EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Red Squirrel on August 09, 2015, 10:47:17 am
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I eventually want to convert my server UPS system to be full dual conversion, basically 48v rectifiers connected to 48v inverters, with a 48v battery bank connected to the same bus to float (voltage would be regulated at 54v). If the rectifiers fail the batteries just take over the load and it's seamless.
Say I was to setup a 48v solar power system, how would I go about injecting the output power into the dual conversion system so that the rectifiers only supply the power needed to keep up with the load? Is this even possible? Obviously need a charge controller as well, but I don't just connect that straight to the bus I imagine as it wont stop the rectifiers from supplying all the power and not taking advantage of the solar.
I'm far from ever designing this, it's just a question I've always had in my head and I'm curious what would be required to make such a system work. Basically how do you put two energy supplies in parallel but give one priority and other one only supplies what the primary can't.
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How are you going to set up a 48V PV installation. Each PV panel is around 30V?
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...Basically how do you put two energy supplies in parallel but give one priority and other one only supplies what the primary can't.
Battery chargers - and solar charge controllers - are primarily constant current power supplies with an output voltage limit; they will automatically parallel without problem even if their output current ratings are wildly different.
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The "rectifiers" are AC-DC converters and supply whatever power they must to keep their outputs at their specified output voltage. As long as your load power is greater than your PV output, you simply need to make your PV output into a constant-current one as MagicSmoker said: whatever power the PV system provides is that much less power draw seen by the rectifiers.
If the PV system might provide more power than what gets used in some cases, then you need to make sure your PV output has a voltage limit that is closely matched to the rectifiers' unloaded output.
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How are you going to set up a 48V PV installation. Each PV panel is around 30V?
They do make 12 volt (nominal) panels so 4 of those in series then group in parallel. Not that familiar with charge controllers but I would imagine the fancier ones would have a float setting you can set. So because it's a constant current device the system would pull all the power from there first? Is the output of a charge controller typically constant current though? As it is normally designed to stay within a certain voltage range to charge the batteries effectively. In the case of a telco setup you typically just keep everything at float and run an equalize cycle every now and then. Some of our COs will equalize after a power outage as well.
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Is the output of a charge controller typically constant current though?
The main phase of most battery charge cycles is current-limited to whatever the battery can safely handle (or less for increased cycle lifespan) until the end-of-charge voltage is reached.
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Most of the time the batteries would be in float mode though, they are simply part of the 48v (54v actual) bus and if the power supply fails then batteries are drawn from naturally. Does this change anything? Basically the charge controller's DC output would be powering all the equipment, and I want the rectifiers to only supply any extra power required to keep up if the equipment is drawing more than solar is producing. Batteries are to only absorb a very small amount of current to stay topped up. Should the power go out and solar is not keeping up, then it would start to absorb extra power from the batteries.
Since I'm not planing on doing this for quite a while, and I'd probably want to do it small scale as a test before going large scale, I'm open to any recommended reading material to make me understand better how I'd make this work. It's kinda an oddball setup so hard to search for.
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If the charger is a simple constant voltage float unit, this should be fairly easy to achieve, at least conceptually. You will probably want to use an MPPT controller to regulate the output of the panels, rather than a shunt or on/off switching regulator. Now, set the MPPT to regulate at the desired battery float voltage, and adjust the mains charger to slightly below this (say 0.1V; this figure may need adjusting depending on regulation characteristics and wiring resistance). The batteries should be sized to safely absorb the full output of the PV panels, or the MPPT output should be appropriately current limited.
Whenever the MPPT is regulating, the error amp in the mains charger will see the voltage as too high, and not attempt to supply any current. If the computers start to draw more power that the solar panels can provide (or the sun goes behind a cloud), the MPPT will drop out of regulation, the bus voltage will fall slightly, and the mains charger will start to pick up the slack. If the mains goes out, the voltage will fall some more, and the batteries will start to deliver power.
Note however that there will be numerous practical issues if you are trying to modify an existing UPS - not the least of which is that many cheap UPSs provide no isolation between the battery and incoming mains supply. You might be better off building up a system out of separate modules.
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That is good to know, seems like the easiest bet. I kinda thought of that but was not sure if it was that simple. I know in some situations you can't do that as the higher voltage will try to feed into the lower voltage source, but I imagine telco grade rectifiers would not be susceptible to that, they are designed to run in parallel redundant systems so they have to account for slight voltage variations. At least that's my guess, something I'd obviously find out when I'm in the market for them.
So set rectifiers to 54.0 volts, set solar to 54.1 volts then load + batteries will draw from the higher voltage first. 54v is float for a 48v system so don't have to worry about limiting current. Batteries will draw what they need. I'd probably want to have an equalize function in the system, I'd probably leave that job up to the rectifiers. On a cloudy day I could just run it manually. This is probably something you can setup in the user interface.
Would not be modifying a UPS but rather more or less building my own from parts designed for that purpose, since yeah it could be risky modifying an existing and potentially risking putting power on the grid if not done right or relay fails etc. Basically my plan is a telecom power plant setup. Everything would run off the inverter at all times which is fed by the 48v bus. Beauty of this is complete isolation from the mains so brownouts or very fast flickers or other very noisy situations would not affect the equipment at all, only rectifiers. If rectifiers go offline the batteries naturally take over as they are on the same bus. Obviously there would be fuses, isolation switches, current monitoring, etc as well.
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So set rectifiers to 54.0 volts, set solar to 54.1 volts then load + batteries will draw from the higher voltage first. 54v is float for a 48v system so don't have to worry about limiting current.
What happens when the power goes out on a cloudy day, and then after your batteries have discharged for a while the utility power comes back at the same time the sun does? You now have a condition where both the solar MPPT and the rectifiers are both trying to drive their maximum current into the batteries and load. If your batteries aren't sized to handle that much charging current, then you'll need to incorporate some sort of additional current limiting, preferably on the rectifiers if you want to maximize solar utilization.
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So set rectifiers to 54.0 volts, set solar to 54.1 volts then load + batteries will draw from the higher voltage first. 54v is float for a 48v system so don't have to worry about limiting current.
What happens when the power goes out on a cloudy day, and then after your batteries have discharged for a while the utility power comes back at the same time the sun does? You now have a condition where both the solar MPPT and the rectifiers are both trying to drive their maximum current into the batteries and load. If your batteries aren't sized to handle that much charging current, then you'll need to incorporate some sort of additional current limiting, preferably on the rectifiers if you want to maximize solar utilization.
Wont the batteries only draw what they need though? For example our rectifiers at work have a load of about 1,600 amps but are capable of delivering much more, but the batteries don't draw all the current they can out. Or do you mean when it goes into a faster charge state? I would probably set it up so only one of the sources will do that. Probably solar. Basically if the batteries stay on float they'll charge slower but eventually charge, while if I set up the different charge cycles they'll charge faster but I have to limit current. So I'd probably set the solar charge controller to go through the various charge phases and set rectifiers to always stay at 54 so they don't try to "fight" each other. Idealy this is settings that I would actually be able to configure. Something to look into before I choose rectifier and charge controller.
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If you connect a 54.1V constant voltage source to a battery bank that's discharged down to 48V, the amount of current that flows will only be limited by the impedance of the batteries and the voltage source: I=(54.1-48)/(Zbat+Zsrc). As the batteries charge, their voltage comes up and the current tapers off.
If the current exceeds the capacity of the source, hopefully it will switch to constant current mode rather than cook itself. In that case, the source voltage drops down until the current matches whatever the current limit is. As the batteries charge, the current stays constant, and as the battery voltage comes up the source voltage will as well until it hits the source's voltage set point, at which time it should switch back to constant voltage mode.
In your case, you have a 54.1V (solar) source and a 54V (utility) source. When the solar source goes into current-limited mode, it's output voltage will drop. Once it drops below 54V, the utility source will start supplying current as well, and will attempt to supply enough current to hold the combined output at 54V. If the load is large enough, the utility source will also go into current-limited mode, and the output voltage will drop further. At that point, both the solar and utility sources are supplying their maximum current into the load.
So you'll need to make sure that everything is sized correctly and current/voltage limits and set points are set up to ensure smooth transitions between operating modes without cooking anything.
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54V is almost the power point of a 36V PV array. I run a 36V solar array and I do almost the opposite. I heat water with excess solar. battery charging operates at power point and when voltage goes only slightly above that the power goes into heating water. All very seamless. From a properly sized 36V PV array you could feed directly into a battery through a rectifier, assuming there is always some load on the battery.
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If you connect a 54.1V constant voltage source to a battery bank that's discharged down to 48V, the amount of current that flows will only be limited by the impedance of the batteries and the voltage source: I=(54.1-48)/(Zbat+Zsrc). As the batteries charge, their voltage comes up and the current tapers off.
If the current exceeds the capacity of the source, hopefully it will switch to constant current mode rather than cook itself. In that case, the source voltage drops down until the current matches whatever the current limit is. As the batteries charge, the current stays constant, and as the battery voltage comes up the source voltage will as well until it hits the source's voltage set point, at which time it should switch back to constant voltage mode.
In your case, you have a 54.1V (solar) source and a 54V (utility) source. When the solar source goes into current-limited mode, it's output voltage will drop. Once it drops below 54V, the utility source will start supplying current as well, and will attempt to supply enough current to hold the combined output at 54V. If the load is large enough, the utility source will also go into current-limited mode, and the output voltage will drop further. At that point, both the solar and utility sources are supplying their maximum current into the load.
So you'll need to make sure that everything is sized correctly and current/voltage limits and set points are set up to ensure smooth transitions between operating modes without cooking anything.
Oh I think I see what you're saying. When power comes back on after a long outage, the batteries will pull a lot of current to charge back if they are highly depleted, so if the voltage source can't handle it then it can be damaged or have it's fuse pop etc. If the voltage source is designed to handle that situation and current limits itself or ramps down voltage then I should be ok right? Or should the amount of current going to the batteries still be limited to protect the batteries themselves even when at float? Of course the wiring would be fused and sized accordingly.
54V is almost the power point of a 36V PV array. I run a 36V solar array and I do almost the opposite. I heat water with excess solar. battery charging operates at power point and when voltage goes only slightly above that the power goes into heating water. All very seamless. From a properly sized 36V PV array you could feed directly into a battery through a rectifier, assuming there is always some load on the battery.
Interesting so I could actually skip the charge controller and feed directly? Guess I'd want some kind of isolation though so I'm not feeding power into the panels when their voltage is lower. I'd probably still go with a charge controller to play things safe though, would also allow me to get away with a bigger solar nominal voltage.
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What ajb has said is correct - batteries have a maximum allowable charging current. This might be something in the order of C/5, where C is the rated capacity. Check the datasheet.
Of course, it may be permissible to exceed this temporarily - thermally speaking, charge current is no worse than discharge current, and (lead-acid) batteries can withstand very heavy discharges for short periods - e.g. to operate a starter motor. However, in a situation where the overcurrent is not time limited, it would be safest to stick to the manufacturer's recommendations. Even if you did not cause a hazardous situation, the battery may be degraded by excessive charge current.
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I guess would the best thing be to have it so the rectifiers start at a lower voltage after a power outage? Not sure how configurable these things typically are but they are designed for this type of application so I imagine they would have this built in. So when power comes back it could check the battery voltage and then determine what voltage to start at. So say the batteries are extremely badly discharged and are at 44v it would start the rectifiers at 50 (or something else) volts for an hour before going to 54. Would that take care of avoiding too much current going in the batteries? A standard charger that is designed only for charging batteries without a load being connected can more easily current limit, but a power supply that is also powering the load cannot really do this. Though I guess you could have a shunt on the batteries only to monitor how much current they are drawing and adjust voltage accordingly. The load would then branch off before that shunt so it's not included in the calculation. Does this sound like it would work? Would also allow to more efficiently charge the batteries based on discharge cycle.
Now that I think of it I do notice that when there is a power outage at some of our COs the rectifiers actually come back on at a lower voltage before hitting 54. On the other hand I've also seen where they just go straight to equalize mode. I guess this depends a lot on the type of battery though. Some will have a higher float voltage. Some have slightly different chemistries in how they discharge too, some actually drop to 47 right away when power goes out, which always catches me off guard as that is "dispatch generator ASAP" voltage. But then it starts to ramp up to 50ish before it starts to go down again.
I'm actually open to any good literature on these types of dual conversion setups + addition of alternate energy, I may as well read up on it so I can plan it ahead of time.