A question on how modern bi-directional 3-phase power meters work.  - Page 1
Forum section: electronics / testgear

author: Fflint
author_rank: Frequent Contributor
timestamp: December 27, 2021, 06:40:12 pm
content: I have a theoretical question on how bi-directional 3 phase power meters distinguish split-phase loads from single phase power generation.I have a 3 phase 4 wire power service at my house (24kW, 3*240V). I'm considering installing a PV system and while pondering various options I thought about a problem I can't resolve myself.Specifically please consider this power meter:According to my understanding and this manual excerpt this is how 3 phase meters are connected here.Consider a 400V load (for example a separation transformer connected between ph1 and ph2. This is L1 L2 on second picture). There is a load connected to the transformer resulting in 400W of power being drawn so a current meter on ph1 sees +1A and - 1A on ph2. Current flows from ph1 through the transformer and back into ph2.Now we have scenario 2. A single phase load between ph1 and neutral drawing 1A of current. And a single phase PV generation setup(inverter) connected to ph2. It supplies 1A of current to the grid.so we have - 1A on ph2 again.For the life of me I can't understand how the power meter can distinguish those two scenarios. Could someone, please enlighten me?I have lots of speculative ideas, but I'm really interested, not in speculation how it "could" work, but how it works.One obvious way would be to measure the current on the neutral, but neutral here is ground bonded before it gets to the meter so that's pointless.

author: Alti
author_rank: Frequent Contributor
timestamp: December 27, 2021, 07:29:27 pm
content: Quote from: Fflint on December 27, 2021, 06:40:12 pmFirst of all you ask two unrelated questions.First one mentions distinguishing load from generation. That is quite easy. It is a matter of sensing direction of energy flow.Second question mentions two types of loads: 1*split phase and 2*single phase.Why do you think this power meter is capable of distinguishing those two scenarios? Had there been a 4-pole RCCB that governs I(L1)+I(L2)+I(L3)+I(N)<delta at all times, it could have been quite easy to figure that out. But there is no RCCB on that drawing so my conclusion is - this is not possible (and your assumption is wrong).Also, you mixed 3-conductor 3-phase connection (AFAIK Norway uses this kind of wiring) with 4-conductor 3-phase connection (rest of EU). I suggest you should focus on one thing at a time.QuoteI doubt you have neutral bonded to ground. It is most likely a PEN conductor.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 27, 2021, 10:21:51 pm
content: Quote from: Alti on December 27, 2021, 07:29:27 pmNot quite. Perhaps I could be more specific in my initial description. I thought the bottom part of my post that describes both scenarios in detail was clear enough, but evidently not.My point is how does the meter distinguish a split phase (between two 240V phases resulting in 400V) load that results in 1A flowing towards the load on phase 1, same amount flowing towards the meter on phase 2 and a single phase load on phase 1 plus a single phase source (a generator, an inverter etc) sending 1A to phase 2.As far as I can tell in both scenarios there is exactly same current flowing in the exact same direction. I realise a written description is not ideal so I'll try 2 drawings.Scenario 1:A single 400V load. Current flows from one phase through the load and back into the other phase.Scenario 2:There is one load on phase 1 and one generator on phase 2. Sum of all currents is exactly the same in both scenarios.Hopefully it is clear now that both scenarios appear to be looking exactly the same if we consider current direction as seen by the meter unless I'm missing something. I would be very grateful for letting me know how those two scenarios are distinguished in practice as both are perfectly valid and they must be recognised somehow by the meter.The rest below is irrelevant to the question, but I answer hopefully to clarify.QuoteThere is only one question presenting two scenarios. Hopefully it is explained more clearly now.However, following on.QuoteRCCBs have nothing to do with metering. I fail to see how it could matter in any way. The current delta in scenario 1 is zero (l1=+1, l2=-1, l3= 0, N=0) and in scenario 2 is zero too (l1=+1, l2= - 1, l3=0, N=-1+1=0). I've omitted it for simplicity's sake.QuoteWell, that is what I would think, but the truth is, there are hundreds if not thousands of 4 conductor 3 phase connections that supply electricity in my country(Poland) and throughout Europe. Various types of loads are connected between a phase and N(240V), using all three phases in a star,delta, or between two phases (400V).In recent years many people got PV installations (single phase, or 3 phase) with so called 3-phase bidirectional meters put in place by the electricity supply companies. Those meters use the same connection as previous single direction 3 phase meters and are supposed to be compatible with all old loads. Yet, the instruction manuals to those meters claim they are capable of measuring usage as well as generation on all 3 phases at the same time.QuoteI don't see where I mixed 3 conductor, with 4 conductor connection, but if I did my intention was to only discuss 4 conductor 3 phase system.QuoteSee the drawings I attach to this post. Electricity is supplied to every plot of land here in a form of 4 conductor cable. Then the electrician that builds the connection to the building is supposed to bond the fourth conductor(pen) to ground. From there we get N and PE (this is all before RCCB). Before the RCCB PEN, N, PE are essentially the same potential and are interconnected. Then 5 conductors enter into the building(l1, l2, l3, N, pe) and first thing in the residential panel is a cutoff switch. Then RCCBs etc and the rest. Anyway, I'm not sure this is relevant in any way. I was trying not to overcomplicate.My main point is shown on those 2 drawings. If it was impossible to distinguish those two scenarios than either some people would pay for electricity despite it being matched by their local PV generation. Or other people would get split phase loads for free. I doubt either case happens.

author: bsdphk
author_rank: Regular Contributor
timestamp: December 27, 2021, 10:39:35 pm
content: Quote from: Fflint on December 27, 2021, 10:21:51 pmModern electronic meters measure voltage and current as AC signals at frequencies of several hundred hertz often several kHz.By looking at the signs of the voltage and current for each phase, you can tell if energy is flowing in or out of that phase.Try to work through your own drawings over a full period of your grid, in steps of 30 or 45 degree angles, remembering the signs of the currents at all times, and you can easily see how it works.Some meters measure the current in the neutral, others assume the current in the neutral is the sum of the current in the phases, but the result will be the same either way.What the meter does with all the measurements is entirely up to the software and configuration, and it can go any which way the local electricity code, business rules and what politicians feel like.So the answer you are seeking is not what the meter can do, but what the legalese governing your meter.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 28, 2021, 01:24:41 pm
content: Quote from: bsdphk on December 27, 2021, 10:39:35 pmI decided to simulate both scenarios and to compare the differences. In my simulation the "meter" measures instantaneous voltage and current measurement on each phase. Then it simply multiplies both to get power in that very instant. Interestingly in scenario 1 (a single 400V load between phases) this is how the power flow looks.The chart on the bottom shows (phase 1 voltage * phase 1 current) + (phase 2 voltage * phase 2 current).Then scenario 2. A single load and a single generator.The total power goes between +200W and -200W to average 0.So the meter can distinguish between those 2 scenarios and it only needs a single voltage and current measurement per phase (measured few thousands times per second).