Network and point of common coupling
The source impedance decides two things: how much short-circuit current is available at the (which picks the row of IEEE 519), and how much voltage distortion each harmonic current creates. A soft source, like a standby generator, turns the same harmonic currents into much more voltage distortion.
Supply
Nameplate rating of the transformer feeding the LV main board. Typical distribution units: 1000, 1500, 2000, 2500 kVA.
Short-circuit impedance from the nameplate or test certificate. 5 to 6 % is common for 1 to 2.5 MVA distribution transformers. Use the guaranteed value, not the tolerance band.
Ratio of reactance to resistance. Only the reactance X creates harmonic voltage in this model, so X/R splits %Z into X and R. Leave at the default if you do not have the test report.
The MV network has its own impedance. Adding it lowers Isc slightly and raises THDv slightly. Ask the utility for the maximum fault level at the MV feeder. "No" assumes an infinitely stiff network, which overstates Isc.
The utility's minimum fault level (for example with an MV feeder or primary transformer out). Used in the scenario table below: lower fault level means less Isc and more voltage distortion.
Many buildings have two (or more) transformers feeding one main board through a bus-tie. In parallel their impedances combine like resistors in parallel, so two equal units halve the source impedance and double Isc.
Closed: all units share one busbar (highest Isc). Open: the board is split into sections, each fed by its own transformer, so each section sees the Isc of one unit and carries only part of the load. Utilities and protection studies often require the tie normally open; check both.
Prime or standby rating of the alternator (kVA, not kWe). With several sets in parallel, enter the total of the sets that will be running.
From the alternator data sheet, on the alternator's own kVA base. Typical values sit around 10 to 20 %. Some engineers use the negative-sequence reactance X2 for harmonics; it is usually close to Xd''.
Maximum demand current IL
IEEE 519 uses IL, the maximum demand load current (fundamental) at the PCC under normal operating conditions. For a new building, use the design maximum demand. For an existing site, use the average of the monthly peak demand over the past year. IEEE 519-2022 adds that where 12 months of data are not available, IL is based on the projected 15 or 30 minute demand.
Normal balanced loads make only odd harmonics. Even harmonics come from asymmetric or faulty operation: arc furnaces, half-controlled rectifiers, transformer inrush, a failed thyristor. IEEE 519-2022 limits even orders to 50 % of the Table 2 value up to the 6th and 100 % above the 6th.
Arithmetic adds every load's harmonic currents as if in phase, which is the worst case. The IEC 61000-3-6 general summation law uses exponent 1 below the 5th, 1.4 from the 5th to the 10th and 2 above the 10th, allowing for some phase diversity between different loads.
Operating scenarios: how switching changes the answer
The same building can see very different short-circuit levels as transformers, the bus-tie and the generator switch. Each row is a full recalculation with your load mix. The current limits move with Isc/IL, and THDv rises as Isc falls.
Single line: where the PCC sits
The PCC is taken at the LV main switchboard (transformer secondary). If the utility defines the PCC on the MV side, refer the harmonic currents through the transformer ratio, use the MV fault level and the MV row of Table 1, and confirm the authority's current requirements.
Load builder
Add each group of non-linear and linear loads at the PCC. Power is the electrical input power at maximum demand. Fundamental current is calculated from power and displacement power factor (DPF).
Loads at the PCC
Spectrum
Results against IEEE 519-2022
Current distortion () is checked against Table 2 (by the Isc/IL row) as a percentage of IL. Voltage distortion () is checked against Table 1 by the PCC bus voltage. Odd harmonics from the 3rd to the 49th are modelled.
Compliance summary
Spectrum against the limit
3D harmonic landscape
Each column is one harmonic current. Left to right: harmonic order. Front to back: each load (or before and after the filter). The amber glass bars show the Table 2 limit for each order in amps. Tap a column to read it.
Harmonic table
IEEE 519-2022 limits used
IEEE 519 assesses compliance statistically from measurements over time (short-time and very-short-time percentile values), so a design calculation like this one is a screening check. Even harmonics (when switched on in tab 1) are limited to 50 % of the Table 2 value for h ≤ 6 and 100 % for h > 6, as revised in the 2022 edition. Interharmonics are assessed separately below. Utilities may set their own limits: confirm the authority's current requirements.
Interharmonics (optional)
An is a current at a frequency that is not a whole multiple of the fundamental. They come from cycloconverters, arc furnaces, drives with small DC links and some inverters, and can cause light flicker even at small levels. Enter each component from a measurement or the manufacturer. IEEE 519-2022 Annex A gives recommended, non-prescriptive interharmonic voltage limits based on LED flicker sensitivity; enter the limit agreed for your project.
Active harmonic filter sizing
The has to inject the harmonic current you want to take out of the supply. Each harmonic is first brought down to its own Table 2 limit, then the whole set is scaled down further if needed to reach the target TDD. The RMS of everything removed is the compensation current the filter must deliver.
Sizing inputs
Set this at or below the Table 2 TDD limit for your Isc/IL row. Designing a little below the limit leaves headroom for load growth and spectrum uncertainty.
Allowance for future load, spectrum uncertainty and derating for ambient temperature or altitude. 20 to 30 % is a common starting point; check the manufacturer's derating rules.
The hottest air temperature the filter will breathe, not the outdoor design temperature. GCC electrical rooms are often designed for 40 to 45 °C, and higher if cooling fails. Active filters are rated at a reference ambient and lose output above it.
How much output the filter loses for every degree above its rated ambient. The default is a typical figure only: take the actual curve from the manufacturer's datasheet, which may also set a maximum ambient.
Thinner air cools less, so power electronics lose output at altitude. Most GCC coastal sites are well below 1000 m and need no altitude derating. The default rate is typical only; use the manufacturer's figure.
Before and after
Harmonic currents removed by the filter
Detuned capacitor bank and resonance check
Power factor capacitors and the source inductance form a parallel circuit. If its resonant order lines up with a harmonic the loads produce, the bank amplifies that harmonic. A series reactor () moves the series tuning point below the lowest significant harmonic so the bank looks inductive to the harmonics.
Bank inputs
p is the reactor reactance as a percentage of the capacitor reactance at fundamental frequency. 7 % tunes near the 3.8th harmonic (for networks dominated by the 5th and above). 14 % tunes near the 2.7th harmonic (used where there is significant 3rd harmonic, for example lots of single-phase LED and IT load).
A series reactor raises the voltage across the capacitor to U/(1-p), plus harmonic voltage on top. Detuned banks use capacitors rated above system voltage. These are typical catalogue voltages; check the manufacturer range.
Resonance check
Impedance seen from the PCC against harmonic order
Normalised to the source reactance at fundamental, log scale. The peak marks the parallel resonance. Small resistances are included only to keep the peak finite.
Branch behaviour at each harmonic
Front-end options compared
Every row in your load mix is swapped for each drive front end in turn, with the rest of the loads unchanged. The distortion figures are calculated; the cost, size and loss columns are general relative guidance only.
TDD by option
Waveform: the filter switching on
The supply current is built from the spectrum calculated in tab 3. Switch the active filter on and it injects the opposite of the harmonics it is sized to remove, leaving a cleaner current upstream of the PCC.
Illustrative waveform synthesised from the calculated harmonic magnitudes. Real phase angles depend on the loads, so the exact shape will differ; the RMS values and THD readout are exact for the spectrum shown.
Voltage at the PCC
Voltage distortion follows the current: Vh = Ih × h × Xsource. With the filter on, the remaining harmonic current creates less harmonic voltage.
4D day replay: distortion through 24 hours
The fourth dimension is time. Loads do not run flat out all day, so the harmonic picture changes hour by hour. Press play to replay a day: each hour is recalculated with the loads scaled by a profile. Watch how can rise at light load while falls, because TDD is always divided by the same maximum demand current IL.
Load profiles used (illustrative, edit to suit)
Profiles are typical shapes for a mixed-use building, shown as a percentage of each load's maximum demand. They are illustrative only. For an existing site, a power quality logger over at least a week gives the real picture.
Worked example and formula register
The working below uses the inputs currently set in tabs 1 and 2, so it always matches the results. Change an input and the steps update.
Step-by-step working
Formula register
Engineer's notes
Practical points from design review and site work. They are guidance for asking the right questions, not a substitute for a harmonic study.
Knowledge check
Thirteen questions on the ideas in this lab. Pick an answer to see whether it is right and why.
Answer all thirteen to get your result.