Compare UPS Input Harmonics at Matched Load Points

Highidea's online UPS routes are selected as part of a source-to-load system. A controlled harmonic-data request helps the electrical team compare the offered input behavior under the task the site actually needs.

Identify the quantity before the percentage

Harmonics are components at integer multiples of the fundamental frequency. Their combined magnitude is not generally the arithmetic sum of individual percentages. Keep the instrument's definition, included orders and calculation method with its result.

Names and instrument settings matter. A report using a different normalization must not be silently relabeled to fit a comparison table.

A hypothetical denominator effect

For a simplified illustrative comparison, assume the combined included harmonic-current RMS magnitude is 2 A. At a 20 A fundamental current, the ratio is 2/20 × 100% = 10%. At a 5 A fundamental current, the same assumed harmonic magnitude gives 2/5 × 100% = 40%.

The larger percentage in this example does not mean the absolute harmonic current increased. Real equipment can change both quantities with load; retain the measurements rather than assuming a constant numerator. No value here is a permitted limit or Highidea test result.

This is why two offers measured at different load levels cannot be ranked from the percentages alone.

Match the operating point

Request the same output load fraction and relevant load type, input voltage/frequency and source arrangement, battery charging condition, UPS mode and environmental conditions. Record the measuring location and instrument/method, observation interval and relevant uncertainty.

Charging after an outage can change the input task from the fully charged condition. Likewise, a source with different characteristics can affect the system measurement. Have the responsible electrical team specify the comparison points needed; there is no universal test grid in this article.

Fluke's harmonics application note distinguishes distortion quantities and emphasizes measurement methods and points of common coupling. Its 2015 example limit tables are not used here as current project criteria. Applicable standards, editions and site limits must be determined separately.

Issue one supplier reply schedule

For each requested point, ask for exact model/configuration, output load W/VA and fraction, mode, charging state, source conditions, current/voltage metric and denominator, spectrum or absolute values where required, method/location and evidence reference.

Keep deviations explicit. A missing condition is a clarification item, not a zero value. Where existing data do not cover the requested task, agree appropriate further evidence rather than copying another product's specification.

Electrical connection and measurement work require qualified personnel and an approved safe plan. Do not introduce harmonic loads or alter production input conditions simply to fill the schedule.

Return the result to the system design

The electrical designer reviews the matched data against the actual source and distribution requirements. UPS input data alone cannot size a generator or approve the whole installation; see our generator-system checklist.

Send Highidea the requested operating points and evidence format with the load/source brief so the proposal can address a comparable requirement.

Technical reference

• Fluke: Harmonics Standards application note, 2015, hosted by Transcat.