Harmonic study based on IEEE and UK G5/5

Performing a harmonic study to demonstrate compliance with utility standards is a critical technical process for connecting modern power systems to the grid. With the proliferation of inverter-based resources and non-linear loads, utilities now strictly enforce standards like IEEE 519 or UK G5/5 to ensure power quality and system stability.

Below is a technical guide that outlines the systematic methodology for conducting a harmonic study, based on current international standards.

1. Foundational Concepts and Regulatory Framework

Before beginning the calculations, it is essential to understand the governing standards and the physical phenomena at play.

1.1 Understanding Harmonic Distortion

Harmonics are sinusoidal voltages or currents with frequencies that are integer multiples of the fundamental frequency (por ejemplo, 60 Hz o 50 Hz) . They are generated by non-linear loads such as variable frequency drives, LED lighting, and inverters. These distortions can lead to equipment overheating, transformer losses, and protection mal-operation .

1.2 Applicable Standards

The choice of standard depends on your geographic location and utility requirements:

IEEE Std 519-2014: Used primarily in North America, this standard sets the quality of power at the Point of Common Coupling (PCC). It limits both the Individual Harmonic Distortion (IHD) and the Total Harmonic Distortion (THD) of voltage and current .
Engineering Recommendation G5/5: Mandatory in the UK, this standard requires a more stringent assessment of both incremental (your plant’s contribution) and total (background + incremental) harmonic voltages, often evaluated up to the 100th harmonic .

2. Data Acquisition and System Modeling

The accuracy of a harmonic study is entirely dependent on the quality of input data.

2.1 Utility Data (Point of Common Coupling)

You must obtain the following from the Distribution Network Operator (DNO) or utility:

Background Harmonic Voltages: Typically based on two weeks of measured data at the PCC to capture normal operating variations .
Harmonic Impedance Loci: This describes how the network impedance varies with frequency. It is critical for identifying potential resonance. The preferred format is ungrouped envelopes for each harmonic order .
Grid Strength: Usually expressed as the Short Circuit Ratio (SCR) at the PCC.

2.2 Equipment Data (OEM)

The Original Equipment Manufacturer (OEM) must provide a Norton Equivalent Model of the plant (inverter, drive, etc.). This model, essential for accurate simulation, consists of two parts for each frequency :

Norton Current Source: The magnitude and phase angle of the harmonic current injected by the device.
Norton Impedance: The internal impedance of the device, which affects how it interacts with grid resonances.

Fundamental (50 Hz) Armonía Distorted waveform

3rd 12%

5ª 20%

7ª 10%

11ª 5%

13ª 4%

THD = 23.1% Crest factor = 1.38 Peak = 1.32 podría

Figura 1: Harmonics Example (Interactive)

Distorted waveform resulting from the combination of fundamental and harmonic frequencies (Fully interactive).

3. Step-by-Step Harmonic Analysis Methodology

Once the data is gathered, the study proceeds through a series of analytical steps using specialized software (por ejemplo, ETAP, DIgSILENT PowerFactory).

3.1 Frequency Scan Analysis

The first step is a frequency sweep to identify resonant conditions. The software injects a current of varying frequency and measures the impedance.

Objetivo: Identify parallel (high impedance) and series (low impedance) resonance points.
Risk: If a resonance peak aligns with a characteristic harmonic frequency (por ejemplo, 5ª, 7ª, 11ª), harmonic voltages will be amplified, leading to high distortion .

Impedance |La| (Z) Inductive ωL Capacitive 1/ωC Resonance (300 Hz)

F₁ = 60 Hz — fundamental F_res = 300 Hz — 5th harmonic order La_pico ≈ 38 Z — parallel resonance Q ≈ 8 — quality factor

Figura 2: Harmonics Resonance at the 5th harmonic (example)

Impedance vs. Frequency plot showing a parallel resonance peak.

Using the Norton model of the plant and the grid impedance, calculate the voltage distortion caused only by your new equipment.

3.2 Calculate Incremental Harmonic Voltages

F la r m usted l un : {En}_{h} = {Yo}_{h} \times {La}_{h}

Formula: V_{h} = I_{h} \times Z_{h}

Donde: En_h is the harmonic voltage, Yo_h is the harmonic current, y La_h is the grid impedance at harmonic order h.

Compliance Check: This value must be below the "Incremental Limits" set by the utility or standard (por ejemplo, G5/5 Stage 1 límites) .

3.3 Calculate Total Harmonic Distortion

This combines the incremental contribution with pre-existing background distortion.

Total Voltage Harmonic Distortion (THDv): The root mean square (RMS) of all harmonic voltages, expressed as a percentage of the fundamental voltage.
Compliance Check: The THDv and individual harmonic voltages must remain below the "Total Limits" (por ejemplo, IEEE 519 limits for voltage quality or G5/5 Planning Levels) .

System: 50 Hz 60 Hz Voltage level (IEEE 519):

Measured IHD (%En) IEEE 519 limit G5/5 planning level

THDv = - IEEE 519 THD limit = - Status: -

Figura 3: Voltage Harmonics (Interactive)

Bar chart representation of individual harmonic voltages (VN) as a percentage of the fundamental .

G5/5 and similar standards require checking that the new connection does not negatively affect neighboring customers. This involves simulating the impact at adjacent substations or sensitive locations (hospitals, data centers) .

Plant — Norton model (OEM) Utilidad / grid Point of common coupling Iₙ(h) Norton corriente Zₙ(h) Norton impedancia Iₕ → Vₕ Zg(h) Cuadrícula impedancia Vₕ = Iₙ(h) · Zg(h) · Zₙ(h) / ( Zₙ(h) + Zg(h) ) Simplified when Zₙ >> Zg : Vₕ ≈ Iₙ(h) · Zg(h)

Figura 4: Harmonics Impedance Loci

Impedance loci plotted on the R-X plane, showing how impedance changes with frequency [citation:4].

The final step is to compile the results into a formal report for the utility.

4. Compliance Verification and Reporting

4.1 Comparison with Limits

Create a summary table comparing the calculated values against the standard limits. For IEEE 519, this involves checking:

IEEE 519 Mesa 1: Voltage distortion limits at the PCC.
IEEE 519 Mesa 2: Current distortion limits based on the I_{SC}/I_L ratio (Short circuit current vs. Load current) .

4.2 Mitigation Strategies

If limits are exceeded, the study must propose solutions:

Passive Filters: Tuned to shunt specific harmonic frequencies.
Los filtros activos de armónicos: Inject opposing currents to cancel harmonics.
Impedance Modification: Changing transformer connections or adding reactors to detune the system .

4.3 The Three-Step Verification Guideline

As outlined in recent IEEE guidelines, compliance can be summarized in a three-step process :

1. Measure data at the Point of Interconnection (POI).
2. Perform a statistical evaluation on the data.
3. Compare the results to the correct IEEE limits.