Power infrastructure
Power quality is how closely the supply matches a clean sine wave at the right voltage and frequency. Harmonics are distortion added by non-linear loads such as rectifiers and switch-mode power supplies. A double-conversion UPS isolates the IT load from most of it, but the UPS input, the generator and the rest of the building still see it, and a survey with a power quality meter shows where it comes from.
A voltage within set limits, not a perfect waveform. Hydro-Québec states that its distribution system supplies power within the limits recommended in standard CAN3-C235-83, and asks customers to call if a 120-V supply is not between 110 V and 125 V; at medium voltage, the limits at the connection point are ±6% of rated voltage, usually 25 kV. Within those limits, short disturbances still occur, and the utility notes that waveform distortions are very often caused by high-power equipment in use on the grid. A data-centre UPS is designed to ride through what the utility supplies; the survey shows whether the site's own loads add more.
Currents and voltages at multiples of the supply frequency, added on top of the fundamental. Loads that draw current in pulses rather than smoothly, such as rectifiers and the switch-mode power supplies in servers, create them. Harmonic current flowing through the wiring and transformers causes extra heating and distorts the voltage seen by other equipment on the same source. Cummins lists UPS rectifiers among the data-centre loads that create harmonics, and the IT power supplies among the loads that draw a leading power factor, both of which matter when a generator carries the room.
It cleans its output and, in a modern design, draws a clean input. A double-conversion UPS rebuilds its output from DC, so harmonics on the supply do not reach the IT load; how that works is covered in how a data-centre UPS works. On the input side, Vertiv specifies its Liebert EXM at an input current distortion of less than 5%. Vertiv also explains why a reading can be higher on site: when the voltage source already carries distortion from other non-linear loads on the same supply, the UPS input current can show high distortion that the rectifier did not create. The measurement has to separate what the UPS adds from what it receives.
Because a generator is a weaker source than the utility, and distortion that the grid absorbs can upset it. Cummins notes that data-centre loads differ from building loads in power factor, in non-linear loads leading to harmonic voltage distortion, and in how they accept load steps. It flags light kW load with high leading reactive power as a risk for the generator, and recommends a UPS's generator mode where available. A power quality reading taken on generator power, during a monthly run or a load test, shows how the UPS and the generator behave together; see generator maintenance.
With a recording meter left on the circuit long enough to catch real events. Eaton's published scope of work for UPS performance testing uses a power quality meter to record voltages, currents and harmonic content at the output of the UPS while it is loaded, with any irregularities noted in the report; the same meter at the UPS input and at the main switchgear shows what the site supplies.
Readings over several days matter because many problems are intermittent: a sag when a large load starts, distortion that only appears when a particular system runs, or a swing on generator power that never shows on the utility. A short spot check misses all of them. The survey is often paired with an infrared scan, which finds the hot connections that harmonic heating makes worse. The thermography and power quality service describes how both are delivered.
It depends on the source, which is why the survey comes first. Hydro-Québec notes that solutions must be assessed case by case by a specialist, a master electrician or engineer, weighing cost against the effect on operations. Typical options range from moving or separating the loads that cause distortion, to harmonic filters, to replacing older rectifier designs. On the UPS side, an older UPS with a high-distortion input is often the largest harmonic source in the room, which is one of the questions a UPS modernization plan answers.
Yes. A power quality survey is quoted with the UPS and generator work, so the readings can be taken on utility and on generator power in the same programme. Send the site, the UPS make and model and any symptoms you have seen through the quote request form; for data-centre UPS service, see UPS maintenance and repair for data centres.
Hydro-Québec asks customers to call if it is not between 110 V and 125 V, per standard CAN3-C235-83.
A double-conversion UPS rebuilds its output, so supply harmonics do not reach the IT load; its own input can still draw distorted current, which modern rectifiers keep low.
Vertiv notes that other non-linear loads on the same source can distort the voltage, so the UPS input current shows distortion it did not create.
At the main switchgear, the UPS input and the UPS output, with a recording meter, on utility and on generator power.
Sources, checked 2026-10-04:
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