On 06/26/2011 05:43 PM, Ingo Flaschberger wrote:
2) Allowing transformer fields to collapse. Even in phase, without a delayed transition ATS you can end up with a partially collapsed transformer field with a new field being created at non-ground state. This can cause a transient back wave that can snap circuit breakers. Yep, this one happened to us a few times before we switched to a delayed ATS, was a PITA to debug and resolve.
The collapse can take more than one cycle, especially in a 3-phase transformer... As can the startup transient. In our case, if a big thunderstorm came, we started one generator ahead of time and synced it to the line, then did a 0-delay (overlap) switch. Avoids this problem, though the generator had better be a bunch bigger than the load to bring this off. Short delay ATS (less than 1sec) is a disaster, though. If you always wait till the power actually fails before starting the gen, you should wait 5-10 secs (or more) before putting the load on it, at least if there are any motors involved. (our generators were 1mw brushless ones that took 10-15secs for the voltage to come up anyhow...) And HVAC compressors have their own problems; once fully stopped you have to wait for the liquid to clear the compressor before restarting, or have LOTS of torque (like a car unit) available (and a supply of new belts :-)
a transformer should be switched to the network when phase is at highest/lowest point, not at zero. zero: highist current highest/lowest point: lowest current because it's a coil.
It isn't this simple... Switching on involves transients that overwhelm the sinusoidal waveform for a few cycles. Also the above is only strictly true for an unloaded transformer; if it has a matched resistive load the current and voltage are (mostly) in phase. (leakage inductance notwithstanding, though it isn't very high for high-power transformers.) Unfortunately most larger feed transformers are integrated 3-phase units where the current-voltage curves are shared between windings and get much more complicated, and there is no time (in normal operation) when all the voltages or currents are 0. Also for high enough ratios the interwinding capacitance is important (480 to 120/208 isn't high for this purpose; 8 or 16kv to 120/208/240 is). Big data centers (most bigger buildings) have 2 stages of transformers, one from "distribution" (8, 16, 20, 34kv) to 480 and a transformer per floor from 480 to whatever. Sometimes the generator is between these two and the UPSs are after the floor transformer(s). Big enough UPSs run at 480 so are between also. To complicate things even more, (modern) computers look kind-of capacitive, modern fluorescent lighting (electronic ballasts) looks kind-of capacitive, and motors and most other loads, and older lighting look inductive. That means it is hard to predict how to switch loads; either no-delay (need to sync the generator, though) or lots-of-delay operation is safer; short delay isn't. (and synchronous motors can actually look capacitive if they have enough of a flywheel on them, but the startup transient for a non-VF drive can be a killer.) BTW - in reply to a misconception long before in this thread, 3-phase sync motors self-start easily, and most older single-phase clock motors had enough of a shaded-pole to start in induction-motor mode then transition to sync once close to speed. The means to do this were subtle; sometimes it involved clever multilayer plating on the rotor and/or very clever shaping of the holes in the rotor. What I mean by kind-of capacitive is a bit odd; it looks capacitive on the voltage rise but resistive and/or a little inductive on the peak and fall, and the current is 0 when the voltage is below some threshold. Actually these days the feds (and I believe EC also) spec power-factor correction for switching power supplies so this effect is less. The main way this is arranged is to make sure the input-rectifier filter capacitors are SMALL enough; then have the switcher waveform compensate for the voltage droop. Bigger VF motor controllers do this also. -- Pete
Kind regards, Ingo Flaschberger