Backup Power Transfer Problems

How a Transfer Works

1. The utility fails. The UPS carries the critical load from its battery. A double-conversion (online) UPS does this with no break at the output, since its inverter feeds the load all the time. A standby or line-interactive UPS switches to its inverter with a break of a few milliseconds.
2. The generator starts. The ATS senses undervoltage or loss of a phase, waits out a short delay so that momentary dips do not start the generator, and signals the start. NFPA 110 classifies a system by how fast it must restore power: within 10 seconds for Type 10, the class hospital essential electrical systems use, and within 60 or 120 seconds for Types 60 and 120.
3. The ATS transfers the load. An open-transition switch breaks from the utility before it makes to the generator. A delayed-transition switch holds a timed neutral position in between, so that motor residual voltage can decay. An in-phase monitor waits until the two sources are in sync, close in frequency and phase, before it transfers.
4. The load steps on. Priority loads connect in blocks, and the UPS rectifier walks its load onto the generator over several seconds. Each step dips the generator’s frequency and voltage until the governor and voltage regulator recover.
5. The utility returns. After a retransfer delay, the ATS returns the load to the utility. An open-transition retransfer is a second short outage. The critical load rides through on the UPS, but loads outside it, including cooling, are interrupted, and many must restart. A closed-transition switch parallels the two sources briefly, typically for 100 ms or less, and can do so only when they are in sync.

Throughout, a double-conversion UPS keeps its inverter in sync with its bypass source, so that on an overload, an output fault, or an inverter alarm it can move the load onto the bypass through a static switch with no break. Each transition made without a break depends on sync in the same way: the two sources must match in frequency and phase at the moment the load moves.

Why Transfers Fail

The ATS Fails to Transfer, or Transfers Late

Lost control power, a timer set wrong, a blown sensing fuse, worn contacts, and mechanical faults all leave the load without power for longer than the design intended. The ATS log records what its controller decided. It does not show when each source was actually available or when the load actually saw voltage.

Motors Reconnected Out of Phase

A motor keeps generating a decaying voltage after it is disconnected. Reconnected to a source that is out of phase with that residual voltage, the difference between the two can approach twice rated voltage. The inrush current and torque transient that follow trip breakers and drives, and they can damage couplings and shafts.

Frequency and Voltage Dips on Load Steps

A generator is a far weaker source than the utility. A block load step, especially of motors or of a UPS walking in, dips its frequency and voltage. The generator’s performance class under ISO 8528-5 sets how far and for how long. Drives trip, contactors drop out, and UPS rectifiers reject the source if the dip exceeds what they tolerate. Commissioning on a resistive load bank does not reproduce the dynamics of the facility’s real loads.

The UPS Rejects the Generator

A UPS rectifier qualifies its input on voltage, frequency, and rate of change of frequency. Rectifiers and drives draw constant power, so their current rises as voltage falls, and that negative incremental resistance can destabilize the generator’s voltage regulator while its governor hunts against the rectifier’s walk-in. A generator bus that oscillates, or whose voltage is distorted by the rectifier’s own harmonic current, falls outside the rectifier’s limits. The UPS returns to battery, which unloads the generator. The generator recovers, the UPS walks back on, and the cycle repeats until the battery runs down.

At light load, the UPS input filter can add to the problem. Its capacitance presents a leading power factor, which a generator’s voltage regulator handles poorly and its capability curve limits. The resulting rise or swing in voltage is one more reason for the UPS to reject the source.

The UPS Loses Sync With Its Bypass

A double-conversion inverter tracks its bypass source in frequency and phase, so that a transfer to bypass needs no break. When the bypass source drifts outside the UPS’s synchronization window, or changes frequency faster than its slew-rate limit, the inverter runs on its own clock and the UPS reports that it is out of sync. A transfer to bypass in that state is either blocked or made as an interrupted transfer, with a dead time at the output. The ITIC (CBEMA) curve expects IT equipment to ride through 20 ms at zero voltage, and many power supplies have less margin than that at full load.

On generator, this is the common case. The generator’s frequency wanders more than the utility’s and swings with every load step, so the UPS loses sync at exactly the moment a bypass transfer is most likely to be needed.

Harmonic Distortion on Generator

A generator’s subtransient reactance is several times the impedance of a typical utility source, so the same harmonic current from rectifiers and drives produces several times the voltage distortion. Distortion and commutation notching can upset the generator’s voltage sensing and the loads’ own zero-crossing detection, so equipment that runs well on utility misbehaves on the generator.

The Battery Cannot Carry the Step

An aged battery string or a loose or corroded connection can hold its float voltage and still collapse under the step to full discharge current. A ride-through that should last minutes then ends in a low-DC shutdown within seconds, before the generator is ready. Only a load test or the record of an actual discharge reveals it.

Ground-Fault Trips on Transfer

When the ATS does not switch the neutral but the generator is still bonded as if it were a separately derived source, neutral current can return through more than one path. Ground-fault protection then sees an imbalance and trips, often only during or after a transfer.

The Retransfer Fails

A closed-transition retransfer needs the generator and the utility within a sync window. A generator that hunts, or a utility that is still disturbed, can delay the retransfer or force an open transition. The facility then takes a second outage after it has already ridden through the first.

The Transfer Should Never Have Happened

Many UPS transfers to battery respond to sags the load would have ridden through, because the input tolerance is set tighter than the load requires. Each one cycles the battery and shortens its life. Comparing the depth and duration of every input sag with the transfers it caused shows whether the settings match the site.

What the Evidence Has to Show

Explaining a transfer takes voltage and current at every link (the ATS sources and load side, and the UPS input, output, and bypass), through the event and the cycles around it, on one timebase:

• The phase angle between the sources, and the motor residual voltage, at the moment of transfer
• The transfer gap and the UPS output dead time, to the millisecond
• The generator’s frequency and voltage through each load step and each UPS walk-in, and the time each took to recover
• The frequency and phase of the bypass source against the UPS inverter before a bypass transfer
• The distortion on the generator bus compared with the same loads on utility
• The cycles before the event, which show whether it was building

The records most sites hold fall short of that:

• The UPS and ATS event logs timestamp each device’s own decisions. They do not show what the sources or the output voltage did.
• A power monitoring system reports RMS values every second or every few seconds. A 10 ms dead time or a sub-cycle phase step disappears into the average.
• A triggered power quality meter catches an event only if it is on the right circuit and its threshold fires. The precursors fall before its pre-trigger window, and the window often ends before the retransfer or the generator’s recovery.
• Monthly tests and commissioning load-bank runs prove the system on the day of the test, under that day’s load and conditions.

How Energy Quotient Records a Transfer

EQ Wave™ is a Waveform Measurement Unit (WMU) that records Continuous Point-on-Wave (CPOW) data at 32 ksps on every voltage and current channel, gap-free, with no trigger to set. Placed on the ATS sources and load side and at the UPS input, output, and bypass, it records every transfer, load step, and retransfer in full, including the ones nobody expected. EQ Coherence™ keeps weeks of that record on your premises, beside UPS status and battery telemetry on the same timeline. EQ Syntropy™ relates events across the chain, tying a generator frequency excursion to the out-of-sync alarm it caused, and explains them with the waveforms attached.

The record gives the transfer gap to the sample, the phase difference at the moment of transfer, and every cycle before and after it. Last month’s trip can be examined cycle by cycle, without waiting for it to happen again: whether it followed an out-of-phase transfer, which load step pulled the frequency down, or why the UPS rejected the generator.