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It depends I suppose. There are some inverters that are truly transformerless, by the basic definition of what a transformer is (two windings and a core). Some high voltage PV-inverters (SMA I believe) are like that, it does have a buck and/or boost stage to get it to the roughly 400V or so needed to make 230VAC (RMS), but it is not isolated. So they are transformerless, but not inductorless. (Essentially, the difference is the same as between an auto-transformer and the real deal). For battery inverters, I don't know of any that are truly transformerless. They are only called transformerless by the marketing guys because the transformer is much smaller. The advantage, of course, is that it is cheaper. The disadvantage is that you lose the isolation. Your DC battery is literally connected to the grid, albeit through a long buck/boost path and with alternate polarity... 🙂

Indeed. The combo unit does both, but it is quite common to use two breakers to get the job done.

This is my earth leakage clamp meter. Here I'm showing the rather terrible situation in my own house at the moment. I have a 13mA standing leakage... And this morning I had an oil heater that tripped an RCD repeatedly too. Ignore the ET112 in the picture... that was just the easiest cable I had lying around where I could reach the earth conductor easily. This heater will gradually increase leakage as it heats up, and then trip the RCD.

They do. Well, it depends on the model. Earliest models didn't. Then there was a firmware update and a certain setting could be changed to use the dry-contact relay in the inverter to operate an external bonding relay. And then finally, they changed the relay they use so it could bond TN when running islanded. The bonding relay will generally open before the inverter reconnects to the grid, so I doubt it's a bonding relay issue. When connected to the grid the bonding relay should be open. If not, then that is the cause of the tripping. 99% of the time a tripping RCD is because you have a leak to earth. You simply have to find it. You do this by disconnecting circuits (on the output of the inverter in this case) until the tripping stops. You usually have to disconnect both the live and the neutral, so generally this is best left to an electrician. You switch off all the live breakers, and disconnect all the neutral wires from the neutral bar. Be areful, since those hanging-in-mid-air neutral wires could be carrying live current while you're messing around like this. By bringing the circuits in one by one you can usually find the culprit. You could (should?) also do an insulation resistance test. Again, that helps to find the circuit with the leak. But, in the last 1% of cases, it could be that your "standing leakage" exceeds 15mA-20mA, where most RCDs starts to trip. Most modern appliances leak a small amount to earth, usually because of the EMI filter or surge arrestor that sits between the L, N and E conductors. This is usually in the order of maybe 1 or 2mA, so not a problem, but if you have enough appliances on the RCD it tends to add up and eventually you get nuisance tripping. In this case, you will usually find that the tripping stops if you unplug all your appliances, and comes back once enough of them are plugged back in. I really honestly truly hate chasing RCD trips. I find that an earth leakage clamp meter (essentially an AC clamp meter that can measure the residual current) helps a LOT to chase down such issues. Especially compounding standing leakages... But, as I said... got to start with the 99%. In 99% of cases, it happens because something is wrong...

Well technically you are supposed to pay for a copy... and it is several hundred bucks... but some bootleg copies are around 🙂

I've heard of others in the past who were lucky enough to have a municipality that helped them out. Conlog will help anyone, as long as the guy who owns the meter (the municipality) allows it. The trouble is simply that in some places, Sannieshof comes to mind, you can forget about that...

You're lucky with your municipality. Some areas in the country... the eyes turn a little glassy... and then that's the end of it.

Indeed. And that is also done to improve signal/noise ratio. At zero it becomes horrible. Conceptually it is easier to think in terms of "zero", but of course the default is to import 50W. Spot on. Which is why it helps to increase the setpoint. It gives you a margin to work with. For example, if you set it to 150W, and its been running at roughly 150W for the last 7-8 seconds (the BEC23 has a 15-second window), that means you have 7*150 ~= 1000 joules in "credit" 🙂 I was in a meeting the other day with people who wanted a more accurate meter. The short answer at the end was: If it was cheap and easy, you'd be able to buy one from the shelf.

On the GX device it will tell you what part of the test failed. From the Device List, select the Multi, then Alarm Status, then VE.Bus Error 8/11 report. It will show you which part of the relay test failed. Usually it is the bonding relay test that fails. It tests that there is a TN bond on the input side (which there usually is), then it tests that there is NOT a TN bond on the output side (which is usually where the problem is), and finally it tests that closing the bonding relay actually establishes a bond (also not usually a problem). Depending on what step fails, it usually indicates some sort of issue with the installation. Basically, the same sorts of things that causes RCDs to trip or not trip...

Yeah, I don't actually think the FreedomWon battery has that exact problem. At least, they told me they don't... 🙂 BYD and BlueNova does have this problem. The current sensor is not very accurate low down. For example, with a BYD battery it cannot measure accurately below 2A. Per module, so if you have two modules (as minimum recommended), that's 4A or 200W. Any load below 200W... the battery doesn't even know it is being discharged, so the SOC drifts. Then suddenly a cell voltage drops out, and the BMS has to make a drastic change to its SOC estimate. I'm told FreedomWon has a more accurate current measurement. They use the Orion BMS, so you could just look into that, whether it uses a shunt or a hall effect sensor. So quite probably something else is causing it overestimate its SOC. The fact that the per cell voltages are also drifting indicates something else to be wrong. Unfortunately the only way to fix it that I know is to cycle it several times and see if the cell voltages converge to some ideal. And if not, it becomes a warranty claim.

I agree about the AC/DC coupling, but perhaps to help you get an idea of what we're dealing with: Imagine what happens at night. There is no PV power coming in, it's just an inverter running fro a battery feeding power into the grid, with a closed control loop that runs from a grid meter, attempting to get the meter to zero. The control loop is slow... 🙂

I work from the premise that if 300 joules trips the meter, then it means 300W for 1 seconds, 600W for 0.5 seconds, 1200W for 0.25 seconds. An AC cycle is 0.02 seconds, and I doubt even these pesky prepaid meters measure power to the nearest 20mS. They will average it over a number of cycles, possibly at least 5 or so. That gives you 100ms to work with. So if you can build something that can average power into 100ms buckets and turn on a TRIAC for a few cycles (as many as is required to get the average down to below 300 joules), you're going to be fine. You also don't have to hold it at zero. You can compensate for 1000W going out for 0.5 seconds by dumping 2000W into a kettle for 0.25 seconds. The prepaid meter will probably still see zero if you make your time intervals short enough.

Aaah I get it. You're talking about AC-tied PV. I'm talking about DC-tied... a bit more common in these parts 🙂 I know that in an off-grid system with AC-tied PV the transformer coupling results in a DC voltage increase, this is in fact how the PV-inverter assistant regulates the frequency. It changes the AC frequency relative to the battery voltage. A reduction in load does not necessarily cause a rise in the DC voltage when the PV is DC-tied and the inverter is feeding a constant amount of power into the grid based on a control loop that updates it roughly every 3 seconds, and hence clipping it on the DC side will not work. Messing around on the AC side is your only option, sadly. Commercial energy diverters do exist. In the UK, people use them to turn on the hot water cylinder once the PV starts pushing into the grid.

Oh I know, that's more for the benefits of others who want to dive that deep 🙂 You very probably know more about this skubala than I do... I agree, but it needs to be clipped on the AC side. an energy diverter, which has been mentioned a few times, at least by myself. Like this one. Trouble is, this level of DIY is beyond the reach of most people that have this problem. With such an energy diverter, it essentially does a crude form of the above measurement, taking a number of samples of both current and voltage, averaging, and switching some kind of switch (probably a TRIAC) with it. Because it does not have to be very accurate, nor does it have to do digital communication or calculate actual energy, even a small MCU is fast enough to do the job. The only trouble, as I said, is that DIY needs skill, and whatever is commercially available is 1) not in this country, and 2) not cheap.

Here is a video explaining how to measure power with an oscilloscope, which gives you an idea of what it takes. It gets very technical. But from 7 minutes on, it shows how to use the math function in many modern DSOs to multiply the two measurements. Around 08:30 you can see the red result of the multiplication, and you will note that on every cycle a little bit goes in the reerse direction (where the red line goes under the zero line, reactive power). But you can visually see that the power is on average going in a forward direction. I've actually repeated this on a DSO, with AC power, and it works perfectly. On my scope I can even ask the "math" module to work out the average of that waveform. Now every power meter, including prepaid meters, does this sort of math and then averages it over a certain window of some milliseconds or seconds.

Regretfully this is not what is happening. Simply, the inverter feeds an amount of energy into the grid to cancel out any of your local loads. Then one of those loads switches off, so now the inverter is feeding more energy into the grid than the loads are using. The remainder goes into the grid. It now takes a certain amount of time before the inverter reduces power. This is for two reasons, the one is that closed control loops (which is a deep technical discussion, I mean deeper than I'm already going here, that I want to avoid now) needs to be dampened otherwise they may become unstable. The second is that the grid measurements are only taken about once a second (and the best case is 600ms... which is not much better). The amount of watts multiplied by the number of seconds that this is going on gives you a number of joules (a joule is literally a watt-second). If that number gets too high, the prepaid meter trips. For Most Conlog meters this number is 600 joules (BEC23) or 300 Joules (BEC44). As you can see... WAY to darn sensitive. To expand a bit more, for the benefit of the OP (which is short for original poster... 🙂 ), AC is a bit of a difficult beat to measure. It is not as easy as DC where you just measure which way the current is going, and then you know which way the power is (also) going. With AC the current literally changes direction 100 times a second, and so does the voltage. Now as long as the voltage and the current goes the same way on average, then power is going in the "forward" direction, towards the consumer, but if the current goes in the opposite direction (out of phase, in other words) to the voltage, then power is going towards the grid. Now we get to the cost of the meter part. Cheaper meters (eg the chip in the Sonoff POW) gives you a value averaged over several seconds. Really good meters (Smappee Powerbox) gives you a value every 100ms. The former device costs R350, the latter costs R4500. You can get even more accurate... but then the price goes to 45k and more... and you still have to account for the communication delays, the adjustment delays, etc. And I haven't even gone into the matter of non-linear loads which seriously complicates power measurements... So long story short, to avoid the issue these meters create would require very expensive metering devices and fast control loops. And it's frankly just not worth it.

These prepaid meters are becoming a real problem. The trouble is that even though the system is somewhat slow to back off (7 - 15 seconds), even the strictest speed requirements (parts of Europe wants 90% reduction in <2 seconds) will still trip these meters. The meter manufacturers really need to be a bit more lenient. I understand that you want to sell anti-tamper features to a municipality, but surely it doesn't have to be so darn hairline sensitive. Take for example the Landis GYR meters with the SRE detection feature (significant reverse energy). It can still be programmed to go into tamper mode, but it only triggers if you are feeding hundreds of watts in for many minutes...

I ended up squeezing my eyes shut and clicking the buy button... I bought the whole book. I wish I can say I read it... I've read some of it, but it's in that same space as Stephen Hawking's "A brief history of time", lots of people have it on their shelf, few have read all of it 🙂

Yeah, that's why I just snigger when someone says "transformerless". Usually, what they mean is that it is a high frequency design, so the transformer is 1) not made of iron, and 2) much smaller. Chapter 9 of the Art of Electronics is actually free, and discusses the various topologies. Interesting reading for those that are interested.Summary on page 700 🙂

I just noticed you quoted this and realised I have to disagree with that... Active balancing means you have small isolated DC/DC converters and switching gear so that charge can be sucked off one cell and put into another. Passive balancing means you have resistors across each cell, again with some switching gear, usually MOSFETs, that bleeds off/bypasses some energy from the high cells so that the lower ones can catch up. Active balancing is more expensive but more efficient. Now here is some news for those who don't know.... the bulk of the batteries out there, big names in the industry... have only passive balancing, and they only balance at the top. This means you have to fully charge the battery every now and then, usually once a week or once a fortnight (that's a British term meaning two weeks). It really is good enough. What is more, an active balancer is also limited in what it can do. While it can in theory balance all the time, this doesn't actually work that well. When the battery is being charged or discharged heavily, phantom imbalances show up all over (well not really phantom, but due to the fact that no two cells are alike across their whole range) and if the BMS tried to move charge around it would just be wasting its time. An active balancer can only really balance when the cells are at rest or close to it, and of course right at the top and at the bottom. So again, it really is a good question whether the premium you pay for that active balancer is worth it. Sure it is really nice, and I often rave about batteries where this is done right (such as the Discover AES), but it really isn't essential.

@anotherbrownbear. But he's not around that often, so rather contact him directly.

@ebrsa is one of the old guys around here. He participates far too infrequently these days. And believe me, we've had some disagreements (always respectfully of course) many times over the past 7 years or so 🙂 Let me be a bit more specific just so that there are no misunderstandings. I always say that you get different levels of BMS functionality. On level one you have basic protection. If a cell is low or if another cell is high, then the BMS disconnects. On this level there isn't even cell balancing, and though you may not believe it, there are some very very low end battery packs (usually for toys, not serious stuff) that have no more than this most basic of protections. On level 2, you get cell balancing. At this level there is still no comms with anything. Level 3, basic protection features in the shape of a relay that opens/closes to indicate that you should stop charging or discharging. This is very very basic comms, and is useful because you can avoid that absolute shutdown that leaves the entire house in the dark. Victron Smart Lithium batteries are on this level. Level 4, SOC tracking. Level 5 adds full communications where the battery can mandate a charge and discharge rate. Now my point in this whole long thread was simply that you don't need level-5 capability to run an LFP battery. But you need at the very least level 2 for this game. In a properly designed system (where your PV is sized correctly to the battery capacity), a BMS never has to intervene. This is how you want it. Now with all of that said, you must also look at the fine print of your battery warranty. Some of them have shorter warranties if you don't have that level 5 charge and discharge control, so again, don't just take my word for the gospel just because I talk a lot 🙂 Also, chemistry play a role. For the longest time (maybe still, I haven't checked lately) LG Resu batteries could not be used off-grid. Why? Cause they are not LiFePO4, they are a different chemistry, and in an off-grid system (or a non-hybrid one) there is no way to control charge and discharge current. Edit: The above "levels" is just my own understanding of the matter. They are not text-book 🙂

You're not reading the full context of my answer. In a properly designed system the BMS never has to intervene. You still need the BMS for safety, and balancing of course. I'm not contesting that. I'm contesting the idea that you cannot possibly run a lithium bank without the highest level of control (aka canbus control) and that's just not true. Lots of people run daly bmses that has no comms. Even victron's smart lithiums essentially have a two-signal BMS.

Dude... I once saw a motorcycle advertised on gumtree. Beautiful machine, almost brand new. The seller explained that his reason for selling is that apparently "Do whatever the f... you want" doesn't mean what he thinks it does.

Correct. Most LFP batteries can handle 1C discharge without a problem, but recommended is C/2 (half that). A kettle takes about 5 minutes or so to boil water, so this should be okay I'd think.