Brownout and Consequences?  - Page 1
Forum section: electronics / general

author: Kevman
author_rank: Regular Contributor
timestamp: March 15, 2016, 05:04:05 pm
content: So I got a text from my sister (at my parent's house) last night saying that all her aquarium pumps quit and the lights were dim.After a while, my dad used his Kill-a-watt to measure several outlets- some were too low to turn the Kill-a-watt on (he said they were dead) and others were reading 74vAlso, supposedly the neighbor's lights were dim too.This being the US, the house has 240v Center Tapped power. But I can't see how only one phase could end up with power, seeing as the whole street was out and the houses don't share the split power. Could something in the house have been pulling that leg down that low?I asked him to turn off the house main breaker until the problem was fixed, but I guess all he did was turn off the well pump and refrigerator. The Internet kept working, which means the wall warts powering the router and modem kept running.The power came back to normal after about an hour and a half, but I can't help but wonder what was happening as the crippled power flowed around the the house. His house has a LOT of 240v circuits- what would have happened when any of the 240v devices came on? I guess power would flow from the higher phase through the appliance, and into the other phase? I guess if the other leg was truly 100% unpowered, wouldn't that cause the devices in the other leg to start receiving power through the 240v device? I guess then, if there was a voltage on the other leg, it would bring it DOWN. I'm not quite sure I grasp what was going on...I assume that breakers would have tripped if anything behaved too poorly, but it seems to me like a lot of things saw a lot of stress...Do you think there will be short term or long term problems stemming from the house receiving something like 50% voltage for 90 minutes? I've seen people talking about brownouts but never ones that last more than a few seconds.Oh the excitement that living in the middle of nowhere gets you

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: March 15, 2016, 05:07:27 pm
content: Perhaps a broken neutral back to the transformer centre tap? Then your two split phases wouldn't be an equal 120v unless you have exactly equal loads on each.

author: AF6LJ
author_rank: Unknown
timestamp: March 15, 2016, 05:10:17 pm
content: I have seen exactly that happen. One side of the transformer develops a bad connection while the other side and neutral are okay. The condition to really be worried about is when the neutral develops a high resistance. That is when the imbalance of the load in a US home can destroy things on the lightly loaded side of the 220V + neutral service.

author: SeanB
author_rank: Super Contributor
timestamp: March 15, 2016, 05:25:41 pm
content: Same on a 3 phase 230V system with a poor neutral. The phase with the highest load has the lowest voltage, but the phase with the lightest load will tend towards the full 400VAC. Had that happen to me last year when the Izinyoka went and stole the neutral and earth wiring from the substation ( twice.....) and I measured around 274VAC on my phase. went to the meter room and saw the 3 phases as 170VAC, 60VAC and 300VAC, after turning off the mains in my place, though a MOV did die warning me about the fault, and the UPS was doing its best bucking to drop the voltage. Turned off the entire block, and got on the phone to tell the Metro about the fault. Took them a full day to repair that, replacing all the cabling and the busbars from the junction of the transformer aluminium cable to the ends of the cabling still intact. Then they eventually installed new steel doors, an alarm system, monitoring and tear gas dispensers inside the old Tram substation, and new barbed wire. Ran the rest of the day on a generator.

author: Kevman
author_rank: Regular Contributor
timestamp: March 15, 2016, 05:47:13 pm
content: Not possible here since the whole street had the problem. The houses are too far apart to have >1 on a transformer.

author: SeanB
author_rank: Super Contributor
timestamp: March 15, 2016, 06:06:44 pm
content: Quote from: Kevman on March 15, 2016, 05:47:13 pmSo you have a feeder ( 11kv, 6.6kV or 2kV, depending on the utility and the loading in the area) with a 3 wire high voltage line running along the right of way in the area. What happened is that, on your feed line segment of the network to the substation upstream of you ( typically a 33kV input and multiple medium voltage feeders for the district) there was a fault to ground, probably enough to trip an automatic overcurrent circuit ( the lights went out for a half second), but which was enough to blow the one phase fuse of the local line feed, leaving the 3 phase line with only 2 phases. This then resulted in the voltage across the 2 phases ( the transformers are connected primary side across phase to phase, the secondary only is grounded at the pole to make the neutral and the protective earth) being much lower than usual, and depending on the load one phase would be very low ( the one with the highest load) leading to your brownout. The other lightly loaded phase might have had a high voltage or close to normal.Another method is a single wire feeder, with local ground returns for the high voltage per pole, where if the ground on the primary goes open it will still work ( 11kV will track across a wooden pole nicely) with a very loud noise on the pole and low voltage, or if you are unlucky your supply rises to 11kV and burns out every wire in the house, often along with the house.

author: AF6LJ
author_rank: Unknown
timestamp: March 15, 2016, 08:01:04 pm
content: Quote from: Kevman on March 15, 2016, 05:47:13 pmDefine too far apart.

author: AF6LJ
author_rank: Unknown
timestamp: March 15, 2016, 08:02:33 pm
content: Quote from: SeanB on March 15, 2016, 06:06:44 pm

author: Kevman
author_rank: Regular Contributor
timestamp: March 15, 2016, 11:16:39 pm
content: Quote from: AF6LJ on March 15, 2016, 08:01:04 pm2 Hundred yards or so on average between houses. I'm 100% sure that they are that my parents are the ONLY house on the transformer, but a couple of houses in the neighborhood that are closer do share. You can see the conduit that runs from the meter, underground and straight up to the pole into the transformer. There's also a transformer on each of the next door neighbor's poles.The local lines only provide single phase power to the area- there's no three phase as there's nothing on the street that needs it.The only thing that they have found to have failed is an LED light bulb. Seems lucky to me.

author: AF6LJ
author_rank: Unknown
timestamp: March 16, 2016, 01:22:58 pm
content: Quote from: Kevman on March 15, 2016, 11:16:39 pmYup sounds like something on the high side.

author: SeanB
author_rank: Super Contributor
timestamp: March 16, 2016, 06:45:51 pm
content: Local feed is 3 phase, as that way they only have to run 3 wires at high voltage, and if there is an industrial park built in the area then the wiring is already there to supply them, plus a 3 phase primary side supply can use thinner conductors for the prospective load, and the balancing is easy to do simply by doing a rotation along phases every few houses so that the load is spread out over all 3 phases. Plus it also means the substastion load is automatically balanced, even if a new development is done anywhere and connects to a feeder, wheras a single phase load line means that there would be a sudden large load on the one phase only, leading to increased loss in the substation.Incidentally the 3 11kV lines are deliberately not ground referenced except by a large reactor, called a Peterson coil, so that a single phase to ground fault can be tolerated without blowing a fuse. Tough if there is a second fault, or a phase to phase fault, it will cause an overcurrent to be detected by a relay in the substation, which will open a circuit breaker for a half second, giving enough time for the now well cremated animal or branch to fall off the line. There is a relay in the circuit with a 3 slot wheel, which increments each time the circuit operates, so that you get 3 strikes for fault clearing, after which it stays open and requires manual reset as there is a hard fault. You had a branch cause 2 phase wires touch, which tripped the power, but the current in the fault was high enough to blow a single 11kV cutout fuse, which tripped open dropping the phase. The fault was probably registered by a power monitor either in the substation showing a current imbalance on the one phase, or by an end of line monitor that stopped sending a heatbeat signal that the 3 phases were present at the end of the line.

author: Helix70
author_rank: Unknown
timestamp: March 16, 2016, 08:56:31 pm
content: Quote from: SeanB on March 16, 2016, 06:45:51 pmPerhaps means a delta - star transformer. When one phase is faulted, the other two phases swing around to 180 degrees out of phase and supply the fault. Typically a recloser will be configured for 4 sequence shots to lock out, with two slow, two fast to allow wildlife and such to be burned off. Earth faults are isolated from the primary, and need to be protected at the secondary in the neutral line connected to earth.

author: Fank1
author_rank: Regular Contributor
timestamp: March 16, 2016, 09:48:34 pm
content: Three phase on the street, individual single phase transformers fed hot to neutral from different phases.Neutral was open or had a bad connection.

author: 0b01010011
author_rank: Regular Contributor
timestamp: March 16, 2016, 09:50:30 pm
content: We had something similar once - most things failed except the TV standby light was still on, and some CFLs were flickering. After checking the fusebox for sanity I measured the voltage and it was down to 36VAC on what should be 240VAC.Only lasted a few minutes and didn't cause any damage - would have turned the non-SMPSU based stuff like the fridge etc off though if it lasted any longer - I am not sure if a much lower then normal voltage could be damaging in that it could just cause the motor to stall and heat up.