AC generator shut down under load?

SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
What happens when an AC generator is manually shut down under load?

Specifically, what happens to the AC output and to the loads or battery charger when the generator is suddenly turned off or runs out of fuel?

Of course the "proper way" to turn off a generator is to first open the circuit breaker and then turn off the ignition. If your inverter system has an auto-gen-start it will typically disconnect the generator input first and then turn off the generator.

I've been working on a system with two seperate battery inverter systems both fed from the same generator and both are able to automatically start the generator. The trouble is that if system "A" calls for the generator to run, system "B" sees it as though the generator has been manually started and stopped. System "B" has recently been tripping it's main DC breaker when system "A" auto-stops the generator. I watched this with a DC clamp meter and saw a DC surge nearly 2x the nominal inverter rating as the generator was shutting down. (Batteries at 100% SOC)

I made some phone calls from the site and got some explanations but I still don't understand well enough to explain it. Following the advice of those who know more than myself I tightened the generator input parameters (volts, Hz and allowable back feed time). For now it is a wait-and-see situation to find out if it works.

I suppose this could be an issue for any system where the device that auto starts the generator does not also first disconnect the generator before shutting it down. I believe that some generators do this as part of their auto cool down circuit but it is often part of an integrated automatic transfer switch.

Any comments are much appreciated,

Thanks in advance,

-Alex Aragon

Comments

  • Ralph DayRalph Day Solar Expert Posts: 873 ✭✭✭
    Re: AC generator shut down under load?

    Alex,
    You're correct in that the device auto starting/shutting down the generator disconnects "itself" from the generator. I hear that when my SW4048 initiates and shuts down the generator (relay de-latches the ac input within 1 or 2 cycles, no flickers). With the same equipment if you shut down (or run out of fuel) the generator manually the inverter chases the sagging voltage and frequency and it's messy...brownout, flickering lights, moaning motors.

    I don't see any way to avoid your problem unless either/both of the inverters controlled a relay latching power to the inverters...almost instant power drop.

    Ralph
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: AC generator shut down under load?
    Ralph Day wrote: »
    I don't see any way to avoid your problem unless either/both of the inverters controlled a relay latching power to the inverters...almost instant power drop.

    The hope is that by tightening the inverter's input parameters it will disconnect from the generator more quickly than it would with the default settings. If i remember right, he default settings were; minimum 80vac, max 150vac, min 50Hz, Max 70Hz. (+/-)
    I changed them to; min 105, max 138, min 58Hz, Max 64Hz.

    I wonder how much quicker your SW would "delatch" from the gen if you tightened your input parameters. The SW's default settings are tighter than the Sunny Islands. I'll have to check it out on an SW next time I get a chance. It'll be interesting to see if it makes any noticable difference. BTW, (as I'm sure you know, Ralph) even if you do not use the Auto Start settings, just having the controls connected allows you to use the "manual run/stop" function on the inverter's control panel thereby providing the "delatch" before the generator is shut down.

    I've heard that this issue is often the cause of failure of the charger in some inverter/chargers. I can't say with certianty but I believe it is true that most failed chargers I've seen were in systems where the generator was mostly switched manually.

    Fortunately most inverter/chargers alow some delay before they connect thus allowing the power to stabilize a bit before connecting and hopefully normal operation allows for the charger to not be so heavily loaded at shut down.

    What of "out board" chargers? (Like IBE transformer type chargers.) It is common around here for installers to put a delay circuit in that allows 20-30 seconds warmup time but it still does not delatch before gen shut down.

    It seems like it could be possible to "hot rod" a typical grid-back-up-generator's automatic transfer switch to allow for some shut down delay.:confused:

    -Alex
  • techntrektechntrek Solar Expert Posts: 1,366 ✭✭✭
    Re: AC generator shut down under load?

    Sure, by adding a delay timer so the shutdown signal would immediately trigger the transfer switch but allow the engine to still run, preferrably for more than a minute to allow cooldown.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AC generator shut down under load?

    To tell the truth, it isn't really much of a problem if the gen shuts down under load.
    The caveats as far as the generator is concerned are how much load and how often it happens.
    From the loads' point of view its a matter of what kind of load. Sensitive electronics may have issues with falling Voltage and fluctuating frequency, but they really shouldn't suffer any damage from it. It happens quickly, like a utility line going down.

    This is not the same as a generator running out of spec supplying incorrect Voltage/frequency for minutes at a time.

    An occasional generator 'stall' is nothing to worry about.
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: AC generator shut down under load?

    An occasional generator 'stall' is nothing to worry about.

    Trying to start under load is a much more significant concern! A good generator may have circuitry to unload during start regardless of what you do externally.
    SMA SB 3000, old BP panels.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AC generator shut down under load?
    inetdog wrote: »
    Trying to start under load is a much more significant concern! A good generator may have circuitry to unload during start regardless of what you do externally.

    Absolutely. Trying to get one to wind up with a load across the output ... instant high Amps. Not good.
  • bill von novakbill von novak Solar Expert Posts: 711 ✭✭✭✭
    Re: AC generator shut down under load?
    inetdog wrote: »
    Trying to start under load is a much more significant concern! A good generator may have circuitry to unload during start regardless of what you do externally.

    Agreed - but most systems take some time (in some cases up to a minute) to sync up to a generator AC signal. My SW4024 takes a good 30 seconds. In such cases turn-off is the traumatic event.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: AC generator shut down under load?

    As far as the generator itself is concerned, if the load is heavy, loading the engine heavily and is shut down under those conditions, it can be hard on the engine, specifically the valves, which, especially the exhaust valve, can be red hot. It's much better to let the engine run for a minute or so with no load, or very light load, to give the valves a chance to cool somewhat before shutdown so the valve seals don't cook.
    Startup under load can also be hard on the engine, loading it heavily before the oil is flowing properly to protect the engine.
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: AC generator shut down under load?
    What happens when an AC generator is manually shut down under load?

    Specifically, what happens to the AC output and to the loads or battery charger when the generator is suddenly turned off or runs out of fuel?

    Of course the "proper way" to turn off a generator is to first open the circuit breaker and then turn off the ignition. If your inverter system has an auto-gen-start it will typically disconnect the generator input first and then turn off the generator.

    I've been working on a system with two seperate battery inverter systems both fed from the same generator and both are able to automatically start the generator. The trouble is that if system "A" calls for the generator to run, system "B" sees it as though the generator has been manually started and stopped. System "B" has recently been tripping it's main DC breaker when system "A" auto-stops the generator. I watched this with a DC clamp meter and saw a DC surge nearly 2x the nominal inverter rating as the generator was shutting down. (Batteries at 100% SOC)

    I made some phone calls from the site and got some explanations but I still don't understand well enough to explain it. Following the advice of those who know more than myself I tightened the generator input parameters (volts, Hz and allowable back feed time). For now it is a wait-and-see situation to find out if it works.

    I suppose this could be an issue for any system where the device that auto starts the generator does not also first disconnect the generator before shutting it down. I believe that some generators do this as part of their auto cool down circuit but it is often part of an integrated automatic transfer switch.

    Any comments are much appreciated,

    Thanks in advance,

    -Alex Aragon

    Hi Alex,

    Going back to the original problem description, since the thread seems to have wandered off into the area of damage to the generator rather than effects on connected loads:

    I have several questions/suggestions:

    1. Did the problem happen when neither A nor B was calling for generator start? That is, if you really manually started and stopped the generator? Or did the problem actually occur when A suddenly turned off its load on the generator in preparation to sending the stop signal? Both are possible and the proper corrective action will depend on which is the true cause.

    In one case, the problem is the deviation of the generator output from nominal as it shuts down. In the other case the problem may be the behavior of the generator's output regulation under a sudden shift in load.

    2. The behavior of a generator which uses an inverter for output and therefore whose frequency does not vary with rotational speed will be different from a generator which requires engine speed regulation to control frequency and uses some other means of regulating the output voltage. The regulator and throttle settings for a given output can interact if the load suddenly changes or if the regulator tries to maintain output voltage as the generator slows down in response to the stop signal.
    SMA SB 3000, old BP panels.
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: AC generator shut down under load?
    inetdog wrote: »
    Going back to the original problem description, since the thread seems to have wandered off into the area of damage to the generator rather than effects on connected loads:
    The "wandering" is great thought provoking educational stuff. Don't anybody quit interjecting the BTWs. But thanks for remembering to address the original questions too.
    inetdog wrote: »
    1. Did the problem happen when neither A nor B was calling for generator start? That is, if you really manually started and stopped the generator? Or did the problem actually occur when A suddenly turned off its load on the generator in preparation to sending the stop signal?

    Hm. Good point and question. I suppose that most of the time "A" calls for the generator. The main exception is when the generator mechanic is doing his monthly services. I'm pretty sure he starts and stops the generator on the generators own control panel. I will have to check to see if the logs show that the failures coincide with the servicing dates.

    Come to think of it, when I did my test with the clamp meter, the mechanic had manually started and stopped the gen (he did not use the emergency shut off) so neither inverter system had done an automatic "delatch" before shut down. System "A" has not had any such problems to date so I was not watching it. It stands to reason this would be a worse situation than it ever has during "normal operation". When the mechanic restarted the gen after the shutdown test there was a backfire and the generator would not restart. The diagnostic panel showed a "throtle position sensor failure" or something to that effect. The mechanic believes that the backfire was caused by the "tpsf" . Do you think it might have been caused by fuel which was still being fed into the generator after the ignition was shut off?
    inetdog wrote: »
    2. The behavior of a generator which uses an inverter for output and therefore whose frequency does not vary with rotational speed will be different from a generator which requires engine speed regulation to control frequency and uses some other means of regulating the output voltage. The regulator and throttle settings for a given output can interact if the load suddenly changes or if the regulator tries to maintain output voltage as the generator slows down in response to the stop signal.

    I've noticed that manually shutting down an 'inverter generator' does not cause as much power disruption. I had figgured that this was why. Is there also a better "surge" response with 'inverter generators'?

    -Alex
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: AC generator shut down under load?
    Hm. Good point and question. I suppose that most of the time "A" calls for the generator. The main exception is when the generator mechanic is doing his monthly services. I'm pretty sure he starts and stops the generator on the generators own control panel. I will have to check to see if the logs show that the failures coincide with the servicing dates.

    Come to think of it, when I did my test with the clamp meter, the mechanic had manually started and stopped the gen (he did not use the emergency shut off) so neither inverter system had done an automatic "delatch" before shut down. System "A" has not had any such problems to date so I was not watching it. It stands to reason this would be a worse situation than it ever has during "normal operation". When the mechanic restarted the gen after the shutdown test there was a backfire and the generator would not restart. The diagnostic panel showed a "throtle position sensor failure" or something to that effect. The mechanic believes that the backfire was caused by the "tpsf" . Do you think it might have been caused by fuel which was still being fed into the generator after the ignition was shut off?

    I've noticed that manually shutting down an 'inverter generator' does not cause as much power disruption. I had figgured that this was why. Is there also a better "surge" response with 'inverter generators'?

    -Alex

    I can't really say about the backfire. Not my field of expertise. I have seen recommendations on large lawn tractor and mower engines to run their speed down to idle at least briefly before shutting them off. It may be for other reasons than just cool down.

    The inverter generator has the potential to deliver better surge performance for two reasons, but there are other factors that may make a particular inverter generator better or worse than a comparable direct generator:
    1. The inverter input circuit may have some energy storage capability of its own. This would only affect an extremely brief surge, like a capacitor charging in a power supply connected as a load.
    2. In a way similar to an MPPT controller, the inverter circuit may have the ability to get the maximum amount of power the generator part can possibly deliver from its rotational energy and engine performance to handle the surge, even as the generator slows down under the surge load.
    SMA SB 3000, old BP panels.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: AC generator shut down under load?
    inetdog wrote: »
    I can't really say about the backfire. Not my field of expertise. .

    Backfire on shutdown of a hot engine is caused by the super hot valves igniting the fuel mixture. Had the engine been run at no load, or idle for a bit to allow things to cool down before being shut off, this wouldn't happen.
  • Ralph DayRalph Day Solar Expert Posts: 873 ✭✭✭
    Re: AC generator shut down under load?

    "even if you do not use the Auto Start settings, just having the controls connected allows you to use the "manual run/stop" function on the inverter's control panel thereby providing the "delatch" before the generator is shut down. "

    Yep, that's what I do. Makes for a clean 1 cycle disconnect. But you have to shut off the generator before the inverter latches on again. Anytime power is sensed on AC2inhot the inverter will accept and start charging/passing through to loads. That's ok, the T240 autotransformer is a few feet from the inverter and can be shut off before re-latching occurs...then you go out and shut down the generator which has had a couple of minutes to cool down run.

    Ralph
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: AC generator shut down under load?
    Backfire on shutdown of a hot engine is caused by the super hot valves igniting the fuel mixture. Had the engine been run at no load, or idle for a bit to allow things to cool down before being shut off, this wouldn't happen.

    This backfire was upon restart about 2 minutes after the previous 'test shutdown'. It's a propane generac. I pretty sure it is 60 or 70k with a 10cyl Ford opeating at +/- 6000'.

    -Alex
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AC generator shut down under load?

    So what you've got here is two inverters off one standard fixed RPM generator. One of them tells it to start/stop and the other goes along for the ride. When the gen stops, the second inverter sees a momentary surge in DC to the battery causing the circuit protection to trip. It shouldn't do that, and you certainly don't want it to.

    What's going on? Gen runs down, frequency goes low, Voltage goes low. This really shouldn't cause the DC charge current on inverter #2 to go high. Can you tell us the make/model of the inverters? If it's "Xantrex XW" then I'll bet the load sharing function is involved: sense AC power going low, tries to make up for it from DC. Just a guess at this point.

    Certainly tightening the allowable gen range for Voltage and frequency should help: the inverter should 'drop' the gen before the DC current goes too high. But let's see what inverters we're dealing with.

    Some clever person here will think up a way to rig a latching relay transfer switch so that only the inverter calling for the generator is fed AC on autostart. (I'm also assuming two different battery banks.)
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: AC generator shut down under load?
    So what you've got here is two inverters off one standard fixed RPM generator. One of them tells it to start/stop and the other goes along for the ride. When the gen stops, the second inverter sees a momentary surge in DC to the battery causing the circuit protection to trip. It shouldn't do that, and you certainly don't want it to.

    The surge is from the batteries.
    Can you tell us the make/model of the inverters? ....(I'm also assuming two different battery banks.)

    These are SMA Sunny Islands. Seperate battery banks.
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: AC generator shut down under load?
    The surge is from the batteries.



    These are SMA Sunny Islands. Seperate battery banks.

    So with separate CC/Inverters and separate battery banks, the only common link between the two systems is on the generator AC side.
    SMA SB 3000, old BP panels.
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: AC generator shut down under load?
    inetdog wrote: »
    So with separate CC/Inverters and separate battery banks, the only common link between the two systems is on the generator AC side.

    Correct.

    The PV in this system is all fed through AC coupled Sunny Boy inverters.
  • mike95490mike95490 Solar Expert Posts: 7,307 ✭✭✭✭
    Generators shut down under load, can loose the residual magnetism that provides the self-excitation.  Re-Flashing the winding will re-magnetize the core and then you are fine.
    http://www.dieselduck.info/machine/03 electricity/flashing_generator.htm


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