Power quality management: Engineering boundaries of voltage sags, UPS, and critical process continuity ( part 2)

2. UPS capacity is not simply added together from the nameplate values, but is calculated based on “energy deficit + dynamic peak capacity + end-of-life margin”.

During short-term disturbances, the energy required by critical loads is provided by the UPS, DC bus capacitors, flywheel, BESS, DVR, or the tool’s internal ride-through module. The most basic energy relationship is:

Where is the critical load power, is the required support time, and is the conversion efficiency. If relying on DC bus capacitor support, the approximate calculation is as follows:

This explains why many tools’ internal power supplies can handle 200 ms-level sags, but require fab-level support for 1-second-level deep sags. Capacitor energy storage varies with the square of the voltage, and the remaining energy decreases rapidly during deep sags; at the same time, the dynamic peak values of VFD, RF, vacuum pumps, and servo systems are often not covered by steady-state power.

3. Voltage sags can also be amplified by system impedance, transformer wiring, and tool rectifiers.

The voltage drop of a three-phase low-voltage feeder can be quickly estimated using the following formula:

The lower the short-circuit capacity, the longer the feeder, and the more synchronously started loads, the worse the voltage recovery observed at the tool terminal. A common misconception in advanced fabs is that they only check voltage quality at the PCC or the low-voltage side of the main transformer, without simultaneously sampling at the PDU , tool power box, UPS output, DVR output, and critical VFD input. What truly causes tool trips is often not the sag depth at the plant entrance, but the residual waveform after passing through the bus, STS, PDU, harmonic filters, and isolation transformer.

2. Fab power quality systems must be stratified by load consequence, not by equipment price.

An advanced fab’s power system typically consists of external dual/multi-circuit incoming lines, a main substation, a medium-voltage ring network or main/standby busbars, low-voltage distribution, UPS/DVR/STS, PDUs, tool hook-ups, power monitoring, and FMCS/SCADA. The design focus is not on connecting all loads to the UPS, but rather on determining the engineering status of each type of load during disturbances.

1. Critical load grading: First define “what can be lost,” then decide “what to protect.”

2. Sub-Fab Interface: Power disturbances can have a reverse impact on the main Fab via the utility.

Many power-related incidents ultimately result in losses not originating from the electrical system itself, but rather from the transmission of sub-Fab utility instability to the tooling. For example, a PCW pump speed drop can cause localized temperature fluctuations, a scrubber control power interruption can lead to exhaust interlocks, a CDA dryer or N2 purge valve assembly reset can result in compromised tool safety conditions, and a short-term FFU/MAU speed drop can alter the differential pressure gradient. Fab-level designs must place the power system and utility system within the same event tree.