The old website

From The Alternative Power Network
Jump to navigation Jump to search

What was it about?

The majority of the pages was about how to connect to the grid made for SHA2017 safely and how to build a grid yourself. Some test results on cheap Chinese DC-DC converters, some tips on how to generate electricity or heat and some user projects were explained on other pages. Let's try to sum it all up below.

Safety third, err, first!

While high voltage grids are more efficient in transporting power over long distances, it makes the system way more complex and many safety measures must be implemented to avoid shocks and risk of fire. Not the way to go on a damp camping field and inside tents. A very low voltage on the other hand is much safer, but cable losses are much greater or, to minimize those losses, cables need to be very thick and expensive. This is because, at the same power, current rises when voltage drops. A 60W laptop charges needs about 0.2A at 300V. At 12V this would be 5A, 25 times as much. Simply said, a cable can be seen as a resistor, cable loss can be calculated at P=I^2*R, for the same cable a current increase of 25 times would mean a power loss factor of 625 times more! You should therefore use much thicker cables at low voltages.

We wanted to be somewhere in the middle, safe but without to much losses. Our grid voltage was specced on 42V +/- 15%. Why 42? It's the answer to the Ultimate Question of Life, the Universe, and Everything! That and voltages below 60VDC (IEC 62368-1:ES1) are believed safe to touch in most conditions. At 42V+15% were at about 48V, leaving some room for safety systems to kick in and leaves a little bit for AC ripple too. Looking back at SHA2017, 48V could have been used too, with maybe a bit tighter +/-10% voltage range. This had maybe helped a bit in finding suitable gear to hook up to the grid. You can keep that in mind if you want to build your own grid.

So, now what, we want to build a grid, what's needed? Basically just some cables, fuses, and maybe a system that dissipates some power if the voltage rises too much. Cables can be thin for short distances or low currents and must be thicker when greater distances have to be crossed, high currents are possible or when voltage drop needs to be low. Loads that need a large startup current, like big motors, also need bigger cables or a buffer of some kind nearby the load.

About the cables, an example:

  • A piece of wire with 2 2.5mm2 copper conductors of 10m long has a resistance of about 0.17 Ohms. The voltage on one end is 42V, on the other end a 120Vac 2000W tea kettle is connected. The kettle has a resistance of about 7.2 Ohms. In total the resistance is 7.37 Ohms resulting in a current draw of 5.7 Amps. The voltage drop percentage as well as the cable power loss percentage is calculated by dividing the total resistance with the cable resistance, about 2.3%. Total power used is about 240W with 5.5W of it being transformed into heat inside the cable. To warm up a liter of water from 10 to 100 degrees Celsius you need about 100Wh. So you can have half a liter of tea in about 15 minutes if the insulation on the kettle is not too bad. But if you have had a 50m 1.0mm2 aluminium cable, can you still have tea? Well, yes, but it would take much longer as your cable has a resistance of 3,2 Ohms, resulting in only about 4A of current and 170W of total power use of which about 50W is lost inside the cable. Still you could have that tea in about half an hour or so.
  • If you have a load that needs a 50A startup current and has a running current of about 4A (let's say a 160W fridge driven by a 42-230V 2000W DC-AC sine-wave inverter) you must choose your cable with much more care. You probably don't want the voltage to drop more than 10%. That means the cable resistance has to be 9 times smaller than the equivalent startup resistance seen at the input of the inverter. The input resistance at 42V-10% equals (42-4.2)/50A = 0,76 Ohms. That means that now your 10m 2.5mm cable is not close to what you need, it could even result in fire. You'll need at least a 6mm2 cable of 10m for this! To cross 50m you'll need an enormous 35mm2 cable. Or you can place a battery pack nearby the fridge that can handle the peak current easily, that way you can use a cable that is much smaller, a 2.5mm copper cable of 50m is now suitable for the task. For those kinds of loads, decentralization is the key. Put your battery packs nearby the most demanding loads, it keeps the grid more stable and it saves on cable costs and on cable losses!

Fuses? Please use them wisely! DC is more tricky to fuse right because of sustained arcs. With AC, arcs are normally extinguished every 10ms (at 50Hz), with DC, they won't stop until the distance is increased enough. At higher voltages this problem becomes more severe. Always look at the DC rating of the fuses! If there is no DC rating in the datasheet or written on the device, do not use it! There are some nice 60 or 72V Polyfuses/PTC fuses/thermistor fuses you can use, Elmark has a nice affordable range of DC circuit breakers (C61DC/C62DC) or just use sand filled fuse cartridges. When selecting a proper fuse, also make sure you check the breaking capacity. If the breaking capacity of the fuse is lower than the maximum current the system at that point can deliver when shorted, do not use that fuse!

Mechanical switches on high current DC, please avoid them if possible! They have the same arcing problem as fuses, and since switches are turned on and off all the time this can be a real fire hazard! A 230Vac 16A switch may only be able to switch 24Vdc at 10A. Always check the datasheet! For DC it is better to switch electronically. A small switch, with a series resistor to limit the current to a few milliamps, can be used to turn on a large MOSFET. A DC contactor with the correct voltage rating (sometimes fitted with magnetic blowouts) can also be used. It has the advantage of disconnecting mechanically, but it is slower and uses a lot of power when engaged. If you really want to use small switches for small devices on 12Vdc or higher, always put the right fuse for that device in front of it and use switches capable of switching at least 10 times as much current on 230Vac!

System tips and tricks

At SHA, sadly we did not have a battery backup system. We had a lot of solar panels though, a few nice distribution cabinets and some more stuff including a backup power supply capable of delivering about 16000W of power. Overkill? Yes... :)

For a battery system, you could use anything from lead-acid, lithium, sea-salt batteries or whatever you like. Just keep in mind to treat them right, use a good charge/discharge safety system! And when using Lithium batteries, please be nice to them and only use "the middle part". Not literally, but if you generally keep the charge of the cells above 20 and below 85%, they last up to 6 times longer! Especially when you keep them cool. And if you put a battery away for a long time, discharge it a bit to below 60% and store it in a cool place! And don't forget to check regularly if they are still charged above 20%. If not, recharge to 60%.

At SHA we had mostly solar power during the day and "just as dirty as the rest" power during the night. The main power supply was set to 42V, so whenever the power demand was greater than the power the solar panels could provide, the power supply started to deliver. The voltage of the grid was used as a simple means to determine if clean power was available. Anything above 42V means there is clean power available, below that it is probably a mix or downright dirty. To keep the grid from oscillating we used capacitor banks of 0.176F, located in each of the cabinets. These capacitor banks also doubled as a high-current delivering system to trip a circuit breaker in case of an over-voltage situation. Normally a battery pack can be used for those things.