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We have a 100kW 3Φ 208V generator and ATS operating in North America. The system as installed does not allow for a test where the load can be placed on generated power when utility is available. We have solicited quotes to modify the system to allow such testing.

The suggestion from two licensed electricians who have bid the project has been to interrupt the sense circuit which the generator uses to detect utility power. The ATS contactor is two-position three-pole (single coil). I asked if the lack of "dead time" where the load is disconnected from either supply would cause problems, and got what I would best describe as a shrug.

Since the generator is not phase synchronized with utility power, I imagine this would cause... interesting things to occur. I'm not so concerned about lighting or electronics - but synchronous motors (HVAC / Elevator) do seem like they would have quite a bit of stored energy. On the other hand, I can envisage this to be equivalent to starting from a stopped state or slip as one would see from a VFD.

Should a rapid (~50ms) loaded cut between two unsynchronized sources be expected to cause problems other than the brief outage for an average office building?

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  • \$\begingroup\$ I hope I am not using a computer when this happens, I would guess a lot will shut down and lose work. \$\endgroup\$
    – Gil
    Apr 13 '21 at 0:50
  • \$\begingroup\$ @Gil, agreed. I'm fine with things restarting - not as fine with fuses blowing or breakers tripping. Very not fine with dead equipment. \$\endgroup\$
    – Mitch
    Apr 13 '21 at 2:04
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I remember that GE's Multilin SR750 feeder protection relay provides synchronous bus transfer. We could set maximum voltsge and angle differences between a bus transfer could be activated. It ensures break before make transfer. Many other suppliers may offer similar features.

It is also not advisable to parallel two informers even momentarily as the circuit breakers for the incoming feeders may not be designed to handle fault currents from two sources.

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ATS are designed to be break-before make, to avoid the case where the genset is connected to the utility (a very bad thing for many reasons, and generally not allowed.)

When the ATS transfers, it would be as if the supply were interrupted briefly. You mention 50ms dropout time. This is several cycles. There's no guarantee that the phase alignment of the transfer between the utility and genset will be smooth, unless the ATS and genset were designed to do that.

What's in an 'average' office building nowadays that will care? It will include some critical IT stuff like servers, network and telephone gear, not to mention life safety and other building management.

On the IT front, PSUs in computers will not hold up for 50ms. ATX (PC) power for example will only hold up for 17ms, basically one missing AC cycle. Good design will have mission-critical IT gear on a UPS, regardless of having onsite generation capability.

For other mechanical and building infrastructure gear, check with the manufacturers. They should be able to restart correctly after a dropout.

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  • \$\begingroup\$ I am not concerned that the supplies would be connected together – I understand that. The question is what happens since the supplies are unsynchronized. How can I estimate when that would be an issue? \$\endgroup\$
    – Mitch
    Apr 13 '21 at 12:55
  • \$\begingroup\$ Consider for example two extremes: one where this system only powers resistive loads, the other only powering inductive loads. The resistive loads I expect would be fine at any level. I imagine there is some level where the inductive load when switched out of phase would trip or damage something. \$\endgroup\$
    – Mitch
    Apr 13 '21 at 12:56
  • \$\begingroup\$ Inductive load is probably the wrong term: spinning loads? Flywheel loads? Synchronous motor loads? Not sure. \$\endgroup\$
    – Mitch
    Apr 13 '21 at 13:04

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