I used a AC/DC converter 230VAC to 400VDC but given the lenght of the tether and cable section the voltage drop was quite significant. In my case, the tether had to be this long due to a internal pipeline inspection. You should consider how long your tether REALLY NEED to be. Do not add extra for “nice to have”
My topside was 1,5KW but due to the voltage drop the outcome in the subsea PDU is less, quite less. The PDU had a 400v/24v DC/DC converter.
I then changed the PDU converters to 600v/24v DC/DC to decrease the voltage drop and that was a significant improvment. Downside was that the DC converter was larger with large capasitors and I have a fear that they will not withstand great pressure (My electronics are oil compensated)
For our current newbuild the tether power cable section will be 2*2mm^2 and the voltage will again be 400VDC. The lenght of the tether is redused to 250m .
In the end it boils down to your tether. 4,5kw topside will not help unless you have tether capasity.
DC does not incur the line charging losses that AC does. A typical ROV tether is very different from HVAC transmission lines, in that the wires in the tether are much closer together. That means that your tether is a not-insignificant capacitor. Every time your AC supply reverses polarity, you need to charge that capacitor up the other way, and that is an extra current burden that you need to both supply, and take into account when sizing your tether’s power conductors. DC, on the other hand, only has to charge up the tether cap once at power-up. Generating your HVDC supply is easy, too, simply by putting several isolated SMPSU in series (with bypass diodes across each supplie’s output). With AC, you have two options: install a transformer on your vehicle, or use a variable output rectifier. The former ends up being too heavy most of the time, and the latter means that you have no isolation between your tether and the vehicle. Isolated DC-DC converters, on the other hand, are plentiful, quite affordable
The AC/DCconsideration is pasted from a different forum. But yes, DC is the answer.