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I am a technician for a security and access control systems company, and we are experiencing a strange interaction between the clients diesel generator and the UPS unit we installed to act as a handover supply to the clients CCTV system during power outages.

The issue to our best understanding is that when the mains supply is lost, the UPS takes over the supply to the equipment while the generator switches on, with the generator supplying the power, the UPS hands the equipment off to it. With the addition of the equipment, the load on the generator supply increases causing a voltage drop, which cause the UPS to take over the supply again, the generators supply stabilizes, the UPS hands off again and the cycle repeats. This switching between supplies causes our CCTV equipment to reboot constantly, which is an issue because we are not monitoring or recording footage.

Another theory we have is that the output of the generator is just unstable and fluctuates enough between cycles that it cause the UPS to take over the supply, though we are not entirely sure on this.

So, how exactly would we go about fixing this issue? We have "loadshedding" which results in the municipal supply being cut for 6 to 8 hours at a time, as often as every day, so the CCTV equipment has to run on the generator, and because it is CCTV we have to have the UPS in place to prevent any downtime. If anyone can suggest a fix, or a piece of equipment we could put in place, that would be great.

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4 Answers 4

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Check generator fuel filters, a blocked filter will cause the machine to fail to maintain speed when the load is applied and will cause this.

How much other load is there on the genny? Adding a few kVA of fixed load to an otherwise unloaded machine will likely help.

If the gear is rebooting due to UPS switching, then the UPS is not suitable for that equipment, some gear with very poor hold up really needs a online double conversion UPS which does not have a switching action between battery and mains, a separate issue to your genny problems, but something you may wish to look into.

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  • \$\begingroup\$ Thank you for the reply, I will do some research into the type of UPS you mentioned and see if it is viable. As for the load on the generator, I believe that it runs a handful of lights, our CCTV equipment and some admin PC's that shouldn't draw all that much... \$\endgroup\$
    – DJBlom
    Mar 11, 2021 at 19:37
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I'd suggest that you first characterise each major element of the system. As a suggestion that sounds a bit noddy and basic, but at the very least it will inform your investigations and build confidence in your conclusions.

How much current does your installation draw? What is the typical load power factor (phase shift)? Are these within the specs for the generator and the UPS?

How does the generator react to changing load? Is its output stable under all loading conditions within its specification, including the load power factor (reactive loading). This is crucial, as your best-guess scenario suggests that it is not.

Is the UPS a switchover type or a constant-sharing type? If switchover, how is the supply transition smoothed?

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  • \$\begingroup\$ The UPS is a switching type, and I believe the transition is smoothed using a capacitive circuit, though I can't be sure as I have never opened one up and inspected it thoroughly enough to say for sure. I will look at your suggestions for checking the parameters of the installation and ensuring that they are within the specs of the generator and the UPS, I appreciate the feedback. \$\endgroup\$
    – DJBlom
    Mar 11, 2021 at 19:45
  • \$\begingroup\$ As mentioned in the answer, it is extremely important that you check power ratings of the generator and ups. Additionally to verify if the generator can actually deliver such power, prepare a dummy load(can be a set of ordinary filament bulbs) and verify that the generator and ups both can actually deliver the power needed. This would help you confirm the hypothesis. \$\endgroup\$
    – NoumanQ
    Mar 12, 2021 at 8:58
  • \$\begingroup\$ Additionally there are online UPS systems that have donot switchover at all. The power is always flowing through the UPS with no switchover. For applications where a <50ms interruption cannot be tolerated, it is a good idea to consider using such type of UPS. \$\endgroup\$
    – NoumanQ
    Mar 12, 2021 at 9:01
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I saw some issues in UPS fed by diesel generators: when some big motors started/stopped, the UPS input was not charging battery because input voltage or frequency was out of range. Check if these parameters UPS can be adjusted in your UPS, and increase if possible.

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Most UPSes are not designed to work from a generator, regardless of load. Generators vary the frequency and sinewave shape as they attempt to regulate the output. Or put another way, generators are not good at holding the frequency and sinewave shape as loads change.
Most UPSes monitor multiple items, each of which can be a basis for switching over to battery power.

  1. Line Frequency
  2. sine wave shape
  3. Line Voltage (RMS)
  4. Line Voltage (Peak)

There might be others, but these are the ones I'm aware of. You can test your setup by simply running your generator with the UPS plugged into it (no other loads). See if the UPS behaves. I'm betting that it won't. Most UPSes will simply oscillate back and forth until their batteries are dead.

To solve this, one needs to have what is called an inverter output from the generator. Honda makes some
Unfortunately I have not seen any systems where you can add an inverter to an existing generator.

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  • \$\begingroup\$ Thank you for the response, I appreciate it. Using an inverter was something we considered but as you said, it isn't something we can add in to an existing setup... \$\endgroup\$
    – DJBlom
    Mar 12, 2021 at 5:42
  • \$\begingroup\$ Typical modern UPS systems are not as sensitive to frequency as much as voltage. This is because typically the AC input to UPS is usually rectified into a DC signal in the very initial stages of conversion \$\endgroup\$
    – NoumanQ
    Mar 12, 2021 at 9:03

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