Feedback about a 3 phase isolated monitoring circuit - Page 1
Forum section: electronics / projects

author: Shay
author_rank: Regular Contributor
timestamp: September 10, 2023, 07:37:44 pm
content: Hey,I'd like to monitor each phase of a 3-phase AC system.The goal is to detect when one (or more) of the phases has dropped in voltage (e.g., failed).It should be reliable, yet cheap and isolated.The exact voltage of the phase does not matter; what matters is if it drops below a certain threshold.I came up with this idea:https://i.gyazo.com/483254c8e25c9eb651d77c8acc939094.png(also attached)Principles of operation:L1_IN, L2_IN, and L3_IN are the phases. Assume that a neutral line is not given.C2/C3/C4 are connected in a "star" arrangement, thus providing a virtual "netural" line that makes it possible to easily monitor each phase and also adds some line impedance, which is good for current limiting.D1/D2/D3/D5 is a bridge rectifier. This rectifies the voltage to DC, and it goes through two zener diodes in series: 160V and 138V (298V total). Once the rectified voltage is above 298V (ideally it sits around 310V peak for 220V RMS), current can flow through the opto-isolator diode. This pulls the M1 gate low, and OUT_L1 goes high to VCC.Let's assume the phase voltage dropped to something like 290V at peak, basically below the zener voltage:Now no current can flow through the zener (expect small leakage, which isn't enough to pull R1 low enough), the M1 gate is still high, and OUT_L1 is low.In simulation, it works OK. I haven't checked the circuit in real life yet.I'd like to hear any feedback, improvements, or alternative solutions that are better.Thanks!

author: Benta
author_rank: Super Contributor
timestamp: September 10, 2023, 09:00:49 pm
content: Could work, L haven't analysed it in detail.But it's quite complex, compared to measuring directly between the phases, and your "pseudo-zero" (the star point between the caps) changes/moves with the phase voltages. For instance, if one phase drops out completely, you'll have 200-0-200 Vpp measured at the two remaining phases.

author: Shay
author_rank: Regular Contributor
timestamp: September 10, 2023, 09:18:25 pm
content: Quote from: Benta on September 10, 2023, 09:00:49 pmHow'd you implement measurment between the phase (e.g delta)?As for your second note, Yes, this is an issue that needs to be fixed. Thank you for your insight.

author: Benta
author_rank: Super Contributor
timestamp: September 10, 2023, 10:14:53 pm
content: Somewhat like what you're doing right now.In a normally running system, you'd have:U - V = 400 VV - W = 400 VW - U = 400 VBut say the W phase drops out. Then you'd have:U - V = 400 V and V - W and W - U much lower (if the phase is completely disconnected you'd have the 2 x 200 V mentioned before).This needs a bit of combinatorial logic effort, but that's on the low voltage side, so it's much easier to do. And you can use your three caps to drop the current to the LED circuits, instead of burning power in the Zeners/resistors.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 10, 2023, 10:36:56 pm
content: Quote from: Benta on September 10, 2023, 10:14:53 pmCaps will save power, but will not give the voltage threshold effects, the OP wanted a 290V offset.

author: Benta
author_rank: Super Contributor
timestamp: September 10, 2023, 11:08:36 pm
content: Quote from: PCB.Wiz on September 10, 2023, 10:36:56 pm...in the original design. Lots of ways to skin this cat.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 11, 2023, 12:22:26 am
content: Quote from: Shay on September 10, 2023, 07:37:44 pmIf you want a voltage offset, and just want a pass / fail you could also look at DIACs + modest Cap + DC optocouplers, or AC optocouplers (two LEDs )https://www.lcsc.com/products/Diacs-Trigger-Diode_379.htmlhttps://www.st.com/content/st_com/en/search.html#q=DIAC-t=products-page=1Those can be ~ 6% tolerant, and fire both ways, so your bridge parts go away.You choose a low current model, allowing higher dropper/divider resistors, for least power loss, and the DIAC firing drives the LED in the opto.