Quick Generator THD Screening Calculator — Estimate Distortion from Nonlinear Load %

This article provides a rapid THD screening calculator methodology for nonlinear load distortion and measurement.

Engineers can estimate harmonic content quickly to prioritize mitigation or detailed analysis and allocate resources.

Quick Generator THD Screening Calculator – Estimate Voltage Distortion from Nonlinear Load

Generator rated apparent power (kVA)

Generator line-to-line voltage (V)

Nonlinear load apparent power (kVA)

Estimated nonlinear-load current THD (%)

Advanced options

Generator short-circuit ratio Isc/Irated (pu) 4 (typical) 3 5 6 Custom

Custom Isc/Irated (pu)

Harmonic attenuation factor (–) 1 accounts for possible resonances (rare, use with caution)." >

Voltage THD planning limit (%)

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Enter generator and nonlinear-load data to estimate voltage THD at the generator terminals.

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Calculation method and formulas (approximate screening model)

This calculator provides a quick screening estimate of voltage total harmonic distortion (VTHD) at a generator bus caused by a nonlinear three-phase load. It assumes a stiff fundamental voltage and uses the generator short-circuit strength versus the nonlinear load current.

• Generator rated line current:
  Generator_I_rated (A) = S_gen (kVA) × 1000 / (sqrt(3) × V_ll (V)
• Generator short-circuit current (initial symmetrical):
  I_sc (A) = Isc_over_Irated (pu) × Generator_I_rated (A)
• Nonlinear load fundamental current:
  I_load (A) = S_nonlinear (kVA) × 1000 / (sqrt(3) × V_ll (V)
• Nonlinear load RMS harmonic current:
  I_harm_rms (A) = I_load (A) × THD_I (%) / 100
• Approximate bus voltage THD (screening):
  V_THD_est (%) ≈ [I_harm_rms / I_sc] × 100 × Attenuation_factor
• Generator short-circuit strength relative to nonlinear load:
  Isc_over_Iload = I_sc / I_load

This is a simplified screening approach: it treats all harmonics as lumped RMS current and uses the generator short-circuit current as a proxy for source impedance. It is suitable to quickly check if a detailed harmonic study may be needed; it is not a replacement for a full frequency-domain harmonic analysis.

Item	Typical value / guideline	Comment
LV generator Isc/Irated	3–6 pu	Based on subtransient reactance; larger units often stronger (higher ratio).
6‑pulse VFD input current THD	30–45%	Unfiltered, at rated load, with stiff supply.
12‑pulse VFD input current THD	8–15%	Phase-shifting transformer.
Active-front-end drive current THD	3–8%	Depends on tuning and filters.
Planning limit for voltage THD at LV	5% VTHD	Common planning target aligned with IEEE 519 for LV systems.
Rule-of-thumb Isc/Iload for nonlinear loads	> 20	Typically gives acceptable voltage distortion for many loads.

Technical FAQ – Quick generator THD screening

Does this calculator replace a detailed harmonic study?

No. It is a rapid screening tool based on simplified relationships between nonlinear load current, generator short-circuit strength, and resulting voltage distortion. For critical installations, high harmonic levels, or multiple harmonic sources, a detailed harmonic study with frequency-dependent models is recommended.

What generator short-circuit ratio (Isc/Irated) should I use?

If a manufacturer value is available (e.g. based on subtransient reactance Xd''), use that value. For many LV diesel generators a range of 3–6 pu is common. Larger, more robust machines may have higher values. When uncertain, using 3–4 pu is conservative for screening.

How should I estimate the nonlinear-load current THD (%)?

Use manufacturer data, test reports, or typical values for the technology: 30–45% for standard 6‑pulse drives, 8–15% for 12‑pulse drives, and 5–10% for active-front-end or well-filtered rectifiers at nominal load. For mixed nonlinear loads, a conservative approach is to use the highest dominant THD source.

When should I be concerned about the estimated voltage THD result?

If the estimated voltage THD approaches or exceeds your planning limit (e.g. 5% at LV) or if Isc/Iload is relatively low (e.g. below 20), it is advisable to perform a more detailed harmonic analysis and consider mitigation options such as filters, derating, or stronger generator capacity.

Fundamental concept and screening objective

Screening for total harmonic distortion (THD) aims to estimate voltage distortion produced by nonlinear loads quickly. The goal is to determine whether detailed harmonic analysis or mitigation is required based on readily available measurements and reasonable network assumptions.

Definitions and units

• Fundamental RMS voltage (V1): RMS magnitude of the fundamental frequency component, typically 50 Hz or 60 Hz, measured in volts (V).
• Harmonic current Ih: RMS current of harmonic order h (h = 2, 3, 4, ...), measured in amperes (A).
• Source impedance Zs: The equivalent Thevenin impedance seen by the load at the point of common coupling (PCC), magnitude in ohms (Ω).
• Voltage harmonic Vh: RMS contribution of a given harmonic order to voltage at PCC, in volts (V).
• THD (voltage): Total harmonic distortion of voltage, defined relative to fundamental V1, often expressed as a percentage (%).

Key formulas for quick THD screening

All formulas below are presented using plain characters and arithmetic operators to ensure readability and implementability in simple calculators.

Voltage harmonic contribution from harmonic current

For each harmonic order h, approximate the voltage harmonic as:

where:

• Vh = voltage contribution at harmonic h (RMS, V)
• Ih = load harmonic current at order h (RMS, A)
• |Zs_h| = magnitude of source impedance at harmonic h (Ω)

Typical assumption for screening: use magnitude |Zs_h| approximated by short-circuit ratio (SCR) or percentage impedance converted to ohms at the system nominal voltage.

Total harmonic distortion (voltage) approximate formula

Compute voltage THD using RMS sum of harmonic voltages relative to fundamental:

Expressed as percentage:

where V1 is the measured or expected fundamental RMS voltage.

Alternate formulation using common impedance model

If source impedance is approximately constant across harmonic orders for screening (|Zs_h| ≈ |Zs|):

Note: sqrt( I2^2 + I3^2 + ... + In^2 ) is the current THD numerator (harmonic current RMS).

Determining source impedance for screening

Accurate Zs can be difficult to obtain quickly. Use practical approximations:

1. Derive from short-circuit current at PCC: |Zs| = V1 / ISC (magnitudes consistent).
2. Use typical X/R ratios and convert percent impedance to ohms for distribution systems.
3. Apply conservative bounds: use worst-case higher |Zs| (weaker grid) for conservative screening.

Example conversions and typical values

Common system voltages and conversion from percent impedance (%Z) to ohms:

Notes:

• Exact conversion depends on base apparent power chosen. Above values illustrate order-of-magnitude behavior for screening.
• For single-phase screening at 230 V, convert orderly by using V1 = 230 V in formulas.

Typical harmonic current spectra for common nonlinear loads

For screening, use representative harmonic current spectra per load type. These are normalized to the load fundamental current (I1) and expressed as percentage of I1.

These percentages are typical normalized harmonic current magnitudes used for screening. Convert to absolute Ih by multiplying by measured or estimated I1.

Step-by-step screening calculator workflow

1. Gather inputs:
   • V1 (fundamental voltage RMS at PCC)
   • I1 (fundamental RMS current of the nonlinear load) or rated load current
   • Representative harmonic current spectrum (Ih/I1 for h = 2..n)
   • Estimate of source impedance |Zs_h| for relevant harmonic orders or a simplified constant |Zs| value
2. Compute Ih = (Ih/I1) * I1 for each harmonic order.
3. Compute Vh = Ih * |Zs_h| for each harmonic order.
4. Compute THD_V = sqrt(sum(Vh^2)) / V1 and convert to percent.
5. Compare THD_V(%) to guideline thresholds (e.g., IEEE 519, EN 50160) to determine pass/fail or need for mitigation.

Real-world example 1: Single industrial VFD on LV feeder

Scenario:

• 400 V three-phase network, fundamental V1 = 400 V (line-to-line), consider phase voltage V1_phase = 230 V for per-phase calculation if modeling phase-wise.
• VFD rated current I1 = 100 A (fundamental RMS per phase for a 3-phase VFD).
• Harmonic spectrum approximated from table for 6-pulse VFD.
• Assume medium-strength feeder with |Zs| ≈ 0.2 Ω at relevant harmonics (conservative).

Step 1: Convert normalized Ih/I1 to absolute Ih (per phase) using data from table. Use phase-to-neutral fundamental V1_phase = 230 V for voltage comparison.

Assumed harmonic percentages for VFD (6-pulse) from table:

• I2 = 2% of I1 -> I2 = 0.02 * 100 A = 2 A
• I3 = 40% -> I3 = 0.40 * 100 A = 40 A
• I5 = 60% -> I5 = 0.60 * 100 A = 60 A
• I7 = 30% -> I7 = 0.30 * 100 A = 30 A
• I9 = 15% -> I9 = 15 A
• I11 = 8% -> I11 = 8 A
• I13 = 4% -> I13 = 4 A

Step 2: Compute Vh = Ih * |Zs| (using |Zs| = 0.2 Ω constant approximate). Use phase voltage basis.

• V2 = 2 A * 0.2 Ω = 0.4 V
• V3 = 40 A * 0.2 Ω = 8.0 V
• V5 = 60 A * 0.2 Ω = 12.0 V
• V7 = 30 A * 0.2 Ω = 6.0 V
• V9 = 15 A * 0.2 Ω = 3.0 V
• V11 = 8 A * 0.2 Ω = 1.6 V
• V13 = 4 A * 0.2 Ω = 0.8 V

Step 3: Compute RMS sum of harmonic voltages:

Compute values:

• 0.4^2 = 0.16
• 8.0^2 = 64
• 12.0^2 = 144
• 6.0^2 = 36
• 3.0^2 = 9
• 1.6^2 = 2.56
• 0.8^2 = 0.64

Assessment:

• THD_V ≈ 6.96% at PCC due to this single VFD and assumed feeder impedance.
• For many networks, voltage THD below 8% is within common distribution tolerances, but IEEE 519 recommends voltage THD limits depending on system voltage and connection. Verify against local limits.
• If the feeder were weaker (higher |Zs|), THD would increase proportionally. If |Zs| = 0.5 Ω, THD_V scales by 2.5x -> ~17.4% (unacceptable without mitigation).

Real-world example 2: Office building with mixed nonlinear loads

Scenario:

• 230 V single-phase distribution for floors, V1 = 230 V.
• Aggregate fundamental current I1_total = 150 A (combined loads) at peak daytime.
• Load mix: 60% SMPS/PCs, 30% LED lighting, 10% small VFDs.
• Assume representative harmonic spectra from table and use weighted average spectrum for aggregate.
• Assume relatively stiff supply with |Zs| = 0.05 Ω at harmonics (strong distribution).

Step 1: Build weighted harmonic percentages (per phase) for aggregate:

Compute weighted percentage for each harmonic order h as 0.6*(SMPS%) + 0.3*(LED%) + 0.1*(VFD%). Use example numbers from the typical table.

• I3 = 0.064 * 150 A = 9.6 A
• I5 = 0.174 * 150 A = 26.1 A
• I7 = 0.093 * 150 A = 13.95 A
• I9 = 0.045 * 150 A = 6.75 A
• I11 = 0.023 * 150 A = 3.45 A
• I13 = 0.0115 * 150 A = 1.725 A

• V3 = 9.6 * 0.05 = 0.48 V
• V5 = 26.1 * 0.05 = 1.305 V
• V7 = 13.95 * 0.05 = 0.6975 V
• V9 = 6.75 * 0.05 = 0.3375 V
• V11 = 3.45 * 0.05 = 0.1725 V
• V13 = 1.725 * 0.05 = 0.08625 V

Step 4: RMS sum and THD calculation:

• 0.48^2 = 0.2304
• 1.305^2 = 1.7030
• 0.6975^2 = 0.4865
• 0.3375^2 = 0.1139
• 0.1725^2 = 0.0298
• 0.08625^2 = 0.0074

Sum ≈ 2.5719

Assessment:

• Aggregate voltage THD is negligible (≈0.7%) due to strong supply impedance and distributed loads.
• Even during peak occupancy, the building is unlikely to require harmonic mitigation for voltage distortion alone.
• If the same loads were connected to a weaker feeder with |Zs| = 0.2 Ω, THD would scale to ≈2.8% — still modest but worth checking local limits and equipment immunity.

Uncertainty, sensitivity, and conservative practices

Screening calculations are sensitive to two primary inputs: harmonic current magnitudes and source impedance. To ensure safe decisions:

• Perform sensitivity runs using a range of |Zs| (e.g., base, weak, very weak grid) to bound THD results.
• Run scenarios with higher harmonic percentages to account for worst-case behavior (e.g., aggregated operating modes).
• When phase imbalance, resonance, or interactions between multiple nonlinear loads are possible, escalate to detailed harmonic power flow or time-domain simulation.

Screening thresholds and triggers for detailed study

Use these practical triggers to decide when to perform full frequency-domain or time-domain analysis:

• Estimated voltage THD > 5%: recommend detailed study and consider mitigation options.
• Estimated voltage THD between 2% and 5%: monitor and evaluate cumulative conditions; consider spot measurements.
• Estimated voltage THD < 2%: low priority for immediate mitigation, but verify if sensitive equipment is present.
• Presence of interharmonics, flicker, or significant odd/even harmonic asymmetry: escalate to specialized analysis regardless of THD.

Mitigation options summary for screening results

• Passive filters: tuned or broadband; effective if resonance risks are assessed.
• Active harmonic filters: provide dynamic compensation and reduce multiple harmonics simultaneously.
• Phase-shifting transformers, multi-pulse rectifiers: reduce characteristic low-order harmonics (e.g., 6-pulse vs 12-pulse).
• Distributed mitigation: apply smaller filters across multiple loads rather than a single large filter at PCC.
• Increase short-circuit strength (reduce |Zs|): utility/system-level action where feasible.

Standards, normative guidance and authoritative references

Screening and limits for harmonic distortion should reference internationally recognized standards and guidance. Key documents and resources:

• IEEE Std 519-2014 — IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems. Authority on harmonic limits at PCC and generator buses. https://standards.ieee.org/standard/519-2014.html
• IEC 61000-3-6 — Electromagnetic compatibility (EMC) — Limits for harmonic emissions to the public low-voltage power supply system. https://webstore.iec.ch/publication/5962
• EN 50160 — Voltage characteristics of electricity supplied by public distribution systems. https://standards.cencenelec.eu/dyn/www/f?p=204:110:0::::FSP_PROJECT:17147&cs=1E2D3F9A1C4A2D6
• NIST Guide to Industrial Power Quality and Harmonics — Practical guidance and measurement considerations. https://www.nist.gov/
• Manufacturer application notes for VFDs, UPS, and LED drivers — source-specific harmonic spectra and mitigation recommendations (consult vendor technical literature).

Practical measurement tips for validating screening estimates

1. Use a true-RMS power quality analyzer with harmonic capture up to at least the 50th harmonic for 50 Hz systems (or higher if needed).
2. Record both current and voltage harmonics at PCC to assess actual coupling and verify Zs-derived estimates.
3. Measure under representative operating conditions: peak, average, and light-load periods to understand variability.
4. Ensure proper connection and calibration of probes; harmonic current clamps and voltage probes must be accurate at higher frequencies.

Common pitfalls

• Underestimating source impedance leads to optimistic THD results; always test a conservative range.
• Ignoring resonance: passive filters or network impedances can create amplification at particular harmonic orders.
• Using only THD as a metric: specific harmonic orders (e.g., 3rd, 5th, 7th) and interharmonics can have disproportionate impacts despite acceptable THD.
• Neglecting system unbalance: single-phase concentrated nonlinear loads can cause negative- and zero-sequence harmonics with different network effects.

Implementation considerations for a quick online calculator

Key inputs to include in a practical web or spreadsheet-based calculator:

• System voltage V1 (phase or line basis)
• Load fundamental current I1 or load rating
• Choice of load type(s) with default harmonic spectra (editable)
• Source impedance entry options:
  • Direct Zs magnitude per harmonic (Ω)
  • % impedance or short-circuit MVA entry to compute Zs
  • Preset grid strength options (weak/typical/strong)
• Ability to compute per-harmonic Vh, overall THD_V, and sensitivity analysis for multiple |Zs| values

Recommended workflow for engineers using the screening calculator

1. Start with conservative assumptions for |Zs| to capture worst reasonable scenario.
2. Use vendor-supplied harmonic data when available; otherwise use representative templates.
3. Perform sensitivity sweeps over |Zs| and load aggregation levels.
4. If screening suggests THD_V exceeds threshold, schedule broadband harmonic measurements and a detailed harmonic study.
5. Document assumptions and results for traceability and discussion with utilities or stakeholders.

Summary of recommended threshold values for screening decisions

Final technical recommendations and next steps

• Use the screening calculator as a first-line triage tool to prioritize measurement and mitigation activities.
• Always cross-check screening outputs with at least one field measurement if mitigation or contractual limits are implicated.
• For critical installations (data centers, hospitals, sensitive manufacturing), adopt conservative thresholds and plan detailed harmonic studies early.
• Maintain records of measured harmonic spectra, grid impedance estimates, and mitigation actions for compliance and long-term planning.

References and further reading: IEEE Std 519-2014; IEC 61000 series on EMC; EN 50160; vendor application notes for specific load types; NIST power quality resources.