Best approach to switch two unsynchronized AC power sources with SSR/IGBT - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: December 21, 2023, 10:23:03 am
content: Hi folks.I have a question about what is the best approach to switch over between two AC power sources, phases are not synchronizedThe switching device is a hybrid relay, a combination of IGBT and Relay. So the switching time can be in a single µs order.Load is common house equipment, so it will contain everything, active PFC, universal motors, induction motors, and capacitor power supplies.Switching should be totally invisible to the users, with no light blip, and no device effects.I'm thinking about two options,1) switching when both voltages are equal - this has no gap, but is it safe for electronic power supplies?2) turn off the first at ZC and wait for ZC on the second one to turn it on - this will cause a small gap and might need some snubber to provide a path for the current

author: Circlotron
author_rank: Super Contributor
timestamp: December 21, 2023, 10:59:42 am
content: If a transformer gets two successive half cycles of the same polarity it will saturate and pull massive current. Good way to blow up one of those standby UPSs that don't have the inverter output synced with the mains.

author: Miyuki
author_rank: Frequent Contributor
timestamp: December 21, 2023, 01:15:46 pm
content: That is a good point.As I am looking it up, it seems like a long-solved problem in the industry.They keep track of applied V·s and turn the backup power to either complete the cycle or to balance it out whatever is closer at a given moment

author: coromonadalix
author_rank: Super Contributor
timestamp: December 21, 2023, 05:15:56 pm
content: what i've seen so far, they use zero crossing detection, get ready for the next one and kick in ??

author: Jwillis
author_rank: Super Contributor
timestamp: December 21, 2023, 07:16:30 pm
content: You need an Automatic Synchronizer similar to thishttps://www.leybold-shop.com/automatic-synchronizer-74504.html. There are less expensive ones depending on your requirements.

author: CaptDon
author_rank: Super Contributor
timestamp: December 22, 2023, 03:47:55 am
content: Tell us more about the supplies. Why are they unsyncronized? Is one of them a U.P.S.? Most U.P.S. units can be syncronized to the local line and stay in phase so when the line dies the switchover is instant. Why would you switch if both are active? What voltage 120, 240? How far apart are they in frequency? Just 1/10th of a hertz or two to three hertz? Tell me more, I am very familiar with powerhouse sync including hydro turbines and fossil fuel turbines and bringing them into step with the grid. E.M.D. locomotives had an option to output over 1 megawatt 3 phase 60Hz at 900 R.P.M. for use as emergency commercial power for hospitals etc. I think there was a 480 / 1260 voltage option.

author: Miyuki
author_rank: Frequent Contributor
timestamp: December 22, 2023, 09:53:33 am
content: The main purpose is to switch between 3-phase and 1-phase power. It is many single-phase devices but split between phases to balance load.50Hz 230VSo they cannot be synchronized in its nature

author: CaptDon
author_rank: Super Contributor
timestamp: December 22, 2023, 02:47:49 pm
content: If you combine the loads of three phases into one phase is it true you will need 1.73X as much current from the single phase supply? Is your single phase supply up to the task? What are the sources of the two different supplies? Is one from the mains and the other from a generator or inverter? I am not understanding why you have a need to switch from a 3 phase source to a single phase source. In any event it looks like you will need to delay at least one full cycle and perhaps 2 cycles for safety. Many devices powered by SMPS like computers, televisions, etc. can survive a 2 cycle hit without rebooting. Fully loaded motors are going to do a little dance on their shock mounts for sure. And of course any true 3 phase load such as 3 phase motors must be disconnected from single phase power. Is this for a home at maybe 10-15KW max or industry where we are talking about 100's of kilowatts? Sounds like in any event the most sensitive equipment will need to be powered from their own dedicated U.P.S. with single phase inputs. Basically a run-of-the-mill standard 500VA to 1500VA type unit. Can you give any more info on this project?

author: Miyuki
author_rank: Frequent Contributor
timestamp: December 22, 2023, 02:58:53 pm
content: It is for a houseThe primary source is single phase inverter running from a battery (solar-fed)3-phase power is a form gird as a backup and is used directly for the motors if needed (it is needed only occasionally)

author: vk6zgo
author_rank: Super Contributor
timestamp: December 22, 2023, 03:16:23 pm
content: If you aren't going to synchronise your sources, you could do what was commonly done with EPPs at TV transmitter sites.Just do a straight "Crash! bang! thud!" changeover without trying any half-arsed methods.Really large changeover contactors can achieve worst case changeover times of the order of five to ten milliseconds, which is unlikely to upset most devices.Electric motors have so much inertia that they will hardly notice, & most SMPS will ride over the gap.

author: CosteC
author_rank: Regular Contributor
timestamp: December 22, 2023, 05:52:02 pm
content: I was going to advice synchronisation, but it is impossible as it is 1 phase to 3 phase system...Then I propose to consider synchronisation only for most important loads there blip is to be avoided. Lights can blip I supposeheaters do not care by definition.Clearly you also do not have any 3 phase motors to worry about as well as no larger transformers.Btw hybrid switch with IGBT? Thyristor/triac would be simpler, wouldn't it?

author: vk6zgo
author_rank: Super Contributor
timestamp: December 22, 2023, 11:08:47 pm
content: Quote from: CosteC on December 22, 2023, 05:52:02 pmI would go with plain old electromechanical.